Knowledge Management System Of Institute of process engineering,CAS
|关键词||两性金属 锂电池 电极材料 高附加值 高性能|
两性金属化合物在锂电池电极材料中的应用是两性金属高值化研究的重要方向。本论文以钒、钛、钴基两性金属氧化物、复合物和氟化物为切入点，围绕两性金属高性能锂电池电极材料的制备与调控，依次合成了马格涅利型V4O7正极材料、NaxV3O8型碱金属钒酸盐正极材料、钒酸钠/石墨烯复合负极材料、钒钛复合柔性负极材料和新型氟化钴/碳纳米管复合正极材料。论文通过对上述材料在三维结构设计、晶体结构调控、自组装赋存形态、电化学储锂特性、SEI膜表界面特性等方面的考察，明晰了上述材料的应用基础性问题，主要包括：厘清材料储锂和失效机理，分析材料构效关系，强化材料电化学界面稳定性，重构兼容性更好的电极-电解液体系。论文解决了材料电极反应可逆性差、储锂循环稳定性差等诸多弊端，达到了增强材料储锂能力的目标，取得的部分结果如下：（1）发展了乙二醇/水混合溶剂热体系，制备出形貌可控、晶型可控的钒氧化物。实验以偏钒酸铵为前驱体，通过控制二元混合溶剂的配比，达到了调节反应体系还原性和准乳液赋存形态的目的。实验分别制备出VO2(B)三维微米球、V4O7纳米十字架、无定型微米球等多种产物。其中，本论文首次发展了制备混价V4O7晶体的新方法并首次报道了V4O7晶体的电化学储锂特性：相比传统的V2O5和VO2(B)，V4O7的充放电循环稳定性、大倍率循环稳定性均显著提升（可逆容量超过300 mA h g-1，百圈循环的容量保持率达到95 %）。（2）碱金属离子插层有利于提高钒氧化物晶格的稳定性。本实验采用溶胶-凝胶法，可控制备出含钠量不同的NaxV3O8型（X=0.55 或0.95）正极材料。通过比较两种产物在可逆容量、循环稳定性、脱嵌锂电极反应等方面的电化学性能，得到了二者在电化学反应阻力、晶格稳定性等方面的显著差异：富含晶格缺陷的贫钠型Na0.55V3O8材料经多次充放电循环后，非晶化转变显著，导致电化学反应电阻显著升高。实验进一步讨论了贫钠型Na0.55V3O8材料中晶格缺陷对于钠离子插层的钒青铜材料的电化学储锂特性的影响。优化后的富钠型Na0.95V3O8材料可逆容量高（301 mA h g-1），倍率性能和循环稳定性均优于贫钠型材料。（3）本论文发展了钒酸钠/石墨烯复合负极材料，将钒酸钠材料应用到了转换型锂电池负极材料领域。在水热环境中，实验探明了原钒酸钠前驱体溶液在石墨烯表面的自组装过程；并通过控制水热反应温度和时间，可控合成了具有枝晶状三维纳米结构的复合材料。实验证明了钒酸钠枝晶是沿石墨烯表面二维生长，且枝晶间具有高度交联性。特殊的三维形貌满足了电子、离子的快速传输，所获得的结构具有高可逆容量（789 mA h g-1），长循环稳定性良好（1000圈容量保持率96 %）。（4）利用同轴静电纺丝法高通量制备出TiO2/VO2同步包埋的中空碳纳米纤维，改善了纤维表面修饰法电化学活性物质负载率低的问题。制备出的复合材料具有多级孔结构：纤维与纤维之间形成孔洞空隙，单根纤维具有中空空腔、因溶剂挥发在单根纤维外壳产生介孔。多级孔结构有利于电解液的渗透、锂离子的嵌入和电子的传输。复合负极材料具有良好的倍率性能及高倍率长程稳定性（20 C电流密度充放电1000圈，容量保持率为94 %）。通过对复合负极材料电极储锂反应的考察，实验揭示其电容—嵌锂双重电子转移机制。（5）揭示了氟化钴转换型正极材料的新失效机理，首次实现了氟化钴正极材料的稳定可逆储锂循环。实验将氟化钴纳米颗粒负载在碳纳米管纤维表面，构建出氟化钴/碳纳米管复合材料模型。通过对模型体系SEI膜表界面特性的分析，实验探明了在氟化物体系中，不同电解液的SEI膜成膜能力以及膜成分的区别。实验发现：氟化钴正极材料在传统的EC/DMC/DEC三元电解液中易溶解；针对氟化钴体系，三元电解液SEI膜成膜能力差，正负极表面生成的SEI膜不完整，不能有效抑制电极活性物质的泄漏。通过电解液的优化，实验发现： FEC/EMC电解液成膜性能好，SEI膜薄且完整。搭配FEC/EMC电解液后的氟化钴复合电极体系，电极循环稳定性得到极大的改善：电极可逆储锂容量高达 350 mA h g-1；在1 A g-1充放电电流密度下，复合材料万圈循环容量为69 mA h g-1，容量保持率超过 50 %。
The application of amphoteric metal compounds is one of the most important research topics on the electrode materials synthesis for lithium-ion batteries (LIBs) with the aspect of value-added application of amphoteric metal compounds. In order to prepare amphoteric metal compounds electrodes with high performance for LIBs, in this thesis, much attention has been paid on the investigation of basic issues, including morphology design, crystal structure manipulation, structure-effect relationship, electrochemical properties evaluation and the degradation mechanism. Amphoteric metals of vanadium, titanium and cobalt are selected and focused in the form of their oxides, compounds and fluorides. Specifically, advanced electrode materials, such as Magneli-type V4O7 cathode, sodium-rich NaxV3O8 cathode, sodium vanadate/graphene nanocomposite anode, TiO2/VO2-impregnated hollow carbon nanofibrous anode and conversion-type cobalt(II) fluoride/carbon nanotube composite cathode, have been developed in this study and systematically investigated with their capability of lithium-ion storage. The typical strategy for the cell performance improvement has been extabilished on the basis of 3D structure design, self-assembly behavior analysis, lithiation/delithiation process explanation, electrochemical property evaluation and post-motern investigation on the physichemical characteristics of formed SEI surface and interface. Therefore, upgradation on both the reversibility of electrode reaction and cycle stability was revealed with the benchmark on explaining the intrinsic degradation mechanism, effectively intensifying the surface or interface stability and re-building the electrolyte-electrode system with higher compatibility. The main achievement is summarized as following:(1) The Eg/water binary solvothermal system has been developed to controllably prepare vanadium oxide with different crystal structure and architecture. In this study, NH4VO3 was selected as the precursor. By controlling the ratio between Eg and water, the reducibility and self-assembly behavior of the solvothermal media was varied and able to prepare different products in the form of 3D VO2(B) microsphere, V4O7 nanocross and amorphous microsphere and ect. In particular, the first success of preparing V4O7 through solvothermal system was revealed and first investigation of V4O7 with its lithium-ion storage capability was reported. By comparison with conventional V2O5 and VO2(B) material, V4O7 exhibited much improved cycle stability especially under high rate (the reversible capacity of 300 mA h g-1 and 95 % capacity retention after 100 cycles).(2) The cycle stability of NaxV3O8 was investigated with different amount of alkali ioninsertion (the value of X in the NaxV3O8). In this study, sol-gel method with subsequent thermal treatment was applied to prepared specific NaxV3O8-type cathode material (X=0.55 and 0.95, repectively). Effect of crystal defect was emphasized on the lithium-ion storage properties of reversible capacity, cycle stability, lithiation/delithiation mechanism for both sodium-defect Na0.55V3O8 and sodium-rich Na0.95V3O8. By sharp comparison, sodium-rich Na0.95V3O8 showed higher structure stability upon lithium-ion insertion and therefore gave rise to much lower electrochemical resistance with good rate performance and high reversible capacity (301 mA h g-1). On the contrary, the obvious amorphorism of sodium-defect Na0.55V3O8 resulted in poor cell performance during cycling. (3) Sodium vanadate/graphene nanocomposite was developed and for the first time, applied as conversion-type anode material for LIBs. In this part, the self-assembly behavior of sodium vanadate precursor on the graphene nanoflakes was systematically investigated and carefully explained through hydrothermal process. By controlling the reaction time and temperature, novel structure of dendritic b-NaV6O15 and a-NaV2O5 was originally grown along the surface of graphene nanoflakes with the highly-branched and interconnected characteristic. Such architecture allowed significant feature of buffering the structure expansion during lithium-ion conversion, shortening the lithium ion diffusion paths and satisfacting the fast electron transfer. As a result, high reversible capacity (789 mA h g-1) was achieved with 96 % capacity retention after 1000 cycles. (4) The protocol of co-axial electrospinning technique was proposed to fabricate TiO2/VO2-impregnated hollow carbon nanofibrous anode with high throughput. The mass loading of active materials approached 60 %, a significant improvement of the low mass loading issue derived from current surface modification technique. The prepared nanofibrous anode enabled hierarchical porous structure. The pores involved were categorized as: nano-size space by the interconnection of nanofibers, the hollow cavity of single nanofiber and the meso-pores on the shell layer of single nanofiber. These pores facilitated fast electrolyte penetration, lihthium-ion insertion and electron transfer, allowing high rate performance and excellent cycle stability with 94 % capacity retention after 1000 cycles under extremely high rate of 20 C. Based on the investigation on electrochemical properties, the capacitance-intercalation mechanism for electron storage was explained. (5) The first reversible lithium-ion storage with conversion-type CoF2(II) was fulfilled and the new mechanism for the capacity degradation was proposed and largely optimized. In this part, CoF2(II) nanoparticles was pre-coated on the surface of carbon nanotubes fabric (CNTs). The CoF2(II)/CNTs nanocomposite was set as the structural benchmark to evaluate the capability of SEI formation of different electrodes. To be specific, the dissolution of CoF2 was revealed with the EC/DMC/DEC ternary electrolyte. It indicated that the imcompleted SEI failed to offer sufficient protection to the active materials. On the contrary, the optimization of electrolyte suggested the high performance of cell with binary FEC/EMC electrolyte, giving rise to reversible 350 mA h g-1 and over 50 % capacity retention after 10,000 cycles under 1 A g-1, which was the state-of-the-art performance of CoF2(II) so far. The investigation of FEC/EMC-derived SEI demonstrated a better protection due to its better physical and chemical properties.
|王欣然. 两性金属化合物在高性能锂离子电池电极材料制备中的应用[D]. 北京. 中国科学院研究生院,2016.|
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