Mechanism of ion transport in PEO/LiTFSI complexes: Effect of temperature, molecular weight and end groups
Résumé
The conductivity and viscosity of PEO/LiTFSI complexes are determined as a function of temperature, molecular weight (Mn) and the end group nature in view of the design of future polymer electrolytes. The results show the crucial role of the end groups on the dynamics of polymers at low Mn. A new method is proposed to estimate the glass transition temperature variation as function of Mn and end groups using conductivity data. The conductivity and viscosity plotted at constant friction factor follow a master curve which suggests that the main impact of end groups is to modify the available free volume which governs in turn the segmental dynamics. The anion and cation conductivities are separated using the cationic transport number obtained by pfg-NMR. Finally, an empirical equation based on Rouse dynamics taking into account the effect of the end groups is proposed. It reproduces with a good degree of accuracy the conductivities over the whole temperature and Mn ranges. In agreement with molecular dynamic simulations, at high Mn the limiting step is the jump of the lithium ion from one coordination site to another and is not influenced by the dynamics of the PEO chain reptation, whereas at low Mn the transport is mainly ensured by a vehicular mechanism.