Abstract
The sodiation mechanism of TiO2 anatase was thoroughly investigated via X-ray absorption spectroscopy under operando conditions. The data set was analysed via an innovative and smart approach based on chemometric tools that allows the unbiased and reliable extraction of the maximum amount of meaningful information. The resulting data analysis reveals that the electrochemical sodiation mechanism is mainly based on the reduction of Ti4+ to Ti3+, going along with the irreversible amorphisation of the pristine anatase structure. At least one semi-amorphous intermediate is formed during the first discharge, whose local structure resembles those obtained at the end of the charge.
Highlights
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]
TiO2 anatase electrode was mounted in special electrochemical cell and cycled vs. Na while X-ray absorption spectroscopy (XAS)
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]
Summary
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]. 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]. While Kim et al [12] proposed the reversible intercalation of sodium ions in anatase, Passerini and coworkers suggested a much more complex mechanism
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