One of the technological barriers to electrification of transport is the insufficient storage capacity of the Li-ion batteries on which the current electric cars are based. The lithium-sulfur (Li-S) battery is an advanced technology whose successful commercialization can lead to significant gains in the storage capacity of batteries and promote wide-spread adoption of electric vehicles. Recently, important Li-S intermediates, including polysulfides, S 3•-, and Li 2 S, have been shown to present unique X-ray absorption near edge structure (XANES) features at the sulfur K-edge. As a result, a combination of XANES characterization with electrochemistry has the potential to contribute to the understanding of Li-S chemistry. In this study, we present an operando XANES cell design, benchmark its electrochemical and spectroscopic performance, and use it to track reaction intermediates during the discharge of the battery. In particular, by employing electrolyte solvents with either a high or a low dielectric constant, we investigate the influence of the solvent on the conversion of polysulfide species to Li 2 S. Our results reveal that Li 2 S is already formed after ∼25-30% discharge in both types of electrolyte solvents, but that further conversion of polysulfides to Li 2 S proceeds more rapidly in a solvent with a low dielectric constant. Electric vehicles represent a promising alternative to conventional transport based on an internal combustion engine and, when combined with renewable energy sources, have the potential to decrease worldwide CO 2 emissions by 20-30%.1,2 In recent years, they have been gaining popularity in the form of passenger cars, with the number of sold units surpassing the 100,000 mark in 2012.2 One of the main technological barriers to an even wider adoption of electric cars and a complete displacement of the CO 2 -emitting vehicles is the insufficient storage capacity of the current battery technology, which leads to a limited vehicle driving range.2,3 The useable energy density of state-of-the-art lithium-ion batteries employed in recent electric vehicles amounts to only ≈120 Wh/kg system , 4 which corresponds to a practical driving range of 150-200 km. If one succeeds in the development of sufficiently durable high-voltage or high-energy cathode materials and combines them with silicon anodes, an energy density of ≈250 Wh/kg system would be in reach. 4 An alternative promising battery concept is the lithium-sulfur (Li-S) battery.3,5-7 Current development in Li-S battery research includes successful commercialization of Li-S cells with 260 Wh/kg system energy density by Sion Power corporation.8 This value already exceeds the DOE target for 2022 (250 Wh/kg system ) 9 and has the potential to also comply with the system cost target of 125 USD/kWh 9 because of the abundance and low cost of sulfur. To achieve even greater gains in the performance and, most importantly, to improve the cycle life of Li-S batteries, it will be necessary to gain a deeper mechanistic understanding of the 16 e -/S 8 in...
Although the “brown‐ring” ion, [Fe(H2O)5(NO)]2+ (1), has been a research target for more than a century, this poorly stable species had never been isolated. We now report on the synthesis of crystals of a salt of 1 which allowed us to tackle the unique bonding situation on an experimental basis. As a result of the bonding analysis, two stretched, spin‐polarised π‐interactions provide the Fe–NO binding—and challenge the concept of “oxidation state”.
Nitrosyl–metal bonding relies on the two interactions between the pair of N–O‐π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation‐state determination ambiguous. However, apart from M‐N‐O‐angle or net‐charge considerations, IUPAC′s “ionic approximation” is a useful tool to reliably classify nitrosyl metal complexes in an orbital‐centered approach.
Die Bindung eines Nitrosyl‐Liganden an ein Zentralmetallatom wird durch die Wechselwirkung zwischen den beiden N−O‐π*‐ und zwei Metall‐d‐Orbitalen bestimmt. Diese beiden (Rück‐)Bindungen sind weitgehend kovalent, wodurch deren Zuordnung bei der Oxidationsstufenbestimmung erschwert ist. Dabei erweist sich IUPACs “ionische Näherung” als wirksames Werkzeug, um auf der Grundlage der beteiligten Orbitale eine schlüssige Zuordnung zu erreichen – und zwar ohne den M‐N‐O‐Winkel oder Nettoladungen heranzuziehen.
Obwohl die Erforschung des Chromophors des “braunen Rings”, das Ion [Fe(H2O)5(NO)]2+ (1), länger als ein Jahrhundert zurückreicht, gelang die Isolierung dieser nur wenig stabilen Spezies bisher nicht. Nun aber eröffnet die Kristallisation eines Salzes von 1 die Möglichkeit, die besondere Bindungssituation auf experimenteller Grundlage zu analysieren. Das Ergebnis, eine Fe‐NO‐Bindung durch zwei gestreckte, spinpolarisierte π‐Wechselwirkungen, ist eine Herausforderung des Konzeptes der Oxidationsstufe.
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