Electrochemistry of Pyrite‐Based Cathodes for Ambient Temperature Lithium Batteries

The charge and discharge mechanisms of and cells near room temperature are studied by in situ x‐ray diffraction and in situ 57 Fe Mössbauer spectroscopy. The electrochemical behavior of these cells are compared, and their cell reactions are described. For , the cycling behavior of is reversible. However, for , structural decomposition of occurs, and disproportionation to nonstoichiometric and S is observed. Similarly, when cells are recharged to 2.8V, the cathode is a mixture of and S. Examination of the mixture by differential scanning calorimetry shows that and S react to form above 200°C. This explains why high temperature cells cycle reversibly while ambient temperature cells show poor reversibility.