Calcium-ion batteries represent a promising alternative to the current lithium-ion batteries. Nevertheless, calcium-ion intercalating materials in nonaqueous electrolytes are scarce, probably due to the difficulties in finding suitable host materials. Considering that research into calcium-ion batteries is in its infancy, discovering and characterizing new host materials would be critical to further development. Here, we demonstrate FeV3O9∙1.2H2O as a high-performance calcium-ion battery cathode material that delivers a reversible discharge capacity of 303 mAh g−1 with a good cycling stability and an average discharge voltage of ~2.6 V (vs. Ca/Ca2+). The material was synthesized via a facile co-precipitation method. Its reversible capacity is the highest among calcium-ion battery materials, and it is the first example of a material with a capacity much larger than that of conventional lithium-ion battery cathode materials. Bulk intercalation of calcium into the host lattice contributed predominantly to the total capacity at a lower rate, but became comparable to that due to surface adsorption at a higher rate. This stimulating discovery will lead to the development of new strategies for obtaining high energy density calcium-ion batteries.

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Calcium-ion batteries (CIBs) are among the promising alternatives to overcome the limitation of lithium-ion batteries in current use. Compared with lithium, calcium is environmentally friendly, reliable, safe, and abundant in resources. Despite the development of intercalation-type cathode materials for CIBs in its infancy, the number of newly discovered materials has remarkably increased in the last few years. In this Review, we present the recent accomplishments and challenges in the development of cathode materials for nonaqueous CIBs, classified by the constituent anion type: oxides, polyanions, and others (chalcogenides, fluorides, and nitrides), and further subdivided based on Ca diffusion dimensionality (one-, two-, and three-dimensions). Each of the materials is presented, emphasizing structural aspects, electrochemical properties, intercalation mechanisms during cycling, and problems to be solved. Finally, this Review concludes by providing overview and perspectives on each type of materials. To date, the observed capacities are still far below the theoretically expected doubled capacity due to the divalency of calcium. Nevertheless, the research progress during the past few years suggests that unexplored opportunities for discovering new cathode materials with improved performances are wide open. This Review will help researchers easily grasp the overall accomplishments and challenges of the CIB cathode materials, stimulating further development.

Magnesium batteries have emerged as a promising alternative to lithium‐ion batteries due to their theoretical high energy density and abundant magnesium resources. Vanadium dioxide, VO2(B), has been reported as a high‐capacity cathode material for magnesium batteries. However, the electrochemical intercalation mechanism requires further elucidation due to a limited understanding of the structure‐property relationship. In this study, we re‐evaluated the magnesium storage capability of the material, with a particular focus on the influence of water content in nonaqueous electrolytes. The higher discharge capacity of 250 mAh g−1 is achieved exclusively in the wet electrolyte with 650 ppm water content. A significantly lower capacity of 51 mAh g−1 was observed in the dry electrolyte solution containing 40 ppm water content. Through X‐ray structural and elemental analyses, as well as magnesium‐ion diffusion pathway analysis using bond‐valence‐energy‐landscape calculations, the restricted capacity was clarified by examining the reaction mechanism. According to this study, the impressive capacity of magnesium‐ion battery cathodes may be exaggerated due to the involvement of non‐magnesium‐ion insertion unless the electrolytes′ water content is appropriately regulated.