Fig. 4. (a) SAED and (b) HRTEM images of NaFeO₂ at the pristine stage (left) and after 1 cycle (right) between 2.0 and 4.5 V at C/10 rate. (a) and (b) are reprinted with permission from ref. 102. Copyright 2016 American Chemical Society. (c) HAADF images of the fully charged NaFeO₂ electrode (left) and corresponding line profile (right) in the red rectangular region. (d) EELS spectra of Fe in pristine NaFeO₂, in the Fe layers and in the Na layers of NaFeO₂ charged to 4.4 V. (e) HAADF images (left) and EELS line-scanned contour map (right) along the Fe atoms (the red arrow). (f) In situ XRD patterns of NaFeO₂ during the charge and discharge process. Pathways for direct Fe migration from the O_Fe to the T_Na sites in (g) pristine NaFeO₂ and (h) Na_0.33FeO₂. The yellow, brown, red, blue and green balls in the structural schematic are for Na, Fe, O, V_Fe and the Fe migration trajectory, respectively. (c)–(g) are reprinted with permission from ref. 103. Copyright 2018 Elsevier.