| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 103 === The advance in high energy density, superior rate capacity, high capacity and long life cycle made lithium ion batteries become the dominant power source for portable electronic devices. Nevertheless, there are several challenges that need to be addressed in the improvement of lithium ion batteries such as improvement of high-temperature performance and minimizing capacity fading during prolonged charge-discharge cycling. Lithium salts are one of the significant elements of LIBs that play a fundamental role in the performance and the safety of the battery. In this study, we developed new potential replacements of LiPF6 salt based on bis(trifluoroborane)-benzimidazole as parent structure. The oxidative stability, film forming and gas releasing ability of the selected designed salt will also be studied.
The effects of different substitution on imidazole hydrogen with electron withdrawing group (-CH3, -CF3 or -C2F5) and benzene ring with electron donating group (-F, -CHO, -CN, -SO2CH3 and -NO2) on parent structure are studied with respect to ion pair dissociation energies and intrinsic anion oxidation potential of the molecule. Based on our calculations, we have found that, ion pair dissociation energies and intrinsic anion oxidation potentials of the anions mainly affected by the position and type of substituents introduced on the parent structure. Compared to -CH3, substitution at C2 position of the parent benzimidazole (B-) moiety by -CF3 results an increase in anion oxidation stability. However, we observed a negligible change in intrinsic anion oxidation potential as the length of the fluoroalkyl group increased to -C2F5. The most promising anions are generated by considering double-substitution at C2 and C5 positions. Among the possible anions, bis(trifluoroborane)-5-nitro-2-(trifluoromethyl) benzimidazolide (BTNTB-), with the calculated intrinsic anion oxidation potential of 5.50 V vs. Li+/Li, can be considered as a potential candidate for high voltage Li-ion battery.
In order to improve the performance of lithium ion batteries, new anion structures are proposed by considering mono-substitution at R1 position with electron donating group (-NH2, -OCHF3, -CH3) and electron withdrawing group (-CF3, -CN). The combination of different molecule substituted on R1, R2, R3, R4, and R5 to form double and triple substitution was also studied. Ion dissociation energy and intrinsic anion oxidative stability of the molecule were used to investigate the effects of different molecule substitution. Based on our calculation, we found that the position and type of molecule substituents on the parent structure affect the ion dissociation energies and intrinsic oxidative potentials.
Compared to electron donating group, electron withdrawing group resulted in an increase in anion oxidation stability. Simillary, electron donating group substituted on R1 combined with different molecule on R2, R3, R4, and R5 resulted in a lower oxidation potential compared to electron withdrawing group. The most promising anion are generated by substituted electron withdrawing group substitution at meta position. Among the possible anions, bis(trifluoroborane)-7,5,2-tricyanobenzimidazole (BZ3C-) with oxidation potential of 5.72 V vs. Li+/Li which has higher oxidation potential compared to bis(trifluoroborane)-5-nitro-2-(trifluoromethyl) benzimidazolide (BTNTB-). The effects of BTTB- on the oxidative decomposition of ethylene carbonate (EC) are also studied. It is found that the presence of BTTB- anion significantly reduces EC oxidation stability. Compared to PF6- and BOB-, which are commonly used salts, in BTTB-e based electrolytes, EC would undergo oxidative decomposition by overcoming a relatively high energy barrier to form a thermodynamically stable product, CO2 and acetaldehyde.
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