Reversible AlCl4−/Al2Cl7− conversion in a hybrid Na–Al battery

Owing to the larger radius of sodium ion as compared to that of lithium ion, the kinetics of transport and electrochemical reactions of Na+ ions in the cathode is more sluggish. This results in poor rate performance and inferior long-term cycling stability of sodium-ion batteries. Herein, an Al cathode coordinated with ionic liquid catholyte consisting of 1-ethyl-3-methylimidazolium chloride (EMImCl) and aluminum trichloride (AlCl3) is designed to pair with Na metal anode to produce a high-performance rechargeable sodium battery. The obtained batteries reach a high voltage of 2.1 V and show excellent long-term cycling stability with 92% capacity retention over 1000 cycles at 100 mA g−1 at 60 °C. Furthermore, the electrochemical processes on Al cathode are revealed dependent on the conversion mechanism of AlCl4−-Al2Cl7− redox couple. This Na–Al hybrid battery offers an idea for addressing affordable and reliable large-scale energy storage.