Cathode Engineering for High Energy Density Aqueous Zn Batteries

Frequent safety accidents of lithium-ion batteries (LIBs) originating from the utilization of flammable electrolytes urges the battery community to develop a safe substitute. This safety background is a boom for aqueous batteries (ABs) which employ aqueous electrolytes to address safety concerns. Recently, ABs have experienced a rapid advance because various battery chemistries have been successively developed, e.g., aqueous Zn batteries (AZBs), aqueous LIBs, aqueous sodium-ion battery, etc. Impeded by the narrow voltage window of aqueous electrolytes, however, the majorities of cathode materials with high operation potential employed in traditional nonaqueous batteries are excluded from the range of ABs cathodes, leading to a low energy density. Directly using metal as an anode is likely to improve the energy density, whereas most of the reported metal anodes, e.g., lithium, sodium, magnesium, etc., cannot run in aqueous electrolytes. One exceptional case is the Zn metal anode that permits theoretically high energy density AZBs due to triple merits: (1) the Zn metal anode exhibits a low redox potential (-0.76 V vs standard hydrogen electrode, SHE), taking the best advantage of the limited voltage window of aqueous electrolytes; (2) Zn metal anode with mild protection can easily maintain its chemical stability in aqueous medium; (3) Zn metal anode releases a high specific capacity of 820 mAh g(-1). AZBs thus exhibit a rapid development, especially in developing high specific capacity cathode materials such as MnO2 and V2O5, and the corresponding structure modification. Despite these spurring achievements, the overall energy density of the whole AZB device is still unsatisfactory. In this Account, we initially present the energy density state of AZBs, where a detailed discussion is given to the energy bottleneck of current cathode materials. Meanwhile, the corresponding strategies that are considered as the first-stage attempt to enhance energy density are discussed, including mediating interlayer spacing, introducing oxygen vacancy, and using high-voltage cathode materials. Due to the unsatisfactory energy density, we then propose a systemic methodology of cathode engineering to renew the energy blueprint of AZBs. Specifically, we show the high possibility of employing conversion-type cathodes with the capability of multiple-electron transfer reaction, e.g., sulfur, selenium, iodine, etc., to remarkably enhance the energy density of AZBs. In addition, strengthening the utilization of cathode active material such as the activation, stabilization, or introduction of metal active centers is highlighted as a branch of cathode engineering to address the energy density issue of AZBs. Finally, we attempt to summarize the remaining challenges and possible solutions to address the energy density issue of AZBs, such as reducing the proportion of electrochemically inactive materials, increasing the cathode loading mass, and avoiding the excessive usage of Zn anode. Overall, we believe this Account can shed light on the promising directions to design a practical high energy density AZBs.