The modification of properties of poly(ethylene oxide) (PEO) was performed by adding different content of salt lithium bis(oxalate)borate to polymer blend PEO/NaAlg with the aim of improving conductivity of solid polymer electrolyte (SPE) due to the lowering of its crystalline phase. Only some films of polymer blend PEO/NaAlg complexed with lithium bis(oxalate)borate have been successfully prepared by solution cast technique. Fourier transform infrared spectroscopy (FT-IR) confirmed the interaction between the ether group of PEO and the Li + cation from LiBOB. Shifts of certain vibration bands and changes in their intensities were observed. Differential scanning calorimetry (DSC) has been applied to investigate the influence of lithium bis(oxalate)borate on the thermal properties and crystallinity of PEO. The addition of salt has a effect on the thermal proporties of PEO while reducing its crystalinity, which is most pronounced in the composite with the highest salt content. By using non-isothermal thermogravimetry (TG), the effect of added salt on thermal stability of PEO/NaAlg/LiBOB composite was investigated. PEO/NaAlg-LiBOB composites degrades throught maximum eight degradation steps. The salt impairs the thermal stability of the initial polymer blend, with the influence of salt being most pronounced at the highest salt content in the composite.