Flower-like NiCo₂S₄ nanosheets with high electrochemical performance for sodium-ion batteries

A three-dimensional flower-like NiCo₂S₄ formed by two-dimensional nanosheets is synthesized by a facile hydrothermal method and utilized as the anode for sodium-ion batteries. Studies have shown that materials can achieve the best performance under the ether-based electrolyte system with voltage ranging from 0.3 to 3 V, which could effectively avoid the dissolution of polysulfides and over-discharge of the material. Here, sodium storage mechanism and charge compensation behaviors of this ternary metal sulfide are comprehensively investigated by ex situ X-ray diffraction. Moreover, ex situ Raman spectra, ex situ X-ray photoelectron spectroscopy and transmission electron microscopy measurements are used to related tests for the first time. Additionally, quantitative kinetic analysis unravels that sodium storage partially depends on the pseudocapacitance mechanism, resulting in good specific capacity and excellent rate performance. The initial discharge capacity is as high as 748 mAh·g^-1 at a current density of 0.1 A·g^-1 with the initial coulomb efficiency of 94%, and the capacity can still maintain at 580 mAh·g^-1 with the Coulomb efficiency close to 100% after following 50 cycles. Moreover, by the long cycle test at a high current density of 2 A·g^-1, the capacity can still reach at 376 mAh·g^-1 after over 500 cycles.