Electrochemical cell

An electrochemical cell having a lithium anode, a thionyl chloride depolarizer and a sulphur dioxide passivation control agent which further includes having the pressure relieved to substantially reduce the internal pressure of the cell. The internal cell pressure is relieved by venting for sufficient time at an elevated temperature to reduce the internal cell pressure to less than five psi at room temperature, preferably by a plurality of venting cycles and a temperature ranging from room temperature to the elevated temperature. Normally, the elevated temperature ranges from at least 100.degree. F. to greater than 150.degree. F.

BACKGROUND OF THE INVENTION 
Substantial development effort has been expended on thionyl 
chloride/lithium batteries because of the extremely high energy contained 
in this electrochemical couple. One advantage of the cell is that thionyl 
chloride is a liquid and does not cause substantial pressure in the cell. 
However, thionyl chloride has been found to have substantial difficulty as 
a depolarizer in cells which are stored for any length of time, 
particularly at high temperature because of lithium negative electrode 
passivation. The use of sulphur dioxide as a passivation control agent has 
been found to prevent this disability at high temperature after long 
storage times and such a system is disclosed in U.S. Patent application 
Ser. No. 731,064, filed Oct. 8, 1976 now U.S. Pat. No. 4,309,490 issued 
Jan. 5, 1982 and assigned to the same assignee as the present invention. 
That system, however, because of the gaseous nature of sulphur dioxide, 
requires a substantially stronger cell to deal with the internal pressure, 
particulary after the cell has been discharged. Typical cells have up to 
fifty-five or more psig internal pressure upon completion of discharge at 
room temperature. At elevated temperatures, of course the pressure is 
substantially greater. 
SUMMARY OF THE INVENTION 
It has now been discovered that a safe active low pressure lithium thionyl 
chloride battery, which is able to take the advantage of the sulphur 
dioxide passivation control feature to permit rapid activation after long 
or high temperature storage, can be manufactured. The cell of the present 
invention includes the concept of venting the cell at elevated 
temperature, preferably, to relieve the pressure in the cell after the 
sulphur dioxide has had the opportunity to assist in controlling the 
passivating film growth at the anode, which happens almost 
instantaneously. In the preferred embodiment, the cell is vented for 
sufficient times to reduce the internal cell pressure to less than five 
psig at room temperature. The elevated temperature at which the cell is 
vented should be at least 100.degree. F., and preferably at least 
160.degree. F. The venting can be accomplished in a single bleeding of the 
pressure from the cell or it may be accomplished by a plurality of venting 
cycles at temperatures ranging from room temperature to the elevated 
temperature previously mentioned. When practiced, the plurality of venting 
cycles is oftentimes sufficient to lower the internal cell pressure to 
less than one atmosphere at room temperature.

DETAILED DESCRIPTION OF THE INVENTION 
As has been mentioned above, the invention comprises the venting of the 
pressure caused by the addition of sulphur dioxide to lithium thionyl 
chloride battery. It has been discovered that the efficiencies and 
improvements found by the addition of sulphur dioxide in permitting 
operation of cells which have been stored at a high temperature or for a 
long period of time or both can be achieved by the addition of sulphur 
dioxide and are not materially affected or taken away by venting the gases 
after the cell is assembled with the sulphur dioxide present. 
Nine cells were constructed to obtain basic data relative to the effect of 
bleeding at various temperatures prior to discharge of the cell. All nine 
cells were eventually discharged at room temperature. However, bleeding or 
venting of the cell was accomplished at both 110.degree. F. and 
165.degree. F. in the manner set forth below. Three cells were bled at 
165.degree. F. and three more were bled at 110.degree. F. The remaining 
three cells served as controls and were not bled. One control cell was 
discharged immediately at 75.degree. F. A second control cell was stored 
unbled at 110.degree. F. and the third control cell was stored unbled at 
165.degree. F. The storage of all cells was four weeks. 
Three cells were then evaluated by venting the pressure in the cells at a 
temperature of 110.degree. F. for approximately fifteen minutes, that is 
until the cell pressure could no longer be reduced by leaving the fill 
valve slightly cracked. The cells were then stored at 110.degree. F. for a 
week and the process was repeated three additional times for a total of 
four cycles. During the period of time when the fill valve was closed, 
slight pressure increases were observed with the passage of time. 
Another set of three cells were tested by storing at 165.degree. with 
bleeding cycles of approximately fifteen minutes every week for a period 
of four weeks. 
All of the cells were then reduced to room temperature for discharged 
testing. The internal pressure of all of the 165.degree. F. bled cells 
were negative, that is it was less than one atmosphere. Two of the three 
cells bled at 110.degree. F. had negative cell pressure or less than one 
atmosphere while the final or third 110.degree. bled cell has an internal 
pressure of approximately 2 psig at room temperature. The cells were then 
discharged under load until the substantial voltage drop occurred 
indicating a depletion of the cell. The capacity of the cell was measured 
in terms of ampere hours of current. In the unbled controls, the internal 
pressure of the discharged cells was between forty-five to fifty-five psig 
at room temperature. The 110.degree. F. bled cells had a pressure of 
twenty to twenty-five psig and the 165.degree. F. bled cells were in a 
range of five to ten psig. Substantial reduction in the pressure permitted 
the design of much safer cells. 
Presented below in Table I are the results of the tests previously 
described. As can be seen, the discharge pressure is substantially reduced 
using the process of this invention, thereby enabling more efficient 
construction of cells. All of the tests indicated that the passivation 
activity of the sulphur dioxide was accomplished equally in all of the 
cells whether or not a bleeding cycle or a venting cycle was employed. 
TABLE I 
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TEMPERATURE STARTING PRESSURE 
END OF LIFE PRESSURE 
CELL NO. 
STORAGE - 
BLEEDG 
AT ROOM TEMP. 
AT ROOM TEMP. 
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1 75.degree. F. 
-- 20 psig 40 psig 
2 110.degree. F. 
-- 50 psig 50 psig 
3 165.degree. F. 
-- 30 psig 50 psig 
4 -- 110 -2 psig 25 psig 
5 -- 110 -2 psig 22 psig 
6 -- 110 +2 psig 22 psig 
7 -- 165 -5 psig 10 psig 
8 -- 165 -5 psig 7 psig 
9 -- 165 -10 psig 10 psig 
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Presented below in Table II are the results of the discharge of the cells 
previously described herein. As can be seen, the ampere hour capacity is 
roughly equivalent for all of the cells, indicating that the bleeding had 
no adverse effect on performance while substantially reducing the pressure 
of the cell upon discharge. 
TABLE II 
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TEMPERATURE 
CELL NO. STORAGE / BLEEDG CAITY A HR. 
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1 75 -- 11,372 
2 110 -- 12,160 
3 165 -- 11,700 
4 -- 110 12,840 
5 -- 110 11,740 
6 -- 110 13,160 
7 -- 165 11,240 
8 -- 165 10,560 
9 -- 165 10,800 
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