A simple visualization method is developed to enable the imaging of the distribution and agglomeration of ceramic fillers in composite polymer electrolytes by scanning electron microscopy (SEM). Solvent-free composite electrolytes based on poly(ethylene oxide) (PEO), LiBF4 and SiO2 particles are used as examples. The SiO2 particles are stained by a heavy element (Pb) to enhance the imaging contrast between the particles and their surroundings. Modified SiO2 from a silanization reaction where most of the surface OH groups has been replaced by Si-phenyl groups is also used to reduce the Lewis acid–base interactions between the ceramic fillers, the polymer and the Li salt. Although nearly the same particle size and particle distribution are found for SiO2 and modified SiO2 in PEO, the two composite electrolyte systems exhibit remarkably different electrochemical transport properties. The difference can, therefore, be unambiguously attributed to the benefit of Lewis acid–base interactions.