Quaternary Ammonium Lignin Enabling High‐Na + ‐Conductivity Gel Electrolyte and NaF‐Rich SEI Toward High‐Safety Sodium Metal Batteries

ABSTRACT Sodium metal batteries (SMBs) using gel polymer electrolytes (GPEs) are considered promising candidates for low‐cost, high energy density, and high‐safety energy storage devices. However, a persistent challenge lies in simultaneously achieving high Na + conductivity of GPEs and a stable solid electrolyte interphase (SEI) on a sodium metal anode. Herein, a quaternary ammonium lignin (QAL) incorporated poly(vinylidene fluoride‐co‐hexafluoropropylene) based GPE is designed to synergistically achieve high Na + conductivity and a stable SEI. The electron‐deficient quaternary ammonium groups of QAL strongly anchor anions and promote sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) dissociation, achieving high Na + conductivity. Meanwhile, the π‐conjugated backbone of QAL enables efficient charge transfer from the unreacted region to the reactive site, providing sufficient charge for TFSI − anion decomposition and promoting the formation of a stable NaF‐rich SEI. As anticipated, the Na||Na symmetric batteries based on the designed GPEs demonstrate stable cycling over 5000 h, and the gel‐state Na||Na 3 V 2 (PO 4 ) 3 full batteries deliver high‐capacity retention of 96.9% after 4700 cycles at 10C. The gel‐state pouch full batteries exhibit excellent stability under mechanical abuse (folding, puncture, and cutting) and over a wide temperature range (−20°C to 60°C). This work synergistically achieves high Na + conductivity and a stable NaF‐rich SEI, paving the way for the development of stable and high‐safety SMBs.