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

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