SEM image of the cross section of photo-thermally reduced graphene shows an expanded structure. The graphene sheets are spaced apart with an inter-connected network allowing for greater electrolyte wetting and lithium ion access for efficient high rate performance in lithium ions batteries. |
Engineering
researchers at Rensselaer Polytechnic Institute made a sheet of paper
from the world’s thinnest material, graphene, and then zapped the paper
with 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 10 times faster than conventional graphite
anodes used in today’s lithium-ion (Li-ion) batteries.
Rechargeable
Li-ion batteries are the industry standard for mobile phones, laptop
and tablet computers, electric cars, and a range of other devices. 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 it
takes about an hour to charge your mobile phone or laptop battery, and
why electric automobile engines cannot rely on batteries alone and
require a supercapacitor for high-power functions such as acceleration
and braking.
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
realizing this grand goal. Such batteries could also significantly
shorten the time it takes to charge portable electronic devices from
phones and laptops to medical devices used by paramedics and first
responders.
“Li-ion
battery technology is magnificent, but truly hampered by its limited
power density and its inability to quickly accept or discharge large
amounts of energy. By using our defect-engineered graphene paper in the
battery architecture, I think we can help overcome this limitation,”
said Koratkar, the John A. Clark and Edward T. Crossan Professor of
Engineering at Rensselaer. “We believe this discovery is ripe for
commercialization, and can make a significant impact on the development
of new batteries and electrical systems for electric automobiles and
portable electronics applications.”
Results of the study were published this week by the journal ACS Nano.
Koratkar
and his team started investigating graphene as a possible replacement
for the graphite used as the anode material in today’s Li-ion batteries.
Essentially a single layer of the graphite found commonly in our
pencils or the charcoal we burn on our barbeques, graphene is an
atom-thick sheet of carbon atoms arranged like a nanoscale chicken-wire
fence. In previous studies, Li-ion batteries with graphite anodes
exhibited good energy density but low power density, meaning they could
not charge or discharge quickly. This slow charging and discharging was
because lithium ions could only physically enter or exit the battery’s
graphite anode from the edges, and slowly work their way across the
length of the individual layers of graphene.
Koratkar’s
solution was to use a known technique to create a large sheet of
graphene oxide paper. This 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
literally 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.
The
researchers quickly learned this damaged graphene paper performed
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 used the cracks and pores as shortcuts
to move quickly into or out of the graphene—greatly increasing the
battery’s overall power density. Koratkar’s team demonstrated how their
experimental anode material could charge or discharge 10 times faster
than conventional anodes in Li-ion batteries without incurring a
significant loss in its energy density. Despite the countless microscale
pores, cracks, and voids that are ubiquitous throughout the structure,
the graphene paper anode is remarkably robust, and continued to perform
successfully even after more than 1,000 charge/discharge cycles. The
high electrical conductivity of the graphene sheets also enabled
efficient electron transport in the anode, which is another necessary
property for high-power applications.
Koratkar
said the process of making these new graphene paper anodes for Li-ion
batteries can easily be scaled up to suit the needs of industry. The
graphene paper can be made in essentially any size and shape, and the
photo-thermal exposure by laser or camera flashes is an easy and
inexpensive process to replicate. The researchers have filed for patent
protection for their discovery. The next step for this research project
is to pair the graphene anode material with a high-power cathode
material to construct a full battery.
Along
with Koratkar, co-authors of the paper are Rensselaer graduate students
Rahul Mukherjee, Abhay Varghese Thomas, and Ajay Krishnamurthy, all of
the Department of Mechanical, Aerospace, and Nuclear Engineering (MANE).
The
study was funded by the National Science Foundation, and supported by
Koratkar’s John A. Clark and Edward T.Crossan Endowed Chair
Professorship at Rensselaer. Koratkar is a professor in MANE and the
Department of Materials Science and Engineering at Rensselaer. He is
also a faculty member of the university’s Center for Future Energy
Systems and the Rensselaer Nanotechnology Center.
Photo-thermally reduced graphene as high power anodes for lithium ion batteries
Source: Rensselaer Polytechnic Institute
