In recent years high-concentration liquid electrolytes (HCEs) received much attention in the battery research due to the evident performance advancement achieved in lithium-based batteries. HCEs offer lots of advantages comprising, among others, broad electrochemical stability window, current collector protection, high safety, an effective solid electrolyte interface (SEI) formation, and therefore a high rate capability and stable galvanostatic cycling.[1] However, due to the high conducting salt content, HCEs are very viscous thus resulting in low ionic conductivity. This is why the concept of localized high-concentration electrolytes (LHCEs) was developed and first published by Chen et al. in 2018.[2] Here, a third component, the diluent, is added to the system to reduce the viscosity without participating in the ion solvation. Thus, the local environment of the ions, which is responsible for the described benefits of a HCE, remains unchanged and the main disadvantage, the low ionic conductivity, is improved.

However, the structural heterogeneity of the LHCEs and the complexity of their composition require further research to gain a profound understanding of the structure and ion dynamics in order to reach the full potential of this type of electrolyte formulation. Therefore, in this work a LHCE consisting of the conducting salt lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI), the solvent 1,2-dimethoxyethane (DME), and the diluent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was investigated by systematically varying the molar ratios of the three components in order to gain information about the structure and lithium dynamics in the formulation. A combination of selected complementary spectroscopic methods, namely NMR, Raman, and impedance spectroscopy was used to investigated the electrolytes. To further analyze the influence of the anion on the solvation, the obtained results were compared to an electrolyte formulation containing lithium bis(fluorosulfonyl)imide (LiFSI) as the conducting salt. It was shown that this small variation already has a significant impact on the structure of the salt/diluent-interface influencing the lithium coordination and dynamics in the electrolyte formulation.

References
[1] Y. Yamada, J. Wang, S. Ko, E. Watanabe, A. Yamada, Nat Energy 4 (2019), 269–280.
[2] S. Chen, J. Zheng, D. Mei, K. S. Han, M. H. Engelhard, W. Zhao, W. Xu, J. Liu, J.-G. Zhang, Adv. Mater. 30 (2018), 1706102.