| It is always the main research direction for lithium ion battery to improve its performance and reduce the cost of electrode materials. Spinel LiMn2O4, with high working voltage, good safety performance, low production cost, and environmental friendliness, is becoming one of the most promising cathode materials for lithium-ion battery in the 21st century. But its low specific capacity and poor electrochemical cycling performance limits its large-scale application. Therefore, the key to extensive application is to improve the electrochemical performance of LiMn2O4 materials and to explore new synthesis and modification methods, it is becoming a research hotspot for spinel LiMn2O4 materials. In order to obtain spinel LiMn2O4 materials with excellent physical and electrochemical performance, this paper, from the perspective of industrialization consideration, adopts the method of slurry spray drying to prepare spherical chromium doped spinel lithium manganese oxide, and conducts researches on some surface coating modification.Firstly a preliminary study about the process conditions of slurry spray drying for spherical lithium manganese oxide is carried out in this paper. The results show that using starch as a binder, spray dried particles gain good sphericity, hard to be broken. Controling solid content at 50%, the slurry is even with good fluidity, and the average size of spray dried particles is about 20?m. The controlling of the temperature during the spray drying process exerts a direct influence on the physical properties of dried samples. The optimal roasting temperature for lithium manganese oxide from dried spherical precursor is 770?, where the initial discharge capacity of the obtained samples is 122.1 mAh·g-1, with an 89.8% capacity retention after cycling for 30 times. Compared with common drying methods, the spherical products prepared by slurry spray drying have great advantages in morphology, electrochemical impedance, discharge capacity and cycling properties than by common drying.Secondly, the effects of different manganese sources on the properties of lithium manganese oxide prepared by slurry spray drying are studied systematically in this paper. Structure and component of electrolytic MnO2 (EMD) are optimized through heat and acid treatment respectively. The results show that after proper heat treatment at 300-400?, the structure of EMD can be optimized, the lithium intercalation and the successful transformation to lithium manganese oxide are facilitated, and the initial discharge capacity and cycle performance of the obtained products are improved. Impure contents in EMD such as sodium and sulfur can be reduced effectively by acid treatment, and so can the impure contents in produced lithium manganese oxide. While the initial discharge capacity of the product is improved, the cycle performance needs to be enhenced. Refined structure of chemical manganese dioxide (CMD) is similar to that of EMD after heat treatment at 360?. The initial discharge capacity of spinel lithium manganese oxide obtained from CMD is 120.0 mAh·g-1, and the discharge capacity is 100.8 mAh-g-1 after cycling for 40 times, a capacity retention of 84.0%. Electrochemical performance of lithium manganese oxide obtained from Mn3O4 is excellent, initial discharge capacity at 0.2C is 127.9 mAh·g-1, and discharge capacity at 2C is 124.7 mAh·g-1, with an 84.0% capacity retention after cycling for 40 times. But its physical performance is dissatisfactory. Compared physical and electrochemical performance of lithium manganese oxide prepared from EMD, CMD, and Mn3O4, EMD after heat treatment at 300-400?is the best manganese source.Then spherical chromium doped spinel lithium manganese oxide prepared by slurry spray drying method is studied thoroughly. It is found that chromium ion from soluble chromium compounds such as chromium acetate and chromium sulfate can dipping into the pore of manganese dioxide, so that even precursor can be formed in the mixing process. Therefore, compared with that from chromium oxide, products obtained from soluble chromium compounds have smaller crystal lattice parameters and volume, and smaller charge transfer impedance and electrochemical polarization. With chromium acetate as a chromium source and the chromium content x= 0.04, the obtained lithium manganese oxide show best high-temperature electrochemical performance. The initial discharge capacity is 117.2 mAh·g-1, and the capacity retention is 93.3% after cycling for 30 times when charging and discharging at 55?.Finally, in order to further improve the cycle performance of lithium manganese oxide under high temperature, three coating methods are adopted. Among them, the water sol-gel coating method is low in cost and friendly to environment, and can produce well-coated products. The initial discharge capacity of products coated with 1% Al2O3 by the water sol-gel method is 114.5 mAh·g-1, and capacity retention is 93.6% after cycling for 50 times at high temperature. The low-heating solid-phase method is simpler in process, and lower in energy consumption, thus more suitable for industrialization. With the same Al2O3 coating content at 1%, the initial discharge capacity for coated products by this method is 115.6 mAh·g-1, and capacity retention after cycling for 50 times is 92.8% at high temperature. Besides coating with oxide, this paper also makes an initial try to coat phosphate by polyol synthesis method, which achieves good results. Phosphate-coated materials show better and much improved cycle stability at high temperature. Among them is lithium cobalt phosphate coated chromium doped lithium manganese oxide, whose initial discharge capacity is 117.8 mAh·g-1, and the capacity after 50 times is 114.0 mAh·g-1, a capacity retention of 96.8% at high temperature. The initial discharge capacity of uncoated materials is 117.1 mAh·g-1, with capacity retention at 90.3%. Compared with this, coated materials gain a great improvement in their cycling stabilization performance. |
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