## 3 Results and Discussion
### Electrochemical Characterization  
Figure 4 shows selected discharge-charge curves at 0.155 A g\({}^{-1}\) current rate, for all the ceria anodes studied. Table 2 gathers the corresponding first cycle capacity values and the ranges of capacity values displayed in each case (i.e., minimum and maximum values obtained in the range of cycles explored in each case). Figure S4 (Supplementary Materials file) includes a comparison of discharge and charge capacity values vs. cycle number in the first 50 cycles for all the ceria anodes. Figure 3: High-magnification images, corresponding ED patterns and particle size distribution for (a) Ce-pH7, (b) Ce-pH13 and (c) Ce-CMK3 samples. Figure 4: Selected discharge–charge curves at 0.155 A g\({}^{-1}\) current rate for (a) Ce-pH13, (b) CepH7, (c) Ce-ox, (d) Ce-PMMA, (e) Ce-CMK3 and (f) Ce-ref. In all the cases, the first discharge cycle displays a much greater capacity values than those obtained in following cycles. This behavior is typically obtained in previously reported cycling curves and can be related to the formation of a solid electrolyte interphase (SEI) film and the insertion of Li\({}^{+}\) into CeO\({}_{2}\)[28]. Indeed, the development of such a film protects the device from further capacity losses in the successive cycles. This phenomenon is characteristic of various electrochemical metal-ion systems, e.g., Li-ion and Na-ion batteries [29]. The Ce-ref, Ce-PMMA and Ce-pH7 anodes display moderate capacity values close to 60 mA h g\({}^{-1}\) at 0.155 A g\({}^{-1}\) current density, which is comparable to values previously obtained for dense CeO\({}_{2}\) particles [20]. The Ce-ox and Ce-pH13 anodes present higher capacity values of about 100 mA h g\({}^{-1}\) at the same intermediate current rate, also in the range of typical values obtained for bare CeO\({}_{2}\) samples [30]. Among the different anodes studied, the highest performance is clearly displayed by the Ce-CMK3 sample, which presents capacity values of about 200 mA h g\({}^{-1}\) through the first 45 cycles. This response is more performative than that previously reported for bare ceria samples [30] and ceria anodes prepared on porous carbon templates [23]. Nevertheless, if the application in a real device was foreseen, the electrochemical performance of this bare ceria should be enhanced by processing of the anode. Indeed, as indicated above, in the present work, we aimed to comparatively assess the characteristics of different bare ceria materials in order to select the most adequate for further preparation of different composites, mainly with non-toxic and abundant iron and manganese oxides. In this sense, the Ce-CMK3 anode is revealed as the most adequate. In Figure 5, the cyclovoltammetry (CV) curves (from 1 st to 4 th cycles) in the voltage region of 0.01-3.0 V vs. Li/Li\({}^{+}\) at a 0.1 mV/s rate for the Ce-CMK3 anode are shown. As can be appreciated, the first cycle is partially irreversible because of side reactions, such as electrolyte decomposition and the formation of the SEI film, as indicated before in previous works [31]. The CV curves quickly become stable, and the discharge-charge curves almost overlap with each other, indicating that the electrochemical reactions are highly reversible for our ceria anode. A strong cathodic peak is observed at the first cycle at \(\sim\)0.7 V, which could be ascribed to the formation of SEI film and the insertion of Li\({}^{+}\) into CeO\({}_{2}\), as previously suggested [28]. Thus, in following cycles, it reduces in intensity and shifts toward \(\sim\)0.5 V. Although the response in the first cycle is somewhat rough, a broad anodic peak at \(\sim\)1.2 V could be related to the extraction of Li\({}^{+}\) from the CeO\({}_{2}\) host. This broad signal is sharpened and shifted toward \(\sim\)1.4 V, indeed corresponding to the delithiation process, in coherence with previous reported data for ceria anodes [22; 28].