Calcium ion batteries (CIBs) have attracted much attention as next-generation batteries to replace lithium ion batteries (LIBs) because the theoretical capacity of CIBs is twice that of LIBs. This doubled capacity can be explained by the difference between monovalent and divalent ions. In addition, CIBs possess advantages such as a lower cost and higher safety because calcium is more abundant than lithium and because CIBs have a higher melting point than LIBs. However, there is one major obstacle to the application in CIBs. There is no suitable electrode material in which calcium ions can be inserted and extracted reversibly because of the relatively large ionic radius of calcium ions (112 pm) as compared to that of lithium ions (76 pm).