Spatial Quantification of Microstructural Degradation during Fast Charge in 18650 Lithium-Ion Batteries through Operando X-ray Microtomography and Euclidean Distance Mapping

Fast charging at rates above 1C aggressively accelerates structural degradation induced by increases in local temperature and inhomogeneous transport of charge. At the micron scale, the first indication of damage is irreversible expansion of the electrode layers. Electrode damage often involves void formation between the active material and conductive binder matrix. Quantification of this evolution must be carried out in real time and, thus, nondestructively. We report operando X-ray microtomography of cylindrical cells under fast-charge cycling. Two 18650 batteries were measured during cycling after antecedent fast charging cycles to track morphological damage at different points of battery life. A method of deep learning segmentation was used to objectively quantify the electrode degradation. Using Euclidean distance mapping, electrode dilation and voids were spatially resolved. Highly reversible trends in dilation were quantified during charge/discharge in the anode layers with irreversible increases in electrode voids. Anode voids showed clear localization within the first 10 μm near the current collectors, indicating delamination that spread upon further cycling. The cathode dilation trended opposite to the anode with higher fluctuations and an overall decrease in cathode voids. Insight into how fast charging induces structural damage better informs research into fast-charge protocols and battery chemistries.