Surprising effect of nanoparticle inclusion on ion conductivity in a lithium doped organic ionic plastic crystal

Doping lithium bis(trifluoromethanesulfonyl)amide (Li[NTf@d2]) into the N-ethyl,N'-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([C@d2mpyr][NTf@d2]) plastic crystal material has previously indicated order of magnitude enhancements in ion transport and conductivity over pure [C@d2mpyr][NTf@d2]. Recently, conductivity enhancements in this ionic plastic crystal induced by SiO2 nanoparticles have also been reported. In this work the inclusion of SiO@d2 nanoparticles in Li ion doped [C@d2mpyr][NTf@d2] has been investigated over a wide temperature range by differential scanning calorimetry (DSC), impedance spectroscopy, positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, NMR spectroscopy and scanning electron microscopy (SEM). Solid state @u1H NMR indicates that the addition of the nanoparticles increases the mobility of the [C@d2mpyr] cation and positron lifetime spectroscopy (PALS) measurements indicate an increase in mean defect size and defect concentration as a result of nanoparticle inclusion, especially with 10 wt% SiO@d2. Thus, the substantial drop in ion conductivity observed for this doped nanocomposite material was surprising. This decrease is most likely due to the decrease in mobility of the [NTf@d2] anion, possibly by its adsorption at the SiO@d2/ grain boundary interface and concomitant decrease in mobility of the Li ion.