Synthesis Modification And Electrochemical Properties Of Electrodes For Lithium Ion Battery

In this paper, LiMn2O4and LiFeP04were synthesized and modified as the cathode materials.The transition metal sulfide (MoS2) and insert compounds MxMoS were prepared and studied as novel anode material. The crystal structure and morphology of the expected compounds were characterized by XRD,TEM and SEM techniques. The electrochemical properties of these compounds were also studied also. It was found that the modification by doping for the cathode materials could improve efficiently electrochemical performances of designed cathode materials, which could be applied in lithium ion battery in near future. The main points are as follows:(1) A rheological phase method was used to synthesize spinel LiMn2O4and several different doped compounds Li1.02MxMn195O4.yFy (M=Co, Y and Ga).In this route, LiAc·2H2O、MnAc2·4H2O、Co(CH3COO)2·4H2O、(NH4)2GaF5·H2O,Ga2O3and Y2O3were selected as starting materials, citric acid as chelating agent. The structure and morphology of the samples were characterized by XRD and SEM. As the cathode materials of lithium ion batteries, their charge and discharge properties and cycle performances were also studied. The results show that the doped samples had the higher crystallinity, spinel structure, and smaller cell parameters to compare with LiMn2O4.Although the initial discharge capacity of the doped samples declined a little, the cycle performances of doped Li-Mn-spinel had been improved greatly, particularly the sample Li1.02Mn1.95Co0.02Y0.01Ga0.01O3.97F0.03had the best cycle stability, the initial discharge capacity was128mAh/g, which could maintain at119mAh/g after100cycles(capacity loss was about7%). moreover, it still have very good cycle stability when charge-discharge at higher current density. It was found also that suitable doping can obviously reduce the electrochemical impedance of Li-Mn-spinel cathode material. Therefore, it can effectively improve the electrochemical properties and increase the cycle stability of the LiMn2O4. The multielement doping with cations and anions is effective method to improve electrochemical properties. So the nonstoichiometric spinel Li1.02Mn1.95Co0.02Yo.01Ga0.01O397F0.03could be used as a promising cathode material which will take replace of LiCoO2for the commercial production in lithium ion battery. (2) The olivine LiFePO4and doped LixM0.01FePO4(M=Mo、W、Mn、Y) compounds were successfully synthesized via rheological phase meyhod. During this route, LiAc·2H2O、 FePO4·4H2O、(NH4)6Mo7O24·4H2O、(NH4)6W7O24·4H2O、MnAc2·4H2O and Y2O3were used as raw materials, PEG as the reducing agent and carbon sources. The structure and morphology of the powders were characterized by XRD and TEM. The XRD results show that doping with small amount of metal ions does not affect the crystalline structure of the LiFePO4, only impact on crystal cell parameters. It was found that all the crystal cell parameters and volumes of the samples doped are increased, the expansion of crystal cell volume increased the space of Li+access, it was more advantageous to the intercalation and deintercalation of Li+. The TEM showed that the particles were irregular ellipse, and the particles surface coated with carbon films. The modification by doping has little effects on the morphology. The results of electrochemical tests also indicated that the electronic conductivity and the electrochemical performance had been greatly improved to be compared with undoped LiFePO4, especially the sample LixMn0.01FePO4, at low current density, its initial charge and discharge capacity can reach167,161mAh/g, very close to the theory capacity of LiFePO4(170mAh/g), after100cycles the discharge capacity can maintain at153mAh/g, the loss was only4.97%, also can maintain good electrochemical performance at a high current densities. Except for the single doping with the metal ions can modified, the multielement doping with metal ions would create the ideal effect. To improve the performance of LiFePO4as cathode material, composites LiFe1-xGax(PO4)1-xF3x(x=0.02,0.06,0.10,0.14) with Ga3+and F-dopant were synthesized by reheological phase reaction method (RPR). The samples were characterized by X-ray diffraction (XRD), and transmission electron microscope (TEM), and their electrochemical properties were investigated by A.C. impedance and charge/discharge tests. The results indicate that the samples by doping Ga3+and F-show a well-ordered olivine phase, a little Ga3+and F dopant considerably improves the electrochemical performance of LiFe1-xGax(PO4)1-xF3x, especially at high charge/discharge rate, which is ascribed to the enhancement of the electronic inductivity by multielement doping. The LiFe0.94Ga0.06(PO4)0.94F0.18(x=0.06) behaved the best electrochemical performance among LiFe1-xGax(PO4)1-xF3x. At the0.1C rate, the initial reversible specific capacity of the LiFe0.94Ga0.06(PO4)o.94F0.18was155mAh/g, and remained at147mAh/g after 100cycles; at the1C rate, the initial reversible specific capacity of the sample was142mAh/g, and remained at130mAh/g after100cycles. In conclusion,the intrinsic conductivity of LiFePO4were obviously increased by doping with ions and leads the electrochemical performance to be improved. The suitable modification for LiFePO4would make doped LiFePO4to be ideal cathode for lithium battery application.(3) MoS2and its insert compounds MxMoS2(M=Li、Na; x=1,1.5,2) were successfully synthesized by a novel rheological phase method,(NH4)6Mo7O24·4H2O, CS(NH2)2, H2C2O4·2H2O、LiAc·2H2O and Na2CO3were chosen for starting materials, oxalic acid as reducing agent and PH regulator. The structure and morphology of the compounds were characterized by XRD and TEM, the electrochemical properties of the samples as anode materials of lithium ion batteries were investigated comparatively by charge-discharge curves, cyclability and A.C. impedance. The results showed that the MxMoS2compounds still exists the low angle diffraction peak (002) and the layered structure, only the layer spacing had been increased, the crystal cell parameters a didn’t have a obvious changes but the crystal cell parameters c was increased obviously; After intercalated the reunion degree between the nanoparticles reduced, the smaller the x grain and the better the dispersion. Compared with the electrochemical properties of MoS2, electrochemical performance of MxMoS2materials had been significantly improved, the conductivity of the electrodes was enhanced, the polarization effect was weaken, which make the cycle performance of MoS2to be improved. Among them with LiMoS2performance is the best. In0.01V～3.00V potential window, current density is40mA/g, the initial discharge capacity of LiMoS2was941mAh/g, reversible capacity was828mAh/g, after50cycles remained at637mAh/g. The materials has high capacities and good cycle properties, in addition, synthesis method is simple and suitable for large scale production, therefore, modified transition metal sulfide is expected to be applied in lithium ion battery industry.