Plain white office paper could be the basis for efficient batteries. Scientists have converted sheets of them into efficient electrical storage devices using ink loaded with carbon nanotubes. This new spin on an ancient invention is the latest in a line of research striving to incorporate paper into batteries to reduce their weight, one of battery technology's major shortcomings.
To trim weight, researchers have tried several approaches, including the use of thin films of materials laid down as inks. The appeal of paper for centuries—its porous microscopic structure, which makes it ideal for holding onto inks—now intrigues researchers for modern applications, not to mention that paper is also a flexible, lightweight, affordable, well-established technology that is used everywhere and could be made from renewable sources.
To devise the novel paper batteries, materials scientist Yi Cui of Stanford University and his colleagues coated plain copy paper with black ink made with single-walled carbon nanotubes, which are electrically conductive pipes only billionths of a meter wide. Positive and negative electrodes—cathodes and anodes—were then applied as slurries dried on the nanotube-impregnated paper. (The cathodes were made from lithium manganese oxide nanorods, and the anodes made either from nanopowders of lithium titanium oxide or nanowires with cores of carbon covered with shells of silicon.)
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The batteries were then dipped in an electrolyte of lithium hexafluorophosphate solution to connect the electrodes and sealed in a pouch. In this setup the nanotubes collected current from each electrode. "It provides a nice example of the combination of high tech with low tech," says materials scientist John Rogers at the University of Illinois at Urbana–Champaign, who did not take part in this research.
Close to conventional storage cells
These paper batteries, described online December 7 in the Proceedings of the National Academy of Sciences , were as good as conventional ones in terms of energy storage and recharging-cycle life. Based on their experiments, the researchers say that incorporating carbon nanotube paper into conventional rechargeable batteries could reduce their weight by up to 20 percent.
This reduction could help make electric and hybrid vehicles more feasible and could lead to longer-lasting mobile phones, laptops and other portable electronics. "The fabrication of inexpensive, flexible and lightweight batteries could revolutionize portable electronics," remarks materials scientist Francesco Stellacci of the Massachusetts Institute of Technology, who did not participate in this study. At large scales, they could also inexpensively help solar and wind farms store what energy they generate, Cui adds.
The carbon nanotubes bonded very strongly to the paper, obviating the need for adhesives that decrease performance and significantly increase production costs, Cui and his colleagues note. The battery could also bend and curl without losing its ability to conduct a charge and "can be easily laminated into flexible computers to power the devices," he suggests. The team noted in their paper that its technique is easily scalable for mass production, and that the ink could even be painted on with brushes, if desired.
Paper trail
These new devices are the latest in a series of paper batteries that have emerged in recent years. For instance, in 2007 biopolymer expert Robert Linhardt of Rensselaer Polytechnic Institute in Troy, N.Y., and his colleagues made batteries with a composite of multiwalled carbon nanotubes and paper as the cathode, a lithium metal film as the anode and aluminum foil as the current collector. They carried out experiments where sweat or blood operated as their electrolytes, suggesting they could find uses in bodily implants.
Nanotechnologist Maria Strømme of Uppsala University in Sweden and her colleagues have also devised batteries with a paper made from seaweed. Because such paper had 100 times more surface area than that made from wood, it can hold dramatically more power, they reported in the October 14 Nano Letters.
One concern about the new sheets is their electrical resistances, which are some 10 times or more than those of the metal foils used as current collectors in conventional batteries. Such resistance slows the delivery of power. Cui suggests incorporating metal nanowires into their devices to lower resistance, thereby helping provide more electrical oomph.
Another major obstacle to implementing these findings is the current high price of carbon nanotubes. "However, carbon nanotube price will continuously drop as production ramps up," Cui notes. "The conductive paper concept can also be realized with other nanomaterials with potentially low cost, such as graphene."
