(637h) Characterizing Ion Transport in Non-Aqueous Electrolyte Solutions for Li Metal and Li-Ion Batteries

Ionic conductivity in current state-of-the-art liquid electrolytes is primarily driven by the movement of the anion rather than the electrochemically active Li⁺ ion. The ionic concentration gradients that result from passing current through such an electrolyte extend into the porous electrodes and generate overpotentials that limit the rate and efficiency of charging as well as increase the risk of short-circuiting through Li plating and dendrite growth. The severity of these concentration gradients is directly influenced by the transference number (t₊), which describes the ratio of current carried by the electrochemically active Li⁺ ion to the total current passed. Rigorously measuring the complete set of transport properties of concentrated liquid electrolytes under battery-relevant operating conditions is difficult and is largely unexplored in experimental liquid electrolyte literature. Complete transport characterization is particularly challenging for novel electrolytes and additives where cost or synthetic complexity limit the volume of available electrolyte. In particular, small volume characterization is necessary for polyelectrolyte solutions, where Li⁺-neutralized polyanions are used as the salt in a liquid solvent, in order to study many different polymer concentrations and molecular weights. Major challenges limiting full ion transport characterization in liquid systems include significant reactivity and corrosion of Li metal in the presence of electrolyte solvents, large and unstable interfacial impedance, and difficulty in establishing sufficiently small concentration gradients to ensure measurement validity while maintaining high signal to noise ratio data.

In this work, we will discuss efforts to understand and address these challenges. We rigorously measured liquid electrolyte transport properties, including ideal solution t₊, total salt diffusion coefficients, activity coefficients, and the true t₊ under battery-relevant conditions using Newman’s concentrated solution theory framework. While our long-term objective is to characterize polyelectrolyte solutions, we initially attempt to develop the methodology on the well-studied liquid electrolyte system of LiPF₆ in an ethylene carbonate:ethyl methyl carbonate blend. Using rigorous statistical analysis and propagation of error methods, we found that transport coefficients obtained via these measurements were consistently not in agreement with those reported in the literature. We believe the discrepancy can largely be attributed to parasitic corrosion reactions at the lithium electrodes, which appear to dominate at the low current densities necessary to maintain sufficiently small concentration gradients. Additional studies incorporating stabilizing additives indicate that simply improving Li stripping/plating coulombic efficiency does not allow for accurate transport coefficient measurements. This reveals a major roadblock in characterizing liquid electrolyte systems, as methods that rely on Li metal stripping/plating do not readily result in reliable liquid electrolyte transport coefficients, unlike similar methods for polymer electrolytes. These results have important implications for the electrolyte engineering field that often relies on similar tests to screen electrolyte candidates.