Variation of the mesh size of PEO-based networks filled with TFSILi: from an Arrhenius to WLF type conductivity behavior

The properties of ionically conducting membranes based on polyethylene oxide (PEO) networks crosslinked with urethane functions, containing lithium bis(trifluoromethane sulfone imide) (TFSILi) were studied. The temperature and salt concentration influence on the ionic conductivity of these materials was analyzed. The best conductivity value measured at 30°C was 7×10−5 S cm−1 at O/Li=43 for a PEO molecular weight between crosslinks of 2000 g mol−1. The curves obtained in the Arrhenius coordinates were typical of a William Landel Ferry (WLF) like behavior when the molecular weight of the PEO between crosslinks was 1000 or 2000 g mol−1. However, the network based on PEO600 followed an Arrhenius behavior, typical of a network where segmental movements are prevented. A strong interaction between TFSILi, ethylene oxide units and urethane crosslinks functions at high urethane concentration explains these results. This finding demonstrates that the two types of conductivity behavior (Arrhenius or WLF) can be found for the same type of polymer electrolyte depending on the predominant elementary conductivity process.