摘要
近些年来,锂离子电池正极材料LiFePO4因具有资源丰富、价格便宜、热稳定性、循环性能较好、环境相容性及安全性能好等优点备受关注。但LiFePO4的电导率、Li+化学扩散系数均较低,振实密度不高成为其应用的主要障碍,而且制约了其在充放电过程中的比容量和倍率性能的发挥。通过在合成LiFePO4的原料中添加碳及掺杂离子、优化合成工艺及制备纳米颗粒等方法来改善其电化学性能是行之有效的方法。本文在前人研究的基础上,开展了以下工作:
1.以FeC2O4-2H2O为铁源,乙炔黑、多孔碳、石墨,葡萄糖、蔗糖、淀粉,聚乙烯醇、酚醛树脂及环氧树脂为碳源,采用固相反应法制备LiFePO4/C复合材料,初步探讨了碳的结合方式对LiFePO4导电机制的影响。在九种碳源中,聚乙烯醇热解后合成的LiFePO4/C复合材料电导率高达5.76×10-2S·cm-1,其放电比容量,倍率性能及循环性能最佳,这可能与C(碳)网在LiFePO4颗粒的“面”包裹结构为其充放电过程中电子的运输提供了通道有关。
2.以球形FePO4-2H2O为铁源,采用碳热还原法制备了LiFePO4正极材料,并对锂源、合成温度及碳含量对LiFePO4性能的影响进行了研究:锂源对于LiFePO4正极材料的形貌、电导率、振实密度及充放电比容量都有较大的影响,以LiOHH2O为原料合成的试样其倍率性能较好,试样在C/20、C/10、C/5及1.0C倍率下的放电比容量为142.56、122.39、84.50及65.56mAh-g-’;合成温度对试样的电子电导率影响不大,但对试样的微观结构影响较大,从而显著地影响试样的充放电特性,650℃为较为合适的合成温度;碳含量可以显著影响试样的电导率,过多的碳的加入对于试样电化学的提高作用不明显,其加入量的合理值在5wt.%。
3.采用Mg2+、Cr3+及Ti4+为掺杂离子制备了一系列掺杂的LiFePO4试样,并对LiFePO4及其掺杂化合物进行了第一性原理计算研究:掺杂提高了材料的电导率,其数量级在~10-4S.cm-1,掺杂有利于提高材料的振实密度,提高材料的倍率放电性能。研究结果表明,在Fe位每单位LiFePO4分子式摩尔掺杂量为0.02atom的Ti试样的电化学性能最好,以1C倍率进行充、放电时,放电容量为98.74mAh.g-’,放电倍率增加到3.0C时,试样的放电容量为83.08mAh-g-’,其倍率性能较好。通过掺杂试样的电子结构分析,掺杂降低了LiFePO4的能隙值,有利于提高材料的电子电导率,而且高价离子掺杂可能在一定程度上提高其离子电导率。
4.以FePO4/PANI前躯体为原料制备了纳米级的LiFePO4正极材料,并对LiFePO4进行了碳包覆和Ti掺杂改性研究:合成纳米材料缩短了固相反应中离子的扩散路径,有利于掺杂离子进入LiFePO4晶格,并增加了掺杂元素均匀分布的几率;Ti掺杂纳米级的LiFePO4/C试样在提高电子电导率数值上的作用不是很明显,但在一定程度上提高了离子扩散的速度;试样LiFe0.96Ti0.02PO4/C获得了良好的碳包覆结构,纳米级颗粒大小均匀,其结晶度高,晶粒尺寸小,电化学性能得到很大的改善,具有优异的平台性能和倍率性能,试样在C/10、C/2、1.0C、3.0C及5.0C倍率下的放电比容量为159.02、154.43、148.49、136.88及121.95mAh-g-’。
Recently, lithium iron phosphate cathode material has been paid more attention due to the advantages of rich resources, cheap price, thermal stability, excellent cycling performance, environmental compatibility as well as good security performance. But lithium iron phosphate suffers from poor electric conductivity, lower chemical diffusion coefficient and bad tap density, which are large obstacles to restrain its application, specific capacity and capability during charge and discharge process. Through adding carbon in the raw material during the synthesis process, doping ion, optimizing the synthetic process and preparation of nanometer particles are effective methods of to improve their electrochemical performance of LiFePO4. On the basis of previous studies, this paper carried out the following job:
1.The LiFePO4/C composite materials were synthesized by solid-state reaction using Iron(Ⅱ) oxalate dehydrate as iron source and adding acetylene black, porous carbon, graphite, glucose, sucrose, starch, polyvinyl alcohol, phenol resin and epoxy resin as the carbon sources to discuss the influence of combination carbon way on conductive mechanism of lithium iron phosphate. During the nine carbon sources, the LiFePO4/C composite material prepared by paralysis of polyvinyl alcohol obtained high electric conductivity value with1.88×10-1S·cm-1and has the best performances of discharge capacity, capability as well as cycling properties. It may relate with transferring channels provide for electronic transport during in charging and discharging process because of the carbon network is face package structure in lithium iron phosphate.
2. Cathode material lithium iron phosphate were synthesized by carbon reduction method using spherical ferric phosphate dehydrate as iron source and the lithium source synthesis temperature and carbon content on the influence of lithium iron phosphate properties are studied.The morphology, electronic conductivity, tapping density and charge discharge capacity has more influence by lithium sources. The samples prepared by lithium hydroxide has the good rate performance with the discharge capacity of142.56,122.39,65.56,84.50mAh·g-1at the current of C/20, C/10, C/5and1.0C rate. The synthesis temperature greatly influenced on the microstructure of samples and has little effect on the electric conductivity, thus the sample's charge and discharge characteristics significantly affects by microstructure and electronic conductivity. The650degree centigrade was the reasonable temperature.The value of electric conductivity about lithium iron phosphate is not proportional relations with the content of carbon. An excess of adding of carbon will not always benefit for the electrochemical of samples and the suitable amount of addition was the five percentages in weight.
3. A series of doping lithium iron phosphate samples were synthesized by magnesium, titanium and chromium as the doping elements. Lithium iron phosphate and its doping compounds were studied by the first principle calculation. The results indicate that the electric conductivity of the material was improved significantly with the order of magnitude in10-4S·cm-1. Doping improves the tapping density and discharging rate capability of the materials. The sample doped0.02atom titanium per mole Fe sites has the best electrochemical properties especially the rate capability of which the discharging capacity was98.74mAh·g-1at the current of1.0C rate and83.08mAh·g-1at the current of3.0C rate. Doping reduces energy band gap of lithium iron phosphate and enhances the electric conductivity of the materials by analysis of the electronic structure of the sample. The high valence ion doping has certain degree boost the ion conductivity.
4. The nanometer scale lithium iron phosphate cathode material was synthesized modifying by carbon coating and titanium doping with FePO4/PANI precursor as raw materials. Synthesizing nanometer materials shortens the ion diffusion pathway during solid phase reaction, has the advantage of doping ion enters the lattice of lithium iron phosphate and increases the even distribution probability of doping element. The nanometer LiFePO4/C composite doped by titanium has little effect on the electric conductivity, but in certain degree increased the ion diffusion velocity. Sample LiFeo.96Tio.o2P04/C abtained the excellent cabon coating struture and the nanometer particle size uniform with high crystalline degree and tiny grain size. The electrochemical properties has been increased significantly and has the outstanding platform performance and rate capability, with the discharging capacity159.02,154.43,148.49,136.88and121.95mAh·g-1at the current of C/10,C/2,1.0C3.0C and5.0C rate, respectively.
引文
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