Improved Electrode Processing and Uniformity with Discrete Carbon Nanotube Aqueous Binder Additives
Authors: Meyers Jeremy P , Peddini Sateesh , Jungman Matthew , Finlayson Malcolm , Bhat Vinay , Gazda Jerzy , Bosnyak Clive 
Published: February 2020

The US Department of Energy has set a goal of doubling the energy density of batteries for vehicle applications by 2022, to spur innovation towards extended battery run time and electric vehicle range, and to reduce cost of cells to levels where economic factors favor mass adoption of electrified vehicles[1]. Novel high-performance active materials can be used to improve energy density, but provide a unique set of challenges, such as accommodating high volume changes associated with the insertion process or matching insertion potentials to the stability of acceptable electrolytes. One of the most straightforward ways to improve energy density is to manufacture thicker electrodes still capable of high active material utilization and robust to high-power charge and discharge. This requires careful optimization of the microstructure of the finished electrode, excellent adhesion to the current collector, and the possibility of creating defect-free structures as the electrodes are processed[2].

In order to make a reliable electrode with acceptable power and energy density, the active materials and electrolyte must be paired with an appropriate binder system. In traditional Li-ion cells, PVDF in NMP has been used in the electrode slurry-making processes. To ease processing requirements, cost and environmental issues, manufacturers are gradually moving away from PVDF and using aqueous base materials such as carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)[3-4]. Molecular Rebar®, which are discrete carbon nanotubes made by Molecular Rebar Design, LLC, are a good candidate for additives to improve mechanical properties and transport properties in the finished electrodes.

While carbon nanotubes have been used in a variety of composite mesostructured applications, the production of nanotubes at the industrial scale frequently result in bundles of tubes that behave more like weak-porous-stress concentration centers than reinforcing components in the composite. Black Diamond Structures is commercializing products based on Molecular Rebar®, a technology platform produced with industrial-scale process that disentangles and functionalizes stock carbon nanotubes and opens the ends of the tubes. The process also cleans the residual carbon and the residual catalyst contamination. These tubes that are individualized and permanently dispersed in liquid medium, can then be added to lithium electrode slurries as, for example, an aqueous dispersion appropriate for meeting requirements of existing roll-to-roll manufacturing process lines.

Graphite anodes produced with addition of Molecular Rebar® to both the CMC and the SBR components of the binder mixture show enhanced mechanical strength, and much cleaner processing. We observe improved pasting, uniform drying and thixotropy which facilitates binder and conductive additive retention in the bulk of the paste thus preventing migration to the surface. In addition, we observe no edge flaking or shedding during cutting or slitting and more uniform interconnectivity between graphite particles. Side-by-side micrographs in Figure 1 show the improved edge performance and reduced shedding of slit electrodes with (left) and without (right) the nanotube additives.

Figure 2 shows half-cell performance of cells made with Molecular Rebar® in the CMC and SBR components of the binder, suggesting that the nanotubes improve capacity retention and uniformity at high C-rates. Coin cell data is shown, but pouch cell performance testing is underway. Performance analysis suggests that much of the improvement is due to enhanced mass transport of lithium ions through the thickness of the anode layer, resulting in a more uniform utilization of the electrode at longer discharge times. More detailed characterization work is underway.

Figure 3 shows SEM micrographs taken of the electrodes after cycling, revealing a more homogeneous and uniform SEI layer in the binders containing Molecular Rebar®, suggesting improved cycleability and more uniform access to the particles during the charge and discharge processes.

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[1] “EV Everywhere Grand Challenge Blueprint,” U.S. Department of Energy, December 2013, http://www.energy.gov/sites/prod/files/2014/02/f8/eveverywhere_blueprint.pdf.

[2] “Thick Electrodes for High Energy Lithium Ion Batteries,” Madhav Singh, Jörn Kaiser, and Horst Hahn, Journal of the Electrochemical Society, 162 (2), A1196-A1201 (2015).

[3] “Effect of Carboxymethyl Cellulose on Aqueous Processing of Natural Graphite Negative Electrodes and their Electrochemical Performance for Lithium Batteries,” Jin-Hyon Lee, Ungyu Paik,, Vincent A. Hackely, and Young-Min Choi, Journal of the Electrochemical Society, 152 (9) A1763-A1769 (2005).

[4] “Optimization of ratio and amount of CMC/SBR binder for a graphite anode,” Honghe Zheng, Gao Liu, Xiangyun Song, Paul Ridgeway and Vince Battaglia, Abstract #200, 218th ECS Meeting.

Figure 1