X-ray imaging and micro-spectroscopy unravel the role of zincate and zinc oxide in the cycling of zinc anodes in mildly acidic aqueous electrolytes.

Sustainability, environmental and safety concerns raised by the increasing demand of batteries are driving research towards post-lithium technologies. Rechargeable Zn batteries are strong candidates, but still not practically viable, owing to the extensively studied, but poorly understood unstable behavior of Zn metal upon discharge-charge cycling. This limiting factor warrants more fundamental investigations and the present report provides the lacking molecular-level information on the Zn-based compounds forming at the electrode/electrolyte interface as a result of electrochemical cyclic in weakly acidic aqueous electrolyte. The results are obtained using ex situ X - ray absorption spectromicroscopy maps, modelled mathematically and complemented with cyclic voltammetry, symmetric-cell tests and electron microscopy. We have identified the role of the zincate precipitation resulting from local alkalinization during recharge, combined with additional zincate formation and decomposition to zinc oxide during discharge. The mathematical model allowed a transparent interpretation of morphochemical changes observed. The synergy of these processes leads to electrochemical localization effects, resulting in the formation of a complexly structured and low conductive ZnO-based template, that might play a role in driving shape changes. [Display omitted] • Soft X-ray absorption spectromicroscopy (XAS) gives insight for Zn electrochemistry. • Zn anode cycling in mildly acidic aqueous electrolyte can generate zincates. • Parasitic HER yields space distribution of Zn species in mildly acidic electrolyte. • Chemical-state distribution of discharged and cycled Zn anodes, disclosed by XAS. • Homogeneous primary current distribution cannot avoid mesoscopic heterogeneities. [ABSTRACT FROM AUTHOR]