【作者】 朱华；

【导师】 张文魁； 夏阳；

【作者基本信息】 浙江工业大学 ， 材料工程， 2015， 硕士

【摘要】 目前商业用锂离子电池正极材料能量密度低是制约其实际应用的主要瓶颈,本文以高能量密度Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂为研究对象,针对其首次充放电效率低、循环稳定性差、倍率性能不佳,以及过渡金属离子易溶解于电解液等问题,通过对比和调控制备工艺,并采用LiMnPO₄对其表面修饰,从而实现Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂电化学性能改性。本文主要内容如下:首先采用共沉淀法和燃烧法制备了Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂材料,并对所得样品的形貌、微结构和电化学性能进行比较。共沉淀法以过渡金属醋酸盐（Co（CH3COO）2·4H2O、Mn（CH3COO）2·4H2O和Ni（CH3COO）2·4H2O）水溶液为前驱体溶液,氨水作为络合剂,氢氧化钠作为沉淀剂,保持反应液pH≥11,获得过渡金属前驱体产物。再以LiOH·H2O为锂源,将上述前驱体产物与LiOH·H2O球磨混合。最后,通过热处理获得Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂。燃烧法是以无水乙醇为溶剂,分别将化学计量比的Co（CH3COO）2·4H2O、Mn（CH3COO）2·4H2O、Ni（NO3）2·6H2O和LiNO3溶于无水乙醇中,通过二次热处理工艺得到Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂。实验结果表明:采用燃烧法和共沉淀法均可获层状结构有序的Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂。通过燃烧法制备的产物粒径为50-200nm,其颗粒尺寸较共沉淀法获得的材料明显要小（共沉淀法粒径为100-300 nm）。但是燃烧法产物团聚现象较为严重,使得颗粒比表面积极具下降,故不利于锂离子扩散。在室温条件下,共沉淀法制备的样品首次充放电比容量分别为442mAhg-1和287 mAhg-1,首次库伦效率为65%;燃烧法制备的样品首次充放电容量约为438 mAhg-1和267 mAhg-1,首次库伦效率为61%,均低于共沉淀法获得的样品。在倍率测试中,随着倍率不断提高,共沉淀法样品倍率性能优势明显。在5 C（1500 mAg-1）条件下测试,其可逆比容量仍保持在90 mAhg-1。电化学交流阻抗谱显示共沉淀法样品的电化学反应阻抗较小,且锂离子扩散速率较高。综合来看,共沉淀法较燃烧法更适于Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂合成。采用LiMnPO₄纳米颗粒对共沉淀法获得的Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂进行表面修饰,以期改善其循环稳定性和倍率性能。通过TEM、SEM、XRD等表征方法对材料的结构、形貌、晶型以及元素分布进行分析,证明LiMnPO₄纳米颗粒成功修饰在Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂的表面,且复合材料的颗粒尺寸未发生明显变化。在室温条件下,当LiMnPO₄添加量为3 wt%时,所得样品具有最优的电化学性能,首次库伦效率高达76.1%。在电流密度为0.1C时,循环80次后容量仍然能达到206 mAhg-1,循环稳定性也明显提高;在电流密度为5 C时,其可逆比容量仍然高达117 mAhg-1,倍率性能得到大幅提升。电化学交流阻抗测试表明,经LiMnPO₄表面修饰的Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂具有较小的电化学反应阻抗,且锂离子扩散速率也高于纯Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂样品。更多还原

【Abstract】 Low energy density is the bottleneck problem of t he cathode materials in commercial lithium- ion batteries for their practical applications. Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ with high energy density is considered as a promising cathode material that is selected as research object. However, the practical application of Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ still is hampered by its intrinsic shortcomings, such as the low initial charge-discharge efficiency, poor cycling stability and rate-capability. Moreover, the transition metal ions can be dissolved easily in the electrolyte. In order to solve the above problems, in this thesis, co-precipitation method and alcohol-combustion method have been both used to prepare Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples. Meanwhile, LiMnPO₄ was introduced to make the surface modification on Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples. The main contents are summarized as follows:Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ samples were prepared by co-precipitation method and alcohol-combustion method. The morphology, microstructure and electrochemical performance of the as-prepared two samples were systematically made a comparison. For the co-precipitation route, ammonia solution was chosen as the base solution and complexing agent. And NaOH solution was used as the precipitator. In more detail, NaOH solution and the mixed solution containing Mn（Ac）2·4H2O, Ni（Ac）2 ·4H2O and Co（Ac）2·4H2O were dropwise added into ammonia solution simultaneously to prepare precursor. The pH of mixed solution was kept at 11. The n the precursor mixed with a stoichiometric amount of LiO H·H2O by ball milling. Finally, the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ sample was obtained by calcining the aforementioned mixture. In the procedure of alcohol-combustion method, a stoichiometric of Co（CH3COO）2·4H2O, Mn（CH3COO）2·4H2O, Ni（NO3）2·6H2O and LiNO3（10% excess） were dissolved in the anhydrous ethanol. Then the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ was prepared via calcining the compound. The results demonstrated that the samples prepared by co-precipitation and alcohol-combustion methods both have the same order layered structure, consisting Li2 MnO3 and LiMO2 phases. The sample prepared by alcohol-combustion has the smaller particle size ranging from 50 to 200 nm. However, the particles were seriously aggregated, and the surface area decreased, which will restrict the diffusion of Li- ion. O n the contrary, the sample prepared by co-precipitation has bigger particle size, however, no visibly aggregation can be detected. The electrochemical performance clearly indicated that the initial charge-discharge efficiency of co-precipitated sample is higher than the sample prepared by alcohol-combustion. The first discharge capacity of co-precipitated sample is 287 mAhg-1 at current density of 30 mAg-1, while the capacity of another sample is 267 mAhg-1. But the cyclic performance of two samples is similar. Additionally, the co-precipitated sample exhibited better rate capability that the discharge capacity at 5 C（1500 mAhg-1） remains 90 mAhg-1. According to EIS results, it is easily found that the charge-transfer resistance of the co-precipitated sample is smaller than the sample prepared by alcohol-combustion method. Meanwhile the Li- ion diffusion rate of co-precipitated sample is also larger than the sample prepared by alcohol combustion.In order to further improve the cyclic stability and rate capability of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂, the olivine LiMnPO₄ nanoparticles were introduced to modify the surface of Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂. The phase, microstructure, morphology and element distribution of the LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ sample were measured by powder X-ray diffraction（XRD）, scanning electron microscopy（SEM） and transmission electron microscopy（TEM） equipped with an energy dispersive spectroscopy（EDS） detector. The morphology of LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ sample was similar with the pure one. And particles size did not change obviously. The electrochemical performance tests demonstrated that that Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ modified by 3 wt% LiMnPO₄ had the best electrochemical properties. Its initial charge-discharge efficiency was improved to 76.1%. Meanwhile its cycling stability also was enhanced. The discharge capacity remained 206 mAhg-1 after 80 cycles at 0.1 C. Even at the current density of 1500 mAg-1, it also delivered a high capacity of 117 mAhg-1. In addition, EIS results also confirmed that the charge transfer resistance and Li- ion migration ability of LiMnPO₄@Li[Li_0.2Mn_0.54Co_0.13Ni_0.13]O₂ were better than pure one.更多还原