Electrode-Scale Heterogeneity in Lithium-Ion Batteries Unveiled via Operando Optical Microscopy

Abstract

With the increase of requirements for battery supplies in various areas, the necessity of superior battery system is also becoming larger. The superior system includes various factors such as higher energy density, safety, faster charging rate, and price. There have been a lot of research and developments to fulfill those requirements. However, it is significantly important to understand the phase separation, or phase heterogeneity, because inhomogeneous reaction would affect the cell property and performance, that cannot be predicted under the point of stable thermodynamics. When dealing with heterogeneity, the improvement of battery system would be able to fully show its potential as an identical superiority. Electrode-scale heterogeneity, particularly depth-dependent reaction distribution, is a critical yet often overlooked factor in lithium-ion battery performance. While many studies have focused on particle- level intercalation and transport mechanisms, the impact of depth heterogeneity on overall electrode behavior has not been thoroughly investigated. Particle-level and electrode-level behavior are connected deeply and affect each other, which means it is also important to fill the empty understanding of electrode-level heterogeneity. This study highlights the importance of understanding how lithium-ion distribution varies across electrode depth and how it influences key electrochemical properties. Using operando optical microscopy, we directly visualized lithium-ion transport behavior within graphite-based electrodes. It is advantageous in the point of its ability to distinguish the local change on the electrode during cell operation. First, pristine graphite exhibited severe depth heterogeneity, where lithium reactions were concentrated near the electrode surface, leading to inefficient material utilization and lithium dendrite formation under high current densities. To further examine this effect, we evaluated silicon-graphite, which showed alleviation of depth-heterogeneity, and applied this effect as phosphorus-coated graphite (GPC) electrodes. Compared to silicon-graphite electrode, which partially improved lithium distribution, phosphorus nanolayer coatings facilitated more uniform lithiation throughout the electrode by enhancing lithium-ion diffusivity and suppressing reaction localization. This indicates that modifying depth heterogeneity plays a significant role in material design These findings demonstrate that depth heterogeneity is a fundamental characteristic of battery operation that must be considered alongside particle-level properties. By establishing a direct link between depth-dependent reaction distribution and overall electrode behavior, this study emphasizes the necessity of incorporating electrode-scale insights into battery material and structural design strategies. Addressing depth heterogeneity is essential for improving the efficiency, stability, and longevity of next-generation lithium-ion batteries.