context: Herein, we fabricated two artificial SEIs on Na metal anodes via similar chemical replacement reactions between Na metal and SnCl4 liquid or SnCl2 additive dissolved in diethylene glycol dimethyl ether (DGM). According to X-ray photoelectron spectroscopy (XPS) depth-profiling results, the SnCl4 liquid treatment leads to a uniform and high ionic diffusion artificial SEI consisting of a simultaneously generated Na–Sn alloy and NaCl, whereas the SnCl2 additive results in a disordered and heterogeneous artificial SEI (Na–Sn alloy, NaCl, Sn metal, organo-chloride, ROCO2Na and RCH2ONa) due to the heterogeneous reaction between the Na metal anode and SnCl2/DGM solvent. It is found that the cycling performance of the SnCl4–Na electrodes is highly superior to that of the SnCl2–Na electrodes, which is mainly ascribed to the uniformity and ionic diffusion distinction of the two artificial SEIs. The importance of homogeneous and high ionic diffusion properties for the artificial SEI is also demonstrated by simulation results. Overall, the artificial SEI of SnCl4–Na electrodes can afford three key advantages: (i) the simple and controllable fabrication leads to a homogeneous artificial SEI consisting of uniformly distributed Na–Sn alloy and NaCl, avoiding the non-uniform products caused by the side reactions that commonly exist in electrolyte additive methods; (ii) both Na–Sn alloy and NaCl possess much higher diffusion coefficients than organic components (ROCO2Na and RCH2ONa) and conventional SEIs, enabling fast ion diffusion and restraining the initiation of Na dendrites; (iii) the high Young's moduli of the Na–Sn alloy and NaCl can also help suppress the growth of Na dendrites. With the above excellent properties, the SnCl4–Na electrode provides an ultralong cycling stability with a stable voltage polarization (∼100 mV) for 4000 h with a cycling capacity of 3 mA h cm−2 at 2 mA cm−2. Moreover, the SnCl4–Na electrode also shows excellent cycling stability for ∼1500 h even under rather tough conditions (5 mA h cm−2, 5 mA cm−2). In addition, benefiting from the durable SEI layer, a SnCl4–Na|FeS2 full cell can deliver a stable capacity of ∼350 mA h g−1 for 380 cycles.
answer: Na metal