Abstract
The charge and discharge mechanisms of 
and 
cells near room temperature are studied by in situx‐ray diffraction and in situ57Fe 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.