A coating improves the lifetime for lithium batteries

Materials World magazine
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27 Sep 2019

A new coating may make lithium batteries safer and longer lasting. Idha Valeur reports.

A coating that will improve the lifetime of lightweight lithium batteries and make them more stable could aid in developing next-generation electric vehicles. The coating has been created by researchers from Stanford University, USA, in collaboration with SLAC National Accelerator Laboratory, also in the USA.

Lithium metal batteries can hold approximately a third more power per half a kilo than lithium-ion batteries, as well as being lighter because the anode is made from lithium instead of heavier graphite. This makes the lithium metal battery a preferred option for most electronic devices.

The hurdle is making the batteries more reliable. To do this, the research team developed the dynamic single-ion-conductive network, a polymer coating that actively prohibits dendrites from forming, which could lead to overheating and batteries exploding.

Stanford University PhD student, David Mackanic, explained that the team has invented a coordination complex. He compared the material to an everyday polymer and said the main difference is the length of the polymer chains - instead of having long polymer chains, the network is made up of short molecular linkers connecting metal nodes. ‘A good way to visualise it is picturing a molecular fishing net. The linkers in the network are based on a fluorinated compound that is similar to Teflon, and the metal nodes are comprised of aluminium atoms,’ Mackanic told Materials World.

The researchers chose three specific properties for the components making up the network. ‘First, the material is flowable and self-healable. As a battery charges and discharges, the surfaces of the electrodes can become rough and puncture the battery. Because our material can flow, it covers the rough patches and prevents them from growing. This leads to more uniform battery operation,’ Mackanic said.

‘The second thing it does is facilitate fast lithium-ion conduction. Because our coating allows for lithium to transport to the electrode efficiently, it minimises reactive hot spots that can cause the formation of lithium dendrites. The third property is the ability to protect the electrode from side-reaction with the electrolyte.’

To test the battery, the team combined the coated anodes with readily available parts to create a working battery and compared the performance against standard lithium metal cells. After 160 cycles, they found that the coated cells produced 85% of the power than that of the first cycle, while the uncoated ones delivered about 30%.

Combustion issue

Not only did the polymer coating extend the battery life during lab testing, the researchers also addressed the combustion issue by limiting the needle-like structure known as dendrites from forming.

‘Normally, when lithium ions in a battery are plated onto a lithium metal anode, they deposit in a rough, filamentous manner,’ Mackanic said. ‘These rough deposits lead to the growth of what is called a lithium dendrite, which can grow rapidly, puncture through the battery separator, and cause an internal short circuit. This short circuit will cause an instant discharge of the battery, leading to rapid heating, and ultimately combustion of the flammable battery components.

‘With the DSN coating, we suppress the growth of dendritic lithium. Our coating allows for more uniform lithium growth, preventing dendrites from growing and short-circuiting the cell,’ he added.

Energising

At present, research is still being conducted at laboratory scale with coin cell batteries, but progress is being made towards commercialisation. ‘We are still investigating more materials to identify the optimal properties of the coating. However, we are also in collaboration with other researchers to scale-up this technology and to try the DSN coating in commercial-size lithium-ion batteries,’ Mackanic said.

The team is working on applying the coating to larger-scale battery systems, as well as testing it in various battery applications to make sure it works efficiently in all battery chemistries and form factors.

According to Mackanic, the DSN coating would be able to extend the lifetime of a lithium metal battery by 30-100%, making these batteries more cost-effective.

He added that the end goal is to meet the targets set by the Battery500 consortium, which aims to develop a battery that has 500Wh/kg at the pack level. ‘This will likely require the use of lithium metal,’ he said. ‘In addition to the DSN coating, we will continue to develop materials that enable efficient use of the lithium metal anode and usher in an era of higher-performing batteries.’