Sodium induced morphological changes of carbon coated TiO2 anatase nanoparticles - High-performance materials for Na-ion batteries

The most promising candidate as an everyday alternative to lithium-ion batteries (LIBs) are sodium-ion batteries (NIBs). This is not only due to Na abundance, but also because the main principles and cell structure are very similar to LIBs. Due to these benefits, NIBs are expected to be used in applications related to large-scale energy storage systems and other applications not requiring top-performance in terms of volumetric capacity. One important issue that has hindered the large scale application of NIBs is the anode material. Graphite and silicon, which have been widely applied as anodes in NIBs, do not show great performance. Hard carbons look very promising in terms of their abundance and low cost, but they tend to suffer from instability, in particular over the long term. In this work we explore a carbon-coated TiO2 nanoparticle system that looks very promising in terms of stability, abundance, low-cost, and most importantly that safety of the cell, since it does not suffer from potential sodium plating during cycling. Maintaining a nano-size and consistent morphology of the active material is a crucial parameter for maintaining a well-functioning cell upon cycling. In this work we applied Anomalous Small Angle X-Ray Scattering (ASAXS) for the first time at the Ti K-edge of TiO2 anatase nanoparticles on different cycled composite electrodes in order to have a complete morphological overview of the modifications induced by sodiation and desodiation. This work also demonstrates for the first time that the nanosize of the TiO2 is maintained upon cycling, which is in agreement with the electrochemical stability.