【作者】 孙伟；

【导师】 赵新红； 李小成；

【作者基本信息】 兰州理工大学 ， 化学工程领域， 2016， 硕士

【摘要】 在锂离子电池中,由于硅的理论容量比较高(最高的理论比容量为4200mA g-1),受到人们的广泛关注。但是由于电极材料在循环过程中,会出现粉碎现象,导致循环性能急剧下降。通过引入导电性高、柔韧性好、机械强度大的石墨烯为第二相,与硅形成复合材料,可以解决以上难题,使得硅基负极材料的电化学性能显著得提高。本论文通过不同的制备方法,成功的制备出不同结构的硅@石墨烯复合材料。利用XRD、SEM、TEM、TG和AFM等技术对材料的微观结构和形貌进行分析,并分别测试了其电化学性能。论文主要内容如下:1、水热法是制备三维自组装石墨烯最常用、最简单的方法。本文采用一步水热法,将石墨烯与硅纳米颗粒(经多巴胺修饰)制备成三维复合材料,经碳化后得到目标产物硅@无定形碳@三维石墨烯结构(Si@a-C@3DGO)。经多巴胺表面修饰后的硅粉在水中的分散性得到明显的改善,进而改善了硅纳米颗粒在复合材料中的分散性;三维石墨烯中的孔道结构可以有效的缓解硅在脱嵌锂过程中产生的体积效应,同时极大的降低了电荷转移电阻。因此,Si@a-C@3DGO电极在500mA g-1的电流密度下,首次放电容量为1703.2mAh g-1,并且具有良好的循环稳定性,但是倍率性能比较差。2、受豌豆荚结构的启发,本文提出了中空的硅@无定形碳@石墨烯纳米卷结构(记为:Si@a-C@GNSs),经无定型碳包覆的硅纳米颗粒(记为Si@a-C)能够均匀的分散在石墨烯纳米卷通道中,并且在石墨烯与Si@a-C纳米颗粒之间留有足够的空间。将多巴胺修饰过的Si纳米颗粒分散液与氧化石墨烯混合并经冷冻干燥、高温退火处理,制得具有豌豆荚状的Si@a-C@GO复合材料。对于高性能的Si负极材料来说,该结构具有许多独特的电化学特性。例如:为电子和离子提供有效的通道;可以有效的缓冲Si的体积变化,结构稳定;具有柔韧性的石墨烯壳层可有效的抑制Si表面形成新的固体电解质相界面(solid electrolyte interphase,SEI)膜。基于以上优点,S i@a-C@GO电极在0.3 A g-1的电流密度下,最高比容量达到2243 mA h g-1,并且具有较好的倍率性能(947.8mA h g-1,电流密度3 A g-1)和较好的循环稳定性(110循环后,容量保留率为91.5%,电流密度2 A g-1),展现出杰出的电化学性能。更多还原

【Abstract】 Lithium- ion batteries(LIBs) are the power source of cho ice not only for portable electronics devices but a lso ever- growing electr ic ve hic les. Among a ll anode mater ia ls, Si is regarded as one of most promis ing candidates due to its abundance in nature, low d ischarge potentia l, and highest know n theoretic d ischarge capacit y(~ 4200 mA h g-1 vs 372 mA h g-1 of commerc ial graphite). However, the large volume expansio n of Si(300%) upon lit hiat ion/de lit hiat ion processes leads to serious capacit y perfor mance fad ing due to the pulverizat ion and thus ind uced electr ical contact loss of Si w ith curre nt collector. Cons idering the high conduct ivit y, excellent fle xibility and mecha nical strength of graphe ne as well as high theoretic capacit y of Si, in this thesis, two kinds of 3D S i/ graphene compounds were successfully synt hesized. The ir structura l characterizat ions and e lectrochemica l perfor mances were syste mat ically analyzed and evaluated. The main contents are listed as follows:A three- dimens io nal macroporous s ilicon/ graphene compos ite has b een synt hes ized via a s imple but cost-effect ive one- step hydrotherma l met hod by us ing graphene oxide and dopamine decorated silicon na nopartic les as raw materia ls fo llowed by a therma l annealing process. O wing to the good dispersib ilit y of dopamine decorated Si nanopartic les in water, the result ing 3D macroporous silicon/ graphene compos ite shows the unifor m d istr ibut io n of S i na nopartic les along the backbone of the 3D graphe ne oxide after hydrother ma l process, superior to that of silicon/ graphene composite w it h prist ine Si na nopartic les as raw materia ls w here serious S i aggregat ions were c learly observed. The porous struct ure of 3D graphene can e ffective ly buffer t he volume cha nge o f s ilicon d ur ing t he lit hiat ion and delithiat io n process, and simultaneous ly reduce the charge transfer resista nce. As a consequence, the prepared 3D macroporous Si@ a-C @ GO e lectrode exhib ited an init ia l high d ischarge capacit y of 1703.2 mA h g- 1 at 500 m A g-1 wit h good cyc ling stabil ity.2. Inspired by bean pod structure, we synthes ized a three- dimens io nal void- conta ining Si@ amorphous carbon@ graphene nanosheet scrolls(Si@ a- C@ GNSs) by freeze- drying the mixture suspens ion o f mildly reduced graphene oxide and dopamine decorated Si na nopart ic les, fo llowed by a ther mal annea ling process. In this structure, Si@ a-C nanopartic les homo geneous ly disperse w ithin the ga ller y of graphene na noscroll, lea ving large a mounts of voids between graphe ne and Si@ a-C. In terms o f s ilicon anode electrode mater ials, t he prepared pea pod- like Si@ a-C@ GNS nanostr ucture has several adva ntages as compared wit h other Si/graphe ne composites. First, t he tub ular shape of 3D GN S geometry configurat ion provides an effect ive access for ions/e lectron transportation. Second, the large amounts of vo ids in Si@ a- C@ GNS structure can effect ive ly buffer the tre mendous volume change of silico n nanopartic les dur ing the lithiat io n/delit hat ion process and guarantee the integr it y of the electrode dur ing the cycling. Third, the flexib le tubular shaped GN S outer shell layer and amorpho us inner carbon layer provides a dua l protection for ins ide Si nanopartic les, and thus effective ly prevent the repeat deposit ion of so lid electrolyte inter face layer on sur face of ins ide Si na nopartic les. Thanks to these extraordinary advantages, the prepar ed Si@ a-C@ GNS- based electrode exhibited a high capacity about 2243 mA h g-1 at 0.3A g-1, excellent rate capabilit y(947.8 mA h g-1 at 3A g-1) and good cyc ling stabilit y(91.5% capacit y retention after 110 cycles at 2A g-1).更多还原