You may have already realized that building a commercially viable battery is not as easy as simply slapping together the component materials. There is both science and art involved in mixing the particle components together, having the right particle size distribution and the manner in which the electrodes are fabricated. In this 2-part blog series, we share insights into cause and effect relationships between battery materials and electrode fabrication on cell performance. In part 1 of this blog series, we discussed the effect of cathode and anode material, electrolyte and binder. In this post, we discuss the impact of conductive aids, current collectors, separators and slurries. Many of the principles discussed are chemistry-agnostic and can be applied to almost any battery type. You can always refer back to our post on battery terminology or our earlier post on power density vs energy density for background information.
Secondary Cell Diagram (Image Source: Barrie Lawson / Wikimedia Commons)
These are minor (2-3%) but important components of electrodes, typically high surface area carbons, such as carbon black (CB), fibers, or even nanotubes ($$), with a bit of graphite added in. These are necessary to promote cathodic ion (in Li-ion) or electron (in alkaline) conductivity. Surprisingly, a small amount of CB is needed in graphite anodes. CB is much smaller than graphite particles, and tends to “decorate” graphite, which prevents the relatively flat particles from packing into a dense mass that leaves no room for Li+ diffusion. (Excessively dense packaging reduces void volume available for electrolyte and this is detrimental especially for high power applications). Li metal phosphates, easily fragmented into submicron domains, are more likely to employ milled carbon nanotubes, since conductive aids must be smaller than the active ingredient to achieve the desired effect. As mentioned in part 1 of this post, the soft graphite (~4-5%) in alkaline cell cathodes also serves as a binder.
As suggested by their name, current collectors facilitate the flow of electrons to and from the battery into the electronic circuit as they are needed (for recharge) or generated (discharge), respectively. Alkaline cells use metal spikes of brass, zinc, or related alloys, and NiMH incorporates a rare earth alloy anode. Li-ion cells can be fancier: besides Al foil for the cathode and Cu foil for the anode, metal meshes (Ni, Al) and woven graphite are sometimes found.
Absorbent paper separators have been in use since Volta discovered the battery. Inexpensive and stable in strong base, paper is the material of choice for any cell with hydroxide electrolyte. Not unexpectedly, Li-ion uses higher technology, and there are numerous choices. The “traditional” separator is polyethylene or polypropylene film, 15-50 µm thick and infused with submicron pores that are filled with electrolyte in the assembled cell. These are designed to soften and collapse the pores if cell temperature gets above ~125 0C, which stops cell operation and prevents thermal runaway. More recent versions include woven polymers, for better porosity with rapid discharge capability, and coatings or loadings of nano-sized ceramic powders, for additional strength and thermal stability.
The common thread with electrode slurries is that all particle surfaces must be wetted, else ion transport is compromised and cell capacity and lifetime are less than optimal. For alkaline and NiMH electrodes, and Li-ion graphitic anodes, this is an easy operation: place the ingredients in a bowl, turn on the planetary mixing blades, and come back after lunch to start cell assembly. Li-ion cathodes are more demanding, however: high-shear mixing is required to coat carbon on the particle surfaces (for ion diffusion into the particle), a difficult combination of art and science that takes several iterations to define for each combination of materials. Both Li-ion electrode slurries are produced under vacuum, which lowers the solvent surface tension and hastens wetting. Rule of thumb: 1 hour of vacuum blending produces the same result as 10 hours of atmospheric mixing. Solvent and binder content must be adjusted (more trial-and-error iterations) to insure viscosity is optimized for extrusion into the cell casing or coating on the current collector.
Hopefully at this point, you have walked away with some insights on the many process and component variables that impact battery performance. As always feel free to contact us for battery development questions or inquiries. You can also check out our battery fundamentals or battery resources sections for useful information.