Comparison of Amorphous TiO2 with Anatase Thin-Films Grown By ALD in Terms of Rate Performance and Capacity for Li+-Insertion

In this work we compare amorphous TiO₂ to anatase thin-films for their Li⁺ insertion/extraction properties. For this, we deposited ultra-thin (35 nm) amorphous TiO₂  and anatase films by atomic layer deposition on a TiN current collector. Such thin films are interesting since they have the same length scale as nanostructured particles or other nanostructures, e.g. nanotubes, but with the additional benefit of offering a well-defined geometry that allows for a more precise determination of the underlying lithium insertion/extraction kinetics. Furthermore, such ultra-thin electrodes might find application in 3D (micro-)batteries.

The amorphous and anatase films were characterized using cyclic voltammetry and galvanostatic charging/discharging. The electrochemical properties of 35 nm amorphous TiO₂ and anatase films are shown in figure 1. The cyclic voltammograms (CV, see figure 1a) performed at 10 mV/s shows distinction between lithium insertion/extraction of the amorphous and anatase films. The average of the insertion and extraction peak potentials is 1.65 V vs Li⁺/Li and 1.99 V vs Li⁺/Li for amorphous and anatase, respectively.  Also, the peaks in the CV for amorphous TiO₂ are less sharp compared to anatase, suggesting a different intercalation mechanism.

Galvanostatic charge/discharge experiments show a significantly increased capacity for amorphous compared to anatase (see figure 1b). At 1C, the amorphous TiO₂ film has a capacity of 1.9 µAh/cm² compared to 1.2 µAh/cm² for the anatase film. Also, at higher C-rates the amorphous TiO₂ film shows high retention of the capacity. For example, at 20C, 63% of the capacity at 1C is retained, compared to 30% for anatase. These results show the potential benefits of using amorphous TiO₂ compared to anatase thin-films due to its significantly higher capacity and superior rate performance.