Research progress on structural regulation and advanced characterization of electric double layer at Zn anode interface of aqueous Zn-ion batteries

<sec><p indent="0mm">Resource depletion and environmental degradation have driven the development of renewable energy sources, such as solar and wind power, to replace fossil fuels. However, the inherent intermittency of these renewable energy sources poses a significant challenge to the power grid, necessitating the development of large-scale energy storage systems. Recently, rechargeable aqueous Zn batteries using Zn metal as the anode have been revisited as a potential solution for grid energy storage due to the advantages of non-flammability, environmental benignity, low cost, etc. Unlike most metals including Li, Na, K, Mg, Al, etc., Zn metal can be directly applied as the anode for aqueous batteries and also exhibits a high theoretical capacity of 819 mAh g⁻¹, superior volumetric capacity of 5854 mAh cm⁻³ and low redox potential of <sc>−0.762 V</sc> vs. the standard hydrogen electrode. The performance of aqueous Zn-ion batteries, however, remains unsatisfactory for their further commercial application due to uncontrollable dendrite growth and interfacial side reactions. Specifically, the deposition-stripping process of Zn metal is controlled by the Zn anode-electrolyte interface, where the structural evolution of the electric double layer (EDL) is the critical issue for the excellent electrochemical performance of zinc anodes. The EDL is the region where electrochemical reactions occur, and it plays a critical role in electrochemical energy conversion and storage. Previous reports reveal that EDL is crucial to the formation of solid electrolyte interphases and conventional electrochemical double layer capacitors. Generally, the dipolar H₂O molecules exist in the inner Helmholtz layer of the EDL, participating in the interfacial reactions during battery operation. Consequently, uncontrollable electrochemical and chemical corrosion reactions continuously occur, significantly degrading the battery performance. Moreover, the de-solvation process of [Zn(H₂O)₆]²⁺ and the Zn deposition process both occur in the EDL, determining the electrochemical reactions at the surface of the Zn anode and the morphology of Zn deposition. Modifying the EDL structure can greatly influence the efficiency, stability, and overall performance of the battery system. In recent years, researchers have made some achievements in the research of Zn anodes by proposing various modification strategies in Zn anode-electrolyte interface modification. However, due to the limitation of characterization methods, the reaction mechanism of the EDL still remains unclear. </sec><sec> In this work, we systematically investigate the structural evolution mechanism, interfacial reaction mechanisms regulation strategies, electrochemical and <italic>in-situ</italic> spectral characterization methods, and theoretical simulations of the EDL at zinc anode-electrolyte interfaces. Moreover, we introduce novel perspectives for the investigation of the EDL at the Zn anode-electrolyte interface in the future. These include, for example, a multi-scale study on the evolution of the EDL at the zinc anode through the integration of electrochemical, spectroscopic, and theoretical modeling methodologies. Furthermore, we propose to examine the evolution behavior of the EDL at the Zn anode-electrolyte interface under practical application conditions within Ah-scale Zn batteries. These research endeavors will offer references for the practical deployment of aqueous Zn-ion batteries. </sec>