Photothermally Powered Wearable Thermal Battery Through Segmental Polymer Motion‐Driven, Solid‐Solid Phase Change with Durable and High Energy Density

ABSTRACT Thermal batteries, designed for storing and on‐demand releasing of heat, require multifunctional materials that can directly charge using sunlight and provide high energy density and power density over repeated charge–discharge cycles. Accordingly, we describe a hybrid polymer through a single‐pot cross‐linking of soft, flexible polyethene glycol (PEG) chains with thermally rigid polystyrene co‐allyl alcohol framework, in the presence of graphene oxide (GO). The resulting graphene oxide incorporated poly (ethylene glycol)‐toluene diisocyanate—poly(styrene‐co‐allyl alcohol) (GO‐PTP) exhibits a reversible, crystalline‐amorphous phase transition between 323 and 333 K, with an enthalpy of 121 J/g. Uniform incorporation of GO within PTP enhances its thermal conductivity by 2.5 times (@4 wt.% loading of GO) and, importantly, supports direct photo‐thermal charging with excellent heat‐storage efficiency (97%). Detailed temperature‐dependent diffraction and NMR analysis reveal a constrained (−15°C to +15°C) and thermally‐labile (>+15°C) PEG‐chain dynamics. Importantly, this overcomes a fundamental mutual exclusivity with both high energy density (50 Wh/kg) and excellent power density (129 W/kg), with stable behavior over 300 continuous cycles, surpassing DoE targets for phase‐change based thermal storage. Finally, seamless inclusion of GO‐PTP onto cotton, wearable patch that rapidly charges under sunlight (∼360 s) and discharges in dark (∼900 s), achieving personalized thermal comfort.