As opposed to the standard graphite anode used for lithium (Li) ion batteries (LIBs), a standard anode material for sodium (Na) ion batteries (NIBs) has not yet been reported. Black phosphorus is potentially very attractive as an anode material for NIBs, as it has a layered structure similar to graphite but a greater interlayer distance. In this work, we propose an atomistic mechanism for the sodiation of black phosphorus, based on first-principle calculations. The layered structure of black phosphorous is maintained up to the composition of Na 0.25 P, with one-dimensional sodiation (an intercalation process) occurring in the interlayer spaces of the black phosphorus, resulting in sliding of the phosphorene layers because one Na atom tends to bind to four P atoms. At Na levels beyond Na 0.25 P, the intercalation process changes to an alloying process. Sodiation exceeding the critical composition leads to breaking of P-P bonds and eventual formation of an amorphous phase from the layered Na x P structure. After the P-P bonds in the layered Na x P structure are broken, in a progress in which staggered P-P bonds are preferentially broken rather than planar P-P bonds, P 2 dumbbells are generated. As sodiation proceeds further, most of the P 2 dumbbells become isolated P atoms. Thus, in the amorphous Na 3 P phase, only low-coordinate P components such as isolated atoms (primarily) and dumbbells are found. We expect that our comprehensive understanding of the sodiation mechanism in black phosphorus will provide helpful guidelines in designing new types of black phosphorus anodes to obtain better performing NIBs.

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We propose the ReaxFF reactive force field as a simulation protocol for predicting the evolution of solid-electrolyte interphase (SEI) components such as gases (CH, CO, CO, CH, and CH), and inorganic (LiCO, LiO, and LiF) and organic (ROLi and ROCOLi: R = -CH or -CH) products that are generated by the chemical reactions between the anodes and liquid electrolytes. ReaxFF was developed from ab initio results, and a molecular dynamics simulation with ReaxFF realized the prediction of SEI formation under real experimental conditions and with a reasonable computational cost. We report the effects on SEI formation of different kinds of Si anodes (pristine Si and SiO), of the different types and compositions of various carbonate electrolytes, and of the additives. From the results, we expect that ReaxFF will be very useful for the development of novel electrolytes or additives and for further advances in Li-ion battery technology.

Using first-principles calculations, we describe and compare atomistic lithiation, sodiation, and magnesiation processes in black phosphorous with a layered structure similar to graphite for Li-, Na-, and Mg-ion batteries because graphite is not considered to be an electrode material for Na- and Mg-ion batteries. The three processes are similar in that an intercalation mechanism occurs at low Li/Na/Mg concentrations, and then further insertion of Li/Na/Mg leads to a change from the intercalation mechanism to an alloying process. Li and Mg show a columnar intercalation mechanism and prefer to locate in different phosphorene layers, while Na shows a planar intercalation mechanism and preferentially localizes in the same layer. In addition, we compare the mechanical properties of black phosphorous during lithiation, sodiation, and magnesiation. Interestingly, lithiation and sodiation at high concentrations (Li2P and Na2P) lead to the softening of black phosphorous, whereas magnesiation shows a hardening phenomenon. In addition, the diffusion of Li/Na/Mg in black phosphorus during the intercalation process is an easy process along one-dimensional channels in black phosphorus with marginal energy barriers. The diffusion of Li has a lower energy barrier in black phosphorus than in graphite.

Liquid CBN (carbon-boron-nitrogen) hydrogen-storage materials such as 3-methyl-1,2-BN-cyclopentane have the advantage of being easily accessible for use in current liquid-fuel infrastructure. To develop practical liquid CBN hydrogen-storage materials, it is of great importance to understand the reaction pathways of hydrogenation/dehydrogenation in the liquid phase, which are difficult to discover by experimental methods. Herein, we developed a reactive force field (ReaxFFCBN) from quantum mechanical (QM) calculations based on density functional theory for the storage of hydrogen in BN-substituted cyclic hydrocarbon materials. The developed ReaxFFCBN provides similar dehydrogenation pathways and energetics to those predicted by QM calculations. Moreover, molecular dynamics (MD) simulations with the developed ReaxFFCBN can predict the stability and dehydrogenation behavior of various liquid CBN hydrogen-storage materials. Our simulations reveal that a unimolecular dehydrogenation mechanism is preferred in liquid CBN hydrogen-storage materials. However, as the temperature in the simulation increases, the contribution of a bimolecular dehydrogenation mechanism also increases. Moreover, our ReaxFF MD simulations show that in terms of thermal stability and dehydrogenation kinetics, liquid CBN materials with a hexagonal structure are more suitable materials than those with a pentagonal structure. We expect that the developed ReaxFFCBN could be a useful protocol in developing novel liquid CBN hydrogen-storage materials.

A six-dimensional intermolecular potential energy surface for a rigid methane (CH4) and carbon dioxide (CO2) dimer was developed from the counterpoise-corrected supermolecular approach at the CCSD(T) level of theory. A total of 466 grid points distributed to 46 orientations were calculated from the complete basis set limit extrapolation based on up to aug-cc-pVQZ basis set. A modified site-site pair potential function was proposed for rapid representation of the high level ab initio calculations. A nonadditive three-body interaction was represented by the Axilrod-Teller-Muto expression for mixtures with the polarizability and the London dispersion constant of each molecule. Second to fourth virial coefficients of CH4 and CO2 mixtures were calculated using both the Mayer sampling Monte Carlo method and the present potential functions. The virial equation of state derived from these coefficients was used to predict the pVT values and showed good agreement with experimental data below 200 bar at 300 K. The vapor-liquid coexistence curves of pure CH4, CO2 and their mixtures were presented with the aid of Gibbs ensemble Monte Carlo simulations. The predicted tie lines agreed with the experimental data within the uncertainties up to near the critical point.

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Molecular dynamics (MD) simulations using the reactive force field (ReaxFF) have been performed to elucidate the underlying water-induced disruption mechanism of several prototypical interpenetrated MOFs (IRMOF-9, IRMOF-13, and SUMOF-4). Through the comparison to the corresponding noninterpenetrated MOFs (IRMOF-10 and IRMOF-14), for both the interpenetrated and noninterpenetrated MOFs, structural collapse was always accompanied by the dissociation of the water molecules, with the produced OH– and H+ forming chemical bonds with the Zn2+ ion and O atom of the ligand, respectively. However, the water stability of the interpenetrated MOFs is less than that of the corresponding noninterpenetrated structures. The reasons for the differences between the MOFs in the resistance to water attack are clarified. The water resistance of the noninterpenetrated MOFs is mainly attributed to the strength of the Zn–Oligand, but, the hydrogen bond has little effect. However, a trade-off between the strength of the Zn–Oligand bond and the hydrogen bond determines the water stability of the interpenetrated MOFs. We expect that our understanding of the water-disruption mechanisms of MOFs will provide helpful guidance for the design of MOFs with a high water-resistance. Additionally, this work shows that ReaxFF simulations could be a useful technique for predicting the hydrothermal stability of MOFs.