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 (P3̄M1) to FeS2 (PA3̄) during the charging process. LiFeS2 is the most stable phase structure. The structure of Li2FeS2 after charging was FeS2 (P3̄M1). 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 Li0.25FeS2 to FeS2 and followed from Li0.375FeS2 to Li0.25FeS2. The electrical properties of LixFeS2 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.