Discussion
It is generally appreciated that the electro-chemo-mechanical processes in lithium ion batteries are ubiquitous across a wide range of length scales. While the transformation in the lattice structure at the atomic scale is often regarded as the root causes of a range of degradation phenomenon, the complexity is further amplified at the mesoscale (within the secondary particles) and the macroscale (at the electrode level). To truly understand the intertwined, heterogeneous and multiscale electro-chemo-mechanical coupling effects in composite battery electrodes, imaging techniques that can resolve features at different length scales are often combined. More specifically, it is very desirable to conduct electrode scale imaging with nanometric resolution. Such dataset could offer unprecedented amount of structural information, which could facilitated sophisticated and valuable statistical analysis.
In this work, we started at the atomic scale and used TEM to visualize the coexisting layered and rocksalt lattice structures over a cracked area. The local lattice deformation can serve as nucleation points for further development of the structural defects. Coupled with nanoscale x-ray spectro-microscopy and electron microscopy, we then reconstruct the mesoscale morphological and chemical transformations in the NMC622 secondary particles under fast charging conditions. We quantify the electrode and particle morphology and, subsequently, elucidate two stages of the chemomechanical transformation at the secondary particle level: 1) the host material’s local lattice structural transition and 2) liquid electrolyte infiltration that forms new diffusion pathway for lithium ions. Finally, we conducted hard x-ray phase contrast nano-tomography of the cathode electrode with nanometric spatial resolution. We extracted the complicated morphological degradation profile across the complete electrode slab and presented the depth dependent trend of particle fracturing as well as its lateral complexity. We developed a method to reconstruct the 3D topological representation of the local current density distribution. Our results suggest that the  electrode’s active materials contributes to the cell level chemistry differently in time and location. The presented findings and their implications offer insights into designing chemomechanically robust battery particles and formulating fast charge capable electrodes.