Amorphous titanium niobium oxides (TNOs) with varying ratios of Ti and Nb (Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7) are presented as promising anode materials for Li ion batteries. The capacity of the TNO materials is seen to be equivalent to, or larger than, that of the binary oxides, with average volumetric capacities over the first 10 cycles of 717, 1,039 and 925 mAh cm−3 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of 0.2 A cm−3, compared to 720 mAh cm−3 and 425 mAh cm−3 for amorphous TiO2 and Nb2O5. Using densities estimated with X-ray reflectometry, these are equivalent to gravimetric capacities of 231, 335, 319 mAh g−1 for amorphous Ti4Nb2O13, Ti2Nb2O9 and TiNb2O7, respectively at a current density of ~70 mA g−1, compared to 257 mAh g−1 and 137 mAh g−1 for amorphous TiO2 and Nb2O5 at a current density ~80 mA g−1 and ~50 mA g−1, respectively. We discuss how rate capability varies with varying ratios of Ti and Nb and relate this to electrochemical parameters determined by the potentiostatic intermittent titration technique. Our findings reveal that the rate capability of the films is dominated by the diffusion resistance, RD, a composite parameter linked to the insertion rate and diffusion coefficient of Li, leading to a conclusion that the rate retention of the thin films is dominated by the density of insertion sites and the insertion reaction more generally.
- High-rate energy storage
- Potentiostatic intermittent titration technique (PITT)
- Amorphous titanium niobium oxide
- Diffusion coefficient
- Insertion reaction kinetics