The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal–air batteries and direct solar water splitting. Cost-efficient ABO₃ perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe⁴⁺-based quadruple perovskite CaCu₃Fe₄O₁₂ has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ and the gold standard RuO₂. The covalent bonding network incorporating multiple Cu²⁺ and Fe⁴⁺ transition metal ions significantly enhances the structural stability of CaCu₃Fe₄O₁₂, which is key to achieving highly active long-life catalysts.

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.

In this work, we propose and examine a battery system with a new design concept. The battery consists of a non-noble polyvalent metal (such as Ca, Mg, Al) combined with a positive electrode already well-established for lithium ion batteries (LIBs). The prototype demonstrated here is composed of a Mg negative electrode, LiFePO₄ positive electrode, and tetrahydrofuran solution of two kinds of salts (LiBF₄ and phenylmagnesium chloride) as an electrolyte. The LIB positive-electrode materials such as LiFePO₄ can preferentially accommodate Li⁺ ions; i.e., they work as a “Li pass filter”. This characteristic enables us to construct a septum-free, Daniel-battery type dual-salt polyvalent-metal storage battery (PSB). The presented dual-salt PSB combines many advantages, e.g., fast diffusion of Li⁺ ions in the positive electrode, high cyclability, and a high specific capacity of lightweight polyvalent metals. The concept is expected to allow the design of many combinations of dual-salt PSBs having a high energy density and high rate capability.

Magnesium-ion batteries (MIBs) with a Mg-metal negative electrode are expected to combine high energy density and high electromotive force, owing to the divalent ion careers and its low redox potential. However, it has been reported to date that the cell voltage of MIBs is not high enough (∼1.5 V), being far below that of lithium-ion batteries (LIBs) (4–5 V). In this work, we have investigated the potentiality of Mg–Co–O and Mg–Ni–O complex oxides as the positive electrode for MIBs, which are composed of these positive electrodes and a Mg negative electrode in acetonitrile with Mg(ClO₄)₂ salt as an electrolyte. These MIBs can exhibit a relatively high open circuit voltage, OCV, and can light a blue diode after charge. However, as the combination of the acetonitrile electrolyte and metal Mg can yield the passivation on the surface of the Mg electrode, we have also checked these materials for a well-established Li ion battery system, and confirmed that the charged battery can show high OCV. In order to attain such a high cell voltage, it would be significant to exploit the unstabilized ion after charge in the host complex oxide.

Ever-proposed descriptors of catalytic activity for the oxygen evolution reaction (OER) were systematically investigated. A wide variety of stoichiometric perovskite oxides ABO₃ (A = Ca, Sr, Y, La; B = Ti, V, Cr, Mn, Fe, Co, Ni, Cu) were examined as OER catalysts. The simplest descriptor, e_g, electron number of transition-metal ions at the B-site, was not applicable for OER overpotentials (η) of the compounds tested in this study. Another descriptor, oxygen 2p band center relative to Fermi energy (ε_2p), was not necessarily adequate for the most part of perovskite oxides. Eventually, a recently proposed descriptor, charge-transfer energy (Δ), displayed a linear relationship with η most reasonably. Because Δ values were obtained from theoretical calculations, not only by spectroscopic experiments, systematic exploration for a wide range of compounds including hypothetical ones could be allowed. This finding proposes the charge-transfer energy as the most helpful descriptor for the design of perovskite oxide catalysts for the OER.