Lithium-protecting polymer layer for an anode-less lithium metal secondary battery and manufacturing method

Abstract

Provided is a lithium secondary battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between the cathode and the anode, wherein the anode comprises: (a) An anode current collector, initially having no lithium or lithium alloy as an anode active material when the battery is made and prior to a charge or discharge operation; and (b) a thin layer of a high-elasticity polymer in ionic contact with the electrolyte and having a recoverable tensile strain from 2% to 700%, a lithium ion conductivity no less than 10.sup.−8 S/cm, and a thickness from 0.5 nm to 100 μm. Preferably, the high-elasticity polymer contains a cross-linked network of polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof in the cross-linked network of polymer chains.

Claims

1. A lithium secondary battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between said cathode and said anode, wherein said anode comprises: a. An anode current collector, initially having no lithium or lithium alloy as an anode active material supported by said anode current collector when the battery is made and prior to a charge or discharge operation; and b. a thin layer of a high-elasticity polymer in ionic contact with said electrolyte and having a recoverable tensile strain from 2% to 700%, a lithium ion conductivity no less than 10.sup.−8 S/cm at room temperature, and a thickness from 0.5 nm to 100 μm, wherein said high-elasticity polymer further comprises an ionic liquid solvent dispersed therein, wherein the ionic liquid solvent is selected from a room temperature ionic liquid having a cation selected from tetraalkylammonium, di-, tri-, or tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, trialkylsulfonium, or a combination thereof.

2. The lithium secondary battery of claim 1, wherein said high-elasticity polymer contains a cross-linked network of polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof in said cross-linked network of polymer chains.

3. The lithium secondary battery of claim 1, wherein said high-elasticity polymer contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

4. The lithium secondary battery of claim 1, wherein said high-elasticity polymer contains from 0.1% to 50% by weight of a lithium ion-conducting additive dispersed therein, or contains therein from 0.1% by weight to 10% by weight of a reinforcement nano filament selected from carbon nanotube, carbon nano-fiber, graphene, or a combination thereof.

5. A lithium secondary battery comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between said cathode and said anode, wherein said anode comprises: a. An anode current collector, initially having no lithium or lithium alloy as an anode active material supported by said anode current collector when the battery is made and prior to a charge or discharge operation; and b. a thin layer of a high-elasticity polymer in ionic contact with said electrolyte and having a recoverable tensile strain from 2% to 700%, a lithium ion conductivity no less than 10.sup.−8 S/cm at room temperature, and a thickness from 0.5 nm to 100 μm, wherein said high-elasticity polymer further comprises an ionic liquid solvent dispersed therein wherein the ionic liquid solvent is selected from a room temperature ionic liquid having an anion selected from B(CN).sub.4.sup.−, CH.sub.3BF.sub.3.sup.−, CH.sub.2CHBF.sub.3.sup.−, CF.sub.3BF.sub.3.sup.−, C.sub.2F.sub.5BF.sub.3.sup.−, n-C.sub.3F.sub.7BF.sub.3.sup.−, n-C.sub.4F.sub.9BF.sub.3.sup.−, PF.sub.6.sup.−, CF.sub.3CO.sub.2.sup.−, CF.sub.3SO.sub.3.sup.−, N(SO.sub.2CF.sub.3).sub.2.sup.−, N(COCF.sub.3)(SO.sub.2CF.sub.3).sup.−, N(SO.sub.2F).sub.2.sup.−, N(CN).sub.2.sup.−, C(CN).sub.3.sup.−, SCN.sup.−, SeCN.sup.−, CuCl.sub.2.sup.−, AlCl.sub.4.sup.−, F(HF).sub.2.3.sup.−, or a combination thereof.

6. The lithium secondary battery of claim 1, wherein said high-elasticity polymer further contains a reinforcement material dispersed therein wherein the reinforcement material is selected from a polymer fiber, a glass fiber, a ceramic fiber or nano-flake, a graphene sheet, a carbon fiber, a graphite fiber, a carbon nano-fiber, a graphite nano-fiber, a carbon nanotube, a graphite particle, an expanded graphite flake, an acetylene black particle, or a combination thereof.

7. The lithium secondary battery of claim 1, wherein said high-elasticity polymer further contains a lithium salt dispersed therein and said lithium salt is selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-methanesulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphate (LiPF.sub.3(CF.sub.2CF.sub.3).sub.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.

8. The lithium secondary battery of claim 1, wherein the high-elasticity polymer forms a mixture or blend with 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.

9. The lithium secondary battery of claim 1, wherein said electrolyte is selected from organic liquid electrolyte, ionic liquid electrolyte, polymer gel electrolyte, solid-state electrolyte, quasi-solid electrolyte having a lithium salt dissolved in an organic or ionic liquid with a lithium salt concentration higher than 2.0 M, or a combination thereof.

10. The lithium secondary battery of claim 1, wherein said high-elasticity polymer has a lithium ion conductivity from 10.sup.−5 S/cm to 10.sup.−2 S/cm.

11. The lithium secondary battery of claim 1, wherein said cathode active material is selected from an inorganic material, an organic material, a polymeric material, or a combination thereof, and said inorganic material does not include sulfur or alkali metal polysulfide.

12. The lithium secondary battery of claim 11, wherein said inorganic material is selected from a metal oxide, metal phosphate, metal silicide, metal selenide, transition metal sulfide, or a combination thereof.

13. The lithium secondary battery of claim 11, wherein said inorganic material is selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, lithium metal silicide, or a combination thereof.

14. The lithium secondary battery of claim 11, wherein said inorganic material is selected from a lithium transition metal silicate, denoted as Li.sub.2MSiO.sub.4 or Li.sub.2Ma.sub.xMb.sub.ySiO.sub.4, wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb is selected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y≤1.

15. The lithium secondary battery of claim 12, wherein said metal oxide contains a vanadium oxide selected from the group consisting of Li.sub.xVO.sub.2, Li.sub.xV.sub.2O.sub.5, Li.sub.xV.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.7, Li.sub.xV.sub.4O.sub.9, Li.sub.xV.sub.6O.sub.13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

16. The lithium secondary battery of claim 12, wherein said metal oxide or metal phosphate is selected from a layered compound LiMO.sub.2, spinel compound LiM.sub.2O.sub.4, olivine compound LiMPO.sub.4, silicate compound Li.sub.2MSiO.sub.4, Tavorite compound LiMPO.sub.4F, borate compound LiMBO.sub.3, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

17. A method of manufacturing a lithium battery, said method comprising: (a) Providing a cathode active material layer and an optional cathode current collector to support said cathode active material layer; (b) Providing an anode current collector, but no lithium metal or lithium alloy as an anode active material in an anode; (c) Providing an electrolyte and an optional separator electrically separating the anode and the cathode; and (d) Providing an anode-protective layer of a high-elasticity polymer disposed between said anode current collector and said electrolyte or separator and dispersing an ionic liquid solvent in a network of chains of said high-elasticity polymer, wherein said high-elasticity polymer has a recoverable tensile elastic strain from 2% to 700%, a lithium ion conductivity no less than 10.sup.−8 S/cm at room temperature, and a thickness from 0.5 nm to 100 μm, wherein said ionic liquid solvent is selected from a room temperature ionic liquid having a cation selected from tetraalkylammonium, di-, tri-, or tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, trialkylsulfonium, or a combination thereof.

18. The method of claim 17, wherein said high-elasticity polymer contains a cross-linked network of polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof in said cross-linked network of polymer chains.

19. The method of claim 17, wherein said high-elasticity polymer contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

20. The method of claim 17, wherein said high-elasticity polymer further comprises from 0.1% to 50% by weight of a lithium ion-conducting additive dispersed therein, or contains therein from 0.1% by weight to 10% by weight of a reinforcement nano filament selected from carbon nanotube, carbon nano-fiber, graphene, nano clay flake, or a combination thereof.

21. The method of claim 17, wherein said high-elasticity polymer is mixed with a lithium ion-conducting additive to form a composite wherein said lithium ion-conducting additive is dispersed in said high-elasticity polymer and is selected from Li.sub.2CO.sub.3, Li.sub.2O, Li.sub.2C.sub.2O.sub.4, LiOH, LiX, ROCO.sub.2Li, HCOLi, ROLi, (ROCO.sub.2Li).sub.2, (CH.sub.2OCO.sub.2Li).sub.2, Li.sub.2S, Li.sub.xSO.sub.y, or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

22. The method of claim 17, wherein said high-elasticity polymer further comprises a lithium salt selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-methanesulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphate (LiPF.sub.3(CF.sub.2CF.sub.3).sub.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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 Schematic of a prior art lithium metal battery cell, containing an anode layer (a thin Li foil or Li coating deposited on a surface of a current collector, Cu foil), a porous separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown.

(2) FIG. 2 Schematic of a presently invented lithium metal battery cell (upper diagram) containing an anode current collector (e.g. Cu foil) but no anode active material (when the cell is manufactured or in a fully discharged state), a high-elasticity polymer-based anode-protecting layer, a porous separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown. The lower diagram shows a thin lithium metal layer deposited between the Cu foil and the polymer layer when the battery is in a charged state.

(3) FIG. 3(A) Representative tensile stress-strain curves of lightly cross-linked ETPTA polymers.

(4) FIG. 3(B) The specific intercalation capacity curves of four lithium cells: 2 cells each having a cathode containing LiV.sub.2O.sub.5 particles (one cell having an ETPTA polymer protective layer and the other not) and 2 cells each having a cathode containing graphene-embraced LiV.sub.2O.sub.5 particles (one cell having an ETPTA polymer protective layer and the other not).

(5) FIG. 4(A) Representative tensile stress-strain curves of PVA-CN polymer films.

(6) FIG. 4(B) The specific capacity values of two lithium-LiCoO.sub.2 cells (initially the cell being lithium-free) featuring (1) high-elasticity PVA-CN layer at the anode and (2) no polymer protection layer at the anode, respectively.

(7) FIG. 5(A) Representative tensile stress-strain curves of PETEA polymer films.

(8) FIG. 5(B) The discharge capacity curves of two coin cells having a NCM532-based of cathode active materials: (1) having a protective layer of high-elasticity PETEA; and (2) no anode-protecting layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) This disclosure is related to a lithium secondary battery, which is preferably based on an organic 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 disclosure is not limited to any battery shape or configuration or any type of electrolyte.

(10) The disclosure provides a lithium secondary battery, comprising a cathode, an anode, and an electrolyte or separator-electrolyte assembly disposed between the cathode and the anode, wherein the anode comprises: (a) an anode current collector, initially having no lithium or lithium alloy as an anode active material when the battery is made and prior to a charge or discharge operation and (b) a thin layer of a high-elasticity polymer having a recoverable tensile strain from 2% to 700%, a lithium ion conductivity no less than 10.sup.−8 S/cm at room temperature, and a thickness from 1 nm to 100 μm (preferably thinner than 10 μm), wherein the high-elasticity polymer is disposed between the current collector and the electrolyte or separator-electrolyte assembly layer. The current collector may be a Cu foil, a layer of Ni foam, a porous layer of nano-filaments, such as graphene sheets, carbon nanofibers, carbon nano-tubes, etc. forming a 3D interconnected network of electron-conducting pathways.

(11) Preferably, this anode-protecting layer is different in composition than the electrolyte used in the lithium battery and the protective layer maintains as a discrete layer (not to be dissolved in the electrolyte) that is disposed between the anode current collector and the electrolyte (or electrolyte-separator layer) when the battery cell is manufactured.

(12) We have discovered that this protective layer provides several unexpected benefits: (a) the formation of dendrite has been essentially eliminated; (b) uniform deposition of lithium back to the anode side is readily achieved; (c) the layer ensures smooth and uninterrupted transport of lithium ions from/to the anode current collector surface (or the lithium film deposited thereon during the battery operations) and through the interface between the current collector (or the lithium film deposited thereon) and the protective layer with minimal interfacial resistance; and (d) cycle stability can be significantly improved and cycle life increased.

(13) In a conventional lithium metal cell, as illustrated in FIG. 1, the anode active material (lithium) is deposited in a thin film form or a thin foil form directly onto an anode current collector (e.g. a Cu foil) before this anode and a cathode are combined to form a cell. The battery is a lithium metal battery, lithium sulfur battery, lithium-selenium battery, etc. As previously discussed in the Background section, these lithium secondary batteries have the dendrite-induced internal shorting and “dead lithium” issues at the anode.

(14) We have solved these challenging issues that have troubled battery designers and electrochemists alike for more than 30 years by developing and implementing a new anode-protecting layer between the anode current collector and the electrolyte (or electrolyte/separator).

(15) This protective layer comprises a high-elasticity polymer having a recoverable (elastic) tensile strain no less than 2% (preferably no less than 5%) under uniaxial tension and a lithium ion conductivity no less than 10.sup.−8 S/cm at room temperature (preferably and more typically from 1×10.sup.−5 S/cm to 5×10.sup.−2 S/cm).

(16) As schematically shown in FIG. 2, one embodiment of the present disclosure is a lithium metal battery cell containing an anode current collector (e.g. Cu foil), a high-elasticity polymer-based anode-protecting layer, a porous separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector (e.g. Al foil) supporting the cathode active layer is also shown in FIG. 2.

(17) High-elasticity polymer refers to a polymer that exhibits an elastic deformation of at least 2% when measured under uniaxial tension. In the field of materials science and engineering, the “elastic deformation” is defined as a deformation of a material (when being mechanically stressed) that is essentially fully recoverable and the recovery is essentially instantaneous upon release of the load. The elastic deformation is preferably greater than 5%, more preferably greater than 10%, furthermore preferably greater than 30%, and still more preferably greater than 100%.

(18) It may be noted that FIG. 2 shows a lithium battery that does not contain a lithium foil or lithium coating at the anode (only an anode current collector, such as a Cu foil or a graphene/CNT mat) when the battery is made or when the battery is completely discharged. The needed lithium to be bounced back and forth between the anode and the cathode is initially stored in the cathode active material (e.g. lithium vanadium oxide Li.sub.xV.sub.2O.sub.5, instead of vanadium oxide, V.sub.2O.sub.5; or lithium polysulfide, instead of sulfur). During the first charging procedure of the lithium battery (e.g. as part of the electrochemical formation process), lithium comes out of the cathode active material, migrates to the anode side, passes through the high-elasticity polymer layer and deposits on the anode current collector. The presence of the presently invented high-elasticity polymer layer enables the uniform deposition of lithium ions on the anode current collector surface. Such a battery configuration avoids the need to have a layer of lithium foil or coating being present during battery fabrication. Bare lithium metal is highly sensitive to air moisture and oxygen and, thus, is more challenging to handle in a real battery manufacturing environment. This strategy of pre-storing lithium in the lithiated (lithium-containing) cathode active materials, such as Li.sub.xV.sub.2O.sub.5 and Li.sub.2S.sub.x, makes all the materials safe to handle in a real manufacturing environment. Cathode active materials, such as Li.sub.xV.sub.2O.sub.5 and Li.sub.2S.sub.x, are typically not air-sensitive.

(19) As the charging procedure continues, more lithium ions get to deposit onto the anode current collector, forming a lithium metal film or coating. During the subsequent discharge procedure, this lithium film or coating layer decreases in thickness due to dissolution of lithium into the electrolyte to become lithium ions, which would create a gap between the current collector and the protective layer if the protective layer were not elastic. Such a gap would make the re-deposition of lithium ions back to the anode impossible during a subsequent recharge procedure. We have observed that the high-elasticity polymer is capable of expanding or shrinking congruently or conformably with the anode layer. This capability helps to maintain a good contact between the current collector (or the lithium film deposited on the current collector surface) and the protective layer, enabling the re-deposition of lithium ions without interruption.

(20) The protective layer is able to be kept in place, prevent gaps, and allow the lithium layer to be deposited on the anode current collector via a compressive force which keeps the elastic material pressed against the current collector to prevent gaps, but which still allows lithium to deposit on the current collector. The elastic layer compresses or extends to maintain its position, electrical contact, and allow for the lithium layer to both be deposited and depleted.

(21) The high-elasticity polymer may be impregnated or infiltrated with an ionic liquid. The ionic liquid is composed of ions only. Ionic liquids are low melting temperature salts that are in a molten or liquid state when above a desired temperature. For instance, an ionic salt is considered as an ionic liquid if its melting point is below 100° C. If the melting temperature is equal to or lower than room temperature (25° C.), the salt is referred to as a room temperature ionic liquid (RTIL). The IL salts are characterized by weak interactions, due to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and asymmetry (cation).

(22) A well-known ionic liquid is formed by the combination of a 1-ethyl-3-methyl-imidazolium (EMI) cation and an N,N-bis(trifluoromethane)sulfonamide (TFSI) anion. This combination gives a fluid with an ionic conductivity comparable to many organic electrolyte solutions, a low decomposition propensity and low vapor pressure up to −300-400° C. This implies a generally low volatility and non-flammability and, hence, a much safer electrolyte for batteries.

(23) Ionic liquids are basically composed of organic or inorganic ions that come in an unlimited number of structural variations owing to the preparation ease of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide and hexafluorophosphate as anions. Based on their compositions, ionic liquids come in different classes that include three basic types: aprotic, protic and zwitterionic types, each one suitable for a specific application.

(24) Common cations of room temperature ionic liquids (RTILs) include, but are not limited to, tetraalkylammonium, di, tri, and tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILs include, but are not limited to, BF.sub.4.sup.−, B(CN).sub.4.sup.−, CH.sub.3BF.sub.3.sup.−, CH.sub.2CHBF.sub.3.sup.−, CF.sub.3BF.sub.3.sup.−, C.sub.2F.sub.5BF.sub.3.sup.−, n-C.sub.3F.sub.7BF.sub.3.sup.−, n-C.sub.4F.sub.9BF.sub.3.sup.−, PF.sub.6.sup.−, CF.sub.3CO.sub.2.sup.−, CF.sub.3SO.sub.3.sup.−, N(SO.sub.2CF.sub.3).sub.2.sup.−, N(COCF.sub.3)(SO.sub.2CF.sub.3).sup.−, N(SO.sub.2F).sub.2.sup.−, N(CN).sub.2.sup.−, C(CN).sub.3.sup.−, SCN.sup.−, SeCN.sup.−, CuCl.sub.2.sup.−, AlCl.sub.4.sup.−, F(HF).sub.2.3.sup.−, etc. Relatively speaking, the combination of imidazolium- or sulfonium-based cations and complex halide anions such as AlCl.sub.4.sup.−, BF.sub.4.sup.−, CF.sub.3CO.sub.2.sup.−, CF.sub.3SO.sub.3.sup.−, NTf.sub.2.sup.−, N(SO.sub.2F).sub.2.sup.−, or F(HF).sub.2.3.sup.− results in RTILs with good working conductivities.

(25) RTILs can possess archetypical properties such as high intrinsic ionic conductivity, high thermal stability, low volatility, low (practically zero) vapor pressure, non-flammability, the ability to remain liquid at a wide range of temperatures above and below room temperature, high polarity, high viscosity, and wide electrochemical windows. These properties, except for the high viscosity, are desirable attributes when it comes to using an RTIL as an electrolyte solvent in a rechargeable lithium cell.

(26) When an ionic liquid alone is used as a solvent to dissolve lithium salt therein, the resulting electrolyte is non-flammable and safe. By impregnating the high-elasticity polymer with such an ionic liquid solvent, with or without a lithium salt dissolved therein, one can produce a safe and highly effective protective layer for the anode.

(27) The presently invented lithium secondary batteries can contain a wide variety of cathode active materials. The cathode active material layer may contain a cathode active material selected from an inorganic material, an organic material, a polymeric material, or a combination thereof. The inorganic material may be selected from a metal oxide, metal phosphate, metal silicide, metal selenide, transition metal sulfide, or a combination thereof.

(28) The inorganic material may be selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, lithium metal silicide, or a combination thereof.

(29) The cathode active material layer may contain a metal oxide containing vanadium oxide selected from the group consisting of Li.sub.xVO.sub.2, Li.sub.xV.sub.2O.sub.5, Li.sub.xV.sub.3O.sub.8, Li.sub.xV.sub.3O.sub.7, Li.sub.xV.sub.4O.sub.9, Li.sub.xV.sub.6O.sub.13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x≤5.

(30) The cathode active material layer may contain a metal oxide or metal phosphate, selected from a layered compound LiMO.sub.2, spinel compound LiM.sub.2O.sub.4, olivine compound LiMPO.sub.4, silicate compound Li.sub.2MSiO.sub.4, Tavorite compound LiMPO.sub.4F, borate compound LiMBO.sub.3, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

(31) The lithium secondary battery may be a lithium-sulfur battery, wherein the cathode comprises a lithium polysulfide.

(32) Preferably and typically, the high-elasticity polymer has a lithium ion conductivity no less than 10.sup.−5 S/cm, more preferably no less than 10.sup.−4 S/cm, further preferably no less than 10.sup.−3 S/cm, and most preferably no less than 10.sup.−2 S/cm. In some embodiments, the high-elasticity polymer is a neat polymer having no additive or filler dispersed therein. In others, the high-elasticity polymer is a polymer matrix composite containing from 0.1% to 50% (preferably 1% to 35%) by weight of a lithium ion-conducting additive dispersed in a high-elasticity polymer matrix material.

(33) The high-elasticity polymer can have a high elasticity (elastic deformation strain value >2%). An elastic deformation is a deformation that is fully recoverable and the recovery process is essentially instantaneous (no significant time delay). The high-elasticity polymer can exhibit an elastic deformation from 5% up to 1,000% (10 times of its original length), more typically from 10% to 800%, and furthermore typically from 50% to 500%, and most typically and desirably from 70% to 300%. 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%).

(34) In some preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof, in the cross-linked network of polymer chains. These network or cross-linked polymers exhibit a unique combination of a high elasticity (high elastic deformation strain) and high lithium-ion conductivity.

(35) In certain preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate (PETEA) chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

(36) Typically, a high-elasticity polymer is originally in a monomer or oligomer states that can be cured to form a cross-linked polymer that is highly elastic. Prior to curing, these polymers or oligomers are soluble in an organic solvent to form a polymer solution. An ion-conducting or electron-conducting additive may be added to this solution to form a suspension. This solution or suspension can then be formed into a thin layer of polymer precursor on a surface of an anode current collector. The polymer precursor (monomer or oligomer and initiator) is then polymerized and cured to form a lightly cross-linked polymer. This thin layer of polymer may be tentatively deposited on a solid substrate (e.g. surface of a polymer or glass), dried, and separated from the substrate to become a free-standing polymer layer. This free-standing layer is then laid on a lithium foil/coating or implemented between a lithium film/coating and electrolyte or separator. Polymer layer formation can be accomplished by using one of several procedures well-known in the art; e.g. spraying, spray-painting, printing, coating, extrusion-based film-forming, casting, etc.

(37) For instance, ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, chemical formula given below), along with an initiator, can be dissolved in an organic solvent, such as ethylene carbonate (EC) or diethyl carbonate (DEC). Then, the ETPTA monomer/solvent/initiator solution may be cast to form ETPTA a monomer/initiator layer on a glass surface. The layer can then be thermally cured to obtain a thin layer of a high-elasticity polymer. The polymerization and cross-linking reactions of this monomer can be initiated by a radical initiator derived from benzoyl peroxide (BPO) or AIBN through thermal decomposition of the initiator molecule. The ETPTA monomer has the following chemical formula:

(38) ##STR00001##

(39) As another example, the high-elasticity polymer for anode lithium foil/coating protection may be based on cationic polymerization and cross-linking of the cyanoethyl polyvinyl alcohol (PVA-CN, Formula 2) in succinonitrile (SN).

(40) ##STR00002##

(41) The procedure may begin with dissolving PVA-CN in succinonitrile (NCCH.sub.2CH.sub.2CN) to form a mixture solution. This is followed by adding an initiator into the mixture solution. For instance, LiPF.sub.6 can be added into the PVA-CN/SN mixture solution at a weight ratio (selected from the preferred range from 20:1 to 2:1) to form a precursor solution. Then, the solution may be deposited to form a thin layer of reacting mass, PVA-CN/LiPF.sub.6, which is subsequently heated at a temperature (e.g. from 75 to 100° C.) for 2 to 8 hours to obtain a high-elasticity polymer. During this process, cationic polymerization and cross-linking of cyano groups on the PVA-CN may be initiated by PF.sub.5, which is derived from the thermal decomposition of LiPF.sub.6 at such an elevated temperature.

(42) It is essential for these materials to form a lightly cross-linked network of polymer chains. In other words, the network polymer or cross-linked polymer should have a relatively low degree of cross-linking or low cross-link density to impart a high elastic deformation.

(43) The cross-link density of a cross-linked network of polymer chains may be defined as the inverse of the molecular weight between cross-links (Mc). The cross-link density can be determined by the equation, Mc=ρRT/Ge, where Ge is the equilibrium modulus as determined by a temperature sweep in dynamic mechanical analysis, p is the physical density, R is the universal gas constant in J/mol*K and T is absolute temperature in K. Once Ge and p are determined experimentally, then Mc and the cross-link density can be calculated.

(44) The magnitude of Mc may be normalized by dividing the Mc value by the molecular weight of the characteristic repeat unit in the cross-link chain or chain linkage to obtain a number, Nc, which is the number of repeating units between two cross-link points. We have found that the elastic deformation strain correlates very well with Mc and Nc. The elasticity of a cross-linked polymer derives from a large number of repeating units (large Nc) between cross-links. The repeating units can assume a more relax conformation (e.g. random coil) when the polymer is not stressed. However, when the polymer is mechanically stressed, the linkage chain uncoils or gets stretched to provide a large deformation. A long chain linkage between cross-link points (larger Nc) enables a larger elastic deformation. Upon release of the load, the linkage chain returns to the more relaxed or coiled state. During mechanical loading of a polymer, the cross-links prevent slippage of chains that otherwise form plastic deformation (non-recoverable).

(45) Preferably, the Nc value in a high-elasticity polymer is greater than 5, more preferably greater than 10, furthermore preferably greater than 100, and even more preferably greater than 200. These Nc values can be readily controlled and varied to achieve different elastic deformation values by using different cross-linking agents with different functionalities, and by designing the polymerization and cross-linking reactions to proceed at different temperatures for different periods of time.

(46) Alternatively, Mooney-Rilvin method may be used to determine the degree of cross-linking. Crosslinking also can be measured by swelling experiments. In a swelling experiment, the crosslinked sample is placed into a good solvent for the corresponding linear polymer at a specific temperature, and either the change in mass or the change in volume is measured. The higher the degree of crosslinking, the less swelling is attainable. Based on the degree of swelling, the Flory Interaction Parameter (which relates the solvent interaction with the sample, Flory Huggins Eq.), and the density of the solvent, the theoretical degree of crosslinking can be calculated according to Flory's Network Theory. The Flory-Rehner Equation can be useful in the determination of cross-linking.

(47) The high-elasticity polymer may contain a simultaneous interpenetrating network (SIN) polymer, wherein two cross-linking chains intertwine with each other, or a semi-interpenetrating network polymer (semi-IPN), which contains a cross-linked polymer and a linear polymer. An example of semi-IPN is an UV-curable/polymerizable trivalent/monovalent acrylate mixture, which is composed of ethoxylated trimethylolpropane triacrylate (ETPTA) and ethylene glycol methyl ether acrylate (EGMEA) oligomers. The ETPTA, bearing trivalent vinyl groups, is a photo (UV)-crosslinkable monomer, capable of forming a network of cross-linked chains. The EGMEA, bearing monovalent vinyl groups, is also UV-polymerizable, leading to a linear polymer with a high flexibility due to the presence of the oligomer ethylene oxide units. When the degree of cross-linking of ETPTA is moderate or low, the resulting ETPTA/EGMEA semi-IPN polymer provides good mechanical flexibility or elasticity and reasonable mechanical strength. The lithium-ion conductivity of this polymer is in the range from 10.sup.−4 to 5×10.sup.−3 S/cm.

(48) The aforementioned high-elasticity polymers may be used alone to protect the lithium foil/coating layer at the anode. Alternatively, the high-elasticity polymer can be mixed with a broad array of elastomers, electrically conducting polymers, lithium ion-conducting materials, and/or strengthening materials (e.g. carbon nanotube, carbon nano-fiber, or graphene sheets).

(49) A broad array of elastomers can be mixed with a high-elasticity polymer to form a blend, co-polymer, or interpenetrating network that encapsulates the cathode active material particles. 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-E1), 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.

(50) 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.

(51) In some embodiments, a high-elasticity polymer can form a polymer matrix composite containing a lithium ion-conducting additive dispersed in the high-elasticity polymer matrix material, wherein the lithium ion-conducting additive is selected from Li.sub.2CO.sub.3, Li.sub.2O, Li.sub.2C.sub.2O.sub.4, LiOH, LiX, ROCO.sub.2Li, HCOLi, ROLi, (ROCO.sub.2Li).sub.2, (CH.sub.2OCO.sub.2Li).sub.2, Li.sub.2S, Li.sub.xSO.sub.y, or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

(52) In some embodiments, the high-elasticity polymer can be mixed with a lithium ion-conducting additive, which contains a lithium salt selected from lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium borofluoride (LiBF.sub.4), lithium hexafluoroarsenide (LiAsF.sub.6), lithium trifluoro-methanesulfonate (LiCF.sub.3SO.sub.3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF.sub.3SO.sub.2).sub.2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), lithium nitrate (LiNO.sub.3), Li-fluoroalkyl-phosphate (LiPF.sub.3(CF.sub.2CF.sub.3).sub.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.

(53) The high-elasticity polymer may form a mixture, blend, or semi-interpenetrating network with 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 high-elasticity polymer may form a mixture, co-polymer, or semi-interpenetrating network with 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.

(54) Unsaturated rubbers that can be mixed with the high-elasticity polymer 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), and 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 bond particles of a cathode active material by one of several means; e.g. spray coating, dilute solution mixing (dissolving the cathode active material particles in an uncured polymer, monomer, or oligomer, with or without an organic solvent) followed by drying and curing.

(55) 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.

Example 1: Some Examples of Ionic Liquid Solvents Used

(56) Preferred Ionic Liquid Solvents (Alone or with a Lithium Salt Dissolved Therein) Used to infiltrate or impregnate a lightly cross-linked polymer network may be selected from a room temperature ionic liquid (RTIL) having a cation selected from tetraalkylammonium, di-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, or dialkylpiperidinium. The counter anion is preferably selected from BF.sub.4.sup.−, B(CN).sub.4.sup.−, CF.sub.3CO.sub.2.sup.−, CF.sub.3SO.sub.3.sup.−, N(SO.sub.2CF.sub.3).sub.2.sup.−, N(COCF.sub.3)(SO.sub.2CF.sub.3).sup.−, or N(SO.sub.2F).sub.2.sup.−. Particularly useful ionic liquid-based electrolytes (ionic liquid-based lithium salt dissolved in an ionic liquid solvent) include: lithium bis(trifluoro methanesulfonyl)imide in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethane sulfonyl)imide (LiTFSI in BEPyTFSI), N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP.sub.13TFSI) containing LiTFSI, or N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI) containing LiTFSI.

(57) A wide range of lithium salts can be used as the lithium salt dissolved in an ionic liquid solvent. The following are good choices for lithium salts that tend to be dissolved well in selected ionic liquid solvents: lithium borofluoride (LiBF.sub.4), lithium trifluoro-methanesulfonate (LiCF.sub.3SO.sub.3), lithium bis-trifluoromethyl sulfonylimide (LiN(CF.sub.3SO.sub.2).sub.2 or LITFSI), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF.sub.2C.sub.2O.sub.4), and lithium bisperfluoroethy-sulfonylimide (LiBETI). A good electrolyte additive for helping to stabilize Li metal is LiNO.sub.3.

Example 2: Anode-Less Lithium Battery Containing a High-Elasticity Polymer-Protected Anode

(58) The ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, Sigma-Aldrich) was dissolved in a solvent mixture of ethylene carbonate (EC)/diethyl carbonate (DEC), at a weight-based composition ratio of the ETPTA/solvent of 3/97 (w/w). Subsequently, benzoyl peroxide (BPO, 1.0 wt. % relative to the ETPTA content) was added as a radical initiator to allow for thermal crosslinking reaction upon deposition on a Cu foil surface. This layer of ETPTA monomer/initiator was then thermally cured at 60° C. for 30 min to obtain a protective layer. An ionic liquid (PP.sub.13TFSI) was then sprayed over this protective layer and heated to permeate into the polymer network.

(59) On a separate basis, some amount of the ETPTA monomer/solvent/initiator solution was cast onto a glass surface to form a wet film, which was thermally dried and then cured at 60° C. for 30 min to form a film of cross-linked polymer. In this experiment, the BPO/ETPTA weight ratio was varied from 0.1% to 4% to vary the degree of cross-linking in several different polymer films. Some of the cured polymer samples were subjected to dynamic mechanical testing to obtain the equilibrium dynamic modulus, Ge, for the determination of the number average molecular weight between two cross-link points (Mc) and the corresponding number of repeat units (Nc), as a means of characterizing the degree of cross-linking. The typical and preferred number of repeat units (Nc) is from 5 to 5,000, more preferably from 10 to 1,000, further preferably from 20 to 500, and most preferably from 50 to 500.

(60) Several tensile testing specimens were cut from each cross-link film and tested with a universal testing machine. The representative tensile stress-strain curves of four BPO-initiated cross-linked ETPTA polymers are shown in FIG. 3(A), which indicate that this series of network polymers have an elastic deformation from approximately 230% to 700%. The above values are for neat polymers without any additive. The addition of up to 30% by weight of a lithium salt typically reduces this elasticity down to a reversible tensile strain in the range from 10% to 100%.

(61) For electrochemical testing, the working electrodes (cathode layers) were prepared by mixing 85 wt. % LiV.sub.2O.sub.5 or 88% of graphene-embraced LiV.sub.2O.sub.5 particles, 5-8 wt. % CNTs, and 7 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 Al 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.

(62) 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 LiPF.sub.6 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 cell featuring high-elasticity polymer binder and that containing PVDF binder were evaluated by galvanostatic charge/discharge cycling at a current density of 50 mA/g using an Arbin electrochemical workstation.

(63) Summarized in FIG. 3(B) are the specific intercalation capacity curves of four lithium cells: 2 cells each having a cathode containing LiV.sub.2O.sub.5 particles (one cell having a cross-linked ETPTA polymer-based lithium metal anode-protecting layer and the other not) and 2 cells each having a cathode containing graphene-embraced LiV.sub.2O.sub.5 particles (one cell having a cross-linked ETPTA polymer-based lithium anode-protecting layer and the other not). As the number of cycles increases, the specific capacity of the un-protected cells drops at the fastest rate. In contrast, the presently invented cross-linked ETPTA polymer protection layer provides the battery cell with a significantly more stable and high specific capacity for a large number of cycles. These data have clearly demonstrated the surprising and superior performance of the presently invented cross-linked ETPTA polymer protection approach.

(64) The high-elasticity cross-linked ETPTA polymer protective layer appears to be capable of reversibly deforming to a great extent without breakage when the lithium foil decreases in thickness during battery discharge. The protective polymer layer also prevents the continued reaction between liquid electrolyte and lithium metal at the anode, reducing the problem of continuing loss in lithium and electrolyte. This also enables a significantly more uniform deposition of lithium ions upon returning from the cathode during a battery re-charge; hence, no lithium dendrite. These were observed by using SEM to examine the surfaces of the electrodes recovered from the battery cells after some numbers of charge-discharge cycles.

Example 3: High-Elasticity Polymer Implemented in the Anode of a Lithium-LiCoO.SUB.2 .Cell (Initially the Cell being Lithium-Free)

(65) The high-elasticity polymer for anode protection was based on cationic polymerization and cross-linking of the cyanoethyl polyvinyl alcohol (PVA-CN) in succinonitrile (SN). The procedure began with dissolving PVA-CN in succinonitrile to form a mixture solution. This step was followed by adding an initiator into the solution. For the purpose of incorporating some lithium species into the high elasticity polymer, we chose to use LiPF.sub.6 as an initiator. The ratio between LiPF.sub.6 and the PVA-CN/SN mixture solution was varied from 1/20 to 1/2 by weight to form a series of precursor solutions. Subsequently, these solutions were separately spray-deposited to form a thin layer of precursor reactive mass onto a Cu foil. The precursor reactive mass was then heated at a temperature from 75 to 100° C. for 2 to 8 hours to obtain a layer of high-elasticity polymer adhered to the Cu foil surface.

(66) Additionally, some amount of the reacting mass, PVA-CN/LiPF.sub.6, was cast onto a glass surface to form several films which were polymerized and cross-linked to obtain cross-linked polymers having different degrees of cross-linking. Tensile testing was also conducted on these films and some testing results are summarized in FIG. 4(A). This series of cross-linked polymers can be elastically stretched up to approximately 80% (higher degree of cross-linking) to 400% (lower degree of cross-linking).

(67) FIG. 4(B) shows that the cell having an anode-protecting polymer layer offers a significantly more stable cycling behavior. The high-elasticity polymer appears to act to isolate the liquid electrolyte from the subsequently deposited lithium coating, preventing continued reaction between the liquid electrolyte and lithium metal.

Example 4: Li Metal Cells Containing a PETEA-Based High-Elasticity Polymer-Protected Anode

(68) For preparing as an anode lithium metal-protecting layer, pentaerythritol tetraacrylate (PETEA), Formula 3, was used as a monomer:

(69) ##STR00003##

(70) In a representative procedure, the precursor solution was composed of 1.5 wt. % PETEA (C.sub.17H.sub.20O.sub.8) monomer and 0.1 wt. % azodiisobutyronitrile (AIBN, C.sub.8H.sub.12N.sub.4) initiator dissolved in a solvent mixture of 1,2-dioxolane (DOL)/dimethoxymethane (DME)(1:1 by volume). The PETEA/AIBN precursor solution was cast onto a lithium metal layer pre-deposited on a Cu foil surface to form a precursor film, which was polymerized and cured at 70° C. for half an hour to obtain a lightly cross-linked polymer. An ionic liquid electrolyte (LiTFSI in BEPyTFSI) was then painted over this polymer film, allowing time for permeation into the network of polymer chains.

(71) Additionally, the reacting mass, PETEA/AIBN (without conductive additive), was cast onto a glass surface to form several films that were polymerized and cured to obtain cross-linked polymers having different degrees of cross-linking. Tensile testing was also conducted on these films and some testing results are summarized in FIG. 5(A). This series of cross-linked polymers can be elastically stretched up to approximately 25% (higher degree of cross-linking) to 80% (lower degree of cross-linking)

(72) Commercially available NCM-532 powder (well-known lithium nickel cobalt manganese oxide), along with graphene sheets (as a conductive additive), was then added into an NMP and PVDF binder suspension to form a multiple-component slurry. The slurry was then slurry-coated on Al foil to form cathode layers.

(73) Shown in FIG. 5(B) are the discharge capacity curves of two coin cells having the same cathode active material, but one cell having a high-elasticity polymer-protected anode and the other having no protective layer. These results have clearly demonstrated that the high-elasticity polymer protection strategy provides excellent protection against capacity decay of an anode-less lithium metal battery. Both coin cells shown in FIG. 5(B) were anode-less.

(74) The high-elasticity polymer appears to be capable of reversibly deforming without breakage when the anode layer expands and shrinks during charge and discharge. The polymer also prevents continued reaction between the liquid electrolyte and the lithium metal. No dendrite-like features were found with the anode being protected by a high-elasticity polymer. This was confirmed by using SEM to examine the surfaces of the electrodes recovered from the battery cells after some numbers of charge-discharge cycles.

Example 5: Effect of Lithium Ion-Conducting Additive in a High-Elasticity Polymer

(75) A wide variety of lithium ion-conducting additives were added to several different polymer matrix materials to prepare anode protection layers. The lithium ion conductivity values of the resulting polymer/salt complex materials are summarized in Table 1 below. We have discovered that these polymer composite materials are suitable anode-protecting layer materials provided that their lithium ion conductivity at room temperature is no less than 10.sup.−6 S/cm. With these materials, lithium ions appear to be capable of readily diffusing through the protective layer having a thickness no greater than 1 μm. For thicker polymer films (e.g. 10 μm), a lithium ion conductivity at room temperature of these high-elasticity polymers no less than 10.sup.−4 S/cm would be required.

(76) TABLE-US-00001 TABLE 1 Lithium ion conductivity of various high-elasticity polymer composite compositions as a shell material for protecting anode active material particles. Sample Lithium-conducting Elastomer Li-ion conductivity No. additive (1-2 μm thick) (S/cm) E-1b Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 70-99% PVA-CN 2.9 × 10.sup.−4 to 3.6 × 10.sup.−3 S/cm E-2b Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 65-99% ETPTA 6.4 × 10.sup.−4 to 2.3 × 10.sup.−3 S/cm E-3b Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 65-99% ETPTA/EGMEA 8.4 × 10.sup.−4 to 1.8 × 10.sup.−3 S/cm D-4b Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 70-99% PETEA 7.8 × 10.sup.−3 to 2.3 × 10.sup.−2 S/cm D-5b Li.sub.2CO.sub.3 + (CH.sub.2OCO.sub.2Li).sub.2 75-99% PVA-CN 8.9 × 10.sup.−4 to 5.5 × 10.sup.−3 S/cm B1b LiF + LiOH + Li.sub.2C.sub.2O.sub.4 60-90% PVA-CN 8.7 × 10.sup.−5 to 2.3 × 10.sup.−3 S/cm B2b LiF + HCOLi 80-99% PVA-CN 2.8 × 10.sup.−4 to 1.6 × 10.sup.−3 S/cm B3b LiOH 70-99% PETEA 4.8 × 10.sup.−3 to 1.2 × 10.sup.−2 S/cm B4b Li.sub.2CO.sub.3 70-99% PETEA 4.4 × 10.sup.−3 to 9.9 × 10.sup.−3 S/cm B5b Li.sub.2C.sub.2O.sub.4 70-99% PETEA 1.3 × 10.sup.−3 to 1.2 × 10.sup.−2 S/cm B6b Li.sub.2CO.sub.3 + LiOH 70-99% PETEA 1.4 × 10.sup.−3 to 1.6 × 10.sup.−2 S/cm C1b LiClO.sub.4 70-99% PVA-CN 4.5 × 10.sup.−4 to 2.4 × 10.sup.−3 S/cm C2b LiPF.sub.6 70-99% PVA-CN 3.4 × 10.sup.−4 to 7.2 × 10.sup.−3 S/cm C3b LiBF.sub.4 70-99% PVA-CN 1.1 × 10.sup.−4 to 1.8 × 10.sup.−3 S/cm C4b LiBOB + LiNO.sub.3 70-99% PVA-CN 2.2 × 10.sup.−4 to 4.3 × 10.sup.−3 S/cm S1b Sulfonated polyaniline 85-99% ETPTA 9.8 × 10.sup.−5 to 9.2 × 10.sup.−4 S/cm S2b Sulfonated SBR 85-99% ETPTA 1.2 × 10.sup.−4 to 1.0 × 10.sup.−3 S/cm S3b Sulfonated PVDF 80-99% ETPTA/EGMEA 3.5 × 10.sup.−4 to 2.1 × 10.sup.−4 S/cm S4b Polyethylene oxide 80-99% ETPTA/EGMEA 4.9 × 10.sup.−4 to 3.7 × 103.sup.4 S/cm

(77) In conclusion, the high-elasticity polymer-based anode-protecting layer strategy is surprisingly effective in alleviating the problems of lithium metal dendrite formation and lithium metal-electrolyte reactions that otherwise lead to capacity decay and potentially internal shorting and explosion of the lithium secondary batteries. The high-elasticity polymer is capable of expanding or shrinking congruently or conformably with the anode layer. This capability helps to maintain a good contact between the current collector (or the deposited lithium film during the charging procedure) and the protective layer, enabling uniform re-deposition of lithium ions without interruption.