# The Progress of Cobalt-Based Anode Materials for Lithium Ion Batteries and Sodium Ion Batteries
## 3 Preparation Methods of Synthesizing Cobalt-based Active Materials
### Heat Treatment  
As a high-temperature treatment, heat treatments can be used independently and combined with other synthesis methods, such as ball milling, electrostatic spinning and hydrothermal/solvothermal, to provide a special reaction condition for intermediate processes like sulfuration and carbonization. Lots of cobalt-based electrode materials with high performances have been successfully synthesized through heat treatments [68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79].  
Figure 5: Morphology characterization of cobalt-based composites. (**a**) Schematic evolution of the Co\({}_{3}\)O\({}_{4}\)@CNT composites. TEM images (**b**,**c**) and HRTEM image (**d**) of the Co\({}_{3}\)O\({}_{4}\)@CNT composites [63]. TEM (**e**) and HRTEM (**f**) images of Sn-Co@C-2; STEM image (**h**) and corresponding elemental mapping images (**h**–**l**) of Sn-Co@C-2 [64].  
[PERSON]'s group successfully distributed CoSe nanoparticles into the porous carbon polyhedral (CoSe@PCP) through a two-step heat treatment [68]. During the first heat treatment, the precursor ZIF-67 was firstly converted to Co@PCP, including the carbonization of the organic linkers and reduction of the Co ion. Subsequently, CoSe@PCP was finally synthesized by selenization of Co@PCP at a high temperature (Figure 6a). As shown in Figure 6b, the resulting CoSe powders were uniformly embedded into porous carbon without aggregation. [PERSON] and coworkers prepared a 3D spongy CoS\({}_{2}\)/C anode material through a heating precursor accompanied by a hydrothermal process [12]. Figure 6c delivers a TEM image of the synthesized spongy CoS\({}_{2}\)/C with CoS\({}_{2}\) nanoparticles (\(\sim\)20 nm) homogeneously distributed in the porous carbon. [PERSON] and colleagues realized accurate control over the synthesis of multi-shelled Co\({}_{3}\)O\({}_{4}\) hollow microspheres by adjusting the solution and subsequent heating in the air [69]. The TEM and HRTEM images displayed in Figure 6d,e clearly reveal the morphology of triple-shelled Co\({}_{3}\)O\({}_{4}\) hollow microspheres. [PERSON]'s group firstly synthesized a unique core-shell porous FeP@CoP phosphide microcube interconnected by reduced graphene oxide nanosheets (RGO@CoP@FeP) through a facile phosphorylation process [70]. In Figure 6f, the SEM image of RGO@CoP@FeP reveals a typical hierarchical structure of wrinkled RGO interconnected microcubes. [PERSON]'s group synthesized micro-scaled spherical CoSn\({}_{2}\)/Sn alloy through a sintering process, employing a stoichiometric amount of SnO\({}_{2}\), Co\({}_{3}\)O\({}_{4}\) and C as raw materials [22]. SEM observation of CoSn\({}_{2}\)/Sn alloy reveals that the particles were composed of lots of small grains, as shown in the inset of Figure 6g. [PERSON] et al. developed a facile heat treatment, composed of pyrolysis and sulfurization, to synthesize hollow Co\({}_{9}\)S\({}_{8}\)/N-C composites [28]. The SEM image of hollow Co\({}_{9}\)S\({}_{8}\)/N-C composites displayed in Figure 6h reveals the thickness of the wall was approximately 70 nm. Co/(NiCo)Se\({}_{2}\) box-in-box structures were firstly fabricated by [PERSON]'s group via a selenization process at Ar/H\({}_{2}\), applying well-known ZIF-67 as a template [71]. As shown in Figure 6i, the thickness of the outer Ni-Co selenide shell is about 60 nm, while the inner CoSe\({}_{2}\) shell shows a very uneven thickness.