Improving Conversion Kinetics of Sodium Polysulfides through Electron Spillover Effect with V/Co Dual-Atomic Site...

Room-temperature sodium─sulfur (RT/Na─S) batteries, with a theoretical capacity of 1672 mAh g⁻¹, face challenges such as the insulating nature of sulfur and slow redox kinetics, particularly during complex liquid-solid (Na₂S₄→Na₂S₂) and solid-solid (Na₂S₂→Na₂S) conversions. Herein, vanadium-cobalt (VCo) diatomic sites implanted in vacancy-rich N-doped MXene (VCo DACs/N-MXene) are introduced to address these issues. The N-bridged VCo diatomic pairs are demonstrated and their strong electronic interactions are also validated through experimental and theoretical analyses. The RT/Na─S battery with optimized VCo DACs/N-MXene delivers an average capacity of 1255.3 mAh g⁻¹ at 0.1 C and remarkable cycling stability, with only ≈0.001% capacity decay per cycle over 1500 cycles at 1 C. DFT calculations reveal that VCo diatomic sites enhance reaction kinetics by reducing the Gibbs free energy for polysulfide conversions, notably reducing the solid-solid conversion energy barriers from 1.17/0.96 eV for V/Co SACs/N-MXene to 0.53 eV for VCo DACs/N-MXene. XANES and DFT analyses attribute this improvement to a unique electron spillover effect, facilitating efficient electron transport during charge and discharge. This work highlights the potential of optimizing electronic configurations and coordinating environments to activate bidirectional kinetics with improved capacity and longevity of RT/Na─S batteries.