# The Progress of Cobalt-Based Anode Materials for Lithium Ion Batteries and Sodium Ion Batteries
## 4 Application of Cobalt-Based Anode Materials in LIBs/SIBs
### Cobalt Sulfate and Its Composites  
Cobalt sulfide has been widely studied as an anode material for LIBs/SIBs owing to its high theoretical capacity, satisfactory electrical conductivities and fair thermal stabilities. Similar to cobalt oxides, the main obstacle to the development of cobalt sulfide is the severe volume fluctuation during periodical cycling [14; 172; 173; 174; 175; 176; 177; 178; 179; 180; 172; 173; 174; 175; 176; 177; 178; 179; 181; 182; 183; 184; 185; 186]. He and his colleagues synthesized self-assembled CoS\({}_{2}\) nanoparticles wrapped by CoS\({}_{2}\)-quantum-dots-anchored graphene nanosheets (denoted as CoS\({}_{2}\) NP@G-CoS\({}_{2}\) QD) and tested as an anode material for LIBs [187]. As displayed in Figure 11a, the CoS\({}_{2}\) NP@G-CoS\({}_{2}\) QD delivered a high specific capacity of 831 mA h g\({}^{-1}\) at 1 A g\({}^{-1}\) after 300 cycles. Meanwhile, the nearly 100% coulombic efficiency over all 300 cycles also indicates the excellent cycle performance of this material. The enhanced cycle performance can be ascribed to the presence of few-layer graphene, which can not only improve the electrical conductivity of this composite but also alleviate the volume fluctuation and aggregation of CoS\({}_{2}\) (Figure 11b). [PERSON]'s group synthesized a sponge-like composite assembled with Co\({}_{9}\)S\({}_{8}\) quantum dots embedded in a carbon matrix, which was wrapped by rGO [172]. When utilized as an anode material for SIBs, the reaction mechanism was studied by an ex-situ XRD pattern in Figure 11c. Upon sodiation, the peaks of Co\({}_{9}\)S\({}_{8}\) gradually disappeared until the discharge was completed. At the same time, the peaks of the Na\({}_{2}\)S and Co phases increasingly appeared owing to the reduction reaction from Co\({}_{9}\)S\({}_{8}\) to Co and Na\({}_{2}\)S. During the subsequent charge process, an inverse reaction, including the disappearance of the Na\({}_{2}\)S and Co phases and the reappearance of Co\({}_{9}\)S\({}_{8}\), occurred. This situation indicates the high reversibility of Co\({}_{9}\)S\({}_{8}\) quantum dots during the initial charge-discharge process. [PERSON]'s group built a Co@rGO composite assembled by CoS nanoplates decorated on rGO and tested as an anode material for SIBs [173]. To investigate the industry application of this composite, a full cell was fabricated with Na\({}_{3}\)VPO\({}_{4}\)@C nanowires as the cathode (Figure 11d). As shown in Figure 11e, the reversible capacity of 290 mA h g\({}^{-1}\) was obtained after 100 cycles, suggesting the superior cycle performance of a CoS@rGO/ Na\({}_{3}\)VPO\({}_{4}\) (NVP)@C full battery. The alight red LED (inset of Figure 11e) fully demonstrates the practical application of a CoS@rGO/NVP@C full battery. This excellent electrochemical performance resulted from the novel structure and the presence of rGO. [PERSON]'s group reported a CoS\({}_{2}\) nanobubble hollow prism and tested it as an anode material for LIBs [174]. This sample delivered remarkable specific capacities of 910, 778, 681 and 470 mA h g\({}^{-1}\) at the current densities of 200, 500, 1000 and 5000 mA g\({}^{-1}\), respectively (Figure 11f). It is worth mentioning that the capacity recovered to 864 mA h g\({}^{-1}\) when the current density was returned to 200 mA g\({}^{-1}\), indicating the outstanding robustness of the electrode. [PERSON]'s group synthesized hollow nanospheres assembled of mesoporous Co\({}_{9}\)S\({}_{8}\)[175]. When utilized as an anode material of LIBs, an excellent specific capacity was maintained as 896 mA h g\({}^{-1}\) even after 800 cycles at 2 A g\({}^{-1}\) (Figure 11g). The superior cycle performance resulted from the remarkable charge-transfer kinetics. Furthermore, [PERSON] and coworkers prepared a one-dimensional multiwalled carbon nanotube@a-C@Co\({}_{9}\)S\({}_{8}\) nanocomposites (MWCNT@a-C@Co\({}_{9}\)S\({}_{8}\)) as an anode material for advanced LIBs [5]. In Figure 11h, a high reversible capacity of 1065 mA h g\({}^{-1}\) was obtained after 700 cycles at a current density of 2 A g\({}^{-1}\).