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.
The goal of a polymer dielectric material with high energy density, high power density and excellent charge-discharge efficiency for electric and hybrid vehicle use has been achieved by a team of Penn State materials scientists. The key is a unique three-dimensional sandwich-like structure that protects the dense electric field in the polymer/ceramic composite from dielectric breakdown.
Their results are published today (8/22/16) in the Proceedings of the National Academy of Sciences (PNAS).
“Polymers are ideal for energy storage for transportation due to their light weight, scalability and high dielectric strength,” says Qing Wang, professor of materials science and engineering and the team leader. “However, the existing commercial polymer used in hybrid and electric vehicles, called BOPP, cannot stand up to the high operating temperatures without considerable additional cooling equipment. This adds to the weight and expense of the vehicles.”
The researchers had to overcome two problems to achieve their goal. In normal two-dimensional polymer films such as BOPP, increasing the dielectric constant, the strength of the electric field, is in conflict with stability and charge-discharge efficiency. The higher the field, the more likely a material is to leak energy in the form of heat. The Penn State researchers originally attacked this problem by mixing different materials while trying to balance competing properties in a two-dimensional form. While this increased the energy capacity, they found that the film broke down at high temperatures when electrons escaped the electrodes and were injected into the polymer, which caused an electric current to form.
“That’s why we developed this sandwich structure,” Wang says. “We have the top and bottom layers that block charge injection from the electrodes. Then in the central layer we can put all of the high dielectric constant ceramic/polymer filler material that improves the energy and power density.”
The outer layers, composed of boron nitride nanosheets in a polymer matrix, are excellent insulators. While the central layer is a high dielectric constant material called barium titanate.
“We show that we can operate this material at high temperature for 24 hours straight over more than 30,000 cycles and it shows no degradation,” Wang says.
Comparison to BOPP
A comparison of BOPP and the sandwich structure nanocomposite, termed SSN-x, in which the x refers to the percentage of barium titanate nanocomposites in the central layer, shows that at 150 degrees C, SSN-x has essentially the same charge-discharge energy as BOPP at it typical operating temperature of 70 degrees C. However, SSN-x has several times the energy density of BOPP, which makes SSN-x highly preferable for electric vehicle and aerospace applications as an energy storage device due to the ability to reduce the size and weight of the electronics significantly while improving system performance and stability. The elimination of bulky and expensive cooling equipment required for BOPP is an additional bonus.
“Our next step is to work with a company or with more resources to do processability studies to see if the material can be produced at a larger scale at a reasonable cost,” Wang says. “We have demonstrated the materials performance in the lab. We are developing a number of state-of-the-art materials working with our theory colleague Long-Qing Chen in our department. Because we are dealing with a three-dimensional space, it is not just selecting the materials, but how we organize the multiple nanosized materials in specific locations. Theory helps us design materials in a rational fashion.”