Slurry Solid Fraction: A Key Processing Parameter for Performance Optimisation of Si-Graphite Electrodes for Li-Ion Batteries

Slurry solid fraction is often treated as an innocuous battery electrode processing parameter at the laboratory scale. In fact, articles that put a number to the water content of their slurries are few and far between. However, recent studies from our group have shown that the slurry solid fraction can have a significant impact on the electrochemical performances of the resulting electrodes. The present research aims to highlight the importance of optimising this parameter by demonstrating its impact throughout the electrode preparation and testing processes of Si-graphite electrodes for Li-ion batteries. Silicon, graphite, a conductive additive of graphene nanoplatelet (GnP) and a partially neutralized PAA-based binder (PAH0.8Na0.2, pH ≈ 4, Mw = 1084k, 393k or 7.6k g/mol) were dispersed in a variable amount of deionized water, yielding slurries of different solid fractions according to equation 1: Eq. 1 SF = (mSi + mGr + mGnP) / mSlurry An initial impact of the slurry solid fraction can be seen in the adsorption of the binder onto the active material and conductive additive in aqueous conditions. Gel permeation chromatography of the polymer remaining in the liquid phase after slurry dispersion reveals a strong preferential adsorption of high-molecular weight PAA, mainly on the silicon particles, that is even more pronounced at higher solid fraction. This localized polymer distribution affects its ability to fulfill its functions as a binder and an artificial solid-electrolyte interphase (SEI), which can later be seen in the irreversible capacity loss that arises from both electrical disconnections and SEI formation during cycling. The solid fraction is also a principal determinant of the rheological properties of the slurry, namely its viscosity under shearing and storage and loss moduli. Shear-thinning behavior is favourable to ensure homogeneous dispersion of matter in the electrode slurry and avoid creating surface defects during the coating process. Increasing the solid fraction leads to shear-thickening behavior in slurries with high-molecular weight binders. As such, large agglomerates of silicon and multiple surface defects are observed in these electrodes. On the other hand, the storage and loss moduli of the slurry will determine its stability during the drying process where there is a risk of sedimentation. The effects of the slurry solid fraction can also be seen in the dried electrodes. For example, the mechanical properties (hardness, elasticity) of the electrodes made with low-solid fraction slurries are comparatively poor, as measured by nanoindentation. Conversely, 4-point probe testing shows that electrodes made with high-solid fraction slurries are more resistive due to the presence of large silicon particles covered in a high concentration of polymer. The numerous impacts of the slurry solid fraction across the electrode preparation and testing processes culminate in a strong dependence of the resulting electrochemical performances on this often-neglected parameter. An optimal solid fraction is determined for the given materials. Formulations with different binder molecular weights are also compared at different solid fractions to illustrate the importance of this optimization step in drawing accurate and meaningful conclusions on the materials at study. References: Ligneel, E.; Lestriez, B.; Hudhomme, A.; Guyomard, D. Effects of the Solvent Concentration (Solid Loading) on the Processing and Properties of the Composite Electrode. J. Electrochem. Soc. 2007, 154 (3), A235. https://doi.org/10.1149/1.2431316. Porcher, W.; Lestriez, B.; Jouanneau, S.; Guyomard, D. Design of Aqueous Processed Thick LiFePO4 Composite Electrodes for High-Energy Lithium Battery. J. Electrochem. Soc. 2009, 156 (3), A133. https://doi.org/10.1149/1.3046129. Xiong, J.; Dupré, N.; Mazouzi, D.; Guyomard, D.; Roué, L.; Lestriez, B. Influence of the Polyacrylic Acid Binder Neutralization Degree on the Initial Electrochemical Behavior of a Silicon/Graphite Electrode. ACS Appl. Mater. Interfaces 2021, 13 (24), 28304–28323. https://doi.org/10.1021/acsami.1c06683.