Conductivity Improvement of LiBF4 Containing Electrolyte for Enhanced Application in Lithium-Ion Batteries

Abstract

In view of a possible cost reduction and safety improvement of Lithium-Ion-Batteries (LIBs), the exchangeability of the common electrolyte salt lithium hexafluorophosphate LiPF6 with lithium tetrafluoroborate LiBF4 was investigated.[1,2] Replacement of LiPF6 with LiBF4 was considered due to the salt’s superior thermal stability and moisture stability compared to LiPF6. While LiBF4 was repeatedly studied for application in LIBs, low conductivity compared to analogous electrolyte solutions with LiPF6 is referred to as one of the main drawbacks of LiBF4 electrolytes.[1,3] These differences in electrolyte solution conductivity are usually related to an increased tendency for ion pair formation between the [BF4]– anion and Li+.[1,4] To overcome the strong ion-pairing between Li+ and [BF4]–, the conductivity of a LiBF4 electrolyte solution was improved by introduction of different amounts of 1,2-dimethoxyethane (DME) as bidentate ligand into the commercially available electrolyte solvent L57 (ethyl carbonate : ethylmethyl carbonate, 30 : 70, by wt%). The solvent modification was carried out in consideration of the published superior oxidative stability of lithium associated oligoethers compared to non-associated analogues.[5] All electrolyte solutions were investigated for their electrochemical behavior and their performance in Lithium-Ion-Battery (LIB) cells with commercially available cell components. All results were referred to analogous measurements with LiPF6 and LiBF4 electrolyte solution in the unmodified L57 solvent. Electrochemical characterization was performed by conductivity measurements and cyclic voltammetry. Battery cell experiments were carried out for Lithium-Metal cells with excess of electrolyte solution and Lithium-Ion cells. Lithium nickel cobalt manganese oxide (NCM622) electrodes were implemented as positive electrodes and Lithium-Ion cells were assembled with graphite electrodes as negative electrodes. All battery cells were investigated for their ambient temperature cycle life. Furthermore, ambient temperature C-rate stability of Li-Ion cells was examined. In view of the electrochemical behavior, conductivity of LiBF4 electrolytes significantly increases with the introduction of DME to the chosen electrolyte solvent, which is shown in the figure presenting the temperature dependent conductivities of the investigated electrolyte solutions. This conductivity enhancement is decreasing towards the contribution of more than two equivalents of DME per Li+ ion in the electrolyte solution. Cyclic voltammetry experiments of the electrolyte solutions show negligible effects on reductive stability of the electrolyte solution system, while the respective oxidative stability distinctly decreases with the addition of DME into the electrolyte solvent. A decreased oxidative stability is considered to induce parasitic reactions at the positive electrode, which presumably can be affected by change of positive electrode material. Despite the differences in conductivity, battery cell experiments present LiBF4 as a comparable electrolyte salt to LiPF6, with a performance difference that could possibly be overcome by utilization of the right electrolyte additive(s). The investigated oligoether containing electrolyte solutions show inferior battery performance compared to the unmodified carbonate-based electrolyte solvent. Since the battery cycling experiments were prepared at the oxidative stability limit of these electrolyte solutions, these electrolyte solutions might still create promising battery cell performance with a change of positive electrode material. In order to further investigate the ion pair association of lithium cations and tetrafluoroborate anions, the utilized system was additionally investigated spectroscopically and with quantum chemical calculations on the basis of density functional theory. Spectroscopic characterization was carried out with help of stimulated spin echo experiments in nuclear magnetic spectroscopy and Raman spectroscopy measurements. Both spectroscopies present changes in the associated structure of lithium cation and tetrafluoroborate anions by addition of DME to the commercial electrolyte solvent. For further conclusions, these results were compared with the calculations prepared for the electrolyte solution system. In summary, spectroscopic characterization and quantum chemical calculations indicate a modification of the associated structures in solution by the addition of DME to the chosen electrolyte solvent. By comparison of all collected results, research concerning the application of LiBF4 electrolytes should be continued with adaption of electrolyte solution additives or positive electrode material, respectively.[1] C. Daniel, J. O. Besenhard (Hrsg.) Handbook of battery materials, Wiley-VCH-Verl., Weinheim, 2011.[2] M. Winter, R. J. Brodd, Chem. Rev. 2004, 104, 4245.[3] R. Korthauer (Hrsg.) Handbuch Lithium-Ionen-Batterien, Springer Vieweg, Berlin, Heidelberg, 2013.[4] H. Tsunekawa, A. Narumi, M. Sano, A. Hiwara, M. Fujita, H. Yokoyama, J. Phys. Chem. B 2003, 107, 10962.[5] K. Yoshida, M. Nakamura, Y. Kazue, N. Tachikawa, S. Tsuzuki, S. Seki, K. Dokko, M. Watanabe, J. Am. Chem. Soc. 2011, 133, 13121. Figure 1