碳纳米管掺杂锂离子电池材料的制备及电化学性能研究
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摘要
在已知的嵌锂型锂离子二次电池正极材料中,橄榄石型LiFePO4因其制备成本低、循环寿命长、安全性能高和对环境友好等优点而被认为是锂离子动力电池的最佳正极材料之一。然而,LiFePO4具有较低的电子导电率和离子传导率,这一缺陷阻碍了LiFePO4的商业化发展。目前,对LiFePO4的锂离子传导机制研究都采用传统的方法,例如循环伏安法和交流阻抗法等,但是这些方法对于嵌入型电极,特别是LiFePO4嵌入型两相转化的电极过程,是否适用,还未有人系统地研究过,本文从LiFePO4的嵌锂机制出发,系统地研究了LiFePO4的锂离子传导机制。另外,当前在改善LiFePO4的电子导电率和离子传导率方面已经取得了较大的进展,但对碳纳米管包覆改性方面还未有全面的研究,本文对碳纳米管包覆改性LiFePO4做了比较全面的研究。主要的研究成果如下。
研究了磷酸铁锂的电极动力学过程,电化学可逆性能是评价电池材料的重要参数。本文采用循环伏安法测定的电势扫描曲线来判断交换电流密度的大小,从而简便地判断电池材料的电化学可逆性能的优劣。
研究了循环伏安法(CV)、恒电位间歇滴定法(PITT)、恒电流间歇滴定法(GITT)和交流阻抗法(EIS)四种常用的锂离子扩散系数的测定方法,结果表明,掺杂碳纳米管有利于提高磷酸铁锂的锂离子扩散系数。通过对不同测定方法的比较分析发现其中的不足之处。循环伏安法不能测定任意充放电状态下锂离子扩散系数,其余三种测定不同嵌锂程度的锂离子扩散系数均出现极值现象,这与实际的磷酸铁锂充放电曲线相矛盾。针对此现象,本文根据磷酸铁锂的锂离子嵌锂机制,即锂离子在正极材料的迁移过程与非平衡载流子迁移过程相似,引入非平衡锂离子概念,提出了测定锂离子扩散系数的修正方法。求解恒流放电时的稳态扩散方程,可得到非平衡锂离子浓度与电流成正比。根据LiFePO4材料中的锂离子浓度与电势差的关系,可以近似得到非平衡锂离子浓度变化与电势差变化的关系式,利用电流阶跃稳态法可以测得浓度变化引起的电势差,然后求得锂离子的电化学扩散系数。采用修正的方法测定的LiFePO4/MWCNTs的锂离子扩散系数与CV、PITT和GITT方法测试值的数量级大致相同,但不会出现极值现象,因此更能体现出锂离子在LiFePO4正极材料的动力学行为。研究得出的结论能为磷酸铁锂的制备、改性和测试提供理论上的指导。
研究了高能球磨法对磷酸铁锂进行碳纳米管包覆改性。掺杂不同管径碳纳米管的LiFePO4电极具有良好的充放电特性和稳定的充放电平台。其中,掺杂碳纳米管管径为60-100nm、长度为L=1-2umm、纯度>95wt%、掺入比例为10%的电极材料在容量和循环性能上表现最优。在常温0.1C充放电,该电极充电比容量达到136mAh/g,放电比容量为129mAh/g,效率为94.8%。经20循环后放电比容量为123.5mAh/g,容量损失4%,循环10次后,容量趋于稳定。
研究了球磨时间优化,掺杂相同的碳纳米管,12h球磨制备的材料充放电比容量最大,分别为132.8mAh/g和126.3mAh/g,首次充放电库伦损失4.9%,结果表明,延长球磨时间能更有效地分散碳纳米管。但是16h球磨的材料电化学性能反而下降,原因高能球磨会破坏碳纳米管,球磨时间过长会破坏碳纳米管越多,在碳纳米管的断裂部位的碳原子会于锂离子发生不可逆反应,导致电化学性能下降。
研究了超声波振荡分散法对磷酸铁锂进行碳纳米管包覆改性。采用的碳纳米管参数为管径φ=60-100nm,长度L=1-2umm,纯度>95wt%,掺入比例为10%。SEM测试显示,超声波功率越大,碳纳米管分散性越好。电化学测试表明,电极材料的可逆性能随超声波振荡功率增大而增加；在室温下(20℃)振荡60min,100%功率制备的材料充放电比容量最大,分别为138.1mAh/g和132.2mAh/g,首次充放电库伦损失4.3%。
研究了超声波振荡环境温度优化。SEM测试显示,环境温度越大,碳纳米管分散性越好。电化学测试表明,在20℃至60℃范围内,电极材料的可逆性能随振荡环境温度增大而增加,充放电比容量增加,60℃至80℃时材料可逆性能不再增加,充放电比容量反而减少,随着温度升高,首次库伦损失有所增大。出现这种现象的原因是因为碳管分散使得活性增加,部分碳原子与锂离子结合较稳定,导致放电时不能脱锂。60℃制备的材料充放电比容量最大,分别为145.4mAh/g和137.9mAh/g,首次充放电库伦损失5.2%。因此,考虑到整体性能和节能效果,超声波制备LiFePO4/MWCNTs的最佳温度是60℃附近。
研究了多壁碳纳米管酸处理,结合超声波振荡分散法对磷酸铁锂进行碳纳米管包覆改性。碳纳米管的规格为直径60-100nm,长度为5-15μ m,纯度为95wt%。采用超声波技术将酸处理后的碳纳米管分散于LiFePO4材料中,碳纳米管的掺入比例为10%。碳纳米管的SEM测试图片显示,随着酸浓度的增加,团聚在一起的碳管逐渐分离,碳纳米管在外观上看起来逐渐细化；经过酸处理,碳纳米管团聚体中的杂质被除掉,从而纯度更高。电化学测试表明,酸浓度越大,LiFePO4/MWCNTs电极材料的可逆性能越好,首次充放电容量也越大。采用最大的硫酸浓度(7.2mol/L)和最大的硝酸浓度(6mol/L)的混酸处理的碳纳米管,掺杂制备的LiFePO4/MWCNTs电极材料性能最优,0.1C首次充放电容量分别为144.4mAh/g和138.3mAh/g,首次库伦损失为4.1%。
研究了酸处理温度的优化,硫酸浓度(7.2mol/L)和硝酸浓度(6mol/L)的混酸,采用回馏装置进行酸处理。碳纳米管的SEM测试图片显示,加热酸处理后的碳纳米管基本上完全纯化并分离。电化学测试表明,40℃酸处理的碳纳米管对LiFePO4的电化学性能提高最大,表现出的可逆性能最好,其O.1C首次充放电容量分别为150.2mAh/g和143.8mAh/g,首次库伦损失为4.3%,去掉碳纳米管非活性物质充放电比容量最高可以达到166.8mAh/g和159.8mAh/g,分别是理论值的98%和94%；50次循环充放电后,其0.5C放电容量仍然能保持在132.8mAh/g;1C充放电容量分别为130.1mAh/g和119.8mAh/g,去掉碳纳米管非活性物质比容量达到144.6mAh/g和133.1mAh/g。过高温度酸处理碳纳米管反而不利于提高LiFePO4的电化学性能,原因是硝酸可以使碳管表面形成羟基、羧基等含氧官能团,在一定的程度上可以改善碳纳米管的分散性能,有利于提高电极的导电性能。但是,含氧官能团会与锂离子发生不可逆反应。温度越高,含氧官能团越多,材料的电化学性能也就下降。
研究了微波固相合成法对磷酸铁锂进行碳纳米管包覆改性。原材料为草酸亚铁(FeC2O4·2H2O)、碳酸锂(Li2CO3)和磷酸二氢氨(NH4H2PO4),三者均为分析纯,碳纳米管的规格为管径φ=60-100nm,长度L=1-2um,纯度>95wt%。采用球磨法碳纳米管分散于原材料中,掺入比例为合成LiFePO4的10%。材料SEM测试显示,随着辐射时间的增加,材料颗粒逐渐增大,碳纳米管也逐渐嵌入进材料颗粒之中。XRD测试结果显示,微波功率100%,辐射时间大于9min时,开始出现Fe2P相。电化学测试结果表明,微波功率100%辐射9min制备的LiFePO4/MWCNTs材料可逆性能最好,首次充放电容量最大,分别为126.0mAh/g和110.4mAh/g,首次库伦损失为12.4%。
研究了微波功率和辐射时间搭配优化,设定功率和时问的规则是功率与时间之积大致相等。SEM测试显示,80%功率辐射12min的材料比100%功率辐射9mmin的材料细微颗粒更丰富,60%功率辐射15min和40%功率辐射22min制备的材料表现出未完全反应的迹象。电化学测试结果表明,80%功率辐射12min的材料可逆性能最好,首次充放电比容量最大,分别为141.9mAh/g和133.9mAh/g,首次库伦损失为5.6%,去掉碳纳米管非活性物质充放电比容量为157.7mAh/g和148.7mAh/g。经过50次循环后,放电容量为116.0mAh/g,容量损失为7.4%。1C充放电容量分别为114.4mAh/g和106.9mAh/g,去掉碳纳米管非活性物质比容量达到133.9mAh/g和130.6mAh/g。因此,微波辐射功率足够大时才能达到前驱体的反应温度,在达到反应温度后,辐射时间越长,前驱体反应越充分。
对比了不同方法制备的LiFePO4/MWCNTs电化学性能,从碳纳米管的分散处理来看,碳纳米管酸处理结合超声波分散制备的LiFePO4/MWCNTs电化学性能最好,其它依次为单独超声波法,球磨法和微波固相合成法。对于微波固相合成法,尚需进一步研究如何减小材料颗粒和保持碳纳米管比例。
Among the well-known Li-inserted cathode materials for lithium ion secondary battery, the olivine-type LiFePO4is considered as one of the most promising cathode material for lithium-ion power battery because of its low cost, good cycling stability, better thermal stability, excellent security and environment-friendly performance. However, LiFePO4has a main drawback of low electronic and ionic conductivity that hinders it to be commercialized. Currently, some traditional methods as CV, PITT, GITT or EIS had been used to studied lithium-ion conduction mechanism in LiFePO4. But, the feasibility of these methods used to study the Li-inserted electrodes, especially the LiFePO4electrode with a process of two-phase transformation, had not been systematically studied. In this article, the lithium-ion conduction mechanism was systematically studied based on the Li-inserted process of LiFePO4. In addition, the electronic and ionic conductivity of LiFePO4had been improved largely. But the performance of LiFePO4coated with carbon nanotube was not studied comprehensively. In this paper, comprehensive research to LiFePO4coated with carbon nanotubes had been done. The main resuits and conclusions were shown as following.
Study on the electrode kinetics of LiFePO4. Potential scanning curves got by cyclic voltammetry could be used to judge the value of the Exchange current density to determine the reversible properties of the material.
Research a variety of traditional techniques used to make a determination of lithium ion diffusion coefficient, and point out the shortcomings. After in-depth study, we found that the lithium ions transferring into cathode material were similar as the nonequilibrium carrier in semiconductor. The inpouring Li-ions can be called nonequilibrium Li-ions. Than the diffusion equation should be revised. The electrochemical diffusion coefficient, named as D, can be worked out approximately in terms of the relationship between the nonequilibrium Li-ions and the relevant potential difference. The diffusion coefficient got by this approach better reflects the kinetics of LiFePO4cathode materials. The conclusion of this study can provide guides in theory for the preparation, modification and testing methods of LiFePO4. With the new method the D was determined as9.21×10-ncm2/s for LiFePO4doped with10%MWCNTs. The order of this value was almost consistent with the one got by CV, PITT and GITT. But it was kept invariable VS. x in LixFePO4.
The LiFePO4/MWCNTs composite was synthesized by ball milling. The XRD spectrum demonstrated that MWCNTs didn't change the olivine structure of LiFePO4and the SEM showed that MWCNTs decentralized into the grains of LiFePO4. The result of electrochemical test indicated that the composite with the MWCNTs in size of60-100nm and length of1-2um exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were136mAh/g and129mAh/g respectively at0.1C ate in room temperature. The first capacity lost was5.2%. The difference between charge-discharge platforms of this composite was the least compared to the others. This result showed that the material had the largest electrochemical diffusion coefficient of lithium. At the same time, the capacity of the composite only lost4.0%after10cycles.
Study on optimization of milling time. The LiFePO4/MWCNTs milled12h with the same carbon nanotubes exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were132.8mAh/g and126.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.9%. This shows that the extension of ball milling time can be more effectively dispersed carbon nanotubes. Due to destruction of carbon nanotubes, the electrochemical properties of16h milling material rather than was decline. The breaking part of the carbon nanotube could be combined with lithium ion. The irreversible combination could come down the electrochemical performance.
Study on supersonic dispersion method for LiFePO4with10%carbon nanotubes in size of60-100nm, length of1-2um, and purity of>95wt%. The SEM test showed that the greater supersonic power the better dispersion effect of carbon nanotubes. The LiFePO4/MWCNTs dispersed60min at100%power in20℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were138.1mAh/g and132.2mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%.
Study on optimization of ultrasonic ambient temperature. The SEM tests showed that the greater the ambient temperature the better dispersion effect of carbon nanotubes. Electrochemical tests showed that the reversible performance of material increaseed within the range of20℃to60℃and kept constant from60℃to80℃. The capacity was diminished when ambient temperature was greater than60℃. This result could be explained by the increased activity of carbon. Some of the active carbon atoms could be combined with lithium ion. The irreversible combination could come down the capacity. The LiFePO4/MWCNTs dispersed60min at100%power in60℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were145.4mAh/g and137.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.2%.
The effect of the LiFePO4mixed with10%acidized MWCNTs by supersonic dispersed method were investigated. The MWCNTs in size of60-100nm, length of5-15μm, purity of>95wt%were treated by different concentration acid mixed with sulfuric acid and nitrate. The SEM tests of carbon nanotubes showed that the swarm of carbon nanotube was separated as the concentrations increasing. The impurities in carbon nanotube were got rid of after acid treatment. The LiFePO4mixed with the MWCNTs acidized by mixture of sulfuric acid(7.2mol/L) and nitrate(6mol/L) exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were144.4mAh/g and138.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.1%.
Study on optimization of acid temperature. The MWCNTs in size of60-100nm, length of5-15℃m, purity of>95wt%were treated by the mixture of sulfuric acid(7.2mol/L) and nitric acid(6mol/L) using reflux device. The SEM test of carbon nanotube showed that the carbon nanotubes had been purified and separated entirely after heated acid-treated. The LiFePO4mixed with the MWCNTs acidized at40℃ambient temperature exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were150.2mAh/g and143.8mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were166.8mAh/g and159.8mAh/g,98%and94%of the theory value respectively. After50cycles, discharge specific capacity still remained in132.8mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were130.1mAh/g and119.8mAh/g respectively. Then a conclusion would be made that the high temperature acid treatment diminished the electrochemical performance of LiFePO4. The reason was that nitric could oxidize carbon nanotube to hydroxyl and carboxyl functional groups in surface. Such groups containing oxygen would improve the dispersion property of carbon nanotubes to a certain extent, and then improve the conductivity of LiFePO4. However, the oxygen-containing functional groups would be combined with lithium ion.
The effect of LiFePO4/MWCNTs synthesized by microwave was investigated. The MWCNTs in size of60-100nm, length of1-2um, were doped by ball milled in raw material mixed with FeC2O4·2H2O, Li2CO3and NH4H2PO4. The ratio of the MWCNTs to the final composite was10%. The SEM test has shown that, with the time of microwave radiation being prolonged, the size of the material particles increased. And the carbon nanotubes were embedded into the material particles. The XRD test results showed that while the time of microwave radiation at100%power was greater than9min, Fe2P began to emerge. The LiFePO4/MWCNTs heated9min at100%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were126.0mAh/g and110.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was12.4%.
Study on optimization of the settings of microwave oven's time and power. The rule of settings kept the product of time and power in equal. The SEM tests show that the material heated12min by80%power and9min by100%power contained finer particles.The LiFeP/VMWCNTs heated12min at80%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were141.9mAh/g and133.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.6%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were157.7mAh/g and148.7mAh/g,92.7%and87.4%of the theory value respectively. After50cycles, discharge specific capacity still remained in116.0mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were114.4mAh/g and106.9mAh/g respectively. Therefore, microwave radiation at80%power was large enough to reach the reaction temperature. When reaction temperature is reached, the longer to be radiated, there was more resultant.
研究了磷酸铁锂的电极动力学过程,电化学可逆性能是评价电池材料的重要参数。本文采用循环伏安法测定的电势扫描曲线来判断交换电流密度的大小,从而简便地判断电池材料的电化学可逆性能的优劣。
研究了循环伏安法(CV)、恒电位间歇滴定法(PITT)、恒电流间歇滴定法(GITT)和交流阻抗法(EIS)四种常用的锂离子扩散系数的测定方法,结果表明,掺杂碳纳米管有利于提高磷酸铁锂的锂离子扩散系数。通过对不同测定方法的比较分析发现其中的不足之处。循环伏安法不能测定任意充放电状态下锂离子扩散系数,其余三种测定不同嵌锂程度的锂离子扩散系数均出现极值现象,这与实际的磷酸铁锂充放电曲线相矛盾。针对此现象,本文根据磷酸铁锂的锂离子嵌锂机制,即锂离子在正极材料的迁移过程与非平衡载流子迁移过程相似,引入非平衡锂离子概念,提出了测定锂离子扩散系数的修正方法。求解恒流放电时的稳态扩散方程,可得到非平衡锂离子浓度与电流成正比。根据LiFePO4材料中的锂离子浓度与电势差的关系,可以近似得到非平衡锂离子浓度变化与电势差变化的关系式,利用电流阶跃稳态法可以测得浓度变化引起的电势差,然后求得锂离子的电化学扩散系数。采用修正的方法测定的LiFePO4/MWCNTs的锂离子扩散系数与CV、PITT和GITT方法测试值的数量级大致相同,但不会出现极值现象,因此更能体现出锂离子在LiFePO4正极材料的动力学行为。研究得出的结论能为磷酸铁锂的制备、改性和测试提供理论上的指导。
研究了高能球磨法对磷酸铁锂进行碳纳米管包覆改性。掺杂不同管径碳纳米管的LiFePO4电极具有良好的充放电特性和稳定的充放电平台。其中,掺杂碳纳米管管径为60-100nm、长度为L=1-2umm、纯度>95wt%、掺入比例为10%的电极材料在容量和循环性能上表现最优。在常温0.1C充放电,该电极充电比容量达到136mAh/g,放电比容量为129mAh/g,效率为94.8%。经20循环后放电比容量为123.5mAh/g,容量损失4%,循环10次后,容量趋于稳定。
研究了球磨时间优化,掺杂相同的碳纳米管,12h球磨制备的材料充放电比容量最大,分别为132.8mAh/g和126.3mAh/g,首次充放电库伦损失4.9%,结果表明,延长球磨时间能更有效地分散碳纳米管。但是16h球磨的材料电化学性能反而下降,原因高能球磨会破坏碳纳米管,球磨时间过长会破坏碳纳米管越多,在碳纳米管的断裂部位的碳原子会于锂离子发生不可逆反应,导致电化学性能下降。
研究了超声波振荡分散法对磷酸铁锂进行碳纳米管包覆改性。采用的碳纳米管参数为管径φ=60-100nm,长度L=1-2umm,纯度>95wt%,掺入比例为10%。SEM测试显示,超声波功率越大,碳纳米管分散性越好。电化学测试表明,电极材料的可逆性能随超声波振荡功率增大而增加；在室温下(20℃)振荡60min,100%功率制备的材料充放电比容量最大,分别为138.1mAh/g和132.2mAh/g,首次充放电库伦损失4.3%。
研究了超声波振荡环境温度优化。SEM测试显示,环境温度越大,碳纳米管分散性越好。电化学测试表明,在20℃至60℃范围内,电极材料的可逆性能随振荡环境温度增大而增加,充放电比容量增加,60℃至80℃时材料可逆性能不再增加,充放电比容量反而减少,随着温度升高,首次库伦损失有所增大。出现这种现象的原因是因为碳管分散使得活性增加,部分碳原子与锂离子结合较稳定,导致放电时不能脱锂。60℃制备的材料充放电比容量最大,分别为145.4mAh/g和137.9mAh/g,首次充放电库伦损失5.2%。因此,考虑到整体性能和节能效果,超声波制备LiFePO4/MWCNTs的最佳温度是60℃附近。
研究了多壁碳纳米管酸处理,结合超声波振荡分散法对磷酸铁锂进行碳纳米管包覆改性。碳纳米管的规格为直径60-100nm,长度为5-15μ m,纯度为95wt%。采用超声波技术将酸处理后的碳纳米管分散于LiFePO4材料中,碳纳米管的掺入比例为10%。碳纳米管的SEM测试图片显示,随着酸浓度的增加,团聚在一起的碳管逐渐分离,碳纳米管在外观上看起来逐渐细化；经过酸处理,碳纳米管团聚体中的杂质被除掉,从而纯度更高。电化学测试表明,酸浓度越大,LiFePO4/MWCNTs电极材料的可逆性能越好,首次充放电容量也越大。采用最大的硫酸浓度(7.2mol/L)和最大的硝酸浓度(6mol/L)的混酸处理的碳纳米管,掺杂制备的LiFePO4/MWCNTs电极材料性能最优,0.1C首次充放电容量分别为144.4mAh/g和138.3mAh/g,首次库伦损失为4.1%。
研究了酸处理温度的优化,硫酸浓度(7.2mol/L)和硝酸浓度(6mol/L)的混酸,采用回馏装置进行酸处理。碳纳米管的SEM测试图片显示,加热酸处理后的碳纳米管基本上完全纯化并分离。电化学测试表明,40℃酸处理的碳纳米管对LiFePO4的电化学性能提高最大,表现出的可逆性能最好,其O.1C首次充放电容量分别为150.2mAh/g和143.8mAh/g,首次库伦损失为4.3%,去掉碳纳米管非活性物质充放电比容量最高可以达到166.8mAh/g和159.8mAh/g,分别是理论值的98%和94%；50次循环充放电后,其0.5C放电容量仍然能保持在132.8mAh/g;1C充放电容量分别为130.1mAh/g和119.8mAh/g,去掉碳纳米管非活性物质比容量达到144.6mAh/g和133.1mAh/g。过高温度酸处理碳纳米管反而不利于提高LiFePO4的电化学性能,原因是硝酸可以使碳管表面形成羟基、羧基等含氧官能团,在一定的程度上可以改善碳纳米管的分散性能,有利于提高电极的导电性能。但是,含氧官能团会与锂离子发生不可逆反应。温度越高,含氧官能团越多,材料的电化学性能也就下降。
研究了微波固相合成法对磷酸铁锂进行碳纳米管包覆改性。原材料为草酸亚铁(FeC2O4·2H2O)、碳酸锂(Li2CO3)和磷酸二氢氨(NH4H2PO4),三者均为分析纯,碳纳米管的规格为管径φ=60-100nm,长度L=1-2um,纯度>95wt%。采用球磨法碳纳米管分散于原材料中,掺入比例为合成LiFePO4的10%。材料SEM测试显示,随着辐射时间的增加,材料颗粒逐渐增大,碳纳米管也逐渐嵌入进材料颗粒之中。XRD测试结果显示,微波功率100%,辐射时间大于9min时,开始出现Fe2P相。电化学测试结果表明,微波功率100%辐射9min制备的LiFePO4/MWCNTs材料可逆性能最好,首次充放电容量最大,分别为126.0mAh/g和110.4mAh/g,首次库伦损失为12.4%。
研究了微波功率和辐射时间搭配优化,设定功率和时问的规则是功率与时间之积大致相等。SEM测试显示,80%功率辐射12min的材料比100%功率辐射9mmin的材料细微颗粒更丰富,60%功率辐射15min和40%功率辐射22min制备的材料表现出未完全反应的迹象。电化学测试结果表明,80%功率辐射12min的材料可逆性能最好,首次充放电比容量最大,分别为141.9mAh/g和133.9mAh/g,首次库伦损失为5.6%,去掉碳纳米管非活性物质充放电比容量为157.7mAh/g和148.7mAh/g。经过50次循环后,放电容量为116.0mAh/g,容量损失为7.4%。1C充放电容量分别为114.4mAh/g和106.9mAh/g,去掉碳纳米管非活性物质比容量达到133.9mAh/g和130.6mAh/g。因此,微波辐射功率足够大时才能达到前驱体的反应温度,在达到反应温度后,辐射时间越长,前驱体反应越充分。
对比了不同方法制备的LiFePO4/MWCNTs电化学性能,从碳纳米管的分散处理来看,碳纳米管酸处理结合超声波分散制备的LiFePO4/MWCNTs电化学性能最好,其它依次为单独超声波法,球磨法和微波固相合成法。对于微波固相合成法,尚需进一步研究如何减小材料颗粒和保持碳纳米管比例。
Among the well-known Li-inserted cathode materials for lithium ion secondary battery, the olivine-type LiFePO4is considered as one of the most promising cathode material for lithium-ion power battery because of its low cost, good cycling stability, better thermal stability, excellent security and environment-friendly performance. However, LiFePO4has a main drawback of low electronic and ionic conductivity that hinders it to be commercialized. Currently, some traditional methods as CV, PITT, GITT or EIS had been used to studied lithium-ion conduction mechanism in LiFePO4. But, the feasibility of these methods used to study the Li-inserted electrodes, especially the LiFePO4electrode with a process of two-phase transformation, had not been systematically studied. In this article, the lithium-ion conduction mechanism was systematically studied based on the Li-inserted process of LiFePO4. In addition, the electronic and ionic conductivity of LiFePO4had been improved largely. But the performance of LiFePO4coated with carbon nanotube was not studied comprehensively. In this paper, comprehensive research to LiFePO4coated with carbon nanotubes had been done. The main resuits and conclusions were shown as following.
Study on the electrode kinetics of LiFePO4. Potential scanning curves got by cyclic voltammetry could be used to judge the value of the Exchange current density to determine the reversible properties of the material.
Research a variety of traditional techniques used to make a determination of lithium ion diffusion coefficient, and point out the shortcomings. After in-depth study, we found that the lithium ions transferring into cathode material were similar as the nonequilibrium carrier in semiconductor. The inpouring Li-ions can be called nonequilibrium Li-ions. Than the diffusion equation should be revised. The electrochemical diffusion coefficient, named as D, can be worked out approximately in terms of the relationship between the nonequilibrium Li-ions and the relevant potential difference. The diffusion coefficient got by this approach better reflects the kinetics of LiFePO4cathode materials. The conclusion of this study can provide guides in theory for the preparation, modification and testing methods of LiFePO4. With the new method the D was determined as9.21×10-ncm2/s for LiFePO4doped with10%MWCNTs. The order of this value was almost consistent with the one got by CV, PITT and GITT. But it was kept invariable VS. x in LixFePO4.
The LiFePO4/MWCNTs composite was synthesized by ball milling. The XRD spectrum demonstrated that MWCNTs didn't change the olivine structure of LiFePO4and the SEM showed that MWCNTs decentralized into the grains of LiFePO4. The result of electrochemical test indicated that the composite with the MWCNTs in size of60-100nm and length of1-2um exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were136mAh/g and129mAh/g respectively at0.1C ate in room temperature. The first capacity lost was5.2%. The difference between charge-discharge platforms of this composite was the least compared to the others. This result showed that the material had the largest electrochemical diffusion coefficient of lithium. At the same time, the capacity of the composite only lost4.0%after10cycles.
Study on optimization of milling time. The LiFePO4/MWCNTs milled12h with the same carbon nanotubes exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were132.8mAh/g and126.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.9%. This shows that the extension of ball milling time can be more effectively dispersed carbon nanotubes. Due to destruction of carbon nanotubes, the electrochemical properties of16h milling material rather than was decline. The breaking part of the carbon nanotube could be combined with lithium ion. The irreversible combination could come down the electrochemical performance.
Study on supersonic dispersion method for LiFePO4with10%carbon nanotubes in size of60-100nm, length of1-2um, and purity of>95wt%. The SEM test showed that the greater supersonic power the better dispersion effect of carbon nanotubes. The LiFePO4/MWCNTs dispersed60min at100%power in20℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were138.1mAh/g and132.2mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%.
Study on optimization of ultrasonic ambient temperature. The SEM tests showed that the greater the ambient temperature the better dispersion effect of carbon nanotubes. Electrochemical tests showed that the reversible performance of material increaseed within the range of20℃to60℃and kept constant from60℃to80℃. The capacity was diminished when ambient temperature was greater than60℃. This result could be explained by the increased activity of carbon. Some of the active carbon atoms could be combined with lithium ion. The irreversible combination could come down the capacity. The LiFePO4/MWCNTs dispersed60min at100%power in60℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were145.4mAh/g and137.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.2%.
The effect of the LiFePO4mixed with10%acidized MWCNTs by supersonic dispersed method were investigated. The MWCNTs in size of60-100nm, length of5-15μm, purity of>95wt%were treated by different concentration acid mixed with sulfuric acid and nitrate. The SEM tests of carbon nanotubes showed that the swarm of carbon nanotube was separated as the concentrations increasing. The impurities in carbon nanotube were got rid of after acid treatment. The LiFePO4mixed with the MWCNTs acidized by mixture of sulfuric acid(7.2mol/L) and nitrate(6mol/L) exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were144.4mAh/g and138.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.1%.
Study on optimization of acid temperature. The MWCNTs in size of60-100nm, length of5-15℃m, purity of>95wt%were treated by the mixture of sulfuric acid(7.2mol/L) and nitric acid(6mol/L) using reflux device. The SEM test of carbon nanotube showed that the carbon nanotubes had been purified and separated entirely after heated acid-treated. The LiFePO4mixed with the MWCNTs acidized at40℃ambient temperature exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were150.2mAh/g and143.8mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were166.8mAh/g and159.8mAh/g,98%and94%of the theory value respectively. After50cycles, discharge specific capacity still remained in132.8mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were130.1mAh/g and119.8mAh/g respectively. Then a conclusion would be made that the high temperature acid treatment diminished the electrochemical performance of LiFePO4. The reason was that nitric could oxidize carbon nanotube to hydroxyl and carboxyl functional groups in surface. Such groups containing oxygen would improve the dispersion property of carbon nanotubes to a certain extent, and then improve the conductivity of LiFePO4. However, the oxygen-containing functional groups would be combined with lithium ion.
The effect of LiFePO4/MWCNTs synthesized by microwave was investigated. The MWCNTs in size of60-100nm, length of1-2um, were doped by ball milled in raw material mixed with FeC2O4·2H2O, Li2CO3and NH4H2PO4. The ratio of the MWCNTs to the final composite was10%. The SEM test has shown that, with the time of microwave radiation being prolonged, the size of the material particles increased. And the carbon nanotubes were embedded into the material particles. The XRD test results showed that while the time of microwave radiation at100%power was greater than9min, Fe2P began to emerge. The LiFePO4/MWCNTs heated9min at100%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were126.0mAh/g and110.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was12.4%.
Study on optimization of the settings of microwave oven's time and power. The rule of settings kept the product of time and power in equal. The SEM tests show that the material heated12min by80%power and9min by100%power contained finer particles.The LiFeP/VMWCNTs heated12min at80%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were141.9mAh/g and133.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.6%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were157.7mAh/g and148.7mAh/g,92.7%and87.4%of the theory value respectively. After50cycles, discharge specific capacity still remained in116.0mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were114.4mAh/g and106.9mAh/g respectively. Therefore, microwave radiation at80%power was large enough to reach the reaction temperature. When reaction temperature is reached, the longer to be radiated, there was more resultant.
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