Transformation of Undesired Li 2 CO 3 into Lithiophilic Layer Via Double Replacement Reaction for Garnet Electrolyte Engineering

Garnet‐type solid‐state electrolytes (SSEs) are a remarkable Li‐ion electrolyte for the realization of next‐generation all‐solid‐state lithium batteries due to their excellent stability against Li metal as well as high ionic conductivities at room temperature. However, garnet electrolytes always contain undesired and hardly removable Li 2 CO 3 contaminations that have persistently large resistance and unstable interface contact with Li metal. This is a critical bottleneck for the practical application of garnet electrolytes. Here, we design a novel strategy to completely root out Li 2 CO 3 both inside and on the surface of garnet. This is achieved by a so‐called double replacement reaction between Li 2 CO 3 and SiO 2 during one‐step hot press process for garnet electrolyte densification. It leads to in‐situ transformation of Li x SiO y (LSO) mostly locating around the grain boundaries of garnet. Due to the higher ion conductivity and better electrochemistry stability of LSO than Li 2 CO 3 , the modified garnet electrolyte shows much improved electrochemical performance. Moreover, the wettability between modified garnet electrolyte and lithium metals was significantly enhanced in the absence of surface Li 2 CO 3 . As a proof of concept, an assembled Li symmetric cell with modified garnet electrolyte displays a high critical current density (CCD) of 0.7 mA cm −2 and a low interfacial impedance (5 Ω cm 2 ) at 25 °C. These results indicate that the upcycling of Li 2 CO 3 is a promising strategy to well‐address the degradation and interfacial issue associated with garnet electrolytes.