Molten salts are used to produce aluminium, about 50Mt/year, magnesium, sodium, lithium and in some nuclear reactors so that plants are available on an industrial scale. In the Hall-Heroult cell for the production of aluminium a carbon based anode is used and the cathode is a molten layer of aluminium supported on a carbon/graphite base. The applied voltage is about 4.5 V and the current density is about 1A/cm2 at 950oC. As well as the cathodic deposition of aluminium, sodium is also deposited from the electrolyte of alumina dissolved in cryolite (Na3AlF3) which dissolves in the aluminium at around 80 ppm and gradually intercalates into the graphite.
In the laboratory, when pure sodium is forced into graphite by the electrolysis of sodium chloride, it was found that the graphite disintegrated due to the fact that the diameter of the sodium (380 pm) is greater than the interlayer spacing of 335 pm in graphite. If the same experiment is performed using lithium chloride, lithium, diameter 334 pm, is intercalated into the graphite and, depending on the grain size of the graphite, the temperature and the current density, nanoscrolls or nanoparticles are formed. The scrolls, rather than consisting of concentric tubes, consist of a single sheet of rolled up graphene. It is also possible to fill the scrolls with low melting point metals or metals that that form a liquid alloy with lithium by co-depositing the metal with the lithium. These filled nanoscrolls have been used as anodes in lithium ion batteries where normally the large volume changes that occur on the charging or discharging cause the metal to decrepitate and lose electrical contact. With the metal filled scrolls, the lithium is able to diffuse around the scroll and the volume changes can be accommodated by the scroll unrolling.
Graphene has been described as the wonder material but its wide application has been restricted by its expense and lack of commercial production. A natural follow on from the production of nanoscrolls by the intercalation of lithium is to intercalate a smaller atom, namely hydrogen (diameter 106 pm) into the graphite by using an electrolyte that contained protons. Instead of scrolls, individual high quality graphene sheets were formed at a significant rate (50kg/m2 of graphite/day).
Graphene produced by this route has been used to wrap silicon nanoparticles for use as anodes in lithium ion batteries greatly increasing the capacity of the anode, in supercapacitors in conjunction with carbon nanotubes to generate high capacities and rapid rates of charge and discharge and as a means of strengthening alumina.
One interesting aspect of these novel methods of producing carbon nanoproducts was the observation of nanoparticles of lithium carbonate trapped in graphene cages. When samples with this structure were heated in air, at atmospheric pressure, is it was found that perfectly shaped nanodiamonds were formed which have numerous applications.
Industrial graphite, which formed the starting material for this interesting research, costs a few dollars/kg yet the present costs of nanotubes, graphene and nanodiamonds is, at least, an order of magnitude larger. As mentioned at the beginning of this article, aluminium is produced on a large scale for a relatively modest cost ($2/kg) using electricity and carbon and alumina, as the starting materials. The quantity of carbon that is consumed in the Hall-Heroult cell is similar to that in the work reported in this paper and the applied voltages and current densities are almost identical. It does seem possible that the techniques and equipment, developed over decades in the metallurgical industries, can be used to produce nanoscrolls, nanoparticles, graphene and nanodiamonds on an industrial scale for modest cost thereby increasing the number of applications for these innovative materials.
Ali Reza Kamali, Derek Fray
Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, UK
The electrochemical interaction between graphite and molten salts to produce carbon nanostructures is reviewed. It is demonstrated that, depending on the conditions, it is possible to electrochemically convert graphite in molten salts to either carbon nanoparticles and nanotubes, metal-filled carbon nanoparticles and nanotubes, graphene or nanodiamonds.
The application of metal-filled carbon nanotubes as anodes in lithium-ion batteries is reviewed. Surprisingly, this method of preparation is relatively simple and very similar to the mass production of aluminium in molten sodium aluminium fluoride–alumina mixtures, which is performed economically on a tonnage scale, indicating that it may be possible to apply it for the production of novel carbon nanostructures.Go To Journal of Materials Science