Charge fluctuation drives anion rotation to enhance the conductivity of Na11M2PS12 (M = Si, Ge, Sn) superionic conductors

Abstract

Solid superionic conductors exhibit good battery safety and stability, promising to replace organic liquid electrolytes. However, a comprehensive understanding of the factors determining high ion mobility remains elusive. Experiments have confirmed that the Na₁₁Sn₂PS₁₂ superionic conductor has high room temperature Na⁺-ion conductivity; excellent phase stability has been demonstrated in a solid-state electrolyte. The PS₄ anion rotation exists in Na₁₁M₂PS₁₂-type superionic conductors, but this rotation is affected by the isovalent cation substitutions of the M site. In combination with ab initio molecular dynamic simulations and joint time correlation analysis of the AIMD data, we show that the transport of Na⁺ ions is directly enhanced by the charge fluctuation in their tetrahedral MS₄ anions that comprise the framework. The fundamental reason for the charge fluctuation is the material structure forming a micro-parallel capacitor with MS₄ anions, which governs the differential capacitance. Our study provides a fundamental and comprehensive understanding of the structure-controlled charge transfer of Na₁₁M₂PS₁₂-type material and can guide solid-state battery optimization and design.