The Dual Role of Ga in LLZTO: Improved Interface Contact versus Accelerated Failure

The cubic phase Li₇La₃Zr₂O₁₂ (c-LLZO) has garnered significant attention as a crucial solid-state electrolyte material for high-energy-density Li metal batteries due to its exceptional chemical stability and safety characteristics. Ga-doped LLZO (Ga-LLZO) has emerged as one of the leading systems owing to its superior ionic conductivity; however, the interfacial instability between Ga-LLZO and Li metal and its underlying mechanisms remain inadequately understood. This study systematically modulated the Ga content in Li_6.4Ga_xLa₃Zr_2-(0.6-3x)Ta_0.6-3xO₁₂ (Ga_xTa_0.6-3x). The aim was to investigate the ionic conductivity enhancement mechanism in Ga-LLZTO and analyze the failure behavior of its interface with Li metal during a high-temperature coating. Ultimately, this work seeks to determine the optimal Ga content for achieving a stable Li metal interface. The results reveal that the improvement in room-temperature ionic conductivity of Ga-LLZTO is attributed to Coulombic repulsion between Ga³⁺ and Li⁺. This enhancement effect is only effective below 45 °C. At elevated temperatures, Li⁺ possesses a sufficient intrinsic driving force for migration within the electrolyte lattice, while Ga³⁺ occupation of Li sites conversely impedes carrier mobility. Furthermore, the reaction between Ga-LLZTO and Li metal leads to the formation of the Li-Ga alloy and LiGaO₂ phases. When the highly electronically conductive alloy phase forms at the electrolyte/Li metal interface, it promotes uniform electron distribution, reduces polarization voltage, and improves interfacial contact. Conversely, accumulation of Li-Ga alloy phases within the electrolyte bulk induces electrolyte failure. The results demonstrate a critical Ga doping threshold at x = 0.10. Within this threshold range, the critical current density (CCD) exhibits a negative correlation with Ga doping content. The Li||Ga_0.10Ta_0.30||Li symmetric cell demonstrates stable cycling for 537 h at a current density of 0.1 mA cm^-2. The formation of detrimental Li₂GaO₂, Li₅GaO₄, and Li-Ga alloy phases within the electrolyte bulk induces soft short circuits, resulting in cell failure through a mechanism distinct from conventional Li dendrite penetration. The LiFePO₄||Ga_0.10Ta_0.30||Li full cell achieves a discharge capacity of 144.1 mAh g^-1 after 100 cycles with a capacity retention of 92.8%. In conclusion, this work clarifies the regulation mechanism of Ga-LLZTO stability and reveals the dual role of Li-Ga alloys. These findings provide a theoretical foundation for the rational application of Ga-doped garnet-type solid electrolytes.