Pseudocapacitive Charge Storage Dynamics of Vanadium Carbide MXene in Water-In-Salt Calcium-Ion Electrolyte

Vanadium carbide (V₂CT_x) MXene is known to exhibit high charge storage capacities in both acidic and basic electrolytes due to pseudocapacitance. However, it exhibits capacitive type charge storage in neutral aqueous electrolytes due to the formation of electrical double layers by the hydrated cations. In this work, pseudocapacitive charge storage of V₂CT_x MXene in water-in-salt (WIS) calcium ion electrolyte is reported for the first time. Thanks to the suppressed water activity of the WIS electrolyte, the operational potential window of V₂CT_x MXene is found to be 1.3 V, significantly superior to that of dilute electrolytes. Indeed, such an extended potential window enables the additional charge storage capacity offered by V³⁺/V⁴⁺ redox couple, confirmed through ex situ X-ray photoelectron spectroscopy. Partial anodically oxidized V₂CT_x MXene shows a specific capacitance of 380 F g^-1 in 5 m calcium bis(trifluoromethanesulphonyl)imide (Ca(TFSI)₂) aqueous electrolyte at a scan rate of 2 mV s^-1. Charge storage dynamics of V₂CT_x MXene in Ca-ion electrolyte is visualized through constructing 3D Bode maps from potential dependent impedance measurements. Furthermore, the assembled symmetric V₂CT_x // V₂CT_x full device operates in the voltage window of 1.4 V in WIS electrolyte with good cycling stability for 10 000 charge-discharge cycles.