A functional self-assembled layer with preferential sodium adsorption sites for ultra-stable sodium metal batteries

Abstract

Generating a resilient and stable solid electrolyte interphase (SEI) is critical to enable dendrite-free sodium (Na) metal anodes; however, sustaining SEI stability at high current densities remains a significant obstacle. Here, an advanced ex-situ structural design composed of Na₂Se and nickel metal (Na₂Se/Ni) is proposed to achieve an interfacial regulating ionic flux system and obstruct dendrite growth. The highly sodiophilic Ni metal provides preferential sites for Na adsorption, preventing fracture and reducing volume expansion, while the Na₂Se component offers pathways to fast migration of Na⁺ through the artificial layer. The designed SEI layer exhibits high mechanical stability, significant ionic conductivity, low nucleation overpotential, and low activation energy, which facilitate the redistribution of Na⁺ flux and promote homogeneous Na deposition. Additionally, theoretical calculations suggest that the Na₂Se/Ni layer can reduce the kinetic barriers for rapid Na⁺ transport, resulting in decreased impedance. Consequently, the symmetric NS@Na cells demonstrate an extended cycle life (3600 h at 2 mA cm⁻²/1 mAh cm⁻², 3900 h at 1 mA cm⁻²/1 mAh cm⁻², and 750 h at 4 mA cm⁻²/1 mAh cm⁻²) with low voltage hysteresis. Furthermore, the full cell, coupled with a Na₃V₂(PO₄)₃ (NVP) cathode, offers an exceptionally long lifespan (5000 cycles) at 5C with a capacity of >90 mAh g⁻¹. This work paves the way for a new and promising approach to stabilizing sodium metal anodes with straightforward and cost-effective interfacial layers.