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Zinc‐ion batteries (ZIBs) are promising grid‐scale energy storage devices owing to their low cost, high energy/power densities, high safety, benign environmental impact, etc. Among various cathode materials, ZnMn2O4 spinel has attracted attention because of its high theoretical capacity (448 mAh g−1) associated with the two‐electron redox reaction of Mn ions (2+/4+), a higher voltage (≈1.4 V vs Zn/Zn2+) than V2O5‐based cathodes (≈1.0 V), and better cyclability among manganese oxide‐based cathodes. However, so far only the one‐electron reaction of Mn ions is used with ZnMn2O4 spinel (≈224 mAh g−1), impairing its attractive features. In this study, the two‐electron reaction is successfully enabled by synthesizing ultrasmall ZnMn2O4 spinel nanoparticles (≈5 nm) composited with graphene (US‐ZMO/G) via a rapid room‐temperature alcohol reduction process, achieving the reversible capacity of 445 mAh g−1 at the second cycle. As far as it is known, the US‐ZMO/G composite achieves the highest gravimetric energy/power densities among cathodes for ZIBs. The combination of high capacity and high voltage enables an outstanding energy density approaching that of lithium‐ion batteries. [ABSTRACT FROM AUTHOR]

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 Mg 2+ conductivity. Here, we fabricate an ultraporous (>500 m 2 g -1 ) and ultrasmall (<2.5 nm) cubic spinel MgMn 2 O 4 (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 Mg 2+ insertion/deintercalation in the MMO and the reversible transformation between the cubic spinel and cubic rock-salt phases.

Simple oxide materials, typically, WO 3 , 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 Bi 2 Na 0.5 La 0.5 TiWO 9 (BNLTW) and Bi 2 LaTi 1.5 W 0.5 O 9 (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 Bi 2 W 2 O 9 (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 (W 6+ /W 5+ 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 cm 2 /C in the prepared perovskite-based EC device, which is comparable to that of a typical WO 3 EC material.