碳热还原法合成磷酸亚铁锂及其电极过程研究

【作者】 龚本利；

【导师】 刘素琴；

【作者基本信息】 中南大学 ， 应用化学， 2007， 硕士

【摘要】 以自制的FePO₄、氢氧化锂和葡萄糖为原料，采用碳热还原法合成LiFePO₄／C复合正极材料。初步研究了碳热还原反应的机理，考察了合成工艺条件对材料结构及电化学性能的影响，并研究了其电极过程动力学。主要内容如下：采用热重—差热（TG-DTA）、元素分析（EA）、红外光谱（FTIR）和X射线衍射（XRD）等技术研究了碳热还原反应的机理，分析了焙烧温度对产物组成的影响。研究发现，250℃前生成还原所需碳，300℃左右反应物不历经中间相而一步生成LiFePO₄。300～500℃下样品中都存在着杂相Li₃PO₄和三价铁，600℃下得到不含杂质的LiFePO₄。系统研究了碳热还原法制备LiFePO₄／C时，焙烧温度、焙烧时间、碳源及产物中碳含量等因素对材料结构、形貌、导电率、振实密度及电化学性能的影响。结果表明，最佳的工艺条件为：以葡萄糖为碳源，原料混合物先以250r·min^-1球磨2h，然后在600℃下焙烧24h，产物中碳含量为8.31％。样品以0.1C电流充放电，首次充放电比容量分别为152.5、151.8mAh·g^-1，充放电效率接近100％；在连续以0.1C、0.5C、1C分别循环30、40、50次中，首次放电比容量分别为151.8、135.5、125.7mAh·g^-1，平均每个循环容量衰减分别为0.17‰、0.32‰、0.38‰。采用循环伏安（CV）、电位阶跃计时电流法（PSCA）、电化学阻抗谱（EIS）等技术研究了LiFePO₄／C的电极过程动力学。循环伏安研究表明，电极具有较好的脱、嵌锂可逆性，氧化、还原过程中Li⁺的固相平均扩散系数（D₀）分别为4.753×10¹²、5.889×10^-12cm²·S^-1。由PSCA计算出Li⁺的固相扩散系数值在2.65×10^-13～9.84×10^-11cm²·S^-1之间，并且在电位平台附近（3.45V）下有一个最小值2.65×10^-13cm²·S^-1。电化学阻抗技术研究表明，LiFePO₄／C电极的表面膜电阻R_f随着充电先略有增加，在3.4V下有最大值，然后又下降至稳定值；正极表面膜电容值C_f随着上升先减小后增大，最小值在3.4V下出现。R_Li、C_Li随电位变化也有相似的变化规律。LiFePO₄／C的电荷转移电阻R_ct在3.4V处有一个最小值。更多还原

【Abstract】 In this paper, LiFePO₄/C composite cathode materials synthesized bycarbothermal reduction method using as-prepared FePO₄、LiOH-H₂O andglucose as raw materials were reported. The reaction mechanism andconditions were also investigated. Besides, the electrode process kineticsof LiFePO₄/C was studied. The main points can be summarized asfollows.The mechanism of carbothermal reduction reaction was investigatedby Thermogravimetry-Differental Thermal Analysis （TG-DTA）, ElementalAnalysis （EA）、Fourier Transfer Infrared Spectroscopy （FTIR） and X-rayDiffraction （XRD）, and the effect of calcination temperature on thecompostion of the products was also analysized. The results show theneeded reduction carbon generates at 250℃, and LiFePO4 is obtained at300℃without any intermediate phase. At 300、400、500℃, Li₃PO₄ andFe³⁺ as impurity phases exist in the samples and the higher temperatureleads to less impurity phases. The sample calcined at 600℃has noimpurity.The effects of calcination temperature、calcination time、carbonsource and carbon content on structure、morphology、conductivity、tapdensity and electrochemical performance were systematically investigated.The optimal conditions of preparing LiFePO₄ are as follows: the rawmaterials containing as-prepared FePO₄, glucose, LiOH-H₂O are firstlyball-milled 2h at a rate of 250 r·min^-1, and then calcined 24h at 600℃, andthe carbon content is 8.31%. The first charge and discharge specialcapacities are 152.5、151.8 mAh·g^-1 at 0.1C, whereas the discharge specialcapacities at 0.5C、1C are 135.5、125.7 mAh·g^-1, respectively. The capacityfade per cycle is 0.17‰（at 0.1C and in 30 cycles）、0.32‰（at 0.5C and in40 cycles）、0.38‰（at 1C and in 50 cycles）.The electrode process kinetics of LiFePO₄/C was studied by CyclicVoltammetry （CV）、Potential Step Chronoamperometry （PSCA） andElectrochemical Impedance Spectroscopy （EIS）. CV analysis indicatesthat LiFePO₄/C is quasi reversible and Li-ion diffusion coefficients in oxidation、reduction process are 4.753×10^-12、5.889×10^-12cm²·s^-1,respectively. The Li-ion diffusion coefficients calculated by PSCA rangefrom 2.65×10^-13 to 9.84×10^-11cm²·s^-1, which has a minimum(2.65×10^-13cm²·s^-1) next to the potential plateau （3.45V）. The EIS revealsthat the film resistance of the LiFePO₄/C electrode （R_f） increases withcharge, and reaches a maximum at 3.4V, and then drops. The filmcapacitance （C_f） of LiFePO₄/C electrode decreases firstly and thenincreases. R_Li、C_Li have similar changes with potentials. The chargetransfer resistance (R_ct) has a minimum at 3.4V.更多还原