University of Delaware Research Group: Containing the formation of harmful crystals in lithium metal batteries

From smart phones to electric cars, many of today's technologies use lithium-ion batteries. This means that consumers must keep the convenience of the charger. The iPhoneX battery lasts only 21 hours of talk time, while the Tesla S model has a range of 335 miles - which means you can get from Newark, Delaware to Providence, Rhode Island City, but not all the way to Boston with one charge.

Scientists around the world - even John Goodenough, the inventor of lithium-ion batteries - are looking for ways to make rechargeable batteries safer, lighter and more powerful.

Now, an international research team led by Bingqing Wei, professor of mechanical engineering at the University of Delaware, director of the Fuel Cell and Battery Center, is working to lay the foundation for the wider use of lithium metal batteries. This will be more than the lithium-ion batteries commonly used in consumer electronics more capacity. The team developed a method to mitigate dendrite formation in lithium metal batteries, which they described in a paper published in NanoLetters.

Lithium metal battery promise (and trap)

In a lithium ion battery, the anode or current generating side is made of a material such as graphite to which lithium ions are bonded. Lithium ions flow to the cathode or collector side.

In a lithium metal battery, the anode is made of lithium metal. Electrons flow from the anode to the cathode to generate electricity. Rechargeable batteries made of lithium metal have many promises because lithium is the most electrically conductive metal and has a very high capacity.

"In theory, lithium metal is one of the best choices for batteries, but it is difficult to deal with in practice," Wei said.

Lithium metal batteries have so far been inefficient, unstable, and even have fire hazards. Their performance is hindered by lithium dendrites, which look like small stalagmites made from lithium deposits. As the battery is used, lithium ions will collect on the anode. Over time, lithium deposits become uneven, resulting in the formation of these dendrites, which can cause short circuits in the battery.

New understanding

Research groups around the world have tried various techniques to inhibit the formation and growth of these dendrites. After studying the literature, Wei found that almost all applied techniques can be understood under a protective umbrella: the introduction of a layer of porous material into the system prevents the dendrites from accumulating on the anode.

The team used mathematical modeling to find that porous materials inhibit dendritic initiation and growth. The dendrites that are indeed formed are 75% shorter than the dendrites formed in systems lacking a porous membrane. To further prove this finding, the team created a diaphragm made of porous silicon nitride filaments, less than one in a million of each diaphragm. They then integrated the film into a lithium metal battery in the battery and ran for 3,000 hours. No dendrites grew up.

"This basic understanding may not be limited to the silicon nitride we use," Wei said. “Other porous structures may also do this.”

More importantly, this principle can also be extended to other battery systems, such as zinc or potassium-based batteries, he said.

“In this field of metal batteries, this is the latest understanding,” he said. “This is a job that can have a major impact.”

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