4d orbital ruthenium doping enables high-capacity and stable α-MnO₂ cathodes for aqueous zinc-ion batteries

The electrochemical performance of α-MnO₂ 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. Although ion doping can alleviate these issues, conventional 3d transition metal dopants usually provide only modest improvement in electrochemical performance. In this work, we explore the possibility and demonstrate that ruthenium ions can act as an alternative dopant to address these challenges in a synergistic way. Ru³⁺/Ru⁴⁺ ions have highly delocalized 4d orbitals and high-bond-energy Ru-O bonds, which can modify the electronic structure of α-MnO₂ effectively. As a result, the electrochemical performance of the ruthenium-doped α-MnO₂ (RMO) cathode is enhanced showing a specific capacity of 360.1 mAh g^-1 at 0.1 A g^-1 and good cycling stability with 76.7% capacity retention after 5000 cycles at 3 A g^-1, which is significantly higher than that of 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 MnO₂ and enhance its potential as a cathode material in aqueous zinc-ion batteries.