A Molecular Dynamics study of the influence of side-chain length and spacing on lithium mobility in non-crystalline LiPF6·PEOx; x = 10 and 30

Molecular Dynamics (MD) simulation techniques have been used to investigate systematically how the length and spacing of poly(ethylene oxide) (PEO) side-chains along a PEO backbone influence ion mobility for two different salt concentrations. This is of fundamental relevance to the design of new polymer electrolytes for battery applications. The salt used has been LiPF6 in concentrations corresponding to Li:EO ratios of 1:30 and 1:10. The MD box contained PEO backbones of 89–343 EO units to which 3, 6, 7, 8, 9 and 15 EO unit side-chains were added. The selected spacings along the backbone between the PEO side-chains attachment points were 5, 10, 15, 20 and 50 EO units. The backbone and all side-chains were methoxy end-capped, and the simulations were all made at 293 K. Ion mobilities have been estimated from the variation of mean-square-displacement with time, and have been analysed in relation to chain dynamics, cross-linking and ion pairing. Comparisons are also made with the results of simulated PEO systems without side-chains and/or without salt. It is found that, at a higher concentration, many short side-chains give the highest ion mobility, while the mobility is highest for side-chain lengths of 7–9 EO units at the lower concentration.