Tunable Pseudocapacitance in 3D TiO2-δ Nanomembranes Enabling Superior Lithium Storage Performance.

Nanostructured TiO2 of different polymorphs, mostly prepared by hydro/solvothermal methods, have been extensively studied for more than a decade as anode materials in lithium ion batteries. Enormous efforts have been devoted to improving the electrical conductivity and lithium ion diffusivity in chemically synthesized TiO2 nanostructures. In this work we demonstrate that 3D Ti3+-self-doped TiO2 (TiO2-δ) nanomembranes, which are prepared by physical vapor deposition combined with strain-released rolled-up technology, have a great potential to address several of the long-standing challenges associated with TiO2 anodes. The intrinsic electrical conductivity of the TiO2 layer can be significantly improved by the in situ generated Ti3+, and the amorphous, thin TiO2 nanomembrane provides a shortened Li+ diffusion pathway. The fabricated material shows a favorable electrochemical reaction mechanism for lithium storage. Further, post-treatments are employed to adjust the Ti3+ concentration and crystallinity degree in TiO2 nanomembranes, providing an opportunity to investigate the important influences of Ti3+ self-doping and amorphous structures on the electrochemical processes. With these experiments, the pseudocapacitance contributions in TiO2 nanomembranes with different crystallinity degree are quantified and verified by an in-depth kinetics analysis. Additionally, an ultrathin metallic Ti layer can be included, which further improves the lithium storage properties of the TiO2, giving rise to the state-of-the-art capacity (200 mAh g-1 at 1 C), excellent rate capability (up to 50 C), and ultralong lifetime (for 5000 cycles at 10 C, with an extraordinary retention of 100%) of TiO2 anodes.