Conversion-type Cathode Materials for Aqueous Zn Metal Batteries in Non-Alkaline Aqueous Electrolytes: Progress, Challenges, And Solutions.

Aqueous Zn metal batteries are attractive as safe and low-cost energy storage systems. At present, due to the narrow window of the aqueous electrolyte and the strong reliance of the Zn 2+ ion intercalated reaction on the host structure, the current intercalated cathode materials exhibit restricted energy densities. In contrast, cathode materials with conversion reactions can promise higher energy densities. Especially, the recently reported conversion-type cathode materials that function in non-alkaline electrolytes have garnered increasing attention. This is because the use of non-alkaline electrolytes can prevent the occurrence of side reactions encountered in alkaline electrolytes and thereby enhance cycling stability. However, there is a lack of comprehensive review on the reaction mechanisms, progress, challenges, and solutions to these cathode materials. In this review, four kinds of conversion-type cathode materials including MnO 2 , halogen materials (Br 2 and I 2 ), chalcogenide materials (O 2 , S, Se, and Te), and Cu-based compounds (CuI, Cu 2 O, Cu 2 S, CuO, CuS, and CuSe) are reviewed. First, the reaction mechanisms and battery structures of these materials are introduced. Second, the fundamental problems and their corresponding solutions are discussed in detail in each material. Finally, future directions and efforts for the development of conversion-type cathode materials for aqueous Zn batteries are proposed. This article is protected by copyright. All rights reserved.