Conversion-type
anode materials for sodium ion batteries have received
extensive attention because of their relatively high theoretical capacity.
However, multiple challenging obstacles stand in the way of their
commercial application, especially their poor cycling stability resulting
from the bad reversibility of the conversion reaction. Herein, Ni–Co
bimetal sulfide was selected and investigated with the goal of improving
the reversibility of the conversion reaction owing to the similarity
of Ni and Co. As expected, when three-dimensional hierarchical Ni@NiCo2S4 (NiCo2S4 nanowires growing
on the Ni foam) was applied as the free-standing anode for sodium
ion batteries, it demonstrated high capacity and excellent cycling
stability (90.65%, 100 cycles) compared with those of monometallic
sulfides. Various characterization [in situ X-ray diffraction (XRD),
ex situ XRD, ex situ X-ray photoelectron spectroscopy, FESEM mapping,
and high-resolution transmission electron microscopy] tests confirmed
that the Ni–Co alloy was formed during the discharge process
and effectively prevented the crystalline grain growth of conversion
reaction products, improving the reaction kinetics and reversibility.