Liquid-State Cathode Enabling a High-Voltage and Air-Stable Fe-Al Hybrid Battery

Aluminum (Al) is an ideal anode material in low-cost battery system for energy storage, with high theoretical capacities. However, the sluggish Al3+-involved kinetics challenges the selection of common cathode materials (Al3+ intercalation or conversion). Herein, a redox-active Fe-Cl complex serves as the liquid-state cathode to couple with a low-cost Al anode, which synergizes the advantages of redox flow batteries and Al rechargeable batteries. The interplay of Fe-Cl coordinated formula and electrochemical properties are revealed for the first time. It is found that [Fe2Cl7](-) molecule has a high voltage versus Al anode (1.3 V), and the novel Fe-Al hybrid battery fulfills a capacity of 1.6 mAh cm(-2) (20 Ah L-1) record high in a coin cell among Al-based batteries. Furthermore, the energy efficiency, which is a vital parameter to evaluate the energy cost of the energy storage technology, reaches 85% (superior to most Al-based batteries) and an average of 70% over approximate to 900 h cycling. Particularly, the unique air-stable character enables normal operation of the battery assembled in ambient air. This work establishes a new application scenario for Al anode toward low-cost large-scale energy storage.