To examine their stability at low potential, their powder is mixed with carbon and then cast onto a Cu foil as the working electrode for CV tests in 0.01~3.0 V vs Li + /Li (Figure ). For LHTO-pre, the CV (Figure ) shows several strong cathodic peaks during the first scan, which can be attributed to the reduction of absorbed water, nano-confined water, as well as the formation of solid electrolyte interface (SEI) on the surface of LHTO-pre (Figure , Supporting Information). The weakening of these peaks in the subsequent cycles suggests the formed SEI to some extent mitigate the further decomposition of the LHTO-pre. For LHTO, the reduction peaks during the first cycle are much weaker than those in LHTO-pre (Figure , f), indicating the nano-confined pseudo-water is more stable than the nano-confined water. To examine if a stable SEI can form on the surface of LHTO, the cell is polarized to 0 V and the current response is recorded. After polarization for 20000 sec, the current decreases to below 10 -2 A g -1 , which suggests the side reaction becomes negligible. It should be noted despite the LHTO being relatively stable at low potential, the reduction of Ti 4+ renders it electronic-conductive and fails it as an SE. Therefore, similar to perovskite-type Li3xLa2/3-x□1/3-2xTiO3 (LLTO), an electronic insulator, e.g., polyethylene oxide (PEO), is still needed to avoid direct contact between Li metal and LHTO if it is used in a solid-state battery.