Ultra-Stable Anode-Free Na Metal Batteries Enabled by Al2O3-Functionalized Separators

The development of anode-free sodium metal batteries (AFSMBs) offers a promising pathway to achieve ultrahigh energy density and cost efficiency inherent to conventional sodium ion/metal batteries. However, irreversible Na plating/stripping and dendritic growth remain critical barriers. Herein, we demonstrate that separator engineering is a pivotal strategy for stabilizing AFSMBs. Through systematic evaluation of four separators—2500 separator (PP), 2325 separator (PP/PE/PP), glass fiber (GF), and an Al2O3-coated PE membrane, we reveal that the Al2O3-coated separator uniquely enables exceptional interfacial kinetics and morphological control. Na||Na symmetric cells with Al2O3 coated separator exhibit ultralow polarization (4.5 mV) and the highest exchange current density (1.77 × 10−2 mA cm−2), while the anode-free AlC-NFPP full cells retain 91.6% capacity after 150 cycles at 2C. Specifically, the Al2O3 coating homogenizes Na+ flux, promotes dense and planar Na deposition, and facilitates near-complete stripping with minimal “dead Na”. This work establishes ceramic-functionalized separators as essential enablers of practical high-energy AFSMBs.