The kinetics and reaction mechanism of titanium dioxide and silicon negative electrodes for lithium-ion batteries

Abstract

Lithium-ion batteries (LIBs) are energy storage•conversion devices, which utilize reversible electrochemical reactions on anode and cathode, storing chemical energy and converting it to electrical energy. Until now, LIBs usage has been limited to energy sources for small IT equipments. LIBs for the future, however, have far more possibilities to be applied in extended fields, such as electric vehicles, and energy storage systems. Thus, LIBs energy density and rate performance should be enhanced. To achieve these goals, methods to use the materials with high energy densities like Si more effectively, such as improving the rate performances by coating conductive material and controlling structure of active materials, have been studied extensively. Other areas of studies, for example, understanding the reaction mechanism or segmentizing the components which can affect kinetics, are also necessary. Nevertheless, these kinds of studies are less common in the research area. In the first part of study, Li ion diffusion in electrolyte and its effect of kinetics was investigated. The mesoporous TiO2 was chosen because mesoporous structure is known to have facile Li ion diffusion through electrolyte, and because other parameters related to volume expansion along the reaction can be eliminated with TiO2 system, facilitating investigation of Li ion diffusion. 3-dimmensional mesoporous TiO2 particles with three different pore sizes and two different particle sizes were synthesized using nano-silica as a template. Moreover, the structure of synthesized materials was analyzed with transmission electron microscopy, x-ray diffraction, small angle x-ray diffraction, and nitrogen adsorption/desorption isotherm. Using the synthesized materials, electrochemical experiments were conducted. There was no difference in electrochemical performances between the synthesized materials with 1 M concentration electrolyte. On the other hand, Li ion depletion occurs inside of the active materials depending on the size of the active materials and the pore sizes with 0.1 M concentration electrolyte. Such result reflects that the particle size and pore size are influencial parameter that affects Li ion diffusion, but their effects are negligible in 1 M concentration electrolyte. Based on the acquired results, the mathematical model with Thiele modulus was set up, and the boundaries of Li ion depletion inside of the particle was predicted using the model. In the second part of this study, the reaction mechanism and the kinetics of Si, which has high theoretical capacity, were investigated. The reaction between Si and Li was widely reported to progress through solid-solution reaction. Recently, however, there were some reports that insist that the reaction of Li-Si is two-phase reaction, and the reaction mechanism of Li-Si requires some clarification. The main obstacle for analyzing the reaction mechanism of Li-Si is due to the amorphous structure which silicon transforms into during the reaction. Thus, routinely used analytic methods have limitations to be used in Li-Si system. In this study, the reaction between Li and Si was analyzed with electrochemical method and thermodynamic relations, which is independent of the structure of material. The result concludes that the reaction between Li and Si proceeds through two steps of two-phase reaction. This result was confirmed by showing the trend of the diffusion coefficient change of Li in Si is similar to that of the materials with two-phase reaction. Also, it is logically more plausible to understand the reaction between Li and Si with two-phase reaction. The electrochemical performances for Si were measured with various current densities, and it was discovered that two different two-phase reactions had different kinetic properties. The cause was investigated using electrochemical impedance spectroscopy. Two resistances related to charge transfer and solid-electrolyte interphase were extracted from the impedance data, and it was found that these two resistances are not dominant parameters that determine the kinetics of two reactions. Based on these evidences, among the three kinetic parameters, diffusion in electrolyte, surface reaction, and bulk diffusion, this study concluded that the rate determining step is the diffusion in bulk phase. Furthermore, the strategy to use Si more effectively was proposed, and confirmed with experiments. Through this study, the reaction mechanism of active material and the important kinetic parameters were clarified. These findings give some clues to effectively designing active materials depending on the environment of the reactions.