【作者】 史月丽；

【导师】 江利；

【作者基本信息】 中国矿业大学 ， 矿物材料工程， 2013， 博士

【摘要】 高能锂离子电池是未来新能源发展的重要技术方向之一。目前负极材料已经在硅、合金材料等方面获得较大突破，可逆容量高达4200mAh g-1。而目前商品化的锂离子电池多基于脱嵌锂机制，一般采用层状LiCoO2和LiNiO2、尖晶石LiMn2O4和橄榄石LiFePO4等材料为正极，这些材料在脱嵌锂过程中一个3d金属只交换一个电子，理论质量比容量较低。如LiCoO2, LiNiO2, LiMn2O4and LiFePO4理论容量分别为274,148,184和170mAhg-1，实际容量只有150mAh g-1，已不能满足日益增长的能量密度需求，急需发展新的正极材料。过渡金属氟化物因具有较高的工作电压，并可通过可逆的转化反应实现更高的能量密度，而被认为是未来极具竞争力的正极材料。但由于氟化物的极性较强，带隙较宽，电子电导率极低，造成电极极化严重；同时转化反应前后伴随着明显的体积变化，影响电池循环性能。本文将不同类型的氟化物与多种导电剂复合，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，探讨这类材料电极的电极动力学过程及其电极界面反应过程，阐明其电化学循环过程中的容量衰减的机理。主要研究内容和结果如下：(1)以NiF2为研究对象，采用高能球磨法制备了NiF2/C复合正极材料，首周放电容量高达1100mAh g-1，第十周后，容量为200mAh g-1。EIS结果揭示NiF2/C电极的Nyquist图的主要特征，依次为高频区半圆(HFS)、中频区半圆(MFS)和低频区圆弧或直线(LFS/L)。根据阻抗谱特征并结合CV和充放电研究结果，本文提出了相应的等效电路并获得了较好的拟合结果，研究发现，各部分区域的阻抗均与极化电位的变化相关，其中HFS主要与锂离子通过电极表面的SEI膜过程有关，LFS/L则反应了电荷传递的过程。而MFS则反映了电极内部的氟化物与导电剂形成的肖特基接触阻抗，导电剂的选择对金属氟化物电极内阻具有显著的影响。并提出了NiF2/C电极转化反应的模型。(2)以FeF3为研究对象，分别选择不同的导电剂(炭黑石墨、TiO2、MoS2或V2O5)与商品化FeF3球磨制备复合材料。FeF3复合电极的充放电实验结果表明，在4.5~1.5V之间，以10mA g-1的电流密度充放电时，FeF3/C电极首次放电容量可达712mAh g-1，100周后放电容量仍具有266mAh g-1，保持率为37%；FeF3/TiO2/C电极首次放电容量为340mAhg-1，20周后放电容量244mAh g-1，保持率为71.7%；FeF3/MoS2/C复合电极首次放电容量为423mAh g-1，100周后放电容量45mAh g-1，保持率仅为10.6%。FeF3/V2O5/C复合电极充放电电流密度为20mA g-1，首次放电容量为490mAh g-1，100周后放电容量34mAhg-1，保持率仅为7%。综合比较，FeF3/C复合电极的容量较高，循环性能较好，FeF3/TiO2/C次之，FeF3/V2O5/C最差。上述复合材料的EIS结果与NiF2/C电极类似，均由HFS、MFS和LFS/L三部分组成，但对应的肖特基接触阻抗有较大差异，其中FeF3/MoS2/C复合电极材料的肖特基接触阻抗最小，肖特基电阻是影响电极电化学性能的一个重要因素。综合分析实验结果，提出了FeF3/C电极转化反应的模型。(3)采用液相法分别合成了FeF3和FeF3/TiO2材料，并通过高能球磨法制备了FeF3/C与FeF3/TiO2/C复合材料。FeF3/C复合电极在4.5~1.5V、以71.2mA g-1电流密度充放电时，首次放电容量为293mAh g-1，30周后放电容量170mAh g-1，保持率为58.0%。FeF3/TiO2/C复合电极在4.5~1.5V、充放电电流密度为10mA g-1的首次放电容量为504mAh g-1，30周后放电容量124mAh g-1，容量保持率为24.6%，20mA g-1电流密度下的容量则略低。EIS结果表明，所合成的FeF3/C和FeF3/TiO2/C复合材料电极的Nyquist图在整个测试频率范围内均由三部分组成，即HFS、MFS和LFS/L，与商品化材料的EIS结果一致。依据拟合结果，放电过程中，FeF3/C电极的SEI膜电阻为8.97~21.98，肖特基接触电阻为13.42~36.01；而合成FeF3/TiO2/C复合电极的SEI膜电阻为14.76~33.20，肖特基接触电阻为70.52~846.30，显示FeF3/TiO2/C复合材料的界面电阻较大，接近商品化材料，说明TiO2较炭黑能产生更高的肖特基接触电阻，一定程度上降低了材料的电化学性能。(4)本文还研究了CuF2正极材料的电化学性能。采用高能球磨法制备CuF2/MoO3/C和CuF2/C的复合材料。充放电研究结果表明，组合导电剂(MoO3和炭黑)较单一导电剂(炭黑)能更好地提高CuF2的电化学活性。CuF2/MoO3/C复合电极的EIS基本特征依然为典型的“三个半圆”，即HFS、MFS和LFS/L。其中MFS对应了CuF2和MoO3及C之间的接触阻抗，低频区半径巨大的圆弧表明反应过程电荷传递电阻较大，尽管MoO3和炭黑的引入，一定程度上改善了CuF2不良的导电性能，减小了电极极化，但对缓减反应产物造成的电化学失活，其效果仍然不佳。该论文有图113幅，表18个，参考文献169篇。更多还原

【Abstract】 The high-energy lithium-ion battery is one of the important technical directions for newenergy development in the future. At present, many breakthroughs have been achieved in anodedevelopment, especially in silicon and alloy materials. The reversible capacity of anode hasreached as high as4200mAh g-1. A wide range of layered intercalation compounds such asLiCoO2, LiMn2O4, LiNiO2and LiFePO4, etc. have been developed as cathode materials forcurrent commercial LIBs. However, these compounds exchange only one electron per a3Dmetal, corresponding to a limited capacity. Ror example, the theory capacities of LiCoO2,LiMn2O4, LiNiO2and LiFePO4are274,184,148and170mAh g-1, respectively. However, theactual capacities are only150mAh g-1. Thus, it is necessary to explore new redox mechanismsand electrode materials to meet the requirements.With higher working voltage and energy density obtained through reversible conversionreaction, transition metal fluorides are regarded as potential cathode material for futurelithium-ion batteries. Despite the above advantages, there are also several problems to be solvedfor transition metal fluoride. First, fluoride generally has stronger polarity, wider band gap andlower electronic conductivity, which would lead to serious electrode polarization problems.Secondly, the conversion reactions of fluoride are accompanied by obvious volume changes,which will affect the cyclic performance of batteries. In order to search for cathod materials ofhigh density, different kind of conductive agents are mixed with fluoride in this paper. Theelectrochemical impedance spectroscopy (EIS) techniques are used to explore the electrodekinetic processes and the electrode interface performance.The main research content and results are as follows:(1) The commercial NiF2/C composites were prepared by milling with conductive agent(graphite and carbon black) in a ratio of5:3:1(w/w) for3h in a high-energy milling machine at500rpm.And its discharge capacity in the first cycle reached up to1100mAh g-1. However,after tenth cycles, the capacity is200mAh g-1. The main Nyquist characteristic of NiF2/Celectrode recorded by EIS showed a semicircle in both high and middle frequencyregion(HFS,MFS), and an arc or a line in the low-frequency region(LFS/L). An equivalentcircuit was proposed according to the different impedance response time of each part inside theelectrode, and the fitting results are satisfactory. This study reveals that, the impedance of eachpart is related to voltage changes, particularly HFS is commonly attributed to the process oflithium ions migrating through SEI film, while LFS/L is ascribed to charge transfer process onthe electrolyte-electrode interface. MFS reflects the Schottky Contact resistance formed byfluoride and conductive agent inside the electrode, which doesn’t exist in lithium intercalation compound electrodes, indicating that the choice of conductive agent has significant influenceon the resistance of the metal fluoride electrode. And the NiF2/C electrode reaction model wasproposed.(2) The commercial FeF3composites were prepared by milling FeF3with differentconductive agents (carbon black, TiO2, MoS2and V2O5) in a ratio of5:3(w/w) for3h in ahigh-energy milling machine at500rpm.The charge/discharge tests showed that the initial specific discharge capacity of FeF3/Celectrode was close to712mAh g-1at the current density of10mA g-1during4.5~1.5V. After100cycles, the discharge capacity was266mAh g-1with the capacity retention of37%; the firstdischarge capacity of FeF3/TiO2/C electrode is close to340mAh g-1, and reduced to244mAhg-1after20cycles, with the capacity retention of71.7%; as for FeF3/MoS2/C compositeelectrode, the first discharge capacity was423mAh g-1, after100circles, only45mAh g-1ofthe capacity remained and the capacity retention is10.6%; The first circle discharge capacity ofFeF3/V2O5/C electrode was490mAh g-1at the current density of20mA g-1, and34mA g-1after100circles, with a capacity retention of only7%. According to the above results, FeF3/Celectrode has the highest capacity and the best cyclic performance, FeF3/TiO2/C electrode isnext and FeF3/V2O5/C electrode is the worst. The EIS results of the above composite electrodesare similar to that of NiF2/C electrode, and all are composed of three parts: HFS, MFS andLFS/L. However, there is difference in the correspon ding Schottky contactimpedance.Schottky contact impedance of FeF3/MoS2/C composite electrode materials is theminimal. Schottky resistance is one of the important factors which affect electrochemicalperformance of the electrode. According to comprehensive analysis of the experimental results,FeF3/C electrode reaction model was proposed.(3) FeF3/C and FeF3/TiO2of different morphologies were synthesized respectively throughtwo liquid phase methods. Then FeF3/C and FeF3/TiO2/C compounds were prepared byhigh-energy ball milling. The FeF3/C electrode is charge/discharged at a current density of71.2mA g-1between4.5and1.5V. The first circle discharge capacity of the electrode is293mAhg-1,34mAh g-1after30circles, and the capacity retention is58.0%. The FeF3/TiO2/C electrodeis charge-discharged at10mA g-1between4.5and1.5V with initial discharge capacity of504mAh g-1,124mAh g-1after30circles, and the capacity retention of24.6%. When cycled underthe current density of20mA g-1, the capacity became a little lower.The EIS results show that, the Nyquist plots of FeF3/C electrode and FeF3/TiO2/Ccompound electrode are both consisted of three parts, namely HFS, MFS and LFS/L. This EISresult is similar to that of the commercial material. According to the fitting results, in thedischarge process the SEI resistance of the synthesized FeF3/C electrode ranges from8.97to 21.98, and its Schottky contact resistance is between13.42to36.01. As to the FeF3/TiO2/Ccompound electrode, the SEI resistance ranges from14.76to33.20, and Schottky contactresistance from70.52to846.30. It is seen that the interfacial impedance is larger for thiscompound material, similar to the commercial material, indicating TiO2leads to larger Schottkycontact resistance than carbon black, therefore resulting in decline in the electrochemicalperformance of electrode.(4) Furthermore, the electrochemical performance of CuF2as cathode material forlithium-ion battery was discussed in this paper. The CuF2/MoO3/C and CuF2/C compoundmaterials were prepared by high-energy ball milling. And then Galvanostatic discharge/chargemeasurements were carried out in the Neware battery test system at0.01C (7.12mA g-1)between1.5V and4.5V versus Li+/Li at room temperature.The results indicate that thecombination of conduct agents (MoO3and carbon black) has better improvement on theelectrochemical activity of CuF2than single carbon black. The EIS of CuF2/MoO3/C compoundelectrode remains typical three parts, namely HFS, MFS and LFS/L. MFS is related tocontacting resistance among CuF2, MoO3and C. LFS/L reflects the large charge transferresistance during the reaction process.Although the introduction of MoO3and carbon black canimprove the conductivity of CuF2to a certain extent and reduce the polarization of electrodes,there is little improvement in relieving the electrochemical inactivation of reaction products inconclusion.In this paper, there are113figures,18tables and169reference articles.更多还原