# The Progress of Cobalt-Based Anode Materials for Lithium Ion Batteries and Sodium Ion Batteries
## 2 Li\({}^{+}\)/Na\({}^{+}\) Storage Mechanism
### Li+/Na+ Storage Mechanism in Co\({}_{x}\)A\({}_{y}\) (A=O, S, P and Se)  
Co\({}_{x}\)A\({}_{y}\) electrode materials consist of cobalttous oxides, cobalttous sulfides, cobalttous phosphide and cobalttous selenide. Li\({}^{+}\)/Na\({}^{+}\) storage mechanism in Co\({}_{x}\)A\({}_{y}\) is the conversion reaction that eventually  
Figure 2: Li\({}^{+}\) and Na\({}^{+}\) storage in the Sn-Co alloy anode. (**a**) Charge-discharge profiles of Sn-Co@polymethyl methacrylate (PMMA) for the initial three cycles in lithium ion batteries (LIBs) [40]. (**b**) Cyclic voltammetry (CV) curves of SnCo@C in sodium ion batteries (SIBs) for the initial three cycles [41]. (**c**) and (**d**) Charge-discharge curves of Sn@C and SnCo@C at different cycles in SIBs, respectively [41]. forms Co and Li\({}_{x}\)A or Na\({}_{x}\)A [14; 15; 20; 46; 47; 48; 46]. The typical reactions of Co\({}_{x}\)A\({}_{y}\) with Li\({}^{+}\) is displayed in Table 1. It should be noted that some temporary intermediate reactions may occur during the discharge process of Co\({}_{x}\)A\({}_{y}\), as depicted in the following equations:  
\[\mathrm{Co_{x}A_{y}+Li^{+}+e^{-}\to LiCo_{x}A_{y}} \tag{5}\]  
or  
\[\mathrm{Co_{x}A_{y}+Li^{+}+e^{-}\to Co_{x}A_{y-1}+LiA} \tag{6}\]  
Furthermore, the sodium storage reactions are similar to lithium in terms of the conversion reaction accompanied by temporary intermediate reactions. [PERSON]'s group (Figure 3a) investigated the sodium storage behavior of shale-like Co\({}_{3}\)O\({}_{4}\) by cyclic voltammetry (CV) from 0.005 to 2.9 V at 0.1 mV/s [35]. During the first discharge process, they found an intermediate reaction initially happened at 0.76 V, with Co\({}_{3}\)O\({}_{4}\) reduced to CoO and Na\({}_{2}\)O, provisionally. Subsequently, the resulting CoO intermediate phase was further converted (0.39 V) to metallic Co and Na\({}_{2}\)O. Upon oxidation, metallic Co was step-by-step oxidized from CoO to Co\({}_{3}\)O\({}_{4}\) with the extraction of Na\({}^{+}\) from Na\({}_{2}\)O. Two main anodic peaks centered at 0.88 and 1.65 V corresponding to the reversible formation of CoO and Co\({}_{3}\)O\({}_{4}\), respectively. The sodium storage reactions of the shale-like Co\({}_{3}\)O\({}_{4}\) are as follows:  
\[\mathrm{Co_{3}O_{4}+2 Na^{+}+2e^{-}\to 3 CoO+Na_{2}O} \tag{7}\]  
\[\mathrm{3 CoO+6 Na^{+}+6e^{-}\to 3 Co+3 Na_{2}O} \tag{8}\]  
[PERSON] et al. investigated the Na\({}^{+}\) storage mechanism of CoP by using ex-situ TEM and CV tests [27]. In Figure 3b, two peaks centered at 0.2 and 0.05 V can be attributed to the stepwise formation of Na\({}_{x}\)P and Na\({}_{3}\)P during the first cathodic scan. During the subsequent charge process, a stepwise extraction of Na\({}^{+}\) occurred along with the decomposition of Na\({}_{3}\)P, characterized by two anodic peaks located at 0.5 and 1.5 V. It should be noted that a partial Na\({}_{3}\)P transformed to P during the decomposition of Na\({}_{3}\)P. The corresponding sodiation/desodiation reactions are as follows:  
\[\mathrm{CoP+3 Na^{+}+3e^{-}\to Co+Na_{3}P} \tag{9}\]  
\[\mathrm{Na_{3}P\to P+3 Na^{+}+3e^{-}} \tag{10}\]  
\[\mathrm{P+3 Na^{+}+3e^{-}\leftrightarrow Na_{3}P} \tag{11}\]  
[PERSON] and coworkers investigated the electrochemical reactions of CoS\({}_{2}\)/rGO by CV curves of the initial two cycles (Figure 3c) [49]. The peak located at 1.4 V can be ascribed to the intercalation of Na\({}^{+}\) into the CoS\({}_{2}\) crystals in the first discharge process. The resulting intermediate Na\({}_{x}\)CoS\({}_{2}\) suffered a further conversion reaction to form Na\({}_{2}\)S and metallic Co at 0.8 V. Upon desodiation, two main oxidation peaks at 1.6 and 2.2 V can be due to the reversible formation of Na\({}_{x}\)CoS\({}_{2}\) and CoS\({}_{2}\), respectively. The corresponding electrochemical reactions are as follows: