Tailoring surface terminals of Ti3C2Tx MXene microgels via interlayer domain-confined polyphosphate ammonium for flexible supercapacitor applications

The MXene material shows potential for flexible energy storage devices due to its high pseudocapacitance and mechanical flexibility. However, MXene nanosheet self-stacking and -F terminal functional groups can hinder active site and ion dynamics, thus affecting the energy storage performance. Herein, we present an interlayer domain-confined strategy that involves the introduction and crosslinking of ammonium polyphosphate (APP) confined to the interlayer of Ti3C2Tx MXene sheets, which induces gelation of the MXene to form a 3D network structure and enlarge their interlayer spacing. And then the cross-linked APP is converted into nitrogen and phosphorus terminals on Ti3C2Tx MXene via thermal treatment. The nitrogen terminals in the form of the pyrrole nitrogen and pyridine nitrogen, and phosphorus terminals of P-O bonds enhance the activity of the redox reaction and the dynamics of the ions, resulting in the boosted energy storage capacity of MXene. The density functional theory (DFT) calculations reveal that nitrogen-phosphorus co-doping modulates the surface electronic states of MXene and contributes to the increase of the electrical conductivity of Ti3C2Tx MXene. As a supercapacitor electrode, Ti3C2Tx MXene with N/P terminals exhibits a higher specific capacitance of 597.8 F/g (1777F cm(-3)) than the raw Ti3C2Tx MXene (384 F/g). In addition, the quasi-solid state flexible symmetric supercapacitor assembled by Ti3C2Tx MXene with N/P terminals can achieve an energy density of 12.5 Wh kg(-1) at a power density of 250 W kg(-1). Therefore, this work provides a new strategy for terminal modification of MXene and high-performance MXene supercapacitors.