Reversible Calcium Metal Anodes Enabled by Asymmetric Solvation Effect in Hybrid Ca‐Na Organoborate Electrolytes

Abstract

ABSTRACT Calcium metal batteries represent a promising frontier for high‐energy‐density energy storage, yet their practical application is hindered by sluggish kinetics and unstable electrolyte‐metal interfaces. Here, we report an electrolyte design strategy based on an asymmetric solvation effect by a hybrid Ca 2+ /Na + organoborate electrolyte that simultaneously regulates solvation chemistry and interphase formation. By introducing monovalent co‐cations and strongly coordinating tetra(3,3,3‐trifluoropropoxy)borate anion, an asymmetric solvation structure is constructed in which electrochemically active Ca 2+ occupies an off‐center position. This configuration significantly breaks the centrosymmetry of the Ca 2+ solvation sheath, leading to an intensified dipole moment and a disruption of the uniform electrostatic shielding, which selectively activates the Ca 2+ for efficient desolvation. Consequently, dense Ca deposition (>10 mA h cm −2 ) is achieved with low overpotential, high reversibility (∼95%), and long‐term stability. Interfacial analysis reveals that the asymmetric solvation environment drives preferential anion decomposition, yielding a polymeric‐polycrystalline interphase composed of CaH 2 /CaO and boron‐containing polymeric ether species that effectively protects the Ca anode. When paired with cathodes under a restricted negative‐to‐positive ratio of 7.2, the Ca/Na/Btfp electrolyte enables stable full‐cell operation to exceed 57 cycles. This strategy is also transferable to Mg metal battery, highlighting its potential as a general electrolyte design principle for multivalent metal batteries.