Facile Multivalent Redox Chemistries in Water-in-Bisalt Hydrogel Electrolytes for Hybrid Energy Storage Full Cells

High-capacity electrode materials have been investigated to overcome the low energy density of electrochemical capacitors, but there are still issues arising from the trade-off between charge storage capacity and kinetics, efficiency, and stability. Herein, we describe multivalent sulfur redox chemistry for the high power and energy efficiency of hybrid energy storage full cells, where nitrogen-incorporated nanoporous carbon/nanosulfur (N-NC/nS) and lithium manganese oxide are configured into negative and positive electrodes, respectively, using water-in-bisalt (WIBS)-soaked poly(acrylic acid) hydrogel electrolyte. As confirmed by the major contribution of surface redox capacity to the total capacity, low activation energy, high exchange current density, and fast charge transfer, the N-NC/nS achieves facile surface redox kinetics arising from the hierarchical porosity, nanoscale confinement of nS, and high ionic conductivity of WIBS hydrogels. The resulting full cells deliver capacitor-like high power density of 15.7 kW kg–1, along with an energy density of 30.1 Wh kg–1, 78.7% retention over 2000 cycles, and an energy efficiency of 98%.