Ionic Conductivity of Lithium Phosphides

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 (Li3P, LiP, Li3P7, Li3P11, LiP7) and metastable (Li4P(3), Li5P4, LiP5) compounds was studied as a function of temperature. In some compounds we found have high ionic conductivity at room temperatures (10-3-10-2 S cm(-1)). Structures with the lowest ionic conductivity are LiP, Li3P11, and LiP7, in which diffusion is negligible in the whole temperature range 300-500 K. In Li3P, Li3P7, and Li4P3, LiP, there is the 3D diffusion of Li atoms, while in Li(5)P(4 )the 2D mechanism prevails, and in LiP5 and LiP(7 )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.