【作者】 刘辉；

【导师】 解晶莹；

【作者基本信息】 中国科学院研究生院（上海微系统与信息技术研究所） ， 材料物理与化学， 2008， 博士

【摘要】 橄榄石结构的LiFePO₄作为新型锂离子电池正极材料具有高安全价、长寿命、低成本和环境友好等特点,因而成为目前电池界竞相开发与研究的热点。本文以合成高性能的LiFePO₄/C复合材料作为研究目标,比较系统地从材料合成及改性方法、导电网络构建、结构特征、电化学性能及结构-性能关系等方面进行了研究。首先考察了不同比表面积碳源对碳热还原过程制备LiFePO₄/C复合材料结构和性能的影响。结果表明,采用不同比表面积的碳导电剂前驱物对LiFePO₄高温固相反应过程中的晶粒长大活化能进行了计算,探讨了比表面积差异对LiFePO₄晶粒大小的影响。研究表明,较大比表面积的碳导电添加剂能够明显抑制LiFePO₄晶粒的长大,所得产品粒径较小,电化学性能变好。基于LiFePO₄材料“纳米-微米”化的结构设计思路,以乙二醇甲醚和水为溶剂,采用新的溶胶-凝胶方法制备了具有纳米碳导电网络分布的LiFePO₄/C复合材料。利用XRD、FE-SEM、EDS、HR-TEM对其进行了结构和形貌分析。所得材料一次粒子为纳米晶LiFePO₄,二次粒子为微米级的纳米团簇体。蔗糖分解产生的碳原位包覆在LiFePO₄晶粒表面,形成薄碳膜,厚度在5～8nm之间,晶粒与晶粒之间通过碳膜相互连接,使二次粒子内部形成导电网络。不同含碳量复合材料的比表面积测试结果表明,LiFePO₄表面薄碳膜为多孔结构,Li⁺迁移阻力较小。通过改变二次造粒的条件可以对材料的粒径进行调控,从而优化其振实密度。电化学测试表明,该复合材料具有优良的倍率性能和循环稳定性,10C倍率下充放电首次容量为108 mAh·g^-1,电位平台3.15V左右,循环50次后,容量保持率为95.4%。论文提出了在LiFePO₄正极材料中构建复合导电网络的材料制备思路,并通过凝胶前驱体辅助碳热还原法制备得到新型LiFePO₄/C+Fe₂P复合材料。分别考察了热处理温度和碳添加量对复合材料结构和性能的影响。实验发现,Fe₂P在LiFePO₄中所占比例对复合材料的电化学性能具有较大影响。较低的热处理温度（700℃）,碳过量3wt.%,Fe₂P比例为3.8%时,材料能表现出最优的电化学性能。我们还详细考察了Fe₂P的生成对复合材料充放电循环过程和高温（55℃）电化学性能的影响。充放电循环过程中,不同电位下电极材料的XRD结果表明,Fe₂P相在复合材料中结构稳定,不参与Li⁺脱嵌过程,为非电化学活性材料。相比常温,其高温循环稳定性较差。结合TGA、XRD和FE-SEM分析确定了较低热处理温度下Fe₂P相的生成反应。尖晶石型Li₄Ti₅O₁₂是目前锂离子电池负极热点研究材料,针对其电导率低、振实密度不高的问题,首次采用流变相法合成了具有窄粒径分布的Li₄Ti₅O₁₂/C复合负极材料。复合负极材料的充放电容量和倍率性能较纯相Li₄Ti₅O₁₂材料有明显提高。将此负极材料与LiFePO₄正极材料组装成电池,Li₄Ti₅O₁₂/LiFePO₄全电池较之MCMB/LiFePO₄全电池具有较平的充放电电压平台和无SEI膜生成。更多还原

【Abstract】 Olivine structure lithium iron phosphate,LiFePO₄,has recently attracted significant interest due to its high-safety,long cycle life,low cost and environmental benignity components.With aim to synthesize LiFePO₄/C composite with good electrochemical properties,many studies have been carried out including:synthesis and modification method,conductive webs build,structure properties, electrochemical properties and structure-properties relationship.In this study,the CTR method was employed to synthesize the carbon-coated LiFePO₄ using three different surface area carbon including acetylene black,VXC and BP2000 carbon black.The purpose of current investigation was to find the effects of different specific surface area carbon on the structure and electrochemical properties of LiFePO₄.According to active energy calculation of crystalline growth, the relationship between specific and particle size has been investigated.The results showed that the carbon with higher specific surface area could hinder the growth of LiFePO₄ crystalline clearly,improve the electrochemical kinetic of LiFePO₄ electrode.Based on the material design concept of "nano-micro",Using 2-methoxyethanol-water solution as the media,nano-LiFePO₄/carbon composite cathode material has been synthesized via a simple and new sol-gel route from iron nitrate,lithium dihydrogen phosphate.X-ray diffraction analysis,field emission scanning electron microscopy and transmission electron microscope observations showed that LiFePO₄ with a well-crystallized olivine structure appeared in the heat-treated powder,and primary particles were nanocrystalline,secondary particles were sphere-liked micrometer nano-cluster.Through pyrolysis of the sucrose dispersed in aqueous gelatin,carbon can be in situ coated on the surface of LiFePO₄ crystalline to form thin carbon film（5～8nm）.To connecting between LiFePO₄ crystalline forms conductive webs inter secondary particle.Specific surface area measurement showed that coated carbon film with porous structure on the surface of LiFePO₄ particles did not block the direct contact between the active particles and penetrated electrolyte.According to adjust the secondary particle size using different milling time for heated products,tap density of final products could be optimized. The favorable physical characteristics of the nano-LiFePO₄/carbon composite materials exhibited excellent rate performance and cyclability,delivering a discharge capacity of 103 mAh·g^-1（10 C charge/discharge rate） after 50 cycles.LiFePO₄ materials with in situ formed conductive carbon and Fe₂P phase were synthesized by gel precursor assistant carbothermal reduction method.XRD and SEM-EDX analysis identified the existence of Fe₂P phase,which was produced by the reduction reaction of phosphate and iron oxide in excess of carbon.The results indicated that Fe₂P could optimize the conductive webs by increasing the electronic conductivity so as to promote the electrochemical kinetics.The electrochemical performances of the LiFePO₄/C+Fe₂P powder,synthesized at 700℃,containing 3.8 %of Fe₂P were evaluated using an electrochemical model cell by galvanostatic charge and discharge at different charge/discharge rates.The material achieved capacities of ca.160 mAh·g^-1 at 0.1 C rate and ca.102 mAh·g^-1 at 5 C rate, exhibiting good discharge capacity and rate capability.In addition,the effects of Fe₂P on charge/discharge and electrochemical properties at high temperature（55℃） were investigated.The results showed that Fe₂P was non-active material without structure change during charge and discharge,but the capacity at 55℃faded seriously.Finally,corresponding synthesis mechanism of Fe₂P at relatively low heat-treatment temperatures was obtained by TGA、XRD and FE-SEM.Recently there is increasing interest in spinel Li₄Ti₅O₁₂ as a potential anode material for Li-ion batteries.In order to improve the conductivity and tap density, coating carbon on the surface of Li₄Ti₅O₁₂ was synthesized by a rheological phase method.Its average particle size is about 2.1μm with a narrow size distribution as a result of homogeneous mixing of the precursors.The in situ carbon coating produced by decomposition of PVB played an important role in improving electrical conductivity,thereby enhancing the rate capacity of Li₄Ti₅O₁₂ as anode material in Li-ion batteries.The Li₄Ti₅O₁₂/C composite,synthesized at 800℃for 15 h under argon,containing 0.98 wt.%of carbon,exhibited better electrochemical properties in comparison with the pristine Li₄Ti₅O₁₂,which could be attributed to the enhanced electrical conductive network of the carbon coating on the particle surface.Finally, the electrochemical properties of Li₄Ti₅O₁₂/LiFePO₄ cell initially assessed by CV and charge/discharge measurements showed that it had a flat charge/discharge platform and no SEI film during Li⁺ insertion.更多还原