Titanium Carbide MXene Shows an Electrochemical Anomaly in Water-in-Salt Electrolytes

Identifying and understanding charge storage mechanisms is important for advancing energy storage, especially when new materials and electrolytes are explored.Wellseparated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusioncontrolled electrochemical processes with distinct Faradic charge transfer.Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti 3 C 2 T x MXene in water-in-salt electrolytes.The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets.This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored.This also demonstrates new opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.TEXT Electrochemical energy storage is the key technology determining the future of transportation, mobile communication, and renewable energy utilization. 1, 2Faradaic reduction-oxidation (redox) reactions and the electrostatic formation of electrical double-layers (EDLs) are used to store charge in batteries and EDL capacitors (EDLCs, or supercapacitors), respectively.More recently, materials that exhibit continuous surface redox reactions across a wide range of potentials, known as pseudocapacitive materials, have attracted attention as a way to increase the energy density of supercapacitors with minimal decreases in their power and cycle life.To distinguish between these charge storage mechanisms, electrochemists will primarily use the shape of cyclic voltammograms