## 3 Discussion  
These findings, and in particular the similarities between components #2 and #4, suggest that, in the first part of the discharge, the incipient sodiation induces the formation of a reaction intermediate via the amorphisation of the starting crystalline anatase structure. This partially sodiated species characterised by a local \"TiO\({}_{2}\)-like\" structure, is recovered after one complete electrochemical cycle, in agreement with the findings of previous pair distribution function (PDF) analysis [15]. Component #3, which corresponds to the majority component present at EOD was fitted based on the structure of delafossite (NaTiO\({}_{2}\), ICSD 68872) taking into account a partial occupancy of the Na site of 0.5. In this way, it was possible to obtain an adequate fit, with bond distances very similar to those reported for this structure. Noteworthy are the elevated Debye-Waller factors which indicate a highly disordered semi-amorphous structure. This is well in line with previous findings of a disordered Na\({}_{x}\)TiO\({}_{2}\) phase prevailing at complete sodiation state [14; 15]. The intermediate Component #2 and #4, on the contrary, could be fitted with the structure derived from TiO\({}_{2}\) anatase. However, the introduction of a additional Na-O neighbours was necessary to achieve a satisfactory fit. This intermediate component can hence be understood as an amorphous anatase-like structure containing a non negligible fraction of sodium. Making such transient species visible is one of the assets of the chemometrics supported _operando_ XAS analysis [19]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Component** & **Shell** & **N \({}^{*}\)** & **R\({}_{fit}\) [Å]** & \(\sigma^{2}\) [Å]\({}^{2}\) \\ \hline \multirow{4}{*}{MCR \#1} & Ti-O\({}_{1}\) & 4 & 1.91(1) & 0.0014(6) \\  & Ti-O\({}_{2}\) & 2 & 2.32(2) & 0.0014(6) \\  & Ti-Ti\({}_{1}\) & 4 & 3.02(8) & 0.008(1) \\  & Ti-Ti\({}_{2}\) & 4 & 3.87(8) & 0.008(1) \\  & Ti-O\({}_{3}\) & 8 & 3.79(3) & 0.004(10) \\ \hline \multirow{4}{*}{MCR \#2} & Ti-O\({}_{1}\) & 4 & 1.91(1) & 0.008(1) \\  & Ti-O\({}_{2}\) & 2 & 2.33(1) & 0.008(1) \\  & Ti-Ti\({}_{1}\) & 6 & 3.09(3) & 0.022(3) \\  & Ti-Na & 3 & 3.20(3) & 0.005(2) \\  & Ti-Ti\({}_{2}\) & 3 & 3.89(3) & 0.012(2) \\ \hline \multirow{4}{*}{MCR \#3} & Ti-O\({}_{1}\) & 4 & 2.00(2) & 0.014(1) \\  & Ti-O\({}_{2}\) & 2 & 2.47(2) & 0.014(1) \\ \cline{1-1}  & Ti-Ti\({}_{1}\) & 4 & 3.18(3) & 0.018(3) \\ \cline{1-1}  & Ti-Na & 3 & 3.34(4) & 0.015(4) \\ \cline{1-1}  & Ti-O\({}_{2}\) & 6 & 3.98(5) & 0.024(5) \\ \hline \multirow{4}{*}{MCR \#4} & Ti-O\({}_{1}\) & 4 & 1.97(1) & 0.007(1) \\  & Ti-O\({}_{2}\) & 2 & 2.41(2) & 0.007(1) \\ \cline{1-1}  & Ti-Ti\({}_{1}\) & 4 & 3.07(3) & 0.013(2) \\ \cline{1-1}  & Ti-Ti\({}_{2}\) & 2 & 3.98(3) & 0.013(2) \\ \cline{1-1}  & Ti-Na (O\({}_{3}\)) & 3 & 3.85(3) & 0.023(3) \\ \hline \end{tabular} \({}^{*}\) Amplitude reduction factor was evaluated for pure anatase at the value of 0.8, and then kept fixed for all fits. Coordination numbers were evaluated and then kept fix for the presented fit.  
\end{table} 
Table 1: Ti K-edge EXAFS fitting parameters of MCR-ALS components. Our results also rebut the formation of metallic titanium upon conversion of the initial anatase, as previously proposed by [PERSON] et al. [4] These authors could detect the formation of Ti metal by ex situ X-ray photoelectron spectroscopy (XPS). Considering that such measurements are obtained with the samples submitted to high vacuum conditions, it is conceivable that the slightly reducing conditions in the XPS analysis chamber have induced the further reduction of Ti\({}^{3+}\) to titanium metal in the near surface region. A simplified sum, without mass balance, of the reaction mechanism for the sodiation of TiO\({}_{2}\) anatase based on our and previous studies results can hence be written as follows:  
\[\text{TiO}_{2}+x\text{Na}\xrightarrow{\text{1 stsodium}}\text{ \  
second and the third ionisation chamber.