Time-Resolved Concentration Profiling of LiPF₆ in EC:EMC Electrolytes in Lithium Ion Batteries

Liquid phase electrolyte lithium-ion batteries (LIBs) typically consist of a lithium salt dissolved in a mixture of cyclic and linear alkyl carbonates. Continuum models typically treat the electrolyte co-solvents as a single variable, dubbed the "single-solvent approximation" . However, recent work in the area has called the validity of this asumption into question [1]. Herein, we develop a new continuum model to describe the dynamics of multicomponent electrolyte solutions and directly compare model predictions with experimentally measured concentration profiles which can be obtained using potentiometric magnetic resonance imaging (MRI) [2] and Raman spectroscopy [3] to investigate the validity of this significant assumption. The developed model is applied to a 1M LP57 type electrolyte (1 M LiPF6 in EC:EMC v%:v% 3:7) to better understand the salt and solvent dynamics in commercial LIB electrolytes. This demonstrates that EC and EMC show distinct concentration gradients, challenging the validity of the "single-solvent approximation" which has been widely used to this point in LIB modelling. The developed model was parameterized solely using classical MD simulations for a 1M LP57 type electrolyte. The effects of electrolyte solvation structures are manifested on the continuum scale. Most notably effects arising from Li⁺ ion solvation by EC and EMC molecules are observed. A dragging effect is observed on solvent molecules due to the motion of the Li⁺ ions they solvate, which influences the concentration gradient of the solvent molecules under an applied current. The concentration gradients of both the solvent molecules and salt ions influence the composition of the solid-electrolyte interphases that form in LIBs, making the understanding of how these gradients change under applied currents of crucial importance. These results have provided fundamental insight into how the solvation structure of commercial LP57 electrolytes affects the transport properties and concentration gradients of the electrolyte components, which will inform the development of improved electrolytes for LIB applications.<br/><br/>[1] Andrew A. Wang, Samuel Greenbank, Guanchen Li, David A. Howey, Charles W. Monroe, Current-driven solvent segregation in lithium-ion electrolytes, Cell Reports Physical Science, Volume 3, Issue 9, 2022, 101047, ISSN 2666-3864,<br/>[2] ACS Energy Lett. 2021, 6, 9, 3086–3095 Publication Date:August 15, 2021<br/>[3] Fawdon, J., Ihli, J., Mantia, F.L. et al. Characterising lithium-ion electrolytes via operando Raman microspectroscopy. Nat Commun 12, 4053 (2021). https://doi.org/10.1038/s41467-021-24297-0