Recent progress of flexible aqueous multivalent ion batteries

Flexible aqueous batteries have been thriving with the growing demand for wearable and portable electrical devices. In particular, flexible aqueous multivalent ion batteries (FAMIBs), the charge carriers of which include Zn²⁺, Al³⁺, Mg²⁺, and Ca²⁺, have great potential for development owing to their high safety, high elemental abundance in the Earth's crust, and a multi-electron redox mechanism with a high theoretical specific capacity. Therefore, for a comprehensive understanding of this developing field, it is necessary to summarize the recent research progress of FAMIBs in a timely manner. Herein, the advancements of the state-of-the-art FAMIBs are reviewed, and the prospects toward this field are also proposed. This study focuses on the rational material and configuration design for FAMIBs in recent studies to achieve high battery performances under deformation conditions, which is elaborated on by classification of the anode, cathode, hydrogel electrolyte, and configurations of FAMIBs. Besides, the electrochemical performance of FAMIBs under flexible conditions is also reviewed from the perspective of their working voltage, specific capacity, and cycling stability. Finally, the approaches to improve the performance of FAMIBs are comprehensively evaluated, followed by the outlook on the challenges and opportunities in future development of FAMIBs.