UC Riverside engineers are developing cheap, energy-efficient lithium-ion batteries for electric vehicles from silicon in diatomaceous earth
Researchers at the University of California, Riverside’s Bourns College of Engineering have developed an inexpensive, energy-efficient way to create silicon-based anodes for lithium-ion batteries from the fossilized remains of single-celled algae called diatoms. The research could lead to the development of ultra-high capacity lithium-ion batteries for electric vehicles and portable electronics.
Titled “Carbon-Coated, Diatomite-Derived Nanosilicon as a High Rate Capable Li-ion Battery Anode,” a paper describing the research was published recently in the journal Scientific Reports. The research was led by Mihri Ozkan, professor of electrical engineering, and Cengiz Ozkan, professor of mechanical engineering. Brennan Campbell, a graduate student in materials science and engineering, was first author on the paper.
Lithium-ion batteries, the most popular rechargeable batteries in electric vehicles and personal electronics, have several major components including an anode, a cathode, and an electrolyte made of lithium salt dissolved in an organic solvent. While graphite is the material of choice for most anodes, its performance is a limiting factor in making better batteries and expanding their applications. Silicon, which can store about 10 times more energy, is being developed as an alternative anode material, but its production through the traditional method, called carbothermic reduction, is expensive and energy-intensive.
To change that, the UCR team turned to a cheap source of silicon—diatomaceous earth (DE)—and a more efficient chemical process. DE is an abundant, silicon-rich sedimentary rock that is composed of the fossilized remains of diatoms deposited over millions of years. Using a process called magnesiothermic reduction, the group converted this low-cost source of Silicon Dioxide (SiO2) to pure silicon nano-particles.
“A significant finding in our research was the preservation of the diatom cell walls—structures known as frustules—creating a highly porous anode that allows easy access for the electrolyte”, Cengiz Ozkan said.
This research is the latest in a series of projects led by Mihri and Cengiz Ozkan to create lithium-ion battery anodes from environmentally friendly materials. Previous research has focused on developing and testing anodes from portabella mushrooms and beach sand.
“Batteries that power electric vehicles are expensive and need to be charged frequently, which causes anxiety for consumers and negatively impacts the sale of these vehicles. To improve the adoption of electric vehicles, we need much better batteries. We believe diatomaceous earth, which is abundant and inexpensive, could be another sustainable source of silicon for battery anodes,” Mihri Ozkan said.
In addition to Mihri and Cengiz Ozkan and Campbell, graduate students Robert Ionescu, Maxwell Tolchin, Kazi Ahmed, Zachary Favors, and Krassimir N. Bozhilov, manager of UCR’s Central Facility for Advanced Microscopy and Microanalysis, also contributed to this research.
Although rechargeable batteries in smartphones, cars and tablets can be charged again and again, they don’t last forever. Old batteries often wind up in landfills or incinerators, potentially harming the environment. And valuable materials remain locked inside. Now, a team of researchers is turning to naturally occurring fungi to drive an environmentally friendly recycling process to extract cobalt and lithium from tons of waste batteries.
The researchers will present their work today at the 252nd National Meeting & Exposition of the American Chemical Society (ACS). ACS, the world’s largest scientific society, is holding the meeting here through Thursday. It features more than 9,000 presentations on a wide range of science topics.
“The idea first came from a student who had experience extracting some metals from waste slag left over from smelting operations,” says Jeffrey A. Cunningham, Ph.D., the project’s team leader. “We were watching the huge growth in smartphones and all the other products with rechargeable batteries, so we shifted our focus. The demand for lithium is rising rapidly, and it is not sustainable to keep mining new lithium resources,” he adds.
Although a global problem, the U.S. leads the way as the largest generator of electronic waste. It is unclear how many electronic products are recycled. Most likely, many head to a landfill to slowly break down in the environment or go to an incinerator to be burned, generating potentially toxic air emissions.
While other methods exist to separate lithium, cobalt and other metals, they require high temperatures and harsh chemicals. Cunningham’s team is developing an environmentally safe way to do this with organisms found in nature — fungi in this case — and putting them in an environment where they can do their work. “Fungi are a very cheap source of labor,” he points out.
To drive the process, Cunningham and Valerie Harwood, Ph.D., both at the University of South Florida, are using three strains of fungi — Aspergillus niger, Penicillium simplicissimum andPenicillium chrysogenum. “We selected these strains of fungi because they have been observed to be effective at extracting metals from other types of waste products,” Cunningham says. “We reasoned that the extraction mechanisms should be similar, and, if they are, these fungi could probably work to extract lithium and cobalt from spent batteries.”
The team first dismantles the batteries and pulverizes the cathodes. Then, they expose the remaining pulp to the fungus. “Fungi naturally generate organic acids, and the acids work to leach out the metals,” Cunningham explains. “Through the interaction of the fungus, acid and pulverized cathode, we can extract the valuable cobalt and lithium. We are aiming to recover nearly all of the original material.”
Results so far show that using oxalic acid and citric acid, two of the organic acids generated by the fungi, up to 85 percent of the lithium and up to 48 percent of the cobalt from the cathodes of spent batteries were extracted. Gluconic acid, however, was not effective for extracting either metal.
The cobalt and lithium remain in a liquid acidic medium after fungal exposure, Cunningham notes. Now his focus is on how to get the two elements out of that liquid.
“We have ideas about how to remove cobalt and lithium from the acid, but at this point, they remain ideas,” he says. “However, figuring out the initial extraction with fungi was a big step forward.”
Other researchers are also using fungi to extract metals from electronic scrap, but Cunningham believes his team is the only one studying fungal bioleaching for spent rechargeable batteries.
Cunningham, Harwood and graduate student Aldo Lobos are now exploring different fungal strains, the acids they produce and the acids’ efficiencies at extracting metals in different environments.
Learn more: Fungi recycle rechargeable lithium-ion batteries
The capacity of lithium-ion batteries might be increased by six times by using anodes made of silicon instead of graphite. A team from the Helmholtz-Zentrum Berlin (HZB) Institute of Soft Matter and Functional Materials has observed for the first time in detail how lithium ions migrate into thin films of silicon. It was shown that extremely thin layers of silicon would be sufficient to achieve the maximal load of lithium.
The team was able to show through neutron measurements made at the Institut Laue-Langevin in Grenoble, France, that lithium ions do not penetrate deeply into the silicon. During the charge cycle, a 20-nm anode layer develops containing an extremely high proportion of lithium. This means extremely thin layers of silicon would be sufficient to achieve the maximal load of lithium.
Lithium-ion batteries provide laptops, smart phones, and tablet computers with reliable energy. However, electric vehicles have not gotten as far along with conventional lithium-ion batteries. This is due to currently utilised electrode materials such as graphite only being able to stably adsorb a limited number of lithium ions, restricting the capacity of these batteries. Semiconductor materials like silicon are therefore receiving attention as alternative electrodes for lithium batteries. Bulk silicon is able to absorb enormous quantities of lithium. However, the migration of the lithium ions destroys the crystal structure of silicon. This can swell the volume by a factor of three, which leads to major mechanical stresses.
Observation during charging cycle
Now a team from the HZB Institute for Soft Matter and Functional Materials headed by Prof. Matthias Ballauff has directly observed for the first time a lithium-silicon half-cell during its charging and discharge cycles. “We were able to precisely track where the lithium ions adsorb in the silicon electrode using neutron reflectometry methods, and also how fast they were moving”, comments Dr. Beatrix-Kamelia Seidlhofer, who carried out the experiments using the neutron source located at the Institute Laue-Langevin.
Lithium-rich layer of only 20 nanometer
She discovered two different zones during her investigations. Near the boundary to the electrolytes, a roughly 20-nm layer formed having extremely high lithium content: 25 lithium atoms were lodged among 10 silicon atoms. A second adjacent layer contained only one lithium atom for ten silicon atoms. Both layers together are less than 100 nm thick after the second charging cycle.
Theoretical maximum capacity
After discharge, about one lithium ion per silicon node in the electrode remained in the silicon boundary layer exposed to the electrolytes. Seidlhofer calculates from this that the theoretical maximum capacity of these types of silicon-lithium batteries lies at about 2300 mAh/g. This is more than six times the theoretical maximum attainable capacity for a lithium-ion battery constructed with graphite (372 mAh/g).
Less is more
These are substantial findings that could improve the design of silicon electrodes: very thin silicon films should be sufficient for adsorbing the maximum possible amount of lithium, which in turn would save on material and especially on energy consumed during manufacture – less is more!
Lithium-ion batteries are a rapidly growing energy storage method due to their high energy density, especially in mobile applications such as personal electronics and electric cars. However, the materials currently used in Li-ion batteries are expensive, many of them, like lithium cobalt oxide (belonging to the EU Critical Raw Materials, CRMs), are difficult to handle and dispose of. Additionally, batteries using these materials have relatively short lifetimes.
New novel materials are being developed for next generation Li-ion batteries. One promising anode-cathode material pair is lithium titanate countered by lithium iron phosphate. The raw materials for these components are readily available; and they are safe to use, and easy to dispose of or recycle. And most importantly, batteries manufactured using these materials have significantly longer cycle and calendar lifetimes compared to the current battery technology. However, the main problem of these new materials is their low electric conductivity.
A study by University of Eastern Finland scientists opens up new electricity storage applications. The results were published recently in the Journal of Alloys and Compounds, which has a large audience especially in Asian countries, where most of the Li-ion battery manufacturing takes place currently.
“The electric conductivity problem can be solved by producing nanosized, high surface area crystalline materials, or by modifying the material composition with highly conductive dopants. We have succeeded in doing both for lithium titanate (LTO) in a simple, one-step gas phase process developed here at the UEF Fine Particle and Aerosol Technology Laboratory,” says Researcher Tommi Karhunen.
Pollens, the bane of allergy sufferers, could represent a boon for battery makers: Recent research has suggested their potential use as anodes in lithium-ion batteries.
“Our findings have demonstrated that renewable pollens could produce carbon architectures for anode applications in energy storage devices,” said Vilas Pol, an associate professor in the School of Chemical Engineering and the School of Materials Engineering at Purdue University.
Batteries have two electrodes, called an anode and a cathode. The anodes in most of today’s lithium-ion batteries are made of graphite. Lithium ions are contained in a liquid called an electrolyte, and these ions are stored in the anode during recharging.
The researchers tested bee pollen- and cattail pollen-derived carbons as anodes.
“Both are abundantly available,” said Pol, who worked with doctoral student Jialiang Tang. “The bottom line here is we want to learn something from nature that could be useful in creating better batteries with renewable feedstock.”
Research findings are detailed in a paper that appeared Friday (Feb. 5) in Nature’s Scientific Reports.