CA1315842C

CA1315842C - Nonaqueous cell

Info

Publication number: CA1315842C
Application number: CA000579903A
Authority: CA
Inventors: Laura Estelle Brand; Ignacio Chi; Shelie Monroe Granstaff, Jr.; Brijesh Vyas
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1987-10-13
Filing date: 1988-10-12
Publication date: 1993-04-06
Anticipated expiration: 2010-04-06
Also published as: DE3878527D1; US4753859A; DE3878527T2; JP2618453B2; EP0312236B1; KR890007446A; JPH01128369A; EP0312236A1; KR920003759B1

Abstract

Abstract The present invention relates to a nonaqueous cell. The cell is comprised of a lithium negative electrode, a positive electrode which is comprised of active material and electrolyte. The electrolyte is comprised of a solvent and current carrying species. The solvent is comprised of at least 15 mole percent ethylene carbonate, at least 15 mole percent propylene carbonate and at least 15 mole percent polyethylene glycol dialkyl ether. These three materials combine to provide a cell which has an increased fire hazard safety. Additionally the addition of polyether to the propylene carbonate/ethylene carbonate mixture increases the cycle life of the cell.

Description

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NONAQUEOUS C~LL
Technical Field The invention is a nonaqueous, lithium, rechargeable cell. In particular, ~he invention pertains to the electrolyte composition in such cells.5 Background uf the Invention Lithium nonaqueous batteries have attracted considerable interest in recent years because of their potentially high voltage and high energy coneent per unit weight. A particula~ly challenging problem associated with rechargeable lithium nonaqueous batteries is improvement of their recycling characteristics. It 10 would be desirable for many applications that both efficiency and extent of recyclability be improved. It is desirable to accomplish this while maintaining the high voltage and high energy content charactenstic of primary lithium batteries.One of Ihe major problems in nonaqueous cell design is safety, particularly when the cell is exposed to drastic environmental conditions such as 15 electrically shordng the cell, rapid ~vercharge or Iapid discharge. The basicproblem is the high reactivity of the cell components (e.g., lithium metal) together with cell design ~quirements necessary to produce a çell with good capacity extensive cycle life and reasonable charge/discharge rates.
An important pari of cell design for nona~ueous cells is the 20 ingredients and composition of the electrolyte system. The electrolyte system is made up of organic solvent and current-ca~ying species. Stringent requirements are imposed on solvent and current-carrying species. For example, they must be chemically inert to electrode materials (e.g., lithium metal for the negative electrode, and niobium triselenide for the positive elec~ode material). It should 25 also exhibit sufficient ionic conductivity so as to pelmit reasonable charge and discharge rates. Generally, this requires sigilificant solubili~ of current-ca~ying species in the solvent. Also, the elecaolyte should wet the sepaTator to insure reasonable ionic conductivity thr~ugh dle separator.
Electrolyte systems for nonaqueous batteries have been discussed in a 30 number of references including U.S. patent 3,928,067, issued to J. Broadhead et al. on December 23, 1975, and U.S. patent 3,864,167, iSSlled to J. Broadhead et aL on February 4, 1975. The first patent ('û67) discloses the use of various polyethylene glycol dial3tyl ethers as additives in small amounts (1-5 weight percent) to electrolyte systems for nonagueous batteries to wet the 35 separator and obtain the high charge and discharge rates. The second patent ('167) describes nonaqueous cells with a number of differene positive 1 3 ~ 2

- 2 -electrode materials including niobium triselenide.
Particularly desirable is a lithium, nonaqueous, rechargeable cell with high energy density and cycle capacity which is safe, especially when exposed toextreme conditions such as electrical shorting or high temperatures.
S Summary of the Invention The invention is based on the discovery that substantial amounts of polyethylene glycol dialkyl ethers in the electrolyte of nonaqueous~ lithium cçll prevents explosion on abusive testing (e.g., shorting the cell) and substantially increases the safety of the cell, in addition to improving the cycling efficiency of 10 the cell. The invention is a nonaqueous lithium cell with an electrolyte comprising solvent and current-carrying species. The solvent comprises propylenecarbonate, ethylene carbonate and substantial arnounts of one or more polyethylene glycol dialkyl ethers. Preferred polyethers are polyethylene glycoldimethyl ethers such as triglyme and ~etraglyme. Such nonaqueous cells have a 15 number of advantages over prior art cells including high safety (freedom fromviolent explosion on exposure to extreme conditions), high charge rates and discharge rates, long cycle life and high shelf life. Generally, at least 15 mole percent propylene carbonate, ethylene carbonate and polye~her proves useful in preventing cell explosion during abusive testing. More preferred is at leas~ 25 20 mole percent polyether (pIeferably triglyme or tetraglyme) in the propylene carbonate/ethylene carbonate rnixture. Various current-car~ying species a~e included in the composidon of the electrolyte. Typically, these current-carryingspecies are lithium salts with or without tetra-aL~cylamrnonium salts soluble in the solvent. Typical lithium salts are lithium hexafluoroarsenate and lithium 25 hexafluorophospha~e. Multiple salt electrolytes such as tetra-aL~ylammonium salts plus lithium salts are useful to improve ionic conductivity, especially at low temperatures. The addition of diethylcarbonate may also be use~ul, particularly for low temperature operation. Such electrolyte systems wet separators used in nonaqueous cells and permit high solubility of current-caIrying species so as to30 provide reasonable discharge (and charge) rates, improve cycle per~ormance for the cells and eliminate explosions and reduce fire hazard during abusive testing.
Brief Description of the Drawin~
FIG. 1 shows a rectangular, nonaqueous cell featuring positive electrode, negative electr~de and separator material;

~ 3 -- ~3 (, ~ i ~ d FIG. 2 shows a cylinclrical, nonaqueous cell featuring positive electrode, negative electrode and separator material;
FIG. 3 shows data in graphical form on cycle life of a nonaqueous battery made in accordance with the invention; and S FIG. 4 shows several differential scanning calorimetry curves ~or samples containing lithium and valious electrolyte compositions.
Detailed Description An understanding of dle invention is facilitated by a drawing of the structural formula of the substances discussed in the disclosure.
C~30[CH2CH20]%CH3 Polyethylene glycol dialkyl ether (Glymes) diglyme x = 2 triglyme x = 3 tetraglyme x = 4 CH~ H El H
C

\C/
3d Propylene Carbonate H H H H
C - -- - C

O\ /0 Il 0, ~thylelle Carbonate - 4 - ~ 3 ~ ~ ~ f~ ~

The invention is based on the discovery that the inclusion of substantial arnounts of certain polyethylene glycol diaLlcyl ethers such as polyethylene glycol dimethyl ethers (glymes) in the electrolyte system greatly S improves safety for nonaqueous cells as well as improves the properties of nonaqueous cells ineluding cell capacity and cycle performance. Also discovered was that the inclusion of certain aliphatic diaLkylcarbonates (e.g., diethyl carbonate) in the electrolyte solvent system improves ionic conductivity and cell perfor nance especially at low temperatures. In addition, the use of more than one 10 salt, particularly mix~ures of lithium salts and tetra-aL~cylammonium-type salts imp~oves electrolyte conducti~dty and low temperature performance.
Particularly significant in the invention is the composition o~ the electrolyte system. It has been found that the inclusion of significant arnounts of polyethers such as glymes in the electrolyte system dr3matically reduces the safety lS hazards (e~g. explosion, fire, etc.) associated with sudden shorting and heating of non-aqueous, lithium cells. Amounts grea~er than lS mole percent dramatically reduces safety hazards such as tendency to explode on shorting. Also, the presence of significane amounts of polyethylene glycol dialkyl ethers promotes wetting ofthe separators so as to permit rapid charging and discharging of the non-aqueous20 lithiurn cell.
Extensive tests are carried out to determine the optimum or preferred composition of the electrolyte. At least 15 mole percent of each of the ma~or components is preferred, the major components being ethylene carbonate, propylene carbonate and polyethylene glycol diaLl~yl ethers. Various polyethylene 25 glycol dialkyl ethers are useful. V arious glymes (polye~hylene glycol dimethyl ethers) are useful including diglyme, triglyme, tetraglyme, pentaglyme, hexaglyme and octaglyme. Various other polyethers are also of use including diethylene glycol diethyl ether, die~hylene glycol dibutyl ether, diethylene glycol ethyl-t-butyl ether, diethylene glycol methyl-t-butyl ether, tetraethylene glycol diethyl ether and 30 tetraethylene glycol dibutyl ether. Usually, it is advantageous if the electrolyte remain liquid so that preferred glycol sthers are those that insure that the electrolyte remain liquid.
Mixtures of these ethers are also useful. Most preferred is ~iglyme and tetraglyme and mixtures of these two ethers. More pre~elled is 20 to S0 mole35 percent polyether with 25-40 mole percent most preferred. The preferred amounts of ethylene carbonate are between 25 and 40 mole percent and the preferred ~3~ 3c~

amount of propylene carbonate is from 25 to 40 mole percent. An excellent electrolyte consists essentially of 30 mole percent of triglyme, tetraglyme or mixture of triglyme and tetraglyme, 35 mole percent propylene carbonate and 35 mole percent ethylene carbonate.
Optionally, other solvent ingredients may be included in the electrolyte system. In 5 particular, various additional solvents may be added to decrease viscosity and increase ionic conductivity. Particularly useful are aliphatic dialkyl carbonates with the alkyl substituents having between two and six carbon atoms. Particularly useful is diethyl carbonate because of high stability under conditions of battery operation. Typical additions are from O to 30 mole percent, with 20 mole percent preferred.
The electrolyte includes, in addition to the solvent system described above, various current carrying species such as lithium salts, other soluble salts and tetra-allyl ammonium type salts. Typical lithium salts are LiPF6, LiAsF6, LiS~104, LiCF3S03, LiBF4, LiAlC14, LiI, LiF and LiE3r with LiPF6 and LiAsF6 preferred and LiAsF6 most preferred. Other salts are also useful including tetra-alkylammonium salts with the anion being either hexafluoroarsenate, 15 hexafluorophosphate, tetra auoroborate, perchlorate and halides such as chlorine, bromine and iodine and alkyl groups typically with up to six carbon atoms. Tetrabutylammonium salts and tetraethylammonium salt are preferred because of easy availability, high solubility and good conductivity exhibited with such çlectrolytes.
Particularly useful are electrolytes with more than one salt. Two or more lithium salts 20 may be used (e.g., LiPF6 and LiAsF6). Preferred is the mixture of lithium salt (preferably LiPF6 and/or LiAsF6) and tetra-alkylammonium salts (e.g., one or more of the tetrabutylammonium salts and tetraethylammonium salts). Such a mixture of salts yields exceptionally high charge and discharge rates especially at low temperatures.
Generally, the concentration of current-carrying species may vary over large limits, 25 typically from O.Q5 molar to saturation. Preferred concentrations are often determined by the concentration of maximum conductivity of the electrolyte solution, often around 0.25 to 0.75 oE
the saturation concentration. For example, for lithium salts, such as lithium hexafluoroarsenate and lithium hexafluorophosphate, typical concentrations are 0.4 to 1.5 molar with 0.6 to 1.0 molar preferred. For tetra-allylammonium salts, concentrations between 0.1 and 1.0 molar are 30 ~pical. For mixtures of lithium salts and tetra-allylammonium ~-. t .~

.

~ 3 ~

salts, lithium salt concentrations of 0.4 to 0.8 molar and te~a-alkylammonium salt concentrations of 0.2 to 0.4 molar are prefeTred.
A variety of cell structures may be used in the practice of the invention. Generally, lithium is pre~erred as the active material in the negative S electrode because of high electrode potential although sodium and potassium might be used. Also, a large variety of material can be used as the active material in the positive electrode including transition-metal chalcogenides. Particularlyusefill are a number of positive electrode materials such as NbSe2, NbSe3, MoS2,MoS3, TiS2, TiS3, TaS2, V6Ol3 (stoichiometric and nonstoichiometric), CoV2 and 10 MoO2. Generally, it is desirable to use positive electrodes with high cycle life, high energy density, etc. Particularly useful for these reasons are positive electrodes made ~om NbSe3. This positive electrode, including procedures for preparadon, is shown in U.S. Patent No. 3,864,167, issued to J. Broadhead et al.on Febroary 4, 1975.
Various separator materials are also of use in the practice of the invention including various polymer materials, such as polyethylene and polypropylene generally made in the form of a microporous film. Preferred are various microporous polypropylene separators such as Celgard(~9 2400 and Celgard@~ 2402 made by the Celanese CoIporation.
Various cell structures and sizes may be used in the practice of the invention. A typical rectangular structure is shown in FIG. 1. This figure showsa cross-section of a test cell s~ucture 10 with plastic (polypropylene) holder 11, lithium negative electrode 12 with separator 13 and NbSe3 positive elec~rode 14 with inert spacer material 15 to ensure a close St in the cell holder 11. Metal 25 wires 16 are used to conduct electrical energy out of the cell. The electrodes are covered with electrolyte 17 in accordance with the invendon. Such structures areuseful for commercial cells as well as for evaluating cell components, and electrolyte compositions.
FIG. 2 shows another cell structur~ 20 useful in the practice of the 30 invention. This cell structure is often called the rolled cylindrical cell structure.
Four layers are put together and rolled into a cylindrical shape. The four layers are the negative lithium electrode 21, the separator 22, the positi ve electrode 23 ~e.g., NbSe3) and another separator layer 24. The roll is generally put into a cylindrical container with suitable electrical connections to positive and negative 35 electrodes. The cylindrical container is filled with electrolyte to permit electrochernical action.

- 7 - ~ J ~ ~,r ~
Various tests were carr~ed out to compare the cycle characteristics of lithium cells with only propylene carbonate and ethylene carbonate as solvents in the electrolyte and with a polyether (tIiglyme) in addition to propylene carbonate and ethylene carbonate as the solvent sys~em. FIG. 3 shows the results of one 5 such comparison test. This figure shows data on the cycle characteristics of cells with and without the polyether. Shown in FIG. 3 is a graph of the capacity characteristics of the cells as a function of cycle number for a cell with and without the polyether. The charging current was 200 mA and discharging cuIrent 400 mA. Cycles were carried out between 2.4 and 1.4 volts, and 0.8 molar 10 LiAsF6 was used as the electrolytic salt.
As can be seen by the ~aph, the addition ~ polyether to the propylene carbonatelethylene carbonate mixture dramatically increases the cycle life of the cell. Indeed, it also increases the capacity of the cell over all of its cycle life and results in a cell of great commercial interest.
Data of safety a~e obtained in a variety of ways. It is generally well known that heating lithium metal in the presence of the elec~olyte solvent will eventually lead to chemical reaction. The nature of this reaction with vaIious solvents is parlicularly impor~ane with respect to cell sa~ety.
Differential thermal analysis (DTA) provides an unusually accurate 20 procedure for observing the reaction of organic solvents used in electrolytes with lithiu~rL Experiments were carried out to study dle thermally induced reaction of lithium with vaIious mixed solvent electrolytes using differential scanning calorimetry (DSC). The solvents used are propylene carbonate (PC), ethylene carbonate (EC), 2 methyltetrahydroforan (MeTHF), diethylene carbonate (DEC), 25 triethylene glycol dimethyl ether (triglyme or trig), and tetraethylene glycol dimethyl ether (tetraglyme or tetrag).
A compaTison of the l:)SC curves for four electrolytes is shown in FIG. 4. The main feature for comparison is the temperatur~ of the peak maxima.
In this figure, the peak heights and areas are not normalized for the amount of 30 reactants, which differed gready, and thus, these values cannot be directly compared. Various mixed solvent electrolytes were measured and are shown in FM. 4. The much higher temperature of the peak maximum for the 35PC/35EC/30Trig, 0.8M LiAsF6 electrolyte is an indication that it should behavemore safely under rapid heating abuse of an actual cell, than the other three 35 electrolytes shown. This was verified by a rapid heating test on actual cells using heating rates in the range of 70C min 1.

~ 3 ~ 2 Table 1 lists some features of the reaction exotherm obtained from ~he DSC cu}ves at 10C min~l for various electrolytes. The peak height and total heat are normalized by dividing the weight of lithium consumed in the reaction. The ratio of weight of lithium reacted to weight of electrolyte reacted is also given as 5 an indication of the reactivity of the electrolyte with the lithium. These values for the electrolytes containing DEC and 2-MeTHF are less accurate because of the voladle natu~e of the solvents causing decreased accuracy in the weight and solvent ratio of these electrolytes. The electrolyte containing DE(: had a further complication since it reacted with the lithium below its melting point. This 10 results in an elTor in its heat output as nolmalized to the weight of lithium. Of all the parameters measured the temperature of the peak maximum has been determined to be the most reliable parameter for comparing the relative safety of the different electrolytes. In general, by comparison with cell heating tests, the higher this temperature is, the safer the electrolyte will be in actual cell abuse.
15 The other parameters give useful in~ormation but must be considered caudously and with respect to one another and the peak maximum temperature. The initiadon temperature can be valuaUe for simpler systems such as single solvents, or predominantly one solvent mixn~re. It is less usefill for complicated systems, with more equal solvent ratios and high concentrations of salt, which may show 20 muldple exothermic peaks. The peak heights and total heats per gram of lithium are more difficult to use in comparing different systems. At a given hea~ing rate both of these can be larger, for what may be a less reactive system, because theinitiation and peak maximum temperatures are higher. This would cause the reactions to go faste~ and could involve more overlap of reacdons within one 25 apparent peak. Therefore, the inidation and peak maximum temperatures are considered to be qualitative weighting factors.

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TABLE I
SUMMARY OF DSC EXPERIMENTS AT IO C rnin~l Init. TempPeak HeatPeakTemp.To~l Heat Li Consumed Elect~olytes~(~C) (Wgli ) ( C) (KJ gL} )(gli gSol) 35PC~35EC/30Trig 259.00 -140iO0 263.QO -19.40 0.27 40PC/40EC/20Trig 257.00 -110.00 272.20 -25.20 0.20 70EC/30Trig 269.00 47.00 272.20 -14.60 0.21 70PcRoTng 231.00 -110.00 241.10 -22.00 0.22 lOQTrig 195.00 -102.00 205.70 -15.00 0.32 10 35PC/35EC/3QTetrag 265.00 -142.00 270.50 -20.30 0.19 40PC14QEC/20Tetrag 259.00 -218.00 269.10 -22.10 0.20 SOPC/40EC/lOTe~ag 260.00 -140.00 264.20 -22.10 0.24 lO~Tetrag 154.00 -75.00 183.00 -14.60 0.32 50PC/SOEC 208.00 -108.5Q 221.00 -19.50 0.31 15 60PC/40EC 212.00 -78.00 238.0Q -14.00 û.S2 40PC/40EC/20I)EC 200.00 -165.00 215.00 -34.00 0.20 80PC/20MeT~;224.00 -295.0Q~ 232.00 -25.00 0.26 ~AIl are 0.8 M LiAsF6 except 80PC/20MeTH~ which was 1.2 ~L

Claims

Claims:

1. A nonaqueous cell comprising lithium negative electrode, positive electrode comprising active material and electrolyte comprising solvent and current carrying species characterized in that the solvent comprises at least 15mole percent ethylene carbonate, at least 15 mole percent propylene carbonate and at least 15 mole percent polyethylene glycol dialkyl ether.

2. The cell of claim 1 in which the solvent comprises at least 15 mole percent ethylene carbonate, at least 15 mole percent propylene carbonate and at least 25 mole percent polyethylene glycol dialkyl ether.

3. The cell of claim 2 in which the solvent comprises 25 to 40 mole percent ethylene carbonate, 25 to 40 mole percent propylene carbonate and 25 to 40 mole percent polyethylene glycol dialkyl ether.

4. The cell of claim 1 in which the polyethylene glycol dialkyl ether is polyethylene glycol dimethyl ether.

5. The cell of claim 4 in which the polyethylene glycol dimethyl ether is triglyme.

6. The cell of claim 4 in which the polyethylene glycol dimethyl ether is tetraglyme.

7. The cell of claim 1 in which the current carrying species comprises at least one lithium salt selected from the group consisting of LiPF6, LiAsF6, LiClO4, LiCF3SO3, LiBF4, LiAlCl4, LiI, LiF and LiBr.

8. The cell of claim 7 in which the lithium salt is LiPF6.

9. The cell of claim 7 in which the lithium salt is LiAsF6.

10. The cell of claim 7 in which the concentration of lithium salt is between 0.05M and saturation.

11. The cell of claim 10 in which the concentration of lithium salt is between 0.25 and 0.75 of saturation.

12. The cell of claim 7 in which the current carrying species comprises lithium salt and tetraalkylammonium salt with up to six carbon atoms on the alkyl group.

13. The cell of claim 12 in which the current carrying species comprises lithium salt and one or more tetrabutylammonium salts with anion selected from the group consisting of chlorine, bromine, iodine, hexafluoroarsenate, hexafluorophosphate, tetrafluoroborate and perchlorate.

14. The cell of claim 12 in which the current currying species comprises lithium salt and one or more tetraethylammonium salts with anion selected from the group consisting of chlorine, bromine, iodine, tetrafluoroborate, hexafluorophosphate, hexafluoroarsenate and perchlorate.

15. The cell of claim 12 in which the lithium salt has a concentration between 0.4 and 0.8 molar and tetra-alkylammonium salt has a concentration between 0.2 and 0.4 molar.

16. The cell of claim 1 in which the solvent comprises in addition diethyl carbonate with concentration between 0 and 30 mole percent.

17. The cell of claim 16 in which the concentration of diethyl carbonate is 20 mole percent.