Operando Confocal Microscopy of Structured Battery Electrodes

Fast-charging and high-energy density Li-ion batteries are crucial for accelerating electrification of the automotive sector. However, these batteries have competing requirements: high-energy density battery designs favour high areal loading electrodes, while fast charging requires the opposite to avoid mass transport limitations. Several electrode structuring techniques have been proposed to balance these requirements with the goal of achieving homogeneous state of charge (SoC) at high rates in high-loading electrodes. However, current techniques to study the effect of electrode structuring on heterogeneity within electrodes are expensive and difficult to perform. Here, we focus on operando cross-section confocal microscopy as a simple high-throughput technique to understand the state of charge heterogeneity of electrodes as a function of battery cycling rate. We use laser patterned electrodes as a model system to demonstrate the utility of cross-sectional microscopy for measuring reaction heterogeneity. Optical analysis shows that at moderate rates (∼1 C), there is significant heterogeneity in SoC between the top and bottom of normal electrodes. However, at the same rate the laser processed electrodes do not exhibit significant heterogeneity due to improved ion transport. The direct correlations between optical cross-section imaging and electrochemical performance will help establish confocal microscopy as a tool to optimize electrode structuring.