SINTERED ELECTRODES FOR BATTERIES AND METHOD OF PREPARING SAME

BACKGROUND

The disclosure relates generally to an electrode for a battery and to a method of preparing same. Various types of battery chemistries exist. Lithium-ion batteries are currently a popular chemistry for a variety of applications. For example, lithium-ion battery technology that is based upon liquid carbonate electrolytes and intercalation electrodes are used in a variety of small electronic devices, such as cellular telephones, laptop computers, and cordless power tools, and also in larger applications, such as hybrid and all electric vehicles and to stabilize electric grids at local and national levels under periods of high demand. Despite the wide-ranging adoption and use of lithium-ion batteries, demand still exists for batteries with higher energy density, smaller size, lighter weight, and lower cost.

Moreover, irreversible reactions often facilitated by the liquid electrolyte lead to fade in capacity. Further, the range of temperatures for operation and charging of lithium batteries with liquid electrolyte is restricted. Overheating is damaging to cell lifetime, and to guard against overheating, lithium-ion batteries employ protection circuits to reduce current draw or charging rate. Temperature is actively managed in applications like electric vehicles where the batteries are densely packed in order to minimize volume.

Still further, the ability to charge quickly is valued in consumer electronics, and many recently manufactured devices can be charged to 80% of capacity is less than hour, albeit with potential loss of battery life. The expectation of quick charging is even more pressing for electric vehicles. Even though a typical charging time is around 45 minutes, the expectation is that the battery of an electric vehicle can be charged to full in a time comparable to refueling of a gasoline powered vehicle of around 5 minutes.

SUMMARY

In one aspect, embodiments of the disclosure relate to embodiments of a sintered electrode for a battery. The sintered electrode has a first surface configured to be arranged facing a current collector and a second surface configured to be arranged facing an electrolyte layer. The sintered electrode includes a chalcogenide compound having at least one of an alkali metal or an alkaline earth metal. The sintered electrode has a thickness between the first surface and the second surface of 2 μm to 100 μm and cross-sectional area of the first surface and of the second surface of at least 3 cm². Further, the sintered electrode has a porosity of from 0.1% to 30%.

In another aspect, embodiments of the disclosure relate to a method of preparing a sintered electrode for a battery. In the method, a slurry or paste is provided. The slurry paste includes 40% to 75% by weight of a powder component including a chalcogenide and at least one alkali metal or alkaline earth metal, 1% to 10% by weight of a binder, and 30% to 50% by weight of a solvent. Further, in the method, the slurry is formed into a green tape having a thickness of 2 μm to 100 μm. The green tape is dried such that the green tape contains at most 10% by weight of organic material, and the green tape is sintered at a temperature of from 500° C. to 1350° C. for a time of no more than 60 min to form a sintered tape.

In still another aspect, embodiments of the disclosure relate to a battery. The battery includes a cathode, an electrolyte region, and an anode. At least one of the cathode or the anode is a sintered electrode having a first surface and a second surface. The sintered electrode includes a chalcogenide compound including at least one of an alkali metal or an alkaline earth metal. The sintered electrode has a thickness between the first surface and the second surface of from 2 μm to 100 μm and cross-sectional area of the first surface and of the second surface of at least 3 cm².

In some embodiments, a sintered electrode for a battery, the sintered electrode having a first surface configured to be positioned facing a current collector and a second surface configured to be positioned facing an electrolyte layer, wherein the sintered electrode comprises: a chalcogenide compound comprising at least one of an alkali metal or an alkaline earth metal; wherein the sintered electrode has a thickness between the first surface and the second surface of 2 μm to 100 μm; and wherein the sintered electrode has an open porosity of from 0.1% to 30%.

In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound comprises at least one of lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, or lithium titanium sulfide.

In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound comprises a solid phase of the sintered electrode, wherein the open porosity comprises a pore phase which is continuous within the solid phase. In one aspect, which is combinable with any of the other aspects or embodiments, the pore phase provides fluid communication between the first surface and the second surface. In one aspect, which is combinable with any of the other aspects or embodiments, the sintered electrode comprises a first phase and a second phase wherein the first phase comprises a chalcogenide compound and wherein the second phase is intermixed with the first phase.

In some embodiments, a cathode for a battery, comprises: a sintered electrode having a first surface and a second surface; wherein: the sintered electrode has a thickness between the first surface and the second surface in a range of 2 μm to 100 μm, and the sintered electrode comprises lithium chalcogenide selected from at least one of: lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, lithium nickel manganate, and lithium titanium sulfide (LiTiS₂).

In one aspect, which is combinable with any of the other aspects or embodiments, a charging capacity of the sintered cathode is maintained at greater than 135 mAhr/g after twenty charge-discharge cycles at a C/5 rate and 4.3 V charging potential. In one aspect, which is combinable with any of the other aspects or embodiments, a cross-sectional area of the sintered cathode is at least 3 cm². In one aspect, which is combinable with any of the other aspects or embodiments, a battery comprises: a cathode as described herein; an electrolyte region penetrating a porous region of the cathode; wherein the cathode is a substrate of the battery. In one aspect, which is combinable with any of the other aspects or embodiments, the electrolyte region comprises a solid electrolyte being at least one of: lithium-phosphorous-oxynitride (LiPON), lithium garnet (Li₇La₃Zr₂O₁₂), or lithium phosphosulfide.

In one aspect, which is combinable with any of the other aspects or embodiments, the battery further comprises: a current collector disposed on a surface of the sintered electrode. In one aspect, which is combinable with any of the other aspects or embodiments, the current collector has a thickness of less than 5 μm. In one aspect, which is combinable with any of the other aspects or embodiments, a volume of the battery is less than a volume of a battery comprising a cathode disposed over a substrate. In one aspect, which is combinable with any of the other aspects or embodiments, a capacity of the battery is at least seven-fold higher in volumetric or absolute terms than a capacity of a battery comprising a cathode disposed over a substrate. In one aspect, which is combinable with any of the other aspects or embodiments, a capacity of the battery is at least ten-fold higher on a weight basis than a capacity of a battery comprising a cathode disposed over a substrate.

In some embodiments, a cathode for a battery, comprises: a sintered electrode having a first surface and a second surface; wherein the sintered electrode: has a thickness between the first surface and the second surface in a range of 2 μm to 100 μm, has a cross-sectional area of at least 3 cm², and is a substrate of the battery. In one aspect, which is combinable with any of the other aspects or embodiments, a charging capacity of the sintered cathode is maintained at greater than 135 mAhr/g after twenty charge-discharge cycles at a C/5 rate and 4.3 V charging potential. In one aspect, which is combinable with any of the other aspects or embodiments, a volume of the battery is less than a volume of a battery comprising a cathode disposed over a substrate. In one aspect, which is combinable with any of the other aspects or embodiments, a capacity of the battery is at least seven-fold higher in volumetric or absolute terms than a capacity of a battery comprising a cathode disposed over a substrate. In one aspect, which is combinable with any of the other aspects or embodiments, a capacity of the battery is at least ten-fold higher on a weight basis than a capacity of a battery comprising a cathode disposed over a substrate.

In some embodiments, a method for forming a sintered composition, comprises providing a slurry precursor including a chalcogenide compound; tape casting the slurry precursor to form a green tape; and sintering the green tape at a temperature in a range of 500° C. to 1350° C. for a time in a range of less than 60 min.

In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound is at least 50 wt. % of the total slurry precursor. In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide comprises LCO or LMO. In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide comprises: at least one of NaVPO₄F, NaMnO₂, Na_2/3Mn_1-yMg_yO₂(0<y<1), Na₂Li₂Ti₅O₁₂, Na₂Ti₃O₇, MgCr₂O₄, or MgMn₂O₄. In one aspect, which is combinable with any of the other aspects or embodiments, the slurry precursor further comprises at least one binder, solvent, dispersant, and plasticizer.

In one aspect, which is combinable with any of the other aspects or embodiments, the tape casting comprises: forming the slurry precursor to a sheet configuration having a thickness in a range of 5 μm to 100 μm; and drying the sheet configuration such that a combination of binder, solvent, dispersant, and plasticizer does not exceed 10 wt. % of the dried sheet. In one aspect, which is combinable with any of the other aspects or embodiments, the method further comprises debinding the dried sheet at a predetermined temperature.

In one aspect, which is combinable with any of the other aspects or embodiments, the predetermined temperature is in a range of 175° C. to 350° C. In one aspect, which is combinable with any of the other aspects or embodiments, the step of debinding and the step of sintering is conducted simultaneously. In one aspect, which is combinable with any of the other aspects or embodiments, the method further comprises pyrolyzing organics in the dried sheet at a temperature in a range of 175° C. to 350° C. In one aspect, which is combinable with any of the other aspects or embodiments, the sintering is conducted for a time in a range of less than 45 min. In one aspect, which is combinable with any of the other aspects or embodiments, the sintering comprises: continually feeding the green tape through a sintering chamber at a predetermined rate measured in in/min.

In some embodiments, a method for forming a sintered composition comprises providing a slurry precursor including at least one first metal carbonate compound and at least one second metal carbonate compound; tape casting the slurry precursor to form a green tape; and sintering the green tape at a temperature in a range of 500° C. to 1350° C. for a time in a range of less than 60 min.

In one aspect, which is combinable with any of the other aspects or embodiments, the at least one first metal carbonate compound comprises Li₂CO₃. In one aspect, which is combinable with any of the other aspects or embodiments, the at least one second metal carbonate compound comprises CoCO₃, MnCO₃, Al₂(CO₃)₃, or nickel carbonate. In one aspect, which is combinable with any of the other aspects or embodiments, the sintering comprises: continually feeding the green tape through a sintering chamber at a predetermined rate measured in in/min; and reacting the at least one first metal carbonate compound with the at least one second metal carbonate compound to form a chalcogenide compound. In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound comprises at least one of lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, lithium titanium sulfide, or combinations thereof.

In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound is at least 50 wt. % of the total weight of the sintered composition. In one aspect, which is combinable with any of the other aspects or embodiments, the chalcogenide compound comprises LCO or LMO. In one aspect, which is combinable with any of the other aspects or embodiments, a final thickness of the sintered composition is in a range of 2 μm to 100 μm immediately after the sintering without further processing.

In some embodiments, an energy device comprises a first sintered, non-polished electrode having a first surface and a second surface; a first current collector disposed on the first surface of the first electrode; an electrolyte layer disposed on the second surface of the first electrode; and a second electrode disposed on the electrolyte layer.

In one aspect, which is combinable with any of the other aspects or embodiments, the second electrode is a second sintered, non-polished electrode. In one aspect, which is combinable with any of the other aspects or embodiments, a second current collector is disposed on the second electrode. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode comprises at least one of: lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, lithium nickel manganate, lithium titanium sulfide (LiTiS₂), NaVPO₄F, NaMnO₂, Na_2/3Mn_1-yMg_yO₂(0<y<1), Na₂Li₂Ti₅O₁₂, Na₂Ti₃O₇, MgCr₂O₄, or MgMn₂O₄.

In one aspect, which is combinable with any of the other aspects or embodiments, the second electrode is lithium metal or magnesium metal. In one aspect, which is combinable with any of the other aspects or embodiments, the second electrode is at least one of lithium titanate, lithium niobium tungstate, Na₂Li₂Ti₅O₁₂, or Na₂Ti₃O₇. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode comprises an open porosity of at least 30 vol. %. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode comprises an open porosity of at most 50 vol. %. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode comprises a porosity of no more than 30%. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode comprises a porosity of no more than 30%. In one aspect, which is combinable with any of the other aspects or embodiments, the electrolyte layer has a conductivity of at least 10'S/cm. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode is a substrate of the energy device. In one aspect, which is combinable with any of the other aspects or embodiments, the first electrode has a thickness of greater than 500 μm or less than 10 μm.

In some embodiments, an electrode for an energy device comprises grains of at least one of lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, lithium nickel manganate, lithium titanium sulfide (LiTiS₂), NaVPO₄F, NaMnO₂, Na_2/3Mn_1-yMg_yO₂(0<y<1), Na₂Li₂Ti₅O₁₂, Na₂Ti₃O₇, MgCr₂O₄, or MgMn₂O₄, sintered to one another to form a porous sheet, wherein the porous sheet has a thickness of greater than 500 μm or less than 10 μm, and a porosity of no more than 30%.

Additional features and advantages will be set forth in the detailed description that follows, and, in part, will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework to understand the nature and character of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and the operation of the various embodiments.

FIG. 1 is a schematic, cross-sectional view depicting a lithium-ion battery having a sintered cathode, according to an exemplary embodiment.

FIG. 2 is a schematic, cross-sectional view of a conventional lithium-ion battery.

FIG. 3 is graph of the charge capacity of the battery of FIG. 1 as compared to the charge capacity of the battery of FIG. 2.

FIG. 4 depicts a green tape being sintered and illustrating the issue of flammability according to conventional sintering processes.

FIG. 5 depicts a graph of DSC curves illustrating heat release as a function of temperature during drying/sintering.

FIGS. 6A and 6B depict an image of LCO tape rapidly debound and sintered with a soak temperature of 1100° C. for a time of 20 min and an image of LCO tape rapidly debound and sintered with a soak temperature of 1100° C. for a time of 1.25 min, respectively.

FIG. 7 depicts an exemplary embodiment of a continuously and rapidly sintered tape.

FIG. 8 is a comparison of the XRD spectra for the sintered LCO tape as compared to the as-received LCO powder.

FIG. 9 is an SEM image of an as-fired LCO tape sintered at 1050° C. for 40 min.

FIGS. 10A and 10B are pictures of an electrochemical cell having a sintered cathode according to an exemplary embodiment.

FIG. 11 depicts a graph of charging capacity as a function of the number of charge/discharge cycles for the electrochemical cell of FIGS. 10A and 10B.

FIG. 12 depicts a graph of charging capacity for the electrochemical cell of FIGS. 10A and 10B as compared to other cathode materials having different porosities and thicknesses.

FIGS. 13A and 13B depict SEM images of the microstructure of LCO immediately after sintering and after charge-discharge cycling, respectively.

FIG. 14 illustrates the tape of slip formulation E6 after ignition and burning due to pyrolysis of organics in the dried green tape.

FIG. 15 illustrates a polished, high-resolution scanning electron microscopy (SEM) cross-section of a rapid and continuously sintered LCO cathode.

FIG. 16 illustrates charging capacity as a function of C-Rate for three different testing cells using rapidly-sintered cathode discs of FIG. 15.

FIG. 17 illustrates a polished, high-resolution SEM cross-section of a rapid and continuously sintered NMC 111 cathode.

FIG. 18 illustrates charging capacity as a function of C-Rate for a testing cell using rapidly-sintered cathode discs of FIG. 17.

FIGS. 19A and 19B illustrate polished, high resolution SEM cross-sections of rapidly sintered closed pore LCO cathodes.

DETAILED DESCRIPTION

Referring generally to the figures, various embodiments of a sintered electrode that includes a chalcogenide or fluoride and at least one alkali metal or alkaline earth metal are disclosed. The sintered electrode has a thickness of 2 μm to 100 μm and a cross-sectional area of at least 3 cm². Compared to conventional electrode materials, the sintered electrode can be made much larger and self-supporting than typical thin-film formed electrodes and is usable without any additional finishing techniques, such as grinding or polishing, in contrast to other sintered electrodes. The disclosed sintered electrodes are able to achieve these advantages through a tape manufacturing process that allows for must faster manufacturing speeds of “medium” thickness electrode materials in which processing speed is independent of electrode thickness. That is, the electrodes can be made thicker than conventional electrodes made through thin film techniques and thinner than other sintered electrodes that have to be ground down to usable sizes. Moreover, the electrode can be rapidly sintered in a more economical process than is currently used for manufacturing electrode materials. Indeed, conventional processes typically utilize thin film techniques that are much slower and more difficult to build up thick layers. In this way, the relatively thicker sintered electrodes of the present disclose not only eliminate inactive components, such as mechanical supports, but also increase the charge capacity of the battery. Moreover, the thickness of the electrode and tape-casting manufacturing process allow for electrode materials to be manufactured in a roll-to-roll format.

The sintered electrodes disclosed herein are envisioned to be suitable for a variety of battery chemistries, including lithium-ion, sodium-ion, and magnesium-ion batteries as well those using solid state or liquid electrolyte. Various embodiments of the sintered electrode, manufacturing process, and lithium-ion batteries are disclosed herein. Such embodiments are provided by way of example and not by way of limitation.

As mentioned, various embodiments of a sintered electrode composed of chalcogenide compound including at least one of an alkali metal or alkaline earth metal. A chalcogenide compound refers to an oxide, sulfide, or selenide compound. In other embodiments, the sintered electrode may be a fluoride compound. In embodiments, the chalcogenide includes at least one of lithium, sodium, or magnesium. In embodiments, the chalcogenide also includes at least one transition metal, such as cobalt, manganese, nickel, niobium, tantalum, vanadium, titanium, copper, chromium, tungsten, molybdenum, tin, germanium, antimony, bismuth, or iron. In some examples, a chalcogenide compound refers to a compound comprising an alkali ion and a transition metal.

Exemplary embodiments of a lithium chalcogenide include lithium cobaltite (LCO), lithium manganite spinel (LMO), lithium nickel cobalt aluminate (NCA), lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), lithium cobalt phosphate (LCP), lithium titanate, lithium niobium tungstate, lithium nickel manganate, and lithium titanium sulfide (LiTiS₂), among others. Exemplary embodiments of a sodium chalcogenide include NaVPO₄F, NaMnO₂, Na_2/3Mn_1-yMg_yO₂(0<y<1), Na₂Li₂Ti₅O₁₂, or Na₂Ti₃O₇, among others. Exemplary embodiments of a magnesium chalcogenide include magnesiochromite (MgCr₂O₄) and MgMn₂O₄, among others.

In embodiments, the chalcogenide (or fluoride) is a first phase of the sintered electrode, and the sintered electrode contains at least one other phase (e.g., a second phase, a third phase, a fourth phase, etc.) intermixed with the first phase. In embodiments, the additional phase or phases are selected to provide additional functionality. For example, in an embodiment involving a lithium electrode, a second phase enhances the effective lithium conductivity of the electrode, for example a lithium garnet phase. In an embodiment, the second phase enhances electronic conductivity. The additional phase or phases can be added prior to sintering, or the sintered electrode may contain open porosity that may be infiltrated with the additional phase or phases. In embodiments, the second phase is a spinel that provides additional electronic conductivity.

One advantage of the sintered electrodes disclosed herein is that they can be made larger than conventional electrode materials for batteries, such as those made using thin-film techniques. In embodiments, the sintered electrode has a thickness of from 2 μm to 100 μm. In further embodiments, the sintered electrode has a thickness of from 20 μm to 80 μm, and in still other embodiments, the sintered electrode has a thickness of from 30 μm to 60 μm. Besides being thicker than thin-film electrodes, the sintered electrode can also be made with a relatively larger cross-sectional area. In embodiments, the sintered electrode has a cross-sectional area of at least 3 cm². In further embodiments, the sintered electrode has a cross-sectional area of at least 10 cm², and in still other embodiments, the sintered electrode has a cross-sectional area of at least 100 cm². In embodiments, the sintered electrode has a cross-sectional area of up to 1 m².

The sintered electrode is able to be made larger than conventional thin-film electrodes because the electrode is formed from a tape cast or extruded green tape that is rapidly sintered. In order to form the green tape, a slurry (or paste) is prepared from a powder component, a binder, and a solvent. The powder component includes a powdered compound or powdered compounds containing a chalcogenide and at least one alkali metal or alkaline earth metal. The powdered compounds containing the chalcogenide and the alkali metal or alkaline earth metal may be a single powdered compound. Alternatively or additionally, the compounds can include chalcogenide compound and a compound containing an alkali metal or alkaline earth metal. Further, in embodiments, the powdered compound can further contain a transition metal along with or in a separate compound from the chalcogenide compound and the compound containing an alkali metal or alkaline earth metal.

For example, with respect to a lithium electrode, the powdered compound can be a chalcogenide compound containing lithium and a transition metal, such as LCO or LMO. In another example, one compound can contain the chalcogenide and the compound containing an alkali metal or alkaline earth metal, and another compound can contain a transition metal. For example, with respect to a lithium electrode, the chalcogenide compound can be at least one of Li₂O, Li₂CO₃, LiOH, LiNO₃, lithium acetate (CH₃COOLi), or lithium citrate (Li₃C₆H₅O₇), among others, and the transition metal-containing compound can be at least one of MnO₂, Mn₂O₃, Co₂O₃, CoO, NiO, Ni₂O₃, Fe₂O₃, Fe₃O₄, FeO, TiO₂, Nb₂O₅, V₂O₅, VO₂, Ta₂O₅, or WO₃. In embodiments, the powder component of the slurry or paste (including all powdered compounds) comprises from 40% to 75% by weight of the slurry (or paste). In other embodiments, the powder component comprises from 45% to 60% by weight of the slurry (or paste), and in still other embodiments, the powder component comprises from 50% to 55% by weight of the slurry (or paste).

The slurry (or paste) is provided with a binder that holds the powder component together in the form of the green tape prior to sintering. In embodiments, the binder is at least one of polyvinyl butyral (PVB) (e.g., Butvar® PVB resins, available from Eastman Chemical Company), acrylic polymers (e.g., Elvacite® acrylic resins, available from Lucite International), or polyvinyl alcohol, among others.

The slurry (or paste) is also provided with a solvent in which the powder component and binder are dispersed. In particular, the solvent is selected so as to avoid leaching the alkali metal or alkali earth metal from the chalcogenide compounds in the slurry. Table 2, below, demonstrates leaching characteristics for two solvents with respect to lithium ions, non-polar 1-methoxy-2-propanyl acetate (MPA) and a polar ethanol-butanol mixture. In investigating the leaching characteristics of the two solvents, 200 g of the powdered electrode material identified in Table 2 were mixed with the 200 g of the solvent. The mixture was centrifuged, and the decanted liquid was analyzed for its lithium concentration via induction coupled plasma (ICP) spectroscopy. As shown in Table 2, the polar ethanol-butanol mixture contained a much greater concentration of lithium than the non-polar MPA. Such leaching of the lithium from the ceramics (e.g., LCO, LMO, etc.) can occur as the result of ion exchange or the formation of hydroxides. Once the lithium enters the solvent, there can be several unwanted side-effects. For example, the solubility of the binder may be reduced. Further, the dissolved lithium may interfere with dispersants. Still further, the dissolved lithium may migrate during drying, which may lead to chemical inhomogeneity in the dried tape. Additionally, the chemistry of the inorganic particles themselves is altered. Moreover, reaction with the solvent is time dependent so the slip properties are subject to continuous change and a potentially unstable process.

TABLE 1

Leaching of lithium from electrode

material in non-polar and polar solvents.

electrode

Li Concentration

Material
Solvent
(×10⁻⁶mg/L)

LMO
MPA
<0.005

LMO
MPA
<0.005

LMO
Ethanol-Butanol Mixture
1.61

LMO
Ethanol-Butanol Mixture
1.77

LCO
MPA
<0.005

LCO
MPA
<0.005

LCO
Ethanol-Butanol Mixture
2.05

LCO
Ethanol-Butanol Mixture
2.28

Accordingly, in embodiments, the solvent is selected to be non-polar. In particular embodiments, the non-polar solvent has a dielectric constant at 20° C. of less than 20. In other embodiments, the non-polar solvent has as dielectric constant at 20° C. of less than 10, and in still other embodiments, the non-polar solvent has a dielectric constant at 20° C. of less than 5. Further, in embodiments, the solvent leaches less than 1 ng/L of the alkali metal or alkaline earth metal from the powder component in the slurry. In other embodiments, the solvent leaches less than 0.1 ng/L of the alkali metal or alkaline earth metal from the powder component in the slurry, and in still other embodiments, the solvent leaches less than 0.01 ng/L of the alkali metal or alkaline earth metal from the powder component in the slurry.

In embodiments, the chemistry of the binder may be adjusted to work with non-polar solvents, such as MPA. For example, Butvar® B-79 is a commercially available PVB that has a low concentration of hydroxyl groups from polyvinyl alchohol (11-13% by weight) and, compared to other PVB binders, has a low molecular weight. This allows for ease of dissolution and high solubility to control viscosity and enable a high loading of solids.

In embodiments, that slurry (or paste) may contain other additives that aid in processing. For example, in embodiments, the slurry (or paste) may contain between 0.1% to 5% by weight of a dispersant and/or of a plasticizer. An exemplary dispersant is fish-oil dispersant, and an exemplary plasticizer is dibutyl phthalate. Further, as will be discussed more fully below, the presence of transition metal oxides in the slurry (or paste) can cause a catalytic combustion reaction during sintering. Thus, in embodiments, the slurry (or paste) may contain additives to prevent or reduce the severity of such combustion reactions. In particular, the slurry (or paste) may contain an antioxidant, such as a phenol (e.g., butylated hydroxytoluene (BHT) or alkylated-diphenylamine), or materials with an endothermic decomposition like inorganic carbonates and hydroxides.

The slurry (or paste) is tape cast or extruded into a green tape having the desired thickness of the sintered electrode. As discussed above, the thickness may be in the range of from 2 μm to 100 μm. In embodiments, the green tape is dried to remove a substantial portion of the solvent, leaving primarily the chalcogenide compound containing the alkali metal or alkaline earth metal. In embodiments, drying may occur at ambient temperature or at a slightly elevated temperature of 60° C. to 80° C. (or begin at an ambient temperature and transition to an elevated temperature). Additionally, in embodiments, air is circulated to enhance drying. In embodiments, the amount of organic material remaining after drying is no more than 10% by weight of the dried green tape. Upon drying the green tape is debound and sintered. That is, the green tape is heated to a temperature at which the polymer binder and any other organics are burned off. In embodiments, debinding occurs in the temperature range of 175° C. to 350° C. Thereafter, the dried and debound green tape is sintered. Sintering occurs in the temperature range of 500° C. to 1350° C. Sintering time in this temperature range is less than 60 minutes. In embodiments, sintering time is less than 50 minutes, and in still other embodiments, sintering time is less than 45 minutes. Upon sintering, the sintered electrode has a porosity of no more than 30%. In embodiments, the sintered electrode tape has a porosity of no more than 25%. In other embodiments, the sintered electrode has a porosity of no more than 20%, and in still other embodiments, the sintered electrode has a porosity of no more than 15%. In embodiments, the porosity of the sintered electrode is at least 0.1%. As a result of the sintering process, in embodiments, the sintered electrode has on average a grain size of from 10 nm to 50 μm. In other embodiments, the grain size on average is from 50 nm to 10 μm, and in still other embodiments, the grain size on average is from 100 nm to 1000 nm.

Further, in embodiments, the sintered electrode has an open porosity such that fluid communication is provided between a first surface of the sintered electrode to the other surface. That is, in embodiments, the chalcogenide compound phase comprises a solid phase, and the porosity comprises a second phase in which the second phase is a continuous phase in the solid phase. Additionally, in embodiments, the pores of the sintered electrode tape are substantially aligned to promote ion transport. That is, the pores are aligned along an axis perpendicular to the first and second surfaces. For example, each pore may have a cross-sectional dimension that is longer than any other cross-sectional dimension of the pore, and the longer cross-section dimension is substantially aligned perpendicularly to the first and second surfaces of the electrode, e.g., on average, aligned to within 25° of perpendicular. Advantageously, in contrast to other sintered electrodes, the sintering process described produces a sintered electrode that requires no further finishing, such as mechanical grinding or polishing, prior to incorporating into a battery architecture. In particular, previous sintered electrodes were formed from large discs at much greater thicknesses, e.g., 500 μm to 1 mm, and had to be diced to usable dimensions and ground down to a usable thickness. Such grinding has reportedly only been able to achieve a thickness of about 130 μm, which is the practical limit for electrodes manufactured according to such processes. By tape-casting the electrode, not only is the process made more economical (e.g., no grinding/polishing steps and ability to utilize roll-to-roll fabrication), but also desirable thicknesses of the electrode material can be achieved.

Further, because the sintered electrode is self-supporting, the sintered electrode can be used as a substrate for deposition of additional layers. For example, a metallic layer (e.g., up to 5 μm) can be deposited onto a surface of the sintered electrode to serve as a current collector for a battery. Additionally, in an exemplary embodiment, a solid electrolyte, such as lithium-phosphorous-oxynitride (LiPON), lithium garnet (e.g., garnet LLZO (Li₇La₃Zr₂O₁₂)), or lithium phosphosulfide, may be deposited by RF-sputtering onto the sintered electrode. Alternatively, a thin layer of LiPON solid electrolyte can be applied through ammonolysis of a thin layer of Li₃PO₄or LiPO₃or through reactive sintering. Such processes are envisioned to be faster and potentially less capital intensive than conventional deposition techniques for solid electrolytes. Similarly, a solid electrolyte of lithium garnet (e.g., LLZO) can be applied by sol-gel, direct sintering, and reactive sintering.

Further, as a self-supporting layer, the sintered electrode can provide the basis for an advantaged manufacturing approach for lithium batteries that use a liquid electrolyte. In other words, the cathode (i.e., sintered electrode) is a substrate of the battery. In particular, the sintered electrode can be made in a continuous process and used as a substrate for coating in either batch or roll-to-roll processing. Such processing could allow, for example, metallization of the sintered electrode by sputtering and/or electrolytic deposition to form a metallized sintered electrode. In this way, the thickness of the electrode current collector metal can for a conventional lithium battery can be reduced from the typical thickness of 10-15 μm to less than 5 μm, less than 1 μm, or even less than 100 nm. Further, the metallized sintered electrode can be supplied in piece or roll form as a stand-alone component to a battery cell manufacturer. Advantageously, such metallized sintered electrodes reduce the volume of the cell typically reserved for the current collector, allowing for more active electrode material and higher capacity.

In this regard, the sintered electrode is particularly suitable for use in ion intercalation type batteries. An exemplary embodiment of a lithium-ion battery 10 is shown in FIG. 1. The lithium-ion battery 10 includes a sintered cathode 12, an electrolyte layer or region 14, and an anode 16. In embodiments, the sintered cathode 12 has a thickness of from 2 μm to 100 μm. Additionally, in embodiments, the sintered cathode 12 has a cross-sectional area of at least 3 cm². Advantageously, the sintered cathode 12 mechanically supports the lithium-ion battery 10 such that the sintered cathode 12 is not carried on a mechanical support, such as a zirconia support. The advantage of this architecture is that inactive components are substantially excluded from the battery. That is, while providing the function of a mechanical support, the sintered cathode 12 is still an active component and contributes to the capacity of the battery. Accordingly, the cathode-supported design can give the same overall capacity in a thinner form-factor, or the thickness of the cathode can be increased for a higher net capacity at the same size.

Further, the sintered cathode 12 can be used in both solid-state and liquid electrolyte lithium-ion batteries. In particular, in a solid-state battery, the electrolyte layer 14 includes a solid-state electrolyte (e.g., having a conductivity of >10-6 S/cm), such as LiPON, lithium garnet (e.g., LLZO), or lithium phosphosulfide. More particularly, in a solid-state battery, the electrolyte layer 14 includes a solid electrolyte, such as LiPON, lithium garnet (e.g., LLZO), lithium phosphosulfide, or lithium super ionic conductor (LISICON), with a combination of lithium ion conductivity and thickness such that the area specific resistance is less than about 100 Ωcm². One advantage of LiPON, in particular, is that it is resistant to dendrite formation. In a liquid electrolyte battery, the electrolyte layer 14 includes a liquid electrolyte, such as LiPF₆-DMC (lithium hexafluorophasophate in dimethyl carbonate), and a polymer or ceramic separator to separate the cathode 12 and anode 16. In either case, the sintered cathode 12 increases the charge capacity over conventional lithium-ion batteries.

The battery 10 also includes a first current collector 18 disposed on a first surface of the sintered cathode 12. In the embodiment depicted, a second current collector 20 is disposed on the anode 16; however, in embodiments, the anode may be a metal (such as lithium metal or magnesium metal) in which case a current collector may be excluded. Further, in the embodiment depicted, the battery 10 is encased in a protective coating 22. In embodiments, the first current collector 18 is copper, and the second current collector 20 (when used) is aluminum. The protective coating 22 may be, e.g., parylene.

While the depicted embodiment only includes a sintered cathode 12, the anode 16 may also be a sintered electrode according to the present disclosure. For a lithium-ion battery, the (sintered) cathode 12 may include at least one of lithium cobaltite, lithium manganite spinel, lithium nickel cobalt aluminate, lithium nickel manganese cobalt oxide, lithium iron phosphate, lithium cobalt phosphate, lithium nickel manganate, or lithium titanium sulfide, and the (sintered) anode 16 may include at least one of lithium titanate or lithium niobium tungstate.

Additionally, while a lithium-ion battery is depicted, the battery could instead be based on sodium-ion, calcium-ion, or magnesium-ion chemistries. For a sodium-ion battery, the (sintered) cathode 12 may include at least one of NaMnO₂, Na_2/3Mn_1-yMg_yO₂(0<y<1), or NaVPO₄F, and the (sintered) anode 16 may include at least one of Na₂Li₂Ti₅O₁₂or Na₂Ti₃O₇. For a magnesium-ion battery, the (sintered) cathode 12 may include at least one of MgCr₂O₄or MgMn₂O₄, and the anode 16 may magnesium metal (which could also serve as the current collector 20). Any of the foregoing battery chemistries may utilize a liquid electrolyte comprising a solvent (e.g., DMC) and a salt with a cation matching the intercalant ion. Additionally, for a sodium-ion battery, sodium super ionic conductor (NASICON) may be used as a solid-state electrolyte.

For the purposes of demonstrating the gain in capacity, FIG. 2 provides a schematic cross-section of a conventional solid-state, thin-film micro-battery 100. The micro-battery 100 includes a cathode current collector 102 and an anode current collector 104 deposited onto an inert mechanical support 106. A cathode 108 (e.g., LCO or LMO) is formed onto the cathode current collector 102 and is surrounded by a solid-state electrolyte 110 (e.g., LiPON). An anode 112 is deposited over the electrolyte 110 and over the anode current collector 104. A coating 114 is provided to protect the cathode 108, electrolyte 110, and anode 112. In the conventional battery design, the mechanical support 106 is relied upon for handling during fabrication of the battery 100 and is the platform for the deposition of the cathode 108 and electrolyte 110 layers. The mechanical support 106 typically has a thickness of 50 μm to 100 μm. The mechanical support 106 and the protective coating 114 also provide rigidity in the final package and help prevent damage.

In these conventional batteries 100, the cathode 108 is typically grown to desired thickness by processes such as RF sputtering or pulsed laser deposition. These deposition techniques are another reason why the conventional battery 100 requires the use of mechanical support 106. Such conventional methods produce cathode materials at a rate of <10 μm/hr, which creates a practical and commercial limit to the achievable thicknesses of these conventional cathode materials. As a consequence, thin film micro-batteries have only found applications where small size power sources are needed like smart cards, medical implants, RFID tags, and wireless sensing.

A comparison of the charge capacity of battery 10 of FIG. 1 according to the present disclosure and the charge capacity conventional battery 100 of FIG. 2 is shown in FIG. 3. The comparison is made at nominally identical thicknesses of 80 μm. In particular, the comparison is made between (1) a conventional battery 100 having a 50 μm thick mechanical support 106 of zirconia and a cathode that is 5 μm thick and (2) the presently disclosed battery 10 having a cathode 12 that is 35 μm thick. Notably, the thickness of the cathode 12 of the presently disclosed battery 10 is less than the thickness of the mechanical support 106 of the conventional battery 100, allowing space to be reserved for lithium metal at the anode 16. As can be seen in FIG. 3, the extra thickness of the sintered cathode 12 and removal of the mechanical support 106 provides a seven-fold higher capacity in absolute and volumetric terms, and the capacity is ten-fold greater on a weight basis.

Besides simply allowing for a larger electrode, the sintered cathode 12 of the depicted embodiment also provides structural advantages that increase its charge capacity over conventional cathodes. In a conventional cathode 108, the active cathode particles make point contacts. The cross-sectional areas of the contacts are small and so have a high impedance to movement of lithium ions and electrons. In order to overcome this impedance issue, carbon is added to the electrode as a conductive pathway to facilitate transport of electrons into and out of the active particles, and pore space in the electrodes are infiltrated with liquid electrolyte for fast conduction of lithium ions. The use of carbon in this manner creates a tradeoff between capacity of the batter and charge/charge rate performance. The other issue with the point contacts between the active cathode particles is that they are weak, and so polyvinyl fluoride (PVF) is used to bind the active particles and carbon together to give the structure strength during processing. In contrast, particles in the depicted sintered cathode 12 are bonded to one another, and so, the electronically conductive carbon and binder may be eliminated. In this way, the proportion of space allocated to porosity for movement of lithium ions may be reduced, and more space can be dedicated to active material with a sintered cathode. The inventors estimate that for a given cathode material, the capacity in aggregate can be raised by approximately 30% on the basis of equal cathode thicknesses. Alternatively, the cathode thickness could be reduced by 20-25% while keeping the capacity the same for a more compact battery. As mentioned above, the pores in the sintered cathode 12 can be aligned in the direction of transport of ions to and from the anode so as to enable further improvements in space utilization or to boost power density.

EXPERIMENTAL EXAMPLES

Five exemplary green tapes, including one comparative example (E1), one reference example (E4) and three examples according to the present disclosure (E2, E3, and E5), were prepared by tape casting the slurries described in Table 2. The LMO and LCO powders were obtained commercially from GELON LIB GROUP (Linyi, Shandong, China), and the alumina powder was obtained from Sasol (Houston, Tex.). The polyvinyl butyral binder was Butvar® B-79 obtained commercially from Eastman Chemical Company (Kingsport, Tenn.).

TABLE 2

Formulations of tape casting slurries

Weight Percentages

Slurry Component
E1
E2
E3
E4
E5

LiMn₂O₄(LMO)
48.98
52.58

LiCoO₂(LCO)

52.58

64.96

Alumina

52.58

1-Methoxy-2-propanyl
44.24
42.25
42.25
42.25
32.09

acetate (MPA)

Fish-Oil Dispersant
0.85
0.84
0.84
0.85
0.64

Dibutyl phthalate
0.98
0.84
0.84
0.84
0.64

Polyvinyl butyral binder
4.95
3.49
3.49
3.49
1.69

Total non-volatile organics
12.17
8.95
8.95
8.95
4.35

without MPA

Tapes that contain lithium-ion battery electrode materials with concentrations of organics above 10% by weight are difficult to rapidly debind and fire. In particular, when the concentration of organic material is above 10% by weight, the tape may become flammable and ignite if a critical temperature is exceeded before binder is removed. Once ignited a combustion front propagates and cracks the tape. FIG. 4 depicts tape E1 of Table 2 in which the green tape 200 has entered the sintering chamber 210, and the binder has combusted 220, creating a propagation front 230. As can be seen in Table 2, the organic component of green tape E1 is 12.17% by weight, which is above the threshold of 10% by weight. The flammability of the tape is a process bottleneck that limits the practical rate of continuous debinding and firing to about 60 minutes. As mentioned briefly above, the flammability of green tape E1 results in part from the interaction between the reducible transition metal oxide in the electrode material and the organic materials. In particular, the transition metal oxides speed combustion by acting as a catalyst and by producing oxygen.

The effect is illustrated in differential scanning calorimetry (DSC) traces shown in FIG. 5. As can be seen there, tapes E2 and E3 contain LMO and LCO, respectively, and begin to combust at approximately 80° C. lower than reference tape E4 with alumina (which does not contain a transition metal). In comparison to tape E4, the total amount of heat released for tapes E2 and E3 is greater because LMO and LCO of tapes E2 and E3, respectively, promote more complete combustion of organics while volatile oxides escape the alumina tape E4. However, as can be seen in the trace of tape E5 in which the amount of organic material is below 5% by weight, the heat released becomes comparable to the alumina reference tape E4. That is, for tape E5, the concentration of combustible organics was reduced by more than half to just 4.35% by weight. While the temperature for onset of combustion is the same, the total amount of released heat drops in concert with the change in concentration of organics relative to tape E4. At the lower binder concentration, the tape may be debound and sintered at significantly higher speeds.

Despite the much lower binder concentration, it was surprisingly found that tape E5 was sufficiently strong for release from the polymer carrier film and for manual handling. FIGS. 6 and 7 provide pictures of two examples of tape E5 that were rapidly sintered. The size of each tape was approximately 8 cm long by 4 cm wide. The tape in FIG. 6A was debound by pulling through the furnace at 300° C. in a first pass and then sintering in a second pass at 1100° C. Each pass lasted for ten minutes for a total processing time of 20 min. The example in FIG. 6B was debound and sintered in a single step by pulling into the furnace at 1100° C. at a rate of 32 in/min (total residence time of 75 seconds). The sintered thickness of the tape E5 was nominally 60 μm.

The rapid sintering can be operated in a continuous fashion working with individual pieces or in a roll-to-roll configuration. The process efficiently produces large areas of sintered electrode material with thicknesses relevant for batteries. FIG. 7 is an example of continuously and rapidly sintered tape of the E5 formulation that approximately 40 mm wide by 600 mm long and is 70 μm thick. Debinding and sintering were conducted in a single step with a total residence time of 20 min at a maximum temperature of 1100° C.

Besides lowering the concentration of combustible organics in the green tape, other means to lessen flammability are envisioned. As mentioned above, combustion may be slowed by the addition of a small quantity of antioxidant to the slurry. Additionally, the powder component may be selected based on the ability of the powdered compounds to undergo an endothermic reaction in the temperature range of 200° C. to 300° C. For example, to produce the LCO or LMO, CoCO₃or MnCO₃may be included in the slip along with a balance of Li₂CO₃for reaction to form LiCO₂(LCO) or LiMn₂O₄(LMO) during the sinter step. In other examples, aluminum carbonate (Al₂(CO₃)₃) or nickel carbonate may be included to react with Li₂CO₃. Decomposition of either carbonate takes place between 150° C. and 300° C., and the process is endothermic, thereby inhibiting combustion. In this way, the amount of binder material can be increased as desired, e.g., for strengthening of the green tape.

A coupon of LCO tape composition E5 that measured 90 cm by 40 cm was sintered at 1050° C. in a cycle with a total duration of 40 minutes. The as-sintered tape had a nominal thickness of 60 μm. As shown in FIG. 8, powder x-ray diffraction confirmed that the chemistry and structure of the sintered LCO is similar to that of the as-received LCO. FIG. 9 is an SEM image of the sintered tape's as-fired surface. The porosity was estimated to be 8-10% by image analysis, and the pore structure was determined to be open via a dye test.

The charge capacity of the exemplary tape cast and sintered electrode was determined by preparing an electrochemical cell 300 as shown in FIGS. 10A and 10B. A cathode disc 310 of the sintered electrode was laser cut to a diameter of 8 mm. Electrical connection to the cathode disc 310 was made with a 4 mil platinum wire 320 that was secured by gold ink 330. The gold ink also fully covered one face of the cathode disc 310 as shown in FIG. 10A so as to force transport of lithium ions through the other face shown in FIG. 10B. The anode 340 was selected to be lithium metal, which also served as a reference electrode. The cathode disc 310 was immersed in a solution of 1 M LiPF₆in 50:50 ethylene carbonate and dimethyl carbonate electrolyte (BASF Selectilyte LP-30) such that the pores were filled with the conductive fluid. The sample was subjected to charge-discharge cycling at rates of C/20 to 2 C. Charging was performed under constant current and then constant voltage charging with 4.2 or 4.3 V potentials. Discharge was at constant current and down to 3 V limit. FIG. 11 shows the charge capacity and stability of the sintered LCO cathode disc 310 through twenty charge-discharge cycles at a C/5 rate and 4.3 V charging potential. The capacity closely matches the theoretical value of 140 mAhr/g for LCO, and there is no evidence of fade in capacity as cycles are accumulated. Thus, the tape cast and rapidly sintered LCO showed near theoretical capacity in the testing of the electrochemical cell. The rapidly sintered LCO electrode also retained high capacity as charging speed increased. As shown in FIG. 12, there is little drop in capacity even as charging speed is increased at fixed potentials of 4.2 and 4.3 V up C/2.

The inventors were surprised by the unexpected result that capacity was retained through multiple charge-discharge cycles. A fade or even abrupt failure had been expected from cracking of the brittle sintered electrode. There are two mechanisms known to drive cracking. One is a large bulk strain in the electrode with intercalation and de-intercalation of lithium. Mismatch strains arising from differential states of charge through the thickness of the electrode, especially when charging or discharging at high rates, tend to exceed the strain tolerance of a brittle ceramic. LCO and most other electrode materials, such as LFP, are anisotropic, and so, differential expansion on charge-discharge cycling drives microcracking. The cracks are thought to break up the electrode leading to formation of isolated islands and increase tortuosity for transport of electrons. The microstructure of LCO immediately after sintering is shown in FIG. 13A, and the microstructure of LCO after charge-discharge cycling is shown in FIG. 13B. As can be seen in a comparison of FIGS. 13A and 13B, microcracks have developed after charge-discharge cycling. While such microcracks did not affect the charge capacity of the LCO electrode, the microcracks could be avoided by making the grain size sufficiently small.

The plot in FIG. 12 also makes a comparison with thicker conventionally sintered and machined cathodes. According to the literature, the cathodes were sintered from 12.5 mm diameter pressed pellets of LCO in a bed of the same powder to limit loss of lithium. The heating rate was 9° C./min and soak time was 90 min. The sintered pills were ground to remove a dense outer shell, polished to 5 μm roughness and diced into 2.2 mm square sections. The diced pieces were finished to a thickness down to 260 μm, had two levels of open porosity (13% and 26%), and were evaluated for potential use as high capacity and energy density micro-batteries. The cells utilized liquid electrolyte and conventional porous polymer separator. There were two differences in testing conditions. In particular, the cells were charged at constant current up to a potential of 4.25 V and discharged down to 2.5 V. In the conventionally produced cathode, porosity was important for infiltration by liquid electrolyte to provide a more conductive pathway for lithium ions than is available from the LCO alone. The importance of the pores grows as the thickness of the electrode increases. Even at 26% porosity, the thinnest 260 μm cathode showed a marked drops in charge capacity charging at a rate of C/3. As such, the conventional cathodes present a difficult trade-off between volumetric capacity and charging rate. That is, porosity must be added to reach faster charging rates, but this comes at the expense of capacity.

In contrast, the sintered electrode disclosed herein was made in a thinner form factor of 60 μm without the need to employ costly machining processes. The thickness is not reduced to such an extent that it becomes small in proportion to thicknesses of other components, such as 1-2 μm layer of LiPON electrolyte in a thin film battery. However, its thickness is still roughly a factor of four less than the conventional cathodes referenced in FIG. 12. As such, transport distances for lithium ions and electrons are shortened, and capacity is retained to high charging speeds with little dependence on porosity to hold a lithium conducting electrolyte. The capacity of the sintered cathode at one C is more than 80% of theoretical at a current density of 3.5 mA/cm².

As used herein, the phrase “self-supporting” refers to a structure that is not adhered or supported by an underlying substrate (i.e., inert mechanical support). In some examples, self-supporting sintered electrodes are free-standing, which can be mechanically manipulated or moved without need of a substrate adhered or fixed thereto and which can itself be used as a substrate for deposition of additional layers. Thus, for the embodiments described herein, a self-supporting sintered electrode serves a dual function of being a support upon which additional energy storage elements (e.g., electrolyte layer, current collector, etc.) may be disposed and being an active component (e.g., cathode or anode) of the battery.

As used herein, the phrase “cross-sectional area” refers to a cathode surface area that may be used to support construction of a battery structure. For example, with reference to FIG. 1, the cross-sectional area of sintered cathode 12 is defined by the horizontal length (e.g., width) of the cathode (measured as the length between protective coating 22) and the depth of the cathode (into the page).

Differences between rapid (e.g., <1 hr), continuously-fired sintering processes described herein and batch sintering processes with prolonged heating times (e.g., >10 hrs)

(1) Lithium Volatilization

One challenge in preparing sintered lithium containing materials is a loss of lithium due to volatilization. Vapor pressure of lithium oxides and lithium hydroxides are well-known from literature to be unacceptably high. Loss of lithium increases as sintering temperatures and times increase. Issues associated with loss of lithium are: (i) spatial variations in the chemistry of the product, (ii) compositional variations within the product, (iii) accumulation of lithium species in cooler regions of the furnace, and (iv) reaction of the lithium species with the kiln hardware itself. The variation in chemistry of the product are be damaging to battery performance and shorten lifetime of furnace hardware.

Rapid and continuous sintering processes limit the amount of loss of lithium during sintering, for example, of lithium-containing electrodes such as LiCoO₂(lithium cobaltite, LCO). In one example, green tapes with a dried thickness of about 34 μm were prepared using the slip formulations of Table 3. C1 represents comparative example formed by batch sintering processes with prolonged heating times (e.g., >10 hrs or >20 hrs or >30 hrs).

TABLE 3

Weight Percentages (wt. %)

Slurry Component
Function
E5
E6
E7
C1

LiCoO₂(LCO)
Active cpd
64.96
58.58
—
46.30

Li(NiMnCo)_1/3O₂(NMC)

—
—
63.65
—

1-Methoxy-2-propanyl acetate (MPA)
Solvent
32.09
—
33.28
—

Methyl Ethyl Ketone (MEK)

—
18.56
—
—

Toluene

—
18.56
—
23.15

2-Propanol

—
—
—
23.15

Fish-Oil Dispersant
Dispersant
0.64
—
0.66
—

Rheodol

—
—
—
0.93

Dibutyl phthalate
Plasticizer
0.64
0.80
0.66
—

Di-2-ethylhexyl phthalate

—
—
—
1.85

Hypermer KD-1 (polyester-polyamine)

—
0.80
—
—

Polyvinyl butyral binder, B79
Binder
1.69
—
1.75
—

Polyvinyl butyral binder, B76

—
2.69
—
—

Polyvinyl butyral binder, BM-2

—
—
4.63

Total non-volatile organic in dry tape

4.35
6.84
4.60
13.79

The dried tape of E6 was pulled through a rapid sintering system starting with a binder burn-out zone, comprising two opposed air bearings roughly 10 mm apart. The purpose of the air bearings is to “float” the tape as the binder pyrolyzes to prevent breakage. The burn-out zone measures approximately 300 mm in length and has six temperature controlled segments programmed to produce a linear temperature ramp from 225° C. to 325° C. The furnace for sintering was programmed to a 1050° C. setpoint and measures approximately 1 meter in length. The E6 formulation tape with a length of 400 mm and width of 52 mm was pulled through the rapid sintering system at a rate of 64 mm/min. Total residence time of the tape in the system was less than 21 min.

It is essential for Li-battery cathodes to maintain proper chemistry for at least the following reasons. One, the cathode serves as a source of lithium. The proportion of lithium relative to the transition metal oxide(s)—in this case, the ratio of Li:Co—is critical to formation and preservation of targeted crystal structures—in this case, a layered rock salt structure with a Li:Co close to 1. Changes in the Li:transition metal oxide(s) ratio after sintering were assessed for the E6 tape by standard flame emission spectroscopy (FES) and inductively coupled plasma (ICP) techniques, respectively. Results for the E6 tape are summarized in weight percentages of individual oxides and atomic ratio of lithium-to-cobalt in Table 4. The Li:Co ratio remains close to 1:1 after sintering, as desired for performance of the material as a cathode.

TABLE 4

Raw Powder
Sintered Tape

Li₂O (FES)
14.8
14.7

Co₃O₄(ICP)
79.2
78.1

Li:Co Ratio
1.004
1.011

The short residence time of the LCO in the rapid sintering process limits volatilization of lithium. This is advantageous because cathode sintering may be conducted with minimal change in chemistry. In comparison, the process by which C1 is formed involves long sintering times (e.g., at least 20 hrs) that results in significant opportunities for the loss of lithium and involve addition of excess lithium (lithium carbonate, Li₂CO₃) to compensate for the loss. In one example, preparation of C1 involves disposing a layer of Li₂CO₃atop the LCO tape during sintering. Thus, ratio of Li:Co for C1 in the assembly is 1.4. In another example, lithium carbonate may be added directly to the slip composition for tape casting such that the ratio of Li:Co is 1.4. The additional lithium increases the cost of product manufacturing without addressing issues mentioned above, such as shortened furnace life and inconsistent chemistry of the fired product.

(2) Process Efficiency

Rapid sintering is efficient and enables production of cathode tapes with sizes not limited by furnace, at least in one direction. Rapid and continuous sintering of a green tape comprising the E6 formulation begins with providing the tape with a nominal width of approximately 40-75 mm (e.g., 50 mm) in the green state and initially, conveying through a furnace binder burn-out zone. In the setup, the green ceramic tape is “hitched” to alumina ribbon ceramic approximately 3 meters in length and thereafter threaded through a hot-zone of the sintering furnace. The E6 green tape is pulled through the rapid sintering system at a rate of 50-75 mm/min (e.g., 64 mm/min) using the alumina ribbon. The E6 tape exits the furnace from a soak temperature of 1050-1200° C. (e.g., 1075° C.). Then, the tape is stretched and distorted (though not broken) around the hitches due to constrained sintering and remains connected to the hitch. The fired E6 tape is pulled through the furnace, it is attached (i.e., wound) to a drum.

In the provided rapid sintering process, the green E6 tape simultaneously, and as a single continuous tape, (a) enters the binder-burnout zone, pyrolyzing organic components, (b) sinters in the furnace, (c) cools back to room temperature, and (d) is coiled onto a drum for storage. In some examples, the total length of the wound coiled tape is at least 5 meters. Thus, this process enables continuous rolls of sintered electrodes or cathodes or sheets that may be cut to predesired sizes after exiting the furnace. The process is rapid because only the tape is being heated and cooled and its thermal mass is small. Because the tape is also thin, it is able to flex rather than break in response to thermal gradients. Efficiency, as judged by square area produced per unit of time, in this example is over 2,000 cm²/hr.

The process by which C1 is formed has a much lower efficiency. In one sampling, the C1 green sheet (i.e., tape formed from the C1 slip formulation) may be cut into a 50 mm squares for batch sintering. However, batch sintering is an inefficient usage of energy since heating and cooling of the sintering furnace apparatus requires undue amounts of time for heat transfer to the green tape. As a result, the throughput is less than 1.0 cm²/hr.

(3) Low Flammability Slip Formulations

With respect to the slip formulation of C1, cathode powders catalyze pyrolysis of organics in the dried green tape which, as a result, may ignite and burn under certain conditions such as: (a) the concentration of organics in the dried tape being too high, (b) the pull speed is too rapid, and (c) the tape is overly thick for (a) and (b). The impact of flammability on tape with the E6 formulation is illustrated in FIG. 14 when at least one of conditions (a)-(c) is satisfied. The E6 tape for this example had a green thickness of 66 μm and is shown after being pulled part way into the binder burn-out zone at a speed of 64 mm/min into a linear temperature ramp of 225° C. to 325° C. The tape ignited, burned, and fractured into multiple pieces; burn marks are visible in the broken pieces. By the sintering conditions provided herein, tape formulations E5 and E6 are designed for flammability control in rapid and continuous sintering processes. In one example, E5 and E6 have an organic content in the dried tape of less than 10 wt. % (thereby keeping organic concentration sufficiently low-condition (a)). In contrast, tape formulations based on C1 utilize significantly higher concentrations of organics (Table 3 shows values reaching almost 14 wt. % for C1). The heating rate for sintering in C1 is 200° C./min. For the rapid sintering process used illustrated in FIG. 1, heating rate through the binder-burn out zone is 1250° C./min.

Differences between products produced from rapid (e.g., <1 hr), continuously-fired sintering processes described herein and products produced from batch sintering processes with prolonged heating times (e.g., >10 hrs)

Rapid, continuously-fired sintering processes as described herein may be used to make sintered compositions, electrodes, and cathodes with a range of predetermined thicknesses and predetermined porosities. FIG. 15 illustrates a polished, high-resolution scanning electron microscopy (SEM) cross-section of a rapid and continuously sintered LCO cathode that was pulled at a speed of 8 in/min through a 1075° C. hot-zone. An average thickness is shown to be about 39 μm while the porosity, as determined by image analysis, is about 17%. Cathodes such as the one in FIG. 15 may be substituted for traditionally-calendared cathodes to increase energy density of lithium batteries in multiple ways: (a) the sintered cathode has a lower porosity than a traditionally-calendared electrode, (b) the sintered cathode does not contain inactive polymer binder and carbon conductor, and (c) the sintered cathode serves as a mechanical support, so a thin evaporator or sputtered metallization can be used for current collection and distribution.

Coin cells were prepared from the rapidly-sintered LCO cathode material shown in FIG. 15 to evaluate charging/discharging performance. Cathode disks measuring 12.3 mm in diameter were laser cut from larger sheets. The disks were assembled along with a glass fiber separator (Whatman), a 14 mm diameter lithium metal chip, and electrolyte solution of 1M LiPF₆in 1:1 ethylene carbonate (EC)-dimethyl carbonate (DMC) co-solvent into a 2032 coin cell. Charging performance was measured under constant current and constant voltage conditions from 3.0V to 4.3V at C-rates starting at 0.1 and progressing to 1.0 C, and then returning to 0.1 and 0.3 C to verify capacity was not lost. Charging was stopped once the current reached 10% of the target C-rate value. Discharging was performed at the same rate as charging and under constant current conditions. Three charge-discharge cycles were applied at each C-rate. The cathode weight-specific capacities through the constant current phase of charging are shown in FIG. 16 for three different cells. The sintered LCO cathodes retain more than 80% of the 0.1 C capacity when charged at 1 C. The low C-rate capacity is also recovered. Further improvements to rate performance increase with microstructural optimization, and specifically, reduction of pore size.

The rapid sintering process may also be utilized to make cathodes of other compositions, such as NMC (LiNiMnCoO₂) and NCA (LiNiCoAlO₂). These cathode compositions are advantageous because they have lower concentrations of expensive cobalt. Tapes were made of NMC 111 (Li(NiMnCo)_1/3O₂) cathode material using the E7 slip formation listed in Table 3. The NMC 111 tape was rapidly sintered on a platinum support at 1035° C. using a 64 mm/min pull speed. A high resolution, polished SEM cross-sectional image is presented in FIG. 17. Analysis of the image shows a porosity of about 18% and thickness of 48 μm. Rate performance was tested in the same manner as above using 2032 coin cells. FIG. 18 illustrates charging capacity as a function of C-Rate for a testing cell using rapidly-sintered cathode discs of FIG. 17. The sintered NMC cathodes retain about 80% of the 0.1 C capacity when charged at 1 C.

Another use for the rapid sintering processes of this disclosure is to make closed-pore cathodes, such as for use in an all solid-state battery. In these designs, the cathode serves as an active mechanical support for a thin layer of solid electrolyte (e.g., LiPON) and the current collector layers. The rapid sintering conditions are tailored to yield the lowest possible porosity to maximize energy density without inducing unwanted grain growth that lowers supportive strength. FIGS. 19A and 19B show polished SEM cross-sections of rapidly sintered LCO cathodes having thicknesses of 25.4 μm and 9.5 μm, respectively. The respective sintering soak temperatures were at 1025° C. and 1050° C. and the pull speed was 64 mm/min in both cases. The respective porosities were determined by image analysis to be 7.4 vol. % (FIG. 19A) and 3.4 vol. % (FIG. 19B).

Microcracking either after sintering or during charge and discharge cycling can degrade capacity of the cathode. Microcracking after sintering alone would be detrimental to the strength of the sintered sheet. Critical grain sizes for microcracking of sintered LCO due to thermal expansion anisotropy and anisotropy in lattice expansion on lithium deintercalation are 10.0 μm and 4.0 μm in FIGS. 19A and 19B, respectively. The grain sizes of these closed-pore LCO materials are below the critical sizes. Grain boundaries of a subregion of each of the high resolution images were manually delineated, and automated image analysis software was used to measure grain size. The average respective grain sizes are 2.2 and 2.9 μm and lower than the critical grain sizes.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. In addition, as used herein, the article “a” is intended to include one or more than one component or element, and is not intended to be construed as meaning only one. As used herein, the term “porosity” is described as a percent by volume (e.g., at least 10% by volume, or at least 30% by volume), where the “porosity” refers to the portions of the volume of the sintered article unoccupied by the inorganic material.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosed embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the embodiments may occur to persons skilled in the art, the disclosed embodiments should be construed to include everything within the scope of the appended claims and their equivalents.

SINTERED ELECTRODES FOR BATTERIES AND METHOD OF PREPARING SAME

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