3D graphene modified sphere-like VPO₄/C as a high-performance anode material for lithium-ion batteries

It is still a great challenge to improve the rate capability and cyclability of a new conversion-type anode material, VPO₄, for lithium-ion batteries. In this work, three-dimensional graphene, carbon and in situ formed V₂O₃ with high theoretical capacity are introduced into VPO₄ to improve the capacity, rate performance and cycle performance. The three-dimensional graphene (3DG) modified VPO₄/C (VPO/C@3DG) are prepared by a combination of hydrothermal reaction, freeze-drying and calcination processes, and the VPO/C@3DG composite is investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectrum and electrochemical tests. The results indicate that VPO/C@3DG composite contains VPO₄, V₂O₃, carbon and three-dimensional graphene, and dispersive VPO₄ and V₂O₃ nanoparticles and carbon form carbon coated sphere-like VPO/C particles, and VPO/C particles are covered by three-dimensional graphene. The VPO/C@3DG composite exhibits much higher capacity, better rate capability and cyclability than the control VPO₄/C and three-dimensional graphene (3DG). In the voltage range of 0.01–3.0 V (vs Li⁺/Li), the second cycle discharge capacities of VPO/C@3DG are 654.6, 551.6, 486.8, 420.6 and 343.1 mAh∙g⁻¹ at 0.2, 0.5, 1, 2 and 5C (2750 mA g⁻¹), respectively, significantly higher than 507.8 (0.2C), 384.9 (0.5C), 294.2 (1C), 219.3 (2C) and 124.1 mAh∙g⁻¹(5C) of VPO₄/C and 393.1(0.2C), 298.2 (0.5C), 252.1(1C), 217.8 (2C), 169.7 mAh∙g⁻¹ (5C) of 3DG. After cycled 30 cycles at 0.2C, the discharge capacity of VPO/C@3DG still remains 631.9 mAh∙g⁻¹, while the discharge capacities of VPO₄/C and 3DG are 352.4 and 308.5 mAh∙g⁻¹, respectively, under the same conditions. Even at high current rate of 5C, the discharge capacity of VPO/C@3DG is 338.8 mAh∙g⁻¹ after 100 cycles, significantly higher than that of VPO₄/C (101 mAh∙g⁻¹) and 3DG (142.1 mAh∙g⁻¹). The results indicate that the special composition and microstructure are responsible for the superior electrochemical performance of VPO/C@3DG. The results suggest that introduction of three-dimensional graphene and construction of special microstructure is substantially beneficial for improvement of electrochemical performance of conversion-type electrode materials.