From grid-scale renewable energy storage to mass-produced electric vehicles, the energy infrastructure of tomorrow relies heavily on batteries. High-capacity batteries, in particular, are hot commodities for the future of energy infrastructure.
However, it is not a simple matter to develop breakthroughs in battery technology and engineering. Over the years, lithium ion batteries using tried-and-true materials have been optimized near their theoretical limits. To overcome the structural limitations, new materials need to be tested.
The research and development department of EUROCELL, a next-generation battery manufacturing company based in Osan, South Korea, explores methods to engineer high-capacity anode materials as part of developing high-capacity batteries.
The most common anode material in commercially available Li-ion batteries is graphite. As a battery material, graphite is abundant, inexpensive, and performs reliably. However, the need for higher-capacity batteries continues to grow, leaving graphite—which is optimized to function near the limits of its theoretical capacity—behind.
To overcome this limitation, my team and I have researched an alternative anode material with high energy density: silicon.
Graphite is the most common anode material in commercially available Li-ion batteries because of its good stability and low potential. However, its limited theoretical capacity of 372 mAh/g falls short of the energy density required to satisfy ever-growing demands.
Silicon is one of the most promising anode materials for high-capacity Li-ion batteries. The theoretical capacity of silicon is 4200 mAh/g, which is approximately 11 times higher than that of graphite. However, typically when charging and discharging, silicon particles dramatically expand and contract by up to 400 percent. Addressing this and other serious hurdles has been a focus of our work.
Our research has recently achieved an engineering breakthrough. We have found a new way to harness a combination of commercially available silicon and graphite with surprising results.
In general, it is extremely difficult to create silicon particles smaller than 300 nanometers (nm) through a process of mechanical pulverization. In order to suppress silicon’s severe volume expansion, particles of 100 nm or less must be manufactured. However, high costs of production make this method commercially unavailable.
Our research team engineered a sophisticated ball mill that could successfully pulverize micro-sized silicon to less than 100 nm. Further, we developed a controllable and scalable method to create structured composites by using our advanced ball-milling and spray-drying methods.
Analysis under field emission scanning electron microscope revealed the core-shell structure: Silicon nanoparticles were successfully embedded onto the graphite core and coated with an amorphous carbon shell. This design shortens the distance of the movement of lithium ions to minimize structural stress, resulting in a higher power density than existing artificial graphite. This anode structure then could be fabricated into a pouch-type full cell for field testing, using a standard NCA (lithium nickel cobalt aluminum oxide based) cathode.
A drone flight test recently was conducted using the breakthrough battery design. The successful flight produced impressive results—flight times using the new battery were 155% longer than those using pre-existing batteries of the same volume.
Our lab also is developing the means to conserve raw material waste produced in battery manufacturing.
Generally, flake graphite mined from nature must be processed into spherical graphite in order to be used as an anode material. Unfortunately this process causes environmental pollution, because a large amount of acidic solution is used to increase the purity of graphite. Spherical graphite processing results in huge amounts of raw material waste—up to 70%—and relies intensively on the use of acid, which is also severely dangerous to the environment.
We, on the other hand, have been able to take 70% of the waste described above and reassemble it into more fully usable spherical assembly graphite. Our success shows that the mass production of batteries can be completed in an eco-friendly way, thus minimizing environmental pollution while being economically viable.
A full-cell pouch battery was manufactured by combining the spherical assembly graphite produced through the above-described process with the NCM622 (lithium nickel cobalt manganese oxide based) cathode material. By conducted charging and discharging tests (from 1°C to 10°C conditions), we were able to observe the excellent power density of spherical assembly graphite. The capacity retention rate (ability of the battery to retain stored charge when not plugged in) after an under-six-minute rapid charge was still 81.7% compared to one hour charging.
Our lab’s promising results show that new batteries can be successfully engineered to solve the high-capacity and high-power problems of existing batteries.
*Dr. Byung-gwan Lee is the director of research and development at EUROCELL, INC., a Korean company that produces lithium ion batteries.