Researchers have found that the irregular movement of lithium ions in next-generation battery materials could be reducing their capacity and hindering their performance.
The team, led by the University of Cambridge, tracked the movement of lithium ions inside a promising new battery material in real time.
It had been assumed that the mechanism by which lithium ions are stored in battery materials is uniform across the individual active particles. However, the Cambridge-led team found that during the charge-discharge cycle, lithium storage is anything but uniform. When the battery is near the end of its discharge cycle, the surfaces of the active particles become saturated by lithium while their cores are lithium deficient. This results in the loss of reusable lithium and a reduced capacity.
The research, funded by the Faraday Institution, could help improve existing battery materials and could accelerate the development of next-generation batteries.
Highlights
- Optical scattering microscopy reveals lithium heterogeneity in NMC single crystals
- Charging and discharging in NMC kinetically limited by concentration-depandant diffusivity
- Results explain ∼10% capacity loss in NMC during the first cycle
Context & scale
The mechanism by which lithium ions are stored in high-energy-density lithium-ion battery materials is typically assumed to be uniform across the individual active particles. Here, by using operando optical scattering microscopy and diffusive modeling, the authors directly image and track the buildup of kinetically induced lithium-ion heterogeneity within individual particles during battery operation in one of the most promising next-generation cathode materials based on nickel-rich manganese cobalt oxide (NMC).
The insights provided by this study do not only challenge long-held beliefs but also motivate new approaches to overcome critical capacity losses in high-performance materials, especially as society moves toward fast-charging regimes.
Summary
Understanding how lithium-ion dynamics affect the (de)lithiation mechanisms of state-of-the-art nickel-rich layered oxide cathodes is crucial to improve electrochemical performance. Here, we directly observe two distinct kinetically induced lithium heterogeneities within single-crystal LiNixMnyCo(1−x−y)O2 (NMC) particles using recently developed operando optical microscopy, challenging the notion that uniform (de)lithiation occurs within individual particles. Upon delithiation, a rapid increase in lithium diffusivity at the beginning of charge results in particles with lithium-poor peripheries and lithium-rich cores. The slow ion diffusion at near-full lithiation states—and slow charge transfer kinetics—also leads to heterogeneity at the end of discharge, with a lithium-rich surface preventing complete lithiation. Finite-element modeling confirms that concentration-dependent diffusivity is necessary to reproduce these phenomena. Our results demonstrate how kinetic limitations cause significant first-cycle capacity losses in Ni-rich cathodes.
Electrical vehicles (EVs) are vital in the transition to a zero-carbon economy. Most electric vehicles on the road today are powered by lithium-ion batteries, due in part to their high energy density. However, as EV use becomes more widespread, the push for longer ranges and faster charging times means that current battery materials need to be improved, and new materials need to be identified.
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