Water-in-polymer electrolyte with a wide electrochemical window and recyclability

Aqueous batteries—with water-based electrolytes—provide safe, reliable and affordable energy storage solutions. However, their energy density and cycling life remain uncompetitive owing to the narrow electrochemical window of the aqueous electrolyte. Adding excessive salt to form saturated electrolytes could address this limitation but at other costs. Here we show a water-in-polymer electrolyte that maximizes the amount of water but works across a voltage range as wide as that for highly concentrated electrolytes. At the heart of this formulation is the introduction of a polyacrylamide network that serves to immobilize and thus tame the otherwise reactive H₂O molecules. As a result, our polymerized solid aqueous electrolytes with 4.1 m (18 wt% H₂O) and 7.6 m (11 wt% H₂O) lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt show extended electrochemical windows of 2.7 V and 3.7 V, comparable to those for the 21 m and 40 m saturated counterparts, respectively. The solid-state Li₄Ti₅O₁₂//LiMn₂O₄ cell exhibits stable cycling even under a higher loading of cathode (16 mg cm⁻²) with a lean electrolyte of 7 g Ah⁻¹. In addition, up to 80% of the LiTFSI salt can be recycled and the polymer matrix can also be regenerated. Our electrolyte design represents a substantial step forwards towards more sustainable aqueous batteries.