Experimental and Computational Analysis of the Solvent-Dependent O2/Li+-O2− Redox Couple: Standard Potentials, Coupling Strength, and Implications for Lithium-Oxygen Batteries

Understanding and controlling the kinetics of O[subscript 2] reduction in the presence of Li+-containing aprotic solvents, to either Li+-O[subscript 2]− by one-electron reduction or Li[subscript 2]O[subscript 2] by two-electron reduction, is instrumental to enhance the discharge voltage and capacity of aprotic Li-O[subscript 2] batteries. Standard potentials of O[subscript 2]/Li+-O[subscript 2]− and O[subscript 2]/O[subscript 2]− were experimentally measured and computed using a mixed cluster-continuum model of ion solvation. Increasing combined solvation of Li+ and O[subscript 2]− was found to lower the coupling of Li+-O[subscript 2]− and the difference between O[subscript 2]/Li+-O[subscript 2]− and O[subscript 2]/O[subscript 2]− potentials. The solvation energy of Li+ trended with donor number (DN), and varied greater than that of O[subscript 2]− ions, which correlated with acceptor number (AN), explaining a previously reported correlation between Li+-O[subscript 2]− solubility and DN. These results highlight the importance of the interplay between ion–solvent and ion–ion interactions for manipulating the energetics of intermediate species produced in aprotic metal–oxygen batteries.National Science Foundation (U.S.) (NSF award no. ECS-0335765)China Clean Energy Research Center-Clean Vehicles Consortium (CERC-CVC)United States. Dept. of Energy (Award number DEPI0000012)National Science Foundation (U.S.) (Award number DMR-0819762)Robert Bosch GmbH (Bosch Energy Research Network (BERN) Grant)Skolkovo Institute of Science and Technology (Skoltech-MIT Center for Electochemical Energy Storage

Lithium peroxide crystal clusters as a natural growth feature of discharge products in Li–O2 cells

The often observed and still unexplained phenomenon of the growth of lithium peroxide crystal clusters during the discharge of Li–O2 cells is likely to happen because of self-assembling Li2O2 platelets that nucleate homogeneously right after the intermediate formation of superoxide ions by a single-electron oxygen reduction reaction (ORR). This feature limits the rechargeability of Li–O2 cells, but at the same time it can be beneficial for both capacity improvement and gain in recharge rate if a proper liquid phase mediator can be found

Mechanism of Oxygen Reduction in Aprotic Li–Air Batteries: The Role of Carbon Electrode Surface Structure

Electrochemical oxygen reduction in aprotic media is a key process that determines the operation of advanced metal–oxygen power sources, e.g., Li–O₂ batteries. In such systems oxygen reduction on carbon-based positive electrodes proceeds through a complicated mechanism that comprises several chemical and electrochemical steps involving either dissolved or adsorbed species, and as well side reactions with carbon itself. Here, cyclic voltammetry was used to reveal the effects of imperfections in the planar sp² surface structure of carbon on the Li oxygen reduction reaction (Li-ORR) mechanism by means of different model carbon electrodes (highly oriented pyrolytic graphite (HOPG), glassy carbon, basal, and edge planes of pyrolytic graphite), in dimethyl sulfoxide (DMSO)-based electrolyte. We show that the first electron transfer step O₂ + e^– ⇆ O₂^– (followed by ion-coupling Li⁺ + O₂^– ⇆ LiO₂) does not involve oxygen chemisorption on carbon as evidenced by the independence of its rate on the carbon electrode surface morphology. The second electron transfer leading to Li₂O₂ (Li⁺ + LiO₂ + e^– ⇆ Li₂O₂) is strongly affected by the electrode surface even in highly solvating DMSO. Formation of Li₂O₂ via the electrochemical reaction could be observed only on the nearly ideal basal plane of graphite. In contrast, for more disordered electrode surfaces, (and/or bulk) the only reduction peak revealed on cyclic voltammograms corresponds to LiO₂ formation, supporting that solution-mediated mechanism for Li₂O₂ growth is more favorable in that case. We also show that increased defect concentrations on the carbon electrode surface promote the formation of Li₂CO₃ during ORR, albeit relatively slower than Li₂O₂ formation

Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium–Oxygen Batteries

Fundamental understanding of growth mechanisms of Li₂O₂ in Li–O₂ cells is critical for implementing batteries with high gravimetric energies. Li₂O₂ growth can occur first by 1e^– transfer to O₂, forming Li⁺–O₂^– and then either chemical disproportionation of Li⁺–O₂^–, or a second electron transfer to Li⁺–O₂^–. We demonstrate that Li₂O₂ growth is governed primarily by disproportionation of Li⁺–O₂^– at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li⁺–O₂^–-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li₂O₂ morphologies observed at different discharge overpotentials: formation of large (∼250 nm–1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal–O₂ batteries

Notable reactivity of acetonitrile towards Li2O2/LiO2 probed by NAP XPS during Li–O2 battery discharge

One of the key factors responsible for the poor cycleability of Li–O2 batteries is a formation of byproducts from irreversible reactions between electrolyte and discharge product Li2O2 and/or intermediate LiO2. Among many solvents that are used as electrolyte component for Li–O2 batteries, acetonitrile (MeCN) is believed to be relatively stable towards oxidation. Using near ambient pressure X-ray photoemission spectroscopy (NAP XPS), we characterized the reactivity of MeCN in the Li–O2 battery. For this purpose, we designed the model electrochemical cell assembled with solid electrolyte. We discharged it first in O2 and then exposed to MeCN vapor. Further, the discharge was carried out in O2 + MeCN mixture. We have demonstrated that being in contact with Li–O2 discharge products, MeCN oxidizes. This yields species that are weakly bonded to the surface and can be easily desorbed. That’s why they cannot be detected by the conventional XPS. Our results suggest that the respective chemical process most probably does not give rise to electrode passivation but can decrease considerably the Coulombic efficiency of the battery.This work of A.K-G., J.J.V-V. and D.M.I. was supported by the Russian Ministry of Science and Education (RFMEFI61614×0007) and Bundesministerium für Bildung and Forschung (Project No. 05K2014) in the framework of the joint Russian-German research project “SYnchrotron and NEutron STudies for Energy Storage (SYNESTESia)”. T.K.Z acknowledges Center for Electrochemical Energy of Skolkovo Institute of Science and Technology for financial support. The work of O.O.K., A.I.B and L.V.Y. is performed within the joint project of the Russian Science Foundation (16-42-01093) and DFG (LA655-17/1). We are grateful to HZB for beamtime granted at ISISS and RGBL beamlines. T.K.Z. and A.S.F. thank to the Russian German laboratory at HZB for support provided. Authors are appreciated to Victor Vizgalov for solid electrolyte membrane preparation. Travelling of T.K.Z. was supported by German-Russian Interdisciplinary Science Center (G-RISC).Peer reviewe

Cover Feature: Extended Limits of Reversible Electrochemical Lithiation of Crystalline V 2

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