Charge storage and operando electrochemical dilatometry of MXene electrodes in ionic liquids

This study offers detailed information regarding the charge storage and structural behavior of the titanium (Ti₃C₂T_x) and molybdenum/titanium (Mo₂Ti₂C₃T_x) MXenes in three ionic liquids (ILs) with different cations, i.e., [EMIm][TFSI], [BMP][TFSI] and [C3mpyr][TFSI]. The MXene preparation method, based on the MAX phase delamination by HCl/LiF or HF, affected the physicochemical properties and electrochemical response of electrodes. It was found that the interlayer spacing, as well as chemical properties affected by functionality, play an important role on determining the energy storage mechanism of MXenes. Titanium carbide (Ti₃C₂T_x) was characterized with an extremely delaminated structure and a high amount of surface functional groups, demonstrating hybrid charge storage of both capacitive and faradaic response. On the other hand, the Mo/Ti-based MXene showed a more compact structure with a lower surface accessibility and limited amount of surface functional groups. Therefore, a smaller rate of ionic insertion/deinsertion was observed. Interestingly, operando dilatometry measurements supplied crucial data on volumetric expansion of MXene electrodes under negative and positive polarization with cyclic voltammetry and chronoamperometry. A volumetric expansion (from 0.2 % to ca. 6 % strain) has been proven depending on the MXene preparation method, electrode polarization, potential range, and dimensions of ionic species in ILs.