In various battery and energy storage technologies, much attention is given to the hierarchical structure of the electrodes so that they can efficiently facilitate the movement of the metallic ions (commonly lithium) in and out of the electrodes—the mechanism by which the electrodes charge and discharge. However, one area that doesn’t garner as much publicity as the materials that make up the electrode is the coatings that are often applied to the electrodes to improve their performance and stability of the electrodes. These coatings are also applied to the electrodes used in various processing and industrial environments.
The coating of an electrode is not always a necessity, but it can help to bring certain benefits, and it is these benefits that are the main drivers for someone taking the time and the money to coat the electrodes—as the coating processes required can sometimes be complex and costly, especially with the more advanced coating methods.
The two key drivers for producing new types of electrodes are an increase in the performance/efficiency and an increase in safety. Most people turn to new materials to find these improvements, the coating of the electrodes offers another way for electrode manufacturers to still use the existing (and often cheaper) and more well-established materials. As mentioned, these coatings can be costly if outsourced to other manufacturers, which can be the case if atomic layer coatings are required, as many approaches are tailored to individual needs.
In terms of performance benefits, coated electrodes are known to have much longer lifetimes than their non-coated equivalents—up to double the potential to increase the capacity of a battery they are used in— increase the charging rates of the electrode, and increase the voltage in the electrode without compromising on the safety. The presence of a coating can also prevent a solid electrolyte interphase (SEI) layer forming over time within batteries, or at the very least reduce this layer, both of which prevents the performance of the electrodes (and the host technology) from degrading over time. Specific coatings can also be applied that enhance oxygen evolution reactions (OER) or chlorine evolution reactions (CER) of the electrodes. Because they are specific coatings, various industry standard materials can be applied to achieve the desired effect(s).
From a safety perspective, the presence of the coating can improve the arc stability of the electrode by ionizing the path of the arc, prevent overvoltage and rapid cycling rates from occurring, provide a protective surface against various gaseous and liquid contaminants that could damage the electrode (which makes the electrode more robust), as well as help reduce the concentration of potentially hazardous gases from forming—as many of these gases are actually formed in the cathode. Many of the larger electrodes are used in harsh processing environments, and coatings offer a way of protecting the electrode against a number of processing conditions (including molten slag and metal). The overall effect is a much more stable environment in which the electrodes can operate in, regardless of the application.
Different materials can be used in these coatings that can be deposited in a range of thicknesses depending on the individual requirements. This is often achieved through coating the electrode(s) multiple times. However, there are limits to how thick the coating can be, where the weight of the coating can only be up to 30 percent of the weight of the electrode. This generally translates to a maximum thickness of 3mm for large electrodes. Some of the most common materials used include metals and metal oxides, complexes made of transition metals, mineral compounds, and stable organic materials (such as cellulose). In many cases a number of these different constituents are required to produce a coating that achieves increases in both performance and safety, with many coatings being held together by binding materials that form complexes with the metals in the coating.
In recent years, advances in nanoscale deposition methods, such as atomic layer deposition (ALD), have enabled ultra-thin coatings to be deposited on to the surface of electrodes—be it specific nanomaterials or the fabrication of nano-sized films of varying compositions. Many nanomaterials are known to have excellent electronic conductivity and charge mobility properties, as well as being stable to many harsh chemicals, high temperatures and pressures, and various mechanical deformations. The thinness of these films also stops them from being cumbersome on the outside of the electrode, which in some cases, can cause the movement of the ions to be impeded. One of the key advantages over other types of coatings used on electrodes is that they don’t require multiple constituents to be bonded together. The way in which the nano-deposition methods work, is that the specific elements in the coating can be tailored to produce a chemically specific coating that provides a certain benefit and negates the need for multiple individual constituents in the coating. It is a much more tailored and tunable method; hence it has been gathering much interest over other electrode coating methods, but they do generally come at an increased cost—so they are more suitable for smaller electrodes, whereas other coating methods are generally applied to large electrodes.
Conclusion
Most people think that advances in performance and safety in electrodes stems from the materials used in the electrode, but advances can also be found by coating these electrodes while using existing established materials. Many different materials can be used in these coatings, and many electrode coatings require multiple constituents. One area that doesn’t require multiple constituents are nanomaterial/ultra-thin film coatings (nanocoatings), which are relatively new and have been brought about by advances in nanofabrication methods, and these are a growing class of electrode coatings despite their higher cost.
