Abstract In this work a systematic study of the structure and thermal properites of poly(ethylene oxide) (PEO) based nanocomposites was performed in order to design nanocomposite polymer electrolyte with enhanced ionic conductivity. PEO based nanocomposites with different nanofiller type (sodium montmorillonite, commercially available organoclays Cloisite 10A, Cloisite 15A, Cloisite 20A, Cloisite 25A, Cloisite 93A i Cloisite 30B) were prepared by melt intercalation method. The role of nanofiller is to reduce crystallinity and increase the extent of the amorphous phase of the polymer matrix in order to improve the transfer of cations and ionic conductivity without disruption other properties. Considering the results of structural and thermal characterization, an assessment of the most favourable nanocomposite system of polymers and nanofillers was made for subsequently introduction of Li + ion. The results of the Small angle X-ray scattering (SAXS) analysis showed that PEO was intercalated into montmorillonite layers in all tested samples, which confirmed the formation of nanocomposite structure. Fourier transform infrared spectroscopy (FTIR) analysis showed the existence of an interaction of PEO and nanofiller for all examined nanocomposites. Change in the crystallinity of PEO and distorted helical structure of poly(ethylene oxide) macromolecules in nanocomposites additionally confirmed the formation of intercalated structure. Differential scanning calorimetry (DSC) revealed the influence on thermal transition (glass transition temperature, melting temperature, crystallization temperature) and crystallinity of PEO. The nanofiller NaMMT showed the most pronounced effect on lowering the degree of crystallinity of PEO in nanocomposites. Organically modified montmorillonites at low filler loadings act as nucleation agents that consequently increase the degree of crystallinity of PEO. The non-isothermal thermogravimetric (TG) analysis showed that NaMMT improves PEO thermal stability up to a certain proportion. Organically modified montmorilonites in amounts higher than 10 wt. % strongly reduce the onset degradation temperature. The most pronounced influence was observed in Cloisite 10A based nanocomposites, where the onset degradation temperatures are reduced up to 198 °C compared to the neat PEO. Thermal and structural characterization of above mentioned nanocomposites indicated the greatest potential of the NaMMT as nanofiller and therefore it was selected for the introduction of Li + ion and further analysis. The introduction of lithium ion into the system was carried out by modification of NaMMT by ion exchange method with LiCl resulting in LiMMT. The influence of LiMMT on the structure and thermal properties of PEO/LiMMT nanocomposites prepared with PEO of different molecular weight and the dependence of the results obtained on ionic conductivity have been studied. SAXS and FTIR analysis confirmed the formation of nanostructures in all prepared PEO/LiMMT nanocomposites. The results of the FTIR analysis revealed a more significant interaction of Li + ions with a polymer matrix in nanocomposite prepared with PEO of the high molecular weight compared to those prepared with PEO of the low molecular weight. DSC analysis revealed that the addition of LiMMT significantly reduces the glass transition and the degree of crystallinity of PEO, which created important precondition for improving the ionic conductivity. Polymer nanocomposites prepared with PEO of the higher molecular weights showed lower values of the degrees of crystallinity. The results of non-isothermal TG analysis revealed that the addition of LiMMT reduce the thermal stability of PEO, but still satisfies the thermal stability required for solid polymer electrolyte. The results of non-isothermal TG analysis show that LiMMT affects the mechanism of non-isothermal degradation of PEO. The effect of the nanocomposite structure and the molecular weight of PEO on the potential to improve the ionic conductivity of PEO/LiMMT nanocomposites was determined by electrochemical impedance spectroscopy (EIS). The addition of LiMMT improves the ionic conductivity of all tested PEO/LiMMT nanocomposites. The ionic conductivity depends on the weight fraction of LiMMT in the prepared nanocomposites and the molecular weight of PEO. The highest value of ionic conductivity at room temperature was noted in a nanocomposite prepared with PEO with a molecular weight of 300,000 and 40 wt. % LiMMT. The obtained ionic conductivity was 2,8·10 -6 S cm -1 . The obtained result shows an increase in ionic conductivity for 949 times compared to ionic conductivity of pure PEO. For the PEO/LiMMT series with the highest ionic conductivity, kinetic analysis was performed and kinetic models of the non-isothermal thermal degradation were developed. Based on the results of the kinetic analysis, it was confirmed that the addition of LiMMT affects the mechanism of the non-isothermal thermal degradation of PEO. Unlike the one-step process of non-isothermal decomposition of PEO, the decomposition of PEO3/LiMMT nanocomposites takes place through three stage mechanism: diffusion, n-order reaction with autocatalysis and n-order reaction.