Depth-Dependent Kinetic Limitations and Degradation Mechanisms in High-Loading LiFePO₄ Electrodes

Lithium-ion batteries are integral to the advancement of electric vehicles and energy storage systems. Among cathode materials, lithium iron phosphate (LiFePO₄, LFP) has gained significant attention due to its low cost and stable lifespan. However, the relatively low energy density of LFP presents a critical challenge. To address this, a thick electrode strategy has been proposed, which reduces the proportion of electrochemically inactive components and increases the loading of active material, thereby enhancing energy density. Despite its potential, thick LFP electrodes suffer from severe capacity degradation during cycling, and the underlying mechanisms remain poorly understood. In this study, we compare reference and thick electrodes with active material loadings of 9 mg/cm² and 18 mg/cm², respectively. Through various analysis techniques such as electrochemical tests, scanning electron microscopy, x-ray photoelectron spectroscopy, and synchrotron-based x-ray analyses, we identify that ionic conductivity is the primary kinetic limitation in thick LFP electrodes rather than electronic conductivity, leading to inhomogeneous reactions. Furthermore, side reactions with the electrolyte in the top layer of the thick electrode impose additional kinetic constraints. This work provides critical insights into electrode design strategies and performance optimization for thick LFP electrode systems. (Figure presented.).