硫化锂正极材料的制备及电化学性能

发布时间：2018-05-02 22:13

本文选题：锂硫电池 + 硫化锂　；参考：《浙江大学》2017年硕士论文

【摘要】：为了适应未来社会对经济、环保、高效的能源体系要求,研究开发新型绿色能量存储材料及器件,已成为各国科学家们关注的重点。锂硫电池因具有理论能量密度高(2674Wh·kg-1和2697Wh.L-1)、成本便宜、环境友好等优势,成为最具发展前景的下一代能源存储体系之一。但是正极材料的电绝缘特性、充放电过程中多硫化物的穿梭效应、体积应变以及负极材料锂易生成枝晶等问题,导致锂硫电池的循环稳定性和安全性能较差,严重制约了其产业化进程。硫化锂是硫放电后的终产物,因其本身含有锂源,可以采用非锂负极与其匹配制备全电池,从而提高电池安全性。然而,硫化锂正极材料同样存在导电性差和多硫化物溶解穿梭的问题,本文设计并制备了碳包覆Li2S@C复合正极材料,并采用纳米化策略来进一步改善Li2S正极材料导电性能,使得正极材料的电化学性能得到了大幅改善。主要研究工作如下:(1)以PVP为碳源,利用Li2S和PVP在四氢呋喃溶剂中溶解度的差异,通过调控温度蒸发溶剂,使Li2S和PVP先后析出,之后通过高温退火的方法将PVP分解碳化,得到碳包覆的Li2S复合正极材料。该复合材料表现了较好的电化学性能,在0.2C充放电时,首次放电容量为865.4mAh·g-1 100次循环之后,其容量衰退至593.1 mAh·g-1,库伦效率降至93%。(2)以纳米级聚苯乙烯(PS)微球为形核剂,利用溶液蒸发和化学气相沉积(CVD)相结合的方法制备纳米尺度核壳结构的复合正极材料。因为PS球在高温下会裂解,所以制备得到的复合材料壳层内部有一定空间余量,可以缓解体积效应。复合正极材料表面CVD沉积的碳壳层一方面提高了活性物质Li2S的导电性,另一方面作为阻挡层,可以有效地将充放电过程中产生的多硫化物限制在壳层内部,抑制多硫化物的穿梭效应,提高活性物质的利用率,从而提高材料的循环性能。而纳米化的结构设计有效缩短了锂离子在正极材料中的扩散距离,从而使得整个正极材料的离子和电子传输速度得到提高,继而改善正极材料的倍率性能。在0.2C充放电时,首次放电容量为1121.9mAh·g-1,100次循环之后,其可逆容量仍可保持为804.2mAh·g-1,容量保持率为71.7%,库伦效率维持在96%以上。在1C和2C的高测试电流下,其容量仍可以达到760.7 mAh.g-1和682.9 mAh.g-1。(3)以纳米导电炭黑颗粒为形核剂,利用溶液蒸发和CVD相结合的方法合成类似鸡蛋结构的复合正极材料nano-Li2S@C,其中纳米导电炭黑为蛋黄,Li2S为蛋白,表面包覆层为蛋壳。在纳米化设计和碳包覆协同作用下,电极显示了良好的循环和倍率性能。在0.2C充放电时,首次放电容量为1083.5 mAh·g-1,100次循环之后,其可逆容量仍可保持为893.6mAh·g-1,容量保持率为82.5%,库伦效率维持在96%以上。在1C和2C的高测试电流下,其容量仍可以达到763.5 mAh.g-1 和 625.8 mAh.g-1。
[Abstract]:In order to meet the requirements of economic, environmental protection and efficient energy systems in the future, the research and development of new green energy storage materials and devices has become the focus of attention of scientists all over the world. Lithium-sulfur batteries have the advantages of high theoretical energy density 2674Wh kg-1 and 2697Wh.L-1, low cost and environment-friendly, so they have become one of the most promising next-generation energy storage systems. However, the electrical insulation characteristics of cathode materials, the shuttle effect of polysulfide during charge and discharge, the volume strain and the dendrite formation of lithium in negative electrode materials lead to poor cycle stability and safety performance of lithium sulfur batteries. Seriously restricted its industrialization process. Lithium sulphide is the final product of sulfur discharge. Because it contains lithium source, it can be used to match the non-lithium negative electrode to prepare the whole battery, thus improving the safety of the battery. However, lithium sulphide cathode materials also have the problems of poor conductivity and polysulfide solution shuttling. In this paper, carbon coated Li2S@C composite cathode materials are designed and prepared, and nanocrystalline strategies are adopted to further improve the conductivity of Li2S cathode materials. The electrochemical performance of the cathode material is greatly improved. The main research work is as follows: (1) by using PVP as carbon source and using the difference of solubility of Li2S and PVP in tetrahydrofuran solvent, Li2S and PVP are precipitated successively by adjusting the temperature evaporation solvent, and then the PVP is decomposed and carbonized by high temperature annealing. Carbon coated Li2S composite cathode material was obtained. The composite showed good electrochemical properties. After the first discharge capacity of 865.4mAh g-1 was 100 cycles, the capacity of the composite decreased to 593.1 mAh g -1, and the Coulomb efficiency decreased to 93%. 2) the nanocrystalline polystyrene (PS) microsphere was used as nucleating agent. Nanoscale composite cathode materials with core-shell structure were prepared by the method of solution evaporation and chemical vapor deposition (CVD). Because the PS spheres will be cracked at high temperature, there is a certain amount of space in the composite shell, which can alleviate the volume effect. On the one hand, the carbon shell deposited by CVD on the surface of the composite cathode material improves the conductivity of the active material Li2S, on the other hand, as a barrier layer, the polysulfide produced during charge and discharge can be effectively confined to the inner shell. The shuttle effect of polysulfide was inhibited and the utilization rate of active substances was improved. The nanocrystalline structure design can effectively shorten the diffusion distance of lithium ion in the cathode material, thus improve the ion and electron transmission speed of the whole cathode material, and then improve the rate performance of the cathode material. At 0.2C charge-discharge, after the first discharge capacity was 1121.9mAh g-1100 cycles, the reversible capacity could be maintained as 804.2mAh g-1, the capacity retention rate was 71.7%, and the Coulomb efficiency was above 96%. At high test currents of 1C and 2C, its capacity can still reach 760.7 mAh.g-1 and 682.9 mAh.g-1.3) with nano-conductive carbon black particles as nucleating agent. The composite cathode material nano-Li2S@ C, which is similar to egg structure, was synthesized by the method of solution evaporation and CVD. The nano-conductive carbon black is egg yolk Li2S protein and the surface coating layer is eggshell. Under the synergistic action of nanocrystalline design and carbon coating, the electrode shows good cycling and rate performance. After the first discharge capacity of 1083.5 mAh g-1100 cycles, the reversible capacity of 0.2C charge-discharge can be maintained as 893.6mAh g-1, the capacity retention rate is 82.5, and the Coulomb efficiency is above 96%. Under the high test current of 1C and 2C, its capacity can still reach 763.5 mAh.g-1 and 625.8 mAh.g-1.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM912

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