Simplified Synthesis of Fluoride-Free Ti 3 C 2 T x via Electrochemical Etching toward High-Performance Electrochemical Capacitors.

MXenes have been intensively studied for electrochemical energy storage and other applications. However, time-consuming multistep procedures involving hypertoxic HF or alike are utilized in conventional synthesis methods of MXenes. Besides, -F terminal functional groups inevitably exist in these MXenes, detrimental to supercapacitor and battery performances. Herein, we develop a facile and time-saving electrochemical etching method to synthesize F-free and Cl-containing Ti 3 C 2 T x in a mixed LiOH and LiCl aqueous solution with an etching efficiency of 92.2%. During the synthesis, sonification alone is able to delaminate Ti 3 C 2 T x without using any hazardous organic intercalant. The obtained delaminated Ti 3 C 2 T x flakes are ∼3.8 μm in lateral size and ∼3.9 nm in thickness, and can be stable in an aqueous dispersion for at least 15 days. The filtrated Ti 3 C 2 T x film is 20.5 MPa in tensile strength, 13.4 GPa in Young's modulus, and 1663 S cm -1 in electrical conductivity, and exhibits specific capacitances of 323.7 F g -1 , 1.39 F cm -2 , and 1160 F cm -3 for supercapacitors. Also, a flexible zinc-ion hybrid capacitor with energy density values of 20.8 mWh cm -3 and 249.9 μWh cm -2 is assembled by using the Ti 3 C 2 T x film as the cathode, and can maintain almost all its capacity under bending.