Synergistically Engineering Grains and Grain Boundaries toward Li Dendrite-Free Li₇La₃Zr₂O₁₂

Cation-doped cubic Li₇La₃Zr₂O₁₂ is regarded as a promising solid electrolyte for safe and energy-dense solid-state lithium batteries. However, it suffers from the formation of Li₂CO₃ and high electronic conductivity, which give rise to an unconformable Li/Li₇La₃Zr₂O₁₂ interface and lithium dendrites. Herein, composite AlF₃-Li_6.4La₃Zr_1.4Ta_0.6O₁₂ solid electrolytes were created based on thermal AlF₃ decomposition and F/O displacement reactions under a high-temperature sintering process. When the AlF₃ is thermally decomposed, it leaves Al₂O₃/AlF₃ meliorating the grain boundaries and F^– ions partially displacing O^2– ions in the grains. Due to the higher electronegativity of F^– in the grains and the grain-boundary modification, these AlF₃-Li_6.4La₃Zr_1.4Ta_0.6O₁₂ deliver optimized electronic conduction and chemical stability against the formation of Li₂CO₃. The Li/AlF₃-Li_6.4La₃Zr_1.4Ta_0.6O₁₂/Li cell exhibits a low interfacial resistance of ∼16 Ω cm² and an ultrastable long-term cycling behavior for 800 h under a current density of 200 μA/cm², leading to Li//LiCoO₂ solid-state batteries with good rate performance and cycling stability.