## 2 Results  
TiO\({}_{2}\) anatase electrode was mounted in special electrochemical cell and cycled vs. Na while XAS spectra on Ti K-edge were acquired, as described in Section 5. The evolution of the XANES portion of the XAS dataset along with corresponding electrochemical cycling curve is shown in Figure 1. In particular, the inset in (a) depicts the evolution of the pre-edge features. In the spectrum of the pristine electrode, the pre-edge shows the typical features of anatase structure consisting of 4 peaks commonly labelled A1, A2, A3 and B, and resulting from the octahedral coordination geometry of titanium in the anatase structure [20]. During the sodiation, a shift of the Ti K-edge position to lower energies is observed, reflecting the reduction of Ti\({}^{4+}\) to Ti\({}^{3+}\) which demonstrates the redox activity of the Ti\({}^{4+/3+}\) redox couple, in line with previous ex situ findings [12,18]. Simultaneously, a slight increase of the intensity of the A2 pre-edge peak and a general fading of the other features is observed, which has been attributed to the aggravating distortion of the coordination of the Ti centres with respect to crystalline TiO\({}_{2}\)[18,21]. A similar change in the shape of the pre-edge was already observed in the case of the lithiation of anatase [22,23]. It is noteworthy that, while the main edge position seems to be largely reversed upon desodiation, the changes in the pre-edge features are not, indicating, differently from the case of lithium, that non-reversible structural rearrangements have occurred upon initial sodiation. The complete dataset comprising more than 350 XAS spectra was analysed via Principal Component Analysis revealing the existence of four independent component (see Appendix A, Figure A1). This rank analysis results paired with additional chemical and physical constrains was used as input for MCR-ALS tool which resulted in pure components whose spectra and concentration profile are presented in Figure A2 (cf. Appendix A) and Figure 2, respectively. Figure 1: (**a**) Evolution of _operando_ Ti K-edge XANES of TiO\({}_{2}\) anatase electrode vs. Na. Dashed line indicates the end of first discharge (#184) and dashed-dotted line the end of charge (#264). The inset shows the evolution of the four indicated pre-edge features A\({}_{1}\), A\({}_{2}\), A\({}_{3}\) and B in the range from 4965 to 4975 eV. (**b**) Corresponding electrochemical cycling curve of the first 1,5 electrochemical cycles of TiO\({}_{2}\) vs. Na. Figure 2: Concentration profile of pure spectral components obtained through MCR-ALS during 1.5 electrochemical cycles of TiO\({}_{2}\) vs. Na. The dashed lines indicate end of first discharge (#184) and end of first charge (#264). From the evolution of the concentration of the four pure components, several perceptions can be made. Component #1 is the sole component at the onset of electrochemical cycling and therefore represents the pristine state. Its intensity quickly decreases upon sodiation and does not increase upon inversion of current (charge). This suggests that the pristine state is not recovered after the first complete electrochemical cycle, which reminds of conversion-type anode materials [24]. Instead of a reemerging component #1, a new component #4 is formed during desodiation and reaches its maximum at the end first charge corresponding to the completion of one full electrochemical cycle (spectrum #264). Even though component #3 is dominant at the end of first discharge (spectrum #184), a 20% contribution of component #2 is still present. The latter has a transient nature, as it reaches its maximum intensity about half-way through the discharge reaction and then fades gradually as sodiation continues. To gain insights on the local structure of the absorbing Ti at specific points of the electrochemical reaction, the EXAFS signal of the pure components was fitted and structural parameters about the nature of next neighbouring shells extracted (cf. Figure 3 and Table 1, respectively).