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

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.