Researchers at the Energy Department's National Renewable Energy Laboratory (NREL) are turning to extremely tiny tubes and rods to boost power and durability in lithium-ion batteries, the energy sources for cell phones, laptops, and electric vehicles. If successful, the batteries will last longer and perform better, leading to a cost advantage for electric vehicles.
Transportation and communication around the world increasingly rely on lithium-ion batteries, with cell phones ubiquitous on six continents, and electric vehicles on pace to accelerate from a $1 billion worldwide market in 2009 to $14 billion by 2016, according to analysts Frost and Sullivan.
NREL Scientist Chunmei Ban assembles a lithium-ion battery in the materials lab at the Solar Energy Research Facility at NREL. Photo by Dennis Schroeder, NREL
NREL's Energy Storage group is working with the Energy Department, automotive battery developers, and car manufacturers to enhance the performance and durability of advanced lithium-ion batteries for a cleaner, more secure transportation future, said Energy Storage Group Manager Ahmad Pesaran. "The nanotube approach represents an exciting opportunity — improving the performance of rechargeable lithium-ion batteries while make them last longer," Pesaran said. "Increasing the life and performance of rechargeable batteries will drive down overall electric vehicle costs and make us less reliant on foreign sources of energy."
Scientists at NREL have created crystalline nanotubes and nanorods to attack the major challenges inherent in lithium-ion batteries: they can get too hot, weigh too much, and are less than stellar at conducting electricity and rapidly charging and discharging.
NREL's most recent contribution toward much-improved batteries are high-performance, binder-free, carbon-nanotube-based electrodes. The technology has quickly attracted interest from industry and is being licensed to NanoResearch, Inc., for volume production.
Nanotechnology refers to the manipulation of matter on an atomic or molecular scale. How small? A nanometer is one-billionth of a meter; it would take 1,000 of the nanotubes in NREL's project lined up next to each other to cross the width of a human hair.
Yet, scientists at NREL are able not only to create useful objects that small, but guide their formations into particular shapes. They've combined nanotubes and nanorods in such a way that they can aid battery charging while reducing swelling and shrinking that leads to electrodes with shortened lifetimes.
"Think of a lithium-ion battery as a bird's nest," NREL Scientist Chunmei Ban said. "The NREL approach uses nanorods to improve what is going on inside, while ensuring that the nest remains durable and resilient."
"We are changing the architecture, changing the chemistry somewhat," without changing the battery itself, she said.
NREL's work was supported by the Energy Department's Vehicle Technology Office under the Battery for Advanced Transportation Technologies (BATT) program, which focuses on reducing the cost and improving the performance and durability of the lithium-ion batteries that power electric vehicles.
Carbon Nanotubes Both Bind and Conduct
Typical lithium-ion batteries use separate materials for conducting electrons and binding active materials, but NREL's approach uses carbon nanotubes for both functions. "That improves our mass loading, which results in packing more energy into the same space, so better energy output for the battery," Ban said. "The NREL approach also helps with reversibility—the reversing of chemical reactions that allows the battery to be recharged with electric current during operation. If we can improve durability and reversibility, we definitely save money and reduce cost."
Single-wall carbon nanotubes (SWCNTs) are expensive, but scientists and engineers working in the field are confident that as the use of SWCNT-based electrodes grows wider, their price will fall to a point where they make economic sense in batteries, Ban said.
In a lithium-ion battery, lithium ions move back and forth in the graphite anode through an electrolyte; the ions are injected between the carbon layers of graphite, which is durable but unnecessarily dense. At the same time, electrons flow outside the battery through an electric load from the cathode to the anode. Electrolytes are essential in rechargeable batteries because they close the circuit inside the batteries by allowing ions to transfer; otherwise, the battery can't continue to conduct electricity from the positive to the negative poles and back again.
High-energy materials, such as metal oxides and silicon anodes, have massive volume changes when lithium ions are injected and extracted from the electrode material. They swell and shrink, gather into a cluster and touch each other, shrinking in unison, causing collapse and subsequent cracks that can harm performance, leading to destruction of the electrode and thus lower lifetime.
Certain metal oxides do a better job than graphite of teaming with the electrodes. But while they improve on the energy content and reversing functions, they still contribute to the large expansion in volume and the destruction of the internal structure.
The NREL team turned to iron oxide, which is abundant, safe, inexpensive, and shows great promise. Yet, to be effective, the size of the iron oxide nanoparticles had to be just right—and had to be maintained in a strong matrix that was both flexible and resilient to deal with large volume changes while optimally conducting electricity.