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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 <i>in situ</i> 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

A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate.

As one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3-calcite, aragonite, and vaterite-were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·½H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3