High‐Performance Stretchable Gallium Battery for Wearable Electronics, Through Synthesis of Foam Electrodes

The demand for sustainable and stretchable thin‐film printed batteries for bioelectronics, wearables, and e‐textiles is rapidly increasing. Recently, we developed a fully 3D‐printed soft‐matter thin‐film Ga‐Ag 2 O battery with 3R characteristics: resilient to mechanical strain, repairable after damage, and recyclable. This battery achieved a record‐breaking areal capacity of 26.37 mAh cm −2 , increasing to 30.32 mAh cm −2 after 10 cycles under 100% strain. This performance stems from the synergistic effects of gallium's liquid metal properties and the styrene‐isoprene‐styrene polymer in the anode. Gallium's high specific capacity (1153.2 mAh g −1 ), deformability, and self‐healing abilities, supported by its supercooled liquid phase, significantly enhance the battery's resilience and efficiency. However, the cathode's lower theoretical capacity, due to Ag 2 O (231.31 mAh g −1 ), remains a limitation. Traditional Ag 2 O‐carbon black‐styrene‐isoprene‐styrene cathodes experience rapid capacity decay as only the surface area of the active materials interacts with the electrolyte. To overcome this, we designed a carbon‐filled Ag 2 O foam electrode using a sacrificial sugar template, increasing the effective surface area. This optimization enhanced ion‐exchange efficiency, specific capacity, and cyclability, achieving a specific capacity of 221.16 mAh g −1 . Consequently, the Ga‐Ag 2 O stretchable battery attained a record areal capacity of 40.91 mAh cm −2 —double that of nonfoam electrodes—and exhibited fivefold improved charge–discharge cycles. Using ultrastretchable Ag‐EGaIn‐styrene‐isoprene‐styrene and carbon black‐styrene‐isoprene‐styrene current collectors, the battery's specific capacity increased by 33% under 50% strain. Integrated into a soft‐matter smart wristband for temperature monitoring, the battery demonstrated its promise for wearable electronics.