The charge density of intercalants inside layered birnessite manganese oxide nanosheets determining Zn-ion storage capability towards rechargeable Zn-ion batteries

Rechargeable aqueous Zn-MnO2 batteries have been considered as one of the promising alternative energy technologies due to their high abundance, environmental friendliness, and safety of both Zn-metal anodes and manganese oxide cathodes. Although layer-type MnO2 (delta-MnO2) is one of the most promising intercalation cathode materials, there are some critical drawbacks such as sluggish Zn2+ diffusion kinetics and a phase transition of delta-MnO2 as a result of a strong electrostatic interaction between Zn2+ and the host structure. Herein, we systematically studied the effects of the charge density of pre-intercalated cations in layered MnO2 using Li+, Ca2+, and Al3+ on its structural properties and electrochemical performance as a cathode in aqueous Zn-MnO2 batteries. The results reveal that a small amount of highly charged intercalant can effectively stabilize the MnO2 layers, facilitating the kinetics of Zn2+ intercalation/deintercalation. As a result, Al-MnO2 exhibits superior capacity and a rate capability of 210 mA h g(-1) with 21% capacity retention when the current density is increased from 0.1 to 2 A g(-1), while Ca-MnO2 and Li-MnO2 exhibit 189 and 160 mA h g(-1) with a capacity retention of 17% and 11%, respectively. The superior capacity of Al-MnO2 is attributed to the enhanced redox activity from more Mn electrochemical utilization as confirmed by ex situ X-ray photoelectron spectroscopy. Moreover, the long-term cycling stability evaluated at 2 A g(-1) shows that Al-MnO2 exhibits superior cycling stability with 84% capacity retention over 2000 cycles. As revealed by ex situ X-ray diffraction and theoretical calculations, the highly charged intercalant can minimize the binding energy between Zn2+ and the MnO2 host, alleviating the strong electrostatic attraction that can induce reversible Zn2+ insertion/extraction.

Automated summary

The study shows that inserting highly charged cations in layered MnO2 cathode can stabilize MnO2 layers and enhance the kinetics of Zn2+ intercalation/deintercalation, leading to improved electrochemical performance in aqueous Zn-MnO2 batteries.