【作者】 韩飞；

【导师】 李文翠；

【作者基本信息】 大连理工大学 ， 化学工艺， 2014， 博士

【摘要】 为了满足节能环保的新能源汽车对锂离子动力电池的需求,发展具有长循环稳定性、高可逆容量、良好的安全性能和快速充放电能力的电极材料成为当务之急。根据电极中活性物质的电化学特性定向设计合适的结构以提高锂离子电池的电化学性能,尤其是循环稳定性,是一个具有挑战性的研究课题。鉴于炭材料在能量储存方面的优势,本论文以纳米电极材料的结构设计为导向,制备一系列多孔炭修饰的金属氧化物／硫化物复合负极材料,旨在提高锂离子电池负极材料的循环稳定性,在此基础上,研究材料的结构特点对其电化学性能和反应机理的影响。具体包括如下几个方面：(1)针对Sn02负极材料在充放电过程中体积膨胀大(250%)和导电性低的问题,以薄壁(-2nm)高孔容(2.16cm3g-1)的管状介孔炭为载体,构筑Sn02颗粒尺寸<5nm的Sn02@C复合材料。对Sn02在炭孔道中的填充度进行调变,调变范围为7-27%。当Sn02的负载量高达80wt%时,Sn02纳米颗粒还能高度分散于炭载体的介孔孔道中,且无团聚现象发生。这种管状复合材料表现出高的可逆容量和稳定的循环性能,经过100次循环后,可逆容量为1039mAh g-1,容量保持率为106%。其稳定后的容量高于Sn02的传统理论容量(782mAhg-1),这可能归因于在充放电过程中Sn02与Sn之间发生了可逆的转化反应。(2)为了从本质上降低Sn02的体积膨胀,将体积变化相对较小的ZnO(体积膨胀：103%)引入到Sn02体相中,制备出ZnSn03(体积膨胀：-191%)负极材料。同时,结合炭材料的优势,设计合成了核壳结构的炭包覆ZnSnO3(ZnSnO3@C)纳米方块。其中,ZnSnO3方块的尺寸为37nm,具有无定形结构和丰富的介孔孔道；外部的炭层相互交联,构成连续的电子导通骨架和相互贯通的大孔通道(74nm)。电化学测试结果表明,ZnSnO3@C复合物的储锂反应综合了合金反应和转化反应的特点(Li4.4Sn与LiZn合金可逆地转变为初始的2ZnSnO3),因而可以提供高的可逆容量。经过100次循环后,可逆容量达到1060mAh g-,并且其容量保持率为93%。(3)考虑到电极／电解液表界面的稳定性问题,以过渡金属氧化物Fe2O3为研究对象,探索了提高界面稳定性的方法。为了获取稳定的固体电解质界面(SEI)膜,根据各组分的不同功能,采用纳米工程技术将Fe203纳米颗粒、管状介孔炭载体和导电聚吡咯分层次地组装在一起,构筑了一种多功能复合负极材料。在复合物中,Fe203高度分散于炭载体中,同时导电聚吡咯均匀地包覆在Fe203@C的孔道口和外表面,将Fe2O3@C颗粒桥接起来构成一个大的单元。作为锂离子电池负极材料,聚吡咯包覆的Fe2O3@C表现出稳定的循环性能,100次循环后,容量保持率高达97%。另外,复合材料还具有快速的充放电速度、高的Fe203利用率和大的体积比容量。(4)以负载于管状介孔炭中的硫为起始物质,铜箔代替传统的铝箔作为集流体,依靠恒流充放电过程中的电化学反应在管状介孔炭中原位生成Cu2S纳米颗粒。对反应机理进行研究发现：S颗粒与Li+反应生成的Li2Sn溶解于电解液中变为Sn2-,来自于铜箔的Cu+会与Sn2-反应生成难溶性的CuxS中间产物,随着循环次数的增加,CuxS逐渐转变为最终的Cu2S产物,得到高度分散于管状介孔炭中的Cu2S。这种原位制备的Cu2S/C复合材料表现出稳定的循环性能和优异的倍率性能。在0.2C下循环300次,可逆容量为270mAh g-1,容量保持率为104%。在10C的大电流密度下,可逆容量保持在225mAhg-1左右,是0.2C下可逆容量的86%。更多还原

【Abstract】 To develop power lithium ion batteries (LIBs) for electric vehicles, it is essential to fulfill the requirements of electrode materials for long cycle stability, high reversible capacity, good safety and fast charge/discharge capability. However, how to controllably synthesize a suitable structure for improved performance according the electrochemical characteristics of active materials, especially their cycle stability, remains a big challenge. Considering the unique merits of porous carbons, herein, my research works have focused on designing and synthesizing a series of porous carbon-modified metal oxide/sulfide nanohybrids with greatly enhanced electrochemical cycle performance as anode materials for advanced LIBs. Furthermore, the correlation between the electrochemical properties, reaction mechanism and their structural features have been fundamentally investigated. Specifically, the works include the following parts:(1) Aiming to buffer large volume expansion (250%) and improve electrical conductivity of SnO2anodes, ultrafine SnO2nanoparticles with size between4-5nm were uniformly encapsulated into ordered tubular mesoporous carbon, which possesses thin carbon walls (-2nm) and high pore volume (2.16cm3g-1). The filling ratios of pore channels in carbon matrix were tailored in a large range of7-27%to efficiently utilize the pore space and adequately accommodate the volume change of SnO2. Even when the loading of SnO2was up to80wt%, the nanoparticles were still highly dispersed in the mesopore channels of carbon matrix, and no bulky aggregated were visible. The tubular hybrid exhibited an ultrahigh reversible capacity of1039mAh g-1after100cycles with a high capacity retention of106%, and the stable capacity value was much higher than the conventional theoretical capacity of SnO2(782mAh g-1). The extra reversible capacity came from partial reversiblity of conversion reaction (oxidation of Sn to SnO2).(2) To further reduce volume expansion of SnO2intrinsically, we tried to introduce ZnO having a relatively low volume change of103%into SnO2for the fabrication of ZnSnO3with decreased volume expansion of～191%. Thus, a core-shell carbon-coated ZnSnO3(ZnSnO3@C) anode material has been designed, in which amorphous ZnSnO3nanocubes (size:-37nm) with a well-developed mesoporous structure are coated with a continuous and thin carbon layer. The core-shell nanocubes cross-linked with each other to form a continuous conductive framework and interconnected porous channels with macropores of74nm width. The amorphous ZnSnO3@C exhibited greatly enhanced integration of alloying and conversion reaction processes (reversible transformation of Li4.4Sn and LiZn alloys to pristine), thus achieving a high reversible capacity. Electrochemical results showed that the reversible capacity of such amorphous ZnSnO3@C reached a high value of1060mA h g’1after100cycles with a capacity retention of93%.(3) Considering of the electrode/electrolyte interface stability issues, e.g., transition metal oxide Fe2O3, which suffers a severe capacity fading due to the instability of interfacial solid electrolyte interface (SEI) film. Herein, a nano-engineering strategy has been successfully developed for fabricating a novel multifunctional nanohybrid composed by highly dispersed nanosized Fe2O3particles, tubular mesoporous carbon host and conductive polypyrrole (PPy) sealing layer. Fe2O3particles were well-dispersed within the carbon matrix and PPy is spatially and selectively coated onto the external surface and the pore entrances of the Fe2O3@C, thereby bridging the hybrid particles together into a larger unit. As an anode material for Li-ion batteries, the PPy-coated Fe2O3@C hybrid exhibited stable cycle performance with97%capacity retention after100cycles. Additionally, the PPy-coated Fe2O3@C also possessed fast electrode reaction kinetics, high Fe2O3use efficiency, and large volumetric capacity.(4) An effective strategy has been developed for the in situ generation of a Cu2S/tubular mesoporous carbon hybrid by exploiting the charge-discharge processes of a S/C hybrid on a copper-foil current collector. By studying the reaction mechanism were explored. It is discovered that the dissolved polysulfide ions (from the reaction of pristine sulfur and inserted Li+) were firmly anchored by the copper ions released from the copper foil, which formed insoluble CuxS intermediates, and further converted into final Cu2S product in the mesopore channels of the carbon matrix. The highly dispersed Ou2S in tubular mesoporous carbon displayed excellent lithium storage properties with extremely stable cycle performance of270mAh g-1at0.2C for300cycles and excellent rate capability of225mAh g-1at10C.更多还原