Model-Assisted 4-Electrode Cell Design for Li-Based Electrolyte Characterization

With the emergence of high power Li-ion batteries for electric vehicle (EV) and hybrid electric vehicle (HEV) applications, the need to estimate the transport properties of battery electrolytes becomes evident. The poor transport properties (e.g., diffusivity, ionic conductivity) of non-aqueous electrolytes become a limiting factor, especially when high currents are to be drawn from the battery. Various methods to measure the transport properties of electrolytes have been suggested. To date, most methods involve assembling a 2-electrode symmetric cell with the liquid electrolyte confined between two identical metallic electrodes, applying current pulses for a certain period of time and recording the relaxation of the voltage when the circuit is open until it reaches zero.¹ However, due to the complicated, not well-understood kinetics of electro-deposition/-stripping reactions at the surface of the metallic electrodes, it is not possible to directly relate the cell voltage evolution during the current pulse (closed circuit) or at the onset of the relaxation step (open circuit) to the variations of electrolyte concentration/potential across the cell. This leads to the loss of useful information that might be gained from these experiments and a larger measurement error. Moreover, the constraint of working under open-circuit conditions restricts the characterization to fewer methods including the restricted-diffusion and semi-infinite-diffusion galvanostatic polarization techniques.

In the current work, the possibility of estimating electrolyte transport properties using a 4-electrode symmetric cell is examined. The proposed cell has cylindrical geometry with the working/counter electrodes located at the two ends of the cell and the reference electrodes placed on the inner wall midway between the working/counter electrodes (Fig.1). All electrodes are Li metal foils. Current is applied between the working/counter electrodes and the potential difference between the reference electrodes is recorded. This arrangement enables the response of the cell during both the open-circuit and closed circuit portions of the experiment to be exploited for measurement of the electrolyte transport properties. Since the reference electrodes are not connected to a sink/source of current, the net charge transferred at their interfaces with the electrolyte is zero. However, because of the non-zero width of the reference electrodes lying along the cell, a bipolar effect occurs whereby anodic and cathodic reactions occur simultaneously at different locations along the electrodes depending on the electrolyte potential profile across their width.² Because of the bipolar effect, the reference electrodes locally perturb the concentration and potential of the electrolyte which in turn affects the measured potential difference. We have developed a model based on concentrated-solution theory³ that includes this bipolar effect in order to quantitatively describe the operation of this cell.

In this presentation, we present simulation results for various operating conditions and examine design parameters such as the cell aspect ratio, reference electrode width and spacing for known physico-chemical properties of a binary Li-based electrolyte. Guidelines for assembling such a cell and performing characterization experiments are provided. With further experimental validation of this concept and design, the 4-electrode cell can be extended to include an array of multiple reference electrodes and provide a wealth of knowledge about the potential and species concentration profiles across the cell.

Figure 1: Schematic view of a cylindrical symmetric cell with two reference electrodes located between the working and counter electrodes.

References

1.   A. Nyman, M. Behm and G. Lindbergh, Electrochim. Acta, 53:6356–6365, 2008.

2.   G. Loget, D. Zigah, L. Bouffier, N. Sojic and A. Kuhn, Acc. Chem. Res., 46:2513–2523, 2013.

3.   J. S. Newman and K. E. Thomas-Alyea. Electrochemical Systems. Wiley Interscience, 2004.