| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 103 === Lithium-ion rechargeable batteries are widely used in portable electronic devices as compared to the conventional batteries due to their high energy density and good environmental properties. However, there are still many challenges concerning the safety requirements. Propylene carbonate (PC) and lithium hexafluorophosphate (LiPF6) have been extensively studied as electrolyte compositions for lithium ion battery. At open-circuit voltage and during cell charging, the high potentials of several cathode materials may result in electrolyte oxidation. These highly exothermic electrolyte decomposition reactions are usually accompanied by the decomposition of the cathode material and gas evolution that will cause potential danger and reduced cyclic stability. The presence of trace amount of water is also believed to enhance the electrolyte decomposition reactions.
In this study, we investigate the oxidative decomposition mechanisms of PC/LiPF6 with water using density functional theory methods. The solvent effect is also included using the implicit solvation model with density method. Based on the calculation, there is a significant difference in the oxidative decomposition mechanism of [H2O-PC-PF6] - compared to [PC-PF6]-. For the [PC-PF6]-, HF is formed at the initial oxidation step; whereas, in [H2O-PC-PF6] - the elementary oxidation reactions, such as H3O+ formation and C-C bond-breaking happened depending on the position of water. The presence of H3O+ which could act as an acidic catalyst would accelerate the PC ring open process. The energy barrier for the ring-opening step of [PC-H2O-PF6] was found to be much lower than that of [PC-PF6]. Our calculations suggest that, the initial oxidative decomposition products of PC in the presence of water are acetone radical, propanal radical and carbon dioxide. We also performed the molecular dynamic simulations to confirm the results from DFT calculation.
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