Exploring high-valence element doping in LLZO electrolytes: Effects on phase transition and lithium-ion conductivity

The increasing popularity of electric vehicles and the emergence of 5G technology have created a demand for high-energy-density and safe batteries. Among various solid-state electrolytes, crystalline Li7La3Zr2O12 (c-LLZO) has garnered significant attention due to its exceptional properties. Researchers have explored doping in LLZO to modulate its phase transition and enhance lithium-ion conductivity. However, the mechanism by which doping induces a phase change and affects lithium-ion conductivity is currently unclear. In this study, we investigated LLZO doping and discovered that high-valence-element doping introduces vacancies, destabilizing the tetragonal phase and promoting the formation of the stable cubic phase. Moreover, we found that the introduction of lithium-ion vacancies significantly enhances lithium-ion conductivity. Nevertheless, doping also has adverse effects. Ga and Al dopants occupy the crucial Li24d sites, impeding lithium-ion diffusion and resulting in the closure of diffusion channels. Additionally, doping reduces the effective vacancy concentration within the structure, further restricting lithium-ion conductivity. In summary, our work elucidates the mechanism behind the influence of dopants on lithium-ion diffusion in LLZO.