Crystallization process of Li3PS4 investigated by X-ray total scattering measurement and the reverse Monte Carlo method

Abstract

Solid electrolytes, such as sulfide glasses, are key materials to put all-solid-state Li-ion batteries into practical use. We evaluated the local structure of 75Li2S･25P2S5 (mol%) Li3PS4 glass with a structural phase transition from glass to crystal between 298 K and 523 K by X-ray total scattering data coupled with Reverse Monte Carlo (RMC) modeling. We have obtained structural models consistent with the observed structure factors (Sobs(Q)) of Li3PS4 at different temperatures with a low R-factor Rp value within 2.0%. The RMC results suggest the PS4 anion remains a tetrahedron over the wide range from glass to crystal based on the angular and distance distribution of intra-PS4 SS pairs. On the other hand, the SS correlation of inter-PS4 distances changes from a wider to a narrower distribution during crystallization. The angular distance distribution functions g(r, θ) indicate that the orientation of the PS4 tetrahedra changes from random to long-range ordered as crystallization progresses. SLi partial correlation and the average spatial distribution of P and S show a wider distribution of Li-ions at a higher temperature of the glassy state and the distribution then narrows due to crystallization. The RMC results suggest that PS4 libration plays an important role in Li-ion migration in Li3PS4 solid electrolytes.