Synthesis of carbon-coated FeSe ₂ nano-microspheres with a stable SEI film as anode materials for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) have recently gained significant interest for grid-scale energy storage due to sodium's abundance and their low cost, environmental friendliness, and similarities to lithium-ion batteries (LIBs). Nevertheless, the widespread application of SIBs remains severely constrained by their inadequate rate capability and suboptimal cycling durability-fundamental challenges stemming from the larger ionic radius of Na⁺. This inherent characteristic not only compromises the structural integrity of electrode materials but also leads to significantly sluggish reaction kinetics, thereby hindering efficient energy conversion processes. Herein, N-doped carbon-coated FeSe₂ (FeSe₂@NC) nano-microspheres have been synthesized <i>via</i> solvothermal and selenization techniques. The engineered anode material demonstrates exceptional cycling durability (376.6 mAh g^-1 retention at 5 A g^-1 over 2000 cycles) coupled with practical viability in a full-cell configuration (322.0 mAh g^-1 at 2 A g^-1 when paired with a Na₃V₂(PO₄)₃ cathode), collectively demonstrating considerable promise for large-scale energy storage solutions. In-depth investigation employing <i>ex situ</i> X-ray photoelectron spectroscopy combined with electrochemical measurements demonstrates that the enhanced sodium storage capability of FeSe₂ is attributed to the formation of a stable solid electrolyte interphase, improved Na⁺ diffusion kinetics, and increased electronic conductivity. Distinct from conventional FeSe₂/C composites, the unique nano-microsphere architecture developed herein establishes a more coherent conductive network with uniform carbon encapsulation, enabling efficient volume buffering and enhanced interfacial kinetics, highlighting the morphological merit of our synthesis route for advanced FeSe₂ anodes.