“…6 Cross-sectional SEM images and EDX spectra of Ni foams after oxidation at 1200°C: a, c, e after single-step oxidation and b, d, f after two-step oxidation Ionics nanograveled NiO/Ni electrodes cycled at high discharge rates of 2.5 C indicates that the internal resistance of this cell is large [51]. Subsequently, it is observed that the reversible discharge capacity gradually increased to 1026.20 mA h g −1 at 40th cycles with a coulombic efficiency of 100.25 %, which reveals excellent cyclability of electrode compared with those reported as 680 mA h g −1 after 50 cycles at 2 C rate [52], 865 mA h g −1 after 50 cycles at 1 C rate [53], and some others as reported in [54][55][56]. In addition, the as-prepared nanogravel structured NiO/Ni electrode exhibited better performances than other reported transition metal oxides electrodes, such as CuO [57], Cu 2 O [58], CoO [59,60], Co 3 O 4 [61], and Fe 2 O 3 [62,63].…”