Charge Storage in MXene/Water–in–Salt Electrolyte Systems

MXenes are two‐dimensional (2D) transition metal carbides, nitrides, and/or carbonitrides combining metallic conductivity, redox‐active surfaces, and hydrophilicity. These properties make them highly suitable for aqueous electrochemical energy storage. However, water decomposition imposes a narrow electrochemically stable voltage window (ESW) in conventional aqueous electrolytes, practically limiting their energy density. Water‐in‐salt electrolytes (WiSEs), with very high salt concentrations, provide a radically different approach by suppressing water activity, delaying both hydrogen and oxygen evolution, and expanding the accessible voltage window significantly. This review shows how the solvation environment and interfacial chemistry of WiSEs modify charge‐storage mechanisms in MXene electrodes, including ion desolvation, interlayer intercalation, surface redox accessibility, and interphase formation. Capacitance, voltage stability, and cyclability are critically discussed alongside emerging strategies involving heterostructures and mixed‐salt electrolyte formulations. The key role of electrolytes and their concentration and MXene surface chemistry are discussed, and the remaining challenges toward practical applications are outlined. Overall, WiSE‐MXene systems offer a promising way toward safer, high‐energy aqueous supercapacitors and hybrid energy storage devices.