Reducing charging polarization in quasi-solid-state rechargeable magnesium–oxygen batteries through cathode-localized 2,2,6,6-tetramethylpiperidine-1-oxyl/ionic-liquid redox mediation
Journal
Journal of Colloid and Interface Science
Journal Volume
726
ISSN
0021-9797
Date Issued
2027-01
Author(s)
Sarkar, Ayan
Wu, Yueh-An
Hung, Ling
Fadana, Yuka
Wang, Ching-Ya
Wu, Ching-Chen
Chang, Wen-Sheng
Cherng, Ding-Hwa
Yeh, Ting-Wei
Abstract
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.
Subjects
Overpotential
Oxygen vacancies
Quasi-solid-state
Rechargeable Mg–O2 batteries
TEMPO redox mediator
Publisher
Elsevier BV
Type
journal article