4d Orbital Ruthenium Doping Enables High-Capacity and Stable α-MnO 2 Cathodes for Aqueous Zinc-Ion Batteries

Abstract

The electrochemical performance of α-MnO 2 cathodes in aqueous zinc-ion batteries is limited by their low electronic conductivity, structural instability due to Jahn-Teller distortion, and slow Zn²⁺ diffusion kinetics. Despite the ion doping can alleviate these issues, conventional 3d transition metal dopants usually only provide modest increments in electrochemical performance. In this work, we explore and demonstrate that ruthenium ions can act as an alternative dopant to address these challenges in a synergistic way. Ru 3+ /Ru 4+ ions have highly delocalized 4d orbitals and high-bondenergy Ru-O bonds, which can modify the electronic structure of α-MnO 2 effectively. As a result, the electrochemical performance of ruthenium-doped α-MnO₂ (RMO) cathode presents enhanced electrochemical performance with a specific capacity of 360.1 mAh g⁻ 1 at 0.1 A g⁻ 1 and good cycling stability with 76.7% capacity remaining after 5000 cycles at 3 A g⁻¹, which is significantly higher than pristine α-MnO₂. Density functional theory (DFT) calculations show that ruthenium doping can narrow the bandgap from 0.9296 eV to 0.6997 eV and decrease the Zn²⁺ diffusion energy barrier from 0.68 eV to 0.56 eV. This work demonstrates that 4d transition metal doping can fundamentally improve the physicochemical properties of MnO 2 and enhance its potential as a cathode material in aqueous zinc-ion batteries.