Rationally integrated nickel sulfides for lithium storage: S/N co-doped carbon encapsulated NiS/Cu2S with greatly enhanced kinetic property and structural stability

Nickel sulfides are promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical capacities but suffer from a sluggish kinetic process and poor structural stability. Herein, we develop a rationally integrated strategy for the construction of a S/N co-doped carbon-coated NiS/Cu2S composite (NiS/Cu2S@N/S-C) via a convenient one-step calcination process. Copper can be formed in situ during the lithiation process of the conversion reaction after combining nano-scale nickel sulfide with proper cuprous sulfide, which is conducive to increasing the conductivity, promoting the reversible conversion of the discharge product Li2S, and suppressing the dissolution of polysulfides, leading to the superior kinetic property and structural stability of the electrode. Notably, the S/N co-doped carbon coating with polar C-S or C-N bonds not only enhances the transfer of electrons and ions but also inhibits coarsening, agglomeration of particles and dissolution of polysulfides during lithiation. Furthermore, density functional theory (DFT) calculations show that heterogeneous atomic doping of N and S can enhance the adsorption behavior of carbon sheets to NiS, Cu2S and intermediate discharge products in LIBs, further demonstrating an improvement of lithium storage capability and cycling stability due to the S/N co-doped carbon coating. As a result, NiS/Cu2S@N/S-C possesses a considerable rate capacity of 511.7 mA h g(-1) at 3 A g(-1), and an outstanding stable capacity of 771.7 mA h g(-1) at 2 A g(-1) with a high retention rate of 98% after 1800 cycles. NiS/Cu2S@N/S-C shows remarkable comprehensive performance when compared with most reported nickel sulfide anode materials. This strategy predigests the tedious process of sulfurization and carbon encapsulation and would be of great significance to design nickel sulfide anodes with excellent lithium storage performance.