【作者】 习小明；

【导师】 唐谟堂； 黄焯枢；

【作者基本信息】 中南大学 ， 有色金属冶金， 2007， 博士

【摘要】 本论文提出并研究成功了一种水溶液中制备超微粉体的新方法：多相氧化还原法，该方法是指在水溶液中，对固相进行氧化或还原生成新固相物质的方法（如氢氧化锰氧化生成四氧化三锰），并且当溶液中存在其它合适的离子（如锂离子）时，这些离子可借助于固相发生氧化、还原反应时的高表面反应活性参与化学反应，嵌入固相而形成新的物质（如氢氧化锰在氢氧化锂溶液中氧化生成锰酸锂）。本论文系统研究了多相氧化还原法制备超微氢氧化锰、四氧化三锰、钴酸锂和锰酸锂的机理和工艺。通过对金属与水的锈蚀反应机理研究，发现金属锰与纯水反应时，氧化生成的致密氢氧化锰紧紧包裹在金属锰的表面，阻碍反应的进一步进行；当有铵盐存在时，铵作为主要氢离子载体参与金属锰的反应，生成的氨与溶解的锰离子继而生成锰氨配合离子，促使锰离子从金属锰表面往溶液深处转移，从而使水解的氢氧化锰不能致密地沉积在金属锰表面，所以生成的氢氧化锰疏松、多孔、易脱落，加速氧化反应，最终使金属锰与水的反应进行完全。氢氧化锰氧化生成四氧化三锰的多相反应动力学研究表明，氧化反应的速度受氧气扩散传质控制，在40-60℃范围内，氧化反应的活化能为20．37 kJ／mol，氧化率与时间呈线性关系；通过进一步的实验研究，首次提出了不定量取样分析料浆锰的Mn²⁺／Mn⁴⁺价态比，并推导了氧化率Y（％）与Mn²⁺／Mn⁴⁺比X的定量关系，提出了氧化反应终点的判断方法-pH值判断法，即当悬浮溶液体系pH值迅速下降之时即为氧化反应进行完全的终点。以氢氧化亚钴为钴源，空气、氧气、双氧水、NaClO₃为氧化剂，在氢氧化锂溶液中，采用多相氧化还原法实现了在常压水溶液中合成钴酸锂的反应，探明了钴酸锂的形成机理。研究表明，以空气和氧气作为氧化剂时所得钴酸锂的Li、Co摩尔比较高，且Li、Co摩尔比随着反应温度的升高、反应时间的延长和Li⁺浓度的提高而增大。动力学研究表明，氢氧化亚钴氧化生成钴酸锂的反应在较高温度（>80℃）下是分两阶段进行的：第一阶段是快速氧化生成钴酸锂和四氧化三钴混合物阶段；第二阶段是中间产物四氧化三钴慢速氧化转变成钴酸锂的阶段。当温度较低时以快速氧化反应为主，主要产物是钴酸锂和CoOOH的混合物。生成钴酸锂过程中，形成的不连续产物层使钴酸锂的生成反应得以持续进行，钴酸锂产物聚集形态受搅拌强度的影响较大。在研究确定的最佳合成条件下所得前驱体化学成分一致、粒度均匀、具有良好的烧结活性；并可制备粒度分布窄，结晶度高，加工性能好，振实密度大(>2．5g·cm^-3)球状钴酸锂，产品电化学性能优异，初始放电比容量>149 mAh·g^-1，充放电效率>97％。前8次循环的放电容量平均衰减率仅为0．04％。研究了以高锰酸钾、氧气、空气、双氧水为氧化剂，以氢氧化锰为锰源，通过多相氧化还原法实现了在常压水溶液中合成锰酸锂的反应，探明了其形成机理。XRD、粒度及SEM研究表明，在氧化反应过程中，产物锰酸锂是在氢氧化锰表面原位形成的、并且是非连续产物层，这就使得多相氧化反应可以持续进行。氧化剂的氧化能力的强弱对氧化嵌锂具有重要影响；对于高锰酸钾，适当提高温度和合适的锂离子浓度有利于氧化嵌锂；产物中锰平均价数与氧化反应时间的关系成线性关系。所得锰酸锂前驱体按Li／Mn=0．5经二次配方烧结（820℃，10h）后，可得到粒度均匀、颗粒表面光滑、结晶度高的尖晶石锰酸锂正极材料；其电化学性能良好，首次放电容量为119．2mAh·g^-1，具有良好的循环性能。以MnO₂为锰源、水合肼或亚硫酸钠为还原剂，在氢氧化锂溶液中，采用多相氧化还原法，实现了常压水溶液中合成具有尖晶石结构的锰酸锂，探明了其形成机理。对产物进行XRD、SEM和粒度分析表明，反应过程中锰酸锂是在二氧化锰表面原位形成的，随着反应的进行及嵌锂程度的提高，产物的BET增加，表明EMD的还原嵌锂反应具有自催化特性，自催化特性是导致这一类生成固相产物层的多相氧化还原反应能够持续进行的原因；二氧化锰具有比锰酸锂更好的亲水性，也使得反应过程中形成新的二氧化锰界面优先与溶液接触，促使液一固反应的进行。将二氧化锰还原法合成的锰酸锂前驱体按Li／Mn=0．5进行二次配方后高温烧结（800℃，10h），也得到了结晶度高、电化学性能优良的锰酸锂，其0．2C初始容量达132．7mAh·g^-1，0．5C初始容量为123．9mAh·g^-1。研究表明，采用多相氧化还原法制备超微粉体材料在技术上是可靠的，工艺上是可行，所得产品形貌和粒度易于控制，产品质量好，性能优良，且操作简便，生产成本低。其中超微四氧化三锰、锂离子电池正极材料钻酸锂超微粉体的制备已成功实现了产业化，分别建成了Mn₃O₄超微粉末5000t／a和LiCoO₂正极材料500 t／a的生产线，并取得了很好的经济效益。作为一种水溶液低温合成的软化学方法，多相氧化还原法有望推广应用于其他超微粉体材料的制备。更多还原

【Abstract】 An aqueous-based novel process , multiphase redox method, was conceived and successfully developed for the preparation of ultra-fine powder materials. It is a aqueous-based process to achieve the transformation of solid reactants into solid products through redox reaction at atmospheric condition,such as oxidation of Mn（OH）₂ to form Mn₃O₄. Furthermore some ions existing in the aqueous solution such as Li⁺ may participate in the reaction of redox to form new solid products during the multiphase transformation because of the high reactivity of the in situ produced new surface of the solid products. A typical example is the formation of LiMn₂O₄ from Mn（OH）₂ by oxidation in LiOH aqueous solution. In this paper, the mechanism for the preparation of ultra-fine Mn（OH）₂, Mn₃O₄, LiCoO₂, LiMn₂O₄ by the multiphase redox method was systematically investigated, and the technologies were well developed as well.From the study of the oxidative mechanism of metallic manganese in aqueous solution, it was found that the Mn（OH）₂ produced in pure water is a compact product tightly surrounding the unreacted metallic manganese, which impedes manganese from further reaction with H₂O. However, when there are some ammonia salts present in the aqueous solution as catalyzers, ammonium ion acts as the carrier of hydrogen ion in the reaction, and then the produced manganese ion is combined with ammonia molecules to give manganic ammonia complex ion, which carries manganese away from the surface of metallic manganese to the deeper solution. The manganic ammonia complex ion is then hydrolyzed to form loose and porous manganic hydroxide precipitates near the metallic manganese surface, which can easily fall off. The oxidation is, therefore, accelerated until the completion of the reaction.From the kinetic study of the formation of Mn₃O₄ from Mn（OH）₂ by multiphase oxidative process, it was shown that the reaction rate is controlled by oxygen diffusion. The activated energy of the reaction was measured to be 20.37kJ·mol^-1 in the temperature range from 40 to 60℃, and a linear relationship was established between the oxidation reactivity and the retention time. Further experimental study showed that there is a quantitative relationship between the oxidation degree Y（%） and the Mn²⁺/Mn⁴⁺ mole ratio of the slurry, and thus, a new method for the determination of the oxidation degree was conceived and developed by sampling an unquantitative amount of slurry and then analyzing the Mn²⁺/Mn⁴⁺ mole ratio. The pH method, as a simple and feasible way to accurately determine the oxidation end point, was further built up based on the observation that the pH of the slurry drops sharply at the end point with oxidation degree of 100%.The reaction of the synthesis of LiCoO₂ was achieved by multiphase redox method in aqueous solution at atmospheric condition by using Co（OH）₂ as starting cobalt source and air, O₂, H₂O₂ and NaClO₃ as oxidant. The mechanism of the formation of LiCoO₂ was revealed. It was found that LiCoO₂ with higher Li/Co mole ratio can be obtained by using air or O₂ as oxidant, and the Li/Co mole ratio increases with increasing temperature, extending retention time and raising Li⁺ concentration. The kinetic study revealed that the reaction of the formation of LiCoO₂ from Co（OH）₂ is conducted by two steps at temperature over 80℃: the first step is the fast oxidation to produce a mixture of LiCoO₂ and Co₃O₄, and then the second step is the slow oxidation of the produced CO₃O₄ to form LiCoO₂. Whereas at low temperature, there is only fast oxidation reaction and a mixture of LiCoO₂ and CoOOH is generated. The as-generated discontinuous product keeps the oxidation of Co（OH）₂ going on during the reaction. The aggregated state of the produced LiCoO₂ is greatly affected by the agitation. The LiCoO₂ precursor with a evenly chemical composition and homogeneous particle sizes as well as a good sintering reactivity was obtained under the optimal conditions. A well-crystallized high quality spherical LiCoO₂ positive material with a narrow particle size distribution and good manufacturing properties as well as a high density of >2.5g·cm^-3 was prepared as well and exhibited an excellent charge/discharge performance. The first discharge capacity of >149 mAh·g^-1 with charge/discharge efficiency of >97% was obtained. The average discharge capacity fade in the first eight cycles is only 0.04%.The synthesis of spinel LiMn₂O₄ was conducted in Mn（OH）₂-H₂O-LiOH system by multiphase redox method at atmospheric condition by using Mn（OH）₂ as manganese raw material, and the formation mechanism was studied as well. It was demonstrated by XRD, particle size and SEM measurements that the product LiMn₂O₄ is in situ formed as a discontinuous product layer on the surface of Mn（OH）₂ particles, which results in the reaction taking place continuously. The oxidizing capability of the oxidant has a great influence on the lithium content in the product. A suitable high temperature and appropriate high Li⁺ concentration favor the formation of LiMn₂O₄ with higher Li content. A linear relationship was found between the average manganese quantivalence and the oxidation time. By re-adjusting the Li/Mn mole ratio at Li/Mn=0.5 and then sintered at 820℃for 10 h, a well-crystallized spinel LiMn₂O₄ positive material with homogeneous particle size, smooth particle surface and good electrochemical properties was produced. The first discharge capacity was measured to be 119.2 mAh·g^-1. It demonstrated an excellent electrochemically cycle performance. A spinel LiMn₂O₄ material was also prepared from MnO₂ with hydrazine or sodium sulphite being a reductant in aqueous solution of LiOH at atmospheric condition via the multiphase redox method. The mechanism was investigated. From the microstructure measurement by XRD, particle size and SEM techniques, it was observed that the LiMn₂O₄ is in situ formed on the surface of MnO₂ particles. With an increase in the amount of inserting Li, the BET of the product increases, indicating the self-catalysis of the reduction of MnO₂, which is the main reason resulting in the mutiphase redox reaction occurred. The exposed fresh MnO₂ is easier to contact with solution than LiMn₂O₄ due to its better hydrophilicity. This is another main reason to cause such a multiphase redox reaction with a solid product generated taken place. By re-adjusting the Li/Mn mole ratio at Li/Mn=0.5 and then followed by sintering at 800℃for 10 h, a well-crystallized spinel LiMn₂O₄ positive material with a good electrochemical performance was prepared as well. The first discharge capacity was measured to be 132.7 mAh·g^-1 at 0.2C and 123.9 mAh·g^-1 at 0.5C, respectively.It was proved that the novel method of redox reactions accompanied by multiphase transformation for the preparation of ultra-fine powder materials is technically reliable and practically feasible. The products with high quality and excellent performance can be obtained with particle size and morphology being easily controllable. Simple operation and low production cost are, of course, the other distinct features of the technology. Two commercial production lines with capacities of 5000 t Mn₃O₄ and 500 t LiCoO₂ per year have been established based on the developed new technology, respectively, and a great financial benefit has been made in industry. There is still a great potential to improve the preparation of spinel LiMn₂O₄ and achieve the commercial production of LiMn₂O₄. It is of bright prospect to apply the new technology in the preparation of other ultra-fine powder materials.更多还原