Polyvinylpyridine additives for nonaqueous electrolytes activating lithium rechargeable electrochemical cells

Abstract

An electrolyte comprising an organic solvent, a lithium salt, and a polymer additive comprised of repeating vinyl units joined to one or more heterocyclic amine moieties is described. The heterocyclic amine contains five to ten ring atoms, inclusive. An electrochemical cell is also disclosed. The preferred cell comprises a negative electrode which intercalates with lithium, a positive electrode comprising an electrode active material which intercalates with lithium, and the electrolyte of the present invention activating the negative and the positive electrodes.

Claims

1. An electrolyte for an electrochemical cell, the electrolyte comprising: a) a linear carbonate mixture of dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC), wherein by volume percent, DMC is in the range of from about 5% to about 75%; EMC is in the range of from about 5% to about 50%; and DEC is in the range of from about 3% to about 45%; b) ethylene carbonate (EC) as a cyclic carbonate, wherein the EC is in the range of from about 10% to about 50% by volume percent; c) a lithium salt; and d) poly(2-vinylpyridine-co-styrene) at a concentration of from about 0.05 to less than 1.0 weight percent.

2. The electrolyte of claim 1 wherein the poly(poly(2-vinylpyridine-co-styrene) comprises a molecular weight of 5,000.

3. The electrolyte of claim 1 wherein the lithium salt is selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4, LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3, LiNO.sub.3, LiN (SO.sub.2CF.sub.3) .sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3, LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3,Li SO.sub.3F, LiB (C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures thereof.

4. The electrolyte of claim 1 further comprising an additional additive selected from the group consisting of vinylene carbonate, fluorinated ethylene carbonate, biphenyl, cyclohexyl benzene, and combinations thereof.

5. The electrolyte of claim 1 not including a biphenyl compound.

6. An electrochemical cell comprising: a) a negative electrode comprising an anode active material which intercalates with lithium; b) a positive electrode comprising a cathode active material which intercalates with lithium; c) a separator intermediate the negative and positive electrodes to thereby prevent the electrodes from direct physical contact with each other; and d) an electrolyte activating the negative and positive electrodes, the electrolyte comprising: i) a linear carbonate mixture of dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC), wherein by volume percent, DMC is in the range of from about 5% to about 75%; EMC is in the range of from about 5% to about 50%; and DEC is in the range of from about 3% to about 45%; ii) ethylene carbonate (EC) as a cyclic carbonate, wherein the EC is in the range of from about 10% to about 50% by volume percent; iii) a lithium salt; and iv) poly(2-vinylpyridine-co-styrene) at a concentration of from about 0.05 to less than 1.0 weight percent.

7. The electrochemical cell of claim 6 wherein the poly(2-vinylpyridine-co-styrene) comprises a molecular weight of 5,000.

8. The electrochemical cell of claim 6 wherein the electrolyte does not include a biphenyl compound.

9. The electrochemical cell of claim 6 wherein the anode active material comprises a carbonaceous material selected from coke, graphite, acetylene black, carbon black, glassy carbon, carbon nanotubes, and meso-carbon microbeads.

10. The electrochemical cell of claim 9 wherein the carbonaceous material is fibrous.

11. The electrochemical cell of claim 6 wherein the negative electrode comprises from about 90 to 97 weight percent graphite as the anode active material mixed with from about 3 to 10 weight percent of a fluoro-resin binder.

12. The electrochemical cell of claim 6 wherein the negative electrode comprises a current collector selected from the group consisting of copper, stainless steel, titanium, tantalum, platinum, gold, aluminum, nickel, cobalt nickel alloy, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium-, and molybdenum-containing alloys.

13. The electrochemical cell of claim 6 wherein the positive electrode comprises from about 90 to 97 weight percent LiCoO.sub.2 as the cathode active material mixed with from about 1 to 10 weight percent of a conductive additive and from about 3 to 10 weight percent of a fluoro-resin binder.

14. The electrochemical cell of claim 6 wherein the cathode active material is selected from the group consisting of LiNiO.sub.2, LiMn.sub.2O.sub.4, LiCoO.sub.2, LiCo.sub.0.92Sn.sub.0.08O.sub.2, lithium cobalt nickel oxide, and LiMPO.sub.4 (where M is one or metals from columns 4 through 13 of the Periodic Table of the Elements).

15. The electrochemical cell of claim 6 wherein the lithium salt is selected from the group consisting of LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4, LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3, LiNO.sub.3, LiN (SO.sub.2CF.sub.3) .sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3, LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3, LiSO.sub.3F, LiB(C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures thereof.

16. A method for making an electrochemical cell, comprising the steps of: a) providing a negative electrode of an anode active material which intercalates with lithium; b) providing a positive electrode comprising a cathode active material which intercalates with the lithium; c) disposing a separator intermediate the negative and positive electrodes to thereby prevent the electrodes from direct physical contact with each other; and d) activating the negative and positive electrodes with a nonaqueous electrolyte comprising: i) a linear carbonate mixture of dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC), wherein by volume percent, DMC is in the range of from about 5% to about 75%; EMC is in the range of from about 5% to about 50%; and DEC is in the range of from about 3% to about 45%; ii) ethylene carbonate (EC) as a cyclic carbonate, wherein the EC is in the range of from about 10% to about 50% by volume percent; iii) a lithium salt; and iv) poly(2-vinylpyridine-co-styrene) at a concentration of from about 0.05 to less than 1.0 weight percent.

17. The method of claim 16 including mixing from about 90 to 97 weight percent graphite as the anode active material with from about 3 to 10 weight percent of a binder to thereby provide an anode active material mixture containing the anode active material for the negative electrode.

18. The method of claim 17 wherein the binder is selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE), polyamides, polyimides, and mixtures thereof.

19. The method of claim 16 including contacting the anode active material to a current collector selected from the group consisting of copper, stainless steel, titanium, tantalum, platinum, gold, aluminum, nickel, cobalt nickel alloy, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium-, and molybdenum-containing alloys.

20. The method of claim 16 including mixing from about 90 to 97 weight percent LiCoO.sub.2 as the cathode active material mixed with from about 1 to 10 weight percent of a conductive additive and from about 3 to 10 weight percent of a fluoro-resin binder to thereby provide a cathode active material mixture containing the cathode active material for the positive electrode.

21. The method of claim 16 including providing the poly(2-vinylpyridine-co-styrene) comprising a molecular weight of 5,000.

22. The method of claim 16 including selecting the lithium salt from the group consisting of LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4, LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3, LiNO.sub.3, LiN (SO.sub.2CF.sub.3) .sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3, LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3,LiSO.sub.3F, LiB (C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures thereof.

23. The method of claim 16 wherein the electrolyte does not include a biphenyl compound.

24. The method of claim 16 including selecting the anode active material from the group consisting of coke, graphite, acetylene black, carbon black, glassy carbon, carbon nanotubes, and meso-carbon microbeads.

25. The method of claim 16 including providing the anode active material as a fibrous carbonaceous material.

26. The method of claim 16 including selecting the cathode active material from the group consisting of LiNiO.sub.2, LiMn.sub.2O.sub.4, LiCoO.sub.2, LiCo.sub.0.92Sn.sub.0.08O.sub.2, lithium cobalt nickel oxide, and LiMPO.sub.4 (where M is one or metals from columns 4 through 13 of the Periodic Table of the Elements).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which:

(2) FIG. 1 is a graph of the cyclic performance of an exemplary cell containing an additive of vinylpyridine in comparison to a cell devoid of an electrolyte additive, both according to the prior art, and a cell with poly(2-vinylpyridine) as an electrolyte additive according to the present invention; and

(3) FIG. 2 is a graph of the recoverable capacity of a exemplary cell, according to the present invention, and a prior art cell with no electrolyte additive, both after storage at 37° C. for seven months.

(4) Accordingly, the present invention will be described in connection with preferred embodiments, however, it should be understood that there is no intent to limit the invention to the embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) A secondary electrochemical cell constructed according to the present invention includes an anode active material selected from Groups IA, IIA, or IIIB of the Periodic Table of Elements, including the alkali metals lithium, sodium, potassium, etc. The preferred anode active material comprises lithium.

(6) In secondary electrochemical systems, the anode or negative electrode comprises a material capable of intercalating and de-intercalating the alkali metal, and preferably lithium. A carbonaceous anode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, carbon nanotubes, etc.) which are capable of reversibly retaining the lithium species is preferred. Alternatively, the anode active material may be a metal or metal oxide such as silicon, tin, aluminum, zinc, silver and the various oxides thereof.

(7) Graphite is particularly preferred due to its relatively high lithium-retention capacity. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because they have excellent mechanical properties which permit them to be fabricated into rigid electrodes that are capable of withstanding degradation during repeated charge/discharge cycling. Moreover, the high surface area of carbon fibers allows for rapid charge/discharge rates. Meso-carbon microbeads (MCMB) are one preferred carbonaceous anode material.

(8) The anode for the secondary cell is fabricated by mixing about 90 to 97 weight percent of the carbonaceous anode material with about 3 to 10 weight percent of an electrode binder material, which is preferably a fluoro-resin powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE), polyamides, polyimides, and mixtures thereof. Less than 3 weight percent of the binder provides insufficient cohesiveness to the loosely agglomerated electrode active materials to prevent delamination, sloughing and cracking during electrode preparation and cell fabrication and during cell discharge. More than 10 weight percent of the binder provides a cell with diminished capacity and reduced current density due to lowered electrode active density.

(9) This negative electrode admixture is provided on a current collector selected from copper, stainless steel, titanium, tantalum, platinum, gold, aluminum, nickel, cobalt nickel alloy, highly alloyed ferritic stainless steel containing molybdenum and chromium, and nickel-, chromium-, and molybdenum-containing alloys. The current collector is a foil or screen and contact is by casting, pressing, or rolling the admixture thereto.

(10) The anode component further has an extended tab or lead of the same material as the anode current collector, i.e., preferably nickel contacted by a weld to a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the carbonaceous anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface cell design.

(11) The cathode or positive electrode of a secondary cell preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO.sub.2, LiMn.sub.2O.sub.4, LiCoO.sub.2, LiCu.sub.0.92Sn.sub.0.08O.sub.2 and LiCo.sub.1−xNi.sub.xO.sub.2. In another embodiment, the active material has an olivine or NASICON structure of the formula LiMPO.sub.4 where M is one or metals from columns 4 through 13 of the Periodic Table of the Elements. Preferably, M is iron or vanadium.

(12) Before fabrication into an electrode for incorporation into an electrochemical cell, the lithiated active material (about 90 to 97 weight percent) is preferably mixed with a conductive additive (about 1 to 10 weight percent) and a binder (about 3 to 10 weight percent). In a similar manner as with the anode, suitable conductive additives for the cathode include acetylene black, carbon black, graphite and metals such as nickel, aluminum, titanium and stainless steel in powder form. Suitable fluoro-resin binders are preferably in a powder form and include PTFE, PVDF, ETFE, polyamides, polyimides, and mixtures thereof.

(13) To discharge such secondary cells, lithium ions comprising the cathode are intercalated into the carbonaceous anode by applying an externally generated electrical potential to recharge the cell. The applied recharging electrical potential serves to draw lithium ions from the cathode material, through the electrolyte and into the carbonaceous anode to saturate the carbon comprising the anode. The resulting Li.sub.xC.sub.6 electrode can have an x ranging from about 0.1 to about 1.0. The cell is then provided with an electrical potential and discharged in a normal manner.

(14) An alternate secondary cell construction comprises intercalating the carbonaceous material with lithium before the anode is incorporated into the cell. In that case, the cathode body can be solid and comprise, but not be limited to, such materials as manganese dioxide, silver vanadium oxide, copper silver vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, and fluorinated carbon. However, this approach is compromised by problems associated with handling lithiated carbon outside the cell. Lithiated carbon tends to react when contacted by air.

(15) The secondary cell of the present invention includes a separator to provide physical segregation between the anode and cathode active electrodes. The separator is of an electrically insulative material to prevent an internal electrical short circuit between the electrodes. It also is chemically unreactive with the anode and cathode active materials and both chemically unreactive with and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow therethrough of the electrolyte during the electrochemical reaction of the cell. The form of the separator typically is a sheet that is placed between the anode and cathode. Such is the case when the anode is folded in a serpentine-like structure with a plurality of cathode plates disposed intermediate the anode folds and received in a cell casing or when the electrode combination is rolled or otherwise formed into a cylindrical “jellyroll” configuration.

(16) Illustrative separator materials include fabrics woven from fluoropolymeric fibers of polyethylenetetrafluoroethylene and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film. Other suitable separator materials include non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, a polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), a polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C.H. Dexter, Div., Dexter Corp.).

(17) In accordance with the present invention, an electrolyte for an electrochemical cell is provided comprising an organic solvent, an alkali metal salt, and a polymer additive. In general, the choice of an electrolyte solvent system for activating a lithium electrochemical cell, and particularly a fully charged lithium ion cell is limited due to the high potential of the cathode material (up to 4.3V vs. Li/Li.sup.+ for Li.sub.1−xCoO.sub.2) and the low potential of the anode material (0.01V vs. Li/Li.sup.+ for graphite). According to the present invention, suitable nonaqueous electrolytes are comprised of an inorganic salt dissolved in a nonaqueous solvent and more preferably a lithium salt dissolved in a mixture of organic carbonate solvents comprising dialkyl (non-cyclic) carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC) and ethylpropyl carbonate (EPC), and mixtures thereof, and at least one cyclic carbonate selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC) and vinylene carbonate (VC), and mixtures thereof. Organic carbonates are generally used in the electrolyte solvent system for such cell chemistries because they exhibit high oxidative stability toward cathode materials and good kinetic stability toward anode materials.

(18) Preferred electrolytes according to the present invention comprise solvent mixtures of EC:DMC:EMC:DEC. Most preferred volume percent ranges for the various carbonate solvents include EC in the range of about 10% to about 50%; DMC in the range of about 5% to about 75%; EMC in the range of about 5% to about 50%; and DEC in the range of about 3% to about 45%. Electrolytes containing this quaternary carbonate mixture have a freezing point below −50° C., and lithium ion cells activated with such mixtures have very good cycling behavior at room temperature as well as very good charge/discharge cycling behavior at temperatures below −20° C.

(19) Suitable lithium salts include LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiSbF.sub.6, LiClO.sub.4, LiAlCl.sub.4, LiGaCl.sub.4, LiC(SO.sub.2CF.sub.3).sub.3, LiNO.sub.3, LiN(SO.sub.2CF.sub.3).sub.2, LiSCN, LiO.sub.3SCF.sub.2CF.sub.3, LiC.sub.6F.sub.5SO.sub.3, LiO.sub.2CCF.sub.3, LiSO.sub.3F, LiB(C.sub.6H.sub.5).sub.4, LiCF.sub.3SO.sub.3, and mixtures thereof. Salt concentrations typically range from about 0.8 to about 1.8 molar.

(20) According to the present invention, an electrolyte that is suitable for activating a lithium-ion secondary cell includes a polymer additive comprised of repeating vinyl units joined to one or more heterocyclic amine moieties. The polymer additive may be present at a concentration of from about 0.05 and about 1.0 weight percent of the electrolyte, and it preferably has a molecular weight of at least about 5,000. The heterocyclic amine may contain from five to ten ring atoms, inclusive. One preferred heterocyclic amine is pyridine, with the additive therefore being poly(2-vinylpyridine). Alternatively, the heterocyclic amine of the polymer may be a substituted pyridine, quinoline, imidazole, phthalimide, pyrimidine, piperidine, pyridazine, pyrazine, triazine, triazole, thiazole, and combinations thereof.

(21) Alternatively, the additive may be a copolymer of the general formula:

(22) ##STR00002##
wherein R.sup.1 is a heterocyclic amine containing five or six ring atoms, and R.sup.2 is an aromatic compound, or an acrylate containing at least four carbon atoms. In the copolymer, m is preferably in the range of 20 to 100 mole percent, with n being in the corresponding range of 0 to 80 mole percent. R.sup.1 may be selected from the above cited heterocyclic amines. R.sup.2 may be an aromatic compound or an acrylate containing at least four carbon atoms. Exemplary compounds for R.sup.2 are benzene, toluene, styrene, chlorobenzene, fluorobenzene, chlorotoluene, fluorotoluene, butylmethacrylate, and combinations thereof.

(23) The electrolyte may also contain up to 6 weight percent of an additional additive such as vinylene carbonate, fluorinated ethylene carbonate, biphenyl, and biphenyl derivatives or cyclohexyl benzene to improve the cycle life or the safety of the cell.

(24) The preferred form of the electrochemical cell is a case-negative design wherein the anode/cathode couple is inserted into a conductive metal casing connected to the anode current collector, as is well known to those skilled in the art. A preferred material for the casing is stainless steel, although titanium, mild steel, nickel, nickel-plated mild steel and aluminum are also suitable. The casing header comprises a metallic lid having a sufficient number of openings to accommodate the glass-to-metal seal/terminal pin feedthrough for the cathode. The anode is preferably connected to the case or the lid. An additional opening is provided for electrolyte filling. The casing header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cell is thereafter filled with the electrolyte solution described hereinabove and hermetically sealed, such as by close-welding a stainless steel plug over the fill hole, but not limited thereto. The cell of the present invention can also be constructed in a case-positive design.

(25) Accordingly, the following examples describe the manner and process of an electrochemical cell according to the present invention, and they set forth the best mode contemplated by the inventors of carrying out the invention, but they are not to be construed as limiting.

EXAMPLE

(26) By way of illustration, and in no way intended to be limiting, two exemplary cells according to the present invention were prepared. Exemplary Cell I contained an electrolyte with the polymer additive being poly(2-vinylpyridine). Exemplary Cell II contained an electrolyte with the polymer additive being poly(2-vinylpyridine-co-styrene).

(27) The anode active material in the exemplary cells was a layered carbon material, i.e. graphite. The anode slurry was prepared by kneading the anode active material, conducting carbon and polyvinylidene fluoride binder dissolved in N-methylpyrrolidinone (NMP). The anode contained about 2 weight percent of the conducting carbon and about 5 weight percent of the binder in the solid contents of the slurry. The anode slurry was then coated on both surfaces of a copper foil using wet-coating techniques and passed through a drier to remove the NMP. After drying, the anode was pressed to obtain a coating density of from about 1.25 and about 1.70 grams per milliliter.

(28) The cathode slurry was prepared by mixing LiCoO.sub.2 as the cathode active material, acetylene black as the conducting carbon and polyvinylidene fluoride binder dissolved in N-methylpyrrolidinone (NMP) by using a planetary mixer. The cathode contained about 2 weight percent of the conducting carbon and about 3 weight percent of the binder in the solid contents of the slurry by weight. The cathode slurry was coated onto an aluminum current collector followed by drying and roll pressing of the electrode to obtain a coating density of from about 3.0 to about 3.8 grams per milliliter. Although polyvinylidene fluoride (PVDF) was used as a binder in the cathodes and anodes of the exemplary cells, any polymer that is stable at less than 2 volts versus lithium and is insoluble in the electrolyte could be used.

(29) The anode, cathode and a micro-porous polyolefin separator of polyethylene were stacked to form an electrode assembly, and then placed into flexible packaging film, activated with electrolyte, and hermetically sealed.

Comparative Example

(30) For comparison, two cells were prepared in a similar manner as the above described present invention Cells I and II except Comparative Cell III an electrolyte additive of 2-vinylpyridine monomer and Comparative Cell IV had no polymer additive in its electrolyte.

(31) Cells I to IV were then charged to 4.1V at a IC-rate with a cut-off current at C/20-rate, and discharged to 2.75V at a IC-rate. Table 1 summarizes the first cycle coulomb efficiency and voltage drop of the exemplary present invention Cells I and II and the comparative prior art Cells III and IV.

(32) TABLE-US-00001 TABLE 1 Voltage Voltage Coulomb after after 1 efficiency full month Voltage of first charge storage at drop Cell Additive cycle, % to 4.1 V RT mV/day I poly(2- 88.5 4.081 4.073 0.27 vinylpyridine) II poly(2- 88.6 4.083 4.075 0.27 vinylpyridine- co-styrene) III vinylpyridine 87.0 4.066 4.034 1.07 IV no additive 89.5 4.081 4.066 0.50

(33) Exemplary Cells I and II containing a polymer additive of the present invention had a reduced voltage drop during the one month storage period, and a similar coulomb efficiency compared to the additive free Cell IV. This indicates that the polymer additives in the exemplary cells improved their shelf life. In contrast, the 2-vinylpyridine monomer additive increases the voltage drop of Cell III and reduces its coulomb efficiency.

(34) Without wishing to be bound to any particular theory, it is believed that these diminished discharge characteristics can be attributed to decomposition of the 2-vinylpyridine monomer additive forming a thick insulation layer on the anode surface-electrolyte-interface (SEI). This SEI layer accelerates lithium deposition at the anode, resulting in poor cycle life and a higher self discharge rate due to consumption of additional electrolyte on the anode surface.

(35) FIG. 1 depicts the cyclic performance of the exemplary present invention Cell I and the comparative prior art Comparative Cells III and IV. It can be seen that the exemplary cell activated with an electrolyte containing the poly(2-vinylpyridine) additive according to the present invention showed superior cyclic performance compared to the additive free cell and the cell activated with an electrolyte containing vinylpyridine monomer additive, which showed extremely poor cyclic performance.

(36) FIG. 2 depicts the recoverable capacity of the Cells I to IV after storage at 37° C. for seven months. The cells were discharged and charged three times each month at C/5-rate during storage at 37° C. to obtain the recoverable capacity of the cells, and then stored again at 37° C. at a fully charged state (4.1V). The exemplary present invention Cell I with the electrolyte containing the poly(2-vinylpyridine) additive had almost 90% of recoverable capacity, which is superior to the approximately 86% of recoverable capacity of the additive free cell.

(37) It is, therefore, apparent that there has been provided, in accordance with the present invention, a rechargeable secondary electrochemical cell having an improved electrolyte. The electrolyte includes a polymer additive comprised of repeating vinyl units joined to one or more heterocyclic amine moieties. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations that fall within the broad scope of the appended claims.