Review Background: As human society and economies advance, the issue of energy shortages becomes increasingly urgent. Therefore, the pursuit of clean, renewable energy and the construction of a low-carbon society have garnered worldwide research attention and public interest. Compared to organic electrolytes, aqueous electrolytes offer superior safety, reduced costs, simplified assembly, faster ion migration, and higher ion conductivity. Thus, aqueous batteries are emerging as highly secure energy storage systems with intermittent energy source utilization and sustainable development potential. The prospects for large-scale applications have led to rapid developments in electrode materials with excellent electrochemical performance and novel electrochemical mechanisms. In comparison to conventional LIBs, aqueous zinc-ion batteries (AZIBs) are inherently safer and benefit from their low cost, abundant resources, ease of assembly and recycling, environmental friendliness, and, most importantly, safety. Advanced AZIBs have the potential to replace traditional lithium-ion, nickel-metal hydride, and lead-acid batteries in future automotive, aerospace, and scalable energy storage applications. Among all rechargeable aqueous batteries, AZIBs have attracted widespread attention due to zinc’s unique advantages, including cost-effectiveness (USD $2 kg−1), outstanding theoretical capacity (820 mAh g−1 and 5851 mAh cm−3), low redox potential (−0.76 V vs. standard hydrogen electrode), remarkable safety characteristics, abundant abundance (approximately 300 times more than lithium), significant stability in aqueous environments compared to highly reactive metals, and environmental friendliness. However, AZIBs still face several challenges, such as water evaporation in electrolytes under high-temperature conditions leading to concentration changes, affecting battery performance and stability; passivation products forming on electrode surfaces causing defects in passivation layers, hindering effective interaction between electrodes and electrolytes; hydrogen evolution reactions leading to gas generation, increasing internal battery pressure, affecting safety and stability; formation of zinc dendrites inside the battery causing internal shorts and shortening cycle life; corrosion of zinc anodes and dissolution of cathode active materials resulting in decreased battery capacity; side reactions between electrolytes and cathodes producing harmful by-products, adversely affecting battery stability and lifespan; additionally, slow kinetics of Zn2+ ions potentially limiting battery charge and discharge rates and power density, thereby affecting performance in high-power applications. These factors collectively impact AZIBs negatively, as illustrated in Figure 1.
Review content: To address the obstacles in energy storage technology for AZIBs, this review summarizes a range of advanced aqueous electrolytes currently applied to AZIBs, such as Water-in-Salt Electrolytes (WISEs), Aqueous Eutectic Electrolytes (AEEs), Molecular Crowding Electrolytes (MCEs), and Hydrogel Electrolytes (HGEs) (Figure 2−6). This review first provides an in-depth overview of the basic composition, principles, and unique characteristics of various advanced aqueous electrolytes for AZIBs. Then, it systematically examines the latest research advancements in these advanced aqueous electrolytes. The challenges and bottlenecks associated with these electrolytes are summarized, along with recommendations (Figure 7). The review outlines future directions and potential strategies for developing high-performance AZIBs, which are expected to provide valuable insights for the development of advanced electrolyte systems for next-generation stable and sustainable multivalent secondary batteries.
Fig. 1 Unsolved issues in AZIBs based on conventional aqueous electrolytes, including the volatilization of aqueous electrolytes at high temperature, the freeze of aqueous electrolytes at low temperature, the formation of passivation products, the hydrogen evolution reaction, the growth of zinc dendrites, the corrosion of zinc anode, the side reaction between electrolytes and cathode, and the dissolution of cathodic active materials.
Fig. 2 Schematic illustrations and radar maps of the AZIBs based on various advanced aqueous electrolytes, including WISEs, AEEs, MCEs and HGEs.
Fig. 3 (a) Typical model of solvent structure of WISEs. (b–d) Advantages, challenges and prospectives of WISEs.
Fig. 4 (a) Typical model of solvent structure of AEEs. (b–d) Advantages, challenges and prospectives of AEEs.
Fig. 5 (a) Typical model of solvent structure of MCEs. (b–d) Advantages, challenges and prospectives of MCEs.
Fig. 6 (a) Typical model of solvent structure of HGEs. (b–d) Advantages, challenges and prospectives of HGEs.
Fig. 7 Challenges and prospectives of advanced electrolyte systems for AZIBs