【作者】 杜柯；

【导师】 张宏；

【作者基本信息】 中国科学院研究生院（上海微系统与信息技术研究所） ， 材料物理与化学， 2003， 博士

【摘要】 锂离子电池的迅速发展要求进一步研究和改善正极材料的性能，本文从合成工艺和元素掺杂（替代）的角度，分别对锂离子正极材料LiMn₂O₄和LiCoO₂的性能进行了研究和改进，同时还初步制备和考察了LiMn₂O₄薄膜的结构和电化学性能。 在柠檬酸络合法制备尖晶石型LiMn₂O₄的基础上，采用快速的无焰燃烧办法代替长时间的真空络合过程，从而大大减少了合成时间和降低了设备要求。考察了乙二醇、pH值和焙烧温度对最终产物的结构和电化学性能的影响，实验表明使用适量的乙二醇，控制pH=7左右，经过800℃恒温焙烧24h可以得到结构完整，电化学性能优良的纳米-亚微米大小的LiMn₂O₄晶体。其透射电镜观察到的直径大小为10～100nm，粒度分析仪观察到的团聚颗粒直径大小为6μm左右。循环伏安和交流阻抗实验均表明采用该法合成的LiMn₂O₄与通常固相法合成的LiMn₂O₄电化学特征相似。采用该法合成的LiMn₂O₄具有比由固相法合成的LiMn₂O₄更大的初始容量，可以达到130mAh／g，但容量下降较快。200K-RT的电阻与温度关系的实验表明LiMn₂O₄为半导体，电阻率为10³-10⁶Ωcm。活化能的计算证明当淬火温度高于800℃时，活化能和电导率E_a都明显下降。同时在近室温处存在一个相变点。恒电位阶跃实验显示了Li离子在LiMn₂O₄材料中的嵌入过程是与初始和终点电位所处的相区，即电位阶跃所跨越的相区有关，当电位阶跃发生在一个相区内时，（dI）/（dt）曲线上不出现拐点：当电位阶跃跨越一个两相区时，（dI）/（dt）曲线上出现两个拐点；当电位阶跃跨越两个两相区时，（dI）/（dt）曲线上出现四个拐点。本文认为恒电位阶跃下，LiMn₂O₄中Li离子的输运行为与初始和终端电位所处的平衡相有关，即不仅涉及单相的扩散而且和异相相界面的形成和运动有关。根据恒电位阶跃后期的电流一时间行为，计算得到Li离子的化学扩散系数在10^-12cm²S^-1量级。 为提高柠檬酸无焰燃烧法制备的LiMn₂O₄的循环性能，本文利用柠檬酸无焰燃烧法的均匀性优势，合成了B、Al、Cr、Fe、Co、Ni、Ga掺杂的LiMn₂O₄化合物。充放电循环实验表明除了B，其余掺杂化合物均在一定程度上提高了铿离子正极材料的制备与性能改进LIMnZO;的循环性能。其中Cr是最佳的掺杂元素，XRD显示有少量的Cr进入LIMllZO;晶格，引起其晶格收缩，稳定但没有破坏LIMnZO;晶胞的结构，在容量降低较小的情况下，较大的提高了LIMnZO;的循环性能。这与Cr原子的大小、电子构型以及与Cr一O化学键键能有关。进一步的实验显示LIMn20;在循环中的容量损失主要发生在4V以上部分，这很可能是高电位下，一定量的尖晶石溶解以及结构的收缩而堵塞部分Li离子脱出的结果。而Cr的掺入正是通过稳定结构来抑制了这部分容量的衰减，从而提高了LIMllZO;的循环性能，使得柠檬酸无焰燃烧法合成的掺C:的LIMllZO;既有较高的初始容量，又具有良好的循环性能。 本文首次研究了Pt掺杂对LICOO:电化学性能的影响，观察到生成的LICOOZ一LiZPto3一R多相系统大大提高了Licoo:的大电流放电的性能。其中最初掺入5%Pt而形成的LICoo:基复合体，在扣式Li电池中1 OC放电时的前50次容量保持在87mA川g以上，1 6c的初始容量亦在80 mA份g以上。恒电位阶跃法、恒电流滴定法和交流阻抗法测得的化学扩散系数均显示了Li离子在掺R样品中比在纯LICoOZ中有更大的扩散速度。提出相界增强扩散模型对高倍率性能改善进行解释，并认为正极材料的复合体很可能是提高正极材料的大电流放电能力的尝试方向。 采用溶胶凝胶法旋转涂布工艺，我们在R片上制备了非致密的LIMllZO;薄膜，其晶型完整，表面晶粒大小约为100二，膜的厚度约为200mn。充放电特性与体相材料一致，但具有优异的电化学性能，放电比容量高达75 oA扮（c mZom），可承受lmA/c mZ的电流，此时的放电比容量仍有49pA川（c mZpm）。其极佳的循环性能表现为400次循环之后容量没有衰减。更多还原

【Abstract】 The rapid development of lithium ion battery asks for more clearly understanding about positive electrode materials and improving urgently their electrochemical performance. This dissertation tries to meet partly this goal. The dissertation is involved in the three issues: the synthesis and electrochemical properties of nano- and sub-micrometer LiMii2O4 and its Mn-substituted compounds by a soft chemical method; the improved rate performance in multi-phase LiCoCVbased composite due to Pt additive; LiMii2O4 thin film by a sol-gel technique and its electrochemical properties.A modified citrate route with combustion for LiMii2O4 has been developed, which is simpler, time-saving and cost-cheaper. The effects of glycol addition, pH value of solution on the precursor and calcining temperature on the phase structure and charge-discharge performance have been investigated. It was optimized that the process with suitable amount of glycol, pH value of near 7 and the calcination at 800 for 24h could prepare a single phase LiMn2O4 with the spinel structure. TEM observation showed that the powder size of the typical synthesized product was 10-100nm, but they usually aggregated as granules with the size of about 6 ?m, much less than the commercial LiMnO2 The LiMn2O4 synthesized by such a processing also showed a better electrochemical performance, compared with commercial one, for the electrode application. The initial specific capacity was about 130mAh/g, but its cycleability was worse. Its electrochemical characteristics were investigated by CV and EIS techniques.Constant potential step experiment showed that the Li ion transport behavior was closely connected with the initial potential and potential step. There were three types of current relaxation curve: monotonously, with a maximum-minimum inflection andwith two maximum-minimum inflections. It is preliminarily suggested that the complicated behavior is not only connected with the diffusion in a phase, but also the evolution of phase component and movement of phase boundary during current relaxation process. Under this frame, the understanding of the inflection is given.LiMxMn2.xO4 was also synthesized by the modified citrate route to examine the elemental substitution effect on the electrochemical properties, where M is Ek AK Q\ Fe. Co. NK Ga. Except B, all substituted LiMxMn2-xO4 showed an improved cycling characteristics with a reduced slightly initial reversible capacity, Among them, Cr was the most effective, for example, the loss of discharge specific capacity in first 100 cycles for LiCr0.2Mni g at 0.5C was lower than 0.1% per cycle, however it was 0.23% per cycle for LiMn2O4. It is because of radius of Cr3+ a little less than that of Mn3+, more stable electronic configuration and stronger bond energy with O atom. With Cr substitution, the step for up-plateau around 4.1V and down-plateau around 4.0V was gradually smeared out. The experiment also demonstrated that the lost capacity of LiMCU almost occurred above 4V It may be caused by the Mn dissolving in electrolyte at high potential and the shrinking and/or destroying of spinel lattice at Li-deficient state, which limits the de-intercalating of lithium ion. The substitution of Cr for Mn can weaken those effects to stabilize the Li-deficient structure. The synthesized LiMn2O4 showed similar electrical conductivity behaviors as the one by solid-state reaction process.The influence of the initial addition of Pt (Li:Co:Pt=l:l-x:x) on the microstructure and electrochemical properties of LiCo02-based material was first investigated. A small amount of Pt additive induced Li2PtO3 and Pt in matrix LiCoO2 to form a multi-phase composite. This multi-phase composite showed the improved rate performance and cycleability at high rate in comparison with the single-phase of LiCoO2, for example, at discharge of IOC, the discharge specific capacity for LiCoO2-base sample with x= 5% kept above 87mAh/g after 50 cycle, however, it was lower than 30 mAh/g for a single phase of LiCoO2. The improved rate performance is attributed更多还原