Cathode materials for calcium-ion batteries: Current status and prospects

In the post-lithium-ion battery era, calcium-ion batteries (CIBs) have aroused extensive attention because of their strong cost competitiveness, low standard redox potentials, and high safety. However, the related research is progressing slowly due to the constraints of the development of electrode materials. The large ionic radius of Ca²⁺ especially increases the challenge to design cathode materials for reversible Ca²⁺ uptake/removal. Despite the inspiring achievements, various challenges still need to be further resolved. Here, this review systematically summarizes the recent advances in CIB cathode materials, including Prussian blue and its analogues, metal oxides, metal chalcogenides, polyanionic compounds, and organic materials. We first provide a brief introduction to CIBs and compare their advantages with other battery technologies. Then, preparation methods are introduced, and breakthrough investigations are highlighted. Finally, some possible research directions are discussed to promote the development of this emerging battery technology.