A material derived from trees could potentially replace liquid electrolytes in next-generation batteries.
Brown University
Now, a
research team from Brown University and the University of Maryland has
developed a new material for use in solid-state batteries that’s derived from an
unlikely source: trees.
In research published in the journal Nature, the team demonstrates a solid ion conductor that combines copper with cellulose nanofibrils — polymer tubes derived from wood.
The paper-thin material has an
ion conductivity that is 10 to 100 times better than other polymer ion
conductors, the researchers say. It could be used as either a solid battery
electrolyte or as an ion-conducting binder for the cathode of an all-solid-state
battery.
“In fact, we found this ion conductor achieved a record high ionic
conductivity among all solid polymer electrolytes.”
The
work was a collaboration between Hu’s lab and the lab of Yue Qi, a professor at
Brown’s School of Engineering.
Today’s lithium ion batteries, which are widely used in everything from cellphones to cars, have electrolytes made from lithium salt dissolved in a liquid organic solvent. The electrolyte’s job is to conduct lithium ions between a battery’s cathode and anode.
Liquid electrolytes work pretty well, but they
have some downsides. At high currents, tiny filaments of lithium metal, called
dendrites, can form in the electrolyte leading to short circuits. In addition,
liquid electrolytes are made with flammable and toxic chemicals, which can
catch fire.
Solid
electrolytes have the potential to prevent dendrite penetration and can be made
from non-flammable materials. Most of the solid electrolytes investigated
so far are ceramic materials, which are great at conducting ions but they’re
also thick, rigid and brittle. Stresses during manufacturing as well as
charging and discharging can lead to cracks and breaks.
The
material introduced in this study, however, is thin and flexible, almost like a
sheet of paper. And its ion conductivity is on par with ceramics.
Qi and Qisheng Wu, a senior research associate at Brown, performed computer simulations of the microscopic structure of the copper-cellulose material to understand why it is able to conduct ions so well.
The modeling study revealed
that the copper increases the space between cellulose polymer chains, which
normally exist in tightly packed bundles. The expanded spacing creates what
amount to ion superhighways through which lithium ions can zip by relatively
unimpeded.
“The
lithium ions move in this organic solid electrolyte via mechanisms that we
typically found in inorganic ceramics, enabling the record high ion
conductivity,” Qi said. “Using materials nature provides will reduce the
overall impact of battery manufacture to our environment.”
In addition to working as a solid electrolyte, the new material can also act as a cathode binder for a solid-state battery. In order to match the capacity of anodes, cathodes need to be substantially thicker. That thickness, however, can compromise ion conduction, reducing efficiency.
In order for thicker cathodes
to work, they need to be encased in an ion-conducting binder. Using their new
material as a binder, the team demonstrated what they believe to be one of the
thickest functional cathodes ever reported.
The
researchers are hopeful that the new material could be a step toward making
bringing solid state battery technology to the mass market.
The
research at Brown University was supported by the National Science Foundation (DMR-2054438).
