Appearance of Lithium-Ion Conduction in a La–Li–Co–O Band Insulator: Possible Route to Oxide Electrolyte

For the past decade or so, oxide electrolytes have been more intensively investigated as solid electrolytes instead of liquid ones because of their safer handling, more sophisticated device architectures, and potentially higher energy/power densities. Among them, however, studies on transition metal ion-bearing oxide electrolytes have been so far limited to the family of compounds characterized by d0 valence states such as La2/3–xLi3xTiO3, Li7La3Zr2O12, and LixLa(1–x)/3NbO3. Unfortunately, other families of oxide electrolytes remain unexplored. In the present study, we have employed a new lithium-intercalated oxide La–Li–Co–O known as a band insulator with the valence state 3d6 while intentionally circumventing the conventional design rule based on the use of d0 transition metal ions (e.g., Ti4+, Zr4+, and Nb5+) and could achieve a lithium-ion conductivity of 7 × 10–5 S cm–1 at 140 °C. Moreover, a quasi-one-dimensional pathway for lithium diffusion has been confirmed to be present and responsible for realizing lithium-ion conduction by analyzing neutron powder diffraction data in combination with the maximum entropy method. It is also confirmed that a pseudo-solid-state cell does serve as a secondary battery. Such findings could pave the way for designing new ionic conductors, hopefully leading to the development of a high-performance electrolyte using a d-band insulator.