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Alternative TitleStudy on Preparation and Performance of V-doped Li1.02Fe0.99-xVxPO4/C Composite as Positive-Electrod
Thesis Advisor谭强强  ;  徐宇兴
Degree Grantor中国科学院研究生院
Degree Discipline化学工艺
Keyword锂离子电池   磷酸钒铁锂   掺杂改性   水热法   电化学性能
Abstract锂离子电池与镍氢、镍镉电池相比具有工作电压高、比能量高、自放电率低、循环寿命长、无污染、安全性能好等独特的优势,具有广阔的应用前景。锂电正极材料是决定锂离子电池性能的关键因素之一,对正极材料的研究尤为重要。其中磷酸铁锂(LiFePO4)具有成本低、无污染、来源广泛等优点,尤其是在安全性和稳定性这两方面的突出优势,使其在电动汽车和电动自行车领域有很好的应用前景。但是同时存在电导率低和能量密度较低的缺点,严重限制了LiFePO4的应用。本课题针对以上问题,采用钒掺杂以及碳包覆的手段,以期改善LiFePO4正极材料的容量与电导率特性。主要研究结果如下: 采用固相煅烧法制备了钒掺杂Li1.02Fe0.99-xVxPO4/C复合材料,考察了锂铁比与钒掺杂对材料的电化学性能的影响。适量提高锂铁比有利于提高材料的电化学性能,钒掺杂能明显地改善材料的循环性能;锂与铁、钒的摩尔比为1.02:0.96:0.03的Li1.02Fe0.96V0.03PO4/C性能较好,室温下电导率为8.24×10-5 S/cm,比未掺杂的样品提高了63.4%;在0.1 C、0.2 C、0.5 C、2 C时的放电比容量分别为163.5 mAh/g、156.6 mAh/g、139.7 mAh/g和116.3 mAh/g,然后在0.2 C充放电400次后,容量保持为初始值的95.4%。 利用水热反应结合热处理制备了由纳米颗粒自组装的LiFePO4/C介孔微米球。该方法以廉价的三价铁作为反应物,成本低廉,工艺简单。以葡萄糖做碳源对LiFePO4进行包覆,当包覆碳的质量为LiFePO4的10%时所制备的材料(LFP/10C)具有较好的电化学性能。LFP/10C的比表面积高达39.1 m2/g,电导率为7.38×10-5 S/cm,在0.1 C时,放电比容量高达165.3 mAh/g,接近理论值170 mAh/g ;0.5 C,放电比容量为142 mAh/g;在8 C的高倍率下循环320次后保持初始值的92%,表现出优异的倍率性能。
Other AbstractCompared with the Ni-MH battery and Ni-Cd battery, the lithium-ion (Li-ion) battery is a large-scale-commercialized rechargeable battery system with high energy density, high specific energy, long cycle life, wide operational temperatures and reasonable rate performance. Because the performance of Li rechargeable batteries is critically dependent ong the positive-electrode performance, much research has focused on optimizing positive-electrode material. Among the cathodes, the lithium iron phosphate (LiFePO4, LFP) has the merits of low cost, low toxicity, flat charge-discharge potential, excellent cycle life and high structural stability, and LFP has revealed to be very promising material especically because of its high security and good stability. However, the low intrinsic electronic conductivity (<10-9 S/cm) significantly restricts the performance. In order to improve the electrochemical performance of LiFePO4, V-doped and carbon coated LiFePO4 particles were synthesized. V-doped Li1.02Fe0.99-xVxPO4/C composites were synthesized via high temperature solid state method and among the materials Li1.02Fe0.96V0.03PO4/C showed better electrochemical performance. V-doping didn’t alter the olivine structure of LiFePO4 but enhanced the electrical conductivity of the composite, and thus greatly improved the electrochemical performances. The electronic conductivity of Li1.02Fe0.96V0.03PO4/C can reach a maximum value of 8.24×10-5 S/cm, which is about 63.4% more than the non-doped composite. The initial discharge capacity was 163.5 mAh/g, 156.6 mAh/g, 139.7 mAh/g and 116.3 mAh/g at 0.1 C, 0.2 C, 0.5 C and 2 C rates, respectively. And the capacity can remain 95.4% of initial value after 400 cycles at 0.2 C rate. Mesoporous LiFePO4/C microspheres consisting of LiFePO4 nanoparticles were successfully fabricated by an eco-friendly hydrothermal approach combined with high-temperature calcinations, using cost effective LiOH and Fe3+ salts as raw materials. In this strategy, mesoporous LFP/10C microspheres uniformly coated with carbon (10 weight %) showed the best performance. The electronic conductivity of LFP/10C can reach 7.38×10-5 S/cm. The initial discharge capacity of LFP/10C microspheres as positive-electrode materials for lithium-ion battery could reach 165.3 mAh/g at 0.1 C rate, very close to the theoretical capacity of LiFePO4 due to the large BET surface area, which provides for a huge electrochemically available surface for the active material and electrolyte. The material also shows high rate capability (~142 mAh/g at 0.5 C) and good cycling stability (a capacity retention of 92.2% after 320 cycles at 8 C rate).
Document Type学位论文
Recommended Citation
GB/T 7714
吕诚. 碳包覆钒掺杂磷酸铁锂正极材料的制备与电化学性能研究[D]. 中国科学院研究生院,2014.
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