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

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

Tuning Surface Chemistry of TiC Electrodes for Lithium-Air Batteries

One of the key problems hindering practical implementation of lithium-air batteries is caused by carbon cathode chemical instability leading to low energy efficiency and short cycle life. Titanium carbide (TiC) nanopowders are considered as an alternative cathode material; however, they are intrinsically reactive toward oxygen, and its stability is controlled totally by a surface overlayers. Using photo-emission spectroscopy, we show that lithium-air battery discharge product, lithium peroxide (Li2O2), easily oxidizes clean TiC surface. At the same time, TiC surface, which was treated by molecular oxygen under ambient conditions, shows much better stability in contact with Li2O2 that can be explained by the presence of a surface layer containing a significant amount of elemental carbon in addition to oxides and oxycarbides. Nevertheless, such protective coatings produced by room temperature oxidation are not practically useful as one of its components, elemental carbon, is oxidized in the presence of lithium-air battery discharge intermediates. These results are of critical importance in understanding of TiC surface chemistry and in design of stable lithium air battery electrodes. We postulate that dense, uniform, carbon-free titanium dioxide surface layers of 2-3 nm thickness on TiC will be a promising solution, and thus further efforts should be taken for developing synthetic protocols enabling preparation of TiO2/TiC core-shell structures

Laterally selective oxidation of large-scale graphene with atomic oxygen

Using X-ray photoemission microscopy, we discovered that oxidation of commercial large-scale graphene on Cu foil, which typically has bilayer islands, by atomic oxygen proceeds with the formation of the specific structures: though relatively mobile epoxy groups are generated uniformly across the surface of single-layer graphene, their concentration is significantly lower for bilayer islands. More oxidized species like carbonyl and lactones are preferably located at the centers of these bilayer islands. Such structures are randomly distributed over the surface with a mean density of about 3× 106 cm–2 in our case. Using a set of advanced spectromicroscopy instruments including Raman microscopy, X-ray photoelectron spectroscopy (μ-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved photoelectron spectroscopy (μ-ARPES), we found that the centers of the bilayer islands where the second layer nucleates have a high defect concentration and serve as the active sites for deep oxidation. This information can be potentially useful in developing lateral heterostructures for electronics and optoelectronics based on graphene/graphene oxide heterojunctions.The work is performed within the joint project of the Russian Science Foundation (16-42-01093) and DFG (LA655-17/1). The work of O.O.K. was supported by the Russian Foundation of Basic Researches (individual project 16-33-60229).Peer Reviewe

Reactivity of Carbon in Lithium–Oxygen Battery Positive Electrodes

Unfortunately, the practical applications of Li–O₂ batteries are impeded by poor rechargeability. Here, for the first time we show that superoxide radicals generated at the cathode during discharge react with carbon that contains activated double bonds or aromatics to form epoxy groups and carbonates, which limits the rechargeability of Li–O₂ cells. Carbon materials with a low amount of functional groups and defects demonstrate better stability thus keeping the carbon will-o’-the-wisp lit for lithium–air batteries

Laterally Selective Oxidation of Large-Scale Graphene with Atomic Oxygen

Using X-ray photoemission microscopy, we discovered that oxidation of commercial large-scale graphene on Cu foil, which typically has bilayer islands, by atomic oxygen proceeds with the formation of the specific structures: though relatively mobile epoxy groups are generated uniformly across the surface of single-layer graphene, their concentration is significantly lower for bilayer islands. More oxidized species like carbonyl and lactones are preferably located at the centers of these bilayer islands. Such structures are randomly distributed over the surface with a mean density of about 3× 10⁶ cm^–2 in our case. Using a set of advanced spectromicroscopy instruments including Raman microscopy, X-ray photoelectron spectroscopy (μ-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved photoelectron spectroscopy (μ-ARPES), we found that the centers of the bilayer islands where the second layer nucleates have a high defect concentration and serve as the active sites for deep oxidation. This information can be potentially useful in developing lateral heterostructures for electronics and optoelectronics based on graphene/graphene oxide heterojunction