New in-situ method for surface evolution monitoring during metallic deposition In this work, we are interested in flowing batteries and mainly in flowing zinc air batteries. The current lack of understanding of metallic Zn deposition mechanisms during charging phase contributes to prevent them from being competitive in industry. Currently, literature seems to converge to give the optimized parameters for flat zinc deposition, and avoiding dendrites or boulder-like structures. However, these conditions are often obtained at microscale level, and the scaling up for more macroscopic applications is not trivial. We seek to understand and develop strategies to analyze the evolution of large electrodes.
Beyond the results of the literature, in our work we propose a non-intrusive method to qualify the evolution of the electrode structure through electrochemical measurements based on Tafel analysis. A strong correlation has been identified between the evolution of exchange current density, i0, and the type of morphology that zinc adopts depending on injected current density, electrolyte concentration or electrolyte flow velocity. When zinc deposits in mossy way, its active surface area is higher than when it deposits under a flat way. By choosing cautiously the configuration of electrodes, the gap between them and the initial active surface area, we are able to determine under which type of morphology zinc is depositing inside the cell, without to have to open it or to use x-ray method. This reproducible technique allow also an early detection of defects, thanks to variations which can be compared to reference curves. The exchange current density increases exponentially when zinc deposits in a mossy way while it varies slightly when zinc is flat.
Using this method, we establish a complete phase diagram that gives the nature of the deposited structure at zinc electrode surface depending on parameters such as current intensity or electrolyte flow rate
Due to the macroscopical scale of the electrode in real flowing batteries, heterogeneous growth of zinc (flat and mossy) is observed at the same electrode surface. Limiting current density and current distribution over the electrode surface seem to be involved in these abnormal growth regimes. Thanks to our innovative method, we are able to bring elements of response to identify the nature of these complex mechanisms.
Charge and discharge cycles are performed.
Based on these analyses, we propose more appropriate charging and discharging strategies to increase battery life.
