The development of more sustainable and efficient energy storage solutions is one of the main challenges facing society today. At the heart of these initiatives are lithium-ion batteries, powering electric vehicles and enabling sustainable renewable energy storage systems. Their widespread use is driven by their high energy density, stability, and greater efficiency compared to other technologies.

Transition metals like nickel (Ni), manganese (Mn), and cobalt (Co) are frequently used in lithium-ion battery cathodes because they promote electrochemical reversible redox reactions, providing high energy density and reliable performances. However, these compositions also suffer from voltage fade and structural instability, leading to performance degradation over multiple cycles. Cobalt was thought to play a particular crucial role in stabilizing the layered structure of these cathode materials by improving electronic conductivity. Yet, its high cost, limited supply, and safety concerns have driven researchers to look for alternatives to reduce or eliminate cobalt from cathodes while enhancing battery performance.

This study, led by researchers at the ALBA Synchrotron, used advanced operando X-ray spectroscopy techniques to observe how cobalt removal affects the material at the atomic level, uncovering key structural and electronic transformations in real time. The main innovation lies in the multi-modal experimental approach, where multi-edge operando X-ray absorption spectroscopy coupled the results obtainable from both the x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectral regions. Moreover, the hard x-ray operando absorption data collected at CLAESS beamline have been coupled with high-resolution transmission X-ray microscopy in the soft x-ray energy range conducted at MISTRAL beamlines, in both cases ALBA beamlines. This approach allowed the researchers to better understand the charge compensation mechanisms during cycling and the specific role of each transition metal and oxygen species.

The study compared two lithium-rich NMC cathodes. NMC is a layered oxide material made of nickel, manganese and cobalt. One of the studied cathodes presented a low cobalt content and the other was Co-free variant, both synthesized via co-precipitation followed by a solid-state reaction. These cathodes were assembled into coin-cell batteries to assess their electrochemical performance through charge and discharge cycles. During cycling, the researchers employed operando multi-edge X-ray Absorption Spectroscopy (XAS) to monitor real-time oxidation state changes in transition metals and, indirectly, on the oxygen species. Automated big data post processing has been applied. While the XANES region have been analysed with advanced statistical methods, automated fitting techniques were also exploited to extract complementary quantitative information from the EXAFS signals. Additionally, full-field Transmission X-ray Microscopy (TXM) was used to analyse nanoscale morphological and structural transformations.

The combination of all these techniques provided critical insights into Co’s role in stabilizing the layered structure along cycling and how its absence impacts battery performance. Operando XANES measurements confirmed that in the Co-free material, nickel undergoes a more complete and faster oxidation process during the first charge cycle, which enhances charge compensation and reduces irreversible oxygen loss. The X-ray Microscopy analysis further revealed that removing Co suppresses the formation of the Mn spinel phase in the bulk of the material particles, a key factor in capacity fading. The finding explained the reasons of the observed higher capacity retention over many charging cycles in the Co-free cathode, making it a strong candidate for next-generation lithium-ion batteries.

These findings identify a pathway toward more sustainable, high-performance lithium-ion batteries without relying on scarce and expensive cobalt.

“The study confirms that cobalt-free cathodes can outperform those with cobalt and provides a deeper understanding of the involved mechanisms. The use of advanced X-ray spectroscopy techniques allowed us to further refine cathode materials and enhance their efficiency and longevity”, says Laura Simonelli, group leader at the CLAESS beamline and main author of the study.

As the demand for cleaner energy storage solutions rises, this research contributes to the ongoing development of safer, more affordable, and environmentally friendly battery technologies.

The ability to track at the same time the Ni oxidation and the consequent strains on the Mn-O network, along with their correlation to the local structural evolution and oxygen release, provided in this work a more comprehensive understanding of charge compensation mechanisms.

This study highlights the importance of deepening the correlative approach in battery research in general, integrating systematically multiple measurable features and advanced statistical tools such as MCR. Indeed, subtilities are most likely at the origin of the high variability of functional properties shown by this class of materials. The next step is then to explore how these features correlate in analogous systems to conduct a thorough investigation into the underlying mechanisms driving the observed phenomena.

“Automated big data treatment, including statistical approaches and automated fitting, and the exploitation of AI became then necessary to really tackle the problems and allow a significant acceleration in the developments”, says Oleg Usoltsev, post-doc at the CLAESS beamline.