Although hierarchical engineering has been extensively employed in electrode synthesis, on the basis of our knowledge, there are scarcely any overviews relevant to this field. However, because of the high requirements for precise, controlled synthesis conditions, the poor controllability and reproducibility make it difficult to achieve large-scale preparation and applications. The orderly combination of self-assembly, yolk–shell construction, nanotechnology, and precise atomic-level regulation contribute to an improved use of active material, high thermodynamic stability, promoted reaction dynamics, and superior electrochemical performances of hierarchical electrode materials. Among multitudinous synthetic strategies, hierarchical engineering stands out because of its general applicability for the majority of electrode materials (intercalation/alloying/conversion electrodes) and the competence of structural variability (in macro/nano/atomic level) in response to the unique properties of different materials.
![anode cathode ray experiment anode cathode ray experiment](http://boson.physics.sc.edu/~hoskins/crfig1.gif)
It is therefore crucial to concentrate on the development of tailored electrode materials with robust architectures and expedited Na + diffusion kinetics so as to take SIB energy systems one step closer to practical applications. However, the biggish radius and heavier molar mass of Na + and the lower negative redox potential of Na +/Na give rise to low volumetric/gravimetric energy densities, sluggish reaction dynamics, and an inferior life-span. Sodium-ion batteries (SIBs) have obtained extensive attention as desirable candidates for smart grids and large-scale energy storage systems (ESSs) because they have the conspicuous advantages of resource abundance and competitive price.