The advance of nickel-cobalt-sulfide as ultra-fast/high sodium storage materials: The influences of morphology structure, phase evolution and interface property

Numerous interests have been captured for bimetallic NiCo2S4 ascribed to its excellent electrical conductivity, whilst its sluggish sodium-ion kinetics at high-rate limits the advancement of reversible sodium storage. Herein, NiCo2S4 nanodots (similar to 9 nm) uniformly incorporated with N-doped carbon are prepared (NiCo2S4@NC) through bottom-up strategy from 0D to stable structure. Considering that the suitable ether-based electrolyte (NaCF3SO3/DEGDME) may well promote faster sodium-ion transportation due to flexible one-dimensional chain structure and favorable solvent-salt interaction, and the optimal voltage region (0.4-3.0 V) could effectively successfully sidestep the side reaction and maintain reversible phase transformation. Such multifactors tuned NiCo2S4@NC offers remarkable electrochemical performance as anode for SIBs. It delivers a stable capacity of 570.1 mAh g(-1) after 200 cycles at 0.2 A g(-1), and still retains 395.6 mAh g(-1) at 6.0 A g(-1) after 5,000 loops. Significantly, the mechanism and dynamics explorations by cyclic voltammetry (CV) profoundly reveal the dominant surface-capacitive behaviors of NiCo2S4@NC. A suite of in-situ electrochemical impedance spectroscopy (EIS) analyses further explore the regular dual-interface resistances of NiCo2S4@NC during the sodiation/desodiation process, corresponding to the reversible phase evolution and stable carbon matrix. This systematic study establishes a firm foundation for the later research of TMDs as excellent energy-storage anode materials for SIBs.