Engineering ternary hydrated eutectic electrolytes to realize rechargeable cathode-free aluminum-ion batteries

Aluminum-ion batteries (AIBs) have been highlighted as a promising candidate for large-scale energy storage due to the abundant reserve, low cost, high specific capacity, and good safety of aluminum. However, the development of AIBs is hindered by the usage of expensive, corrosive, and humidity-sensitive AlCl3-based ionic liquid electrolytes. Herein, we develop a low-cost, non-corrosive, and air-stable ternary hydrate eutectic electrolyte (HEE) composed of aluminum nitrate hydrate, manganese nitrate hydrate, and N,N-dimethylacetamide for cathode-free AIBs. The solvation structure of metal cations and water state in the electrolyte are regulated through the coordination of both water and N,N-dimethylacetamide molecules. The unique structural features of the HEE enables reversible deposition/stripping of AlxMnO2 on the cathodic current collector that are confirmed by various in-situ and ex-situ characterizations, ultimately realizing cathode-free AIBs. The architected cathodefree AIB delivers a discharge specific capacity of 361 mAh g- 1 and shows good cycling stability and high air stability. This study provides a valuable insight into the design of rechargeable multi-valent metal ion batteries for advanced large-scale energy storage systems.