Imaging the space-resolved chemical heterogeneity of degraded graphite anode through scanning transmission X-ray microscope

Deeply understanding the chemical heterogeneity of graphite anode is important because it can provide insights into the underlying mechanisms of electrode degradation and failure in battery systems. Herein, the chemical heterogeneity of graphite anode is imaged by X-ray fluorescence mode scanning transmission X-ray microscope (XRF-STXM) to space-resolve the chemical distribution and structure at C, O, F and Na sites. The synchrotron radiation X-ray photoelectron spectroscopy analysis suggests that the solid-electrolyte-interphase (SEI) layer on graphite anodes contains a complex mixture of both organic (-COOR, -C-OR, etc.) and inorganic species (CO₃²⁻, LiF, Li_xPO_yF_z, etc.), which can vary with depth. Furthermore, XRF-STXM imaging combined with X-ray absorption spectra is used to map out the heterogeneity in the chemical spatial distribution of SEI composition and its location dependence, as well as its strong correlation with the distribution of conductive additives/binders and binders decomposition. The distribution of SEI exhibits a distinctive feature, with the CO₃²⁻ and LiF species being significantly thicker at edge plane than at basal plane and the distribution being highly non-uniform. The insights gained from the visualization of the spatial chemical distribution will deepen our understanding of SEI, which is crucial for delineating effective strategies for the development of Li-ion batteries.