Irregular lithium ion movement could be hindering electric battery performance.
Researchers have discovered that the performance and capacity of next-generation battery materials may be hampered by the irregular movement of lithium ions. The team, which was led by the University of Cambridge, monitored the flow of lithium ions in real time inside a prospective new battery material.
It was previously believed that the mechanism by which lithium ions are stored in battery materials is uniform for each active particle. However, the Cambridge-led research discovered that lithium storage is anything but uniform over the charge-discharge cycle.
When the battery is nearing the conclusion of its discharge cycle, the active particles’ surfaces become lithium saturated while their cores are lithium deficient. This causes a reduction in capacity and the loss of reusable lithium.
The Faraday Institution-funded study might contribute to the advancement of existing battery materials and hasten the creation of next-generation batteries. The findings were recently published in the journal Joule.
In order to shift to a zero-carbon economy, electrical vehicles (EVs) are essential. Because of its great energy density, lithium-ion batteries power the majority of electric vehicles currently on the road. However, as EV usage increases, the requirement for greater ranges and quicker charging times necessitates the improvement of existing battery materials as well as the discovery of new ones.
Some of the most promising of these materials are state-of-the-art positive electrode 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 this is linked to their electrochemical performance, are not fully understood, so we cannot yet obtain maximum performance from these materials.
By tracking how light interacts with active particles during battery operation under a microscope, the researchers observed distinct differences in lithium storage during the charge-discharge cycle in 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 modeling, the researchers found that the non-uniformity originates from drastic changes to the rate of lithium-ion diffusion in NMC during the charge-discharge cycle. Specifically, lithium ions diffuse slowly in fully lithiated NMC particles, but the 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 Dr Shrinidhi S. 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 ten percent of their capacity after the first charge-discharge cycle.
“This is significant, considering one industry standard that is used to determine whether a battery should be retired or not is when it has lost 20 percent of its capacity,” said co-first author Dr Chao Xu, from ShanghaiTech University.
The researchers are now seeking new approaches to increase the practical energy density and lifetime of these promising battery materials.
