In-plane isotropic separator-induced highly efficient sodium plating for unlocking the fast-charging capability of anode-free sodium battery at practical conditions

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.