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Abstract Chloroaluminate ionic liquids are commonly used electrolytes in rechargeable aluminum batteries due to their ability to reversibly electrodeposit aluminum at room temperature. Progress in aluminum batteries is currently hindered by the limited electrochemical stability, corrosivity, and moisture sensitivity of these ionic liquids. Here, a solid polymer electrolyte based on 1‐ethyl‐3‐methylimidazolium chloride‐aluminum chloride, polyethylene oxide, and fumed silica is developed, exhibiting increased electrochemical stability over the ionic liquid while maintaining a high ionic conductivity of ≈13 mS cm⁻¹. In aluminum–graphite cells, the solid polymer electrolytes enable charging to 2.8 V, achieving a maximum specific capacity of 194 mA h g⁻¹at 66 mA g⁻¹. Long‐term cycling at 2.7 V showed a reversible capacity of 123 mA h g⁻¹at 360 mA g⁻¹and 98.4% coulombic efficiency after 1000 cycles. Solid‐state nuclear magnetic resonance spectroscopy measurements reveal the formation of five‐coordinate aluminum species that crosslink the polymer network to enable a high ionic liquid loading in the solid electrolyte. This study provides new insights into the molecular‐level design and understanding of polymer electrolytes for high‐capacity aluminum batteries with extended potential limits.