Investigating the Influence of Polymer Binders on Liquid Phase Transport and Tortuosity Through Lithium-Ion Electrodes

Abstract

Quantifying the tortuosity of porous lithium-ion electrodes is important for understanding the rate capability of cells and for optimizing their design, particularly when designing high energy density cells such as those desired for electric vehicles. However, quantifying tortuosity may be difficult, and results often disagree with the commonly used Bruggeman relation. Here, we discuss the observation that PVDF binder, a polymer used to mechanically hold the electrode together, has a direct effect on the rate capability of NMC111 cathodes. Using a pseudo-two-dimensional (P2D) physics-based model of the system, we fit the electrode tortuosity to the cycling data and determine that increased binder volume fraction in an electrode leads to increased electrode tortuosity. Using a TiO2-based blocking electrode, we further support these findings using electrochemical impedance spectroscopy (EIS) measurements and transmission line models. Finally, using pulsed field gradient nuclear magnetic resonance (PFG-NMR) experiments on these blocking electrodes, we propose a mechanism involving liquid phase Li ion “choke-points,” formed by the addition of PVDF binder, which dominates electrode tortuosity. We provide an empirically derived relationship that serves as a binder volume correction to the Bruggeman relation, and this finding motivates further work on the impact of different electrode components on transport through porous electrodes.