Ultrafast\nIon Transport in 2D Confined MXene for Improved\nElectrochemical Performance: Boron-Atom-Substituted −OH Termination

Regulating\nthe surface termination of a confined space to achieve\nultrafast ion transport remains an ongoing challenge. Two-dimensional\n(2D) MXenes possess adjustable structures and interlayer spacing,\nwhich provide an ideal platform for in-depth investigation of ion\ntransport in 2D confined space; however, the strong interaction of\nthe negatively charged terminations in MXenes hinders the transport\nof intercalated cations. In this work, we proposed a strategy that\nprecisely regulates the surface modification of Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXene with the weak polarity\nof boron atoms (SCB-MXene) via the distinct effect of supercritical\nCO<sub>2</sub>. This not only could effectively substitute −OH\ntermination in MXene but also can prevent the loss of −O active\nsites, and then, both ultrafast ion transport and high volumetric\ncapacitance can be achieved simultaneously. Ideally, a volumetric\ncapacitance up to 742.7 C cm<sup>–3</sup> at 1000 mV s<sup>–1</sup> for the SCB-MXene film as pseudocapacitive materials\nthat provides an energy density of 66.3 Wh L<sup>–1</sup> even\nat an ultrahigh power density of 132.5 kW L<sup>–1</sup> is\nobtained, which is a prominent record of energy density and power\ndensity reported up to now. Subsequently, it can be used in large-scale\nenergy storage and conversion devices.