Lithium-ion batteries based on polymer electrolytes have received much attention due to their potential for creating intrinsically safer and more flexible devices. However, their economic and environmental efficiency is one of the important issues in choosing materials to prepare these electrolytes. To overcome these problems, polymer electrolytes based on natural materials such as cellulose have been used due to their cheapness and availability, abundance, compatibility with the environment, and electron-donating groups in their structure. Also, cellulose modification by polymer is an important factor due to the increase of electron-donating groups and improvement of polymer electrolyte flexibility. In this study, a polymer electrolyte was synthesized by grafting ion-conducting segments of poly(ethylene glycol) methyl ether methacrylate (PEGMA) onto cellulose through reversible addition-fragmentation chain-transfer (RAFT) polymerization. As a result, the increase in the PEGMA content led to enhanced ionic conductivity in both the solid and gel states. In solid-polymer electrolyte (SPE) samples, by increasing the PEGMA percentage from 10:1 (PEGMA/DDMAT) (DDMAT = 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid) to 90:1 (PEGMA/DDMAT), the ionic conductivity was increased from 3.9 × 10–5 to 5.9 × 10–4 S cm–1, whereas some gel-polymer electrolyte (GPE) samples showed ionic conductivity values of 2.5 × 10–4 and 2.4 × 10–3 S cm–1, respectively. Also, the prepared films presented good electrochemical properties, including considerable transference number (t+) in the range of 0.35–0.80, a wide electrochemical stability window higher than 4.5 V, and good specific capacity (>330 mA h g–1 with capacity retention higher than 95% after 100 cycles at 0.2 C of LiCoO2/GPEs-SPEs/Gr).