Designing New Conductive Imidazole-Derived Salt for Lithium-Ion Battery Electrolytes: A Computational Approach
In this work, we report designing of an alternative salt derived from bis(trifluoroborane)imidazolide (LiIm(BF3)2) by employing density functional theory (DFT) calculations. New anion structures were generated by considering substitution at the C5 position with electron withdrawing groups (-F, -CF3, -SO3H and -NO2) and electron donating groups (-NH2, -OCH3 and –CH3). By using the optimized anion structures, random lithium ion-pair configurations (around 130 structures for each) were generated and optimized to identify the most stable configurations. The effects of different substituents were investigated with respect to ion dissociation energies, anion oxidative stability, and aromaticity of the molecules. Based on our calculations, we have found that, ion dissociation energies and intrinsic oxidative potentials of the anions were largely affected by the type of substituents introduced on the parent structure. Compared to –PF6 and -Im(BF3)2 (standard anions), –CF3 and -NO2 substituted anions showed an increase in anion oxidation stability. Since, BF3 occupies the strong Lewis basic sites of the imidazole ring, the parent structure itself exhibit a charge delocalization over the aromatic ring, which results in a weak coordinating ability with Li+. With the exception of –F, -CF3 and –CH3, further introduction of substituents at C5 position led to a much lower cation–anion interaction. Among the possible anions studied, 5-nitro-bis(trifluoroborane)imidazolide, with oxidation potentials > 5.7 V vs. Li+/Li , can be considered as a good candidates for high voltage Li-ion battery applications. Moreover, we have also found that the ion-pair dissociation energy, and anion stability of the selected salt are much improved compared to experimentally reported LiIm(BF3)2 and LiPF6.
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