Synthesis, properties, and applications of MXenes and their composites for electrical energy storage

MXenes, a new family of two-dimensional transition metal carbides, nitrides and carbonitrides, have emerged as promising materials for electrical energy storage (EES) systems due to their superior properties, such as high electronic conductivity, excellent mechanical capability, and hydrophily. These properties of MXenes are closely related to their structure and surface functional groups, and directly decided and readily tailored by means of synthesis methods applied. The properties of MXenes have a determining effect on the electrochemical performance of EES systems. This review begins with the intrinsic connections between properties and crystal structure, chemical composition and surface chemistry of MXenes as background. Then, the effects of latest synthesis on MXenes' properties are systematically scrutinized, including the effects of precursors, processing parameters, the etching, delaminating, and compositing strategies of MXenes. Further focus is turned to the state-of-the-art progress of MXenes and their composites acting as cathodes, anodes, current collectors, electrolyte additives, and conductive binder in supercapacitors, monovalent (Li+, Na+, K+, and halogen anion) ion batteries, and multivalent (Zn2+, Mg2+, Ca2+, and Al3+) ion batteries. The synthesis-property-application relationships in MXenes for desired EES devices are highlighted. Finally, the critical challenges and perspectives are discussed for the future development of MXenes in advanced supercapacitors and rechargeable batteries.