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

Abstract 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_1.5Mn_0.61Fe_0.39[Fe(CN)₆]_0.77·H₂O 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.