Energy storage has long been a major issue in solving the energy crisis. Energy, in forms such as wind energy, is quite abundant. However, the wind does not always blow when energy is needed, nor does the sun shine when it is convenient for us. The limiting factor with these "free" forms of energy is storage. One proposed storage solution is in the form of batteries. However, with our current battery technology we are not able to feasibly store energy on the scale that we use it.
One obvious solution, if possible, would be to increase the storage capacity of the batteries. Though no major breakthrough has yet been found, we have make some improvement in recent years through the addition of materials such as carbon nanotubes and graphene. Graphene is a single layer (one atom thick) of graphite, the material often used as pencil lead. Graphene is made of carbon with a hexagonal bond shape. Its electronic properties vary greatly from graphite since charge carriers (electrons and holes) can only move in two rather than three dimensions.  Graphene can be rolled upon itself to form a carbon nanotube. A single rolled sheet of graphene is called a single-wall nanotube, and acts as either a metal or semiconductor based on the orientation in which it rolls. A series of tubes rolled around each other is called a multi-wall nanotube.  Though the basic properties of graphene and nanotubes are well know, there is a significant amount of research still to be done on these materials.
A research group at MIT, led by Dr. Yang Shao-Horn, has recently published the results of their work on carbon nanotube enhanced batteries. In their experiment, the group built batteries with modified cathodes (the positive electrode of the battery) and lithium titanium oxide anodes (the negative electrode of the battery).  The cathode was made entirely out of commercially available multi-wall carbon nanotubes. To fabricate a proper cathode, the group used a method by which they increased the thickness of the cathode layer by layer of carbon nanotubes. The cathodes reported in their paper were a maximum of 3 micrometers thick. These cathodes are miniscule in comparison to traditional lithium-ion electrodes which are on the order of 100 to 200 micrometers thick. 
The electrodes are thin because the current process of making these nanotube electrodes is quite tedious and time consuming. The electrode would be washed in a bath of carbon nanotubes in a solution. It would then be left out to dry, with gravity pulling the nanotubes down through the solution on the electrode and depositing a new layer on the previous top layer of the electrode. The drying time is on the order of 15 minutes, and since each layer of nanotubes is very thin the process had to be repeated about 400 times to create one usable cathode.  To fix this issue, new work is being done to develop a method by which carbon nanotubes are sprayed onto the electrode layer by layer. The group estimates this method could speed up production of nanotube cathodes by about 100 times, allowing for the production of much thicker and more interesting nanotube cathodes. 
The result of building such an electrode is that the nanotube structure has a much larger surface area per volume than do traditional electrode materials. The increased area of exposed electrode surfaces allows more charge to be stored and charge carriers to move faster.  The group estimates that each layer of electrode can store about 200mAh/g of charge and deliver 100kW/kg of power. The group also determined there was minimal degradation of the nanotube electrode after a few thousand cycles of charging and discharging. The group's nanotube and lithium titanium oxide batteries were found to store about five times the amount of charge and deliver about ten times the power of a comparably sized lithium-ion battery. 
A research group at the Pacific Northwest National Laboratory led by Dr. Gary Yang has built battery devices where the traditional lithium titanium oxide electrodes were coated with a sheet of graphene. They showed that the addition of graphene greatly increased the recharge time of the batteries. The group estimated that their modified batteries, with a pre-modified recharge time of about two hours, could be recharged in only ten minutes.  Commercial production of these devices would mark a significant improvement in the performance of lithium-ion batteries.
There are a number of challenges which must be addressed before devices similar to those made by the Shao-Horn group at MIT could be used for commercial purposes. As discussed above, the nanotube electrode must be made many times thicker to deliver the power that many electronic devices require. At the same time, there is a shortage of affordable, high quality multi-wall carbon nanotubes for large scale commercial production.  There is also no guarantee that the properties shown by the group for electrodes on the order of 3 micrometers will scale to devices whose electrodes are on the order of 50 micrometers or greater. Dr. Shao-Horn suggests there is no such theoretical limitation, though there has been no experimental evidence to back this.  Presently, the devices the group makes could be used to power microelectronic devices. However, serious advances in size must be made before such devices could be used to power reasonably sized electronic devices, replace the batteries in electric cars, or store significant amounts of energy produced from wind, solar, or other forms of alternative energy.
© Matthew Yankowitz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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