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Electronic connections between active material particles and the conductive carbon binder domain govern high-energy commercial Li-ion batteries' rate capability and lifetime (LIB). This work develops an in situ electrochemical fluorescent microscopy (EFM) technique that maps fluorescence intensity to these local electronic connections. Specifically, rapid redox kinetics of an electrofluorophore translates to reaction distributions limited by the electronic accessibility of battery electrode regions and individual active material particles. This technique can visualize hot spots, dead zones, and isolated particles on the electrode surface. EFM characterization of a series of LiNi_0.33Mn_0.33Co_0.33O₂electrodes across processing parameters finds a significant negative correlation between the number of disconnected active particles and the rate capability. This low-cost technique provides quantitative mesoscale characterization of commercial LIB electrodes with fast throughput (<60 s) to facilitate rapid research and development and provide manufacturing quality control. 

A new electroanalytical design based on asymmetric interdigitated arrays (IDA) is presented and the effects of asymmetry on the device performance are characterized electrochemically. Varying the collector and generator band widths independently of each other tunes the collection efficiency and redox cycling-induced feedback. These arrays are able to provide low feedback (<10%) while maintaining moderate collection efficiency (25%–40%). Behavior is evaluated experimentally and using a numerical model. The application of the device to detect soluble electrolyte degradation products in nonaqueous lithium-ion and sodium-ion battery electrolytes is demonstrated.