Redox

About: Redox is a research topic. Over the lifetime, 26853 publications have been published within this topic receiving 862368 citations. The topic is also known as: reduction-oxidation & reduction-oxidation reaction.

Towards high throughput screening of electrochemical stability of battery electrolytes.

Abstract: High throughput screening of solvents and additives with potential applications in lithium batteries is reported. The initial test set is limited to carbonate and phosphate-based compounds and focused on their electrochemical properties. Solvent stability towards first and second reduction and oxidation is reported from density functional theory (DFT) calculations performed on isolated solvents surrounded by implicit solvent. The reorganization energy is estimated from the difference between vertical and adiabatic redox energies and found to be especially important for the accurate prediction of reduction stability. A majority of tested compounds had the second reduction potential higher than the first reduction potential indicating that the second reduction reaction might play an important role in the passivation layer formation. Similarly, the second oxidation potential was smaller for a significant subset of tested molecules than the first oxidation potential. A number of potential sources of errors introduced during screening of the electrolyte electrochemical properties were examined. The formation of lithium fluoride during reduction of semifluorinated solvents such as fluoroethylene carbonate and the H-transfer during oxidation of solvents were found to shift the electrochemical potential by 1.5-2 V and could shrink the electrochemical stability window by as much as 3.5 V when such reactions are included in the screening procedure. The initial oxidation reaction of ethylene carbonate and dimethyl carbonate at the surface of the completely de-lithiated LiNi0.5Mn1.5O4 high voltage spinel cathode was examined using DFT. Depending on the molecular orientation at the cathode surface, a carbonate molecule either exhibited deprotonation or was found bound to the transition metal via its carbonyl oxygen.

Highly Reversible Oxygen-Redox Chemistry at 4.1 V in Na4/7−x[□1/7Mn6/7]O2 (□: Mn Vacancy)

Abstract: The tremendous industrial demand resulting from the market penetration of electric vehicles has significantly raised the requirement for rechargeable batteries with higher energy density. One research focus is the development of higher energy positive electrode. Quantitatively, in the conventional charge-compensation regime which relies on the redox capability of transition metals, the theoretical capacity is maximized for the chemical composition of AMO2 (A: alkali metal; M: transition metal). However, recent studies have shown that A-excess transition-metal oxides, A1+xM1−xO2, can deliver large extra capacities exceeding the theoretical limit of the M-redox reaction via additional redox of nonbonding oxygen 2p orbital. Therefore, exploiting additional oxygen redox would significantly contribute to surpassing the current energy-density limit of batteries. In this work, we propose introducing transition metal vacancies (e.g., Ax[yM1−y]O2, : transition metal vacancy) as a possible approach for stable oxygen-redox reactions. The damaging M migration would be suppressed if the ionic radii of A and M differ largely. In this regard, sodium shows larger contrast in ionic radii with 3d transition metals and its compounds look promising. Therefore, we investigate the reversible oxygen-redox capacity of a layered sodium manganese oxide Na2Mn3O7 (Na4/7[1/7Mn 4+ 6/7]O2 in conventional NaxMO2 notation, Figure 1a). Considering the inherent manganese vacancies in the [1/7Mn 4+ 6/7]O2 layers, we expect the oxygens neighboring  to have nonbonding 2p orbitals, leading to an oxygen-redox capacity upon Na extraction. In this study, we show that Na2−xMn3O7 can operate as a highly reversible 4.1 V oxygen-redox cathode (Figure 1b).

Stability of Substituted Phenyl Groups Electrochemically Grafted at Carbon Electrode Surface.

Abstract: The electrochemical reduction of an aryl diazonium tetrafluoroborate salt, dissolved in acetonitrile, at a carbon electrode surface allowed the grafting of aryl groups with the formation of a carbon-carbon bond. Groups such as 4-carboxyphenyl, 4-nitrophenyl, 4-diethylaniline (DEA), and 4-bromophenyl were grafted at a glassy carbon electrode surface. The stability of these grafted groups, present at the glassy carbon electrode surface, was studied at various electrode potentials in aqueous media. In appropriate experimental conditions, the as-grafted groups severely inhibit the cyclic voltammetry response of selected redox probes. Thus, the reappearance and/or increase of an electrochemical response, after polarization, was taken as an indication that a modification of the grafted layer occurred. Our results demonstrated that polarization at very positive (ca. 1.8 V) and negative (ca. -2 V) potentials is needed to observe an electrochemical response. Electrochemical impedance and X-ray photoelectron spectroscopies were also used to investigate the stability of the grafted layers. The impedance data usually tracks fairly well the cyclic voltammetry results, although the former appears to be more sensitive to changes that are occurring upon polarization of the modified electrode. Interestingly, the XPS data indicate clearly that the grafted layer is not always completely removed at the extreme positive and negative potentials investigated. A mechanism was proposed to explain the transformation occurring during polarization of the modified electrode and involves desorption of the substituted aryl groups during the concomitant hydrogen, oxygen, or chlorine evolution and finally leaving close to a covalently bonded monolayer of the grafted species at the electrode surface.