NaF-rich solid electrolyte interphase for dendrite-free sodium metal batteries

Na metal is a promising candidate as anode material owing to its high theoretical energy density and abundance on earth, but suffers from dendrite growth, extremely large volume change, and poor Coulombic efficiency. Herein, a low-cost and effective strategy is developed to discourage the dendrite growth by in situ formation of an artificial NaF-rich protective layer on Na metal. The protective layer facilitates the formation of highly stable, NaF-rich solid electrolyte interphase (SEI) capable of preventing continuous electrolyte depletion during charge/discharge cycles. The depth profiling experiment confirms functional gradient of SEI through its thickness with wealthier NaF species towards the Na metal. The symmetric cells assembled using the Na anode with a protective layer exhibit excellent cyclic stability with low overpotentials of 8, 50, and 70 mV, at areal currents of 1, 5 and 10 mA cm⁻², respectively, thanks to its high dendrite suppression ability as proven by theoretical calculations. The full battery prepared with a Na₃V₂(PO₄)₃ cathode delivers 99% retention of Coulombic efficiency after 400 and 600 cycles at 1C in ether- and carbonate-based electrolytes, respectively. The SEI layer design strategy presented here can shed some important insights into the development of high-performance dendrite-free Na metal batteries and interface engineering for solid electrolytes.