Using carbon nanotubes in lithium batteries can actually increase the batteries’ power tenfold, according to researchers at the Massachusetts Institute of Technology (M.I.T.). Conventional batteries consist of a negative electrode called the anode and a positive electrode called the cathode, which are separated by an electrolyte, an electrically conductive material through which ions can move easily.
When a battery is in use, positively charged lithium ions travel across the electrolyte to the cathode, producing an electric current. An external current then causes the recharged ions to move the opposite way, so they set into the spaces in the porous material of the anode.
However, the M.I.T. research team discovered that substituting a carbon nanotube for one of the electrodes in a lithium battery can boost the amount of power it can deliver for a given weight of material. The team was led by Yang Shao-Horn, associate professor of mechanical engineering and materials science and engineering, in collaboration with Paula Hammond, Bayer chair professor of chemical engineering.
Batteries embedded with the new material demonstrate some of the advantages of both capacitors, which can produce very high power outputs in short bursts, and lithium batteries, which can provide lower power steadily for long periods.
The research team found that a battery can produce power for a given weight of the new electrode material five times more than for conventional capacitors, with the total power delivery rate 10 times greater than that of conventional lithium-ion batteries.
The team attributed the battery’s performance to the good conduction of ions and electrons in the electrode and the efficient lithium storage on the surface of the nanotubes.
In addition to their high power output, the carbon nanotube electrodes showed very good stability over time. There was no detectable change in the material’s performance even after 1,000 cycles of charging and discharging.
To produce the new electrode material, the researchers used a layer-by-layer fabrication process, in which a base material is alternately dipped in solutions that contain carbon nanotubes treated with simple organic compounds, giving the nanotubes either a positive or a negative net charge.
When the layers are alternated on a surface, they self assemble into a tightly bound structure that is porous at the nanometer scale because of the complementary charges, creating a stable and durable film.
In addition, the carbon nanotubes possess many oxygen groups on their surfaces, which can store a large number of lithium ions, allowing the nanotubes to serve as the positive electrode in lithium batteries instead of just the negative electrode.
This electrostatic self-assembly process is the key to the process because ordinarily, carbon nanotubes tend to clump together in bundles, leaving fewer exposed surfaces to undergo reactions.
The team aims to improve the material production process as the method of dipping a substrate into two different solutions is relatively time-consuming. They believe that the process can be modified to allow the spraying of the alternate layers of substrate on a moving ribbon of material. This will eventually allow a continuous manufacturing process for future high volume commercial production.
The new electrodes might find applications in small portable devices, and with further research, they may also lead to improved batteries for larger, more power-hungry applications such as hybrid vehicles, the team said.
The researchers reported their findings in a paper published on June 20 in the journal Nature Nanotechnology.
