FLAME-RESISTANT QUASI-SOLID HYBRID ELECTROLYTE FOR SAFE ANODE-LESS LITHIUM BATTERIES AND PRODUCTION METHOD

Abstract

A rechargeable lithium cell comprising: (a) a cathode having a cathode active material and a first electrolyte in ionic contact with the cathode active material; (b) an anode having an anode current collector but no anode active material and having no lithium metal when the cell is made; (c) an optional porous separator electronically separating the anode and the cathode; and (d) a second electrolyte, comprising a polymer electrolyte in ionic contact with the first electrolyte, wherein the polymer electrolyte is disposed substantially between the anode and the cathode, between the separator and the cathode, and/or between the separator and the anode. The polymer electrolyte substantially does not permeate into the anode or the cathode. Also provided is a method of preparing or operating such an anode-less lithium cell.

Claims

1. A rechargeable lithium cell comprising: (a) A cathode having a cathode active material and a first electrolyte in ionic contact with the cathode active material; (b) an anode having an anode current collector but no anode active material and having no lithium metal when the cell is made; (c) a porous separator electronically separating said anode and said cathode; and (d) a second electrolyte, comprising a polymer electrolyte in ionic contact with the first electrolyte, wherein the polymer electrolyte is disposed substantially between the anode and the cathode, between the separator and the cathode, and/or between the separator and the anode.

2. The rechargeable lithium cell of claim 1, wherein the first electrolyte contains a lithium salt dissolved in a liquid solvent, having a lithium salt concentration C1 from 1.5 M to 14.0 M so that said electrolyte exhibits a vapor pressure less than 0.01 kPa when measured at 20° C., a vapor pressure less than 60% of the vapor pressure of said liquid solvent alone, a flash point at least 20 degrees Celsius higher than a flash point of said liquid solvent alone, a flash point higher than 150° C., or no flash point.

3. The rechargeable lithium cell of claim 2, wherein said first electrolyte further comprises a flame-retardant additive, different in composition than said liquid solvent, and the flame-retardant additive is selected from Hydrofluoro ether (FIFE), Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE), Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi), Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone (PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkylsilane (Si—C), liquid oligomeric silaxane (—Si—O—Si—), Tetraethylene glycol dimethylether (TEGDME), canola oil, or a combination thereof and said additive-to-said liquid solvent ratio in said mixture is from 5/95 to 95/5 by weight.

4. The rechargeable lithium cell of claim 3, wherein said liquid additive-to-said liquid solvent ratio in said mixture is from 15/85 to 85/15 by weight.

5. The rechargeable lithium cell of claim 1, wherein said polymer electrolyte comprises a polymer selected from poly(ethylene oxide), polypropylene oxide, poly(ethylene glycol), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene, cyanoethyl poly(vinyl alcohol), a pentaerythritol tetraacrylate-based polymer, an aliphatic polycarbonate, 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 or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

6. The rechargeable lithium cell of claim 1, wherein said anode or said cathode is substantially free of the polymer electrolyte.

7. The rechargeable lithium cell of claim 1, which is a lithium metal secondary cell, a lithium-ion cell, a lithium-sulfur cell, a lithium-ion sulfur cell, a lithium-selenium cell, or a lithium-air cell.

8. The rechargeable lithium cell of claim 2, wherein said liquid solvent is selected from the group consisting of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), a hydrofluoroether, and combinations thereof.

9. The rechargeable lithium cell of claim 2, wherein 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-phosphates (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid lithium salt, or a combination thereof.

10. The rechargeable lithium cell of claim 2, wherein a molar fraction or molecular fraction of said lithium salt in said first electrolyte is greater than 0.2 and up to 0.99.

11. The rechargeable lithium cell of claim 1, wherein said first electrolyte is selected from a liquid electrolyte, an ionic liquid electrolyte, a polymer electrolyte, a polymer gel electrolyte, an inorganic solid electrolyte, a catholyte, or a combination thereof.

12. The rechargeable lithium cell of claim 11, wherein said ionic liquid 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.

13. The rechargeable lithium cell of claim 11, wherein said ionic liquid is selected from a room temperature ionic liquid having an anion selected from 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−, 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.

14. The rechargeable lithium cell of claim 1, wherein said anode current collector comprises a foil, perforated sheet, or foam of a metal having two primary surfaces wherein at least one primary surface is coated with or protected by a layer of lithiophilic metal, a layer of graphene material, or both.

15. The rechargeable lithium cell of claim 14, wherein the metal foil, perforated sheet, or foam is selected from Cu, Ni, stainless steel, Al, graphene-coated metal, graphite-coated metal, carbon-coated metal, or a combination thereof.

16. The rechargeable lithium cell of claim 14, wherein the lithiophilic metal is selected from Au, Ag, Mg, Zn, Ti, K, Al, Fe, Mn, Co, Ni, Sn, V, Cr, an alloy thereof, or a combination thereof.

17. The rechargeable lithium cell of claim 14, wherein the graphene layer comprises graphene sheets selected from single-layer or few-layer graphene, wherein said few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d.sub.002 from 0.3354 nm to 0.6 nm as measured by X-ray diffraction and said single-layer or few-layer graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 45% by weight of non-carbon elements.

18. The rechargeable lithium cell of claim 17, wherein said non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.

19. The rechargeable lithium cell of claim 1, wherein said separator comprises polymeric fibers, ceramic fibers, glass fibers, particles of a glass or ceramic material, or a combination thereof.

20. A method of producing the rechargeable lithium cell of claim 1, said method comprising: (A) preparing a lithium cell comprising (i) a cathode having a cathode active material and a first electrolyte in ionic contact with the cathode active material, (ii) an anode having an anode current collector but no anode active material and having no lithium metal when the cell is made, (iii) an optional porous separator or ion-permeable membrane electronically separating the anode and cathode, and (iv) a protecting housing that substantially encloses the cathode, the anode, the optional separator, wherein said lithium cell has an unfilled space; and (B) introducing a second electrolyte into said unfilled space, wherein said second electrolyte comprises a polymer electrolyte in ionic contact with said first electrolyte and being disposed between the anode and the cathode, between the separator and the cathode, and/or between the separator and the anode.

21. The method of claim 20, wherein step (A) comprises a procedure of introducing the first electrolyte in the cathode when the cathode is made and before the cathode is enclosed in the protective housing, or a procedure of introducing the first electrolyte in the cathode after the cathode, the anode, and the separator are enclosed in the protective housing.

22. The method of claim 20, wherein said polymer electrolyte comprises a polymer selected from poly(ethylene oxide), polypropylene oxide, poly(ethylene glycol), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene, cyanoethyl poly(vinyl alcohol), a pentaerythritol tetraacrylate-based polymer, an aliphatic polycarbonate, 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 or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

23. The method of claim 20, wherein step (B) comprises a procedure of introducing a precursor monomer, oligomer, or un-cured version of said polymer electrolyte into said unfilled space and then polymerizing and/or curing the precursor inside said battery cell to form said polymer electrolyte.

24. The method of claim 20, wherein said step (A) comprises a procedure (i) of assembling said anode, said cathode, and said separator or membrane, along with a cell housing together to form a dry battery cell having initially no electrolyte therein and a procedure (ii) of introducing a first electrolyte into said dry cell, enabling said first electrolyte to permeate into the cathode; and wherein said step (B) is conducted after procedure (ii).

25. The method of claim 20, wherein said method further comprises, before step (B), a procedure of removing a desired portion of a liquid solvent from said battery cell to create additional unfilled space.

26. The method of claim 20, wherein the first electrolyte has a solution comprising a lithium salt dissolved in a liquid solvent having a salt concentration from 1.5 M to 14.0 M.

27. The method of claim 26, wherein the first electrolyte further comprises a flame-retardant additive that is different in composition than said liquid solvent and is selected from Hydrofluoro ether (HFE), Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE), Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi), Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone (PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkylsilane (Si—C), liquid oligomeric silaxane (—Si—O—Si—), Tetraethylene glycol dimethylether (TEGDME), canola oil, or a combination thereof and said additive-to-said liquid solvent ratio in said mixture is from 5/95 to 95/5 by weight.

28. A rechargeable lithium cell, comprising: (a) a cathode having a cathode active material; (b) an anode having an anode current collector, but no anode active material, and having no lithium metal when the cell is made; and (c) an electrolyte, comprising a polymer electrolyte in ionic contact with the anode and the cathode, wherein the polymer electrolyte comprises a polymer selected from poly(ethylene oxide), polypropylene oxide, poly(ethylene glycol), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene, cyanoethyl poly(vinyl alcohol), a pentaerythritol tetraacrylate-based polymer, an aliphatic polycarbonate, 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 or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] FIG. 1(A) Photos showing the results of a flammability test conducted for various electrolytes with different lithium salt concentrations (intended to serve as a first electrolyte of the presently disclosed lithium cell);

[0073] FIG. 1(B) Photo showing the result of a flammability test for a lower concentration electrolyte but having an additive, 30% FPC; LiTFSI/(DME+DOL+FPC)=0.16 or approximately 2.0 M (intended to serve as a first electrolyte of the presently disclosed lithium cell).

[0074] FIG. 2(A) Structure of an anode-less lithium metal cell (as manufactured or in a discharged state) according to some embodiments of the present disclosure;

[0075] FIG. 2(B) Structure of an anode-less lithium metal cell (in a charged state) according to some embodiments of the present disclosure.

[0076] FIG. 2(C) Structure of an anode-less lithium metal cell (as manufactured or in a discharged state) according to some embodiments of the present disclosure;

[0077] FIG. 2(D) Structure of an anode-less lithium metal cell (in a charged state) according to some embodiments of the present disclosure.

[0078] FIG. 3 Cycling performance (discharge specific capacity and Coulomb efficiency) of an anode-less Li metal cell containing an NMC-622 cathode active material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0079] The present disclosure provides a safe and high-performing lithium battery, which can be any of various types of lithium-ion cells or lithium metal cells. A high degree of safety is imparted to this battery by a novel and unique electrolyte that is essentially non-flammable and would not initiate a fire or sustain a fire and, hence, would not pose explosion danger. This disclosure has solved the very most critical issue that has plagued the lithium-metal and lithium-ion industries for more than two decades.

[0080] As indicated earlier in the Background section, a strong need exists for a safe, non-flammable, yet injectable quasi-electrolyte electrolyte system for a rechargeable lithium cell that is compatible with existing battery production facilities. The electrolyte must be sufficiently high in lithium salt concentration to ensure non-flammability and yet also maintain adequate flowability (fluidity) to enable that the electrolyte can be introduced into dry battery cells. The present disclosure has solved this problem of having two conflicting requirements that appear to be mutually exclusive.

[0081] In certain embodiments, the rechargeable lithium cell comprises: (a) a cathode having a cathode active material and a first electrolyte in ionic contact with the cathode active material; (b) an anode having an anode current collector but no anode active material (no conventional anode active material such as graphite, Si, SiO, Sn, and conversion-type anode materials) and having no lithium metal when the cell is made (there is no lithium metal or anode active material before the cell begins to charge and discharge, hence the name “anode-less” cell); (c) an optional porous separator electronically separating the anode and the cathode; and (d) a second electrolyte, comprising a polymer electrolyte in ionic contact with the first electrolyte, wherein the polymer electrolyte is disposed substantially between the anode and the cathode, between the separator and the cathode, and/or between the separator and the anode. Preferably and typically, the anode and/or the cathode are substantially free of the second electrolyte composition.

[0082] As illustrated in FIG. 2(A), the anode-less lithium cell is in an as-manufactured or fully discharged state according to certain embodiments of the present disclosure. The cell comprises an anode current collector 12 (e.g. Cu foil), a second electrolyte 14 shown to be on two surfaces of a separator 15 (can be just one layer on the anode or the cathode side of the separator), a cathode layer 16 comprising a cathode active material and an electrolyte (first electrolyte), and a cathode current collector 18 that supports the cathode layer 16. There is no lithium metal in the anode side when the cell is manufactured.

[0083] In a charged state, as illustrated in FIG. 2(B), the cell comprises an anode current collector 12, lithium metal 20 plated on a surface (or two surfaces) of the anode current collector 12 (e.g. Cu foil), second electrolyte 14 shown to be on two surfaces of a separator 15, a cathode layer 16 comprising a cathode active material and an electrolyte (first electrolyte), and a cathode current collector 18 supporting the cathode layer. The lithium metal comes from the cathode active material (e.g. LiCoO.sub.2 and LiMn.sub.2O.sub.4) that contains Li element when the cathode is made. During a charging step, lithium ions are released from the cathode active material and move to the anode side to deposit onto a surface or both surfaces of an anode current collector.

[0084] FIG. 2(C) shows the anode-less lithium cell is in an as-manufactured or fully discharged state according to another certain embodiment of the present disclosure. FIG. 2(D) shows a charged state of a cell of the present disclosure. FIG. 2(C) and FIG. 2(D) do not have a separator.

[0085] In certain embodiments, the first electrolyte contains a solution comprising a lithium salt dissolved in a liquid solvent, having a lithium salt concentration C1 from 1.5 M to 14.0 M so that the electrolyte exhibits a vapor pressure less than 0.01 kPa when measured at 20° C., a vapor pressure less than 60% of the vapor pressure of said liquid solvent alone, a flash point at least 20 degrees Celsius higher than a flash point of the liquid solvent alone, a flash point higher than 150° C., or no detectable flash point.

[0086] The first electrolyte may be selected from a liquid electrolyte, an ionic liquid electrolyte, a polymer electrolyte, a polymer gel electrolyte, an inorganic solid electrolyte, a catholyte, or a combination thereof.

[0087] In some embodiments, the first electrolyte and/or the second electrolyte comprises a flame-retardant additive that is different in composition than the liquid solvent and is selected from Hydrofluoro ether (FIFE), Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE), Fluoroethylene carbonate (FEC), Tris(trimethylsilyl) phosphite (TTSPi), Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone (PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkylsilane (Si—C), liquid oligomeric silaxane (—Si—O—Si—), tetraethylene glycol dimethylether (TEGDME), canola oil, or a combination thereof and the additive-to-said liquid solvent ratio in said mixture is from 5/95 to 95/5 by weight.

[0088] One unique feature of the presently disclosed lithium cell is the notion that there is substantially no anode active material and no lithium metal is present when the battery cell is made. The commonly used anode active material, such as an intercalation type anode material (e.g. graphite, carbon particles, Si, SiO, Sn, SnO.sub.2, Ge, etc.), P, or any conversion-type anode material, is not included in the cell. The anode only contains a current collector or a protected current collector. No lithium metal (e.g. Li particle, surface-stabilized Li particle, Li foil, Li chip, etc.) is present in the anode when the cell is made; lithium is basically stored in the cathode (e.g. Li element in LiCoO.sub.2, LiMn.sub.2O.sub.4, lithium iron phosphate, lithium polysulfides, lithium polyselenides, etc.). During the first charge procedure after the cell is sealed in a housing (e.g. a stainless steel hollow cylinder or an Al/plastic laminated envelop), lithium ions are released from these Li-containing compounds (cathode active materials) in the cathode, travel through the electrolyte/separator into the anode side, and get deposited on the surfaces of an anode current collector. During a subsequent discharge procedure, lithium ions leave these surfaces and travel back to the cathode, intercalating or inserting into the cathode active material.

[0089] Such an anode-less cell is much simpler and more cost-effective to produce since there is no need to have a layer of anode active material (e.g. graphite particles, along with a conductive additive and a binder) pre-coated on the Cu foil surfaces via the conventional slurry coating and drying procedures. The anode materials and anode active layer manufacturing costs can be saved. Furthermore, since there is no anode active material layer (otherwise typically 40-200 μm thick), the weight and volume of the cell can be significantly reduced, thereby increasing the gravimetric and volumetric energy density of the cell.

[0090] Another important advantage of the anode-less cell is the notion that there is no lithium metal in the anode when a lithium metal cell is made. Lithium metal (e.g. Li metal foil and particles) is highly sensitive to air moisture and oxygen and notoriously known for its difficulty and danger to handle during manufacturing of a Li metal cell. The manufacturing facilities must be equipped with special class of dry rooms, which are expensive and significantly increase the battery cell costs.

[0091] The anode current collector may be selected from a foil, perforated sheet, or foam of Cu, Ni, stainless steel, Al, graphene, graphite, graphene-coated metal, graphite-coated metal, carbon-coated metal, or a combination thereof. Preferably, the current collector is a Cu foil, Ni foil, stainless steel foil, graphene-coated Al foil, graphite-coated Al foil, or carbon-coated Al foil.

[0092] The anode current collector typically has two primary surfaces. Preferably, one or both of these primary surfaces is deposited with multiple particles or coating of a lithium-attracting metal (lithiophilic metal), wherein the lithium-attracting metal, preferably having a diameter or thickness from 1 nm to 10 μm, is selected from Au, Ag, Mg, Zn, Ti, K, Al, Fe, Mn, Co, Ni, Sn, V, Cr, an alloy thereof, or a combination thereof. This deposited metal layer may be further deposited with a layer of graphene that covers and protects the multiple particles or coating of the lithiophilic metal.

[0093] The graphene layer may comprise graphene sheets selected from single-layer or few-layer graphene, wherein the few-layer graphene sheets are commonly defined to have 2-10 layers of stacked graphene planes having an inter-plane spacing d.sub.002 from 0.3354 nm to 0.6 nm as measured by X-ray diffraction. The single-layer or few-layer graphene sheets may contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 45% by weight of non-carbon elements. The non-pristine graphene may be selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.

[0094] The graphene layer may comprise graphene balls and/or graphene foam. Preferably, the graphene layer has a thickness from 1 nm to 50 μm and/or has a specific surface area from 5 to 1000 m.sup.2/g (more preferably from 10 to 500 m.sup.2/g).

[0095] Another surprising and of tremendous scientific and technological significance is our discovery that the flammability of any volatile organic solvent can be effectively suppressed provided that a sufficiently high amount of a lithium salt and polymer is added to and dissolved in this organic solvent to form a solid-like or quasi-solid electrolyte (e.g. first electrolyte in the cathode). In general, such a quasi-solid electrolyte exhibits a vapor pressure less than 0.01 kPa and often less than 0.001 kPa (when measured at 20° C.) and less than 0.1 kPa and often less than 0.01 kPa (when measured at 100° C.). (The vapor pressures of the corresponding neat solvent, without any lithium salt dissolved therein, are typically significantly higher.) In many cases, the vapor molecules are practically too few to be detected.

[0096] A highly significant observation is that the high concentration of the lithium salt dissolved in an otherwise highly volatile solvent (a large molecular ratio or molar fraction of lithium salt, typically >0.2, more typically >0.3, and often >0.4 or even >0.5) can dramatically curtail the amount of volatile solvent molecules that can escape into the vapor phase in a thermodynamic equilibrium condition. In many cases, this has effectively prevented the flammable gas molecules from initiating a flame even at an extremely high temperature (e.g. using a torch, as demonstrated in FIG. 1(A) and FIG. 1(B)). The flash point of the quasi-solid electrolyte is typically at least 20 degrees (often >50 degrees) higher than the flash point of the neat organic solvent alone. In most of the cases, either the flash point is higher than 150° C. or no flash point can be detected. The electrolyte just would not catch on fire. Furthermore, any accidentally initiated flame does not sustain for longer than 3 seconds. This is a highly significant discovery, considering the notion that fire and explosion concern has been a major impediment to widespread acceptance of battery-powered electric vehicles. This new technology could significantly impact the emergence of a vibrant EV industry.

[0097] In the no flash point conditions of the present application, such as those demonstrated in FIG. 1(A) and FIG. 1(B), contacting the electrolyte with a butane torch does not trigger an ignition that would occur at a flash point. The flame of a butane torch is known to burn at temperatures above 1,400° C. The flame used in no flash point conditions is estimated to have a temperature of 1,400° C.

[0098] However, an excessively high salt concentration could result in an excessively high electrolyte viscosity. When the lithium salt concentration exceeds approximately 3.5 M (molecular ratio or fraction >0.28), it becomes very difficult to inject the electrolyte into a well-packed dry cell to finish the cell production procedure. The injection becomes totally impossible when the salt concentration exceeds 5.0 M (molecular fraction >0.4). This has prompted us to search for solutions to this problem of having two mutually exclusive requirements (high salt concentration for non-flammability and low salt concentration for electrolyte fluidity). After extensive and in-depth studies, we have come to discover that these conflicting issues can be resolved provided certain liquid additives are added to the liquid solvent to form a mixture in which the lithium salt is dissolved to form the electrolyte. There can be one liquid solvent with one liquid additive, one liquid solvent with two liquid additives, two liquid solvents with one liquid additive, etc. in the liquid mixture. There can be multiple liquid solvents mixed with multiple liquid additives.

[0099] FIG. 1(B) is a photo showing the result of a flammability test for a lower concentration electrolyte but having an additive (30% FPC), which is a flame retardant. The electrolyte composition has a molecular ratio of LiTFSI/(DME+DOL+FPC)=0.16 or approximately 2.0 M. Even though this is a relatively low concentration among the group of quasi-solid electrolytes, this electrolyte is non-flammable when a torch was brought to be in touch with the electrolyte. This electrolyte has a relatively low viscosity, having sufficient flowability to enable electrolyte injection during battery manufacturing. Injection of electrolyte becomes more difficult when the salt concentration exceeds 3 M and becomes practically non-feasible when the salt concentration exceeds 5 M.

[0100] The polymer electrolyte preferably 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.

[0101] The polymer electrolyte composition preferably contains a polymer that can be cured or cross-linked. This polymer may be initially in a monomer or oligomer state that remains as a liquid which can be injected into the battery cell and then cured or crosslinked after being injected into the cell. Examples include cyanoethyl poly(vinyl alcohol) (PVACN), a pentaerythritol tetraacrylate (PETEA)-based polymer, poly(vinylene carbonate) (PVC), poly(ethylene carbonate) (PEC), poly(propylene carbonate) (PPC), and poly(trimethylene carbonate) (PTMC).

[0102] The polymer electrolyte in the second electrolyte composition is designed to further reduce the flammability of the battery cell. In the presently disclosed lithium secondary cell, typically and preferably the first electrolyte is incorporated into the battery cell first. This can be conducted by incorporating the first electrolyte into the cathode before the electrodes, along with the porous separator or ion-permeable membrane, are assembled into a cell. Alternatively, the anode, the cathode, and the porous separator or ion-permeable membrane are assembled into a dry cell, which is then injected with the first electrolyte composition. The first electrolyte does not fully occupy the internal space of the dry cell, leaving some room to accommodate the second electrolyte composition. Once an adequate amount of time is allowed for permeation of the first electrolyte to reach the cathode active materials in the cathode, one may then introduce the second electrolyte composition into the cell. Since the internal structures of the cathode has been substantially loaded with the first electrolyte, the second electrolyte tends to stay near the separator or a space between the anode (anode current collector) and the cathode. This feature is also important in terms of isolating any potential fire-causing event (e.g. constraining any potential thermal runaway event inside the anode, not to easily spread into the cathode, and vice versa).

[0103] As indicated earlier, the polymer may be initially in a monomer or oligomer state that remains as a liquid which is capable of being injected and flowed into the battery cell and then cured or crosslinked after being injected into the cell. The polymer electrolyte typically will not permeate into the interior of the anode or the interior of the cathode.

[0104] In addition to the non-flammability and high lithium ion transference numbers, there are several additional benefits associated with using the presently disclosed quasi-solid electrolytes. As one example, the quasi-solid electrolyte can significantly enhance cyclic and safety performance of rechargeable lithium batteries through effective suppression of lithium dendrite growth. It is generally accepted that dendrites start to grow in the non-aqueous liquid electrolyte when the anion is depleted in the vicinity of the electrode where plating occurs. In the ultrahigh concentration electrolyte, there is a mass of anions to keep the balance of cations (Li.sup.t) and anions near metallic lithium anode. Further, the space charge created by anion depletion is minimal, which is not conducive to dendrite growth. Furthermore, due to both ultrahigh lithium salt concentration and high lithium-ion transference number, the quasi-solid electrolyte provides a large amount of available lithium-ion flux and raises the lithium ionic mass transfer rate between the electrolyte and the lithium electrode, thereby enhancing the lithium deposition uniformity and dissolution during charge/discharge processes. Additionally, the local high viscosity induced by a high concentration will increase the pressure from the electrolyte to inhibit dendrite growth, potentially resulting in a more uniform deposition on the surface of the anode. The high viscosity could also limit anion convection near the deposition area, promoting more uniform deposition of Li ions. These reasons, separately or in combination, are believed to be responsible for the notion that no dendrite-like feature has been observed with any of the large number of rechargeable lithium cells that we have investigated thus far.

[0105] As another benefit example, this electrolyte is capable of inhibiting lithium polysulfide dissolution at the cathode of a Li—S cell, thus overcoming the polysulfide shuttle phenomenon and allowing the cell capacity not to decay significantly with time. Consequently, a coulombic efficiency nearing 100% along with long cycle life can be achieved. When a concentrated electrolyte is used, the solubility of lithium polysulfide will be reduced significantly.

[0106] The liquid solvent utilized in the instant electrolytes may be selected from the group consisting of 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutyl ether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene, methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate (VC), allyl ethyl carbonate (AEC), a hydrofloroether (e.g. methyl perfluorobutyl ether, MFE, or ethyl perfluorobutyl ether, EFE), and combinations thereof.

[0107] The lithium salt may be 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-phosphates (LiPF.sub.3(CF.sub.2CF.sub.3).sub.3), lithium bisperfluoro-ethysulfonylimide (LiBETI), lithium bis(trifluoromethanesulphonyl)imide, lithium bis(fluorosulphonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid lithium salt, or a combination thereof.

[0108] 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-based lithium 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).

[0109] Some ILs may be used as a co-solvent (not as a salt) to work with the first organic solvent of the present disclosure. 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)sulphonamide (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 solvent for batteries.

[0110] 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(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide and hexafluorophosphate as anions. Useful ionic liquid-based lithium salts (not solvent) may be composed of lithium ions as the cation and bis(trifluoromethanesulphonyl)imide, bis(fluorosulphonyl)imide and hexafluorophosphate as anions. For instance, lithium trifluoromethanesulfonimide (LiTFSI) is a particularly useful lithium salt.

[0111] 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. 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.

[0112] 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 co-solvent in a rechargeable lithium cell.

[0113] There is also no restriction on the type of the cathode materials that can be used in practicing the present disclosure. For Li—S cells, the cathode active material may contain lithium polysulfide. If the cathode active material includes lithium-containing species (e.g. lithium polysulfide) when the cell is made, there is no need to have a lithium metal pre-implemented in the anode.

[0114] The rechargeable lithium metal or lithium-ion cell featuring an organic liquid solvent-based quasi-solid electrolyte containing a high lithium salt concentration may contain a cathode active material selected from, as examples, 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.

[0115] The disclosure also provides a method of producing the disclosed rechargeable lithium cell, the method comprising:

[0116] (A) preparing a lithium cell comprising (i) a cathode having a cathode active material and a first electrolyte in ionic contact with the cathode active material, (ii) an anode having an anode current collector but no anode active material and having no lithium metal when the cell is made, (iii) an optional porous separator or ion-permeable membrane electronically separating the anode and cathode, and (iv) a protecting housing that substantially encloses the cathode, the anode, the optional separator, wherein the lithium cell has an unfilled space; and

[0117] (B) introducing a second electrolyte into said unfilled space, wherein the second electrolyte comprises a polymer electrolyte in ionic contact with the first electrolyte and being disposed between the anode and the cathode, between the separator and the cathode, and/or between the separator and the anode.

[0118] In certain embodiments, step (A) comprises either a procedure of introducing the first electrolyte in the cathode when the cathode is made and before the cathode is enclosed in the protective housing or a procedure of introducing the first electrolyte in the cathode after the cathode, the anode, and the separator are enclosed in the protective housing.

[0119] The first electrolyte may comprise a solution comprising a lithium salt dissolved in a liquid solvent having a salt concentration from 1.5 M to 14.0 M.

[0120] The first electrolyte may further comprise a flame-retardant additive that is different in composition than said liquid solvent and is selected from Hydrofluoro ether (FIFE), Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE), Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi), Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone (PS), Propene sultone (PES), Alkylsiloxane (Si—O), Alkylsilane (Si—C), liquid oligomeric silaxane (—Si—O—Si—), Tetraethylene glycol dimethylether (TEGDME), canola oil, or a combination thereof and said additive-to-said liquid solvent ratio in said mixture is from 5/95 to 95/5 by weight.

[0121] The polymer electrolyte preferably comprises a polymer selected from poly(ethylene oxide), polypropylene oxide, poly(ethylene glycol), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), poly bis-methoxy ethoxyethoxide-phosphazenex, polyvinyl chloride, polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene, cyanoethyl poly(vinyl alcohol), a pentaerythritol tetraacrylate-based polymer, an aliphatic polycarbonate, 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 or poly(ethylene glycol) methyl ether acrylate, a sulfonated derivative thereof, or a combination thereof.

[0122] In some embodiments, step (B) comprises a procedure of introducing a precursor monomer, oligomer, or un-cured version of the polymer electrolyte into the unfilled space and then polymerizing and/or curing the precursor inside the battery cell to form the polymer electrolyte.

[0123] In some embodiments, step (A) comprises a procedure (i) of assembling the anode, the cathode, and the separator or membrane, along with a cell housing together to form a dry battery cell having initially no electrolyte therein and a procedure (ii) of introducing a first electrolyte into the dry cell, enabling the first electrolyte to permeate into the cathode; and wherein step (B) is conducted after procedure (ii).

[0124] The method may further comprise, before step (B), a procedure of removing a desired portion of a liquid solvent from said battery cell to create additional unfilled space.

[0125] The following examples are presented primarily for the purpose of illustrating the best mode practice of the present disclosure, not to be construed as limiting the scope of the present disclosure.

Example 1: Some Examples of Electrolytes Used in the First Electrolyte Compositions

[0126] A wide range of lithium salts can be used as the lithium salt dissolved in an organic liquid solvent (alone or in a mixture with another organic liquid or an ionic liquid). The following are good choices for lithium salts that tend to be dissolved well in selected organic or 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. Particularly useful ionic liquid-based lithium salts include: lithium bis(trifluoro methanesulfonyl)imide (LiTFSI).

[0127] Preferred organic liquid solvents include: ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), propylene carbonate (PC), acetonitrile (AN), vinylene carbonate (VC), allyl ethyl carbonate (AEC), 1,3-dioxolane (DOL), and 1,2-dimethoxyethane (DME).

[0128] Preferred flame retardant liquid additives are Hydrofluoro ether (FIFE), Trifluoro propylene carbonate (FPC), Methyl nonafluorobutyl ether (MFE), Fluoroethylene carbonate (FEC), Tris(trimethylsilyl)phosphite (TTSPi), Triallyl phosphate (TAP), Ethylene sulfate (DTD), 1,3-propane sultone (PS), Propene sultone (PES), Diethyl carbonate (DEC), Alkylsiloxane (Si—O), Alkylsilane (Si—C), liquid oligomeric silaxane (—Si—O—Si—), Tetraethylene glycol dimethylether (TEGDME), canola oil.

[0129] Preferred ionic liquid solvents 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 solvents include N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethane sulfonyl)imide (BEPyTFSI), N-methyl-N-propylpiperidinium bis(trifluoromethyl sulfonyl)imide (PP.sub.13TFSI), and N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide.

Example 2: Flash Points and Vapor Pressure of Some Solvents and Additives and Corresponding Quasi-Solid Electrolytes with a Lithium Salt Molecular Ratio of x=0.2

[0130] The flash points of several solvents (with or without a liquid flame retardant additive) and their electrolytes having a lithium salt molecular ratio x=0.2 are presented in Table 1 below. It may be noted that, according to the OSHA (Occupational Safety & Health Administration) classification, any liquid with a flash point below 38.7° C. is flammable. However, in order to ensure safety, we have designed our quasi-solid electrolytes to exhibit a flash point significantly, higher than 38.7° C. (by a large margin, e.g. at least increased by 50° and preferably above 150° C.). The data in Table 1 indicate that the addition of a lithium salt to a molecular ratio of 0.2 is normally sufficient to meet these criteria provided a selective additive is added into the liquid solvent. The addition of a second electrolyte, introduced after the first electrolyte was injected and permeated into the cathode, was found to substantially totally suppress the flash point.

TABLE-US-00001 TABLE 1 The flash points and vapor pressures of select solvents and their electrolytes with a lithium salt molecular ratio x = 0.2 (approximately 2.5M). Flash Liquid additive Flash point (° C.) point (additive/solvent = with x = Liquid solvent (° C.) 25/75) 0.2 of (Li salt) DOL 1 none 35 (LiBF.sub.4) (1,3-dioxolane) DOL TEGDME 150 (LiBF.sub.4) DOL 1 none 76 (LiCF.sub.3SO.sub.3) DOL Ethylene sulfate 155 (LiCF.sub.3SO.sub.3) (DTD) DEC (diethyl 33 none 120 (LiCF.sub.3SO.sub.3) carbonate) DEC FPC (Trifluoro >200 (LiCF.sub.3SO.sub.3) propylene carbonate) DMC (Dimethyl 18 none 87 (LiCF.sub.3SO.sub.3) carbonate) DMC hydrofluoro ether >180 (LiCF.sub.3SO.sub.3) EMC (ethyl methyl 23 none 88 (LiBOB) carbonate) EMC MFE >200 (LiBOB) AN (Acetonitrile) 6 none 65 (LiBF.sub.4) AN 1,3-propane sultone 155 (LiBF.sub.4) (PS) AN Canola oil 160 (LiBF.sub.4) EA (Ethyl acetate) + −3 none 70 (LiBF.sub.4) DOL EA + DOL Triallyl phosphate >180 (LiBF.sub.4) (TAP) DME (1,2- −2 55(LiPF.sub.6) dimethoxyethane) DME liquid silaxane 155(LiPF.sub.6) VC (vinylene 53.1 none 65 (LiPF.sub.6) carbonate) VC Alkyylsilane >200 (LiPF.sub.6) (Si—C) * As per OSHA (Occupational Safety & Health Administration) classification, any liquid with a flash point below 38.7° C. is flammable.

Example 3: Pentaerythritol Tetraacrylate (PETEA)-Based Electrolyte for Use in a Second Electrolyte Composition

[0131] A pentaerythritol tetraacrylate (PETEA)-based polymer electrolyte was prepared by gelation of a precursor solution. The precursor solution comprised 1.5 wt % PETEA (C.sub.17H.sub.20O.sub.8) as a monomer and 0.1 wt. % azobisdiisobutyronitrile (AIBN, C.sub.8H.sub.12N.sub.4) as an initiator dissolved in a liquid electrolyte containing 1M bis(trifluoromethane) sulfonamide lithium (LiTFSI) salt in a mixture of 1,2-dioxolane (DOL)/dimethoxymethane (DME) (1:1 by volume) with 1 wt % LiNO.sub.3 additive. This precursor solution was injected into the unfilled space in a battery cell previously loaded with a first electrolyte composition. The precursor solution in the cell was polymerized at 70° C. for half an hour to form the polymer electrolyte in situ.

[0132] The radical polymerization of PETEA was thermally initiated by azobisisobutyronitrile (AIBN). The polymerization reaction occurs in liquid electrolyte. The primary radicals derived via the thermal decomposition of AIBN attack the C═C double bond of the PETEA monomer to create four free radicals on the monomer since PETEA possesses four C═C double bonds to be initiated, followed by the chain growth reaction by sequentially adding PETEA monomers to the active sites (i.e., four free radical ends) of initiated monomer. Finally, a three-dimensional network-like polymerized PETEA is formed in liquid electrolyte.

Example 4: Polymer Electrolyte Based on Cyanoethyl Poly(Vinyl Alcohol) (PVA-CN)

[0133] Cyanoethyl poly(vinyl alcohol) polymer was prepared by gelation of a precursor solution containing 2 wt. % PVA-CN dissolved in a liquid electrolyte that contained 1M LiPF.sub.6 in a mixture solution of ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) with a volume ratio of 1:1:1. The precursor solution was injected into an unfilled space in a battery cell and then heated at a temperature of 70° C. to obtain black PVA-CN based polymer electrolyte.

Example 5: Poly(Vinyl Carbonate)-Based Polymer Electrolyte

[0134] Liquid vinylene carbonate (VC), in the presence of a lithium salt, can be polymerized into poly(vinyl carbonate) (PVCA) catalyzed by a thermally initialized radical initiator. The lithium salt, lithium difluoro(oxalate) borate (LiDFOB) has the combined chemical structures of lithium bis(oxalate) borate and lithium tetrafluoroborate (LiBF.sub.4). In an experiment, 1.43 g LiDFOB was dissolved in to 10 mL VC to obtain a homogeneous and transparent solution (1.0 m LiDFOB in VC, ≈9.6% (w/w)) and then the solution was added with 10 mg AIBN. The precursor solution was injected into an unfilled space in a battery cell, which was maintained at 60° C. for 24 h and 80° C. for 10 h in a vacuum oven to complete polymerization of VC.

Example 6: Preparation of Graphene-Enabled Li.SUB.x.V.SUB.3.O.SUB.8 .Nano-sheets (as a Cathode Active Material in a Rechargeable Lithium Metal Battery) from V.SUB.2.O.SUB.5 .and LiOH

[0135] All chemicals used in this study were analytical grade and were used as received without further purification. V.sub.2O.sub.5 (99.6%, Alfa Aesar) and LiOH (99+%, Sigma-Aldrich) were used to prepare the precursor solution. Graphene oxide (GO, 1% w/v obtained in Example 2 above) was used as a structure modifier. First, V.sub.2O.sub.5 and LiOH in a stoichiometric V/Li ratio of 1:3 were dissolved in actively stirred de-ionized water at 50° C. until an aqueous solution of Li.sub.xV.sub.3O.sub.8 was formed. Then, GO suspension was added while stirring, and the resulting suspension was atomized and dried in an oven at 160° C. to produce the spherical composite particulates of GO/Li.sub.xV.sub.3O.sub.8 nano-sheets. Corresponding Li.sub.xV.sub.3O.sub.8 materials were obtained under comparable processing conditions, but without graphene oxide sheets.

[0136] An additional set of graphene-enabled Li.sub.xV.sub.3O.sub.8 nano-sheet composite particulates was produced from V.sub.2O.sub.5 and LiOH under comparable conditions, but was dried under different atomization temperatures, pressures, and gas flow rates to achieve four samples of composite particulates with four different Li.sub.xV.sub.3O.sub.8 nano-sheet average thicknesses (4.6 nm, 8.5 nm, 14 nm, and 35 nm). A sample of Li.sub.xV.sub.3O.sub.8 sheets/rods with an average thickness/diameter of 76 nm was also obtained without the presence of graphene oxide sheets (but, with the presence of carbon black particles) under the same processing conditions for the graphene-enhanced particulates with a nano-sheet average thickness of 35 nm. It seems that carbon black is not as good a nucleating agent as graphene for the formation of Li.sub.xV.sub.3O.sub.8 nano-sheet crystals. The specific capacities and other electrochemical properties of these cathode materials in anode-less Li metal cells having a graphene-protected Cu foil as the anode current collector were investigated.

Example 7: Preparation of Electrodes for Li-Ion Cells Featuring a Quasi-Solid Electrolyte

[0137] Several dry electrodes containing graphene-enhanced particulates (e.g. comprising lithium cobalt oxide or lithium iron phosphate primary particles embraced by graphene sheets) were prepared by mixing the particulates with a liquid to form a paste without using a binder such as PVDF. The paste was cast onto a surface of a piece of glass, with the liquid medium removed to obtain a dry electrode. Another dry electrode was prepared by directly mixing LiFePO.sub.4 primary particles with graphene sheets in an identical liquid to form a paste without using a binder. Again, the paste was then cast to form dry electrodes. These electrodes were made into full cells containing Ag-coated Cu foil as an anode current collector.

[0138] The first-cycle discharge capacity data of small anode-less lithium metal cells were obtained. The data show that, as compared to the cells featuring a single conventional electrolyte, the Li cells having two electrolytes (one containing a high lithium salt concentration in an organic liquid solvent, plus a second polymer-based electrolyte) typically exhibit a longer and more stable cycling life, experiencing a significantly lesser extent of capacity decay after a given number of charge/discharge cycles. In addition, the cells having two types of electrolytes as herein disclosed did not suffer from any fire or explosion when a nail penetration test was conducted on them. In contrast, the cells containing conventional lithium salt-organic solvent electrolytes all exhibited thermal runaway problems and caught fires.

[0139] It may be further noted that the cathode active material that can be used in the presently invented cell is not limited to lithium cobalt oxide and lithium iron phosphate. There is no particular limitation on the type of electrode active materials that can be used in a Li cell featuring the presently invented quasi-solid electrolyte and an initially lithium metal-free anode.

Example 8: Evaluation of Electrochemical Performance of Various Cells

[0140] Charge storage capacities were measured periodically and recorded as a function of the number of cycles. The specific discharge capacity herein referred to is the total charge inserted into the cathode during the discharge, per unit mass of the composite cathode (counting the weights of cathode active material, conductive additive or support, binder, and any optional additive combined, but excluding the current collector). The specific charge capacity refers to the amount of charges per unit mass of the composite cathode. The specific energy and specific power values presented in this section are based on the total cell weight. The morphological or micro-structural changes of selected samples after a desired number of repeated charging and recharging cycles were observed using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM).

[0141] As an example, the first-cycle efficiency (Coulomb efficiency) of several cells was evaluated using a baseline electrolyte of EC+DEC and two non-flammable electrolytes (NF-1 contains FPC and NF-2 contains FEC). The data have demonstrated that the selected additive can actually increase the Coulomb efficiency of an electrolyte. This is an unexpected and desirable outcome. FIG. 3 shows the discharge curves and Coulombic efficiencies of two anode-less lithium metal cells each containing a first electrolyte (the lithium salt concentrations being 2.4 M, with a flame retardant additive) and a second electrolyte based on PVA-CN discussed above. The first cell has a graphene-protected Cu foil as the anode current collector and the second cell has a neat Cu foil, without a graphene protective layer, as the current collector. These data have demonstrated the significance of having a graphene-based protective layer on a Cu foil surface in imparting cycling stability to the cell.

[0142] In summary, the present disclosure provides an innovative, versatile, and surprisingly effective platform materials technology that enables the design and manufacture of superior lithium metal and lithium-ion rechargeable batteries. The approach of adding a second electrolyte, a polymer-based one, imparts additional safety features to battery cells.