Degradation Mechanism Induced by Depth-Dependent Inhomogeneity in Thick High-Areal-Capacity Graphite Electrode

Abstract

Employing thick electrodes with high active material loading is one of the most practical approaches to enhance the energy density of lithium-ion batteries by fully leverage the potential of electrode materials. However, use of thick electrodes typically leads to a significant decline in electrode performance, accompanied by accelerated electrode degradation. Herein, the degradation mechanism is elucidated in high-loading graphite electrodes, driven by depth-dependent reaction inhomogeneity along the electrode thickness. It is demonstrated that the inhomogeneity is primarily caused by entrapment of lithium ions at the bottom of the electrode, progressively worsening with cycles, and contributes to the generation of current hotspots particularly at the top of the electrode. These hotspots trigger excessive solid electrolyte interphase formation, causing a sharp rise in charge transfer resistance and further exacerbating reaction inhomogeneity. It is further shown that the protection of the electrode surface mitigates the side reactions induced by current hotspots, breaking the negative feedback loop between electrode resistance and reaction inhomogeneity. The negative feedback loop in the degradation mechanism suggests a need for a comprehensive strategy that not only enhances diffusion process commonly targeted for improving thick electrode performance but mitigates the surface reaction for the successful implementation of high-loading electrodes.