Tunable Pseudocapacitance in 3D TiO_2-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 (TiO_2-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.