Silicon (Si) is expected to become the anode material of the next generation of high-performance lithium-ion batteries because of its high specific capacity and rich resources. However, its poor conductivity and serious volume change in the cycle limit its further application. Carbon materials provide silicon with good conductivity and structural stability, so Si/C composites have been widely studied. However, the carbon layer is difficult to with stand the volume change in a long cycle, and the conductivity of carbon is low and unevenly distributed, resulting in low utilization of silicon. As a functional material, vanadium nitride (VN) has high conductivity (1.67×106 S·m-1), good chemical inertia and mechanical stability. The introduction of high conductivity vanadium nitride into silicon carbon composites can improve the electron and ion conduction rate of silicon anode materials and enhance their rate performance. Vanadium nitride with high strength and good mechanical stability can be used as a structural buffer to further effectively alleviate the volume and structural changes and improve the cycle stability of the composites. Vanadium nitride/nano silicon/carbon composite microspheres (Si@VN/C) were prepared by spray drying, carbonization and nitridation. Firstly, commercial nano silicon, NH4VO3 and corn starch were fully mixed in deionized water and dispersed by ultrasound, and then the mixed microsphere precursor was synthesized by spray drying. Then, pre-carbonization, carbonization and nitridation treatment was carried out in a protective atmosphere to decompose corn starch and NH4VO3 to form VN and obtain the final product Si@VN/C microspheres. By designing a unique micro sphere structure, VN and nano silicon were dispersed in the carbon layer. When Si@VN/C microsphere was used as the anode of lithium-ion battery, VN could further improve the conductivity of silicon, effectively restrict the volume change of Si microsphere, and then improve its electrochemical performance. For comparison, the product without NH4VO3 was recorded as Si@C. Using scanning electron microscope (SEM) and transmission electron microscope (TEM), it could be observed that Si@VN/C was a relatively complete three-dimensional spherical structure. When the surface of the microsphere was locally enlarged, it showed that the microsphere was composed of nano Si particles with a particle size of 50~100 nm, and a layer of VN and thin carbon layer with a thickness of about 20 nm were distributed on the surface of the nano Si particles. The thin carbon layer was observed by high resolution TEM (HRTEM). It showed that there were a large number of dark crystal areas in the thin carbon layer, and the lattice distance of 0.342 nm corresponded to the d-spacing of (220) planes of crystalline VN, which proved the successful formation of VN. Energy dispersive spectrometer (EDS) mapping results of each element proved the uniform dispersion of Si and the uniform mixing of each component in precursor material. The appearance of V and N elements proved the successful introduction of V and the in-situ formation of VN. There was a small amount of O element in the material, indicating that the surface of the material was slightly oxidized. X-ray diffraction (XRD) pattern of Si@VN/C could obtain obvious Si crystal diffraction peaks. Raman spectrum of Si@VN/C showed the crystallization peak of Si near 520 cm-1. The two weak peaks at 260 and 930 cm-1 corresponded to two amorphous Si peaks respectively. Two peaks appeared near 1350 and 1590 cm-1, corresponding to D peak and G peak of carbon respectively. The ratio of D peak to G peak intensity (ID/IG) value was 1. The results showed that the graphitization degree of carbon in Si@VN/C was low, and there were a large number of defects in the carbon lattice, which was conducive to the insertion of lithium ions and the infiltration of electrolyte. The elemental composition and valence state were studied by X-ray photoelectron spectroscopy (XPS). In N 1s fine spectrum, the peak at 397.1 eV was attributed to V-N bond, which proved that VN was successfully formed. In V 2p fine spectrum, the peak at 514.5 and 515.7 eV corresponded to V-N and V-O-N bonds, while the peaks with binding energies of 517.8 and 516.8 eV represented vanadium oxides, indicating that a small amount of VN was oxidized. The electrochemical performance of Si@VN/C and Si@C electrodes was evaluated using coin cells. With the introduction of VN, the rate performance of Si@VN/C microspheres was improved accordingly. When the current density increased by 2 A·g-1 from 0.1 A·g-1, the capacity of Si@C decreased to 461.1 mAh·g-1. When the current returned to 0.1 A·g-1, the specific capacity of Si@C and Si@VN/C rose to 955.8 and 1046.1 mAh·g-1 respectively, which proved that Si@VN/C had good electrochemical reversibility. The cyclic charge discharge test of Si@C and Si@VN/C with current density of 0.2 A·g-1 showed that the specific capacity of Si@C decreased seriously during the cycle, while the specific capacity of Si@VN/C remained 818 mAh·g-1 after 130 cycles. The capacity of Si@VN/C electrode was maintained at above 580.5 mAh·g-1 at 0.5 A·g-1 with good cycle stability for over 300 cycles. The electrochemical impedance spectroscopy (EIS) test results of pure Si, Si@C and Si@VN/C showed that Si@VN/C had good conductivity. After 30 cycles of charge discharge cycle, Si@VN/C had the lowest Warsburg impedance (Zw) value, about 11.54 Ω, indicating its high ion diffusion rate and excellent rate performance. This Si@VN/C composite microsphere provided a new idea for alleviating the volume change of silicon and improving the conductivity of silicon anode materials, and provided the possibility for the preparation of lithium-ion batteries with high energy density and high safety in the next step. © 2022, Youke Publishing Co., Ltd. All right reserved.
