Recent Strategies for Lithium-Ion Conductivity Improvement in Li₇La₃Zr₂O₁₂ Solid Electrolytes

The development of solid electrolytes with high conductivity is one of the key factors in the creation of new power-generation sources. Lithium-ion solid electrolytes based on Li₇La₃Zr₂O₁₂ (LLZ) with a garnet structure are in great demand for all-solid-state battery production. Li₇La₃Zr₂O₁₂ has two structural modifications: tetragonal (I41/acd) and cubic (Ia3d). A doping strategy is proposed for the stabilization of highly conductive cubic Li₇La₃Zr₂O₁₂. The structure features, density, and microstructure of the ceramic membrane are caused by the doping strategy and synthesis method of the solid electrolyte. The influence of different dopants on the stabilization of the cubic phase and conductivity improvement of solid electrolytes based on Li₇La₃Zr₂O₁₂ is discussed in the presented review. For mono-doping, the highest values of lithium-ion conductivity (~10^-3 S/cm at room temperature) are achieved for solid electrolytes with the partial substitution of Li⁺ by Ga³⁺, and Zr⁴⁺ by Te⁶⁺. Moreover, the positive effect of double elements doping on the Zr site in Li₇La₃Zr₂O₁₂ is established. There is an increase in the popularity of dual- and multi-doping on several Li₇La₃Zr₂O₁₂ sublattices. Such a strategy leads not only to lithium-ion conductivity improvement but also to the reduction of annealing temperature and the amount of some high-cost dopant. Al and Ga proved to be effective co-doping elements for the simultaneous substitution in Li/Zr and Li/La sublattices of Li₇La₃Zr₂O₁₂ for improving the lithium-ion conductivity of solid electrolytes.