Oil bath chelation-assisted fabrication of nitrogen-doped carbon-coated Ni9S8/Ni3S2 composites for lithium-ion batteries

Dimethylformamide (DMF) and polyvinylpyrrolidone (PVP) were chosen as precursors for the synthesis of a carbon-coated and fully nitrogen-doped Ni₉S₈/Ni₃S₂ nanocomposite denoted as N-NiS-X, which was successfully prepared through a simple oil bath chelation process followed by annealing. The N-NiS-2 electrode revealed optimal electrochemical performance with a sulfur addition of 18.6 mmol. The synthesized composite demonstrated a first-cycle discharge capacity of 1151.3 mAh·g⁻¹ at 50 mA·g⁻¹, with initial Coulombic efficiency measuring 64.4%. Following 500 cycles of galvanostatic charge–discharge testing at 0.5 A·g⁻¹, this prepared electrode maintained 110.1% of its original capacity, which suggested superior kinetic characteristics during electrochemical processes. Electrochemical impedance analysis further demonstrated a reduction in the solution resistance and charge transfer resistance to 5.17 and 32.46 Ω, respectively, highlighting enhanced charge transport capabilities. Consequently, the dual roles of in situ nitrogen doping and carbon coating, which effectively suppress the volume expansion effect of Ni_xS_y, are realized by DMF and PVP as nitrogen and carbon sources, respectively. These functionalities markedly improve the structural integrity and electrical conductivity of materials, thereby highlighting their substantial prospects for commercial applications.