The structure and dynamics of the water/vapor interface is revisited by means of path-integral and second-generation Car-Parrinello ab initio molecular dynamics simulations in conjunction with an instantaneous surface definition [Willard, A. P.; Chandler, D. J. Phys. Chem. B 2010, 114, 1954]. In agreement with previous studies, we find that one of the OH bonds of the water molecules in the topmost layer is pointing out of the water into the vapor phase, while the orientation of the underlying layer is reversed. Therebetween, an additional water layer is detected, where the molecules are aligned parallel to the instantaneous water surface.

Rechargeable lithium−sulfur (Li−S) batteries have drawn significant attention as next-generation energy storage systems. Sulfur-copolymers are promising alternative cathode materials to elemental sulfur in Li−S batteries as they provide high reversible capacity. However, the redox mechanisms of these materials are not well understood owing to the difficulty in characterizing amorphous structures and identifying individual ionic species. Here, we use solid-state NMR techniques together with electrochemistry experiments and quantum calculations to investigate the structural evolution of the prototype S-copolymer cathodes, sulfur−diisopropenylbenzene copolymers (poly(S-co-DIB)), during cycling. We demonstrate that polysulfides with different chain lengths can be distinguished by 13 C and 7 Li NMR spectroscopy, revealing that the structure of the copolymers can be tuned in terms of polysulfide chain lengths and resulting reaction pathways during electrochemical cycling. Our results show that the improved cyclability of these cathodes originates from the role of organic moieties acting as anchors that fixate polysulfides to the polymeric network during cycling, thus preventing their diffusion into the electrolyte. We provide a new methodological concept for the mechanistic studies to track the intermediate species and phase transition in Li−S batteries.

Using density functional theory calculations and ab initio molecular dynamics simulations we have studied the structures and the Raman spectra of Li2S4 clusters, which are believed to be the last polysulfide intermediates before the formation of Li2S2/Li2S during the discharge process in Li-S batteries. Raman spectra have been obtained using a new technique to estimate polarizabilities using Wannier functions. We have observed clear evidence of Li2S4→ Li2S2 transition by studying systematic changes in the simulated Raman spectra of (Li2S4)n, n = 1, 4, and 8 towards that of (Li2S2)8. Furthermore, we have shown that the dominant Raman peak of the Li2S2 cluster at ∼440 cm(-1) arises from sulfur-sulfur stretching mode. This peak has been experimentally observed in the discharged state of Li-S batteries and has also been attributed to the formation of Li2S2. We have also demonstrated that the transition is mainly due to the strong electrostatic interactions between Li2S4 monomers, which results in energy lowering by arranging the local Li(+δ)-S(-δ) dipole moments in an anti-parallel fashion.

No abstract

We investigate the structure and electronic properties of phosphatidylcholine (PC) under different degrees of hydration at the single-molecule and monolayer type level by linear scaling ab initio calculations. Upon hydration, the phospholipid undergoes drastic long-range conformational rearrangements which lead to a sickle-like ground-state shape. The structural unit of the tilted gel-phase PC appears to be a water-bridged PC dimer. We find that hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids and consequently guides the interactions of the lipids with other molecules and the communication between cells.