In-situ construction of Li-Mg/LiF conductive layer to achieve an intimate lithium-garnet interface for all-solid-state Li metal battery

Garnet-type Li6.75La3Zr1.75Ta0.25O12 (LLZTO) is a promising solid electrolyte due to its high ionic conductivity and good stability to Li. However, poor wettability of LLZTO|Li interface causes huge contact impedance and easily inclined formation of Li dendrites. Herein, a 700 nm-thick MgF2 sol film is constructed on the LLZTO pellet by a spin coating method, then an all-inorganic mixed conductive layer of Li-Mg alloy/LiF (LMF) is firstly obtained by the conversion reaction with molten Li, without adding any binder or filmogen with high resistance and low migration rate. Density functional theory (DFT) calculation results show that LMF layer has perfect interfacial contact with LLZTO and Li metal, as well as excellent electronic insulativity and ionic conductivity to prevent electron tunneling and attacking the LLZTO. The interface impedance of Li|LMF@LLZTO|Li symmetrical battery is significantly reduced from 1850 to 25 omega cm2, and an excellent cycle stability over 1000 h is obtained at 0.3 mA cm-2 with a low overpotential of 62 mV. Moreover, the Li|LMF@LLZTO|LiFePO4 full battery shows outstanding rate capacity and cycle performance, revealing the practical feasibility of the interface modification strategy for all-solid-state Li metal battery.