The Nature of Lithium Battery Materials under Oxygen Evolution Reaction Conditions (original) (raw)
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Chemistry of Materials, 2012
The high capacity of the layered Li−excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li−excess transition metal oxides in rechargeable lithium batteries.
Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water
Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water, 2012
The development of water oxidation catalysts (WOCs) to replace costly noble metals in commercial electrolyzers and solar fuel cells is an unmet need that is preventing the global development of hydrogen fuel technologies. Two of the main challenges in realizing catalytic water splitting are lowering the substantial overpotential that is required to achieve practical operating current densities in the O 2 -evolving halfreaction at the anode, and the use of earth-abundant elements for the fabrication of inexpensive electrodes that are free from noble metals. To meet these challenges, molecular catalysts that are based upon the cubic CaMn 4 O x core within photosystem II in photosynthetic organisms, which is the gold standard of catalytic efficiency, have begun to appear. Among solid-state materials, several noble-metal oxides, which include IrO 2 and RuO 2 , are already in use in industrial electrolyzers, but are not globally scalable. Aqueous solutions of cobalt phosphate form water-oxidation catalysts under electrolysis and photolysis that are suitable for the fabrication of noncrystalline electrode materials. Nanocrystalline spinel-phase metal oxides (AM 2 O 4 , M = transition metals) that are comprised of M 4 O 4 cubical subunits and are active water oxidation catalysts have been developed. The catalytic activity of the spinel Co 3 O 4 has been reported for Co 3 O 4 nanorods that are incorporated into SBA-15 silica, as well as Co 3 O 4 nanoparticles that are adsorbed onto Ni electrodes. NiCo 2 O 4 spinel also oxidizes water when the nanoparticles are electrophoretically deposited onto a Ni electrode. Reports that examined the effect of lithium doping on the surface of Co 3 O 4 electrodes in solutions of KOH attributed the higher evolution rate of O 2 to better electrical conductivity. However, the oxidation of water by Co 3 O 4 was strongly dependent on crystallite size and surface area and frequently necessitates high overpotentials and alkaline conditions to accelerate the rate of reaction. In contrast, we recently reported that the catalytically inert spinel LiMn 2 O 4 gives spinel l-MnO 2 , which is an active water oxidation catalyst, upon topotactic delithiation. Thus, the importance of removing the A-site lithium for catalysis by the cubic Mn 4 O 4 core of spinels was revealed. Metal oxides that contain lithium are well-researched cathode materials for lithium-ion batteries. In particular, lithium cobalt oxide has been implemented extensively for this application, and its electrochemical properties have been examined thoroughly. Lithium cobalt oxide occurs as two crystalline polymorphs of identical composition ): cubic spinel-like Li 1+y Co 2 O 4 (Fd3 m; y = 1 or less), simplified as Li 2 Co 2 O 4 and rhombohedral layered LiCoO 2 . Layered lithium cobalt oxide is an effective cathode material for lithium-ion batteries as it has a higher energy density and better stability than the corresponding cubic phase. Herein, we show that of the two, only the cubic phase Li 2 Co 2 O 4 is active in catalyzing the oxidation of water, when driven either electrolytically or by a photochemically generated oxidant.
Development of efficient, affordable electrocatalysts for the oxygen evolution reaction and the oxygen reduction reaction is critical for rechargeable metal-air batteries. Here we present lithium cobalt oxide, synthesized at 400°C (designated as LT-LiCoO 2 ) that adopts a lithiated spinel structure, as an inexpensive, efficient electrocatalyst for the oxygen evolution reaction. The catalytic activity of LT-LiCoO 2 is higher than that of both spinel cobalt oxide and layered lithium cobalt oxide synthesized at 800°C (designated as HT-LiCoO 2 ) for the oxygen evolution reaction. Although LT-LiCoO 2 exhibits poor activity for the oxygen reduction reaction, the chemically delithiated LT-Li 1 À x CoO 2 samples exhibit a combination of high oxygen reduction reaction and oxygen evolution reaction activities, making the spinel-type LT-Li 0,5 CoO 2 a potential bifunctional electrocatalyst for rechargeable metal-air batteries. The high activities of these delithiated compositions are attributed to the Co 4 O 4 cubane subunits and a pinning of the Co 3 þ /4 þ :3d energy with the top of the O 2 À :2p band.
The effect of O2 concentration on the reaction mechanism in Li-O2 batteries
Journal of Electroanalytical Chemistry, 2017
The promising lithium-oxygen battery chemistry presents a set of challenges that need to be solved if commercialization is ever to be realized. This study focuses on how the O 2 reaction path is effected by the O 2 concentration in the electrolyte. An electrochemical quartz crystal microbalance system was used to measure current, potential, and change in
Scientific Reports, 2014
Most Li-O 2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt 3 Co nanoparticles as an effective OER catalyst in Li-O 2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO 2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li 2 O 2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O 2 batteries. L i-ion batteries (LIBs) have penetrated deeply into our everyday lives. They are power sources of various mobile electronics and have also begun to support future green transportation. However, the current LIBs rely on the intercalation mechanism for reversible reactions of Li ions with active materials on both cathode and anode sides, so their practical energy densities are usually limited below 250 Wh kg 21 1,2 . This limitation imposes a significant hurdle for future LIB applications, especially all-electric vehicles (EVs) that require the battery energy density substantially larger than the current values. The energy density of the EV battery is directly related to driving distance per each charge, and its driving mileage will be inevitably assessed in comparison against those of today's combustion engine-based counterparts: ,550 km per each refuel 1-3 .
For the purpose of reducing not only the consumption of natural resources, but also the environmental pollution from internal combustion engines, much effort has been dedicated to developing new energy storage systems (ESSs) and electric vehicles (EVs) powered by batteries. There are several stringent requirements, such as high power/energy density, good safety, and high reliability against external environmental abuse. For next-generation batteries to meet these requirements, the development of a new energy conversion system is crucial. Therefore, lithium-oxygen (lithium-O2 ) batteries have attracted intensive attention, due to their high theoretical energy density, compared with those of gasoline engines. However, present lithium-O2 batteries exhibit low round-trip efficiency and cyclic degradation, thus preventing their commercialization as next-generation power sources. This drawback may be attributed to the high thermodynamic stability of discharge products and their intrinsic insulating character, leading to the surge of polarization in oxygen reduction reactions/ oxygen evolution reactions (ORRs/OERs). To alleviate cyclic degradation and improve round-trip efficiency, it has been reported that the polarization can be reduced by adopting adequate cathode catalysts, based on their surface structures regulating oxygen adsorption. Here we provide and discuss several design strategies for tailoring catalytic materials from a structural and morphological viewpoint, as well as their effect on discharge products.
Electrochemical and structural study of LiCoPO4-based electrodes
Journal of Solid State Electrochemistry, 2004
LiCoPO 4 samples were synthesized by two different techniques (high-temperature solid-state reaction and lower-temperature synthesis using NH 4 Co-PO 4 AEH 2 O as precursor) and tested as cathode materials for 5-V lithium batteries. An irreversible lithium deinsertion was observed for the high-temperature sample. In contrast, the application of lower-temperature synthesis led to a significant improvement of the lithium storage reversibility. Different delithiation mechanisms in LiCoPO 4 were found for the samples obtained by different synthetic techniques. The nature of capacity fading during cycling of the cells is discussed.
The mechanisms of oxygen reduction and evolution reactions in nonaqueous lithium-oxygen batteries
ChemSusChem, 2014
A fundamental understanding of the mechanisms of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in nonaqueous lithium-oxygen (Li-O2) batteries is essential for the further development of these batteries. In this work, we systematically investigate the mechanisms of the ORR/OER reactions in nonaqueous Li-O2 batteries by using electron paramagnetic resonance (EPR) spectroscopy, using 5,5-dimethyl-pyrroline N-oxide as a spin trap. The study provides direct verification of the formation of the superoxide radical anion (O2(˙-)) as an intermediate in the ORR during the discharge process, while no O2(˙-) was detected in the OER during the charge process. These findings provide insight into, and an understanding of, the fundamental reaction mechanisms involving oxygen and guide the further development of this field.
Nature of Li2O2 oxidation in a Li-O2 battery revealed by operando X-ray diffraction
Journal of the American Chemical Society, 2014
Fundamental research into the Li-O2 battery system has gone into high gear, gaining momentum because of its very high theoretical specific energy. Much progress has been made toward understanding the discharge mechanism, but the mechanism of the oxygen evolution reaction (OER) on charge (i.e., oxidation) remains less understood. Here, using operando X-ray diffraction, we show that oxidation of electrochemically generated Li2O2 occurs in two stages, but in one step for bulk crystalline (commercial) Li2O2, revealing a fundamental difference in the OER process depending on the nature of the peroxide. For electrochemically generated Li2O2, oxidation proceeds first through a noncrystalline lithium peroxide component, followed at higher potential by the crystalline peroxide via a Li deficient solid solution (Li(2-x)O2) phase. Anisotropic broadening of the X-ray Li2O2 reflections confirms a platelet crystallite shape. On the basis of the evolution of the broadening during charge, we specul...