Hierarchical Nanocellulose‐Based Gel Polymer Electrolytes for Stable Na Electrodeposition in Sodium Ion Batteries

Abstract Sodium ion batteries (NIBs) based on earth‐abundant materials offer efficient, safe, and environmentally sustainable solutions for a decarbonized society. However, to compete with mature energy storage technologies such as lithium ion batteries, further progress is needed, particularly regarding the energy density and operational lifetime. Considering these aspects as well as a circular economy perspective, the authors use biodegradable cellulose nanoparticles for the preparation of a gel polymer electrolyte that offers a high liquid electrolyte uptake of 2985%, an ionic conductivity of 2.32 mS cm −1 , and a Na + transference number of 0.637. A balanced ratio of mechanically rigid cellulose nanocrystals and flexible cellulose nanofibers results in a mesoporous hierarchical structure that ensures close contact with metallic Na. This architecture offers stable Na plating/stripping at current densities up to ±500 µA cm −2 , outperforming conventional fossil‐based NIBs containing separator–liquid electrolytes. Paired with an environmentally sustainable and economically attractive Na 2 Fe 2 (SO 4 ) 3 cathode, the battery reaches an energy density of 240 Wh kg −1 , delivering 69.7 mAh g −1 after 50 cycles at a rate of 1C. In comparison, Celgard in liquid electrolyte delivers only 0.6 mAh g −1 at C/4. Such gel polymer electrolytes may open up new opportunities for sustainable energy storage systems beyond lithium ion batteries.