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 (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.
ABSTRACT
Despite the successful debut of gold nanoclusters (Au NCs) in solar cell applications, Au NCs, compared to dyes and quantum dots, have several drawbacks, such as lower extinction coefficients. Any modulation of the physical properties of NCs can have a significant influence on the delicate control of absorbance, energy levels, and charge separation, which are essential to ensure high power conversion efficiency. To this end, we systematically alter the optoelectronic structure of Au18(SR)14 by Ag doping and explain its influence on solar cell performance. Our in-depth spectroscopic and electrochemical characterization combined with computational study reveals that the performance-dictating factors respond in different manners to the Ag doping level, and we determine that the best compromise is the incorporation of a single Ag atom into an Au NC. This new insight highlights the unique aspect of NCs-susceptibility to atomic level doping-and helps establish a new design principle for efficient NC-based solar cells.
ABSTRACT
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.
