Surface Modification of Lithium-ion Battery Electrode Materials with Polyvinylidene Difluoride

• Main Authors: Yu-Hsiu Chang, 張瑜修
• Other Authors: Nae-Lih Wu
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/2zj4j9

碩士 === 國立臺灣大學 === 化學工程學研究所 === 107 === Lithium-ion battery (LIB) with high energy density, excellent cycle life and high safety is one of the most promising materials currently used in energy storage devices for electric vehicles. Conventional graphite anodes have limited rate capabilities and safety issues. Therefore, development of a highly efficient electrode material with long-term cycle stability, excellent charge and discharge performance, and high safety is quite important for a high-power LIB. Spinel Li4Ti5O12 (LTO) is a competitive anode material for high-power LIB due to its high safety, excellent rate performance and extremely long cycle stability. However, severe gas evolution can be observed during charging and discharging and storage. Then it becomes a major obstacle to the large-scale application of LTO to LIB. It is necessary to improve the severe gassing reaction in LTO batteries because it not only seriously deteriorates their cycle stability, but also causes serious safety problems. So far, some research reports have mentioned gassing phenomena and a few improvement methods for LTO electrodes. However, there is no detailed study on the mechanism of LTO gassing reactions. Therefore, this study attempts to construct an artificial solid-electrolyte-interface (SEI) layer with a polymeric material, polyvinylidene fluoride (PVDF), to improve the gassing problem of LTO. Gas chromatography-mass spectrometry (GC-MS) was used to observe the in-situ gassing phenomenon of LTO half-cells during charge and discharge. Electrochemical performance, pressure change, gas composition, and other data was compared to clarify the actual reaction situation inside. First, we want to directly process the already-made LTO electrode. Therefore, after preparing LTO electrode, PVDF was coated on the LTO electrode by blade-coating. However, constructing a protective layer only on the surface of the electrode did not provide good protection. Therefore, we impregnated the LTO electrode in a solution of PVDF, and with the effect of negative pressure, the gas was extracted, which promoted two-phase-only condition, so that the solution can be closely contact with the surface of LTO, while PVDF was coated as much as possible on each LTO particle. Since PVDF has strong hydrophobicity and dipole moment, it can avoid residual moisture from reacting with the LTO surface. Moreover, when lithium-ions enter the LTO through this interface, the surrounding electrolyte molecules will be isolated, and unable to react with LTO surface. In addition to a series of electrochemical measurements, the results of GC-MS and the pressure monitor were used to analyze the exact effect of the artificial solid-electrolyte-interface (ASEI) on gassing reactions. Then, investigate into the mechanism and the sequence of gassing reactions. The operando in-situ GC-MS data provides the performance of various gas components during charging and discharging, as well as the relative proportions of the gaseous components in the battery. After comparing the trends of the gaseous components, it can be judged which component of the electrolyte dominated the gas reaction. In addition, the pressure monitor can show the effect of this surface modification on gassing problems. Based on the above results, we gradually understand the cause and effect of gas production, and compounds produced during gassing. It will be more easily for researchers to apply LTO to a variety of applications in the future. And facilitate the successful commercialization of this material.