July 28, 2008

Carbon nanosheets hold more electricity

Electrodes made from single-atom-thick sheets of carbon could boost the storage capacity of lithium batteries several times over. The graphene sheets can store 540 milliampere-hours per gram, roughly twice the capacity of ordinary graphite.

Sandwich layers of carbon nanotubes or carbon buckyballs between the sheets and the storage capacity rises to about 750 milliampere-hours per gram, which is in the high range for experimental battery anode materials. Today's lithium-ion batteries generally have capacities under 200 milliampere-hours per gram.

Research paper:
Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries
Nano Letters, July 24, 2008

Researchers' homepage:
Energy Interface Technology Group, AIST

Related stories and briefs:
Nanotubes pack power -- nanotubes that outperform graphite

Back to ERN July 28/August 4, 2008


July 31, 2008

1. The claimed capacity of 540 mAh/g is actually only 1.45 times, not twice, the capacity of graphite (372). Other materials, such as silicon composites, reach 1100 and more.
2. The voltage is just as important as the capacity. Look at the discharge curve in the original paper and draw a line across from the usefuless limit of 1.5 volts for a lithium negative electrode. Any capacity shown above the line is useless because the difference between it and the 4V positve electrode is too small. Now, looking at the parts of the graphene discharge curves that are below the line, we see only HALF the capacity of graphite!
3. The effects of buckyballs are irrelevant because they are in the 3V region - useless for a negative electrode.
4. The asymmetry of the charge discharge curves suggest that the results are distorted by side reactions and are therefore not a reliable measure of how much lithium is stored.

Professor John R. Owen
University of Southampton



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