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In this study, we investigated the effect of nanostructured Al2O3 particles on Li ion conducting, poly(ethylene oxide) (PEO)-based membranes prepared by electrospinning, solution casting and hot pressing. Pure PEO:LiBF4 solid polymer electrolytes (SPEs) and also plasticizer containing membranes were investigated with various amounts of Al2O3. In a first step, the best-performing composition of pure PEO:LiBF4 concerning the resulting ionic conductivity was identified and used as a standard for further experiments. In the following, the influence of the preparation method, the nature of the Al2O3, and the type of the plasticizer additives on the thermal and electrochemical properties for this standard composition were investigated. The Al2O3 composition was varied between 1 and 5 wt%. The ionic conductivity of bare electrospun PEO:LiBF4 SPE standard material has been improved by a factor ten to 1.9 × 10−6 S cm−1 at T = 293 K when 5 wt% of Al2O3 is added. For solution-casted PEO:LiBF4 standard compositions 18:1 with an initial ionic conductivity of 6.7 × 10−8 S cm−1, the addition of 2 wt% Al2O3 increased the performance to 1.4 × 10−7 S cm−1, both at T = 293 K. If succinonitrile and Al2O3 was admixed to the solution casted standard material, the ionic conductivity was further increased to reach 5.5 × 10−5 S cm−1 at T = 293 K. This material with a composition of 18:3:1 + 2 wt% Al2O3, outperforms the standard material by three orders of magnitude. [ABSTRACT FROM AUTHOR]