Weak Solvation Effect Enhancing Capacity and Rate Performance of Vanadium‐Based Calcium Ion Batteries: A Strategy Guided by Donor Number

Abstract Calcium ion batteries (CIBs) are considered as an important candidate for post‐lithium energy storage devices due to their abundance of resources and low cost. However, CIBs still suffer from slow kinetics due to the large solvation structure and high desolvation energy of Ca²⁺ ions. Here, a solvation regulation strategy based on donor number (DN) is reported to achieve easy‐desolvation and rapid storage of Ca²⁺ in sodium vanadate (Na₂V₆O₁₆·2H₂O, NVO). Specially, the solvent with a low DN, represented by propylene carbonate (PC), forms the first solvation shell of calcium ions with weak binding energy and small shell structure, which facilitates the migration of Ca²⁺ in the electrolyte. More importantly, the low DN solvent is preferentially desolvated at the cathode/electrolyte interface, promoting the insertion of Ca²⁺ into the NVO electrode. Mechanism studies further confirm the highly reversible uptake/release of Ca²⁺ in the NVO cathode, along with the VO distance change in the coordination structure. Therefore, the NVO cathode achieves high capacity (376 mAh g⁻¹ at 0.3 A g⁻¹) and high‐rate performance (151 mAh g⁻¹ at 5 A g⁻¹). The weak solvation effect strategy further improves the electrochemical performance and provides great importance for the design of the long‐term development of CIBs.