Time- and Space-Resolved In Situ X-ray Diffraction Study of Phase Transformations and Redistribution in Zn-Air Battery Anodes

Zn-air primary batteries are widely used, but several challenges remain to make them rechargeable. The anode undergoes a phase transition between pure Zn and ZnO during the operation, with significant changes in the morphology and deposition rates. Using spatially and time-resolved in situ X-ray diffraction, the phase transitions in the anode have been mapped and directly linked to the electrochemical (dis)charge patterns. During discharge, only a small portion of the anode can be accessed at a given time, leading to an increase in over-potential and a limit to the accessible thickness of an electrode. During charge, the ZnO to Zn transformation has been found to happen simultaneously in a much larger portion of the anode, with only a low rise in overpotential. This has led to the conclusion that the limiting factor for an effective secondary Zn-air cell is not the effectiveness of the air catalyst to run the reverse reaction, rather the main challenge is the morphology changes in the anode that over time limit the access to ZnO nucleation sites near the surface, thus increasing the overpotential necessary to initiate the discharge reaction.