【作者】 吴仕明；

【导师】 史鹏飞；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2007， 硕士

【摘要】 锂离子电池橄榄石型正极材料LiFePO₄因其价格低廉、对环境友好、循环性能优良、安全性能突出等优点而成为最具开发和应用潜力的新一代锂离子电池用正极材料。本文采用高温固相法制备了金属离子掺杂的LiFePO₄及LiFePO₄/C复合材料。利用TG-DTA对前驱混合物的作用机理进行了研究,同时利用XRD、SEM和XPS等技术对产物的化学组成和微观形貌进行了分析,并采用恒流充放电和循环伏安技术测试其电化学性能。重点讨论了合成工艺条件对以廉价的Fe₂O₃和Fe₃O₄为铁源,以煤焦油沥青作为导电剂和还原剂的前驱物合成的LiFePO₄/C复合材料的物理和电化学性能的影响。以Fe²⁺化合物作铁源,采用固相法合成了LiFePO₄,并分别对其进行Fe位掺杂(Mg²⁺)和Li位掺杂(Cr³⁺和Zr⁴⁺)。并研究了各种合成条件对金属离子掺杂的LiFePO₄电化学性能的影响,其中包括烧结温度和掺杂比例。研究结果表明,金属离子掺杂的LiFePO₄的电化学性能较差。以价廉的Fe₂O₃为铁源,以煤焦油沥青为还原剂和碳源进行正极材料的合成。研究了煤焦油沥青添加量对材料电化学性能的影响。结果发现,添加17 mass%煤焦油沥青所得材料的电化学性能最好。并考察了前处理工艺、球磨速度和加碳时机的影响,从而确定了200 r/min的球磨速度和在材料制备过程中须将球磨后和预烧后的料进行充分研磨的前处理工艺,而且通过试验找到了最好的添加煤焦油沥青的时间是在球磨后、预烧前。研究了烧结温度、烧结气氛及烧结时间等合成条件对所合成材料的物理和电化学性能的影响。结果发现,高温有利于LiFePO₄晶体的生长,但温度过高又导致LiFePO₄颗粒聚集长大,而且会在产物中生成电化学惰性的Fe₂P。在本文研究的温度范围内,700℃是合成同时具有较高结晶度和较小颗粒尺寸的LiFePO₄的最佳烧结温度。而烧结气氛对产物的纯度和电化学性能等的影响较大,研究结果表明,当烧结时的气氛都仅为氩气时,可以得到高纯度、高结晶度以及高电化学性能的LiFePO₄。700℃下通氩气时,不同烧结时间对所合成材料的电化学性能影响较大,结果发现6 h是本实验条件下高温烧结的最佳时长。XPS测试结果显示,在所得材料表面的Fe和P的价态分布为+2和+5。以Fe₃O₄为铁源,以煤焦油沥青作为还原剂和碳源在700℃下通氢氩混合气成功制备了结晶较好的纯相的LiFePO₄。其0.1 C时的首次放电比容量为112 mAh/g,随着循环的进行其放电比容量逐渐减小,这和经典的LiFePO₄的趋势一致,而且材料在0.2 C和1.0 C也表现出较好的循环稳定性。对所合成的材料进行XPS的测试结果表明,在材料表面不存在Fe³⁺和Fe₂P。研究了烧结气氛和煤焦油沥青添加量对以Fe₃O₄为铁源进行LiFePO₄/C复合材料电化学性能的影响。研究结果表明,当烧结气氛为氩气时所得材料的电化学性能较好,其0.1 C的首次放电比容量达到128 mAh/g并且具有较好的循环稳定性,而且确定了17 mass%作为本实验条件下比较好的煤焦油沥青添加量。通过实验得出并不是碳的添加量越多,材料的性能越好。更多还原

【Abstract】 Lithium ion battery olivine-type cathode material LiFePO₄ is gaining particular interest as a potential candidate cathode material for rechargeable Li-ion batteries because of low cost , environmental benigh, excellent cyclic performance and stand-out safety etc. In this paper, LiFePO₄ dopping with metallic ion and LiFePO₄/C composite cathode materials have been synthesized through solid-state route. Adopting TG-DTA measurement to research on the reaction mechanism of the precursor mixtures, the micro-structures and morphologies of these compounds were investigated by XRD, SEM and XPS, meanwhile. The electrochemical performances have been evaluated by galvanostatic charge-discharge and cyclic voltammetry. The effects of the synthesis parameters on the physico-electrochemical properties of LiFePO₄/C composite cathode materials have been discussed in detail. The materials were synthesized using Fe₂O₃ or Fe₃O₄ as iron source and using asphaltum as both conductive and reductive agent precursor, respectively.Metallic ion dopped LiFePO₄ were synthesized by solid-state reaction using ferrous compound as iron precursor, include dopping in Fe site (Mg²⁺) and Li site (Cr³⁺ and Zr⁴⁺). The effects of some synthesis factors including dopant content and sintering temperature on the electrochemical properties were investigated. The results showed that the electrochemical properties of metallic ion dopped LiFePO₄ were rather bad.The cathode material was synthesized using Fe₂O₃ as iron source and asphaltum as both reductive agent and carbon source. The influences of the asphaltum’s addition on the electrochemical properties of materials were investigated. It was found that 17 mass% would be the best addition of asphaltum in our experimental conditions. We also did some research on the influences of pre-treatment technics, ball-milling speed and the opportunity adding carbon, accordingly confirm the 200 r/min’s ball-milling speed, thoroughly grinding of the material during the synthesis, and the results showed that the best time for adding asphaltum is after ball-milling before pre-sintering. The effects of synthesis conditions such as pyrolyzing temperature, sintering atmosphere and sintering time on the physico-electrochemical properties of the materials prepared were investigated. It was found that increasing the sintering temperature leads to higher crystallinity, but to a larger particle size, and the formation of electrochemically inert Fe₂P in the product. 700℃was the optimum synthetic temperature for LiFePO₄ with perfect crystal and uniform small particle sizes. The sintering atmosphere had great impact on the purity and electrochemical properties of the production, the result showed that LiFePO₄ with high purity, high crystallinity and pefect electrochemical performance would be gained when the sintering atmosphere was all argon. When sintering in argon at 700℃, the effect of sintering time on the electrochemical performance was great, it was found that 6 h was the optimum sintering time under our experiment condition. The results of XPS measurement showed that the valence of element of Fe and P on the surface of materials is +2 and +5, respectively.High crystallinity, pure LiFePO₄/C composite was synthesized at 700℃in Ar/H2 using Fe₃O₄ as iron source and asphaltum as both reductive agent and carbon source. Its initial discharge capacity is 112 mAh/g at 0.1 C rate, and the capacity gradually decreased with cycling, this behavior fitted the trency of conventional LiFePO₄, and the material showed good cyclic stability at 0.2 C and 1.0 C. The XPS results showed that there are no Fe³⁺ and Fe₂P on the surface of the product.The effects of sintering atmosphere and the addition of asphaltum on the electrochemical properties of the materials prepared were researched. It was found that LiFePO₄/C with pefect electrochemical performance would be gained when the sintering atmosphere was all argon, and confirmed that the better content of asphaltum added was 17 mass%. Also, through experiment we could deny that the more carbon we added, the better performance we would get.更多还原