Interface engineering of Li_6.75La₃Zr_1.75Ta_0.25O_{1 2}via in situ built LiI/ZnLi_x mixed buffer layer for solid-state lithium metal batteries

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 Li6.75La3Zr1.75Ta0.25O12 (LLZTO), resulting in a large interfacial impedance. Herein, a LiI/ZnLix 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/ZnLix buffer layer shows an interface impedance of 10.3 Omega cm(2), much lower than that of the cell with bare LLZTO (1173.4 Omega cm(2)). 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.