| Abstract: | Sodium‐ion batteries have been considered one of the most promising power sources beyond Li‐ion batteries. Although the Na metal anode exhibits a high theoretical capacity of 1165 mAh g?1, its application in Na batteries is largely hindered by dendrite growth and low coulombic efficiency. Herein, it is demonstrated that an electrolyte consisting of 1 m sodium tetrafluoroborate in tetraglyme can enable excellent cycling efficiency (99.9%) of a Na metal anode for more than 1000 cycles. This high reversibility of a Na anode can be attributed to a stable solid electrolyte interphase formed on the Na surface, as revealed by cryogenic transmission electron microscopy and X‐ray photoelectron spectroscopy (XPS). These electrolytes also enable excellent cycling stability of Na||hard‐carbon cells and Na||Na2/3Co1/3Mn2/3O2 cells at high rates with very high coulombic efficiencies. |
