Examining Solvent Motion in Polarized Concentrated Electrolytes

Abstract

Historically, ion transport in electrolytes has been described in terms of current – potential response. This behavior is often explained using macroscopic transport properties such as ionic conductivity, transference number, and salt diffusivity.1,2 In contrast, recent X-ray3 and magnetic resonance4 techniques capture ion motions at smaller yet continuum scales. Interestingly, some of these measurements observe the directed motion of solvent in polarized electrolytes, while traditionally solvent is assumed to be a stationary species. Driven by this mismatch of two interpretations, we revisit Newman’s concentrated solution theory1,2,5–8 to examine the role of solvent motion in polarized electrolytes (Figure 1(a)). We use LiTFSI in PEO as an exemplar electrolyte since it is one of the few electrolytes with a full set of transport properties measured previously.9 We find that as the electrolyte is polarized, salt concentration gradients can create a thermodynamic drive for the solvent to travel against the salt motion. Time evolution of voltage (a macroscopic quantity), as well as velocity fields for each species (Figure 1(b)), are examined. We study polarizations at various currents and concentrations to identify when and how the solvent motion contributes to electrolyte response. We find that the solvent, even if it is a charge-neutral species, moves in the direction of ionic current. Apart from these new insights into electrolyte transport dynamics, the results help rationalize recent direct measurements of ion fluxes.Figure 1. (a) A schematic showing physical situation of interest. x=0 is the left interface between Li electrode and electrolyte, while x=L is the right interface. As time passes by, the electrolyte fields evolve as well as the interfaces move equally in the opposite direction of current density, iapp. (b) Example distribution of cation, anion, and solvent velocities in the polarized electrolyte. Directions of these velocities are shown when current flows along the positive coordinate. In this scenario, ions pile up close to the left electrode, while they deplete near the right electrode.(1) Newman, J.; Thomas-Alyea, K. E. Electrochemical Systems; John Wiley & Sons, 2012.(2) Newman, J.; Bennion, D.; Tobias, C. W. Mass Transfer in Concentrated Binary Electrolytes. Berichte der Bunsengesellschaft für Phys. Chemie 1965, 69 (7), 608–612. https://doi.org/10.1002/bbpc.19650690712.(3) Steinrück, H. G.; Takacs, C. J.; Kim, H. K.; MacKanic, D. G.; Holladay, B.; Cao, C.; Narayanan, S.; Dufresne, E. M.; Chushkin, Y.; Ruta, B. et al. Concentration and Velocity Profiles in a Polymeric Lithium-Ion Battery Electrolyte. Energy Environ. Sci. 2020, 13 (11), 4312–4321. https://doi.org/10.1039/d0ee02193h.(4) Bazak, J. D.; Allen, J. P.; Krachkovskiy, S. A.; Goward, G. R. Mapping of Lithium-Ion Battery Electrolyte Transport Properties and Limiting Currents with In Situ MRI. J. Electrochem. Soc. 2020, 167 (14), 140518. https://doi.org/10.1149/1945-7111/abc0c9.(5) Liu, J.; Monroe, C. W. Solute-Volume Effects in Electrolyte Transport. Electrochim. Acta 2014, 135, 447–460. https://doi.org/10.1016/j.electacta.2014.05.009.(6) Dees, D.; Gunen, E.; Abraham, D.; Jansen, A.; Prakash, J. Alternating Current Impedance Electrochemical Modeling of Lithium-Ion Positive Electrodes. J. Electrochem. Soc. 2005, 152 (7), A1409. https://doi.org/10.1149/1.1928169.(7) Pollard, R.; Comte, T. Determination of Transport Properties for Solid Electrolytes from the Impedance of Thin Layer Cells. J. Electrochem. Soc. 1989, 136 (12), 3734–3748. https://doi.org/10.1149/1.2096540.(8) Nyman, A.; Behm, M.; Lindbergh, G. Electrochemical Characterisation and Modelling of the Mass Transport Phenomena in LiPF6-EC-EMC Electrolyte. Electrochim. Acta 2008, 53 (22), 6356–6365. https://doi.org/10.1016/j.electacta.2008.04.023.(9) Villaluenga, I.; Pesko, D. M.; Timachova, K.; Feng, Z.; Newman, J.; Srinivasan, V.; Balsara, N. P. Negative Stefan-Maxwell Diffusion Coefficients and Complete Electrochemical Transport Characterization of Homopolymer and Block Copolymer Electrolytes. J. Electrochem. Soc. 2018, 165 (11), A2766–A2773. https://doi.org/10.1149/2.0641811jes. Figure 1