Increasing electrode thickness confronts a number of challenges including control of pasting rheology, maintaining paste integrity during drying, slitting, and battery assembly operations, as well as uniform electrical conductivity and ion transport.
Aqueous binders can play a meaningful role in lowering cell cost by reducing manufacturing cost as they eliminate the need for complex equipment and also mitigate environmental hazards associated with organic solvents. Aqueous binders are currently used for graphite anodes, but they are also increasingly used in cathodes.
In this paper, we present new results and understanding of aqueous binders in the context of discrete functionalized multiwall carbon nanotubes, MOLECULAR REBAR®, (MR), and their use in developing thicker electrodes.
The discreet carbon nanotubes (MR) have been cleaned of catalytic residue and other carbonaceous debris and opened at both ends to allow electrolyte absorption and transport. They are on average 13nm in external diameter and about 900nm in length. The interior cavity diameter is about 5 nm. The surfaces of these tubes are selectively modified to facilitate their role within electrodes, allowing us to tailor lithium transport characteristics, electron transport, and mechanical properties to the desired battery performance targets. Insights will be provided here on the evolution of multi-scale structure in anodes developed with aqueous binder compositions incorporating various ratios of MR combined with conductive carbon black, graphite particles, and various carboxymethylcellulose (CMC) - styrene butadiene rubber (SBR) ratios. In our process, the MR is dispersed as individual tubes in both CMC and SBR binder components. Their content was varied within each polymer phase to study effects on the microstructure. The CMC-MR composite (0-10% wt. relative to CMC) is found to uniformly coat the graphite particles with thickness approximating individual tube diameters while still maintaining a random distribution of the discrete tubes important to ionic conductivity. The SBR–MR composite (0 -10% wt. MR relative to SBR) forms reinforced, elastic, and when MR is added - electron conductive bridges which connect the CMC coated graphite particles and adhere to current collector. Through formulation of SBR-MR special care is taken to ensure that MR does not agglomerate independently and remains intimately entwined within the consolidated latex agglomerate.
Determinations were made of the adhesive and cohesive strengths of the dried pastes, before and after calendaring, using a newly developed mechanical test that is more accurate than the simple peel test. The nature of paste shedding or delamination (i.e. roll-to-roll and cell coiling process-ability) is controlled by both the magnitude of the adhesion and cohesion strength and their ratio. Failure analyses of the dried electrodes upon slitting or bending were conducted by electron microscopy and will be illustrated. Both the adhesion characteristics of the pastes at the copper current collector surface and the cohesive failure of the graphite-graphite particles were examined. We learned that the adhesive strength of the graphite coated particles to copper is influenced by the CMC-SBR ratio and is enhanced with the presence of MR. Clear evidence will be presented on the relative sequence of failure events in slitting operations. The cohesive bond strength is also a function of the CMC adhesion to graphite and the characteristics of SBR itself bridging the CMC coated graphite particles. Of importance, is that the discrete carbon nanotubes maintain electrical conductivity while stretching those bonds that cannot be obtained with just conductive carbon black. This means that MR additions to CMC-SBR binders result in stronger and tougher formulations that retain electron transport especially under high volumetric strains, such as strains experienced with higher capacity anode materials like high silicon content graphite.
Conventional testing of electrode performance as a function of thickness with the new binder formulations containing MR reveal more consistent behavior and improved cycling of the electrodes. Examination of the solid electrolyte interface layer formation after 300 cycles at room temperature by electron microscopy have shown much smoother, more uniform coverage than graphite anodes without the discrete carbon nanotubes.
The ability to control the placement of discrete, clean, functionalized carbon nanotubes in binder formulations has opened new windows to the design of binders for batteries with improved performance and lower cost. There is a natural extension of the science and technology discussed here to other binders such as polyacrylic acid and to advancing aqueous binders for cathode materials.