Ionic conductivity enhancement at phase boundaries achieved by two-phase coexistence of spinel and rock salt structures: mimetics for the anode material Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>

Oxide solid electrolytes with high Li-ion conductivity are necessary for the realization of highly safe all-solid-state Li-ion batteries. Although oxide materials have been extensively investigated, no material with sufficient properties as a solid electrolyte has yet been achieved. Therefore, enhancing ionic conductivity in solid materials remains an important challenge. In this study, two-phase coexistence ceramics were prepared to mimic the conductivity enhancement observed in the anode material Li4Ti5O12 with spinel and rock salt structures using Li-containing insulating materials. LiFe5O8 and Li3NbO4 are used as model spinel and rock salt-related structures, respectively. A two-phase separated dense ceramic was successfully prepared by using a conventional solid-state reaction. AC impedance measurements revealed a single semicircle component at room temperature, indicating ionic conductivity enhancement induced by two-phase coexistence. The ceramics with two-phase coexistence showed a conductivity on the order of 10−7 S/cm, about nine orders of magnitude higher than that of pure LiFe5O8 or Li3NbO4. These results suggest that the conductivity enhancement can be interpreted as mimetic to that observed in Li4Ti5O12.