Could you imagine a day when you print out an extra paper battery before heading on a road trip?
Researchers at the Nanostructures for Electrical Energy Storage (NEES) have used paper to create a template for a device that combines the properties of a battery -- holds a lot of energy but distributes it slowly -- with those of a capacitor -- holds little energy but distributes it very fast.
Harnessing the power of cellulose. Most paper products are composed of cellulose, a type of natural fiber taken from the cell wall of different plants. Layering these fibers on top of each other creates a distribution of large and small pores. By coating the fibers with different materials, the cellulose-based compound can be used as a cathode in lithium-ion batteries. In general, batteries are made up of an anode and a cathode that sandwich an electrolyte. The charge/discharge rate is determined by how fast the lithium ion can move from the anode to the cathode through the liquid electrolyte, which is influenced by the small micro-pores and larger meso-pores of the cathode.
Xinyi Chen, lead author on the paper, describes the motivation to use templates based on cellulose, a relatively inexpensive battery material. "Much of electrochemical research attempts to optimize the electron transport and lithium ion diffusion in the solid. We wanted to identify a relationship between the meso-porosity in the cellulose and the lithium ion migration in the liquid electrolyte to optimize the electrochemical performance."
To isolate the effects of porosity, Chen and his colleagues coated the cellulose fibers with carbon nanotubes and then with a thin layer of vanadium oxide, or V2O5. The carbon nanotubes quickly conduct electrons away from the interface so that the conduction of electrons is not the limiting factor for performance. The vanadium oxide is coated using atomic layer deposition, which allows extreme control of depositing thicknesses on the order of 10 nanometers, approximately 1/10th the size of a red blood cell! This thin layer of V2O5 shortens the diffusion length of the lithium ions and, thus, the limiting factor becomes the lithium ion movement in the electrolyte.
The researchers compared two versions of this cellulose structure. The first was coated with carbon nanotubes and vanadium oxide. The second was coated with a dense aluminum oxide layer to block the medium-size pores and subsequently coated with carbon nanotubes and vanadium oxide. The results showed a higher charge transfer resistance for the blocked electrode meaning lower performance for a given charge or discharge rate. They also found the unblocked version had a relatively large capacity while also operating under a large range of charge and discharge rates. This performance combines the properties of a battery with those of a capacitor. This combination of behavior was attributed to the thin layer of vanadium oxide that utilizes lithium-ion storage in the bulk as well as on the surface.
"By using a thin layer of vanadium, we take advantage of additional charge storage on the surface through double-layer formation and surface defects. This in turn increases our actual capacity beyond the theoretically calculated value that is based on the bulk lithium ion storage value," said Chen.
What the future holds. By combining the properties of a battery and a capacitor, these devices could be used in applications where it is important for high power and long cycle life, such as in stationary energy storage. Renewable energy technologies such as wind-powered electrical generation need to store their harvested energy at a high charge rate. Distributing this energy efficiently also requires a high rate to match variable energy usage for smart grid management.
The research described in this paper allows us a glimpse into the future of energy storage, such as the development of flexible energy storage. While traditional batteries are limited to small hard cases, these thin, flexible batteries could be integrated into clothing to allow charging on the go!
The next step? Large-scale manufacturing. "This device relies on complex research equipment that is not ideal for commercialization. The logical next step is to work on large-scale deposition. For example, a breakthrough in the area of printing materials on a cellulose substrate would allow mass production," said Chen.
With research like this, it is not so crazy to imagine a day when you print out an extra paper battery before heading out on your road trip. Just don't forget to thank Chen and his colleagues at the Nanostructures for Electrical Energy Storage research center.
