Carboxymethyl cellulose regulated solvation structures enabling long cyclability in aqueous calcium-ion battery

Aqueous calcium-ion battery (ACIB) has emerged as a promising candidate for energy storage devices due to its environment friendly, high safety, and cost-effectiveness. However, traditional aqueous calcium salt solutions contain a large number of active free water molecules, which triggers a range of side reactions, such as hydrogen evolution, electrode dissolution, and weak kinetics of Ca²⁺ diffusion. Here, a green and cost-effective sodium carboxymethyl cellulose (CMC) additive was utilized to change the composition and properties of Ca(NO₃)₂ electrolyte, which effectively improved the electrochemical performance of the Ca(NO₃)₂ electrolyte. DFT calculations and MD simulations demonstrated that CMC disrupts the strong hydrogen bond network architecture within the electrolyte through strong carboxylate coordination. Moreover, CMC reconfigures the solvation structure of Ca²⁺, thereby decreasing the overall rigidity of the solvation shell. Therefore, the assembled battery with MnO₂ electrode demonstrates a capacity retention of 70.4% after 4500 cycles at the current density of 1 A g^-1, and the full cell used activated carbon as anode exhibits a capacity retention of 96.6% after 190 cycles at 1 A g^-1, and the coulombic efficiency is up to 97%. This work provides a low-cost electrolyte additive with an innovative protection strategy for aqueous Ca²⁺ energy storage devices.