| Due to their unique structures, layered ammonium vanadates can be used as electrode materials for lithium-ion battery, supercapacitors, gas sensors and catalysts, presenting a broad prospect. The structure and properties of ammonium vanadates are affected greatly by preparation process and synthesis condition, because the hydrolysis and polycondensation process of VO3- is closely related to the p H value and temperature of the solution. In this thesis, a variety of synthetic methods are employed to control the structure of NH4V3O8 and(NH4)2V10O25·8H2O. The effect of synthesis process on the structure of the products is investigated by means of XRD, SEM, and TEM. Meanwhile, galvanostatic charge-discharge, CV and EIS are used to characterize the electrochemical performances of different samples and study the relationship between the synthetic process and the properties of the products.Four different structures(such as sphericals, nanobelts, nanorods and flakes) NH4V3O8 are controllable synthesized by microwave hydrothermal(MH) method. When the p H value, MH temperature(T), concentration of NH4VO3(c) or reaction time(t) is changed, NH4V3O8 with different structure and electrochemical performance can be obtained. Results show that, pure phase NH4V3O8 can be prepared by MH method at 120~200 ℃for 60~120min when the p H is 3~6, concentration of NH4VO3 is 0.1~0.2 mol·L-1. The best synthesis condition for preparation of NH4V3O8 by MH method is p H=3, c=0.1 mol·L-1, T=150℃, t=60 min. The NH4V3O8 nanobelts prepared under such condition possess uniform size, good dispersion, large length-to-diameter ratio, exhibiting an initial discharge capacity of 332 m Ah·g-1 at 15 m A·g-1 and remain a high capacity of 296 m Ah·g-1 after 20 cycles. The capacity of this material is 279, 248, 217 and 189 m Ah·g-1 at 60, 120, 240 and 300 m A·g-1, respectively.Pure phase NH4V3O8 is prepared by water bath(WB) method at 70~90℃, the p H value has a significant influence on the structure and electrochemical performance of the products. The NH4V3O8 nanorods prepared by WB method at 90 ℃for 2h with p H=3 exhibit the best electrochemical performance among the samples synthesized by WB, which show an initial discharge capacity of 245 m Ah·g-1 at 15 m A·g-1 with a high retention of 96% after 20 cycles. The capacity of this material is 234, 179, 156 and 129 m Ah·g-1 at 60, 120, 240 and 300 m A·g-1, respectively. Its electrochemical performance is much better than that of NH4V3O8 prepared by co-precipitation method, although can’t be comparable to that of NH4V3O8 nanobelts synthesized by MH method. The solvent type also affects the structure of products, the morphology of NH4V3O8 changes from agglomerated nanorods into flowers assembled by nanarods after the addition of ethanol into the system. When ethylene glycol(EG) is added, the product changes into urchin-like(NH4)2V10O25·8H2O assembled by nanarods.In order to improve the structure of NH4V3O8, microwave irradiation(MW) is employed to prepare NH4V3O8. Microwave reduces its generation temperature and NH4V3O8 can be obtained at 65 ℃through MW. By comparing the four samples synthesized at 65~90℃ we know that, with the increase of reaction temperature, the crystallinity and size of the NH4V3O8 increase, and the electrochemical performance of products also changes significantly. The NH4V3O8 nanosheets prepared at 65 ℃exhibit the best electrochemical performance in the four samples, showing a capacity of 255, 242, 221, 208 and 196 m A·g-1 at 15, 30, 120, 240 and 300 m A·g-1, respectively, whose performance is superior to NH4V3O8 nanarods synthesized by WB method.(NH4)2V10O25·8H2O is prepared by WB method with mixture solution of water and EG as solvent. The morphology of the products is different apparently when the solution p H or the addition of EG is changed. Results show that the urchin-like(NH4)2V10O25·8H2O prepared with EG content of 50% at p H=2 possess smaller size and looser structure than other samples, exhibiting a discharge capacity of 428, 335, 294, 231, 199 and 175 m Ah·g-1 at 15, 30, 60, 120, 240 and 300 m A·g-1, respectively. |
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