In situ growth of δ-MnO2/C fibers as a binder-free and free-standing cathode for advanced aqueous Zn-ion batteries†

Manganese dioxide (MnO₂) has been extensively investigated as a cathode material for aqueous zinc ion batteries (AZIBs) due to its high redox potential, substantial theoretical capacity and low cost. However, the challenges of capacity decay and poor structural stability hinder its applications. In this study, we propose a novel approach where δ-MnO₂ is grown in situ on carbon fibers derived from coal (δ-MnO₂@CCFs). The uniform anchoring of δ-MnO₂ on the carbon fibers enhances flexibility and maintains structural stability during cycling. The incorporation of heteroatoms, specifically nitrogen (N), directly leads to the formation of chemical bonds with δ-MnO₂, reducing electrostatic repulsion within the δ-MnO₂@CCFs material, facilitating the insertion and extraction of Zn²⁺ and H⁺ ions. Moreover, the δ-MnO₂@CCFs exhibit excellent electrochemical reversibility even under bending and folding conditions. The δ-MnO₂@CCFs cathode achieves a capacity of 352 mA h g⁻¹ at a current density of 0.5 A g⁻¹ after 100 cycles. Additionally, the assembled flexible Zn//δ-MnO₂@CCFs AZIBs maintain an energy density of 73.7 W h kg⁻¹ after 50 cycles at a current density of 0.1 A g⁻¹.