An In Situ Synchrotron Study of Zinc Anode Planarization by a Bismuth Additive

Abstract

Cyclic voltammetry of zinc plated from flowing alkaline zincate electrolyte with a bismuth additive showed a marked mass transport effect during metal layer deplating. This bismuth was added as Bi 2 O 3 and had a saturated concentration of 26 ppm bismuth. Using a small, transparent window flow cell the mechanism was studied in situ using synchrotron X-rays. X-ray microdiffraction revealed that the metal-electrolyte interface was bismuth rich, and bismuth behaved in a manner similar to a surfactant. Transmission X-ray microscopy revealed that in the presence of bismuth additive, 5 μm raised features on the metal layer were preferentially dissolved during deplating. However, macro-morphology experiments demonstrated that at 26 ppm a detrimental bismuth buildup occurred over many cycles. By reducing additive concentration to 3 ppm a metal layer was planarized compared to a no-additive control, while avoiding the bismuth buildup. These findings suggested that 3 ppm bismuth could be used to planarize zinc metal layers such as those in flow-assisted zinc batteries. However, concentration will need to be well-controlled.Zinc anode batteries are desirable for secondary energy storage due to the water-compatibility, natural abundance, safety, and high energy density of zinc. 1 Zinc anodes are found in several forms, which vary widely in design. Examples include the anodes in the following: metallic zinc flow battery hybrids, pelletized zinc-air battery/fuel cell hybrids, zinc-manganese dioxide batteries, and flow-assisted alkaline nickel-zinc batteries. 2-9 Systems in which a zinc anode is plated onto a current collector from electrolyte under forced convection have received attention for large scale applications, as in the flow-assisted nickel-zinc battery referenced above. 4 A key challenge for these systems is high aspect ratio zinc "dendrites" which progressively form during charge-discharge cycling of the zinc layer. Unless mitigated, these structures cross the electrolyte gap and short the cell.The micro-morphology of these progressive dendrites is generally the mossy form of zinc. Thus they are not like crystalline dendrites or diffusion limited aggregation structures except in their aspect ratio. Strategies to prevent dendrites during metal plating are the subject of frequent investigations, such as varying current density, electrolyte flow rate, and current waveforms. 10-12 These all aim to prevent initial formation of a high aspect ratio profile on the metal layer, as once that has occurred electric field effects will intensify dendrite formation. Additives or leveling agents also work in this way, suppressing deposition at asperity tips. 13 Screening electrolyte additives for use in flow-assisted batteries, we observed that ppm quantities of bismuth oxide provoked a substantial mass transport effect during anodic dissolution of zinc. This lead us to speculate that small quantities of bismuth oxide could act as a "reverse" leveling agent, planarizing zinc layers during discharge of the layer. The action of bismu...

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