Calcium-ion batteries (CIBs) are promising energy storage systems because of their low cost and high theoretical volumetric energy density. However, calcium ion (Ca2+), a divalent cation with a larger ionic radius than lithium ion, forms strong electrostatic interactions with the divalent O2– anion in conventional metal-oxide-based cathode active materials, resulting in capacities less than 55% of the theoretical values and poor rate performance. In this study, Cu3(HHTP)2 (HHTP: 2,3,6,7,10,11-hexahydroxytriphenylene), a two-dimensional metal-organic framework with monovalent O– anions included in its crystal structure, is used as a cathode active material for CIBs, and its performance, as well as its calciation and de-calciation mechanisms, is investigated. Cu3(HHTP)2 has a high initial capacity equal to 86% of the theoretical value and excellent rate performance owing to sufficient redox reaction and smooth calciation, respectively. However, it exhibits poor cycle performance because of insufficient de-calciation during charging, which results from electrostatic interactions between intercalated Ca2+ ions with O– anions in the narrower interlayer spacing of Cu3(HHTP)2 after the first discharge. These findings provide valuable insights for improving the performance of cathode active materials for CIBs.
Wakamatsu et al. (Wed,) studied this question.