Insights into the electrochemical properties of Li2FeS2 after FeS2 discharging

Abstract

All-solid-state lithium–sulfur batteries (ASSLSBs) have high reversible characteristics owing to the high redox potential, high theoretical capacity, high electronic conductivity, and low Li⁺ diffusion energy barrier in the cathode. Monte Carlo simulations with cluster expansion, based on the first-principles high-throughput calculations, predicted a phase structure change from Li₂FeS₂ (PM1) to FeS₂ (PA) during the charging process. LiFeS₂ is the most stable phase structure. The structure of Li₂FeS₂ after charging was FeS₂ (PM1). By applying the first-principles calculations, we explored the electrochemical properties of Li₂FeS₂ after charging. The redox reaction potential of Li₂FeS₂ was 1.64 to 2.90 V, implying a high output voltage of ASSLSBs. Flatter voltage step plateaus are important for improving the electrochemical performance of the cathode. The charge voltage plateau was the highest from Li_0.25FeS₂ to FeS₂ and followed from Li_0.375FeS₂ to Li_0.25FeS₂. The electrical properties of Li_xFeS₂ remained metallic during the Li₂FeS₂ charging process. The intrinsic Li Frenkel defect of Li₂FeS₂ was more conducive to Li⁺ diffusion than that of the Li₂S Schottky defect and had the largest Li⁺ diffusion coefficient. The good electronic conductivity and Li⁺ diffusion coefficient of the cathode implied a better charging/discharging rate performance of ASSLSBs. This work theoretically verified the FeS₂ structure after Li₂FeS₂ charging and explored the electrochemical properties of Li₂FeS₂.