Ultrahigh Areal Capacitance of Flexible MXene Electrodes:\nElectrostatic and Steric Effects of Terminations

Abstract

Two-dimensional\n(2D) Ti₃C₂T<i>_x</i> MXene\nhas shown great potential in the energy storage field,\nand its performance strongly depends on the intercalation of cations.\nTherefore, engineering its interlayer ion channels is the key to enhance\nthe electrochemical performance of Ti₃C₂T<i>_x</i>, but it is challenging due to the restacking\nnature of 2D materials. Herein, an original strategy for in situ introduction\nof large-size and electrostatic −SO₄ termination\nis developed to engineer Ti₃C₂T<i>_x</i> MXene interlayer channels. The chemical binding\nand steric effect of −SO₄ termination ensure a stable\nand expanded interlayer ion channel. The electrostatic effect of −SO₄ benefits electrolyte ion infiltration. Consequently, the\ncapacitance of Ti₃C₂T<i>_x</i> is increased by approximately 66 and 143% compared to those synthesized\nby common methods. The Ti₃C₂T<i>_x</i> electrode exhibits a high areal capacitance of 1399.0 mF\ncm^–2 at 1 mV s^–1, excellent rate\ncapability, and ultralong cycle life without capacitance loss after\n17,200 cycles. The all-solid-state supercapacitor (ASSS) based on\nthe Ti₃C₂T<i>_x</i> delivers\nan ultrahigh areal capacitance of 391.5 mF cm^–2,\nwhich reaches the state-of-the-art level. Moreover, the ASSS shows\nexcellent flexibility and wearable potential. The established strategy\nblazes a new trail to improve the capacitance performance of MXenes.