Strongly-coordinating organoborates with eccentric solvation structure enable secondary calcium metal battery

Rechargeable calcium (Ca) metal batteries are promising candidates for next-generation energy storage owing to calcium's abundance, divalent charge and low redox potential. However, achieving reversible Ca plating/stripping at room temperature remains challenging due to sluggish desolvation kinetics and unstable interfacial chemistry. Here, we design a class of reversible calcium electrolytes using eccentric solvation structure strategy by introducing strongly coordinating monovalent organoborates into calcium-based electrolytes. The tetra(trifluoroethanoloxy)-borate anion enables participation in the primary solvation sheath, while monovalent cations promote the formation of eccentric solvation structures via anion bridging. This solvation environment lowers both the solvation sheath reorganization energy and desolvation energy barrier of Ca²⁺, while simultaneously upshifting the Ca electrode potential, thereby facilitating the formation of an anion-derived CaO/CaH₂-rich polymeric solid electrolyte interphase layer ( ~ 29 nm). The synergy between fast desolvation kinetics and high interfacial stability enables calcium metal half-cells to achieve a coulombic efficiency of 97.65% under 2 mA cm^-2, along with dendrite-free deposition up to 20 mA h cm^-2. Full cells using de-sodiated layered oxide electrode exhibit stable cycling performance under a negative-to-positive ratio of 2.69 and high-rate conditions. This work highlights the role of solvation structure engineering in enabling secondary Ca metal chemistry, providing a rational design framework for advanced multivalent battery electrolytes.