Zn-based energy storage devices have attracted considerable attention lately as a result of its several advantages, including its stability in aqueous electrolytes. The latter overcomes a major limitation of current (Li-ion) and other future technologies (e.g. Na-ion). Nevertheless, while Zn does not react violently with water, H2 evolution still affects the performance of these devices causing self-discharge (corrosion) and decreasing the current efficiency ​[1]​.The use of water-in-salt electrolytes (WISE) has proved to be an efficient way of limiting this side reaction, and at the same time significantly extending the electrochemical stability window both in batteries ​[2,3]​ and supercapacitors​[4]​. However, the high concentration also translates into higher density and viscosity limiting the high-rate performance of supercapacitors. Adding acetonitrile as a co-solvent can not only compensate these negative effects while retaining the non-flammability properties of WISE electrolytes but can also extend the operating temperature range ​[5]​. It also improves the wettability of hydrophobic carbon materials boosting the capacity of this kind of devices ​[6]​. Based on this, in the present work acetonitrile/WISE (AWISE) electrolytes containing both NaClO4 and Zn(ClO4)2 were prepared and characterized. Different salt concentrations and water/acetonitrile ratios were considered and the effect on Zn stability and cyclability was assessed in Zn//Zn symmetric coin-cells. Three-electrode cell measurements were conducted to study the Zn deposition and dissolution behaviour and to determine the electrochemical stability window. The impact of electrolyte composition on Zn morphology was determine from Zn//Zn cells after 20 cycles while deposition coulombic efficiency was estimated in Cu//Zn cells. Finally, activated carbon//Zn devices were assembled and cycled. Results showed that the electrolytes proposed in this study allow for Zn deposition and dissolution without evidence of relevant secondary reactions between -1.6 V to 2.5 V vs Ag/AgCl(sat) when a glassy carbon electrode is used. However, the potential window is limited when stainless steel is used as a cell component (i.e. current collector or coin cells casing). Controlling the amount of electrolyte in the devices it is possible to reach high cycling stability (up to 2000 cycles) and current efficiencies (≈99.9 %) with a 2 V potential window and current densities of 15 A/g. Overall, the approach used seems to be a cheap and efficient way of enhancing Zn-ion supercapacitors performance.