Lithium oxide in the Li(Si)/FeS/sub 2/ thermal battery system

The formation of lithium oxide (Li20) in Li(Si)/FeS2 thermal batteries during the required shelf life of twenty-five years has been identified in previous work as a reaction deleterious to thermal battery performance. This paper gives the results of a study designed to determine performance degradation caused by Li20 and to determine an acceptable level of Li20 that can be used to define required dryness of battery parts and allowable leak rates. Pellets preconditioned with Li20 were used in single cells or in batteries. Their performance was compared with discharges made using pellets with no Li20 added. The actual Li20 present in anode pellets at various stages during fabrication was determined by using 14 Mev neutron activation analysis. The significant results can be summarized as follows: Li20 increases the “wetness” of separator pellets. This effect reaches a plateau at about 2.5 wt. 7; Li20 in a separator pellet which is 65 wt. 7; (LiCI . KC1 eutectic) blended with 35 wt.% MgO. Li20 blended with 44 wt.% Li(Si) causes an abrupt end-of-life on that part of the discharge corresponding to Li21Si8 discharging to Li2Si. This effect always occurs if the LizO weight percentage is as high as 30, and was observed for Li20 weight percentage as low as 15. A layer of Li20 on anode pellets surfaces facing the current collectors causes the same abrupt end-of-life if that layer represents 6-8 wt.7; Li20 in the anode pellet. This work shows that thermal battery production controls should be designed in such a manner that not more than 15 wt. 9; of the Li(Si) is oxidized a t the end of the desired self life. Furthermore, the formation of a Li20 layer equivalent to the oxidation of 6.0 wt.% of the anode on the surface facing the current collector must be prevented. Battery designers must allow for a drop in coulombic efficiency as the Li(Si) reacts, and the effect on performance of Li20 in the separator must be considered.