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

Abstract

Graphite electrode dominates the Li-ion batteries market as a negative electrode material. As a consequence, it is of the utmost importance to understand the origin of its 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. By using X-ray holotomography at large-scale facilities, we probed the microstructure of the electrodes showing the increase of tortuosity as function of the 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 indepth investigation demonstrated that larger heterogeneities in the electrode are associated to biphasic domains. Finally, by correlating all the results, we were able to estimate the local current densities, and demonstrated that effective local C-rate can reach twice the applied C-rate, raising serious concerns about ageing phenomenon in batteries.