Effect of Polyvinylidene Fluoride Binder Distribution on Dry Spray-Coated Graphite Electrodes for Lithium-Ion Batteries for Electric Vehicle Applications
Authors: Barreras Uruchurtu Jesus Alberto , Besnard Nicolas , Paul Clément , Marchal Lauréline , Devisme Samuel , Lestriez Bernard 
Published: December 2024

Dry process manufacturing techniques, which involve fabricating electrodes without the use of solvents and directly depositing electrode materials onto current collectors, have emerged as an alternative to traditional lithium-ion battery (LIB) electrode preparation methods1. In the conventional wet process, solvent-based mixtures, known as slurries, are coated onto metallic current collectors. These slurries typically contain electrochemically active material, conductive additive, polymer binder, and solvent for binder dissolution. For cathodes, polyvinylidene fluoride (PVdF) dissolved in N-methyl-2-pyrrolidone (NMP) is commonly used.2 The growing interest in solvent-free techniques is due to their numerous advantages over traditional methods. For instance, eliminating the use of NMP during cathode production reduces energy consumption by almost 47% during the manufacturing process and decreases total costs by around 15%.3,4 This approach is also relevant for anodes, where a mix of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) dissolved in water is used. The uniform mixing distribution of the binder and CB additive materials throughout the active material is crucial for manufacturing dry-processed LIB electrodes because it can affect the electrode characteristics such as homogeneity of binder distribution, porosity, adhesion, and cohesion between active material and carbon black (CB) conducting agent particles and the current collector, electrical properties, and therefore the battery performance5–10.

For our study, we prepared negative electrode powder mixtures consisting of graphite and a low-content of PVdF through a dry-mixing process at room temperature, varying the mixing time. Then, these powder electrode mixtures were subjected to a high-voltage electrostatic dry coating-spray process and applied onto a carbon-coated copper current collector. Afterwards, the electrodes were calendered at high temperature with a determined applied force.

Our objective was to characterize the mixing quality in the electrode powder mixtures and explore its correlation with electrode quality. To accomplish this, we employed various methods, including notably Scanning Electron Microscopy (SEM) imaging and powder resistivity tests conducted via impedance measurements with precise pressure control. Additionally, image analysis tools were utilized to quantify the fraction of binder distributed on the surface of the graphite particles and differentiate it from the binder fraction forming agglomerates. An example of this quantification can be observed in Fig. 1a-b, where a graphite particle covered by PVdF nanoparticles is depicted, showcasing how the image analysis tool accurately distinguishes between both materials. This allowed for a better understanding and rationalization of the influence of PVdF binder distribution on the properties of the electrodes.

Characterizing the electrodes produced from these powder mixtures was equally important and included a wide range of analyses such as morphology assessment using SEM and Energy Dispersive X-ray Spectroscopy (EDS), as well as surface resistivity measurements. Mechanical properties were evaluated through peel strength tests to measure electrode-current collector adhesion force and nanoindentation tests for hardness, elastic modulus, and plasticity. Electrochemical evaluations were conducted focusing on rate capability to determine the delithiation capacity at various delithiation rates, as well as through long-term cycling measurements.

Our research findings suggest that mixing time, particularly the binder distribution, significantly impacts the morphology, electrical, and electrochemical properties of our electrodes. This exploration enabled us to identify optimal parameters for electrode fabrication, resulting in high-quality electrodes with relatively short production times and favorable electrochemical and mechanical properties. These electrodes exhibit performance comparable to those produced through conventional wet-slurry based methods.

References:

Y. Lu et al., Matter, 5, 876–898 (2022).

M. Armand et al., Journal of Power Sources, 479, 228708 (2020).

D. L. Wood, J. Li, and C. Daniel, Journal of Power Sources, 275, 234–242 (2015).

Y. Liu, R. Zhang, J. Wang, and Y. Wang, iScience, 24, 102332 (2021).

D. Mohanty et al., Journal of Power Sources, 312, 70–79 (2016).

A. Kraytsberg and Y. Ein-Eli, Adv. Energy Mater., 6, 1600655 (2016).

W. B. Hawley and J. Li, Journal of Energy Storage, 25, 100862 (2019).

G. Lenze et al., J. Electrochem.
Soc., 165, A314–A322 (2018).

B. Ludwig et al., Adv Materials Inter, 4, 1700570 (2017).

A. Yonaga et al., Journal of Power Sources, 581, 233466 (2023).

Figure 1