Understanding the fast charging ability limitation in a graphite electrode for Li-ion batteries. Heterogeneities of lithiation in conventional electrodes

Abstract

Graphite electrodes dominate the Li-ion battery market as a negative electrode material. As a consequence, it is of utmost importance to understand the origin of their power limitation for fast charging applications in electric vehicles. We first investigated thick and porous graphite electrodes using rate capability tests by varying the electrode porosity and the salt concentration, showing a first correlation between porosity, salt concentration, and high-power capability. Using X-ray holo-tomography at large-scale facilities, we probed the microstructure of the electrodes, showing the increase of tortuosity as a function of porosity. These gathered results were strengthened by using operando vertical profile high resolution X-ray diffraction. Heterogeneities of lithiation along the electrode thickness were found by varying the electrode porosity and the cycling rate from C/4 to C/1. The highest tortuosity results in large ionic transport hindrance along the electrode thickness. The in-depth investigation demonstrated that larger heterogeneities through the electrode thickness are observed. These heterogeneities depend on the graphite reaction mechanisms and are more pronounced in the biphasic domain ascribed to the flat potential plateau. Finally, by correlating all the results, we were able to estimate the local current densities and demonstrate that the effective local C-rate can reach twice the applied C-rate, raising serious concerns about the ageing phenomenon in batteries.