Role of van der Waals Forces in Thermodynamics and Kinetics of Layered Transition Metal Oxide Electrodes: Alkali and Alkaline-Earth Ion Insertion into V₂O₅

Layered transition metal oxides (LTMOs) have a long tradition of success as effective electrode materials for power storage applications. However, the growing demand for improved technologies has motivated a strong interest in developing new generations of this class of materials. First-principles calculations, in particular density functional theory (DFT), have become an important tool to gain atomic-level understanding and speed up the search of new materials in general. An important structural ingredient of LTMOs is the weak van der Waals (vdW) forces that hold layers together. Unfortunately, conventional DFT approaches have serious shortcomings to treat these dispersion interactions. This is an uneasy position for the role of DFT in describing such layered-type structural materials. Recent exciting developments in DFT allow us now to tackle this problem head on. Here we have employed newly developed vdW-inclusive methods based on improved nonlocal density functionals to thoroughly explore the role of vdW forces in key thermodynamic and kinetic properties of alkali (Li, Na, and K) and alkaline-earth (Mg, Ca, and Sr) ion insertion into α-V₂O₅. We find that vdW forces help to stabilize inserted ions and, therefore, increase average voltages compared to the values obtained with conventional non-vdW-inclusive DFT methods. Added to this, activation energies for ion diffusion significantly increase as a consequence of a proper account for vdW interactions. These results highlight the relevance of vdW forces to ion intercalation and dynamics in LTMOs in general

Electronic Structure of Sodium Superoxide Bulk, (100) Surface, and Clusters using Hybrid Density Functional: Relevance for Na–O₂ Batteries

Clarifying the electronic structure of sodium superoxide (NaO₂) is a key step in understanding the electrochemical behavior of Na–O₂ batteries. Here we report a density functional theory study to explore the effect of atomic structure and morphology on the electronic properties of different model systems: NaO₂ bulk, (100) surface, and small (NaO₂)_<i>n</i> clusters (<i>n</i> = 3–8). We found that a correct description of the open-shell 2p electrons of O₂^– requires the use of a hybrid functional, which reveals a clear insulating nature of all of the investigated systems. This sheds light onto the capacity limitations of pure NaO₂ as a discharge product and highlights the need for developing new strategies to enhance its electron transport in the optimization of Na–O₂ cells

Revealing the Mechanism of Sodium Diffusion in Na_<i>x</i>FePO₄ Using an Improved Force Field

Olivine NaFePO₄ is a promising cathode material for Na-ion batteries. Intermediate phases such as Na_0.66FePO₄ govern phase stability during intercalation-deintercalation processes, yet little is known about Na⁺ diffusion in Na_<i>x</i>FePO₄ (0 < <i>x</i> < 1). Here we use an advanced simulation technique, Randomized Shell Mass Generalized Shadow Hybrid Monte Carlo Method (RSM-GSHMC) in combination with a specifically developed force field for describing Na_<i>x</i>FePO₄ over the whole range of sodium compositions, to thoroughly examine Na⁺ diffusion in this material. We reveal a novel mechanism through which Na⁺/Fe²⁺ antisite defect formation halts transport of Na⁺ in the main diffusion direction [010], while simultaneously activating diffusion in the [001] channels. A similar mechanism was reported for Li⁺ in LiFePO₄, suggesting that a transition from one- to two-dimensional diffusion prompted by antisite defect formation is common to olivine structures, in general

Molecular-Level Understanding of CeO₂ as a Catalyst for Partial Alkyne Hydrogenation

The unique catalytic properties of ceria for the partial hydrogenation of alkynes are examined for acetylene hydrogenation. Catalytic tests over polycrystalline CeO₂ at different temperatures and H₂/C₂H₂ ratios reveal ethylene selectivities in the range of 75–85% at high degrees of acetylene conversion and hint at the crucial role of hydrogen dissociation on the overall process. Density-functional theory is applied to CeO₂(111) in order to investigate reaction intermediates and to calculate the enthalpy and energy barrier for each elementary step, taking into account different adsorption geometries and the presence of potential isomers of the intermediates. At a high hydrogen coverage, β-C₂H₂ radicals adsorbed on-top of surface oxygen atoms are the initial reactive species forming C₂H₃ species effectively barrierless. The high alkene selectivity is owed to the lower activation barrier for subsequent hydrogenation leading to gas-phase C₂H₄ compared to that for the formation of β-C₂H₄ radical species. Moreover, hydrogenation of C₂H₅ species, if formed, must overcome significantly large barriers. Oligomers are the most important byproduct of the reaction and they result from the recombination of chemisorbed C₂H_<i>x</i> species. These findings rationalize for the first time the applicability of CeO₂ as a catalyst for olefin production and potentially broaden its use for the hydrogenation of polyunsaturated and polyfunctionalized substrates containing triple bonds

Types of maternal behavior.

<p>Distribution of different types of maternal behavior in control (CTR) and early-life stress (ELS) dams during PND 2 to 8.</p

Sign-tracking (ST)–Goal-tracking (GT) score for day 5.

<p>A positive score indicates that ST behavior predominates over GT, whereas a negative score indicates that GT behavior predominates over ST. The results are shown for control (CTR) and early-life stress (ELS) rats of both sexes, exposed to immobilization (IMO) stress or not when adults. ** p < 0.01 versus corresponding CTR; +++ p < 0.001 versus corresponding males. When compared against a “zero” value, the score was only statistically significant for IMO males and females not exposed to ELS ($ p < 0.05; &&& p < 0.001). Means and SEM are shown.</p

Sign-tracking and goal-tracking behavior in day 1.

<p>Time (s) spent in behavior towards the lever (left panels) and time (s) spent inside the magazine during lever presentation (right panels) for day 1. Results are shown for control (CTR) and early-life stress (ELS) rats of both sexes, exposed to immobilization (IMO) stress or not when adults. && p < 0.01 versus non-IMO. The same scale as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0190044#pone.0190044.g005" target="_blank">Fig 5</a> is maintained to allow comparisons between days 1 and 5. In the insets, the same results with different scale are provided. Means and SEM are shown.</p

Sex-dependent impact of early-life stress and adult immobilization in the attribution of incentive salience in rats

<div><p>Early life stress (ELS) induces long-term effects in later functioning and interacts with further exposure to other stressors in adulthood to shape our responsiveness to reward-related cues. The attribution of incentive salience to food-related cues may be modulated by previous and current exposures to stressors in a sex-dependent manner. We hypothesized from human data that exposure to a traumatic (severe) adult stressor will decrease the attribution of incentive salience to reward-associated cues, especially in females, because these effects are modulated by previous ELS. To study these factors in Long-Evans rats, we used as an ELS model of restriction of nesting material and concurrently evaluated maternal care. In adulthood, the offspring of both sexes were exposed to acute immobilization (IMO), and several days after, a Pavlovian conditioning procedure was used to assess the incentive salience of food-related cues. Some rats developed more attraction to the cue predictive of reward (sign-tracking) and others were attracted to the location of the reward itself, the food-magazine (goal-tracking). Several dopaminergic markers were evaluated by in situ hybridization. The results showed that ELS increased maternal care and decreased body weight gain (only in females). Regarding incentive salience, in absolute control animals, females presented slightly greater sign-tracking behavior than males. Non-ELS male rats exposed to IMO showed a bias towards goal-tracking, whereas in females, IMO produced a bias towards sign-tracking. Animals of both sexes not exposed to IMO displayed an intermediate phenotype. ELS in IMO-treated females was able to reduce sign-tracking and decrease tyrosine hydroxylase expression in the ventral tegmental area and dopamine D1 receptor expression in the accumbens shell. Although the predicted greater decrease in females in sign-tracking after IMO exposure was not corroborated by the data, the results highlight the idea that sex is an important factor in the study of the long-term impact of early and adult stressors.</p></div

Expression of dopaminergic markers.

<p>Tyrosine hydroxylase (TH) expression in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNpc) and D1 dopamine receptors in the nucleus accumbens shell (AcbS) in arbitrary units (AU). The results are shown for control (CTR) and early-life stress (ELS) rats of both sexes, exposed to immobilization (IMO) stress or not when adults. * p < 0.05 versus corresponding CTR; + p < 0.05 versus corresponding males; & p < 0.05, && p < 0.01 versus corresponding non-IMO. Means and SEM are shown.</p

Maternal behavior measures.

<p>Left panels: Number of episodes of arched-back (A), licking-grooming (B) and “off” nest (C) behaviors during postnatal days (PND) 2 to 8, and on PND 13 and PND 18 (sum of all the times of the day analyzed). Right panels: Number of episodes of licking-grooming (D), arched-back (E) and off-nest (F) behaviors at 4 different times of day (sum of PND 2 to 8). The results are shown for control (CTR) and early-life stress (ELS) dams. ** p < 0.01 and *** p < 0.001 versus CTR. Means and SEM are shown.</p