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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). 1. D. Andre et al., J. Mater. Chem. A, 3, 6709–6732 (2015). 2. J. R. Croy et al., J. Phys. Chem. C, 117, 6525–6536 (2013). 3. Z. Q. Deng and A. Manthiram, J. Phys. Chem. C, 115, 7097–7103 (2011). 4. P. Rozier and J. M. Tarascon, J. Electrochem. Soc., 162, A2490–A2499 (2015). 5. F. La Mantia, F. Rosciano, N. Tran, and P. Novák, J. Appl. Electrochem., 38, 893–896 (2008). 6. A. R. Armstrong et al., J. Am. Chem. Soc., 128, 8694–8698 (2006). 7. B. Strehle et al., J. Electrochem. Soc., 164, A400–A406 (2017). 8. C. Genevois et al., J. Phys. Chem. C, 119, 75–83 (2015). 9. J. Hong, H. Gwon, S.-K. Jung, K. Ku, and K. Kang, J. Electrochem. Soc., 162, A2447–A2467 (2015). 10. J. Bréger et al., J. Solid State Chem., 178, 2575–2585 (2005). 11. C. A. Murray et al., J. Appl. Crystallogr., 50, 172–183 (2017). Figure 1

The use of in-situ X-Ray Powder diffraction for the study of new battery materials has grown in recent years, providing an insight into the effects that charge/discharges cycles have on cathode materials. These experiments are often limited to short periods of time and therefore only allow for one full charge/discharge cycle/multiple fast cycles to be observed. The result is that material characteristics may be well understood for the first cycle but little is known about the long term processes that ultimately lead to failure of the cell over longer timescales. The Long Duration Experiment (LDE) Facility on Beamline I11 at Diamond Light Source is the first of its kind, housing experiments that require periodic, in-situ monitoring over periods of months to years. It is an ideal facility for the long term study of battery materials, allowing data to be collected over many cycles. To conduct these experiments a new cell had to be developed that would allow the transmission of X-rays, while also remaining impermeable to air and moisture for a long period of time. Previous in-situ diffraction experiments of battery materials have generally involved using unconventional cells. While these are effective, they are often not stable for long periods of time and are not true representations of real-life battery systems. By modifying a standard CR2032 coin cell case, we have been able to produce a cell that fits this purpose. Laser thinning was used to reduce the thickness of a small section of the cell to 50μm, allowing sufficient X-ray transmission without compromising the integrity of the cell.

The Long Duration Experiment (LDE) Facility on Beamline I11 at Diamond Light Source is the first of its kind, allowing in situ synchrotron X-ray powder diffraction (SXPD) data to be collected on a weekly basis for extended periods of time (a few months up to a few years). The facility is capable of housing multiple experiments simultaneously, and even though data are only collected at weekly intervals the experiments remain in place and all sample environments (high/low temperature, humidity, electrochemical cycling) are maintained throughout the duration of the experiment. The increasing demand for large-scale energy storage (e.g. electric vehicles) means that new materials, with much higher Li+ storage capabilities and longer lifetimes than are currently available are required to meet this demand. Many promising candidates for such materials are known but before these can become commercially available their chemical/structural stability and electrochemical cycling performance need to be well understood. In-situ powder diffraction has proven to be an extremely useful technique for just this, providing an insight into the complex chemical process that take place during charge/discharge cycling. Standard in situ experiments using existing facilities are, however, often limited to short periods of time, from a few hours up to a few (2-3) days, and therefore, only allow the first few cycles (often at accelerated charging rates) of the material to be observed. The result is that the material characteristics may be well understood for the first cycle but little is known about the long term processes that ultimately lead to failure of the cell over longer timescales. With a specially designed coin cell holder (Figure 1 inset) and dedicated multi-channel Arbin battery cyclers it is an ideal facility for the long term study of both existing and novel battery materials. This setup allows data to be collected over many hundreds of cycles at slower, more realistic, rates without the need for accelerated cycling therefore better simulating battery use under real-life conditions. To achieve this, a new in situ cell was developed through the modification of a standard CR2032 coin cell which allows for the transmission of X-rays while retaining the integrity and impermeability of the stainless steel case and therefore allowing the cell to perform well for long periods of time. Details of the facility and the modified coin cells will be presented along with preliminary results of the first long term battery studies, of a selection of Li-based electrodes (commercial and novel materials), to demonstrate the capabilities of this facility and the quality of the data produced. Figure 1

The use of in-situ X-Ray Powder diffraction for the study of new battery materials has grown in recent years, providing an insight into the effects that charge/discharges cycles have on cathode materials. These experiments are often limited to short periods of time and therefore only allow for one full charge/discharge cycle/multiple fast cycles to be observed. The result is that material characteristics may be well understood for the first cycle but little is known about the long term processes that ultimately lead to failure of the cell over longer timescales. The Long Duration Experiment (LDE) Facility on Beamline I11 at Diamond Light Source is the first of its kind, housing experiments that require periodic, in-situ monitoring over periods of months to years. It is an ideal facility for the long term study of battery materials, allowing data to be collected over many cycles. To conduct these experiments a new cell had to be developed that would allow the transmission of X-rays, while also remaining impermeable to air and moisture for a long period of time. Previous in-situ diffraction experiments of battery materials have generally involved using unconventional cells. While these are effective, they are often not stable for long periods of time and are not true representations of real-life battery systems. By modifying a standard CR2032 coin cell case, we have been able to produce a cell that fits this purpose. Laser thinning was used to reduce the thickness of a small section of the cell to 50μm, allowing sufficient X-ray transmission without compromising the integrity of the cell.