Atomic Substitution Enabled Synthesis of Vacancy-Rich Two-Dimensional Black TiO _{2- x} Nanoflakes for High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg ²⁺ ions, the insertion of Mg ²⁺ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO _2-x (B-TiO _2-x ) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS ₂ nanoflakes as precursors. We find out that the presence of OVs in B-TiO _2-x electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg ²⁺ ion storage. The vacancy-rich B-TiO _2-x nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.