Understanding Voltage Hysteresis for High-Energy-Density Li–S Batteries

Li–S batteries are promising candidates for next-generation energy storage technologies owing to their high theoretical capacity and low weight and the wide availability of S. The addition of Se to S is considered a rational design principle to regulate the polarization of Li–S cells intrinsically. Moreover, the electrochemical utilization of solid-state Li2–xS (0.0 ≤ x ≤ 1.0) provides sufficiently high theoretical specific capacity (836 mA h g–1) and long-term stability. However, solid-state Se-doped Li–S compounds during (de)lithiation have not been studied in detail. Therefore, we performed combined experimental and theoretical studies to reveal the reduction of polarization by Se doping owing to multiple factors that were previously assumed to be negligible. Experimentally, the polarization reduction in Se-doped Li2S is dependent on the electronic, ionic, and thermodynamic properties of the Se dopant. Theoretically, Se doping simultaneously lowers the formation energy, bond symmetry of Li–S(Se), energy required for structural changes, and electronic stability, resulting in the reduction of polarization. Our concrete understanding of the two types of Li–S electrodes can aid the design of advanced high-energy solid-state Li–S batteries.