Estimation of electric field effects on the adsorption of molecular superoxide species on Au based on density functional theory.

Superoxide species are key intermediates in the oxygen reduction reactions (ORR) that occur at the cathodes of aprotic metal-air batteries. Herein we report a DFT study of the effects of an externally applied electric field (ε) on the stability of various molecular superoxide species, including MO2 (M = Li, Na, K) and O2-, on gold surfaces, which shows that the stability of such species on Au electrodes can be materially affected by the presence of an electric field and solvent molecules, suggesting that such effects should be included in the first-principles modeling of cathode reactions in metal-O2 cells. In the ε range of ±0.4 V Å-1, the stability of MO2 species is found to vary by up to |0.4| eV on Au(111) and |0.2| eV on Au(211) in vacuo, which is larger than the field effects on the stability of O and OH, key intermediates in the ORR by hydrogen. An aprotic solvent such as dimethyl sulfoxide (DMSO), considered here via the inclusion of explicit DMSO molecules above the Au surfaces, stabilizes all three MO2 species at zero fields and amplifies the field effects on the stability of MO2, on both Au surfaces. The variations in the stability of the molecular MO2 species with ε, which have small polarizabilities, are closely approximated by the first-order Stark effect (µ0·Îµ, where µ0 is the static surface dipole moment induced by adsorption at ε = 0 V Å-1). The superoxide anion (O2-) that has been identified on an under-coordinated Au site has a larger polarizability than the MOx species and a µ0 that is opposite in sign to those of the metal MO2 species, which results in larger errors by the first-order approximation, although its stability varies only moderately under positive electric fields of up to 0.4 V Å-1.

Mechanistic origin of low polarization in aprotic Na-O2 batteries.

Research interest in aprotic sodium-air (Na-O2) batteries is growing because of their considerably high theoretical specific energy and potentially better reversibility than lithium-air (Li-O2) batteries. While Li2O2 has been unequivocally identified as the major discharge product in Li-O2 batteries containing relatively stable electrolytes, a multitude of discharge products, including NaO2, Na2O2 and Na2O2·2H2O, have been reported for Na-O2 batteries and the corresponding cathodic electrochemistry remains incompletely understood. Herein, we provide molecular-level insights into the key mechanistic differences between Na-O2 and Li-O2 batteries based on gold electrodes in strictly dry, aprotic dimethyl sulfoxide electrolytes through a combination of in situ spectroelectrochemistry and density functional theory based modeling. While like Li-O2 batteries, the formation of oxygen reduction products (i.e., O2-, NaO2 and Na2O2) in Na-O2 batteries depends critically on the electrode potential, two factors lead to a better reversibility of Na-O2 electrochemistry, and are therefore highly beneficial to a viable rechargeable metal-air battery design: (i) only O2- and NaO2, and no Na2O2, form down to as low as ∼1.5 V vs. Na/Na+ during discharge; (ii) solid NaO2 is quite soluble and its formation and oxidation can proceed through micro-reversible EC (a chemical reaction of the product after the electron transfer) and CE (a chemical reaction preceding the electron transfer) processes, respectively, with O2- as the key intermediate.

Amorphous Li2 O2 : Chemical Synthesis and Electrochemical Properties.

When aprotic Li-O2 batteries discharge, the product phase formed in the cathode often contains two different morphologies, that is, crystalline and amorphous Li2 O2 . The morphology of Li2 O2 impacts strongly on the electrochemical performance of Li-O2 cells in terms of energy efficiency and rate capability. Crystalline Li2 O2 is readily available and its properties have been studied in depth for Li-O2 batteries. However, little is known about the amorphous Li2 O2 because of its rarity in high purity. Herein, amorphous Li2 O2 has been synthesized by a rapid reaction of tetramethylammonium superoxide and LiClO4 in solution, and its amorphous nature has been confirmed by a range of techniques. Compared with its crystalline siblings, amorphous Li2 O2 demonstrates enhanced charge-transport properties and increased electro-oxidation kinetics, manifesting itself a desirable discharge phase for high-performance Li-O2 batteries.

Covalent hypercoordination: can carbon bind five methyl ligands?

C(CH3)5(+) is the first reported example of a five-coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis-violating "electron-deficient bonds" between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5(+) fleeting existence near 0 K.

Correlation effects on the relative stabilities of alkanes.

The "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3-alkyl-alkyl interactions-including methine, methylene, and methyl groups as alkyl entities-present in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (∼1.5-3.0 Å) electron correlation. Localized molecular orbital second-order Møller-Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.

Unlocking the energy capabilities of micron-sized LiFePO4.

Utilization of LiFePO4 as a cathode material for Li-ion batteries often requires size nanonization coupled with calcination-based carbon coating to improve its electrochemical performance, which, however, is usually at the expense of tap density and may be environmentally problematic. Here we report the utilization of micron-sized LiFePO4, which has a higher tap density than its nano-sized siblings, by forming a conducting polymer coating on its surface with a greener diazonium chemistry. Specifically, micron-sized LiFePO4 particles have been uniformly coated with a thin polyphenylene film via the spontaneous reaction between LiFePO4 and an aromatic diazonium salt of benzenediazonium tetrafluoroborate. The coated micron-sized LiFePO4, compared with its pristine counterpart, has shown improved electrical conductivity, high rate capability and excellent cyclability when used as a 'carbon additive free' cathode material for rechargeable Li-ion batteries. The bonding mechanism of polyphenylene to LiFePO4/FePO4 has been understood with density functional theory calculations.

A Hückel theory perspective on Möbius aromaticity.

Heilbronner's Hückel molecular orbital treatment of Möbius 4n-π annulenes is revisited. When uneven twisting in π-systems of small Möbius rings is accounted for, their resonance energies become comparable to iso-π-electronic linear alkenes with the same number of carbon atoms. Larger Möbius rings distribute π-twisting more evenly but exhibit only modest aromatic stabilization. Dissected nucleus independent chemical shifts (NICS), based on the LMO (localized molecular orbital)-NICS(0)π index confirm the magnetic aromaticity of the Möbius annulenes considered.

Why do two π-electron four-membered Hückel rings pucker?

Notwithstanding their two (i.e., 4n + 2) π electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C(4)F(4)(2+)) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH(3) in C(4)(SiH(3))(4)(2+)) enhance hyperconjugation and increase the ring inversion barriers substantially.