What could be reducing the capacity and performance of state-of-the-art battery materials for electric vehicles (EVs)? According to EU-backed researchers, it’s the irregular movement of lithium ions.
Most EVs are powered by lithium-ion batteries that have one of the highest energy densities of any battery technology today. However, as more petrol car owners go over to EVs, the demand for longer driving ranges and faster charging times will increase, propelling the need for new and better battery materials.
Advanced layered lithium nickel-rich oxides are among the most promising battery materials and widely used in luxury EVs today. Understanding their working mechanisms, and especially lithium ion transport, is crucial for improving their electrochemical performance. However, to date these mechanisms are not fully understood.
Research supported by the EU-funded SOLARX, MULTILAT and BATNMR projects has shown that, unlike what was previously believed, lithium ion storage in battery materials across individual active particles is far from uniform. In fact, the project team found that the irregular movement of lithium ions in a next-generation cathode material based on nickel-rich manganese cobalt oxide (NMC) could be reducing battery capacity and impeding performance. The study has been published in the journal ‘Joule’.Using operando optical scattering microscopy and diffusive modelling, the researchers tracked how light interacts with active particles during battery operation. They noticed clear differences in lithium storage during the charge-discharge cycle in NMC particles. “This is the first time that this non-uniformity in lithium storage has been directly observed in individual particles,” states PhD student and co-first author Alice Merryweather of SOLARX, MULTILAT and BATNMR projects’ host University of Cambridge in a news item posted on the university’s website. “Real time techniques like ours are essential to capture this while the battery is cycling.”
As reported in the news item, the above non-uniformity in lithium storage has been attributed to the drastic changes that the rate of lithium-ion diffusion undergoes in NMC during the charge-discharge cycle. Lithium ions diffuse slowly in fully lithiated NMC particles, but when some lithium ions are removed from the particles, diffusion increases rapidly. The fast diffusion observed upon delithiation – at the beginning of the charge – results in active particles with lithium-deficient surfaces and lithium-rich cores. In contrast, the slow ion diffusion in fully lithiated NMC particles – near the end of battery discharge – results in particles with lithium-rich surfaces and lithium-poor cores.
“Our model predicted lithium distributions accurately and captured the degree of heterogeneity observed in experiments,” notes co-first author Dr Shrinidhi Pandurangi, also from the University of Cambridge. “These predictions are key to understanding other battery degradation mechanisms such as particle fracture.”
The lithium heterogeneity at the end of discharge provides an explanation for why nickel-rich cathodes lose about 10 % of their capacity after the first charge-discharge cycle. “This is significant, considering one industrial standard that is used to determine whether a battery should be retired or not is when it has lost 20 percent of its capacity,” observes co-first author Dr Chao Xu of ShanghaiTech University, China, who participated in the study while based at Cambridge.
The insights gained in the study supported by SOLARX (Photon Management for Solar Energy Harvesting with Hybrid Excitonics), MULTILAT (Multi-phase Lattice Materials), and BATNMR (Development and Application of New NMR Methods for Studying Interphases and Interfaces in Batteries) pave the way for new approaches to overcoming capacity losses and increasing the lifetime of high-performance battery materials.
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