【作者】 陈宏浩；

【导师】 周运鸿；

【作者基本信息】 武汉大学 ， 物理化学， 2005， 硕士

【摘要】 锂离子电池由于具有高电压、能量密度大、自放电小、无记忆效应等优点,近十几年来取得了飞速的发展,在笔记本电脑、移动电话、摄录机等移动电子终端设备领域占据了主导地位。同时,它在军事、航天和电动汽车等领域的应用也日益受到重视。 到目前为止,大多数产业化的锂离子电池所采用的正极材料为LiCoO₂,由于钴的资源相对贫乏、价格高、毒性较大,且只能提供150mAh/g左右的实际容量,极大地制约了锂离子电池的进一步发展和应用;LiNiO₂虽然价格相对便宜,而且比容量较高,但是合成计量比的LiNiO₂非常困难,同时由于脱锂状态下的Li_1-xNiO₂热稳定性差会给电池带来严重的安全隐患,因此,这种材料的实际应用还面临诸多问题。 由于以上两种材料各自的缺陷,研究者们在它们的基础上合成了LiNi_1-yCo_yO₂系列材料。与LiCoO₂和LiNiO₂材料相比,它不仅容量得到提高,循环性能良好,而且通过对其进一步掺杂,还能有效的改善其电化学性能及安全性能。故对于LiNi_1-yCo_yO₂系列材料的研究,受到人们的广泛关注。 本文通过结构分析以及电化学手段,全面研究了流变相法合成LiNi_xCo_1-xO₂时条件的控制,并在此基础上,对掺杂型LiNi_0.85Co_0.15O₂正极材料的结构、表面性质、热稳定性以及电化学性能进行了分析,研究内容可以分为以下几个部分: 1) 采用流变相法合成LiNi_1-yCo_yO₂系列材料,优化合成条件。 对采用流变相法合成的前体进行烧结处理,得到LiNi_1-yCo_yO₂。通过Ni、Co配比和烧结温度的改变对合成条件加以优化。XRD和电化学测试结果表明,在氧气氛围中,800℃下烧结6小时得到的LiNi_0.85Co_0.15O₂材料具有完整的层状结构和优良的电化学性能。首次放电容量可以达到198.2 mAh/g,经过20周循环后容量保持率仍然维持在90%左右。与高温固相法相比,采用流变相法合成的样品无论是在结构的完整性还是电化学性能上都有明显改善。 2) 对掺铝层状锂镍钴氧化物的研究。 通过流变相法合成了系列掺铝LiNi_0.85Co_0.15-yAl_yO₂材料,初步考察了铝掺杂对LiNi_0.85Co_0.15O₂材料结构、电化学性能以及热稳定性的影响。XRD实验结果表明,随着掺铝量的增加,材料的晶胞参数a、c值增大,并且由于Al原子取代了更多还原

【Abstract】 In recent decade, lithium ion batteries have been rapidly developed because of their advantages on high cell voltage, high energy density, slow self-discharge, no-memory effect and etc. Till now, lithium ion batteries have occupied the main status in small portable electronic equipment, such as laptop computers, mobile phones and vidicons. At the same time, lithium ion batteries show promising prospects of application in electric vehicle, space technique and military field.Up to now, most commercialized lithium ion batteries use lithium cobalt oxides, LiCoO₂, as their positive material. But it suffers from high price and high toxicity. LiNiO₂ has been ever considered to be a promising substitute, which is with a larger discharge capacity and lower price. However, the preparation difficulties and poor safety limits its practical application. Basing upon the above two materials, LiNi_1-yCo_yO₂ was exploited. Comparing with its end compound-LiCoO₂ and LiNiO₂, LiNi_1-yCo_yO₂ shows a higher capacity and better cycleability, moreover its electrochemical performance and safety property can be further improved through doping treatment. So it won great attention since it was firstly proposed.In this thesis, a rheological phase method was adopted in the sample preparation. The influences of the synthetic condition were comprehensively studied through structural and electrochemical characterization. Basing on the establishment of the optimum preparation condition, several doping phases were further obtained. Their structure character, surface property, thermal stability and electrochemical performance was investigated. And the main results are as follows: i. A series of layered LiNi_1-yCo_yO₂ compounds was synthesized through rheological phase method and an optimum preparation condition was established.LiNi_1-yCo_yO₂ was obtained through the calcination of the precursor that was prepared by rheological phase method. An optimum preparation condition wasdetermined through the structural and electrochemical analysis of the sample that was obtained under different calcinations temperature and with different Co/Ni molar ration. The result of XRD measurement and cycling test shows that the resulting LiNio.85Coo.15O2 sample presents an ordered layered structure and best electrochemical performance when the precursor was calcined in oxygen atmosphere for 6h. The first discharge capacity of LiNio.85Coo.15O2 reaches 198.2 mAh/g, and more than 90% initial capacity can be retained after 20 cycles. Comparing with the sample synthesized by the solid-state method, the sample prepared by the rheological phase method shows an obviously improved structural and electrochemical property.ii. Investigation on Al-doped LiNio.85Coo.i5-yAly02 phaseA series of LiNio85Coo.i5-yAly02 samples was prepared through rheological phase method. The effect of Al-doping on the structure, electrochemical character and thermal property was discussed. It was found from structure analysis that the lattice parameters "a" and "c" of LiNio85Coo.i5-yAly02 increase with the increasing of y value, and a corresponding slight increase in the c/a ratio was also observed. The variation results from the substitution of Al for Co. In the DSC measurement, it was noticed that the Al-doping phase exhibited an improved thermal stability. It can be explained by the following reasons. The first, the structural stability of the de-lithiated phase is enhanced and some unfavorable phase transformation is depressed after Al is introduced in the framework of LiNio.85Coo.15O2; the second, the existence of A12O3 facilitates the heat release considering its good heat conductibility. Comparing to the undoped LiNio.85Coo.15O2, the Al-doped phase shows an improved cycleability. The smaller capacity fading suggests that Al-doping did favor the maintenance of the layer crystal structure during repeated cycling.iii. Investigation on other doping phase- LiNio.85Coo.i25Mo.o2502（M=Ca, Ga, Ti）LiNio.85Coo.i25Mo.o2502 （M=Ca, Ga, Ti） was also synthesized through the rheological phase method. CV and AC impedance technique was applied to examine the Li+ intercalation/de-intercalation behavior. CV measurement shows that dopingLiNio.85Coo.15O2 with Ga3+ or Ti4+ can depress the phase transformation that occurs during the charge and discharge process, especially for the Ti-doped phase. It also implies that Ga3+ or Ti4+ can enter into the crystal cell and the resulting variation in microstructure brings about the difference in Li+ insertion/ejection process. According to the result of CV experiment, it is suggested that Ca2+ is unable to be introduced into the framework of LiNio.g5Coo.15O2 due to its large ion radius, instead it exists as CaO. Although the mechanism for the notable improved cycleabilty for LiNio.85Coo.i5.x02-xCaO phase is not clear yet, it is speculated from the preliminary analysis of EIS that the role of CaO is similar to that of the coating layer. The existence of CaO is in favor of the formation of a stable interface between the LiNio.85Coo.15O2 electrode and the electrolyte, thus the decomposition of the electrolyte can be greatly avoided, which finally results in the enhancement of the cycle life of LiNixCoi.xO2.material.更多还原