【作者】 蒋庆来；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 电化学工程， 2011， 博士

【摘要】 提高电池性能和降低电极材料的成本一直是锂离子电池的主要研发方向。尖晶石型LiMn2O4具有工作电压高、安全性能好、生产成本低、对环境友好等特点,是21世纪最具发展前景的锂离子电池正极材料之一。但尖晶石型LiMn2O4的比容量较低,电化学循环性能较差阻碍其规模化应用。所以如何通过改性提高尖晶石LiMn2O4材料的电化学性能是其得以广泛应用的关键。因此,探寻新的合成及改性方法成为了尖晶石LiMn2O4材料改性研究的热点。为了获得物理加工和电化学性能优异的锰酸锂正极材料,本文从工业化角度出发,提出采用浆料喷雾干燥法制备球形掺铬锰酸锂,并进行了系列表面包覆改性研究。首先对浆料喷雾干燥法制备球形锰酸锂的工艺条件进行了初步研究。结果发现,采用淀粉做为粘结剂时,喷雾干燥后样品颗粒球形度好,不易破碎；控制制浆固含量为50%时,所得浆料均匀、流动性好,得到喷雾干燥产物粉体平均颗粒大小在20μm左右；喷雾干燥过程中温度的控制直接影响到干燥效果和干燥样品的物理性质。干燥后得到球形前驱体制备锰酸锂的最佳焙烧温度为770℃,此时得到样品的初始放电容量为122.1 mAh·g-1,循环30次后容量保持率为89.8%。与普通干燥法相比,浆料喷雾干燥法制备的球形材料在形貌、电化学阻抗、放电容量和循环性能等方面有较大的优势。其次,系统研究了不同锰源对浆料喷雾干燥法制备锰酸锂性能的影响。通过热处理和酸处理的方式对现有电解二氧化锰(EMD)进行结构和组份优化。结果发现,将EMD在300-400℃进行适当的热处理,可以优化其结构,使其有利于嵌锂并顺利实现向锰酸锂的转化,得到的产物在初始放电容量和循环性能上都有所提高。而酸处理能有效减少EMD中钠、硫杂质的含量,使产物锰酸锂中杂质含量也降低,产物的初始放电容量得到提高,但是循环性能还需要改善。精制化学二氧化锰(CMD)的结构与360℃热处理后EMD相似。CMD制备得到尖晶石锰酸锂的初始放电容量为120.0 mAh·g-1,循环40次后放电容量保持为100.8 mAh·g-1,容量保持率为84.0%。四氧化三锰做锰源制备的锰酸锂电化学性能最佳,0.2C首次放电容量达127.9mAh·g-1,2C放电容量为124.7 mAh·g-1,40次充放电循环后容量保持率达到97.6%,但是物理性能相对较差。综合比较EMD、CMD和四氧化三锰等锰源制备的锰酸锂的物理和电化学性能,300-400℃热处理后EMD应用前景最佳。然后深入研究了浆料喷雾干燥法制备掺铬锰酸锂,发现采用醋酸铬和硫酸铬等可溶性铬盐,铬离子可以浸渍进入二氧化锰空隙,在混合过程即可形成均匀的前驱体,因此得到的样品与采用三氧化二铬制备的样品相比晶格参数和晶胞体积小,电荷转移阻抗小,电化学极化小。使用醋酸铬为铬源,掺铬量x=0.04时,制得的锰酸锂高温电化学性能最佳,在55℃条件下进行充放电测试,得到初始放电容量为117.2 mAh·g-1,循环30次后容量保持率为93.3%。最后,为了更进一步改善锰酸锂的高温循环性能,分别采用三种包覆方法对上述制得的球形掺铬锰酸锂进行了表面包覆改性研究。其中,水溶胶法包覆氧化铝,成本低,环境友好,得到产物包覆效果好,包覆1%的Al2O3得到的包覆产物初始放电容量为114.5 mAh·g-1,高温循环50次后容量保持率为93.6%。低热固相法包覆氧化铝工艺更简单,能耗更低,更适合于工业放大,同样包覆1%的Al2O3得到的包覆产物初始放电容量为115.6 mAh·g-1,50次高温循环容量保持率为92.8%。除了包覆氧化物外,还首次对多元醇法包覆磷酸盐进行了尝试,并取得了较好的成效,磷酸盐包覆材料表现出更好的稳定性,使得材料的高温循环稳定性得到较大提高,其中包覆了磷酸钴锂的球形掺铬锰酸锂样品,其首次放电比容量为117.8 mAh·g-1,经50次高温循环后放电比容量仍然保持为114.0 mAh·g-1,容量保持率提高为96.8%。相比未包覆材料初始放电容量为117.1 mAh·g-1,50次高温循环后容量保持率只有90.3%,包覆后材料的循环稳定性能都得到较大提高。更多还原

【Abstract】 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 20μm. 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.更多还原