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 beta-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 gamma-Li3PS4 phase at room temperature. The resultant Li+ conductivity at room temperature was also higher than that expected from interpolation of results for nondoped beta-Li3PS4. It is believed that the structural changes produced by the divalent cation doping contribute to this increase in conductivity. The stability of the beta-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 beta-Li3PS4. This series of experiments involving the synthesis and analyses of beta-Li3PS4 with divalent cation doping provides a way to enhance Li+ conductivity through structural modification and optimization.