Preparation And Li-ion Insertion/Extraction Mechanism Of Novel Vanadate Compounds As High Performance Cathode Materials

There has been great interest in synthesizing vanadium oxides and their derivatives as electrode materials for Li-ion batteries because of their low cost, easy synthesis and high capacity. Vanadium-related resources in China are rich, however, the comprehensive utilization is still too low. Therefore, it is meaningful to develop the high capacity vanadium-related compounds as novel electrode materials for Li-ion batteries. It's well known that poor cycling stability and rate capability are the main two problems for vanadium-related compounds, which limit their further application. This thesis focuses on the trivanadium compounds with relatively high structure stability. At first, lithium trivanadate and sodium trivanadate with high electrochemical performance were obtained by optimizing the preparation strategies. As follows, NH4+group was used to replace Li+in V3O8-layered structure to form NH4V3O8, a new cathode material candidate for LiV3O8. The as-prepared NH4V3O8exhibited a high discharge capacity and excellent long-term cycling life. The lithium ion insertion/extraction mechanisms of all three kinds of materials were emphasized. The main research work and results are shown as follows:A novel two-step method combining the hydrothermal process and the following solid state reaction was designed to obtain LiV3O8nanosheets in this thesis. Firstly, well-dispersed (NH4)0.5V20s nanosheets were prepared by a simple hydrothermal approach. After the calcination of the mixture of the intermediate and LiOH, the LiV3O8nanosheets were yielded. The morphology of the as-prepared product was investigated by FE-SEM, TEM, AFM, etc. The nanosheets with the thickness of20-50nm showed the best rate capability for LiV3O8that have even been reported to date. The discharge capacities of148.7and105.8mAh g-1were retained at5C and10C, respectively. Meantime, LiV3O8nanosheets exhibited the excellent cycling stability with the capacity retention of84.1%and85.3%at300and1000mA g-1, respectively. The excellent rate performance should be due to the unique nanosheet morphology. A possible mechanism model was proposed to explain the reason why the electrode showed the better cycling stability at high rates in comparison with that at low current density.In order to address the poor cycling stability for vanadates, NaV3Og was investigated when Na+was used to substitute the Li+in LiV3O8. A simple hydrothermal method using V2O5and NaOH as raw materials was employed to obtain NaV3O8·xH2O nanowires. The effect of thermal treatment on the structure, morphology and electrochemical performance of as-prepared products was investigated. The nanowires showed a diameter of60-100nm and a length of up to5micrometers. Appropriate thermal treatment could effectively improve the cycling performance, although the discharge capacity was sacrificed to some extent. Na2V6O16·0.86H2O after heat treatment under300℃delivered an initial specific discharge capacity of235.2mAh g-1at30mA g-1, with a capacity retention of91.1%after30cycles. Long cycling test was demonstrated by the retention of90.4%and94.4%at150and300mA g"1after80cycles, respectively. Good rate capability was also achieved for this material. It is proposed that the improved cycling stability of the electrode after thermal treatment is mainly attributed to the removal of a part of crystal water, accompanied with certain structural arrangement.To further improve the rate performance, Na1.08V3O8nanosheets were prepared by the hydrothermal process combining the following solid state reaction. Ultra-thin and well-dispersed features with the mono-layer thickness of less than10nm were demonstrated for nanosheets. The BET specific surface area was9.5m2g-1. The formation mechanism of nanosheets involved the fusion and conversion of nanorods. When used as cathode material for Li-ion battery, the nanosheets showed superior rate capability, with the discharge capacities of ca.200.0,131.3,109.9,93.8and72.5mAh g"1at0.4,10,20,30and50C, respectively, which was the best value for all carbon-free coated vanadates and much better than those of most reported carbon-coated vanadates. Excellent cycling stability without considerable capacity loss over200cycles was observed at600and1000mA g-1. Cyclic voltammetry (CV) results revealed that the Li-ion diffusion coefficient in Na1.08V3O8nanosheets was-10-9cm2s-1. It is believed that the unique nanosheet morphology as well as its intrinsic structure feature greatly facilitates the kinetics of Li-ion diffusion and excellent structure stability, thus resulting in superior electrochemical performance.On the basis of the above-mentioned work, NH4V3O8was proposed as a novel cathode material for Li-ion batteries for the first time. Much work has been carried out. The effect of experimental conditions, including reaction time and pH value on the structure, morphology and electrochemical performance was optimized. And the behavior of NH4+and Li ion insertion/extraction mechanism were also studied. It was found that the NH4V3O8possessed good lithium ion insertion/extraction ability and NH4+could not be extracted in the charge process. NH4V3O8nanosheets prepared by the hydrothermal reaction with the pH of4and reaction time of24h showed the best electrochemical performance. The nanosheets showed the high reversible discharge capacity and superior cycling stability, probably owning to their unique nanosheet morphology and formed molecular H-bond in crystal. A maximum discharge capacity of353.2mAh g-1was exhibited at30mA g"1and202.1mAh g-1discharge capacity was maintained well over100cycles at300mA g-1. Even at600mA g-1, no capacity fading was observed over200cycles. A slight structure arrangement should occur during the prolonged cycling since the lithium ion intercalation/deintercalation plateaus were meliorated, which was further confirmed by FT-IR and CV results. Note that the capacity decreased to202.5mAh g-1at300mA g-1, indicating the inferior rate performance.NH4O3carbon nanotubes (CNTs) composites were synthesized by one-step hydrothermal method and the influence of the coated amount of CNTs was studied. The CNTs were clearly observed on the surface of modified NH4V3O8. It was found that incorporation of CNTs could result in the impurity of (NH4)0.5V205.0.5wt%CNTs coated composite showed the best electrochemical performance. It delivered a discharge capacity of226.2mAh g-1with excellent capacity retention of97%after100cycles at150mA g-1,45mAh g-1larger than that of the pristine one. The greatly improved electrochemical performance of NH4V3O8should be attributed to incorporation of CNTs, which facilitated the interface charge transfer and Li+diffusion. NH4V3O8nanorods with further improved rate capability were hydrothermally prepared in the presence of sodium dodecyl benzene sulfonate (SDBS). The diameter of nanorods is about30nm and the length is less than1μm. The BET surface area is15.1m2g-1. In comparison with the flakes prepared without surfactant, the nanorods are better suited as lithium inserting electrode material, with superior lithium ion insertion/deinsertion plateaus, higher discharge capacity and better cycling stability.(NH4)o.5V205nanobelts and nanosheets were synthesized, respectively. The diameter of the nanobelts was50-200nm and length was about several micrometers. The reversible lithium intercalation behavior was evaluated. It delivered an initial specific discharge capacity of225.2mAh g-1at15mA g-1and still maintained a high discharge capacity of197.5mAh g-1after11cycles. Cycling stability with the capacity retention of81.9%after100cycles at150mA g-1was much better than that in literature. Interestingly, the excess120mAh g-1capacity in the first charge process was observed, most of which could be attributed to the extraction of NH4+group, verified by Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) results. Meanwhile, XRD results showed that the structure of lithium inserted electrode changed during lithium ion insertion and extraction, but the structure change was reversible. It is interesting to note that the thickness of the as-prepared nanosheets was less than1.5nm, which was equivalent to the c axis value of single unit-cell. It was found that the amount of the added oxalic acid influenced the electrochemical performance of the as-prepared materials. The optimized electrode exhibited the superior cycling stability, without capacity fading after200cycles at0.5C, which was probably due to the unique nanosheet morphology and molecular H bond in the crystal.