Synthesis and Comprehensive Analytical Study of β-Li₃PS₄ Stabilization by Ca- and Ba-Codoped Li₃PS₄

Sulfide-based solid electrolytes with high Li⁺ conductivity, such as Li₃PS₄, are key materials for the realization of all-solid-state Li⁺ batteries. One approach to achieving high Li⁺ conductivity is to combine crystalline-phase stabilization at high temperatures with the introduction of defects at room temperature. In this work, this approach was verified by codoping Li₃PS₄ with two kinds of divalent cations. The resulting structural changes were comprehensively investigated both experimentally and computationally. The high-temperature β-Li₃PS₄ phase of Li₃PS₄ could be stabilized at room temperature by adjusting the amount of Ca or Ba doping. The synthesized samples doped with divalent cations were found to have conductivities about 2 orders of magnitude higher than that of the γ-Li₃PS₄ phase at room temperature. The resultant Li⁺ conductivity at room temperature was also higher than that expected from interpolation of results for nondoped β-Li₃PS₄. It is believed that the structural changes produced by the divalent cation doping contribute to this increase in conductivity. The stability of the β-Li₃PS₄ phase with divalent cation doping was also demonstrated using density-functional-theory calculations for models with equivalent compositions to the synthesized samples. The Li⁺ positions obtained by structural optimization calculations showed the presence of diverse and disordered Li sites in the Ca-doped lattice. Such structural changes can contribute to cascade processes involving Li⁺ collisions, referred to as the "billiard-ball" mechanism, which cannot occur in nondoped β-Li₃PS₄. This series of experiments involving the synthesis and analyses of β-Li₃PS₄ with divalent cation doping provides a way to enhance Li⁺ conductivity through structural modification and optimization.