Observing asymmetric structural changes during cycling of NMC622 batteries through operando X-ray diffraction

Currently, lithium-ion batteries are the leading technology for energy storage, finding application in electric devices and vehicles, medical equipment and intermittent energy storage systems. This extensive use, however, also requires a high throughput of material resources, and extending lithium-ion battery lifetimes would alleviate the strain on the supply chain. X-ray spectroscopy and scattering methods are ideal for probing through an outer surface at the micrometer length scale, allowing one to observe chemical processes and structural changes within the electrode. In this work, we perform in-house operando X-ray diffraction with novel 3D-printable cells, achieving an increase in signal intensity compared to standard setups by using aluminized Kapton current collectors. The increased resolution helps to clearly identify asymmetric changes in lattice parameters during charging and discharging, signifying for the first time a discrepancy in lithium migration kinetics during cycling of an NMC battery. This behavior is also confirmed by impedance spectroscopic cycling data, reiterating that the delithiation step is more abrupt than the lithiation. Ultimately, the significantly larger local structural changes that occur during delithiation emphasize that the charge step, specifically, should be investigated as a more likely source for critical lattice defects. • High-resolution in-house operando X-ray diffraction on Li ion batteries. • Low incident angles achieved using a novel cell design. • Lithiation depends on direction of charge diffusion, elucidating failure mechanisms.