Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

Since their discovery in 2011, MXenes have attracted considerable interest in the fields of energy storage due to their unique combination of properties, such as metallic conductivity, hydrophilic nature, large interlayer spacing, and rich surface chemistries. Although there have been extensive reports on MXenes, the most distinguishing features of MXenes have been ignored, namely larger, adjustable interlayer spacing and rich surface chemistry (adsorbed functional groups on the surface and surface electronegativity), which make MXenes different from other two-dimensional materials. By changing the type of active materials, substituting graphene with MXenes to prepare MXene-based composites in a sequential manner is easy, which does not make much sense for the development of MXenes. Based on this view as well as considering the distinguishing nature of MXenes, we mainly discuss the various preparation methods of MXenes and their stability, and we summarize their applications in high-capacity anodes, metal anodes, and lithium-sulfur batteries, particularly pillared MXenes. We highlight the viewpoints on the basis of their components (individual MXenes or MXene-based composites) and adjustable interlayer engineering by using the pillaring technology, which are based on the distinguishing properties of MXenes. Finally, conclusions and perspectives, together with potential proposals for MXenes and their composites in the energy storage field, are also outlined.