A 1,000-cycle lithium-sulfur battery could quintuple the range of electric vehicles

A diagram of the battery shows how lithium ions can return to the lithium electrode while lithium polysulphides cannot cross the membrane separating the electrodes. Also, the spiky dendrites growing from the lithium electrode cannot short the battery by piercing the membrane and reaching the sulfur electrode. Image credit: Ahmet Emre, Kotov Lab

A new bio-inspired battery membrane has enabled a battery with five times the capacity of the industry standard lithium-ion design to operate for the thousands of cycles required to power an electric car.

A network of aramid nanofibers, recycled from Kevlar, can enable lithium-sulfur batteries to overcome their Achilles’ heel of lifespan – the number of times they can be charged and discharged – a team has shown. from the University of Michigan.

“There are a number of reports claiming several hundred cycles for lithium-sulfur batteries, but this comes at the expense of other parameters: capacity, charge rate, resiliency and safety. The challenge these days is to manufacture a battery that increases the cycle rate from the previous 10 cycles to hundreds of cycles and meets many other requirements, including cost,” said Nicholas Kotov, Irving Langmuir University Professor Emeritus of Chemical Sciences and Engineering , who directed the search.

“The biomimetic engineering of these batteries integrates two scales: molecular and nanometric. For the first time, we integrated the ion selectivity of cell membranes and the toughness of cartilage. Our integrated systems approach has allowed us to meet the great challenges of lithium-sulfur batteries.

Previously, his team relied on arrays of aramid nanofibers impregnated with an electrolyte gel to stop one of the main causes of short life cycles: dendrites that grow from electrode to electrode, piercing the diaphragm. The tenacity of the aramid fibers stops the dendrites.

But lithium-sulfur batteries have another problem: small molecules of lithium and sulfur form and flow to the lithium, attach themselves and reduce the capacity of the battery. The membrane was to allow lithium ions to flow from lithium to sulfur and vice versa, and to block lithium and sulfur particles, called lithium polysulphides. This ability is called ion selectivity.

“Inspired by biological ion channels, we designed lithium ion highways where lithium polysulfides cannot pass the tolls,” said Ahmet Emre, postdoctoral researcher in chemical engineering and co-first author of the paper in NatureCommunications.

At the start of the experiment, lithium polysulfides were only found on the left side of the battery cell, both for the industrial Celgard membrane (left) and for the UM aramid nanofiber membrane (at right). Credit: Ahmet Emre, Kotov Lab

Lithium ions and lithium polysulfides are similar in size, so it was not enough to block lithium polysulfides by creating small channels. Mimicking the pores of biological membranes, the UM researchers added electrical charge to the pores of the battery membrane.

They did this by exploiting the lithium polysulfides themselves: they stuck to the aramid nanofibers and their negative charges repelled the lithium polysulfide ions that continued to form at the sulfur electrode. . However, positively charged lithium ions could pass freely.

Half an hour later, the Celgard membrane (on the left) lets out lithium polysulphides. However, the UM membrane (right) completely blocks lithium polysulfides 96 hours later. Image credit: Ahmet Emre, Kotov Lab

“Achieving record levels for multiple parameters for multiple material properties is what is needed now for car batteries,” Kotov said. “It’s kind of similar to gymnastics for the Olympics – you have to be perfect all around, including the durability of their production.”

As a battery, Kotov says the design is “almost perfect”, with its capacity and efficiency approaching theoretical limits. It can also withstand the extreme temperatures of automotive life, from the heat of charging in direct sunlight to the cold of winter. However, real-world cycle life may be shorter with fast charging, closer to 1,000 cycles, he says. This is considered a ten year lifespan.

Along with their greater capacity, lithium-sulfur batteries have durability advantages over other lithium-ion batteries. Sulfur is much more abundant than cobalt in lithium-ion electrodes. In addition, the aramid fibers of the battery membrane can be recycled from old bulletproof vests.

The research was funded by the National Science Foundation and the Department of Defense. The team studied the membrane at the Michigan Center for Materials Characterization. The University of Michigan has patented the membrane and Kotov is developing a company to commercialize it.

Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering and Professor of Chemical Engineering, Materials Science and Engineering, and Macromolecular Science and Engineering.

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