Calcium/metal sulfide battery development program. Progress report, October 1979-September 1980

Abstract

A Ca-Al-Si/FeS 2 cell has been developed and has exhibited reasonably stable capacity through 3200 h of operation. This system is expected to be capable of meeting the ultimate performance goals (i.e., 160 W-h/kg) of this development program. Further tests of this cell system in the coming year will better define its ultimate performance capabilities. SUMMARY Separator Chemistry. A reaction between the Ca-Si negative electrode and the boron-nitride (BN) separator material has limited the cycle life and performance of calcium cells. Chemical tests showed that the reaction could be eliminated by preventing the formation of high-calcium-activity compounds such as Ca 2 Si. Lower activity compounds, such as CaSi and CaAl 2 , did not react with BN. Negative-Electrode Development. A high-performance negative electrode with a composition of CaAl 1. 2 Si 0 .4 was developed in cyclic voltammetry experiments. According to the separator chemistry tests, this alloy will not react with BN separators. Positive-Electrode Development. Tests of the FeS electrode indicated acceptable utilization (652) at a current density of 40 mA/cm 2. Cyclic voltammetry tests were used to evaluate two types of carbon current collectors for FeS 2 electrodes. A finely divided carbon (E-Coke Flour) appeared to be superior to carbon foam. Electrolyte Development. The effects of electrolyte composition on the performance of FeS2 electrodes were evaluated by cyclic voltametry. Variations in the concentration of sodium, potassium, and barium ions had no effect; higher calcium-ion concentrations were beneficial. A ternary LiCl-KCl-CaF 2 electrolyte for use with CaF 2 powder separators was subjected to a preliminary cell test and gave encouraging results. Cell Development. Eight cell tests were conducted to evaluate various possible solutions to the problem of BN-separator degradation. The CaAl1.2Si0.4/FeS2 cell test was highly successful. The cell, which employed BN-felt separators, operated for over 3200 h with only a slight decline in cell capacity.