Interface and Substrate Design Enhance the Stability of Sodium Metal Anodes

Sodium metal batteries are plagued by the uncontrollable growth of sodium dendrites, which leads to short circuits and limits their practical applications. This work enhances the stability of the sodium metal anode through a combined strategy of constructing the solid electrolyte interphase (SEI) and modifying the current collector with hard carbon materials for reversible Na deposition. Therefore, the combination of these two strategies could enable a stable Na metal anode in a gel electrolyte. Through this methodology, we could achieve a stable anode-less battery. SnF₂ initiates the in situ polymerization of tetrahydrofuran (THF) and participates in constructing a fluoride- and Na_xSn_y-rich SEI. Consequently, the PTHF electrolyte demonstrates a high Na⁺ transference number of 0.88, excellent oxidative stability with an electrochemical window extended to 4.5 V, and remarkable cycling stability, enabling a Na||Na symmetric cell operated for 1700 h at a current density of 0.5 mA cm^{- 2}. Furthermore, the introduction of micron-sized hard carbon (µHC) further enhances the sodiophilicity of the current collector. Consequently, an anode-less sodium metal battery assembled with the µHC substrate and PTHF gel electrolyte exhibits excellent long-term cycling stability, retaining 80% of its capacity after 180 cycles at room temperature and 84% after 200 cycles at -10°C.