We have been conducting research to explore new materials and their use in batteries by pursuing the phenomenon of high ion diffusion in solids. Lithium ion conducting materials have been particularly explored because of their potential application to all-solid-state batteries with high energy density. In addition to an overview of lithium ionic conductors, we will review the material search process for Li₁₀GeP₂S₁₂, which exhibits particularly high ionic conductivity. We will also review the research that clarified the formation region of this material, investigated the ionic conduction mechanism, and applied this material to all-solid-state batteries.

All-solid-state Li–S batteries with Li₂S cathode active materials have attracted considerable attention owing to their high theoretical energy densities; however, the improvement of electron, ion conductivity and the activation of the redox reaction of Li₂S is challenging owing to its insulating behavior. The addition of multivalent cations (such as Mg²⁺, Ca²⁺, Al³⁺, and Y³⁺) to Li₂S results in a higher discharge capacity, thereby improving the ionic conductivity and reactivity of Li₂S. These cations can form solid solutions or nanocomposites, and may also exhibit catalytic activity. The ionic radii of Mg²⁺ (0.57 Å) and Al³⁺ (0.35 Å) are similar to that of Li⁺ (0.59 Å); therefore, these additives are expected to form solid solutions. The ionic radius of Ca²⁺ (1.0 Å) is larger than that of Li⁺ (0.59 Å), resulting in the formation of CaS–Li₂S nanocomposites. Y³⁺ tends to segregate at the grain boundaries, while Y³⁺ on the surface of Li₂S particles exhibits catalytic effects. This review summarizes the effects of multivalent cation doping on the activation of Li₂S-based cathode materials.

The effect of Al addition in ZnO varistors revealed to be the suppression of grain growth, not the lowering of the electric resistance of ZnO. This Al effect can explain the change in the E-J characteristics by the following three points. (i) The grain boundary becomes thinner and the proportion of boundary layers with a thickness of 10 nm or less increases, which enlarges the effective cross-sectional area of the tunnel current generation. (ii) The delay in densification during the sintering makes it easier for pores to remain at the grain boundary, which increases discontinuities of boundary layers between ZnO grains and increases leakage current, resulting in reduced non-ohmic properties in the low current range. (iii) Abnormal grain growth of ZnO is suppressed and the particle size becomes more uniform, which improves non-ohmic characteristics. This paper proposes a new barrier model for the conduction mechanism of ZnO varistors, based on the tunneling effect and taking into account the influences caused by microstructural inhomogeneities specific to polycrystalline ceramics, such as discontinuities in grain boundary layers and variations in ZnO grain size.

Mechanical properties of Li_10.35Ge_1.35P_1.65S₁₂(LGPS) solid electrolytes with different grain sizes were investigated via indentation tests. Hand-milled LGPS (HM LGPS, d₅₀:1.32 µm) and wet-milled LGPS (WM LGPS, d₅₀:0.46 µm) powders were uniaxially pressed to obtain pellet samples. The HM LGPS and WM LGPS pellets had a similar bulk density of 1.63 g cm^–3. At the initial loading/unloading, the WM LPGS pellet showed a large deformation owing to a decrease in cavities compared with the HM LGPS. In the subsequent cycles, both pellets exhibited elastic deformation behavior in the pressure range up to 20 N. The elastic modulus and relative residual depth of HM LGPS and WM LGPS were 21.7 GPa and 0.41 GPa and 0.75 and 0.71, respectively. This result revealed that WM LGPS is more elastically deformable and less plastically deformable than HM LGPS, which could be associated with the grain boundary strengthening interpreted by the Hall-Petch relation. Based on these mechanical properties, the superior cycle stability at the In-Li electrode/WM LGPS electrolyte interface during charging/discharging was discussed. Controlling the mechanical properties of sulfide solid electrolytes by the grain size is important for suppressing physical degradation in all-solid-state batteries.