## 3 Results and Discussion
### Electrochemical Characterization
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Q\({}_{d}\) 1 st Cycle (mA h g\({}^{-1}\))** & **Q\({}_{d}\) at 0.155 A g\({}^{-1}\)** \\  & **(mA h g\({}^{-1}\))** * \\ \hline Ce-pH13 & 184 & 91 (15)–103 (2) \\ Ce-pH7 & 193 & 24 (50)–67 (2) \\ Ce-ox & 227 & 85 (30)–102 (2) \\ Ce-PMMA & 108 & 43 (25)–56 (2) \\ Ce-CMK3 & 401 & 212 (45)–226 (2) \\ Ce-ref & 179 & 58 (50)–85 (2) \\ \hline \hline \end{tabular}
* The numbers in bold and brackets refer to corresponding cycle.  
\end{table} 
Table 2: First cycle discharge capacity (Qd) and range of capacity values displayed for the ceria samples. Several redox processes were reported in the last years to probably operate in the CeO\({}_{2}\)-Li system. For example, in the previous work of [PERSON] et al. [32], the authors took into account, as examples, the processes operating in CuO and Co\({}_{3}\)O\({}_{4}\) anodes in which the oxidation states of the metal cations reduce to zero from the highest value after reaction with Li (i.e., conversion anodes). Thus, admitting that basic electrochemical reaction of ceria in LIBs was still unknown, they started from the initial hypothesis of considering the ceria anode as a conversion anode. The reactions of the reversible lithiation/delithiation process in this case would be  
\[\text{CeO}_{2}+4\text{ Li}^{+}+4\text{ e}^{-}\ \leftrightarrow\text{ Ce}+2\text{ Li}_{2}\text{O} \tag{1}\]  
However, from in situ transmission electron microscopy analysis, a reversible phase transformation between fluorite CeO\({}_{2}\) and cubic Ce\({}_{2}\)O\({}_{3}\) was eventually concluded:  
\[2\text{CeO}_{2}+2\text{Li}^{+}+2\text{e}^{-}\ \leftrightarrow\text{ Ce}_{2}\text{O}_{3}+\text{Li}_{2}\text{O} \tag{2}\]  
This conclusion was drawn from experimental data obtained in a nano LIB device constructed by CeO\({}_{2}\)/graphene electrode, a layer of Li\({}_{2}\)O solid electrolyte, and a bulk Li metal (counter electrode inside the TEM). As similar results of in situ experiments on carbon nanotubes and graphene nanoribbons were obtained, it was concluded that this could be an intrinsic property regarding the interaction between lithium and exposed graphite basal planes. However, several years later, both [PERSON] et al. [33] and [PERSON] et al. [28], from SXRD, high resolution XPS and DFT, proved that the lithiated ceria phases, in which the reduction of Ce(IV) to Ce(III) takes place, own the same fluorite structure. In both cases, the anode composites included different carbonaceous additives, such as those employed in our case. The obtained CV profiles were also similar to those we obtained. The corresponding reaction is  
\[\text{CeO}_{2}+\text{x Li}^{+}+\text{x e}^{-}\ \leftrightarrow\text{Li}_{ \text{x}}\text{CeO}_{2} \tag{3}\]  
We, therefore, assumed this mechanism as operative in our case, although the small amount of active material (0.5-3 mg in conventional Swagelok-type cells) impeded us to unambiguously determine whether the post-cycling structure was maintained or not. In this process, the fluorite structure of CeO\({}_{2}\) is maintained, whereas a lattice parameter expansion of less than 1.9% takes place for x values of about 0.375. Interestingly, after the  
Figure 5: CV curves (from 1 st to 4 th cycles) for the Ce-CMK3 anode in the voltage range of 0.01–3.0 V vs. Li/Li\({}^{+}\) at 0.1 mV/s rate. first cycle, the CV curves for the battery with the Ce-CMK3 anode become smooth and coincident, suggesting a charge-discharge process with good reversibility. Aiming to further analyze the performance of the Ce-CMK3 anode, a long-term cycling experiment was carried out. Figure 6 shows the charge-discharge capacities through cycling up to 550 cycles at different current rates and selected E vs. Q curves. Although the capacity values are achieved at moderate compared to previous reported nanoarchitectured ceria anodes [20; 22], it must be underlined that the Ce-CMK3 anode displays excellent cyclability particularly at moderate high current densities.