Structure and Diffusion of Ionic PDMS Melts

Ionic polymers exhibit mechanical properties that can be widely tuned upon selectively charging them. However, the correlated structural and dynamical properties underlying the microscopic mechanism remain largely unexplored. Here, we investigate, for the first time, the structure and diffusion of randomly and end-functionalized ionic poly(dimethylsiloxane) (PDMS) melts with negatively charged bromide counterions, by means of atomistic molecular dynamics using a united atom model. In particular, we find that the density of the ionic PDMS melts exceeds the one of their neutral counterpart and increases as the charge density increases. The counterions are condensed to the cationic part of end-functionalized cationic PDMS chains, especially for the higher molecular weights, leading to a slow diffusion inside the melt; the counterions are also correlated more strongly to each other for the end-functionalized PDMS. Temperature has a weak effect on the counterion structure and leads to an Arrhenius type of behavior for the counterion diffusion coefficient. In addition, the charge density of PDMS chains enhances the diffusion of counterions especially at higher temperatures, but hinders PDMS chain dynamics. Neutral PDMS chains are shown to exhibit faster dynamics (diffusion) than ionic PDMS chains. These findings contribute to the theoretical description of the correlations between structure and dynamical properties of ion-containing polymers.

A theoretical study on Na+ solvation in carbonate ester and ether solvents for sodium-ion batteries

The electrochemical performance of sodium-ion batteries is strongly related to the electrolyte solvents. Na⁺ solvation in commonly used carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) as well as in ether solvents such as 1,3-dioxolane (DOL), tetrahydrofuran (THF), and dimethoxyethane (DME) is studied by the density functional theory for sodium-ion batteries. It is indicated that the thermodynamic equilibrium is reached when forming 4sol-Na⁺ in the EC, PC, DMC, EMC, DEC, and THF solvents by spontaneous stepwise solvation reactions, and the formation of 3sol-Na⁺ complexes will reach thermodynamic equilibrium in DOL and DME at room temperature. It is demonstrated that Na⁺ is more easily solvated by the carbonate ester-based solvents EC, PC, DEC, DMC and EMC compared with that for the ether-based solvents DOL, THF and DME. In addition, the cyclic carbonate ester solvents more easily form a solvation-Na⁺ complex compared with the linear carbonate ester solvents, and THF is the easiest to form the solvation-Na⁺ complex among the three ether-based solvents. It is also indicated that the C[double bond, length as m-dash]O and carbonate C-O bond stretching vibrations in carbonate ester solvation complexes move to higher and lower frequencies, respectively, with the decrease in Na⁺ concentration. In addition, the C-O stretching vibrations with or without Na⁺ interactions in the ether solvation complexes shift to higher and lower frequencies, respectively, and the shift in frequency is not obvious after forming the maximum innermost solvation shell.

The effect of different organic solvents and anion salts on sodium ion storage in cylindrical carbon nanopores

In this article fully atomistic Molecular Dynamics simulations were employed to study the behaviour of electrolyte salts and different organic solvents in cylindrical carbon nanotubes, in order to reveal the storage mechanism.