Transition-Metal Migration in Lithium-Rich Layered Oxides – a Long-Duration Synchrotron Study
Authors: Kleiner Karin Maria , Strehle Benjamin , Buchberger Irmgard , Baker Annabelle , Day Sarah , Tang Chiu C. , Gasteiger Hubert A. , Piana Michele 
Published: February 2020

Lithium-rich layered oxides offer an 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 OEMS and TEM studies, this process is limited to near-surface regions of the particles.7,8 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 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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Figure 1