Hydrothermal Synthesis And Electrochemical Performances Of Anode Materials For Li-Ion Batteries

Li-ion batteries have been widely used in portable electronics such as mobile communication devices due to their high energy density, friendly-environment and long cycle life. Li-ion batteries are the main development trends for power devices because of their excellent energy performances, which lets their use for electric vehicles (EVs) and hybrid-electric vehicles (HEVs) an fierce competition at present in the world, it also an main industrialization direction of our new power vehicles. Graphite materials, with the merit of excellent cycle performance, have been widely used as negative electrode materials of Li-ion batteries, but the electrochemical capacity is relatively low (its theoretical value 372 mAh/g). The new generation Li-ion batteries (especially power battery) require electrode materials with higher performances in specific capacity and cycling durability. So it is very important to develop alternative anode materials with higher specific capacity and better cycling durability for the new generation Li-ion batteries. In this thesis, transition metal disulfides, metal oxides, and their nanocomposites combined with carbon with different morphology and microstructure (three-dimensional (3D) flowerlike MoS2, two dimensional (2D) MoS2 nanosheet crystals/amorphous carbon composites, porous SnO2 particles and SnS2 nanoflakes) were prepared by improved hydrothermal synthesis. Their morphology, microstructure and electrochemical performances of Li-ion intercalation were investigated.(1) As attractive novel environmentally friendly solvents, room-temperature ionic liquids (RIILs) have been used as new reaction media or additive for reactants and morphology templates for the products. Three-dimensional (3D) flowerlike MoS2 was fabricated by hydrothermal synthesis assisted with an ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. The influences of heat treatment on the structures and electrochemical performances of 3D flowerlike MoS2 were discussed.3D flowerlike nanostructure could provide larger electrode/electrolyte interface area and more Li-ion diffusion path, and also accommodate the structure strains caused by the lithium insertion-extraction. The electrochemical tests showed that the 3D flowerlike nanostructural MoS2 exhibited better electrochemical lithiation/delithiation capacity and cycling durability than MoS2 nanosheets prepared without IL. Although the MoS2 samples with high crystallinity could be gained by their annealing at 800℃in the atmosphere of N2/H2, their electrochemical lithiation/delithiation reversible capacity and cycle durability decreased.(2) MoS2/carbonous nanocomposites were synthesized by facile hydrothermal route employing sulfocarbamide, sodium molybdate and D-grouse as starting materials, and 2D-MoS2 nanosheet crystals/amorphous carbon composites were obtained after their calcinations at 800℃. XRD analysis showed the diffraction peak of MoS2 (002) plane in annealed MoS2/C composites isn't detected, only two XRD peaks attributed to (100) and (110) plane of MoS2 are found. This means the crystal growth in c axis was intensively restrained, only the crystallinity of ab plane was improved. This result indicates that MoS2 possess the two dimensional nanosheet crystals characteristics. The carbonous materials in composites suppress the crystal growth of MoS2 in c axis during the heat-treating. This kind of 2D MoS2 nanosheet crystals possess the more diffusion path and shorter diffusion distances for Li-ion, thus have higher electrochemical lithiation/delithiation capacity. Moreover, the composite with carbon also enhance the stabilities of 2D MoS2 nanosheet crystals and electrode during charge/discharge cycling; make the composite possess excellent cycling performance. The electrochemical testing results showed that the 2D-MoS2 nanosheet crystals/amorphous carbon composites exhibited highly electrochemical lithiation/delithiation reversible capacity and excellent cycle durability for Li-ion intercalation electrode materials, the reversible capacity of 1065 mAh/g could be achieved and 1011 mAh/g is remained after 120 cycles with the capacity retention of 94.9%.(3) The decomposition of the oxalates can be used as precursor to prepare metal oxides. The advantage of this synthesis process is simple and the composition easy to control, and that the performances of the products are stabile and repeatable. Porous SnO2 was synthesized by hydrothermal process using tin oxalate as precursor and NaC103 as oxidizer, the influences of oxidizer on the morphology and electrochemical performances were discussed. Owing to the decompose of oxalates, porous structure is formed in the product which enlarge the electrode/electrolyte contact area and favor the transference of Li-ion; this porous structure also accommodate the volume change during the Li-ion intercalation/delithiation process and improve the cycle durability at larger current density.(4) More recently, small biomolecule-assisted synthesis methods have been a new focus in the preparation of nanomaterials. SnS2 nanoplates were fabricated by hydrothermal synthesis assisted with small biomolecule L-cysteine employing SnCl4·2H2O as starting materials. It was found the mol ratio of L-cysteine to Sn4+ had great impact on the structures and morphology. The product are mixture of SnO2 and SnS2 when the mol ratio of L-cysteine:Sn4+=2, while SnS2 nanoplates could be get when the mol ratio of L-cysteine:Sn4+ between 4 and 6. The electrochemical performances of SnS2 nanoplates are relate to the precursor concentration, the sample prepared in lower concentration exhibit higher capacity retention. SnS2 nanoplates prepared with mol ratio L-cysteine to Sn4+ of 4 and 12.5 mmol/L Sn4+ produces the initial discharge capacity of 479.6 mAh/g between 0.01 V and 1.5 V at the current density of 100 mA/g, and 438.6 mAh/g is remained after 50 cycles with the capacity retention of 91.5%. Nanotubes of W/C composite were synthesized through the reduction of WO3/C with hydrogen at 800℃, which were synthesized via a facile hydrothermal route. The forming mechanism also been discussed. W/C nanotubes exhibit the initial discharge capacity of 457 mAh/g between 0.01 V and 3.0 V at the current density of 100 mA/g, and 448.2 mAh/g is remained after 100 cycles with the capacity decaying of 0.116% per cycle.