In the past, it was thought that the mechanism of lithium ion storage in battery materials was consistent for each active particle. However, the research led by Cambridge University found that lithium was unevenly stored during the charge-discharge cycle.
When the battery discharge cycle comes to an end, the surface lithium of active particles is saturated, while the core lithium is lacking. This will lead to capacity reduction and loss of reusable lithium. This research may contribute to the progress of existing battery materials and accelerate the invention of the next generation battery. The research results were recently published in Joule magazine.
In order to transform into a zero-carbon economy, it is very important to develop electric vehicles. Because of its huge energy density, lithium-ion batteries provide power for most electric vehicles on the road at present. However, with the increasing use of electric vehicles, it is necessary to improve the existing battery materials and discover new materials in order to obtain longer cruising range and faster charging time.
The most promising of these materials is the most advanced cathode material, called layered nickel-rich lithium oxide, which is widely used in advanced electric vehicles. However, their working mechanism, especially the transmission of lithium ions under actual working conditions, and the relationship between this and electrochemical properties are not completely clear, so we can't get the maximum performance from these materials.
In the latest research, by tracking the interaction between light and active particles during the operation of the battery under a microscope, researchers at Cambridge University observed that the lithium storage capacity of nickel-rich manganese cobalt oxide (NMC) was obviously different during the charge-discharge cycle.
"This is the first time that the inhomogeneity of lithium storage has been directly observed in a single particle," said AliceMerryweather, the first author from the chemistry department of Youssef Hamid, Cambridge University. "Real-time technology like ours is very important to capture this situation during the battery cycle."
Combined with experimental observation and computer modeling, the researchers found that this inhomogeneity stems from the drastic change of lithium ion diffusion rate in NMC during charge and discharge cycles. Specifically, lithium ions diffuse slowly in fully lithiated NMC particles, but once some lithium ions are extracted from these particles, the diffusion will be significantly enhanced.
More importantly, the inhomogeneity of lithium at the end of discharge determines one of the reasons why nickel-rich cathode materials usually lose about 10% capacity after the first charge-discharge cycle. The researchers said: "This is important because the industry standard for determining whether the battery should be retired is that it has lost about 20% of its capacity."
Through this latest research, scientists will hopefully seek new ways to improve the actual energy density and life span of these promising battery materials.