【作者】 吴超；

【导师】 章新喜； 庄全超；

【作者基本信息】 中国矿业大学 ， 矿物材料工程， 2014， 博士

【摘要】 简单过渡金属氧化物如MnO2、α-Fe2O3、Fe3O4、Cr2O3、Co3O4、MnO、Cu2O因能提供高达700mAh/g以上的可逆容量而受到广泛的关注，是极具潜力的新一代锂离子电池电极材料。其中，铁的氧化物（Fe2O3、Fe3O4）作为锂离子电池负极材料因具有较高的理论比容量和廉价、环境友好等优点受到较多的研究。但是铁的氧化物导电性能较差和在充放电过程中体积变化较大，用作负极时出现很差的循环性能和倍率性能，进而限制了铁的氧化物作为负极材料的应用。最常见的，也是最有效的解决方法是与碳材料进行复合或者制备具有特殊形貌结构的材料。基于以上两点，本文通过将不同种类的碳材料与铁的氧化物进行复合及制备特殊形貌结构铁的氧化物，旨在寻求此类高能密度正极材料的改性方案；重点运用电化学阻抗谱技术，探讨电极动力学过程及其电极界面的性能，寻求此类电极的容量衰减的机理。主要研究内容和结果如下：（1）利用高温固相反应法制备α-Fe2O3/C复合材料。运用X射线衍射（XRD）、扫描电子显微镜、充放电测试、电化学阻抗谱对其结构和电化学性能进行了表征。充放电测试结果显示，α-Fe2O3/C电极循环50周时可逆充电容量为935.3mAh/g，循环性能较商品化α-Fe2O3有显著改善。电化学阻抗谱测试结果显示，α-Fe2O3/C电极在首次嵌锂过程中分别出现了锂离子通过固体电解质相界面膜（SEI膜）的迁移、材料的电子电导率、电荷传递过程相关的半圆，并详细分析了它们的变化规律。（2）采用水热合成的方法分别制备了α-Fe2O3/GNS、α-Fe2O3/CNTs复合材料和亚微米颗粒α-Fe2O3，系统研究了不同碳源对α-Fe2O3的形貌、结构和电化学性能影响。测试结果表明，α-Fe2O3/GNS和α-Fe2O3/CNTs电极有较高的可逆容量、倍率性能及在大电流下较长的循环寿命。复合材料电化学性能的提高归结于三个方面：一方面碳材料可以缓解由体积变化产生的应力及活性颗粒团聚现象；另一方面，复合材料具有的较大的表面积，使得电极/电解液接触充分；此外，碳材料可以提高电极的电子电导率。（3）采用水热法制备了空心纳米结构的α-Fe2O3。随着反应时间的延长，α-Fe2O3出现了从棒状到管状的演变过程。通过对这一系列产物进行表征，得出管状的形成是由棒状从两端开始“溶解”再结晶的过程，且“溶解”的方向是沿着[001]晶向指数（C轴）。采用了不同反应物浓度（PO43-）制备α-Fe2O3，随着PO43-浓度的减少，α-Fe2O3出现了从桶状到环状的演变过程。通过对这一系列产物进行表征，得出阴离子（PO43-和SO42-）对产物形貌的调控作用是有所差别的。即，PO43-易于控制前驱体生长，SO42-更倾向于加速α-Fe2O3的“溶解”过程。对所制备的α-Fe2O3进行了电化学性能测试，表明管状的α-Fe2O3有最好的电化学性能。经过65周循环之后，纳米管状的α-Fe2O3电极可逆容量为1131mAh/g，容量保持率在83%。且在不同充放电电流下，管状的α-Fe2O3具有较好可逆容量和倍率性能，这与其特殊结构密切相关。（4）采用水热法制备了Fe@Fe2O3核壳纳米颗粒与GNS、CNTs复合材料，Fe@Fe2O3/GNS电极在100mA/g下经过90周循环后，仍有959.3mAh/g的可逆容量，容量保持率在86.4%。在大电流密度下，经过280周循环后，Fe@Fe2O3/GNS电极的可逆容量仍然有515mAh/g。电化学阻抗谱测试结果显示，在首次嵌锂过程中，EIS的Nyquist图出现三个半圆，即高频区域的一个圆弧(HFA)，中频区域的一个半圆(MFS)和低频区域的一个半圆(LFS)，并对每部分的归属进行了探讨，详细分析了它们的变化规律。在100mA/g下经过60周循环后，Fe@Fe2O3/CNTs电极仍有702.7mAh/g的可逆容量。Fe@Fe2O3/CNTs电极具有较好的倍率性能，且在大电流充放电下，仍然具有较好的可逆容量。电化学阻抗谱测试结果显示，金属Fe和CNTs的存在有利于降低锂离子通过SEI膜和电荷传递电阻，进而使得Fe@Fe2O3/CNTs复合材料具有较好的电化学性能。（5）采用溶剂热合成的方法合成了Fe3O4-HSs和Fe3O4-HSs/CNTs复合材料。在100mA/g下，Fe3O4-HSs/CNTs电极循环70周后，可逆容量高达1153.8mAh/g，容量保存率在87.8%；在10.0A/g大电流下，Fe3O4-HSs/CNTs电极经过350周长周期循环后，可逆容量仍然能够保持在552.7mAh/g。采用水热、固相烧结的合成方法分别制备了Fe3O4/CNTs和Fe3O4/C复合材料。充放电测试显示：Fe3O4/CNTs、Fe3O4/C和商品化Fe3O4电极的首次放电容量分别为1421mAh/g、1651mAh/g和2104mAh/g，循环到55周时可逆容量分别为1030mAh/g、513mAh/g和280mAh/g。EIS测试表明，Fe3O4/CNTs电极在首次放电过程中，出现了高频区域与SEI膜相关的一个半圆，中频区域与电荷传递过程相关的一个半圆，低频区域与相变电阻相关的一个圆弧。更多还原

【Abstract】 Simple transition metal oxides (such as MnO2、α-Fe2O3、Fe3O4、Cr2O3、Co3O4、MnO、Cu2O) have long been intensively investigated as possible candidates for thenext generation anode materials in lithium ion batteries (LIBs), because of their highreversible capacities (700mAh/g). Among the transition metal oxides, iron oxides(α-Fe2O3、Fe3O4) are extensively investigated as anode materials for LIBs due to theirhigh theoretical capacity, inexpensive materials and environment friendly, and so on.However, the conductivity of the iron oxides is low, and the lithiation of iron oxidesusually leads to huge volume changes, and consequently, resulting in poor cyclingstability and rate capability, which hamper the applications of iron oxides as the anodematerials. To circumvent these obstacles, the most common and effective method is tosynthesis of iron oxides/carbon composites and iron oxides with special morphologies.Based on the above two points, the different types of carbon materials were mixedwith iron oxides, and iron oxides with special morphologies were synthesized in thispaper, in order to search for anode materials of high density. The electrochemicalimpedance spectroscopy (EIS) techniques were used to explore the electrode kineticprocesses and the electrode interface performance. The main research content andresults are as follows:(1) The α-Fe2O3/C composites were prepared by high-temperature solid-statereaction. The structure and electrochemical performance of the composites werecharacterized by X-ray diffraction (XRD), scanning electron miscroscopy (SEM),charge/discharge test and electrochemical impedance spectroscopy (EIS). Theelectrochemical test results indicated that the α-Fe2O3/C composites showed areversible charge capacity of935.3mAh/g after50cycles, and had better cycleperformance compared with commercial α-Fe2O3. Electrochemical impedancespectroscopy test indicated that there appeared three semicircles respectivelyrepresenting the Li-ion migration in solid electrolyte interface film (SEI film),electrical conductivity and charge transfer in the first lithiation, and their evolutiveprinciples were also investigated.(2) The α-Fe2O3/GNS、α-Fe2O3/CNTs hybrid materials and α-Fe2O3microparticleswere synthesized by a facile hydrothermal method, respectively, and the carbon motifefforts on the morphology, structure and electrochemical performance were studiedsystematically. The results showed that the α-Fe2O3/GNS、α-Fe2O3/CNTs electrodes exhibited a large reversible capacity and rate capability, especially excellent long-lifecycling performance at a high current. The improvements can be due to the CNTs inthe3D network, which several functions, including1) alleviating the mechanicalstress caused by the severe volume change and preventing the aggregation betweenthe active materials;2) providing large reaction surface and favoring the efficientelectrode/electrolyte interface contact;3) increasing the electronic conductivity ofelectrodes by forming3D conductive network.(3) The α-Fe2O3hollow nanostructures were prepared by a facile hydrothermalmethod. As the reaction time increases, α-Fe2O3underwent an evolution fromspindlelike precursors to nanotubes. Based on evidence from the abovetime-dependent morphology evolution evidence, the formation process of thenanotubes can be proposed as taking place by “dissolution” of the spindle-likeprecursors from the tips toward the interior along the axis, resulting in rod-likecrystals, semi-nanotubes and eventually hollow nanotubes, which follows apreferential dissolution along the [001] direction of nanotubes (C axis). Furthermore,the experiments were conducted with a fixed mass of sulfate ions and ferric ions butvarious quantities of phosphate ions (PO43-), and a series of nanostructure includingshort nanotubes, very short nanotubes, and nanorings were obtained. The resultsshowed that the roles that phosphate and sulfate ions played in the formation of thehollow nanostructure should be different, namely, phosphate ions played a moreimportant role than sulfate ions in the formation of the precursors in the early stage ofα-Fe2O3formation process, while sulfate ions favored the dissolution of α-Fe2O3dueto their coordination effect with ferric ions, resulting in the formation of ananostructure with hollow interior. Electrochemical measurements indicated that theα-Fe2O3nanotubes showed the best electrochemical performance, that the α-Fe2O3nanotubes displayed a large reversible capacity of1131mAh/g at100mA/g after65cycles, which was83%retention of the first charge capacity. Addition, the α-Fe2O3nanotubes presented the highest lithium storage capacity and best rate capacity atvarious rates, due to its special structure.(4) The Fe@Fe2O3core-shell nanoparticles anchored on graphene or CNTs hadbeen firstly synthesized by using a facile hydrothermal reaction. The galvanostaticcycling test showed that the Fe@Fe2O3/graphene electrode displayed a reversiblecharge capacity of959.3mAh/g up to90cycles at a current density of100mA/g,which was86.4%retention of the first charge capacity. At high current of5C, the Fe@Fe2O3/graphene electrode remained at515mAh/g after280cycles. Furthermore,the first lithiation process of Fe@Fe2O3/graphene electrode was studied byelectrochemical impedance spectroscopy (EIS) at different potentials. There appearedthree semicircles respectively representing the Li-ion migration in solid electrolyteinterface film (SEI film) and contact problems, electrical conductivity and chargetransfer in the first discharge process, and the change of kinetic parameters forlithiation process of Fe@Fe2O3/graphene electrode as a function of potential wasdiscussed in detail.The results showed that the Fe@Fe2O3/CNTs electrode exhibited a reversiblecapacity of702.7mAh/g up to60cycles at a current density of100mA/g, whichdisplayed much rate capability, especially, a large reversible capacity at high current.EIS tests showed that the Fe@Fe2O3/CNTs electrode had much smaller SEI resistanceand charge-transfer resistance due to the CNTs and Fe metal in the Fe@Fe2O3/CNTscomposites, resulting in the improvement of the electrochemical performance of theFe@Fe2O3/CNTs composites.(5) The Fe3O4-HSs and Fe3O4-HSs/CNTs hybrid materials were synthesized bysolvothermal method, respectively. After70cycles, the Fe3O4-HSs/CNTs electrodeexhibited a reversible capacity of1153.8mAh/g, which was87.8%retention of thefirst reversible capacity. Even at10.0A/g, the reversible capacity of Fe3O4-HSs/CNTselectrode remained552.7mAh/g after350cycles.The Fe3O4/CNTs hybrid material was synthesized by hydrothermal reaction.Charge-discharge tests showed that the initial discharge was1421mAh/g forFe3O4/CNTs composites,1651mAh/g for Fe3O4/C composites and2194mAh/g forcommercial Fe3O4, and the reversible capacity was1030mAh/g for Fe3O4/CNTscomposites,513mAh/g for Fe3O4/C composites and280mAh/g for commercialFe3O4after55cycles. The main Nyquist characteristic of Fe3O4/CNTs electroderecorded by EIS showed both high and middle frequency region, and an arc in thelow-frequency region, which respectively representing the Li-ion migration in solidelectrolyte interface film (SEI film), charge transfer and phase transformation in thefirst lithiation.更多还原