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Diamond Light Source stands as one of the leading research facilities globally, driving scientific advancements. The 14,000th paper published as a result of innovative experiments undertaken at the UK’s national synchrotron highlights the profound impact science can have in addressing the world's most urgent challenges. A team of researchers from WMG at the University of Warwick, in collaboration with academic partners in the Faraday Institution’s Degradation and FutureCat projects, has conducted a ground-breaking study that bridges the gap between academic models and real-world battery performance.
Their work, recently published in Joule, used a trio of synchrotron techniques – Resonant Inelastic soft X-ray Scattering (RIXS), hard and soft X-ray absorption spectroscopy (XAS) - to investigate the charge compensation mechanism of lithium-ion (Li-ion) battery cathodes during high-voltage operation. This study demonstrates that oxygen plays a significant role through a metal-ligand redox process, emphasising the importance of focusing on surface passivation strategies to mitigate oxygen reactivity with electrolytes, reducing degradation and enhancing safety. By using pilot-line fabricated pouch cells, this work aligns fundamental research with commercial applications and offers crucial insights to improve energy density and cycling stability.
The global transition to a low-carbon future requires the development of low-cost, reliable and long-range electric vehicles, with Li-ion batteries playing a crucial role. However, traditional models of the electronic charge compensation mechanism in layered metal oxide cathodes are insufficient for developing next-generation batteries with increased energy density through high-voltage operation.
Prof Louis Piper explained:
When we talk about trying to increase the energy density of a battery, what that means is being able to remove as many electrons as possible, In a lithium-ion battery, Li-ions move between the anode and a cathode, releasing an equal number of electrons as current. In a commercial layered metal oxide battery, we can pull out about two-thirds of the accessible lithium ions, and therefore, two-thirds of the available electrons. That means the battery is always below its theoretical capacity, but it’s engineered that way to prevent the degradation that occurs when you pull more out. Replacing cobalt with nickel increases the practical capacity of the battery, but it pushes it closer to the point where you see accelerated degradation. Traditional models attribute charge compensation solely to transition metal oxidation, but if that’s true then why does replacing cobalt with nickel change things, and why do we have more problems with safety and oxygen loss as we increase the energy density? We need a better understanding of the metal-ligand redox process to develop safe, stable, higher performance Li-ion cells.
WMG is home to a Battery Scale-Up pilot facility, a suite of cell production equipment covering the full production process cell assembly and testing. It allows researchers to manufacture battery cells in a variety of different formats.
Prof Piper says:
A lot of battery research uses experimental half cells rather than cells that are close to real-world batteries. And the results of those experiments don’t match what we’re seeing in commercial cells. What we wanted to do here was to use the same X-ray techniques to investigate the role that oxygen is playing in these conventional materials, and the way that we do that is to create NMC811 // graphite full pouch cells in our pilot facility. So we can bring what is essentially a commercial battery to Diamond and see how NMC811 cathodes change during charge and discharge.
The team used two Diamond beamlines to build up a complete picture of what was happening inside the pouch cell. Using RIXS on I21 explored the role of oxygen in the bulk of the materials, with hard XAS on B18 focusing on the nickel edge in the bulk. Soft XAS modes were also employed at I21 for insights into the metal and oxygen atoms at the surface. Theoretical spectral simulations explained the changes in electron density shown in the experimental results.
Their results showed how electrons are largely removed from the oxygen orbitals, more than the transition metal orbitals, when lithium ions are extracted from the cathode, and hence responsible for energy storage. The experiments also highlight that the NMC811 cathodes are stable in the bulk during high voltage cycling despite the oxygen participation. Degradation effects occur instead at the surface via reactive oxygen reacting with the liquid electrolyte. That means that future strategies to minimise degradation and increase lifespan and safety can focus on surface stabilisation.
Prof Piper adds:
Completing these studies has demonstrated the kind of science we can do with the cells we build in our pilot facility. We’re building 2.5 Amp cells and combining operando X-ray studies at Diamond with operando neutron studies at the Isis Neutron and Muon Source, and that gives us exciting insights into how these batteries behave in real-world conditions, and how we can develop the next-generation of safe and affordable, longer-lasting batteries.
To find out more about the beamlines or discuss potential applications, please contact Principal Beamline Scientist (PBS) Kejin Zhou ([email protected]) for I21 and Diego Gianolio ([email protected]) for B18.
Ogley MJW et al. Metal-ligand redox in layered oxide cathodes for Li-ion batteries. Joule (2024). DOI:10.1016/j.joule.2024.10.007.
image credits: this article: 10.1016/j.joule.2024.10.007, under the terms of the Creative Commons CC-BY license
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