Synergistically Engineering Grains and Grain Boundaries toward Li Dendrite-Free Li 7 La 3 Zr 2 O 12 .

Cation-doped cubic Li 7 La 3 Zr 2 O 12 is regarded as a promising solid electrolyte for safe and energy-dense solid-state lithium batteries. However, it suffers from the formation of Li 2 CO 3 and high electronic conductivity, which give rise to an unconformable Li/Li 7 La 3 Zr 2 O 12 interface and lithium dendrites. Herein, composite AlF 3 -Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 solid electrolytes were created based on thermal AlF 3 decomposition and F/O displacement reactions under a high-temperature sintering process. When the AlF 3 is thermally decomposed, it leaves Al 2 O 3 /AlF 3 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 AlF 3 -Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 deliver optimized electronic conduction and chemical stability against the formation of Li 2 CO 3 . The Li/AlF 3 -Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 /Li cell exhibits a low interfacial resistance of ∼16 Ω cm 2 and an ultrastable long-term cycling behavior for 800 h under a current density of 200 μA/cm 2 , leading to Li//LiCoO 2 solid-state batteries with good rate performance and cycling stability.