To examine the conductivity of the synthesized Li-H-Ti-O ternary compounds containing different amounts of confined water, the LHTO-pre, LHTO, and LTO powders are first cold-compressed into pellets in a die (Figure , Supporting Information). To measure the Li + conductivity, a Li|PEO|pellet|PEO|Li sandwich cell is made, and then direct current (DC) measurement is performed with the same cell at different temperatures between 60 °C to 80 °C. The result shows the Li-ion conductivities of LHTO-pre and LHTO at 60 °C are 1.12 ×10 -7 S cm -2 and 3.74 ×10 -7 S cm -2 , respectively, which is 2 orders of magnitude higher than that of the water-free LTO (2.68 ×10 -9 S cm -2 ) (Figure ). To measure the electronic conductivity of the sample, an ss|pellet|ss sandwich cell is made (ss refers to stainless steel) for DC measurement. The result shows the electronic conductivity of LHTO is about ~10 -12 S cm -2 (Figure ), which is 5 orders of magnitude lower than its ionic conductivity. These results confirm the suitability of LHTO as a SE. Its conductivity is comparable to amorphous LiPON or solid polymer electrolytes . Since both LHTO-pre and LHTO contain confined water in their crystal structure, it remains a question how the confined water would affect their electrochemical stability. Here slow-scan cyclic voltammetry (CV) is performed to assess their electrochemical stability . To examine the stability of these compounds at high potential, their powder is mixed with carbon and then cast onto an Al foil as an electrode. The electrode is then used as the working electrode in an Electrode|liquid electrolyte|Li cell for CV tests in 1.0~4.5 V vs Li + /Li (Figure ). A pair of redox peaks is observed between 1.0~2.3 V for all three materials, which corresponds to the redox of Ti 4+ /Ti 3+ . If the Ti 4+ is reduced to Ti 3+ , the electronic conductivity will increase rapidly . At above 2.3 V, an obvious peak appears for LHTOpre starting from 3.5 V, which can be attributed to the oxidation of nano-confined water. In contrast, no clear redox peaks can be observed in 2.3~4.0 V for LHTO and LTO (Figure ). Besides the highvoltage stability, the reversibility for ionic transport should also be considered for a SE. After the first activation process, the curves from the 2 nd to 5 th cycles overlap quite well (Figure ), demonstrating the LHTO structure is quite stable at 1.0~4.5 V as an ionic conductor.