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 LiSn-LiF interface layer formed on the surface of garnet-type LiLaZrTaO (LLZT) through the reaction between a SnF coating layer and lithium metal enables stable, high-rate cycling for LLZT-based batteries. The interface layer possesses a nano-mosaic structure of LiSn nanoparticles and surrounding LiF, enabling fast lithium ion conduction. Meanwhile, ion insulating LiCO on the surface of LLZT pellets is completely removed by SnF 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, LiFePO||SnF-LLZT||Li cells show superior cycle performance over 200 cycles at 1 C (272 μA cm) with a capacity of 140.6 mAh g (94.6% retention) at 30°C. Even at 2 C a capacity of 102.9 mAh g remains after 200 cycles. This work provides an optimal interfacial structure to enhance lithium ion migration between garnet electrolytes and lithium metal and paves the way for developing high-performance solid-state batteries.