氢热还原法制备锂离子电池正极材料LiFePO_4
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摘要
高功率、大容量、长寿命锂离子电池的广泛市场，如电动助力车电源、发光二极管矿灯电源等，促使新型、廉价锂离子电池正极材料的研发。LiFePO_4正极材料正是迎合了这一需求，而受到业内的关注。
本论文采用两段式高温固相反应的氢热还原法，制备锂离子电池正极活性材料LiFePO_4。用Fe_2O_3、Fe_3O_4、FePO_4三种化合物为铁源，用蔗糖、柠檬酸、淀粉为碳源，用MgO为Mg~(2+)掺杂原料，用纯H_2为还原气和保护气，经300℃-350℃和600℃-700℃两个温度段的高温固相反应，制备出以下三类锂离子电池用正极材料：具有橄榄石单相结构的LiFePO_4、包覆碳的LiFePO_4／C、包覆碳并且掺杂镁的LiFe_xMg_(1-x)PO_4／C。采用粉末X射线衍射(XRD)、扫描电镜(SEM)、电化学阻抗谱(EIS)、循环伏安技术(CV)和实验电池性能测试等手段，对所制材料的结构、形貌、电化学性能进行了比较研究，结论如下：
1、用氢热还原法可以制备出具有橄榄石结构的LiFePO_4单相晶体，其空间群为Pmnb。后段烧结温度为650℃，所制备出的LiFePO_4有一定的活性。
2、少量碳的引入对LiFePO_4的橄榄石结构不产生影响，碳颗粒均匀分散于LiFePO_4晶体颗粒之间，使LiFePO_4颗粒粒径减小，增强了LiFePO_4颗粒之间的导电性。实验电池充放电测试结果表明，包覆碳的LiFePO_4首次放电时间较未包覆碳的LiFePO_4长。
3、镁掺杂更有利于生成粒径小且均匀的LiFe_xMg_(1-x)PO_4；碳的包覆使LiFe_xMg_(1-x)PO_4／C颗粒粒径减小，且对其晶体结构没有影响。由扫描电镜图看出750℃下烧结的LiFe_xMg_(1-x)PO_4／C材料粉末发生了不同程度的团聚，粉体的粒径均大于2μm，大部分为柱形大颗粒。而650℃下烧结的LiFe_xMg_(1-x)PO_4／C的晶体则颗粒均匀，平均粒径约1μm，样品的粒径较小且产物表面粗糙，具有丰富的微结构，比表面积增大了很多。掺杂Mg~(2+)并包覆碳的LiFe_xMg_(1-x)PO_4／C在0.01C和0.2C进行放电，放电倍率增大时放电时间有很大增长，且与LiFePO_4、LiFePO_4／C相比首次放电时间较长，实验结果证明包覆碳和掺杂镁的LiFe_xMg_(1-x)PO_4／C要比没包覆碳，或只包覆碳的材料电化学性能要好。
4、与碳热还原法相比，氢热还原法制备的LiFe_xMg_(1-x)PO_4／C晶体的晶格常数较小；两种样品的循环伏安曲线都反映出一定的可逆性，但氢热还原法所制LiFe_xMg_(1-x)PO_4／C的氧化还原峰电位差值更小。
5．不同有机物还原剂制备的正极材料LiFe_xMg_(1-x)PO_4／C均属于橄榄石型晶体结构，CV结果表明柠檬酸做还原剂所制LiFe_xMg_(1-x)PO_4／C的可逆性较好。
6．从掺锂与掺镁样品循环伏安图看出，掺镁样品扫描所得的峰面积较掺锂样品的峰面积大。但掺锂样品的氧化还原峰电位差值更小。
The development of newer and cheaper cathode material in lithium-ion battery, was urged by the market needs for it with high power, large capacity and long cyclic number, such as the power source for dynamoelectric help bicycle and for mineral light using LED.
In this thesis, LiFePO_4 was synthesized for lithium ion battery cathode material, using the hydrogen thermal reduction method which contains high temperature solid reactions in two different temperature ranges. In this thesis, Fe_2O_3 Fe_3O_4 and FePO_4 were used as iron sources compounds, sucrose citric acid and starch were used as carbon sources compounds, MgO as Mg~(2+) additive sources compound, pure hydrogen gas were used as reducing and protective gas. Three different cathode materials for lithium ion battery were synthesized undergone high temperature solid reactions occurred in two temperature ranges of 300℃-350℃and 600℃-700℃, they are, LiFePO_4 pure phase of olivine structure without any additive, LiFePO_4/C pure phase of olivine structure only with carbon coat, LiFe_xMg_(1-x)PO_4/C pure phase of olivine structure with carbon coating and Mg additive. Different methods for preparing LiFePO_4 and the performance of the target products were compared, from the views of crystal structure, particle morphology and electrochemical performance, by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammegram (CV), electrochemical impedance spectra (EIS) and charge-discharge performance of simulated cell. The results are showed as follow,
1. The olivine structure lithium iron(II) phosphate (LiFePO_4) could be synthesized by hydrogenthermal reduction. Its crystal structure is belong to the space group Pmnb. The LiFePO_4 synthesized at 650℃had a certain discharging capability.
2. A few carbon additive didn't influence the structured of LiFePO_4, also they reduced the particle size of LiFePO_4 and uniformly dispersed among the particle of the crystal, which strengthened the electricity of the particle. The results of electrochemical capability testing showed that the initial discharging time of carbon-coated LiFePO_4 was longer than the initial discharging time of the uncoated material.
3.Doping Mg~(2+) was propitious to synthesizing small and uniform LiFe_xMg_(1-x)PO_4; From SEM we can see that coating carbon reduced the particle size and didn't influence the crystal structure , LiFe_xMg_(1-x)PO_4/C material sintered at 750°C aggregated in a different extent, the particle size was above 2μm, most of them were large column. While LiFe_xMg_(1-x)PO_4/C crystals sintered at 650℃were uniform, the average particle size was about 1μm, the samples had small particle size and coarse surface, and they have abundant microstructure, specific area had a considerable increasing. Compared with the initial discharging time at 0.01C and 0.2C of LiFePO_4 and LiFePO_4/C, the discharging time of LiFe_xMg_(1-x)PO_4/C are longer. The experiment result shown that the LiFe_xMg_(1-x)PO_4/C had the best electrochemical performance
4. Compared with the LiFe_xMg_(1-x)PO_4/C synthesized by carbon thermal reaction, the LiFe_xMg_(1-x)PO_4/C synthesized by hydrogen thermal reduction method has smaller crystal lattices; the cathode peaks and anode peaks of the CV are very sharp ,also the potential difference of between two peaks is very small. But the value of cathode peaks and anode peaks of the CV of LiFe_xMg_(1-x)PO_4/C material synthesized by hydrogenous reduction are more close to theoretical value.
5. LiFe_xMg_(1-x)PO_4/C cathode materials synthesized with various organic carbon are olivine-type structure, the result of CV indicate that cell system formed by LiFe_xMg_(1-x)PO_4/C material synthesized with citric acid as reducing agent and Lithium metal is commendably reversible.
6. As can be seen from the CV graphs, compared with the peak area of the CV Li~+ -adulterating material scanning, the peak area of the CV Mg~(2+)-adulterating sample scanning are larger. The cathode peaks and anode peaks of the CV Mg~(2+)-adulterating sample scanning are sharper. But the value of cathode peaks and anode peaks of the CV of Li~+-adulterating material scanning are more close to theoretical value.
本论文采用两段式高温固相反应的氢热还原法，制备锂离子电池正极活性材料LiFePO_4。用Fe_2O_3、Fe_3O_4、FePO_4三种化合物为铁源，用蔗糖、柠檬酸、淀粉为碳源，用MgO为Mg~(2+)掺杂原料，用纯H_2为还原气和保护气，经300℃-350℃和600℃-700℃两个温度段的高温固相反应，制备出以下三类锂离子电池用正极材料：具有橄榄石单相结构的LiFePO_4、包覆碳的LiFePO_4／C、包覆碳并且掺杂镁的LiFe_xMg_(1-x)PO_4／C。采用粉末X射线衍射(XRD)、扫描电镜(SEM)、电化学阻抗谱(EIS)、循环伏安技术(CV)和实验电池性能测试等手段，对所制材料的结构、形貌、电化学性能进行了比较研究，结论如下：
1、用氢热还原法可以制备出具有橄榄石结构的LiFePO_4单相晶体，其空间群为Pmnb。后段烧结温度为650℃，所制备出的LiFePO_4有一定的活性。
2、少量碳的引入对LiFePO_4的橄榄石结构不产生影响，碳颗粒均匀分散于LiFePO_4晶体颗粒之间，使LiFePO_4颗粒粒径减小，增强了LiFePO_4颗粒之间的导电性。实验电池充放电测试结果表明，包覆碳的LiFePO_4首次放电时间较未包覆碳的LiFePO_4长。
3、镁掺杂更有利于生成粒径小且均匀的LiFe_xMg_(1-x)PO_4；碳的包覆使LiFe_xMg_(1-x)PO_4／C颗粒粒径减小，且对其晶体结构没有影响。由扫描电镜图看出750℃下烧结的LiFe_xMg_(1-x)PO_4／C材料粉末发生了不同程度的团聚，粉体的粒径均大于2μm，大部分为柱形大颗粒。而650℃下烧结的LiFe_xMg_(1-x)PO_4／C的晶体则颗粒均匀，平均粒径约1μm，样品的粒径较小且产物表面粗糙，具有丰富的微结构，比表面积增大了很多。掺杂Mg~(2+)并包覆碳的LiFe_xMg_(1-x)PO_4／C在0.01C和0.2C进行放电，放电倍率增大时放电时间有很大增长，且与LiFePO_4、LiFePO_4／C相比首次放电时间较长，实验结果证明包覆碳和掺杂镁的LiFe_xMg_(1-x)PO_4／C要比没包覆碳，或只包覆碳的材料电化学性能要好。
4、与碳热还原法相比，氢热还原法制备的LiFe_xMg_(1-x)PO_4／C晶体的晶格常数较小；两种样品的循环伏安曲线都反映出一定的可逆性，但氢热还原法所制LiFe_xMg_(1-x)PO_4／C的氧化还原峰电位差值更小。
5．不同有机物还原剂制备的正极材料LiFe_xMg_(1-x)PO_4／C均属于橄榄石型晶体结构，CV结果表明柠檬酸做还原剂所制LiFe_xMg_(1-x)PO_4／C的可逆性较好。
6．从掺锂与掺镁样品循环伏安图看出，掺镁样品扫描所得的峰面积较掺锂样品的峰面积大。但掺锂样品的氧化还原峰电位差值更小。
The development of newer and cheaper cathode material in lithium-ion battery, was urged by the market needs for it with high power, large capacity and long cyclic number, such as the power source for dynamoelectric help bicycle and for mineral light using LED.
In this thesis, LiFePO_4 was synthesized for lithium ion battery cathode material, using the hydrogen thermal reduction method which contains high temperature solid reactions in two different temperature ranges. In this thesis, Fe_2O_3 Fe_3O_4 and FePO_4 were used as iron sources compounds, sucrose citric acid and starch were used as carbon sources compounds, MgO as Mg~(2+) additive sources compound, pure hydrogen gas were used as reducing and protective gas. Three different cathode materials for lithium ion battery were synthesized undergone high temperature solid reactions occurred in two temperature ranges of 300℃-350℃and 600℃-700℃, they are, LiFePO_4 pure phase of olivine structure without any additive, LiFePO_4/C pure phase of olivine structure only with carbon coat, LiFe_xMg_(1-x)PO_4/C pure phase of olivine structure with carbon coating and Mg additive. Different methods for preparing LiFePO_4 and the performance of the target products were compared, from the views of crystal structure, particle morphology and electrochemical performance, by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammegram (CV), electrochemical impedance spectra (EIS) and charge-discharge performance of simulated cell. The results are showed as follow,
1. The olivine structure lithium iron(II) phosphate (LiFePO_4) could be synthesized by hydrogenthermal reduction. Its crystal structure is belong to the space group Pmnb. The LiFePO_4 synthesized at 650℃had a certain discharging capability.
2. A few carbon additive didn't influence the structured of LiFePO_4, also they reduced the particle size of LiFePO_4 and uniformly dispersed among the particle of the crystal, which strengthened the electricity of the particle. The results of electrochemical capability testing showed that the initial discharging time of carbon-coated LiFePO_4 was longer than the initial discharging time of the uncoated material.
3.Doping Mg~(2+) was propitious to synthesizing small and uniform LiFe_xMg_(1-x)PO_4; From SEM we can see that coating carbon reduced the particle size and didn't influence the crystal structure , LiFe_xMg_(1-x)PO_4/C material sintered at 750°C aggregated in a different extent, the particle size was above 2μm, most of them were large column. While LiFe_xMg_(1-x)PO_4/C crystals sintered at 650℃were uniform, the average particle size was about 1μm, the samples had small particle size and coarse surface, and they have abundant microstructure, specific area had a considerable increasing. Compared with the initial discharging time at 0.01C and 0.2C of LiFePO_4 and LiFePO_4/C, the discharging time of LiFe_xMg_(1-x)PO_4/C are longer. The experiment result shown that the LiFe_xMg_(1-x)PO_4/C had the best electrochemical performance
4. Compared with the LiFe_xMg_(1-x)PO_4/C synthesized by carbon thermal reaction, the LiFe_xMg_(1-x)PO_4/C synthesized by hydrogen thermal reduction method has smaller crystal lattices; the cathode peaks and anode peaks of the CV are very sharp ,also the potential difference of between two peaks is very small. But the value of cathode peaks and anode peaks of the CV of LiFe_xMg_(1-x)PO_4/C material synthesized by hydrogenous reduction are more close to theoretical value.
5. LiFe_xMg_(1-x)PO_4/C cathode materials synthesized with various organic carbon are olivine-type structure, the result of CV indicate that cell system formed by LiFe_xMg_(1-x)PO_4/C material synthesized with citric acid as reducing agent and Lithium metal is commendably reversible.
6. As can be seen from the CV graphs, compared with the peak area of the CV Li~+ -adulterating material scanning, the peak area of the CV Mg~(2+)-adulterating sample scanning are larger. The cathode peaks and anode peaks of the CV Mg~(2+)-adulterating sample scanning are sharper. But the value of cathode peaks and anode peaks of the CV of Li~+-adulterating material scanning are more close to theoretical value.
引文
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[2] Thackeray M M, David W I F, Goodenough J B. High-temperature lithiation ofα-Fe_2O_3: A mechanistic study[J]. Solid State Chem., 1984, 55: 280;
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