Synthesis, Structure, and Electrochemical Performance of Bi-induced Stabilization of MnO2 Cathodes for Use in Highly Acidic Aqueous Electrolytes (pH <2)

Abstract

MnO₂ cathode materials are widely studied in alkaline and neutral aqueous electrolytes. In these mediums, the MnO₂ cathode shows suboptimal performance limited by dissolution and electrochemically inactive compound formation, leading to capacity fading. This study explores the enhancement of MnO₂ cathode performance through Bi³⁺ ion doping (0, 1, 2.5, 5, and 10 mol%) in a highly acidic electrolyte (pH < 2). By incorporating up to 10 mol% Bi ions into the MnO₂ structure, we significantly improved specific capacity and capacity retention stability. Energy-dispersive X-ray spectroscopy (EDX) analysis revealed a uniform dispersion of Bi³⁺ ions throughout the MnO₂ cathode after electrochemical cycling, contributing to performance enhancements. X-ray photoelectron spectroscopy (XPS) results indicated that Bi³⁺ ion concentration from 1 to 10 mol% stabilises Mn³⁺ within the MnO₂ lattice. Also, Bi³⁺ ion doping promotes the formation of a 2×2 tunnel structured α-MnO₂ phase. Electrochemical impedance spectroscopy results demonstrated a reduction in double-layer and overall bulk capacitance. These findings suggest that Bi³⁺ ion doping effectively enhances MnO₂ electrochemical performance and could enhance its use in aqueous metal-ion batteries.