| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 105 === The composition and morphology of solid electrolyte interphase (SEI) plays a significant role on the performance of lithium ion batteries. The presence of trace impurities such as water causes decomposition of LiPF6. The decomposed product further reacts with water and leading to form acid. The deposition of ionic insulating products on SEI and the reaction of acid with SEI components cause SEI deterioration. In this work, electrolyte additive and electrochemical process have been developed to reduce water/acid impurities and minimize their effect to overall degradation of cell system. In addition, synergetic effect of electrolyte additives is achieved for efficient and effective SEI former to enhance performance of graphite electrode for wide temperature range. The effect of electrolyte additives and electrochemical process on the performance of graphite electrode, surface composition and electrolyte stability have been investigated in detail.
Considering the Lewis-base feature of N-Si bond, 1-(Trimethylsilyl)imidazole (1-TMSI) is proposed as a novel water scavenging electrolyte additive to suppress LiPF6 decomposition. The scavenging ability of 1-TMSI and beneficiary interfacial chemistry between the MCMB electrode and electrolyte are studied through a combination of experiments and density functional theory (DFT) calculations. NMR analysis indicated that LiPF6 decomposition by water was effectively suppressed in the presence of 0.2 vol % 1-TMSI. XPS surface analysis of MCMB electrode showed that the presence of 1-TMSI reduced deposition of ionic insulating products caused by LiPF6 decomposition. The results showed that the cells with 1-TMSI additive have better performance than the cell without 1-TMSI by facilitating the formation of solid-electrolyte interphase (SEI) layer with better ionic conductivity.
Synergetic effect of electrolyte additives was explored. 4-(Chloromethyl)-1,3,2-dioxathiolane-2-oxide (CMDO) was prepared to improve the properties of commonly used electrolyte constituents - ethylene carbonate (EC), and fluoroethylene carbonate (FEC). The formation of efficient passivation layer in Propylene Carbonate (PC)-based electrolyte for MCMB electrode was investigated. The addition of CMDO resulted in a much less irreversible capacity and induces thin SEI formation. In fact, the combination of the three additives played a key role to enhance reversible capacity of MCMB electrode at lower or ambient temperature. The electrochemical measurement analysis showed that the SEI formed by using a mixture of the three additives gave better intercalation-deintercalation of lithium ions.
The electrochemical treatment process was performed by applying voltage to avoid water impurity from organic electrolyte. After electrochemical treatment, the electrolyte with intentionally added water showed the measured water amount was significantly decreased. In addition, the electrochemical performance test of Li/MCMB cells showed that the electrochemical treatment has a positive effect on the electrolyte to improve the performance of Li/MCMB cells. The electrochemical treatment method is promising in avoiding water impurity from organic electrolyte.
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