Electrochemistry of Pyrite‐Based Cathodes for Ambient Temperature Lithium Batteries

The charge andn ischarge. 57 mechanisms. , of Li/FeS2 and Li/L12FeS2 cells near room temperature are studied by in situ x-ray diffraction a d in sztu Fe Mossbauer spectroscopy. The electrochemical behavior of these cells are compared, and their cell reactions are described. For 0 x- 0. 8, the cycling behavior of Li2ₓFeS2 is reversible. However, for x 0. 8, structural decomposition of Li2-xFeS2 occurs, and disproportionation to nonstoichiometric FeS (FeS, ) and S is observed. Similarly, when Li/FeS2 cells are recharged to 2. 8V, the cathode is a mixture of FeS, and S. Examination of the mixture by differential scanning calorimetry shows that FeS, and S react to form FeS2 above 200 ~ This explains why high tempera-ture LEFeS2 cells cycle reversibly while ambient emperature Li/FeS2 cells show poor reversibility. Lithium-alloy/iron-sulfide secondary batteries, operated at 400 ~ have been studied extensively for the develop-ment of high power cells for vehicle propulsion (1). Several studies have shown that during the charge and discharge of these batteries, intermediate phases such as Li3Fe2S4 and Li2FeS2 are formed (1-3). Due to the excellent revers-ible behavior of these high temperature cells, chemically