MXenes and their composites as electrodes for sodium ion batteries

Sodium-ion batteries (SIBs) are attractive for energy storage applications owning to the abundant raw resources and low cost, supplementing the pervasive lithium-ion batteries, particularly for the large scale and stationary applications. Among various active materials, 2D MXenes are considered as promising electrode materials for SIBs due to their excellent physical/chemical characteristics, such as high conductivity, various chemical compositions, tunable structure, and abundant surface functional groups. In this review, the structure and properties of MXenes are initially introduced and elaborated, followed by discussion of influence factors on electrochemical properties. The state-of-the-art progress in the design and synthesis of MXenes and their composites, together with their properties and electrochemical performance of SIBs are then comprehensively summarized. The corresponding Na+ storage mechanisms are also analyzed to establish connections between synthesis methods, material properties, and device's performance. Finally, a summary and perspectives are presented for possible further development of MXene-based SIBs.