Water-Soluble Binders for Aqueously Processed Li-Ion Batteries

As Li-ion battery technology is developed for the automotive sector, more environmentally friendly processing methods must be employed. The commonly used binder, PVDF, is being phased out due to legislation banning PFAS substances, providing an opportunity to re-evaluate the solvent. Aqueous processing with hydrophilic polymer binders is an emerging alternative to the conventional NMP/PVDF solvent/binder system for Li-ion batteries, offering advantages; namely nontoxic, unrestricted use, and cost-effectiveness. Aqueous binders can offer improvements in performance over typically processed cathodes with NMP, providing an opportunity to explore novel solvent/binder systems.

To screen for potential aqueous cathode binders, the chemical affinity of polymers to carbon black was quantified via adsorption measurements. Polyethylene oxide (PEO) was the most adsorbing homopolymer, and is cheap, commercially available, and biodegradable. Early work with PEO resulted in low-capacity battery cells, likely because the molecular weight of the binder was not taken into consideration, resulting in low-viscosity slurries outside the optimal range for cathode formation. To test this hypothesis, two high molecular weights of PEO were selected to evaluate the impact of viscosity on slurry structure. Various concentrations of each PEO were chosen to match the viscosity of two polymer solutions with different molecular weights, thereby determining whether slurry structure is strictly affected by viscosity or whether the molecular weight of the polymer has a role, presumably through the thickness of the adsorbed layer. Varying the polymer concentration in the slurry enabled control over the percolation of carbon black. To assess PEO as a potential binder for high-nickel cathodes, coin cells were assembled. Despite PEO-containing slurries appearing smooth and homogeneous to the eye, the PEO cathodes performed worse than those made with NMP/PVDF.

The chemical affinity between the binder and carbon black was shown to correlate with the distribution of carbon black flocs, enabling tuning of 3D carbon black networks in the slurry. The distribution of carbon black was shown to affect both the cathode porosity and the contact resistance, which in turn affected the electrochemical performance of cathodes, likely due to more conductive pathways formed via better carbon black distribution in the slurry. Electrochemical characterization revealed that the solid cathodes had varying microstructures, indicating that cathode particles were not uniformly dispersed within the matrix of CBD, which was attributed to the distribution of carbon black in the dried cathode. The chemical affinity of the polymer binder to carbon black was therefore shown to predict the electrochemical performance of the cell.

Polymers with strong affinities for carbon black are hypothesised to result in better electrochemical performance, but adsorption is limited by the fact that hydrophobic polymers do not dissolve in water. To overcome this, an amphiphilic block copolymer (pluronic F68) was chosen to control the dispersion of carbon black in the slurry, yet its low viscosity renders it unsuitable for processing by itself. Therefore, to further study its effect on electrochemical performance, a series of blends were prepared with a high molecular weight PEO as a viscosity modifier. A series of formulations were developed that maximize the mass fraction of active material whilst ensuring enough binder was present to disperse the carbon black in water. By carefully controlling the viscosity, it was shown that electrochemical cell performance correlates strongly with the pluronic content.

It has been demonstrated that aqueously processed high-nickel cathodes can show comparable or better performance than cathodes processed conventionally with NMP, if the chosen binders have a strong chemical affinity to the conductive additive. Furthermore, amphiphilic block copolymers can be used for aqueous processing, as their hydrophobic block can adsorb onto the carbon surface, and their hydrophilic chains can disperse the flocs in water, resulting in improved carbon distribution compared to other aqueous binders. Both the solids loading and viscosity of the polymer blend can dictate carbon black distribution in the slurry and the dry electrode. This research demonstrates that aqueous-processed battery half-cells can achieve the same performance as those processed with the conventional organic solvent when the formulation is adapted to compensate for the hydrophobicity of the conductive additive.