【作者】 国海鹏；

【导师】 贾梦秋；

【作者基本信息】 北京化工大学 ， 材料物理与化学， 2008， 硕士

【摘要】 提高电池性能和降低电极材料的成本一直是锂离子电池的主要研发方向。目前已大规模商业化的正极材料LiCoO₂,因其存在价格昂贵和毒性等问题,故研发者一直致力于寻找其替代材料。具有优异性能的镍钴锰三元过渡金属复合氧化物被认为是最有可能替代LiCoO₂的正极材料,不同镍钴锰配比的LiNi_1-x-yCo_xMn_yO₂被广泛研究。本文在综合文献,考查锂离子电池正极材料研究进展的基础上,通过实验优选Ni、Co、Mn三者的配比,选取了电化学性能优良且成本较低的层状LiNi_1/2Co_1/6Mn_1/3O₂材料为研究重点,采用形貌表征、热重分析、结构分析和电化学性能测试等实验手段,从合成方法、形貌及粒径分布、结构特征、电化学性能等多方面对LiNi_1/2Co_1/6Mn_1/3O₂材料进行了系统深入的研究,制备出性能良好的正极材料。本实验选择以氨配合氢氧化物共沉淀法合成Ni_1/2Co_1/6Mn_1/3（OH）₂前驱体,前驱体与LiOH·H₂O在高温条件下固相反应制备LiNi_1/2Co_1/6Mn_1/3O₂的合成路线。讨论了溶液的pH值、加氨量、反应时间对前驱体物理性质的影响和对目标产物LiNi_1/2Co_1/6Mn_1/3O₂电化学性能的影响,探索出适宜于空气气氛条件下制备目标产物的前驱体Ni_1/2Co_1/6Mn_1/3（OH）₂的工艺条件为:pH=12,NH₃/M=2.25,反应时间t＞12h。在高温固相反应制备目标产物的条件的讨论中,通过热重分析确定了预处理温度为450℃,以正交实验讨论了预处理时间、焙烧温度、焙烧时间、配锂量对材料的容量的影响,优化固相合成条件为:n（Li）/n（Ni+Mn+Co）=1.05,450℃下预处理4～8h,850℃下焙烧12h。针对普通共沉淀法合成的材料首次充放电效率不高的问题,提出通过Co²⁺浓度递增的金属离子混合溶液分次共沉淀,控制NaOH溶液和金属离子溶液滴加速度和滴加的时间及反应体系的pH值等合成的条件,使生成的Ni_1-x-yCo_xMn_y（OH）₂中Co的含量递增并逐步沉积包覆在原沉淀的表面来合成前驱体,并将前驱体与LiOH·H₂O在空气气氛中高温条件下通过固相反应合成LiNi_1/2Co_1/6Mn_1/3O₂,使其形成Co元素由内到外递增的含量梯度,并通过表征确定,通过该方法合成的LiNi_1/2Co_1/6Mn_1/3O₂具有良好的形貌、晶型和电化学性能,并证明了梯度的存在有利于提高首次充放电效率,增加可逆容量,提高放电平台。800℃下合成梯度材料在2.5～4.2V以0.1C倍率充放电,首次放电效率和首次放电容量,分别为177.8 mA·h·g^-1,93.3%,多次循环后容量仍保持在171.2 mA·h·g^-1,高于商业化的LiCoO₂(140～150 mA·h·g^-1)。对LiNi_1/2Co_1/6Mn_1/3O₂进行了F^-掺杂改性,结果表明,在不牺牲容量的情况下,产物的振实密度得以提升,达到2.41 g·cm^-1,接近商业化LiCoO₂。更多还原

【Abstract】 Intensive research and development work has been conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials.LiCoO₂ has been used a major cathode material in commercial lithium 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.Ternary transition metal oxide,Li-Ni-Co-Mn-O was regarded as the possible substitute for LiCoO₂,owing to its excellent performance.Extensive researches have been made to LiNi_1-x-yCo_xMn_yO₂ at different ratio of Ni,Co and Mn.This work emphasized layed LiNi_1/2Co_1/6Mn_1/3O₂ material possessing superior performance and lower cost,based on scientific literature review and experiments verification.The synthesis,morphology,crystal structure and electrochemical behaviors of layered LiNi_1/2CO_1/6Mn₍1/3_O2 cathod material were studied in detial using X-ray diffraction,scanning electron microscope, galvanostatic charge-discharge tests and electrochemical impedance spectroscopy. LiNi_1/2Co_1/6Mn_1/3O₂ was synthesized by mixing Ni_1/2Co_1/6Mn_1/3（OH）₂ with LiOH·H₂O followed by heat-treatment.The precursor Ni_1/2Co_1/6Mn_1/3（OH）₂ was prepared via hydroxide co-precipitation using ammonia as complexant. The effect of synthesis method and synthesis conditions,such as the pH value of the aqueous solution,dosage of ammonia,time of reaction were studied. The results indicated that the optimum conditions for Ni_1/2Co_1/6Mn_1/3（OH）₂ were that the pH value was 10.5,the NH₃/M was 2.25 and the reaction time was 12h.In the optimization for high temperature solid state reaction of Ni_1/2Co_1/6Mn_1/3（OH）₂ and LiOH·H₂O,pre-heated temperatrue was selected at 450℃by thermogravimetric analysis and the effects of pre-heated time, calcination temperature,calcination time and Li/（Ni+Mn+Co）ratio on the electrochemical properties of LiNi_1/2Co_1/6Mn_1/3O₂ were characterized by orthogonal experiments.The results showed that LiNi_1/3Co_1/3Mn_1/3O₂ which were synthesized from the mixture of Ni_1/2Co_1/6Mn_1/3（OH）₂ precursor and LiOH·H₂O（Li/（Ni+Mn+Co）=1.05）sintered at 850℃for 12h after pre-heat 4～8h at 450℃in the air,exhibited better electrochemical performance.Multiple co-precipitation route was devised to improve the efficiency of the first charge-discharge cycle.Gradient precursor Ni_1/2Co_1/6Mn_1/3（OH）₂ was prepared via multiple coprecipitation of solutions with different composition of Ni²⁺,Co²⁺,Mn²⁺.The deposition with lower Co content was covered by Ni_1-x-yCo_xMn_y（OH）₂ with higher Co content in appropriate conditons.Gradient LiNi_1/2CO_1/6Mn_1/3O₂ in which the content of Co was increasing from interior to exterior was received by solid state reactions of this kind of precursor and LiOH·H₂O.Tests showed that the gradient material possessed perfect layered crystal structure and eximious electrochemical performance.Not only the first discharge specific capacity of the gradient material was higher than the homogeneous material,but the cyclic stability of the gradient material is improved greatly.The gradient material sintered at 800℃delivered high discharge specific capacity of 171.2 mA·h·g^-1after multiple cycles,which was higher than that of commercialized LiCoO₂.The gradient LiNi_1/2Co_1/6Mn_1/3O₂ material was modified by doping F^-. The results illuminated that the tap density of the material doped F^- was improved to approximately 2.41 g·cm^-1,closed to commercialized LiCoO₂.更多还原