An in Situ-Formed Mosaic Li₇Sn₃/LiF Interface Layer for High-Rate and Long-Life Garnet-Based Lithium Metal Batteries

Garnet-type solid electrolytes are suitable for solid-state batteries with a lithium metal anode, but it is challenging to fabricate garnet-based lithium metal batteries with a long cycle life at high rates. This study demonstrates that a mosaic Li₇Sn₃/LiF interface layer formed in situ on the surface of garnet-type Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZT) through the reaction between a SnF₂ coating layer and a lithium metal enables stable, high-rate cycling for LLZT-based batteries. The interface layer possesses a nanomosaic structure of Li₇Sn₃ nanoparticles and surrounding LiF, enabling fast lithium-ion conduction. Meanwhile, ion insulating Li₂CO₃ on the surface of LLZT pellets is completely removed by SnF₂ during the formation of the interface layer, which reduces the ion diffusion barrier from LLZT to the lithium anode. Benefiting from the advantageous interface layer, LiFePO₄||SnF₂-LLZT||Li cells show superior cycle performance over 200 cycles at 1 C (272 μA cm^-2) with a capacity of 140.6 mAh g^-1 (94.6% retention) at 30 °C. Even at 2 C, a capacity of 102.9 mAh g^-1 remains after 200 cycles. This work provides an optimal interfacial structure to enhance lithium-ion migration between garnet electrolytes and a lithium metal and paves the way for developing high-performance solid-state batteries.