Ether-based electrolytes with excellent reductive stability are compatible with sodium (Na) metal anodes, which enables stable cycling for Na metal batteries even in an anode-free configuration. However, the practical applications of anode-free sodium batteries (AFSBs) with a high theoretical energy density are restricted by the low-rate capability and limited cycle life. Here we demonstrate that the mechanical properties of the separators, which have been overlooked in previous studies, can significantly affect the cycling stability of AFSBs due to the intrinsic softness of Na and the large volume variation of AFSBs during Na plating/stripping. By using various separators including polypropylene (PP), polyethylene (PE), PP/PE/PP tri-layer, and aluminum oxide-coated separators, we find that the balanced elastic moduli of the separator along the machine direction and transverse direction are crucial for enabling highly efficient Na plating and unlocking the 4 C fast-charging capability of the AFSBs at practical conditions including a high cathode active mass loading (13.5 mg/cm²), lean electrolyte addition (8.8 μL/cm²), and no pre-sodiation process. This study provides an important separator design principle for the development of high-rate and long-cycle-life AFSBs.

Lithium-oxygen (Li-O₂) batteries have a great potential in energy storage and conversion due to their ultra-high theoretical specific energy, but their applications are hindered by sluggish redox reaction kinetics in the charge/discharge processes. Redox mediators (RMs), as soluble catalysts, are widely used to facilitate the electrochemical processes in the Li-O₂ batteries. A drawback of RMs is the shuttle effect due to their solubility and mobility, which leads to the corrosion of a Li metal anode and the degradation of the electrochemical performance of the batteries. Herein, we synthesize a polymer-based composite protective separator containing molecular sieves. The nanopores with a diameter of 4 Å in the zeolite powder (4A zeolite) are able to physically block the migration of 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) molecules with a larger size; therefore, the shuttle effect of TEMPO is restrained. With the assistance of the zeolite molecular sieves, the cycle life of the Li-O₂ batteries is significantly extended from ~ 20 to 170 cycles at a current density of 250 mA·g⁻¹ and a limited capacity of 500 mAh·g⁻¹. Our work provides a highly effective approach to suppress the shuttle effects of RMs and boost the electrochemical performance of Li-O₂ batteries.