K-Mn3O4-NCs@PANI nanochains for high-rate and stable aqueous zinc-ion batteries: A doping and morphology-tailored synthesis strategy

Abstract

Aqueous zinc ion batteries (AZIBs) are promising energy storage solutions due to their high energy density and safety. However, developing cathode materials that offer both high energy density and durability for Zn²⁺ ions storage remains challenging. Manganese (Mn) oxide-based cathodes have been developed for AZIBs due to their high discharge voltage and desirable capacity, but face challenges like poor conductivity, slow reaction kinetics, and dissolution during cycling. Doping, morphology/structure design, and protective layers are effective for enhancing the structure, conductivity, and electronic properties of Mn-based oxides. A synthetic strategy combining these methods for Mn₃O₄ cathodes is proposed for AZIBs. K⁺ ions doping in Mn₃O₄ (K-Mn₃O₄) can regulate local electronic structure, induce oxygen vacancies, improve conductivity, and provide more active sites for Zn²⁺ ions diffusion. Additionally, K-Mn₃O₄ nanochain (K-Mn₃O₄-NCs), with a unique chain-like nanostructure (NCs) and high aspect ratio, synthesized via Mn²⁺ ions chelation with nitrilotriacetic acid (NTA) and calcination, show reduced interparticle contact resistance, shorter Zn²⁺ ions diffusion length, and faster reaction kinetics. Meanwhile, the in-situ polymerized polyaniline (PANI) layer on K-Mn₃O₄-NCs shields against corrosion (K-Mn₃O₄-NCs@PANI), connects 1D K-Mn₃O₄-NCs into a continuous conductive network, suppresses volume expansion, and improves stability. Electrochemical analysis shows that K-Mn₃O₄-NCs@PANI exhibits higher stability and faster reaction kinetics due to a reduced bandgap, increased oxygen defects, and less coulombic repulsion between Zn²⁺ ions and Mn oxide hosts. The K-Mn₃O₄-NCs@PANI cathode achieved a high capacity of 510 mAh/g at 0.1 A/g and excellent rate capacity of 203.2 mAh/g at 5 A/g. After 20,000 cycles, it maintained a capacity of 90.3 mAh/g at 5 A/g, showing exceptional long-term stability with a minimal decay rate of 0.026 ‰ per cycle.