Owing to its high ionic conductivity (10−4 - 10−3 S cm−1), excellent stability toward lithium and a wide electrochemical window, the garnet-type solid-state electrolyte has been regarded as one of the most promising solutions to the viability of lithium metal-based batteries. However, the poor interfacial contact between the garnet and the lithium metal leads to an ultrahigh interfacial resistance (∼1000 Ω cm2) and uneven current distribution. In this work, the issue is effectively addressed by coating the garnet pellet with a SnO2 nanolayer. Attributed to the lithiation reaction between the lithium metal and SnO2, an ion-conducting interlayer is generated automatically between the garnet and lithium metal, which reduces their interfacial resistance from 1019 to 153 Ω cm2. A Li | SnO2-garnet | Li symmetric cell is then built and achieves a stable lithium plating/stripping for more than 900 h at 0.2 mA cm−2 without short circuit. Moreover, in a Li | SnO2-garnet | LiFePO4 full battery test, the SnO2 nanolayer enables the full battery to be stably operated for over 100 cycles at 0.3 C with a capacity retention of 98.6%, whereas the control group using bare garnet is unable to work at this high rate.