Molecular engineering of renewable cellulose biopolymers for solid-state battery electrolytes

As the most abundant and renewable biopolymer, cellulose has found applications in a range of fields such as healthcare, packaging, electronics and environmental remediation, contributing to the transition towards sustainability. Here we apply a green and scalable process transforming cellulose to a robust electrolyte exhibiting lithium (Li) ion conductivity of 1.09 × 10⁻³ S cm⁻¹ with a transference number of 0.81 and mechanical strength of 12 MPa. Our process takes advantage of the rich hydroxyl groups in the cellulose which are replaced by phthalic anhydride through an esterification reaction to form cellulose phthalate (CP). Combined experimental and theoretical analyses reveal that the introduction of phthalate groups is essential to not only ensure effective multi-oxygen interaction with Li ions to create fast ion transportation channels, but also facilitates the intermolecular hydrogen bond responsible for the impressive mechanical properties. The CP biopolymer film is even compatible with most commercial cathode materials, and our solid-state Li/CP/LiFePO₄ cells show better performance and notably good stability over 1,000 cycles than that of a baseline Li-ion cell with a flammable organic liquid electrolyte. Our study unlocks the enormous potential of cellulose utilization in batteries and opens an avenue for the development of abundant and sustainable solid-state electrolytes.