LAYERED ELECTRODE WITH HIGH RATE TOP LAYER

FIELD

This disclosure relates to systems and methods for electrochemical cells. More specifically, the disclosed embodiments relate to multilayered electrodes for electrochemical cells.

INTRODUCTION

Environmentally friendly sources of energy have become increasingly critical, as fossil fuel-dependency becomes less desirable. Most non-fossil fuel energy sources, such as solar power, wind, and the like, require some sort of energy storage component to maximize usefulness. Accordingly, battery technology has become an important aspect of the future of energy production and distribution. Most pertinent to the present disclosure, the demand for secondary (i.e., rechargeable) batteries has increased.

Various combinations of electrode materials and electrolytes are used in these types of batteries, such as lead acid, nickel cadmium (NiCad), nickel metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).

SUMMARY

The present disclosure provides systems, apparatuses, and methods relating to electrochemical cells having one or more multilayered electrodes.

In some embodiments, a multilayered electrode may include: a current collector substrate; and an active material composite layered onto the substrate, wherein the active material composite comprises: a first layer adjacent the current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and a second layer adjacent the first layer and including a plurality of second active material particles configured to have a second solid state diffusivity; wherein the first solid state diffusivity is less than the second solid state diffusivity.

In some embodiments, an electrochemical cell having one or more multilayered electrodes may include: a first electrode separated from a second electrode by a liquid-permeable separator; and an electrolyte disposed generally throughout the first and second electrodes; the first electrode comprising a first current collector substrate and a first active material composite layered onto the first current collector substrate, wherein the first active material composite comprises: a first layer adjacent the first current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and a second layer adjacent the liquid-permeable separator and including a plurality of second active material particles configured to have a second solid state diffusivity; the second electrode comprising a second current collector substrate and a second active material composite layered onto the second current collector substrate, wherein the second active material composite comprises: a third layer adjacent the second current collector substrate and including a plurality of third active material particles configured to have a third solid state diffusivity; and a fourth layer adjacent the liquid-permeable separator and including a plurality of fourth active material particles configured to have a fourth solid state diffusivity; wherein the first solid state diffusivity is lower than the second solid state diffusivity; and wherein the third solid state diffusivity is higher than the fourth solid state diffusivity.

Features, functions, and advantages may be achieved independently in various embodiments of the present disclosure, or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an illustrative electrochemical cell.

FIG. 2 is a schematic sectional view of a portion of an electrochemical cell having a first illustrative multilayered electrode, depicted accepting lithium ions in a lithiation process.

FIG. 3 is a schematic sectional view of a portion of an electrochemical cell having a second illustrative multilayered electrode, depicted releasing lithium ions in a delithiation process.

FIG. 4 is a schematic sectional view of an illustrative electrochemical cell having one multilayered electrode and one homogeneous electrode, in accordance with aspects of the present disclosure.

FIG. 5 is a schematic sectional view of another illustrative electrochemical cell having one multilayered electrode and one homogeneous electrode, in accordance with aspects of the present disclosure.

FIG. 6 is a schematic sectional view of another illustrative electrochemical cell having one multilayered electrode and one homogeneous electrode, in accordance with aspects of the present disclosure.

FIG. 7 is a schematic sectional view of another illustrative electrochemical cell having one multilayered electrode and one homogeneous electrode, in accordance with aspects of the present disclosure.

FIG. 8 is a schematic sectional view of an illustrative electrochemical cell having two multilayered electrodes, in accordance with aspects of the present disclosure.

FIG. 9 is a schematic sectional view of another illustrative electrochemical cell having two multilayered electrodes, in accordance with aspects of the present disclosure.

FIG. 10 is a schematic sectional view of another illustrative electrochemical cell having two multilayered electrodes, in accordance with aspects of the present disclosure.

FIG. 11 is a schematic sectional view of another illustrative electrochemical cell having two multilayered electrodes, in accordance with aspects of the present disclosure.

FIG. 12 is an illustrative sectional view of another illustrative electrochemical cell having two multilayered electrodes, in accordance with aspects of the present disclosure.

FIG. 13 is a flow chart depicting steps of an illustrative method for manufacturing electrodes and electrochemical cells of the present disclosure.

FIG. 14 is a schematic diagram of an illustrative manufacturing system suitable for carrying out steps of the manufacturing method of FIG. 13.

DETAILED DESCRIPTION

Various aspects and examples of electrochemical cells having multilayer electrodes, as well as related methods, are described below and illustrated in the associated drawings. Unless otherwise specified, an electrochemical cell in accordance with the present teachings, and/or its various components, may contain at least one of the structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein. Furthermore, unless specifically excluded, the process steps, structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein in connection with the present teachings may be included in other similar devices and methods, including being interchangeable between disclosed embodiments. The following description of various examples is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. Additionally, the advantages provided by the examples and embodiments described below are illustrative in nature and not all examples and embodiments provide the same advantages or the same degree of advantages.

This Detailed Description includes the following sections, which follow immediately below: (1) Definitions; (2) Overview; (3) Examples, Components, and Alternatives; (4) Advantages, Features, and Benefits; and (5) Conclusion. The Examples, Components, and Alternatives section is further divided into subsections A through D, each of which is labeled accordingly.

Definitions

The following definitions apply herein, unless otherwise indicated.

“Comprising,” “including,” and “having” (and conjugations thereof) are used interchangeably to mean including but not necessarily limited to, and are open-ended terms not intended to exclude additional, unrecited elements or method steps.

Terms such as “first”, “second”, and “third” are used to distinguish or identify various members of a group, or the like, and are not intended to show serial or numerical limitation.

“AKA” means “also known as,” and may be used to indicate an alternative or corresponding term for a given element or elements.

“Coupled” means connected, either permanently or releasably, whether directly or indirectly through intervening components.

“Providing,” in the context of a method, may include receiving, obtaining, purchasing, manufacturing, generating, processing, preprocessing, and/or the like, such that the object or material provided is in a state and configuration for other steps to be carried out.

“Active material fraction” means the mass of active material divided by the total mass of an electrode (or a cell).

“Active volume fraction” means the volume of active material divided by the total volume of an electrode (or a cell).

“NCA” means Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO₂).

“NMC” or “NCM” means Lithium Nickel Cobalt Manganese Oxide (LiNiCoMnO₂).

“LFP” means Lithium Iron Phosphate (LiFePO₄).

“LMO” means Lithium Manganese Oxide (LiMn₂O₄).

“LNMO” means Lithium Nickel Manganese Spinel (LiNi_0.5Mn_1.5O₄).

“LCO” means Lithium Cobalt Oxide (LiCoO₂).

“LTO” means Lithium Titanate (Li₂TiO₃).

“NMO” means Lithium Nickel Manganese Oxide (Li(Ni_0.5Mn_0.5)O₂).

“Li” means lithium.

“Li+” or “Li-ion” means lithium ion.

“SSD” means solid-state diffusivity. In some examples, the solid-state diffusivity of a material may refer to a solid-state diffusivity per micron or a solid-state diffusivity per radial micron. In some examples, the solid-state diffusivity of a particle may refer to a bulk ability to accept lithium. For example, a particle having a high solid-state diffusivity may have a lower time to fully lithiate than a particle having a low solid-state diffusivity.

“High nickel content cathodes” are cathodes having a stoichiometric nickel percentage greater than or equal to 80%. In contrast, “low nickel content cathodes” are cathodes having a stoichiometric nickel percentage less than 70%.

“Single crystal” materials are monocrystalline particles having long-range order in their atomic structure within the entire bulk of the material particle. In contrast, “polycrystalline” material particles comprise a plurality of monocrystalline “grains”, each roughly ˜1 μm or less in size, that together make up a particle including “grain boundaries” disposed between grains.

Overview

In general, electrodes and electrochemical cells including electrodes according to the current disclosure may include two or more layers, each layer having tailored material properties, to achieve a desired polarization profile within the electrode. Electrochemical cells according to the present disclosure may include bipolar electrochemical cells such as batteries, redox supercapacitors, and/or the like. Electrodes according the present disclosure may be suitable for use in a lithium ion battery cell.

Electrodes according to the current disclosure may be structured to have multiple layers containing selected active materials having different solid-state diffusion coefficients, different lithiation energies, and/or different particle sizes. Including two or more layers having different material properties may improve rate capabilities of the entire electrode bulk upon lithiation or delithiation.

Gradient fields (e.g., concentration gradients within the electrolyte) naturally form in non-equilibrium conditions, such as upon charging or discharging of a cell. Gradient fields are especially likely to form when such charging or discharging is conducted at increased rates (e.g., during fast charging). These gradient fields result in polarization within the cell. While this is an inevitable phenomenon in any Li-ion battery, excessive polarization is detrimental to performance. For example, excessive polarization can lead to low utilization of battery capacity before threshold cutoff voltages are reached. After the threshold cutoff voltages are reached, a cell is charged in a slow, constant-voltage phase with an exponentially decreasing current, reducing charge rates. In some examples, excessive polarization can result in unwanted lithium plating reactions on the anode surface. This type of plating severely impairs performance and poses a safety risk.

Accordingly, to counteract the natural gradient fields that would otherwise form in traditional battery electrodes, electrodes of the present disclosure comprise active materials spatially oriented within the thickness of an electrode bulk in a strategic manner. Active materials may be oriented based on solid-state diffusivity, free energy to delithiate, material particle size, and/or other suitable factors which may affect electrode polarization. Electrodes and electrochemical cells according to the present disclosure may be configured to achieve fast charging rates when compared with conventional devices.

In some examples, an electrode may include a first layer including first active materials having lower solid-state diffusivity coefficients situated closer to the current collector, and a second layer including second active materials having higher solid-state diffusivity coefficients situated closer to the separator. This arrangement may enable the second layer to partially or completely delithiate prior to delithiation of the first layer, improving electrode delithiation rates.

One such example is a cathode including a first active material having a lower solid-state diffusivity (SSD) situated closer to the current collector, and a second active material having a higher solid-state diffusivity situated closer to the separator. In other words, the SSD of the first active material is lower than the SSD of the second active material. In this example, lithium ions may preferentially delithiate in areas of the cathode closest to the separator, thereby improving charge rates. This configuration may mitigate the oversaturation of lithium ions within pores of the cathode electrode bulk, which may otherwise cause polarization and premature arrival at the threshold cutoff voltage (typically 4.2V). This configuration may further avoid charge repulsion between lithium ions within the cathode, as the second layer may preferentially delithiate before the first layer.

Other ways of improving delithiation rate capabilities of an electrode (e.g., in a fast-charging cathode) include situating active materials having smaller particle sizes closer to the separator so as to counteract the naturally forming gradient fields.

In some examples, an electrode may include a first layer including first active materials having higher solid-state diffusivity coefficients situated closer to the current collector, and a second layer including second active materials having lower solid-state diffusivity coefficients situated closer to the separator. This arrangement may enable the first layer to preferentially lithiate before the second layer, reducing lithium plating and increasing electrode lithiation rates.

One such example is an anode including a first layer including a first active material having a higher solid-state diffusivity situated closer to the current collector, and a second layer including a second active material having a lower solid-state diffusivity situated closer to the separator. In other words, the SSD of the first active material is higher than the SSD of the second active material. In this example, lithium ions may preferentially lithiate (e.g., intercalate or alloy) in areas of the anode closest to the current collector, which may reduce anode polarization and lithium plating. In some examples, an electrode may have first active materials that require less energy to lithiate situated closer to the current collector, and second active materials that require more energy to lithiate situated closer to the separator. This arrangement enables the electrode to lithiate in a “backfill” manner. In other words, the electrode has a reaction front that proceeds from the current collector toward the separator, as opposed to the opposite way around (in non-optimized electrodes).

One such example is an anode that has a first active material having a higher lithiation voltage (with respect to Li/Li+) situated closer to the current collector and a second active material having a lower lithiation voltage (with respect to Li/Li+) situated closer to the separator. In this example, the anode is optimized for improved lithiation properties (e.g., upon charging of the Li-ion cell) to accept Li-ions at an increased charging rate.

Similarly, an illustrative cathode comprises a first active material with a lower lithiation voltage (with respect to Li/Li+) disposed closer to the current collector, and a second active material with a higher lithiation voltage (with respect to Li/Li+) situated closer to the separator. In this example, the cathode is optimized for improved lithiation properties (e.g., upon discharging of the Li-ion cell) to accept Li-ions at an increased discharge rate.

Other ways of improving lithiation rate capabilities of an electrode may include situating active materials having smaller particle sizes closer to the current collector so as to counteract the naturally forming gradient fields.

An electrode having multiple layers may have regions of lower and higher lithium ion accepting capability, such that the overall electrode has an increased lithium-accepting capability as compared with a homogeneous electrode of an equivalent loading, thickness, and/or chemistry. Additionally, an electrode having multiple layers may have regions of lower and higher lithium donating capability, such that the overall electrode has increased lithium-donating capability as compared with a homogeneous electrode of an equivalent loading, thickness, and/or chemistry. By having an electrode with multiple layers in an electrochemical cell, the cell may exhibit increased power density on charging or discharging depending on which electrode(s) (i.e., cathode, anode, or both) feature multiple layers, and depending on how the multiple layers in the electrodes are configured.

Layers within electrodes may be differentiated using one or more of several methods. The first two are based on the active materials utilized. The third is based on the particle sizes of those active materials. First, each layer may have a different solid state diffusion coefficient. Second, each layer may have a different energy of lithiation or delithiation. Third, each layer may have a different distribution of particle sizes.

In some examples, an electrode has first active materials that have a comparatively higher energy density and a comparatively lower stability situated closer to the current collector, and second active materials that have a comparatively lower energy density and a comparatively higher stability situated closer to the separator.

One such example is a cathode that has a first active material having a higher nickel percentage situated closer to the current collector and a second active material having a lower nickel percentage situated closer to the separator, wherein each nickel percentage is a stoichiometric ratio between nickel and a total of transition metal elements included in the active material particles. The first nickel percentage may be configured to be greater than the second nickel percentage. In some examples, the first nickel percentage may be configured to be at least 80%. In some examples, the second nickel percentage may be less than 70%. In some examples, the active particles may comprise NCA-type materials, which include transition metal oxides having nickel, cobalt, and aluminum as their primary transition metal elements. In some examples, the active particles may comprise NMC, which is a transition metal oxide having nickel, manganese, and cobalt as its primary transition metal elements.

In some examples, active material particles of the first active material and the second active material each have a tailored crystalline structure. The first active material particles may be polycrystalline, with each active material particle comprising a plurality of monocrystalline grains. The second active material particles may be monocrystalline, with each active material particle comprising a single crystal. In some examples, monocrystalline active materials have a higher solid-state diffusivity than polycrystalline active materials. In some examples, the second active material particles may be polycrystalline.

An electrode may have a thickness defined as the distance along a direction perpendicular to the plane of a current collector to which the electrode is coupled, measured from the current collector to an opposing major surface of the electrode. The opposing major surface (AKA the “upper” surface) may be substantially planar. This upper surface of the electrode may mate with a separator, a gel electrolyte, or a solid electrolyte when the electrode is included in a cell. In some examples, an electrode described herein and having multiple layers may have a thickness between approximately 10 μm and approximately 200 μm. Each layer of an electrode may also have a thickness, defined in the same direction as that of the electrode and measured between opposing faces of the layer.

In some examples, an electrochemical cell according to the present disclosure may include a first electrode and a second electrode, with a separator disposed between the two electrodes. In some examples, the first electrode may be a cathode and the second electrode may be an anode. In some examples, one or both electrodes may include multiple layers including selected active materials having different solid-state diffusivity coefficients, different lithiation energies, and/or different particle sizes.

In some examples, the electrochemical cell may include electrodes selected to produce a fast charging cell. The electrochemical cell may therefore include a cathode configured to delithiate quickly and an anode configured to lithiate quickly.

EXAMPLES, COMPONENTS, AND ALTERNATIVES

The following sections describe selected aspects of illustrative electrodes and electrochemical cells, as well as related systems and/or methods. The examples in these sections are intended for illustration and should not be interpreted as limiting the scope of the present disclosure. Each section may include one or more distinct embodiments or examples, as well as contextual or related information, function, and/or structure.

A. Illustrative Electrodes and Electrochemical Cells

As shown in FIGS. 1-3, this section describes illustrative electrodes and electrochemical cells in accordance with aspects of the present disclosure. FIG. 1 is a schematic sectional diagram of an illustrative electrochemical cell, and FIGS. 2 and 3 are schematic sectional diagrams of two different types of illustrative multilayer electrodes suitable for use in an electrochemical cell.

Referring now to FIG. 1, an electrochemical cell 100 is illustrated in the form of a lithium-ion battery. Electrochemical cell 100 includes a positive and a negative electrode, namely a cathode 102 and an anode 104. The cathode and anode are sandwiched between a pair of current collectors 106, 108, which may comprise metal foils or other suitable substrates. Current collector 106 is electrically coupled to cathode 102, and current collector 108 is electrically coupled to anode 104. The current collectors enable the flow of electrons, and thereby electrical current, into and out of each electrode. An electrolyte 110 disposed throughout the electrodes enables the transport of ions between cathode 102 and anode 104. In the present example, electrolyte 110 includes a liquid solvent and a solute of dissolved ions. Electrolyte 110 facilitates an ionic connection between cathode 102 and anode 104.

Electrolyte 110 is assisted by a separator 112, which physically partitions the space between cathode 102 and anode 104. Separator 112 is liquid permeable, and enables the movement (i.e., flow) of ions within electrolyte 110 and between each of the electrodes. In some embodiments, electrolyte 110 includes a polymer gel or solid ion conductor, augmenting or replacing (and performing the function of) separator 112.

Cathode 102 and anode 104 are composite structures, which comprise active material particles, binders, conductive additives, and pores (i.e., void space) into which electrolyte 110 may penetrate. An arrangement of the constituent parts of an electrode is referred to as a microstructure, or more specifically, an electrode microstructure.

In some examples, the binder is a polymer, e.g., polyvinylidene difluoride (PVdF), and the conductive additive typically includes a nanometer-sized carbon, e.g., carbon black or graphite. In some examples, the binder is a mixture of carboxyl-methyl cellulose (CMC) and styrene-butadiene rubber (SBR). In some examples, the conductive additive includes a ketjen black, a graphitic carbon, a low dimensional carbon (e.g., carbon nanotubes), and/or a carbon fiber.

In some examples, the chemistry of the active material particles differs between cathode 102 and anode 104. For example, anode 104 may include graphite (artificial or natural), hard carbon, titanate, titania, transition metals in general, elements in group 14 (e.g., carbon, silicon, tin, germanium, etc.), oxides, sulfides, transition metals, halides, and/or chalcogenides. On the other hand, cathode 102 may include transition metals (for example, nickel, cobalt, manganese, copper, zinc, vanadium, chromium, iron), and their oxides, phosphates, phosphites, and/or silicates. In some examples, the cathode may include alkalines and alkaline earth metals, aluminum, aluminum oxides and aluminum phosphates, halides and/or chalcogenides. In an electrochemical device, active materials participate in an electrochemical reaction or process with a working ion to store or release energy. For example, in a lithium-ion battery, the working ions are lithium ions.

Electrochemical cell 100 may include packaging (not shown). For example, packaging (e.g., a prismatic can, stainless steel tube, polymer pouch, etc.) may be utilized to constrain and position cathode 102, anode 104, current collectors 106 and 108, electrolyte 110, and separator 112.

For electrochemical cell 100 to properly function as a secondary battery, active material particles in both cathode 102 and anode 104 must be capable of storing and releasing lithium ions through the respective processes known as lithiating and delithiating. Some active materials (e.g., layered oxide materials or graphitic carbon) fulfill this function by intercalating lithium ions between crystal layers. Other active materials may have alternative lithiating and delithiating mechanisms (e.g., alloying, conversion).

When electrochemical cell 100 is being charged, anode 104 accepts lithium ions while cathode 102 donates lithium ions. When a cell is being discharged, anode 104 donates lithium ions while cathode 102 accepts lithium ions. Each composite electrode (i.e., cathode 102 and anode 104) has a rate at which it donates or accepts lithium ions that depends upon properties extrinsic to the electrode (e.g., the current passed through each electrode, the conductivity of the electrolyte 110) as well as properties intrinsic to the electrode (e.g., the solid state diffusion constant of the active material particles in the electrode; the electrode microstructure or tortuosity; the charge transfer rate at which lithium ions move from being solvated in the electrolyte to being intercalated in the active material particles of the electrode; etc.).

During either mode of operation (charging or discharging) anode 104 or cathode 102 may donate or accept lithium ions at a limiting rate, where rate is defined as lithium ions per unit time, per unit current. For example, during charging, anode 104 may accept lithium at a first rate, and cathode 102 may donate lithium at a second rate. When the second rate is lesser than the first rate, the second rate of the cathode would be a limiting rate. In some examples, the differences in rates may be so dramatic as to limit the overall performance of the lithium-ion battery (e.g., cell 100). Reasons for the differences in rates may depend on a solid state diffusion coefficient of lithium ions in an active material particle; an energy required to lithiate or delithiate a quantity of lithium-ions per mass of active material particles; and/or a particle size distribution of active material within a composite electrode. In some examples, additional or alternative factors may contribute to the electrode microstructure and affect these rates.

Turning to FIG. 2, a schematic sectional view of a portion of an electrochemical cell 200 is depicted. Cell 200 has a multilayered electrode 202, shown accepting lithium ions 220 and 222 during a lithiation process. Cell 200 is an example of electrochemical cell 100 of FIG. 1, and includes a separator 212, an electrolyte 210, and a current collector 206. Electrode 202 may be a cathode or an anode, and includes a first layer 230 and a second layer 232. First layer 230 is adjacent current collector 206; second layer 232 is located adjacent (intermediate) the first layer and separator 212. For consistency, all examples of the present disclosure follow a similar convention, where the “first” layer is defined adjacent the current collector and the “second” layer is defined adjacent the separator. First layer 230 and second layer 232 may each be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 206.

In the present example, electrode 202 is depicted as accepting lithium, for example under a constant potential or constant current, such that lithium ions 220 and 222 are induced to react (e.g., intercalate) with active material present within first layer 230 and second layer 232. Lithium ions 220 and 222 migrate toward current collector 206 under diffusive and electric field effects. In this example, ion 220 follows a path 224 within electrolyte 210, through separator 212, second layer 232, and a portion of first layer 230, until it lithiates an active material particle within first layer 230. In contrast, lithium ion 222 follows a path 226 within electrolyte 210, through separator 212 and a portion of second layer 232, until it lithiates an active material particle within second layer 232.

In general, path 224 of the ion traveling through the separator to active material within the first layer will be longer than path 226 of the ion traveling through the separator to active material within the second layer. Additionally, the ion on path 224 travels a longer distance while in second layer 232 than does the ion on path 226.

In a standard electrode, one consequence of the disparity in path lengths 224 and 226 is that a residence time in the second layer is likely to be greater than a residence time in the first layer for a given lithium ion. Another consequence of the disparity in path lengths 224 and 226 is that a lithium ion entering electrode 202 is more likely to react with an active material particle within second layer 232 than first layer 230. Accordingly, a gradient reaction field may be generated in such electrodes, which may negatively impact cell performance by: (1) a polarization overpotential in electrolyte 210 leading to parasitic energy losses within the electrochemical cell; and (2) underutilization of active material of first layer 230 compared to the active material of second layer 232 (causing, e.g., lower apparent lithium-ion battery capacity and/or longer time to compete acceptance of lithium by electrode 202 at lower power).

However, in the present example, the disparity in path lengths and resulting gradient reaction field is at least partially mitigated by electrode 202 having a first active material included in first layer 230 and a second active material included in second layer 232. The first active material is configured to be different from the second active material, such that at least one of the following is true:

(i) the first active material is chemically different from the second active material and a free energy per mole to lithiate (AKA free energy to lithiate, or FEL) the first active material is lower than a free energy to lithiate the second active material;

(ii) a solid state diffusion (SSD) coefficient of the first active material is greater than a solid state diffusion coefficient of the second active material; and/or

(iii) a particle size distribution of the first active material includes particles that are substantially smaller than a particle size distribution of the second active material.

Where item (i) is true (i.e., lower FEL in the first layer), the longer path is mitigated by a sequential reaction timeline, where lithiation of first layer 230 preferentially commences at a time before lithiation of second layer 232. This arrangement enables the electrode to lithiate in a “backfill” manner. In other words, the electrode has a reaction front that proceeds from the current collector toward the separator, as opposed to the opposite way around (e.g., in electrodes having a single active material layer and/or an opposite layer configuration).

Furthermore, in examples where either or both of items (ii) (greater SSD coefficient in first layer) and/or (iii) (smaller particle size in first layer) is true, the advantage of the sequential reaction timeline is improved by increasing utilization of the active material of first layer 230 prior to onset of lithiation of the active material of second layer 232 (and thus prior to onset of the gradient reaction field with its associated disadvantages).

In this example, a thickness of second layer 232 is chosen to be equal to or less than a selected maximum thickness. The maximum thickness is determined by the microscopic architecture of second layer 232, i.e., active material particles with distinct shapes and sizes arranged in a particular way in three-dimensional space. The factors that describe this microscopic architecture include a distribution of the active material particle sizes, a porosity, and a tortuosity within the second layer. If second layer 232 has a thickness greater than the maximum thickness, transport through the second layer to the first layer may become so tortuous that the benefit of properties (i), (ii) and (iii) above are negated.

In examples where electrode 202 is an anode within the cell, lithiation of first layer 230 preferentially commences at a time before lithiation of second layer 232. This mitigates, at least in part, the gradient field and disparity in path lengths lithium ions must travel throughout electrode 202. A lithium ion battery having an anode with a layered configuration similar to that of electrode 202 is capable of exhibiting increased charge rate acceptance compared with a conventional lithium ion battery having a typical anode with a substantially homogeneous microstructure throughout its thickness. Such a battery is also capable of exhibiting increased charge rate acceptance compared with a lithium ion battery having an anode with a layered configuration inverse to that of electrode 202. The inverse configuration is also likely to have a charge acceptance performance inferior to a conventional lithium ion battery having a typical homogeneous anode microstructure. This anode design consideration is useful for designing a lithium ion battery capable of being charged at increased rates. Mitigating the onset of a significant gradient reaction field on the anode also helps prevent metallic lithium deposition (i.e., lithium plating) due to overpolarization upon charging.

Electrode 202 may be constructed as an anode using any suitable materials configured to produce an anode having a lower free energy to lithiate and/or a higher solid state diffusivity in the first layer than in the second layer. Recall that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises one or more of a hard carbon (or additional non-graphitic carbon), silicon monoxide, other silicon oxides, titanium dioxide, titanate, graphene, and/or an alloying material (e.g., tin, silicon, germanium, or the like), and the second active material of the second layer comprises graphitic carbons. In some examples, the first active material of the first layer comprises titanium dioxide or titanate, and the second active material of the second layer comprises one or more of a hard carbon (or additional non-graphitic carbon), graphitic carbons, silicon monoxide, other silicon oxides, graphene, and/or an alloying material (e.g., tin, silicon, germanium, or the like).

In some examples, an anode version of electrode 202 may be constructed using materials configured to produce an anode having a higher free energy to lithiate (FEL) in the second layer than in the first layer (e.g., to facilitate backfill lithiation). Recall again that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises silicon oxide(s) blended with a hard carbon (or additional non-graphitic carbon) or with graphitic carbons, and the second active material of the second layer comprises graphitic carbons. An anode including active material particles comprising silicon oxide(s) may have benefits over an anode including alternative active material particles, namely that lithiation of SiO₂involves electrochemical reduction of lithium ions and SiO₂into a variety of products, such as Li₂Si₂O₅, Li₄Si₄O₄, Li₂O, Si, Li_xSi, and/or the like, improving electrode capacity. Furthermore, inclusion of SiOx and graphitic carbon in a single electrode may exhibit differences in lithiation and/or delithiation energies between anode layers which may be greater than differences between two different carbonaceous materials included in a single electrode.

In examples where electrode 202 is a cathode within the cell, lithiation of first layer 230 commences at a time before lithiation of second layer 232. This mitigates, at least in part, the gradient field and disparity in path lengths lithium ions must travel throughout electrode 202. A lithium ion battery having a cathode with a layered configuration similar to that of electrode 202 is capable of exhibiting increased discharge rate capability compared with a conventional lithium ion battery having a typical cathode with a substantially homogeneous microstructure throughout its thickness. Such a battery is also capable of exhibiting increased discharge rate performance compared with a lithium ion battery having a cathode with a layered configuration inverse to that of the electrode 202. The inverse configuration is also likely to have a discharge rate performance inferior to a conventional lithium ion battery having a typical homogeneous cathode microstructure. This cathode design consideration is useful for designing a lithium ion battery capable of being discharged at increased rates.

Electrode 202 may be constructed as a cathode using any suitable materials configured to produce a cathode having a lower free energy to lithiate and/or a higher solid state diffusivity in the first layer than in the second layer. Again, recall that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises LFP, and the second active material of the second layer comprises one or more of NMC, NCA, LCO, and LMO. In some examples, the first active material of the first layer comprises one or more of NMC and NCA, and the second active material of the second layer comprises LMO and/or LCO. Turning now to FIG. 3, a schematic sectional view of a portion of an electrochemical cell 300 is depicted. Cell 300 has a multilayered electrode 302, shown donating lithium ions 320 and 322 during a delithiation process. Cell 300 is an example of electrochemical cell 100 of FIG. 1. The electrochemical cell includes a separator 312, an electrolyte 310, and a current collector 306. Electrode 302 may be a cathode or an anode, and includes a first layer 330, and a second layer 332. Per the convention described above, first layer 330 is adjacent to current collector 306, and second layer 332 is disposed adjacent (intermediate) the first layer and separator 312. First layer 330 and second layer 332 may each be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 306.

In the present example, electrode 302 is depicted donating lithium, for example under a constant potential or constant current, such that lithium ions 320 and 322 are induced to react (e.g., deintercalate) and are released from active material present within first layer 330 and second layer 332. Lithium ions 320 and 322 migrate toward separator 312 under diffusive and electric field effects. Lithium ion 320 is shown delithiated (released) from an active material particle within first layer 330, then following a path 324 within electrolyte 310 through a portion of first layer 330, second layer 332, and separator 312. In contrast, lithium ion 322 is shown delithiated from an active material particle within second layer 332, then following a path 326 within electrolyte 310 through a portion of second layer 332 and separator 312.

In general, path 324 of lithium ion 320 traveling from within first layer 330 to separator 312 will be longer than path 326 of lithium ion 322 traveling from within second layer 332 to separator 312. Furthermore, a first distance between the start of path 324 and the separator is greater than a second distance between the start of path 326 and the separator.

In a standard electrode, one consequence of these differences in paths 324 and 326 is that lithium ion 320 experiences charge repulsion effects from lithium ion 322, thereby inhibiting travel of lithium ion 320 to the separator, causing charge build-up within the electrode. Accordingly, a gradient reaction field may be generated, negatively impacting performance by: (1) a polarization overpotential in the electrolyte leading to parasitic energy losses within the cell; and (2) starvation of lithium ions in the electrolyte (causing, e.g., lower apparent lithium-ion battery capacity and/or longer time to complete the release of lithium by the electrode, at lower power).

However, in the present example, the disparity in path lengths and resulting gradient reaction field is at least partially mitigated by electrode 302 having a first active material included in first layer 330 and a second active material included in second layer 332. The first active material is configured to be different from the second active material, such that at least one of the following is true:

(i) the first active material is chemically different from the second active material and a free energy per mole to delithiate (AKA free energy to delithiate, or FED) the first active material is higher than a free energy to delithiate the second active material;

(ii) a solid state diffusion (SSD) coefficient of the second active material is greater than a solid state diffusion coefficient of the first active material; and/or

(iii) a particle size distribution of the first active material includes particles that are substantially larger than a particle size distribution of the second active material.

Where item (i) is true (i.e., greater FED in the first layer), the longer path (charge repulsion of lithium ions intermediate to the separator) is mitigated by a sequential reaction timeline where delithiation of the second layer commences at a time before delithiation of the first layer. Furthermore, in examples where either or both of items (ii) (greater SSD coefficient in second layer) and/or (iii) (i.e., smaller particle size in second layer) is true, the advantage of the sequential reaction timeline is improved by maximizing depletion of the active material of the second layer 332 prior to onset of delithiation of the active material of the first layer 330 (and thus prior to onset of the gradient reaction field with its associated disadvantages).

In this example, a thickness of second layer 332 is chosen to be equal to or less than a selected maximum thickness. The maximum thickness is determined by the microscopic architecture of second layer 332, i.e., active material particles with distinct shapes and sizes arranged in a particular way in three-dimensional space. The factors that describe this microscopic architecture include a distribution of the active material particle sizes, a porosity, and a tortuosity within the second layer. If second layer 332 has a thickness greater than the maximum thickness, transport through the second layer to the separator may become so tortuous that the benefit of properties (i), (ii) and (iii) above are negated.

In examples where electrode 302 is an anode within the cell, delithiation of second layer 332 commences at a time before delithiation of first layer 330. This mitigates, at least in part, the gradient field and disparity in path lengths lithium ions must travel throughout electrode 302. A lithium ion battery having an anode with a layered configuration similar to that of electrode 302 is capable of exhibiting increased discharge rate capability compared with a conventional lithium ion battery having a typical anode with a substantially homogeneous microstructure throughout its thickness. Such a battery is also capable of exhibiting increased discharge rate capability compared with a lithium ion battery having an anode with a layered configuration inverse to that of electrode 302. The inverse configuration is also likely to have a discharge rate performance inferior to a conventional lithium ion battery having a typical anode with a substantially homogeneous microstructure. This anode design consideration is useful for designing a lithium ion battery capable of being discharged at increased rates.

Electrode 302 may be constructed as an anode using any suitable materials configured to produce an anode having a higher free energy to delithiate and a lower solid state diffusivity in the first layer than in the second layer. Recall that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises graphitic carbons, and the second active material of the second layer comprises one or more of a hard carbon (or additional non-graphitic carbon), silicon monoxide, other silicon oxides, graphene, titanium dioxide, titanate, and/or an alloying material (e.g., tin, silicon, germanium, or the like). In some examples, the first active material of the first layer comprises one or more of a hard carbon (or additional non-graphitic carbon), graphitic carbons, silicon monoxide, other silicon oxides, and/or an alloying material (e.g., tin, silicon, germanium, or the like), and the second active material of the second layer comprises one or more of titanium dioxide or titanate.

In some examples, an anode version of electrode 302 may be constructed using materials configured to produce an anode having a lower free energy to lithiate (FEL) in the second layer than in the first layer (e.g., to facilitate prevention of lithium plating). Recall again that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises graphitic carbons, and the second active material of the second layer comprises silicon oxide(s) blended with a hard carbon (or additional non-graphitic carbon) or with graphitic carbons. An anode including active material particles comprising silicon oxide(s) may have benefits over an anode including alternative active material particles, namely that lithiation of SiO₂involves electrochemical reduction of lithium ions and SiO₂into a variety of products, such as Li₂Si₂O₅, Li₄Si₄O₄, Li₂O, Si, Li_xSi, and/or the like, improving electrode capacity. Furthermore, inclusion of SiOx and graphitic carbon in a single electrode may exhibit differences in lithiation and/or delithiation energies between anode layers which may be greater than differences between two different carbonaceous materials included in a single electrode.

In examples where electrode 302 is a cathode within the cell, delithiation of second layer 332 commences at a time before delithiation of first layer 330. This mitigates, at least in part, the gradient field, oversaturation of lithium ions within pores of the cathode electrode bulk, and disparity in path lengths lithium ions must travel throughout electrode 302. A lithium ion battery having a cathode with a layered configuration similar to that of electrode 302 is capable of exhibiting increased charge rate capability compared with a conventional lithium ion battery having a typical cathode with a substantially homogeneous microstructure throughout its thickness. Such a battery is also capable of exhibiting increased charge rate acceptance compared with a lithium ion battery having a cathode with a configuration inverse to that of electrode 302. The inverse configuration is also likely to have a charge rate performance inferior to a conventional lithium ion battery having a typical cathode with a substantially homogeneous microstructure. This cathode electrode design consideration is useful for designing a lithium ion battery capable of being charged at increased rates.

Electrode 302 may be constructed as a cathode using any suitable materials configured to produce a cathode having a lower solid state diffusivity and/or having a higher free energy to delithiate in the first layer than in the second layer. Again, recall that the first layer is defined adjacent the current collector and the second layer is defined adjacent the separator. In some examples, the first active material of the first layer comprises a transition metal oxide. In some examples, the second active material of the second layer comprises a transition metal oxide.

Different cathode active materials may provide different levels of stability and battery capacity, depending on properties of their transition metal elements. Cathode active materials may include “solid-solution” or structured composite materials including two or more elements, with each element providing specific structural and functional properties. In some examples, cathode active particles include nickel-containing transition metal oxides such as NCA-type materials, which are transition metal oxides having nickel, cobalt, and aluminum as their primary transition metal elements; NMC, which is a transition metal oxide having nickel, manganese, and cobalt as its primary transition metal elements; and/or any nickel-containing transition metal oxide suitable for inclusion in a cathode of an electrochemical cell.

Stoichiometric percentages of transition metals within the active particles may be tailored to produce active materials having desired properties. Stoichiometric percentages referred to below describe a percentage of a specified transition metal in a stoichiometric ratio between transition metal elements of the active particles. Typical cathodes having NCA-type active particles have a stoichiometric nickel percentage greater than or equal to 80%. NMC811 is a similar transition metal oxide suitable for use in cathodes, and has a stoichiometric ratio of approximately 80% nickel, 10% manganese, and 10% cobalt. High nickel content cathodes, such as those including NMC811 and NCA active particles including at least 80% nickel, have a high specific capacity when compared with cathodes having a stoichiometric nickel percentage less than 70% (AKA low nickel content cathodes). Lithiated transition metal oxide cathode materials generally include layered crystalline structures, which intercalate or deintercalate lithium in interplanar spaces between the layers. Increasing nickel content within a lithiated transition metal oxide generally results in increasing interstitial sites available for intercalation, which increases specific capacity of the cathode active material and therefore energy density of an electrochemical cell including the cathode.

However, high nickel content cathodes are more unstable than low nickel content cathodes, and may react with an electrolyte, especially at high temperatures and/or high states of charge (e.g., high degree of delithiation). High nickel content cathodes may also have poor cycle life performance due to side reactions with bulk electrolyte which may take place on cathode surfaces. Low nickel content cathodes have increased stability when compared with high nickel content cathodes, but may have low specific capacity.

The instability of high nickel content cathodes and cathode materials is due to the reactivity of nickel oxides. For example, in discharged NMC cathode material, nickel, manganese, and cobalt form a crystal structure with lithium atoms intercalated into interstitial spaces within the crystal structure. Nickel and Cobalt are the electrochemically active materials in NMC cathode material, which are present as Ni²⁺ and Co³⁺ in the discharged (AKA lithiated) cathode. During battery charging, lithium ions are extracted from the cathode. To compensate for this change in charge, Ni²⁺ oxidizes to Ni³⁺ and Ni⁴⁺, and Co³⁺ oxidizes to Co⁴⁺. This charging process increases cathode instability in two ways. First, Ni⁴⁺ is highly reactive, especially in a fully charged battery. Oxidation of Ni⁴⁺ causes adjacent electrolyte to oxidize, irreversibly consuming lithium and/or increasing charge transfer impedance of the active material. Electrolyte contacting Ni⁴⁺ may break down and react parasitically with the nickel ions. Second, oxidation of nickel and cobalt changes the crystal lattice structure of the NMC cathode material. Manganese and cobalt provide a majority of the structure of NMC due to the reactivity of nickel. Increasing the stoichiometric percentage of nickel therefore decreases stability, while increasing the stoichiometric percentages of manganese and cobalt increase stability. This relationship between high nickel content and instability is similar in NCA and other nickel-based transition metal oxides.

Operation of an electrochemical cell or battery including a cathode disproportionately uses portions of the cathode disposed adjacent to the separator (hereby referred to as the “top portion”). It can therefore be beneficial to include stable active materials in the top portion of the electrode to increase the cycle life of the electrochemical cell.

Accordingly, in some examples, the first active material of the first layer is selected for its high specific capacity, and the second active material of the second layer is selected for its electrochemical stability and its ability to shield the first layer from electrolyte included in the electrochemical cell. In some examples, the first active material has a stoichiometric nickel percentage greater than or equal to 70%. In some examples, the first active material has a stoichiometric nickel percentage greater than or equal to 80%. In some examples, the first active material comprises NMC811 and/or NCA having a nickel percentage greater than or equal to 80%. In some examples, the first active material comprises NMC811 and/or NCA having a nickel percentage greater than 80%. In some examples, the second active material of the second layer has a stoichiometric nickel percentage of less than 70%. In some examples, the second active material of the second layer comprises NMC622.

This configuration of layers may allow the first (AKA bottom) layer to provide the benefits of high nickel content cathodes, such as high specific capacity, while mitigating some of their disadvantages, such as poor cycle life performance. The second (AKA top) layer may provide a barrier to shield the first layer from electrolyte bulk surrounding the separator, preventing side reactions, especially in conditions where no load is placed on the battery. In these no-load conditions, especially in the case of a high state-of-charge cell under no load, there is no flux of lithium ions within the cell. High nickel content materials in the bottom layer therefore have a small amount of lithium available with which to react (e.g., less than 1% of overall available lithium). In contrast, materials in the top layer are immediately adjacent to the separator, which is generally a reservoir of lithium and electrolyte in cells including liquid and/or gel electrolytes. In batteries where lithiation potentials between the top and bottom layers are similar, the top layer may be utilized at a greater rate than the bottom layer, increasing battery safety. This greater utilization of the top layer is due to the reaction gradient within an electrochemical cell under load conditions proceeding from the separator to the current collector.

Additional factors related to electrochemical cell function may also affect side reactions (i.e., reactions between cathode active particles and electrolyte). Voltage of the cell (e.g., potential energy) or state of charge is the greatest contributor to side reactions. The higher the voltage of the cell (e.g., 4.2 V), the higher the nickel oxidation state, and therefore the tendency of the cathode active material to form nickel oxides. Another factor affecting rate of side reactions is heat within the cell (e.g., kinetic energy of molecules). An overall surface area of active particles within the cell may affect side reaction rates, as a greater surface area correlates with a greater number of reaction sites for side reactions. A rate of charging and discharging within the cell may affect side reactions, as increased current densities within the cell may cause active particle cracking, especially within cathode materials. Cracking may further increase exposed surface area of active materials, increasing side reactions. Increased charging and discharging rates generally increase cell temperatures due to resistive heating, which further contributes to cell degradation due to side reactions. Impurities within electrode materials and electrolyte can also cause increased degradation rates.

Cathode active particles may have tailored crystallinities to reduce side reactions due to exposed surface area and to reduce cracking. Cathode active particles may comprise single crystal or polycrystalline materials. Single crystal materials include monocrystalline particles having long-range order in their atomic structure within the entire bulk of the material particle. Polycrystalline material particles include a plurality of small monocrystalline particles, or grains, which may be roughly 1 μm or less in size. The grains collectively form a polycrystalline particle having grain boundaries, which has a reduced exposed surface area for side reactions when compared with the grains as independent particles. Single crystal material particles are generally synthesized to have increased monocrystalline particle sizes when compared to the grains. In some examples, single crystal material particles may have particle sizes (e.g., D50) between 4-8 μm. Polycrystalline material particles generally have particle sizes (e.g., D50) between 6-25 μm. Generally, any cathode active material may exist as either a single crystal or polycrystalline material. Polycrystalline particles may be susceptible to interparticle cracking, which may negatively affect cathode cycle life, whereas single crystal materials may be more stable. However, single crystal materials are generally more difficult to synthesize or manufacture and therefore more expensive.

Generally, single crystal materials have higher solid-state diffusivities than polycrystalline materials. As electrode 302 may be constructed as a cathode using any suitable materials configured to produce a cathode having a lower solid state diffusivity and/or having a higher free energy to delithiate in the first layer than in the second layer, the top layer may include single crystal active particles and the bottom layer may include polycrystalline active particles. In some examples, increasing a percentage of single-crystal materials within an electrode layer may increase the solid-state diffusivity of the electrode layer. Including single crystal materials in the top layer may reduce interparticle grain cracking associated with lower cycle life. In some examples, the top layer may include polycrystalline active particles. In some examples, the bottom layer may include single crystal and/or polycrystalline active particles.

Some illustrative material combinations exhibiting the benefits described above are listed here. In some examples, first active material particles 212 may comprise polycrystalline NMC811 and second active material particles 222 may comprise polycrystalline NMC622. In some examples, first active material particles 212 may comprise polycrystalline NCA having a nickel percentage greater than or equal to 80% and second active material particles 222 may comprise polycrystalline NMC622. In some examples, first active material particles 212 may comprise polycrystalline NMC811 and second active material particles 222 may comprise single crystal NMC622. In some examples, first active material particles 212 may comprise polycrystalline NCA having a nickel percentage greater than or equal to 80% and second active material particles 222 may comprise single crystal NMC622.

With respect to the electrode of FIG. 2, whether an anode or a cathode, the first active material particles of the first layer may have a first distribution of sizes (e.g., by volume) smaller than a second distribution of sizes (e.g., by volume) of the second active material particles of the second layer. In some examples, the first distribution may be smaller than the second distribution by having a median particle size (e.g., by volume) smaller than a median particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be smaller than the second distribution by having a mean particle size (e.g., by volume) smaller than a mean particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be smaller than the second distribution by having one or more modes of particle size (e.g., by volume) smaller than a lowest mode of particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be smaller than the second distribution by having a tenth percentile of the first distribution smaller than a tenth percentile of the second distribution.

With respect to the electrode of FIG. 3, whether an anode or a cathode, the first active material particles of the first layer may be comparatively larger than the second active material particles of the second layer. In some examples, the first active material particles of the first layer may have a first distribution of sizes (e.g., by volume) larger than a second distribution of sizes (e.g., by volume) of the second active material particles of the second layer. In some examples, the first distribution may be larger than the second distribution by having a median particle size (e.g., by volume) larger than a median particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be larger than the second distribution by having a mean particle size (e.g., by volume) larger than a mean particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be larger than the second distribution by having one or more modes of particle size (e.g., by volume) larger than a lowest mode of particle size (e.g., by volume) of the second distribution. In some examples, the first distribution may be larger than the second distribution by having a tenth percentile of the first distribution larger than a tenth percentile of the second distribution.

Additional aspects and features of multilayer electrodes are presented below without limitation as a series of paragraphs, alphanumerically designated for clarity and efficiency. Each of these paragraphs can be combined with one or more other paragraphs, and/or with disclosure from elsewhere in this application, in any suitable manner. Some of the paragraphs below expressly refer to and further limit other paragraphs, providing without limitation examples of some of the suitable combinations.

A0. An electrode comprising:

a current collector substrate; and

an active material composite layered onto the substrate, wherein the active material composite comprises:

- a first layer adjacent the current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and
- a second layer adjacent the first layer and including a plurality of second active material particles configured to have a second solid state diffusivity;

wherein the first solid state diffusivity is greater than the second solid state diffusivity.

A1. The electrode of A0, wherein the first layer further includes a first energy to lithiate per mole and the second layer further includes a second energy to lithiate per mole, and wherein the first energy to lithiate per mole is less than the second energy to lithiate per mole.

A2. The electrode of A0 or A1, wherein the electrode is an anode.

A3. The electrode of A2, wherein the first active material particles consist essentially of hard carbon, and the second active material particles consist essentially of graphitic carbon.

A4. The electrode of A2, wherein the first active material particles consist essentially of hard carbon and silicon monoxide, and the second active material particles consist essentially of graphitic carbon.

A5. The electrode of A2, wherein the first active material particles consist essentially of lithium titanate.

A6. The electrode of A0 or A1, wherein the electrode is a cathode.

A7. The electrode of A6, wherein the first active material particles consist essentially of lithium iron phosphate.

A8. The electrode of A7, wherein the second active material particles comprise an oxide.

A9. The electrode of paragraph A0, A1, A2, or A6, wherein a first average volumetric size of the first active material particles is smaller than a second average volumetric size of the second active material particles.

A10. The electrode of any of paragraphs A0 through A9, wherein the first active material particles are held together by a first binder and wherein the second active material particles are adhered together by a second binder.

B0. An electrode comprising:

a current collector substrate; and

an active material composite layered onto the substrate, wherein the active material composite comprises:

- a first layer adjacent the current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and
- a second layer adjacent the first layer and including a plurality of second active material particles configured to have a second solid state diffusivity;

wherein the first solid state diffusivity is less than the second solid state diffusivity.

B1. The electrode of B0, wherein the first layer further includes a first energy to delithiate per mole and the second layer further includes a second energy to delithiate per mole, and wherein the first energy to delithiate per mole is greater than the second energy to delithiate per mole.

B2. The electrode of B0 or B1, wherein the electrode is an anode.

B3. The electrode of B2, wherein the first active material particles consist essentially of graphitic carbon, and the second active material particles consist essentially of hard carbon.

B4. The electrode of B2, wherein the first active material particles consist essentially of graphitic carbon, and the second active material particles consist essentially of hard carbon and silicon monoxide.

B5. The electrode of B2, wherein the second active material particles consist essentially of lithium titanate.

B6. The electrode of B0 or B1, wherein the electrode is a cathode.

B7. The electrode of B6, wherein the second active material particles consist essentially of lithium iron phosphate.

B8. The electrode of B7, wherein the first active material particles comprise an oxide.

B9. The electrode of paragraph B0, B1, B2, or B6, wherein a first average volumetric size of the first active material particles is larger than a second average volumetric size of the second active material particles.

B10. The electrode of any of paragraphs B0 through B9, wherein the first active material particles are held together by a first binder and wherein the second active material particles are adhered together by a second binder.

B11. The electrode of paragraph B6, wherein the first active material particles comprise a transition metal oxide and wherein the second active material particles comprise a transition metal oxide.

B12. The electrode of paragraph B11, wherein the first active material particles comprise a nickel-containing transition metal oxide and have a first stoichiometric nickel percentage and wherein the second active material particles comprise a transition metal oxide and have a second stoichiometric nickel percentage, and wherein the first stoichiometric nickel percentage is greater than the second stoichiometric nickel percentage.

B13. The electrochemical cell of any of paragraphs B1 through B12, wherein the second active material particles comprise a single crystal material.

B. Illustrative Cells Having One Homogeneous Electrode and One Multilayer Electrode

As shown in FIGS. 4-7, this section describes illustrative electrochemical cells having one homogeneous electrode (e.g., an electrode formed as a single layer extending from the separator to the corresponding current collector substrate) and one multilayer electrode in accordance with aspects of the present disclosure.

FIG. 4 is a schematic sectional view of an illustrative electrochemical cell 400 having a multilayered cathode 402 and a homogeneous anode 404. Electrochemical cell 400 is an example of electrochemical cell 100 of FIG. 1, and cathode 402 is an example of electrode 202 of FIG. 2. Cell 400 includes a separator 412, an electrolyte 410, and current collectors 406 and 408. Electrolyte 410 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 412 separates and electronically insulates the electrodes from each other.

Homogeneous anode 404 includes a single layer adjacent to current collector 408 and separator 412. Anode 404 is coated on current collector 408 in such a way that all parts of the electrode are substantially similar in terms of their chemistry (e.g., of active material particles, binder, conductive additive, etc.), and microstructure (e.g., of active mass fraction, porosity, tortuosity, etc.) within the volume of the electrode composite. Anode 404 may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 408.

As mentioned above, multilayer cathode 402 is an example of electrode 202. Accordingly, the components and characteristics of cathode 402 are substantially identical to corresponding components and characteristics of electrode 402. Multilayer cathode 402 includes a first layer 430 and a second layer 432. First layer 430 is adjacent to current collector 406, and second layer 432 is located adjacent (intermediate) the first layer and separator 412. First layer 430 and second layer 432 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 406.

In the present embodiment, first layer 430 includes first active material particles and second layer 432 includes second active material particles. In some examples, first active material of first layer 430 has a greater active volume fraction and/or active mass fraction than active material of second layer 432. In some examples, active material of first layer 430 has a lower active volume fraction and/or active mass fraction than active material of second layer 432.

Characteristics regarding material composition and size distributions of the layers of cathode 402 are substantially as described with respect to electrode 202. Subsequently, in some examples, a solid state diffusivity of the first active material of first layer 430 may be greater than a solid state diffusivity of the second active material of second layer 432. Furthermore, in some examples, a free energy to lithiate the first active material of first layer 430 may be lower than a free energy to lithiate the second active material of second layer 432. Additionally, a first average volumetric size of the active material particles of first layer 430 may be smaller than a second average volumetric size of the active material particles of second layer 432. This configuration of layers may result in first layer 430 preferentially lithiating before second layer 432. This may result in an increased lithiation rate over homogeneous cathodes or cathodes having a configuration inverse to the configuration of cathode 402, and therefore an increased discharge rate of an electrochemical cell including the cathode.

FIG. 5 is a schematic sectional view of an illustrative electrochemical cell 500 having a multilayered cathode 502 and a homogeneous anode 504. Electrochemical cell 500 is an example of electrochemical cell 100 of FIG. 1, and cathode 502 is an example of electrode 302 of FIG. 3. Cell 500 includes a separator 512, an electrolyte 510, and current collectors 506 and 508. Electrolyte 510 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 512 separates and electronically insulates the electrodes from each other.

Homogeneous anode 504 includes a single layer adjacent to current collector 508 and separator 512. Anode 504 is coated on current collector 508 in such a way that all parts of the electrode are substantially similar in terms of their chemistry (e.g., of active material particles, binder, conductive additive, etc.), and microstructure (e.g., of active mass fraction, porosity, tortuosity, etc.) within the volume of the electrode composite. Anode 504 may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 508.

As mentioned above, multilayer cathode 502 is an example of electrode 302. Accordingly, the components and characteristics of cathode 502 are substantially identical to corresponding components and characteristics of electrode 302. Multilayer cathode 502 includes a first layer 530 and a second layer 532. First layer 530 in the present example is adjacent to current collector 506, and second layer 532 is located adjacent (intermediate) the first layer and separator 512. First layer 530 and second layer 532 may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 506.

In the present embodiment, first layer 530 includes first active material particles and second layer 532 includes second active material particles. In one example, first active material of the first layer 530 has a greater active volume fraction and/or active mass fraction than active material of the second layer 532. In another example, active material of the first layer 530 has a lower active volume fraction and/or active mass fraction than active material of the second layer 532.

Characteristics regarding material composition and size distributions of the layers of cathode 502 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 530 may be less than a solid state diffusivity of the second active material of second layer 532. Additionally, a free energy to delithiate the first active material of first layer 530 may be greater than a free energy to delithiate the second active material of second layer 532. Additionally, a first average volumetric size of the active material particles of first layer 530 may be greater than a second average volumetric size of the active material particles of second layer 532. This configuration of layers may result in second layer 532 preferentially delithiating before first layer 530. This may result in an increased delithiation rate over homogeneous cathodes or cathodes having a configuration inverse to the configuration of cathode 502, as lithium ions released from second layer 532 may have reduced charge repulsion with lithium ions released from first layer 530. An electrochemical cell including cathode 502 may therefore have an increased charge rate when compared with cells including homogeneous cathodes and/or cathodes having an inverse configuration.

FIG. 6 is a schematic sectional view of an illustrative electrochemical cell 600 having a homogeneous cathode 602 and a multilayer anode 604. Electrochemical cell 600 is an example of electrochemical cell 100 of FIG. 1, and anode 604 is an example of electrode 202 of FIG. 2. Cell 600 includes a separator 612, an electrolyte 610, and current collectors 606 and 608. An electrolyte 610 enables the transport of ions between cathode 602 and anode 604, and a liquid permeable polymer separator 612 separates and electronically insulates the electrodes from each other.

Homogeneous cathode 602 includes a single layer adjacent to current collector 606 and separator 612. Cathode 602 is coated on current collector 606 in such a way that all parts of the electrode are substantially similar in terms of their chemistry (e.g., of active material particles, binder, conductive additive, etc.), and microstructure (e.g., active mass fraction, porosity, tortuosity, etc.) within the volume of the electrode composite. Cathode 602 may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 606. First layer 640 and second layer 642 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 608.

As mentioned above, multilayer anode 604 is an example of electrode 202. Accordingly, the components and characteristics of anode 604 are substantially identical to corresponding elements and characteristics described above with respect to electrode 202. Multilayer anode 604 includes a first layer 640 and a second layer 642. First layer 640 is adjacent to the current collector 608, and second layer 642 is disposed adjacent and intermediate the first layer and separator 612.

In the present embodiment, first layer 640 includes first active material particles and second layer 642 includes second active material particles. In some examples, first active material of the first layer 640 has a greater active volume fraction and/or active mass fraction than active material of the second layer 642. In some examples, active material of the first layer 640 has a lower active volume fraction and/or active mass fraction than active material of the second layer 642.

Characteristics regarding material composition and size distributions of the layers of anode 604 are substantially as described with respect to electrode 202. Subsequently, a solid state diffusivity of the first active material of first layer 640 may be greater than a solid state diffusivity of the second active material of second layer 642. Furthermore, a free energy to lithiate the first active material of first layer 640 may be lower than a free energy to lithiate the second active material of second layer 642. This configuration of layers may result in first layer 640 preferentially lithiating before second layer 642. This may result in an increased lithiation rate, and therefore an increased cell charge rate, over homogeneous anodes or anodes having a configuration inverse to the configuration of anode 604.

FIG. 7 is a schematic sectional view of an illustrative electrochemical cell 700 having a homogeneous cathode 702 and a multilayered anode 704. Electrochemical cell 700 is an example of electrochemical cell 100 of FIG. 1, and anode 704 is an example of electrode 302 of FIG. 3. Cell 700 includes a separator 712, an electrolyte 710, and current collectors 706 and 708. An electrolyte 710 enables the transport of ions between cathode 702 and anode 704 and a liquid permeable polymer separator 712 separates and electronically insulates the electrodes from each other.

Homogeneous cathode 702 includes a single layer adjacent to current collector 706 and separator 712. Cathode 702 is coated on current collector 706 in such a way that all parts of the electrode are substantially similar in terms of their chemistry (e.g., of active material particles, binder, conductive additive, etc.), and microstructure (e.g., of active mass fraction, porosity, tortuosity, etc.) within the volume of the electrode composite. Cathode 702 may be substantially planar, with thicknesses measured relative to a direction perpendicular to the current collector 706.

As mentioned above, multilayer anode 704 is an example of electrode 302. Accordingly, the components and characteristics of anode 704 are substantially identical to corresponding elements and characteristics described above with respect to electrode 302. Multilayer anode 704 includes a first layer 740 and a second layer 742. First layer 740 is adjacent to current collector 708, and second layer 742 is located adjacent (intermediate) the first layer and separator 712. First layer 740 and second layer 742 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 708.

In the present embodiment, first layer 740 includes first active material particles and second layer 742 includes second active material particles. In some examples, first active material of first layer 740 has a greater active volume fraction and/or active mass fraction than active material of second layer 742. In some examples, active material of first layer 740 has a lower active volume fraction and/or active mass fraction than active material of second layer 742.

Characteristics regarding material composition and size distributions of the layers of anode 704 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 740 may be less than a solid state diffusivity of the second active material of second layer 742. Furthermore, a free energy to delithiate the first active material of first layer 740 may be higher than a free energy to delithiate the second active material of second layer 742. This configuration of layers may result in first layer 740 preferentially delithiating before second layer 742. This may result in an increased delithiation rate, and therefore an increased cell discharge rate, over homogeneous anodes or anodes having a configuration inverse to the configuration of anode 704.

C. Illustrative Cells Having Two Multilayer Electrodes

As shown in FIGS. 8-12, this section describes several illustrative electrochemical cells wherein both electrodes have multiple layers configured to provide substantive advantages over known cell designs.

FIG. 8 is a schematic sectional view of an illustrative electrochemical cell 800 having a multilayered cathode 802 and a multilayered anode 804. Electrochemical cell 800 is an example of electrochemical cell 100 of FIG. 1, and cathode 802 and anode 804 are both examples of electrode 202 of FIG. 2. Cell 800 includes a separator 812, an electrolyte 810, and current collectors 806 and 808. Electrolyte 810 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 812 separates and electronically insulates the electrodes from each other.

As mentioned above, multilayer cathode 802 and multilayer anode 804 are each an example of electrode 202. Accordingly, the components and characteristics of cathode 802 and anode 804 are substantially identical to corresponding elements and characteristics described above with respect to electrode 202.

Cathode 802 includes a first layer 830 and a second layer 832. First layer 830 is adjacent current collector 806, and second layer 832 is located adjacent (intermediate) the first layer and separator 812. First layer 830 and second layer 832 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 806.

First layer 830 includes first active material particles and second layer 832 includes second active material particles. In some examples, first active material of first layer 830 has a greater active volume fraction and/or active mass fraction than active material of second layer 832. In some examples, active material of first layer 830 has a lower active volume fraction and/or active mass fraction than active material of second layer 832.

Characteristics regarding material composition and size distributions of the layers of cathode 802 are substantially as described with respect to electrode 202.

Subsequently, a solid state diffusivity of the first active material of first layer 830 may be greater than a solid state diffusivity of the second active material of second layer 832. Furthermore, a free energy to lithiate the first active material of first layer 830 may be lower than a free energy to lithiate the second active material of second layer 832. Additionally, a first average volumetric size of the active material particles of first layer 830 may be smaller than a second average volumetric size of the active material particles of second layer 832. Cathode 802 may therefore have an increased lithiation rate when compared with homogeneous cathodes and/or cathodes having an inverse configuration.

Anode 804 includes a first layer 840 and a second layer 842. First layer 840 is adjacent current collector 808, and second layer 842 is adjacent (intermediate) the first layer and separator 812. First layer 840 and second layer 842 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to the current collector 808.

First layer 840 includes first active material particles and second layer 842 includes second active material particles. In some examples, first active material of first layer 840 has a greater active volume fraction and/or active mass fraction than active material of second layer 842. In some examples, active material of first layer 840 has a lower active volume fraction and/or active mass fraction than active material of second layer 842.

Characteristics regarding material composition and size distributions of the layers of anode 804 are substantially as described with respect to electrode 202. Subsequently, a solid state diffusivity of the first active material of first layer 840 may be greater than a solid state diffusivity of the second active material of second layer 842. Furthermore, a free energy to lithiate the first active material of first layer 840 may be lower than a free energy to lithiate the second active material of second layer 842. Anode 804 may therefore have an increased lithiation rate when compared with homogeneous anodes and/or anodes having an inverse configuration.

FIG. 9 is a schematic sectional view of an illustrative electrochemical cell 900 having a multilayered cathode 902 and a multilayered anode 904. Electrochemical cell 900 is an example of electrochemical cell 100 of FIG. 1, and cathode 902 and anode 904 are both examples of electrode 302 of FIG. 3. Cell 900 includes a separator 912, an electrolyte 910, and current collectors 906 and 908. Electrolyte 910 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 912 separates and electronically insulates the electrodes from each other.

As mentioned above, multilayer cathode 902 and multilayer anode 904 are each an example of electrode 302. Accordingly, the components and characteristics of cathode 902 and anode 904 are substantially identical to corresponding elements and characteristics described above with respect to electrode 302.

Cathode 902 includes a first layer 930 and a second layer 932. First layer 930 is adjacent current collector 906, and second layer 932 is adjacent (intermediate) the first layer and separator 912. First layer 930 and second layer 932 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 906.

First layer 930 includes first active material particles and second layer 932 includes second active material particles. In some examples, first active material of first layer 930 has a greater active volume fraction and/or active mass fraction than active material of second layer 932. In some examples, active material of first layer 930 has a lower active volume fraction and/or active mass fraction than active material of second layer 932.

Characteristics regarding material composition and size distributions of the layers of cathode 902 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 930 may be less than a solid state diffusivity of the second active material of second layer 932. Furthermore, a free energy to delithiate the first active material of first layer 930 may be greater than a free energy to delithiate the second active material of second layer 932. Additionally, a first average volumetric size of the active material particles of first layer 930 may be greater than a second average volumetric size of the active material particles of second layer 932. Cathode 902 may therefore have an increased delithiation rate when compared with homogeneous cathodes and/or cathodes having an inverse configuration.

Anode 904 includes a first layer 940 and a second layer 942. First layer 940 is adjacent current collector 908, and second layer 942 is adjacent (intermediate) the first layer and separator 912. First layer 940 and second layer 942 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 908.

First layer 940 includes first active material particles and second layer 942 includes second active material particles. In some examples, first active material of first layer 940 has a greater active volume fraction and/or active mass fraction than active material of second layer 942. In some examples, active material of first layer 940 has a lower active volume fraction and/or active mass fraction than active material of second layer 942.

Characteristics regarding material composition and size distributions of the layers of anode 904 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 940 may be less than a solid state diffusivity of the second active material of second layer 942. Furthermore, a free energy to delithiate the first active material of first layer 940 may be greater than a free energy to delithiate the second active material of second layer 942. Anode 904 may therefore have an increased delithiation rate when compared with homogeneous anodes and/or anodes having an inverse configuration.

FIG. 10 is a schematic sectional view of an illustrative electrochemical cell 1000 having a multilayered cathode 1002 and a multilayered anode 1004. Electrochemical cell 1000 is an example of electrochemical cell 100 of FIG. 1, multilayer cathode 1002 is an example of electrode 202 of FIG. 2, and multilayer anode 1004 is an example of electrode 302 of FIG. 3. Cell 1000 includes a separator 1012, an electrolyte 1010, and current collectors 1006 and 1008. Electrolyte 1010 enables the transport of ions between electrodes, and a liquid permeable polymer separator 1012 separates and electronically insulates the electrodes from each other.

As mentioned above, multilayer cathode 1002 is an example of electrode 202, and multilayer anode 1004 is an example of electrode 302. Accordingly, the components and characteristics of cathode 1002 are substantially identical to corresponding elements and characteristics described above with respect to electrode 202, and the components and characteristics of anode 1004 are substantially identical to corresponding elements and characteristics described above with respect to electrode 302.

Cathode 1002 includes a first layer 1030 and a second layer 1032. First layer 1030 is adjacent current collector 1006, and second layer 1032 is adjacent (intermediate) the first layer and separator 1012. First layer 1030 and second layer 1032 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1006.

First layer 1030 includes first active material particles and second layer 1032 includes second active material particles. In some examples, first active material of first layer 1030 has a greater active volume fraction and/or active mass fraction than active material of second layer 1032. In some examples, active material of first layer 1030 has a lower active volume fraction and/or active mass fraction than active material of second layer 1032.

Characteristics regarding material composition and size distributions of the layers of cathode 1002 are substantially as described with respect to electrode 202.

Subsequently, a solid state diffusivity of the first active material of first layer 1030 may be greater than a solid state diffusivity of the second active material of second layer 1032. Furthermore, a free energy to lithiate the first active material of first layer 1030 may be lower than a free energy to lithiate the second active material of second layer 1032. Additionally, a first average volumetric size of the active material particles of first layer 1030 may be smaller than a second average volumetric size of the active material particles of second layer 1032. Cathode 1002 may therefore have an increased lithiation rate when compared with homogeneous cathodes and/or cathodes having an inverse configuration.

Anode 1004 includes a first layer 1040 and a second layer 1042. First layer 1040 is adjacent current collector 1008, and second layer 1042 is adjacent (intermediate) the first layer and separator 1012. First layer 1040 and second layer 1042 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1008.

First layer 1040 includes first active material particles and second layer 1042 includes second active material particles. In some examples, first active material of first layer 1040 has a greater active volume fraction and/or active mass fraction than active material of second layer 1042. In some examples, active material of first layer 1040 has a lower active volume fraction and/or active mass fraction than active material of second layer 1042.

Characteristics regarding material composition and size distributions of the layers of anode 1004 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 1040 may be less than a solid state diffusivity of the second active material of second layer 1042. Furthermore, a free energy to delithiate the first active material of first layer 1040 may be greater than a free energy to delithiate the second active material of second layer 1042. Anode 1004 may therefore have an increased delithiation rate when compared with homogeneous anodes and/or anodes having an inverse configuration.

As cathode 1002 may have an increased lithiation rate and anode 1004 may have an increased delithiation rate, electrochemical cell 1000 may have an improved discharge rate when compared with electrochemical cells including homogeneous electrodes or alternative configurations of multilayered electrodes.

FIG. 11 is a schematic sectional view of an illustrative electrochemical cell 1100 having a multilayered cathode 1102 and a multilayered anode 1104. Electrochemical cell 1100 is an example of electrochemical cell 100 of FIG. 1, multilayer cathode 1102 is an example of electrode 302 of FIG. 3, and multilayer anode 1104 is an example of electrode 202 of FIG. 2. Cell 1100 includes a separator 1112, an electrolyte 1110, and current collectors 1106 and 1108. Electrolyte 1110 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 1112 separates and electronically insulates the electrodes from each other.

As mentioned above, multilayer cathode 1102 is an example of electrode 302, and multilayer anode 1104 is an example of electrode 202. Accordingly, the components and characteristics of cathode 1102 are substantially identical to corresponding elements and characteristics described above with respect to electrode 302, and the components and characteristics of anode 1104 are substantially identical to corresponding elements and characteristics described above with respect to electrode 202.

Cathode 1102 includes a first layer 1130 and a second layer 1132. First layer 1130 is adjacent to current collector 1106, and second layer 1132 is adjacent (intermediate) the first layer and separator 1112. First layer 1130 and second layer 1132 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1106.

First layer 1130 includes first active material particles and second layer 1132 includes second active material particles. In some examples, the first active material of first layer 1130 has a greater active volume fraction and/or active mass fraction than active material of second layer 1132. In some examples, the active material of first layer 1130 has a lower active volume fraction and/or active mass fraction than active material of second layer 1132.

Characteristics regarding material composition and size distributions of the layers of cathode 1102 are substantially as described with respect to electrode 302. Subsequently, a solid state diffusivity of the first active material of first layer 1130 may be less than a solid state diffusivity of the second active material of second layer 1132. Furthermore, a free energy to delithiate the first active material of first layer 1130 may be greater than a free energy to delithiate the second active material of second layer 1132. Additionally, a first average volumetric size of the active material particles of first layer 1130 may be greater than a second average volumetric size of the active material particles of second layer 1132. Cathode 1102 may therefore have an increased delithiation rate when compared with homogeneous cathodes and/or cathodes having an inverse configuration.

Anode 1104 includes a first layer 1140 and a second layer 1142. First layer 1140 is adjacent to current collector 1108, and second layer 1142 is adjacent (intermediate) the first layer and separator 1112. First layer 1140 and second layer 1142 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1108.

First layer 1140 includes first active material particles and second layer 1142 includes second active material particles. In some examples, the first active material of first layer 1140 has a greater active volume fraction and/or active mass fraction than the active material of second layer 1142. In some examples, the active material of first layer 1140 has a lower active volume fraction and/or active mass fraction than active material of the second layer 1142.

Characteristics regarding material composition and size distributions of the layers of anode 1104 are substantially as described with respect to electrode 202.

Subsequently, a solid state diffusivity of the first active material of first layer 1140 may be greater than a solid state diffusivity of the second active material of second layer 1142. Furthermore, a free energy to lithiate the first active material of first layer 1140 may be lower than a free energy to lithiate the second active material of second layer 1142. Anode 1104 may therefore have an increased lithiation rate when compared with homogeneous anodes and/or anodes having an inverse configuration.

As cathode 1102 may have an increased delithiation rate and anode 1104 may have an increased lithiation rate, electrochemical cell 1000 may have an improved charge rate when compared with electrochemical cells including homogeneous electrodes or alternative configurations of multilayered electrodes.

FIG. 12 is a schematic sectional view of an illustrative electrochemical cell 1200 having a multilayered cathode 1202 and a multilayered anode 1204. Electrochemical cell 1200 is an example of electrochemical cell 100 of FIG. 1, cathode 1202 is an example of electrode 302 of FIG. 3, and anode 1204 is an example of electrode 202 of FIG. 2. Cell 1200 is also an example of electrochemical cell 1100 of FIG. 11 (see above).

Cell 1200 includes a separator 1212, an electrolyte 1210, and current collectors 1206 and 1208. Electrolyte 1210 enables the transport of ions between the electrodes, and a liquid permeable polymer separator 1212 separates and electronically insulates the electrodes from each other.

As mentioned above, multilayer cathode 1202 is an example of electrode 302, and multilayer anode 1204 is an example of electrode 202. Accordingly, the components and characteristics of cathode 1202 are substantially identical to corresponding elements and characteristics described above with respect to electrode 302, and the components and characteristics of anode 1204 are substantially identical to corresponding elements and characteristics described above with respect to electrode 202.

Cathode 1202 includes a first layer 1230 and a second layer 1232. First layer 1230 is adjacent current collector 1206, and second layer 1232 is adjacent (intermediate) the first layer and separator 1212. First layer 1230 and second layer 1232 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1206.

First layer 1230 includes first active material particles and second layer 1232 includes second active material particles. In the present example, first active material of first layer 1230 has a smaller active volume fraction and/or active mass fraction than active material of the second layer 1232. Additionally, the first active material particles of first layer 1230 have a first distribution of sizes smaller than a second distribution of sizes of the second active material particles of second layer 1232. The first distribution may be smaller than the second distribution by having a median particle size smaller than a median particle size of the second distribution.

In this example, the first active material of first layer 1230 consists essentially of LMO and the second active material of second layer 1232 consists essentially of NMC. Accordingly, a free energy to delithiate the first active material of first layer 1230 (at ˜3.9 V vs. Li/Li+) is greater than a free energy to delithiate the second active material of second layer 1232 (at ˜3.8 V vs. Li/Li+). Additionally, a solid state diffusivity of the first active material of first layer 1230 is greater than a solid state diffusivity of the second active material of second layer 1232.

Anode 1204 includes a first layer 1240 and a second layer 1242. First layer 1240 is adjacent current collector 1208, and second layer 1242 is adjacent (intermediate) the first layer and separator 1212. First layer 1240 and second layer 1242 each may be substantially planar, with thicknesses measured relative to a direction perpendicular to current collector 1208.

First layer 1240 includes first active material particles and second layer 1242 includes second active material particles. In the present example, first active material of first layer 1240 has an active volume fraction approximately equal to the active volume fraction of the active material of second layer 1242. Additionally, the first active material particles of first layer 1240 have a first distribution of sizes smaller than a second distribution of sizes of the second active material particles of second layer 1242.

In this example, the first active material of first layer 1240 includes one or more of a hard carbon (e.g., a non-graphitic carbon) and silicon monoxide, and the second active material of second layer 1242 includes graphitic carbons. Accordingly, a free energy to lithiate the first active material of first layer 1240 is lower than a free energy to lithiate the second active material of second layer 1242. Furthermore, a solid state diffusivity of the first active material of first layer 1240 is greater than a solid state diffusivity of the second active material of second layer 1242.

Additional aspects and features of electrochemical cells having one or more multilayer electrodes are presented below without limitation as a series of paragraphs, alphanumerically designated for clarity and efficiency. Each of these paragraphs can be combined with one or more other paragraphs, and/or with disclosure from elsewhere in this application, in any suitable manner. Some of the paragraphs below expressly refer to and further limit other paragraphs, providing without limitation examples of some of the suitable combinations.

C0. An electrochemical cell comprising:

a first electrode separated from a second electrode by a liquid-permeable separator; and

an electrolyte disposed generally throughout the first and second electrodes;

the first electrode comprising a first current collector substrate and an active material composite layered onto the first current collector substrate, wherein the active material composite comprises:

- a first layer adjacent the first current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and
- a second layer adjacent the liquid-permeable separator and including a plurality of second active material particles configured to have a second solid state diffusivity;

wherein the first solid state diffusivity is greater than the second solid state diffusivity.

C1. The electrochemical cell of paragraph C0, wherein the first active material particles are configured to have a first free energy to lithiate and the second active material particles are configured to have a second free energy to lithiate, and wherein the first free energy to lithiate is less than the second free energy to lithiate.

C2. The electrochemical cell of C1, wherein the second electrode is substantially homogeneous.

C3. The electrochemical cell of paragraph C0, C1, or C2, wherein the first electrode is an anode.

C4. The electrochemical cell of C3, wherein the first active material particles consist essentially of hard carbon, and the second active material particles consist essentially of graphitic carbon.

C5. The electrochemical cell of C3, wherein the first active material particles consist essentially of hard carbon and silicon monoxide, and the second active material particles consist essentially of graphitic carbon.

C6. The electrochemical cell of C3, wherein the first active material particles consist essentially of lithium titanate.

C7. The electrochemical cell of C0, C1, or C2, wherein the first electrode is a cathode.

C8. The electrochemical cell of C7, wherein the first active material particles consist essentially of lithium iron phosphate

C9. The electrochemical cell of C8, wherein the second active material particles comprise an oxide.

C10. The electrochemical cell of any of paragraphs C0 through C9, wherein a first average volumetric size of the first active material particles is smaller than a second average volumetric size of the second active material particles.

C11. The electrochemical cell of any of paragraphs C0 through C10, wherein the first active material particles are adhered together by a first binder and the second active material particles are adhered together by a second binder.

D0. An electrochemical cell comprising:

a first electrode separated from a second electrode by a liquid-permeable separator; and

an electrolyte disposed generally throughout the first and second electrodes;

- a first layer adjacent the first current collector substrate and including a plurality of first active material particles adhered together by a first binder, the first active material particles configured to have a first solid state diffusivity; and
- a second layer adjacent the liquid-permeable separator and including a plurality of second active material particles configured to have a second solid state diffusivity;

wherein the first solid state diffusivity is lower than the second solid state diffusivity.

D1. The electrochemical cell of D0, wherein the first active material particles have a first free energy to delithiate and the second active material particles have a second free energy to delithiate, and wherein the first free energy to delithiate is greater than the second free energy to delithiate.

D2. The electrochemical cell of D0 or D1, wherein the second electrode is substantially homogeneous.

D3. The electrochemical cell of D0, D1, or D2, wherein the first electrode is an anode.

D4. The electrochemical cell of D3, wherein the first active material particles consist essentially of graphitic carbon.

D5. The electrochemical cell of D3, wherein the first active material particles consist essentially of hard carbon, and the second active material particles consist essentially of lithium titanate.

D6. The electrochemical cell of D0, D1 or D2, wherein the first electrode is a cathode.

D7. The electrochemical cell of D6, wherein the first active material particles consist essentially of lithium manganese oxide, and the second active material particles include nickel.

D8. The electrochemical cell of D7, wherein the second active material particles consist essentially of lithium nickel cobalt aluminum oxide.

D9. The electrochemical cell of D0, D1, or D2 wherein a first average volumetric size of the first active material particles is greater than a second average volumetric size of the second active material particles.

D10. The electrochemical cell of any of paragraphs D0 through D9, wherein the first active material particles are adhered together by a first binder and the second active material particles are adhered together by a second binder.

D11. The electrochemical cell of any of paragraphs D0 through D10, wherein the second electrode is formed as a single layer extending from the separator to a second current collector substrate.

D12. The electrochemical cell of paragraph D6, wherein the first active material particles comprise a transition metal oxide and wherein the second active material particles comprise a transition metal oxide.

D13. The electrochemical cell of paragraph D6 or D12, wherein the first active material particles comprise a nickel-containing transition metal oxide and have a first stoichiometric nickel percentage and wherein the second active material particles comprise a transition metal oxide and have a second stoichiometric nickel percentage, and wherein the first stoichiometric nickel percentage is greater than the second stoichiometric nickel percentage.

D14. The electrochemical cell of any of paragraphs D1 through D13, wherein the second active material particles comprise a single crystal material.

E0. An electrochemical cell comprising:

a first electrode separated from a second electrode by a liquid-permeable separator; and

an electrolyte disposed generally throughout the first and second electrodes;

the first electrode comprising a first current collector substrate and a first active material composite layered onto the first current collector substrate, wherein the first active material composite comprises:

- a first layer adjacent the first current collector substrate and including a plurality of first active material particles configured to have a first solid state diffusivity; and
- a second layer adjacent the liquid-permeable separator and including a plurality of second active material particles configured to have a second solid state diffusivity;

the second electrode comprising a second current collector substrate and a second active material composite layered onto the second current collector substrate, wherein the second active material composite comprises:

- a third layer adjacent the second current collector substrate and including a plurality of third active material particles configured to have a third solid state diffusivity; and
- a fourth layer adjacent the liquid-permeable separator and including a plurality of fourth active material particles configured to have a fourth solid state diffusivity;

wherein the first solid state diffusivity is lower than the second solid state diffusivity; and

wherein the third solid state diffusivity is higher than the fourth solid state diffusivity.

E1. The electrochemical cell of E0, wherein the third layer further includes a first energy to lithiate per mole and the fourth layer further includes a second energy to lithiate per mole, and wherein the first energy to lithiate per mole is lower than the second energy to lithiate per mole.

E2. The electrochemical cell of E0 or E1, wherein the first electrode is a cathode.

E3. The electrochemical cell of any of paragraphs E0 through E2, wherein the second electrode is an anode.

E4. The electrochemical cell of E2, wherein the first active material particles consist essentially of lithium manganese oxide, and the second active material particles include nickel.

E5. The electrochemical cell of E2 or E4, wherein the second active material particles consist essentially of lithium nickel cobalt aluminum oxide.

E6. The electrochemical cell of any of paragraphs E0 through E5, wherein a first average volumetric size of the first active material particles is greater than a second average volumetric size of the second active material particles.

E7. The electrochemical cell of paragraph E2, wherein the first active material particles comprise a transition metal oxide and wherein the second active material particles comprise a transition metal oxide.

E8. The electrochemical cell of paragraph E7, wherein the first active material particles comprise a nickel-containing transition metal oxide and have a first stoichiometric nickel percentage and wherein the second active material particles comprise a transition metal oxide and have a second stoichiometric nickel percentage, and wherein the first stoichiometric nickel percentage is greater than the second stoichiometric nickel percentage.

D. Illustrative Method and Device for Manufacturing Multilayered Electrodes

This section describes steps of an illustrative method 1300 for forming an electrode including multiple layers; see FIGS. 13-14.

Aspects of electrodes and manufacturing devices described herein may be utilized in the method steps described below. Where appropriate, reference may be made to components and systems that may be used in carrying out each step. These references are for illustration, and are not intended to limit the possible ways of carrying out any particular step of the method.

FIG. 13 is a flowchart illustrating steps performed in an illustrative method, and may not recite the complete process or all steps of the method. Although various steps of method 1300 are described below and depicted in FIG. 13, the steps need not necessarily all be performed, and in some cases may be performed simultaneously, or in a different order than the order shown.

Step 1302 of method 1300 includes providing a substrate. In some examples, the substrate comprises a current collector, such as current collectors 206, 306 (and others) described above. In some examples, the substrate comprises a metal foil.

Method 1300 next includes a plurality of steps in which at least a portion of the substrate is coated with an active material composite. This may be done by causing the substrate to move past an active material composite dispenser (or vice versa) that coats the substrate as described below. The composition of active material particles in each active material composite layer may be selected to achieve the benefits, characteristics, and results described herein.

Step 1304 of method 1300 includes coating a first layer of a composite electrode on a first side of the substrate. In some examples, the first layer may include a plurality of first particles adhered together by a first binder, the first particles having a first average particle size (or other first particle distribution).

The coating process of step 1304 may include any suitable coating method(s), such as slot die, blade coating, spray-based coating, electrostatic jet coating, or the like. In some examples, the first layer is coated as a wet slurry of solvent, e.g., water or NMP(N-Methyl-2-pyrrolidone), binder, conductive additive, and active material. In some examples, the first layer is coated dry, as an active material with a binder and/or a conductive additive. Step 1304 may optionally include drying the first layer of the composite electrode.

Step 1306 of method 1300 includes coating a second layer of a composite electrode, on the first side of the substrate, onto the first layer, forming a multilayered (e.g., stratified) structure. The second layer may include a plurality of second particles adhered together by a second binder, the second particles having a second average particle size (or other second particle distribution).

In some examples, steps 1304 and 1306 may be performed substantially simultaneously. For example, both of the active material slurries may be extruded through their respective orifices simultaneously. This forms a two-layer slurry bead and coating on the moving substrate. In some examples, difference in viscosities, difference in surface tensions, difference in densities, difference in solids contents, and/or different solvents used between the first active material slurry and the second active material slurry may be tailored to cause interpenetrating finger structures at the boundary between the two active material composite layers. In some embodiments, the viscosities, surface tensions, densities, solids contents, and/or solvents may be substantially similar. Creation of interpenetrating structures may be facilitated by turbulent flow at the wet interface between the first active material electrode slurry and the second active material electrode slurry, creating partial intermixing of the two active material electrode slurries.

To ensure proper curing in the drying process, the first layer (closest to the current collector) may be configured to be dried from solvent prior to the second layer (further from the current collector) so as to avoid creating skin-over effects and blisters in the resulting dried coatings.

Method 1300 may optionally include drying the composite electrode in step 1308, and/or calendering the composite electrode in step 1310. In these optional steps, both the first and second layers may experience the drying process and the calendering process as a combined structure. In some examples, steps 1308 and 1310 may be combined (e.g., in a hot roll process). In some examples, drying step 1308 includes a form of heating and energy transport to and from the electrode (e.g., convection, conduction, radiation) to expedite the drying process. In some examples, calendering step 1310 is replaced with another compression, pressing, or compaction process. In some examples, calendering the electrode may be performed by pressing the combined first and second layers against the substrate, such that electrode density is increased in a non-uniform manner, with the first layer having a first porosity and the second layer having a lower second porosity.

Turning to FIG. 14, an illustrative system 1400 suitable for use with method 1300 will now be described. In some examples, a slot-die coating head with at least two fluid slots, fluid cavities, fluid lines, and fluid pumps may be utilized to manufacture a battery electrode featuring multiple active material composite layers. System 1400 includes a dual-cavity slot-die coating head configured to manufacture electrodes having two layers. In some examples, additional cavities may be utilized to create additional layers. System 1400 is a manufacturing system in which a foil substrate 1402 (e.g., current collector substrate 206, 306, etc.) is transported by a revolving backing roll 1404 past a stationary dispenser device 1406. Dispenser device 1406 may include any suitable dispenser configured to evenly coat one or more layers of active material slurry onto the substrate, as described with respect to steps 1304 and 1306 of method 1300. In some examples, the substrate may be held stationary while the dispenser head moves. In some examples, both may be in motion.

Dispenser device 1406 may, for example, include a dual chamber slot die coating device having a coating head 1408 with two orifices 1410 and 1412. A slurry delivery system supplies two different active material slurries to the coating head under pressure. Due to the revolving nature of backing roll 1404, material exiting the lower orifice or slot 1410 will contact substrate 1402 before material exiting the upper orifice or slot 1412. Accordingly, a first layer 1414 will be applied to the substrate and a second layer 1416 will be applied on top of the first layer.

Accordingly, corresponding steps of method 1300 may be characterized as follows. Causing a current collector substrate and an active material composite dispenser to move relative to each other, and coating at least a portion of the substrate with an active material composite, using the dispenser. Coating, in this case, includes: applying a first layer of slurry to the substrate using a first orifice or slot of the dispenser, and applying a second layer of a different slurry to the first layer using a second orifice or slot of the dispenser.

ADVANTAGES, FEATURES, AND BENEFITS

The different embodiments and examples of the electrodes and electrochemical cells described herein provide several advantages over known solutions for improving cell charge and discharge rates. For example, illustrative embodiments and examples described herein prevent charge repulsion between lithium ions moving between electrodes. This may reduce polarization and prevent lithium starvation within electrodes.

Additionally, and among other benefits, illustrative embodiments and examples described herein allow increased battery utilization, as a greater percentage of battery active materials may be used before reaching the battery cut-off voltage.

No known system or device can perform these functions. However, not all embodiments and examples described herein provide the same advantages or the same degree of advantage.

CONCLUSION

The disclosure set forth above may encompass multiple distinct examples with independent utility. Although each of these has been disclosed in its preferred form(s), the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense, because numerous variations are possible. To the extent that section headings are used within this disclosure, such headings are for organizational purposes only. The subject matter of the disclosure includes all novel and nonobvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. Other combinations and subcombinations of features, functions, elements, and/or properties may be claimed in applications claiming priority from this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.

LAYERED ELECTRODE WITH HIGH RATE TOP LAYER

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