In a paper published in the journal Joule, the researchers explain that it had been assumed that the mechanism by which lithium ions are stored in battery materials is uniform across the individual active particles. What they found, however, was that during the charge-discharge cycle, lithium storage is anything but uniform.
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 results in the loss of reusable lithium and a reduced capacity.
In particular, by tracking how light interacts with active particles during battery operation under a microscope, the scientists observed distinct differences in lithium storage during the charge-discharge cycle in nickel-rich manganese cobalt oxide (NMC).
Combining the experimental observations with computer modelling, the researchers noticed 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,” co-first author Shrinidhi S. Pandurangi said in a media statement.
“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.”
According to Pandurangi, 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.
In his view and that of his colleagues, such a finding is very important considering one industry standard that is used to determine whether a battery should be retired or not is when it has lost 20% of its capacity.