Make it flow from solid to liquid: Redox-active electrofluids for intrinsically stretchable batteries

High-capacity stretchable batteries are crucial for next-generation wearables to enable long-term operation and mechanical conformability with the human user. In existing stretchable battery designs, increasing the active material to yield higher capacity often leads to thicker and stiffer solid electrodes with poor mechanical properties. Here, we present a concept that transfers the physical property of a battery electrode from a conventional solid into a fluid state. The mechanical and electrochemical properties of the electrode rely on the viscosity of fluids rather than Young's modulus of solids. Fluids conform easily into any shape with minimal force, making them intrinsically deformable. This decouples the electrochemical and mechanical property of the redox-active electrofluid, leading to higher capacities with more active material loading without stiffening the cell. The cell showed excellent capacity retention over 500 charge-discharge cycles and mechanical robustness up to 100% strain. Our work provides a technological solution for stretchable batteries that balances capacity and mechanical performance.