High‐Performance Co‐Solvent Engineering Electrolyte for Obtaining a High‐Voltage and Low‐Cost K⁺ Battery Operating from −25 to 50 °C

High‐safety potassium‐ion batteries (HPIBs) are highly intriguing owing to their green energy, low cost, high voltage, noncombustible, and simple assembly. However, most high‐voltage HPIBs use water‐in‐salt electrolytes (WISE), which lead to several problems, such as a high viscosity, which significantly reduces the performance and increases the cost of HPIBs, thus impeding their development. Unfortunately, studies regarding HPIB electrolytes remain limited, further limiting the development of HPIBs. Herein, a co‐solvent engineering electrolyte (4.0 m KOTf in a mixture of propylene carbonate (PC) and H 2 O with a volume ratio of 5.0:1.0) featuring low‐cost (1/4 of WISE) and high‐performance (45.43 mS cm −1 ) characteristics is proposed, which not only achieves a wide electrochemical stability window by reducing the activity of H 2 O, but also adjusts the solvation structure of K + . Consequently, the HPIBs assembled via co‐solvent engineering electrolyte demonstrated a high energy density of 88.05 Wh kg −1 , and sufficiently operated at rates of 0.50–10.0 A g −1 over a wide temperature range (−25–50 °C). This study provides a promising means for developing high‐voltage HPIBs.