Role of Critical Oxygen Concentration in the β-Li₃PS_4–xO_x Solid Electrolyte

Lithium superionic conductors are the critical enabling component for next-generation all-solid lithium-ion batteries. In particular, the β polymorph of Li3PS4 has attracted major interest due to its combination of excellent ionic conductivity and passivating interfacial stability with Li. In this work, we systematically investigated the effect of oxygenation in β-Li3PS4 to further enhance its ionic conductivity and electrochemical stability using density functional theory calculations and ab initio molecular dynamics simulations. We predict that a maximum ionic conductivity of 1.52 mS cm–1 (and minimum activation energy) can be achieved at x = 0.25 in Li3PS4–xOx which is about 7 times higher than that of β-Li3PS4. This increase in ionic conductivity can be attributed to the flattening of the potential energy surface due to the diversification of the Li chemical environments by the S–O mixed-anionic framework, resulting in a change from quasi-2D to 3D Li diffusion. We highlight that the spatial localization of the electrostatic potential is a qualitative descriptor to assess the migration barrier of the charge carrier in the S–O mixed framework. These microscopic analyses shed light on the role of critical oxygen concentration to tune the rate-performance of mixed-anion lithium superionic conductors.