OPERANDO VISUALIZATION OF DYNAMIC HETEROGENEITY IN LI-ION BATTERIES

Lithium-ion batteries have become a foundational technology for modern energy systems, enabling the rapid growth of electric vehicles and the integration of renewable energy into the grid. As these applications expand, increasing demands for higher energy density, faster charging, and longer lifetimes have increased the need to understand and mitigate degradation. However, many of the processes that limit battery performance originate from spatially heterogeneous behavior within electrodes that is not captured by conventional, spatially averaged measurements. In this work, a cross-sectional operando imaging platform is developed to directly visualize lithiation dynamics within graphite electrodes under representative electrochemical conditions. By integrating high-resolution visible light microscopy with synchronized electrochemical measurements and quantitative image analysis, lithiation behavior is resolved continuously from the electrode scale down to individual particles and basal planes. At the electrode level, lithiation is shown to proceed non-uniformly, with phase transitions initiating near the separator and propagating toward the current collector. Depth-resolved analysis reveals that successive staging transitions propagate at decreasing rates, indicating increasingly transport-limited behavior. At the particle scale, significant heterogeneity is observed, with localized nucleation and non-uniform propagation pathways governing lithiation dynamics. At the basal plane scale, a transition from relatively uniform behavior in early stages to strongly localized, front-like propagation in later stages is directly visualized. A key finding of this work is the observation of transition-dependent lithiation pathways, including a reversal in propagation behavior between the 3L→2 and 2→1 transitions. This behavior suggests that lithiation is governed not only by transport limitations, but also by local structural evolution and transformation history. A mechanism is proposed in which variations in lithium ordering and domain structure influence subsequent phase transitions. These results demonstrate that lithiation in graphite is inherently heterogeneous across multiple length scales and cannot be fully described using spatially averaged models. More importantly, they show that macroscopic degradation is rooted in localized processes at the grain and particle level. The cross-sectional operando platform developed here provides a framework for directly linking electrochemical response to spatially resolved behavior and offers a versatile tool for studying a wide range of battery materials. Ultimately, this work advances the understanding of lithium-ion battery operation and provides insight critical to improving performance, reliability, and lifetime in next-generation energy systems.