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

Abstract

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.