Do Binders Form Surface Films on Active Particles in Li-Ion Electrodes? Revealing Nanoscopic Surface Binder Morphology in Graphitic Electrodes Using Chemical Staining and High-Resolution Energy-Selective Backscattered Electron Imaging
Authors: Zankowski Stanislaw , Grant Patrick S. 
Published: December 2024

Although polymeric binders constitute only a small fraction (< 5% volume) of Li-ion electrodes, they are an essential component of Li-ion batteries, binding otherwise loose active particles to each other and to the current collector. Yet, despite more than 30 years of development of Li-ion batteries, the exact morphology that binders adopt on and around the active particle surfaces in the electrodes remains extremely difficult to assess, similarly to the fraction of active particle surface covered by the binder. It is already well established that binder chemistry strongly influences key battery surface reactions, such as Li insertion/deinsertion or formation of solid electrolyte interface (SEI).1,2 Also, binder chemistry can be a decisive factor in reaching long cycle stability over rapid failure of, for example, silicon-based electrodes.3 Therefore, quantifying surface coverage of the binders on active particles remains the missing piece for fully understanding binder roles in batteries, and can open new avenues for improving lifetime and rate performance of current and next generation electrodes.

In this work, we present a newly established method for large-scale, high-resolution imaging and quantification of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes. The imaging was enabled using highly selective chemical staining of the binders with high atomic number elements that act as excellent contrast agents during backscattered electron (BSE) imaging. Following staining of the binders, we imaged them using a variant of BSE known as energy-selective backscattered electron imaging (EsB), which operates at low beam voltage and can provide nanometer-level resolution by selective energy filtering of backscattered electrons. By acquiring images at different beam voltages, we selectively captured different fine-scale morphological details of the binder on and around the graphite particles. We revealed binder/carbon additive agglomerates, islands of non-dispersed SBR nanoparticles and, for the first time, details of a continuous thin layer formed by CMC and SBR on the graphite surface that spans across the entire graphitic electrode. Using a combination of Monte Carlo simulations and high resolution cross-sectional EsB imaging, we estimated the thickness of this surface film to be only 10 nanometers. Because of such small thickness, the film adopts the morphology of the underlying graphite surface, which is why it cannot be observed using conventional secondary electron imaging, even at very low beam voltages. Using quantitative image analysis, we also characterized electrodes before and after calendering, finding that calendering shatters the previously-continuous binder film, leaving only 30% of graphitic surfaces covered by irregular patches of the binder layer. We also verified our results by analyzing commercial graphitic electrodes used in electric vehicle batteries, finding identical binder morphologies inside these electrodes.

Considering the likely impact of binder coverage on e.g. active particle stability, SEI formation or distribution of local current density during electrochemical cycling, we suggest that this new ability to rapidly image and quantify binder coverage on active particle surfaces will open new avenues to optimize Li-ion electrodes, bridging the gaps between the chemistry, processing and performance of binder-active particle interfaces.

(1) Fedorova, A. A.; Levin, O. V.; Eliseeva, S. N.; Katrašnik, T.; Anishchenko, D. V. Investigating the Coating Effect on Charge Transfer Mechanisms in Composite Electrodes for Lithium-Ion Batteries. Int. J. Mol. Sci.
2023, 24.

(2) Young, B. T.; Nguyen, C. C.; Lobach, A.; Heskett, D. R.; Woicik, J. C.; Lucht, B. L. Role of Binders in Solid Electrolyte Interphase Formation in Lithium Ion Batteries Studied with Hard X-Ray Photoelectron Spectroscopy. J. Mater. Res.
2019, 34, 97–106.

(3) Jung, C. H.; Kim, K. H.; Hong, S. H. Stable Silicon Anode for Lithium-Ion Batteries through Covalent Bond Formation with a Binder via Esterification. ACS Appl. Mater. Interfaces
2019, 11, 26753–26763.

Figure 1. Combined secondary electron image (left) and color-enhanced EsB image (right) of an internal part of uncalendered graphitic electrode with chemically stained CMC binder (colored green in the EsB image).

Figure 1