Exploring the Effect of Cation Vacancies in TiO 2 : Lithiation Behavior of n-Type and p-Type TiO 2 .

TiO ₂ offers several advantages over graphite as an anode material for Li-ion batteries (LIBs) but suffers from low electrical conductivity and Li-diffusion issues. Control over defect chemistry has proven to be an effective strategy to overcome these issues. However, defect engineering has primarily been focused on oxygen vacancies (V _O ). The role of another intrinsic TiO ₂ vacancy [i.e., titanium vacancies (V _Ti )] with regard to the Li ⁺ storage behavior of TiO ₂ has largely evaded attention. Hence, a comparison of V _O - and V _Ti -defective TiO ₂ can provide valuable insight into how these two types of defects affect Li ⁺ storage behavior. To eliminate other factors that may also affect the Li ⁺ storage behavior of TiO ₂ , we carefully devised synthesis protocols to prepare TiO ₂ with either V _O (n-TiO ₂ ) or V _Ti (p-TiO ₂ ). Both TiO ₂ materials were verified to have a very similar morphology, surface area, and crystal structure. Although V _O provided additional sites that improved the capacity at low C-rates, the benefit obtained from over-lithiation turned out to be detrimental to cycling stability. Unlike V _O , V _Ti could not serve as an additional lithium reservoir but could significantly improve the rate performance of TiO ₂ . More importantly, the presence of V _Ti prevented over-lithiation, significantly improving the cycling stability of TiO ₂ . We believe that these new insights could help guide the development of high-performance TiO ₂ for LIB applications.