The present invention provides a process for the preparation of solid polymer electrolytes using ionic liquids useful for electrochromic devices or electrochromic windows, which used an insitu polymerized solid polymer electrolyte encompassing an ionic liquid. Therefore the electrolyte is free from leakage, corrosion, side reaction and moisture sensitivity. The synthesis of polymeric electrolyte comprising of Ionic liquid containing at least one of the anions selected from the group of trifluoromethylsulfonate (CF3SO3−), bis(trifluoromethylsulfonyl)imide [(CF3SO2)2N−] and bis(perfluoroethylsulfonyl)imide [(C2F5SO2)2N−], while varying the alkyl chain of the imidazolium ring is described and a simple method for preparing Ionic liquids has also been discussed. A method for preparing substantially pure molten salts i.e. ionic liquids is also described. Another objective of the present invention is to provide an electrochromic device utilizing these systems.