The electrochemical stability window (EW) of polyether based salt-in-polymer electrolytes was investigated using density functional theory (DFT) calculations. The electrolyte systems investigated consisted of polyethylene oxide (PEO) in either lithium-bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium-hexafluorophosphate (LiPF6) salt and the EW was determined by performing calculations of reduction and oxidation potentials of isolated ethylene oxide (EO) oligomer and the respective salt species in a continuum solvent. The simulations suggest that the cathodic limit of the polymer-salt system is defined by the reduction potential of the salt anion and that both salt anions considered are unstable against Li anode. The anodic limit is defined by EO and it is stable against most commercial cathodes. Including explicit salt molecules in the calculations shows that the predicted EW is changed by ∼0.4 V. The calculations further reveal that the EW is dependent on the type of the salt molecule. Perfluoropolyether, a perfluorinated analog of PEO that has lower reduction potential and higher oxidation potential in isolation as compared to PEO, improved both oxidation and reduction stability of the polymer-salt system. Substitution of other functional groups to PEO improved the electrochemical stability to potentially accommodate higher voltage window requirements.