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磷酸盐系锂离子电池正极材料的制备与低温性能研究
刘志强
学位类型硕士
导师康雪雅
2012-05
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业材料物理与化学
关键词锂离子电池 正极材料 低温性能 复合材料 磷酸盐系材料
摘要随着锂离子电池应用的日趋广泛,限制其性能的关键因素——正极材料的研究成为关注的焦点。磷酸盐系正极材料以其结构稳定、安全性好、资源丰富等优点备受关注。作为其典范,LiFePO4和Li3V2(PO4)3固存的电子电导率低、离子扩散速率低,大电流充放电性能差的缺点,目前已能通过表面碳包覆、元素掺杂、控制颗粒大小等途径解决;而与环境适应能力直接相关的低温电化学性能却成为人们关注的新的焦点,有待进一步研究。本文通过优化合成工艺,采用草酸螯合辅助碳热还原法制备出了室温性能优异的LiFePO4/C、Li3V2(PO4)3/C及xLiFePO4•yLi3V2(PO4)3/C系列材料,通过恒流充放电测试、EIS、CV等方法对其在不同温度下的电化学性能进行了系统研究。同时,利用SEM、XRD等测试手段对样品材料的晶体结构和微观形貌进行了表征。 草酸螯合辅助碳热还原法是在碳热还原法基础上,通过在固相研磨体系中加入草酸作为螯合剂,以促进原料颗粒的分散,使其混合的更为均匀。在高温烧结过程,有机碳源分解为无定形碳,包覆在晶体颗粒的表面抑制了颗粒的长大,同时也提高了材料的导电性,该法制备出的样品材料具有良好的室温性能。其中,2 C倍率下LiFePO4/C、Li3V2(PO4)3/C及40LiFePO4•Li3V2(PO4)3/C的放电比容量分别稳定在130.5 mAh g-1、120.3 mAh g-1和140.1 mAh g-1,50次充放电循环后容量基本没有衰减。 在对室温性能研究的基础上,本文又重点研究了LiFePO4/C、Li3V2(PO4)3/C及不同比例的xLiFePO4•yLi3V2(PO4)3/C复合材料在不同温度下的电化学性能。结果表明:样品材料的电化学性能和温度之间呈非线性关系,温度越低电化学性能越差;受晶体结构的影响,Li3V2(PO4)3/C低温性能优异,而LiFePO4/C较差;xLiFePO4•yLi3V2(PO4)3/C复合材料受两相复合效应和元素掺杂效应的影响,可以显著提高LiFePO4/C的低温性能;对于xLiFePO4•yLi3V2(PO4)3/C材料,在Li3V2(PO4)3相含量较低时元素掺杂效应起主导作用,而在Li3V2(PO4)3相含量较高时两相复合效应起主导作用。 利用不同温度下的电化学阻抗谱(EIS)测试及等效电路拟合结果分析了限制Li3V2(PO4)3/C低温电化学性能的关键因素,计算了不同温度下材料内部Li+的扩散系数DLi,电解液的离子电导率σLi。研究结果表明:在电极动力学参数中Rct、DLi、σLi随温度的迅速变化是引起材料低温性能下降的关键因素,也是导致材料电化学性能和温度之间非线性关系的主要原因。
其他摘要As cathode materials for lithium batteries, Li transition metal phosphates have been investigated intensively among researchers because of their outstanding advantages. So far, much attention has been given to improve their basic electrochemical behavior at ambient and high temperatures. However, the problem of low temperature performance has attracted more and more attention in many high technology applications, such as military and aerospace missions. Herein, a valuable and systematic study to investigate the synthesis and low-temperature electrochemical performances of Li transition metal phosphates was conducted. We attempted to improve low-temperature performances by optimizing the process of synthesis and preparing two-phase composite materials. The samples were synthesized by chelation assisted carbothermic method to evaluate the electrochemical performances at different temperatures (-40 to 26 °C). The microstructures of the samples also were characterized using various microscopic techniques and correlated with their different temperature electrochemical performances. On the basis of the synthesis optimization, a systematic research has been done about the basic electrochemical behavior at ambient temperature. LiFePO4/C, Li3V2(PO4)3/C and xLiFePO4•yLi3V2(PO4)3/C composites were prepared by chelation assisted carbothermic method using C2H2O4 as the chelating reagent to promote the dispersion of the particles of raw materials. The samples showed good performances at room temperature, and the stable specific discharge capacities at 2 C rate of LiFePO4/C, Li3V2(PO4)3/C and 40LiFePO4•Li3V2(PO4)3/C is 130.5 mAh g-1, 120.3 mAh g-1, 140.1 mAh g-1, the performance without decay after 50 cycles. The following conclusions can be drawn through the systematic research about low-temperature electrochemical performances of Li transition metal phosphates: (a) the electrochemical performances of the samples are closely related with the temperature, the lower the operation temperature is, the poorer the performances are. There is a nonlinear relationship between the performance and temperature, which should be attributed to the nonlinear change of Rct, DLi and σLi with temperature. (b) LiFePO4/C have a poor low temperature performance, but the low temperature performances of LiFePO4 could be enhanced effectively by doping with Lithium vanadium phosphate phase. (c) Li3V2(PO4)3/C have a good low temperature performance, which should be attributed to the structure of Li3V2(PO4)3 consists of a 3D framework. The nonlinear relationship between the performance and temperature of Li3V2(PO4)3/C is more obvious, which is manifested through the critical temperature range in the capacity-temperature curve at the same rate.
文献类型学位论文
条目标识符http://ir.xjipc.cas.cn/handle/365002/4379
专题材料物理与化学研究室
作者单位中国科学院新疆理化技术研究所
推荐引用方式
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刘志强. 磷酸盐系锂离子电池正极材料的制备与低温性能研究[D]. 北京. 中国科学院研究生院,2012.
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