Red Phosphorous‐Derived Protective Layers with High Ionic Conductivity and Mechanical Strength on Dendrite‐Free Sodium and Potassium Metal Anodes

Abstract Sodium metal anodes are ideal candidates for advanced high energy density Na metal batteries. Nevertheless, the unstable solid electrolyte interphase (SEI), the uncontrollable dendrite growth, and low Coulombic efficiency during cycling have prevented their applications. Herein, a high‐performance Na anode is achieved by introduction of an ex situ artificial Na 3 P layer on the surface via a simple red phosphorus pretreatment method. The artificial SEI layer possesses high ionic conductivity and high Young's modulus, which regulates uniform deposition of ions and prevents the dendrite growth. Benefiting from these merits, the Na||Na cells with the protected layers demonstrate excellent electrochemical performance (780 h at 1.0 mA cm –2 , 1.0 mAh cm –2 ). When assembled into a full battery with a Na 3 V 2 (PO 4 ) 3 cathode, the Na metal battery exhibits a long lifespan of 400 cycles at 15 C and a high rate capacity of ≈53.2 mAh g –1 at 30 C. In addition the red P pretreatment method can be applied to potassium metal anodes. Outstanding performance is also achieved in K||K cells with the formation of a K x P y protecting layer (550 h at 0.5 mA cm –2 , 0.5 mAh cm –2 ). The artificial P‐derived protection approach can also be extended to solid‐state alkali metal batteries with high power density and energy density.