U of Waterloo makes major battery breakthrough
Staff
Electronics CEL<div
Professor Linda Nazar, graduate student David Xiulei Ji and
postdoctoral fellow Kyu Tae Lee is one of the first to demonstrate
robust electrochemical performance for a lithium-sulphur battery. The
finding is reported today in the on-line issue of Nature Materials.
The
prospect of lithium-sulphur batteries has tantalized chemists for two
decades, and not just because successfully combining the two
chemistries delivers much higher energy densities. Sulphur is cheaper
many other materials currently used in lithium batteries. It has always
showed great promise as the ideal partner for a safe, low cost, long
lasting rechargeable battery, exactly the kind of battery needed for
energy storage and transportation in a low carbon emission energy
economy.
“The difficult challenge was always the cathode,
the part of the battery that stores and releases electrons in the
charge and recharge cycles,” said Dr. Nazar. “To enable a reversible
electrochemical reaction at high current rates, the electrically-active
sulphur needs to remain in the most intimate contact with a conductor,
such as carbon.”
The Canadian research team leap-frogged the
performance of other carbon-sulphur combinations by tackling the
contact issue at the nanoscale level. Although they say the same
approach could be used with other materials, for their proof of concept
study they chose a member of a highly structured and porous carbon
family called mesoporous carbon. At the nanoscale level, this type of
carbon has a very uniform pore diameter and pore volume.
Using
a nanocasting method, the team assembled a structure of 6.5 nanometre
thick carbon rods separated by empty three to four nanometre wide
channels. Carbon microfibres spanning the empty channels kept the voids
open and prevented collapse of the architecture.
Filling the
tiny voids proved simple. Sulphur was heated and melted. Once in
contact with the carbon, it was drawn or imbibed into the channels by
capillary forces, where it solidified and shrunk to form sulphur
nanofibres. Scanning electron microscope sections revealed that all the
spaces were uniformly filled with sulphur, exposing an enormous surface
area of the active element to carbon and driving the exceptional test
results of the new battery.
“This composite material can
supply up to nearly 80 percent of the theoretical capacity of sulphur,
which is three times the energy density of lithium transition metal
oxide cathodes, at reasonable rates with good cycling stability,” said
Dr. Nazar.
What is more, the researchers say, the high
capacity of the carbon to incorporate active material opens the door
for similar “imbibed” composites that could have applications in many
areas of materials science.
The research team continues to
study the material to work out remaining challenges and refine the
cathode’s architecture and performance.
