LAUSR

Zn metal batteries (ZnBs) are safe and sustainable because of their operability in aqueous electrolytes, abundance and recyclability. However, the thermodynamic instability of Zn metal in aqueous electrolytes is a major bottleneck for its commercialisation. As such, Zn deposition (Zn2+ → Zn(s)) is continuously accompanied by hydrogen evolution reaction (HER) (2H+ → H2 ) and dendritic growth that further accentuates the HER. Consequently, the local pH around Zn electrode increases and promotes the formation of inactive and/or poorly conductive Zn passivation species (Zn + 2H2 O → Zn(OH)2 + H2 ) on the Zn. This aggravates the consumption of Zn and electrolyte and degrades the performance of ZnB. To propel HER beyond its thermodynamic potential (0V vs. SHE at pH = 0), the concept of water-in-salt-electrolyte (WISE) has been employed in ZnBs. Since the publication of the first article on WISE for ZnB in 2016, this research area has progressed continuously. This research provides an overview on this topic and discusses the promising research directions for accelerating the maturity of ZnBs. This review briefly describes the current issues with conventional aqueous electrolyte in ZnBs, including a historic overview and basic understanding of WISE. Furthermore, this review details the application scenarios of WISE in ZnBs with the description of various key mechanisms (e.g., side reactions, Zn electrodeposition, anions or cations intercalation in metal oxide or graphite, ion transport at low temperature). This article is protected by copyright. All rights reserved.