Unlocking the Impact of Electrolytes on Charge Storage in High‐Energy‐Dense Supercapacitors

Abstract Safe electrolytes with a broad voltage window and high surface area, hierarchical porous activated carbon (HAC) electrodes are essential for developing high energy dense Electrochemical double‐layer capacitors (EDLCs) and durable Aqueous Zinc‐ion hybrid capacitors (AZHCs). Mindful of this, herein, the charge storage potency of HAC electrodes is unveiled in water‐in‐salt (WIS) 15m Lithium nitrate (LiNO 3 : 2 V), ionic liquids, ILs 1‐ethyl‐3‐methylimidazolium trifluoromethane sulfonate ([EMIM][OTf]:3 V) for EDLCs and quasi‐solid state electrolytes Polyvinyl‐Zinc trifluromethane sulfonate (PVA‐(Zn(CF 3 SO 3 ) 2 : 1.6 V) for ZHCs. WIS electrolytes with locked free water molecules allow broad voltage and significant ion storage performance. The intrinsic broad voltage window, non‐flammable nature of the ILs improve the energy density and power delivery of EDLCs. Additionally, the choice of polymer‐based hydrogel is one of the facile strategies to develop stable, dendrite‐free and durable ZHCs. Assembled coin cell devices show energy densities of 42, 77, and 78.8 Wh kg −1 at 1 A g −1 and power outputs of 12.8, 9.9, and 8 kW kg −1 at 10 A g −1 for WIS, IL‐based EDLCs and AZHCs respectively. These devices demonstrate low self‐discharge and long cycle life offering promising directions for next generation energy‐dense EDLCs and durable ZHCs.