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

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.