【作者】 李喆；

【导师】 陈岗；

【作者基本信息】 吉林大学 ， 材料物理与化学， 2012， 博士

【摘要】 锂离子电池作为一种重要的电化学能量储存与转换装置，已经广泛地应用在手机、笔记本电脑、电动汽车以及混合电动汽车等领域，成为缓解能源危机与环境污染等问题的一种有效途径。锂离子电池的性能在很大程度上取决于正极材料的性能，以LiCoO₂为代表的传统正极材料由于资源紧缺、原料成本高和对环境有污染等缺点在工业化生产中受到制约。近年来，富锂层状正极材料以其低廉的成本，高的能量密度和良好的安全性能成为了人们研究的重点。本文以富锂层状正极材料Li[Li_{（1/3-x/3）}Co_xMn_{（2/3-2x/3）}]O₂为研究对象，针对材料特殊的充放电机理，深入地研究了材料的内部结构、动力学性能和电化学反应机制等问题。首先，我们采用溶胶凝胶法合成了亚微米尺寸的Li[Li_0.2Co_0.4Mn_0.4]O₂材料，利用拉曼散射确定了富锂层状正极材料具有单相固溶体结构而不是LiCoO₂和Li2MnO3两种物质的纳米复合结构。恒电流间歇滴定法（GITT）定量计算表明，材料的锂离子扩散系数在首次充电至4.45V时达到最小值10^-19cm²s^-1，说明此时材料中氧气的释放、锂离子的脱出、以及结构的重排是动力学受限的过程。随着材料结构重排的结束，在首次充电完成时，形成新的层状结构Li_xMO₂，同时扩散系数明显增大。其次，我们研究了材料在低温下的电化学性能。电化学阻抗谱研究表明材料在-20°C下电荷转移阻抗急剧增加，同时，X-射线光电子能谱及循环伏安分析表明材料中的Mn⁴⁺离子基本未被激活。上述原因导致材料在低温下电化学容量降低。最后，我们通过对材料在不同截止电压下进行充放电测试，发现在2.0-4.6V电压区间内，材料在首次充电过程中的氧气释放反应基本结束，首次放电容量高达213mAh g^-1，同时循环性能相对稳定。因此，这个区间是富锂层状正极材料适宜的充放电电压区间。此外，我们还对材料进行了首次高电压及低倍率的预充电处理。通过对材料在第一次充电过程中进行高压预充电处理，激活了材料中的Mn⁴⁺离子，使其参与到后续电化学过程中。在此基础上，将材料在低电压范围内进行充放电循环，可在获得较高容量的同时，显著提升材料的循环稳定性。通过这种预充电技术，使富锂层状正极材料在目前商用电解液允许的电压范围内最大限度地发挥了其高比容量特性，有力地促进了材料的实际应用。更多还原

【Abstract】 Great efforts have been done on lithium-ion batteries since SONYintroduced the first commercial lithium-ion battery in1991. Nowadays,conventional cathode materials such as layered LiCoO₂, spinel LiMn2O4, andolivine LiFePO4have been successfully applied in lithium ion batteries. Thesecathode materials typically deliver capacities of100-160mAh g-1. To satisfythe requirements for electric vehicles and hybrid electric vehicles, it is desiredto develop new cathode materials with superior performance such as highercapacities, lower cost, and better thermal stability. Recently, a series ofLi-riched layered materials, which can be simplified as Li[Li_xM_1-x]O₂（M=Mn,Ni, Co, Cr）, have been studied as promising cathode materials for nextgeneration lithium ion batteries because of their low cost, low toxicity, highdischarge capacities. However, there are lots of arguments on true crystalstructure of such materials, not enough lucubrating on kinetics character andmechanism during the charge-discharge processes. Moreover, Li-richedlayered materials have to be charged over4.5V for the high specific energy,but the commercial electrolyte could not worke for long at such high voltage.Therefore, this work is devoted to research on the charge mechanism, crystalstructure and the electrochemical properties of Li[Li_{（1/3-x/3）}Co_xMn_{（2/3-2x/3）}]O₂ cathode materials.Firstly, we successfully prepared Li[Li_0.2Co_0.4Mn_0.4]O₂at900°C for15hours using the sol-gel method. We studied the structure properties of thematerial using varies techniques including X-ray diffraction（XRD）, Fouriertransform infrared spectroscopy （FTIR） and Raman scattering，based on whichwe determined Li[Li_0.2Co_0.4Mn_0.4]O₂material was a solid solution rather thana composite of nano Li2MnO3and LiCoO₂. The additional shoulder band at670cm-1should be attributed to the local distortion, because that the Li+andMn4+superlattice was substituted by Co³⁺. FTIR and X-ray photoelectronspectrum （XPS） showed there was a little Li2CO3in the surface layer of thematerials resulting from the oxidation reaction of the residual Li.Secondly, we studied the electrochemical kinetics of the material usingGalvanostatic intermittent titration technique （GITT） and Electrochemicalimpedance spectroscopy （EIS）. It is observed that the material showed lowestLi⁺diffusion coefficients (10^-19cm²s^-1), increased resistance of solidelectrolyte interface （SEI） film and huge charge-transfer resistance at the firstcharge plateau because of the high kinetic barriers associated with theconcurrent Li+diffusion，oxygen loss and structural rearrangement. Thesecomplicated reactions slowed down the electrochemical kinetics process. Atthe end of first charge, the material changed to a LixMO2layered structure,and the diffusion coefficients increased in evidence.Then, we compared the electrochemical properties of the material at-20°C and room temperature by charge-discharge cycling, cyclic voltammetry（CV）, XPS and EIS. The resultes showed that the Mn4+was actived at roomtemperature, and deoxidized to Mn3+during the discharge process. The Mn³⁺was prone to dissolve into the electrolyte, resulting in bad cycle performance. EIS study showed that the charge transfer resistance severely increased withthe temperature dropped to-20°C, which was due to insufficient oxygen loss,and the material showed a low capacity. XPS and CV analysis showed that theinactive Mn⁴⁺ions in the electrode suppressed the dissolution of manganeseand the Jahn-Teller distortion of the material lattice, both of which resulted instable structure and excellent cycle life at low temperature.Finally, we studied the electrochemical properties at different voltagewindows, our experiments showed more sufficient oxygen loss and higherdischarge capacities were obtained when the material charged to4.8V, but thebig charge transfer resistance at high voltage decreased the cycle performance.The material showed a high discharge capacity of213mAh g-1and excellentcycle life in the voltage window of2.0-4.6V, so we determined this voltagewindow was a proper choice for Li-riched layered material. Moreover, weprecharged the materials with high voltage and low rate before cycling. CVshowed that the Mn4+was actived during the precharge treatment, andparticipated in the flowing cycles in the narrow voltage window, so thedischarge capacity and cycle performance were enhanced. This prechargetreatment technique brought high discharge capacity in the voltage window,which was fit for the commercial electrolyte for Li-ion Batteries.On all accounts, this work gives us a comprehensive understanding on thepreparation of Li[Li_{（1/3-x/3）}Co_xMn_{（2/3-2x/3）}]O₂cathode materials, as well as itsstructural and electrochemical properties.更多还原