Designed Synthesis And Properties Of Vanadium Oxide-Based Materials For Energy Storage

The development of high-performance energy storage devices is considered as an effective way to alleviate the growing energy demand and environmental concerns by realizing the efficient utilization of renewable and clean energy sources such as solar and wind.The electrode material is the one of the main factors that determine the performance of the electrochemical energy storage device,thus much attention has been brought to the development of environmental friendly electrode materials with high capacity and high stability.Vanadium has variable oxidation states,making it realizable for multi-electrons electrochemical reactions and thus leading to the high theoretical capacities of vanadium oxides.In this thesis,several kinds of vanadium oxide-based electrode materials for supercapacitors and zinc ion batteries were designed and synthesized.The efficiencies of ion and electron diffusion and the cycling stability of the materials were improved by different strategies including morphological control,structure optimization,interface modification and ion doping.The effects of the materials’ structure on their electrochemical performances were also studied.The main results are as follows:（1）The NH4VO3 crystals with controlled morphologies were synthesized through the recrystallization method,and were converted into porous V₂O₅.The growth mechanism of the crystals at different conditions was discussed,and the impact of the calcination temperatures and morphologies on electrochemical properties of products used as battery-type electrode for supercapacitors was studied.Rhombohedral V₂O₅ exhibited higher ion diffusion coefficient and lower charge transfer resistance because of its high density pore structure,and showed a high specific capacitance of 601 F·g-1 at 1 A·g-1.While flower-like V₂O₅ exhibited the worst cycling stability because of its unstable crystal structure.（2）A-phase VO2 and carbon composites with core-shell structure were prepared through a hydrothermal method using glucose and V₂O₅ nanowire as raw materials to improve the conductivity and cycling stability of VO2（A）.The capacitance of the cmposites at 1 A·g-1 was improved from 70 F·g-1 of pure VO2（A）to 144 F·g-1.A flexible asymmetric supercapacitor device was fabricated using VO2（A）@C and activated carbon.Though using LiCl/PVA gel electrolyte instead of Na2SO4 aqueous electrolyte led to a reduced capacity because of a smaller working voltage range,the cycling stability of the device can be efficiently improved.（3）The conductivity and cycling stability of the V2O3 and amorphous carbon composites were improved by optimizing the core-shell structure to the highly dispersed structure.The capacitance of the composites at 0.5 A·g-1 was improved from 193 F·g-1 to 458.6 F·g-1.Besides,75.6%of the initial capacitance was remained after 2000 cycles in aqueous electrolyte.Asymmetric supercapacitors assembled by highly dispersed composites and active carbon demonstrated a capacitance of 262 mF·cm-2 at 5 mV·s-1,as well as excellent flexibility and cycling performance.（4）Al-ions were introduced into hydrated vanadium oxides with chemical bonds by the hydrothermal method,and the product was used as the cathode material for zinc ion batteries.After modification,the expanded lattice spacing enables fast ion migration,and the introduction of low-valance vanadium and oxygen vacancies were beneficial for charge transfer and redox reactions,thus the capacity and structural stability were both improved.The specific capacity was improved from 275 to 380 mAh·g-1 at 50 mA·g-1 after introducing Al-ions,and the capacity did not decrease after 3000 cycles.The energy storage mechanism was detailed analyzed by the ex-situ characterizations of the charged and discharged electrode materials.