Energetic All‐Polymer Fiber Batteries Enabled by Interface‐Interlocked Water‐In‐Network Electrolytes for Wearable Electronics

ABSTRACT All‐polymer batteries configured in a fibrous form by integrating polymeric electrodes with aqueous quasi‐solid polymer electrolytes (AQPEs) represent a promising solution for wearable electronics with safety and sustainability. However, their practical development is constrained by the narrow electrochemical stability window (ESW) of AQPEs and the structural instability of electrode–electrolyte interfaces under deformation. Here, we present a “water‐in‐network” (WIN) electrolyte that precisely engineers water activity and dynamics by modulating the crosslinking density of the polymer network. We unanticipatedly discover a distinct “water confinement” effect wherein water activity exhibits a non‐monotonic dependence on crosslinking density, originating from the structural evolution of the network. Benefiting from this mechanism, the ESW significantly expands to 3.4 V, endowing an all‐polymer sodium‐ion fiber battery with an energy density of 92.4 Wh kg −1 . Crucially, in situ interfacial polymerization following electrolyte pre‐infiltration forms a mechanically interlocked electrode–electrolyte interface, effectively suppressing water‐induced delamination and preserving 78% capacity after 12 000 bending cycles.When seamlessly woven into a shirt, the fiber batteries can power the fabric‐based chemical sensor, enabling real‐time, on‐body health monitoring across various activities.