A molecular dynamics study of ion-conduction mechanisms in crystalline low-Mw LiPF6·PEO6

Abstract

Molecular dynamics (MD) simulation has been used to probe ion-conduction mechanisms in crystalline LiPF₆·PEO₆ for smectic- and nematic-ordered models of methyl-terminated short-chain monodisperse poly(ethylene oxide) chains with the formula CH₃–(OCH₂CH₂)₂₃–OCH₃; M_w = 1059. The effect of aliovalent substitution of the PF₆⁻ anion by ca. 1% SiF₆²⁻ has also been studied. External electric fields in the range 3–6 × 10⁶ V m⁻¹ have been imposed along, and perpendicular to, the chain direction in an effort to promote ion transport during the short time-span of the simulation. Ion-migration barriers along the polymer channel are lower for the nematic models than for the smectic, with anions migrating along the channels more readily than Li-ions. Ion mobility within the smectic interface could also be confirmed, but at a higher field-strength threshold than along the chain direction. Li-ion migration within the smectic plane appears to be suppressed by ion pairing, while Li-ion transport across the smectic gap is facilitated by uncoordinated methoxy end-groups. Interstitial Li-ions introduced into the PEO channel through SiF₆²⁻ doping are also shown to enhance Li-ion conduction.