Visualizing ion diffusion in battery systems by fluorescence microscopy: A case study on the dissolution of LiMn₂O₄

Fundamental understanding of ionic transport is critical to developing rechargeable batteries with high performance. While various techniques have been developed to characterize ionic transport in solid battery electrodes, little has been done to visualize ion movement in the liquid electrolyte, since it is difficult to realize high temporal, spatial and concentration resolutions simultaneously in liquids. Fluorescence imaging has the capability to detect ions in liquid with high resolution (< 1 s, < 300 nm and < 1 M), and it is widely used in biomedical studies. However, it has been rarely applied to battery studies. Here we show that using an ion indicator, the dissolution of trace amount of Mn from LiMn₂O₄, a common Li-ion battery cathode material, can be visualized under a fluorescence microscope. Moreover, important physical parameters, such as the dissolution rate and ionic diffusivity, can be extracted quantitatively from the fluorescence images. These results also show that nanoscale Al₂O₃ coating can effectively suppress Mn dissolution, which is consistent with past studies. This study demonstrates the capability of fluorescence-imaging based techniques for battery studies, which could help gain more insight on the behavior of ions in battery systems and develop better battery materials.