CA1210056A - Electrochemical cells having low vapor pressure complexed so.sub.2 electrolytes - Google Patents

Abstract

ELECTROCHEMICAL CELLS HAVING LOW VAPOR PRESSURE

A non-aqueous electrochemical cell having a solid active cathode, an active metal anode and a low vapor pressure highly conductive electrolyte comprising a liquid solvate-complex of sulfur dioxide (SO2) and an alkali or alkaline earth metal salt soluble therein such as those having a Group 3A element halide anion with the ratio of salt to SO2 in said electrolyte ranging from about 1:1 to 1:7.

Description

5~;

This invention relate6 to electrolytes for non-aqueous electrochemical cells and more particularly to such electrolytes contalning ~ulfur dioxide (S02).
Sulfur dioxlde, though a poor electrolyte ~olvent has nevertheless been wlde~y utilized ln non-aqueous electrochemical cells in the dual functlon of cathode depolarlzer and electrolyte solvent because of its high energy density and high rate capability. Without its functioning as the cathode depolarizer 52 has rarely been utilized as an electrolyte solvent alone since in addition to its poor solvating properties S02 has several other shortcomings which are generally related to its being a gas at room temperature and pressure (B.P. 10C). For proper utilization the gaseous S02 is converted into a liquid under conditions of low temperature and/or high pressure and must be maintained in such liquid form by constant pressuri~ation. As a result, cells havlng S02 therein have had the attendant disadvantage of requiring expensive reinforced cell containers and hermetic seals reslstant to ~he volatile S02.
Additionally, expense was further incurred AS a result of the need for the initial llquefaction o~ the S02 and the 6pecial handling required particularly wlth respect to the filling of thP cells ~ith the volatile liquid S02. The safety aspects of cells containing S02 were also somewhat of a problem since required safety venting mechanisms, while providing protection, nevertheless operated by the at~ospheric expulsion of noxious gaseous S02.
In order to aid in electrolyte solvation and to reduce the high vapol pressure of the S02, cells have generally contained organic cosolven~s 6uch as acetonitrile ~ith the S02. ~owever~ despite ~he presence of the vapor pressure reducing organic cosolvents, the cells nevertheless remained highly pressuri~ed with its at~endant dlsadvantages. Furthermore, organic cosolvents such as the aforementioned acetoni~rlle generally precluded efficient cell cycling and occasionally in ~hemselves posed potential safety problems when the cells were abused. Electrolyte salts which were found to readily dissolve in the S02 without the necess$ty for organic cosolvents either provided poorly operatlng cells e.g.
LiAlC14 or were generally prohibltively expensive e.g. clovoborate salts such as Li2BloCllo. Furthermore, such salts when dissolved in the liquified SO2 though alleviating hazard problems associated with or~anic co-solvents did not alleviate the problems caused by ~he high vapor pressure of the sulfur dioxide.
It is an object of the present invention to provide a non-aqueous cell having a low vapor pressure electrolye based upon S0~ and a method for the preparation of such electrolyte.
It is a further obJect of the present invention to provide such electrolyte having a ~ery high conductivity and thus being sui~able for high rate applications; being economical with respect to both component materials and the preparation thereof; being a llquid at normal temperature and pressure; being chemically stable in non-aqueous cell environments;
being suitable for secondary or rechargeable cell applications; being suitable as an electrolyte over a wide temperature range; and being without free organic materials and therefore being safer than conventional organic electrolytes.

These and other ob~ects, features and advantages of the present invention will become more evident from the following discussion and drawings in which:
Figure 1 is a vapor pressure comparison of a prior art S02 electrolyte and an electrolyte of the present invention, Figure 2 is a graph of the conductivities of various electrolytes of the present invention at various temperatures;
Figure 3 is a graph of the cycling characteristics Df a cell having the electrolyte of the present invention;

Figures 4-7 are discharge curves of various cells having the electrolyte of the present invention. Figure 5 also depicts a charging curve for one embodiment; and Q5~;

Figure 8 is a graph of the polarization characteristics of a cell having an electrolyte of the present invention.
Generall~ the present invention compri6es a low vapor pTessure (below 2 atm at room temperature and preferably below 1 atm) high ronductivity electrolyte based upon S02; a method for the preparation thereo; and both non-aqueous primary and secondary cells ha~ing active metal anodes such as of alkali or alkaline earth metal including mixtures and alloys thereof and containing such low vapor pressure electrolyte.
The electrolyte 4f the present invention is comprised of a tightly bound solvate-complex of S02 and an alkali or alkaline earth metal salt soluble therein such as those wherein the anion of the salt is comprised of a Group 3A (of the Periodic TablP) ~lement halide. The ~7roup 3A elements which are preferred for the salt are boron, aluminum, gallium and indium and the preferred alkali and alkaline earth metals are lithium, sodium and calcium. Examples of preferred Group 3A salts suitable for complexing with the S02 include LiAlC14, LiGaC14, LiBF4, ~iBC14, LiInC14, NaAlC14, NaGaC14, Na8F4, NaBC14, NaInC14, Ca(AlC14)2, Ca(GaC14)2, Ca~BF4)2 Ca(BC14)2, Ca(~nC14)2, Sr~AlC14)2, Sr(GaC14)2, Sr(BF4)2, Sr(BC14)2, Sr(InC14)2 and mixtures thereof. Non-Group 3A salts sultable for complexing with the S02 include Li3SbC16 and LiSbC16. Salts which do not complex with the S02 may however be dissolved in the solvate-complex if desired.

The solvate-complexlng of the S02 and the salt is dependent upon equivalent ratios of the materials rather than mole ratios with such difference being apparent with respect to, for example, alkaline earth metal salts which generally contain two equivalents per mole. The equivalen~ ra~ios range from abou~ 1:1 to 1:4 (sal~:S02~ and because of such variation the combinatlon of the salt snd S02 is demonstrated as belng in the nature of a complex rather than a reaction-formed n~w compound. Though the actual complexing of the sal~ to S02 is generally up to a ratio of 1:4 ~salt:S02), addi~ion of uncomplexed S02 to the cell in an amount of up to about 1:7 (salt:total S02) ~ill not generally ~2~Qa!56 de,.i~entaily pressurize che cell 2t room temperature. h'ith sai. tO S02 ra;ios above l:/ not onl~ does the cell becor.~e de.rimen~ally p.essurized (above about 2 at~.), the conductivlty of the electrolyte is 21so reduced thereby. Electrolytes such as the l molar LiAlC14 in 5O2~(equivalent r2.io of about 1:22 of Li.~lCl4:SO2) as described in U.S. Patent ~o.
3,493,433 2re highly pressurized (about 3.5 atm.) and are in fact described in s2id patent as being utilized at tem?eratures between -10C to -30C.
Because of its being in unavailable complexed form wi~h only a minimal, if any, content of uncomplexed SO2, the SO2 in the electrolyte does not function as the active cathode de?ol2rizer. rne electrolyte o, the present invention therefore has its main utility in eiectrochemical cells having solid cathode depolarizers. Such cathodes include CuCl2, CuO, CuS, ~L~O2, Cr3O~, V205 as well as other metal halides, oxides, chromates, vanadaces, titanates, tungstates, chalco~enides and active non-~etal cathodes such as organic conductive pol~;mers such as poly-acetvlene, poly-?-phenylene, polyphen~lene sulfide and various carbon com~ounàs such as C S and C~ , x n where "~" is in the ran~e o from 4 to 50, and "n" i.s a finite but large number.

rnough it would have been e~pected that the solid cathodes would provide cells having reduced hi~h rate capability when compared to cells having fluid cathode depolari~ers such as S02 such reduction is in fact minimized by the une~pectedly very high conductivity of the electrolyte of the present in~ention. Furthermore, advantages of a substantially non-pressurized system, particularl,v with respect to increased safety ~ore than co~pensates for any reduction in high rate capability.
rne solvate-comple~ electrolyte of the present invention is pre?ared by re2cting the SO2 with the al'.iali or al~aline earth metal sal~ in the requisite equivalent ratios. Such reaction ~ay be effected by substantiallv saturatinr ]iqulfied SO2 with the salt to the requisite equivalent ra.ios. It is however preferr2d that the salt be reacted with the SO~

in gaseous form such as by passing a stream of dry S02 through the salt whereby an exo.hermic reaction occurs wlth the formation of a liquid ~Z1~6 solvate-complex. The resultant liquid has a low vapor pressure ( ~10 psi at 20 C wlth a B.P. of about 40~C) and can be handled as a llquld in contrast to liquified S02 (B.P. 10C) which mu6t be specially handled as a volatile material. Figure 1 compares the vapor pressure of prior art S02 c~nt~1n~ng electrolyte (Curve A) and a LlAlC14 3-5 S02 solvate-complex (Curve B) at various temperatures. At room temperature (20~C) prior art S02 electrolytes have high vapor pressures (about 50 psi or about 3 1/2 atm) whereas the ~olvate-complex has a vapor pressure of about 10 psi, well below atmospheric pressure of 15 psi. Pressure rise of the solvate-complex electrolyte is logarithmical with a rise in temperatu~e to a pressure of 60 psl at 80~C. This is ln further marked contrast to the 260 psl at ~0C of the prior art S02-containing electrolytes.
It is no~ed that the S02 need not be direcely reacted with tne salt per se bu~ may in fact be reacted with for example theLewis acid and base components of the ~alt whereby the 6alt and ~he liquid solvate-complex are simultaneously produced. For exa~ple, a stream of dry S0~
may be passed ehrough a LiAlC14 salt or a 1:1 stolchiometrlc mixture of the Lewis acid and base components thereof, LiCl and AlC13~ to produce the same liquid LlAlC14 xS02 solvate-complex, with l'x" having been determined as rangin~ from abou~ 1 to 4 (equivalentbase5 ). Continued bubbling of S02 therethrough results in a higher value for "~" in the range. Lower values for "x" may be obtained by evaporating some of the S02 from the liquid solvate-complex. Addition of excessive S02 as described above ~herein the ratio of total S02 to salt exceeds 7:1, however, forms an unde6irable pressuri2ed(above about 2 atm) electrolyte.
The conductivity at room temperature of a solvate-complex electrolyte of LiAlC14 3.5 S02 has been discovered ~o be about 1 x 10 lohm lcm 1 which is the highest conduceivity observed to date ln any non-aqueous electrolyte. ~he electrolyte of the present ~nvention has been found to be very 6table wlth lithium anDdes and has been further found to enable lithium to be electrochemically plated and stripped therein with efficiencies of over 97% even over extended cycling reglmens whereby it is an eminently ~2~ S~

suitable electrolyte for rechargeable llthium or other alkali or alkaline earth metal cells.
A solvate-complex of NaAlCl~ 2.8 S02 while having somewhat lD~er conductivity of 8 x 19 2ohm cm 1 (but still very high) has the advantage of good low temperature operabillty e.g. conductivlty of 2 x 10 2ohm lcm 1 even at -30 C- In contrast to the LiAlC14 xS02 solvate-complexes which freeze at temperatures between about -8DC to -15DC the NaAlC14 xS02 solvate-complexes freeze at about -44C and are more ~ultable when low temperature operation is desired. A solvate-complex of Ca(AlC14)2- 2.75 S02 whlch has a conductivity of 1.7 x 10 ohm lcm 1 at room temperature and about 10 ohm cm at ~35C does not ln face freeze but rather becomes immobile at about -509C. It can accordingly be admlxed with other ~olvate-complex salts cuch as LiAlC14 3.5 S02 to provide an electrolyte having both high conduceivity and extremely low snd high temper~ture capability.
Alternatively3 it hss been discovered that the lncorporation of additive amounts ~f inorganic solvenes 6uch as SOCl~, S2C12, SC12, S02Cl~ admixtures thereof to the solvate-complex electrolyte also serves to enhance low temperature capability. For example, a mixture of 90%
(by weight) LlAlC14 2.7 S02 and 10% SOC12 (containing 1 M LiAlC14) freezes ~t abDut -25C with a conductivity of about 1.8 x ~0 2ohm lcm 1 at -20DC.
Though llthium ha~ been found to be ~table ln ~he psesence of, for example, the NaAlC14 and Ca(AlC14)2 solvate~complex elec~rolytes, in secondary or rechargeable cell applications it i6 preferred that the complexed 6alt contains cations ~orrespondlng to the anode metal.
For primary cell applications other salts ~uch as the aforementioned sod$um and calcium salt6 m~y be af~ectlvely util~zed ~ith lithium anodes particularly ln provlding enhanced low temperature capability.
and preferably Addltionally/for primary cell applications electroly~e ~alts normally not ~oluble ln S02 alone m~y be utilized by ctoichiome~ric complexing with both S02 and an organic COBolvent ~uch as ace~onitrile, ethers such :"~

as dimeth~ thane; propylene carbonate and the like. Such salts include LiBr, LiC104, LiAsF6 and LiPF6. The organi~ cosolvent theTeby ~akes ~uch salts sDluble in the S02. The or~anic cosolvent is present only in suf-ficient quantity to co-complex the salt with the hazards of free organic materials not being present. Soluble salts ~ay similarly be utillzed with such co-complex.
The fact that the electrolyte of ehe present inves~ion has a low vapor pressure despite lts S0~ component results in seversl very important economic and safety beneflt~. Cells made therewlt}l need ~ot be reinforced or otherwise nade resistant to pressurized contents. Ventlng, lf at all necessary as a safety precaution does noe result in the emission of rapidly spreading noxious 52 fumes. Her~etic seals ~or the cells are not as susceptlble ~o degradation becau3e of the bound state of the nor~ally corrosive S02 and in fact more economical seals suitable for non-pressurized cells ~ay be utili~ed. Except for the initial relatively simple procedure of forming the liquld ~olvste-complex electrolyte as described above no special hand~in~ or storage is required in contrast to the handling of uncomplexed volatile S02. Filling of cells with the elec~rolyte is simply wi~h a stable liquid as opposed to filling cells with a volatlle pressuri2ed liquid 6uch as S02.
In order ~o more fully lllustrate the properties and benefleial aspects of the electrolyte of the precent 4nvention the following examples are presented. It is understood, however, ~hat such examples are illustrative in nature and are not to be construed BS llmiting the present inventlon. Unless otherwi6e indlcated all parts are parts by weight.

Stoichiometric amounta o~ LlCl and AlC13 were placed in a gla~s vessel and dry S02 was passed through the salt particles. A clear liquld solvate ~omplex of LlAlC14 S02 was formed rapidly wieh the ~eneration of heat and after cooling to room te~perature the molar or equivalent ratio of LlAlC14 eO S02 in ~he ~olvate-co~plex was determlned to b~ 1:3.1. Contlnued bubbling of the dry S02 through the LiCl and the AlC13 provided another clear liquid sol~a~e-complex having an equivalent ratio of LiAlC14 to S02 o~ 1:3.5~ Evaporation of ~ome S02 from the LiAlC14 3.1 S02 solvate-complex provided another clear llquid solvate-complex having an equivalent ratio of LiAlC14 to S02 of 1:2.6. The conductivities of the three l~quid 601va~e-complexes at varlous tempera-tures were measured as shown ~n Figure 2 as curves C, D and E respectively.
The conductivltles obtained were the highest ever obtalned for non-aqueous electrolytes. Additionally, llthium metal 6tored in the electrolytes for periods in excess of four weeks s~owed no corrosion thereby indicating the stabllity of such solvate-comple~ elect~olytes in lithium containing cells.

A NaAlC14 2.~ S02 solvate-complex was formed as in Example 1 but wi~h NaCl instead of LiCl with its conductivi~y at various temperatures shown i~ Figure 2 as Cur~e F.

A Ca~AlC14)2 2.75 S02-equivalent ratio ~Ca(AlCl4)2 5.5 S02 molar ratio) solv3te-complex ~as formed as in Example 1 but with a stoichio-metric ratio of CaCl2:2AlC13. The conductivity at various temperatures is shown in Fi~ure 2 as Curve G.

:~0 A ~ixed solvaee complex of ~LiAlC14 + 4Ca(AlC14)2) 5.6 S02 ~molar ratio) was made by passing S02 throu~h AlC13, LiCl and CaC12 ~molar ratio of 9:1:4). The resultant llquid solva~e-complex provi~ed conductivi~ies at various temperatures as shown in Figure 2 as Curve H.

A mi~ure of 90% LiAlC14 2.7 S02 ~olvate-complex and 10% 1 M
LlAlC14 in SOC12 wa~ made and provided conduct~vitie~ a~ varlous ~emperatures as shown ln Flgure 2 as Curve I.

,.

~2~5~

A cathode limited electrochemical cell was constructed with a 2 gram cathode of 60% CuC12, 30% graphite and 10% polytetrafluoroethylene (PTFE) pressed on a nickel grid, 1" (2.5 cm) x 1.6" ~4.1 cm) sandwiched by two lithium anodes each of si~llar dimensions. The cell was filled with the LiAlC14 . 3.1 S02 ~olvate-complex electrolyte of Example 1 snd dlschareed at the rate of 40 ma (2 ma/cm2) to a 2.6 volt cutoff and charged at a rate of 20 ma (1 ma/cm~ to 4.05 volts on a continuous cycling regimen. The cell has cycled for sbout 350 cycles at close to 1O07D of the one electron transfer csthcde capacity (Li ~ OuCl~ CuCl +
LiCl). The charging and diEcharging curves for cycles nos. six, 173 and 230 are shown in Flgure 3. Cumulative capacity so far is 72 Ahrs with about 36 Li t~rnovers (97% eff. on the anode). The lnitial theoretical primary capacity was 0.24 Ahr.

Three cells were made as in Example 6 bu~ with ~he solvate-complex electrolyte, (LiAlCl~ ~ 4Ca(AlC14)2)- 5.6 S02 of Example 4. The cells were discharged at rates of 20 ma, 40 ma and 60 ma respectively with dischar~e results as shown in Figure 4.
F.XAMPL~ 8 A cell as in Example 6 was made but with the ~olvate complex electrolyte of Example 5 (90% LlAlC14 2.7 S02 and 10~ lM LiAlC14 in SOC12). The cell was cycled at 40 ma (2 ma/cm ) di~charge 20 ma ( 1 ma/cm ) charge and the discharge-charge of the sixth cycle ls shown in Flgure S.

A cell as in ~xample 6 ~a6 made but with a 3 gram cathode comprlsed of 60% CuO, 30% graphite and 10% PTFE. The cell ~as discharged at 40 ma with results as shown ln Figure 6.

~2~56 Example 10 A cell as ln Example 6 was made bu~ with a 3 ~ram cathode comprised of 60% CuS, 30% graphlte and lOZ PTFE. The cell was dl~charged at 40 ma with results as shown in Figure 7~
Exa~ple ll A spirally wound Li/MnO2 cell having electrodes 6.25" (15.9 cm) x 1"
(2.5 c~) of lithlum and MnO2 with the cell being cathode limited to a theoretical capacity of 0.75 Ahrs was filled with the LlAlC14-3.1 S02 elec-trolyte of Example 1. The cell showed an initial open circuit voltage of 4.0 volts. The cell was then dlschar~ed at 85 ma and delivered a capacity of 0.56 Ahr to a voltage cutoff of 2.0 voles. The cell was char~ed at 40 ma for 14 hours and discharged again at 40 ma delivering O.40 Ah on the ~econd discharge. Figure 8 shows the polarization of the cell during discharge and char~e.
Effective utilization of the 1GW vapor pressure elec~rolyte of the present invention in Li/MnO2 cells may alleviate problems relatlng to the reactive electrolyte salts and volatile solvents generally utilized in such electrochemical cell systems such as lithium perchlorate and dimeth-oxyethane by simply repalcing them ~ith the relatively safe electrolyte of the present invention.
From the above examples it is evident tha~ the solvate-complex electro-lytes of the present lnvention provide for substantially non-pressurized cells whlch are efficien~ly rechargeabl~ and that 60me embodi~ents have conductivities well above those of previously kno~n non-aqueous electrolytes which did not generally have room temperature conductivities above about 5 X lO ohm cm It is understood that the above examples are illus~ratlve in nature and that changPs in the cathode materials, electrolyte composition and ratios as well as the cell systems in whlch they are utiliæed may be made without departin~ fro~ the scope of the present invention as defined in the following claims.

Claims (14)

What is claimed is:

1. A non-aqueous, electrochemical cell having an active anode, a solid active cathode and a liquid electrolyte characterized in that said electrolyte consists essentially of a low vapor pressure, liquid solvate-complex of a) sulfur dioxide and b) one or more alkali or alkaline earth metal salts soluble in said sulfur dioxide and also capable of forming a complex therewith, and wherein the equivalent ratio of said one or more salts to SO2 in said liquid electrolyte ranges from 1:1 to 1:7.

2. The cell of claim 1 wherein said one or more salts contains Group 3A element halide anions.

3. The cell of claim 1 wherein one or more salts are selected from the group consisting of LiAlCl4, LiGaCl4, LiBF4, LiBCl4, LiInCl4, NaAlCl4, NaGaCl4, NaBF4, NaBCl4, NaInCl4, Ca(AlCl4)2, Ca (GaCl4)2, Ca(BF4)2, Ca(BCl4)2, Ca(InCl4)2, Sr (AlCl4)2, Sr(GaCl4)2, Sr(BF4)2,Sr(BCl4)2, Sr(InCl4)2, Li3SbCl6, LiSbCl6 and mixtures thereof.

4. The cell of claim 1 wherein said solid active cathode is comprised of a material selected from the group consisting of metal halides, oxides, chromates, vanadates, titanates, tungstates, chalcogenides, polyacetylene, poly-p-phenylene, polyphenylene sulfide, CxS and CFn , where "x"
is in the range of from 4 to 50, and "n" is a finite but large number.

5. The cell of claim 1 wherein said active metal anode is comprised of lithium.

6. The cell of claim 1 wherein said equivalent ratio ranges from 1:1 to 1:4.

7. The cell of claim 1 wherein said electrolyte is further comprised of an organic electrolyte which with said SO2 is co-complexed with said one or more salts.

8. The cell of claim 1 wherein said cell further includes an inorganic solvent selected from the group consisting of SOCl2, SO2Cl2, S2Cl2, SCl2 and mixtures thereof.

9. A method of making the liquid solvate complex of claims 1,2 or 3 comprising the steps of contacting gaseous SO2 with said one or more salts in solid form and forming and removing the liquid solvate complex.

10. A method of making the liquid solvate complex of claims 1, 2 or 3 comprising the steps of contacting gaseous SO2 with the Lewis acid and base components of said one or more salts in solid form, forming said salt and said liquid solvate-complex and removing said liquid.

11. A low vapor pressure, non-aqueous electrochemical cell comprising a lithium anode, a solid active cathode and a liquid electrolyte comprised of SO2 complexed with one or more salts selected from the group consisting of LiAlC14, NaAlC14 and Ca(AlC14)2 wherein the equivalent ratio of said one or more salts to SO2 in said liquid electrolyte ranges from 1:1 to 1:4.

12. The low vapor pressure electrochemical cell of claim 11 wherein said solid active cathode is comprised of a member selected from the group consisting of CuCl2, CuO, CuS and MnO2.

13. The cell of claim 12 wherein said solid active cathode is comprised of CuCl2.

14. A non-aqueous electrochemical cell comprising a lithium anode, a CuCl2 cathode and a liquid electrolyte consisting essentially of SO2 complexed with LiAlCl4.