Concentration and Velocity Profiles in a Polymeric Lithium-Ion Battery Electrolyte
Authors: Steinrück Hans-Georg , Takacs Christopher J , Kim Hong-Keun , Mackanic David , Holladay Benjamin , Cao Chuntian , Narayanan Suresh , Dufresne Eric , Chushkin Yuriy , Ruta Beatrice , Zontone Federico , Will Johannes , Borodin Oleg , Sinha Sunil , Srinivasan Venkat , Toney Michael F 
Published: December 2020

Predictive and quantitative understanding of ion and mass transport in liquid and polymeric electrolytes is at the heart of electrochemistry. Here, the goal is to accurately simulate the performance of an electrochemical cell, for which the temperature- and concentration-dependent transport coefficients of a given electrolyte, as well as the thermodynamic mean salt molal activity coefficient, must be known. These are the conductivity, σ, the salt diffusion coefficient, D, and the cation transference number, t+. The transport coefficients must hence be extracted with high accuracy from experimental measurements. The transference number is defined as the ratio of current carried by the cation to the total electric current. Its importance with regards to energy density and power density was recognized in the early 1990s by Doyle, Fuller and Newman (1). Nevertheless, researchers still argue about transference number values, even in baseline systems such as lithium bistrifluoromethanesulfonimidate (LiTFSI) in Poly(ethylene oxide) (PEO) (2). The transference number can be measured via the steady-state Bruce-Vincent polarization method (3, 4), the Balsara-Newman method (5), as well as pulsed field gradients NMR (pfg-NMR) (6) and pulsed field gradients electrophoretic NMR (e-NMR) (2). Despite extensive efforts towards unified results of these approaches, in particular as a function of ion concentration, a clear picture has not yet emerged.

Towards this end, we developed an alternative approach towards determining the ion transport properties. Specifically, we directly and operando measured precise microscopic and macroscopic physical properties of the electrolyte upon cell polarization in a Li/electrolyte/Li cell, combined this with calculations via concentrated solution theory continuum modelling (CM), and rationalized our findings with microscopic insight from molecular dynamics (MD) simulations. We utilized a well-studied benchmark model system electrolyte consisting of PEO and LiTFSI at Li+ to EO molar ratio of r = 0.1. Under constant voltage polarization, we directly measured the velocity associated with electrolyte and ions via heterodyne synchrotron X-ray photon correlation spectroscopy (XPCS), and the TFSI- concentration gradient from electrode to electrode via X-ray absorption microscopy (XAM). The significance of our results lies in the unification of microscopic and macroscopic predictions from simulation with experimental measurements as well as the self-consistent determination of a concentration-independent transference numbers of approximately 0.2. Our study paves the way for further length- and time-scale bridging understanding of ion transport.

1. M. Doyle, T. F. Fuller, J. Newman, The Importance of the Lithium Ion Transference Number in Lithium Polymer Cells. Electrochimica Acta 39, 2073-2081 (1994).

2. M. P. Rosenwinkel, M. Schönhoff, Lithium Transference Numbers in PEO/LiTFSA Electrolytes Determined by Electrophoretic NMR. Journal of The Electrochemical Society 166, A1977-A1983 (2019).

3. J. Evans, C. A. Vincent, P. G. Bruce, Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. Polymer 28, 2324-2328 (1987).

4. P. G. BRUCE, C. A. VINCENT, STEADY STATE CURRENT FLOW IN SOLID BINARY ELECTROLYTE CELLS. J. Electroanal. Chem. 225, 1-17 (1987).

5. D. M. Pesko, S. Sawhney, J. Newman, N. P. Balsara, Comparing Two Electrochemical Approaches for Measuring Transference Numbers in Concentrated Electrolytes. Journal of The Electrochemical Society 165, A3014-A3021 (2018).

6. D. M. Pesko et al., Negative transference numbers in poly(ethylene oxide)-based electrolytes. Journal of the Electrochemical Society 164, E3569-E3575 (2017).