Visualising and Characterising Zinc Ion Transport for Zinc Ion Batteries By Fluorescence Microscopy

Zinc has been regarded as a promising anode material for aqueous batteries in view of its advantages including high specific capacity, abundance and intrinsic safety. Aqueous zinc ion batteries have attracted growing attention as a potential alternative to lithium ion batteries, especially for medium and large-scale energy storage. Aqueous zinc ion batteries consist of a zinc anode and a zinc intercalating cathode in a zinc-salt-containing electrolyte and use zinc ions as charge carriers. The electrolyte transporting zinc ions between the anode and the cathode plays an essential role in determining battery performance and life. Visualising ion transport in the electrolytes of zinc ion batteries can give insights into electrolyte dynamics as well as battery processes. Yet, this is limited by the spatial-temporal resolution of the existing techniques. Moreover, most of the existing in-situ visualisation techniques are very expensive and not easily accessible. Fluorescence microscopy provides a powerful tool for probing tiny structures and tracking species in real time. It relies on fluorescence which occurs when molecules absorb light with a certain wavelength followed by the re-emission of light with a longer wavelength. These excitation and emission wavelengths are usually unique fingerprints of certain substances. Fluorescent sensors, with high sensitivity of fluorescence assays, have been widely exploited as a useful tool for species detection and mechanistic studies in biology and chemistry. In this study, a novel microfluidics-based fluorescence microscopy platform is developed for visualising and characterising zinc ion transport in the electrolytes of zinc ion batteries. The platform is calibrated by comparing the measurement results with the literature data. Key transport properties of zinc ions are quantified under different electrolyte conditions. The developed platform is demonstrated to be a simple and versatile tool for electrolyte characterisation, which provides key information guiding future electrolyte development and battery performance improvement.