Long term galvanostatic charge/discharge cycling of oxygen deficient, carburized and self‐organized titanium dioxide (TiO2) nanotubes (NTs) in sodium ion (Na) batteries (SIBs) are subject to a significant self‐improving charge storage behavior. Surface reactions upon sodiation of carburized NTs form acicular surface films that can be reversibly cycled. We show that, alongside organic species from the decomposition of the electrolyte, mainly inorganic compounds, such as Na2O2 and Na2CO3, are the main constituents. These components possess a characteristic acicular morphology. Na2O2 is found to form upon sodiation and converted to NaO2 upon desodiation. This, in combination with its pseudo‐capacitive charge storage characteristics, explains the excellent rate capability measured for TiO2‐x‐C NTs. The observed high reversibility of this surface chemistry is also essential for the fast kinetics and the high capacity retention found in the system. Our findings point to a more general Na‐ion storage mechanism, that is potentially relevant to other transition metal oxides also.

Excellent, self‐improving sodiation rate capabilities in combination with high capacity retention upon galvanostatic charge/discharge cycling are found for oxygen‐deficient, carburized, and self‐organized titanium dioxide (TiO2−x) nanotubes (NTs). The sodiation mechanism is attributed to the formation of an acicular surface film as the active storage material with sodium (Na) peroxide (Na2O2) being the main component. Whether the proposed surface chemistry is unique for TiO2 NTs or serves as a common scheme for Na‐ion storage at metal oxide surfaces, in general, is not clear by now. Herein, three different materials, titanium(IV) oxide in the anatase and rutile phase and molybdenum(IV) oxide, are investigated in a planar electrode geometry toward their capability for Na‐ion storage. It is shown that all three materials under investigation demonstrate a significant progression of capacity increase upon cycling in combination with the formation of a Na‐oxide containing surface film. These “self‐improving” characteristics are found to significantly enhance the Na‐ion storage performance of the electrodes during long‐term galvanostatic cycling in a Na‐containing electrolyte.