【作者】 李靖；

【导师】 郑洪河；

【作者基本信息】 苏州大学 ， 凝聚态物理， 2013， 硕士

【摘要】 橄榄石型LiFePO₄具有较高比容量、优异的循环性能、价格低廉、无污染、环境友好等诸多的优点，使其成为最具潜力的商业化锂离子电池正极材料，特别是将其应用到电动汽车方面。本文以材料的实际应用为出发点，以水热法为基础，通过碳包覆来改善材料较低的电子电导率，成功合成了电化学性能优异的石墨烯与LiFePO₄的复合材料，此外还研究有机溶剂和离子液体对于水热过程的影响。本文以LiOH、FeSO₄、H3PO₄为原料，成功合成了LiFePO₄正极材料，通过XRD、SEM表征发现材料具有较高的结晶度，其粒径分部均一（200⁵00nm），进一步对水热法制备工艺优化后，材料表现出优异的电化学性能。其最佳的合成条件为：反应溶液pH=8，温度T=200℃，时间t=600min，可逆比容量依次为146mAh g^-1（0.1C）、137mAh g^-1（1C）、131mAh g^-1（2C）、128mAh g^-1（5C）。以水热法制备的LiFePO₄为原材料，使用三种不同的含碳前躯体作为碳源研究了碳包覆过程对于LiFePO₄/C的电化学性能影响，结果表明：以蔗糖作为碳源在750℃下煅烧5h得到的LiFePO₄/C复合材料显示出非常均一的碳包覆层，其复合材料表现出优异的电化学性能。在碳包覆过程中，通过引入微量的Fe²⁺成功合成了原位催化生长的LiFePO₄与石墨烯的复合材料，通过SEM和HRTEM测试发现其表面包覆层是一层石墨化碳层厚度约为2.5nm，而且生长出来的石墨烯将颗粒与颗粒相互连接，构成了三维的导电网络，进一步通过CV、Raman、EIS、恒流充放电等电化学测试都证实了LiFePO₄与石墨烯的复合材料具有非常好的电子导电率和锂离子迁移率，其复合材料表现出非常优异的倍率性能和长期循环性能。在水热法制备LiFePO₄过程中添加少量离子液体（HMIMBr）有利于材料的成核过程，使纳米粒子在溶液中具有很好的稳定性。XRD的分析结果显示，其暴露出的晶面是沿着锂离子的传输方向，有利于锂离子的嵌入和脱出；而使用有机溶剂EC和PC辅助水热法合成LiFePO₄正极材料，虽然其长期循环性能有所提高，但是其大倍率放电性能略有下降。更多还原

【Abstract】 Due to the various advantages of high specific capacity, excellent cycle performance,low cost, non-toxicity, environment friendly and so on, LiFePO₄is considered as one of themost potential lithium-ion battery cathode materials for commercial use, especially theapplication to electric vehicles. For the purpose of application, based on the hydrothermalmethod, we improved the low electronic conductivity by carbon coating, and obtained theLiFePO₄/graphene composite of excellent electrochemical performance successfully. Inaddition, we also tried to improve the electrochemical performance of LiFePO₄by addingdifferent organic solvents and ionic liquids during the hydrothermal process.Oliver-structured LiFePO₄cathode material was prepared by hydrothermal methodwith the starting materials LiOH, FeSO₄and H3PO₄. As is shown by XRD and SEM, themorphology showed a uniform partial size around200⁵00nm with high crystalline,resulting in excellent electrochemical performance. The optimized experimental conditionswere pH=8, T=200℃, t=600min.We studied the effect of carbon coating process using three carbon precursors ascarbon sources on the electrochemical performance LiFePO₄/C composites. The resultsrevealed that LiFePO₄/C composite showed a uniform carbon layer when sucrose was usedas carbon source with sintering temperature at750℃for5h. The LiFePO₄/C compositesshowed an excellent electrochemical performance.In-situ catalytic LiFePO₄/graphene composite was successfully synthesized by addinga few Fe²⁺into the carbon coating process. The SEM and HRTEM graphs showed that thesurface coating layer was a graphitic carbon layer around2.5nm and the graphene growingfrom the carbon layer connected the particles with each other, constructing a three- dimensional conductive network. CV, Raman, EIS and galvanostatic tests and othermeasuring methods have further proved that the LiFePO₄/graphene composites owedexcellent electronic conductivity and ionic conductivity, therefore the LiFePO₄/graphenecomposite displayed a prominent rate performance and excellent cycle life.A small addition of ionic liquid HMIMBr to the hydrothermal synthesis process ofLiFePO₄is beneficial to the stability of nano-particles in the solution. The XRD resultsillustrated that the exposed crystal face is ac plane, indicating that the thinnest part of theparticle is along the transportation path of lithium ions, stimulating the easy intercalationand deintercalation of lithium ions. LiFePO₄synthetized with the assistance of organicsolvents EC and PC showed improvement on the cycle performance to some extent whilethe rate discharge capability was slightly weakened.更多还原