Designing New Conductive Imidazole-Derived Salt for Lithium-Ion Battery Electrolytes: A Computational Approach

Ever since the commercialization of the rechargeable Li-ion cell, LiPF₆ is by far the most widely used electrolyte salt. It can produce electrolyte solutions with a superior ion conductivity, good electrochemical oxidation stability, and adequate thermal stability when dissolved in binary or ternary solvent systems of cyclic carbonates. However, LiPF₆is also reported to have series problems that may lead to performance and safety issues (1). In order to circumvent these issues, researches have focused on designing new lithium salts which have thermal and electrochemical stability, high ionic conductivity and easy synthesis route (2-6).

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.

References

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6. J. Scheers, L. Niedzicki, G. Z. Zukowska, P. Johansson, W. Wieczorek and P. Jacobsson, Phys Chem Chem Phys, 13, 11136 (2011).