Solid lithium-conducting nanocomposite polymer electrolytes based on poly(oxyethylene) (POE) were prepared from high aspect ratio cellulosic whiskers and lithium imide salt, LiTFSI. The cellulosic whiskers were extracted from tunicate -a sea animal- and consisted of slender parallelepiped rods that have an average length around 1 μm and a width close to 15 nm. High performance nanocomposite electrolytes were obtained. The filler provided a high reinforcing effect while a high level of ionic conductivity was retained with respect to unfilled polymer electrolytes. Cross-linking and plasticizing of the matrix as well as preparation of the composites from an organic medium were also investigated.