Exploring the Effect of Cation Vacancies in TiO₂: Lithiation Behavior of n-Type and p-Type TiO₂

TiO2 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 (VO). The role of another intrinsic TiO2 vacancy [i.e., titanium vacancies (VTi)] with regard to the Li+ storage behavior of TiO2 has largely evaded attention. Hence, a comparison of VO- and VTi-defective TiO2 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 TiO2, we carefully devised synthesis protocols to prepare TiO2 with either VO (n-TiO2) or VTi (p-TiO2). Both TiO2 materials were verified to have a very similar morphology, surface area, and crystal structure. Although VO 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 VO, VTi could not serve as an additional lithium reservoir but could significantly improve the rate performance of TiO2. More importantly, the presence of VTi prevented over-lithiation, significantly improving the cycling stability of TiO2. We believe that these new insights could help guide the development of high-performance TiO2 for LIB applications.