Low‐Concentration Hydrogel Polyelectrolyte with In Situ Formed Interphases Enables 2.7 V Aqueous Pouch Cell

Abstract To maximize energy output, aqueous zinc‐based batteries are pushed to extreme potentials far beyond thermodynamic stability limit of aqueous electrolytes. Interphases at interfaces of electrodes/electrolytes are needed to make aqueous batteries stable and reversible kinetically. The absence of interphases at interfaces of hydrogel/electrodes severely impedes aqueous batteries operating in extreme potentials, although low‐concentration hydrogels are regarded as promising polyelectrolytes. Herein, a type of initiator system as cross‐linking agent is developed that makes hydrogel polyelectrolyte spontaneously in situ form interphases at 25 °C and darkness. This widens the electrochemical stability window of hydrogel polyelectrolyte to above 3.5 V. Also, the initiator system makes the designed hydrogel become a single ion conductor, increasing ionic conductivity to ≈280 mS cm −1 . This hydrogel pushes a zinc//LiMn 2 O 4 pouch cell to operate stably in a voltage window of 0.2 to 2.7 V and deliver high energy densities (e.g., 471 Wh kg LiMn2O4 −1 at 2.2 mg LiMn2O4 cm −2 ; 203 Wh kg LiMn2O4 −1 , 7.1 mg LiMn2O4 cm −2 ) and excellent rate capabilities (e.g., 410 mAh g −1 at 0.11 C, 50 mAh g −1 at 57 C, 2.2 mg LiMn2O4 cm −2 ; 175 mAh g −1 at 0.7 C, 70 mAh g −1 at 7.5 C, and 7.1 mg LiMn2O4 cm −2 ).