WO1996029750A1

WO1996029750A1 - Organic carbonate additives for nonaqueous electrolyte in alkali metal electrochemical cells

Info

Publication number: WO1996029750A1
Application number: PCT/US1996/003118
Authority: WO
Inventors: Hong Gan; Esther S. Takeuchi
Original assignee: Wilson Greatbatch Ltd.
Priority date: 1995-03-17
Filing date: 1996-03-06
Publication date: 1996-09-26
Also published as: AU5419896A

Abstract

An alkali metal, solid cathode, nonaqueous electrochemical cell capable of delivering high current pulses, rapidly recovering its open circuit voltage and having high current capacity, is described. The stated benefits are realized by dissolving organic additives, preferably carbonate additives in the electrolyte. The carbonate additives have an O-X bond with a dissociation energy less than about 80 kcal/mol on either one or both sides of a carbonyl group wherein X is selected from the group consisting of C, O and N.

Description

ORGANIC CARBONATE ADDITIVES FOR NONAQUEOUS ELECTROLYTE IN ALKALI METAL ELECTROCHEMICAL CELLS

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to a nonagueous electrolyte alkali metal electrochemical cell, and in particular, a lithium cell designed for high current pulse discharge applications.

2. Description of the Prior Art

It is well known that the anode surface film, known as solid-electrolyte interphase (SEI) , plays a very important role in the discharge performance of either a primary or a secondary alkali metal electrochemical cell, and in particular, a lithium cell. A good SEI benefits cell performance, including high discharge capacity, long useful life, better rechargeability for secondary cells and little or no voltage delay during high current pulse applications. Since 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 low potential and high reactivity towards organic electrolytes, much effort has been focused on modification of the chemical composition and morphology of the anode surface film.

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. As a result, higher impedance builds up inside the cell due to this surface layer formation which often results in lower discharge voltage and lower cell capacity. In the case of a cell subjected to high current pulse discharge, large voltage delay and voltage drop may occur and limit the effectiveness of the cell under these conditions. PCMJS96/03118

- 2 -

The surface film also affects the efficiency and cyclability of secondary alkali metal cells.

One of the known solutions to the above problem is to use C0₂ saturated electrolyte solutions. Cycling efficiency is improved dramatically in secondary cell systems having a lithium anode and activated with C0₂ saturated electrolytes (V.R. Koch and S.B. Brummer, Electrochimica Acta, 1978, 23, 55-62; U. S. Patent No. 4,853,304 to Ebner et al.; D. Aurbach, Y. Gofer, M. Ben-Zion and P. Aped, J. Electroanal . Chem. 1992, 339, 451-471) . The same effect is also known for lithium intercalated carbon anode secondary batteries (D. Aurbach, Y. Ein-Eli, 0. Chusid, Y. Carmeli, M. Babai and H. Yamin. J. Electrochem . Soc. 1994, 141, 603-611) . In spite of the success of C0₂, its use in electrochemical cells is limited due to the difficulty in controlling its concentration. To achieve the best results, high pressures of up to 50 psig have to be used, which further detracts from the practicality of this additive.

Instead of C0₂, the present invention is directed to dissolving organic additives in the electrolyte to change the anode surface film. The carbonate additives have weak 0-X bonds, which when dissolved in the activating electrolyte effect the anode surface film to desirably change its ionic conductivity.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the performance of a primary alkali metal electrochemical cell, and more particularly, a lithium electrochemical cell by dissolving in the nonaqueous electrolyte solution a combination of organic additives. A further object of this invention is to provide the present electrolyte solution in operative association with an electrochemical system incorporated in a defibrillator battery to minimize or eliminate voltage delay under high current pulse discharge conditions. The concept of this invention is likewise applicable to secondary alkali metal electrochemical cells. The aforementioned objects are achieved by the addition of organic additive, preferably carbonate additives dissolved in the electrolyte solution of an alkali metal electrochemical cell. Unlike C0₂ used in the prior art cells, the present invention uses solid organic additives which provide ease in handling and in electrolyte preparation. The additives are designed to interact with the alkali metal anode, and particularly with a lithium anode to form a protective anode surface layer which improves the discharge performance of the cell, and minimizes or even eliminates voltage delay in high current discharge conditions. BRIEF DESCRIPTION OF DRAWINGS

Fig. l is a graph showing the pulse discharge curve 10 of a control electrochemical cell activated with an electrolyte comprising an alkali metal salt dissolved in an aprotic organic solvent devoid of an organic additive according to the present invention.

Fig. 2 is a graph showing the pulse discharge curve 20 of an electrochemical cell activated with an electrolyte comprising an alkali metal salt dissolved in an aprotic organic solvent including an organic additive according to the present invention.

Fig. 3 is a graph showing the pulse discharge curve 30 of an electrochemical cell similar to that discharged to generate the graph shown in Fig. 2 but without the organic additive.

Figs. 4 to 6 are graphs showing discharge curves of electrochemical cells having 800 mAh of capacity removed before testing and including no additive, 0.01M BSC additive and 0.03M DBC additive, respectively. Figs. 7 to 9 are graphs showing discharge curves of electrochemical cells having 1200 mAh of capacity removed before testing and including no additive, 0.01M BSC additive and 0.03M DBC additive, respectively. 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 drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrochemical cell of the present invention includes an anode selected from Group IA 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 lithium-aluminum with the aluminum comprising from between about 0% to about 50% by weight of the alloy.

The greater the amount of aluminum present by weight in the alloy the lower the energy density of the cell.

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. 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 element, a metal oxide, a mixed metal oxide, a metal sulfide or carbonaceous compounds, and combinations thereof. Suitable cathode materials include silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, carbon and fluorinated carbon. The solid cathode exhibits excellent thermal stability and is generally safer and less reactive than a non-solid cathode.

Preferably, the solid cathode material comprises a mixed metal oxide formed by the chemical addition, reaction or otherwise intimate contact or by a thermal spray coating process of various metal sulfides, metal oxides or metal oxide/elemental metal combinations. The materials thereby produced contain metals and oxides of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the Periodic Table of Elements, which includes the noble metals and/or their oxide compounds. By way of illustration, and in no way intended to be limiting, an exemplary cathode active material comprises silver vanadium oxide having the general formula Ag_xV₂O_y in any one of its many phases, i.e. β-phase silver vanadium oxide having in the general formula x = 0.35 and y = 5.18, γ-phase silver vanadium oxide having in the general formula x = 0.74 and y = 5.37 and e-phase silver vanadium oxide having in the general formula x = 1.0 and y = 5.5, and combination and mixtures of phases thereof.

The cell of the present invention includes a separator to provide physical separation between the anode and cathode active electrodes. The separator is of electrically insulative material to prevent an internal electrical short circuit between the electrodes, 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 non-woven glass, polypropylene, polyethylene, glass fiber material, ceramics, polytetrafluorethylene membrane commercially available under the designations ZITEX (Chemplast Inc.), polypropylene membrane, commercially available under the designation CELGARD (Celanese Plastic Company Inc.) and DEXIGLAS (CH. Dexter, Div. , Dexter Corp.). 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.

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, dimethyl carbonate (DMC) , 1,2-dimethoxyethane (DME) and others. High permittivity solvents include propylene carbonate (PC) , ethylene carbonate (EC) , acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, 7-butyrolactone (GBL) and N-methyl- pyrrolidinone (NMP) and others. Suitable nonaqueous solvents are substantially inert to the anode and cathode electrode materials and a preferred solvent comprises a 50/50 mixture (by volume) of propylene carbonate (PC) and dimethoxyethane (DME) . The preferred electrolyte comprises an inorganic salt having the general formula MMF₆ wherein M is an alkali metal similar to the alkali metal comprising the anode and M is an element selected from the group consisting of phosphorous, arsenic and antimony. Examples of salts yielding MF₆ are: hexafluorophosphate (PF₆) , hexafluoroarsenate (AsF₆) and hexafluoroantimonate (SbF₆) . Preferably the electrolyte comprises at least one ion-forming alkali metal salt of hexafluorophosphate, hexafluoroarsenate or hexafluoroantimonate dissolved in a suitable organic solvent wherein the ion-forming alkali metal is similar to the alkali metal comprising the anode. Thus, in the case of an anode comprising lithium, the alkali metal salt of the electrolyte of the present invention comprises lithium hexafluorophosphate, lithium hexafluoroarsenate or lithium hexafluoroantimonate dissolved in a suitable solvent mixture. Other inorganic salts useful with the present invention include LiBF₄, LiCI0₄ and LiCF₃S0₃, and mixtures thereof. In the present invention, the addition of organic additives containing an O-X bond having a dissociation energy less than about 80 kcal/mol (X can be C, 0 or N) on either one or both sides of a carbonate group improves the high current pulse discharge performance of the alkali metal cells. The greatest effect is found when di- (N-succinimidyl) carbonate, benzyl-N- succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof are used as additives in the electrolyte. The presence of a weak 0-X bond in these additives is crucial for improved performance of the alkali metal cells, and particularly lithium cells.

Due to the low 0-X bond dissociation energy, these additives can compete effectively with electrolyte solvents or solutes to react with the lithium anode.

Increased amounts of lithium carbonate are believed to be deposited on the anode surface to form an ionically conductive protective film. As a consequence, the chemical composition and perhaps the morphology of the anode surface protective layer is changed with concomitant benefits to the cell's discharge characteristics.

In the present invention, the anode is lithium metal and the cathode is preferably the transition mixed metal oxide AgV₂0₅._s (SVO) . The preferred electrolyte is l.OM to 1.4M LiAsF₆ dissolved in an aprotic solvent mixture comprising at least one low viscosity solvent and one high permittivity solvent. The low viscosity solvent is preferably selected from organic esters, ethers and dialkyl carbonates. The high permittivity solvent is preferably selected from cyclic carbonates, cyclic esters and cyclic amides. The concentration of the above mentioned organic additives should preferably be in the range of between about 0.001M to about 0.1M. The positive effects of these additives have been achieved both at room temperature as well as at 37°C. 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 or aluminum, but not limited thereto, so long as the metallic material is compatible for use with components of the cell. 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 from between about 0% 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 described hereinabove and hermetically sealed such as by close-welding a stainless steel ball over the fill hole, but not limited thereto. This 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.

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

EXAMPLE I Lithium anode material and silver vanadium oxide cathode material were each pressed on titanium current collector screens connected to nickel leads. A prismatic cell stack assembly of a single anode and a single cathode with two layers of polypropylene separator sandwiched between the two electrodes was prepared. The thusly constructed electrode assembly was placed in a glass vessel sealed with a rubber septum and activated with a nonaqueous electrolyte. The electrochemical cell assemblies were then divided into ten cell groups.

The nonaqueous electrolyte was prepared by dissolving LiAsF6 salt in each of the solvent mixtures listed in Table 1.

Table 1 Electrolyte Conductivity

Electrolyte Solvent Conductivity [LiAsF₆] (Ratio) (mmho/cm)

1 l . OM PC(50)/DME(50) 17.30

2 1 .2M PC(30)/DMC(70) 14.02 3 3 1 1..44MM PC(20)/MA(80) 24,

4 l . OM GBL(40)/DME(60) 21.

5 1 . 3M NMP(20)/DME(80) 20,

PC = propylene carbonate DME = 1,2-dimethoxyethane DMC = dimethyl carbonate MA = methyl acetate GBL = 7-butyrolactone NMP= -methyl-pyrrolidinone

After the electrolyte solutions cooled to room temperature, the respective organic additives listed in table 2 were added to the test cells and the solutions were further diluted to the desired concentration.

Table 2 Organic Additives in Test Cells

Cell Additive Group Concentration Additive Electrolyte

1 O.OIM DCS 1

0.05M DCS

0.01M DCS

7 0.01M BCS

8 0.01M BCS

0.01M BCS

10 0.01M DBC

DSC = di- (N-succinimidyl) carbonate BSC = benzyl-N-succinimidyl carbonate DBC = dibenzyl carbonate

To demonstrate the benefits in cell performance attributable to the organic additives of the present invention, reference test cells from each of the ten cell groups were also activated with electrolytes similar to those listed in Table 1, but without the present organic additives.

All the cells, i.e., those having the organic additives of the present invention dissolved in the respective electrolytes and the reference test cells, . were discharged under a IK ohm load for l hour and then discharged under a 2K ohm load. These cells received once every two days an application of a pulse train consisting of four 17.7 mA/cm², 10 second pulses with 15 seconds rest between each pulse. Four pulse trains were applied to each cell. In general, voltage delay in the first pulse can be observed for all of the above reference test cells in the third and fourth pulse trains. Voltage delay is defined as pulse one end potential minus pulse one minimum potential as shown in Fig. 1 wherein in the pulse discharge curve 10, pulse one end potential is indicated at 12 and pulse one minimum potential is indicated at 14. A positive value indicates the existence of voltage delay. The larger this value, the larger the voltage delay. The beneficial effects of the organic additives on the voltage delay are thus obtained by comparing the results of test cells having the organic additives of the present invention dissolved in the electrolyte solution with respect to the reference cells at pulse trains 3 and 4. The results from pulse train 4 are summarized in Table 3. Table 3 Effect of Organic Additives on Test Cell Performance: Voltage Delay in Fourth Pulse Train (V)

Cell Without Additive With Additive

GrOUp (Plend-Plmin) (Plend-Plmin'

1 0.08 (0.08)a 0.00 (0.02).

2 (0.08)_b (0.02)_b

3 0.04 0.00

4 0.24 0.09

5 0.36 0.25

6 (0.11)_c (0.06)_c

7 0.13 0.00

8 0.36 0.29

9 (0.11)_c (0.05)_c

10 0.13 0.07

(a) Small voltage delay at 37°C.

(b) Result at 37°C. More Λ /^•oltage delay is in pulse trains 2 and 3. (c) Pulse current density is 7.8 mA/cm².

The results from Table 3 clearly indicate the beneficial effect on cell pulse discharge performance of the organic carbonate additives of the present invention dissolved in the electrolyte solutions of the various cell groups. Voltage delay is eliminated in Cell Groups 1, 3 and 7. In all other cell groups, voltage delay is also decreased by addition of the present carbonate additives. More importantly, the improvements in minimizing voltage delay in the above example was achieved without any other detrimental effects on cell discharge performance. In addition to the noted improvements in voltage delay, it was discovered that the carbonate additives (DSC, BSC) are also useful in preventing decrease in cell potential. Alkali metal cells activated with electrolytes including these additives always have equal or higher prepulse discharge potential than the cells without them. This is especially the case when relatively more reactive solvents are used, such as GBL, NMP and MA. Higher cell capacity, thus, is expected. The data of prepulse potentials from cell groups 5, 8 and the control cell are listed in Table 4.

Table 4

Pre-Pulse Potential (V)

C Ceellll I Pulse Pulse Pulse Pulse

Group Train 1. Train 2. Train 3. Train 4

Control 3.14 2.68 2.32 2.04

5 3.13 2.74 2.42 2.12

8 3.16 2.77 2.45 2.14

EXAMPLE II 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 icroporous 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 casenegative configuration and activated with an electrolyte comprising l.OM LiAsF₆ dissolved in a 50/50 mixture (by volume) of propylene carbonate and 1,2-dimethoxyethane. Some of the cells in this example were provided with

0.01M DSC dissolved in the electrolyte while reference cells were built without the dissolved organic additive. These cells were partially discharged under 200 ohm for 70 hours at 37°C. After storing on open circuit at 37°C for 8 weeks, they received an application of a pulse train consisting of four 18.4 mA/cm², 10 second pulses with 15 second rest between each pulse at 37°C. The voltage delay results are listed in Table 5 and are graphically shown in Figs. 2 and 3, wherein in the pulse discharge curve 20 (Fig. 2) , the pulse one end potential is indicated at 22 and the pulse one minimum potential is indicated at 24. In the pulse discharge curve 30 shown in Fig. 3, the pulse one end potential is indicated at 32 and the pulse one minimum potential is indicated at 34.

Table 5 Voltage Delay (V) Data From Hermetic Cells at 37°C

Effect of DSC Additive

Without DSC Additive With DSC Additive

'Plend^"-?lmin' (P-en ^" P_nώι) 0.163* 0.116*

(a) Data average from 8 cells

For those cells including the DSC additive, the improvement in cell performance is clearly evident in terms of decreased voltage delay during high current pulse discharge.

EXAMPLE III Hermetically sealed Li/SVO defibrillator cells were constructed similar to those described in Example II and activated with an electrolyte comprising l.OM LiAsF₆ dissolved in a 50/50 mixture (by volume) of propylene carbonate and 1,2-dimethoxyethane. Some of the cells were provided with 0.01M BSC dissolved in the electrolyte while some of the cells were provided with 0.03M DBC additive dissolved in the electrolyte. Reference cells for the two cell groups were built without organic additives in the electrolyte. These cells were further divided into two sub-groups. One group was partially discharged under 200 ohm for about 56 hours to remove 800 mAh capacity. The second sub-group cells were discharged under 200 ohm for about 87 hours to remove 1200 mAh capacity. After storing on open circuit at 37°C for 8 weeks, they received an application of a pulse train consisting of four 18.4 mA/cm², 10 second pulses with 15 second rest between each pulse at 37°C. The voltage delay results are listed in Table 6 and are graphically shown in Figs. 4 to 9. Thus, the pulse discharge curve 40 (Fig. 4) was constructed from a Li/SVO cell activated with an electrolyte of l.OM LiAsF₆ dissolved in a 50/50 mixture (by volume) of propylene carbonate and 1.2-dimethoxyethane without any additive therein and after 800 mAh of capacity was removed, wherein the pulse one end potential is indicated at 42 and the pulse one minimum potential is indicated at 44. The pulse discharge curve 50 in Fig. 5 was constructed from a cell similar to that used in Fig. 4 and activated with an electrolyte of 1.0 LiAsF₆ dissolved in a 50/50 mixture (by volume) of PC/DME with 0.01M BSC additive and after 800 mAh of capacity was removed, wherein the pulse one end potential is indicated at 52 and the pulse one minimum potential is indicated at 54. And, the pulse discharge curve 60 in Fig. 6 was constructed from a similar electrochemical system as that used in Fig. 5 except that the electrolyte contained 0.03M DBC additive dissolved therein and after 800 mAh of capacity was removed, wherein the pulse one end potential is indicated at 62 and the pulse one minimum potential is indicated at 64.

The pulse discharge curve in Fig. 2 was constructed from a Li/SVO cell activated with an electrolyte of l.OM, LiAsF₆ dissolved in a 50/50 mixture (by volume) of PC/DME without any additive therein and after 1200 mAh of capacity was removed, wherein the pulse one end potential is indicated at 72 and the pulse one minimum potential is indicated at 74. The pulse discharge curve 80 in Fig. 8 was constructed from a similar electrochemical system as that used in Fig. 7 except that the electrolyte contained 0.01M BSC additive dissolved therein and after 1200 mAh of capacity was removed, wherein the pulse one end potential is indicated at 82 and the pulse one minimum potential is indicated at 84. And, the pulse discharge curve 90 in Fig. 9 was constructed from an electrochemical system similar to those used in Figs. 7 and 8 except that the electrolyte contained 0.03M DBC additive dissolved therein and after 1200 mAh of capacity was removed, wherein the pulse one end potential is indicated at 92 and the pulse one minimum potential is indicated at 94.

Table 6 Voltage Delay (V) Date From Hermetic Cells at 37°C Effect of BSC and DBC Additives

Organic Capacity Without Additive' With Additive Additive Removed (P.end^'P-min) 'P.end^"P-min' BSC 800 mAh 0.068 0.003 1200 mAh 0.034 0.000

DBC 800 mAh 0.068 0.000 1200 mAh 0.034 0.000

(a) Both cell groups have the same control cells

For those cells with activated with an electrolyte including the BSC and DBC additives, the improvement in cell performance is clearly evident in terms of voltage delay elimination during high current pulse discharge. EXAMPLE IV Organic carbonate additives are also found to be beneficial to low rate lithium batteries with carbon monofluoride as a cathode material. Hermetically sealed LI/CF_X batteries were constructed in a similar fashion to those described in Example II (except using a non-woven polypropylene separator instead of microporous membrane polypropylene separator) and activated with an electrolyte comprising l.OM LiBF₄ dissolved in GBL. Some of the cells were provided with O.OlM DSC dissolved in the electrolyte while reference cells were built without any organic additive in the electrolyte. These cells were further divided into three sub-groups. These three sub-groups were discharged under constant loads of 2.OK ohm, 5.IK ohm and 10.OK ohm, respectively. The results are summarized in Tables 7, 8 and 9.

Table 7 Li/CF, Cell Potential (V) vs. Time (Days) Under 2.OK Ohm Load Discharge at 37°C Cell Potential

(V)*

Time (days) 2 15 30 44 60 80 Reference 2.753 2.804 2.786 2.748 2.660 1.219

O.OlM DSC 2.771 2.815 2.801 2.762 2.681 1.476

(a) Average of three cells.

Table 8 Li/CF_X Cell Potential (V) vs. Time (Days) Under 5.IK Ohm Load Discharge at 37°C

Cell Potential (V)'

Time (days) 2 30 61 96 131 156

Reference 2.186 2.853 2.826 2.798 2.771 2.690

O.OlM DSC 2.862 2.877 2.852 2.822 2.792 2.695 (a) Average of three cells.

Table 9 Li/CF_X Cell Potential (V) vs. Time (Days)

Under 10.OK Ohm Load Discharge at 37°C

Cell Potential (V)*

Time (days) 2 26 61 96 131 156

Reference 2.858 2.878 2.879 2.871 2.858 2.848 O.OlM DSC 2.917 2.910 2.909 2.902 2.887 2.877

(a) Average of three cells.

For cells with DSC additive in the electrolyte, the improvement in cell performance is clearly indicated in terms of higher discharge potential throughout the test. Higher cell capacity (mAh) and higher deliverable energy (mWh) are thus expected with the added organic carbonate DSC additive in the electrolyte.

It is believed that the organic carbonates of the present invention are reduced to form a product which deposits on the anode surface. This surface film is ionically more conductive than the film formed in the absence of additives and it is responsible for the increased cell performance. Since lithium carbonate is known to form a good ionic conductive film on lithium surfaces, the reductive cleavage of the 0-X bond (X = C,0 or N) in the carbonate additives of the present invention may produce lithium carbonate as the final product. 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.

Claims

What is claimed is:

1. An electrochemical cell comprising an alkali metal anode; a cathode; an electrolyte operatively associated with the anode and the cathode, the improvement in the cell comprising an additive dissolved in the electrolyte, wherein the additive includes an 0-X bond having a dissociation energy less than about 80 kcal/mol on either one or both sides of a carbonyl group, and wherein X is selected from the group consisting of C, 0 and N.

2. The electrochemical cell of claim l wherein the additive is selected from the group consisting of di- (N-succinimidyl) carbonate, benzyl-N-succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof.

3. The electrochemical cell of claim 1 wherein the anode is of an alkali metal, the electrolyte is a nonaqueous electrolyte and there is dissolved therein an alkali metal salt similar to the alkali metal comprising the anode.

An electrochemical cell, which comprises:

a) an anode comprising an alkali metal which is electrochemically oxidizable to form metal ions in the cell upon discharge to generate electron flow in an external electrical circuit connected thereto; b) a cathode comprising a cathode active material wherein the electron flow is generated by intercalation of the metal ions formed by oxidation of the alkali metal anode into the cathode active material; and c) a nonaqueous electrolyte operatively associated with the anode and the cathode, the electrolyte comprising an alkali metal salt dissolved therein, wherein the alkali metal salt is similar to the alkali metal comprising the anode, the improvement in the electrolyte comprising an additive having an 0-X bond on either one or both sides of a carbonyl group, wherein the 0-X bond has a dissociation energy less than about 80 kcal/mol and wherein X is selected from the group consisting of C, 0 and N.

5. The electrochemical cell of claim 4 wherein the additive is selected from the group consisting of di- (N-succinimidyl) carbonate, benzyl-N-succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof.

6. The electrochemical cell of claim 4 wherein the nonaqueous electrolyte comprises a low viscosity solvent selected from the group consisting of an ester, an ether and a dialkyl carbonate, and mixtures thereof.

7. The electrochemical cell of claim 6 wherein the low viscosity solvent is selected from the group consisting of 1,2-dimethoxyethane, dimethyl carbonate, methyl acetate, tetrahydrofuran, diglyme, triglyme and tetraglyme, and mixtures thereof.

8. The electrochemical cell of claim 4 wherein the nonaqueous solvent comprises a high permittivity solvent selected from the group consisting of a cyclic carbonate, a cyclic ester and a cyclic amide, and mixtures thereof.

9. The electrochemical cell of claim 8 wherein the high permittivity solvent is selected from the group consisting of propylene carbonate, ethylene carbonate, γ-butyrolactone, N-methyl-pyrrolidinone, dimethyl ' sulfoxide, acetonitrile, dimethyl formamide and dimethyl acetamide, and mixtures thereof.

10. The electrochemical cell of claim 9 wherein the cathode comprises a cathode active material selected from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, cobalt oxide, nickel oxide, carbon, fluorinated carbon, titanium disulfide and copper vanadium oxide, and mixtures thereof.

11. The electrochemical cell of claim 4 wherein the alkali metal salt comprising the electrolyte solution is selected from the group consisting of LiPF₆, LiAsF₆, LiSbF₆ LiBF₄, LiC10₄ and LiCF₃S0₃, and mixtures thereof.

12. An electrochemical cell, which comprises: a) an anode comprising an alkali metal which is electrochemically oxidizable to form metal ions in the cell upon discharge to generate electron flow in an external electrical circuit connected thereto; b) a cathode comprising a cathode active material wherein the electron flow is generated by intercalation of the metal ions formed by oxidation of the alkali metal anode into the cathode active material; and c) an electrolyte operatively associated with the anode and the cathode, the electrolyte comprising at least one ion-forming alkali metal salt selected from one of the group consisting of hexafluoroarsenate, hexafluorophosphate and hexafluoroantimonate, and mixtures thereof, wherein the alkali metal salt is similar to the alkali metal comprising the anode; and d) an additive dissolved in the electrolyte having weak 0-X bond with a dissociation energy less than about 80 kcal/mol on either one or both sides of a carbonyl group, wherein X is selected from the group consisting of C, 0 and N.

13. The electrochemical cell of claim 12 wherein the additive is selected from the group consisting of di- (N-succinimidyl) carbonate, benzyl-N-succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof.

14. A method for reducing voltage delay in an electrochemical cell activated with a nonaqueous electrolyte which comprises: a) providing an anode comprising an alkali metal; b) providing a solid cathode of electronically conductive material; c) activating the electrochemical cell with the nonaqueous electrolyte operatively associated with the anode and the cathode, the nonaqueous electrolyte comprising an alkali metal salt dissolved therein, wherein the alkali metal of the salt is similar to the alkali metal comprising the anode; and d) dissolving an additive into the electrolyte, the additive having an 0-X bond on either one or both sides of a carbonyl group, wherein the 0-X bond has a dissociation energy less than about 80 kcal/mol and wherein X is selected from the group consisting of C, 0 and N.

15. The method of claim 14 including selecting the additives from the group consisting of di- (N-succinimidyl) carbonate, benzyl-N-succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof.

16. The method of claim 14 providing the nonaqueous electrolyte comprising a low viscosity solvent and selecting the low viscosity solvent from the group consisting of an ester, an ether and a dialkyl carbonate, and mixtures thereof.

17. The method of claim 14 providing the nonaqueous electrolyte comprising a high permittivity solvent and selecting the high permittivity solvent from the group consisting of a cyclic carbonate, a cyclic ester and a cyclic amide, and mixtures thereof.

18. A method for reducing voltage delay in an electrochemical cell using a nonaqueous electrolyte, which comprises: a) providing the anode comprising an alkali metal; b) providing a solid cathode of electronically conductive material; c) activating the electrochemical cell with the nonaqueous electrolyte operatively associated with the anode and the cathode, the nonaqueous electrolyte comprising at least one ion-forming alkali metal salt selected from one of the group consisting of hexafluorophosphate, hexafluoroarsenate and hexafluoroantimonate, and mixtures thereof, wherein the alkali metal salt is similar to the alkali metal comprising the anode; and d) dissolving an additive in the electrolyte, the additive having an 0-X bond on either one or both sides of a carbonyl group, wherein the 0-X bond has a dissociation energy less than about 80 kcal/mol with X selected from the group consisting of C, 0 and N.

19. The method of claim 18 including selecting the additives from the group consisting of di- (N-succinimidyl) carbonate, benzyl-N-succinimidyl carbonate and dibenzyl carbonate, and mixtures thereof.