【作者】 李晓；

【导师】 王煜；

【作者基本信息】 重庆大学 ， 化学， 2016， 硕士

【摘要】 近年来,伴随科学技术的不断创新,便携式电子设备、电动运输等新兴领域得到了飞速发展。因此,这些新产品对于电动能源的需求和依赖与日俱增。锂离子电池作为一种新能源设备正是凭借其超高的容量密度、杰出的功率密度以及稳定的循环性能等诸多优势得以脱颖而出,获得世界范围内的广泛关注。众所周知,锡基负极材料作为下一代锂离子电池的负极材料得到了广大科研工作者的一致认可。那是因为其理论容量高达992 mA h g-1,这几乎是现如今使用的商业化石墨负极材料(372 mA h g-1)的两倍多。所以,锡基负极材料的改性,尤其是对其形貌的调控得到了广泛的研究和报道。目前,合成形貌多变、纯度高且结晶性良好的晶体材料的主要方法是水热合成法。本论文中的双壳层SnO2@C纳米空心球、单壳层SnO2@C纳米空心球和双壳层SnO2纳米空心球都是采用多步水热合成的方法制得。合成采用了模板法,其中核-壳结构的SiO2模板发挥重要的作用。随后,我们将这些合成的锡基材料应用到锂离子电池上并测试了它们的电化学性能。课题中得到的主要结论如下:(1)通过对核-壳结构SiO2的形貌表征,我们推测出了SiO2胶球向核-壳结构转变的可能原理。从形貌的变化过程可以清楚的看到SiO2内部核的消融与外部壳层的生长这两个过程是单独且并发的。在整个过程中,NaBH4起到了至关重要的作用。反应开始,NaBH4会和H2O发生缓慢的反应,并产生出NaBO2和H2。一方面,这一反应会使体系呈碱性,因此SiO2很容易在碱性溶液中发生溶解,比如SiO2在NaOH溶液中的溶解。另一方面,单硅酸盐和聚硅酸盐类大量释放到体系中,最终会达到过饱和的程度。同时,NaBO2的量也会伴随着NaBH4的反应而增加。这些导致SiO2再次沉淀到正在溶解的内部核表面,形成壳层结构。(2)双壳层SnO2@C纳米空心球具有诸多杰出性能。内部活性壳层的引入可以提高材料的体积比容量。双碳层包覆不仅可以保护材料结构的完整性而且可以增加材料的导电性。更重要的是材料本身又具有多孔结构,既可以提供电解液自由穿梭壳层内外的通道,又可以缓解电极反应带来的体积效应。同时,材料具有较大的比表面积,可以很大程度上提高活性材料与电解液的接触面积,也可以提供更多的锂离子附着点,这样就可以提高材料的实际容量。很多优点都得到了实验的验证。本论文制备的双壳层SnO2@C纳米空心球相对于单壳层SnO2@C纳米空心球和双壳层SnO2纳米空心球具有更加优越的倍率性能、更加稳定的循环性能(循环100圈后具有911 mA h g-1的容量)和更高的比容量。更多还原

【Abstract】 Nowadays, with the fast development and vast demand in portable electronic devices and vehicular transport, lithium ion batteries have attracted intensive attention because of their high energy density, superior power density and enhanced cyclability. As we all know, Sn-based anode materials are regarded as a promising candidate for the next generation of lithium ion batteries(LIBs),due to its high theoretical capacity 992 mA h g-1 which is about twice than commercial graphite anode materials(372 mA h g-1).Therefore, Sn-based materials with different novel morphologies have been widely studied and reported.It is well known that hydrothermal method is one of promising approaches to synthesize different materials with varied morphology, high purity and perfect crystallization. In this paper, the double-shelled SnO2@C hollow spheres, single-shelled SnO2@C hollow spheres and double-shelled SnO2 hollow spheres were synthesized by multi-step hydrothermal methods. In the process, the core-shell SiO2 hollow spheres are the crucial template. Subsequently, the as-synthesized tin-based materials have been applied as anode materials of lithium ion batteries and the main results are as follows:(1)On the basis of our observations, we can conceive the mechanism of the transformation from solid silica spheres to hollow structures. Obviously, core-dissolution and shell-regrowth processes are concurrent but separate during the reaction to yield. In the process of the whole reaction, the NaBH4 is crucial. It is well known that the reaction between water and Na BH4 slowly generates sodium metaborate NaBO2 and H2. On the one hand, in the beginning, the solution is alkaline because of the reaction of water and NaBH4.Therefore, the silica is dissolved into solution easily, which is similar to process of silica dissolved in NaOH solution. On the other hand, monosilicate and polysilicate species are released into the solution, which eventually becomes supersaturated. At the same time, the concentration of NaBO2 also increases gradually as a result of the decomposition of NaBH4, thus causing the silicate species to precipitate and redeposit on the core surfaces.(2) The novel double-shelled SnO2@C hollow spheres possess numerous outstanding properties. Its inner active hollow spheres can effectively improve the tag energy density and its double-shelled carbon wrapping outside of each SnO2-shell can protect the integrity of structure as well as enhance electrical conductivity. This is a significant improvement in the design and synthesis of multi-shell or core-shell functional materials. More valuably, the porous hollow structure can not only facilitate liquid electrolyte fast diffusion into the double-shelled spheres but also buffer large volume changes during lithium ions insertion/extraction. Importantly, due to the prominent dispersibility and high specific surface area, the SnO2@C spheres can provide sufficient contact areas between active materials and electrolyte so as to improve its electrochemical performance. As a result, the obtained double-shelled SnO2@C nanocomposites exhibit excellent rate capability, enhanced cyclability(911 mA h g-1 after 100 circles) and high specific energy density, which are much better than single-shelled SnO2@C hollow spheres and double-shelled SnO2 hollow spheres.更多还原