Low Ca concentration doping enhances the mechanical properties and ionic conductivity of NaPS superionic conductors based on first-principles.

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.