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

Abstract

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.