## 3 Results and Discussion
### Electrochemical Properties  
The lithium storage properties of MgCo\({}_{2}\)O\({}_{4}\) nanosheets at a calcination temperature of 600 \({}^{\circ}\)C (MCO-600) was evaluated by cyclic voltammetry (CV) at the scan rate of 0.5 mV s\({}^{-1}\) in the voltage range of 0.01-3.0 V, and the first four cycles of CV curves are shown in Figure 4a. The first cathodic peak centered at -0.75 V could be ascribed to the formation of the solid electrolyte interphase (SEI) layer, which corresponds to the reduction of Co\({}^{3+}\) to metallic Co\({}^{0}\)without Mg\({}^{2+}\) reduction to metallic Mg\({}^{0}\) because of the strong bond energy in MgO [23]. In the following anodic polarization process, one broad peak centered at around 2.12 V could be ascribed to oxidation of Co to Co\({}^{2+}\). At the same time, the peak current in the initial cathodic scan is much higher than those in subsequent three cathodic scans, the reason may be that a dramatic reaction comes up when the electrolyte contacts fresh active material in the charge-discharge process, along with fast formation of SEI which is hardly dissolved [26]. In the subsequent three CV cycles, they have similar cathodic peak and anodic peak at -1.06 V and -2.16 V, respectively, indicating excellent cycling performance of MgCo\({}_{2}\)O\({}_{4}\)[27]. In comparison to the initial CV cycle that has a cathodic peak at 0.75 V and an anodic peak at 2.12 V, the latter three cycles have smaller electrochemical polarization, which indicates that there are better and steady electrochemical kinetics after the initial cycle. On the other hand, the peak area at cathodic scan and anodic scan are similar, which in a certain extent indicates there may be reversible capacity and high coulombic efficiency during charge-discharge cycles. On the basis of the cyclic voltammograms, the entire electrochemical process is classified as follows [22; 23]:  
\[\mathrm{MgCo_{2}O_{4}+6 Li^{+}+6e^{-}\to 2 Co+2 Li_{2}O+MgO} \tag{1}\]  
\[2\mathrm{Co+2 Li_{2}O\leftrightarrow 2 CoO+4 Li^{+}+4e^{-}} \tag{2}\]  
\[2\mathrm{CoO+2/3 Li_{2}O\leftrightarrow 2/3C_{03}O_{4}+4/3 Li^{+}+4/3e^{-}} \tag{3}\]  
Figure 4b shows the charge and discharge curves of MCO-600 electrode for the first three cycles at the current density of 200 mA g\({}^{-1}\). In the first cycle, the specific charge and discharge capacities are about 916.2 mAh g\({}^{-1}\) and 1173.8 mAh g\({}^{-1}\), respectively. Accordingly, the initial coulombic efficiency is -78.1%. The charge and discharge capacities are higher than the theoretical capacity of MgCo\({}_{2}\)O\({}_{4}\), and extra capacity can be ascribed to SEI formation and the decomposition of electrolyte [23]. In the subsequent cycles, the coulombic efficiency easily reached -93.3% and 96.6% at the 2 nd and 3 rd cycles, respectively. Figure 4c gives a comparison of reversible capacity versus cycle number for MgCo\({}_{2}\)O\({}_{4}\) annealed at different temperatures (500, 550, 600, and 650 \({}^{\circ}\)C). The initial specific discharge capacities of MgCo\({}_{2}\)O\({}_{4}\) prepared at 500, 550, 600, and 650 \({}^{\circ}\)C, are 983.1, 844.3, 1173.8, and 1030.3 mAh g\({}^{-1}\), respectively. As we can see, the specific capacities decrease in the initial cycles, and then increase gradually, which have the same trend as results reported elsewhere [22]. After cycling for 50 cycles, the discharge capacity of MCO-500, MCO-550, MCO-600, and MCO-650 are 1023.4, 720.9, 1254.3, and 990.4 mAh g\({}^{-1}\), respectively, and the corresponding capacity retention ratesare104.1%, 85.4%, 106.9%, and 96.1%, respectively. Obviously, as compared to MCO-500, MCO-550, and MCO-650, the MCO-600 shows the highest discharge capacity and superior cycling stability, which may be attributed to the unique porous nanosheet structure of MgCo\({}_{2}\)O\({}_{4}\) calculated at 600 \({}^{\circ}\)C. With such an optimal structure, the MCO-600 may not only provide sufficient reaction active sites for Li storage, but also could cope with the volume change during cycling [22].