Naturally superionic polymer electrolyte of macromolecular lignin for all-solid-state sodium-ion batteries at room temperature

Solid polymer electrolytes (SPEs) that offer superior safety, mechanical strength and flexibility are crucial for advancing next-generation sodium-ion batteries (SIBs). Conventional SPEs often display temperature sensitivity, leading to relatively low ionic conductivity at room temperature (RT). Herein, lignin-based SPEs (LG-SPEs) are created by solvation and desolvation of lignin and sodium bis(fluorosulfonyl)imide (NaFSI). Theoretical calculations reveal that lignin (containing rich functional groups) and FSI⁻ molecules facilitate the movement of Na-ions within the electrolyte by minimizing steric hindrance and offering migration sites. Consequently, LG-SPEs demonstrate an enhanced ionic conductivity of 3.4 × 10⁻⁴ S cm⁻¹ at RT, with a Na-ion transfer number as high as 0.53. The assembled all-solid-state SIB comprising Na₃V₂(PO₄)₃/LG-SPE/NaTi₂(PO₄)₃ exhibits excellent electrochemical performance at RT, achieving a specific capacity of 95 mA h g⁻¹ and retaining 82 % of its capacity after 200 cycles at 0.1 C. This work presents an environmentally friendly and straightforward methodology for developing high-performance SPEs at RT, while also opening up new avenues for the valorization of lignin.