TEM was employed to probe into the morphology and structure
of MgCo2O4 NCAs. As showed in Fig. 4a and b, the MgCo2O4
nanocones has an inconspicuous tip, its length is about 2.1 mm, and
its diameter decreases from 90.8 to 46.8 nm, confirming the coneshape
of MgCo2O4, which are consistent with the SEM images. The
MgCo2O4 nanocones are composed of plentiful small nanoparticles(5e10 nm in diameter, Fig. 4c). These nanoparticles with different sizes accumulate together to form a porous architecture, thus
improving the surface area and electrochemical performance of the
material. The high-resolution TEM image in Fig. 4d shows that the
MgCo2O4 nanocones are crystalline with a lattice spacing of
0.286 nm, which agrees well with the theoretical interplane
spacing of MgCo2O4 (220) planes [39].