Tech Briefs Magazine - February 2022 - BT-22

manipulation approaches, which have
focused on depositing conductive polymers
on the metal's surface.
Commercial electrolytes are a combination
of carefully selected molecules.
Using NMR and computer simulations,
the researchers were able to understand
how the electrolyte formulations
improve lithium metal battery performance
at the molecular level. The team is
now testing alkali metal additives that
stop the formation of deleterious surface
layers in combination with more traditional
additives that encourage the
growth of conductive layers on lithium
metal. They are also actively using NMR
to directly measure the rate of lithium
transport through this layer.
    
   

Material Derived from Trees Could Replace Liquid
Electrolytes
The material could pave the way for better, safer solid-state batteries.
Brown University, Providence, RI, and University of Maryland, College Park, PA
n pursuit of batteries that deliver more
power and operate more safely, researchers
are working to replace the liquids
commonly used in today's lithium-ion
batteries with solid materials. Researchers
have now developed a new material for use
in solid-state batteries that's derived from
an unlikely source: trees.
The team demonstrated 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. 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.
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 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 that can catch fire.
Solid electrolytes have the potential to
prevent dendrite penetration and can be
made from nonflammable materials. Most
of the solid electrolytes investigated so far
are ceramic materials, which are good 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 new materi22
I
a
Cellulose
nanofibril
200 nm
Fibre
d = 20-50 μm
Elementary fibril
d = 1.5-3.5 nm
Nanofibril
d = 10-15 nm
Molecular chain
d < 1 nm
HO
Cu
O
OH
Li
Li
HO
Li
O
Cu
O OH
Li
Li
O
Li
O
O
Li
Li
HO
Cu
OH
O
Li
Li
Li
Li
HO
Cu
O
OH
Li
Li
b
This work
Crosslinked polymer
PEO-inorganic
High Li concentration
Structure and ion-transport performance of the Li-Cu-CNF solid-state ion conductor. (Photo:
Brown University)
al, however, is thin and flexible - almost
like a sheet of paper - and its ion conductivity
is on par with ceramics.
Computer simulations of the microscopic
structure of the copper-cellulose material
were performed 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 ion superhighways through
which lithium ions can zip by relatively
unimpeded.
In addition to working as a solid electrolyte,
the new material can also act as a
cathode binder for a solid-state battery. To
match the capacity of anodes, cathodes
need to be substantially thicker. That thickness,
however, can compromise ion conduction,
reducing efficiency. For thicker cathodes
to work, they need to be encased in an
ion-conducting binder. Using the new
material as a binder, the team demonstrated
what they believe to be one of the thickest
functional cathodes ever reported.
     
  
Battery & Electrification Technology, February 2022

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Tech Briefs Magazine - February 2022

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