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Operando X-ray diffraction unveils a common degradation path for battery electrode materials

Operando X-ray diffraction has been used to unveil the root cause of degradation in LiNixMnyCozO2 compounds, commonly used as positive electrodes in lithium-ion batteries. By probing various nickel contents and charging rates, it was found that these materials undergo a common degradation mechanism due to chemical instability during lithium depletion, which correlates with structural distortions, stress, and cracking.

LiNixMnyCozO2 (NMCxyz) compounds are layered transition metal oxides (TMO) typically used as cathode materials in lithium (Li)-ion batteries. In these materials, Li+ ions are reversibly deintercalated or intercalated between the TMO layers during charge and discharge, inducing changes in the crystal lattice parameters. The excellent energy density of NMCs makes them highly suitable for long-range electric vehicles. However, they degrade more quickly than less energy-dense materials like LiFePO4. Nickel-rich compositions, such as NMC622, NMC811, and LiNiO2 (LNO), offer higher energy densities because more Li ions can be exchanged within the same electrochemical potential stability window, defined by the carbonate electrolytes. Yet, these compositions suffer from accelerated capacity loss during cycling.

The (de)lithiation mechanism in NMCs is still debated, with some studies reporting a continuous evolution typical of a solid-solution mechanism, while others indicate discontinuous lattice parameter changes, suggesting successive phase transitions. The phase

transitions have been implicated in capacity loss. Therefore, understanding the lithiation mechanism in NMCs is crucial to identifying the root cause of degradation and finding ways to mitigate it.

In this study, NMC622, NMC811, and LNO were analysed using operando X-ray diffraction under different charging rates (C-rate). To investigate the (de)lithiation mechanism near equilibrium – using very low electric currents to minimize kinetic limitations (charging over 100 hours) – laboratory X-ray diffraction was employed. Phase transitions were observed only in LNO. Faster C-rates, ranging from C/5 up to 2C (charging times of 5 hours to 30 mins, respectively), were measured at beamline BM32 as part of the Battery Pilot Hub. Care was taken to minimize radiation damage that could affect peak broadening measurements [1]. Additionally, regardless of the C-rate or nickel content, the evolution of the lattice parameters was consistent, indicating a common delithiation mechanism. Quantitatively, all compounds exhibited a maximum TMO interlayer distance at 40% Li content, where O-O repulsion is least compensated by the covalency of the Ni-O bond (Figure 66a). These findings resolve the “solid-solution versus phase transitions” debate.

Closer analysis of the diffraction data revealed significant peak broadening in the 〈00l〉 reflections towards the end of charge. This anisotropic broadening is likely caused by heterogeneous lithium distribution within the NMC. As Li content falls below 40%, the lattice parameters become increasingly sensitive to lithium concentration, particularly along the c-axis (the layer stacking direction). Even small spatial variations in lithium content lead to large distributions in lattice parameters, which are

Fig. 66: a) Crystallographic distances in NMCs versus lithium concentration (cLi) showing transition metal (TM) and inter-layer (IL) distances (a and c/3, respectively). b) Anisotropic strain versus cLi for different nickel content and charge rates. Strain grows

exponentially below a critical cLi value of 0.4 (red zone).