AbstractMultivalent‐ion batteries are emerging as low‐cost, high energy density, and safe alternatives to Li‐ion batteries but are challenged by slow cation diffusion in electrode materials due to the high polarization strength of Mg‐ and Al‐ions. In contrast, Ca‐ion has a low polarization strength similar to that of Li‐ion, therefore a Ca‐ion battery will share the advantages while avoiding the kinetics issues related to multivalent batteries. However, there is no battery known that utilizes the Ca‐ion chemistry due to the limited success in Ca‐ion storage materials. Here, a safe and low‐cost aqueous Ca‐ion battery based on a highly reversible polyimide anode and a high‐potential open framework copper hexacyanoferrate cathode is demonstrated. The prototype cell shows a stable capacity and high efficiency at both high and low current rates, with an 88% capacity retention and an average 99% coloumbic efficiency after cycling at 10C for 1000 cycles. The Ca‐ion storage mechanism for both electrodes as well as the origin of the fast kinetics have been investigated. Additional comparison with a Mg‐ion cell with identical electrodes reveals clear kinetics advantages for the Ca‐ion system, which is explained by the smaller ionic radii and more facile desolvation of hydrated Ca‐ions.