Preparation and Electrochemical Characteristics of the Co-Doped Li7La3Zr2O12 Solid Electrolyte with Fe3+ and Bi3+

Solid-state electrolytes (SSEs) have emerged as the most promising alternative to liquid electrolytes in batteries due to their enhanced stability and safety. Among these, the garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte has attracted significant research interest due to its wide electrochemical stability window and good air stability. However, the ionic conductivity of LLZO is lower due to its high sintering temperature and unstable phase structure. In this study, Li_6.4+xFe_0.2La₃Zr_2-xBi_xO₁₂ (x = 0, 0.05, 0.1, 0.15) solid electrolytes were synthesized using a conventional solid-state reaction method by co-doping LLZO with Fe³⁺ and Bi³⁺ ions. Compared with pure LLZO, doping with Fe³⁺ effectively stabilizes the cubic phase, thereby enhancing the ionic conductivity. Moreover, Bi³⁺ doping significantly lowers the sintering temperature of the electrolyte, which in turn reduces energy consumption during the processing. The co-doping of Fe³⁺ and Bi³⁺ not only improves the density of the LLZO electrolyte, achieving a relative density of up to 95%, but also increases the ionic conductivity, with a maximum value of 7.57 × 10^-4 S·cm^-1 observed at the optimal composition (Li_6.4+xFe_0.2La₃Zr_2-xBi_xO₁₂, x = 0.1).