Electrode coatings in batteries facilitate the electrochemical reactions that are necessary to deliver electrical energy. The binders used in these coatings are typically only a small percentage of the overall weight of a battery, but they are essential to battery cell construction and delivering a range of benefits, from safety enhancements and energy density to capacity and more.
Lithium-ion batteries have a few key components: a cathode (positive electrode), an anode (negative electrode), a separator and an electrolyte mixture as a conductor. The electrochemical reactions that generate voltage and current are facilitated at the coated electrodes, where reduction and oxidation reactions occur.
Most lithium-ion batteries share a similar design, which consists of:
a metal oxide-based positive electrode (cathode) that is coated onto an aluminum current collector
- a negative electrode (anode) made from carbon/graphite coated onto a copper current collector
- a polyolefin membrane separator that is also sometimes coated
- electrolytes made of lithium salt in organic solvents
During discharge, the ions flow from the anode (oxidation) to the cathode (reduction) through the electrolyte and separator. Charging the battery reverses the direction and the ions flow from the cathode to the anode.
In a lithium battery, binders have multiple tasks. They must hold the coating particles together and assist in adhering the coating to the metal or separator membrane. The binder also:
- Aids in film formation
- Helps form a good particle dispersion in solvent or water. The binder must help the coating disperse to deliver a uniform slurry and discrete particles in the cathode and anode
- Remain stable inside the harsh environment of a battery, where multiple reactions happen. The binder can’t break apart or be destroyed in this difficult environment.
Binders can use either solvent-based or water-based technologies. Solvent-based binders require added costs for recovery, collection and environmental compliance. Water-based binders are often preferred for cost and environmental reasons.
When choosing a binder, there are several factors to consider. As mentioned, a binder must perform a number of tasks and do them well—disperse, adhere, bind particles, survive in a harsh environment… plus, it must facilitate the performance of the battery as it is charged and discharged. A battery can slowly lose capacity over cycling and charging/discharging hundreds or thousands of times. A binder plays an important role in maintaining capacity. With a cell phone, a reduction of battery capacity over time could be a nuisance, but with an electric car, it’s more than a nuisance. The binder can influence the capacity and stability of a battery, and ultimately battery life.
Binders must also have a certain degree of pliability so they don’t crack or develop defects. If there is brittleness, it can create problems during manufacturing or battery assembly during which a coated electrode is wound or wrapped.
Smaller, Lighter, More Powerful
As battery manufacturers continue to develop more powerful, more stable and smaller/lighter batteries, Lubrizol is committed to assist. Through our expertise and capabilities, we're focused on enabling enhancements to battery life and the ultimate cost of the battery through advancements in binders/dispersants for anodes, cathodes and separators. This is evident in mobile phones and laptop computers as they continue to get thinner and lighter thanks to battery technology. They are also evident in electric vehicles, where battery weight and size are shrinking while the drivable range on a single charge continues increasing.