Reducing charging polarization in quasi-solid-state rechargeable magnesium-oxygen batteries through cathode-localized...

Rechargeable magnesium-oxygen (Mg-O₂) batteries offer high theoretical energy density, but the electronically insulating and kinetically persistent MgO discharge product imposes severe charging polarization. Here, an aliquot of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) dissolved in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI) is localized directly at a ruthenium nanoparticle-decorated multi-walled carbon nanotube (Ru/CNT) cathode in a Mg-O₂ battery employing poly(vinylidene fluoride-co-hexafluoropropylene)-based quasi-solid-state electrolyte, concentrating mediator activity at the oxygen electrode while partially limiting crossover to the magnesium anode. Screening TEMPO concentrations from 0 to 1 M identifies 0.75 M as optimal. Relative to a mediator-free control of identical architecture, this composition lowers the average terminal overpotential from 1.73 to 1.0 V (minimum 0.77 V), raises the cumulative round-trip energy efficiency from 37.3% to 53.9%, extends capacity-limited cycling from 39 to 49 cycles at 100 mA g^-1 and 500 mAh g^-1, and increases the maximum discharge capacity from ∼12,000 to ∼20,000 mAh g^-1 while holding the deep-recharge voltage largely below 2.2 V. Controls under argon and on ruthenium-free cathodes establish that TEMPO acts as an oxygen-coupled charging mediator rather than a parasitic capacity source or a substitute for ruthenium catalysis. Electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption near-edge spectroscopy, synchrotron diffraction, and electron microscopy identify an amorphous, defect-perturbed MgO-like deposit containing oxygen-vacancy-type centers. Hybrid density functional calculations suggest that such vacancies introduce occupied mid-gap states that may act as donor levels for electron transfer to TEMPO⁺ during charging. Residual crossover persists, identifying cathode-side mediator confinement as the principal remaining challenge for redox-mediated quasi-solid-state divalent metal-oxygen batteries.