Synthesis And Electrochemical Properties Of Vanadium Based Oxides Nanomaterials For Lithium/sodium Ion Batteries

Due to the properties of high energy density, high power density, long cycle life and low environmental pollution, lithium-ion batteries（LIBs） have been widely applied to mobile phones, laptops, tablet computers and other portable electronic devices. With the increasing demand of high-performance electronic devices and the development of new energy vehicles, there is a growing need for LIBs with high capacity. The key issue of improving the energy density of LIBS is to develop the electrode materials with high specific capacity. As a kind of cathode materials for LIBs, vanadium based oxides attract world-wide attention because of their high capacity, low cost and abundant resources. However, the low lithium diffusion coefficient and the moderate electronic conductivity of the vanadium based oxides hinder the application of these materials. At present, nanostructuring strategy, surface modification and dopant manipulation can enhance the electrochemical performance of electrode materials. Therefore, it would be the essential issue of how to apply these approaches to improve the electrochemical performance of the vanadium based oxides.In this paper, nanostructuring strategy, surface modification and dopant manipulation are introduced to improve the electrochemical performance of the vanadium based oxides materials. This paper investigates the effects of these three approaches on the electrochemical property of the vanadium based oxides and first proposes the application of the synergetic effects of both surface modification and dopant manipulation to optimize the electrochemical performance. In addition, we design a capable vanadium oxides cathode for sodium ion batteries（SIBs） and discuss the influence of the structure and morphology on the sodium ion insertion/extraction. The main research content of this paper are as follows:Firstly, V₂O₅ nanoparticles are fabricated by employing low-priced rice husk carbon as templates. During the preparation process, the interior of V₂O₅, appears V⁴⁺ ion auto-doping phenomenon, which would enhance its electronic conductivity. Moreover, compared with commercial V₂O₅ micrometre particles, nanoparticles provide a shorter transportation pathway and more active sites for the electron and lithium ion. Thus, the materials present a high capacity of 229 m Ah/g after 50 cycles under the current density of 300 m A/g.Secondly, H₂V₃O₈ nanowires wrapped by reduce graphene oxide（RGO） are synthesized successfully through a hydrothermal process. Because of the use of highly conducting graphene as the transportation pathway of electrons, the electronic conductivity of materials has been remarkably improved. Meanwhile, graphene has the function of inhibiting structure collapse while in the charging and discharging process. The first discharging and charging capacity of H₂V₃O₈/RGO can run up to 268 m Ah/g and 256 m Ah/g respectively, and its coulombic efficiency is up to 95.5%. The material shows high discharging capacity of 117 m Ah/g at the current densities of 1 A/g, without obvious capacity fading after 50 cycles.Thirdly, an Al³⁺-doped V₂O₅/RGO nanocomposite is prepared to improve the electrochemical performance of V₂O₅. Al³⁺ doping can improve the structure stability of materials and the diffusion coefficient of lithium ion, while graphene modification can raise the electronic conductivity. The synergetic effects of Al³⁺ doping and graphene modification are established, which makes Al_0.16V₂O₅/RGO present a favorable electrochemical performance. The sample presents a high discharge capacity of 274 m Ah/g and a remarkable capacity retention of 90% after 50 cycles. Additionally, under current density of 3 A/g, a discharge capacity of 123 m Ah/g is achieved, suggesting a capable rate ability.At last, a sponge-like V₂O₅ nanomaterial is prepared for the sodium ion insertion/extraction with the width of several micrometers and thickness of approximately 15 nm. The V₂O₅ nanosheets have a large interlayer space which is favorable for charge-transfer reaction. V₂O₅ nanosheets display a high discharge capacity of 216 m Ah/g at a current density of 20 m A/g, and 178 m Ah/g after 20 cycles. It retains 73% the initial capacity after 100 cycles at a current density of 100 m A/g.In summary, this work provides a comprehensive investigation on the method to improve the electrochemical performance and the mechanism of lithium/sodium ion insertion/extration. It also offers a necessary theoretical and technical guidance for the research of vanadium based oxides cathode.