Preparation of Li2S-FePS3 composite positive electrode materials and their electrochemical properties

For an attempt to incorporate phosphorous ions into the Fe-containing Li2S, we have prepared Li2S-FePS3 composite positive electrode materials using the combination process of the thermal heating and the mechanical milling. The XRD results showed that the Li2S-FePS3 composite samples consisted of low-crystalline Li2S and small amounts of FeP as impurity. The electrochemical tests demonstrated that the Li2S-rich composite sample cells (Li2S:FePS3 = 4:1 mol) showed relatively high initial discharge capacity (ca. 780 mAh·g− 1) without any pre-cycling treatments. This makes a clear contrast to the Fe-containing Li2S (Li2S-FeSx composite) sample cells, which showed the initial discharge capacity of ca. 330 mAh·g− 1 and it was enlarged to ca. 730 mAh·g− 1 after the stepwise pre-cycling treatment. Ex-situ XRD and XAFS measurements showed the reversible changes of the peaks ascribed to Li2S and the local structures around S atoms during cycling. The incorporation of phosphorous ions into the Fe-containing Li2S was effective for improving the structural reversibility against Li extraction/insertion reactions, resulting in the improved electrochemical performance of the cells.