Solid Polymer Electrolytes with Enhanced Electrochemical Stability for High-Capacity Aluminum Batteries

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 approximate to 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-1 at 66 mA g-1. Long-term cycling at 2.7 V showed a reversible capacity of 123 mA h g-1 at 360 mA g-1 and 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. This work develops an ionic liquid-based solid polymer electrolyte with an inorganic additive for aluminum-graphite batteries, showing improved electrochemical stability over conventional ionic liquid electrolytes to enable approximate to 30% higher specific capacities. The origins of the improved performance are elucidated via variable-rate cyclic voltammetry and solid-state nuclear magnetic resonance spectroscopy, revealing insights into the electrolyte formation process and changes to the electrolyte ion speciation.image