High-Power and Ultrastable Aqueous Calcium-Ion Batteries Enabled by Small Organic Molecular Crystal Anodes

Calcium ion batteries (CIBs) are pursued as potentially low-cost and safe alternatives to current Li-ion batteries due to the high abundance of calcium element. However, the large and divalent nature of Ca²⁺ leads to strong interaction with intercalation hosts, sluggish ion diffusion kinetics and low power output. Herein, a small molecular organic anode is reported, tetracarboxylic diimide (PTCDI), involving carbonyl enolization (C-O↔C-O⁻) in aqueous electrolytes, which bypasses the diffusion difficulties in intercalation-type electrodes and avoid capacity sacrifice for polymer organic electrodes, thus manifesting rapid and high Ca storage capacities. In an aqueous Ca-ion cell, the PTCDI presents a reversible capacity of 112 mAh g⁻¹, a high-capacity retention of 80% after 1000 cycles and a high-power capability at 5 A g⁻¹, which rival the state-of-the-art anode materials in CIBs. Experiments and simulations reveal that Ca ions are diffusing along the a axis tunnel to enolize carbonyl groups without being entrapped in the aromatic carbon layers. The feasibility of PTCDI anodes in practical CIBs is demonstrated by coupling with cost-effective Prussian blue analogous cathodes and CaCl₂ aqueous electrolyte. The appreciable Ca storage performance of small molecular crystals will spur the development of green organic CIBs.