Interfacial modulation with homogeneous gallium phosphide protective layer enables dendrite-free and superior stable sodium metal anode

Sodium metal is heralded as a premier anode candidate poised to supplant lithium in next-generation rechargeable batteries due to its abundant availability, cost-effectiveness, and superior energy density. Due to the highly reactive nature of metallic sodium, an unstable solid electrolyte interphase (SEI) forms spontaneously on the Na metal anode. This instability leads to non-uniform sodium deposition during cycling, promoting dendrite growth and the accumulation of “dead” sodium. As a result, the cycling lifespan is significantly reduced, creating further complications. Herein, a facile in situ artificial interfacial layer of gallium phosphide (GaP) has been successfully constructed on the surface of sodium metal via a one-step method. This novel GaP protective layer, uniformly and densely distributed, effectively mitigates the instability of the sodium metal anode during the stripping/deposition process, resulting in enhanced structural integrity and the absence of dendritic growth. The Na/GaP symmetric cell exhibits low polarization voltage and a decreased energy barrier for Na⁺ diffusion during cycling, enabling stable operation for over 1200 h at a current density of 0.5 mA cm⁻² (1 mAh cm⁻²). The inhibitory effect of the GaP interfacial layer on dendrite formation and the uniform deposition enhancement during the stripping and deposition processes of sodium metal anode were verified through in situ optical microscopy, along with complementary ex situ scanning electron microscope (SEM) and x-ray photoelectron spectroscopy (XPS) characterizations. After 1100 cycles at a high current rate of 5 C, a full cell made with a Na₃V₂(PO₄)₃ (NVP) cathode and a Na/GaP anode exhibits a reversible capacity of 90 mAh g⁻¹. The NVP||Na/GaP complete cell also produces a remarkable energy density of 352.2 Wh kg⁻¹. This work offers unique insights for the facile construction of mono-component sodium metal anode interfacial coatings and their related applications.