On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCo₂O₄, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via “intercalation and push‐out” process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo₂O₄, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg²⁺/Mg, and similarly it occurs around 2.3 V vs. Mg²⁺/Mg with the valence change from Mn(III) to Mn(II) in MgMn₂O₄, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg₂X₂O₄ or MgX₃O₄ (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn₂O₄ and MgCr₂O₄ can be demagnesiated to some extent owing to the robust host structure of Mg_1−xX₂O₄, where the Mg extraction/insertion potentials for MgMn₂O₄ and MgCr₂O₄ are both about 3.4 V vs. Mg²⁺/Mg. Especially, the former “intercalation and push‐out” process would provide a safe and stable design of cathode materials for polyvalent cations.

The layered-to-spinel phase transformation in Li ₂ MnO ₃ during the initial charge process occurs by a two-phase reaction process within a single particle.

High energy-density rechargeable batteries are strongly demanded from the viewpoint of energy and environmental concern. This work is devoted to fundamental electrochemistry on a novel concept of rechargeable batteries, “rocking-chair type” Mg–Li dual-salt batteries (DSBs), where both Mg and Li cations are carrier ions. In this system, dangerous dendritic growth is drastically suppressed by co-electrodeposition of Mg and Li, and Mg–Li alloys can be used as anode materials with high electrical capacities. As a DSB cathode material that can accommodate both Mg and Li cations, we use a spinel oxide MgCo₂O₄, in which an eccentric insertion mechanism, the “intercalation & push-out” process, occurs. Mg insertion occurs at 2.9 V vs. Mg²⁺/Mg and Li insertion occurs at 3.1 V vs. Li⁺/Li, being consistent with ab initio calculations, and its capacity approximately amounts to 150–200 mA h g⁻¹. In the combination of MgCo₂O₄ and Mg₅₀Li₅₀ alloys, the cell voltage during discharge is as high as about 2–3 V. The concept of rocking-chair type DSB systems provides a new strategy for future safe rechargeable batteries combining high energy/power densities.

Despite the fact that phase-change materials are widely used for data storage, no consensus exists on the unique mechanism of their ultrafast phase change and its accompanied large and rapid optical change. By using the pump-probe observation method combining a femtosecond optical laser and an x-ray free-electron laser, we substantiate experimentally that, in both GeTe and ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ crystals, rattling motion of mainly Ge atoms takes place with keeping the off-center position just after femtosecond-optical-laser irradiation, which eventually leads to a higher symmetry or disordered state. This very initial rattling motion in the undistorted lattice can be related to instantaneous optical change due to the loss of resonant bonding that characterizes GeTe-based phase change materials. Based on the amorphous structure derived by first-principles molecular dynamics simulation, we infer a plausible ultrafast amorphization mechanism via nonmelting.