【作者】 王国宝；

【导师】 王先友；

【作者基本信息】 湘潭大学 ， 应用化学， 2010， 硕士

【摘要】 LiFePO₄材料具有原料来源丰富、价格便宜、环境友好、理论容量高、循环性能和热稳定性能好等优点,被认为是最有发展前途的锂离子电池正极材料之一。但是,LiFePO₄正极材料的低电子电导率和锂离子扩散速率,导致其高倍率性能不佳,影响该材料在锂离子电池中的应用。本论文以提高LiFePO₄的电子电导率和锂离子扩散速率从而提高材料的电化学性能,以及降低生产成本为主要目的,通过便于工业化生产的高温固相法制备LiFePO₄/C复合材料及其离子掺杂复合材料,利用X-射线衍射（XRD）、扫描电镜（SEM）等物理测试方法及电化学测试技术,系统地研究焙烧温度、反应时间、铁源、碳源、Mn2+掺杂量等条件对复合材料的物理性能和电化学性能的影响,并优化合成工艺条件,为LiFePO₄/C复合材料工业化提供参考。LiFePO₄的成本和性能是决定该材料在锂离子电池中应用的关键。本论文以廉价的FeSO₄·7H₂O为铁源,采用固相法合成了锂离子电池正极材料—橄榄石型LiFePO₄/C复合材料。利用XRD和SEM及电化学测试技术研究合成条件对LiFePO₄/C复合材料的结构、形貌、电化学性能的影响。结果表明,焙烧温度、反应时间和碳源均对材料的性能有较大的影响。其中,用蔗糖作碳源,在700℃煅烧15 h制得的样品具有均一的橄榄石型结构和优良的电化学性能。研究了以FeC₂O₄·2H₂O为铁源,固相法合成条件对LiFePO₄/C复合材料的物理性能和电化学性能的影响,并优化合成条件。并且考察了廉价的FeSO₄·7H₂O和昂贵的FeC₂O₄·2H2_O两种铁源在各自优化条件下合成的LiFePO₄/C复合材料的结构、形貌和电化学性能的差异。结果表明,以硫酸亚铁为铁源合成的LiFePO₄/C在2.3⁴.2 V电压范围内,0.1C、0.5C和1C倍率的放电比容量分别稳定在150 mAh/g、140 mAh/g和130 mAh/g左右,容量与草酸亚铁为铁源合成的LiFePO₄/C的容量相当。但前者在5C倍率放电时,30次循环后,容量仍高达105.2 mAh/g,而后者30次循环后,容量仅为95.7 mAh/g,并且前者的循环性能和大倍率放电性能均优于后者。利用高温固相法合成了一系列Mn²+掺杂化合物LiMnyFe₁-yPO₄ （y=0.2, 0.4, 0.6, 0.8）,考察了Mn²+掺杂量对材料性能的影响。结果表明,样品中锰的含量对样品的形貌影响较小,所有的LiMnyFe1-yPO4材料在放电过程中均出现3.5 V和4.0 V两个电压平台,Mn2+的掺杂减小了氧化还原峰电位差,改善了材料的电化学性能,其中LiMn₀.2Fe₀.8PO₄样品具有最佳的电化学性能。同时,在选取最佳掺杂量后,我们通过Mn2+掺杂和碳包覆相结合的方法,制备出了LiMn0.2Fe0.8PO4/C复合材料,并探讨了复合材料的性能。研究表明,LiMn₀.2Fe₀.8PO₄/C样品具有较高的放电比容量,较佳的倍率性能及优良的循环性能。更多还原

【Abstract】 LiFePO4 is considered as one of the most promising cathode materials for lithium-ion batteries, due to its advantages such as abundant raw materials, low cost, environmental friendliness, high theoretical capacity, better cycle performance and excellent thermal stability. However, its low electron conductivity and slow Li-ion diffusion result in poorer high rate capability and limits its application in lithium batteries. Therefore, improving the electron conductivity and Li-ion diffusion rate, and reducing the cost of LiFePO4 are the aim of our study. In this dissertation, LiFePO4/C and doped-LiFePO4/C were prepared by solid state reaction convenient to large-scale production and investigated by X-ray diffraction （XRD）, scanning electron microscope （SEM） and electrochemical tests, and the effects of clacination temperature, reaction time, iron source, carbon source, and the quantity of dope Mn（II） on the physical and electrochemical performance of LiFePO4/C or doped-LiFePO4/C composite cathode material for lithium-ion batteries were studied. The preparation of LiFePO4/C and doped-LiFePO4/C was optimized and exemplified for large-scale production. The cost and performance of lithium iron phosphate are the key of application as cathode material for lithium-ion batteries. In this paper, LiFePO4/C, a composite cathode material for lithium-ion battery, was prepared by a solid-state synthesis method using low-cost FeSO4·7H2O as iron source. The structure, morphology and electrochemical performance of the prepared LiFePO4/C were investigated by XRD, SEM and electrochemical tests. The results showed that the influences of calcination temperature, reaction time and carbon source on the performance of the LiFePO4/C composite cathode material are prominent. Sucrose as carbon precursor, the LiFePO4/C synthesized by calcination of raw materials at 700℃for 15 h had olivine structure and best electrochemical performance. Expensive FeC2O4?2H2O as iron source, the effects of the preparation conditions on the physical and electrochemical performance of the LiFePO4/C were studied, and the optimum conditions was obtained. The differences of the LiFePO4/C synthesized by FeSO4?7H2O and LiFePO4/C by FeC2O4?2H2O in structure, morphology and electrochemical performance were evaluated. The results showed that the capacities in the voltage range from 2.3 to 4.2V of LiFePO4/C synthesized by FeSO4?7H2O were 150, 140, 130 mAh/g at the current rates of 0.1C, 0.5C and 1C, respectively, which closed to the capacities of LiFePO₄/C synthesized by FeC2O₄·2H2O at the corresponding current rates. But the cycle performance and the capacity of the former at 5C were better than that of the latter, the capacity of the former still remained 105.2 mAh/g after 30 cycles at 5C, while the capacity of the latter was just only 95.7 mAh/g.A series of Mn-doped lithium iron phosphate LiFe₁-xMn_xPO₄ （x=0.2, 0.4, 0.6, 0.8） were prepared by high temperature solid state reaction and the effects of the quantity of doped Mn on the performance of the LiFe₁-xMn_xPO₄ were studied. The result indicated that the Mn content of the sample had less impact on morphology of the samples, and all LiFe₁-xMn_xPO₄ samples exhibited two discharge voltage plateaus 3.5 and 4.0V, and the voltage difference of the redox peaks of LiFe₁-xMn_xPO₄ is smaller than that of LiFePO₄, indicating the better electrochemical performance, and LiMn0.2Fe0.8PO₄ exhibited the best electrochemical performance among all the LiFe₁-xMn_xPO₄ samples. Additionally, LiMn0.2Fe0.8PO₄/C was synthesized by solid state reaction in optimum conditions and its electrochemical performance was studied. The results demonstrated that LiMn0.2Fe0.8PO₄/C composite exhibited higher discharge capacity, better rate capability and longer cycle life.更多还原