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

Francielli S Genier,Ian D Hosein

doi:10.1021/acs.macromol.1c01028

Abstract

Alternatives to lithium-ion batteries are extensively studied due to concerns about lithium availability considering the constant increase in demand for rechargeable batteries. While several studies have investigated different liquid electrolytes, the transport of those ions in their polymer counterparts remains understudied. In this work, we used molecular dynamics simulations to characterize the main factors that affect sodium-ion transport in two polymer hosts: poly(ethylene oxide) (PEO) and poly(tetrahydrofuran) (PTHF). We analyzed the influence of oxygen density in each chain and its effect on diffusivity, conductivity, and cation–anion interactions. It is inferred that the weaker coordination in PTHF resulted in differences in the Na+-transport mechanism, with interchain hopping being more prominent in PTHF than in PEO. The faster diffusion observed in PTHF was, however, hindered by the significantly larger formation of ion clusters in the PTHF electrolyte, which could lead to smaller transference numbers in battery settings. These findings elucidate the fundamental influences and correlations of varied polymer ether content to ion coordination and transport, which can inform on novel syntheses that improve polymer electrolyte properties.

Highlights

The establishment of sustainable energy sources highly depends on efficient energy-storage devices
The forcefield PCFF+ with rescaled charges has been used in poly(ethylene oxide) (PEO)−lithium salts electrolytes
We assumed this forcefield to be suitable for PEO and PTHF with sodium salt simulations as well

Summary

Introduction

The establishment of sustainable energy sources highly depends on efficient energy-storage devices. From electric cars to nationwide transmission grids, the demand for improved batteries continues to grow. 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. The lack of solvents prevents potential leakage, flammability issues, and enhances overall safety.[1−5] 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. Understanding the ion diffusion mechanisms in the polymer of interest is crucial to improving conductivity.[2,3,6]

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