Low Ca2+ concentration doping enhances the mechanical properties and ionic conductivity of Na3PS4 superionic conductors based on first-principles

Abstract

The mechanical strength and ionic conductivity of sulfide solid electrolytes have received widespread attention for their application in solid sodium batteries. Herein, first-principles calculations are used to determine the properties, including the electronic, mechanical and ionic transport properties, of Na₃PS₄ sulfide solid electrolytes doped with low and high Ca ion concentrations. Our theoretical results demonstrate that low Ca ion concentrations can be easily doped in tetragonal and cubic phases (t-Na₃PS₄ and c-Na₃PS₄) and create a suitable number of Na vacancies based on the formation energy analysis. Furthermore, the calculated density of states and charge density differences indicate that the surrounding electronic environment is changed, and Ca–S ionic bonds are formed in Na₃PS₄ with Ca-doping. In addition, the improved ductility and mechanical strength of c-Na₃PS₄ and t-Na₃PS₄ achieved by low-concentration Ca doping may help suppress dendritic growth and electrode deformation. Finally, sodium ion migration in Ca-doped Na₃PS₄ is described with the aid of the CI-NEB method, and it is found that the migration energy barriers are less than those of pure Na₃PS₄, which suggests that the sodium ion conductivity can be effectively improved by doping with low Ca²⁺ concentrations. The present work improves the understanding of the influence of doping on the performance of solid electrolytes and provides a feasible framework for the future design of high-performance solid electrodes.