【作者】 刘桥生；

【导师】 王海辉；

【作者基本信息】 华南理工大学 ， 化学工程， 2010， 硕士

【摘要】 锂离子二次电池具有高的体积能量密度和质量能量密度,且工作电压高,无记忆效应,循环寿命长,已成为21世纪电池的主体。尖晶石LiMn2O4作为锂离子电池正极材料,具有廉价、毒性低、环境友好以及Mn资源丰富等优势而受到广泛关注。本文概述了锂离子电池的工作原理、结构优势和正极材料的的研究进展,介绍了尖晶石锰酸锂的研究现状。研究了合成LiMn2O4的两种新方法。针对其循环过程中发生的容量衰减问题,分别采用了Fe元素掺杂、改进合成方法和表面包覆,对尖晶石LiMn2O4正极材料进行了改性研究。通过EDTA溶胶凝胶法制备了正极材料LiMn2O4和LiFexMn2-xO4 (x=0, 0.05)的超细粉体,采用热分析测试,X射线衍射等表征手段和伏安循环等电化学性能测试确定了锰酸锂的较优合成工艺,研究表明采用600 oC合成的尖晶石LiMn2O4材料具有很高的质量比容量(0.5 C,120.2 mAh·g-1),50次循环后容量保持为初始容量的70%。通过Fe部分掺杂后,比容量虽然稍有下降为113.6 mAh·g-1,但材料的循环性能得到了很大提高,50次循环后的比容量仍有首次容量的83%。以廉价的Li2CO3和MnO2为原料,以毒性很低的有机溶剂丙酮为液相分散剂,通过液相活化和机械活化,采用两步煅烧使反应物更均匀混合,制得了结晶度高的,纯相的尖晶石锰酸锂正极材料。经过电化学性能表征得出合成的材料不仅具有高的质量比容量(0.2 C,3.3 - 4.4 V电压范围内,初始放电容量为117 mAh·g-1)和循环稳定性(40次循环后容量仍保持为首次容量的94%),同时材料的倍率性能相比传统固相法也有极大提高。改进固相法合成尖晶石锰酸锂因其合成工艺简单易操作,所用合成原材料价格低廉,同时制得的LiMn2O4表现出了优秀的电化学性能,所以此方法具备商业化应用的很大潜力。研究了通过液相沉淀法在LiMn2O4表面包覆Al2O3后,对材料高温电化学性能的影响。采用缓冲溶液液相沉淀法,Al(OH)3能非均匀的成核,从而使得LiMn2O4表面形成的Al2O3层很薄,因此不仅没有对Li+的脱嵌造成阻碍,同时有效的保护了LiMn2O4表面不受电解液的侵蚀,减少了Mn的溶解,从而提高的材料的高温循环稳定性。更多还原

【Abstract】 Lithium ion batteries have become one of the most important batteries in 21th century because of its high volume and mass energy density, high working voltage, long life and little self discharge. As one of the cathode materials for lithium ion batteries, spinel LiMn2O4 attracts great attention due to its low cost, abundant Mn resource, low toxicity and environmental friendly nature. The principle of charge & discharge for lithium ion batteries, its structure and the research progress on cathode materials was presented in this paper. Two new methods were developed to synthesize LiMn2O4. In order to overcome the severe capacity fading which limits the commercial application of lithium ion batteries, Fe was adopted to partially substitute of Mn and Al2O3 was used for the surface modification.The super-fine powders of spinel LiFexMn2-xO4 (x=0, 0.05) were synthesized using EDTA- citric acid (EDTA-CA) sol-gel method with manganese acetate, lithium nitrate as raw materials. The precursor and synthesized powders of LiMn2O4 were characterized by TG-DSC, XRD, CV and galvanostatic charge and discharge. It is found that the optimal sintering temperature is 600 oC. At room temperature, the initial discharge capacity of the half cell with LiMn2O4 cathode material is 120.2 mAh·g-1 at the current density of 0.5 C and the capacity can remain 70% of initial capacity after 50 cycles. By Fe doping, the initial discharge capacity decreased to 113.6 mAh·g-1, however, the LiFe0.05Mn1.95O4 material performed improved cycle ability, maintaining 83% of initial capacity after 50 cycles. An improved solid state reaction method was also developed, in which low toxicity acetone was used as a dispersant making the Li2CO3 and MnO2 to mix homogeneously.Furthermore, we adopted two-step calcinations to achieve a high crystalline spinel LiMn2O4 which played an important role in the electrochemical property. The electrochemical characterizations indicated that the spinel LiMn2O4 prepared by the improved solid state reaction method shows a higher capacity, and better cycle stability. It has an initial capacity of 117 mAh·g-1 at a current density of 0.2 C between 3.3 - 4.5 V and good cycle stability, maintaining 94% of its initial capacity after 40 cycles. Moreover, the LiMn2O4 prepared by the improved method shows better discharge ability at high rates than that prepared by the conventional method. This method has a great potential for the commercial preparation of LiMn2O4 because it can produce the LiMn2O4 with enhanced capacity and cycle life by a simple way with low cost of raw materials.In order to improve the cycle performance of material at high temperature, the LiMn2O4 was modified by coating its surface with a thin layer of amorphous A12O3. In buffer solution, Al(OH)3 can be heterogeneous nucleation, therefore after heat treatment, a thin layer of A12O3 could form on the surface of the spinel. Obviously, coating the surface of LiMn2O4 with A12O3 can modify the properties of its surface, which is exposed to the electrolyte solution and avoid the parasite reactions. A12O3 can also capture the HF from electrolyte, which reacts with the LiMn2O4 and accelerate the dissolution of Mn, then the cycle stability of the spinel material was improved.更多还原