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 (δ-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 δ-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.