Visualizing Correlation between Electrode-Level Heterogeneity and Li Plating in Graphite Anodes via Operando Side-View Optical Microscopy

Abstract

Reaction heterogeneity at electrode scale plays a critical role in governing the performance, degradation, and safety of lithium-ion batteries, particularly under fast-charging conditions. While lithium plating has traditionally been associated with operating parameters such as C-rate and temperature, its strong coupling with internal reaction heterogeneity arising from electrode architecture remains insufficiently understood. In this work, we systematically investigate electrode-scale reaction heterogeneity using electrode porosity as a key structural parameter and elucidate its direct relationship with lithium plating behavior in graphite anodes. Graphite electrodes with identical areal capacity were fabricated with controlled porosities through systematic calendering, enabling isolation of porosity-induced transport effects. Electrochemical characterization revealed that decreasing porosity intensifies diffusion limitations, leading to increased overpotential, earlier transition to constant-voltage charging, and accelerated capacity fade, particularly under high C-rate operation. Differential capacity (dQ dV⁻¹) analysis demonstrated pronounced peak broadening and suppression of distinct staging transitions with decreasing porosity, indicating increasingly heterogeneous lithiation behavior at the electrode scale. Complementary in situ X-ray diffraction further confirmed the coexistence of multiple graphite intercalation phases in low-porosity electrodes, even at high states of charge, highlighting incomplete and spatially non-uniform phase evolution. To directly visualize reaction heterogeneity, operando optical microscopy was employed as a non- destructive, spatially resolved diagnostic tool. By leveraging lithiation-induced color evolution of graphite and quantitative analysis using the CIELAB color space, depth- and areal-resolved phase distributions were extracted in real time. Porosity 40% electrodes exhibited relatively uniform lithiation across both depth and surface directions, whereas porosity 30% electrodes developed pronounced reaction localization near the electrode surface, which intensified with increasing C-rate. Importantly, lithium plating was preferentially observed in regions exhibiting strong reaction heterogeneity, demonstrating that plating is not solely determined by global operating conditions but is critically governed by localized transport limitations and reaction non-uniformity. By correlating electrochemical signatures, structural evolution, and operando visualization, this study establishes reaction heterogeneity as a key intermediate factor linking electrode design parameters to lithium plating formation. These findings highlight the necessity of heterogeneity-aware electrode design, emphasizing that porosity optimization must balance energy density and transport uniformity to suppress localized current concentration and irreversible lithium deposition. The insights provided herein offer a framework for designing durable, fast-charging-capable lithium-ion battery electrodes through informed control of electrode architecture.