Fast Anion\nRedox by Amorphization in Sodium-Ion Batteries

Sodium-ion batteries have attracted considerable attention\nfor\nenergy storage in microgrids and other applications because of their\nhigh energy density as well as low cost. Although the anionic redox\nreaction provides high capacity, the slow kinetics associated with\nlarge structural changes are considered problematic. This study focuses\non accelerating anionic redox reactions via amorphization, selecting\nNa<sub>3</sub>FeS<sub>3</sub> composition in terms of glass-forming\nregions, multielectron redox reactions, and high Na diffusivity. Mechanochemically\nsynthesized Na<sub>3</sub>FeS<sub>3</sub> features an amorphous structure\ncomprising a local structure similar to that of crystalline Na<sub>3</sub>FeS<sub>3</sub>. The single-compound electrode without any\nadditives for increasing the ionic or electronic conductivity operates\nat 30 °C, and the charge–discharge capacity improves upon\namorphization, achieving a complete two-electron anodic redox reaction\nat 60 °C. Voltage profiles obtained from experiments and first-principles\ncalculations show that amorphization changes the charge–discharge\nreaction mechanism from that of a two-phase coexistence to that of\na solid solution. Impedance analysis and computational modeling of\nthe charge–discharge process demonstrate that amorphization\nsuppresses the abrupt structural reconstruction associated with the\ncrystalline two-phase coexistence reaction and creates random conduction\npathways that are absent in the crystalline form, leading to fast\nanionic redox reactions. The activation of anion redox by amorphization\nin this study provides a novel strategy for utilizing this type of\nredox for high-capacity storage batteries.