Nonequilibrium Phase Transitions in Amorphous and Anatase TiO2 Nanotubes

The electrochemical lithiation/delithiation behavior of self-organized amorphous and anatase titanium dioxide (TiO2) nanotubes (NTs) is analyzed by means of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The bulk lithiation properties are governed by the different phase transitions in amorphous and anatase TiO2. While in the case of amorphous nanotubes the phase transition only leads to a thermodynamic limitation of the bulk Li content, it additionally limits the lithiation kinetics for the anatase case. This kinetic constraint is found to originate from under­lithiation of the anatase TiO2–x bulk caused by the instant first phase transition during lithium insertion. Together with the surface lithiation properties, it leads to different lithiation characteristics. Amorphous nanotubes are characterized by a reversible surface chemistry and thus pseudocapacitive lithiation/delithiation behavior. As a result, amorphous TiO2 nanotubes show higher overall capacities due to the contribution of surface lithiation, higher capacity retention, higher rate capability, and higher Coulombic efficiencies at high C-rates, even though at the lowest applied lithiation potential of 1.1 V, slightly more lithium is inserted into the bulk of anatase TiO2–x nanotubes under quasi steady-state conditions.