Lithium-rich layered oxides offer an extraordinarily high gravimetric capacity of more than 250 mAh g^-1, which makes this class of materials attractive as cathode material in future automotive applications.¹ However, the materials suffer from several drawbacks, such as low initial coulombic efficiency, poor capacity retention and voltage fading upon cycling.² While a lot of efforts have been put into the modification of the materials with minor success, no matter whether the modification were bulk- or surface-related,^3,4 less attention has been paid to understand the underlying capacity fading mechanism. Initially, the performance drop was ascribed to oxygen release from the host structure.^5,6 According to more recent Online-Electrochemical-MS⁷ and TEM⁸ studies, this process is limited to near-surface regions of the particles. However, an uneven increase of overpotentials during charge and discharge, and changes/shifts of the peaks in the differential capacity plots suggest bulk effects as the main reason of the poor cycling stability.² Although changes in the geometry of the unit cell were not evidenced by powder or neutron diffraction so far,⁸ a more detailed diffraction study is still missing. This should include the analysis of transition metal migration during long-term cycling, which is frequently called spinel and/or rock-salt transformation in the literature,⁹ and was also proposed by theoretical studies to be the reason for the performance drop.¹⁰

Aiming at this, we cycled the lithium-rich layered oxide, x Li₂MnO₃∙(1-x) LiNi_aCo_bMn_cO₂ (a+b+c=1), versus metallic lithium at a low C-rate of C/5 (50 mA g^-1) at the Long-Duration-Experiment facility of Beamline I11 at Diamond Light Source¹¹ using a custom-made pouch cell design. We collected XRD patterns every week, alternatingly in the charged and discharged state, from several cells at an interval of 15 cycles per week (more than 7 weeks in total). On the collected long-term synchrotron in situ data we performed Rietveld analysis and detected an imbalanced electron density in difference Fourier mapping, using an ordered model structure. We quantified the transition metal migration upon cycling (i. e., the transition metal disorder) by detailed reflection profile analysis. Our data demonstrate experimentally for the first time that the transition metal migration in lithium-rich layered oxides proceeds upon cycling from the octahedral transition metal sites (Figure 1A) via tetrahedral sites in the lithium layer (Figure 1B) into octahedral lithium sites (Figure 1C). Such migration is irreversible (at least partially) and it is correlated to the irreversible discharge voltage fade. From the collected XRD data, we cannot conclude if the origin of the voltage fade is thermodynamic or kinetic, but various hypotheses will be discussed. Furthermore, a comparison with other layered metal oxides will be performed.

Acknowledgement: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry and batteries.

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