Researchers have found that the performance and capacity of next-generation battery materials can be affected by the uneven movement of lithium ions. The team, led by the University of Cambridge, monitored the flow of lithium ions in the potentially new battery material in real time.
So far it has been assumed that the mechanism for storing lithium ions in the battery material is the same for each active particle. However, Cambridge-led research found that lithium storage is far from seamless across charge cycles.
As the battery nears the end of its discharge cycle, the surface of the active particles becomes saturated with lithium while the core is depleted of lithium. This results in reduced capacity and loss of reusable lithium.
Research funded by the Faraday Institute could help improve existing battery materials and accelerate development of the next generation of batteries. The results were recently published in the journal Joule.
Electric vehicles (EVs) are critical to the transition to a zero carbon economy. Due to their high energy density, lithium-ion batteries power most of the electric vehicles currently on the road. However, as the use of electric vehicles increases, the demand for longer ranges and shorter charging times requires upgrading existing battery materials as well as the invention of new battery materials.
Some of the most promising materials are the cutting-edge positive electrode materials known as nickel-lithium rich oxide coatings, which are widely used in premium electric vehicles. However, their mechanism of action, especially the transport of Li-ions under practical operating conditions and how it relates to their electrochemical performance, is not fully understood, so we have not been able to achieve maximum efficiency from these materials.
By observing under a microscope how light interacts with active particles during battery operation, the researchers observed a marked difference in lithium storage during a charge cycle in nickel-rich cobalt oxide (NMC).
“This is the first time this non-uniformity of lithium storage has been directly observed in individual particles,” said lead author Alice Merriweather of the Yusuf Hamid Chair in Chemistry at Cambridge. The ability to capture this while the battery is still running depends on real-time methods like ours.
The researchers combined experimental observations with computer models and found that the inhomogeneity was caused by drastic changes in the diffusion rate of lithium-ion in the NMC during charge-discharge cycles. In particular, lithium ions slowly diffuse into fully lithified NMC particles, but diffusion increases rapidly after some lithium ions are extracted from these particles.
“Our model highlights how the diffusion of lithium-ion in the NMC varies during the early stages of charging,” said co-author Dr. Srinidi S. Pandurangi from the Department of Engineering, Cambridge. “Our model accurately predicts the distribution of lithium and captures the degree of heterogeneity observed in experiments. This prediction is key to understanding other battery degradation mechanisms, such as B. particle degradation.
Importantly, the heterogeneity of lithium observed at the end of discharge is one reason why nickel-rich cathode materials typically lose about ten percent of their capacity after the first charge-discharge cycle.
According to co-author Dr. Chao Xu of the Shanghai Technological University, “this is significant because the industry standard is used to determine whether or not a battery should be recalled when it has lost 20 percent of its capacity.”
Researchers are now exploring new approaches to increase the practical energy density and lifetime of this promising battery material.