Characterizing Ion Transport in Non-Aqueous Electrolyte Solutions for Li-Ion and Li- Metal Batteries
Authors: Bergstrom Helen K , Fong Kara D. , McCloskey Bryan D. 
Published: December 2020

A majority of ionic conductivity in current state-of-the-art liquid electrolytes results from the movement of the anion rather than the electrochemically active Li+ ion. Passing current through such an electrolyte can cause severe ionic concentration gradients to extend into porous electrodes, which at high currents can reduce active material utilization and result in high overpotentials that cause deleterious side reactions to occur (e.g., Li plating on graphite electrodes). 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. Non-aqueous polyelectrolyte solutions (PESs) are a promising, but underexplored, route to high transference number electrolytes that demonstrate potential for both high ionic conductivity and t+. This combination is achieved by anchoring the anion to a polymer backbone – allowing for high t+ by slowing down the motion of the electrochemically inactive anion – while maintaining high ion conductivity through improved ion dissociation and solvent-mediated Li+ transport. New measurement techniques are necessary to rigorously understand ion-transport and structure-property relationships in this new class of electrolytes.

Rigorously measuring the complete set of transport properties of concentrated liquid electrolytes under battery-relevant operating conditions is difficult and sparsely explored 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, in order to study many different polymer concentrations and molecular weights. Major challenges limiting full ion transport characterization in liquid systems include significant 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 presentation, we will discuss efforts to understand and address these challenges. Using techniques amenable to small electrolyte volumes, we rigorously measured liquid electrolyte transport properties, including the ideal solution Bruce-Vincent transference number, total salt diffusion coefficients, activity coefficients, and the true transference number under battery-relevant conditions using Newman’s concentrated solution theory framework. While our long-term objective is to fully characterize Li-triflimide-appended 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, we found that transport coefficients obtained via these measurements were consistently not in agreement with those reported in the literature and prone to significant variation depending on data fitting methodology. We believe the discrepancy can largely be attributed to parasitic corrosion reactions and large interfacial resistances at the lithium electrodes, which appear to dominate at the low current densities necessary to maintain sufficiently small concentration gradients. These experimental findings are supported by COMSOL Multiphysics modeling studies. Additional experimental 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 solid polymer electrolytes. These results have important implications across the electrolyte engineering field that often relies on similar tests to screen electrolyte candidates.