Nitrogen and Sulfur Vacancies in Carbon Shell to Tune Charge Distribution of Co6Ni3S8 Core and Boost Sodium Storage

Recently, the metal sulfide-carbon nanocomposites have been suggested as a low-cost alternative to lithium ion batteries, but commercial application is seriously hindered by their relatively inferior cyclic performance. Herein, N and S vacancies in an N,S co-doped carbon (NSC) shell for anchoring a new bimetallic sulfide core of Co6Ni3S8 using Co-Ni-alginate biomass are introduced. The obtained Co6Ni3S8/carbon aerogels (Co6Ni3S8/NSCA) exhibit excellent sodium-ion storage properties, high reversible capacity (568.1 mAh g(-1) at 1 A g(-1)), and an excellent cycle stability (94.4% after 300 cycles). Density functional theory calculation results disclose that nitrogen and sulfur vacancies in the carbon shell can enhance the binding between the Co6Ni3S8 core and NSC shell, ensuring an improved structural and electrochemical stability. In addition, an increased adsorption energy of Na+ (-1.88 eV) and a decreased barrier energy for Na+ diffusion (0.46 eV) are observed indicating a fast Na+ diffusion process. The powder X-ray diffraction refinement confirms that the lattice parameters of Co6Ni3S8 extend to 0.9972 nm compared with Co9S8 (0.9928 nm), suppressing the volume expansion in Na+ diffusion processes.