【作者】 林燕；

【导师】 潘洪革；

【作者基本信息】 浙江大学 ， 材料学， 2009， 博士

【摘要】 橄榄石结构LiFePO₄因低廉的价格、无污染且循环性能、安全性能优良等优点而成为极具开发和应用潜力的新一代锂离子电池用正极材料。但该材料较低的电导率及锂离子扩散能力使其在动力型电池用正极材料上受到极大限制,因而改善LiFePO₄的大电流放电能力是目前锂离子电池研究领域的热点之一,也是实现LiFePO₄在动力电池上实用化的关键。本文采用溶胶凝胶法合成LiFePO₄/C材料,采用XRD/Rietveld、SEM、TEM、元素分析、BET、Raman光谱等材料分析方法以及恒电流充放电、循环伏安（CV）、电化学阻抗谱（EIS）、等电化学测试技术,对合成过程中采用的铁源(Fe³⁺和Fe²⁺)、络合剂及碳源（柠檬酸和乙二醇）、溶胶pH值、干凝胶的预处理、烧结的温度、烧结时间和烧结气氛（氮氢混合气中氢气含量）等关键工艺参数对LiFePO₄/C材料结构和电化学性能的影响进行了系统研究,获得了高性能LiFePO₄/C材料的制备工艺和参数,揭示了LiFePO₄/C材料的结构和倍率性能的关系及LiFePO₄/C材料的结构和电化学性能（尤其是高倍率性能）的关键影响因素。研究结果表明,本文溶胶凝胶法的制备工艺及参数能成功制备适量碳包覆的LiFePO₄/C正极材料,材料的结构和形貌可通过改变相应制备工艺和参数进行调节和优化。通过调节和控制制备工艺,可获得原位碳包覆量较少（～2 wt.%）、颗粒细小和具有适量高电导率的铁磷化合物含量的LiFePO₄/C材料。研究获得的LiFePO₄/C正极材料其0.1 C、1 C、5 C和10 C容量可分别达到157、124、92和70 mAh/g。本文研究的LiFePO₄/C正极材料均具有良好的循环稳定性,但其倍率性能受材料中LiFePO₄活性物质的含量、颗粒尺寸及其分散程度、铁磷化合物含量和LiFePO₄的结晶完整性等因素的影响,各因素的作用程度还与材料的放电倍率有关。在较低的放电倍率下,LiFePO₄的含量、结晶的完整性和颗粒尺寸对其容量起到关键作用,LiFePO₄结晶完整、颗粒尺寸较细小及较高的活性物含量是LiFePO₄/C材料具有较高容量的关键。随着放电倍率的增加,适量高电导率的铁磷化合物相可显著提高LiFePO₄的利用率,对提高其倍率性能起到关键作用。在LiFePO₄/C具有较小颗粒尺寸的条件下,少量的Fe₂P相（～2wt.%）即可明显提高材料的高倍率性能。即使在样品颗粒尺寸相对较大时,适量的Fe₂P也能较明显地改善材料的高倍率性能。在一定范围内,Fe₂P相含量的增加有利于提高LiFePO₄的倍率性能,但Fe₂P含量过高,降低了有效活性物质的质量,使材料的容量降低,其含量宜低于10 wt.%。交流阻抗谱及循环伏安测试结果显示,材料的动力学性能也随LiFePO₄/C颗粒的细化和原位铁磷化合物相的适量增加而明显提高,与其较高放电倍率下的容量特性相一致。更多还原

【Abstract】 Olivine structured LiFePO₄ is a promising cathode material for the next generation of lithium ion batteries for its low cost,environmental benign,good cycling performance and safety,etc.However,its intrinsic poor electrical conductivity and diffusion capability of lithium ion greatly hinder its application in the high power rechargeable battery.The improvement of its high-rate performance is key important to realize the application in high power rechargeable batteries.In this thesis,a sol-gel method was used to synthesize carbon coated LiFePO₄（LiFePO₄/C） materials.The effects of the synthesis technique and parameters, including the valence of the iron source(Fe³⁺ and Fe²⁺),chelating agents and carbon sources （citric acid and ethylene glycol）,the pH value of the sol,pre-treatment of the dried gel,the sintering temperature,time and atmosphere（N₂ with different content of H₂） on the structure and electrochemical properties of the LiFePO₄/C were systematically investigated by means of XRD/Rietveld,SEM,TEM,elemental analysis,etc.and galvanostatic charge-discharge, EIS,CV etc.The relationships between the structure and electrochemical properties of LiFePO₄/C materials,and the key factors that influence the structure and the rate capability, especially the high-rate capability of LiFePO₄/C,have been revealed.The results of the investigation show that LiFePO₄/C materials can be successfully prepared by the present synthesis parameters,and the structure and morphology of the materials have been optimized by modifying the synthesis parameters.LiFePO₄/C materials with low amount of carbon coating（2 wt%）,small particle size and suitable amount of high electrical conductive phases of iron phosphides have been obtained,showing capacities of 157、124、92 and 70 mAh/g at 0.1 C,1 C,5 C and 10 C,respectively.The LiFePO₄/C materials in the present work all display good cycling performance.The rate capacities are affected by the content,the crystallization,the particle size and its distribution of LiFePO₄,and the content of iron phosphides.However,the above factors act differently on the LiFePO₄/C materials at various discharge rates.At the low discharge rate, the content,crystallization and the particle size of LiFePO₄ play key effect on the capacity. High content,well crystallization and small particle size of LiFePO₄ lead to the high capacity. But with the discharge rate increasing,certain amount of high electronic-conductive iron phosphides becomes key important on the improvement of the high rate capability.Iron phosphides can greatly enhance the utilization of LiFePO₄ at high discharge rate.Combing with the small size of LiFePO₄/C,low amount of Fe₂P of 2 wt%can obviously increase the rate capability.Even when the particle size of LiFePO₄ is a bit larger,certain content of Fe₂P can also improve the high rate performance pronouncedly.The high rate capability of LiFePO₄/C material increases with the increase of the Fe₂P content within a certain range. However,too much Fe₂P decreases the capacity due to the decrease of the fraction of the active material of intercalation/deintercalation of lithium ion.EIS and CV studies confirm that the kinetic property of LiFePO₄/C is obviously improved by the reduction of the particle size and the proper amount of iron phosphides,which is consistent with the high rate performance of LiFePO₄/C.更多还原