【作者】 刘智敏；

【导师】 刘业翔； 胡国荣；

【作者基本信息】 中南大学 ， 有色冶金， 2009， 博士

【摘要】 开发高性能、低成本的新型电极材料一直是锂离子电池的研究方向。多元过渡金属氧化物正极材料LiNi_xCo_1-2xMn_xO₂与LiCoO₂比较,价格相对低廉,质量比容量相当,热稳定性能和循环性能更好,被认为是最具开发应用前景,现实替代LiCoO₂的新型正极材料之一,目前该材料已经逐步走向市场,应用领域不断扩大。然而,LiNi_xCo_1-2xMn_xO₂材料属于多元复合氧化物,制备过程较LiCoO₂复杂。本文以制备高性能可商业化应用的LiNi_xCo_1-2xMn_xO₂系列材料为目标,选取低Co含量的LiNi_0.4Co_0.2Mn_0.4O₂材料为主要研究对象,在合成方法上寻求探索和突破,采用喷雾热分解法和控制结晶法两种技术路线合成层状结构材料LiNi_0.4Co_0.2Mn_0.4O₂。同时,分析LiNi_xCo_1-2xMn_xO₂系列材料由于过渡金属元素含量的变化带来材料性能上的差异,以及阴阳离子掺杂对材料在高截止电压下循环性能的影响。论文主要的研究工作如下:（1）喷雾热分解机理模型的建立和实验条件研究。针对合成过程易产生空心破碎粒子,降低材料振实比重的现象,在分析喷雾热分解固体颗粒形成过程步骤的基础上,建立关键控制步骤的数学模型,并提出合成高比重实心粒子的机理模型。机理模型认为,溶剂从液滴表面蒸发液滴收缩变小的过程中,在液滴表面达到过饱和浓度开始出现晶核（t=（?）₁）和液滴表面壳体完全形成（t=（?））两个时刻,如液滴中心浓度达到溶液的饱和平衡浓度,即C_A（0,（?）₁）≥C_AE,液滴才能形成实心颗粒。而后,通过不同的工艺条件小试实验,探讨喷雾热分解过程中影响粉体形貌的各种因素。研究表明,在前驱溶液中添加一定量的尿素,经热分解过程在液滴内分解成的OH^-和NH₄⁺有利于生成成核晶种,从而有利于实心粒子的形成。（2）率先进行了喷雾热分解技术在锂离子电池正极材料制备领域应用的工程化实践。在小试实验确定优化合成工艺条件的基础上,自行设计一套喷雾热分解中试设备,并进行放大试验,研究合成条件对LiNi_0.4Co_0.2Mn_0.4O₂性能的影响,探索喷雾热分解技术合成LiNi_0.4Co_0.2Mn_0.4O₂材料产业化应用的可行性。试验结果表明,经喷雾热分解合成的前驱体,可以在较低的后续烧结温度（900℃）,较短的烧结时间（6h）制备出晶型完备,电化学性能优良的正极材料LiNi_0.4Co_0.2Mn_0.4O₂。（3）控制结晶法制备LiNi_0.4Co_0.2Mn_0.4O₂中试生产技术研究。采用氢氧化物控制结晶法合成LiNi_0.4Co_0.2Mn_0.4O₂时,主要探讨前驱体的合成条件对形貌、粒度、振实比重、比表面积等物理性能的影响。pH值、氨水含量对前驱体微观晶粒形貌影响显著,提高pH值和合成温度,增加反应溶液中的氨水含量,有利于提高前驱体的振实比重,减小比表面积。研究锂化烧结温度和气氛对材料性能的影响,最佳的烧结制度为:100℃/h升温至750℃保温10h而后再升温至950℃保温10h。在此实验基础上,形成了整套控制结晶法合成LiNi_0.4Co_0.2Mn_0.4O₂中试生产技术,包括原料物理化学指标、工艺技术参数、前驱体合成和锂化合成的主要设备。（4）系统研究了LiNi_xCo_1-2xMn_xO₂（x=0.25,0.33,0.4,0.5）系列材料性能上的差异。采用喷雾热分解技术合成不同组分的LiNi_xCo_1-2xMn_xO₂（x=0.25,0.33,0.4,0.5）系列材料,研究组分的变化对材料结构和电化学性能的影响。Co含量的增加有利于提高材料的二维层状结构特征及结构的稳定性,降低金属阳离子的混排现象,改善材料的循环性能和倍率性能。提高截止电压LiNi_xCo_1-2xMn_xO₂系列材料可获得更高的比容量,但是在循环过程中的衰减加剧。（5）研究Ti、F掺杂对层状LiNi_0.4Co_0.2Mn_0.4O₂材料电化学性能的影响。Ti、F掺杂可明显改善LiNi_0.4Co_0.2Mn_0.4O₂在高截止电压下的循环性能。掺杂材料Li_0.4Co_0.2Mn_0.4-xO_2-y（x=0.02,y=0.06）在2.5～4.7V电压区间首次容量达193.5 mAh·g^-1,40次循环容量保持在91.5%。更多还原

【Abstract】 Intensive research and development work is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electode materials.Multi-element transition metal oxides of LiNi_xCo_1-2xMn_xO₂ is considered as a potential and active one of the new cathode materials of Li-ion batteries to replace LiCoO₂,for its inexpensive,which also has higher specific capacity,better thermal stability and better cyclic performance compared with LiCoO₂.Recently,this material begins to be commercially employed and has been applied in more and more fields.The multiple complex oxides of LiNi_xCo_1-2xMn_xO₂ have more complicated preparation than LiCoO₂.In this dissertation,the research aims at synthesizing LiNi_xCo_1-2xMn_xO₂ that can reach a level of commercial application.The layer-structured material of LiNi_0.4Co_0.2Mn_0.4O₂ with low Co content is selected to be emphases of the research works.To make a breakthrough in prepared method, the materials had been synthesized by two different routes as spray pyrolysis and co-precipitated metal hydroxide by controlling crystallization.In addition,the effect of different transition-metal content on the structural and electrochemical characteristics had been studied.The influence of doping anion and cation on the cyclic performance at higher cut-off voltage were also investigated.The main results are as follows:（1） The mechanism model of spray pyrolysis was established and the experimental conditions was researched.To overcome low tap density caused by the hollow and crushing phenomenon,in this work,on the basis of studying the process stages of the particle formation in spray pyrolysis,the mathematical model of the key controlling stages was established and the mechanism model of forming the filled dense particle was proposed.Mechanism model is described as follows:during the solvent evaporation from a free liquid surface,at the moments of the solute concentration on the droplet surface reaching its critical supersaturation（t=（?）₁） and full crust forming（t=（?））,if the droplet center concentration reached equilibrium saturation of solute,the liquid droplet would form the filled dense particle.The effects on the particle morphology were also studied by small experimental with different synthesis conditions.Moreover,the experimental results showed that addition of polymeric precursor solutions containing urea,which decomposed and produces OH^- and NH₄⁺ as the precipitation during spray pyrolysis process,was advantageous to formation of filled dense particle.（2） The engineering practice of spray pyrolysis in the field of preparing lithium-ion battery cathode material were implemented firstly.To explorate the feasiblitity of realizing industrial applications of spray pyrolysis,based of the small experiment,the pilot experiment was implemented on pilot experimental platform designed by ourself and the effect of synthesis conditions on the performance of LiNi_0.4Co_0.2Mn_0.4O₂ was studied.The test result indicated,when the precursor prepared by spray pyrolysis was used as primal materials,LiNi_0.4Co_0.2Mn_0.4O₂ with the perfect crystal and excellent electrochemical performance could be synthesized by low temperature（900℃） and short sintering time（6h）.（3） The pilot technique of LiNi_0.4Co_0.2Mn_0.4O₂ was also synthesized by co-precipitated metal hydroxide by controlling crystallization.The effect of operating conditions on precursor morphology,particle size,tap density and specific surface area was investigated.PH and ammonia content significantly influenced micro-morphology of precursor.With increasing pH,preparing temperature and ammonia content in reaction solution,tap density of precursor was increased and specific surface area was decreased. Meanwhile,influence of sintering temperature and sintering atmosphere of lithiation sinter on material performance was investigated.The optimized sintering conditions was obtained as follows:At the elevating rate of 100℃/h,the material was sintered at 750℃for 10h,then,raise to 950℃for 10h.On this basis,a pilot-production technique of co-precipitated metal hydroxide by controlling crystallization was developed to prepare LiNi_0.4Co_0.2Mn_0.4O₂.The technique includes:the physical & chemical index,technology technique parameter and main equipment.（4） A systematic study of the performance difference on LiNi_xCo_1-2xMn_xO₂ is conducted.The series composites of LiNi_xCo_1-2xMn_xO₂（x=0.25,0.33,0.4,0.5） were synthesized by spray pyrolysis.The influence of different transition-metal content on the structural and electrochemical characteristics were studied.The increase of Co content improved characteristics of two-dimensional layered structure and structure stablitity,reduced the degree of the cation mixing and improved the cyclic performance and rate capability.The capacity of LiNi_xCo_1-2xMn_xO₂ increases with higher cut-off voltage,but the capacity retention in the cyclic tests become worse.（5） The effects of doping F and Ti on the structure and electrochemical performance of LiNi_0.4Co_0.2Mn_0.4O₂ were studied. The results indicated that the material with doping F and Ti could help to improve the cyclic performance at higher cut-off voltage. LiNi_0.4Co_0.2Mn_0.38Ti_0.02O_1.94F_0.06 exhibited the specific capacity of 193.5 mAh·g^-1,and its capacity retained 91.5%after cycling 40 times in the voltage range of 2.5～4.6V.更多还原