【作者】 王翠莲；

【导师】 张建民； 陈卫华；

【作者基本信息】 郑州大学 ， 物理化学， 2013， 硕士

【摘要】 随着全球经济的发展,环境恶化和资源紧缺问题在全世界范围内越来越受到人们的关注,发展高效环保的新能源一直是各个国家的努力方向。锂离子电池以其功率密度大、毒性小、无公害等优点在电子便携设备、航空航天、军事等领域得到了普遍的应用,并且已经开始用于节能环保型的轻型电动车、新能源汽车领域,这在很大程度上能够有效的缓解环境和资源的问题。但是,为了进一步拓展锂离子电池在相关领域的广泛应用,其功率密度、能量密度等有待提高。由于锂离子电池构造上存在正极材料限容的特点,因此,正极材料在锂离子电池各组成部件中起着至关重要的作用,从而对正极材料的研究显得尤为必要。新一代层状富锂固溶体材料Li[LixM1-x]O2(M为一种或几种过渡金属离子),由于其较高的放电比容量、较好的循环稳定性能以及新的充放电机制,被认为是前景良好的正极材料。基于此类材料本文的主要研究工作分为以下几个部分：(一)首先通过常规共沉淀方法合成氢氧化物前驱体Mn0.67Ni0.33(OH)2,然后与含锂化合物混合并通过固相反应合成层状富锂正极材料Li[Li0.2Ni0.26Mn0.54]O2。物理表征表明,所制备的材料具有典型的层状α-NaFeO2结构,由不规则的谷粒状颗粒组成；电化学测试结果表明,所制备的材料的首次放电比容量为212mAh/g,30次循环后材料的容量保持率为81%,且倍率性能较差。(二)在常压水浴条件下对上述共沉淀方法所制备的前驱体Mn0.67Ni0.33(OH)2进行C02+阳离子交换处理,在前驱体中引入钴元素,从而调节其电化学性能。在常压低温水浴条件下,使得Co2+与前驱体Mn0.67Ni0.33(OH)2发生离子交换。经过混锂煅烧后,制得含Co三元富锂材料Li[Li0.2Ni0.26Mn0.54Cox]O2。研究结果表明,随着水浴温度的升高,进入材料中的Co的量逐渐增加,对应材料的电化学性能也逐渐得到改善。这是由于交换进去的Co2+可以参与可逆的氧化还原反应,提高材料的比容量；同时也能降低Ni的含量,进而减弱Li/Ni混排阻碍Li+可逆脱嵌的现象,提高材料的循环稳定性。(三)在高压水热条件下,提高阳离子交换反应的温度,在前驱体Mn0.67Ni0.33(OH)2中引入Co2+,混锂煅烧后获得含Co三元富锂材料Li[Li0.2Ni0.26Mn0.54Cox]O2。这是鉴于常压水浴条件下的限制因素和该条件下制备的系列材料所呈现的随温度的增加综合性能逐渐变好的变化规律而做的改性研究。结果表明,所制备的材料的首次放电比容量、循环稳定性及倍率性能均优于不含Co的二元材料Li[Li0.2Ni0.26Mn0.54]02;从110℃到150℃,随着反应温度的升高,各个温度对应的材料中Co的含量增加,综合性能也逐渐得到提高；当温度从150℃继续增加至160℃、180℃时,Co含量出现了先增加后降低的趋势,对应材料的电化学性能却呈现逐渐降低的趋势。可见,尽管150℃对应材料中Co含量不是最高,但材料的综合性能在150℃时达到了最优,其首次放电比容量、循环稳定性以及倍率性能均优于其他条件相同时水浴中制备的性能最优的材料。得出以上结果,一方面是因为引入的Co既能能起到活性元素的作用,也能削弱Ni离子占据Li层时对Li+扩散的阻碍作用,同时也有助于稳定材料的层状结构；另一方面是因为,在高压水热条件下可以得到具有更高结晶度的含Co富锂材料；当然,当Co的量增加到一定程度时会导致Mn的量过多减少时,材料的MnO6结构框架反而受到破坏,因此材料的电化学性能反而会变差。(四)采用阳离子交换方法在水热条件下制备粒径均匀、堆积致密的球形前驱体MnzNi1-z(OH)2,混锂煅烧后制备了系列Ni、Mn二元层状材料,探索了不同混锂量对材料电化学性能的影响。结果显示,随着混锂量的增加,材料的比容量呈现逐渐增加的趋势,当混锂量增加到一定程度时,对应材料的首次充电过程中出现了层状富锂材料λLi2MnO3·(1-λ)LiMO2(M=Ni、Co、Mn、Cr中的一种或其任意组合)的特征电压平台。据此可知,通过此种方式很有可能得到形貌良好的层状富锂材料。然而,所得材料的比容量偏低。因此,为了使用此种方式得到结晶良好、电化学性能优秀的层状富锂材料,有必要对可能影响材料性能的其他条件(比如混锂研磨的方式、研磨时间的长短、材料的烧成制度等)进行探索。更多还原

【Abstract】 Along with the development of global economy, the problems in environment degradation and the shortage of resources have aroused the global attention. Many countries are trying their best to develop efficient and environmentally friendlynew energy, in the hope of alleviating effectively or solving the problems in environment and resources. With the advantages, such as large power density, low toxicity and rare pollution, lithium ion batteries can alleviate effectively the problems in environment and resources to a great extent for its wide use in portable electronic devices, aerospace, military areas and its tentative use in light electric vehicle(LEV), new energy automobile and so on. However, lithium ion batteries must be improved in the density and power density to be applied widely. In addition, cathode material plays a vital role in comprehensive performance of the lithium ion battery, especially in capacity, so it is very necessary to do more research on cathode for lithium ion battery.The layered Li-rich Li1+2M1-zO2, which be of new generation cathode (M is one or more than one kind of transition metal elements, z≥0), has attracted a wide attention of many researchers for its high discharge capacity, excellent cycleability and new charge and discharge mechanism and is regarded as a promising cathode material for future Li-ion battery. Based on such materials, the main contents of this thesis are as follows:(1) Layered Li-rich Li[Li0.2Ni0.26Mn0.54]O2cathode material was firstly synthesized via conventional co-precipitation and then solid state reaction method. Physical characterization revealed that the prepared sample behaves the typical layered a-NaFeO2structure and an irregular morphology, consisted of small grains. The electrochemical tests indicated that the material provides a discharge specific capacity of212mAh/g in first cycle. After30cycles, the discharge capacity remains81%of that of the first cycle. However, the prepared material shows a poor rate capability.(2) Under ordinary pressure and water bath condition, the as-prepared coprecipitation precursor Mn0.67Ni0.33(OH)2was treated using Co(NOs)2solution via cation exchange method, and then series of Co-cotaining layered Li-rich material Li[Li0.2Ni0.26Mn0.54Cox]O2were obtained. The research results proved that the initial discharge capacity becomes bigger and cycling stability becomes better with the increasing of temperature. This is closely accordant to the increasing Co content entering into Li[Li0.2Ni0.26Mn0.54]02as temperature increases. On one hand, this law can be attributed to that the Co can act as active material to make a bigger specific capacity, and on the other hand, Co can reduce the Ni content to weaken the disordering between Li and Ni in the metal layers and lithium layers, which may cause the phenomenon of Ni ion occupying in lithium layer to block the pathway of Li+diffusion, and improve the cycle stability of the material.(3) Under hydrothermal condition, as well as high pressure and higher temperature, the precursor Mno.67Nio.33(OH)2was treated using Co(NO3)2solution via cation exchange, and then series of Co-cotaining layered Li-rich materials Li[Li0.2Ni0.26Mn0.54Cox]O2were prepared. It came out that all the Li-rich Co-containing materials prepared under hydrothermal condition offere a bigger initial discharge capacities, better cycle stability and rate performance than the material Li[Li0.2Ni0.26Mn0.54]O2without Co. As hydrothermal temperature increases to150℃from110℃, the Co content in corresponding materials increases and the comprehensive properties of the obtained materials get better. When temperature increases to160℃and continues to increase to180℃from150℃, the Co content increases first and then decreases. However, the electrochemical properties of materials become worse gradually. It can thus be concluded that, in spite of without the most Co content, the material obtained at150℃shows the biggest initial discharge capacity, as well as the best cycle stability and rate performance. All the properties of these materials are superior to the optimum Co-containing Li-rich material obtained at water bath temperature. On the one hand, the above results can be attributable to that participation of Co can play the role of active elements and weaken the disordering between Li and Ni in the metal layers and lithium layers to prevent the Ni ion occupying in lithium layer to block the pathway of Li+diffusion. On the other hand, this is because, under high pressure hydrothermal condition, Co-cotaining material with a higher crystallinity can be obtained. However, when Mn content is insufficient to support MnO6octahedral structure framework, owing to the Co content increasing to to a certain value, the electrochemical performance will get worse instead.(4) Quasi spherical MnzNi1-z(OH)2with uniform size was prepared via similar cation exchange method mentioned above, and then series of layered materials containing different content of Ni and Mn were prepared. And the effect of different amount of lithium on the performance was investigated. It turned out that the initial discharge capacity increases as the amount of Li increases, and also a better cycling stability appeares. When the Li content increases to a certain extent, the obtained material exhibits the inherent characteristic of layered Li-rich λLi2MnO3-(1-λ,)LiMO2(M=Ni、Co、Mn、Cr or any combination of them) in the initial charging curve. It proved that layered Li-rich material with a fine morphology is likely to be prepared by the method mentioned above. However, the materials obtained showes a poor comprehensive performance, especially a lower specific capacity. It seemed that, to obtain the Li-rich layered material with a fine morphology and an excellent electrochemical property, it is very necessary to make more attempts and explorations, such as grinding way and time, sintering system and so on.更多还原