Increased Disorder at Graphite Particle Edges Revealed by Multi-length Scale Characterization of Anodes from Fast-Charged Lithium-Ion Cells

Fast charging of lithium-ion cells increases voltage polarization of the electrodes and creates conditions that are favorable for Li-deposition at the graphite anode. Repeated fast charging induces changes in the capacity-voltage profiles and increases the probability of lithium-plating on the electrode. This higher probability results from structural, morphological and chemical modifications that are revealed by multi-length scale characterization of graphite anodes extracted from discharged lithium-ion cells, previously charged at rates up to 6 C. The distinct differences between anodes with lithium-plating and as-prepared electrodes are clearly seen in analytical electron microscopy data. Scanning electrode microscopy (SEM) images show that the fast-charged anode is significantly thicker, apparently because of the electrolyte reduction/hydrolysis products that accumulate in electrode pores. High resolution electron microscopy (HREM) images reveal wavy graphite fringes near the particle edges. Analysis of scanning electron nanodiffraction (SEND) data reveal higher d-spacings and greater lattice rotations, indicating disorder in the graphite near the particle edges that extend about 20 nm into the bulk. The extent of this disorder is greater near larger internal pores, highlighting nanoscale heterogeneities within particles. As graphite lithiation occurs primarily through edge planes, this permanent disorder would hinder Li+ intercalation kinetics and favor Li0 plating during repeated cycling.