Enabling high ionic conductivity in yttrium-based lithium halide electrolytes by composition modulation for all-solid-state batteries

Chloride electrolytes have again become a focus of research in recent years due to the oxidative stability at high potential. Y-based chlorides such as Li3YCl6 have a considerable ionic conductivity of ∼10−4 S/cm. The ionic conductivity of a solid-state electrolyte (SSE) is closely related to its crystal structure. Low lithium concentration composition is beneficial for lithium-ion conduction structure. Due to the presence of more free octahedral sites, the crystal structure with Pnma space group has a better ion diffusion compared to the crystal structure with P-3m1 space group with hexagonal close-packed structure (hcp). In addition to this, the doping of cations with higher valence states results in more lithium vacancy compensation in the crystal structure, which is a significant modification to improve the ionic conductivity of the chloride. Herein, ab initio molecular dynamics (AIMD) simulations were performed to investigate the effects of the lithium-deficient state configuration and the cation Nb5+ doping modification on the ion conduction of LiaYClb solid-state electrolytes. The lithium-deficient state composition structure has shifted the material space group structure from P-3m1 to Pnma, which facilitates the diffusion of lithium ions. The Nb5+ doping has increased the chance of lithium ion co-diffusion and disordered the lithium ion sites near the Y(Nb) cation site, resulting in a lower ab-plane diffusion barrier and a significant improvement of lithium ion migration. A series of lithium-deficient state compositions and Nb-doped chlorides were synthesized. Li2.31Y0.98Nb0.02Cl5.31 is obtained by solid sintering preparation with an ionic conductivity close to 1.0 × 10−3 S/cm and high electrochemical stability. The full cell of Li2.31Y0.98Nb0.02Cl5.31 matched with bare LiCoO2 maintains stability for 100 cycles.