FIELD

The present disclosure relates generally to the field of lithium-ion batteries and, in particular, to composite particulates for use in the anode of a lithium-ion battery.

BACKGROUND

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 may be expressed as LixC6, 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 purpose of the charge transfer between an anode and a cathode. 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 subsequent 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 Qircan 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 LiaA (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), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, as schematically illustrated inFIG. 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 nanoparticles. 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-conductive to lithium ions (e.g., ceramic or oxide coating). Ideally, the protective material should meet the following requirements: (a) The protective material must be lithium ion-conducting as well as initially electron-conducting (when the anode electrode is made) and be capable of preventing liquid electrolyte from being in constant contact with the anode active material particles (e.g. Si). (b) The protective material should also have high fracture toughness or high resistance to crack formation to avoid disintegration during 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 combined protective material-anode material structure must allow for an adequate amount of free space to accommodate volume expansion of the anode active material particles when lithiated. The prior art protective materials all fall short of these requirements. Hence, it is 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 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 a specific object of the present disclosure to meet these needs and address the issues associated the rapid capacity decay of a lithium battery containing a high-capacity anode active material.

SUMMARY

The present disclosure provides multi-functional composite particulates for a lithium battery, wherein at least one of the composite particulates has a diameter from 100 nm to 50 μm and comprises a polymer electrolyte, comprising from 0.1% to 40% by weight of a lithium salt dissolved or dispersed in the polymer electrolyte, and one or a plurality of primary particles of an anode active material that are encapsulated by, embedded in, dispersed in, or bonded by the polymer electrolyte having a lithium ion conductivity from 10−8to 5×10−2S/cm, wherein the primary particles have a diameter or thickness from 0.5 nm to 20 μm and occupy a weight fraction from 5% to 98% based on the total weight of the composite particulate. The lithium salt is preferably in the amount from 1% to 30% by weight based on the total composite particulate weight. The multi-functional particulate may have a diameter preferably and typically from 500 nm to 50 μm, and more preferably and typically from 1 μm to 30 μm.

In certain embodiments, the polymer electrolyte comprises a polymer selected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO), poly(ethylene glycol) (PEG), 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), cyanoethyl poly(vinyl alcohol) (PVACN), a pentaerythritol tetraacrylate (PETEA)-based polymer, an aliphatic polycarbonate (including poly(vinylene carbonate) (PVC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and poly(trimethylene carbonate) (PTMC)), a single Li-ion conducting solid polymer electrolyte with a carboxylate anion, a sulfonylimide anion, or sulfonate anion, a crosslinked electrolyte of poly(ethylene glycol)diacrylate (PEGDA) or poly(ethylene glycol)methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

It may be noted that the polymers in the above list are commonly used as an ingredient in a gel polymer electrolyte or solid polymer electrolyte of a lithium-ion battery cell. These polymers, however, have not been previously used in a composite particulate as herein disclosed.

The lithium salt is preferably selected from lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium borofluoride (LiBF4), lithium hexafluoroarsenide (LiAsF6), lithium trifluoro-methanesulfonate (LiCF3SO3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF3SO2)2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF2C2O4), lithium nitrate (LiNO3), Li-fluoroalkyl-phosphate (LiPF3(CF2CF3)3), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

In the multi-functional composite particulates, the polymer electrolyte may be further impregnated with an organic liquid solvent, an ionic liquid, or a combination thereof. Such a liquid may be pre-impregnated into the electrolyte polymer during or after the composite particulates are made, but prior to being incorporated in the anode electrode and the fabrication of the battery cell. In certain embodiments, the liquid solvent or ionic liquid may get permeated into the polymer of the particulate from the liquid electrolyte of the cell per se.

The composite particulate is preferably further encapsulated by a shell of a conducting material selected from a graphene, carbon (e.g. carbon nanoparticles, carbon black, acetylene black, carbon nanotube, carbon nano-fiber, etc.), graphite (e.g. expanded graphene flakes, exfoliated graphite worms, nano-scaled or sub-micron-scaled graphite particles), conducting polymer, metal, composite, or a combination thereof. Preferably, the shell has an electrical conductivity from 10−8S/cm to 103S/cm, more preferably from 10−5S/cm to 102S/cm and further preferably at least 10−2S/cm. The composite shell may include a polymer matrix composite (e.g. rubbery or elastomeric matrix, thermoplastic matrix, thermoset matrix, interpenetrating network matrix, etc.), metal matrix composite (e.g. Al, Zn, etc.), ceramic matrix composite (e.g. metal oxide and metal sulfide matrix), glass matrix composite, carbon matrix composite (e.g. amorphous carbon, CVD carbon, PVD carbon, sputtering carbon, polymeric carbon, etc.), and organic matrix composite. The shell preferably has a thickness from 0.34 nm (thickness of a pristine graphene) to 10 μm.

In some embodiments, the composite further comprises graphene sheets (inside the particulate core, in the encapsulating shell, or both) selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, nitrogenated graphene, hydrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.

In some embodiments, the polymer electrolyte further comprises a reinforcement material or additive selected from carbon nanotubes, carbon nano-fibers, carbon or graphite fibers, graphene sheets, expanded graphite flakes, polymer fibrils, glass fibers, ceramic fibers, metal filaments or metal nano-wires, whiskers, carbon black, acetylene black, needle coke, carbon particles, graphite particles, or a combination thereof. The preferred amount of the reinforcement material or additive is from 1% to 30% by weight based on the total composite particulate weight.

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 having at least 60% by weight of lithium element therein; and (h) combinations thereof. The Li alloy may contain from 0.1% to 10% by weight of a metal element selected from Zn, Ag, Au, Mg, Ni, Ti, Fe, Co, V, Al, or a combination thereof.

The anode active material may contain a prelithiated Si, prelithiated Ge, prelithiated Sn, prelithiated SnOx, prelithiated SiOx, prelithiated iron oxide, prelithiated Mn3O4, prelithiated Co3O4, prelithiated Ni3O4, lithium titanate, lithium niobite, or a combination thereof, wherein x=1 to 2.

The primary particles of anode active material may be in a form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, or nanohorn having a thickness or diameter from 0.5 nm to 100 nm. In some embodiments, at least one of the primary anode active material particles is coated with a layer of carbon, graphite, or graphene.

In certain embodiments, the composite particulate further comprises from 0.1% to 40% by weight of a lithium ion-conducting additive dispersed in the conducting polymer gel network. The lithium ion-conducting additive may be 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, 1≤y≤4.

The disclosure also provides a powder mass comprising presently invented multi-functional composite particulates and a battery anode comprising this powder mass. These individual particulates in the powder mass are isolated and separated from one another (not chemically bonded together by a binder or by the network of cross-linked polymer chains prior to being incorporated into an anode electrode). These individual particulates may be bonded with a resin binder, along with an optional conductive additive, to make an anode electrode. Also provided is a battery containing the battery anode, which is a lithium-ion battery, lithium metal battery, lithium-sulfur battery, lithium-air battery, or lithium-selenium battery.

The present disclosure also provides a method of producing multi-functional composite particulates comprising a lithium salt and one or multiple anode active material particles embedded in, dispersed in, encapsulated by, or bonded by a polymer electrolyte material for a lithium battery. The method comprises (A) dispersing a plurality of primary particles of an anode active material having a diameter or thickness from 0.5 nm to 20 μm and a lithium salt in an uncured conjugated polymer, reactive monomer, or growing oligomer in a liquid state (e.g. an oxidative initiator, a cross-linking agent, and a monomer in a liquid state) to form a reactive slurry; (B) forming the slurry into micro-droplets and curing the polymer or polymerizing the reactive monomer or growing oligomer to form the multi-functional composite particulates.

The step (B) of micro-droplet formation may be conducted by operating a procedure selected from pan-coating, air-suspension coating, centrifugal extrusion, vibration-nozzle encapsulation, spray-drying, coacervation-phase separation, interfacial polycondensation or interfacial cross-linking, in-situ polymerization, matrix polymerization, extrusion and palletization, or a combination thereof. In certain preferred embodiments, step (B) comprises conducting spray-drying, fluidized bed coating, or air-suspension coating to produce the multi-functional particulates.

In certain embodiments of the disclosure, the method of producing multi-functional particulates of conducting composites for a lithium battery may comprise: (a) dispersing a plurality of primary particles of an anode active material, having a diameter or thickness from 0.5 nm to 20 μm, and a desired amount of a lithium salm into a precursor liquid (precursor to a polymer electrolyte) to form a suspension or slurry; and (b) forming the suspension or slurry into micro-droplets and, concurrently or sequentially, polymerizing and/or crosslinking the precursor to form the multi-functional composite particulates.

The step of forming the suspension into micro-droplets may include operating a procedure selected from pan-coating, air-suspension coating, centrifugal extrusion, vibration-nozzle encapsulation, spray-drying, coacervation-phase separation, interfacial polycondensation or interfacial cross-linking, in-situ polymerization, matrix polymerization, extrusion and palletization, or a combination thereof.

In some embodiments, the precursor liquid may contain a monomer, an initiator or catalyst, a crosslinking agent, an oxidizer and/or dopant. During or after the micro-droplet formation procedure, one may initiate the polymerization and/or crosslinking reactions to produce linear-chain polymer, branched chain polymer, or a cross-linked network of polymer chains in the droplets. In the particulate, the primary particles of anode active material (e.g. Si nanowires or SiO particles) and some optional reinforcement materials and/or additive, are embedded in or encapsulated by the electrolyte polymer chains.

In certain preferred embodiments, anode active material primary particles are porous having surface pores, internal pores, or both surface and internal pores.

The anode active material is preferably in a form of nanoparticle, nanowire, nanofiber, nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, or nanohorn having a thickness or diameter from 0.5 nm to 100 nm.

In certain embodiments, the method further comprises a step of incorporating the composite particulates into an anode electrode. The method may also further comprise a step of combining the anode electrode, a cathode electrode, and an electrolyte to form a lithium battery cell.

The present disclosure also provides a powder mass containing the invented composite particulates. In some embodiments, the invented method further comprises a step of making the invented multiple composite particulates into a powder mass as a product. Also provided is a battery anode containing the invented composite particulate described above. The disclosure further provides a battery containing such a battery anode. Thus, the method may further contain a step of incorporating the composite particulates into an anode and a further step of combining the anode, a cathode and electrolyte into a battery cell. The battery may be a lithium-ion battery, lithium metal secondary battery, lithium-sulfur battery, lithium-air battery, or lithium-selenium battery.

In some embodiments, the particles of anode active material contain pre-lithiated particles. In other words, before the electrode active material particles (such as Si or SnO2) are combined with a sacrificial material and embraced by graphene sheets, these particles have been previously intercalated with Li ions (e.g. via electrochemical charging) up to an amount of 0.1% to 30% by weight of Li. Such a pre-lithiating step may be conducted after the porous anode particulates are made.

In some embodiments, the primary particles of anode active material contain particles pre-coated with a coating layer of a conductive material selected from carbon, pitch, carbonized resin, a conductive polymer, a conductive organic material, a metal coating, a metal oxide shell, graphene sheets, or a combination thereof. The coating layer thickness is preferably in the range from 1 nm to 20 μm, preferably from 5 nm to 10 μm, and further preferably from 10 nm to 1 μm.

