Effects of Fe/ Mo co-doping on the structure and ionic conductivity of garnet-type Li7La3Zr2O12 solid-state electrolyte

Garnet-type lithium lanthanum zirconium oxide (Li 7 La 3 Zr 2 O 12 , LLZO) has two polymorphs: the tetragonal phase (t-LLZO) and the cubic phase (c-LLZO). c-LLZO possesses superior ionic conductivity and thermal stability. However, c-LLZO tends to transform into t-LLZO at room temperature, which reduces its conductivity. Additionally, conventional cation doping strategies often induce grain coarsening and poor sintering performance. Herein, Fe/Mo co-doped LLZO (Li 6.4-2 x Fe 0.2 La 3 Zr 2- x Mo x O 12 , x = 0, 0.1, 0.2, 0.3) is synthesized via the sol-gel method followed by sintering. The effects of Fe/Mo co-doping on the stability of the cubic phase and ionic transport are investigated. Results reveal that Mo 6+ substitution for Zr 4+ causes lattice contraction (approximately 6.9 ‰), thereby inhibiting impurity phase formation and stabilizing the cubic phase. Simultaneously, Fe/Mo co-doping improves sintering activity, resulting in refined grains (D50 = 20 μm) and high densification (>95% of theoretical density). Electrochemical tests confirm that the electrolyte sheet with a Mo doping concentration of x = 0.2 achieves peak ionic conductivity (σ = 1.97 × 10 −3 S cm −1 ). A Li-Li symmetric cell equipped with Li 6 Fe 0.2 La 3 Zr 1.8 Mo 0.2 O 12 electrolyte exhibits an interface impedance as low as 71.4 Ω cm 2 and a critical current density of 0.5 mA cm −2 , demonstrating effective suppression of lithium dendrite growth through improved interfacial stability.