Figure 6.

Structural characterization and electrochemical performance analysis of metal oxide yolk-shell structured HCM-Fe₃O₄@void@N–C nanospheres as an anode in SIBs: (a) field emission scanning electron microscope (FESEM) of the material, (b) transmission electron microscopy (TEM) of the material, (c) scanning transmission electron microscopy (STEM) of the material, (d–g) energy dispersive X-ray (EDX) elemental mappings of a HCM-Fe₃O₄@void@N–C nanosphere, (h) representation of the sodiation and desodiation processes of CC-Fe₃O₄ and their characteristic volume increases, (i) cyclic voltammetry curves from 1st to 4th cycles for voltage range from 0.01–3.0 V. It is worth noting that the first cycle shape differs from the other cycles because of side reactions and Solid Electrolyte Interphase (SEI) formation, (j) charge–discharge curves for different current densities of HCM-Fe₃O₄@void@N–C nanospheres, (k) representation of the sodiation and desodiation processes of HCM-Fe₃O₄, (l) specific capacity representation versus cycle number for current density of 160 mAg⁻¹ of the following materials: (A) HCM-Fe₃O₄@void@N–C, (B) HCM-Fe₃O₄@void@C, (C) HCM-Fe₃O₄@C, (D) HCM-Fe₃O₄, and (E) CC-Fe₃O₄ nanospheres, and (m) representation of the sodiation and desodiation processes of HCM-Fe₃O₄@void@N–C nanospheres, showing the SEI film, which accommodates the volume expansion characteristic of Fe₃O₄ species. The bars of (c–g) are 100 nm. Reprinted with permission from [51]. Copyright 2019 Elsevier B.V. All rights reserved.