Ambipolar Electrochemistry of Pre-Intercalated Ti3C2Tx MXene in Ionic Liquid Electrolyte

Cation intercalation with or without redox remains the dominant charge storage mechanism for two-dimensional (2D) Ti3C2Tx MXene. Anion-based charge storage remains unexplored due to intrinsic negative surface charge of MXenes preventing spontaneous intercalation of anions and irreversible oxidation of Ti at anodic potentials in aqueous electrolytes. In this work, we report on the ambipolar electrochemical behavior of the Ti3C2Tx in ionic liquid electrolyte over a 2.5 V electrochemically stable window. The experiments are conducted on a thin Ti3C2Tx film current collector coated with an electroactive layer of small flakes (similar to 150 nm) of Ti3C2Tx pre-intercalated with 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide (EMIM-TFSI) ionic liquid. Couples of redox peaks with a very small potential separation during the voltage sweep are observed at high negative (-0.75 V vs. Ag wire) and high positive (+0.75 V vs. Ag wire) potentials. Our experimental electrochemical data combined with density functional theory (DFT) calculations suggest feasibility of pseudo-intercalation of TFSI anions between Ti3C2Tx flakes. This study provides a pathway for elucidating anion intercalation for different MXene chemistries in solvent-free electrolytes, which can lead to development of MXene based energy storage devices with improved performance.