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

Abstract

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 (V-O). The role of another intrinsic TiO2 vacancy [i.e., titanium vacancies (V-Ti)] with regard to the Li+ storage behavior of TiO2 has largely evaded attention. Hence, a comparison of V-O- and V-Ti-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 V-O (n-TiO2) or V-Ti (p-TiO2). Both TiO2 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 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 highperformance TiO2 for LIB applications.