Room Temperature Calcium‐Ion Battery Enabled by Calcium Citrate as a Calcium Salt

ABSTRACT This study explores the development of calcium‐ion batteries (CIBs) by focusing on the design of a novel electrolyte to overcome key challenges, such as poor calcium plating/stripping. Hydrated vanadium pentoxide (H‐V₂O₅) served as the cathode, and graphite acted as the anode. Tricalcium dicitrate tetrahydrate (Ca‐citrate) salt was used for the first time in an ethylene carbonate/propylene carbonate (EC/PC) solvent system. Ca‐citrate showed marked improvement in ionic conductivity (up to 2.6 × 10 −1 S/cm) and electrochemical stability (~6 V) relative to traditional Ca‐nitrate (2.5 V) and also displayed better capacity retention. To solve the solubility limitations of Ca‐citrate in EC/PC, highly diluted (0.001 M) solution of EC/PC and modified electrolytes with cosolvents like poly(ethylene glycol) 200 (PEG 200) and trifluoroacetic anhydride (TFA) were studied. PEG 200 increased solubility and stability through hydrogen bonding, while TFA increased ionic mobility but decreased electrochemical stability. The PEG‐modified electrolyte achieved a stability window of ~4 V with a charge/discharge capacity of ~198 mAh/g. Additionally, using ethylene glycol (EG) as an alternative solvent increased the electrochemically reversible, soluble, and capacitive Ca‐citrate (up to 708 mAh/g, 85 mA/g). EG‐based electrolyte showed high columbic efficiency (~99%–100%) and stable interfacial behavior.