All-solid-state batteries incorporating lithium metal anode have the potential to address the energy density issues of conventional lithium-ion batteries that use flammable organic liquid electrolytes and low-capacity carbonaceous anodes. However, they suffer from high lithium ion transfer resistance, mainly due to the instability of the solid electrolytes against lithium metal, limiting their use in practical cells. Here, we report a complex hydride lithium superionic conductor, 0.7Li(CB 9 H 10 )–0.3Li(CB 11 H 12 ), with excellent stability against lithium metal and a high conductivity of 6.7 × 10 −3 S cm −1 at 25 °C. This complex hydride exhibits stable lithium plating/stripping reaction with negligible interfacial resistance (<1 Ω cm 2 ) at 0.2 mA cm −2 , enabling all-solid-state lithium-sulfur batteries with high energy density (>2500 Wh kg −1 ) at a high current density of 5016 mA g −1 . The present study opens up an unexplored research area in the field of solid electrolyte materials, contributing to the development of high-energy-density batteries.

High-temperature (high-T c ) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state 1,2 . On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates 1 . Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides 3-6 . Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped

Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H − ) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H − conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH 3 (LaH 3 −2 x O x ) has an optimum ionic conductivity of 2.6 × 10 −2 S cm −1 , which to the best of our knowledge is the highest H − conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H − trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H − , and the framework comprising densely packed H − in LaH 3 − 2 x O x are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H − conduction.