In Situ Formed Surface Layer to Improve the Air Stability of LLZTO and Its Contact with the Li Metal

Garnet-type Li7La3Zr2O12(LLZO) is regarded as one of the most promising solid-state electrolytes due to its good thermal stability, high mechanical strength, wide electrochemical window, and good stability against Li metal. However, Li and LLZO have poor physical contact, leading to a large interfacial resistance and poor electrochemical performance. It is recognized that LLZO is intrinsic lithiophilic. But LLZO easily reacts with H2O and CO2in air and generates Li2CO3and other inevitable surface contaminants. The contaminated surface results in poor interfacial wettability against Li metal. Herein, an air-stable surface layer is introduced to Li6.4La3Zr1.4Ta0.6O12(LLZTO) by simply reacting LLZTO with a LiPF6-based liquid electrolyte. During the reacting process, Li2CO3is removed and a lithiophilic and ion-conductive modified layer is formed. The layer helps to build a uniform and intimate contact between Li and LLZTO. The interfacial resistance is reduced to 15.675 Ω cm2, less than one fifth of that between bare LLZTO and Li. Benefiting from the favorable interface, stable Li plating and stripping can be conducted for over 1000 h at a current density of 0.22 mA cm–2. Even after being exposed to air for 3 days, the interfacial resistance of surface-modified LLZTO against Li remains as low as 36.29 Ω cm2and performs stable Li plating and stripping for 1000 h. Moreover, the quasi-solid-state Li metal batteries of Li/LLZTO/LiCoO2, Li/LLZTO/LiNi0.8Co0.1Mn0.1O2,and Li/LLZTO/LiNi0.5Mn1.5O4exhibit excellent cycling performances, which confirms the feasibility of the strategy in the practical manufacture.