The conductivity of poly(ethylene oxide) (PEO) based electrolytes can be enhanced by addition of ceramic fillers. We study the properties of composite host matrices obtained by adding to PEO a lithium triborate glass prepared by sol-gel. By means of X-ray diffraction and C-13 NMR at the magic angle, we show that there is no chemical interaction among the PEO chains and the filler. Lithium cations enter the PEO structure and increase the polymer amorphous phase by creating transient cross-links. The conductivity data are analyzed in terms of the Meyer-Neldel equation.