Preparation Of Vanadium-Based Oxides And Their Sodium Ion Storage Properties

Lithium ion batteries（LIBs）have been recognized as the dominant energy storage devices for portable consumer electronics and electric cars in practical terms nowadays.However,with the exploitation of lithium resources in the earth’s crust,the cost of LIBs is gradually increasing.In contrast,the reserves of sodium in the crust and seawater are more abundant.Thus,sodium ion batteries（SIBs）have received widespread attention as a new type of cost energy storage device.However,the Na⁺with large radius leads to unsatisfied ion（de）intercalation behavior during charging and discharging processes.As one of the core of the battery,the negative electrode faces many challenges in the selection of materials.Vanadium-based oxides can be used as a negative electrode material with great potential for sodium ion batteries because of its high theoretical capacity,high abundance and low cost.However,the intrinsic low conductivity and relatively large volume change of vanadium-based oxides are two main obstacles to fullling the energy storage requirements,and reasonable structural design is the key to improving its electrochemical performance.In this paper,surface modification and multi-dimensional assembly are introduced to improve the electrochemical performance of the vanadium-based oxides materials.The main research works of this paper are as follows:Firstly,carbon-coated V₂O₃ materials with glucose as a carbon source were prepared by hydrothermal reaction and subsequent calcination treatment.Then,the influence of different hydrothermal reaction time and glucose addition on the material was explored.The results show that when the reaction time is 24 h and the amount of glucose added is 0.16 g,V₂O₃/C has the most excellent electrochemical performance.When the current density is 0.1 A·g^-1,it still has a specific discharge capacity of 294 m Ah·g^-1 after 100 cycles.In addition,the V₂O₃/C anode material also has excellent rate capability.In addition,the V₂O₃/C anode material also has excellent rate capability.When the current densities were 0.1,0.2,0.5 and 1 A·g^-1,the V₂O₃/C electrode exhibited high reversible capacity of 380,324,294 and 255 m Ah·g^-1,respectively.The reversible capacity of 205 m Ah·g^-1can be achieved even at a large current density of 2 A·g^-1.When the current density returns to 0.1 A·g^-1,the reversible specific capacity of V₂O₃/C can be maintained at 351 m Ah·g^-1.It can be seen that carbon is a fast electron transport medium,which plays a certain role in improving the sodium ion storage performance of vanadium oxide.However,the 0D nanostructure of carbon-coated V₂O₃ is not suitable for ultra-long-term cycling with large volume expansion.To this end,two-dimensional graphene is introduced into the matrix,which is expected to alleviate this problem.At the same time,considering the small volume expansion rate and fast ion diffusivity of amorphous materials,graphene-modified VO_x were prepared by hydrothermal reaction and subsequent calcination treatment.Then,the effect of different graphene additions on the material was explored.By comparison,it is found that the performance of the sample（VO_x/r GO-5%）is the best when the mass ratio of graphene is 5%.When the current density is 0.5 A·g^-1,it still has a specific discharge capacity of 185 m Ah·g^-1after 400 cycles.In addition,the VO_x/r GO-5%anode material also has excellent rate capability.When the current densities were 0.1,0.2,0.5 and 1 A·g^-1,the VO_x/r GO-5%electrode exhibited high reversible capacity of 336,258,230 and 211 m Ah·g^-1,respectively.Even at a high current density of 2 A·g^-1,it can still reach a reversible capacity of 189m Ah·g^-1.When the current density returns to 0.1 A·g^-1,the reversible specific capacity of VO_x/r GO-5%can still be maintained at 286 m Ah·g^-1.It can be seen that the cycling stability of vanadium oxide modified by an appropriate amount of graphene can be effectively improved.However,with the intercalation and deintercalation of Na⁺,the open structure of the composite cannot completely limit the volume change and self-aggregation of 0D nanoparticles.Using graphene,MXene nanosheets and nanotubes embedded in V₂O₃ nanoparticles to assemble into a multi-dimensional nanocomposite material（V₂O₃-C-NTs-MXene/r GO）to solve the above problems.Embedding 0D nanoparticles into 1D nanotubes greatly reduced their self-aggregation.At the same time,the addition of r GO and MXene improves the overall electronic conductivity of the material.The multi-dimensional structure assembled with nanotubes not only effectively prevents the accumulation of lamellae,but also reduces the volume expansion generated during the cycle.The unique structure of V₂O₃-C-NTs-MXene/r GO makes it exhibit excellent electrochemical performance when used as the negative electrode of sodium ion battery.When the current density is 0.5 A·g^-1,it still has a high specific discharge capacity of 233 m Ah·g^-1 after 1000 cycles.The capacity retention rate of V₂O₃-C-NTs-MXene/r GO is as high as 96.8%,which shows superior long-cycle stability.When the current densities were 0.1,0.2,0.5 and 1 A·g^-1,the electrode exhibited high reversible capacity of 327,264,239 and 215 m Ah·g^-1,respectively.Even at a high current density of 2 A·g^-1,it can still reach a reversible capacity of 215 mAh·g^-1.When the current density returns to 0.1 A·g^-1,the reversible specific capacity of V₂O₃-C-NTs-MXene/rGO can still be maintained at 305 mAh·g^-1.