July 28, 2008
Carbon nanosheets hold more
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.
Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable
Lithium Ion Batteries
Nano Letters, July 24, 2008
Technology Group, AIST
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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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