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Thesis Advisor谭强强 ; 徐宇兴
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline材料工程
Keyword锂离子电池 磷酸锰铁锂 包覆改性 固相二次烧成法 电化学性能

锂离子电池正极材料磷酸铁锂(LiFePO4)和磷酸锰锂(LiMnPO4)具有安全性高、稳定性强、原料来源广泛和环境友好等优点,在动力和储能领域有着广阔的应用前景。然而,由于LiFePO4的工作电压较低、LiMnPO4导电性差等问题,导致其比能量较低,阻碍了其在新能源汽车、电子通讯和储能等领域的广泛应用和推广。本课题针对以上问题,通过改进烧成工艺、元素掺杂和表面包覆等途径,以期提高LiMn1-xFexPO4导电性能及电化学性能。本研究采用固相烧成法制备了LiMn1-xFexPO4/C复合电极材料,并采用DTG-TG、XRD、SEM、XPS、电池测试系统和电化学工作站等系统考察了烧成温度、烧成时间和锰铁比对电极材料的晶体结构、微观形貌和电化学性能的影响规律。结果表明:在750 ℃和氩气气氛条件下,烧成时间10 h,且锰铁比为7:3、1:1和3:7时,所得LiMn1-xFexPO4/C复合电极材料性能较优,物相结构为纯橄榄石型结构,二次颗粒粒径为1-10 μm,0.1 C倍率下放电,比容量分别达到98.0、103.8和116.5 mAh/g。其中LiMn0.5Fe0.5PO4/C复合电极材料的二次颗粒平均粒径为2.49 μm;室温导电率可达8.32×10-6 S/cm,锂离子传导率达到1.63×10-12 cm2/s;该电极材料在0.05 C倍率下放电,其比容量达到115.9 mAh/g,放电比能量为423.4 Wh/kg,平均工作电压可达3.65 V,Fe的引入可以明显提高材料的电化学性能。采用固相二次烧成法制备了核壳结构LiMn0.7Fe0.3PO4/LiFePO4/C复合电极材料,系统考察了烧成温度、烧成时间和LiFePO4包覆对其形貌、结构和电化学性能的影响。结果表明:采用LiFePO4包覆可以明显改善复合电极材料的电化学性能,包覆前后锂离子传导率由9.31×10-13 cm2/s增加到1.71×10-12 cm2/s,提高了84%;核壳结构LiMn0.7Fe0.3PO4/LiFePO4/C复合电极材料的电导率较LiMn0.5Fe0.5PO4/C提高了142%,达到2.01×10-5 S/cm;在0.1 C倍率下放电比容量高达136.6 mAh/g,比能量可达506.9 Wh/kg。

Other Abstract

Li-ion batteries positive materials lithium iron phosphate (LiFePO4) and lithium manganese phosphate (LiMnPO4) have lots of advantages, such as abundant resource, high safety, excellent stability and no pollution to the environment,which has been widely used in power batteries and energy storage area. However, the low working voltage of LiFePO4 and low electrical conductivity and lithium ion conductivity of LiMnPO4 affect its widespread commercial application and promotion in new energy vehicles, electronic communications, energy storage, and so on. In order to solve these problems and enhance the electrical conductivity and electrochemical properties of LiMn1-xFexPO4, methods such as improving firing process, element doping and surface coating were under taken.LiMn1-xFexPO4/C composites positive materials were synthesized via solid state firing method. The effects of different heating temperatures, different calcine times and the Mn/Fe ratios on crystal structure, morphology and electrochemical performance were systematically investigated by DTG-TG、XRD、SEM、XPS、battery test system and electrochemical workstation. The results show that the LiMn1-xFexPO4/C composites with the Mn/Fe ratio of 7:3, 1:1 and 3:7, heat-treated at 750 oC under argon atmosphere for 10 h exhibit best electrochemical performance, which have pure olivine structure with particle size from 1 to 10 μm, and speical discharge capacity of 98.0、103.8 and 116.5 mAh/g at a discharge rate of 0.1 C. Among them, the composites of LiMn0.5Fe0.5PO4/C has a average particle size of 2.49 μm; the electrical conductivity and Li-ion conductivity in room temperature are 8.32×10-6 S/cm and 1.63×10-12 cm2/s, respectively; the discharge capacity and energy density are 115.9 mAh/g and 423.4 Wh/kg at a discharge rate of 0.05 C, with an average working voltage of 3.65 V, and the introduce of iron significantly improved the electrochemical performance of materials.LiMn0.7Fe0.3PO4/LiFePO4/C composites positive materials with core-shell structure were successfully fabricated by solid-state double-firing method. The effects of different heat treatment temperatures, different heat treatment times and LiFePO4-coated on crystal structure, morphology and electrochemical performance were systematically investigated. The results demonstrate that it is benefical for the electrochemical performance of composite to be coated by LiFePO4 and the capacity of Mn in the composite can be adequately utilized after coated with LiFePO4, the Li-ion conductivity is increased from 9.31×10-13 cm2/s to 1.71×10-12 cm2/s by 84%, compared with non-coated composite. The as-prepared LiMn0.7Fe0.3PO4/LiFePO4/C electrode also shows better performance than the LiMn0.5Fe0.5PO4/C, the conductivity is increased by 142% and reach 2.01×10-5 S/cm; when cycle at a discharge rate of 0.1 C, the specific discharge capacity and discharge energy density could reach 136.6 mAh/g and 506.9 Wh/kg, respectively. 

Document Type学位论文
Recommended Citation
GB/T 7714
闫博. 磷酸铁锂包覆磷酸锰铁锂正极材料的制备与电化学性能研究[D]. 北京. 中国科学院研究生院,2016.
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