Preparation of Li2S-FeSxComposite Positive Electrode Materials and Their Electrochemical Properties with Pre-Cycling Treatments

Li2S-FeSx (x = 1, 2) composite positive electrode material, consisted of low-crystalline Li2S, was prepared by the combination process of the thermal heating and the mechanical milling. The electrochemical tests demonstrated that the Li2S-rich composite sample (Li2S : FeS = 4 : 1 mol) cells showed relatively high initial charge capacity (ca. 600 mAh·g−1). The discharge capacity was ca. 330 mAh·g−1 with the normal electrochemical cycling, and it was enlarged to ca. 730 mAh·g−1 after the stepwise pre-cycling treatment. Ex situ XRD measurements showed the improved reversibility of the Li2S peaks for Li extraction/insertion reactions after the stepwise pre-cycling treatment. Also, ex situ S and Fe K-edge XAFS measurements indicated the improved reversibility of the valence states and local structures around S and Fe atoms after the stepwise pre-cycling treatment. Thus, the stepwise pre-cycling treatment was effective for improving the structural reversibility of the Li2S-FeSx composites, particularly with Li2S-rich composition, for Li insertion/extraction reactions, resulting in the improved electrochemical performances of the cells.