Ultrafast Ion Transport in 2D Confined MXene for Improved Electrochemical Performance: Boron-Atom-Substituted -OH Termination.

Regulating the surface termination of a confined space to achieve ultrafast ion transport remains an ongoing challenge. Two-dimensional (2D) MXenes possess adjustable structures and interlayer spacing, which provide an ideal platform for in-depth investigation of ion transport in 2D confined space; however, the strong interaction of the negatively charged terminations in MXenes hinders the transport of intercalated cations. In this work, we proposed a strategy that precisely regulates the surface modification of Ti₃C₂T_x MXene with the weak polarity of boron atoms (SCB-MXene) via the distinct effect of supercritical CO₂. This not only could effectively substitute -OH termination in MXene but also can prevent the loss of -O active sites, and then, both ultrafast ion transport and high volumetric capacitance can be achieved simultaneously. Ideally, a volumetric capacitance up to 742.7 C cm^-3 at 1000 mV s^-1 for the SCB-MXene film as pseudocapacitive materials that provides an energy density of 66.3 Wh L^-1 even at an ultrahigh power density of 132.5 kW L^-1 is obtained, which is a prominent record of energy density and power density reported up to now. Subsequently, it can be used in large-scale energy storage and conversion devices.