A photonic hydrogel for health self-monitoring of solid-state electrolytes in zinc-air batteries

The solid-state electrolyte is one of the critical components of flexible zinc-air batteries, directly determining its discharge performance via water content. However, the health monitoring of solid-state electrolytes has long been neglected in practical applications due to the unique “half-open” structure of zinc-air batteries. Here, we first report a solid-state electrolyte health monitoring strategy based on photonic metamaterials. The double-network (DN) hydrogels formed by polyacrylamide (PAM) and sodium alginate (SA) were combined with photonic crystals (PhCs) self-assembled from polymethyl methacrylate (PMMA) nanoparticles to monitor the water content of solid-state electrolytes. The results demonstrate that the dehydration of solid-state electrolytes leads to the shrinkage of the gel volume and the structural color changes (blue shift) of PhCs, real-time monitoring the health of PAM-SA electrolyte and reflecting ionic conductivity of the electrolyte via optical signals. Moreover, a flexible zinc-air battery with the electrolyte of 275 mS cm^–1 exhibits high charging-discharging efficiency (>80%), and the battery can work at the low temperature of -20 ℃. This work provides a strategy for the self-monitoring of solid-state batteries.