in order to prevent short circuits in batteries, porous separator membranes are often placed between a battery's electrodes. There is typically a tradeoff involved, since these separators must simultaneously prevent leakage current between electrodes while allowing ions to pass through the porous channels to generate current. Conventionally, these membranes are made of synthetic materials, such as polymers.
In a new study published in Nano
Letters, researchers from the Ulsan National Institute of Science and
Technology (UNIST) in South Korea have designed a cellulose nanomat, or
"c-mat," separator membrane that contains a thin layer of nanoporous
plant cellulose on top of a thick macroporous polymer layer.
By finely tuning the thicknesses of the
two layers, the researchers were able to design a separator membrane that
delicately balances the tradeoff between preventing leakage current and
supporting fast ion transport.
With its tiny pores, the nanoporous
cellulose layer prevents leakage current between electrodes,
preventing short circuits. On the other hand, the macroporous polymer layer's
porous channels are too large to prevent leakage
current between electrodes, but their large size enables them to
function like "ionic highways" to rapidly transport charges.
The new separator has another major
advantage: At high temperatures (60 °C), batteries with the new separator
membranes have an 80% capacity retention after 100 cycles, whereas batteries
with typical commercial polymer separators maintain just 5% of their initial
capacity after 100 cycles at the same temperature.
The researchers explain that the large
capacity loss in the commercial batteries at high temperature occurs due to
unwanted side reactions between lithium salts and water, which produces harmful
byproducts such as manganese ions. The nanoporous cellulose-based
layer of the new separator membranes has a manganese-chelating ability, so that
it binds to the manganese ions and prevents them from participating in the
reactions that cause capacity loss. In addition, the macroporous polymer layer
captures the acidic reactants that produce the manganese ions, resulting in
fewer of these ions in the first place.
"We demonstrate in this work that
the chemically active cellulose-based c-mat separator can mitigate the
manganese ion-induced adverse effects," coauthor Sang-Young Lee, Professor
at UNIST's School of Energy and Chemical Engineering, told Phys.org.
"This enables a remarkable improvement in the high-temperature cycling
performance far beyond that which is attainable with conventional membrane
technologies."
In the future, the researchers plan to
modify the separators for potential use in next-generation rechargeable
batteries such as sodium-ion, lithium-sulfur, and metal-ion batteries.
"The c-mat separator is expected
to be used for next-generation high-performance batteries with high temperature
stability—for example, in large-sized batteries for electric vehicles and
grid-scale electricity storage systems," Lee said.
In addition to its use as a battery
separator membrane, the c-mat separator also has potential applications in
membranes for desalination systems, as well as for ecofriendly sensors for
heavy metal ions.
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