In addition, sodium‐ion storage mechanisms along with the relation between the structures and electrochemical performance are also intensively revealed. Particularly, the application of pure MXenes and MXene‐based composite materials in the electrodes for SIBs, SSBs, and SICs are both introduced, and the corresponding effective methods to optimize their performance are highlighted and discussed in depth ( Figure 1). We put the emphasis on their synthesis conditions, structures, ion intercalation chemistries, and detailed sodium‐ion storage performances based on the experimental and theoretical investigations. In this review, the recent research work and progress carried out on the MXene‐based materials for sodium‐ion storage are systematically and comprehensively summarized. What is more, the unique structures and synergistic effects are beneficial for the electrochemical performance. For another, secondary materials are expected to prevent the aggregation of individual nanosheets. For one thing, MXenes can offer the intertwined conductive network and then significantly increase the electronic conductivity. ] Therefore, many researchers have focused on designing the MXene‐based composite materials. To address these issues, many strategies including single‐/few‐layer MXenes, expanded interlayer spacing, 3D porous structures have been proposed to accelerate the electrochemical kinetics and enhance the capability.Īdditionally, compared with the existing anode materials for SIBs, pure MXene electrodes do not perform the satisfying reversible capacity, limiting their further application in energy storage fields. However, experimentally, MXenes have the tendency to aggregate or stack, which impedes the charge transport through the electrodes, resulting in limited capacity values. Simultaneously, the monolayer bare MXenes or O‐terminated MXenes also exhibit low diffusion barrier and open‐circuit voltage (OCV) for sodium ions, suggesting that they are expected to be the promising anode materials with high capacities and good rate capabilities. For example, sodium ions can be well adsorbed on monolayer bare MXenes or O‐terminated MXenes due to the good negative adsorption energies but cannot well absorbed on the monolayer F‐terminated and OH‐terminated MXenes. ] the surface termination groups have a great effect on the properties and performance of MXenes. ] Based on the density functional theory (DFT) calculations, [ In fact, many research have been conducted on MXenes as electrode materials for sodium‐ion‐based devices. ] electromagnetic interference shielding, [
Owing to their outstanding metallic conductivity, tunable surface chemistry, and 2D layered structure, MXenes have been considered as the promising candidates for supercapacitors, [ ] The MXenes family shares a general composition of M n +1X nT x, where M represents a transition metal like Ti, V, Mn, Mo, Nb, Cr, Sc, etc., X is C and/or N, and T x stands for terminal surface groups ‐O, ‐F, and/or ‐OH, which is typically prepared by selectively etching of A layers, such as Al, Si, Ga, from the corresponding M n +1AX n phases. Recently, a new large group of 2D transition metal carbides, carbonitrides, and nitrides labeled as MXenes has attracted tremendous attention. ] Thus, new chemical structure and architecture of sodium accommodable materials should be developed to improve the efficiency of sodium storage. ] Therefore, the sodium‐ion‐based devices, such as sodium‐ion batteries (SIBs), sodium–sulfur batteries (SSBs), and sodium‐ion capacitors (SICs), always suffer from the low reversible capacity and poor cycling stability. Unfortunately, compared with lithium, sodium has larger ion radius (0.102 nm), higher standard reduction potential (−2.71 V vs standard hydrogen electrode (SHE)), and lower electronegativity (0.93), leading to the sluggish sorption or/and insertion kinetics and large volume expansion. Sodium‐ion storage is the strong alternative to lithium‐ion storage for large‐scale renewable energy storage systems due to the similar physical/chemical properties, higher elemental abundance, and lower supply cost of sodium to lithium.
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