Dilute Aqueous Hybrid Electrolyte with Regulated Core-Shell-Solvation Structure Endows Safe and Low-Cost Potassium-Ion Energy Storage Devices

High-concentration “water-in-salt” (WIS) electrolytes with the wider electrochemical stability window (ESW) can give rise to safe, non-flammable, and high-energy aqueous potassium-ion energy storage devices, thus highlighting the prospect for applications in grid-scale energy storage. However, WIS electrolytes usually depend on highly concentrated salts, leading to serious concerns about cost and sustainability. Here, an aqueous low-concentration-based potassium-ion hybrid electrolyte is demonstrated with the regulated core-shell-solvation structure by using an aprotic solvent, i.e., trimethyl phosphate, to limit the water activity. This aqueous hybrid electrolyte has a low salt concentration (1.6 mol L −1 ) of potassium trifluoromethanesulfonate but with an expanded ESW up to 3.4 V and the nonflammable property. Based on this dilute aqueous hybrid electrolyte, electrochemical double-layer capacitors are capable of working within a large voltage range (0–2.4 V) at a wide range of temperatures from −20 to 60 °C. An aqueous potassium-ion battery consisting of an organic 3,4,9,10-perylenetetracarboxylic diimide anode, Prussian blue K 1.5 Mn 0.61 Fe 0.39 [Fe(CN) 6 ] 0.77 ·H 2 O cathode and this dilute aqueous hybrid electrolyte can operate well at rates between 0.2 and 4.0 C and deliver a high energy density of 66.5 Wh kg −1 as well as a durable cycling stability with a capacity retention of 84.5% after 600 cycles at 0.8 C.