反尖晶石结构多孔SnFe2O4的设计合成及其作为锂离子电池负极材料的储锂性能研究

Tin-based oxides anodes for lithium ion batteries (LIBs) have attracted increased attention due to their super specific capacity and environmental protection. Unfortunately, very large initial capacity losses (~50%) and large changes in the unit cell volume (as high as 300% in some cases) are detrimental to the long-term Li cyclability since it will give rise to “electrochemical pulverization” of the active material of the electrode, and eventually leads to the electrode disintegration and capacity fading upon long-term cycling. In order.to solve or reduce the above problems, the inverse spinel SnFe2O4 is chose as the active materials of anode for LIBs, which can intercalate of more Li+ ions into the lattice of inverse spinel structure than that of spinel structure materials; and then the Sn2+ and Fe3+ of SnFe2O4 can react with Li+ and converse to metallic Sn and Fe, with the formation of Li2O; finally, metallic Sn can alloy with Li+ and.forming Li4.4Sn. Thus the theoretical capacity of SnFe2O4 should be higher than other tin oxides. In this work, various synthesis methods are applied to prepare the inverse spinel structure of SnFe2O4 with different particle size and porous size. The synthesis methods are precipitation-self assembling method, precipitation-soft template and (the different micelle formed by surfactants) and solvothermal method. The very small nanosize SnFe2O4 will be benefit to enhance the reversibility of reaction of Li2O and Sn changing to Li and SnO due to a large volume fraction of interfaces between Li2O and nanosized Sn. Moreover, the porous and hollow structure of SnFe2O4 can buffer the unit cell volume changes that occur during alloying dealloying reactions to some extent, thereby minimizing the capacity fading. On the other hand, incorporation of carbonaceous materials into the porous or hollow spaces or coating carbon shell can further minimizing the volume expansion of the active materials. These help in improving the electronic conductivity of the composite, and can help in absorbing some of the volume changes of the main alloy-forming metal, also act as catalysts for better Li cycling. The investigations are focused on the influence of the synthesis process of SnFe2O4, incorporation of carbonaceous materials on the lithium storage electrochemical performances. Furthermore, the lithiation/delithiation mechanism of inverse spinel SnFe2O4 will be clarified by the advanced testing technology and equipment, and elucidate the influence of crystal structure, particle size, morphology and structure, and incorporation of carbonaceous materials or carbon shells on lithium storage performance of SnFe2O4. In addition, the kinetics of electrode reaction process is investigated in detail, and the diffusion coefficient of Li+ and the active energy of lithiation/delithiation in the inverse spinel SnFe2O4 will be determined. This work will provide important insight into the rational design of high-performance tin oxide electrodes for Li-ion batteries.

锡氧化物锂离子电池负极材料因高比容量、环境友好备受关注。但首次充放电不可逆以及电极材料体积严重膨胀导致容量迅速衰减。为此，本项目以反尖晶石结构SnFe2O4为研究对象，首次进行储锂机制和电极反应动力学研究。反尖晶石结构可嵌入更多Li+，并与之发生转换和合金反应，三种方式协同作用将提供比单一氧化物更高的比容量。采用沉淀-自组装、沉淀-软模板法和溶剂热法合成多种尺度的多孔SnFe2O4。期望通过控制粒子小尺度和空心（孔）结构解决首次充放电不可逆和体积膨胀问题，从而提高材料库仑效率和循环稳定性；同时通过不同形式的碳复合进一步抑制体积膨胀，提高材料稳定性。借助现代测试技术研究SnFe2O4的储锂机制，阐明晶格结构、粒子尺寸、孔结构、复合碳对储锂性能的影响。同时研究电极反应动力学，测定Li+在反尖晶石结构中的扩散系数及嵌入脱出反应的活化能等动力学参数。该研究将为锂离子电池负极材料的开发奠定理论基础。

锡基锂离子电池负极材料因高比容量、环境友好备受关注。但其首次充放电效率低、容量衰减迅速。本项目以反尖晶石结构SnFe2O4 (SFO) 为研究对象，完成以下内容：.（1）制备不同形貌、尺寸的SFO.分别以吐温-20、PVP、SDBS、CTAB为软模版，沉淀法合成了球形和多面体状SFO。以兼具沉淀剂与形貌导向剂的弱碱（盐）柠檬酸钠、NaAc、Na3PO4、尿素等合成了纳米球层状堆积、纳米片花状堆积结构的SFO。以不同醇溶剂热法合成了纳米球、长方体、八面体、纤维球、微纳笼型、微米球型SFO。.（2）制备SFO@不同碳类材料的复合物.合成了SFO与N/S共掺杂石墨烯、石墨烯、无定形碳、石墨烯状多孔碳、N掺杂碳等复合材料。.（3）电化学性能研究.研究了上述SFO和复合材料半电池的循环寿命、倍率性能、阻抗、循环伏安，以及特定材料的全电池性能。.（4）SFO的储锂机制和储锂动力学.采用原位、非原位技术，研究了SFO充放电过程结构、形貌变化；Li+脱嵌的速控步、活化能等。利用DFT理论计算了SFO的晶格参数、带隙宽度、不同晶面对Li+的吸附行为。.得到主要结果：.（1）SFO的倍率和容量稳定性规律为纳米球>多面体>纳米花>微纳笼>微米球。尺寸越小，孔容积越高，容量越高，首圈效率越高。.（2）纳米级材料循环前期容量剧烈衰减后又趋于平稳，归因于纳米材料较高的表面赝电容效应以及各形貌的SFO经电化学研磨后趋向均匀的小尺寸纳米球需较长活化过程。.（3）碳类复合材料解决了纯SFO前期容量急剧衰减问题。归因于碳材料的导电性、对SFO材料膨胀的缓冲、提供Li+脱嵌活性位点等。.（4）证实了Li+在SFO的脱嵌机制为Li+优先吸附在(111)晶面嵌入反尖晶石晶格，发生转换和合金反应。.（5）SFO的嵌锂活化能：Li+通过SEI 膜的能垒、电荷传递能、扩散能等略高于碳复合材料。大比表面材料为扩散和表面赝电容共同控制；小比表面材料为扩散控制，扩散系数与材料结构有关。.（6）达到了预期目标的电化学关键数据.纯SFO纳米球：0.2 Ag-1，首效77.4%，200次后容量1450 mAhg-1；2 Ag-1，首效73%，1000次后570 mAhg-1；倍率性能6 Ag-1，容量481 mAhg-1，返回0.2 Ag-1循环200次，1200 mAhg-1.