Lithium heterogeneity explains why nickel-rich cathode materials typically lose ten percent of their capacity after the first charge-discharge cycle.
The move to electric vehicles (EVs) means that the search is on for more efficient battery materials. Some of the most promising materials are layered lithium nickel-rich oxides, which are widely used in premium EVs. However, the mechanism by which these batteries operate, particularly the lithium-ion transport under normal operating conditions, and how this is linked to their electrochemical performance, is not fully understood.
In an effort to better understand and improve the performance of next-generation battery materials, researchers at University of Cambridge, UK, have tracked how light interacts with active particles during battery operation under a microscope.
The research, funded by the Faraday Institution and published in the journal Joule lays to rest the assumption that the mechanism by which lithium ions are stored in battery materials is uniform across the individual active particles.
When the battery is near the end of its discharge cycle, the surfaces of the active particles become saturated by lithium, while their cores are lithium deficient. This, the research team says, leads to a loss of reusable lithium and a reduced capacity.
By combining experimental observations with computer modelling, the researchers found that the non-uniformity stems from the fact that lithium ions diffuse slowly in fully lithiated nickel-rich manganese cobalt oxide (NMC) particles, but the diffusion is significantly enhanced once some lithium ions are removed from these particles. Lithium heterogeneity, the researchers said, explains why nickel-rich cathode materials typically lose ten percent of their capacity after the first charge-discharge cycle.
‘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’ said co-first author Alice Merryweather, from Cambridge University’s Yusuf Hamied Department of Chemistry.
Co-first author, Dr Shrinidhi S Pandurangi, from the University of Cambridge Department of Engineering, added: ‘Our model provides insights into the range over which lithium-ion distribution in NMC varies during the early stages of charging. 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.’
The researchers are now seeking new approaches to increase the practical energy density and lifetime of the new battery materials.
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