Research finds key to lithium-nickel-oxide battery wear

UK Scientists have taken steps to understand and overcome the challenges associated with Ni-rich cathode materials used in lithium-ion batteries. These materials could achieve both high voltages and capacities, but their practical applications have been hindered by structural instabilities and loss of oxygen. The research findings, published in Joule, revealed that ‘oxygen hole’ formation — when an oxygen ion loses an electron — plays a crucial role in the degradation of LiNiO₂ cathodes, accelerating the release of oxygen which can then further degrade the cathode material.

Using a set of computational techniques on UK regional supercomputers, the researchers examined the behaviour of LiNiO₂ cathodes as they are charged. They found that during charging the oxygen in the material undergoes changes while the nickel charge remains unchanged. Research co-author Professor Andrew J. Morris, from the University of Birmingham, said the charge of the nickel ions remains around +2, regardless of whether it’s in its charged or discharged form. At the same time, the charge of the oxygen varies from -1.5 to about -1. “This is unusual; the conventional model assumes that the oxygen remains at -2 throughout charging, but these changes show that the oxygen is not very stable, and we have found a pathway for it to leave the nickel-rich cathode,” Morris said.

The researchers compared their calculations with experimental data and found that their results aligned with what was observed. They proposed a mechanism for how oxygen is lost during this process, involving the combination of oxygen radicals to form a peroxide ion, which is then converted into oxygen gas, leaving vacancies in the material. This process releases energy and forms singlet oxygen, a highly reactive form of oxygen.

First author Dr Annalena Genreith-Schriever from the University of Cambridge said that adding dopants that reduce oxygen redox, while promoting transition-metal redox particularly at the surface, mitigating the generation of singlet oxygen, allows the researchers to enhance the stability and longevity of lithium-ion batteries.

Lithium-ion batteries are used for various applications because of their high energy density and rechargeability, but challenges associated with the stability of cathode materials have hindered their performance and lifespan.

“Oxygen loss drives accelerated degradation in Ni-rich lithium-ion batteries. This works explains its origin and is critical for understanding how to engineer solutions to improve battery life,” said Louis Piper, co-author and Professor of Battery Innovation, WMG, University of Warwick.

Image credit: iStock.com/Mark Hochleitner