Recent development of MXenes and their composites in electrochemical energy storage: Current status, challenges and future prospects

Recently, two-dimensional (2D) layered MXenes family which exhibited high electronegativity, mechanical strength, tunable surface chemistry, etc. have become a hotspot research topic of materials science for energy storage application.The discovery of MXene has opened an exciting area of research in energy storage materials and attracted interest among researchers. Till now noteworthy researches have been performed on the synthesis and application of MXene-based composite materials for energy storage. Supercapacitors and batteries are wellknown devices in the group of energy storage applications to address the current requirement. Here, we have systematically reviewed the progressive developments of MXene and latest approaches for synthesis of different kinds of MXene via various routes and their composite formation utilized for supercapacitor and secondary batteries. Firstly, the fundamentals about structural configuration of MXene are summarized and the positions of atoms in MXene are outlined. Various routes for preparation of MXene are described along with the attachment of surface functional groups. Subsequently, numerous typical cases of supercapacitor and batteries are evaluated based on the structural arrangement, compositional variation, selection of specific MXene, and electrochemical performance. The energy performance studies on MXene-based composite reveals that increase in the interlayer spacing, altering the surface functionality, and synthesis of few-layered MXene are highly adoptable for enhancing the storage capability. In the end, challenges/issues and future prospects associated with MXene as energy electrodes are discussed and opinions are suggested in detail. We believe that this comprehensive review will summarize and deliver new visions for the improvement in the study of MXene and their applied use in energy storage devices.