Intedrcalation in Two-Dimensional Transition Metal Carbides and Nitrides(MXenes) toward Electrochemical Capacitor and Beyond

Since its discovery in 2011, the emerging family of two-dimensional(2D)transition metal carbides, nitrides, and carbonitrides(denoted as MXenes)has shown tremendous promise in the field of energy storages, especially electrochemical capacitors(ECs). The intercalation of foreign species,including but not limited to proton, cations, organic ions, and solvents, is demonstrated as one of the dominant facts during the energy storage process of MXenes, through which interlayer spacing of MXenes can be reversibly tuned, followed with reversible redox reactions on the surface of MXenes. Such a mechanism provides MXenes extremely high capacitance up to 1500 F cm, in aqueous acidic electrolyte. In addition, because of the versatile terminations on their surface, the intercalation of ions into MXenes can simultaneously induce the reversible transformation of their electronic structure to trigger some other phenomenon, for example, electrochromic effect, which has great potential on electrochromic smart window—an extended application of ECs. Accordingly, regulating and facilitating the intercalation in MXenes is of great significance for MXene-based ECs. In this review, we summarize the recent progress on the intercalation in MXenes towards ECs, discussing on the intercalated species, working mechanisms,and methods to promote the intercalation. Furthermore, we prospect the future research directions of intercalation of MXenes in ECs, such as the combination of simulation and experiment on finding the best intercalation species, precisely controlling the interlayer spacing and beyond, to boost the electrochemical performance of MXene toward practical applications and multi-functional devices.