Operando Bragg Coherent Diffraction Imaging of Commercial NMC811 Electrodes

<br/>Commercial candidates for electric vehicle batteries include Ni-rich Li(Ni0.8Mn0.1Co0.1)O2 (NMC811) cathodes due to their high specific capacity (200 mAh/g) and reduced cobalt content, which has positive socio-economic repercussions. Despite all of the advantages, these materials suffer from a range of degradation modes, many of which are associated with the redox and crystallographic behavior at high states of charge and are thought to be the cause of rapid capacity fade. NMC811 suffers from anisotropic changes in the crystal structure during cycling, which induce strain and can then lead to issues such as particle crack formation. Gathering data at multiple length scales is imperative for understanding the discrete mechanisms at play. Bragg Coherent Diffraction Imaging (BCDI) is a key tool for obtaining data at the nano-scale by providing high spatial resolution images of individual particles. BCDI also affords the unique opportunity to resolve the strain of individual crystals at the picometer scale, producing data with the highest quantifiable imaging resolution of battery electrodes and allowing for the detection of evolving defect modes such as shearing and stacking faults in the crystals during cycling.<br/>NMC811 typically employs the archetypal cathode particle morphology, consisting of a secondary particle agglomerate (~ 10 μm) made up of nanometer-sized primary particles. It is widely accepted that crack formation occurs at grain boundaries between primary particles. One mitigation method is to switch to a single crystal morphology, reducing the number of grain boundaries within particles and therefore improving overall performance. However, there is still a limited understanding of the subtle mechanistic differences between the two morphologies during cycling.<br/>In this work, Operando BCDI was employed to study both polycrystalline and single crystal NMC811 electrodes in both pristine and aged states. Non-uniform distributions of inter- and intracrystal strain were observed for both polycrystalline and single crystal morphologies, even in the pristine state. The nucleation of crystal splitting was observed for a pristine single crystal during cycling, which could subsequently result in intergranular cracking. This feature is only resolvable with extremely high-resolution techniques such as BCDI. This work ultimately adds to the understanding of the central role of crystal-scale dynamics during cycling and the potential ramifications for a cell's electrochemical performance, leading to informed material design.