UC Berkeley

Imposing a steady ionic current through an electrolyte results in the formation of salt concentration gradients that compromise battery performance. The limiting current is usually defined as the current at which the salt concentration at the cathode approaches zero. Higher currents cannot be imposed on the cell as larger concentration gradients are unsustainable. We study the limiting current in electrolytes comprising a perfluorinated oligomer, C8-DMC, and lithium bis(fluorosulfonyl)imide salt in symmetric lithium cells. The time-dependence of the potential, which increases as salt concentration gradients develop, was also measured. Both steady-state and transient behaviors are modeled using Newman's concentrated solution theory; transport and thermodynamic parameters needed to perform the calculations were measured independently. The limiting current is a nonmonotonic function of salt concentration in both theory and experiment. The model shows that at low salt concentrations (below 0.88 mol/kg solvent), the concentration at the cathode approaches zero at limiting current. In contrast, at high salt concentrations (above 0.88 mol/kg solvent), the concentration at the anode approaches the solubility limit (2.03 mol/kg solvent). The experimentally determined salt concentration at which the limiting current is maximized is in excellent agreement with theoretical predictions made without resorting to any adjustable parameters.