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Bryan McCloskeyDept. of Chemical and Biomolecular EngineeringUniv. of CA – BerkeleyEnergy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory
Conventional Li-ion battery electrolytes have been designed to optimize numerous desirable properties, including interfacial and thermal stability, conductivity, and low flammability. However, all Li+-bearing electrolytes still possess low Li+ transference (t+) numbers, where current passed through them is primarily carried by the counteranion, resulting in large concentration gradients that limit battery performance, particularly at high discharge and charging rates. The development of high t+ electrolytes—those in which most (or all) current is carried by the Li+ ion—could enable safer battery cycling, faster charging rates, and thicker, more energy-dense cathode designs in Li-ion batteries.
This presentation will outline our attempts to develop polymer-based high t+ electrolytes wherein anions are appended to a polymer (polyanions) and neutralized with Li ions. In this configuration, Li ions, when appropriately solvated, have hydrodynamic radii much smaller than the polymer’s radius of gyration, ostensibly allowing them to diffuse or migrate faster than their appended counteranions, and hence enable high t+ electrolytes. Ultimately, I show this picture to be oversimplified, and that anion-anion and cation-anion correlations severely limit the t+ of high conductivity polyanionic solutions.
Initially, I will present studies that highlight the inherent tradeoff between segmental motion and ion content that has generally limited room temperature conductivity of dry polymer electrolyte systems. We show that blending of a small molecule diluent breaks the apparent tradeoff between segmental dynamics and charge carrier concentration in dry polymer electrolytes, motivating the pursuit of polyelectrolyte solutions (charged polymers dissolved in solvent) as potential high t+ electrolytes. In the second part of the presentation, I discuss our efforts to understand and characterize the full transport properties of a model, monodisperse, battery-relevant oligomeric polyelectrolyte system. Intriguingly, for polyelectrolytes with larger molecular weights, we observe that the average velocity of Li ions in an electric field is in the direction opposite than expected, evidence of a negative t+ due to correlated motion through Li ion condensation on the polyelectrolyte chain. While this result is a negative outcome in our pursuit of high t+ electrolytes, we anticipate many of the capabilities and insights gleaned throughout our studies to be of general use to those studying novel, concentrated electrolyte solutions where strong, non-ideal ion-ion correlations are observed.