Anion Identity and Time Scale Affect the Cation Insertion Energy Storage Mechanism in Ti₃C₂T_x MXene Multilayers

While our understanding of energy storage mechanisms for Ti3C2Tx MXene multilayer (ML) sheets is emerging, it still remains unclear as to how anions affect cation insertion and whether the energy storage mechanism changes over varying time scales. Here, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry are coupled with in situ gravimetric electrochemical quartz crystal microbalance with dissipation (EQCM-D) measurements of binder-free Ti3C2Tx MLs in different aqueous electrolytes. EIS frequency-dependent mass changes associated with the transfer of hydrated cations are revealed, in which the shortest insertion time is associated with the LiCl electrolyte. The capacitance of Ti3C2Tx MLs in LiCl is 40% higher than in LiTFSI despite having the same inserting cation because different amounts of water accompany the Li+ cations. This study demonstrates that although cations are primarily responsible for energy storage mechanisms in Ti3C2Tx MLs, the anion and the time scale of observation can have strong effects.