Ionic Conductivity of Lithium Phosphides

Abstract

We comprehensively study the ionic conductivity in lithium phosphides, promising materials for energy storage applications, by using a combination of first-principles computations and machine learning interatomic potentials. Using the quasiharminic approximation, we calculated convex hulls of the Li-P system at various temperatures and the temperature-composition phase diagram was obtained, delineating the stability regions of each phase. The ionic conductivity of stable (Li (Formula presented.) P, LiP, Li (Formula presented.) P (Formula presented.), Li (Formula presented.) P (Formula presented.), LiP (Formula presented.)) and metastable (Li (Formula presented.) P (Formula presented.), Li (Formula presented.) P (Formula presented.), LiP (Formula presented.)) compounds was studied as a function of temperature. In some compounds we found have high ionic conductivity at room temperatures (10 (Formula presented.) –10 (Formula presented.) S cm (Formula presented.)). Structures with the lowest ionic conductivity are LiP, Li (Formula presented.) P (Formula presented.), and LiP (Formula presented.) in which diffusion is negligible in the whole temperature range 300–500 K. In Li (Formula presented.) P, Li (Formula presented.) P (Formula presented.), and Li (Formula presented.) P (Formula presented.), LiP, there is the 3D diffusion of Li atoms, while in Li (Formula presented.) P (Formula presented.) the 2D mechanism prevails, and in LiP (Formula presented.) and LiP (Formula presented.) the 1D mechanism was observed. This study may provide insights for the development of Li-P materials in lithium ion and lithium metal battery applications.