Synthesis And Electrochemical Performances Of New Lithium-ion Battery Cathode Materials Li₃V₂(PO₄ )₃ And Li[Ni_xLi_1/3-2x/3 Mn _2/3-x/3)O₂

Rechargeable lithium-ion batteries have been widely applied in many portable electronic devices, and begin to enter the battery market of high capacity, such as hybrid electric vehicles.The commercial cathode materials could not gratify the requirements of high capacity and low cost of some electronic devices.As polyanion cathode materials for lithium ion batteries, lithium metal phosphates such as LiMPO₄ (M=Fe, Co, Ni, Mn,…) and Li₃M₂(PO₄)3 (M=V, Fe) have attracted many research interests due to their high specific capacity, low cost and good cycling performance compared to other spinel-type or layered structured cathode materials.But so far only olivine-type lithium iron phosphate (LiFePO₄) has been subjected to intensive studies and commercialized, monoclinic lithium vanadium phosphate, Li3V2(PO₄)3, which has higher specific capacity is merely conceptual.In recent years, high lithium manganese-base cathode material Li[Ni_xLi_1/3-2x/3Mn_2/3-x/3]O₂ also receives significant attention as cathode material for lithium ion batteries, owing to its low cost, excellent electrochemical performance and environment friendliness . Therefore, the study and application of new cathode materials Li3V2(PO₄)3 and Li[Ni_xLi_1/3-2x/3Mn_2/3-x/3]O2 for lithium-ion batteries have great importance to alleviate environmental pollution and energy crisis.In Chapter 3, monoclinic Li₃V_2-xSn_x(PO₄)₃/C (x=0.00, 0.10, 0.15, 0.20, 0.25, 0.40) compounds were synthesized by a sol-gel method using LiOH·H₂O, NH4VO3, NH₄H₂PO₄, SnCl4·5H₂O and citric acid as starting materials. Here citric acid was not only employed as a chelating agent which could provide the mixing of cations at the molecular level in a sol–gel process, but also as a carbon source which could prevent the oxidation of vanadium ions and afford the network structure of carbon for electronic conduction.SEM results indicated that particles of all samples were irregular, non-uniform, and some of them were agglomerated. It could be found that some particles merged with each other to form a porous network.This microstructure could not only allow the electrolyte to penetrate into the positive materials and promote good electronic contact among the particles, but also provide good accommodation of volume changes without fracture during cycling, good electrical connection with the current collector and high efficiency for electron transportation.The electrochemical tests indicated that Li₃V1.8Sn_0.2(PO₄₎3/C exhibited the lowest charge-transfer resistance, highest lithium diffusion coefficient and discharge capacity.In the voltage region of 3.0-4.8 V, Li3V1.8Sn_0.2(PO₄)₃/C presented an initial discharge capacity of 157 mAh/g at rate of 0.1 C. Even after 50 cycles at _0.2 C discharge rate, the material still could deliver a discharge capacity of 122 mAh/g, much better than that of the other samples. Kinetic mechanism of the Li₃V₂-xSnx(PO₄)3/C with different Sn contents was also studied.The results indicated that the extraction and insertion of the previous two lithium ion was two-phase reaction process. The extraction and insertion of the first lithium ion was corresponding to Li₃V₂(PO₄)3 - Li₂V₂(PO₄)₃, the second lithium ion was corresponding to Li2V2(PO₄)3 - LiV₂(PO₄)₃.Nevertheless, the inserting of the third lithium ion was solid solution reaction.In Chapter 4, high lithium manganese-base cathode material Li[Ni_0.2Li_0.2Mn0.6]O₂ was prepared by high temperature calcining the mixture of lithium resource and precursor which was synthesized via a co-coprecipitation method. XRD results suggested that all the strong diffraction peaks could be indexed to a hexagonalα-NaFeO₂ structure(space group R-3m). Several weak peaks between 21 and 25°,which could not be matched to R-3m symmetry,were consistent with the LiMn6 cation ordering that occured in the transition metal layers of Li₂MnO₃. The secondary particles had an irregular morphology with particle size in the range of 1-2μm by SEM. The long voltage plateau at 4.5 V of the initial charge reaction in the voltage region of 2.0-4.8 V, which was absent during the subsequent charge, was consistent with the irreversible removal of Li₂O from Li₂MnO₃ component. At a current rate of 60 mA/g under room temperature, the reversible capacity was measured to be 120 mAh/g after 50 cycles. When the discharge rate approached 300 mA/g, the discharge capacity decreased to 68.9 mAh/g, but the specific capacity was over 66.7 mAh/g and the capacity retention was 96.8% after 437 cycles.