## 3 Results and Discussion
### Electrochemical Characterization
Thus, after 400 cycles at different rates, the capacity values are practically recovered when cycling again at 0.31 A g\({}^{-1}\) up to 500 cycles (Figure 6b), with efficiencies close to 100%. The improved cyclability was previously connected to microstructural aspects. Thus, small particle size or porous nature of the material could increase the electrode-electrolyte surface area and increase the amount of oxygen vacancies, leading to a better electrochemical response [21]. In this sense, the enhanced long-term cyclability of Ce-CMK3 anode could be related to its microstructure features. Although it does not present particularly small particle size when compared to the other prepared anodes, the particles do not aggregate, and probably the Li\({}^{+}\) and e\({}^{-}\) pathways are favored. Indeed, a deeper inspection from HRTEM analysis reveals interesting microstructural details of Ce-CMK3 particles, as shown in Figure 7. First, a nanoarchitecture regarding the particles from the surface to the bulk can be appreciated (Figure 7a,b), i.e., an outer part of 3-4 nm is visible. Second, as mentioned above, particles do not aggregate as they do in the other ceria samples. Instead, particles are well interconnected. Figure 6: Long-term cycling for Ce-CMK3 anode: (**a**) Q vs. cycle number at different current rates and (**b**) selected E vs. Q curves at 0.31 A g\({}^{-1}\). Figure 7: High magnification HRTEM images showing Ce-CMK3 particles in detail: arrows indicate nanoarchitecture regarding surface particle in (**a,b**); and contact between particles in (**c,d**). On the other hand, SEM images also evidence particularities of the Ce-CMK3 anode. Figure 8 shows several micrographs at different magnifications. In them, the existence of great pores of about 200 nm is apparent throughout all the sample. A closer inspection evidences the structure of such cavities (Figure 8c). Thus, the well-connected particles form holes which are separated from each other by about 1 \(\upmu\)m. These formations are probably the result of the calcination process applied to the Ce-CMK3 sample for oxidizing the mesoporous carbon employed in the synthesis method. In summary, the Ce-CMK3 anode presents several microstructural peculiarities which could be connected to its enhanced cyclability. In this sense, considering that CeO\({}_{2}\) material is non-toxic, low cost and environmentally friendly, our nanostructured bare CeO\({}_{2}\) anode is a promising candidate to improve the capacity values by doping. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app12010022/s1](https://www.mdpi.com/article/10.3390/app12010022/s1)]([https://www.mdpi.com/article/10.3390/app12010022/s1](https://www.mdpi.com/article/10.3390/app12010022/s1)), Figure S1: Rietveld profiles for (a) Ce-pH7 and (b) Ce-pH-13. Red points refer to the experimental data, black lines to the calculated profile, blue line indicate the difference between them and vertical green lines indicate the Bragg reflections, Table S1: Rietveld factors from the refinements for the ceria anodes. Figure S2: Representative EDX spectra for the ceria anodes. Figure S3: Representative FTIR spectra for the ceria anodes. Figure S4: Comparative of charge (pale color) and discharge (intense color) capacity values during the first 50 cycles at 0.155 A g\({}^{-1}\) current rate for all the ceria anodes. **Author Contributions:** Conceptualization, I.A.-S. and F.L. ; methodology, M.L.L. and B.S. ; software, N.B. and M.P.P. ; validation, F.J.L. and D.A.-D.; investigation, I.A.-S. and M.L.L. writing--original draft preparation, I.A.-S. and F.L. ; writing--review and editing, M.L.L. and B.S. ; visualization, F.L. ; supervision, M.L.L. ; project administration, M.L.L. ; funding acquisition, I.A.-S. All authors have read and agreed to the published version of the manuscript.