Tailoring Na-ion flux homogenization strategy towards long-cycling and fast-charging sodium metal batteries

Sodium metal batteries (SMBs) are promising candidates for next-generation energy storage devices owing to their excellent safety performance and natural abundance of sodium. However, the insurmountable obstacles of dendrite formation and quick capacity decay are caused by an unstable and inhomogeneous solid electrolyte interphase that resulted from the immediate interactions between the Na metal anode and organic liquid electrolyte. Herein, a customised glass fibre separator coupled with chitosan (CS@GF) was developed to modulate the sodium ion (Na⁺) flux. The CS@GF separator facilitates the Na⁺ homogeneous deposition on the anode side through redistribution at the chitosan polyactive sites and by inhibiting the decomposition of the electrolyte to robust solid electrolyte interphase (SEI) formation. Multiphysics simulations show that chitosan incorporated into SMBs through the separator can make the local electric field around the anode uniform, thus facilitating the transfer of cations. Na|Na symmetric cells utilising a CS@GF separator exhibited an outstanding cycle stability of over 600 h (0.5 mA cm⁻²). Meanwhile, the Na|Na₃V₅(PO₄)₃ full cell exhibited excellent fast-charging performance (93.47% capacity retention after 1500 cycles at 5C). This study presents a promising strategy for inhibiting dendrite growth and realizes stable Na metal batteries, which significantly boosts the development of high-performance SMBs.