Aging effects of anatase TiO2 nanoparticles in Li-ion batteries.

Anatase TiO2 nanoparticles with a diameter of 5 nm have been investigated as a negative intercalation electrode material for Li-ion batteries. The focus was on the stability upon cycling within four different potential ranges, namely from 1.5, 1.2, 1.0 and 0.7 V vs. Li/Li(+) as the lower potential limit to 3.0 V vs. Li/Li(+) as the upper potential limit. While a lower cut-off potential allows for a higher amount of charge stored, the irreversible processes induce a faster fading of the specific charge. Galvanostatic cycling (GC), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) experiments suggest that SEI formation has a negligible contribution to the irreversible processes. It appears more plausible that an irreversible degradation of the bulk phase occurs, leading to a decrease in the amount of active sites. Moreover, it has been observed that this degradation appears as an anodic shift of the thermodynamic potential of (de-)intercalation of Li-ions in the TiO2 structure. The shift is caused by a change in the activity of Li-ions in the solid phase, which is driven by changes in the ionic atmosphere of the crystal.