RESUMO
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.
RESUMO
The nature of precursors employed in the synthesis of lithium-ion battery cathode materials is a crucial performance-dictating factor. Therefore, it is of great importance to establish a way to manipulate the precursor and seek a comprehensive understanding of its influence on the electrochemical behavior of a targeted electrode material. A thermal route is herein demonstrated for the synthesis of lithium-excess LiMn2 O4 (LMO) by exploiting an intriguing thermal phenomenon, thermally induced grain fining, and sheds light on how it affects the mechanism and kinetics of lithiation, and, furthermore, the electrochemical behavior of LMO. Detailed insights into the lithiation mechanism and kinetics reveal that the use of a finely grained, porous Mn3 O4 , which possesses an open crystal structure, is a key to the success of incorporating excess Li. In addition, this in-depth electrochemical investigation verifies a very recent theoretical prediction of faster Li diffusion kinetics enabled by excess Li.
RESUMO
Despite the many benefits of hierarchical nanostructures of oxide-based electrode materials for lithium-ion batteries, it remains a challenging task to fully exploit the advantages of such materials partly because of their intrinsically poor electrical conductivities. The resulting limited electron supply to primary particles inside secondary microparticles gives rise to significant variation in the lithium-ion (Li+) storage capability within the nanostructured particles. To address this, facile annealing, where in situ generated carbon-coated primary particles were assembled into porous microagglomerates, is demonstrated to prepare nanostructured titanium dioxide (TiO2). A systematic study on the effect of the carbon coating reveals that it is exclusively governed by the characteristics of the TiO2/carbon interface rather than by the nature of the carbon coating. Depending on their number, oxygen vacancies created by carbothermal reduction on the TiO2 surface are detrimental to Li+ diffusion in the TiO2 lattice, and structural distortion at the interface profoundly influences the Li+ (de)intercalation mechanism. This new insight serves as a stepping stone toward understanding an important yet often overlooked effect of the oxide/carbon interface on Li+ storage kinetics, thereby demanding more investigations to establish a new design principle for carbon-coated oxide electrode materials.
RESUMO
A thermal reaction route that induces grain fracture instead of grain growth is devised and developed as a top-down approach to prepare nanostructured oxides from bulk solids. This novel synthesis approach, referred to as the sequential oxygen-nitrogen exchange (SONE) reaction, exploits the reversible anion exchange between oxygen and nitrogen in oxides that is driven by a simple two-step thermal treatment in ammonia and air. Internal stress developed by significant structural rearrangement via the formation of (oxy)nitride and the creation of oxygen vacancies and their subsequent combination into nanopores transforms bulk solid oxides into nanostructured oxides. The SONE reaction can be applicable to most transition metal oxides, and when utilized in a lithium-ion battery, the produced nanostructured materials are superior to their bulk counterparts and even comparable to those produced by conventional bottom-up approaches. Given its simplicity and scalability, this synthesis method could open a new avenue to the development of high-performance nanostructured electrode materials that can meet the industrial demand of cost-effectiveness for mass production.
