Li-ion batteries get a boost from tiny tubes, rods

Tiny tubes and rods are becoming the go-to solution for boosting power and durability in Li-ion batteries. Researchers at the Energy Department's National Renewable Energy Laboratory (NREL) said that if successful, the batteries will last longer and perform better, leading to a cost advantage for electric vehicles.

They have created crystalline nanotubes and nanorods to deal with the devils inherent in Li-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 towards 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.

"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."

Carbon Nanotubes Both Bind and Conduct

Typical Li-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."

In a Li-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.

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."

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