Lithium-ion battery performance is strongly influenced by the ionic conductivity of the electrolyte, which depends on the speed at which Li ions migrate across the cell and relates to their solvation structure. The choice of solvent can greatly impact both the solvation and diffusivity of Li ions. In this work, we used first-principles molecular dynamics to examine the solvation and diffusion of Li ions in the bulk organic solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and a mixture of EC and EMC. We found that Li ions are solvated by either carbonyl or ether oxygen atoms of the solvents and sometimes by the PF6(-) anion. Li(+) prefers a tetrahedrally coordinated first solvation shell regardless of which species are involved, with the specific preferred solvation structure dependent on the organic solvent. In addition, we calculated Li diffusion coefficients in each electrolyte, finding slightly larger diffusivities in the linear carbonate EMC compared to the cyclic carbonate EC. The magnitude of the diffusion coefficient correlates with the strength of Li(+) solvation. Corresponding analysis for the PF6(-) anion shows greater diffusivity associated with a weakly bound, poorly defined first solvation shell. These results can be used to aid in the design of new electrolytes to improve Li-ion battery performance.

This article reviews recent advances in the development of reactive empirical force fields or potentials. In particular, we compare two widely used reactive potentials with variable-charge schemes that are desirable for multicomponent or multifunctional systems: the ReaxFF (reactive force field) and charge-optimized many-body (COMB) potentials. Several applications of these approaches in atomistic simulations that involve metal-based heterogeneous systems are also discussed.

Contact electrification and the triboelectric effect are complex processes for mechanical-to-electrical energy conversion, particularly for highly deformable polymers. While generating relatively low power density, contact electrification can occur at the contact–separation interface between nearly any two polymer surfaces. This ubiquitousness of surfaces enables contact electrification to be an important phenomenon to understand energy conversion and harvesting applications. The mechanism of charge generation between polymeric materials remains ambiguous, with electron transfer, material (also known as mass) transfer, and adsorbed chemical species transfer (including induced ionization of water and other molecules) all being proposed as the primary source of the measured charge. Often, all sources of charge, except electron transfer, are dismissed in the case of triboelectric energy harvesters, leading to the generation of the “triboelectric series”, governed by the ability of a polymer to lose, or accept, an electron. Here, this sole focus on electron transfer is challenged through rigorous experiments, measuring charge density in polymer–polymer (196 polymer combinations), polymer–glass (14 polymers), and polymer–liquid metal (14 polymers) systems. Through the investigation of these interfaces, clear evidence of material transfer via heterolytic bond cleavage is provided. Based on these results, a generalized model considering the cohesive energy density of polymers as the critical parameter for polymer contact electrification is discussed. This discussion clearly shows that material transfer must be accounted for when discussing the source of charge generated by polymeric mechanical energy harvesters. Thus, a correlated physical property to understand the triboelectric series is provided.

No abstract

Corrosion processes occurring in aqueous solutions are critically dependent upon the interaction between the metal electrode and the solvent. In this work, the interaction of a nickel substrate with water molecules has been investigated using reactive force field (ReaxFF) molecular dynamics simulations. This approach was originally developed by van Duin and co-workers to study hydrocarbon chemistry and the catalytic properties of organic compounds. To our knowledge, this method has not previously been used to study the corrosion of nickel. In this work, we studied the interaction of 480 molecules of water (ρ = 0.99 g·cm(-3)) with Ni(111) surfaces at 300 K. The results showed that a water "bilayer" was adsorbed on the nickel surface. In the absence of an applied electric field, no dissociation of water was observed. However, the nickel atoms at the surface were charged positively, whereas the first water layer was charged negatively, indicating the formation of an electric double layer. To study the corrosion of nickel in pure water, we introduced an external electric field between the metal and the solution. The electric field intensity varied between 10 and 20 MeV/cm. The presence of this electric field led to oxidation of the metal surface. The structural and morphological differences associated with the growth of this oxide film in the presence of the electric field were evaluated. The simulated atomic trajectories were used to analyze the atomic displacement during the reactive process. The growth of the oxide scale on the nickel surface was primarily due to the movement of anions toward the interior of the metal substrate and the migration of nickel toward the free surface. We found that increasing the electric field intensity sped up the corrosion of nickel. The results also showed that the oxide film thickness increased linearly with increasing electric field intensity.

No abstract

Reactive molecular dynamics studies of stress corrosion properties of Ni metal nanoplate structures in pressurized water at different temperatures, chemical environments and mechanical boundary conditions have been performed. The results indicate reduction of dislocation nucleation barriers due to the water reactions on material surfaces, simulated loading rate and elevated temperature, resulting in reduction of material strength and ductility. It is also found that pre-oxidized surfaces yield increased initial dislocation nucleation barriers. Likewise, significant effects of stress triaxiality on strength, ductility and surface reactivity are also observed by comparison of boundary conditions. Size dependent structure failure modes and distributions of stress and strain have been explored. A possible Ni dissolution mechanism is identified and validated using DFT calculations and metadynamics simulations.

researchers aim to magnify the triboelectric charge on polymer surfaces. Polymer triboelectrification can be enhanced in several ways, including surface functionalization, [8] adjustment of the electronic and physicochemical properties between contacting materials, [9][10][11][12][13][14][15] or by increasing the specific contact area via nanostructuring. [16,17] Surface contact electrification can be observed also in nature. Spider ballooning is one of the most exciting natural phenomena. Using electrified strands of silk, [18] spiders can travel in the airstream for distances of hundreds of kilometers. [19] Spider silk is electrified due to contact and friction with airborne particles or during the spinning process. [20] To ensure the electrostatic flight, holding the weight of a spider, even in the absence of any aerodynamic lift, a strong surface charge is required.To understand how silk achieves this strong surface charge, understanding of its macromolecular structure is required. Spider silk has a hierarchical structure composed of highly ordered macromolecular inclusions which are interconnected by disordered elastomeric chains. [21] This macromolecular structure may lead to stress concentration, which we hypothesize is (partially) responsible for the strong surface charge.Recently, it has been found that the surface electrification of polymers is strongly connected to their physicochemical Triboelectrification of polymers enables mechanical energy harvesting in triboelectric generators, droplet generators, and ferroelectrets. Herein, triboelectric polymers, inspired by the ordering in spider-silk, with strongly enhanced contact electrification are presented. The ordering in polyether block amide (PEBA) is induced by the addition of inorganic goethite (α-FeOOH) nanowires that form H-bonds with the elastomeric matrix. The addition of as little as 0.1 vol% of α-FeOOH into PEBA increases the surface charge by more than order of magnitude (from 0.069 to 0.93 nC cm -2 ). The H-bonds between α-FeOOH and PEBA promote the formation of inclusions with higher degree of macromolecular ordering, analogous to the structure of spider silk. The formation of these inclusions is proven via nanoindentation hardness measurements and correlated with H-bond-induced chemical changes by Fourier transform infrared spectroscopy and direct scanning calorimetry. Theoretical studies reveal that the irregularity in hardness provides stress accumulation on the polymer surface during contactseparation. Subsequent molecular dynamic studies demonstrate that stress accumulation promotes the mass-transfer mechanism of contact electrification. The proposed macromolecular structure design provides a new paradigm for developing materials for applications in mechanical energy harvesting.