Unlocking the Potential of Phosphate-Based Weakly Coordinating Anions as Electrolyte Components for Calcium-Ion Batteries

Development of calcium-ion batteries (CIBs) is strongly hindered by the limited availability of electrolytes that enable reversible plating and stripping of Ca²⁺ ions while maintaining interfacial and electrochemical stability. Motivated by this need for chemically distinct and less fluorinated weakly coordinating anions, we present a density functional theory (DFT) evaluation of hexacoordinated phosphate/phosphorate anions as prospective electrolyte components for CIBs. Surface electrostatic analysis indicates diffuse negative potential distributions consistent with weakly coordinating anion (WCA)-like behavior, broadly comparable to fluorinated alkoxy borate and aluminate anions. Their electrochemical viability was further assessed using frontier molecular orbital (HOMO/LUMO) descriptors, binding energies, and oxidation/reduction limits to estimate electrochemical stability windows (ESW) in representative solvents. Across the series, oxygen-rich chelating frameworks generally exhibit weaker Ca²⁺-anion association and enhanced oxidative robustness, which is consistent with the calculated ESW values. Overall, the phosphate-based anions fall within the performance window of previously investigated WCA families, supporting their potential as a chemically distinct class of electrolyte additives for CIBs. Among the anions examined, TOP and TCP show higher ESW, with TOP in ethylene carbonate (EC) emerging as the best combination under the descriptors considered, making it a viable electrolyte component.