Abstract
With the urgent need to move away from fossil fuels towards more sustainable technologies, alternative battery technologies are needed to alleviate the existing supply and safety concerns associated with Li-ion batteries1. Metal-air batteries are one such alternative that hold promise thanks to their high theoretical specific energies2. Whilst zinc-air batteries have one of the lowest theoretical specific energies (~1350 Wh kg-1 for zinc-air versus 5200 Wh kg-1 for Li-air2), zinc is an abundant element and there are few safety concerns associated with this type of battery. Furthermore, the theoretical specific energy of a zinc-air cell is still around three times higher than Li-ion (upwards of 270 Wh kg-1)3. Currently, primary zinc-air cells are commercialised for application in hearing aids, but the commercialisation of secondary cells is still limited by poor cycle lifetimes and degradation of the anode4.
Thus, alongside the need to develop better bifunctional catalysts, improvements need to be made to the anode to avoid dendrite formation and improve the reversibility between zinc and zinc oxide (ZnO) during cycling; there is significant volume expansion and morphology change during reaction from zinc to ZnO. In order to visualise the changes occurring in zinc-air battery electrodes, X-ray computed tomography (CT) has been used in a number of studies5-7. It is non-destructive and internal features of the sample can be visualised and measured without the need to open the sample. This allows for time-resolved quantification of changes occurring to the various phases within zinc-air cells, like zinc or ZnO.
In this work, we explore the morphologies of different anode electrode types, including foils, free-standing electrodes and slurries, to highlight the various existing anode design concepts for secondary zinc-air cells. Imaging methods, including X-ray CT and scanning electron microscopy (SEM), are used to visualise the structure of these electrodes, and quantitative analysis allows for the morphology to be correlated to their electrochemical performance. We discuss the advantages and disadvantages of the different electrode types and, finally, share our perspectives on the future of anode design for zinc-air cells.
References
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