【作者】 高飞；

【导师】 唐致远；

【作者基本信息】 天津大学 ， 电化学， 2008， 博士

【摘要】 随着各种便携式电子产品日益普及,电池作为一种携带方便的电源设备日益受到关注。LiFePO₄作为新一代锂离子电池正极材料,其理论比容量为170mAh·g^-1,电压平台为3.4V（相对于Li/Li⁺）,具有价格低廉,环保,热稳定好,安全性高,循环性能优越等优点,被认为是锂离子电池理想的正极材料。本文以LiFePO₄为研究对象,以提高其电化学性能为主要目的,采用固相法分别合成LiFePO₄,LiFePO₄/C,Li_1-x+δTi_xFe_1-yMn_yPO₄/C等材料。主要利用XRD,SEM,CV,EIS,电池充放电等测试手段研究了合成工艺条件,碳包覆,体相金属离子掺杂,粒径控制等对LiFePO₄材料性能的影响。在此基础上对LiFePO₄/C材料的扩散系数和电荷转移电阻等动力学过程参数进行了研究。研究表明,合成温度对产物的形貌、结晶度均有影响,最佳合成温度为650℃。通过不同掺碳量对LiFePO₄/C材料的放电性能及振实密度影响的研究,最优化的碳含量应控制在3-5 wt.%。利用改进的三段高温煅烧固相法合成LiFePO₄/C材料,相对于两步高温煅烧固相法合成的相同含碳量的材料（共加入6wt.%葡萄糖）,三步煅烧法合成材料的振实密度较高,达到了1.35 g·cm^-3;0.2C首次放电比容量达到135.8 mAh·g^-1,0.2C循环20次容量保持99.4%;5C放电比容量达到129.8mAh·g-1,5C循环20次,容量仍保持99.4%,即每个循环损失容量约0.03%。LiFePO₄/C材料进行体相掺杂并制备相应的Li_1-x+δTi_xFe_1-yMn_yPO₄/C复合材料。XRD分析结果显示,掺杂后的样品属于单一的橄榄石型晶体结构,没有明显的杂质峰,少量的Ti和Mn离子进入晶格,晶型完整、单一,掺杂不会改变LiFePO₄的晶型结构。Ti、Mn的掺杂达到比较好的效果。有效控制LiFePO₄的粒子尺寸是改善LiFePO₄中Li⁺的扩散能力的关键。利用机械球磨12h和气流粉碎方式均能得到粒径均匀的LiFePO₄材料,并具有良好的倍率放电性能和循环性能。更多还原

【Abstract】 Along with the increasing popularization of the portable electronic products, battery as a portable power source receives high attention in the world. LiFePO₄ is emerging as a promising cathode material for lithium-ion batteries because of low cost and environmental compatibility. In addition, LiFePO₄ has a large theoretical capacity of 170mAh·g⁽-1_, a flat discharge potential of 3.4V versus Li/Li+, the good cycle stability, and the excellent thermal stability.In this paper, LiFePO₄, LiFePO₄/C and Li_1-x+δTi_xFe_1-yMn_yPO₄/C cathode materials were synthesized by solid-state reaction. The effects of thermal processing conditions, carbon coating, ion-doping and particle size on the phase-purity, particle size, morphology and electrochemical performance of the materials are investigated by X-ray, SEM, CV, EIS. The dependence of kinetics and lithium ion diffusion coefficient of LiFePO₄/C electrode is also evaluated by means of the EIS.The results indicate that the synthesis temperature and carbon-coating affect the morphology, the crystallinity, tap density and discharge performance of the products, and the best synthesis temperature is 650℃. The most optimized carbon-coating content should be controlled in the 3-5 wt%.The tap density of LiFePO₄/C samples synthesized by three-step solid-state method is higher than that by two-step solid-state method, reaching to 1.35 g·cm^-3. The discharge capacity of LiFePO₄/C is 135.8mAh·g^-1 at the C/5 rate, and the capacity remained 99.4% after 20 cycles. Even at the 5C rate, a discharge capacity of 129.8mAh·g?1 is obtained with only a capacity fading of 0.03% per cycle.In order to improve the performance of LiFePO₄ cathode materials, composites Li_1-x+δTi_xFe_1-yMn_yPO₄/C with Ti⁴⁺ and Mn4+ dopant were synthesized. The XRD analysis shows that the samples by doping Ti⁴⁺ and Mn4+ are pure well-ordered olivine phase with no impurities. And the Ti⁴⁺ and Mn4+ ion dopant considerably improves the electrochemical performances of Li_1-x+δTi_xFe_1-yMn_yPO₄/C.To control the particle size of the LiFePO₄/C samples is the key to improve the Li+ diffusion. The samples LiFePO₄ dealt with ball-milling for 12h or airflow milling display uniform particle size and exhibit excellent cycle life.更多还原