Reaction heterogeneity at the electrode scale. The mesoscale structural and chemical complexity can be further amplified at the electrode level. In a real-life battery cathode electrode, secondary particles are embedded in the porous carbon matrix with a rather high mass loading of ~10 mg/cm2 or greater [ref]. Close packing of active materials is essential to ensure the desired volumetric density. In addition to accommodating the active particles, the porous carbon matrix is also responsible for providing the electric contact throughout the electrode. The liquid electrolyte soaks the porous matrix and provides an interconnected tortuous diffusion network for the lithium ions. The mismatch in the local electric and ionic conductivity, both of which could change as a function of time and position, can lead to reaction heterogeneity at the electrode scale, which is closely relevant to the real-life cell aging and even failure.  
We show in Figure 2 the rendering of the x-ray phase contrast tomographic results of the NMC622 electrode that was cycled for 10 times at a rate of 5C. For penetrating the thick electrode, the x-ray imaging experiment was carried out at high energy (17 keV), which, unfortunately, sacrifices the absorption contrast and, thus, accentuates the need for the sensitivity improvement with phase contrast methodology\cite{Hubert_2018}\cite{Fang_2016}. Figure 2a is an overview of the electrode (phase contrast micro tomography data), with a magnified region of interest (ROI) shown in Figure 2b and a selected slice through the center of the ROI shown in Figure 2c (70 nm pixel size). Three of the representative particles are highlighted in red (severely damaged), green (mildly damaged), and blue (least damaged). The corresponding central slices of these particles are shown in Figures 2d-2f. Further comparison of the electrode with different cycling history (10 cycles vs. 50 cycles) is shown in Figure 2g, suggesting that the particle cracking continues to develop upon electrochemical cycling.