【作者】 李翔；

【导师】 周豪慎； 何平；

【作者基本信息】 南京大学 ， 材料物理与化学， 2016， 硕士

【摘要】 对化石能源依赖所造成的能源安全和环境污染等问题限制了人类社会的可持续发展。在住宅能量存储、电动汽车驱动和智能电网等方面我们急切需要能量密度高、原材料成本低廉的能源。锂离子电池被认为是开发和利用可再生清洁能源的有力技术。锂离子电池的电化学性能主要由正极材料所限制,在众多正极材料中,锰基材料由于储量丰富、对环境友好而被认为有很好的应用前景。锰基过渡金属氧化物具有两种结构,即层状结构和尖晶石结构。层状结构的代表为xLi2MnO3·(1-x)LiNi0.5Mn0.5O2,尖晶石结构的代表为LiNi0.5Mn1.5O4,其中前者有很高的放电容量,后者有很高的工作电压,这都使这两种材料具有很高的能量密度。但是,材料的成分、结构和性能的关系以及制备工艺对材料电化学性能的影响尚没有研究的很清楚。本文即采用共沉淀法制备xLi2MnO3·(1-x)LiNi0.5Mn0.5O2和固相合成法来制备LiNi0.5Mn1.5O4,同时探究了材料成分、结构、制备工艺与电化学性能的关系：第一：在制备层状富锂锰基材料xLi2MnO3·(1-x)LiNi0.5Mn0.5O2时,我们用Ni、Mn等过渡金属的硫酸盐作为原料,在适当pH值和沉淀剂下制备出前驱体,再配以适当比例的锂盐进行煅烧,得到成品。我们探究了x取不同值(x=0,0.2,0.3,0.5,0.7)时材料的电化学性能的差别。通过XRD精修数据发现,合成的实际样品和目标样品是基本吻合的。对材料进行的电化学测试表明,富锂锰基材料的电化学性能与Li2MnO3的含量有很大的关系。随着Li2Mn03组分的提高,材料的电化学性能也在提升,但当x=0.5时,材料电化学性能达到最优,此时若再增加Li2Mn03的含量，反而会起到反作用,可能是由于锂离子的脱嵌或过多活化的Li2Mn03使材料的结构改变所致。为了探究x=0.5时样品电化学性能优异的原因,我们对0.5Li2Mn03·0.5LiNi0.5Mn0.5O2做了原位XRD测试,结果表明,4.5 V的氧化峰不仅仅在第一圈存在,在后续的循环中也可以观察到。而且,峰(003)的变化可以表示晶胞的c轴的变化,所以我们通过峰(003)的偏移推测出在放电过程中,锂离子不是单嵌入锂层的,还有一部分嵌入了过渡金属层,说明材料的电化学性能很可能跟少量可逆的Li2Mn03组分有关。第二：在制备高电位材料LiNi0.5Mn1.5O4中,我们用MnO2、Ni(Ac)2·4H2O和Li(Ac)·2H2O作为原料借助球磨辅助的方法制得样品。继而考查了煅烧温度和冷却条件对产品的电化学性能的影响。温度测试中,我们将样品在750℃～950℃中煅烧并进行相同的电化学测试。结果显示,样品的循环性能和倍率性能随着温度的提升而明显提高,但是温度的提高也使材料发生明显的氧缺陷。在随后的探究中,我们用950℃为温度标准,用不同速度和气氛进行冷却,结果发现,缓慢冷却和在氧气气氛下冷却可以明显弥补高温环境带来的氧缺失,材料的循环性能和倍率性能都有明显的提升,综合几组对比实验,我们可以发现氧气中煅烧冷却的样品的性能是最好的,无论从放电容量还是从循环性和循环效率来讲。综合考虑,在氧气中煅烧冷却的样品和在空气中以10℃ min-1降温的样品都有很优秀的电化学性能,在工业制备上,需要综合成本、设备、制备方法的难易等因素,在取舍上我们可以给出相对高的煅烧温度和相对缓的冷却过程,这样可以使制备方法和材料性能结合的最好。但也有一定的问题需要我们探讨。比如说第一圈都会发生过充现象,从而影响后续的性能,这样的差异部分原因在于电解液的氧化分解,但还需要进一步探究。本论文探究了层状富锂锰基材料和尖晶石高电位材料的组分、结构和电化学性能的影响,旨在对影响材料的电化学性能的因素有一定的了解。更多还原

【Abstract】 The high energy density of Li-ion batteries have facilitated the development of mobile electronic equipment and attracted people’s attention due to its advantages towards renewable energy, low cost, safe characters, environment friendly and so on. Moreover, the high energy and power density Li-ion batteries (LlBs) are the key technique of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs). To address the capacity dilemma, researchers have turned their focus on the Li-rich Mn-based layered oxides and LiNi0.5Mn1.5O4 as the alternative promising cathode materials.The Li-rich Mn-based layer materials xLi2MnO3·(1-x)LiNi0.5Mn0.5O2 (x=0～0.7) were successfully synthesized and systematically studied as one of the potential high capacity cathode materials for Li-ion batteries. The experimental results show that the relative content of Li2MnO3 for xLi2MnO3·(1-x) LiNi0.5Mn0.5O2 have a large impact on the structure and electrochemical properties. During rate tests, the specific capacities under different charging rate for each sample roughly improved with the increase of Li2MnO3 containment (equal to the x value) and reaching to the maximum around x= 0.5. However, keep on increasing Li2MnO3 content (x> 0.5), the electrochemical performance of the materials became worse which maybe caused by the structural change during the process of Li+ extraction/insertion. This tendency also existed during cycling test up to 50 cycles. Further analysis of sample xLi2MnO3·(1-x) LiNi0.5Mn0.5O2 (x=0.5) using in-situ XRD as implement disclosed the advanced electrochemical performance can be ascribed to the reversible of Li2MnO3. Thereby, these results indicate the electrochemical performance of xLi2MnO3·(1-x) LiNi0.5Mn0.5O2 could be improved by optimized the content of Li2MnO3, leading to the potential use as high capacity Li-ion batteries.Spinel compound LiNi0.5Mn1.5O4 with high capacity and high rate capability was synthesized by solid-state reaction. The MnO2、Ni(Ac)2·4H2O and LiAc·2H2O as a starting material by means of ball-milling auxiliary sample preparation. Then examines the calcination atmosphere, temperature and cooling conditions on the electrochemical properties of the product. We have measured samples through constant current charge-discharge tests. It is easy to see that with the calcination temperature increasing, the capacity of discharge of the samples is improved, and the sample exhibits a typical cubic spinel structure with a space group of Fd3m. Electrochemical tests demonstrate that the sample obtained possesses high capacity and excellent rate capability. When being discharged at a rate of 0.1 C after 8 cycles, the as-prepared LiNi0.5Mn1.5O4 powders can deliver a capacity of 114.5 mAh g-1. The calcined under an oxygen atmosphere or slow cooling treatment can significantly improve the cycling performance,which shows to be a promising cathode material for high-power battery.更多还原