Mapping Distribution of Modern Binders in Graphitic Li-Ion Electrodes Enables Optimization of Electrodes Structure

Despite their low fraction (&lt;5 % volume) in Li-ion electrodes, polymeric binders are a critical component of Li-ion batteries. Binders provide cohesion and adhesion of electrode particles to one another and to current collectors, and stabilize active particles against volume changes upon cycling. But binders also impede transport of Li⁺ in the electrode pore space and, therefore, co-determine overall battery rate performance and lifetime. During high rate drying of cast electrodes, re-precipitated binder is prone to migrate from the current collector (where needed most) towards the electrode free surface (adding to ionic resistance). Consequently, slow electrode drying to maintain optimal binder distribution can become the bottleneck to high throughput, low-cost electrode fabrication. Up to now, however, practical binder optimization studies in negative Li-ion electrodes have been hindered by the lack of a convenient method to map the aqueous-processable binder used in most modern graphite electrodes that comprises a mixture of sodium-carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).<br/><br/>We have developed two methods of independently labelling CMC and SBR using aqueous Cu²⁺ ions and gaseous Br₂ vapour, respectively, and tracing their distribution in electrodes using energy dispersive X-ray spectroscopy (EDX). Immersing electrodes in aqueous Cu²⁺ solution results in instant formation of an insoluble Cu-CMC complex, while exposing the electrodes to Br₂ vapour results in rapid bromination of SBR C=C bonds. Owing to the specificity of reactions, both binder components can be precisely mapped using EDX for copper and bromine. Additionally, bromination allows rapid (few seconds) detection of binder using backscattered electron imaging for quick assessment and feedback of binder distribution. Enabled by these new labelling techniques, we present two exemplary studies on binder agglomeration from sub-optimally mixed slurries, and on mitigating binder migration during fast electrode drying using a novel phase inversion approach. We show how improved CMC/SBR binder control can enhance the electrochemical performance of negative Li-ion electrodes.