Regulating the Electric Double Layer via Cellulose Acetate Enabling Stable and Long-Lifetime Sodium Metal Batteries

Sodium metal batteries hold great promise for high-energy density and low-cost energy storage. However, notorious sodium (Na) dendritic growth and severe parasitic reactions on the Na metal anode raise safety concerns. Regulating the electric double layer (EDL) structure adjacent to the Na metal anode via electrolyte additives offers an efficient strategy, although it is insufficiently explored. Herein, cellulose acetate (CLA), a biomass-derived natural polymer, was demonstrated as a functional electrolyte additive for EDL regulation. By preferentially adsorbing within the inner Helmholtz plane, CLA molecules competitively coordinate Na⁺ with solvent molecules via abundant acetate groups, displacing reactive desolvated solvents from the anode surface while simultaneously homogenizing Na⁺ flux and facilitating desolvation kinetics within the EDL. This dual role synergistically inhibits solvent decomposition and promotes the uniform Na deposition, thereby suppressing the dendrite growth. Consequently, a long cycling life of nearly 7000 h (∼292 days) at 0.5 mA cm^-2 was achieved for the Na||Na symmetric cell, and a high average Coulombic efficiency of 99.97% over 1200 cycles was delivered at 2 mA cm^-2 for the Na||Al cell. Furthermore, the Na₃V₂(PO₄)₃||Na battery with a CLA-containing electrolyte displayed a high specific capacity of 94 mAh g^-1 and retained 80.4% capacity after 2000 cycles at a high discharge rate of 5 C. This work demonstrates a sustainable EDL regulating strategy using biomass-derived additives to stabilize Na metal anode, offering a versatile approach for manipulating interfacial electrochemistry in next-generation rechargeable batteries.