Low temperature one-step synthesis of carbon co-encapsulated NiS2, NiS and S8 nanocrystals by electrophilic oxidation of nickelocene

• A novel synthesis method for carbon encapsulated nanocrystals is proposed.
• The synthesis temperature and time are only 200 °C and 1 h, respectively.
• Electrophilic oxidation of metallocene is a general route for core–shell structure.
• Graphitizable carbon can form even at 200 °C in an oxidation environment.
• The NiS2&NiS@C electrodes show good cycling behavior and rate capability.

A novel method is developed for the synthesis of carbon co-encapsulated near-equiaxed NiS2, NiS and S8 nanocrystals with core–shell structure by a one-step solid state reaction of nickelocene with ammonium persulphate at 200 °C in an autoclave. These nanocrystals have a narrow size distribution ranging from 5 to 19 nm and the median size is 10.99 nm. Both amorphous and graphitizable carbon shell are formed, encapsulating hundreds of nanocrystals as a whole and providing exclusive rooms for these nanocrystals separated from each other. The interaction of nickel with cyclopentadienyl ligands through π bond results in the formation of core–shell structure. The carbon encapsulated NiS2 and NiS nanocrystal anodes retain a reversible capacity of 517 and 326 mA h/g after 50 cycles at a current density of 208 and 1480 mA/g, respectively.