<p>The performance and lifetime of next-generation rechargeable batteries are limited not only by their materials, but also by our incomplete understanding of how ions move through their complex, porous architectures during operation. Spatial heterogeneities in ion transport—long assumed to be negligible in models—can silently degrade capacity, efficiency, and safety. Here, we present an operando optical platform based on tilted fiber Bragg grating sensors and infrared fiber spectroscopy to resolve lithium-ion and sodium-ion transport dynamics at high perpendicular spatial and temporal resolution. This dual-modality approach complementarily tracks electrolyte refractive index and solvent coordination under comparable electrochemical cycling conditions, revealing previously hidden concentration anomalies at charge and discharge endpoints and during voltage-plateau transitions. These anomalies correlate with mass-transport overpotentials and deviate from predictions of the&#xa0;Doyle–Fuller–Newman model, indicating spatially non-uniform current distributions that challenge the long-standing assumption of reaction homogeneity in battery modeling. The compact, non-invasive, and readily integrable fiber-based strategy offers a pathway toward more durable, efficient, and sustainable energy-storage technologies.</p>