Rational Design of an Artificial SEI: Alloy/Solid Electrolyte Hybrid Layer for a Highly Reversible Na and K Metal Anode.

The practical application of a Na/K-metallic anode is intrinsically hindered by the poor cycle life and safety issues due to the unstable electrode/electrolyte interface and uncontrolled dendrite growth during cycling. Herein, we solve these issues through an reaction of an oxyhalogenide (BiOCl) and Na to construct an artificial solid electrolyte interphase (SEI) layer consisting of an alloy (NaBi) and a solid electrolyte (NaOCl) on the surface of the Na anode. As demonstrated by theoretical and experimental results, such an artificial SEI layer combines the synergistic properties of high ionic conductivity, electronic insulation, and interfacial stability, leading to uniform dendrite-free Na deposition beneath the hybrid SEI layer. The protected Na anode presents a low voltage polarization of 30 mV, achieving an extended cycling life of 700 h at 1 mA cm in the carbonate-based electrolyte. The full cell based on the NaV(PO) cathode and hybrid SEI-protected Na anode shows long-term stability. When this strategy is applied to a K metal anode, the protected K anode also reaches a cycling life of over 4000 h at 0.5 mA cm with a low voltage polarization of 100 mV. Our work provides an important insight into the design principles of a stable artificial SEI layer for high-energy-density metal batteries.