Intercalation-type positive electrode materials for nonaqueous calcium-ion batteries

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.