NREL tapped the unique properties of SWCNTs to address the challenges of heat, weight, and discharging all at once. "We use the carbon nanotube in this flexible network to make a conductive rope-like wrap," Ban said. So, when there is shrinkage, those wraps allow the electrons to reach the iron oxide and continue on the conductive path unabated. Using nanoparticles shortens diffusion length, enhancing the capability of fast charging and discharging. Using abundant inexpensive material means less need for such expensive metals as cobalt, currently used in lithium ion batteries' cathodes, lowering overall cost."
Building Better Anodes and Cathodes
The SWCNT with iron oxide solution produced a power density triple that of graphite, which means strong performance while eliminating much of the weight of a battery that depends on graphite. To get there, it was essential that the iron oxide particles be distributed uniformly within the encircling nanotubes.
Ban and NREL colleague Zhuangchun Wu used hydrothermal synthesis and vacuum filtration to build lithium-ion anodes that don't require the typical binders (the adhesion strength that allows the battery to endure charge-discharge cycling) yet have high capacity. The first step was to make iron oxide nanorods as precursors for making electrodes. Ban and her colleagues discovered that at 450°C, annealing the iron hydroxide nanorods with SWCNTs would produce iron oxide. And, the SWCNTs contributed just 5% to the weight. Not only did the SWCNTs actually facilitate the formation of the iron oxide particles, but they ensured excellent physical and electrical contact between the two materials.
For cathode electrodes, they embedded NMC—lithium nickel manganese cobalt oxide—in the nanotubes, causing the nanoparticles to become very conductive. The resulting nanocomposite retains 92% of its original capability to store and conduct electrical charges even after 500 cycles of charging and recharging.
Expertise in Wet-Chemistry Synthesis Guided the Ideal Shapes
It's not as easy as simply putting nanomaterials into batteries, Ban said. "You need a special process to make it work." Ban and her NREL colleagues Wu and Anne Dillon used a vacuum filtration process to combine inexpensive iron oxide with carbon nanotubes.
Ban brought her experience in wet-chemistry synthesis to the challenge of influencing the shapes of the nanomaterials to make them in the form of rods. "We know how to change the synthesis conditions to direct the design or realize the structure and shape of nanomaterials," Ban said.
They chose a rod shape because they thought that would integrate well with the nanowires and curvatures of nanotubes, wrapping around them to create a robust electrode.The unusually long and very flexible strands of the nanomaterials are crucial to the superior features of the electrodes. They attach intimately to the particles, and their porosity allows for ideal diffusion.
A Rechargeable Battery That Lasts
The innovative electrodes conceived by NREL can mean superior capacity, performance, and safety for lithium-ion batteries.
David Addie Noye, who founded NanoResearch, Inc., with a plan to commercialize proven nanoscience innovations, visited NREL, saw the process, and decided to license the technology. The nanomaterial chemistry innovation and manufacturing process innovation that results in binderless electrodes "is a game changer because it helps solve a fundamental problem the lithium-ion battery industry has not been able to solve for decades," he said.
The improvements in the lithium-ion batteries offered by NREL's approach also can make a difference in portable consumer electronics, such as laptops, tablets, cell phones, and portable media, as well as the stationary energy storage devices that will become increasingly important as more variable-generation renewable energy enters the grid.
"We aren't making a new battery, but we're changing the architecture somewhat by using SWCT wrapped metal oxide anodes," Ban said. "By so doing, we improve the mass loading, energy output per weight, and volume." The process ensures a faster charge, and that's what is most essential to manufacturers and their customers. That means fewer trips to the recharging station, and a battery that keeps going and going and going.