Synthesis And Electrochemical Properties Of Several Composite Cathode Materials For Lithium Ion Battery

Compared with conventional rechargeable batteries such as Ni-Cd battery, Ni-MH battery, Pb-PbO2 battery and so on, lithium ion batteries exhibit more obvious advantages with respect to energy density, rate capability and charge-discharge performance. Moreover, lithium ion batteries also exhibit the advantages of low self-discharge rate, long cycling life and environmental friendliness. Recently, lithium ion battery has been extensively used in minitype portable electronic devices and has been undergoing its applications in electric vehicles. The improvement of lithium ion battery technology is mainly depended on the development of electrode materials, and nowadays the main attack is further improving electrochemical properties and decreasing costs. Because the electrode materials occupy the majority of costs in lithium ion battery, it is important to investigate and modify the electrode materials.The development of lithium ion batteries and cathode materials were reviewed in detail. The aims of the present study were to investigate the preparation processes, modification of materials, structural characterization and electrochemical properties of several composite materials as cathode materials for lithium ion batteries. The microstructures and morphologies of materials were investigated by means of thermogravimetry/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), scanning electronic microscope (SEM) and transmission electron microscopic (TEM). The electrochemical performances of materials were investigated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS).The Li-V-O composite (nLi:=1:3.5) was synthesized by a hydrothermal method and post-treated at 300℃. XRD analysis confirms that the composite is a binary LiV3O8-V2O5 composite system with the formula of 0.8LiV3O8·0.2V2O5. The composite consists of small laminar nanocrystallites with numerous cavities between the stacked laminar nanocrystallites. The initial discharge capacities of LiV3O8 and 0.8LiV3O8·0.2V2O5 samples were 276 and 365mAh/g. after 20 cycles, the discharge capacities of the two samples were 197.6 and 304mAh/g, respectively. Smaller capacity loss indicates that the capacity retention of the composite is superior to that of bare LiV3O8 cathode. Smaller capacity loss for the composite implies that the porous structure of the composite can not only provides good channels for Li+ transfer but also buffers the volume change for Li+ intercalation/deintercalation and keeps the structural integrity during the charge and discharge processes.Li-Ni-Sb-O composite materials were synthesized by two-step calcination method. XRD analysis confirms that the composite (nLi:nNi:nSb=3:2:1) is the mixture of LiNi1-ySbyO2 and LiSbO3 with the formula of LiNi0.72Sb0.28O2·0.05 LiSbO3. It is shown that the crystal lattice parameters (a, c) of the Sb-doped compound are bigger than those of pure LiNiO2. LiNiO2 was made up of large and irregularly shaped particles and has a broad particle size distribution, and the Sb-doped compound consists of spherical-like nanoparticles with a mean grain size of 50nm. The compound exhibits excellent capacity retention during the charge-discharge processes due to its reinforced structural stability, and a discharge capacity of 101.5mAh/g is still obtained in the voltage range of 2.5-4.5V after 20 cycles. Thermal analysis further confirms the structural stability of LiNi0.72Sb0.28O2 is superior to that of pure LiNiO2.Li-Mn-Sb-O composites (nLi:nMn:nSb=2:3:1) were synthesized via ball milling technique and solid state reaction method. XRD analysis confirms that the as-prepared composite is binary LiMn2O4-LiSbO3 system. The composite calcinated at 700℃consists of spherical-like nanoparticles with average size of 50nm. Galvanostatic charge-discharge cycle tests show that the initial discharge capacity of the composite is 106mAh/g and can attain 94.7mAh/g after 20 cycles. The addition of Sb5+ ions expand the unit cell volume and provide inactive interconnected network, which are beneficial for Li+ intercalation/deintercalation and keeping the structural integrity during the charge and discharge processes.LiNi3/6Mn2/6V1/6O2 cathode material was prepared by the rheological method. The parameters of LiMn1/2Ni1/3V1/6O2 sample calcined at 800℃are a=2.869A, c=14.211A, The values of c/a (4.953, close to 5) and I(003)/I(104) (1.430, much more than 1.0) show that the obtained LiMn1/2Ni1/3V1/6O2 sample has a highly ordered layered structure. XPS results show that the valence states of Ni, Mn and V in LiNi3/6Mn2/6V1/6O2 are mainly +2,+4 and +4, respectively. Mn and V mutually act as active components for the charge-discharge capacity while Mn provides a stable framework during cycling. The initial charge and discharge capacities of LiNi3/6Mn2/6V1/6O2 cathode material are 172.5 and 142mAh/g, and the discharge capacity can attain 130.3mAh/g after 20 cycles. The capacity loss is only 8.2%, indicative the excellent cycleability of as-prepared material.MoO3 material was prepared by solid state reaction method, and MoO3/SiO2 composite material was also prepared using MoO3 and Si(OC2H5)4 as the starting materials. All the diffraction peaks from the pattern of MoO3 can be readily indexed to the pure phase of a-MoO3 with the orthorhombic structure. No peaks from other phases have been detected from the pattern of MoO3/SiO2, indicating that SiO2 presents as amorphous material. The initial discharge and charge capacities of MoO3 are 286 and 243.7mAh/g, the initial discharge and charge capacities of Mo03/Si02 are 257.8 and 225.8mAh/g. The coulombic efficiency of MoO3/SiO2 (87.6%) was higher than that of MoO3 (85.2%), indicative of the better reversibility of Li+ intercalation/deintercalation reaction for MoO3/SiO2 electrode material. SiO2 coating over MoO3 can reduce the direct contact of electrode material with electrolyte and suppress the dissolution reaction of metal ions in electrode material.