Molecular dynamics studies of poly(ethylene oxide) based electrolytes

Introduction

Solid electrolytes for the Lithium-Ion Polymer Battery can be produced by mixing a lithium salt, typically LiPF₆ or LiBF₄, into poly(ethylene oxide) (PEO), -(CH₂CH₂O)n-. However, such electrolytes only exhibit adequate ionic conductivity (>10^-4 S/cm) at temperatures above 70°C, where the polymer becomes amorphous. The conventional wisdom has been that the high degree of local order (“crystallinity”) is the reason for the poor ionic conductivity at ambient temperatures. Much attention has therefore been devoted to the task of increasing the amorphous content of the PEO electrolyte at ambient temperatures.

===Methodology - Molecular Dynamics simulations===

In a Molecular Dynamics (MD) simulation, atomic motion is described in classical mechanics terms by solving Newton’s equations of motion for each atom in a fixed simulation box. This procedure generates a sequence of snapshots which together constitutes a “movie” of the simulated system on the atomic scale. The massive computer time needed to solve these equation for a large number of particles means that such movies are generally fairly short – within the nanosecond regime for some thousands of atoms. All that is needed to solve the equations of motion are the masses of the particles and the potentials describing the interactions between them - the so-called Force Field. This is normally generated by quantum mechanics calculations.

===Crystalline polymer electrolytes===

Crystalline phases of polymer electrolytes were long regarded as insulators. This view has been overturned to some extent during recent years by the demonstration of ionic conductivity in the complexes LiXF₆PEO₆ for X = P, As or Sb. Although the conductivity is still relatively low in these materials, they nevertheless show ca. ten times higher ionic conductivity than their amorphous counterparts. The LiXF₆PEO₆ complexes also display very fascinating structures – the materials are composed of coaxial hemi-helices of PEO, which pairwise form cylindrical channels containing the lithium ions coordinated to ether-oxygen chains; the anions lie outside the hemi-helical pairs, with no directstructural interaction with the lithium ions (see Fig. 1). Within our group, we have made the first MD survey of this LiPF₆PEO₆ system to try to find the source of this unorthodox behaviour.

The effect of side-chains

The addition of side-chains to the polymer backbone prevents PEO from crystallising, but also changes the local mobility in the surroundings. We find that side-chains can act as "paddle-wheels" or "rotors", thereby facilitating Li-ion motion. Within our group, we use MD simulations to study these phenomena on the atomic and molecular scale. We have varied side-chain length and separation for a number of PEO-based system using different salt concentrations.

Nano-particles in polymer electrolytes

Inorganic nano-particles can both prevent PEO from crystallisation ("nano-plasticising"), but also modify the local structure and dynamics of the surrounding polymer and the solvated ions. Our group has investigated a number of PEO systems with Al₂O₃ nano-particles additives with MD techniques.