Electrolyte for use in an all inorganic rechargeable cell and lithium inorganic cell containing the improved electrolyte

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates in general to an improved electrolyte for use in all inorganic rechargeable cell and to an all inorganic rechargeable cell including the improved electrolyte and in particular to an improved electrolyte for use in a lithium/SO 2 rechargeable cell and to a lithium/SO 2 rechargeable cell including the improved electrolyte.

An all inorganic Li/SO 2 rechargeable cell generally includes a lithium anode, a porous carbon cathode and a stable highly conductive liquid electrolyte complex LiAlCl 4 --xSO 2 . This complex is rapidly formed when the commonly used LiAlCl 4 electrolyte salt reacts with SO 2 gas or SO 2 liquid. Although the initially formed liquid complex has the formula LiAlCl 4 --3SO 2 , additional lowered LiAlCl 4 salt concentration can be achieved at will by diluting the above electrolyte with additional SO 2 . Thus, electrolyte with formulas LiAlCl--xSO 2 where x is at least 3 can be prepared. The starting electrolyte LiAlCl 4 --3SO 2 has a very high specific conductivity of 0.1 S/cm. The fact that this is the highest known conductivity for any ambient temperature non aqueous electrolyte makes this general class of electrolytes extremely valuable.

One difficulty with the use of this electrolyte is however, that during cell discharge, the insoluble reduction product is deposited in the porous carbon cathode. Simultaneously, some of the LiAlCl 4 electrolyte salt is also precipitated in the porous carbon cathode. The coprecipitation of electrolyte salt is partially responsible for the severely reduced capacity currently plaguing this system.

The general object of this invention is to provide an improved lithium inorganic rechargeable cell system. A more particular object is to minimize or even eliminate LiAlCl 4 coprecipitation and thus improve cell capacity.

It has been found that the aforementioned objects can be attained by adding a suitable cosolvent to the electrolyte to allow excess LiAlCl 4 that would normally precipitate to dissolve. The requirements of an effective cosolvent are:

(A) The cosolvent should be capable of rapidly dissolving the LiAlCl 4 salt and, in order to minimize IR losses, must in the process form a reasonably conducting electrolyte solution.

(B) The cosolvent should be extremely miscible with liquid SO 2 .

(C) The cosolvent must not become easily oxidized during cell charging which typically takes place at 4.0 volts relative to lithium.

(D) The cosolvent should not become chemically degraded in the presence of AlCl 3 or chlorine, both of which are formed during cell charging.

A particularly effective cosolvent has been found to be sulfuryl chloride, SO 2 Cl 2 . One would not expect that the addition of SO 2 Cl 2 as a cosolvent would be able to improve cell capacity in Li/SO 2 rechargeable cells since the specific conductivity of pure SO 2 Cl 2 is very poor. That is, pure SO 2 Cl 2 is one million times less conductive than is the LiAlCl 4 --3SO 2 electrolyte.

Experimental results were obtained in small laboratory cells. The cells include a lithium anode, a porous carbon cathode made from high surface area carbon black and a LiAlCl 4 electrolyte salt.

The following electrolytes were tested: (1) LiAlCl--3SO 2 ; (2) LiAlCl 4 --3SO 2 +10% SO 2 Cl 2 ; (3) LiAlCl 4 --3SO 2 +20% SO 2 Cl 2 ; and (4) LiAlCl 4 --SO 2 Cl 2

The results of these experiments are shown in the following Table:

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LiAlCl.sub.4 --3SO.sub.2

9 14

LiAlCl.sub.4 --3SO.sub.2 + 10% SO.sub.2 Cl.sub.2

32 20

LiAlCl.sub.4 --3SO.sub.2 + 20% SO.sub.2 Cl.sub.2

52 18

LiAlCl.sub.4 --SO.sub.2 Cl.sub.2

20 0

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The results clearly show that Li/LiAlCl 4 --3SO 2 cells constructed with 10% and 20% added SO 2 Cl 2 had longer capacities than either Li/LiAlCl 4 --3SO 2 cells or even Li/LiAlCl 4 --SO 2 Cl 2 cells. It is clear that on the TENTH CYCLE, the advantage of the addition of low percentages of SO 2 Cl 2 is evident. Cells containing more than 50% added SO 2 Cl 2 could not be cycled ten times.

We wish it to be understood that we do not desire to be limited to the exact details as described for obvious modifications will occur to a person skilled in the art.