Ilmenite nanoflowers as anode material for SIBs: the thin red line between morphology control and decomposition

In the search for new electrode materials for rechargeable ion batteries and capacitors, FeTiO3 ilmenite has been recently proposed as anode based on the possible conversion reaction: FeTiO3 + (2+x)Na+ +3e- → Fe + NaxTiO2 + Na2O resulting in high theoretical capacity combined with high sustainability and affordability of the pristine material that can be found as abundant mineral [1,2]. Although after these first report various works have appeared in literature on the use of ilmenite as anode material, the electrode reaction is still not completely clear and further investigations are needed. Promising performances have been reported for the hydrothermally-treated material for which a peculiar nano-flower morphology has been reported. The beneficial effect of this morphology has been demonstrated by comparison between the electrochemical performance of the spherical particles and the nanoflowers powders. At the same time, it cannot be neglected that the hydrothermal treatment can affect not only the morphology but also the structure and composition of the ilmenite. In fact, hydroxide based hydrothermal treatment under strong conditions has been proposed as a cost-effective method for the decomposition of ilmenite and subsequent production of titania at industrial scale. Here we report an accurate investigation of the effect of the hydrothermal treatment on the composition and crystal structure of the ilmenite powders. We also suggest a possible decomposition mechanism based on ex-situ XRD powder diffraction, Raman spectroscopy, SEM-EDX and thermal analysis measurements. The electrochemical results obtained for the hydrothermally-treated material are then revalued in the perspective of the occurrence of material decomposition.