Nitrogen-doped carbon shell-confined Ni3S2 composite nanosheets derived from Ni-MOF for high performance sodium-ion battery anodes

Highlights: • Ni₃S₂ embedded in N-doped carbon nanosheets exhibit superior cycling and rate performances as SIBs anodes. • The method of constructing N-doped carbon shell by PPy contributes to design SIBs anodes with enhancing conductivity. • The nano-sized structure accelerates reaction kinetics and minimizes stress/strain caused by volume changes. • The N-doped carbon nanosheets connect the transmission pathway of the electrons, and buffer the volume change. -- Abstract: Nickel sulfides are suitable anode materials for sodium-ion batteries (SIBs) because of their moderate cost and high theoretical capacity. Nevertheless, low cycling stability and rate performance caused by volume expansion and inferior electronic conductivity during the charge/discharge process still hamper their development. Herein, Ni₃S₂ nanoparticles uniformly embedded in N-doped carbon nanosheets (Ni₃S₂@C) are synthesized by sulfuration of a Ni-based metal-organic framework (Ni-MOF), followed by coating with polypyrrole (PPy). The PPy protects Ni₃S₂ particles from aggregation, and is converted to N-doped carbon shell during the annealing process. The nano-sized structure accelerates reaction kinetics and minimizes stress/strain caused by volume changes. The N-doped carbon nanosheets connect the transmission pathway of the electrons, and buffer the volume change during the electrochemical reaction. As expected, NiSx@C-600 delivers superior sodium storage performance with a high discharge capacity of 432.8 mA h g⁻¹ at 0.2 A g⁻¹ over 100 cycles and remarkable rate capacity of 371.6 mA h g⁻¹ at a high rate of 6.4 A g⁻¹. To the best of our knowledge, the obtained nanosheets exhibit the best rate performance among current Ni₃S₂ composites. The proposed method for enhancing conductivity and doping heteroatom by using PPy provides a novel insight to design SIB anodes with superior performance.