The viability of next generation lithium and beyond-lithium battery technologies hinges on the development of electrolytes with improved performance. Comparing electrolytes is not straightforward, as multiple electrochemical parameters affect the performance of an electrolyte.Additional complications arise due to the formation of concentration gradients in response to dc potentials. We propose a modified version of Ohm's law to analyze current through binary electrolytes driven by a small dc potential. We show that the proportionality constant in Ohm's law is given by the product of the ionic conductivity, κ, and the ratio of currents in the presence (i ss ) and absence (i Ω ) of concentration gradients, ρ +¿ ¿ . The importance of ρ +¿ ¿ was recognized by J. Evans, C.A. Vincent, and P.G. Bruce [Polymer 28, 2324[Polymer 28, (1987]. The product κ ρ +¿ ¿ is used to rank order a collection of electrolytes. Ideally, both κ and ρ +¿ ¿ should be maximized, but we observe a trade-off between these two parameters, resulting in an upper bound. This trade-off is analogous to the famous Robeson upper bound for permeability and selectivity in gas separation membranes. Designing polymer electrolytes that overcome this trade-off is a worthwhile but ambitious goal.
The limiting current is an important transport property of an electrolyte as it provides an upper bound on how fast a cell can be charged or discharged. We have measured the limiting current in lithium-lithium symmetric cells with a standard polymer electrolyte, a mixture of poly(ethylene oxide) and lithium bis(trifluoromethane) sulfonamide salt at 90°C. The cells were polarized with increasing current density. The steady-state cell potential was a smooth function of current density until the limiting current was exceeded. An abrupt increase in cell potential was taken as an experimental signature of the limiting current. The electrolyte mixture was fully characterized using electrochemical methods to determine the conductivity, salt diffusion coefficient, cation transference number, and thermodynamic factor as a function of salt concentration. We used Newman's concentrated solution theory to predict both cell potential and salt concentration profiles as functions of position in the cell at the experimentally applied current density. The theoretical limiting current was taken to be the current at which the calculated salt concentration at the cathode was zero. We see quantitative agreement between experimental measurements and theoretical predictions for the limiting current. This agreement is obtained without resorting to any adjustable parameters.
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