Role of divalent cation (Ba) substitution in the Li+ ion conductor LiTi2(PO4)3: a molecular dynamics study

Abstract

The derivatives of LiTi₂(PO₄)₃ are promising electrolytes for solid-state batteries. An extensive molecular dynamics study is performed employing a refined set of potential parameters to understand the influence of Ba substitution on Li⁺ ion conductivity in Ba_x/2Li_1−xTi₂(PO₄)₃ (0.0 ≤ x ≤ 0.83). The refined set of potential parameters reveals the structural and dynamical properties of Ba_x/2Li_1−xTi₂(PO₄)₃ which are consistent with experimental results. In the presence of Ba²⁺, the system endures a persistent competition between the generation of vacant Li-sites and blocking of Li⁺ ion paths. The diffusivity of Li⁺ ions enhances with x and increases one order of magnitude higher at x = 0.67, where the creation of vacant Li-sites mainly drives the diffusion. This trend is similar to the experimental report. However, for x > 0.67 compositions, the blocking of the Li⁺ ion path dominates in the presence of immobile Ba²⁺ ions, resulting in a reduction of Li⁺ ion diffusion. The present study also proposes an ordered substitution of Ba²⁺ ions at crystallographically identified Li1-sites, where an extra Li-site generation is identified at higher compositions. In this case, the vacancy strongly dominates over Li⁺ ion path blocking, resulting in the possibility to achieve even higher Li⁺ ion diffusion. The creation of extra Li-sites and mechanism of Li⁺-ion transport are studied systematically with varying compositions. Further insight into Li⁺ ion transport is gained by constructing a three-dimensional density map and determining the free energy barrier and clustering of Li⁺ ion probability density. And the factors affecting the cation diffusion are also systematically investigated.