## 3 Results and Discussion  
Morphological and Electrochemical Characterization of NiS, Co\({}_{3}\)S\({}_{4}\), and NiCo\({}_{2}\)S\({}_{4}\)  
XRD patterns of NiS, Co\({}_{3}\)S\({}_{4}\), and NiCo\({}_{2}\)S\({}_{4}\) prepared by MISR are shown in Figure 2a. There were four obvious cobalt sulfide diffraction peaks at \(2\theta=19.0^{\circ}\), \(31.3^{\circ}\), \(36.8^{\circ}\), and \(51.3^{\circ}\) on the Co\({}_{3}\)S\({}_{4}\) curve, which corresponded to the plane reflections of (111), (220), (311), and (400) for the spinel Co\({}_{3}\)S\({}_{4}\) (JCPDS: 42-1448), respectively. No peaks of other phases appeared in the spectrum, demonstrating that the sample was of high purity. For the NiS curve, the presence of diffraction peaks at \(2\theta\) of \(19.3^{\circ}\), \(34.1^{\circ}\), and \(60.3^{\circ}\) indicated the successful preparation of NiS (JCPDS: 86-2281). In addition, the XRD curve of NiCo\({}_{2}\)S\({}_{4}\) composites also showed characteristic peaks of NiS with noticeably lower intensity but shifted to higher positions than those of pure NiS, indicating that Co was doped into the NiS lattice. Figure 2b presents the cycling performance of the prepared materials to explore the effects of Co doping on the electrochemical performance of NiS at different molar ratios. It shows that Co\({}_{3}\)S\({}_{4}\) had the lowest specific capacity (SC) while NiCo\({}_{2}\)S\({}_{4}\) had the highest SC, but the cycling performance of NiS was the poorest with only 26.2% of its initial capacity retained after 1000 cycles. Therefore, when NiS was doped with Co, not only its specific capacity but also its cycling performance were greatly improved, which can be attributed to the synergistic interaction between Co\({}_{3}\)S\({}_{4}\) and NiS in the NiCo\({}_{2}\)S\({}_{4}\) composites. The optimal mole ratio of Ni\({}^{2+}\)Co\({}^{2+}\) was found to be 2:1 in this work. Under the designed voltage window of 0-0.5 V, the electrochemical performance of NiS, Co\({}_{3}\)S\({}_{4}\), and NiCo\({}_{2}\)S\({}_{4}\) as working electrodes was tested at a scan rate of 5 mV s\({}^{-1}\). Their obvious difference from the nearly ideal rectangular CV curves of the electric double-layer capacitance was that unique redox peaks were displayed in each of the cyclic voltammetry curves in Figure 2c, indicating the typical Faraday behavior of battery-type electrodes [34]. The redox peak pairs of NiCo\({}_{2}\)S\({}_{4}\), NiS, and Co\({}_{3}\)S\({}_{4}\) electrodes appeared at 0.43/0.18 V, 0.47/0.25 V, and 0.31/0.17 V, respectively, which agreed well with the reversible Faraday reactions between Co\({}^{2+}\)/Co\({}^{3+}\)/Co\({}^{4+}\) and Ni\({}^{2+}\)/Ni\({}^{3+}\) combined with OH\({}^{+}\) anions, and the charge-discharge mechanism of NiCo\({}_{2}\)S\({}_{4}\) is listed as follows in Equations (5) and (6) [35]. Obviously, the integrated area enclosed by the CV curve of the NiCo\({}_{2}\)S\({}_{4}\) composite material was the largest, which indicates that the NiCo\({}_{2}\)S\({}_{4}\) composite material has the largest specific capacity [36]. \[\text{NiCo}_{2}\text{S}_{4}+\text{OH}^{-}+\text{H}_{2}\text{O}\leftrightarrow \text{NiSOH}+2\text{CoSOH}+\text{e}^{-} \tag{5}\]  
\[\text{CoSOH}+\text{OH}^{-}\leftrightarrow\text{CoSO}+\text{H}_{2}\text{O}+ \text{e}^{-} \tag{6}\]  
An EIS test was accomplished in the specified frequency range of 0.01-100 kHz to further explore the electron transfer characteristics of the metal sulfide electrode materials. Figure 2d presents the Nyquist fitting plots of NiCo\({}_{2}\)S\({}_{4}\), NiS, and Co\({}_{3}\)S\({}_{4}\). Due to the Faraday reactions of the electrochemical system, its bulk resistance \(R\)s could be determined by the value of the intersection of the EIS curve and the \(X\) axis in the high-frequency region, and the charge transfer resistance \(R\)ct could be determined by the diameter of the semicircle in the Nyquist diagram [37,38].