Researchers from Caltech’s campus and JPL have worked together to develop a technique for applying graphene to lithium-ion battery cathodes, which will increase the lifespan and functionality of these popular rechargeable batteries, according to a study published in the Journal of The Electrochemical Society on November 1st, 2024.
Graphene Encapsulated Nanoparticles (GEN). Nanoparticle silica (SiO2) is encapsulated with graphene using the Caltech low temperature process. The GEN is then dry-coated onto the cathode of a lithium-ion battery to improve performance. Image Credit: David Boyd
As a result of these efforts, a promising discovery has been made that could enhance the performance of lithium-ion batteries and lessen dependency on cobalt, an element often found in lithium-ion batteries but challenging to acquire sustainably.
David Boyd, a senior research scientist at Caltech, has spent the last ten years developing methods for producing graphene, a sheet of carbon that is only one atom thick, extremely strong, and more electrically conductive than materials like silicon. Boyd and associates found high-quality graphene could be made at room temperature in 2015. Previously, temperatures as high as 1,000 degrees Celsius were needed to produce graphene.
Following this achievement, researchers began looking for new applications for graphene. Boyd recently teamed up with Will West, a technologist at JPL, which Caltech handles for NASA. West specializes on electrochemistry, specifically the development of improved battery technologies. Boyd and West set out to discover if graphene could improve lithium-ion batteries. They have demonstrated that it can.
Demonstrating a reliable trend in battery-cell performance requires consistent materials, consistent cell assembly, and careful testing under a range of conditions. It is fortunate that the team was able to do this work so reproducibly, although it took some time to be sure.
Brent Fultz, Barbara and Stanley R. Rawn, Jr., Professor of Materials Science and Applied Physics, California Institute of Technology
The lithium-ion battery, which was initially introduced to the market in 1991, has transformed how we utilize electricity in our everyday lives. From cell phones to electric vehicles, we rely on lithium-ion batteries as a low-cost, energy-efficient, and, most crucially, rechargeable energy source when on the road.
Despite its achievements, lithium-ion battery technology still has opportunity for advancement.
Tesla engineers want a cost-effective battery that can charge quickly and operate for a longer period of time between charges. That's called the charge-rate capability.
David Boyd, Research Scientist, California Institute of Technology
Will West, a technologist at JPL, added, “The more times you can charge a battery over its lifetime, the fewer batteries you have to use. This is important because lithium-ion batteries make use of limited resources and disposing of lithium-ion cells safely and effectively is a very challenging task.”
The performance of lithium-ion batteries over numerous cycles of usage and charging is a crucial characteristic. The cathode and anode, the battery's two ends, generate chemical energy that is then transformed into electrical energy to power the battery. The anode’s and cathode’s chemicals may not entirely return to their initial state as they operate over time.
Transition metal dissolution from the cathode material is a frequent issue. It is more severe in cathode materials with a high manganese content, but less so in a high cobalt level.
Boyd added, “As a result of unwanted side-reactions that occur during cycling, transition metals in the cathode gradually end up in the anode where they get stuck and reduce the performance of the anode.”
This transition metal dissolution (TMD) is responsible for the use of costly cobalt-bearing cathodes rather than low-cost cathodes with a high manganese concentration.
Another difficulty for lithium-ion batteries is that they require expensive, scarce metals that are not necessarily mined ethically. A significant portion of the world's cobalt supply is concentrated in the Democratic Republic of the Congo, and much of it is extracted by so-called artisanal miners: freelance workers, including children, who perform dangerous and demanding physical labor for little to no pay.
The quest has been on for solutions to improve battery performance while lowering or eliminating the usage of cobalt and preventing TMDs.
Enter graphene. Engineers have previously discovered that carbon coatings on a lithium-ion battery’s cathode might delay or stop TMD, but establishing a method to apply these coatings proved problematic.
“Researchers have tried to deposit graphene directly onto the cathode material, but the process conditions typically needed to deposit graphene would destroy the cathode material. We investigated a new technique for depositing graphene on the cathode particles called dry coating. The idea is that you have one 'host' substance of large particles and a 'guest' substance of tiny particles. By mixing them under certain conditions, the system can undergo a phenomenon known as 'ordered mixing' in which the guest particles uniformly coat the host particles,” Boyd added.
Dry-coating technology has been used in the pharmaceutical business since the 1970s to protect tablets from moisture, light, and air, extending their shelf life.
Boyd added, “This is a good idea we might be able to use with graphene! We can first manufacture graphene guest particles—graphene encapsulated nanoparticles (GEN)—using our room-temperature method, and then dry coat a very small amount of it (1 percent in weight) onto the host cathode material so that graphene effectively covers and protects the cathode.”
Dry coating the cathode with a graphene composite worked well in the experiment. The graphene covering significantly reduced TMD while also doubling cell cycle life and allowing the batteries to run over a somewhat wider temperature range than previously achievable. This result startled the researchers.
It was anticipated that only a continuous covering could inhibit TMD, while a dry particle-based coating could not. Furthermore, because graphene is a kind of carbon, it is readily available and environmentally friendly.
This approach provides extra benefits to the battery industry.
Boyd added, “Battery factories are very expensive. A lot of money has been invested into them. So, it is very important that improved battery technologies are scalable and can fit into the workflows of existing battery manufacturing. We can take almost any cathode material and add in just a small amount of our GEN, run it for a few minutes in the dry mixer, and it will reduce transition metal dissolution and improve charge-rate capacity.”
Boyd concluded, “This is also an advance for coating technologies in general. It opens up a lot of possibilities for the use of dry coatings.”
Boyd; Fultz; Cullen M. Quine (PhD '23); Caltech staff research scientist Channing Ahn; and West and Jasmina Pasalic at JPL are the study co-authors.
Lewis and Diane van Amerongen, as well as Charles Fairchild, generously financed the research. The equipment was supplied by Graph Energy Inc. NASA financed the experiments conducted at JPL.
Journal Reference:
Boyd, D. A. et. al. (2024) Suppression of Transition Metal Dissolution in Mn-Rich Layered Oxide Cathodes with Graphene Nanocomposite Dry Coatings. Journal of The Electrochemical Society. doi.org/10.1149/1945-7111/ad867f