| 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.
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