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.