Selectivity for intercalated ions in MXene toward a high-performance capacitive electrode

Abstract

The intercalation of foreign ions has been demonstrated to be one of the effective methods to improve the electrochemical performance of Ti3C2Tx MXene. However, whether different intercalated ions have diverse impacts on the MXene structure and electrochemical properties is not well understood. Herein, we comprehensively investigated the structural features and electrochemical performances of a series of ions, including Li+, Na+, K+, Cs+, Zn2+, Mg2+, SO42−, and OH− intercalated MXenes. Different ions are observed to exhibit variations in the intercalation effect because of the differences in their sizes, charge numbers, and chemical features. Among the investigated intercalation agents, CsOH composed of monovalent Cs+ with a large size and strong base anion OH− exhibits a superior intercalation effect, where a large interlayer space accommodating a large amount of free water, a high oxidation state of Ti and a preferred surface with more O-containing functional groups for redox reaction is created, intensifying the pseudocapacitive H+ intercalation. Therefore, CsOH-treated MXene exhibits a significant enhancement in gravimetric capacitance (i.e., approximately 725% of the original) and more than 100% capacitance retention over 20,000 cycles. This paper provides a guideline for the rational design and construction of high-capacitance MXene and MXene-based hybrid electrodes in aqueous electrolytes.