Impacts of GeO₂ modification on microstructure, electrical and interface performance of Ta-doped garnet electrolytes

Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) electrolytes have attracted tremendous attentions in recent years. In this work, a novel strategy is proposed to simultaneously address their grain boundary problems and the interface issues with Li anode by introducing GeO₂ additive in sintering of LLZTO ceramics. GeO₂ consumes Li₂O, forming Li₄(Ge₅O₁₂)/Li₂O eutectic melt phase in grain boundary regions. Li-Ge-O melt contributes primarily in (i) regulating growth and combination of LLZTO grains, (ii) constructing consecutive Li⁺ conduction pathways within the electrolytes, (iii) promoting Li wettability on ceramic's surface and (iv) suppressing growth of lithium dendrites. Therefore, the ionic conductivity is increased to 6.23 × 10⁻⁴ S·cm⁻¹ and the activation energy is reduced to 0.29 eV. The compact bonding without any modification layer is formed between Li and the electrolyte. The Li|LLZTO@Li₄(Ge₅O₁₂)/Li₂O|Li symmetrical cell cycles stably for over 400 h at the current density of 0.1 mA·cm⁻² and 300 h at the current density of 0.2 mA·cm⁻².