Redox-coupled ZnCl2 water-in-salt electrolyte enables high capacity and long-term cycling stability in Ti3C2Tx MXene supercapacitors

Water-in-salt electrolytes coupled with redox species (RC-WiSEs) can be promising candidates for 2D Ti3C2Tx MXene supercapacitors due to their expanded electrochemical stability window (ESW) and enhanced diffusion-controlled contributions. Herein, the incorporation of RC-ZnCl2 WiSE into a Ti3C2Tx MXene supercapacitor enables a wide ESW of 1.6 V, along with high capacity and ultrastability. The Ti3C2Tx MXene supercapacitor delivers a maximum energy density of 36.7 Wh kg-1 and a power density of 3065 W kg-1. Notably, the device retains approximately 80% of its capacity even after 100,000 continuous galvanostatic charge–discharge cycles and also maintains stable capacitive performance over a broad temperature range from -5 °C to 45 °C. A thorough in situ Raman and UV-Vis spectroelectrochemical investigation, combined with electrochemical analysis, reveals that redox species directly regulate interfacial chemistry, thereby enhancing the capacity and stability of the electrode. This work demonstrates the effectiveness of RC-WiSE systems and encourages their use to enhance the charge storage capacity and cycling stability of MXene-based systems.