Using synchrotron surface X-ray diffraction, we investigated the atomic structures of the interfaces of a solid electrolyte (Li 3 PO 4 ) and electrode (LiCoO 2 ). We prepared two types of interfaces with high and low interface resistances; the low-resistance interface exhibited a flat and well-ordered atomic arrangement at the electrode surface, whereas the highresistance interface showed a disordered interface. These results indicate that the crystallinity of LiCoO 2 at the interface has a significant impact on interface resistance. Furthermore, we reveal that the migration of Li ions along the interface and into grain boundaries and antiphase domain boundaries is a critical factor reducing interface resistance. KEYWORDS: X-ray crystal truncation rod scattering, thin film, all-solid-state battery, solid-electrolyte/electrode interface, LiCoO 2 , Li 3 PO 4 , interface resistance

Solid-state Li batteries containing Li(NiMn)O as a 5 V-class positive electrode are expected to revolutionize mobile devices and electric vehicles. However, practical applications of such batteries are hampered by the high resistance at their solid electrolyte/electrode interfaces. Here, we achieved an extremely low electrolyte/electrode interface resistance of 7.6 Ω cm in solid-state Li batteries with Li(NiMn)O. Furthermore, we observed spontaneous migration of Li ions from the solid electrolyte to the positive electrode after the formation of the electrolyte/electrode interface. Finally, we demonstrated stable fast charging and discharging of the solid-state Li batteries at a current density of 14 mA/cm. These results provide a solid foundation to understand and fabricate low-resistance electrolyte/electrode interfaces.

Anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with a Bi square net is known to show an insulator–metal transition by substituting RE. In this study, La2O2Bi polycrystals with different oxygen nonstoichiometry were synthesized. As the amount of oxygen in La2O2Bi increased, the c-axis length was expanded due to the generation of an additional 4e site for excess oxygen, while the a-axis length remained almost constant, indicating the separation of Bi square nets by oxygen intercalation. Concomitantly, transformation of insulating La2O2Bi into metallic La2O2Bi occurred with the change in carrier polarity from the n- to p-type. Despite its polycrystalline form, La2O2Bi with the largest amount of oxygen showed a rather high hole mobility of 85 cm2 V−1 s−1 among other layered oxypnictides and oxychalcogenides.

Intermetallic Ru 2 Sn 3%δ was found to show high thermoelectric performance of ZT > 0.3 at temperatures around 660 K. The transport properties, combined with the electronic structure calculation, indicate that Ru 2 Sn 3 is a semi-metal with contrasting dispersions between the hole and electron bands, which generate a large thermoelectric power of S > 100 µV/K. A low thermal conductivity κ of around 1 W m %1 K %1 was observed above room temperature, which is likely due to the strong lattice anharmonicity associated with the martensitic transformation.