GB2124821A - Electrochemical cells having low vapor pressure complexed SO2 electrolytes - Google Patents

Abstract

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 3 element halide anion, with the equivalent ratio of salt to SO2 in said electrolyte ranging from about 1:1 to 1:7.

Description

SPECIFICATION Electrochemical cells having low vapor pressure complexed SO2 electrolytes This invention relates to electrolytes for nonaqueous electrochemical cells and more particularly to such electrolytes containing sulfur dioxide (SO2).

Sulfur dioxide, though a poor electrolyte solvent has nevertheless been widely utilized in non-aqueous electrochemical cells in the dual function of cathode depolarizer and electrolyte solvent because of its high energy density and high rate capability. Without its functioning as the cathode depolarizer SO2 has rarely been utilized as an electrolyte solvent alone since in addition to its poorsolvating properties SOP has several other shortcomings which are generally related to its being a gas at room temperature and pressure (B.P. 1 00C). For proper utilization the gaseous 502 is converted into a liquid under conditions of low temperature andlor high pressure and must be maintained in such liquid form by constant pressurization.As a result, cells having SO2 therein have had the attendant disadvantage of requiring expensive reinforced cell containers and hermetic seals resistant to the volatile SO2. Additionally, expense wasfurther incurred as a result ofthe need for the initial liquefaction ofthe SO2 and the special handling required particularly with respect to the filling of the cells with the volatile liquid SO2. The safety aspects of cells containing SO2 were also somewhat of a problem since required safety venting mechanisms, while providing protection, nevertheless operated by the atmospheric expulsion of noxious gaseous 502.

In ordertoaid in electrolytesolvation and to reduce the high vapor pressure of the SO2, cells have generally contained orga nrc cosolvents such as aceto nitrilewith the SO2. However, despite the presence of the vapor pressure reducing organic cosolvents, the cel is nevertheless remained highly pressurized with its attendant disadvantages. Furthermore, organic cosotvents such as the aforementioned acetonitrile generally precluded efficient cell cycling and occa sionally in themselves posed potential safety problemswhenthe cellswereabused.Electrolyte salts which were found to readily dissolve in the SO2 without the necessìtyfororganiccosolvents either provided poorlyoperating cells e.g. LiAlCl4orwere generally prohibitively expensive e.g. clovoborate salts such as Li2B1ocllo. Furthermore, such salts when dissolved in the liquified 502 though alleviating hazard problems associated with organic co-solvents did notalleviatethe problems caused bythe high vapor pressure ofthe sulfur dioxide.

It is an object of the present invention to provide a non-aqueous cell having a low vapor pressure electro- lyte based upon SO2 and a method for the preparation of such electrolyte.

It is a further object of the present invention to provide such electrolyte having a very high conductiv ity and thus being suitable for high rate applications; being economical with respect to both component materials and the preparation thereof; being a liquid at normal temperature and pressure; being chemical ly 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 withoutfree organic materials and therefore being saferthan conventional organic electrolytes.

These and other objects, features and advantages of the present invention will become more evident from thefollowing discussion and drawings in which: Figure 1 is a vapor pressure comparison of a prior art SO2 electrolyte and an electrolyte of the present invention, Figure 2 is a graph of the conductivities ofvarious electrolytes ofthe present invention at various temperatures; Figure 3 is a graph ofthe cycling characteristics of a cell having the electrolyte of the present invention; Figures 4-7 are discharge curves of various cells having the electrolyte ofthe present invention. Figure 5 also depicts a charging curve for one embodiment; and Figure 8 is a graph of the polarization characteristics of a cell having an electrolyte of the present invention.

Generally the present invention comprises a low vapor pressure (below 2 atm at room temperature and preferably below 1 atm) high conductivity electrolyte based upon SO2; a method for the preparation thereof; and both non-aqueous primary and secondarycells having active metal anodes such as of alkali or alkaline earth metal including mixtures and alloys thereof and containing such lowvapor pressure electrolyte. The electrolyte of the present invention is comprised of a tightly bound solvate-complex of 802 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 (ofthe PeriodicTable) element halide.The Group 3A elements which are preferredforthe 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 SOP include LiAlCI4, LiGaCI4, LiBF4, LiBCI4, LilnCI4, NaAlCl4, NaGaC4, NaBF4, NaBCl4, NaInCI4, Ca(AICI4)2, iCa(GaCI4)2, Ca(BF4)2, Ca(BCI4)2, Ca(lnCl4)2, Sr(AlCl4)2, Sr(GaCl4)2, Sr(BF4)2, Sr(BCl4)2, Sr(lnCl4)2 and mixtures thereof.Non-Group 3A salts suitable for complexing with the 502 include Li3SbCl6 and LiSbCl6. Salts which do not complex with the SO2 may however be dissolved in the solvate-complex if desired.

The solvate-complexing of the SO2 and the salt is dependent upon equivalent ratios ofthe materials rather than mole ratios with such difference being apparent with respectto, for example, alkaline earth metal salts which generally contain two equivalents per mole. The equivalent ratios range from about 1:1 to 1:4 (salt:802) and because of such variation the combination ofthe salt and 802 is demonstrated as being in the nature of a complex rather than a reaction-formed new compound. Though the actual complexing of the salt to SOP is generally up to a ratio of 1:4 (salt:SO2), addition of uncomplexed 502 to the cell in an amount of up to about 1:7 (salt:total SO2) will not generally detrimentally pressurize the cell at room temperature. With salt to SO2 ratios above 1:7 not only does the cell become detrimentally pressurized (above about 2 atm.),the conductivity of the electro lyte is also reduced thereby. Electrolytes such as the 1 molar LiAICI4 in 802 (equivalent ratio of about 1:22 of LiAICI4:SO2) as described in U.S.Patent No. 3,493,433 are highly pressurized (about 3.5 atm.) and are in fact described in said patent as being utilized at temperatures between -100Cto -300C. Because of its being in unavailable complexed form with only a minimal, if any, content of uncomplexed 802,the 802 in the electrolyte does not function as the active cathode depolarizer. The electrolyte of the present invention therefore has its main utility in electrochemical cells having solid cathode depolarizers.Such cathodes include CuCI2, CuO, CuS, MnO2, Cr308, V205 as well as other metal halides, oxides, chromates, vanadates, titanates, tungstates, chalcogenides and active nonmetal cathodes such as organic conductive polymers such as pòlyacetylene, poly - p - phenylene, polyphenylene sulfide and various carbon compounds such as CxS and CFn.

Though itwould have been expected thatthe solid cathodes would provide cells having reduced high rate capability when compared to cells having fluid cathode depolarizers such as 502 such reduction is in fact minimized by the unexpectedly very high conductivity of the electrolyte of the present invention.

Furthermore, advantages of a substantially nonpressurized system, particularly with respect to increased safety more than compensates for any reduction in hgih rate capability.

The solvate-complex electrolyte of the present invention is prepared by reacting the 802 with the alkali or alkaline earth metal salt in the requisite equivalent ratios. Such reaction may be effected by substantially saturating liquified 802 with the salt to the requisite equivalent ratios. It is however preferred thatthe salt be reacted with the 802 in gaseous form such as by passing a stream of dry 802 through the saltwhereby an exothermic reaction occurs with the formation of a liquid solvate-complex.The resultant liquid has a low vapor pressure ( < 10 psi at 200 C with a B.P. ofabout400C) and can be handled as a liquid in contrast to liquified 802 (B.P. 1 00C) which must be specially handled as a volatile material. Figure 1 compares the vapor pressure of prior art SOP containing electrolyte (Cu rve A) and a LiAlCl4.3.5 SO2 solvate-complex (Curve B) at various temperatures. At room temperature (20 C) prior art SO2 electrolytes have high vapor pressures (about 50 psi or about 3 1/2 atm) whereas the solvate-complex has a vapor pressure of about 10 psi, well below atmospheric pressure of 15 psi.Pressure rise ofthe solvatecomplex electrolyte is logarithmical with a rise in temperature to a pressure of 60 psi at 80 C. This is in further marked contrastto the 260 psi at 800C of the priorart8Orcontaining electrolytes.

It is noted that the SOP need not be directly reacted with the salt per se but may in fact be reacted with for example the Lewis acid and base components of the saltwherebythe salt and the liquid solvate-complex are simultaneously produced. For example, a stream of dry 802 may be passed through a LiAICI4 salt or a 1:1 stoichiometric mixture of the Lewis acid and base components thereof, LiCl and AlCl3, to produce the same liquid UAlCl4.x8O2 solvate-complex, with "x" having been determined as ranging from about 1 to 4 (equivalent bases). Continued bubbling of SO2 there- through results in a higher value for "x" in the range.

Lowervaluesfor "x" may be obtained by evaporating some ofthe 802 from the liquid solvate-complex.

Addition of excessive 802 as described above wherein the ratio of tota 1802 to salt exceeds 7:1, however, forms an undesirable pressurized (above about 2 atm) electrolyte.

The conductivity at room temperature of a solvatecomplex electrolyte of LiAlCl4.3.5 802 has been discovered to be about 1 x 10-1 ohm-1cm-1 which is the highestconductívityobserved to date in any nonaqueous electrolyte. The electrolyte of the present invention has been found to be very stable with lithium anodes and has been furtherfound to enable lithium to be electrochemically plated and stripped therein with efficienciesofover97% even over extended cycling regimens whereby it is an eminently suitable electrolyte for rechargeable lithium or other alkali or alkaline earth metal cells.

A solvate-complex of NaAICI4.2.8 802 while having somewhat lower conductivity of 8 x 10-2ohm-1cm-1 (but still very high) has the advantage of good low temperature operability e.g. conductivity of 2 x 1 0#2ohm#1cm#1 even at -300C. In contrast to the LiAICI4-xSO2 solvate-complexes which freeze at temperatures between about -8 C to -1 50C the NaAICI4-xSO2 solvate-complexes freeze at about -44QC and are more suitable when lowtemperature operation is desired.A solvate-complex of Ca(AlCl4)2#2.75 802 which has a conductivity of 1.7 x 10-2ohm-1cm-1 at room temperature and about 10 3Ohm~'cm~' at -35 C does not in fact freeze but rather becomes immobile at about -500C. It can accordingly be admixed with other solvate-complex salts such as LiAlCl4.3.5 802 to provide an electrolyte having both high conductivity and extremely low and hightemperature capability.

Alternatively, it has been discovered that the incorporation of additive amounts of inorganic solvents such as SOCl2, S2CI2, SCI2, SO2CI2 admixtures thereof to the solvate-complex electrolyte also serves to enhance lowtemperature capability. For example, a mixture of 90% (by weight) LiAIC42.7 802 and 10% SOCl2 (containing 1 M LiAlCI4) freezes at about -250C with a conductivity of about 1.8 x 10-2ohm-1cm-1 at -200C.

Though lithium has been found to be stable in the presence of, for example, the NaAlCl4 and Ca(AlCl4)2 solvate-complex-eiectrolytes, in secondary or rechargeable cell applications it is preferred that the complexed salt contains cations corresponding to the anode metal.

For primary cell applications other salts such as the aforementionedsodium and calcium salts may be effectively utilized with lithium anodes particularly in providing enhanced lowtemperature capability Addi- tionally and preferably for primary cell applications electrolyte salts normally not soluble in 802 alone -may be utilized by stoichiometric complexing with both 802 and an organic cosolvent such as acetonit rile; ethers such as dimethoxyethane; propylene carbonate and the like. Such salts include LiBr, LiClO4, LiAsF6and LIPFB.The organic cosolvent thereby makes such salts soluble in the 802. The organic cosolventis present only in sufficient quantity to co-complexthe salt with the hazards offree organic materials not being present. Soluble salts may similar ly be utilized with such co-complex.

Thefactthatthe electrolyte ofthe present invention has a low vapor pressure despite its 802 component results in several very important economic and safety benefits. Cells made therewith need not be reinforced or otherwise made resistant to pressurized contents.

Venting, if at all necessary as a safety precaution does not result in the emission of rapidly spreading noxious 802 fumes. Hermetic seals for the cells are not as susceptible to degradation because of the bound state ofthe normally corrosive 802 and infact more economical seals suitable for non-pressurized cells may be utilized. Exceptforthe initial relatively simple procedure offorming the liquid solvate complex electrolyte as described above no special handling or storage is required in contrasttothe handling of uncomplexed volatile Soys. Filling of cells with the electrolyte is simply with a stable liquid as opposed to filling cells with a volatile pressurized liquid such as 802.

In orderto morefullyillustratetheproperties and beneficial aspects ofthe electrolyte ofthe present invention the following examples are presented. It is understood, however,thatsuch examplesareillus trative in nature and are notto be construed as limiting the present invention. Unless otherwise indicated all parts are parts by weight EXAMPLE 1 Stoichiometric amounts of UCI and AICI3 were placed in a glass vessel and dry 802 was passed through the salt particles. A clear liquid solvate complex of LiAIC4-SO2 was formed rapidly with the generation of heat and after cooling to room tempera turethe molar or equivalent ratio of LiAICl4to 802 in the solvate-complex was determined to be 1:3.1.

Continued bubbling ofthe dry 802through the LiCI and the AICI3 provided anotherclearliquid solvate complex having an equivalent ratio of LiAlCl4 to 802 of 1:3.5. Evaporation of some SO2 from the LiAICI4-3.1 802solvate-complex provided another clear liquid solvate-complex having an equivalent ratio of LiAICI4 to 802 of 1:2.6. The conductivities ofthethree liquid solvate-complexes at varioustemperatures were measured as shown in Figure 2 as curves C, D and E respectively. The condu#tivities obtained were the highest ever obtained fcEr non-aqueous electrolytes.

Additionally, lithium metal stored in the electrolytes for periods in excess affour weeks showed no corrosion thereby indicating the stability of such solvate-complex electrolytes in lithium containing cells.

EXAMPLE 2 A NaAlCl4#2.8 802 solvate-complex was formed as in Example 1 butwith NaCI instead of LiCI with its conductivity atvarious temperatures shown in Figure 2 as Curve F.

EXAMPLE 3 A Ca(AlCl4)2#2.75 S02-equivalent ratio (Ca(AlCl4)2.5.5 802 molar ratio) solvate-complex was formed as in Example 1 but with a stoichiometric ratio of CaCl2:2AICl3. The conductivity at various temperaturves is shown in Figure 2 as Curve G.

EXAMPLE 4 A mixed solvate complex of(LiAICl4 + 4Ca(AlCl4)2)-5.6 802 (molar ratio) was made by passing 802 through AICI3, LiCI and CaCI2 (molar ratio of 9:1:4). The resultant liquid solvate-complex provided conductivities at various temperatures as shown in Figure 2 as Curve H.

EXAMPLE 5 A mixture of 90% LiAlCl4#2.7 802 solvate-complex and 10% 1 M LiAICI4 in SOCI2was made and provided conductivities at various temperatures as shown in Figure2 as Curve I.

EXAMPLE 6 A cathode limited electrochemical cell was constructed with a 2 gram cathode of 60% CuCI2, 30% graphite and 10% polytetrafluoroethylene (PTFE) pressed on a nickel grid, 1" (2.5cm) x 1.6" (4.1 cm) sandwiched bytwo lithium anodes each of similar dimensions. The cell was filled with the LiAlCl4-3.1 S02solvate-complex electrolyte of Example 1 and discharged atthe rate of 40 ma (2 ma/cm2) to a 2.6 volt cutoff and charged ata rate of 20 ma (1 ma/cm2) to 4.05 volts on a continuous cycling regimen.The cell has cycled for about 350 cycles at close to 100% of the one electron transfer cathode capacity (Li + CuCI2 < CuCI + UCl).Thecharging and discharging curvesforcycles nos. six, 173 and 230 are shown in Figure 3. Cumulative capacity so far is72 Ahrs with about36 Li turnovers (97% eff. onthe anode). The initial theoretical primary capacity was 0.24 Ahr.

EXAMPLE 7 Three cells were made as in Example 6 butwith the solvate-complex electrolyte, (LiAICI4 + 4Ca(AlCl4)2)~5.6 802 of Example 4. The cells were discharged at rates of 20 ma, 40 ma and 60 ma respectively with discharge results as shown in Figure 4.

EXAMPLE 8 A cell as in Example 6 was made butwith the solvate complex electrolyte of Example 5 (90% LiAlCI4-2.7 802 and 10% 1 M LiAICI4 in SOC12). The cell was cycled at40 ma (2 ma/cm) discharge 20 ma (1 ma/cm2) charge and the discharge-charge of the sixth cycle is shown in Figure 5.

EXAMPLE 9 A cell as in Example 6 was made but with a 3 gram cathode comprised of 60% CuO, 30% Graphite and 10% PTFE. The cell was discharged at 40 ma with results as shown in Figure 6.

EXAMPLE 10 A cell as in Example 6 was made but with a 3 gram cathode comprised of 60% CUS, 30% graphite and 10% PTFE. The cell was discharged at 40 ma with results as shown in Figure 7.

EXAMPLE 11 Aspirallywound Li1MnO2ceIl having electrodes 6.25" (15.9cm) X 1" (2.5cm) of lithium and MnO2 with the cell being cathode limited to a theoretical capacity of 0.75 Ah rs was filled with the LiAlCl4#3. 1 SO2 electrolyte of Example 1. The cell showed an initial open circuit voltage of 4.0 volts. The cell was then discharged at 85 ma and delivered a capacity of 0.56 Ahrto a voltage cutoff of 2.0 volts. The cell was charged at 40 ma for 14 hours and discharged again at 40 ma delivering 0.40 Ah on the second discharge.

Figure 8 shows the polarization of the cell during discharge and charge.

Effective utilization of the low vapor pressure electrolyte of the present invention in Li/MnO2 cells may alleviate problems relating to the reactive electrolyte salts and volatile solvents generally util ized in such electrochemical cell systems such as lithium perchlorate and dimethoxyethane by simply repalcing them with the relatively safe electrolyte of the present invention.

From the above examples it is evidentthatthe solvate-complex electrolytes of the present invention provide for substantially non-pressurized cells which are efficiently rechargeable and that some embodiments have conductivities well above those of previously known non-aqueous electrolytes which did not generally have room temperature conductivi ties above about 5 x 102ohm4cm#1.

It is understood thatthe above examples are illustrative in nature and that changes in the cathode materials, electrolyte composition and ratios as well as the cell systems in which they are utilized may be made without departing from the scope of the present invention as defined in the following claims.

Claims (13)

1. Anon-aqueous, electrochemical cell having an active metal anode, a solid active cathode and a liquid electrolyte characterized in that said electrolyte is comprised 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 offorming a complex therewith, and wherein the equivalent ratio of said one or more salts to 802 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 3 element halide anions.

3. The cell of claim 1 wherein the said one or more salts are selected from the group consisting of LiAlCI4, Lilac4, LiBF4, LiBCl4, LilnCI4, NaAIC4, NaGaCI4, NaBF4, NaBCl4, NalnCI4, Ca(AlCI4)2, Ca(GaCl4)2, Ca(BF4)2, Ca(BCI4)2, Ca(lnC14)2, Sr(AIC4)2, Sr(GaC14)2, Sr(BF4)2, Sr(BCI4)2, Sr(lnC14)2, Li3sbcl6, Li8bCl5 and mixtures thereof.

4. The cell ofany of claims 1-3 wherein said solid active cathode comprises a material selected from metal halides, oxides, chromates, vanadates, titanates, tungstates, chalcogenides, polyacetylene, polyp-phenylene, polyphenylene sulfide, CxS and CFn.

5. The cell of any of claims 14wherein said active metal anode comprises lithium.

6. The cell of any of claims 1-5 wherein said equivalent ratio ranges from 1:1 to 1:4.

7. The cell of any of claims 1wherein said electrolyte further comprises an organic electrolyte which with said 802 is co-complexed with said one or more salts.

8. The cell of any of claims 1-7 wherein said cell further includes an inorganicsolvent selected from SOCI2, SO2CI2, 82CI2, SCI2 and mixtures thereof.

9. A method of making the liquid solvate complex setforth in claim 1,2 or 3 comprising the steps of contacting gaseous 802 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 com plex set forth in claim 1, 2 or 3 comprising the steps of contacting gaseous 802 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 electroch mical cell comprising a lithium anode, a solid active cathode and a liquid electrolyte comprised of Sic)2 complexedwithoneor more salts selected from LiAICI4, NaAICI4 and Ca(AIC4)2 wherein the equiva lent ratio of said one or more salts to 802 in said liquid electrolyte ranges from 1:1 to 1:4.

12. The cell of claim 11 wherein said solid active cathode is comprised of a member selected from CuCI2, CuO, CuS and MnO2.

13. An electrochemical cell substantially as set forth in any of the foregoing examples.