Ultrafast High-Capacity Calcium-Ion Battery Based on Efficient Calciation into the Two-Dimensional Structure of a Cu 3 (2,3,6,7,10,11-hexahydroxytriphenylene) 2 Cathode

Calcium-ion batteries (CIBs) are promising energy storage systems because of their low cost and high theoretical volumetric energy density. However, calcium ion (Ca 2+ ), a divalent cation with a larger ionic radius than lithium ion, forms strong electrostatic interactions with the divalent O 2– 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, Cu 3 (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. Cu 3 (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 Ca 2+ ions with O – anions in the narrower interlayer spacing of Cu 3 (HHTP) 2 after the first discharge. These findings provide valuable insights for improving the performance of cathode active materials for CIBs.