Functionalized mesoporous silica MCM-41 in poly(ethylene oxide)-based polymer electrolytes: NMR and conductivity studies

A novel method for the introduction of mesoporous silica MCM-41 modified with functional hydrophilic groups into PEO-based polymer electrolytes is reported. The effect of addition of mesoporous silica MCM-41 with surface modification of (3-glycidyloxypropyl)trimethoxysilane (GLYMO) to poly(ethylene oxide) (PEO) complexed with LiClO4 has been explored by multinuclear solid-state NMR, alternating current (AC) impedance, powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) measurements. The presence of a small quantity (5 ∼ 10 wt.%) of GLYMO modified MCM-41 enhances the ionic conductivity of the resulting composite electrolyte as compared to that of the parent PEO/LiClO4 electrolyte. The enhancement in conductivity is directly correlated with the improved compatibility between PEO and surface modified MCM-41 as a result of blending PEO with GLYMO group. Addition of high concentration (∼ 20 wt.%) of surface modified MCM-41, however, leads to a decrease in the ionic conductivity of the composite electrolyte. 7Li static NMR shows that the 7Li line width narrowing begins at temperature much lower than the glass transition temperature of PEO chains, suggesting the presence of an additional conduction mechanism with lithium ions moving along (both interior and external) the mesoporous channels of MCM-41. This additional conduction mechanism is unique for the composite electrolytes doped with mesoporous silica MCM-41. Variable temperature proton decoupled 7Li NMR spectra reveal that at least two different lithium environments are present in the composite electrolyte, serving as an evidence for the existence of interaction between lithium cation and MCM-41 surface.