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

The cubic phase Li 7 La 3 Zr 2 O 12 (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 6.4 Ga x La 3 Zr 2–(0.6–3 x ) Ta 0.6–3 x O 12 (Ga x Ta 0.6–3 x ). 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 Ga 3+ 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 Ga 3+ 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 LiGaO 2 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 0.10 Ta 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 Li 2 GaO 2, Li 5 GaO 4, 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 LiFePO 4 ||Ga 0.10 Ta 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.