Roadmap for flexible solid-state aqueous batteries: From materials engineering and architectures design to mechanical characterizations

Aqueous batteries (ABs) have been regarded promising candidates for large-scale energy-storage applications due to their low-cost, high-safety, ease-of-fabrication, and high ionic conductivity. In contrast to standard rigid battery devices, flexible batteries can retain their functionality under deformation such as bending, twisting, rolling, or stretching. Therefore, the flexible solid-state ABs (FSABs) accelerates their practical application in wearable electronics. To date, numerous studies have focused on the optimization of the electrolyte, the electrode design, and the battery preparation processes to enhance both electrochemical performance and mechanical robustness. Although some reviews mention FSABs in a wider context, no exclusive review on FSABs for wearable electronics exists. Such a review is presented here, containing all aspects of the engineering, design and characterization of FSABs. The review presented gives an ample introduction to the basic principles of the energy storage mechanisms, the evaluation of the flexibility, and the design principles of FSABs. Furthermore, the recent progress in the electrochemical performance and mechanical flexibility of FSABs and their for practical applications in wearable electronic devices are comprehensively summarized. Finally, our insights regarding major challenges and prospective solutions in future research are provided to guide the further development of this fascinating and fast-evolving research area of FSABs.