Enhancing Li+ Transference Number by the Formation of Clusters with Heterogeneous Li+ Coordination
Anne HOCKMANN1, Diddo DIDDENS2, Isidora CEKIC-LASKOVIC2, Monika SCHÖNHOFF1
1University of Münster, Münster, Germany
2Helmholtz Institute Münster (IEK-12), Forschungszentrum Jülich GmbH, Münster, Germany
In terms of developing energy storage systems with energy density higher than the state-of-the-art lithium ion batteries, lithium sulfur batteries stand for a promising alternative. Nonetheless, they suffer from safety issues and capacity fading due to lithium dendrite growth and polysulfide dissolution. To overcome these drawbacks, Suo et al. developed an electrolyte with a conducting salt/solvent ratio > 1 (by volume and/or by weight), known as solvent-in-salt (SiS) electrolyte. This SiS electrolyte reduces the lithium dendrite growth and polysulfide dissolution leading to a high coulombic efficiency and long cycle life of a battery. Moreover, they reported a high Li+ transport number of 0.7.[1,2] However, to explain fast Li+ transport, the underlying structure, composition, and dynamics of this electrolyte formulation has to be investigated and understood.
In this study, the conducting salt concentration in an electrolyte consisting of lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI), 1,2-dimethoxyethane (DME), and 1,3-dioxolane (DOL) was systematically increased up to the solvent-in-salt regime. In order to explain the increasing transport number reported by Suo et al., NMR and Raman spectroscopy analysis was conducted and complemented by molecular dynamics (MD) simulations. By monitoring the changes in the chemical shift of the NMR signals as well as the changes in the Raman bands of the anion and solvent molecules, a basic stochiometric model of the Li+ coordination shell was introduced and confirmed by the MD simulation results. Using this model and additional results obtained by MD simulations, a deepened understanding about the structure and dynamics in this SiS electrolyte formulation was gained thus providing possible explanation for the high transport number of Li+ in the SiS regime.
References
[1] L. Suo, YS. Hu,H. Li, M. Armand, L. Chen, Nat Commun 4 (2013), 1-9.
[2] V. A. Azov, K. S. Egorova, M. M. Seitkalieeva, A. S. Kashin, V. P. Ananikov, Chem. Soc. Rev. 47 (2018), 1250-1284.