Layered materials for supercapacitors and batteries: Applications and challenges

Layered materials displaying a unique anisotropic structure with strong in-plane bonds but weak interaction between layers have been widely investigated as electrodes for batteries and supercapacitors. However, the limited capacity and sluggish ion diffusion impede their satisfaction of the requirements for higher energy and power density. Much effort has been expended and many new developments have been achieved in recent years. This review provides a critical overview of the current progress on these topics in different layered materials. It systematically summarized the application and improvement strategies of several typical layered materials, i.e., graphite, black phosphorus, transition metal dichalcogenides, MXene, layered oxide/hydroxides, nanosheets, and nanosheet-derived layered materials as electrodes of lithium ion batteries, sodium ion batteries, supercapacitors, and Li-S batteries. For each layered material, current methods such as expanding the interlayer spacing, tuning the surface group, changing the chemical composition, co-intercalation of the electrolyte molecules, nanosheet heterostructures, etc., were discussed based on their influences on stability, ion diffusion, phase change, capacity, and voltage. We highlighted the importance of nanosheet heterostructures and interlayer modification as a generally promising direction. It is believed that molecule-level electrode design (structure and functionality) is significant for the energy storage of layered materials in the future.