Structural comparison of NCM622, NM64 and Co-NM64. (a) SEM images of Co-NM64. (b) EDS line profile along the arrow. (c) EDS mapping of Co in the cross section. (d–f) XRD diffraction and refinement of NCM622 (d), NM64 (e) and Co-NM64 (f). (g1–i1) Surface scanning transmission electron microscopy under high-angle annular dark-field mode (HAADF-STEM) image of NCM622 (g1), NM64 (h1) and Co-NM64 (i1). (g2–i2) Bulk HAADF-STEM image of NCM622 (g2), NM64 (h2) and Co-NM64 (i2). (j) HAADF-STEM image of Co-NM64 marked by yellow and green boxes. (k–l) Corresponding HAADF signal profiles of the yellow and green boxes in (j). Scale bars, 200 nm (a), 2 μm (an inset), 50 nm (b,c), 2 nm (g–j).