Effects of Solvent Polarity on Li-ion Diffusion in Polymer Electrolytes: An All-Atom Molecular Dynamics Study with Charge Scaling

Abstract

We herein report an all-atom molecular dynamics study on the role of solvent polarity for Li + diffusion in polymer electrolytes using PEO–LiTFSI (poly(ethylene oxide)–lithium bis(trifluoromethane)sulfonimide) as a model system. By separating the effect of T g and the effect of solvent polarity in our simulations, we show that the maximum in the diffusion coefficient of Li + with respect to the dielectric constant of polymer solvent ε p is due to transitions in the transport mechanism. In particular, it is found that the frequent interchain hopping involves the coordination of both PEO and TFSI. This optimal solvating ability of PEO at an intermediate value of ε p leads to the fast ion conduction. These findings highlight the synergetic effect of solvent polarity and bond polarity on Li-ion diffusion in solid polymer electrolytes.

Importance of the ion-pair lifetime in polymer electrolytes

“…Kubo conductivity σ G-K when modulating the solvent polarity as seen in Fig.1b. Both of these results support our previous observations of the effect of solvent polarity on the Li + transportation in PEO-LiTFSI systems41 and agree with other recent studies of polymer electrolyte systems 42,43. …”

The solvation structure, transport properties and reduction behavior of carbonate-based electrolytes of lithium-ion batteries

“…70 A less used approach is to scale the point charge of solvent molecules to modulate the solvent polarity. 71,72 Thus far, we have shown that MCEs exhibit a higher diffusivity, while the EC-base and ECF electrolytes exhibit a higher ion dissociation rate and a higher ratio of ionic conducting species.…”

Effect of Coordination Behavior in Polymer Electrolytes for Sodium-Ion Conduction: A Molecular Dynamics Study of Poly(ethylene oxide) and Poly(tetrahydrofuran)

“…Frequency of PEO and PTHF chains visited by cations. A Na + ion is considered to have visited a chain if it coordinated with a polymer oxygen for at least 5% of simulation time (∼5 ns), as suggested by Gudla et al …”

“…The molecular dynamics package LAMMPS was used in the equilibration and production runs, with the universal forcefield PCFF+. As discussed in the previous MD studies, nonpolarizable simulation models are not recommended for ionic systems due to their overestimation of the interaction between ions. ,,,, However, polarizable models are very demanding computationally. To mitigate this issue, point charge rescaling in classical force fields has been proposed as a way to reproduce transport properties in systems containing salts and organic electrolytes .…”

“…Solid polymer electrolytes (SPEs), e.g., solvent-free electrolytes, represent a promising alternative to conventional liquid ones due to their potential to allow high-energy-density batteries via the use of metal anodes. Furthermore, the lack of solvents prevents potential leakage, flammability issues, and enhances overall safety. − However, challenges to the application of SPEs still need to be addressed, notoriously their reduced ionic conductivities at room temperature. In SPEs, the ionic conductivity is greatly influenced by the ion-transport properties of the polymer host, as well as the strength of the salt–polymer interactions.…”