Method for reducing voltage delay in an alkali metal electrochemical cell activated with a nonaqueous electrolyte having a sulfate additive

[0002] This application is a continuation-in-part of application Ser. No. 09/009,557, now U.S. Pat. No. 6,013,394 to Gan et al.

[0003] 1. Field of the Invention

[0004] The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to an alkali metal cell suitable for current pulse discharge applications with reduced or no appreciable voltage delay.

[0005] 2. Prior Art

[0006] The voltage response of a cell which does not exhibit voltage delay during the application of a short duration pulse or pulse train has distinct features. First, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse, and second, the minimum potential of the first pulse in a series of pulses is higher than the minimum potential of the last pulse. FIG. 1 is a graph showing an illustrative discharge curve 10 as a typical or “ideal” response of a cell during the application of a series of pulses as a pulse train that does not exhibit voltage delay.

[0007] On the other hand, the voltage response of a cell which exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied. FIG. 2 is a graph showing an illustrative discharge curve 12 as the voltage response of a cell that exhibits both forms of voltage delay.

[0008] The initial drop in cell potential during the application of a short duration pulse reflects the resistance of the cell, i.e., the resistance due to the cathode, the cathode-electrolyte interphase, the anode, and the anode-electrolyte interphase. In the absence of voltage delay, the resistance due to passivated films on the anode and/or cathode is negligible. However, the formation of a surface film is unavoidable for alkali metal, and in particular, lithium metal anodes, and for lithium intercalated carbon anodes, due to their relatively low potential and high reactivity towards organic electrolytes. Thus, the ideal anode surface film should be electrically insulating and ionically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. In the event of voltage delay, the resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation which often results in reduced discharge voltage and reduced cell capacity. In other words, the drop in potential between the background voltage and the lowest voltage under pulse discharge conditions, excluding voltage delay, is an indication of the conductivity of the cell, i.e., the conductivity of the cathode, anode, electrolyte, and surface films, while the gradual decrease in cell potential during the application of the pulse train is due to the polarization of the electrodes and electrolyte.

[0009] Thus, the existence of voltage delay is an undesirable characteristic of alkali metal/mixed metal oxide cells subjected to current pulse discharge conditions in terms of its influence on devices such as medical devices including implantable pacemakers and cardiac defibrillators. Voltage delay is undesirable because it limits the effectiveness and even the proper functioning of both the cell and the associated electrically powered device under current pulse discharge conditions.

[0010] The present invention is directed to the provision of sulfate additives in the electrolyte of an alkali metal electrochemical cell to beneficially modify the anode surface film. The sulfate additives preferably include at least one unsaturated hydrocarbon containing a C(sp²or sp³)—C(sp³) bond unit having the C(sp³) carbon directly connected to the —OSO₃— functional group, or are a silyl sulfate or a tin sulfate, and are provided as a co-solvent with commonly used organic aprotic solvents. The organic sulfate additives are in a condensed phase which makes them easy to handle in electrolyte preparation. When used as a co-solvent in an activating electrolyte, the sulfate additives interact with the alkali metal anode to form an conically conductive surface protective layer thereon. The conductive surface layer improves the discharge performance of the alkali metal electrochemical cell and minimizes or even eliminates voltage delay in the high current pulse discharge of such cells.

[0011] The object of the present invention is to improve the pulse discharge performance of an alkali metal electrochemical cell, and more particularly a primary lithium electrochemical cell, by the provision of at least one of a family of sulfate additives as a co-solvent in the cell's activating nonaqueous electrolyte solution. Due to the high reduction potentials of the sulfate group vs. lithium, the sulfate additives can compete effectively with the other electrolyte co-solvents or the solute to react with the lithium anode. Lithium sulfate or the lithium salt of sulfate reduction products are believed to be the major reaction products. These lithium salts are believed to deposit on the anode surface to form an ionically conductive protective film thereon. As a consequence, the chemical composition and perhaps the morphology of the anode surface protective layer is changed, and this proves beneficial to the discharge characteristics of the cell. The thusly fabricated cell exhibits reduced or no appreciable voltage delay under current pulse discharge usage, which is an unexpected result.

[0012] These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.

[0013] FIG. 1 is a graph showing an illustrative pulse discharge curve 10 of an exemplary electrochemical cell that does not exhibit voltage delay.

[0014] FIG. 2 is a graph showing an illustrative pulse discharge curve 12 of an exemplary electrochemical cell that exhibits voltage delay.

[0015] FIG. 3 is a graph showing the pulse discharge voltage curves of a lithium/silver vanadium oxide cell activated with an electrolyte devoid of an alkyl sulfate additive during the application of four 10 second pulses (23.2 mA/cm²) with a 15 rest between each pulse.

[0016] FIG. 4 is a graph showing the pulse discharge voltage curves of a lithium/silver vanadium oxide cell activated with an electrolyte having bis(trimethylsilyl) sulfate (BTMSS) dissolved therein during the application of a pulse train similar to that used to construct the graph of FIG. 3.

[0017] As used herein, the term “pulse” means a burst of electrical current of a greater amplitude than that of a pre-pulse current immediately prior to the pulse. A pulse train consists of at least two pulses of electrical current delivered in relatively short succession with or without open circuit rest between the pulses. Voltage delay is calculated as the pulse end potential minus the pulse minimum potential.

[0018] The electrochemical cell of the present invention includes an anode selected from Group IA, IIA or IIIB of the Periodic Table of Elements, including lithium, sodium, potassium, etc., and their alloys and intermetallic compounds including, for example Li—Si, Li—B and Li—Si—B alloys and intermetallic compounds. The preferred anode comprises lithium, and the more preferred anode comprises a lithium alloy, the preferred lithium alloy being a lithium-aluminum alloy. The greater the amount of aluminum present by weight in the alloy, however, the lower the energy density of the cell.

[0019] The form of the anode may vary, but preferably the anode is a thin metal sheet or foil of the anode metal, pressed or rolled on a metallic anode current collector, i.e., preferably comprising nickel, to form an anode component. In the exemplary cell of the present invention, the anode component has an extended tab or lead of the same material as the anode current collector, i.e., preferably nickel, integrally formed therewith such as by welding and contacted by a weld to a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface cell design.

[0020] The cathode is preferably of a solid material and the electrochemical reaction at the cathode involves conversion of ions which migrate from the anode to the cathode in atomic or molecular forms. The solid cathode material may comprise a metal, a metal oxide, a mixed metal oxide, a metal sulfide or a carbonaceous compound, and combinations thereof. The metal oxide, the mixed metal oxide and the metal sulfide can be formed by the chemical addition, reaction, or otherwise intimate contact of various metal oxides, metal sulfides and/or metal elements, preferably during thermal treatment, sol-gel formation, chemical vapor deposition or hydrothermal synthesis in mixed states. The active materials thereby produced contain metals, oxides and sulfides of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII, which includes the noble metals and/or other oxide and sulfide compounds.

[0021] One preferred mixed metal oxide has the general formula SM_xV₂O_ywherein SM is a metal selected from Groups IB to VIIB and VIII of the Periodic Table of Elements, wherein x is about 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula. By way of illustration, and in no way intended to be limiting, one exemplary cathode active material comprises silver vanadium oxide (SVO) having the general formula Ag_xV₂O_yin any one of its many phases, i.e., β-phase silver vanadium oxide having in the general formula x=0.35 and y=5.8, γ-phase silver vanadium oxide having in the general formula x=0.80 and y=5.40 and ε-phase silver vanadium oxide having in the general formula x=1.0 and y=5.5, and combination and mixtures of phases thereof. For a more detailed description of such a cathode active material reference is made to U.S. Pat. No. 4,310,609 to Liang et al., which is assigned to the assignee of the present invention and incorporated herein by reference.

[0022] Another preferred composite cathode active material includes V₂O_zwherein z≦5 combined with Ag₂O with the silver in either the silver(II), silver(I) or silver(0) oxidation state and CuO with the copper in either the copper(II), copper(I) or copper(0) oxidation state to provide the mixed metal oxide having the general formula Cu_xAg_yV₂O_z, (CSVO). Thus, this composite cathode active material may be described as a metal oxide-metal oxide-metal oxide, a metal-metal oxide-metal oxide, or a metal-metal-metal oxide and the range of material compositions found for Cu_xAg_yV₂O_zis preferably about 0.01≦x≦1.0, about 0.01≦y≦1.0 and about 5.01≦z≦6.5. Typical forms of CSVO are Cu_0.16Ag_0.67V₂O_zwith z being about 5.5 and Cu_0.5Ag_0.5V₂O_zwith z being about 5.75. The oxygen content is designated by z since the exact stoichiometric proportion of oxygen in CSVO can vary depending on whether the cathode material is prepared in an oxidizing atmosphere such as air or oxygen, or in an inert atmosphere such as argon, nitrogen and helium. For a more detailed description of this cathode active material reference is made to U.S. Pat. No. 5,472,810 to Takeuchi et al. and U.S. Pat. No. 5,516,340 to Takeuchi et al., both of which are assigned to the assignee of the present invention and incorporated herein by reference.

[0023] Additional cathode active materials include manganese dioxide, lithium cobalt oxide, lithium nickel oxide, copper vanadium oxide, vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, and fluorinated carbon, and mixtures thereof. Preferably, the cathode comprises from about 80 to about 99 weight percent of the cathode active material.

[0024] Cathode active materials prepared as described above are preferably mixed with a binder material such as a powdered fluoro-polymer, more preferably powdered polytetrafluoroethylene or powdered polyvinylidene fluoride present at about 1 to about 5 weight percent of the cathode mixture. Further, up to about 10 weight percent of a conductive diluent is preferably added to the cathode mixture to improve conductivity. Suitable materials for this purpose include acetylene black, carbon black and/or graphite or a metallic powder such as powdered nickel, aluminum, titanium and stainless steel. The preferred cathode active mixture thus includes a powdered fluoro-polymer binder present at about 3 weight percent, a conductive diluent present at about 3 weight percent and about 94 weight percent of the cathode active material. The cathode active mixture may be in the form of one or more plates operatively associated with at least one or more plates of anode material, or in the form of a strip wound with a corresponding strip of anode material in a structure similar to a “jellyroll”.

[0025] In order to prevent internal short circuit conditions, the cathode is separated from the Group IA, IIA or IIIB anode material by a suitable separator material. The separator is of electrically insulative material, and the separator material 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. Illustrative separator materials include woven and non-woven fabrics of polyolefinic fibers or fluoropolymeric fibers including polyvinylidene fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene laminated or superposed with a polyolefinic or a fluoropolymeric microporous film. Suitable microporous films include a polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), 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.). The separator may also be composed of non-woven glass, glass fiber materials and ceramic materials.

[0026] The form of the separator typically is a sheet which is placed between the anode and cathode electrodes and in a manner preventing physical contact therebetween. 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.

[0027] The electrochemical cell of the present invention further includes a nonaqueous, ionically conductive electrolyte operatively associated with the anode and the cathode electrodes. The electrolyte serves as a medium for migration of ions between the anode and the cathode during the electrochemical reactions of the cell and nonaqueous solvents suitable for the present invention are chosen so as to exhibit those physical properties necessary for ionic transport (low viscosity, low surface tension and wettability). Suitable nonaqueous solvents are comprised of an inorganic salt dissolved in a nonaqueous solvent and more preferably an alkali metal salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent including organic esters, ethers and dialkyl carbonates, and mixtures thereof, and a high permittivity solvent including cyclic carbonates, cyclic esters and cyclic amides, and mixtures thereof. Low viscosity solvents include tetrahydrofuran (THF), methyl acetate (MA), diglyme, triglyme, tetraglyme, 1,2-dimethoxyethane (DME), dimethyl carbonate (DMC), diisopropylether, 1,2-diethoxyethane, 1-ethoxy,2-methoxyethane, diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC), and mixtures thereof. High permittivity solvents include propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL) and N-methyl-pyrrolidinone (NMP), and mixtures thereof.

[0028] The preferred electrolyte comprises an inorganic alkali metal salt, and in the case of an anode comprising lithium, the alkali metal salt of the electrolyte is a lithium based salt. Known lithium salts that are useful as a vehicle for transport of alkali metal ions from the anode to the cathode include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiO₂, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄and LiCF₃SO₃, and mixtures thereof. Suitable salt concentrations typically range between about 0.8 to 1.5 molar, and a preferred electrolyte for a lithium/transition metal oxide electrochemical cell includes LiAsF₆or LiPF₆dissolved in a 50:50 mixture, by volume, of PC and DME.

[0029] A distinguishing characteristic of a primary lithium metal cell, in comparison to that of a secondary lithium ion cell, is that the lithium anode material is consumed during discharge. In other words, the surface of the anode is shrinking or fading away from the electrode/electrolyte interphase during discharge. This is especially the case during high current pulse discharge when the anode surface passivation film breaks up to expose a fresh lithium surface. At this stage, a new passivation film forms and its chemical composition and surface morphology depends on the competition reactions of the existing components in the electrolyte, which, in turn, depends on the cell depth of discharge.

[0030] In a lithium/silver vanadium oxide cell, there are two characteristic voltage plateaus in the discharge profile. The first one occurs at about 3.2V and the second plateau occurs at about 2.6V. At the 3.2V discharge plateau, Li/SVO cells are known to be relatively stable and exhibit consistent long term pulse discharge behavior. This indicates that the lithium anode is effectively passivated by the electrolyte. However, when the cell is discharged to the 2.6V plateau, voltage delay is present in long term pulse discharge tests.

[0031] It has been determined that voltage delay in an alkali metal/transition metal oxide cell is caused by the growth of anode surface impedance. The chemical composition and the passivation ability of the anode surface layer have changed in the time that the Li/SVO cell has discharged from the 3.2V plateau to the 2.6V plateau. Consequently, the anode surface impedance increases over time and results in voltage delay during high current pulse discharge in long term tests.

[0032] In the absence of a sulfate additive to the electrolyte according to the present invention, the anode surface passivation film, which reforms on the newly exposed, fresh lithium surface during discharge, becomes more resistive tonically, or does not effectively passivate the anode electrically, and therefore, additional amounts of electrolyte decomposition occur on the anode surface. In turn, the impedance of the anode passivation layer increases over long term discharge tests, which results in unacceptable voltage delay and decreased pulse minimum potentials over those experienced without the undesirable anode surface passivation film. This is the reason for voltage delay during high current pulse discharge, especially in long term tests when the cell has been depleted of 40% to 70% of its capacity.

[0033] According to the present invention, the undesirable or “bad” passivation film.formed on the lithium anode at the 2.6V depth of discharge in a Li/SVO cell is prevented by the presence of an electrically insulating and ionically conductive or “good” passivation layer formed on the anode surface. This passivation layer does not break up, or does not break up easily, during high current pulse discharge. If the passivation film does break up, it is readily reformed before an undesirable passivation layer replaces it. Thus, it is believed that exposure of fresh lithium is prevented or minimized during discharge.

[0034] An electrically insulating and tonically conducting or “good” passivation layer is formed on the lithium anode in accordance with the present invention by the provision of at least one sulfate additive as a co-solvent in the electrolyte solution of the previously described alkali metal electrochemical cell. The sulfate additive preferably has the general formula R¹OS (═O)₂(OR²), wherein R¹and R²are the same or different and they can both be a hydrogen atom or an organic group containing 1 to 12 carbon atoms, and wherein at least R¹has the structure (R³) (R⁴) (R⁵)C— if R²≠H with at least R³being an aromatic substituent or an unsaturated organic or inorganic group and wherein if any of the remaining groups of R⁴and R⁵is a saturated organic group, the saturated organic group contains 1 to 11 carbon atoms.

[0035] The greatest effect is found when diallyl sulfate, methyl benzyl sulfate, ethyl benzyl sulfate, propyl benzyl sulfate, butyl benzyl sulfate, pentyl benzyl sulfate, methyl allyl sulfate, ethyl allyl sulfate, propyl allyl sulfate, butyl allyl sulfate, pentyl allyl sulfate, mono-methyl sulfate, mono-ethyl sulfate, mono-butyl sulfate, mono-propyl sulfate, mono-pentyl sulfate, mono-allyl sulfate, mono-benzyl sulfate, dibenzyl sulfate, mono-trimethyltin sulfate, mono-triethyltin sulfate, mono-tripropyltin sulfate, mono-tributyltin sulfate, mono-tripentyltin sulfate, bis(trimethyltin) sulfate, bis(triethyltin) sulfate, bis(tripropyltin) sulfate, bis(tributyltin) sulfate, bis(tripentyltin) sulfate, mono-trimethylsilyl sulfate, mono-triethylsilyl sulfate, mono-tripropylsilyl sulfate, mono-tributylsilyl sulfate, mono-tripentylsilyl sulfate, bis(trimethylsilyl) sulfate, bis(triethylsilyl) sulfate, bis(tripropylsilyl) sulfate, bis(tributylsilyl) sulfate and bis(tripentylsilyl) sulfate and mixtures thereof are used as additives in the electrolyte.

[0036] The above described compounds are only intended to be exemplary of those that are useful with the present invention, and are not to be construed as limiting. Those skilled in the art will readily recognize sulfate compounds which come under the purview of the general formula set forth above and which will be useful as additives for the electrolyte to reduce voltage delay according to the present invention.

[0037] While not intending to be bound by any particular mechanism, it is believed that due to the presence of the sulfate additive being a silyl sulfate or a tin sulfate or comprising at least one unsaturated hydrocarbon containing a C(sp²or sp³)—C (sp³) bond unit having the C(sp³) carbon directly connected to the —OSO₃— functional group, the bond between oxygen and at least one of the group R¹and R²is readily severed and the sulfate intermediate is able to compete effectively with the other electrolyte solvents or solutes to react with lithium and form a sulfate salt, i.e., lithium sulfate, or the lithium salt of a sulfate reduction product on the surface of the anode. The resulting salt is more conductive than lithium oxide which may form on the anode in the absence of the organic sulfate additive. In fact, it is believed that the lithium sulfate or the lithium salt of a sulfate reduction product on the surface of the anode provides for the existence of charge delocalization due to resonance equilibration at the anode surface. This equilibration allows lithium ions to travel easily from one molecule to the other via a lithium ion exchange mechanism. As a result, beneficial ionic conductance is realized.

[0038] During discharge, the lithium metal electrode is shrinking while the surface film moves continuously toward the lithium metal surface by following the lithium ion exchange mechanism. This process prevents the break up of the surface film and avoids the exposure of fresh lithium metal to the electrolyte. As a result, voltage delay in a lithium anode/transition metal oxide cell is minimized and even eliminated using electrolytes containing sulfate additives according to the present invention. As a consequence, the chemical composition and perhaps the morphology of the anode surface protective layer is believed to be changed with concomitant benefits to the cell's discharge characteristics.

[0039] In the present invention, the anode is lithium metal and the cathode is preferably the transition mixed metal oxide AgV₂O_5.5(SVO). The preferred electrolyte is 1.0M to 1.2M LiAsF₆dissolved in an aprotic solvent mixture comprising at least one of the above listed low viscosity solvents and at least one of the above listed high permittivity solvents. The preferred aprotic solvent mixture comprises a 50/50, by volume, mixture of propylene carbonate and dimethoxyethane. The concentration of the above discussed sulfate additives according to the present invention should preferably be in the range of between about 0.001M to about 0.20M. The positive effects of these additives in reducing voltage delay in a pulse discharging alkali metal cell have been achieved both at room temperature as well as at temperatures up to about 37° C. This makes the novel electrolyte solution of the present invention particularly useful for activating an alkali metal/transition metal oxide cell incorporated into an implantable medical device such as a cardiac defibrillator to minimize or even eliminate voltage delay under high current pulse discharge conditions.

[0040] As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. From time to time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected. Reduction and even elimination of voltage delay during a current pulse application is important for proper device operation and extended device life.

[0041] The assembly of the cell described herein is preferably in the form of a wound element cell. That is, the fabricated cathode, anode and separator are wound together in a “jellyroll” type configuration or “wound element cell stack” such that the anode is on the outside of the roll to make electrical contact with the cell case in a case-negative configuration. Using suitable top and bottom insulators, the wound cell stack is inserted into a metallic case of a suitable size dimension. The metallic case may comprise materials such as stainless steel, mild steel, nickel-plated mild steel, titanium, tantalum or aluminum, but not limited thereto, so long as the metallic material is compatible for use with components of the cell.

[0042] The cell header comprises a metallic disc-shaped body with a first hole to accommodate a glass-to-metal seal/terminal pin feedthrough and a second hole for electrolyte filling. The glass used is of a corrosion resistant type having up to about 50% by weight silicon such as CABAL 12, TA 23 or FUSITE 425 or FUSITE 435. The positive terminal pin feedthrough preferably comprises titanium although molybdenum, aluminum, nickel alloy, or stainless steel can also be used. The cell header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cathode lead is welded to the positive terminal pin in the glass-to-metal seal and the header is welded to the case containing the electrode stack. The cell is thereafter filled with the electrolyte solution comprising at least one of the sulfate additives described hereinabove and hermetically sealed such as by close-welding a stainless steel ball over the fill hole, but not limited thereto.

[0043] The above assembly describes a case-negative cell, which is the preferred construction of the exemplary cell of the present invention. As is well known to those skilled in the art, the exemplary electrochemical system of the present invention can also be constructed in a case-positive configuration.

[0044] The following example describes the manner and process of an electrochemical cell according to the present invention, and sets forth the best mode contemplated by the inventors of carrying out the invention, but it is not to be construed as limiting.

[0045] Lithium anode material was pressed on nickel current collector screen and silver vanadium oxide cathode material was pressed on titanium current collector screen. A prismatic cell stack assembly configuration with two layers of microporous membrane propylene separator sandwiched between the anode and cathode was prepared. The electrode assembly was then hermetically sealed in a stainless steel casing in a case-negative configuration. Three cells were activated with the standard electrolyte consisting of 1.0M LiAsF₆dissolved in a 50:50, by volume, mixture of PC and DME without an organic sulfate additive (Group 1). Three cells were activated with the same electrolyte used to activate the Group 1 cells and further containing 0.01M of bis(trimethylsilyl) sulfate (BTMSS), as set forth in Table 1.

[0046] A constant resistive load of 3.57 kΩ was applied to all of the cells for 21 hours during an initial pre-discharge burn-in period. The pre-discharge burn-in period depleted the cells of approximately 1% of their theoretical capacity. Following burn-in, the cells were subjected to acceptance pulse testing consisting of four 10 second pulses (18.4 mA/cm²) with a 15 second rest between each pulse. Then, the cells were further pre-discharged under a 200 Ω load to remove about 600 mA-hrs of capacity.

[0047] After pre-discharge, the cells were discharged under a load of 11 kΩ at 37° C. and a pulse train 1 consisting of four 10 second pulses (18.4 mA/cm²) was immediately applied. The average discharge readings for the pre-pulse potentials and pulse 1 to 4 minimum potentials during pulse train 1 for the groups 1 and 2 cells are summarized in Table 2.

[0048] The data in Table 2 clearly demonstrate that both groups of cells exhibited no voltage delay. Voltage delay is calculated as pulse 1 end potential minus pulse 1 minimum potential. Group 2 cells with the sulfate additive exhibited slightly lower pulse minimum potentials than that of the group 1 cells in all four pulses.

[0049] After 38 days of discharge on the 11 kΩ load, another pulse train (pulse train 2) was applied. The results are summarized in Table 3 and in FIGS. 3 and 4. FIG. 3 is a graph showing the pulse train of an exemplary one of the prior art group 1 cells wherein curve 20 was constructed from pulse train 2. FIG. 4 is a graph showing the pulse train of an exemplary one of the present invention group 2 cells wherein curve 22 was constructed from pulse train 2.

[0050] The data in Table 3 clearly demonstrate the sulfate additive effect. Both groups of cells presented similar pre-pulse potentials. However, for group 1 cells (without electrolyte additive) the pulse minimum potentials are in the order of:

[0051] P1min<P2min<P3min<P4min

[0052] While for group 2 cells (with 0.01M BTMSS additive) the pulse minimum potentials is in the order of:

[0053] P1min>P2min>P3min˜P4min

[0054] The recovery of voltages (group 1 cells) after high current pulsing represent the existence of voltage delay due to the formation of an ionically less conductive anode surface passivation layer. In the presence of the dialkyl sulfate additive (group 2 cells), the anode passivation layer is chemically modified to be ionically more conductive. As a result, no voltage delay was observed.

[0055] Comparing the average pulse minimum potential of both groups of cells (subtracting the average pulse minimum potentials for the group 1 cells from the group 2 cells), the group 2 cells exhibited higher potentials in pulse 1 and pulse 2, and slightly lower potentials in pulse 4, as set forth in Table 4.

[0056] While not intended to be bound by any particular theory, it is believed that the formation of S(═O₂) (O—Li)_n(OR)_mwherein n=1 or 2 and m=O or 1, or its reaction products, deposited on the anode surface is responsible for the improved performance of an alkali metal/transition metal oxide cell and, in particular, a Li/SVO cell. If the R group in the sulfate additive is hydrogen (acidic proton), it will react with lithium metal to form an O—Li bond directly. In addition, if one of the R groups is activated (benzyl for example), the O—R bond is relatively weak. During reduction, the O—R bond breaks to form a product containing the S—O—Li salt group. The sulfate intermediate is then able to compete effectively with the other electrolyte solvents or solutes to react with lithium and form a sulfate salt, i.e., lithium sulfate, or the lithium salt of a sulfate reduction product on the surface of the anode. The resulting sulfate salt is ionically more conductive than lithium oxide which may form on the anode in the absence of the organic sulfate additive. As a consequence, the chemical composition and perhaps the morphology of the anode surface protective layer is believed to be changed with concomitant benefits to the cell's discharge characteristics. This is believed to be the reason for the observed improvements in the lithium ion cells, as exemplified by those having the BTMSS additive.

[0057] The concentration limit is between about 0.001M to 0.40M. The beneficial effect of the alkyl sulfate will not be apparent if the additive concentration is less than about 0.001M. On the other hand, if the additive concentration is greater than about 0.40M, the beneficial effect of the additive will be cancelled by the detrimental effect of higher internal cell resistance due to the thicker anode surface film formation and lower electrolyte conductivity. For BTMSS, the concentration limit is about 0.02M.

[0058] Thus, the existence of voltage delay is due to the formation of an anode surface passivation layer that is ionically less conductive than either the anode material itself or the electrolyte solution. In the presence of an alkyl sulfate additive according to the present invention, the anode passivation layer is chemically modified to be ionically more conductive than the passivation layer formed without the benefit of the organic sulfate additive. It is believed that due to the presence of the —OSO₃— functional group, the reductive cleavage of at least one of the OR¹and OR²bonds in the sulfate additives of the present invention may produce lithium sulfate or the lithium salt of a sulfate reduction product on the anode surface. This surface film is ionically more conductive than the film formed in the absence of the additives and it is believed responsible for the increased cell performance, especially during pulse discharge applications. As a consequence, diminished voltage delay results when an alkali metal/transition metal oxide couple activated with a nonaqueous organic solvent having a sulfate additive dissolved therein according to the present invention is subjected to a pulse discharge application.

[0059] It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.