Fast Anion Redox by Amorphization in Sodium-Ion Batteries

Sodium-ion batteries have attracted considerable attention for energy storage in microgrids and other applications because of their high energy density as well as low cost. Although the anionic redox reaction provides high capacity, the slow kinetics associated with large structural changes are considered problematic. This study focuses on accelerating anionic redox reactions via amorphization, selecting Na3FeS3 composition in terms of glass-forming regions, multielectron redox reactions, and high Na diffusivity. Mechanochemically synthesized Na3FeS3 features an amorphous structure comprising a local structure similar to that of crystalline Na3FeS3. The single-compound electrode without any additives for increasing the ionic or electronic conductivity operates at 30 °C, and the charge–discharge capacity improves upon amorphization, achieving a complete two-electron anodic redox reaction at 60 °C. Voltage profiles obtained from experiments and first-principles calculations show that amorphization changes the charge–discharge reaction mechanism from that of a two-phase coexistence to that of a solid solution. Impedance analysis and computational modeling of the charge–discharge process demonstrate that amorphization suppresses the abrupt structural reconstruction associated with the crystalline two-phase coexistence reaction and creates random conduction pathways that are absent in the crystalline form, leading to fast anionic redox reactions. The activation of anion redox by amorphization in this study provides a novel strategy for utilizing this type of redox for high-capacity storage batteries.