First Principles Study of the Phase Stability, the Li Ionic Diffusion, and the Conductivity of the Li10GexMo1−xP2S12 of Superionic Conductors

Abstract

Using first-principles density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we performed this study on the phase stability, the intrinsic redox stability, and the Li+ conductivity of Li10GexMo1−xP2S12 (x = 0~1) superionic conductors. Molybdenum (Mo) is expected to replace expensive germanium (Ge) to lower tmaterial costs, reduce sensitivity to ambient water and oxygen, and achieve acceptable Li+ conductivity. The ab initio first principle molecular dynamics simulations show that room-temperature Li+ conductivity is 1.12 mS·cm−1 for the Li10Ge0.75Mo0.25P2S12 compound, which is comparable to the theoretical value of 6.81 mS·cm−1 and the experimental measured one of 12 mS·cm−1 of the Li10GeP2S12 (LGPS) structure. For Li10GexMo1−xP2S12 (x = 0, 0.25, 0.5 and 1) compounds, the density of states and the projection fractional wave state density were calculated. It was found that when Ge atoms were partially replaced by Mo atoms, the band gap remained unchanged at 2.5 eV, but deep level defects appeared in Mo-substituted compounds. Fortunately, this deep level defect is difficult to ionize at room temperature, so it has no effect on the electronic conductivity of Mo substitute compounds, making Mo substitution a suitable solution for electrolyte materials. The projection fractional wave state density calculation shows that the covalent bond between Mo and S is stronger than that between Ge and S, which reduces the sensitivity of Mo-substituted compounds to water and oxygen contents in the air. In addition, the partial state density coincidence curve between Li and S elements disappears in the 25% Mo-substituted compound with energies of 4–5 eV, indicating that the Li2S by-product is decreased.