Lithium ion batteries (LIBs) are currently one of the preferred technologies to store electrical energy. However, the worldwide availability of Li is limited, and it is questionable whether the rising energy storage demands can be fulfilled by LIBs in the future . Sodium ion battery (SIB) technology could provide an alternative, as Na is readily available, cheap, and environmentally friendly. SIBs additionally allow Al to be used as current collector instead of Cu, which is heavier and more expensive. Recently, the excellent performance of a SIB anode based on TiO2 nanoparticles has been demonstrated [2,3]. In contrast to the well-optimized electrochemical behavior of state-of-the-art anodes, however, there is still a lack of understanding of the mechanisms involved in the performance of these nanoparticle anodes. In particular, it is challenging to unravel the structural and electronic changes of the anatase TiO2 nanoparticles and the loss of crystallinity of the anodic nanomaterial upon Na uptake. For instance, it has been shown that initial sodiation leads to an irreversible capacity loss of nearly 40% .
X-ray absorption near edge spectroscopy (XANES) of the Ti K-edge is ideal for studying the electronic and geometrical structure around the probed Ti atom and its nearest neighbors and, hence, can reveal effects of sodium intercalation. However, ex situ experiments performed on disassembled electrodes can raise questions about the relevance of the results due to the expected changes of the electrode material due to air exposure, sample transport, and preparation. To gain direct, relevant insights into the intercalation process of Na into the TiO2 nanoparticle anode material, we designed an operando XANES experiment using a modified coin cell equipped with an x-ray transparent Kapton® window and a 6 μm-thick Al foil current collector. This setup allowed the measurement of the Ti K-edge (at the HZB BESSY II synchrotron source in the HiKE endstation  at the KMC-1 beamline) of the TiO2 electrode while cycling the coin cell battery, i.e. de/sodiating the electrode.
The Ti K absorption edge reveals the average Ti oxidation state of the TiO2 anode material, which changes during the sodiation from the expected +4 (Ti4+) oxidation state in the original anatase structure to values below +3. The operando results of the Ti K-edge show, that the oxidation state increases during the desodiation but does not reach Ti4+ again due to the presence of irreversibly intercalated sodium. The study is also focused on the evolution of the pre-edge structure of the Ti K-edge during the dis/charging. In the initial sodiation cycle, the pre-peak changes from the characteristic four-peak feature related to anatase  to a structure dominated by a single peak, hinting at a change in the number of nearest neighbors around the probed Ti atoms (from six to five or even four neighbors) . The single peak intensity increases during the first desodiation, suggesting that the anode material does not completely recover the original anatase structure after the first full dis/charging cycle. The subsequent cycling shows the same spectroscopic trend on the pre-edge feature, revealing a stable intercalation process after the irreversible structural rearrangement of the TiO2 during the first sodiation process.
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