Synergistic Interfacial Blocking and Water Activity Suppression in Water-in-Salt Electrolytes toward High-Energy Aqueous Supercapacitors

Electrolyte optimization is critical for broadening the electrochemical stability window (ESW) and enhancing the capacitance of aqueous supercapacitors (ASCs) for high energy storage. High-concentration water-in-salt (WIS) electrolytes effectively suppress water activity to widen the ESW, but they provide limited improvement in capacitance. To solve this trade-off without disrupting the intrinsic solvation structure of WIS electrolytes, we describe an electrolyte system created by adding zwitterionic charge-neutral glycylglycine (GG) with stable electrode adsorption to a 27 m potassium acetate (KAc) WIS electrolyte, aiming to synergistically broaden the ESW and improve the ASC capacitance. Theoretical and experimental results indicate that GG does not significantly disrupt the first solvation structure of K +, although it introduces a limited perturbation to the bulk electrolyte environment, which may explain why the changes in the ionic conductivity and viscosity of the electrolyte are not substantial. Instead, GG preferentially adsorbs onto activated carbon (AC) electrode surfaces through multifunctional interactions, occupying the inner Helmholtz plane and physically isolating water molecules from the electrode interface, which further contributes to enhanced electrode capacity. As a result, the ASCs with the KAc/GG electrolyte show an extended ESW from 2.0 to 2.4 V. At a current density of 5 A g –1, the AC electrode achieves a specific capacitance of 367.24 F g –1, with a capacitance retention of over 90% after 10,000 cycles. This effective approach offers a viable pathway for the development of high-energy-density ASCs, supporting their potential applications in portable electronics and grid-scale energy storage.