Iron-Doped Nanorods of MnO2 For Applications in Zinc-Ion Batteries

Abstract

Manganese dioxide materials exhibit an unstable crystal structure due to the Jahn–Teller effect and the disproportionation reaction of Mn³⁺ to Mn²⁺/Mn⁴⁺. Therefore, how to regulate and control the electronic state of Mn in MnO₂ materials and achieve higher structural stability constitutes the cornerstone for large-scale applications. Ferrous iron possesses strong reducibility, while manganese dioxide has oxidation properties, and both undergo redox reactions. During the reaction process, some electrons acquired by manganese ions are reduced, and their average valence state decreases, which leads to a change in the chemical bonds between manganese ions and oxygen ions, resulting in oxygen ions being more readily separated from the crystal lattice and thus forming oxygen vacancies. In this paper, through a hydrothermal reaction, the average valence state of manganese is lowered, the proportion of Mn³⁺ is increased, and the exposed proportion of adsorbed oxygen is enhanced by the reaction of ferrous iron with potassium permanganate, subsequently constructing nanorod manganese dioxide rich in oxygen vacancies. When utilized as cathodes of aqueous zinc-ion batteries, it offers a specific capacity as high as 192.24 mAh g^–1 at 2 A g^–1 and retains 172 mAh g^–1 after 1000 cycles at 1 A g^–1. This indicates that a more stable structure can restrain the structural damage caused by the cyclic intercalation/extraction of H⁺ and Zn²⁺, and more oxygen vacancies can promote electrode reaction kinetics. Moreover, the assembly of the soft-pack battery demonstrates its flexible performance, which paves the way for the large-scale energy storage production and application of zinc-ion batteries.