WO2001052341A1

WO2001052341A1 - Perfluoroalkanesulfonate salts in electrochemical systems

Info

Publication number: WO2001052341A1
Application number: PCT/US2000/015149
Authority: WO
Inventors: Steven J. Hamrock; Alan D. Fanta; William M. Lamanna; Bryan J. Johnson; Steven D. Boyd; Hiroshi Shimada; Phat T. Pham
Original assignee: 3M Innovative Properties Company
Priority date: 2000-01-11
Filing date: 2000-06-01
Publication date: 2001-07-19
Also published as: AU2000253140A1

Abstract

Electrolyte compositions and electrochemical systems containing such compositions are described where the electrolyte includes in a matrix material a combination of a conductive imide or methide salt and a perfluoroalkanesulfonate additive salt. The compositions reduce high temperature capacity fade, aluminium current collector corrosion and improve safety while maintaining and improving conductivity, stability, and compatibility with other cell components.

Description

PERFLUOROALKANESULFONATE SALTS IN ELECTROCHEMICAL SYSTEMS

Field of the Invention This invention relates to perfiuoroalkanesulfonate salts which, when used as additives to short chain imide and methide conductive electrolyte salts in electrochemical systems improve high temperature capacity fade, reduce aluminum current collector corrosion and improve safety by reducing thermal runaway or total exothermic energy.

Background of the Invention In recent years, highly conductive lithium salts such as lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate and lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(perfluoroethanesulfonyl)imide and lithium tris(trifluoromethanesulfonyl)methide have found frequent use in liquid, polymer and gel electrolytes for lithium primary and secondary batteries. See, for example, Kirk-Othmer's Encyclopedia of Chemical Technology, Fourth Edition, 3, 1016— 1018 (1992) and 1107-1109; and 15, 446-447 (1995). Typically, liquid electrolytes for lithium batteries are made by dissolving lithium salt(s) of choice in anhydrous polar aprotic liquid solvent(s) at a Li⁺ molar concentration of around 0.5-2.0 M to produce a homogeneous solution having good conductivity and stability. The solvent must be sufficiently polar to effectively dissolve and dissociate the electrolyte salt, yet the solvent must be aprotic, i.e., free of any active hydrogen, to prevent reaction with the anode, which contains lithium metal or a form of carbon, such as graphite, intercalated with lithium. Lithium hexafluorophosphate (LiPF₆) is known as the major salt used in the cylindrical rechargeable lithium ion battery industry. It offers good performance characteristics but suffers from being thermally and hydrolytically sensitive. As a result LiPF₆ requires special handling in shipping and manufacturing processes. Furthermore thermal degradation of LIPF₆ produces PF₅ which is a toxic gas. Therefore an electrolyte which has thermal and hydrolytic stability provides a desired performance characteristic in the market. Lithium bis(perfluoroethanesulfonyl)imide (BETI) developed by 3M has excellent thermal and hydrolytic stability, good ion conductivity, and electrochemical stability. However, BETI cells exhibit large capacity fade at 60°C with 72 hours storage at full charge. Certain salts are known to corrode aluminum at low voltages. Such corrosion can be detrimental to lithium in cells. Additional inhibitors used to prevent aluminum corrosion by the salts would improve the performance of such cells. Finally thermal runaway of lithiated graphite anodes has been known to create safety problems in lithium ion batteries. It would be advantageous to reduce thermal runaway by appropriate choice of electrolyte salt.

Thus there remains a need for a method to improve high temperature capacity fade , aluminum corrosion and safety of electrochemical systems incorporating imide and methide salts.

Summary of the Invention We have found that use of certain fluorinated sulfonate salts as additives to short chain imide and methide conductive electrolyte salts in electrochemical systems reduces high temperature capacity fade and reduces aluminum current collector corrosion, and improves safety by reducing thermal runaway or total exothermic energy. Accordingly, the present invention provides for an improved electrolyte composition which includes:

(a) a conductive salt including a cation selected from the group consisting of an alkali metal; an alkaline earth metal; a Group IIB metal; a Group IIIB metal; a transition metal; a rare earth metal; a nitrogen onium cation, such as tetraalkylammonium, trialkylammonium, N-alkylpyridinium and N,N'-dialkyl- imidazolium, in which alkyl has 1 to 4 carbon atoms; and a proton; and an anion selected from the group consisting of an anion of the formula

in which Rn and R ₂ are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and RQ having a total of up to 5 carbon atoms; and Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms; an anion having a formula R_f4R_f5N-(CF₂)_n'SO -X^~, or

in which X^~ is -NSO₂R_f4 or

(R_f6SO₂)-C-(SO₂R_f7);

Z is -CF₂- -O-

-NRfB _;

or -SF₄-; R_f4 and Rβ , independently, are -C_mF_2m+1 , or -(CF₂)_q-SO₂-X ; R_f6 and R_f7 , independently, are -C_mF_2m+ι , -(CF₂)₄-SO₂-X^",

'N-(CF₂)„- or

R, β - / ^R'\

Z N-(CF₂)„-

in which R_re is -C_mF_2m+ι , or -(CF₂)_q-SO₂-X^_; Rf_6' and Rn_' , independently, are perfluoroalkylene moieties having the formula -C_rF_2r-; n' is 1-4; r is 1-4; m is 1- 4; and q is 1-4; a bis-fluoroalkanesulfonyl methide anion R_f"-SO -C^_(R)-SO₂-R_f ^πι in which R_f ¹¹ and R_f ¹¹¹ , independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R_f ¹¹ and R_f ^{1 π} is up to 5 carbon atoms, and R is H, CN, F, alkyl of 1 to 6 carbon atoms, phenyl or phenyl substituted by alkyl of 1 to 4 carbon atoms; a tris-(perfluoroalkanesul_fonyl)methide anion of the formula ^~C(SO₂R_f ^II)(SO₂R_f ^III)(SO₂R_f ^IV) in which R_f ^π, R_f ^πι, and R_f ^IV, independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R_f ¹¹, R_f ¹¹¹ and R_f ^IV are up to 6 carbon atoms; and conductive salts including cyano-substituted amide and methide anions of the formula:

R-[Q-A^~(C ]y

wherein R is a fluorine atom, a monovalent or divalent non-fluorinated or fluorinated straight or branched, saturated or unsaturated aliphatic group having 1 to 4 carbon atoms, a cycloaliphatic group of 3 to 6 carbon atoms, a cycloaliphatic— aliphatic group in which the aliphatic group has 1 or 2 carbon atoms, in which the carbon chain of the aliphatic or cycloaliphatic groups is uninterrupted or interrupted by a catenary heteroatom and which the aliphatic or cycloaliphatic group is unsubstituted or substituted by a halogen atom; y is 1 or 2, and A is C or N; when A is C, n is 2 and the compound is a methide; when A is N, n is 1 and the compound is an amide; Q is a linking group selected from -SO₂- and -C(O)-, and

(b) a sulfonate additive salt of the formula

R_fSO₃M wherein M is a cation; Rt is a straight or branched perfluoroalkyl group of 4 to 16 carbon atoms, a perfluorocycloalkyl group or a perfluorocycloalkyl perfluoroalkyl group of 3-7- ring carbon atoms and 1-4 carbon atoms on the alkyl chain, which perfluoroalkyl or perfluorocycloalkyl group may contain one or more heteroatoms; R_t may optionally be partially fluorinated with a maximum of 20% of the non-fluorine substituents being hydrogen, and wherein the molar ratio of conductive salt to additive salt is between about 99.9:0.1 to about 70:30.

In a second aspect, the invention features an electrochemical system that includes at least one positive electrode, at least one negative electrode, and an electrolyte comprising the combination of an imide or methide conductive salt and additive salt as described above, where the molar ratio of conductive salt to additive salt may range between about 99.9:0.1 to 70:30, preferably between about 99:1 to 90:10. A third aspect of the invention includes a method of improving high temperature capacity fade of electrochemical systems by incorporating into an electrolyte composition in a matrix material containing an imide or methide conductive salt up to 30 mole %, based on total salt content, of a fluorinated sulfonate additive salt. Surprisingly we have found that the use of said additive salts of this invention in combination with the conductive imide or methide salts significantly reduces the high temperature capacity fade of the latter salts on repeated cycling at high temperatures. When employing said additive salt of this invention, capacity fade in cells during high temperature cycling and storage (e.g., 60°C or higher at full state of charge (4.2 V)) is reduced when compared to similar cells containing no additive salt. Overall, this additive salt/conductive salt combination gives electrolyte salt performance comparable to LiPF₆, without the drawbacks of hydrolytic and thermal instability inherent in the PF₆ ^~ anion.

A fourth aspect of the invention includes a method of improving aluminum corrosion in electrochemical systems by incorporating into an electrolyte composition in a matrix material containing an imide or methide conductive salt up to 30 mole %, based on total salt content, pf a fluorinated sulfonate additive salt. Corrosion of aluminum current collectors is greatly reduced when employing said additive salt of this invention. For example in combination with BETI repassivation potential of the cell is increased to over 4.5 volts, thus greatly reducing the corrosion current at high positive potentials.

A fifth aspect of the invention includes a method of reducing thermal runaway or exothermic energy in electrochemical systems by incorporating into an electrolyte composition in a matrix material containing a conductive salt up to 30 mole %, based on total salt content, of a fluorinated sulfonate additive salt.

Surprisingly, we have further found that in lithium ion battery electrolytes comprising short chain bis(perfluoroalkanesulfonyl)imide conductive salts, preferably bis(perfluoroethanesulfonyl)imide conductive salts, incorporation of said additive salts of this invention, preferably at concentrations of 10% by weight or more based on the weight of the conductive salt, which can improve the safety and performance of the battery. Lithium ion battery safety is a critical issue, due to the potential of the highly reactive battery components to undergo thermal runaway and resultant battery explosion. Graphite electrodes which are intercalated with lithium (i.e., lithiated graphite) have similar chemical and electrochemical characteristics to lithium metal. As such, these electrodes are very reactive and exhibit an exothermic reaction with the electrolyte at elevated temperatures, temperatures which may be encountered in the battery under severe use conditions or during an electrical short. Incorporating long chain perfluorinated sulfonate additive salts of this invention in electrolytes comprising short chain bis(perfluoroalkanesulfonylimide) conductive salts greatly reduces exotherm energies produced at the electrode/electrolyte interface when such a battery reaches temperatures of up to 200°C.

Thus, a further aspect of the present invention includes a method of improving the safety and performance of an electrochemical system, e.g. a battery or rechargeable battery, particularly a lithium ion battery, by employing an electrolyte composition which incorporates an effective amount or more of a fluorinated sulfonate additive salt as described above containing a conductive salt having a cation as described above, but preferably lithium, and an anion of the formula

in which Rn and Rf2 are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and Rβ taken together having a total of up to 5 carbon atoms; Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms.

We have found that by adding said additive salt of the present invention to an imide or methide conductive salt in an electrolyte composition, one or more of the following advantages can result: • provides all of the requisite functions of a battery electrolyte salt including: solubility, ionic conductivity, chemical and thermal stability, etc.;

• reduces high temperature capacity fade;

• reduces corrosion of aluminum 'current collectors; • improves safety;

• may lower surface tension thus allowing better wetting of battery component materials and expanding the range of electrolyte compositions available;

• can be used in small quantity additions to existing electrolyte formulations to enhance performance; and

• provides for potential applications in a variety of battery systems and capacitors.

Brief Description of the Drawing The FIGURE is a cut-away view of a lithium-ion battery.

Detailed Description The present invention relates to electrolyte compositions useful in electrochemical systems such as batteries, e.g. primary and secondary (rechargeable) batteries, double-layer capacitors, supercapacitors, fuel cells, electroplating and electrorefining systems and the like. The electrolyte compositions include combinations of certain fluorinated sulfonate salts with conductive imide or methide salts which reduce high temperature capacity fade, reduce aluminum current collector corrosion and improve safety while exhibiting good conductivity, stability and compatibility with other cell components.

The fluorinated sulfonate additive salts useful in this invention are depicted by the formula:

wherein M is a cation; n is an integer from 1 to 3;

R_f is preferably a straight or branched perfluoroalkyl group of 4 to 16 carbon atoms, a perfluorocycloalkyl group or a perfluorocycloalkyl perfluoroalkyl group of 3-7 ring carbon atoms and 1-4 carbon atoms on the alkyl chain, which perfluoroalkyl or perfluorocycloalkyl group may contain one or more heteroatoms R_f may optionally be partially fluorinated with a maximum of 20% of the non- fluorine substituents being hydrogen.

Preferably, M^"" is a cation of an alkali metal, an alkaline earth metal, a transition metal, a rare earth metal, a Group IIB metal or a Group IIIB metal, a nitrogen onium cation, or a proton; more preferably, M^1"" is a cation of an alkali metal; most preferably, M^1"" is a lithium cation.

Suitable fluorinated lithium sulfonate surfactant salts include, for example, C₈Fι₇SO₃Li. Other cations may replace the lithium cation, such as Na⁺, K⁺, Mg⁺², Ca⁺², Ba⁺², Al⁺³, La⁺³, Eu⁺³, Sm⁺³, and H⁺. A nitrogen onium cation includes, for example, a tetraalkylammonium, trialkylammonium, N-alkyl-pyridinium and an N,N'-dialkylimidazolium in which alkyl has 1 to 4 carbon atoms, such as (C₂H₅)₄N⁺, (CH₃)₄N⁺,

The electrolyte composition of the present invention includes a conductive salt different from the sulfonate additive salt. Typically any conventional conductive salt known for electrochemical systems may be used. For example, a conductive salt may include: a cation selected from the group consisting of an alkali metal; an alkaline earth metal; a Group IIB metal; a Group IIIB metal; a transition metal; a rare earth metal; a nitrogen onium cation such as tetraalkylammonium, trialkylammonium, N-alkylpyridinium and N,N'-dialkylimidazolium; and a proton; and an anion selected from the group consisting of an anion of the formula

in which Rn and R ₂ are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with R and R_f2 having a total of up to 5 carbon atoms; and Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms; an anion having a formula R_f4R_f5N-(CF₂)_n'SO₂-X^~;

in which X is -NSO₂Rf₄ or

I (R_f6SO₂)-C-(SO₂R );

Z is -CF₂- -O-,

I

-NRffl , or -SF4-; Rf4 and R_t-₅ , independently, are -C_mF_2m+r or -(CF₂)_q-SO₂-X^~; R« and R_t , independently, are -C_mF_2m+ι , -(CF₂) -SO₂-X^~,

R . ^V N-(CF₂)„-- or

Rβ is -C_mF _m+ι , or -(CF₂)_q-SO₂-X^~; Rrø and R_fr , independently, are perfluoroalkylene moieties having the formula -C_rF_2r-; n' is 1-4; r is 1-4; m is 1— 4; and q is 1-4; a bis-fluoroalkylsulfonyl methide anion R_f ^π-SO₂-C^~(R)-SO₂-R_f ^iπ in which R_f ¹¹ and R_f ^iπ , independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of Rf and R_f ¹¹¹ is up to 5 carbon atoms and R is H, CN, F, alkyl of 1 to 6 carbon atoms, phenyl or phenyl substituted by alkyl of 1 to 4 carbon atoms; a tris-(perfluoroalkanesulfonyl)methide anion of the formula

^~C(SO₂R_f ^II)(SO₂R_f ^III)(SO₂R_f ^IV) in which R_f ^ιr, R_f ^πι, and R_f ^IV, independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R_f ¹¹, R_f ¹¹¹ and R_f ^{1 v} are up to 6 carbon atoms; and cyano-substituted amide and methide anions of the formula: R-[Q-A-(CN)_n]_y

wherein R is as defined hereafter; y is 1 or 2, and A is C or N; when A is C, n is 2 and the compound is a methide; when A is N, n is 1 and the compound is an amide; Q is a linking group selected from -SO - and -C(O)— . Suitable monovalent or divalent organic R groups include a fluorine atom, a hydrocarbon or a fluorinated hydrocarbon group. Preferably, R includes a monovalent or divalent non-fluorinated or fluorinated straight or branched, saturated or unsaturated aliphatic group having 1 to 4 carbon atoms, a cycloaliphatic group of 3 to 6 carbon atoms, a cycloaliphatic-aliphatic group in which the aliphatic group has 1 or 2 carbon atoms, in which the carbon chain of the aliphatic or cycloaliphatic groups is uninterrupted or interrupted by a catenary heteroatom and which the aliphatic or cycloaliphatic group is unsubstituted or substituted by a halogen atom. More preferably, R is a monovalent perfluoroalkyl group of from 1 to 4 carbon atoms. If divalent, a preferred R group is either perfluoroethylene, perfluoropropylene or perfluorobutylene, e.g., -(CF₂)_n-, where n = 2-4.

A more preferred conductive salt includes one that has an anion of the formula:

((R_nSO₂)(R_f2SO₂)N)- or

in which Rn and Rβ are each independen ly a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and Rβ having a total of up to 5 carbon atoms;

Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms. Most preferred conductive salts are lithiuni bis(perfluoromethanesulfonyl)imide, lithium tris(perfluoromethanesulfonyl)methide, lithium bis(perfluoroethanesulfonyl)imide, lithium (perfluorobutanesulfonyl)(perfluoromethanesulfonyl)imide, or a mixture thereof. A particularly preferred electrolyte composition of the present invention involves incorporating, preferably at concentrations of 10% by weight or more based on the weight of the conductive salt, a fluorinated sulfonate additive salt as described above in combination with a matrix material containing a conductive salt having a cation, as described above, preferably lithium, and an anion of the formula

in which Rn and R( are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and R_β having a total of up to 5 carbon atoms;

Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms. A preferred conductive salt is lithium bis(perfluoroethanesulfonyl)imide, or lithium(perfluorobutanesulfonyl)(perfluoromethanesulfonyl)imide.

This particular electrolyte composition has been found especially effective in improving the safety (i.e., preventing thermal runaway) and performance (i.e., excellent electrolyte conductivity, high repassivation potential, and minimal capacity fade during high temperature storage and/or cycling) of lithium ion batteries.

In general, the above described bis(perfluoroalkanesulfonyl)imide and cyclic perfluoroalkylenedisulfonylimide conductive salts are known and can be prepared from the following methods by the reaction of perfluoroalkanesulfonyl fluorides, e.g. R_πSO₂F and RβSO₂F, or perfluoroalkylenedisulfonyl fluoride,

FSO₂RβSO₂F, with anhydrous ammonia. Symmetrical imides in which R and R_f2

* are the same can be prepared in a single step using a non-nucleophilic base such as triethylamine as shown in Scheme I, whereas unsymmetrical imides in which R and R_f2 are different must be prepared in two steps as shown in Scheme II.

SCHEME I

Et₃N 2R_π SO₂F + NH₃ > E^NH^{1" ~}N(SO₂R_fl)₂ + 2Et₃NH⁺ F^"

SCHEME II

Ether H⁺

R_nSO₂F + 3NH₃ > NH₄ ^{+ ~}NH(SO₂Rn') + NH₄ ⁺ F^" -» R_flSO₂NH₂

Et₃N

R_fiSO₂NH₂ + R_c'SO₂F > Et₃NH^{+ ~}1N(SO₂Rn)(SO₂R_Q') + Et₃NHT

Synthesis of Methides

Preferred methides can be prepared according to the procedure shown below RQY + N≡C-CH₂-C≡N + 2B -» RQC^~(C≡N)₂ Bit + BfiT Y~

Synthesis of Amides Preferred amides can be prepared according to the procedure shown below:

RQY + H₂N-C≡N + 2B -» RQN (C≡N) BH + BH" Y^~

where Y is a leaving group such as halogen or tosylate, and B is a non- nucleophilic base such as a tertiary amine, e.g. triethylamine or pyridine. The reaction can also use inorganic bases, such as, for example, solid anhydrous alkali metal carbonates.

In general, the above-described cyano-containing methides and amides containing perfluorosulfonylalkyl groups can be prepared from the reaction of fluoroalkylsulfonyl fluorides, R_fSO₂F, with anhydrous malononitrile and cyanamide, respectively, in the presence of a non-nucleophilic base. This synthetic procedure is described in Scheme 1 of United States Patent Application Serial No. 08/577,425 for making (bis)fluoroalkylsulfonylimides, which is herein incorporated by reference, wherein either the malononitrile or the cyanamide is substituted for the fluoroalkylsulfonamide. The intermediate non-nucleophilic base cation— containing methide or amide salt can be converted to the desired . cation salt (typically lithium) via standard methods known in the art. Obvious variations of this synthetic procedure can be used to make methides and amides containing other R groups, as described in United States Patent Application Serial Number 08/937,519.

Cyclic perfluoroalkylenedisulfonylimide salts can be prepared as described in U.S. Pat. No. 4,387,222, incorporated herein by reference.

Perfluoroalkanesulfonyl fluorides and perfluoroalkylenedisulfonyl fluorides used as precursors to the imide salts of this invention can be prepared by a variety of methods known in the art as described, for example, in U.S. Pat. Nos. 3,542,864; 5,318,674; 3,423,299; 3,951,762; 3,623,963; 2,732,398, S. Temple, J Org. Chem., 33(1), 344 (1968), and D.D. DesMarteau, Inorg. Chem., 32, 5007 (1993), all of which are incorporated herein by reference. U.S. Pat. No. 2,732,398, issued January 24, 1956 to 3M and incorporated herein by reference discloses how to make perfluorinated sulfonyl fluorides of the formula R_tSO₂F, and how to make sulfonic acids and metal salts therefrom, such as the lithium salts. To form the electrolyte composition, the conductive and sulfonate additive salts are, optionally, mixed together with a matrix material, such that the salts are at least partially dissolved or dispersed in each other or in the matrix material. The salts are preferably employed at a concentration such that the conductivity of the electrolyte solution is at or near its maximum value, although a wide range of other concentrations will also serve.

An electrolyte composition may contain as high as 100% total salt concentration where the mixture of conductive and sulfonate salts are inherently liquid at ambient temperature, e.g. 20°C or higher. Such "ionic liquid electrolytes" are described in U.S. Patent No. 5,827,602 which reference is incorporated herein. Such ionic liquids may find application in electrochemical systems such as non- aqueous batteries, electrochemical capacitors, electroplating, and the like.

A matrix material may be used to dissolve the salts and may be in the form of a solid, liquid, gel or a liquid impregnated porous membrane. For battery applications, the matrix material is chosen to provide the particular conductance, viscosity, mechanical strength, reactivity and stability desired for the electrolyte. Suitable matrix materials for preparing electrolyte solutions can be liquid, polymeric or mixtures of polymer and liquid. Examples of suitable solid matrix materials include polymers and copolymers such as polyethers like poly(ethylene oxide), polyesters, polyacrylates, polyphosphazenes, polysiloxanes, poly(propylene oxide), fluoropolymers (e.g., poly(vinylidene fluoride)), and poly(acrylonitrile), as well as the polymers and copolymers described in Armand et al., U.S. Pat. No. 4,505,997, incorporated herein by reference, and mixtures thereof. The polymers may be used in cross-linked or uncross-linked form and plasticized. Such materials are generally dry, i.e., have a water content less than about 100 ppm, preferably less than about 50 ppm.

Mixtures of matrix materials can be employed and are sometimes preferred in tailoring the matrix material's properties to provide optimum performance. In general, the amount of matrix material is selected such that the total salt concentration ranges from about 0.1M (moles per liter) to about 2.0M, preferably about 1 M. Preferably, the conductive salt concentration in the electrolyte is from about 0.5 to 1.5M, and the surfactant salt concentration in the electrolyte is from about 10 to 250 millimoies per liter.

In batteries comprising a highly reducing electrode (such as lithium metal) and a liquid matrix material, the liquid is preferably a nonaqueous, polar, aprotic, organic solvent. Such liquids are generally dry, i.e., have a water content less than about 100 ppm, preferably less than about 50 ppm. Examples of suitable aprotic liquids include linear ethers such as diethyl ether, diethylene glycol dimethyl ether, and 1.2-dimethoxyethane; cyclic ethers such as tetrahydrofuran, 2- methyltetrahydrofuran, dioxane, dioxolane, and 4-methyldioxolane; esters such as methyl formate, ethyl formate, methyl acetate, dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, and butyrolactones (e.g. gamma butyrolactone); nitriles such as acetonitrile and benzonitrile; nitro compounds such as nitromethane or nitrobenzene; amides such as N,N- dimethylformamide, N,N-diethylformamide, and N-methylpyrrolidinone; sulfoxides such as dimethyl sulfoxide; sulfones such as dimethylsulfone; tetramethylene sulfone, and other sulfolanes; oxazolidinones such as N-methyl— 2— oxazolidinone and mixtures thereof. Maximum conductivities of the electrolyte salts of this invention in typical nonaqueous, polar, aprotic liquid media (e.g., propylene carbonate) are generally in the range of 0.1-20 mS (milliSiemens) at room temperature, preferably greater than 1 mS.

A preferred electrochemical system of the present invention relates to a battery that includes at least one cathode, at least one anode, a separator and liquid electrolyte comprising certain fluorinated sulfonate additive salts, conductive salts and aprotic solvents.

The electrodes (i.e., anode and cathode) of, for example, a lithium battery generally consist of a metallic foil or particles of active material blended with a conductive diluent such as carbon black or graphite bound into a plastic material binder. Typical binders include polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene (EPDM) terpolymer, and emulsified styrene-butadiene rubber (SBR), and the binder may be cross-linked^ The binder may also be, for example, a solid carbon matrix formed from the thermal decomposition of an organic compound. The metallic foil or composite electrode material is generally applied to an expanded metal screen or metal foil (preferably aluminum, copper or nickel) current collector using a variety of processes such as coating, casting, pressing or extrusion.

Examples of suitable battery anodes include lithium metal, lithium metal alloys., sodium metal, carbon-based materials such as graphite, coke, carbon fiber, pitch, transition metal oxides (such as LiTi₅Oι₂ and LiWO₂), and lithiated tin oxide. In the case of lithium ion batteries, the lithium may be intercalated into a host material such as carbon (i.e., to give lithiated carbon) or carbon alloyed with other elements (such as silicon, boron and nitrogen), a conductive polymer, or an inorganic host that is intercalatable (such as Li_xTi₅Oι₂). The material comprising the anode may be carried on foil (e.g., nickel and copper) backing or pressed into expanded metal screen and alloyed with various other metals.

Examples of suitable cathode materials include graphite, amorphous carbon, Li_xCoO₂, Li_xNiO₂, Co-doped Li_xNiO₂, Li_xMn₂O₄, Li_xMnO₂, V₂O₅, V₆Oι₃, LiN₃O₈, Ba₂SmΝiO₅, SmMnO₃, Sm₃Fe₅O₁₂, EuFeO₃, EuFe₅Oι₂, EuMnO₃, LaNiO₃, La₂CoO₄ and LaMnO₃ (including the charged and discharged forms of these materials), and conducting polymers such as polypyrrole, polysulfides and polyvinylferrocene. In primary batteries, the cathode can be fluorinated carbon (e.g., (CF)_n), SO₂Cl₂, Ag₂V Oπ, Ag₂CrO , sulfur, polysulfide, and an O₂ or SO₂

» electrode. Lithium batteries and supercapacitors usually contain a separator to prevent short-circuiting between the cathode and anode. The separator usually consists of a single-ply or multi-ply sheet of microporous polymer (typically polyolefin, e.g., polyethylene, polypropylene, or combinations thereof) having a predetermined length and width and having a thickness of less than 10 mils (0.025 cm). For example, see U.S. Pat. Nos. 3,351,495 (Larsen et al.), 4,539,256 (Shipman et al.), 4,731,304 (Lundquist et al.) and 5,565,281 (Yu et al.). The pore size in these microporous membranes, typically about 5 microns in diameter, is sufficiently large to allow transport of ions but is sufficiently small to prevent cathode/anode contact, either directly or from particle penetration or dendrites which can form on the electrodes.

The invention is illustrated further by, but is not intended to be limited to, the following examples. EXAMPLES

Examples 1-3 and Comparative Examples C1-C3

This series of examples was run to demonstrate the improved high temperature capacity fade performance achieved when various amounts of lithium perfluorooctanesulfonate salt (PFOS) are incorporated into an electrolyte comprising lithium bis(perfluoroethanesulfonyl)imide (BETI). LiPF₆ (Cl), a commonly used electrolyte salt, was used as a reference. BETI alone (C2) was used as a control. PFOS (C3) alone was used as second control. BETI plus PFOS at various levels (Examples 1-3) demonstrates the additive. effect in reducing capacity fade. Results are presented in Table 1 below.

The half-cell "1225" coin cell used for this set of experiments was constructed as follows. Lithium metal was used as the anode and SFG44 graphite was used as the cathode. The SFG44 cathode composition, consisting of 85% (wt) SFG44 graphite (available from Timcal Co., West Lake, Ohio), 3% (wt) Super P (available from MMM carbon, Antwerp, Belgium), 12% (wt) polyvinylidene fluoride resin (PVDF) (available from Elf Atochem North America) and 0.1% (wt) oxalic acid, was coated onto a copper substrate, dried at 120°C under vacuum for 12 hours, and was cut into circular disks having an area of 0.44 cm². Electrolyte solutions were prepared by dissolving dry conductive salts (either BETI or LiPF₆, having a water content of less than 30 ppm as determined by Karl Fisher titration) at 1M concentration in a dry solvent blend consisting of 50/50 (vol/vol) ethylene carbonate/dimethyl carbonate (available from Grant Chemicals, San Leandro, California). PFOS was added to a 1M BETI electrolyte at 10% by weight based on the weight of the conductive salt. Coin cells were then assembled in the following order: (1) stainless steel can top, (2) copper current collector, (3) lithium anode, (4) PE 9711 Cotran membrane, (5) polypropylene gasket, (6) about 30 μL of electrolyte, (7) SFG 44 cathode, (8) copper current collector and (9) stainless steel can bottom. Graphite lithiation was done by a discharge, charge, and final discharge process. Each cell was then stored for 24 hours at room temperature to achieve equilibrium. A constant current discharge and charge of 0.45 mA/cm with voltage limits of 0.0 V and 2.0 V, respectively, was utilized. Table 1 Capacity Retention

Examples 5, 6 and Comparative Examples C4, C5

This series of examples was run to demonstrate the improved high temperature capacity fade performance achieved when 10% (wt%) PFOS is incorporated into an electrolyte comprising tris(trifluoromethanesulfonyl)methide salt (LiC(SO₂CF₃)₃) (C4) or a cyclic imide C5. The methide alone (C4) or the cyclic imide alone (C5) were used as controls. Examples 5 and 6 demonstrate the additive effect in reducing capacity fade. Results are presented in Table.2 below.

The half cells were constructed and tested as in examples 1-3 and comparative examples C1-C3 above. Table 2 Capacity Retention

^♦average of 10 cycles at room temperature Examples 7-9 and Comparative Example C6

This series of examples was run to demonstrate the improved repassivation potential achieved when various levels of PFOS salt are incorporated into an electrolyte comprising BETI.

In Comparative Example C6, the repassivation potential for 1M BETI in a 50/50 (vol) blend of ethylene carbonate/dimethyl carbonate was measured (no PFOS salt was added to the electrolyte).

In Examples 7-9 various levels of PFOS by weight were added to 1M BETI in a 50/50 (vol) blend of ethylene carbonate/dimethyl carbonate. The repassivation potential and corrosion current was measured for each electrolyte as described in the Repassivation Potential Test Procedure.

Results are presented in Table 3 below.

Table 3 Repassivation Potential

Examples 10-11 and Comparative Example C7

This exothermic energy test was run to simulate the Underwriter's Laboratory hot box test, which measures a battery's performance after exposure to 150°C storage conditions. Specifically, 10%) straight (Example 10) and branched chain (Example 11) PFOS were evaluated as additives to an electrolyte containing (C₂F₅SO )₂N- Li⁺ (BETI) to determine their effect on exothermic energies measured up to 200°C in a half-cell "1225" coin cell - relative to control electrolyte containing BETI (Comparative Example C7).

The half-cell "1225" coin cell used for this set of experiments was constructed as follows. Lithium metal was used as the anode and SFG44 graphite was used as the cathode. The SFG44 cathode composition, consisting of 85% (wt) SFG44 graphite (available from Timcal Co., West Lake, Ohio), 3% (wt) Super P (available from MMM carbon, Antwerp, Belgium), 12% (wt) polyvinylidene fluoride resin (PVDF) (available from Elf Atochem North America) and 0.1 % (wt) oxalic acid, was coated onto a copper substrate, dried at 120°C under vacuum for 12 hours, and was cut into circular disks having an area of 0.44 cm². Electrolyte solutions were prepared by dissolving dry BETI having a water content of less than 30 ppm as determined by Karl Fisher titration) at 1M concentration in a dry solvent blend consisting of 50/50 (vol/vol) ethylene carbonate/dimethyl carbonate (available from Grant Chemicals, San Leandro, California). PFOS salts were added to a 1M BETI electrolyte at 10% by weight based on the weight of BETI. Coin cells were then assembled in the following order: (1) stainless steel can top, (2) copper current collector, (3) lithium anode, (4) PE 9711 Cotran membrane, (5) polypropylene gasket, (6) about 30 μL of electrolyte, (7) SFG 44 cathode, (8) copper current collector and (9) stainless steel can bottom. Graphite lithiation was done by a discharge, charge, and final discharge process. Each cell was then stored for 24 hours at room temperature to achieve equilibrium. A constant current discharge and charge of 0.45 mA/cm² with voltage limits of 0.0 N and 2.0 V, respectively, was utilized.

After the discharge/charge/discharge cycle was run, each coin cell was taken to an Argon-filled dry box and the lithiated graphite electrodes were removed and cut into circular samples 2.9 mm in diameter. The pieces were weighed and then placed into sealed aluminum sample pans, and the pans with samples were transferred to an Argon-filled differential scanning calorimetry

(DSC) chamber. DSC measurements were conducted at a 10°C/min heating rate from room temperature (usually about 27°C) up to 500°C.

The data in Table 4 show that the addition of PFOS salt to the 1M BETI electrolyte decreased the amount of exothermic energy released.

Table 4 Exothermic Energies

Examples 12-13 and Comparative Examples C2. C8 and C9

This series of examples was run to demonstrate the improved high temperature capacity fade performance achieved when 10% of lithium perfluorobutanesulfonate salt (PFBS) or 10%o of lithium perfluorodecanesulfonate salt (PFDS) are incorporated into an electrolyte comprising lithium bis(perfluoroethanesulfonyl)imide (BETI). BETI alone (C2) was used as a control. PFBS (C8) and PFDS (CIO) alone were used as additional controls. BETI plus PFBS or PFDS at 10% (Examples 12-13) demonstrates the additive effect in reducing capacity fade. Results are presented in Table 5 below. Cell construction and cycling were done as described in Examples 1-3.

Table 5 Capacity Retention

SYNTHESIS. SOURCE OF FLUORINATED IMIDE AND SULFONATE SALTS

Li⁺ PF_ή ^"

Li⁺ PF₆ ^~ (high purity, battery grade) was purchased from Hashimoto Chemical Co., Ltd. through Biesterfeld U.S., Inc., a U.S. distributor.

(C F SO_{7 z}N^" Li⁺

(C₂F₅SO₂)₂N^~ Li⁺ was prepared using the procedure described in Example 3 of U.S. Pat. No. 5,652,072, which is herein incorporated by reference. CsFπSOiLi

C₈Fι SO₃Li used was material commercially available from 3M as FC-94™ which contains about 30% branched and 70% straight chain material.

C₄F₉SO₃Li and Cι₀F₂ιSO₃Li were prepared using the procedure described in Example 5 of U.S. Pat. No. 2,732,398 for the octyl material by substituting butylsulfonyl chloride and decylsulfonylchloride respectively for octanesulfonyl chloride. The mixture resulting from the electrochemical fluorination was then subjected to the following hydrolysis procedure. The sulfonyl fluorides were hydrolyzed by mixing them with IP A (about 10-20% by weight sulfonyl fluoride) and then adding 1 equivalent of 1M LiOH (high purity, Aldrich). This was stirred until it formed a single phase (usually overnight) and dried under a stream of nitrogen followed by vacuum.

Branched and straight CSFJ SOTLJ

A mixture of branched and straight C₈F₁₇SO₃Li was made from the

> hydrolyzed mixture as described in Example 5 of U.S. Pat. No. 2,732,398. The straight chain material was separated from the mixture of straight chain and branched chain by cooling the hydrolyzed mixture to -30C and filtering the crystallized straight chain isomer from the liquid mixture through a glass frit at -30°C. The solid was washed with a small amount of cold acetone. This procedure was repeated. The remaining uncrystallized material had an increase (about 35%> from about 30%) amount of branched sulfonate. The ratio of straight to branched was determined by F19 NMR. The straight chain material used was <5% branched and the branched chain material used was >35% branched (see CsEπSO LTL

BATTERY SEPARATORS The membranes employed in the following test methods were prepared as follows: For comparative examples C1-C3 and examples 1-3:

PP 1 1 17-19 Membrane Preparation

A polypropylene resin, available from Union Carbide under a designation DS 5D45 with a melt flow index of 0.65 dg/min (ASTM D1238, Condition I), was fed into the hopper of a 40 mm twin-screw extruder. Mineral oil, available under a trade designation Amoco White Mineral Oil #31 and having a viscosity of 60 centistokes (ASTM D445 at 40°C) (available from Amoco Petroleum Products, Oak Brook, Illinois), was introduced into the extruder through an injection port at a rate to provide a composition of 60%) by weight of the polymer and 40%) by weight mineral oil. The composition also contained 0.28% Millad™ 3905 (available from Milliken & Co., Spartanburg, South Carolina) nucleating agent. The overall feed rate was 11.35 kg/hr. The polymer was heated to 271 °C in the extruder to melt and, after mixing with oil, the temperature was maintained at 177°C during the extrusion. The melt was extruded through a 38.1 cm-wide coat hanger slit die and cast onto a casting wheel maintained at 60°C. The cast film was extracted with HCFC-123 (LNertrel™ 423, C₂HF₃C1₂, duPont) to remove mineral oil, then oriented 2.7 to 1 in the machine direction at 90°C and 1.5 to 1 in the cross- direction at 138°C.

PE 9711 Cotran Membrane Preparation A polyethylene resin, available from Fina Chemicals under a designation

Fina 1285 with a melt flow index of 8 dg/min (ASTM D 1238-90, Condition F), was fed into the hopper of a 40 mm twin-screw extruder. Mineral oil, available under a trade designation Witco Protol and having a viscosity of 36 centistokes (ASTM D445 at 40°C) (available from Witco Corp., Greenwich, Connecticut) was introduced into the extruder through an injection port at a rate to provide a composition of 39% by weight of the polymer and 61%) by weight mineral oil. The overall feed rate was 10.7 kg/hr. The polymer was heated to 271°C in the extruder to melt and, after mixing with oil, the temperature was maintained at 204°C during the extrusion. The melt was extruded through a 38.1 cm-wide coat hanger slit die and cast onto a casting wheel maintained at 66°C. The cast film was extracted with HCFC-123 to remove mineral oil, then oriented 2.3 to 1 in the machine direction at 35°C and 2.2 to 1 in the cross-direction at 102°C.

For all other examples a SETELA® E25MM microporous polyethylene film (Mobil Chemical Company Films Division, Pittsford, ΝY) was used. PREPARATION OF ELECTRODES Cathode

A mix was prepared by blending 44 g of LiCoC^ (FMC, Bessemer City, North Carolina) with 5.0 g of VXC72 conductive carbon (Cabot Corp., Billesica, Massachusetts) and 1.0 g of Kynar 461 polyvinylidene fluoride resin (Elf Atochem North America, Philadelphia, Pennsylvania) in a small food processor for 2 minutes. The food processor is a small single-speed common household food processor. Portions of the resulting mix were pressed into pellets using a pellet die and a Carver press at about 2000 pounds of force. The pellet die is a steel cylinder body 3 cm diameter by 2.5 cm height with a central bore 7.5 mm in diameter and a steel pin 3.75 cm in length and 7.47 cm in diameter with flat ends. The body of the die was placed on a flat surface and loaded with a measured weight of mix. The pin was inserted and the whole assembly placed in the Carver press. The resulting pellet electrodes averaged 7.6 mm in diameter, 0.28 mm in height and 33 mg in weight. The cathode was dried at 100°C in a vacuum oven for 17 hours prior to assembly.

Anode

Anode pellets were prepared in a manner identical to the cathode procedure except that the ingredients of the mix were 21 g of XP3 petroleum coke (Conoco, Ponca City, Oklahoma), 1.2 g of Super S conductive carbon (MMM Carbon, Brussels, Belgium), and 3 g of Kynar 461 polyvinylidene fluoride. The resulting pellet electrodes averaged 7.7 mm in diameter, 0.35 mm in height and 27 mg in weight. The cathode was dried at 100°C in a vacuum oven for 17 hours prior to assembly.

TEST METHODS

Coin Cell Charge/Discharge Cycling To demonstrate electrolyte performance in a real test battery, charge/discharge cycling tests were run with a "1225" size coin cell, measuring capacity using a commercial battery tester available from Maccor Inc., Tulsa, Oklahoma. The "1225" coin cell stack assembly was constructed as shown in the Figure. A stainless steel top 1, 12 mm in diameter, was placed on a horizontal surface flat side down. On this base were stacked in order a 31 mil (0.80 mm) thick copper disk anode current collector 2 and an anode 3 prepared as described above. 240 μl of the test electrolyte solution (consisting of test salt(s) dissolved in a 50/50 (vol) blend of ethylene carbonate/dimethyl carbonate, dried to a water content of no greater than 50 ppm, as determined by Karl Fischer titration) was applied to the graphite side of the anode surface, then two layers of #9711 polyethylene, 2.2 mils, separator 4 (prepared as described above) were placed on the wet anode surface. A polypropylene spacer gasket 5 was inserted. To complete the assembly, cathode 6 prepared as described above was placed on the stack, 240 μl of additional electrolyte were added, followed by a 20 mil (0.51 mm) thick aluminum disk cathode current collector 7 and a chromium steel can cell 8. The assembly was then crimp sealed to complete the fabrication of the "1225" coin cell.

The constructed "1225" coin cell was then cycled at room temperature using a Maccor® Series 2000 battery tester (available from Maccor Inc., Tulsa OK) with appropriate current and voltage range operated with generation 3.0 software designed to charge the cell at a current density of no greater than 2 mA cm under a constant voltage of 4.2 V, followed by discharge under a constant current of lmA/cm², with two 30 minute interrupts (i.e., no current flow, circuit opened up) when the cell voltage reached 3.8 N and 3.0 N, respectively; total discharge time was typically 3 hours

For a given electrolyte formulation, the cells were cycled 15 times at room temperature. Then the cells were brought to 60 C and two cycles were run at charge (4.2V) and discharge (open circuit for 72 hours- full charge storage). After the 72 hour storage period at 60 C with open circuit, the cells were discharged and two additional charge/discharge cycles were run at 60°C under the same current rate and limited voltage conditions as were run during the 15 cycles at room temperature. The cells were then returned to room temperature and cycled again at least 5 times at room temperature. The process of cycling and storage at 60°C followed by cycling at room temperature was repeated up to 6 times (i.e. up to 6 "macrocycles" as referenced in the Tables) for each electrolyte combination, and the % capacity retention after the second up to the sixth macrocycle was calculated as a percentage of discharge capacity (mAh/g) measured after the first cycle.

The impedance characteristics of the cells before cycling were determined using a Princeton Applied Research Model 273 potentiostat and a Solatron® FRA SI 1260 frequency response analyzer equipped with an 11287 electrochemical interphase (both available from Solatron, a Division of Solatron Group Ltd., Houston, TX). Scans were made at an amplitude of 5 millivolts over a frequency range of 100,000 Hz to 0.1 Hz.

Repassivation Potential Test Procedure

The repassivation potential of the candidate salt was measured using a cyclic voltammetry test employing aluminum as a working electrode, using the technique generally described in Bard and Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley and Sons, New York, 1980, pp. 350- 353. The repassivation potential is an excellent predictor of the degree of corrosion to be expected when aluminum is used in an electrode, especially as a current collector.

For each cyclic voltammetry measurement, a three-electrode cell was used, having polished aluminum as the working electrode, metallic lithium as the reference electrode and platinum plate as the counter electrode. The aluminum electrode consisted of a 99.9% pure aluminum rod inserted into a polytetrafluoroethylene sleeve to provide a planar electrode having an area of 0.07 cm². Prior to running each cyclic voltammetry test, the native metal oxide layer was removed from the aluminum electrode by polishing the electrode with 3 μm aluminum oxide paper using heptane as a lubricant. A lithium wire inserted in a luggin glass capillary served as a reference electrode, and a 10 cm platinum flag was used as the auxiliary electrode.

After polishing, the three electrodes and a glass cell for holding the electrolyte were all placed in an argon dry box (water and oxygen level less than 1 ppm), and the three electrodes were connected to a potentiostat. Each electrolyte salt to be evaluated was dissolved at 1M concentration in a 1 : 1 (vol) blend of ethylene carbonate: dimethyl carbonate to form the test electrolyte (containing less than 50 ppm water, as determined by Karl Fischer titration), and 10 mL of each test electrolyte was placed in the glass cell. A scan at the rate of approximately 0.5 mV/sec was taken from 1 V up to at least 5 V (vs. the reference electrode), followed by gradually returning the potential to 4 V, and the current was measured as a function of voltage potential. The repassivation potential was defined as that voltage at which the measured current of the hysteresis loop fell precipitously back to a value close to the currents measured during the early part of the forward scan (i.e., the point of inflection on the curve). The corrosion current was also measured at each repassivation potential.

Claims

WE CLAIM:

1. An electrolyte composition comprising:

(a) a conductive salt comprising a cation selected from the group consisting of an alkali metal; an alkaline earth metal; a Group IIB metal; a Group IIIB metal; a transition metal; a rare earth metal; a nitrogen onium cation, and a proton; and an anion selected from the group consisting of an anion of the formula

((R_flSO₂)(R_GSO₂)N)- or

in which Rn and RQ are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with R_\ and Re having a total of up to 5 carbon atoms; and Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to

2 carbon atoms, with Rβ having a total of up to 4 carbon atoms; an anion of the formula R_f4R_f5N-(CF₂)n'SO₂-X', or

in which X is -O , -N^~SO₂R_f , or

I

(R_f6SO₂)-C--(SO₂R_f7);

Z is -CF₂- -O-,

I

-NRffi ,

or -SF₄-; R_f4 and Rβ , independently, are -C_mF_2m+ι' or -(CF₂)_q-SO₂-X ; i R_f6 and Rn , independently, are -C_mF_2m+ι , -(CF₂)₄-SO₂-X , Rf.

^ N-(CF₂)„.- or n /

in which R is -C_mF_2m+ι, or -(CF₂)_q-SO₂-X^~; Rrø and R^. , independently, are perfluoroalkylene moieties having the formula -C_rF_2r-; n' is 1-4; r is 1-4; m is 1-4; and q is 1-4; a bis-fluoroalkylsulfonyl methide anion R_f ¹ -SO₂-C^"(R)-SO₂-R_f ^m in which Rf¹¹ and R_^m , independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R₄ ¹¹ and R ^π is up to 5 carbon atoms, and R is H, CN, F, alkyl of 1 to 6 carbon atoms, phenyl or phenyl substituted by alkyl of 1 to 4 carbon atoms; a tris-(perfluoroalkanesulfonyl)methide anion of the formula ^"C(SO₂R_f ^II)(SO₂R_f ^III)(SO₂R_f ^IV) in which R_f ^π, R_f ^m, and R_f ^IV, independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R ¹, R ¹¹ and R_f ^IV are up to 6 carbon atoms; a cyano-substituted amide or methide anion of the formula

R-[Q-A(CN)_n]_y where R is a fluorine atom, a monovalent or divalent non- fluorinated or fluorinated straight or branched, saturated or unsarurated aliphatic group having 1 to 4 carbon atoms, a cycloaliphatic group of 3 to 6 carbon atoms, a cycloaliphatic-aliphatic group in which the aliphatic group has 1 or 2 carbon atoms, in which the carbon chain of the aliphatic or cycloaliphatic groups is uninterrupted or interrupted by a catenary heteroatom and which the aliphatic or cycloaliphatic group is unsubstituted or substituted by a halogen atom; y is 1 or 2; A is C or N; and when A is N, n is 1, and when A is C, n is 2; and Q is a linking group selected from -SO₂- and -C(O)-; (b) an additive sulfonate salt of the formula

R_fSO₃M wherein M is a cation; Rf is a straight or branched perfluoroalkyl group of 4 to 16 carbon atoms, a perfluorocycloalkyl group or a perfluorocycloalkyl perfluoroalkyl group of 3-7 ring carbon atoms and 1-4 carbon atoms on the alkyl chain, which perfluoroalkyl or perfluorocycloalkyl group may contain one or more heteroatoms, or R_f is a partially fluorinated group as above defined with a maximum of 20% of the non-fluorine atoms being hydrogen; and wherein the molar ratio of conductive salt to surfactant salt is between about 99.9:0.1 to about 70:30.

2. The composition of claim 1 , wherein R_f is a straight or branched perfluoroalkyl group of 6 to 10 carbon atoms.

3. The composition of claim 1 , wherein R_f has at least 8 carbon atoms.

4. The composition of claim 1, wherein the additive sulfonate salt has a cation M selected from the group consisting of an alkali metal, an alkaline earth metal, a transition metal, a rare earth metal, a group IIB metal, a Group IIIB metal,

» tetraalkylammonium, N-alkylpyridinium and N,N'-diaIkylimidazolium.

5. The composition of claim 4, wherein the cation is selected from the group consisting of Li⁺, Na⁺, Ca⁺², Ba⁺², Al⁺³, Eu⁺³, Sm⁺³, (CzH^N^1", (CH₃)₄N⁺,

6. The composition of claim 1 , wherein the additive sulfonate salt is

C₈F₁₇SO₃Li

7. ^■ The composition of claim 1 , wherein the molar ratio of conductive salt to additive sulfonate salt is between about 99:1 and about 90:10.

8. The composition of claim 1 , wherein the conductive salt has an anion of the formula: ((RfiSO₂)(R_BSO₂)N)- or

in which R and Rf2 are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and Rβ having a total of up to 5 carbon atoms;

Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms.

9. The composition of claim 1 , wherein the conductive salt is a member selected from the group consisting of lithium bis(perfluoromethanesulfonyl)imide, lithium tris(perfluoromethanesulfonyl)methide, lithium bis(perfluoroethanesulfonyl)imide, lithium (perfluorobutanesulfonyl)(perfluoromethanesulfonyl)imide, and a mixture thereof.

10. The composition of claim 1 , wherein the conductive salt is lithium bis(perfluoroethanesulfonyl)imide.

11. An electrochemical system comprising: at least one positive electrode; at least one negative electrode; and an electrolyte composition comprising:

(a) a conductive salt comprising: a cation selected from the group consisting of an alkali metal; an alkaline earth metal; a Group IIB metal; a Group IIIB metal; a transition metal; a rare earth metal; a nitrogen onium cation, and a proton; and an anion selected from the group consisting of an anion of the formula

in which Rn and Rβ are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and Rβ having a total of up to 5 carbon atoms; and Rβ is a perfluoroalkylene moiety of 2 to 4 carbon atoms optionally substituted by a straight or branched perfluoroalkyl group of 1 to 2 carbon atoms, with Rβ having a total of up to 4 carbon atoms; an anion having a formula R_f RβN-(CH₂)_π-SO - X-;

Z N-(CF₂)_n>SO₂-X^" Rϋ

in which XT is -O , -N SO₂R_f4 or

(R_f6SO₂)-C--(SO₂R_f7);

Z is -CF₂-, -O-,

-NRffl ,

or -SF₄-; R_f4 and R_f5 , independently, are -C_mF_2m+ι , or -(CF₂)_q- SO₂-X^~; R_f6 and R_f/ , independently, are -C_mF_2m+ι' -(CF₂)₄-SO₂-

RΩ N-(CF₂)„.- or

5 in which R_β is -C_mF_2m+ι , or -(CF₂)_q-SO₂-X^~; R_f6. and R_fy , independently, are perfluoroalkylene moieties having the formula - C_rF_2r-; n' is 1-4; r is 1-4; m is 1-4; and q is 1-4; a bis-fluoroalkylsulfonyl methide anion R_f ^π-SO₂-C^~(R)- SO₂-Rf^IH in which R_f ^π and R_f ^Iπ , independently, are perfluoroalkyl 10 groups having between 1 and 4 carbon atoms, wherein the sum of

R_f ^l and R ^π is up to 5 carbon atoms, and R is H, CN, F, alkyl of 1 to 6 carbon atoms, phenyl or phenyl substituted by alkyl of 1 to 4 carbon atoms; a tris-(perfluoroalkanesulfonyl)methide anion of the formula 15 ^"C(SO₂R_f ⁿ)(SO₂R_f ^πι)(SO₂R_f ^IV) in which R_f", R_f ^ιπ, and R_f ^IV, independently, are perfluoroalkyl groups having between 1 and 4 carbon atoms, wherein the sum of R_f ¹¹, R ^π and R_f ^IV are up to 6 carbon atoms; and a cyano-substituted'amide or methide anion of the formula 20 R-[Q-A(CN)„]_y wherein R is a fluorine atom, a monovalent or divalent non- fluorinated or fluorinated straight or branched, saturated or unsaturated aliphatic group having 1 to 4 carbon atoms, a cycloaliphatic group of 3 to 6 carbon atoms, a cycloaliphatic- 25 aliphatic group in which the aliphatic group has 1 or 2 carbon atoms, in which the carbon chain of the aliphatic or cycloaliphatic groups is uninterrupted or interrupted by a catenary heteroatom and which the aliphatic or cycloaliphatic group is unsubstituted or substituted by a halogen atom; 30 y is 1 or 2;

A is C or N; and when A is N, n is 1, and when A is C, n is 2; and

Q is a linking group selected from -SO₂- and -C(O)-; (b) an additive sulfonate salt of the formula

R_rSO₃M wherein M is a cation;

Rf is a straight or branched perfluoroalkyl group of 4 to 10 carbon atoms, a perfluorocycloalkyl group or a perfluorocycloalkyl perfluoroalkyl group of 3-7 ring carbon atoms and 1-4 carbon atoms on the alkyl chain, which perfluoroalkyl or perfluorocycloalkyl group may contain one or more heteroatoms, or R_f is a partially fluorinated group as above defined with a maximum of 20% of the non-fluorine atoms being hydrogen; and wherein the molar ratio of conductive salt to surfactant salt is between about 99.9:0.1 to about 70:30.

12. The system of claim 11 , which further comprises a non-aqueous matrix material.

13. The system of claim 11 , wherein the molar ratio of conductive salt to additive sulfonate salt is between about 99: 1 and about 90: 10.

14. The system of claim 11 , wherein the additive sulfonate salt has a cation M selected from the group consisting of an alkali metal, an alkaline earth metal, a transition metal, a rare earth metal, a group IIB metal, a Group IIIB metal,

» tetraalkylammonium, trialkylammonium, N-alkylpyridinium, N,N'- dialkylimidazolium, and a proton.

15. The system of claim 14, wherein the cation is selected from the group consisting of Li⁺, Na⁺, Ca⁺², Ba⁺², Al⁺³, Eu⁺³, Sm⁺³, (C₂H₅)₄N⁺, (CH₃)₄N⁺,

16. The system of claim 11 , wherein the additive sulfonate salt is

C₈F₁₇SO₃Li.

17. The system of claim 11 , wherein the electrolyte composition comprises:

(a) a conductive salt at a concentration from about 0.5 to 1.5 mole per liter; and

(b) an additive sulfonate salt at a concentration from about 10 to 100 millimole per liter.

18. The system of claim 11 , wherein the conductive salt has an anion of the formula:

in which Rn and Rβ are each independently a straight or branched perfluoroalkyl group of 1 to 4 carbon atoms, with Rn and Rβ having a total of up to 5 carbon atoms;

19. The system of claim 11 , wherein the conductive salt is lithium bis(perfluoroethanesulfonyl)imide.

20. The system of claim 11 , being a battery or rechargeable battery.

21. The battery or rechargeable battery of claim 20, comprising a lithium-intercalated carbon anode and a metal oxide cathode.

22. A method of improving safety and performance of an electrochemical system which comprises using an electrolyte composition according to claim 1 in said system.

23. The method of claim 22, wherein the system is a battery or rechargeable battery.

24. The method of claim 22, wherein the system is a lithium battery.

25. The method of claim 22, wherein the electrolyte composition comprises 10% by weight or more of the additive sulfonate salt based on the weight of the conductive salt.

26. A method of improving safety and performance of an electrochemical system which comprises incorporating an effective amount of an electrolyte composition according to claim 8.

27. A method of improving aluminum corrosion in an electrochemical system comprising incorporating in a matrix material an electrolyte composition according to claim 1 having up to 30 mole % of sulfonate additive salt, based on total salt content.

28. A method of improving high temperature capacity fade of electrochemical systems comprising incorporating in a matrix material an electrolyte composition according to claim 1 having up to 30 mole %> of the sulfonate additive salt, based on total salt content.