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.
Highlights
The increasing desire to move from fossil fuel-powered internal combustion engines to electric vehicles is driving swift advancements in the performance of Li-ion batteries, which will lead to a more sustainable society (Tarascon and Armand, 2001; Blomgren, 2017)
It is likely that the contribution to the overall charge of the cell during the constant current (CC) charging is larger than for the center of the cell, before they reach the limit of their ability to extract lithium at some point during the CC step, meaning that the remaining charging of the cell during the constant voltage (CV) hold is undertaken by the central locations of the cell (Xu et al, 2021)
Such inhomogeneities in lithiation extent, and effective local C-rate, in the cell during charge, will have implications for the material degradation on cycling, the result of which is evidenced in the accumulation of this effect after 900 cycles from the spatially resolved synchrotron XRD (SXRD) presented here
Summary
The increasing desire to move from fossil fuel-powered internal combustion engines to electric vehicles is driving swift advancements in the performance of Li-ion batteries, which will lead to a more sustainable society (Tarascon and Armand, 2001; Blomgren, 2017). LiNi0.8Mn0.1Co0.1O2, or NMC811, is desirable because of its high specific capacity and the lower ethical impact due to its lower cobalt content (Kim et al, 2018) Despite all of their benefits, NMC811 and other Ni-rich layered materials suffer from poor cycle life relative to their lower-Nicontent analogs (Ryu et al, 2018; Friedrich et al, 2019). Synchrotron XRD (SXRD) has been employed to reveal the distribution of the state of charge and the fatigue of NMC811 cathodes within operating pouch cells prior to, and following, long-term electrochemical aging. The motorized xyz sample stage of the beamline was able to move the cell through the nine acquisition positions, while charging and discharging was conducted allowing for the continuous sequential measurements of all the positions (from position 1 to position 9, and starting the cycle again at position 1, Figure 1). Projections (1,061) were collected over a 180° rotation, which were reconstructed using commercial software (“Reconstructor Scout-and-Scan,” Carl Zeiss, CA, USA) employing filtered-back-projection (FBP) algorithms
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