Synthesis And Improvement Of Spinel LiMn₂O₄ And LiNi_1/3Co (1/3)Mn_1/3O₂

Intensive research and development work has been conducted to further improve the performance of lithium ion batterries and reduce the cost of electrode materials. LiCoO₂ has been used to be a major cathode material in ion battery production. However due to its disavantages of high cost, toxicity and instability at high potential windows, considerable effort has been expended to find possible alternative to LiCoO₂. Spinel LiMn₂O₄ was regarded as one of the most promising cathode materials for lithium-ion batteries in virtue of its obvious advantages such as the abundant and cheap resources, environmental benignity, safety, high voltage and good rate of capability. And they have been extensively investigated as a cathode material for lithium-ion batteries. This dissertation aims to improve the electrochemical performance of spinel LiMn₂O₄ and LiNi_1/3Co_1/3Mn_1/3O₂ material via improving synthetic method and bulk doping.Firstly, a series of spinel Li_1.05M_xMn_1.95-xO₄ (M=Cr, Al, Fe, Zn, Mg) as cathode material for Li-ion battery was synthesized by a solid-state reaction, and its high temperature performance were investigated in detail. Results showed that the substitution of metal-ion for Mn improved the cycle performance of spinel LiMn₂O₄. On one hand, the specific capacity of the doped materials depends on the atomicity of doped metal ion; on the other hand, it also lies on the position of alpha lattice. It is the main reason that the chemical stability of the materials leads to attenuation of the high temperatural specific capacity.Secondly, a series of spinel Li_1.02Mg_xMn_1.98-xO₄ (x=0.03, 0.05, 0.07, 0.09) and Li_1.02Mg_yMn_1.98-yO_4-yF_y(y=0.03, 0.05, 0.07, 0.09)as cathode material for Li-ion battery were synthesized by the high temperature solid-state reaction, its structure, morphology and electrochemical performance were studied by XRD, SEM, CV, EIS and charge-discharge tests. Results showed that the specific capacity and reversibility of the material was increased, the stability of spinel structure and cycle performance were improved by F-, Mg2+ and Li+ multiplex doping. The charge-discharge tests revealed that Li_1.02Mg_0.05Mn_1.93O_3.95F_0.05 has the best cycling performance. At 1/3C rate, its first specific discharge capacity is 115.9 mAh/g and the capacity retention ratio is 96.5% after 40 cycles.Once more, to improve the cycle performance of spinel LiMn₂O₄ as the cathode of 4-V-class lithium secondary batteries, novel spinel phases Li_1.02Mn_1.92Al_0.02Fe_0.02Cr_0.02O_4-xF_x and Li_1.02Mn_1.92Al_0.02Cr_0.02Mg_0.02O_4-xF_x (x=0, 0.06) have been successfully prepared by a conventional solid-state method. The structure and physicochemical properties of these as-prepared powders have been investigated by powder X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge test in detail. For Li_1.02Mn_1.92Al_0.02Fe_0.02Cr_0.02O_3.94F_0.06, the initial discharge capacity is 119.6 and 120.4mAh/g at 25 and 55℃, respectively; the capacity retention ratio is 97.5% and 91.9% after 50 cycles under 1/3C at 25 and 55℃, respectively. The results reveal that the multidoped spinel Li_1.02Mn_1.92Al_0.02Fe_0.02Cr_0.02O_3.94F_0.06 has an excellent capacity and cyclability, which may be contributed to the the multiple cation and anion doping, leading to a more stable spinel framework with good capacity retention rate. Finally, Layered structure LiNi_1/3Co_1/3Mn_1/3O₂ were synthesized by the citric acid sol-gel method. The LiNi_1/3Co_1/3Mn_1/3O₂ calcined at 900℃for 20h shows excellent electrochemical performances with large reversible specific capacity of 165.3mAh/g in the voltage range of 3.0-4.3V and good capacity retention ratio of 94.5% after 30 charge /discharge cycle. We have successfully prepared the layered structure LiNi_1/3Co_1/3Mn_1/3O₂ with Crcontents by the citric acid sol-gel method. Electrochemical test indicated that the cycling performance of LiMn_1/3Ni_1/3-0.05Co_1/3O₂Cr_0.05 can be significantly improved with the increasing of Cr contents, although the initial discharge capacity of the sample has a little decrease.