Elastomer-encapsulated particles of high-capacity anode active materials for lithium batteries

Abstract

Provided is an anode active material layer for a lithium battery. This layer comprises multiple particulates of an anode active material, wherein at least a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a lithium ion conductivity no less than 10−7 S/cm (preferably no less than 10−5 S/cm) at room temperature and an encapsulating shell thickness from 1 nm to 10 μm, and wherein the high-capacity anode active material (e.g. Si, Ge, Sn, SnO2, Co3O4, etc.) has a specific capacity of lithium storage greater than 372 mAh/g (the theoretical lithium storage limit of graphite).

Description

FIELD OF THE INVENTION

The present invention relates generally to the field of rechargeable lithium battery and, more particularly, to the anode active materials in the form of elastomer-encapsulated particles and the process for producing same.

BACKGROUND OF THE INVENTION

A unit cell or building block of a lithium-ion battery is typically composed of an anode current collector, an anode or negative electrode layer (containing an anode active material responsible for storing lithium therein, a conductive additive, and a resin binder), an electrolyte and porous separator, a cathode or positive electrode layer (containing a cathode active material responsible for storing lithium therein, a conductive additive, and a resin binder), and a separate cathode current collector. The electrolyte is in ionic contact with both the anode active material and the cathode active material. A porous separator is not required if the electrolyte is a solid-state electrolyte.

The binder in the binder layer is used to bond the anode active material (e.g. graphite or Si particles) and a conductive filler (e.g. carbon black or carbon nanotube) together to form an anode layer of structural integrity, and to bond the anode layer to a separate anode current collector, which acts to collect electrons from the anode active material when the battery is discharged. In other words, in the negative electrode (anode) side of the battery, there are typically four different materials involved: an anode active material, a conductive additive, a resin binder (e.g. polyvinylidine fluoride, PVDF, or styrene-butadiene rubber, SBR), and an anode current collector (typically a sheet of Cu foil). Typically the former three materials form a separate, discrete anode layer and the latter one forms another discrete layer.

The most commonly used anode active materials for lithium-ion batteries are natural graphite and synthetic graphite (or artificial graphite) that can be intercalated with lithium and the resulting graphite intercalation compound (GIC) may be expressed as Li_xC₆, where x is typically less than 1. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal corresponds to x=1, defining a theoretical specific capacity of 372 mAh/g.

Graphite or carbon anodes can have a long cycle life due to the presence of a protective solid-electrolyte interface layer (SEI), which results from the reaction between lithium and the electrolyte (or between lithium and the anode surface/edge atoms or functional groups) during the first several charge-discharge cycles. The lithium in this reaction comes from some of the lithium ions originally intended for the charge transfer purpose. As the SEI is formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e. these positive ions can no longer be shuttled back and forth between the anode and the cathode during charges/discharges. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer. In addition to SEI formation, the irreversible capacity loss Q_ir can also be attributed to graphite exfoliation caused by electrolyte/solvent co-intercalation and other side reactions.

In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and the like, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions or react with lithium. Among these materials, lithium alloys having a composition formula of Li_aA (A is a metal or semiconductor element, such as Al and Si, and “a” satisfies 0<a≤5) are of great interest due to their high theoretical capacity, e.g., Li₄Si (3,829 mAh/g), Li_4.4Si (4,200 mAh/g), Li_4.4Ge (1,623 mAh/g), Li_4.4Sn (993 mAh/g), Li₃Cd (715 mAh/g), Li₃Sb (660 mAh/g), Li_4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li₃Bi (385 mAh/g). However, as schematically illustrated in FIG. 2(A), in an anode composed of these high-capacity materials, severe pulverization (fragmentation of the alloy particles) occurs during the charge and discharge cycles due to severe expansion and contraction of the anode active material particles induced by the insertion and extraction of the lithium ions in and out of these particles. The expansion and contraction, and the resulting pulverization, of active material particles, lead to loss of contacts between active material particles and conductive additives and loss of contacts between the anode active material and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.

To overcome the problems associated with such mechanical degradation, three technical approaches have been proposed:

• (1) reducing the size of the active material particle, presumably for the purpose of reducing the total strain energy that can be stored in a particle, which is a driving force for crack formation in the particle. However, a reduced particle size implies a higher surface area available for potentially reacting with the liquid electrolyte to form a higher amount of SEI. Such a reaction is undesirable since it is a source of irreversible capacity loss.
• (2) depositing the electrode active material in a thin film form directly onto a current collector, such as a copper foil. However, such a thin film structure with an extremely small thickness-direction dimension (typically much smaller than 500 nm, often necessarily thinner than 100 nm) implies that only a small amount of active material can be incorporated in an electrode (given the same electrode or current collector surface area), providing a low total lithium storage capacity and low lithium storage capacity per unit electrode surface area (even though the capacity per unit mass can be large). Such a thin film must have a thickness less than 100 nm to be more resistant to cycling-induced cracking, further diminishing the total lithium storage capacity and the lithium storage capacity per unit electrode surface area. Such a thin-film battery has very limited scope of application. A desirable and typical electrode thickness is from 100 μm to 200 μm. These thin-film electrodes (with a thickness of <500 nm or even <100 nm) fall short of the required thickness by three (3) orders of magnitude, not just by a factor of 3.
• (3) using a composite composed of small electrode active particles protected by (dispersed in or encapsulated by) a less active or non-active matrix, e.g., carbon-coated Si particles, sol gel graphite-protected Si, metal oxide-coated Si or Sn, and monomer-coated Sn nano particles. Presumably, the protective matrix provides a cushioning effect for particle expansion or shrinkage, and prevents the electrolyte from contacting and reacting with the electrode active material. Examples of high-capacity anode active particles are Si, Sn, and SnO₂. Unfortunately, when an active material particle, such as Si particle, expands (e.g. up to a volume expansion of 380%) during the battery charge step, the protective coating is easily broken due to the mechanical weakness and/o brittleness of the protective coating materials. There has been no high-strength and high-toughness material available that is itself also lithium ion conductive.
  • It may be further noted that the coating or matrix materials used to protect active particles (such as Si and Sn) are carbon, sol gel graphite, metal oxide, monomer, ceramic, and lithium oxide. These protective materials are all very brittle, weak (of low strength), and/or non-conducting (e.g., ceramic or oxide coating). Ideally, the protective material should meet the following requirements: (a) The coating or matrix material should be of high strength and stiffness so that it can help to refrain the electrode active material particles, when lithiated, from expanding to an excessive extent. (b) The protective material should also have high fracture toughness or high resistance to crack formation to avoid disintegration during repeated cycling. (c) The protective material must be inert (inactive) with respect to the electrolyte, but be a good lithium ion conductor. (d) The protective material must not provide any significant amount of defect sites that irreversibly trap lithium ions. (e) The protective material must be lithium ion-conducting as well as electron-conducting. The prior art protective materials all fall short of these requirements. Hence, it was not surprising to observe that the resulting anode typically shows a reversible specific capacity much lower than expected. In many cases, the first-cycle efficiency is extremely low (mostly lower than 80% and some even lower than 60%). Furthermore, in most cases, the electrode was not capable of operating for a large number of cycles. Additionally, most of these electrodes are not high-rate capable, exhibiting unacceptably low capacity at a high discharge rate.Due to these and other reasons, most of prior art composite electrodes and electrode active materials have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction steps, and other undesirable side effects.

Complex composite particles of particular interest are a mixture of separate Si and graphite particles dispersed in a carbon matrix; e.g. those prepared by Mao, et al. [“Carbon-coated Silicon Particle Powder as the Anode Material for Lithium Batteries and the Method of Making the Same,” US 2005/0136330 (Jun. 23, 2005)]. Also of interest are carbon matrix-containing complex nano Si (protected by oxide) and graphite particles dispersed therein, and carbon-coated Si particles distributed on a surface of graphite particles Again, these complex composite particles led to a low specific capacity or for up to a small number of cycles only. It appears that carbon by itself is relatively weak and brittle and the presence of micron-sized graphite particles does not improve the mechanical integrity of carbon since graphite particles are themselves relatively weak. Graphite was used in these cases presumably for the purpose of improving the electrical conductivity of the anode material. Furthermore, polymeric carbon, amorphous carbon, or pre-graphitic carbon may have too many lithium-trapping sites that irreversibly capture lithium during the first few cycles, resulting in excessive irreversibility.

In summary, the prior art has not demonstrated a composite material that has all or most of the properties desired for use as an anode active material in a lithium-ion battery. Thus, there is an urgent and continuing need for a new anode active material that enables a lithium-ion battery to exhibit a high cycle life, high reversible capacity, low irreversible capacity, small particle sizes (for high-rate capacity), and compatibility with commonly used electrolytes. There is also a need for a method of readily or easily producing such a material in large quantities.

Thus, it is an object of the present invention to meet these needs and address the issues associated the rapid capacity decay of a lithium battery containing a high0capacity anode active material.

SUMMARY OF THE INVENTION

Herein reported is an anode active material layer for a lithium battery that contains a very unique class of anode active materials: elastomer-encapsulated particles of an anode active material that is capable of overcoming the rapid capacity decay problem commonly associated with a lithium-ion battery that features a high-capacity anode active material, such as Si, Sn, and SnO₂.

The anode active material layer comprises multiple particulates of an anode active material, wherein a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a lithium ion conductivity no less than 10⁻⁷ e S/cm at room temperature (preferably and typically no less than 10⁻⁶ S/cm, further preferably no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴ S/cm, and most preferably no less than 10⁻³ S/cm) and an encapsulating shell thickness from 1 nm to 10 μm (preferably from 1 nm to 1 μm and more preferably from 1 nm to 100 nm), and wherein said high-capacity anode active material has a specific capacity of lithium storage greater than 372 mAh/g (which is the theoretical capacity of graphite).

In this anode active material layer, the anode active material is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li having at least 60% by weight of lithium element therein; and (h) combinations thereof.

In some preferred embodiments, the anode active material contains a prelithiated Si, prelithiated Ge, prelithiated Sn, prelithiated SnO_x, prelithiated SiO_x, prelithiated iron oxide, prelithiated VO₂, prelithiated Co₃O₄, prelithiated Ni₃O₄, or a combination thereof, wherein x=1 to 2.

It may be noted that pre-lithiation of an anode active material means that this material has been pre-intercalated by or doped with lithium ions up to a weight fraction from 0.1% to 54.7% of Li in the lithiated product.

The anode active material is preferably in a form of nano particle (spherical, ellipsoidal, and irregular shape), nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, nano disc, nano platelet, or nano horn having a thickness or diameter less than 100 nm. These shapes can be collectively referred to as “particles” unless otherwise specified or unless a specific type among the above species is desired. Further preferably, the anode active material has a dimension less than 50 nm, even more preferably less than 20 nm, and most preferably less than 10 nm.

In some embodiments, one particle or a cluster of particles may be coated with or embraced by a layer of carbon disposed between the particle(s) and the elastomeric material layer (the encapsulating shell). Alternatively or additionally, a carbon layer may be deposited to embrace the encapsulated particle or the encapsulated cluster of multiple anode active material particles.

The particulate may further contain a graphite or carbon material mixed with the active material particles and disposed inside the encapsulating shell. The carbon or graphite material is selected from polymeric carbon, amorphous carbon, chemical vapor deposition carbon, coal tar pitch, petroleum pitch, meso-phase pitch, carbon black, coke, acetylene black, activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, natural graphite particle, or a combination thereof.

The anode active material particles may be coated with or embraced by a conductive protective coating, selected from a carbon material, electronically conductive polymer, conductive metal oxide, or conductive metal coating. Preferably, the anode active material, in the form of a nano particle, nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, nano disc, nano platelet, or nano horn is pre-intercalated or pre-doped with lithium ions to form a prelithiated anode active material having an amount of lithium from 0.1% to 54.7% by weight of said prelithiated anode active material.

Preferably and typically, the elastomeric material has a lithium ion conductivity no less than 10⁻⁶ S/cm, more preferably no less than 5×10⁻⁵ S/cm. In some embodiments, the elastomeric material is a neat polymer containing no additive or filler dispersed therein. In others, the elastomeric material is an elastomer matrix composite containing from 0.1% to 50% by weight (preferably from 1% to 35% by weight) of a lithium ion-conducting additive dispersed in an elastomer matrix material.

In some embodiments, the elastomeric material contains a material selected from natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, and combinations thereof.

In some embodiments, the elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_xSO_y, or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group, x=0-1, y=1-4.

In some embodiments, the elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive contains a lithium salt selected from lithium perchlorate, LiClO₄, lithium hexafluorophosphate, LiPF₆, lithium borofluoride, LiBF₄, lithium hexafluoroarsenide, LiAsF₆, lithium trifluoro-metasulfonate, LiCF₃SO₃, bis-trifluoromethyl sulfonylimide lithium, LiN(CF₃SO₂)₂, lithium bis(oxalato)borate, LiBOB, lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium nitrate, LiNO₃, Li-Fluoroalkyl-Phosphates, LiPF₃(CF₂CF₃)₃, lithium bisperfluoro-ethysulfonylimide, LiBETI, lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-based lithium salt, or a combination thereof.

The elastomeric material may contain a mixture or blend of an elastomer and an electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonated versions), or a combination thereof.

In some embodiments, the elastomeric material contains a mixture or blend of an elastomer and a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof. Sulfonation is herein found to impart improved lithium ion conductivity to a polymer.

The present invention also provides a powder mass of an anode active material for a lithium battery, said powder mass comprising multiple particulates wherein at least a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a lithium ion conductivity no less than 10⁻⁷ S/cm at room temperature and an encapsulating shell thickness from 1 nm to 10 μm, and wherein the high-capacity anode active material has a specific capacity of lithium storage greater than 372 mAh/g. The powder mass may further comprise graphite particles, carbon particles, meso-phase microbeads, carbon or graphite fibers, carbon nanotubes, graphene sheets, or a combination thereof. Preferably, the high-capacity anode is prelithiated.

The present invention also provides an anode electrode that contains the presently invented elastomer-encapsulated high-capacity anode material particles, an optional conductive additive (e.g. expanded graphite flakes, carbon black, acetylene black, or carbon nanotube), an optional resin binder (typically required), and, optionally, some amount of the common anode active materials (e.g. particles of natural graphite, synthetic graphite, hard carbon, etc.).

The present invention also provides a lithium battery containing an optional anode current collector, the presently invented anode active material layer as described above, a cathode active material layer, an optional cathode current collector, an electrolyte in ionic contact with the anode active material layer and the cathode active material layer and an optional porous separator. The lithium battery may be a lithium-ion battery, lithium metal battery (containing lithium metal or lithium alloy as the main anode active material and containing no intercalation-based anode active material), lithium-sulfur battery, lithium-selenium battery, or lithium-air battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of a prior art lithium-ion battery cell, wherein the anode layer is a thin coating of an anode active material itself.

FIG. 1(B) Schematic of another prior art lithium-ion battery; the anode layer being composed of particles of an anode active material, a conductive additive (not shown) and a resin binder (not shown).

FIG. 2(A) Schematic illustrating the notion that expansion of Si particles, upon lithium intercalation during charging of a prior art lithium-ion battery, can lead to pulverization of Si particles, interruption of the conductive paths formed by the conductive additive, and loss of contact with the current collector;

FIG. 2(B) illustrates the issues associated with prior art anode active material; for instance, a non-lithiated Si particle encapsulated by a protective shell (e.g. carbon shell) in a core-shell structure inevitably leads to breakage of the shell and that a pre-lithiated Si particle encapsulated with a protective layer leads to poor contact between the contracted Si particle and the rigid protective shell during battery discharge.

FIG. 3 Schematic of the presently invented elastomer-encapsulated anode active material particles (pre-lithiated or unlithiated). The elasticity of the elastomeric shell enables the shell to expand and contract congruently and conformingly with core particle.

FIG. 4 Schematic of four types of elastomer-embraced anode active material particles.

FIG. 5 The specific capacity of a lithium battery having an anode active material featuring elastomer-encapsulated Co₃O₄ particles and that having un-protected Co₃O₄ particles.

FIG. 6 The specific capacity of a lithium battery having an anode active material featuring elastomer-encapsulated SnO₂ particles and that having un-protected SnO₂ particles.

FIG. 7 The specific capacity of a lithium battery having an anode active material featuring elastomer-encapsulated Sn particles, that having carbon-encapsulated Sn particles, and that having un-protected Sn particles.

FIG. 8 Specific capacities of 4 lithium-ion cells having Si nanowires (SiNW) as an anode active material: unprotected SiNW, carbon-coated SiNW, elastomer-encapsulated SiNW, and elastomer-encapsulated carbon-coated SiNW.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed at the anode active material layer (negative electrode layer, not including the anode current collector) containing a high-capacity anode material for a lithium secondary battery, which is preferably a secondary battery based on a non-aqueous electrolyte, a polymer gel electrolyte, an ionic liquid electrolyte, a quasi-solid electrolyte, or a solid-state electrolyte. The shape of a lithium secondary battery can be cylindrical, square, button-like, etc. The present invention is not limited to any battery shape or configuration. For convenience, we will primarily use Si, Sn, and SnO₂ as illustrative examples of a high-capacity anode active material. This should not be construed as limiting the scope of the invention.

As illustrated in FIG. 1(B), a lithium-ion battery cell is typically composed of an anode current collector (e.g. Cu foil), an anode or negative electrode active material layer (i.e. anode layer typically containing particles of an anode active material, conductive additive, and binder), a porous separator and/or an electrolyte component, a cathode or positive electrode active material layer (containing a cathode active material, conductive additive, and resin binder), and a cathode current collector (e.g. Al foil). More specifically, the anode layer is composed of particles of an anode active material (e.g. graphite, Sn, SnO₂, or Si), a conductive additive (e.g. carbon black particles), and a resin binder (e.g. SBR or PVDF). This anode layer is typically 50-300 μm thick (more typically 100-200 μm) to give rise to a sufficient amount of current per unit electrode area.

In a less commonly used cell configuration, as illustrated in FIG. 1(A), the anode active material is deposited in a thin film form directly onto an anode current collector, such as a sheet of copper foil. This is not commonly used in the battery industry and, hence, will not be discussed further.

In order to obtain a higher energy density cell, the anode in FIG. 1(B) can be designed to contain higher-capacity anode active materials having a composition formula of Li_aA (A is a metal or semiconductor element, such as Al and Si, and “a” satisfies 0<a≤5). These materials are of great interest due to their high theoretical capacity, e.g., Li₄Si (3,829 mAh/g), Li_4.4Si (4,200 mAh/g), Li_4.4Ge (1,623 mAh/g), Li_4.4Sn (993 mAh/g), Li₃Cd (715 mAh/g), Li₃Sb (660 mAh/g), Li_4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li₃Bi (385 mAh/g). However, as discussed in the Background section, there are several problems associated with the implementation of these high-capacity anode active materials:

• • 1) As schematically illustrated in FIG. 2(A), in an anode composed of these high-capacity materials, severe pulverization (fragmentation of the alloy particles) occurs during the charge and discharge cycles due to severe expansion and contraction of the anode active material particles induced by the insertion and extraction of the lithium ions in and out of these particles. The expansion and contraction, and the resulting pulverization, of active material particles, lead to loss of contacts between active material particles and conductive additives and loss of contacts between the anode active material and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.
  • 2) The approach of using a composite composed of small electrode active particles protected by (dispersed in or encapsulated by) a less active or non-active matrix, e.g., carbon-coated Si particles, sol gel graphite-protected Si, metal oxide-coated Si or Sn, and monomer-coated Sn nano particles, has failed to overcome the capacity decay problem. Presumably, the protective matrix provides a cushioning effect for particle expansion or shrinkage, and prevents the electrolyte from contacting and reacting with the electrode active material. Unfortunately, when an active material particle, such as Si particle, expands (e.g. up to a volume expansion of 380%) during the battery charge step, the protective coating is easily broken due to the mechanical weakness and/o brittleness of the protective coating materials. There has been no high-strength and high-toughness material available that is itself also lithium ion conductive.
  • 3) The approach of using a core-shell structure (e.g. Si nano particle encapsulated in a carbon or SiO₂ shell) also has not solved the capacity decay issue. As illustrated in upper portion of FIG. 2(B), a non-lithiated Si particle can be encapsulated by a carbon shell to form a core-shell structure (Si core and carbon or SiO₂ shell in this example). As the lithium-ion battery is charged, the anode active material (carbon- or SiO₂-encapsulated Si particle) is intercalated with lithium ions and, hence, the Si particle expands. Due to the brittleness of the encapsulating shell (carbon), the shell is broken into segments, exposing the underlying Si to electrolyte and subjecting the Si to undesirable reactions with electrolyte during repeated charges/discharges of the battery. These reactions continue to consume the electrolyte and reduce the cell's ability to store lithium ions.
  • 4) Referring to the lower portion of FIG. 2(B), wherein the Si particle has been pre-lithiated with lithium ions; i.e. has been pre-expanded in volume. When a layer of carbon (as an example of a protective material) is encapsulated around the pre-lithiated Si particle, another core-shell structure is formed. However, when the battery is discharged and lithium ions are released (de-intercalated) from the Si particle, the Si particle contracts, leaving behind a large gap between the protective shell and the Si particle. Such a configuration is not conducive to lithium intercalation of the Si particle during the subsequent battery charge cycle due to the gap and the poor contact of Si particle with the protective shell (through which lithium ions can diffuse). This would significantly curtail the lithium storage capacity of the Si particle particularly under high charge rate conditions.

In other words, there are several conflicting factors that must be considered concurrently when it comes to the design and selection of an anode active material in terms of material type, shape, size, porosity, and electrode layer thickness. Thus far, there has been no effective solution offered by any prior art teaching to these often conflicting problems. We have solved these challenging issues that have troubled battery designers and electrochemists alike for more than 30 years by developing the elastomer-protected anode active material.

The present invention provides an anode active material layer comprising multiple particulates of an anode active material, wherein a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a lithium ion conductivity no less than 10⁻⁷ S/cm at room temperature and an encapsulating shell thickness from 1 nm to 10 μm, and wherein said high-capacity anode active material has a specific capacity of lithium storage greater than 372 mAh/g (which is the theoretical capacity of graphite).

As illustrated in FIG. 4, the present invention provides four major types of particulates of elastomer-encapsulated anode active material particles. The first one is a single-particle particulate containing an anode active material core 10 encapsulated by an elastomer shell 12. The second is a multiple-particle particulate containing multiple anode active material particles 14 (e.g. Si nano particles), optionally along with other active materials (e.g. particles of graphite or hard carbon, not shown) or conductive additive, which are encapsulated by an elastomer shell 16. The third is a single-particle particulate containing an anode active material core 18 coated by a carbon layer 20 (or other conductive material) further encapsulated by an elastomer shell 22. The fourth is a multiple-particle particulate containing multiple anode active material particles 24 (e.g. Si nano particles) coated with a conductive protection layer 26, optionally along with other active materials (e.g. particles of graphite or hard carbon, not shown) or conductive additive, which are encapsulated by an elastomer shell 28. These anode active material particles can be pre-lithiated or non-prelithiated.

As schematically illustrated in the upper portion of FIG. 3, a non-lithiated Si particle can be encapsulated by an elastomeric shell to form a core-shell structure (Si core and elastomer shell in this example). As the lithium-ion battery is charged, the anode active material (elastomer-encapsulated Si particle) is intercalated with lithium ions and, hence, the Si particle expands. Due to the high elasticity of the encapsulating shell (elastomer), the shell will not be broken into segments (in contrast to the broken carbon shell). That the elastomeric shell remains intact prevents the exposure of the underlying Si to electrolyte and, thus, prevents the Si from undergoing undesirable reactions with electrolyte during repeated charges/discharges of the battery. This strategy prevents continued consumption of the electrolyte to form additional SEI.

Alternatively, referring to the lower portion of FIG. 3, wherein the Si particle has been pre-lithiated with lithium ions; i.e. has been pre-expanded in volume. When a layer of elastomer is encapsulated around the pre-lithiated Si particle, another core-shell structure is formed. When the battery is discharged and lithium ions are released (de-intercalated) from the Si particle, the Si particle contracts. However, the elastomer is capable of elastically shrinking in a conformal manner; hence, leaving behind no gap between the protective shell and the Si particle. Such a configuration is amenable to subsequent lithium intercalation and de-intercalation of the Si particle. The elastomeric shell expands and shrinks congruently with the expansion and shrinkage of the encapsulated core anode active material particle, enabling long-term cycling stability of a lithium battery featuring a high-capacity anode active material (such as Si, Sn, SnO₂, Co₃O₄, etc.).

The anode active material may be selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li; and (h) combinations thereof. Particles of Li or Li alloy (Li alloy containing from 0.1% to 10% by weight of Zn, Ag, Au, Mg, Ni, Ti, Fe, Co, or V element), particularly surface-stabilized Li particles (e.g. wax-coated Li particles), were found to be good anode active material per se or an extra lithium source to compensate for the loss of Li ions that are otherwise supplied only from the cathode active material. The presence of these Li or Li-alloy particles encapsulated inside an elastomeric shell was found to significantly improve the cycling performance of a lithium cell.

Pre-lithiation of an anode active material can be conducted by several methods (chemical intercalation, ion implementation, and electrochemical intercalation). Among these, the electrochemical intercalation is the most effective. Lithium ions can be intercalated into non-Li elements (e.g. Si, Ge, and Sn) and compounds (e.g. SnO₂ and Co₃O₄) up to a weight percentage of 54.68% (see Table 1 below). For Zn, Mg, Ag, and Au encapsulated inside an elastomer shell, the amount of Li can reach 99% by weight.

TABLE 1
Lithium storage capacity of selected non-Li elements.
	Atomic	Atomic
	weight	weight of	Max.
Intercalated	of Li,	active material,	wt. %
compound	g/mole	g/mole	of Li
Li4Si	6.941	28.086	49.71
Li4.4Si	6.941	28.086	54.68
Li4.4Ge	6.941	72.61	30.43
Li4.4Sn	6.941	118.71	20.85
Li3Cd	6.941	112.411	14.86
Li3Sb	6.941	121.76	13.93
Li4.4Pb	6.941	207.2	13.00
LiZn	6.941	65.39	7.45
Li3Bi	6.941	208.98	8.80

The particles of the anode active material may be in the form of a nano particle, nano wire, nano fiber, nano tube, nano sheet, nano platelet, nano disc, nano belt, nano ribbon, or nano horn. They can be non-lithiated (when incorporated into the anode active material layer) or pre-lithiated to a desired extent (up to the maximum capacity as allowed for a specific element or compound.

Preferably and typically, the elastomeric material has a lithium ion conductivity no less than 10⁻⁷ S/cm, more preferably no less than 10⁻⁵ S/cm, further preferably no less than 10⁻⁴ S/cm, and most preferably no less than 10⁻³ S/cm. In some embodiments, the elastomeric material is a neat polymer having no additive or filler dispersed therein. In others, the elastomeric material is an elastomer matrix composite containing from 0.1% to 50% (preferably 1% to 35%) by weight of a lithium ion-conducting additive dispersed in an elastomer matrix material. The elastomeric material must have a high elasticity (high elastic deformation value). An elastic deformation is a deformation that is fully recoverable and the recovery process is essentially instantaneous (no significant time delay). An elastomer, such as a vulcanized natural rubber, can exhibit an elastic deformation from 2% up to 1,000% (10 times of its original length), more typically from 10% to 800%, and further more typically from 50% to 500%, and most typically and desirably from 100% to 500%. It may be noted that although a metal typically has a high ductility (i.e. can be extended to a large extent without breakage), the majority of the deformation is plastic deformation (non-recoverable) and only a small amount of elastic deformation (typically <1% and more typically <0.2%).

A broad array of elastomers, as a neat resin alone or as a matrix material for an elastomeric matrix composite, can be used to encapsulate an anode active material particle or multiple particles. Encapsulation means substantially fully embracing the particle(s) without allowing the particle to be in direct contact with electrolyte in the battery. The elastomeric material may be selected from natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, and combinations thereof.

The urethane-urea copolymer film usually consists of two types of domains, soft domains and hard ones. Entangled linear backbone chains consisting of poly(tetramethylene ether) glycol (PTMEG) units constitute the soft domains, while repeated methylene diphenyl diisocyanate (MDI) and ethylene diamine (EDA) units constitute the hard domains. The lithium ion-conducting additive can be incorporated in the soft domains or other more amorphous zones.

In some embodiments, the elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_xSO_y, or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group, x=0-1, y=1-4.

In some embodiments, the elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive contains a lithium salt selected from lithium perchlorate, LiClO₄, lithium hexafluorophosphate, LiPF₆, lithium borofluoride, LiBF₄, lithium hexafluoroarsenide, LiAsF₆, lithium trifluoro-metasulfonate, LiCF₃SO₃, bis-trifluoromethyl sulfonylimide lithium, LiN(CF₃SO₂)₂, lithium bis(oxalato)borate, LiBOB, lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium oxalyldifluoroborate, LiBF₂C₂O₄, lithium nitrate, LiNO₃, Li-Fluoroalkyl-Phosphates, LiPF₃(CF₂CF₃)₃, lithium bisperfluoro-ethysulfonylimide, LiBETI, lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide, LiTFSI, an ionic liquid-based lithium salt, or a combination thereof.

The elastomeric material may contain a mixture or blend of an elastomer and an electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, derivatives thereof (e.g. sulfonated versions), or a combination thereof.

In some embodiments, the elastomeric material contains a mixture or blend of an elastomer and a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivative thereof (e.g. sulfonated versions), or a combination thereof.

Some elastomers are originally in an unsaturated chemical state (unsaturated rubbers) that can be cured by sulfur vulcanization to form a cross-linked polymer that is highly elastic (hence, an elastomer). Prior to vulcanization, these polymers or oligomers are soluble in an organic solvent to form a polymer solution. Particles of an anode active material (e.g. SnO₂ nano particles and Si nano-wires) can be dispersed in this polymer solution to form a suspension (dispersion or slurry) of an active material particle-polymer mixture. This suspension can then be subjected to a solvent removal treatment while individual particles remain substantially separated from one another. The polymer precipitates out to deposit on surfaces of these active material particles. This can be accomplished, for instance, via spray drying.

Unsaturated rubbers that can be vulcanized to become elastomer include natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR),

Some elastomers are saturated rubbers that cannot be cured by sulfur vulcanization; they are made into a rubbery or elastomeric material via different means: e.g. by having a copolymer domain that holds other linear chains together. Each of these elastomers can be used to encapsulate particles of an anode active material by one of several means: melt mixing (followed by pelletizing and ball-milling, for instance), solution mixing (dissolving the anode active material particles in an uncured polymer, monomer, or oligomer, with or without an organic solvent) followed by drying (e.g. spray drying), interfacial polymerization, or in situ polymerization of elastomer in the presence of anode active material particles.

Saturated rubbers and related elastomers in this category include EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, and protein elastin. Polyurethane and its copolymers (e.g. urea-urethane copolymer) are particularly useful elastomeric shell materials for encapsulating anode active material particles.

Several micro-encapsulation processes require the elastomer materials to be dissolvable in a solvent. Fortunately, all the elastomers used herein are soluble in some common solvents. Even for those rubbers that are not very soluble after vulcanization, the un-cured polymer (prior to vulcanization or curing) can be readily dissolved in a common organic solvent to form a solution. This solution can then be used to encapsulate solid particles via several of the micro-encapsulation methods to be discussed in what follows. Upon encapsulation, the elastomer shell is then vulcanized or cured. Some examples of rubbers and their solvents are polybutadiene (2-methyl pentane+n-hexane or 2,3-dimethylbutane), styrene-butadiene rubber (toluene, benzene, etc.), butyl rubber (n-hexane, toluene, cyclohexane), etc. The SBR can be vulcanized with different amounts sulfur and accelerator at 433° K in order to obtain different network structures and crosslink densities. Butyl rubber (IIR) is a copolymer of isobutylene and a small amount of isoprene (e.g. about 98% polyisobutylene with 2% isoprene distributed randomly in the polymer chain). Elemental sulfur and organic accelerators (such as thiuram or thiocarbamates) can be used to cross-link butyl rubber to different extents as desired. Thermoplastic elastomers are also readily soluble in solvents.

There are three broad categories of micro-encapsulation methods that can be implemented to produce elastomer-encapsulated particles of an anode active material: physical methods, physico-chemical methods, and chemical methods. The physical methods include pan-coating, air-suspension coating, centrifugal extrusion, vibration nozzle, and spray-drying methods. The physico-chemical methods include ionotropic gelation and coacervation-phase separation methods. The chemical methods include interfacial polycondensation, interfacial cross-linking, in-situ polymerization, and matrix polymerization.

Pan-Coating Method:

The pan coating process involves tumbling the active material particles in a pan or a similar device while the encapsulating material (e.g. elastomer monomer/oligomer, elastomer melt, elastomer/solvent solution) is applied slowly until a desired encapsulating shell thickness is attained.

Air-Suspension Coating Method:

In the air suspension coating process, the solid particles (core material) are dispersed into the supporting air stream in an encapsulating chamber. A controlled stream of a polymer-solvent solution (elastomer or its monomer or oligomer dissolved in a solvent; or its monomer or oligomer alone in a liquid state) is concurrently introduced into this chamber, allowing the solution to hit and coat the suspended particles. These suspended particles are encapsulated (fully coated) with polymers while the volatile solvent is removed, leaving a very thin layer of polymer (elastomer or its precursor, which is cured/hardened subsequently) on surfaces of these particles. This process may be repeated several times until the required parameters, such as full-coating thickness (i.e. encapsulating shell or wall thickness), are achieved. The air stream which supports the particles also helps to dry them, and the rate of drying is directly proportional to the temperature of the air stream, which can be adjusted for optimized shell thickness.

In a preferred mode, the particles in the encapsulating zone portion may be subjected to re-circulation for repeated coating. Preferably, the encapsulating chamber is arranged such that the particles pass upwards through the encapsulating zone, then are dispersed into slower moving air and sink back to the base of the encapsulating chamber, enabling repeated passes of the particles through the encapsulating zone until the desired encapsulating shell thickness is achieved.

Centrifugal Extrusion:

Anode active materials may be encapsulated using a rotating extrusion head containing concentric nozzles. In this process, a stream of core fluid (slurry containing particles of an anode active material dispersed in a solvent) is surrounded by a sheath of shell solution or melt. As the device rotates and the stream moves through the air it breaks, due to Rayleigh instability, into droplets of core, each coated with the shell solution. While the droplets are in flight, the molten shell may be hardened or the solvent may be evaporated from the shell solution. If needed, the capsules can be hardened after formation by catching them in a hardening bath. Since the drops are formed by the breakup of a liquid stream, the process is only suitable for liquid or slurry. A high production rate can be achieved. Up to 22.5 kg of microcapsules can be produced per nozzle per hour and extrusion heads containing 16 nozzles are readily available.

Vibrational Nozzle Method:

Core-shell encapsulation or matrix-encapsulation of an anode active material can be conducted using a laminar flow through a nozzle and vibration of the nozzle or the liquid. The vibration has to be done in resonance with the Rayleigh instability, leading to very uniform droplets. The liquid can consist of any liquids with limited viscosities (1-50,000 mPa·s): emulsions, suspensions or slurry containing the anode active material. The solidification can be done according to the used gelation system with an internal gelation (e.g. sol-gel processing, melt) or an external (additional binder system, e.g. in a slurry).

Spray-Drying:

Spray drying may be used to encapsulate particles of an active material when the active material is dissolved or suspended in a melt or polymer solution. In spray drying, the liquid feed (solution or suspension) is atomized to form droplets which, upon contacts with hot gas, allow solvent to get vaporized and thin polymer shell to fully embrace the solid particles of the active material.

Coacervation-Phase Separation:

This process consists of three steps carried out under continuous agitation:

• (a) Formation of three immiscible chemical phases: liquid manufacturing vehicle phase, core material phase and encapsulation material phase. The core material is dispersed in a solution of the encapsulating polymer (elastomer or its monomer or oligomer). The encapsulating material phase, which is an immiscible polymer in liquid state, is formed by (i) changing temperature in polymer solution, (ii) addition of salt, (iii) addition of non-solvent, or (iv) addition of an incompatible polymer in the polymer solution.
• (b) Deposition of encapsulation shell material: core material being dispersed in the encapsulating polymer solution, encapsulating polymer material coated around core particles, and deposition of liquid polymer embracing around core particles by polymer adsorbed at the interface formed between core material and vehicle phase; and
• (c) Hardening of encapsulating shell material: shell material being immiscible in vehicle phase and made rigid via thermal, cross-linking, or dissolution techniques.

Interfacial Polycondensation and Interfacial Cross-Linking:

Interfacial polycondensation entails introducing the two reactants to meet at the interface where they react with each other. This is based on the concept of the Schotten-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom (such as an amine or alcohol), polyester, polyurea, polyurethane, or urea-urethane condensation. Under proper conditions, thin flexible encapsulating shell (wall) forms rapidly at the interface. A solution of the anode active material and a diacid chloride are emulsified in water and an aqueous solution containing an amine and a polyfunctional isocyanate is added. A base may be added to neutralize the acid formed during the reaction. Condensed polymer shells form instantaneously at the interface of the emulsion droplets. Interfacial cross-linking is derived from interfacial polycondensation, wherein cross-linking occurs between growing polymer chains and a multi-functional chemical groups to form an elastomer shell material.

In-Situ Polymerization:

In some micro-encapsulation processes, active materials particles are fully coated with a monomer or oligomer first. Then, direct polymerization of the monomer or oligomer is carried out on the surfaces of these material particles.

Matrix Polymerization:

This method involves dispersing and embedding a core material in a polymeric matrix during formation of the particles. This can be accomplished via spray-drying, in which the particles are formed by evaporation of the solvent from the matrix material. Another possible route is the notion that the solidification of the matrix is caused by a chemical change.

Example 1

Cobalt Oxide (Co₃O₄) Anode Particulates

An appropriate amount of inorganic salts Co(NO₃)₂.6H₂O and ammonia solution (NH₃.H₂O, 25 wt. %) were mixed together. The resulting suspension was stirred for several hours under an argon flow to ensure a complete reaction. The obtained Co(OH)₂ precursor suspension was calcined at 450° C. in air for 2 h to form particles of the layered Co₃O₄. Portion of the Co₃O₄ particles was then encapsulated with a urea-urethane copolymer with the encapsulating elastomer shell thickness being varied from 17 nm to 135 nm.

For electrochemical testing, the working electrodes were prepared by mixing 85 wt. % active material (encapsulated or non-encapsulated particulates of Co₃O₄, separately), 7 wt. % acetylene black (Super-P), and 8 wt. % polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5 wt. % total solid content. After coating the slurries on Cu foil, the electrodes were dried at 120° C. in vacuum for 2 h to remove the solvent before pressing. Then, the electrodes were cut into a disk (ϕ=12 mm) and dried at 100° C. for 24 h in vacuum. Electrochemical measurements were carried out using CR2032 (3V) coin-type cells with lithium metal as the counter/reference electrode, Celgard 2400 membrane as separator, and 1 M LiPF₆ electrolyte solution dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). The cell assembly was performed in an argon-filled glove-box. The CV measurements were carried out using a CH-6 electrochemical workstation at a scanning rate of 1 mV/s.

The electrochemical performance of the particulates of elastomer-encapsulated Co₃O₄ particles and that of non-protected Co₃O₄ were evaluated by galvanostatic charge/discharge cycling at a current density of 50 mA/g, using a LAND electrochemical workstation. The results indicate that the charge/discharge profiles for the encapsulated Co₃O₄ particles and un-protected Co₃O₄ particle-based electrodes show a long voltage plateau at about 1.06 V and 1.10 V, respectively, followed by a slopping curve down to the cut-off voltage of 0.01 V, indicative of typical characteristics of voltage trends for the Co₃O₄ electrode.

As summarized in FIG. 5, the first-cycle lithium insertion capacity is 752 mAh/g (non-encapsulated) and 751 mAh/g (encapsulated), respectively, which are higher than the theoretical values of graphite (372 mAh/g). Both cells exhibit some first-cycle irreversibility. The initial capacity loss might have resulted from the incomplete conversion reaction and partially irreversible lithium loss due to the formation of solid electrolyte interface (SEI) layers.

As the number of cycles increases, the specific capacity of the bare Co₃O₄ electrode drops precipitously. Compared with its initial capacity value of approximately 752 mAh/g, its capacity suffers a 20% loss after 150 cycles and a 35.88% loss after 220 cycles. By contrast, the presently invented elastomer-encapsulated particulates provide the battery cell with a very stable and high specific capacity for a large number of cycles, experiencing a capacity loss of less than 2.7% after 220 cycles. These data have clearly demonstrated the surprising and superior performance of the presently invented particulate electrode materials compared with prior art un-encapsulated particulate-based electrode materials.

It may be noted that the number of charge-discharge cycles at which the specific capacity decays to 80% of its initial value is commonly defined as the useful cycle life of a lithium-ion battery. Thus, the cycle life of the cell containing the non-encapsulated anode active material is approximately 150 cycles. In contrast, the cycle life of the presently invented cells (not just button cells, but large-scale full cells) is typically from 1,000 to 4,000.

Example 2

Elastomer-Encapsulated Tin Oxide Particulates

Tin oxide (SnO₂) nano particles were obtained by the controlled hydrolysis of SnCl₄.5H₂O with NaOH using the following procedure: SnCl₄.5H₂O (0.95 g, 2.7 m-mol) and NaOH (0.212 g, 5.3 m-mol) were dissolved in 50 mL of distilled water each. The NaOH solution was added drop-wise under vigorous stirring to the tin chloride solution at a rate of 1 mL/min. This solution was homogenized by sonication for 5 m in. Subsequently, the resulting hydrosol was reacted with H₂SO₄. To this mixed solution, few drops of 0.1 M of H₂SO₄ were added to flocculate the product. The precipitated solid was collected by centrifugation, washed with water and ethanol, and dried in vacuum. The dried product was heat-treated at 400° C. for 2 h under Ar atmosphere. A dilute elastomer-solvent solution (0.01-0.1 M of cis-polyisoprene in cyclohexane and 1,4-dioxane) was used as a coating solution in an air-suspension method to produce elastomer-encapsulated SnO₂ particles having a shell thickness of 2.3 nm to 124 nm.

The battery cells from the elastomer-encapsulated particulates (nano-scaled SnO₂ particles) and non-coated SnO₂ particles were prepared using a procedure described in Example 1. FIG. 6 shows that the anode prepared according to the presently invented elastomer-encapsulated particulate approach offers a significantly more stable and higher reversible capacity compared to the un-coated SnO₂ particle-based.

Example 3

Tin (Sn) Nano Particles Encapsulated by a Styrene-Butadiene Rubber (SBR)

Nano particles (76 nm in diameter) of Sn were encapsulated with a thin layer of SBR shell via the spray-drying method, followed by curing of the butadiene segment of the SBR chains to impart high elasticity to the SBR. For comparison, some amount of Sn nano particles was encapsulated by a carbon shell. Carbon encapsulation is well-known in the art. Un-protected Sn nano particles from the same batch were also investigated to determine and compare the cycling behaviors of the lithium-ion batteries containing these particles as the anode active material.

Shown in FIG. 7 are the discharge capacity curves of three coin cells having three different Sn particles as the anode active material: elastomer-encapsulated Sn particles, carbon-encapsulated Sn particles, and un-protected Sn particles. These results have clearly demonstrated that elastomer encapsulation strategy provides the very best protection against capacity decay of a lithium-ion battery featuring a high-capacity anode active material. Carbon encapsulation is not good enough to provide the necessary protection.

Example 4

Si Nanowire-Based Particulates

In a typical procedure, approximately 2.112 g of silicon powders (average diameter 2.64 μm) were mixed with 80 ml of a 0.1M aqueous solution of Ni(NO₃).6H₂O and vigorously stirred for 30 min. Then, water was evaporated in a rotary evaporator and the solid remnants were completely dried in an oven at 150° C. The final sample (Ni-impregnated Si powers) was obtained by grinding the solids in a mortar.

Subsequently, 0.03 g of Ni-impregnated Si particles was placed in a quartz boat, and the boat was placed in a tube furnace. The sample was reduced at 500° C. for 4 hours under flowing Ar (180 sccm) and H₂ (20 sccm), then the temperature was raised to 990° C. to catalytically synthesize Si nanowires; Si nanowires were found to emanate from original micron-scaled Si particles. For the purpose of separating Si nanowires, for instance, every 0.1 g of the reacted Si powders was mixed with 10 ml of ethanol and the resulting mixture was sonicated for 1 hour. Subsequently, Si nanowires were separated from the Si powders by centrifuge at 5,000 rpm for 10 min.

Some Si nanowires were encapsulated with cis-polyisoprene elastomer. Some Si nanowires were coated with a layer of amorphous carbon and then encapsulated with cis-polyisoprene elastomer. For comparison purposes, Si nanowires unprotected and protected by carbon coating (but no elastomer encapsulation), respectively, were also prepared and implemented in a separate lithium-ion cell. In all four cells, approximately 25-30% of graphite particles were mixed with the protected or unprotected Si nanowires (SiNW), along with 5% binder resin, to make an anode electrode. The cycling behaviors of these 4 cells are shown in FIG. 8, which indicates that elastomer encapsulation of Si nanowires, with or without carbon coating, provides the most stable cycling response. Carbon coating alone does not help to improve cycling stability by much.

Example 5

Effect of Lithium Ion-Conducting Additive in an Elastomer Shell

A wide variety of lithium ion-conducting additives were added to several different elastomer matrix materials to prepare encapsulation shell materials for protecting core particles of an anode active material. We have discovered that these elastomer composite materials are suitable encapsulation shell materials provided that their lithium ion conductivity at room temperature is no less than 10⁻⁷ S/cm. With these materials, lithium ions appear to be capable of readily diffusing in and out of the encapsulation shell having a thickness no greater than 1 μm. For thicker shells (e.g. 10 μm), a lithium ion conductivity at room temperature no less than 10⁻⁴ S/cm would be required.

TABLE 2
Lithium ion conductivity of various elastomer composite compositions as a shell
material for protecting anode active material particles.
Sample	Lithium-conducting
No.	additive	Elastomer (1-2 μm thick)	Li-ion conductivity (S/cm)
E-1	Li2CO3 + (CH2OCO2Li)2	70-99% polyurethane	2.7 × 10−6 to 1.8 × 10−3 S/cm
E-2	Li2CO3 + (CH2OCO2Li)2	65-99% polyisoprene	6.1 × 10−6 to 3.6 × 10−4 S/cm
E-3	Li2CO3 + (CH2OCO2Li)2	65-99% SBR	6.5 × 10−6 to 5.2 × 10−4 S/cm
D-4	Li2CO3 + (CH2OCO2Li)2	70-99% urethane-urea	7.4 × 10−7 to 4.3 × 10−4 S/cm
D-5	Li2CO3 + (CH2OCO2Li)2	75-99% polybutadiene	8.7 × 10−6 to 3.6 × 10−3 S/cm
B1	LiF + LiOH + Li2C2O4	80-99% chloroprene	8.7 × 10−7 to 2.1 × 10−4 S/cm
		rubber
B2	LiF + HCOLi	80-99% EPDM	2.1 × 10−6 to 8.6 × 10−4 S/cm
B3	LiOH	70-99% polyurethane	2.8 × 10−5 to 1.2 × 10−3 S/cm
B4	Li2CO3	70-99% polyurethane	4.4 × 10−5 to 3.9 × 10−3 S/cm
B5	Li2C2O4	70-99% polyurethane	9.3 × 10−6 to 7.7 × 10−4 S/cm
B6	Li2CO3 + LiOH	70-99% polyurethane	1.4 × 10−5 to 1.6 × 10−3 S/cm
C1	LiClO4	70-99% urethane-urea	4.8 × 10−5 to 2.2 × 10−3 S/cm
C2	LiPF6	70-99% urethane-urea	2.4 × 10−5 to 8.2 × 10−4 S/cm
C3	LiBF4	70-99% urethane-urea	1.2 × 10−5 to 1.2 × 10−4 S/cm
C4	LiBOB + LiNO3	70-99% urethane-urea	6.8 × 10−6 to 1.2 × 10−4 S/cm
S1	Sulfonated polyaniline	85-99% SBR	6.3 × 10−6 to 4.2 × 10−4 S/cm
S2	Sulfonated SBR	85-99% SBR	5.2 × 10−6 to 2.2 × 10−4 S/cm
S3	Sulfonated PVDF	80-99% chlorosulfonated	3.3 × 10−6 to 2.8 × 10−4 S/cm
		polyethylene (CS-PE)
S4	Polyethylene oxide	80-99% CS-PE	4.9 × 10−6 to 3.7 × 10−4 S/cm

Example 6

Cycle Stability of Various Rechargeable Lithium Battery Cells

In lithium-ion battery industry, it is a common practice to define the cycle life of a battery as the number of charge-discharge cycles that the battery suffers 20% decay in capacity based on the initial capacity measured after the required electrochemical formation. Summarized in Table 3 below are the cycle life data of a broad array of batteries featuring presently invented elastomer-encapsulated anode active material particles vs. other types of anode active materials.

TABLE 3
Cycle life data of various lithium secondary (rechargeable) batteries.
			Initial
Sample		Type & % of anode active	capacity	Cycle life
ID	Protective means	material	(mAh/g)	(No. of cycles)
Si-1	SBR-encapsulation	25% by wt. Si nano	1,120	1,230-1,575
		particles (80 nm) + 67%
		graphite + 8% binder
Si-2	Carbon	25% by wt. Si nano	1,242	251
	encapsulation	particles (80 nm)
SiNW-1	Urea-Urethane	35% Si nanowires	1,258	1,455
	encapsulation	(diameter = 90 nm)
SiNW-2	ethylene oxide-	45% Si nano particles,	1,766	1,420 (pre-
	epichlorohydrin	pre-lithiated or non-		lithiated); 1,125
	copolymer	prelithiated (no pre-Li)		(no prelithiation)
VO2-1	Polyurethane	90%-95%, VO2 nano	255	1689
	encapsulation	ribbon
Co3O4-2	Polyisoprene	85% Co3O4 + 8%	720	2,356 (Pre-lithiated);
	encapsulation	graphite platelets +		1,722 (no pre-Li)
		binder
Co3O4-2	No encapsulation	85% Co3O4 + 8%	725	266
		graphite platelets +
		binder
SnO2-2	polybutadiene	75% SnO2 particles	740	1,138
	encapsulation	(3 μm initial size)
SnO2-2	EPDM	75% SnO2 particles	738	3,245 (Pre-Li);
	encapsulation	(87 μm in diameter)		1,856 (non pre-Li)
Ge-1	butyl rubber	85% Ge + 8% graphite	850	1,226
	encapsulation of	platelets + binder
	C-coated Ge
Ge-2	Carbon-coated	85% Ge + 8% graphite	856	120
		platelets + binder
Al-Li-1	Polyurethane	Al/Li alloy (3/97)	2,850	1,544
	encapsulation	particles
Al-Li-2	None	Al/Li alloy particles	2,856	155
Zn-Li-1	Cis-polyisoprene	C-coated Zn/Li alloy	2,626	1,239
	encapsulation	(5/95) particles
Zn-Li-2	None	C-coated Zn/Li alloy	2,631	146
		(5/95) particles

These data further confirm:

• • (1) The elastomer encapsulation strategy is surprisingly effective in alleviating the anode expansion/shrinkage-induced capacity decay problems.
  • (2) The encapsulation of high-capacity anode active material particles by carbon or other non-elastomeric protective materials does not provide much benefit in terms of improving cycling stability of a lithium-ion battery
  • (3) Pre-lithiation of the anode active material particles prior to elastomer encapsulation is beneficial.
  • (4) The elastomer encapsulation strategy is also surprisingly effective in imparting stability to lithium metal or its alloy when used as the anode active material of a lithium metal battery.

Claims

1. An anode active material layer for a lithium battery, said anode active material layer comprising multiple particulates of an anode active material, wherein a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a fully recoverable elastic deformation from 2% to 1000%, a lithium ion conductivity no less than 10−7 S/cm at room temperature, and an encapsulating shell thickness from 1 nm to 10 μm, wherein said thin layer of elastomeric material is capable of expanding and shrinking congruently with expansion and shrinkage of said one or a plurality of particles of said high-capacity anode active material, and wherein said high-capacity anode active material has a specific capacity of lithium storage greater than 372 mAh/g.

2. The anode active material layer of claim 1, wherein said anode active material is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (f) prelithiated versions thereof; (g) particles of Li, Li alloy, or surface-stabilized Li having at least 60% by weight of lithium element therein; and (h) combinations thereof.

3. The anode active material layer of claim 1, wherein said Li alloy contains from 0.1% to 10% by weight of a metal element selected from Zn, Ag, Au, Mg, Ni, Ti, Fe, Co, V, or a combination.

4. The anode active material layer of claim 1, wherein said anode active material contains a prelithiated Si, prelithiated Ge, prelithiated Sn, prelithiated SnOx, prelithiated SiOx, prelithiated iron oxide, prelithiated VO2, prelithiated Co3O4, prelithiated Ni3O4, or a combination thereof, wherein x=1 to 2.

5. The anode active material layer of claim 1, wherein said anode active material is in a form of nano particle, nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, nano disc, nano platelet, or nano horn having a thickness or diameter from 0.5 nm to 100 nm.

6. The anode active material layer of claim 5, wherein said anode active material has a dimension less than 20 nm.

7. The anode active material layer of claim 1, wherein said one or a plurality of particles is coated with a layer of carbon disposed between said one or said plurality of particles and said elastomeric material layer.

8. The anode active material layer of claim 1, wherein said particulate further contains a graphite or carbon material therein.

9. The anode active material layer of claim 8, wherein said graphite or carbon material is selected from polymeric carbon, amorphous carbon, chemical vapor deposition carbon, coal tar pitch, petroleum pitch, meso-phase pitch, carbon black, coke, acetylene black, activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, natural graphite particle, or a combination thereof.

10. The anode active material layer of claim 5, wherein said nano particle, nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, nano disc, nano platelet, or nano horn is coated with or embraced by a conductive protective coating selected from a carbon material, electronically conductive polymer, conductive metal oxide, or conductive metal coating.

11. An anode active material layer for a lithium battery, said anode active material layer comprising multiple particulates of an anode active material, wherein a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material that has a lithium ion conductivity no less than 10−7 S/cm at room temperature, and an encapsulating shell thickness from 1 nm to 10 μm, and wherein said high-capacity anode active material has a specific capacity of lithium storage greater than 372 mAh/g; wherein said anode active material is pre-intercalated or pre-doped with lithium ions to form a prelithiated anode active material having an amount of lithium from 0.1% to 54.7% by weight of said prelithiated anode active material.

12. The anode active material layer of claim 1, wherein said elastomeric material has a lithium ion conductivity no less than 10−5 S/cm.

13. The anode active material layer of claim 1, wherein said elastomeric material has a lithium ion conductivity no less than 10−4 S/cm.

14. The anode active material layer of claim 1, wherein said elastomeric material is a neat polymer having no additive or filler dispersed therein.

15. The anode active material layer of claim 1, wherein said elastomeric material is an elastomer matrix composite containing from 0.1% to 50% by weight of a lithium ion-conducting additive dispersed in an elastomer matrix material.

16. An anode active material layer for a lithium battery, said anode active material layer comprising multiple particulates of an anode active material, wherein a particulate is composed of one or a plurality of particles of a high-capacity anode active material being encapsulated by a thin layer of elastomeric material, wherein said elastomeric material contains a material selected from natural polyisoprene, synthetic polyisoprene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, ethylene propylene rubber, ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, or a combination thereof.

17. The anode active material layer of claim 1, wherein said elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive is selected from Li2CO3, Li2O, Li2C2O4, LiOH, LiX, ROCO2Li, HCOLi, ROLi, (ROCO2Li)2, (CH2OCO2Li)2, Li2S, LixSOy, or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

18. The anode active material layer of claim 1, wherein said elastomeric material is an elastomer matrix composite containing a lithium ion-conducting additive dispersed in an elastomer matrix material, wherein said lithium ion-conducting additive contains a lithium salt selected from the group consisting of lithium perchlorate, lithium hexafluorophosphate, lithium borofluoride, lithium hexafluoroarsenide, lithium trifluoro-metasulfonate, bis-trifluoromethyl sulfonylimide lithium, lithium bis(oxalato)borate, lithium oxalyldifluoroborate, lithium nitrate, Li-fluoroalkyl-phosphates, lithium bisperfluoro-ethysulfonylimide, lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide, an ionic liquid-based lithium salt, and combinations thereof.

19. The anode active material layer of claim 1, wherein said elastomeric material contains a mixture or blend of an elastomer and an electron-conducting polymer selected from polyaniline, polypyrrole, polythiophene, polyfuran, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.

20. The anode active material layer of claim 1, wherein said elastomeric material contains a mixture or blend of an elastomer and a lithium ion-conducting polymer selected from the group consisting of poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), poly bis-methoxy ethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, and combinations thereof.

21. A lithium battery containing an optional anode current collector, the anode active material layer as defined in claim 1, a cathode active material layer, an optional cathode current collector, an electrolyte in ionic contact with said anode active material layer and said cathode active material layer, and an optional porous separator.

22. The lithium battery of claim 19, which is a lithium-ion battery, lithium metal battery, lithium-sulfur battery, lithium-selenium battery, or lithium-air battery.