Batteries made from graphene could power tomorrow’s EVs
23 August 2012
Researchers are using intentionally blemished graphene paper to create an easy-to-make, quick-charging lithium-ion battery with high power density.
SEM image of the cross section of photo-thermally reduced graphene showing the expanded structure (photo courtesy Rensselaer Polytechnic Institute)
Engineering researchers at Rensselaer Polytechnic Institute have made a sheet of paper from the world’s thinnest material, graphene, and then exposed it to a laser or camera flash to blemish it with countless cracks, pores, and other imperfections. The result is a graphene anode material that can be charged or discharged ten times faster than conventional graphite anodes used in today’s lithium-ion (Li-ion) batteries.
While Li-ion batteries have a high energy density and can store large amounts of energy, they suffer from a low power density and are unable to quickly accept or discharge energy. This low power density is why electric vehicle (EV) engines cannot rely on batteries alone and require a supercapacitor for high-power functions such as acceleration.
The Rensselaer research team, led by nanomaterials expert Nikhil Koratkar, sought to solve this problem and create a new battery that could hold large amounts of energy but also quickly accept and release this energy. Such an innovation could alleviate the need for the complex pairing of Li-ion batteries and supercapacitors in electric cars, and lead to simpler, better-performing automotive engines based solely on high-energy, high-power Li-ion batteries.
Koratkar and his team are confident their new battery, created by intentionally engineering defects in graphene, is a critical stepping stone on the path to realising this goal. Such batteries could also significantly shorten the time it takes to charge portable electronic devices from phones and laptops to medical devices.
The graphene oxide paper is about the thickness of a piece of everyday printer paper, and can be made nearly any size or shape. The research team then exposed some of the graphene oxide paper to a laser, and other samples of the paper were exposed to a simple flash from a digital camera.
In both instances, the heat from the laser or photoflash caused mini-explosions throughout the paper, as the oxygen atoms in graphene oxide were violently expelled from the structure. The aftermath of this oxygen exodus was sheets of graphene pockmarked with countless cracks, pores, voids, and other blemishes. The pressure created by the escaping oxygen also prompted the graphene paper to expand five-fold in thickness, creating large voids between the individual graphene sheets.
This damaged graphene paper performs remarkably well as an anode for a Li-ion battery. Whereas before the lithium ions slowly traversed the full length of graphene sheets to charge or discharge, the ions now use the cracks and pores as shortcuts to move quickly into or out of the graphene, greatly increasing the battery’s overall power density.
Despite the countless microscale pores, cracks, and voids that are ubiquitous throughout the structure, the graphene paper anode is remarkably robust, and continues to perform successfully even after more than 1,000 charge/discharge cycles.
Results of the study are published in the journal ACS Nano