摘要
锂离子电池作为储能设备已经广泛的应用于移动电子设备如手机、笔记本电脑等。但是尚不能满足大功率、高能量密度的汽车用动力电池的需求。锂离子电池的性能和成本主要取决于正极材料,目前商业化使用的锂离子电池正极材料LiCoO2、LiMn2O4和LiFePO4等的放电比容量只有110-160mAh g-1,发展高比容量的锂离子电池正极材料已成为电极材料研究的热点之一。富锂层状氧化物xLi2MnO3·(1-x)LiMO2(M=Ni, Co, Mn, Fe, Cr, Ki1/2Mn1/2, Ni1/3Co1/3Mn1/3...)的放电比容量能够达到230-300mAhg-1,远高于商业化的正极材料,同时具有较好的循环稳定性,是一种有发展前景的正极材料；但是富锂层状氧化物较低的首次库伦效率,较差的倍率性能和循环中电压衰减等问题影响了它的发展。目前制备具有良好结晶性和纳米尺寸的富锂层状氧化物,优化富锂层状氧化物中各种化学组分的含量,杂原子掺杂和功能材料表面修饰等手段是改善该材料缺陷的有效途径。本论文选取一种具有较好电化学性能的富锂层状氧化物Li1.2Ni0.13Co0.13Mn0.54O2)作为研究对象,重点对此材料进行了可控制备和组分优化研究,分为以下几个方面：
(1)以P-MnO2纳米棒作为牺牲模板通过固相反应制备了富锂层状氧化物Li1.2Ni0.13Co0.13Mn0.54O2,重点考察了煅烧温度对材料结构和电化学性能的影响。随着煅烧温度的提高,产物的层状有序性明显提升,颗粒尺寸变大,同时颗粒的表面也变得更加光滑。在较低的煅烧温度(750和800℃)下,形成的是Li1.2Ni0.13Co0.13Mn0.54O2纳米棒,而在较高的煅烧温度(850和900℃)下,产物转变为结晶性更好,尺寸更大的多面体纳米颗粒。作为锂离子电池正极材料,材料显示了结构和形貌决定的电化学性能。在850℃的煅烧温度下,产物具有最优的电化学性能：在0.1C的电流密度和2.0-4.7V的电压区间,其首次放电比容量达到239.2mAh g-1;在1000mA g-1电流密度下,放电比容量为92.8mAh g-1;在100mAg-1电流密度下,经过70次循环,容量保持率超过90%。
(2)分别以球形的Ni0.13Co0.13Mn0.54(CO3)0.8和MnO2作为牺牲模板制备了多孔和实心的Li1.2Ni0.13Co0.13Mn0.54O2微球。当以球形碳酸盐作为模板时,由于在反应中C02的逸出,形成的是由尺寸为100-300nm纳米颗粒堆积而成的多孔微球,其比表面积为1.91m2g-1。当以球形MnO2作为牺牲模板时,由于过渡金属元素在固相反应中的化学嵌入,形成的是由纳米颗粒密堆积形成的实心微球,其比表面积为1.41m2g-1。作为锂离子电池正极材料,多孔微球在0.1C的电流密度和2.0-4.8V的电压区间,其首次放电比容量达到255.7mAh g-1;在5C的电流密度下,放电比容量也能达到121.4mAh g-1。但是实心微球在0.1C的电流密度下,其首次放电比容量只有159.9mAh g-1。
(3)通过一个起始的80℃丙烯酸聚合,450℃分解和最后850℃晶化的方法制备了富锂层状Li1.2Ni0.13Co0.13Mn0.54O2纳米颗粒。制备目标产物的颗粒尺寸为173.2±66.2nm,并具有良好的层状结构。通过高分辨透射电子显微镜(HRTEM)和选区电子衍射(SAED)测试表明：产物具有单晶结构和超晶格特征。作为锂离子电池正极材料,产物在20mAg-1的电流密度和2.0-4.8V的电压区间,其首次放电比容量和库伦效率达到290.7mAh g-1和78.3%；在100mAg-1的电流密度下,经过60次循环剩余容量为182.7mAh g-1;在1000mAg-1的电流密度下,其放电比容量也能达到122.5mAh g-1。
(4)通过丙烯酸聚合分解法制备了不同Co含量的0.5Li2MnO3·0.5LiNi1/3+xCo1/3-2xMn1/3-xO2(-1/12≤x<1/12)样品,这些样品呈现由纳米颗粒堆积而成的片状结构,Co含量的增加促使纳米颗粒尺寸的变大。作为锂离子电池正极材料,Co含量的增加略微降低了材料的放电比容量,但起始库伦效率和循环稳定性明显提升。综合成本和电化学性能考虑,最优的x值应该在0到1/12之间,这与目前主要研究的和商业化的富锂层状氧化物的钴含量接近。
(5)通过柠檬酸辅助的溶胶-凝胶法制备了具有良好结晶性的Li1.2Ni0.i3+xCo0.13Mn0.54-xO2(-0.06≤x≤0.06)纳米颗粒。Mn含量的增加提高了产物中Li2MnO3组分的含量和产物颗粒尺寸；Ni含量的增加引起了材料中更强的Li/Ni阳离子混排。当作为锂离子电池正极时,x=0的样品具有最高的起始放电比容量和库伦效率；增加材料的Mn含量能够提高产物的循环稳定性；增加材料的Ni含量能够提高材料的倍率性能和导电性,同时抑制电压衰减。最优的x值应该是-0.03到0.03。此外,考察了截止电压(4.5-4.8V)对Li1.2Ni0.13Co0.13Mn0.54O2纳米颗粒电化学性能的影响,通过降低截止电压能够有效的提高材料的循环稳定,倍率性能；同时抑制电压的衰减,但是却会降低材料的放电比容量和比能量。综合考虑,最优的截止电压应该为4.6-4.7V。
Nowdays, lithium ion battery (LIB) have been widely used in portable electronic devices such as mobile phone and computer. However, they are difficult to meet the demands of high power and energy densities for electric and hybrid electric vehicles. It is well known that, the electrochemical performance and cost of LIB are determined by the cathode materials. Commercial cathode materials such as LiCoO2, LiMn2O4and LiFePO4only have a specific discharge capacity within100-160mAh g-1, which limits the further development of LIB. The exploration and development of cathode materials with higher specific discharge capacity are of great importance. In recent years, the lithium-rich layered oxides in the chemical formula of xLi2MnO3·(1-x)LiMO2(M=Ni, Co, Mn, Fe, Cr, Ni1/2Mn1/2, Ni1/3Co1/3Mn1/3...) have attracted specialists more and more attentions and have been recognized as one of the most promising next generation cathodes due to their high specific discharge capacities of230-300mAh g-1. However, their high irreversible capacity loss in the initial cycle, low rate capability and decrease of discharge midpoint voltage upon cycling still exist. In order to solve or alleviate these shortages, preparation of well-constructed and nanosized materials, optimization of the chemical compositions of lithium rich layered oxides, doping with other ions and surface modification with functional compounds have been successfully used. Herein, a promising lithium-rich layered oxides Li1.2Ni0.13Co0.13Mn0.54O2(0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2) has been selected as research target, and a serie of controlling synthesis and content optimization studies have been carried out, shown as below:
(1) Lithium-rich layered oxides Li1.2Ni0.13Co0.13Mn0.54O2have been successfully prepared by a template directed route using β-MnO2nanorods as sacrificed templates. The influence of calcination temperature on the structures and electrochemical properties of as-prepared materials are systematically studied. The elevated calcination temperature helps to improve the layered structures and particle sizes. At the low temperatures (i.e.,750and800℃), rod-like Li1.2Ni0.13Co0.13Mn0.54O2can be obtained, and which will be transformed into polyhedral Li1.2Ni0.13Co0.13Mn0.54O2nanoparticles at a higher temperature (i.e.,850or900℃). The electrochemical performances of as-prepared Li1.2Ni0.13Co0.13Mn0.54O2are determined by their different particle sizes and layered structures. At the optimal calcination temperature of850℃, the resulting Li1.2Ni0.13Co0.13Mn0.54O2shows the highest discharge capacity of239.2mAh g-1at20mA g-1within2.0-4.7V, and a stable discharge capacity of92.8mAh g-1at1000mA g-1. The good electrochemical performances of850℃sample should be attributed to the better layered structure and/or the more appropriate particle size comparing with those of other samples.
(2) Porous and solid Li1.2Ni0.13Co0.13Mn0.54O2spheres have been successfully prepared by using templates of spherical Ni0.13Co0.13Mn0.54(CO3)0.8and MnO2. Porous Li1.2Ni0.13Co0.13Mn0.54O2spheres obtained from carbonate precursor are composed of well-defined primary nanoparticles with the size of100-300nm, and the porous feature can be visually determined. While the solid Li1.2Ni0.13Co0.13Mn0.54O2spheres are made of tightly clustered nanoparticles. The former porous structures come from the generation of CO2, and the latter results from the excessive ions insertion during calcination. Correspondingly, the porous spheres have higher BET surface area than solid spheres. As lithium ion battery cathodes, the porous spheres exhibit a higher initial discharge capacity of255.7mAh g-1at0.1C between2.0and4.8V. After50cycles, a discharge capacity of177.7mAh g-1could be retained at0.5C. Even at a high charge-discharge rate of5C (1000mA g-1), a specific value of121.4mAh g-1can be reached. By comparison, the solid spheres only deliver initial discharge capacity of159.9mAh g-1at0.1C.
(3) A combination of the primary polymerization of acrylic acid (AA) monomers, the subsequent pyrolysis of PAA polymer gels at450℃and the final high-temperature crystallization (850℃) is used to prepare Li1.2Ni0.13Co0.13Mn0.54O2nanoparicles. The as-prepared Li1.2Ni0.13Co0.13Mn0.54O2nanoparicles have an average particle size of173.2±66.2nm and a good layered structure. HRTEM and SAED results show that, these nanoparticles possess a single-crystalline nature and a superlattice ordering. As LIB cathodes, these single-crystalline nanoparticles can deliver a high initial discharge capacity of290.7mAh g-1and the first cycle coulombic efficiency of78.3%at20mA g-1between2.0and4.8V, remain a reversible value of182.7mAh g-1at100mA g-1over60charge-discharge cycles and reach a capacity of122.5mAh g-1at a high current rate of1000mAg-1.
(4) A series of cathode materials0.5Li2MnO3·0.5LiNi1/3+xCo1/3-2xMn1/3+xxO2(-1/12≤x≤1/12) are prepared by a PAA polymerization-pyrolysis-assisted crystallization route. All powders possess a two-dimensional sheet-like superstructure composed of crystalline nanoparticles, and the added Co content helps to increase the particle size. As LIB cathodes, the samples with an increasing Co content can slightly increase the diacharge capacity, but are unfavorable to the initial coulombic efficiency and cycling stability. The optimal Co contents correspond to the x value of1/12and0, which is closed to the Co content of most used and commercial lithium-rich layered oxides.
(5) The influences of Mn-Ni contents on the structures and electrochemical performances of Li1.2Ni0.13+xCo0.13Mn0.54-xO2(-0.06≤x≤0.06) are clearly studied. A citric acid-assisted sol-gel route has been successfully used for the nanofabrication of these materials. The elevated Mn contents can increase the content of Li2MnO3component and particle sizes, and the elevated Ni contents lead to more Li/Ni cation mixing. As LIB cathodes, the x=0sample shows the highest discharge capacity and coulombic efficiency, the elevated Mn content helps to improve the cycling stability, and the elevated Ni content are favorable to the rate capability, electrochemical conductivity and discharge voltage. The optimal x value should be between-0.03and0.03. Furthermore, the upper cutoff voltage (4.5-4.8V) studies are also carried out for Li1.2Ni0.13Co0.13Mn0.54O2nanoparticles. Based on the total considerations of discharge capacity, discharge energy, cycling stability and rate capability, the optimal upper cutoff coltage should be4.6-4.7V.
引文
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