Spatially Resolved Operando Synchrotron-Based X-Ray Diffraction Measurements of Ni-Rich Cathodes for Li-Ion Batteries

Abstract

Understanding the performance of commercially relevant cathode materials for lithium-ion (Li-ion) batteries is vital to realize the potential of high-capacity materials for automotive applications. Of particular interest is the spatial variation of crystallographic behavior across (what can be) highly inhomogeneous electrodes. In this work, a high-resolution X-ray diffraction technique was used to obtain operando transmission measurements of Li-ion pouch cells to measure the spatial variances in the cell during electrochemical cycling. Through spatially resolved investigations of the crystallographic structures, the distribution of states of charge has been elucidated. A larger portion of the charging is accounted for by the central parts, with the edges and corners delithiating to a lesser extent for a given average electrode voltage. The cells were cycled to different upper cutoff voltages (4.2 and 4.3 V vs. graphite) and C-rates (0.5, 1, and 3C) to study the effect on the structure of the NMC811 cathode. By combining this rapid data collection method with a detailed Rietveld refinement of degraded NMC811, the spatial dependence of the degradation caused by long-term cycling (900 cycles) has also been shown. The variance shown in the pristine measurements is exaggerated in the aged cells with the edges and corners offering an even lower percentage of the charge. Measurements collected at the very edge of the cell have also highlighted the importance of electrode alignment, with a misalignment of less than 0.5 mm leading to significantly reduced electrochemical activity in that area.

Combining multi-modal non-destructive techniques to investigate ageing and orientation effects in automotive Li-ion pouch cells

“…It is known that thermal inhomogeneities in cells can create a variation in local cell impedance and current density 64,65 and in turn cause differences in the SoC across a cell. 66 Higher current densities at the tabs of pouch cells have also been experimentally confirmed 67 and in an in-situ synchrotron X-ray diffraction experiment on a pouch cell the area under the positive tab was found to be of a higher SoC compared to the rest of the cell 5 . In a separate HELM study on a singlephase NMC622 pouch cell, Yu et al found that the edges and corners were of a lower SoC than the middle of cell.…”

“…Misalignment is a problem because it causes a reduction in surface area overlap between the electrode layers, meaning that lithium is left inactive during cycling and in turn causes capacity loss. 5 More importantly, Figures 5b-e show that gas formation related to electrolyte depletion was observed in both R2 and F3 samples after ageing. X-ray imaging and subsequent quantification of liquid electrolyte in this region allow for comparison of electrolyte 'depletion' that can result from different displacements.…”

“…36 One such study on long-duration degradation by Leach et al collected XRD data across nine sites on a single-layer small-format NMC811/graphite pouch cell over 900 cycles. 5 They found that the SoC of the edges and corners of the pouch cell increased slower relative to the centre of the cell on charging and contained a greater proportion of a fatigued phase. The authors speculated that these spatial inhomogeneities may become more pronounced in larger format pouch cells or cylindrical cells.…”

“…Different cell orientations and tab positions can result in a heterogeneous distribution of the SoC, 5 current densities and voltage, which can cause temperature gradients across the battery. 6,7 While battery performance is strongly linked to temperature, 8 temperature gradients will influence ageing mechanisms during long-term usage, 9 Battery ageing characterisation by IR thermography has been covered in several studies: Veth et al 10 highlighted the possibility of ex-situ IR thermography characterisation of large-format pouch cells (high/low current cycled and calendar aged) that have been aged to different degrees.…”

Fluorinated co-solvent electrolytes for high-voltage Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) positive electrodes

“…Nowadays, lithium-ion batteries (LIBs) are the important power sources for portable electronics, electric vehicles, aerospace, and deep-sea diving devices due to their high energy density, acceptable cycle life, and no memory effect (Wang et al, 2020;He et al, 2021). With the continuous expansion of the application fields of LIBs, lots of efforts have been made to further improve their energy density and safety (Leach et al, 2022;Li et al, 2022;Yang et al, 2022). So far, Nickel-rich LiNi x Co y Mn 1-x-y O 2 (NCM for short, x ≥ 0.6) materials, specially, Nickel-rich LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811 for short) positive electrodes are one of the most promising positive electrodes for utilization in the next-generation of lithium-ion batteries (Li et al, 2020;Xue et al, 2021).…”