Bilayer-Separator Strategy Facilitates Uniform and Dense Na Deposition for Stable Anode-Less Sodium Metal Batteries Revealed by EPR imaging

The uneven Na deposition inducing generation of Na dendrites and dead Na, as well as depletion of electrolyte, has been regarded as the biggest obstacle in commercializing anode-less sodium metal batteries (SMBs). This work reveals that the separator plays a critical role in the Na deposition behavior by providing mechanical confinement for Na growth and pathways for Na-ion transport. Conventional polyolefin and glass fiber separators, however, suffer from severe short-circuiting issues. To address this limitation, we propose a novel bilayer-separator strategy for anode-less SMBs. This approach enables long-term cycling with both high Coulombic efficiency and minimal volatility. Furthermore, mechanism research led by electron paramagnetic resonance (EPR) imaging demonstrates that the bilayer-separator strategy facilitates uniform and dense Na plating by creating an evenly pressurized and self-adaptive deposition zone. Additionally, a new “standing corrosion” electrochemical characterization method and direct uneven-Na-supplementation technique have also been successfully developed.