## 1 Introduction  
TiO2 has gained considerable interest as negative electrode material in lithium-ion batteries due to its stable cycling and inherently safe insertion potential [1; 2]. While the latter is preserved for its reaction towards sodium, the underlying redox mechanism is clearly distinct from the lithium insertion reaction [3; 4; 5]. Nevertheless, stable electrochemical cycling with high reversibility has been obtained for the sodiation reaction of anatase, the most redox-active TiO2 polymorph, which continues to nourish performance-orientated research interest [6; 7; 8; 9; 10; 11]. At the same time, the precise reaction mechanism on which this remarkable performance is based is still not completely understood. In practice three different mechanisms have been proposed for the electrochemical sodiation of TiO2 anatase: (i) the redox reaction based on the Ti\({}^{4+}\)/Ti\({}^{3+}\) with no phase change, (ii) the redox reaction based on Ti\({}^{4+}\)/Ti\({}^{3+}\) with the intermediate formation of an amorphous phase, and (iii) a complete conversion reaction mechanism implying the formation of metal Ti nanoparticles [5]. For instance, while [PERSON] et al. [12] simply proposed the reversible intercalation of sodium ions in anatase, [PERSON] and coworkers suggested a much more complex mechanism implying the disproportionation of an intermediate sodium titanate phase into a mixture of new phases including Ti metal and amorphous titanate during the first discharge process, the latter phase being responsible for the further cycling activity of the material [4]. Their findings are largely supported by those of [PERSON] et al., although differing sample and experimental conditions limit comparability [13]. To unveil the structural changes and phase transitions and propose such mechanisms, many different ex situ and in a few cases in situ techniques have been applied. Among them, the widely applied X-ray diffraction (XRD) has proven to be insufficient to closely follow the reaction mechanism, due to the loss of long-range order upon first sodiation. This has lead to ambiguous claims causing much debate. As a result short-range probing techniques such as Raman spectroscopy and synchrotron based pair distribution function were carried out under _operando_ conditions to provide more reliable insights from a local structural perspective [14; 15]. Based on these findings an amorphous Na\({}_{x}\)TiO\({}_{2}\) phase appears to form in the initial sodiation reaction. Upon desodiation amorphisation aggravates and an \"anatase-like\" amorphous structure is recovered. These two phases build the foundation for the reversible redox mechanism. X-ray absorption spectroscopy (XAS) has proven to be particularly useful for analysis of complex reaction mechanism which are accompanied by strong nanostructuring upon electrochemical cycling [16]. Thanks to its element specificity, the needlessness for long-range order and the high penetration of x-rays, it provides reliable bulk information on the electronic and local structure of the absorbing element. In the specific case of anatase vs. sodium, [PERSON] et al. applied it ex situ to follow the evolution of the oxidation state of Ti [12]. In their case, the evolution of the X-ray Absorption Near Edge Structure (XANES) of the XAS and in particular the edge position proved the reversible reduction of part of the Ti\({}^{4+}\) to Ti\({}^{3+}\) during the discharge and its oxidation upon charging, thus excluding the occurrence of a conversion reaction which would have implied the formation of titanium metal nanoparticles [17]. More recently, ex situ XAS was applied by [PERSON] and coworkers to follow the sodiation of anatase nanoparticles [18]. By studying the Extended X-ray Absorption Fine Structure (EXAFS) of the XAS spectra, they propose a rearrangement of the coordination sphere of titanium leading to 5-coordinated Ti. However, their pristine material could not be simply interpreted by assuming only the presence of the anatase structure.