石墨烯基三元正极材料的制备及其电化学性能研究

发布时间：2018-09-04 15:13

【摘要】：锂离子电池被广泛应用在消费类电子产品、新能源汽车、储能电网等领域,已经为人们日常生活,经济发展不可或缺的能源产品。手机和新能源汽车是锂离子电池主要的使用对象,两者发展势头迅猛,这使锂离子电池的需求非常巨大。所以加大对锂离子电池的研发,技术革新是非常必要的。而决定锂离子电池性能最主要的部件就是正极材料。三元锂电池由于具有更高的能量密度,相对较低的生产成本而受到企业和市场的重视。因此,选择三元正极材料作为研究对象。石墨烯自2004年被发现以来由于其诸多优异的性能而受到科研工作者的重视,石墨烯良好的导电性能不仅能用在负极材料上,同样也可以应用在正极材料上,在提升容量的同时也可以提高倍率性能。本文采用物质电中性的原理用溶胶凝胶法合成了5种不同元素配比的三元正极材料,分别为LiNi_(1/3)Co_(1/3)Mn_(1/3)O_2、LiNi_(1/2)Co_(1/3)Mn_(1/4)O_2、LiNi_(0.7)Co_(0.2)Mn_(0.25)O_2、LiNi_(1/4)Co_(1/2)Mn_(1/4)O_2、LiNi_(0.1)Co_(0.6)Mn_(0.25)O_2。通过对这5种材料分别进行XRD、SEM、恒流充放电测试、倍率测试、循环测试、循环伏安测试、电化学交流阻抗测试,来研究材料的结构和电化学性能。从XRD衍射结果看出,这5种材料均具有层状结构。通过电化学测试表明,LiNi_(0.7)Co_(0.2)Mn_(0.25)O_2具有较高的可逆容量和稳定的循环性能,在0.1C下首次放电容量达142mAh/g。然后将LiNi_(0.7)Co_(0.2)Mn_(0.25)O_2于700℃、800℃、900℃合成,考察不同合成温度对材料的影响。测试结果表明,在700℃条件下合成的材料性能最优,材料经过30个循环后,容量保持率为90%。随着合成温度的升高,材料的电化学性能越差。主要是因为合成温度升高会使材料的粒径变大,颗粒分布不均匀,影响了锂离子的迁移过程,从而造成性能下降。将700℃条件下合成的LiNi_(0.7)Co_(0.2)Mn_(0.25)O_2分别与1%、2%、3%的石墨烯通过球磨法进行复合,同时以直接加入1%、2%、3%的石墨烯的正极材料做对比。在0.1C的电流密度下,2.0V到4.5V电压区间内,经过球磨的含量为2%的复合材料首次放电达166mAh/g,比单纯的正极材料提升了近24mAh/g,倍率性能也有所提升。在确定了最佳的石墨烯掺量后,在氮气的保护下通过热处理工艺以进一步加工,处理温度分别为200℃、300℃、400℃。实验结果表明,经过热处理后材料的首次充放电并没有得到提升,反而有点下降。原因可能是石墨烯与正极材料经过热处理后使正极材料的层状结构遭到一定程度的破坏,从而使电化学性能降低。但是通过电化学阻抗测试表明,300℃下得到的复合材料经过50次循环后电荷转移阻抗为70.68m?,低于任何材料循环50次后的电阻值,这说明经过热处理后能改善石墨烯与正极材料的结合性,增强复合材料的电子导电率。
[Abstract]:Lithium ion batteries are widely used in consumer electronics, new energy vehicles, energy storage grid and other fields, which have become indispensable energy products for people's daily life and economic development. Mobile phones and new energy vehicles are the main users of Li-ion batteries, and they are developing rapidly, which makes the demand of Li-ion batteries very great. So increasing the development of lithium ion batteries, technological innovation is very necessary. The main component that determines the performance of lithium ion battery is cathode material. Ternary lithium batteries have attracted much attention from enterprises and markets because of their higher energy density and lower production cost. Therefore, the ternary cathode material is chosen as the object of study. Since its discovery in 2004, graphene has been paid much attention by researchers because of its excellent properties. Graphene can be used not only in negative electrode materials, but also in positive electrode materials. While increasing the capacity, it can also improve the performance of the rate. In this paper, we have synthesized five kinds of ternary cathode materials by sol-gel method, which are LiNi_ (1 / 3) Co_ (1 / 3) Mn_ (1 / 3) O _ 2LiNi_ (1 / 2) Co_ (1 / 3) Mn_ (1 / 4) O _ (2) LiNi_ (0.7) Co_ (0.2) Mn_ (0.25) O _ 2LiNi _ (1 / 4) Co_ (1 / 2) Mn_ (1 / 4) O _ 2Li _ (0. 1) Co_ (0. 6) Mn_ (0. 25) O. The structure and electrochemical properties of the five materials were studied by XRD,SEM, constant current charge-discharge test, rate test, cyclic voltammetry test and electrochemical impedance test. The results of XRD diffraction show that all of the five materials have layered structure. Electrochemical measurements show that LiNi0.7 Co_ _ (0.2) Mn_ _ (0.25) O _ 2 has a high reversible capacity and stable cycling performance, and the initial discharge capacity at 0.1C is up to 142mAh/ g. Then LiNi_ _ (0.7) Co_ _ (0.2) Mn_ _ (0.25) O _ 2 was synthesized at 700 鈩，

本文编号：2222547