Bimetal Synergistic Effect Induced High Reversibility of Conversion-Type Ni@NiCo2S4 as a Free-Standing Anode for Sodium Ion Batteries

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 freestanding 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.