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

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 Ca²⁺/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 Ca²⁺ occupies an off-center position. This configuration significantly breaks the centrosymmetry of the Ca²⁺ solvation sheath, leading to an intensified dipole moment and a disruption of the uniform electrostatic shielding, which selectively activates the Ca²⁺ 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 CaH₂/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.