WO2009049220A1 - Methods of overcharge protection for electrochemical cells - Google Patents

Abstract

The present invention provides methods of protecting and/or preventing the overcharge of an electrochemical cell and methods for recharging a lithium-ion electrochemical cell while chemically limiting cell damage due to overcharging by using lithium imide or lithium methide compounds and derivatives as electrolytes.

Description

METHODS OFOVERCHARGE PROTECTIONFOR ELECTROCHEMICALCELLS

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 60/979,047 filed October 10, 2007, which application is incorporated herein by reference in its entirety and for all purposes.

BACKGROUND OF THE INVENTION

[0002] Safety is a major concern for high energy density batteries such as lithium ion batteries since they are more sensitive to certain types of abuse, particularly overcharge abuse wherein the maximum cell voltage is exceeded during recharge. Overcharge generally occurs when a current is forced through the batteries and the charge delivered exceeds the charge- storing capability of the battery. During overcharge, excessive lithium is extracted (i.e., more de-intercalation than is needed to transfer charge within the normal operating parameters of the battery) from the positive electrode with a corresponding excessive insertion or even plating of lithium at the negative electrode. This can make both electrodes less thermally stable. Overcharge results in heating of the battery since much of the input energy is dissipated rather than stored. The decrease in thermal stability combined with battery heating can lead to thermal runaway and explosion or fire on overcharge, especially because the organic solvents used in the electrolyte are flammable. Overcharge of lithium-ion batteries leads to the chemical and electrochemical side-reactions of battery components, rapid temperature elevation, and can trigger self-accelerating reactions in the batteries and even explosion.

[0003] A redox shuttle is a chemical compound that is incorporated as an overcharge protection mechanism for lithium-ion batteries. Generally, the redox shuttle is electrochemically oxidized at the positive electrode at a potential higher than the maximum working potential of the positive electrode; the oxidized compound migrates to the negative electrode and is electrochemically reduced. The reversible oxidation and reduction of the redox shuttle acts as a "chemical bypass" for the excess charge. If the redox shuttle is effective it will carry the excessive charge through the battery without causing damage Under such a mechanism, the dangerous thermal runaway of the battery may be averted when the battery is subjected to overcharge-abuse. An effective redox shuttle overcharge protection requires that the redox shuttle molecules are mobile in the electrolyte and present in large concentrations. However, typical redox shuttle molecules either do not dissolve in electrolyte at high concentrations and/or it compromises cell performance. Current method of using redox shuttle additives provides only limited concentration and mobility. The use of redox shuttle additives in lithium-ion cells has additional drawbacks, which include the interference of the additives with the formation of solid electrolyte layer (SEI) resulting in reduced battery capacity, cycle life and performance. Furthermore, large amount of additives can lead to poor battery performance.

[0004] Therefore, there is a need to develop other electrolyte based overcharge protection systems that are capable of providing a large concentration of highly mobile reversible redox shuttling molecules that (1) do not compromise cell capacity and performance; and (2) have oxidation potential lower than the decomposition potential of the electrolyte solvent, but higher than the charging cut-off voltage. The present invention satisfies these and other needs.

BRIEF SUMMARY OF THE INVENTION

[0005] The present invention relates to methods for protecting and/or preventing the overcharge of an electrochemical cell. In particular, the present invention provides electrochemical cells having lithium imide or lithium methide salts and derivatives as electrolytes to supply the overcharge protection, wherein the salts and derivatives are substantially free of any redox shuttle additives. Compared to the use of redox shuttle additives, the present invention offers the advantage of a large concentration of highly mobile reversible redox shuttling molecules that (1) do not compromise cell performance; (2) maintain capacity and power capacity of the cell; (3) extend the cell's life; and (4) have oxidation potential lower than the decomposition potential of the electrolyte solvent, but higher than the charging cut-off voltage. Surprisingly, the electrolyte of the present invention provides overcharge protection at a predetermined upper charging voltage, for example, 4.2 - 4.5 V and 4.4 -5.2 V.

[0006] In one aspect, the present invention provides a method for preventing lithium-ion electrochemical cell damage due to overcharge. The method includes providing a negative electrode, a positive electrode having a recharged potential and an electrolyte solution comprising a medium and a lithium compound of formula I:

3_λ

R'-X-(L^I )RXR^J). m

I and combining the negative electrode, positive electrode and electrolyte solution into an electrochemical cell to prevent damage due to overcharge in a lithium-ion cell, wherein the electrolyte solution is substantially free of any redox shuttle additives. The lithium compound i) has an oxidation peak potential above the charging cut-off potential of the positive electrode, ii) undergoes reversible redox reactions at the negative and the positive electrodes and iii) acts as a redox shuttle. The subscript m is 0 or 1. Symbol X is N when m is 0 and X is C when m is 1. Substituents R¹, R² and R³ are each independently an electron- withdrawing group selected from the group consisting of -CN, -SO₂R^a, -SO₂-LZ-SO₂N^' Li⁺SO₂R³, -P(O)(OR^a)₂, -P(O)(R^a)₂, -CO₂R³, -C(O)R³ and -H; with the proviso that R¹ and R² are other than hydrogen when m = 0, and no more than one of R¹, R² and R³ is hydrogen when m = 1. Each R^a is independently selected from the group consisting of Ci_-8 alkyl, Ci- shaloalkyl, Ci_-8 perfluoroalkyl, aryl, optionally substituted barbituric acid and optionally substituted thiobarbituric acid, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl are optionally substituted with a member selected from -O- or -S- to form an ether or a thioether linkage and the aryl is optionally substituted with from 1 -5 members selected from the group consisting of halogen, Ci_-4haloalkyl, Ci_-4perfluoroalkyl, -CN, - SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and -C(O)R^b, wherein R^b is C_1-8 alkyl or Ci_-8 perfluoroalkyl, and L^a is C_1-4perfluoroalkylene.

[0007] In another aspect, the present invention provides a method of designing a lithium-ion electrochemical cell for preventing damage due to overcharge. The method includes selecting an oxidation potential; and selecting a compound based on the oxidation potential, wherein the compound has formula (I).

[0008] In yet another aspect, the present invention provides a method for recharging a lithium-ion electrochemical cell while chemically limiting cell damage due to overcharging. The method includes supplying charging current across a positive electrode and a negative electrode of a lithium-ion rechargeable electrochemical cell containing a charge-carrying electrolyte solution comprising a medium and a lithium compound of formula I. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 illustrates the charge and discharge with 300% overcharge at 1C rate. The upper curve shows charging including overcharging with time. The lower curve shows discharging with time.

[0010] FIG. 2 illustrates the charge and discharge with 300% overcharge at 3C rate. The upper curve shows charging including overcharging with time. The lower curve shows discharge with time.

[0011] FIG. 3 illustrates that the electrochemical cell retains about 88% capacity after 10 cycles of lC/300% overcharge. The thin line shows normal full charge and the thick line shows full discharge.

[0012] FIG. 4 illustrates the electrochemical cell retains 40% capacity after 10 cycles of 3C/300% overcharge. The thick line shows normal full charge and the thin line shows full discharge.

[0013] FIG. 5 illustrates a comparison of overcharge tolerance of LiTFSI (lower curve) and LiPF₆ (upper curve) salts within a 400-second period. The lower curve shows that LiTFSI is able to hold a voltage at about 4.8V.

[0014] FIG. 6 illustrates a comparison of overcharge tolerance of LiTFSI (the curve at about 5V) and LiPF₆ (the curve having a peak at about 14V) salts within 4 hours time frame. LiTFSI is able to hold a voltage at about 4.8V. LiPF₆ shows rapid voltage rise.

[0015] FIG. 7 illustrates the overcharge tolerance of LiTFSI salt over an extended period after overcharge at 1 C rate.

[0016] FIG. 8 illustrates a cyclic voltammetry of LiTFSI undergoing reversible oxidation with a peak at 4.8V.

[0017] FIG. 9 illustrates a cyclic voltammetry of LiTFSI salt. LiPF₆ has a high oxidation potential. The oxidation is not peaked even at 5.5V.

[0018] FIG. 10 illustrates the change of the cell voltage and body temperature during overcharge. DETAILED DESCRIPTION OF THE INVENTION

[0019] The term "alkyl", by itself or as part of another substituent, means, unless otherwise stated, a straight or branched chain hydrocarbon radical, having the number of carbon atoms designated (i.e. Cr₈ means one to eight carbons). Examples of alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl, isobutyl, sec-butyl, n-pentyl, n-hexyl, n-heptyl, n- octyl, and the like.

[0020] The term "alkylene" by itself or as part of another substituent means a linear or branched saturated divalent hydrocarbon radical derived from an alkane having the number of carbon atoms indicated in the prefix. For example, Ci-₆alkylene is meant to include methylene, ethylene, propylene, 2-methylpropylene, pentylene, and the like.

Perfluoroalkylene means to an alkylene where all the hydrogen atoms are substituted by fluorine atoms. Examples of perfluoroalkylene include -CF₂-, -CF₂CF₂-, -CF₂-CF₂CF₂-, - C(CF₃)₂-, -CF₂CF₂CF₂CF₂-, -CF₂CF₂CF₂CF₂CF₂- and the like. Fluoroalkylene includes an alkylene where hydrogen atoms are partially substituted by fluorine atoms. Exemplary fluorealkylenes include -CHCF-, -CF(Cl)-, -CH₂CF₂-, -CF₂-CHFCF₂-, -C(CF₃)(CH₃)-, - CF₂C(F)(Cl)CF₂CF₂-, -CH₂CH₂CH₂CF₂CF₂- and the like.

[0021] The terms "halo" or "halogen," by themselves or as part of another substituent, mean, unless otherwise stated, a fluorine, chlorine, bromine, or iodine atom.

[0022] The term "haloalkyl," are meant to include monohaloalkyl and polyhaloalkyl. For example, the term "Cr₄ haloalkyl" is mean to include trifluoromethyl, 2,2,2-trifluoroethyl, 4- chlorobutyl, 3-bromopropyl, 3-chloro-4-fluorobutyl and the like. The term "fluoroalkyl" is meant to include both perfluoroalkyl and partially fluorinated alkyls. Exemplary fluoroalkyls include -CH₂CF₃, -CHF₂, -CF₃, -CF₂CH₂CF₃ and the like.

[0023] The term "perfluoroalkyl" means an alkyl where all the hydrogen atoms in the alkyl are substituted by fluorine atoms. Examples of perfluoroalkyl include -CF₃, -CF₂CF₃, -CF₂- CF₂CF₃, -CF(CF₃)₂, -CF₂CF₂CF₂CF₃, -CF₂CF₂CF₂CF₂CF₃ and the like.

[0024] The term "aryl" means a monovalent monocyclic, bicyclic or polycyclic aromatic hydrocarbon radical of 6 to 10 ring atoms which is unsubstituted or substituted independently with one to four substituents, preferably one, two, or three substituents selected from alkyl, cycloalkyl, cycloalkyl-alkyl, halo, cyano, hydroxy, alkoxy, amino, acylamino, mono- alkylamino, di-alkylamino, haloalkyl, haloalkoxy, heteroalkyl, COR (where R is hydrogen, alkyl, cycloalkyl, cycloalkyl-alkyl, phenyl or phenylalkyl, aryl or arylalkyl), -(CR'R")_n-

COOR (where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R is hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl aryl or arylalkyl) or -(CR'R")_n-CONR'"R""(where n is an integer from 0 to 5, R' and R" are independently hydrogen or alkyl, and R'" and R"" are, independently of each other, hydrogen, alkyl, cycloalkyl, cycloalkylalkyl, phenyl or phenylalkyl, aryl or arylalkyl). More specifically the term aryl includes, but is not limited to, phenyl, biphenyl, 1-naphthyl, and 2-naphthyl, and the substituted forms thereof.

[0025] Substituents for the aryl groups are varied and are generally groups selected from: - halogen, -OR', -OC(O)R', -NR'R", -SR', -R', -CN, -NO₂, -CO₂R', -CONR'R", -C(O)R', - OC(O)NR⁵R", -NR"C(0)R', -NR"C(0)₂R', -NR'-C(0)NR"R"', -NH-C(NH₂)=NH, - NR'C(NH₂)=NH, -NH-C(NH₂)=NR', -S(O)R', -S(O)₂R', -S(O)₂NR⁵R", -NR⁵S(O)₂R", -N₃, perfluoro(Ci-C₄)alkoxy, and perfluoro(Ci-C₄)alkyl, in a number ranging from zero to the total number of open valences on the aromatic ring system; and where R⁵, R" and R"⁵ are independently selected from hydrogen, C_1-8 alkyl, unsubstituted aryl, (unsubstituted aryl)-d-₄ alkyl, and unsubstituted aryloxy-Ci-₄ alkyl. Preferred substituents for aryls are electronic withdrawing groups selected from halogen, -OR', -R⁵, -CN, -NO₂, -CO₂R⁵, -CONR⁵R", -C(O)R', -S(O)R', -S(O)₂R', -S(O)₂NR⁵R", -NR⁵S(O)₂R", perfluoro(C,-C₄)alkoxy, or perfluoro(C]-C₄)alkyl, in a number ranging from zero to the total number of open valences on the aromatic ring system.

[0026] The term "positive electrode" refers to one of a pair of rechargeable lithium-ion cell electrodes that under normal circumstances and when the cell is fully charged will have the highest potential. This terminology is retained to refer to the same physical electrode under all cell operating conditions even if such electrode temporarily (e.g., due to cell overdischarge) is driven to or exhibits a potential below that of the other (the negative) electrode.

[0027] The term "negative electrode" refers to one of a pair of rechargeable lithium-ion cell electrodes that under normal circumstances and when the cell is fully charged will have the lowest potential. This terminology is retained to refer to the same physical electrode under all cell operating conditions even if such electrode is temporarily (e.g., due to cell overdischarge) driven to or exhibits a potential above that of the other (the positive) electrode. [0028] The term "redox chemical shuttle" refers to an electrochemically reversible moiety that during charging of a lithium-ion cell can become oxidized at the positive electrode, migrate to the negative electrode, become reduced at the negative electrode to reform the unoxidized (or less-oxidized) shuttle species, and migrate back to the positive electrode.

[0029] The term "recharged potential" refers to a potential value measured relative to Li/Li⁺ by constructing a cell containing the positive electrode, a lithium metal negative electrode and an electrolyte but no redox chemical shuttle, carrying out a charge/discharge cycling test and observing the potential at which the positive electrode becomes delithiated during the first charge cycle to a lithium level corresponding to at least 90% of the available recharged cell capacity. For some positive electrodes (e.g., LiFePO₄), this lithium level may correspond to approximately complete delithiation (e.g., to Li₀FePO₄). For other positive electrodes (e.g., some electrodes having a layered lithium-containing structure), this lithium level may correspond to partial delithiation.

[0030] The term "oxidation potential" refers to a potential value, which may be measured in the chosen electrolyte, by measuring current flow vs. voltage using cyclic voltammetry and a platinum or glassy carbon working electrode, a copper counter electrode and a non-aqueous Ag/AgCl reference electrode that has been previously referenced to Li/Li⁺ and determining the potentials V_up (viz., during a scan to more positive potentials) and V_down (viz., during a scan to more negative potentials), relative to Li/Li⁺, at which peak current flow is observed. The potential will be the average of V_up and Vdown-

[0031] The term "overcharge protection potential" refers to the reversible oxidation potential of the electrolyte, such as a lithium compound of formula (I).

[0032] In one aspect, the present invention provides a method for preventing lithium-ion electrochemical cell damage due to overcharge. The method includes providing a negative electrode, a positive electrode having a recharged potential and an electrolyte solution comprising a medium and a lithium compound of formula I:

R'-X-(Li⁺)R²(R³)_m,

(I) and combining the negative electrode, positive electrode and electrolyte solution into an electrochemical cell to prevent damage due to overcharge in a lithium-ion cell, wherein the electrolyte solution is substantially free of any redox shuttle additives. In one embodiment, the electrolyte solution is free of any redox shuttle additives. The lithium compound has an oxidation peak potential above the recharged potential of the positive electrode, undergoes reversible redox reactions at the negative and the positive electrodes, and acts as a redox shuttle. As such, overcharge of the lithium-ion electrochemical cell is counteracted. In some embodiments, the lithium compound has an oxidation potential above the maximum permitted recharged potential of the positive electrode.

[0033] In another aspect, the present invention provides a method for manufacturing a rechargeable lithium-ion electrochemical cell. The method includes assembling a positive electrode having a recharged potential, a negative electrode and a charge-carrying electrolyte solution comprising a medium and a lithium compound of formula I to produce a rechargeable lithium-ion electrochemical cell, wherein the electrolyte solution is substantially free of any redox shuttle additives. In one embodiment, the electrolyte solution is free of any redox shuttle additives. The lithium compound has an electrochemical oxidation onset potential above the recharged potential of the positive electrode, undergoes reversible redox reactions at the negative and the positive electrodes and acts as a redox shuttle. As such, overcharge of the lithium-ion electrochemical cell is counteracted. In some embodiments, the lithium compound has an oxidation potential above the maximum permitted recharged potential of the positive electrode.

[0034] In still another aspect, the present invention provides a method of designing a lithium-ion electrochemical cell for preventing damage due to overcharge. The method includes selecting an over charge protection potential and selecting a compound based on the oxidation potential, wherein the compound has formula (I). In one embodiment, the overcharge protection potential is between about 4.2 V and 4.5 V. In another embodiment, the overcharge protection potential is between about 4.4 V and 5.2 V.

[0035] A simple test to determine whether a salt has reversible overcharge protection (oxidation/reduction) characteristics and assist in the design of the lithium cell involves running cyclic voltammetric scans on a lithium compound in a standard electrolyte solvent (for example, ethylene carbonate (EC)/dimethyl carbonate (DMC) in a certain ratio, such as 3:7 weight ratio) using a Pt working electrode vs. a lithium reference electrode. Other reference electrodes can also be used. A particularly useful lithium compound will exhibit an oxidation current at a suitable overcharge protection potential, e.g., from 0.1 to 2 volts, or from 0.1 to 1 volt or preferably from 0.1 to 0.5 volts above the design voltage of the cell and typically below 5 volts vs. Li. When the oxidation scan is followed by a reduction scan, the lithium compound shows a nearly equivalent reduction current at the original oxidation potential.

[0036] In yet another aspect, the present invention provides a method for recharging a lithium-ion electrochemical cell while chemically limiting cell damage due to overcharging. The method includes supplying charging current across a positive electrode and a negative electrode of a lithium-ion rechargeable electrochemical cell containing a charge-carrying electrolyte solution comprising a medium and a lithium compound of formula I to recharge said lithium-ion electrochemical cell, wherein the electrolyte solution is substantially free of any redox shuttle additives. In one embodiment, the electrolyte solution is free of any redox shuttle additives. The lithium compound has an oxidation peak potential above the recharged potential of the positive electrode, undergoes reversible redox reactions at the negative and the positive electrodes and acts as a redox shuttle.

[0037] In compounds of formula I, the subscript m is 0 or 1 , with the proviso that R¹ and R² are other than hydrogen when m = 0, and no more than one of R¹, R² and R³ is hydrogen when m = 1.

[0038] In compounds of formula I, R¹, R² and R³ are each independently an electron- withdrawing group selected from the group consisting of -CN, -SO₂R^a, -SO₂-L^a-SO₂N^" Li⁺SO₂R³, -P(O)(OR^a)₂, -P(O)(R^a)₂, -CO₂R³, -C(O)R³ and -H. Each R³ is independently selected from the group consisting of C_1-8 alkyl, Ci_-8haloalkyl, Ci_-8 perfluoroalkyl, aryl, optionally substituted barbituric acid and optionally substituted thiobarbituric acid, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl are optionally substituted with a member selected from -O- or -S- to form an ether or a thioether linkage and the aryl is optionally substituted with from 1-5 members selected from the group consisting of halogen, Ci_-4haloalkyl, C₁ ^perfluoroalkyl, -CN, -SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and - C(O)R^b, wherein R^b is Ci_-8 alkyl or Cj_-8 perfluoroalkyl, and L^a is C_Mperfluoroalkylene. The substituents for barbituric acid and thiobarbituric acid include alkyl, halogen,

C,_-4perfluoroalkyl, -CN, -SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and -C(O)R^b. In some embodiments, L³ is -CF₂- or -CF₂-CF₂-. In one embodiment, R¹ is -SO₂R^a In some instances, R¹ is -SO₂(C, _-8perfluoroalkyl). For example, R¹ is -SO₂CF₃, -SO₂CF₂CF₃, - SO₂(perfluoropgenyl) and the like. In some other instances, when m is 0, R¹ is -SO₂(Ci- sperfluoroalkyl) and R² is -SO₂(Ci_-8perfluoroalkyl) or -SO₂(-L^a-SO₂Li⁺)SO₂-R^a, wherein L^a is C_Mperfluoroalkylene and R^a is Ci_-8perfluoroalkyl, wherein one to four carbon-carbon bonds are optionally replaced with -O- to form an ether linkage. For example, each R^a is independently selected from the group consisting of -CF₃, -OCF₃, -CF₂CF₃, -CF₂-SCF₃, -CF₂- OCF₃, -CF₂CF₂-OCF₃, -CF₂-O-CF₂-OCF₂CF₂-O-CF₃, C^fluoroalkyl, perfluorophenyl, 2,3,4- trifluorophenyl, trifluorophenyl, 2,3,5-trifluorophenyl, 2,3,6-trifluorophenyl, 3,4,5- trifluorophenyl, 3,5,6-trifluorophenyl, 4,5,6-trifluorophenyl, trifluoromethoxyphenyl and bis- trifluoromethylphenyl, 2,3-bis-trifluoromethylphenyl, 2,4-bis-trifluoromethylphenyl, 2,5- bis- trifluoromethylphenyl, 2,6-bis-trifluoromethylphenyl, 3,4-bis-trifluoromethylphenyl, 3,5-bis- trifluoromethylphenyl, 3,6-bis-trifluoromethylphenyl, 4,5-bis-trifluoromethylphenyl and 4,6- bis-trifluoromethylphenyl .

[0039] In another embodiment of compounds having formula I, R¹ is -SO₂(C i-sfluoroalkyl). Ci_-8fluoroalkyl includes alkyls having up to 17 fluorine atoms and is also meant to include various partially fluorinated alkyls, such as -CH₂CF₃, -CH₂-OCF₃, -CF₂CH₃, -CHFCHF₂, - CHFCF₃, -CF₂CH₂CF₃ and the like.

[0040] In compounds of formula I, L^a is Ci_-4perfluoroalkylene, such as -CF₂-, -CF₂CF₂-, - CF₂CF₂CF₂-, -CF₂CF₂CF₂CF₂-, -CF₂CF(CF₃)-CF₂- and isomers thereof.

[0041] The symbol X is N when m is 0. X is C when m is 1.

[0042] In certain embodiments, the compounds of formula I is selected from the group consisting of: CF₃SO₂N^"(Li⁺)SO₂CF₃, CF₃CF₂SO₂NXLi⁺)SO₂CF₃, CF₃CF₂SO₂N- (Li⁺)SO₂CF₂CF₃₃ CF₃SO₂NXLi⁺)SO₂CF₂OCF₃₅ CF₃OCF₂SO₂N-(Li⁺)SO₂CF₂OCF₃, C₆F₅SO₂N-(Li⁺)SO₂CF₃, C₆F₅SO₂N-(Li⁺)SO₂C₆F₅, CF₃SO₂NXLi⁺)SO₂PhCF₃, CF₃SO₂C (Li⁺)(SO₂CF₃),, CF₃CF₂SO₂CXLi⁺)(SO₂CF₃),, CF₃CF₂SO₂CXLi⁺)(SO₂CF₂CF₃)₂, (CF₃SOz)₂CXLi⁺)SO₂CF₂OCF₃, CF₃SO₂CχLi⁺)(SO₂CF₂OCF₃)₂, CF₃OCF₂SO₂C- (Li⁺)(SO₂CF₂OCF₃)₂, C₆F₅SO₂CXLi⁺)(SO₂CF₃)₂, (C₆F₅SO₂)₂CXLi⁺)SO₂CF₃, C₆F₅SO₂C^" (Li⁺)(SO₂C₆Fs)₂, (CF₃SO₂)₂CχLi⁺)SO₂PhCF₃ and CF₃SO₂CXLi⁺)(SO₂PhCF₃)₂. In some embodiments, the compounds are preferably CF₃SO₂NXLi⁺)SO₂CF₃, CF₃SO₂C^" (Li⁺)(SO₂CF₃)₂ or C₆F₅SO₂N-(Li⁺)SO₂C₆F₅.

[0043] In one embodiment, compounds of formula I have a subformula (Ia):

(C_1-8fluoroalkyl)SO₂-X-(Li⁺)R²(R³)_m,

Ia where the subscript m is 0 or 1 and the substituents R² and R³ are as defined above in compounds of formula (I). In some embodiments, R² and R³ are each independently -SO₂R^a or -SO₂-L^a-SO₂N^"Li⁺SO₂R^a. In certain instances, R^a is C_1-4perfluoroalkyl.

[0044] In another embodiment, compounds of formula I have a subformula (Ia-I):

(C,.₈fluoroalkyl)SO₂-C^"(Li⁺)R²R³

Ia-I

where the substituents R and R are as defined above in compounds of formula (I). In some embodiments, R² and R³ are each independently -SO₂R³ or -SO₂-L^a-SO₂N^"Li⁺SO₂R^a. In certain instances, R^a is Ci_-4perfluoroalkyl.

[0045] In yet another embodiment, compounds of formula I have a subformula (Ia-2):

(Ci_-8fluoroalkyl)SO₂-N^"(Li⁺)R²

Ia-2

where the substituents are as defined above in compounds of formula (I). In some embodiments, R² is -SO₂R³ or -SO₂-L^a-SO₂N^'Li⁺SO₂R^a. In certain instances, R^a is Ci- ₄perfluoroalkyl.

[0046] In a group of embodiments, compounds of formula I have a subformula (Ib):

R¹NXLi⁺)R²

Ib R¹ and R² are each independently selected from -SO₂R³ or -SO₂-IZ-SO₂NXi⁺SO₂R³, wherein each R^a is independently Ci-sperfluoroalkyl, perfluoroaryl or aryl, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl substituent in R^a is optionally substituted with from 1-5 members selected from the group consisting of halogen, Ci^haloalkyl, Ci_-4perfluoroalkyl, -CN, - SO₂R^b, -CO₂R^b and -C(O)R^b, wherein R^b is Cj.g alkyl, perfluoroaryl or Ci_-8 perfluoroalkyl; and L³ is Ci_-4perfluoroalkylene. L^a is C_Mperfluoroalkylene. In some embodiments, R^a is Q- ₄perfluoroalkyl or perfluoroaryl, wherein one to two carbon-carbon bonds of the perfluoroalkyl are optionally replaced with -O- to form an ether linkage.

[0047] In another group of embodiments, compounds of formula I have a subformula (Ic):

R¹CXLi⁺)R²R³ Ic

R¹, R² and R³ are each independently selected from -SO₂R³ or -SO₂-L^a-SO₂N^"Li⁺SO₂R^a, wherein each R^a is independently Ci-sperfluoroalkyl, perfiuoroaryl or aryl, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl substituent in R^a is optionally substituted with from 1-5 members selected from the group consisting of halogen, C_1-4haloalkyl, Ci ^perfluoroalkyl, -CN, - SO₂R^b, -CO₂R^b and -C(0)R^b, wherein R^b is C_1-8 alkyl, perfiuoroaryl or C_1-8 perfluoroalkyl; and L^a is Ci_-4perfluoroalkylene. L^a is Ci ^perfluoroalkyl ene. In some embodiments, R^a is Cj- ₄perfluoroalkyl or perfiuoroaryl, wherein one to two carbon-carbon bonds of the perfluoroalkyl are optionally replaced with -O- to form an ether linkage.

[0048] In some embodiments, the electrolyte for overcharge protection can be one or more lithium compounds selected from formulas I, Ia, Ia-I, Ia-2, Ib or Ic. The desired oxidation potential for overcharge protection can be achieved by using a mixture of the compounds of formulas I, Ia, Ia-I, Ia-2, Ib or Ic.

[0049] In some embodiments, the compounds of formula I have an oxidation potential from about 100 mV to about 500 mV above the recharged potential or charging potential of the positive electrode. In certain instances, compounds have an oxidation potential from about 100 mV to about 400 mV, 100 mV to about 300 mV or 100 mV to about 200 mV above the recharged potential of the positive electrode.

[0050] In some other embodiments, the compounds of formula I can undergo reversible reduction and oxidation reactions. For example, the compounds can have a reversible peak oxidation potential at about 4.8 V. In certain instances, the compounds can have a reversible peak oxidation potential at about 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.9, 5.0, 5.1 or 5.2 V. In other instances, the compounds have an oxidation potential between about 3.5 V and 4.8 V. In yet other instances, the compounds have an oxidation potential between about 4.0 V and 4.6 V, preferably between about 4.2 V and 4.5 V. In still other instances, the compounds have an oxidation potential between about 4.4 V and 5.2 V.

[0051] In certain embodiments, the compounds of formula I is able to hold a voltage at about 4.8 V during overcharge, thereby providing an overcharge protection to the electrochemical cell. In certain instances, the compounds can hold a voltage at about 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.9, 5.0, 5.1, 5.2, 5.3, 5.5, 5.6, 5.7 or 5.8 V during overcharge of an electrochemical cell. In other instances, the compounds can hold a voltage an between about 3.5 V and 4.8 V. In yet other instances, the compounds can hold a voltage between about 4.0 V and 4.6 V, more preferably between about 4.2 V and 4.5 V. In still other instances, the compounds can hold a voltage between about 4.4 V and 5.2 V.

[0052] In one embodiment, the present invention provides a positive electrode, which includes electrode active materials and a current collector. The positive electrode has an upper charging voltage of 3.5-4.5 volts versus a Li/Li⁺ reference electrode. The upper charging voltage is the maximum voltage to which the positive electrode may be charged at a low rate of charge and with significant reversible storage capacity. In some embodiments, cells utilizing positive electrode with upper charging voltages from 3-5.8 volts, 4.4-5.2 volts or 4.2-4.5 volts versus a Li/Li⁺ reference electrode are also suitable. A variety of positive electrode active materials can be used. Non-limiting exemplary electrode active materials include transition metal oxides, phosphates and sulfates, and lithiated transition metal oxides, phosphates and sulfates. In some embodiments, the electrode active materials are oxides such as LiCoO₂, spinel LiMn₂O₄, chromium-doped spinel lithium manganese oxides

Li_xCr_yMn₂0₄, layered LiMnO₂, LiNiO₂, LiNi_xCθi_-xθ2 where x is 0<x<l, with a preferred range of 0.5<x<0.95, and vanadium oxides such as LiV₂O₅, LiV₆ On, or the foregoing compounds modified in that the compositions thereof are nonstoichiometric, disordered, amorphous, overlithiated, or underlithiated forms such as are known in the art. The suitable positive electrode-active compounds may be further modified by doping with less than 5% of divalent or trivalent metallic cations such as Fe²⁺, Ti²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Mg²⁺, Cr³⁺, Fe³⁺, Al³⁺, Ni³⁺, Co³⁺, or Mn³⁺, and the like. In some other embodiments, positive electrode active materials suitable for the positive electrode composition include lithium insertion compounds with olivine structure such as LiFePO₄ and with NASICON structures such as LiFeTi(SO₄)₃, or those disclosed by J. B. Goodenough in "Lithium Ion Batteries" (Wiley- VCH press, Edited by M. Wasihara and O. Yamamoto). In yet some other embodiments, electrode active materials include LiFePO₄, LiMnPO₄, LiVPO₄, LiFeTi(SO₄)₃, LiNi_xMn_1-x0₂, LiNi_xCθ_yMni_-x-yO₂ and derivatives thereof, wherein x is 0<x<l and y is 0<y<l. In certain instances, x is between about 0.25 and 0.9. In other instances, x is between about 0.3 and 0.8. In yet other instances, x is 1/3. In one instance, x is 1/3 and y is 1/3. Particle size of the positive electrode active material should range from about 1 to 100 microns. In some preferred embodiments, transition metal oxides such as LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_xMni_-x0₂, LiNi_xCo_yMni_-x-yO₂ and their derivatives, where x is 0<x<l and y is 0<y<l. LiNi_xMn i_-x0₂ can be prepared by heating a stoichiometric mixture of electrolytic MnO₂, LiOH and nickel oxide to about 300 to 400 ⁰C. LiNi_xCo_yMni_-x-yO₂ can be prepared by heating a stoichiometric mixture of electrolytic MnO₂, LiOH, nickel oxide and cobalt oxide to about 300 to 500 ⁰C. The positive electrode may contain conductive additives. In one embodiment, the subscripts x and y are each independently selected from 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9 or 0.95. x and y can be any numbers between 0 and 1 to satisfy the charge balance of the compounds LiNi_xMn) _-XO₂ and LiNi_xCOyMn i _-x-y O₂.

[0053] Representative positive electrodes and their approximate recharged potentials include FeS₂ (3.0 V vs. Li/Li⁺), LiCoPO₄ (4.8 V vs. Li/Li⁺), LiFePO₄ (3.45 V vs. Li/Li⁺), Li₂FeS₂ (3.0 V vs. Li/Li⁺), Li₂FeSiO₄ (2.9 V vs. Li/Li⁺), LiMn₂O₄ (4.1 V vs. Li/Li⁺), LiMnPO₄ (4.1 V vs. Li/Li⁺), LiNiPO₄ (5.1 V vs. Li/Li⁺), LiV₃O₈ (3.7 V vs. Li/Li⁺), LiV₆Oj₃ (3.0 V vs. Li/Li⁺), LiVOPO₄ (4.15 V vs. Li/Li⁺), LiVOPO₄F (4.3 V vs. Li/Li⁺), Li₃ V₂(PO₄)₃ (4.1 V (2 Li) or 4.6 V (3 Li) vs. Li/Li⁺), MnO₂ (3.4 V vs. Li/Li⁺), MoS₃ (2.5 V vs. Li/Li⁺), sulfur (2.4 V vs. Li/Li⁺), TiS₂(2.5 V vs. Li/Li⁺), TiS₃ (2.5 V vs. Li/Li⁺), V₂O₅ (3.6 V vs. Li/Li⁺), V₆Oi₃ (3.0 V vs. Li/Li⁺), and combinations thereof.

[0054] A positive electrode can be formed by mixing and forming a composition comprising, by weight, 2-15%, preferably 4-8%, of a polymer binder, 10-50%, preferably 15- 25%, of the electrolyte solution of the invention herein described, 40-85%, preferably 65- 75%, of an electrode-active material, and 1-12%, preferably 4-8%, of a conductive additive. Optionally, up to 12% of inert filler may also be added, as may such other adjuvants as may be desired by one of skill in the art, which do not substantively affect the achievement of the desirable results of the present invention. In one embodiment, no inert filler is used.

[0055] In one embodiment, the present invention provides a negative electrode, which includes electrode active materials and a current collector. The negative electrode comprises either a metal selected from the group consisting of Li, Si, Sn, Sb, Al and a combination thereof, or a mixture of one or more negative electrode active materials in particulate form, a binder, preferably a polymeric binder, optionally an electron conductive additive, and at least one organic carbonate. Examples of useful negative electrode active materials include, but are not limited to, lithium metal, carbon (graphites, coke-type, mesocarbons, polyacenes, carbon nanotubes, carbon fibers, and the like). Negative electrode-active materials also include lithium-intercalated carbon, lithium metal nitrides such as Li_{2 6}Co₀₄N, metallic lithium alloys such as LiAl or Li₄Sn, lithium-alloy-forming compounds of tin, silicon, antimony, or aluminum such as those disclosed in "Active/Inactive Nanocomposites as Anodes for Li-Ion Batteries " by Mao et al. in Electrochemical and Solid State Letters, 2 (1), p. 3, 1999. Further included as negative electrode-active materials are metal oxides such as titanium oxides, iron oxides, or tin oxides. When present in particulate form, the particle size of the negative electrode active material should range from about 0.01 to 100 microns, preferably from 1 to 100 microns. Some preferred negative electrode active materials include graphites such as carbon microbeads, natural graphites, carbon nanotubes, carbon fibers, or graphitic flake-type materials. Some other preferred negative electrode active materials are graphite microbeads such as those produced by Osaka Gas in Japan (MCMB 25-28, 10-28, or 6-28) .

[0056] A negative electrode can be formed by mixing and forming a composition comprising, by weight, 2-20%, preferably 3-10%, of a polymer binder, 10-50%, preferably 14-28%, of the electrolyte solution of the invention herein described, 40-80%, preferably 60- 70%, of electrode-active material, and 0-5%, preferably 1 -4%, of a conductive additive. Optionally up to 12% of an inert filler as hereinabove described may also be added, as may such other adjuvants as may be desired by one of skill in the art, which do not substantively affect the achievement of the desirable results of the present invention. It is preferred that no inert filler be used.

[0057] Suitable conductive additives for the positive and negative electrode composition include carbons such as coke, carbon black, carbon nanotubes, carbon fibers, and natural graphite, metallic flake or particles of copper, stainless steel, nickel or other relatively inert metals, conductive metal oxides such as titanium oxides or ruthenium oxides, or electronically-conductive polymers such as polyacetylene, polyphenylene and polyphenylenevinylene, polyaniline or polypyrrole. Preferred additives include carbon fibers, carbon nanotubes and carbon blacks with relatively surface area below ca. 100 m /g such as Super P and Super S carbon blacks available from MMM Carbon, Brussels, Belgium.

[0058] The current collector suitable for the positive and negative electrodes comprises a carbon sheet selected from a graphite sheet, carbon fiber sheet and carbon nanotubes sheet. Highest conductivity is generally achieved in pure graphite, so it is preferred that the graphite sheeting contain as few binders, additives and impurities as possible in order to realize the benefits of the present invention. Carbon nanotubes can be present from 0.01% to about 99%. Carbon fiber can be in microns or submicrons. Carbon black or carbon nanotubes may be added to enhance the conductivities of the certain carbon fibers.

[0059] The carbon sheet current collector suitable for the present invention may be in the form of a powder coating on a substrate such as a metal substrate, a flexible free-standing sheet, or a laminate. That is the current collector may be a composite structure having other members such as metal foils, adhesive layers and such other materials as may be considered desirable for a given application. However, in any event, according to the present invention, it is the graphite layer, or graphite layer in combination with an adhesion promoter, which is directly interfaced with the electrolyte of the present invention and is in electronically conductive contact with the electrode surface.

[0060] One particularly preferred form of graphite is the flexible low-density graphite sheeting described in J. H. Shane et al., U. S. Pat. No. 3,404,061 which is herein incorporated by reference to the entirety, which offers the chemical, thermal, tensile, and electrical properties normally associated with graphite in combination with a highly desirable enhancement of the mechanical properties of flexibility, compactability, conformability, flexural toughness, and resilience. The flexible graphite sheeting preferred for the practice of the present invention exhibits a bulk density in the range of 0.08-2.25 g/cm³, encompassing that of natural graphite, however the density is preferably 0.8-1.4 g/cm³.

[0061] In some embodiments, the flexible free-standing graphite sheet cathode current collector is made from expanded graphite particles without the use of any binding material. The flexible graphite sheet can be made from natural graphite, Kish flake graphite, or synthetic graphite that has been voluminously expanded so as to have d₀₀₂ dimension at least 80 times and preferably 200 times the original d₀₀₂ dimension. Expanded graphite particles have excellent mechanical interlocking or cohesion properties that can be compressed to form an integrated flexible sheet without any binder. Natural graphites are generally found or obtained in the form of small soft flakes or powder. Kish graphite is the excess carbon which crystallizes out in the course of smelting iron. In one embodiment, the current collector is a flexible free-standing expanded graphite. In another embodiment, the current collector is a flexible free-standing expanded natural graphite.

[0062] The flexible graphite sheeting preferred for the practice of the present invention is characterized by a thickness of at most 250 micrometers, with less than 75 micrometers preferred, and less than 25 micrometers most preferred. The flexible graphite sheeting preferred for the practice of the invention is further characterized by an electrical conductivity along the length and width of the sheeting of at least 100 Siemens/cm (S/cm), preferably at least 500 S/cm, most preferably at least 1000 S/cm measured according to ASTM standard C611-98.

[0063] The flexible graphite sheeting preferred for the practice of the present invention may be compounded with other ingredients as may be required for a particular application, but graphite having a purity of ca. 95% or greater is highly preferred. At a thickness below about 10 μm, it may be expected that electrical resistance could be unduly high, so that thickness of less than about 10 μm is less preferred.

[0064] A binder is optional, however, it is preferred in the art to employ a binder, particularly a polymeric binder, and it is preferred in the practice of the present invention as well. One of skill in the art will appreciate that many of the polymeric materials recited below as suitable for use as binders will also be useful for forming ion-permeable separator membranes suitable for use in the lithium or lithium-ion battery of the invention.

[0065] Suitable binders include, but are not limited to, polymeric binders, particularly gelled polymer electrolytes comprising polyacrylonitrile, poly(methylmethacrylate), poly(vinyl chloride), and polyvinylidene fluoride and copolymers thereof. Also, included are solid polymer electrolytes such as polyether-salt based electrolytes including poly(ethylene oxide)(PEO) and its derivatives, poly(propylene oxide) (PPO) and its derivatives, and poly(organophosphazenes) with ethyleneoxy or other side groups. Other suitable binders include fluorinated ionomers comprising partially or fully fluorinated polymer backbones, and having pendant groups comprising fluorinated sulfonate, imide, or methide lithium salts. Preferred binders include polyvinylidene fluoride and copolymers thereof with hexafluoropropylene, tetrafluoroethylene, fluorovinyl ethers, such as perfluoromethyl, perfluoroethyl, or perfluoropropyl vinyl ethers; and ionomers comprising monomer units of polyvinylidene fluoride and monomer units comprising pendant groups comprising fluorinated carboxylate, sulfonate, imide, or methide lithium salts.

[0066] Gelled polymer electrolytes are formed by combining the polymeric binder with a compatible suitable aprotic polar solvent and, where applicable, the electrolyte salt. PEO and PPO-based polymeric binders can be used without solvents. Without solvents, they become solid polymer electrolytes, which may offer advantages in safety and cycle life under some circumstances. Other suitable binders include so-called "salt-in-polymer" compositions comprising polymers having greater than 50% by weight of one or more salts. See, for example, M. Forsyth et al, Solid State Ionics, 113, pp 161-163 (1998).

[0067] Also included as binders are glassy solid polymer electrolytes, which are similar to the "salt-in-polymer" compositions except that the polymer is present in use at a temperature below its glass transition temperature and the salt concentrations are ca. 30% by weight. In one embodiment, the volume fraction of the preferred binder in the finished electrode is between 4 and 40%.

[0068] Medium includes electrolyte solvents. Electrolyte solvents can be aprotic liquids or polymers. Included are organic carbonates and lactones. Organic carbonates include a compound having the formula: R⁴OC(=O)OR⁵, wherein R⁴ and R⁵ are each independently selected from the group consisting of Ci_-4alkyl and C_3-6cycloalkyl, or together with the atoms to which they are attached to form a 4- to 8-membered ring, wherein the ring carbons are optionally substituted with 1-2 members selected from the group consisting of halogen, Ci. ₄alkyl and Ci_-4haloalkyl. In one embodiment, the organic carbonates include propylene carbonate, dimethyl carbonate, ethylene carbonate, diethyl carbonate, ethylmethyl carbonate and a mixture thereof as well as many related species. The lactone is selected from β- propiolactone, γ-butyrolactone, δ-valerolactone, ε-caprolactone, hexano-6-lactone and a mixture thereof, each of which is optionally substituted with from 1-4 members selected from the group consisting of halogen, Ci_-4alkyl and Ci_-4haloalkyl. Also included are solid polymer electrolytes such as polyethers and poly(organo phosphazenes). Further included are lithium salt-containing ionic liquid mixtures such as are known in the art, including ionic liquids such as organic derivatives of the imidazolium cation with counterions based on imides, methides, PF₆ ^", or BF₄ ^". See for example, MacFarlane et al., Nature, 402, 792 (1999). Mixtures of suitable electrolyte solvents, including mixtures of liquid and polymeric electrolyte solvents are also suitable.

[0069] The electrolyte solution suitable for the practice of the invention is formed by combining the lithium imide or methide salts of compounds of formula I with optionally a co- salt selected from LiPF₆, LiBF₄, LiAsF₆, LiB(C₂O₄)₂, (Lithium bis(oxalato)borate), or LiClO₄, along with the electrolyte solvent by dissolving, slurrying or melt mixing as appropriate to the particular materials. The present invention is operable when the concentration of the imide or methide salt is in the range of 0.2 to up to 3 molar, but 0.5 to 2 molar is preferred, with 0.8 to 1.2 molar most preferred. Depending on the fabrication method of the cell, the electrolyte solution may be added to the cell after winding or lamination to form the cell structure, or it may be introduced into the electrode or separator compositions before the final cell assembly.

[0070] The rechargeable cell optionally contains a separator. The separator suitable for the lithium or lithium-ion battery of the present invention is any ion-permeable shaped article, preferably in the form of a thin film or sheet. Such separator may be a microporous film such as a microporous polypropylene, polyethylene, polytetrafluoroethylene and layered structures thereof. Suitable separators also include swellable polymers such as polyvinylidene fluoride and copolymers thereof. Other suitable separators include those known in the art of gelled polymer electrolytes such as poly(methyl methacrylate) and poly(vinyl chloride). Also suitable are polyethers such as poly(ethylene oxide) and poly(propylene oxide). Preferable are microporous polyolefin separators, separators comprising copolymers of vinylidene fluoride with hexafluoropropylene, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, or perfluoropropyl vinyl ether, including combinations thereof, or fluorinated ionomers, such as those described in Doyle et al., U.S. Pat. No. 6,025,092.

[0071] The Li-ion cell can be assembled according to any method known in the art (see, U.S. Pat. Nos. 5,246,796; 5,837,015; 5,688,293; 5,456,000; 5,540,741; and 6,287,722 incorporated herein by reference). In a first method, electrodes are solvent-cast onto current collectors, the collector/electrode tapes are spirally wound along with microporous polyolefin separator films to make a cylindrical roll, the winding placed into a metallic cell case, and the nonaqueous electrolyte solution impregnated into the wound cell. In a second method electrodes are solvent-cast onto current collectors and dried, the electrolyte and a polymeric gelling agent are coated onto the separators and/or the electrodes, the separators are laminated to, or brought in contact with, the collector/electrode tapes to make a cell subassembly, the cell subassemblies are then cut and stacked, or folded, or wound, then placed into a foil- laminate package, and finally heat treated to gel the electrolyte. In a third method, electrodes and separators are solvent cast with also the addition of a plasticizer; the electrodes, mesh current collectors, electrodes and separators are laminated together to make a cell subassembly, the plasticizer is extracted using a volatile solvent, the subassembly is dried, then by contacting the subassembly with electrolyte the void space left by extraction of the plasticizer is filled with electrolyte to yield an activated cell, the subassembly(s) are optionally stacked, folded, or wound, and finally the cell is packaged in a foil laminate package. In a fourth method, the electrode and separator materials are dried first, then combined with the salt and electrolyte solvent to make active compositions; by melt processing the electrodes and separator compositions are formed into films, the films are laminated to produce a cell subassembly, the subassembly(s) are stacked, folded, or wound and then packaged in a foil-laminate container.

[0072] In one embodiment, the electrodes can conveniently be made by dissolution of all polymeric components into a common solvent and mixing together with the carbon black particles and electrode active particles. For example, a lithium battery electrode can be fabricated by dissolving polyvinylidene (PVDF) in l-methyl-2-pyrrolidinone or poly(PVDF- co-hexafluoropropylene (HFP)) copolymer in acetone solvent, followed by addition of particles of electrode active material and carbon black or carbon nanotubes, followed by deposition of a film on a substrate and drying. The resultant electrode will comprise electrode active material, conductive carbon black or carbon nanotubes, and polymer. This electrode can then be cast from solution onto a suitable support such as a glass plate or a current collector, and formed into a film using techniques well known in the art.

[0073] The positive electrode is brought into electronically conductive contact with the graphite current collector with as little contact resistance as possible. This may be advantageously accomplished by depositing upon the graphite sheet a thin layer of an adhesion promoter such as a mixture of an acrylic acid-ethylene copolymer and carbon black. Suitable contact may be achieved by the application of heat and/or pressure to provide intimate contact between the current collector and the electrode.

[0074] The flexible carbon sheeting, such as carbon nanotubes or graphite sheet for the practice of the present invention provides particular advantages in achieving low contact resistance. By virtue of its high ductility, conformability, and toughness it can be made to form particularly intimate and therefore low resistance contacts with electrode structures that may intentionally or unintentionally proffer an uneven contact surface. In any event, in the practice of the present invention, the contact resistance between the positive electrode and the graphite current collector of the present invention preferably does not exceed 50 ohm-cm², in one instance, does not exceed 10 ohms-cm², and in another instance, does not exceed 2 ohms- cm². Contact resistance can be determined by any convenient method as known to one of ordinary skill in the art. Simple measurement with an ohm-meter is possible.

[0075] The negative electrode is brought into electronically conductive contact with an negative electrode current collector. The negative electrode current collector can be a metal foil, a mesh or a carbon sheet. In one embodiment, the current collector is a copper foil or mesh. In a preferred embodiment, the negative electrode current collector is a carbon sheet selected from a graphite sheet, carbon fiber sheet or a carbon nanotube sheet. As in the case of the positive electrode, an adhesion promoter can optionally be used to attach the negative electrode to the current collector.

[0076] In one embodiment, the electrode films thus produced are then combined by lamination with the current collectors and separator. In order to ensure that the components so laminated or otherwise combined are in excellent ionically conductive contact with one another, the components are combined with an electrolyte solution comprising an aprotic solvent, preferably an organic carbonate as hereinabove described, and a lithium imide or methide salt represented by the formula I.

[0077] In anther aspect, the present invention provides use of an electrolyte in a lithium-ion electrochemical cell for the prevention and/or protection of a lithium-ion electrochemical cell from overcharge damage. Preferably, the electrolyte solution is substantially free or completely free of any redox shuttle additives. The electrolyte is a lithium compound as described herein and in accordance with any of formulas I, Ia, Ia-I, Ia-2, Ib or Ic or a combination thereof. The lithium compound as described herein has an oxidation potential above the maximum permitted recharged potential of the positive electrode, undergoes reversible redox reactions at the negative and the positive electrodes and acts as a redox shuttle. Generally, a lithium-ion electrochemical cell comprises a positive electrode, a negative electrode, an electrolyte solution, a carbon sheet current collector and an ion- permeable separator.

Examples

[0078] The following exemplary embodiments of the invention are presented to explain the invention in more detail, and do not limit the invention.

Example 1

Preparation of a positive electrode film

[0079] A positive electrode film was made by mixing in an organic solvent, such as acetone, 83.5% parts LiMn₂O₄, 6.5 parts Super P carbon black (MMM Carbon), and 10 parts KYNAR FLEX 2801 (Elf Atochem). Films were cast on flexible expandable graphite foil using the doctor blade technique and the acetone evaporated. After densification by passing the electrode through calender rolls, a positive electrode film was obtained with 20 mg/cm² loading.

Preparation of a negative electrode film

[0080] A negative electrode film was made by mixing in acetone 86.7 parts MCMB 2528 (Osaka Gas), 3.3 parts Super P carbon black, 10 parts KYNAR FLEX 2801. After casting, acetone evaporation, and densification, a negative electrode film with 8.5 mg/cm² loading was obtained.

Example 2

Preparation of an electrolyte solution

[0081] Salt (CF₃SO₂)₂NLi (3M Company, MN) was dried under vacuum at 120 ⁰C for 48 hours before use. An electrolyte solution was prepared by dissolving the salt at a concentration of 1.0 M in a solvent mixture of 1 parts by weight ethylene carbonate and 1 part by weight dimethyl carbonate.

Example 3

Preparation of a coin type cell

[0082] A coin type cell battery (diameter 20 mm, thickness 3.2 mm) comprised of a positive electrode and negative electrode as described in Example 3, a commercial Celgard 2320 separator and an electrolyte solution from Example 4 was prepared at room temperature.

Example 4

Capacity confirmation and formation test

[0083] Initial formation charging and discharging of the coin cells were performed using commercial battery tester (Arbin Instruments) at room temperature. The cell was first charged to 4.2 volts (V) at a rate of 0.2 C at constant current. After reaching 4.2 V, the cell was charged at constant 4.2V for additional 3 hours followed by 1 hour rest. The cell was then discharged at a rate of 0.2 C at constant current until the cut-off voltage 2.7V was reached. The above charge-discharge cycle was repeated 5 times. Cell capacity of 6.2 mAh (1C rate = 6.2mA) was obtained.

Example 5

Overcharge protection with (CF₃SO₂)₂NLi (LiTFSI) salt

[0084] An coin cell from Example 5 was tested for overcharge. In each charge/discharge cycle, the cell was charged at a 1C rate for 3 hrs followed by a constant current discharge at 1C rate to 3.0 V (FIG. 1). Alternatively, in each charge/discharge cycle, the cell was charged at a 3C rate for 1 hrs followed by a constant current discharge at 3C rate to 3.0 V ((FIG. 2)). Such a charging protocol effectively overcharged the cell at to 200% above its full charge capacity.

[0085] FIGs. 3 and 4 show the normal charging and discharge curves in the voltage range of 4.2V to 2.5V after the overcharging in FIGs 1 and 2, respectively. FIG. 3 shows the cell retains 88% capacity after 10 cycles of lC/300% overcharge. Fig. 4 shows the electrochemical cell retains 40% capacity after lOcycles of 3C/300% overcharge.

Example 6

Comparison of LiTFSI and LiPF₆ cells

[0086] Coin cells (diameter 20 mm, thickness 3.2 mm) were made where the negative electrode is Li metal disc and positive electrode is pristine graphite foil (no coating). The electrolyte was either LOM LiTFSI in EC/EMC/DMC(2:1 :1 by volume) or l.OM LiPF₆ in EC/EMC/DMC(2: 1 : 1 by vol). Linear sweep voltammetry was performed using a Potentiostat (Gamry). FIG. 5 shows a comparison of a LiTFSI cell with a LiPF₆ cell. LiTFSI cell can hold a voltage at about 4.8 V. In contrast, a LiPF₆ cell does not have such a property.

[0087] FIG. 6 shows a comparison of LiTFSI cell and LiPF₆ cell within 4 hours time frame. LiTFSI cell is able to hold a voltage at about 4.8V. In contrast, LiPF₆ cell does not have such a property.

[0088] FIG. 7 shows the linear sweep voltammetry of LiTFSI cell over a period of 20 hours. LiTFSI cell is able to hold a voltage at about 5.0 V.

[0089] FIG. 8 shows the cyclic voltammetry of LiTFSI cell and LiPF₆ cell. LiTFSI undergoes reversible oxidation with a peak oxidation potential of 4.8 V. In comparison, LiPF₆ does not undergo reversible oxidation and its oxidation potential is not peaked even at

5.5 V.

Example 7

A cylindrical 18650 cell battery

[0090] A cylindrical 18650 cell battery (diameter 17.8 mm, height 65 mm) comprised of a positive electrode, negative electrode, separator and electrolyte was prepared at room temperature. The positive electrode active material consists of 7:3 (by weight) mixture of LiNio.₈Cθo.i₅Alo.o5θ2 and LiNio.₄Co₀.₄Mno,₂0₂. The positive electrode comprises of 94% by weight of the positive active material, 3% by weight of carbon black (conductive agent), and 3% by weight of polyvinylidene fluoride (binder). The positive electrode current collector is a graphite foil current collector. The negative electrode active material comprises of 1 : 1 by weight mixture of graphite SMG/MAGE (Hitachi Chemical). The negative electrode comprises of 92% by weight of the negative active material, 5% by weight of polyvinylidene fluoride (binder) 5% and 3% by weight of carbon black. The negative electrode current collector is a copper current collector. The separator is Celgard^®2320. The electrolyte is 1.2 M Li(CF₃SO₂)₂N and 1 :1 by weight mixture of ethylene carbonate and dimethyl carbonate.

[0091] The cell has rated capacity of 2.2 Ah. Initial charging and discharging of the electrochemical cells were performed according to the constant current charging and discharging method at room temperature. The cell was first charged to 4.3 volts (V) at a rate of 0.1 C at constant current. After reaching 4.3 V, the cell voltage was held at 4.3V until the charging current droped below 0.01C. After 15 minutes rest, the cell was discharged at a constant rate of 0.5 C at constant current until the cut-off voltage 2.75 V. Rated capacity of the cell was confirmed.

Example 8

Cell voltage and temperature profile during an overcharge

[0092] A fully discharged cell was charged at constant 1C rate and charging at 1C rate continues after the cell reached its rated capacity. FIG. 10 shows the cell voltage and body temperature. During overcharge, the cell voltage gradually increased to 5.2 V, then decreased and stabilized at about 4.8V. The cell body temperature reached about 72 °C when the cell's internal current interrupt device was activated, which terminated the charging current. The cell was safe during the overcharge test without fire or smoke. [0093] While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that examples and embodiments described herein are for illustrative purposes only and the invention is not limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.

Claims

WHAT IS CLAIMED IS:

1. A method for preventing lithium-ion electrochemical cell damage due to overcharge, said method comprising:

providing a negative electrode, a positive electrode having a recharged potential and an electrolyte solution comprising a medium and a lithium compound of formula I: R'-X-(Li⁺)R²(R³)_m, I m is 0 or 1 ; R¹, R² and R³ are each independently an electron- withdrawing group selected from the group consisting of -CN, -SO₂R³, -SO₂-L^a-SO₂N^"Li⁺SO₂R^a, -P(O)(OR^a)₂, - P(O)(R^a)₂, -CO₂R^a, -C(O)R³ and -H; with the proviso that R¹ and R² are other than hydrogen when m = 0, and no more than one of R¹ , R² and R³ is hydrogen when m = 1 ; X is N when m is 0; X is C when m is 1; wherein each R^a is independently selected from the group consisting of Ci_-8 alkyl, Ci- shaloalkyl, Ci_-8 perfluoroalkyl, perfluoroaryl, aryl, optionally substituted barbituric acid and optionally substituted thiobarbituric acid, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl in R^a is optionally substituted with from 1-5 members selected from the group consisting of halogen, Ci- ₄haloalkyl, C_Mperfluoroalkyl, -CN, -SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and -C(O)R^b, wherein each R^b is independently Ci_-8 alkyl, perfluoroaryl or Ci- ₈ perfluoroalkyl; and L^a is Ci-₄perfluoroalkylene; and combining the negative electrode, positive electrode and electrolyte solution into an electrochemical cell to prevent damage due to overcharge in a lithium-ion cell, wherein the electrolyte solution is free of any redox shuttle additives and wherein said lithium compound i) has an oxidation peak potential above the maximum permitted recharged potential of the positive electrode, ii) undergoes reversible redox reactions at the negative and the positive electrodes and iii) acts as a redox shuttle, whereby overcharge of the lithium-ion electrochemical cell is counteracted.

2. A method of designing a lithium-ion electrochemical cell for preventing damage due to overcharge, said method comprising: selecting an oxidation potential; selecting a compound based on said oxidation potential, wherein said lithium compound has formula (I): R'-X-(Li⁺)R²(R³)_m, I wherein: m is 0 or 1 ; R , R and R are each independently an electron- withdrawing group selected from the group consisting of -CN, -SO₂R⁸, -SO₂-IZ-SO₂NTi⁺SO₂R⁸, -P(O)(OR⁸)₂, - P(O)(R^a)₂, -CO₂R⁸, -C(O)R³ and -H; with the proviso that R¹ and R² are other than hydrogen when m = O, and no more than one of R¹ , R² and R³ is hydrogen when m = 1 ; X is N when m is 0; X is C when m is 1 ; and wherein each R^a is independently selected from the group consisting of Ci- ₈ alkyl, Ci_-8haloalkyl, Ci_-8 perfluoroalkyl, perfluoroaryl, aryl, optionally substituted barbituric acid and optionally substituted thiobarbituric acid, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl in R^a is optionally substituted with from 1 -5 members selected from the group consisting of halogen, C_Mhaloalkyl, C_1-4perfluoroalkyl, -CN, -SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and -C(O)R^b, wherein R^b is Ci_-8 alkyl, perfluoroaryl or Ci_-8 perfluoroalkyl; and L^a is Ci- ₄perfluoroalkylene;

wherein the electrolyte solution is free of any redox shuttle additives and wherein said lithium compound i) has an oxidation peak potential above the maximum permitted recharged potential of the positive electrode, ii) undergoes reversible redox reactions at the negative and the positive electrodes and iii) acts as a redox shuttle; to recharge said lithium-ion electrochemical cell, whereby overcharge of the lithium-ion electrochemical cell is counteracted.

3. The method of claim 2, wherein the oxidation peak potential is between 4.4 V and about 5.2 V.

4. A method for recharging a lithium-ion electrochemical cell while chemically limiting cell damage due to overcharging comprising:

supplying charging current across a positive electrode and a negative electrode of a lithium-ion rechargeable electrochemical cell comprising a charge-carrying electrolyte solution comprising a medium and a lithium compound of formula I: R'-X-(Li⁺)R²(R³)_m, I . wherein: m is 0 or 1 ; R¹, R² and R³ are each independently an electron- withdrawing group selected from the group consisting of -CN, -SO₂R³, -SO₂-L^a-SO₂N^"Li⁺SO₂R^a, -P(O)(OR^a)₂, - P(O)(R^a)₂, -CO₂R^a, -C(O)R³ and -H; with the proviso that R¹ and R² are other than hydrogen when m = 0, and no more than one of R , R and R is hydrogen when m = 1 ; X is N when m is 0; X is C when m is 1 ; and wherein each R^a is independently selected from the group consisting of Ci- ₈ alkyl, Ci_-8haloalkyl, C_1-8 perfluoroalkyl, perfluoroaryl, aryl, optionally substituted barbituric acid and optionally substituted thiobarbituric acid, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl in R^a is optionally substituted with from 1-5 members selected from the group consisting of halogen, C_1-4haloalkyl, C_Mperfluoroalkyl, -CN, -SO₂R^b, -P(O)(OR^b)₂, -P(O)(R^b)₂, -CO₂R^b and -C(O)R^b, wherein R^b is Ci_-8 alkyl, perfluoroaryl or Ci_-8 perfluoroalkyl; and L^a is Ci- ₄perfluoroalkylene; wherein the electrolyte solution is free of any redox shuttle additives and wherein said lithium compound i) has an oxidation potential above the maximum permitted recharged potential of the positive electrode, ii) undergoes reversible redox reactions at the negative and the positive electrodes and iii) acts as a redox shuttle; to recharge said lithium- ion electrochemical cell, whereby overcharge of the lithium-ion electrochemical cell is counteracted.

5. The method of any of claims 1-4, wherein the lithium compound has a formula: R¹NXLi⁺)R² (Ib) wherein R¹ and R² are each independently selected from -SO₂R^a or -SO₂-L^a- S O₂NXi⁺S O₂R^a, wherein each R^a is independently Ci_-8perfluoroalkyl, perfluoroaryl or aryl, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl in R^a is optionally substituted with from 1 -5 members selected from the group consisting of halogen, Ci_-4haloalkyl, Cj ^perfluoroalkyl, - CN, -SO₂R^b, -CO₂R^b and -C(O)R^b, wherein R^b is Ci_-8 alkyl, perfluoroaryl or Ci_-8 perfluoroalkyl; and L^a is C_Mperfluoroalkylene.

6. The method of any of claims 1-4, wherein the lithium compound has a formula: R¹CXLi⁺)R²R³ (Ic) wherein R¹, R² and R³ are each independently selected from -SO₂R^a or -SO₂- L^a-SO₂N^"Li⁺SO₂R^a, wherein each R^a is independently Ci.sperfluoroalkyl, perfluoroaryl or aryl, wherein at least one carbon-carbon bond of the alkyl or perfluoroalkyl is optionally replaced with -O- to form an ether linkage; the aryl in R^a is optionally substituted with from 1-5 members selected from the group consisting of halogen, Ci^haloalkyl, Ci_-4perfluoroalkyl, -CN, -SO₂R^b, -CO₂R^b and -C(O)R^b, wherein R^b is C,_-8 alkyl, perfluoroaryl or C_[-8 perfluoroalkyl; and L^a is Ci^perfluoroalkylene.

7. The method of any of claims 1-4, wherein R¹ is -SO₂R^a.

8. The method of claim 7, wherein R¹ is -SO₂(C_1-8fluoroalkyl).

9. The method of claim 8, wherein R¹ is -SO₂(Ci_-8perfluoroalkyl).

10. The method of any of claims 1-4, wherein m is 0 and R² is -SO₂R^a or - SO₂(-L^a-SO₂NXi⁺)SO₂-R^a, wherein R^a is Ci_-8fluoroalkyl, Ci_-8perfluoroalkyl, perfluoroaryl or optionally substituted aryl, wherein at least one carbon-carbon bond of Ci_-8fluoroalkyl or Ci- ₈perfluoroalkyl is optionally replaced with -O- to form an ether linkage.

11. The method of claim 10, wherein R^a is C_1-8perfluoroalkyl or perfluoroaryl.

12. The method of any of claims 1-4, wherein m is 1 and R² and R³ are each independently -SO₂R³ or -SO₂(-L^a-SO₂N^"Li⁺)SO₂-R^a, wherein R^a is Ci.gfluoroalkyl, Ci- sperfluoroalkyl, perfluoroaryl or optionally substituted aryl, wherein at least one carbon- carbon bond of Ci_-8fluoroalkyl or Ci_-8perfluoroalkyl is optionally replaced with -O- to form an ether linkage.

13. The method of claim 12, wherein R^a is Ci_-8perfluoroalkyl or perfluoroaryl.

14. The method of any of claims 1-4, wherein each R^a is independently selected from the group consisting of -CF₃, -CF₂CF₃, -CF₂-CF₂CF₃, -CF₂-OCF₃, C₁. sfluoroalkyl, perfluorophenyl, trifluoromethylphenyl and bis-trifluoromethylphenyl.

15. The method of any of claims 1-4, wherein the compound is selected from the group consisting of: CF₃SO₂N^"(Li⁺)SO₂CF₃, CF₃CF₂SO₂NXLi⁺)SO₂CF₃, CF₃CF₂SO₂NXLi⁺)SO₂CF₂CF₃, CF₃SO₂NXLi⁺)SO₂CF₂OCF₃, CF₃OCF₂SO₂N^" (Li⁺)SO₂CF₂OCF₃, C₆F₅SO₂NXLi⁺)SO₂CF₃, C₆F₅SO₂NXLi⁺)SO₂C₆F₅, CF₃SO₂N^" (Li⁺)SO₂PhCF₃, CF₃SO₂C^"(Li⁺XSO₂CF₃)₂, CF₃CF₂SO₂C^"(Li⁺)(SO₂CF₃)₂, CF₃CF₂SO₂C^" (Li⁺)(SO₂CF₂CF₃)₂, (CF₃SOz)₂CXLi⁺)SO₂CF₂OCF₃, CF₃SO₂CχLi⁺)(SO₂CF₂OCF₃)₂, CF₃OCF₂SO₂CXLi⁺)(SO₂CF₂OCF₃)₂, C₆F₅SO₂CXLi⁺)(SO₂CF₃),, (C₆F₅SO₂)₂CχLi⁺)SO₂CF₃, C₆F₅SO₂C^"(Li⁺)(SO₂C₆F₅)₂, (CF₃SO₂)₂CXLi⁺)SO₂PhCF₃ and CF₃SO₂CχLi⁺)(SO₂PhCF₃)₂.

16. The method of claim 15, wherein the compound has the formula selected from CF₃SO₂NXLi⁺)SO₂CF₃, C₆F₅SO₂N-(Li⁺)SO₂C₆F₅ or CF₃SO₂CXLi⁺)(SO₂CF₃)₂.

17. The method of any of claims 1-4, wherein the positive electrode exhibits an upper charging voltage in the range of about 3 to 5.2 volts with respect to a Li/Li⁺ reference electrode.

18. The method of any of claims 1-4, wherein the compound has an oxidation peak potential from about 100 mV to about 500 mV above the upper charging voltage of the positive electrode.

19. The method of any of claims 1-4, wherein the compound has a reversible peak oxidation potential at about 4.8V.

20. The method of any of claims 1-4, wherein the compound is able to hold a voltage at about 4.8 V during overcharge, thereby providing an overcharge protection to the electrochemical cell.

21. The method of any of claims 1-4, wherein the compound undergoes reversible reduction and oxidation reactions.

22. The method of any of claims 1-4, wherein the lithium compound has a concentration from about 0.2 M to about 3 M.

23. The method of any of claims 1-4, wherein the medium is a non- aqueous solvent, a polymer, an ionic liquid or a mixture thereof.

24. The method of claim 23, wherein the medium is a polar aprotic solvent.

25. The method of claim 24, wherein the polar aprotic solvent is selected from the group consisting of an organic carbonate and a lactone.

26. The method of claim 25, wherein the organic carbonate is a compound having the formula: R⁴OC(=O)OR⁵, wherein R⁴ and R⁵ are each independently selected from the group consisting of Ci_-4alkyl and C_3-6cycloalkyl, or together with the atoms to which they are attached to form a 4- to 8-membered ring, wherein the ring carbons are optionally substituted with 1-2 members selected from the group consisting of halogen, C_1-4alkyl and Ci- ₄haloalkyl.

27. The method of claim 26, wherein the carbonate is selected from the group consisting of ethylene carbonate, propylene carbonate, dimethylcarbonate, diethylcarbonate, ethylmethylcarbonate and a mixture thereof.

28. The method of claim 25, wherein the lactone is selected from the group consisting of β-propiolactone, γ-butyrolactone, δ-valerolactone, ε-caprolactone, hexano-6- lactone and a mixture thereof, each of which is optionally substituted with from 1-4 members selected from the group consisting of halogen, Ci.₄alkyl and Ci_-4haloalkyl.

29. The method of any of claims 1-4, wherein the negative electrode comprises a negative electrode active material and a negative electrode current collector in electronically conductive contact with the negative electrode active material.

30. The method of claim 29, wherein the negative electrode active material is selected from the group consisting of graphite microbeads, natural graphites, a flexible expanded graphite, carbon fibers, graphite flakes and carbon nanotubes or a metal selected from the group consisting of Li, Si, Sn, Sb, Al or a combination thereof.

31. The method of claim 29, wherein the negative electrode active material is in a particulate form, said particulate ranging in size from about 0.01 to about 100 micrometers in average equivalent spherical diameter.

32. The method of any of claims 1-4, wherein the positive electrode comprises a positive electrode active material and a carbon sheet current collector in electronically conductive contact with the positive electrode active material, wherein said carbon sheet is a flexible expanded graphite sheet, carbon fiber sheet or a carbon nanotube sheet.

33. The method of claim 32, wherein the positive electrode active material comprises a lithium insertion transition metal oxide, phosphate or sulfate is selected from the group consisting Of LiCoO₂, spinel LiMn₂O₄, chromium-doped spinel lithium manganese oxides, layered LiMnO₂, LiNiO₂, LiNi_xCθi_-xO₂, wherein x is 0<x<l, vanadium oxides, LiFePO₄, LiMnPO₄, LiVPO₄, LiFeTi(SO₄)₃ and LiNi_xCo_yMni_-x-yO₂, wherein x is 0<x<l and y is θ<y<l.

34. The method of claim 33, wherein the positive electrode active material is LiNi_xCθ_yMni_-x-yO₂, wherein x and y are between 0 and 1.

35. The method of claim 34, wherein x is between 0.25 and 0.9.

36. The method of claim 35, wherein x is 1/3 and y is 1/3.

37. The method of claim 33, wherein the positive electrode active material is LiNi_xMn_1-x0₂, wherein x is between 0.3 and 0.8.

38. The method of claim 33, wherein the lithium insertion transition metal oxide, phosphate, or sulfate is a lithium insertion transition metal oxide selected from the group consisting of LiMn₂O₄, LiNiO₂, LiNi_xMni_-x0₂, LiFePO₄, LiMnPO₄ and derivatives thereof, wherein x is 0<x<l .

39. The method of claim 32, wherein the graphite sheet positive electrode current collector has a purity of >95%, a thickness of less than 250 micrometers, a bulk density of 0.08-2.25 g/cc, an electrical conductivity of at least 500 Siemens/cm, and said electronically conductive contact being characterized by a resistance of less than 50 ohm- cm².

40. The method of claim 39, wherein the carbon sheet has a thickness of less than about 50 μm.

41. The method of claim 39, wherein the carbon sheet has a bulk density of about 0.8 to about 1.4 g/cm³.

42. The method of any of claims 1-4, wherein the carbon sheet has an electrical conductivity of at least 100 Siemens/cm in the plane of the sheet.

43. The method of any of claims 1-4, wherein the cell retains at least about 80% capacity when charge and discharged normally after 300% overcharge at 1C rate.

44. The method of any of claims 1-4, wherein the cell retains at least about 40% capacity after when charge and discharged normally after 300% overcharge at 3C rate.