Aiming at this, we quantify the transition metal migration upon cycling by detailed reflection profile analysis and difference Fourier mapping from long-term synchrotron X-ray powder diffraction measurements. The lithium-rich layered oxide, x Li2MnO3 · (1-x) LiNiaCobMncO2 (a+b+c=1), was cycled versus metallic lithium at a low C-rate of C/5 (50 mA g-1) in a custom-made pouch cell design at the long duration experiment facility of beamline I11 at Diamond Light Source.11 XRD patterns were collected every week alternating between the charged and discharged state from several cells at an interval of 15 cycles per week (more than 7 weeks in total). Our data show that during prolonged cycling transition metals can migrate reversibly from normal octahedral sites into adjacent tetrahedral sites in the lithium layer at high states of charge (Figure 1A and B). However, we show also that they further move into octahedral sites in the lithium layer (Figure 1C). This movement is, according to the present study, irreversible. Consequently, the reversible disorder might explain the high initial capacity due to reduced lattice strain, whereas the accumulation of transition metals in the lithium layer probably causes both capacity and voltage fade by blocking the lithium diffusion pathways during cycling.
Acknowledgements: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry and batteries. We also thank Diamond Light Source for access to beamline I11 (Beamtime Award EE14552).
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