【作者】 岳鹏；

【导师】 王志兴；

【作者基本信息】 中南大学 ， 冶金工程， 2013， 博士

【摘要】 层状LiNi1-x-yCoxMnyO2材料兼具了LiNiO2、LiCoO2和LiMnO2三者的优点,成为极具应用前景的锂离子电池正极材料之一。但是,LiNi1-x-yCoxMnyO2依然存在一些问题,如容量衰减较快和倍率性能较差等,从而限制了其应用领域。因此,本文采用不同方法合成两种镍基LiNi1-2xCoxMnxO2(x=0.01和0.02)材料,并通过表面修饰及掺杂等手段,改善电化学性能。采用喷雾干燥法和共沉淀法合成了层状LiNi1-2xCoxMnxO2(x=0.01和0.02),研究合成条件及合成方法对材料结构和电化学性能的影响。通过Rietveld结构精修发现,采用喷雾干燥法合成的LiNi1-2xCoxMnxO2样品阳离子混排程度分别为5.5%(x=0.01)和4.3%(x=0.02)；共沉淀法合成的LiNi1-2xCoxMnxO2样品阳离子混排程度低于喷雾干燥法合成的样品,分别为3.8%(x=0.01)和3.1%(x=0.02)。电化学阻抗与电化学测试表明,共沉淀法合成的LiNi1-2xCoxMnxO2电荷转移阻抗更小,电化学性能优于喷雾干燥法合成的样品。以LiNi0.8Co0.1Mn0.1O2为例,在2.8～4.3V电压范围内,喷雾干燥法合成的LiNi0.8Co0.1Mn0.1O2在0.1C、0.5C、1C、2C和5C的放电容量分别为194.4、177.8、166.5、153.5和127.5mAh g-1;在0.5C、1C和2C循环50次后的容量保持率分别为87.8%、82.9%和80.3%。共沉淀法合成的LiNi0.8Co0.1Mn0.1O2在0.1C、0.5C、1C、2C和5C的放电容量分别为186.6、173.7、165.7、155.8和138.6mAh g-1;在0.5C、1C和2C循环50次后的容量保持率分别为91.1%、86.5%和83.8%。采用喷雾干燥法合成了LiNi1-2xCoxMnxO2/RGO (Reduced Graphene Oxide,还原氧化石墨烯)复合材料。研究结果表明复合材料具有α-NaFeO2型层状结构,且LiNi1-2xCoxMnxO2颗粒被薄层RGO包裹。电化学测试表明LiNi0.6Co0.2Mn0.2O2/RGO复合材料的电化学性能优于LiNi0.6Co0.2Mn0.2O2材料,然而LiNi0.8Co0.1Mn0.1O2/RGO复合材料的电化学性能不如LiNi0.gCo0.1Mn0.1O2。以LiNi0.8Co0.1Mn0.1O2和RGO为原料,采用固相混合法合成了LiNi0.8Co0.1Mn0.1O2/RGO复合材料,其中RGO含量为2%。复合材料0.1C的首次放电容量为192.5mAhg-1,其倍率性能和循环性能均优于LiNi0.8Co0.1Mn0.1O2。电化学阻抗表明RGO的添加降低了复合材料的电荷转移阻抗,有利于改善电极充放电过程中的动力学性能。以LiNi1-2xCoxMnxO2和NH4F为原料,采用低温氟化反应合成了氟掺杂的LiNi1-2xCoxMnxO2-zFz,研究了不同的氟掺杂量对材料结构、元素价态及电化学性能的影响。LiNi1-2xCoxMnxO2-zFz(0≤z≤0.06)样品均为具有层状结构的纯相物质；随着氟掺杂量的提高,阳离子混排程度增加；XPS分析表明材料中过渡金属的元素价态发生变化,以对掺入的氟离子进行电荷补偿,进而引起材料晶格参数的变化。电化学测试表明适量的氟掺杂尽管略微降低了材料的初始放电容量,但改善了材料的倍率性能、循环性能和存储性能。TEM和XRD证实了氟掺杂有利于稳定材料的结构,抑制电解液中HF对材料颗粒表面的侵蚀。而电化学阻抗表明氟掺杂可以有效降低材料的电荷转移阻抗,改善充放电过程中的动力学性能。图99幅,表38个,参考文献271篇。更多还原

【Abstract】 Layered LiNi1-x-yCoxMnyO2materials have been considered as a promising cathode material for lithium ion batteries in that it owns the advantages of LiNiO2, LiCoO2and LiMnO2. However, there are still some obstacles limiting its applications, such as the loss of capacity during cycling and insufficient rate capability. In this thesis, the Nickel-based LiNi1-2xCoxMnxO2(x=0.01and0.02) cathode materials have been synthesized by different methods, and modified to improve its electrochemical performance.Layered LiNi1-2xCoxCoxMnO2(x=0.01and0.02) cathode materials have been synthesized by spray drying and co-precipitation methods. The influences of synthesis conditions and methods on the structural and electrochemical performance of LiNi1-2xCoxMnxO2materials have been investigated. Rietveld refinement showed that the cation mixing degree of LiNi1-2xCoxMnxO2materials prepared by spray drying method were5.5%(x=0.01) and4.3%(x=0.02), LiNi1-2xCoxMnxO2materials prepared by co-precipitation method exhibited lower degree of cation mixing, which were3.8%(x=0.01) and3.1%(x=0.02). Electrochemical test and electrochemical impedance spectroscopy (EIS) showed that LiNi1-2xCoxMnxO2materials prepared by co-precipitation method exhibited lower charge transfer resistance and better electrochemical performance than sample prepared by spray drying method. Take LiNi0.8Co0.1Mn0.1O2materials as example, the discharge capacities of sample prepared by spray drying method were194.4,177.8,166.5,153.5and127.5mAh g-1at0.1C,0.5C,1C,2C and5C between2.8and4.3V. Capacity retentions after50cycles were87.8%,82.9%and80.3%at0.5C,1C and2C, respectively. LiNi0.8Co0.1Mn0.1O2materials prepared by co-precipitation method were186.6,173.7,165.7,155.8and138.6mAh g-1at0.1C,0.5C,1C,2C and5C between2.8and4.3V. Capacity retentions after50cycles were91.1%,86.5%and83.8%at0.5C,1C and2C, respectively.LiNi1-2xCoxMnxO2/RGO (Reduced Graphene Oxide) composites were prepared by a simple spray drying method. XRD showed that composites possessed a typical hexagonal structure. Raman and FTIR confirmed the existence of RGO in the composites. SEM and TEM verified that LiNi1-2xCoxMnO2particles were wrapped with RGO sheets. LiNi0.6Co0.2Mn0.2O2/RGO composite displayed improved electrochemical performance than pristine LiNi0.6Co0.2Mn0.2O2materials. However, LiNi0.8Co0.1Mn0.1O2/RGO composite prepared by spray drying method showed worse electrochemical performance. Thus, using LiNi0.8Co0.1Mn0.1O2and RGO as raw materials, composite with RGO content of2%was prepared by physical mixing method. The composite displayed an initial discharge capacity of192.5mAh g-1, and its electrochemical performance was better than that of pure LiNi0.8Co0.1Mn0.1O2materials. EIS showed that the RGO can greatly reduce the charge transfer resistance, improving the kinetic behaviors during charge/discharge process and affecting the electrochemical performance.Fluorine substituted LiNi1-2xCoxMnxO2-zFz cathode materials have been synthesized by calcining NH4F with corresponding LiNi1-2xCoxMnxO2materials at a relatively low temperature. The structure, ionic valency and electrochemical performance of LiNi1-2xCoxMnxO2-zFz materials have been investigated. LiNi1-2xCoxMnxO2-zFz(0<z<0.06) materials possessed a typical hexagonal structure without impurity. However, the degree of cation mixing increased with the increase of fluorine contents. XPS proved that the valence of transition metals changed with fluorine substitution, which caused the change of lattice constant. Although the fluorine substituted materials showed somewhat lower initial discharge capacity, it exhibited improved cycling performance compared with the pristine material. XRD and TEM provided evidences that fluorine substitution protected the electrode from HF attack and maintained the structure stability of electrode. As shown in EIS, Rct values of fluorine substituted samples were reduced compared with pristine electrode. The small charge transfer resistance suggested that a small amount of fluorine substitution can improve the kinetic behaviors during charge/discharge process, affecting the electrochemical performance.更多还原