Energy-Dispersive X-ray Diffraction: Operando Visualization of Electrochemical Activity of Thick Electrodes

Effective utilization of the electroactive material in thick electrodes could enable Li-based batteries to be developed with higher energy densities while simultaneously reducing the overall cost of the battery. Herein, we explore the lithiation of a multiple electron transfer conversion material, Fe₃O₄, and report tomographic-like mapping of the electroactive material utilization under operando electrochemical lithiation using energy-dispersive X-ray diffraction. A challenge for thick electrodes is limited Li⁺ diffusion resulting in an inability to fully access the active material. Our strategy to surmount this obstacle is the deliberate incorporation of acicular carbon nanotubes to the electrode where the design maximized the electron and ion access to the Fe₃O₄ active material within a thick electrode (â500 μm). Based on whole pattern fitting of the diffraction data, the phase composition was determined with both spatial and temporal resolutions. The data allow identification of the electrochemical conversion products, Li₂O and Fe metal, where interestingly, the Li₂O crystallites increase in size after initial formation. The observation of increased crystallite size of Li₂O after initial formation provides new insight into the time-dependent phenomena of conversion-type active materials and their reversibility. This investigation contributes to our understanding of Li⁺ transport in thick electrodes and provides insight into the design of battery electrodes that facilitate electroactive material utilization of energy storage systems crucial for the development of next-generation batteries.