Complex Hydride‐Based Gel Polymer Electrolytes for Rechargeable Ca‐Metal Batteries

Rechargeable Ca batteries offer the advantages of high energy density, low cost, and earth-abundant constituents, presenting a viable alternative to lithium-ion batteries. However, using polymer electrolytes in practical Ca batteries is not often reported, despite its potential to prevent leakage and preserve battery flexibility. Herein, a Ca(BH₄)₂-based gel-polymer electrolyte (GPE) is prepared from Ca(BH₄)₂ and poly(tetrahydrofuran) (pTHF) and tested its performance in Ca batteries. The electrolyte demonstrates excellent stability against Ca-metal anodes and high ionic conductivity. The results of infrared spectroscopy and ¹H and ¹¹B NMR indicate that the terminal ─OH groups of pTHF reacted with BH₄ ^- anions to form B─H─(pTHF)₃ moieties, achieving cross-linking and solidification. Cyclic voltammetry measurements indicate the occurrence of reversible Ca plating/stripping. To improve the performance at high current densities, the GPE is supplemented with LiBH₄ to achieve a lower overpotential in the Ca plating/stripping process. An all-solid-state Ca-metal battery with a dual-cation (Ca²⁺ and Li⁺) GPE, a Ca-metal anode, and a Li₄Ti₅O₁₂ cathode sustained >200 cycles, confirming their feasibility. The results pave the way for further developing lithium salt-free Ca batteries by developing electrolyte salts with high oxidation stability and optimal electrochemical properties.