Simple oxide materials, typically, WO3, have been conventionally employed for electrochromic (EC) materials because of their high coloration efficiency; however, it is quite difficult to realize multiple coloration because they involve redox reactions due to single ions. On the other hand, multiple oxides are expected to show various colors when applying different voltages due to the diverse structures and combinations of ions; however, multiple oxide-type EC materials are still in the research stage, and the discovery of further EC materials is necessary. Toward the development of multiple oxide-type EC materials, tungsten-containing layered perovskites have been synthesized, and their optical properties have been evaluated. X-ray diffraction and X-ray absorption fine structure analyses revealed that the discovered tungsten-based layered perovskites Bi2Na0.5La0.5TiWO9 (BNLTW) and Bi2LaTi1.5W0.5O9 (BLTW) have an orthorhombic phase with an Aurivillius-type layered perovskite structure. EC devices fabricated with three kinds of perovskites, including well-known Aurivillius-type Bi2W2O9 (BWO), have no absorption in the visible-light region when no voltage is applied, while they show absorption over the whole visible-light spectrum to black when a voltage of +4.5 V is applied. Furthermore, with an applied voltage of −4.5, the transmittance recovered to the same level as the initial state, meaning the EC function is reversible. In this reaction, only tungsten in the perovskite framework acted as a redox-active species (W6+/W5+ redox) without the redox of the other metal ions. From the electrochemical analysis of the EC materials using cyclic voltammetry, redox peaks could be observed at −0.2 to 0.4 V for reduction and +0.1 to +0.3 V for oxidation. Interestingly, the redox potentials are linearly related to the W content in the perovskite unit, indicating that the redox potentials can be tuned by controlling the chemical formula. The coloration efficiency of the BNLTW EC device was the best at 37.1 cm2/C in the prepared perovskite-based EC device, which is comparable to that of a typical WO3 EC material.

Magnesium rechargeable batteries (MRBs) promise to be the next post lithium-ion batteries that can help meet the increasing demand for high-energy, cost-effective, high-safety energy storage devices. Early prototype MRBs that use molybdenum-sulfide cathodes have low terminal voltages, requiring the development of oxide-based cathodes capable of overcoming the sulfide’s low Mg2+ conductivity. Here, we fabricate an ultraporous (>500 m2 g–1) and ultrasmall (<2.5 nm) cubic spinel MgMn2O4 (MMO) by a freeze-dry assisted room-temperature alcohol reduction process. While the as-fabricated MMO exhibits a discharge capacity of 160 mAh g–1, the removal of its surface hydroxy groups by heat-treatment activates it without structural change, improving its discharge capacity to 270 mAh g–1the theoretical capacity at room temperature. These results are made possible by the ultraporous, ultrasmall particles that stabilize the metastable cubic spinel phase, promoting both the Mg2+ insertion/deintercalation in the MMO and the reversible transformation between the cubic spinel and cubic rock-salt phases.

For practical room-temperature magnesium rechargeable battery operation, utilizing the nano-sized MgMn 2 O 4 spinel as cathode material is effective way to have high rate-capabilities; however, electrolyte decomposition at the cathode/electrolyte interface is inevitable. In this work, a Mg-Fe binary oxide is coated onto nano-sized MgMn 2 O 4 to form stable cathode electrolyte interface (CEI). This developed cathode material suppresses the electrolyte decomposition and improves the cyclability at room temperature.

To improve the cycle performance of Mg metal batteries, a spinel-type MgMn2O4 (MMO) as the active material of the cathode was coated with a poly­(styrene-4-sulfonyl trifluoromethylsulfonyl)­imide Mg salt (PSTFSI-Mg) on the MMO surface. Coating was performed by spray drying, freeze drying, simple mixing, and one-pot spray drying. Transmission electron microscopy measurements of the resulting polymer-coated MMO were undertaken to verify the surface and the inner morphology. The results of charge–discharge measurements suggest that the polymer coating on the MMO surface promotes electron transfer between the particles. These results are interested in terms of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the modified polymers obtained from density functional theory (DFT), confirming that the polymer reduces the gap between the valence band maximum of MMO and the HOMO level of the electrolyte solution, suppressing the degradation of the electrolyte.