Influence of Li-site dopants on phase stabilization, lithium distribution, and ionic conductivity in high-entropy Li-garnet solid electrolytes

Abstract

Garnet-type solid electrolytes are promising candidates for solid-state lithium batteries, yet their practical deployment is often limited by secondary phase formation, poor thermal stability during high-temperature processing, and overall conductivity. In this work, we systematically investigate the role of Li-site dopants in governing phase stability, secondary phase evolution, and Li-ion transport in the high-entropy garnet Li_6−xA_xLa₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ (A = Al³⁺, Fe³⁺, Ga³⁺, and Zn²⁺). X-ray diffraction and microstructural analyses reveal strong dopant-dependent differences in thermal stability: while undoped, Al-, and Fe-doped compositions exhibit pronounced secondary phase formation under prolonged sintering, Ga- and Zn-doped garnets maintain single-phase cubic structures with excellent thermal robustness. Neutron diffraction refinements show that all compositions retain the cubic Iad framework but display distinct Li redistributions between tetrahedral 24d and octahedral 96h sites, directly controlling the topology of Li-ion migration pathways. Trivalent dopants enhance ionic conductivity by generating active octahedral vacancies and promoting Li-site disorder, whereas Zn²⁺ produces predominantly dopant-adjacent, mobility-suppressed vacancies despite improved phase stability. These results demonstrate that ionic conductivity in high-entropy garnets is governed not by total vacancy concentration or phase purity alone, but by the balance between thermal stability, secondary phase suppression, and active vacancy topology. This work establishes Li-site chemistry as a critical lever for simultaneously tuning phase stability and ionic transport in high-entropy garnet solid electrolytes.