Interface engineering of Li_6.75La₃Zr_1.75Ta_0.25O₁₂via in situ built LiI/ZnLi mixed buffer layer for solid-state...

Garnet-type solid-state Li metal batteries (SSLMBs) are viewed as hopeful next-generation batteries due to their high energy density and safety. However, the major obstacle to the development of garnet-type SSLMBs is the lithiophobicity of Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO), resulting in a large interfacial impedance. Herein, a LiI/ZnLi mixed ion/electron conductive buffer layer is constructed at the interface by an in situ reaction of molten Li metal with ZnI₂ film. This mixed buffer layer ensures close contact between the Li metal and garnet, significantly reducing interfacial impedance. As a result, the Li symmetrical cell with the LiI/ZnLi buffer layer shows an interface impedance of 10.3 Ω cm², much lower than that of the cell with bare LLZTO (1173.4 Ω cm²). The critical current density (CCD) is up to 2.3 mA cm^-2, and the symmetric cells present a long cycle life of 2000 h at 0.1 mA cm^-2 and 800 h at 1.0 mA cm^-2. In addition, the full cells assembled with the LiFePO₄ cathode show a capacity of 143.9 mA h g^-1 after 200 cycles at 0.5C with a low-capacity decay of 0.021% per cycle. This work reveals a simple, feasible, and practical interface modification strategy for solid-state Li metal batteries.