Electrochemically Activated Charge Storage of Tetraalkylammonium Cations Confined in Non-Delaminated MXene Interlayers.

Layered transition-metal carbides/nitrides (MXenes) are promising electrode materials for electrochemical capacitors because their large active surfaces and open interlayer spaces enable dense and fast charge storage. Understanding how non-delaminated MXene particles undergo interlayer structural evolution during repeated ion insertion/extraction is essential for designing electrodes that utilize nanoscale ion confinement. Although the charge-storage behavior of Lewis-acidic ions such as Li+ and H+ in MXenes has been extensively investigated, that of bulky tetraalkylammonium ions remains largely unexplored. Here, we clarify the charge-storage behavior of tetraethylammonium cations (TEA+) in the interlayer space of non-delaminated Ti3C2Tx MXene. Repeated CV cycling drives TEA+-associated charge storage within nanosized MXene particles, accompanied by progressive interlayer expansion and enhanced specific capacitance. Structural analysis using a simplified geometrical model reveals how the relative sizes of the ion and solvent molecule correlate with the interlayer spacing of MXene. These findings provide insights into confined ion-MXene interlayer structures and offer a strategy for enhancing the charge-storage activity of non-delaminated MXene electrodes with bulky organic cations.