“This is the first time that this non-uniformity in lithium storage has been directly observed in individual particles. Real-time techniques like ours are essential to capture this while the battery is cycling.”Alice Merryweather, University of Cambridge

A team of researchers has found that the irregular movement of lithium ions in next-generation battery materials could be reducing their capacity and hindering their performance. The team, led by researchers at the University of Cambridge in the UK, tracked the movement of lithium ions inside a promising new battery material in real time.

It had always been assumed that the mechanism by which lithium ions are stored in battery materials is uniform across the individual active particles making up those materials. However, the Cambridge-led team found that during the charge-discharge cycle, lithium storage is anything but uniform.

When a lithium-ion battery is near the end of its discharge cycle, the surfaces of the active particles become saturated by lithium while their cores become lithium deficient. This results in the loss of reusable lithium and a reduced capacity.

This research, funded by the Faraday Institution, could help to improve existing battery materials and accelerate the development of next-generation batteries. The researchers report their findings in a paper in Joule.

Electrical vehicles (EVs) are vital in the transition to a zero-carbon economy. Most EVs on the road today are powered by lithium-ion batteries, due in part to their high energy density. But as EV use becomes more widespread, the push for longer ranges and faster charging times means that current battery materials need to be improved, and new materials need to be identified.

Some of the most promising of these materials are state-of-the-art cathode materials known as layered-lithium nickel-rich oxides, which are widely used in premium EVs. However, their working mechanisms, particularly lithium-ion transport under practical operating conditions, and how these mechanisms are linked to electrochemical performance, are not fully understood, so scientists cannot yet obtain maximum performance from these materials.

By tracking how light interacts with active particles during battery operation under a microscope, the researchers were able to observe distinct differences in lithium storage during the charge-discharge cycle in one of these cathode materials – nickel-rich manganese cobalt oxide (NMC).

“This is the first time that this non-uniformity in lithium storage has been directly observed in individual particles,” said co-first author Alice Merryweather from Cambridge’s Yusuf Hamied Department of Chemistry. “Real-time techniques like ours are essential to capture this while the battery is cycling.”

Combining the experimental observations with computer modelling, the researchers found that the non-uniformity originates from drastic changes in the rate of lithium-ion diffusion in NMC during the charge-discharge cycle. Specifically, lithium ions diffuse slowly in fully lithiated NMC particles, but this diffusion is significantly enhanced once some lithium ions are extracted from these particles.

“Our model provides insights into the range over which lithium-ion diffusion in NMC varies during the early stages of charging,” said co-first author Shrinidhi Pandurangi from Cambridge’s Department of Engineering. “Our model predicted lithium distributions accurately and captured the degree of heterogeneity observed in experiments. These predictions are key to understanding other battery degradation mechanisms such as particle fracture.”

Importantly, the lithium heterogeneity seen at the end of discharge establishes one reason why nickel-rich cathode materials typically lose around 10% of their capacity after the first charge-discharge cycle.

“This is significant considering one industrial standard that is used to determine whether a battery should be retired or not is when it has lost 20% of its capacity,” said co-first author Chao Xu from ShanghaiTech University in China.

The researchers are now seeking new approaches to increase the practical energy density and lifetime of these promising battery materials.

This story is adapted from material from the University of Cambridge, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.