【作者】 刘万民；

【导师】 胡国荣； 彭忠东；

【作者基本信息】 中南大学 ， 冶金工程， 2012， 博士

【摘要】 在锂离子电池富镍系LiNiO2基正极材料中,LiNi0.8Co0.15Al0.05O2作为LiNiO2、LiCoO2和LiAlO2三者的固溶体,同时具备了容量高,热稳定性好和价廉低毒等优点,被认为是能够取代LiCoO2的第二代绿色锂离子电池正极材料。但是,LiNi0.8Co0.15Al0.05O2仍然存在充放电过程中容量衰减较快、倍率性能不好和储存性能极差等缺陷,严重阻碍了其大规模的应用。为此,本文采用不同的方法对其进行制备和改性研究,以改善其电化学性能和储存性能。以控制结晶法制备的球形Ni0.8Co0.15Al0.05(OH)2.05为前驱体,采用加压氧化法制备了LiNi0.8Co0.15Al0.05O2正极材料。分析了LiOH·H2O用量、氧气压力、温度和时间等因素对材料结构和电化学性能的影响。在优化工艺条件下,所得LiNi0.8Co0.15Al0.05O2正极材料振实密度为2.57g·cm-3,在2.8～4.3V电位范围内,以0.2C进行充放电,首次放电比容量达到190mAh·g-1,30次循环后容量保持率为91%。采用共氧化-控制结晶法制备了球形Ni0.8Co0.15Al0.05OOH前驱体,详细讨论了Ni0.8Co0.15Al0.05OOH的形成机理,(NH4)2S2O8用量、pH值、温度和时间等因素对前驱体振实密度和镍离子平均氧化态的影响。结果发现,反应初期,以[Ni0.8Co0.5Al0.05(H2O)x-y(NH3)y]2.05+络离子的先沉淀后氧化为主,反应中后期,以[Ni0.8Co0.15Al0.05(H2O)x-y(NH3)y]2.05+络离子的先氧化后沉淀为主,最终得到Ni0.8Co0.15Al0.05OOH。在优化工艺条件下,所得前驱体振实密度达到1.88g.cm-3,镍离子平均氧化态为3。以该前驱体制备的LiNi0.8Co0.15Al0.05O2正极材料几乎不存在阳离子混排,层状结构最完整,显示出了优异的电化学性能,在2.8～4.3V电位范围内,以0.2C充放电,首次放电比容量达到196.8mAh·g-1,50次循环后容量保持率为96.1%。对熔盐法制备LiCoO2表相改性LiNi0.8Co0.15Al0.05O2正极材料作了系统研究。研究结果表明,优化工艺条件下,可在LiNi0.8Co0.15Al0.05O2基体材料表面均匀而又牢固地沉积一层厚度约为50nm的LiCoO2薄膜。3.0wt.%LiCoO2表相改性的LiNi0.8Co0.15Al0.05O2正极材料首次放电比容量高达196.2mAh·g-1,与基体材料相当；50次循环后容量保持率为98.7%,相对于基体材料的96.1%有所提高；而且,改性材料的倍率性能和高温性能均得到了提高。电化学阻抗谱分析表明,LiCoO2有效地覆盖在基体材料的表面,阻碍了Ni3+与电解液的直接接触,减少了Ni3+的还原机会,减少了NiO的生成量,降低了充放电过程中阻抗的增加幅度,改善了材料的电化学性能。而沉淀法制备的LiCoO2表相改性LiNi0.8Co0.15Al0.05O2正极材料,只有少部分LiCoO2沉积在基体材料的表面,大部分LiCoO2自行团聚在一起；相对于基体材料而言,改性材料的循环性能基本没有改变,放电比容量反而降低了。系统地研究了去离子水和乙醇洗涤及LiCoO2表相改性对LiNi0.8Co0.15Al0.05O2正极材料储存性能的影响。研究结果表明,采用低温水洗可以有效地除去LiNi0.8Co0.15Al0.05O2材料表面残留的含锂杂质,对材料的结构和放电比容量几乎不造成影响,且能小幅度地改善材料的储存性能,在空气中储存3个月后,材料的首次放电比容量由洗涤前的127.5mAh·g1增加至洗涤后的160mAh·g-1左右,30次循环后的容量保持率由洗涤前的66.5%增加至洗涤后的82%。水洗后的热处理温度对水洗材料的结构和储存性能有着显著的影响,450～600℃热处理材料因表面生成了NiO而提高了其储存性能,但是放电比容量降低了；而热处理温度过低或过高,所得材料与新制备材料显示出了相似的储存性能。储存过程中的空气湿度对新制备材料有着显著的影响,湿度越大,放电比容量和容量保持率下降越多；但是,空气湿度对低温水洗再450℃热处理过的材料的储存性能影响不大。新制备的LiNi0.8Co0.15Al0.05O2正极材料立即采用乙醇洗涤2次后,亦能有效地除去材料表面的锂杂质,与低温水洗的效果相当。熔盐法制备的3.0wt.%LiCoO2表相改性LiNi0.8Co0.15Al0.05O2正极材料,其储存性能得到了大幅提高。在相对湿度80%的空气中储存3个月后,基体材料的首次放电比容量只有127.5mAh·g-1,30次循环后的容量保持率仅仅66.5%；而改性材料的首次放电比容量高达186.6mAh·g-1,30次循环后的容量保持率达到95.3%,50次循环后的容量保持率仍有90%。XPS分析表明,LiCoO2改性层中的Co3+具有很好的化学稳定性,能耐环境中CO2和H2O的腐蚀；覆盖在基体材料表面后有效地抑制了Ni3+的还原,大大降低了NiO和Li2CO3的生成量,从而改善了LiNi0.8Co0.15Al0.05O2的储存性能。更多还原

【Abstract】 Among the nickel-rich LiNiO2-based cathode materials for lithium-ion batteries, LiNi0.8Co0.15Al0.05O2has been considered as the substitution for LiCoO2in that it owns the advantages of LiNiO2, LiCoO2and LiAlO2, such as high capacity, good thermal stability, low cost and low toxicity. However, the loss of capacity during cycling, insufficient rate capability and poor storage performance have seriously hindered its application in large scale. In this dissertation, in order to improve its electrochemical properties and storage performance, the LiNi0.8Co0.15Al0.05O2cathode material was prepared and modified using different methods.The LiNi0.8Co0.15Al0.05O2cathode material was synthesized under the elevated oxygen pressure from the spherical Ni0.8Co0.15Al0.05(OH)2.05precursor prepared by a controlled crystallization method. The effects of molar ratio of Li to Ni+Co+Al, oxygen pressure, temperature and time on the structure and electrochemical properties of LiNi0.8Co0.15Al0.05O2were examined. The spherical LiNi0.8Co0.15Al0.05O2cathode material with tap density of2.57g·cm-3was prepared under the optimum condition. The initial discharge specific capacity of these powders was190mAh·g-1at0.2C in the voltage range of2.8-4.3V, and91%of the initial discharge capacity was maintained after30cycles.The spherical Ni0.8Co0.15Al0.05O2OOH precursor was synthesized by a co-oxidation-controlled crystallization method. The effects of (NH4)2S2O8, pH value, temperature and time on the precursor’s tap density and nickel ions’ average oxidation state, as well as the precursor’s forming mechanism were studied systematically. In the initial stage, the [Ni0.8Co0.15Al0.05(H2O)x-y(NH3)y]2.05+ions were precipitated and later oxidized. In the middle and final stages, the [Ni0.8Co0.15Al0.05(H2O)x-y(NH3)y]2.05+ions were oxidized and later precipitated. Eventually, Ni0.8Co0.15Al0.05OOH with tap density of1.88g·cm-3and nickel ions’ average oxidation state of three was obtained. The LiNi0.8Co0.15Al0.05O2cathode material prepared from this Ni0.8Co0.15Al0.05OOH precursor had the best-ordered hexagonal layer structure and least cation mixing. The charge-discharge tests demonstrated that these powders exhibited excellent electrochemical properties, with an initial discharge capacity of196.8mAh·g-1and capacity retention of96.1%after50cycles when cycled at a current density of0.2C between2.8and4.3V.The LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material was prepared successfully via a molten salt method. The results showed that a uniform LiCoO2layer with a thickness of～50nm was coated on the surface of LiNi0.8Co0.15Al0.05O2under the optimum condition. The3.0wt.%LiCoO2-coated LiNi0.8Co0.15Al0.05O2showed an initial discharge capacity of196.2mAh·g-1and capacity retention of98.7%after50cycles. While the corresponding values of the bare LiNi0.8Co0.15Al0.05O2were196.8mAh·g-1and96.1%, respectively. Moreover, the coated sample exhibited better rate capability and high-temperature performance than the bare sample. The electrochemical impedance spectroscopy analyses indicated that the LiCoO2layer coated on the LiNi0.8Co0.15Al0.05O2hindered effectively the direct contact between Ni3+and electrolyte, and decrease of NiO amount on the coated material reduced impedance during charge and discharge, which improved the electrochemical performance of LiNi0.8Co0.15Al0.05O2. On the other hand, the LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material prepared by a precipitation method showed lower discharge capacity than the pristine LiNi0.8Co0.15Al0.05O2and the same cycle performance as the bare sample, which was attributed to the formation of the mixture of LiCoO2and LiNi0.8Co0.15Al0.05O2, with a few LiCoO2coated on LiNi0.8Co0.15Al0.05O2.The effects of washing LiNi0.8Co0.15Al0.05O2with deionized water and ethanol, and surface coating with LiCoO2on its storage performance were systematically studied. The results showed that washing with low-temperature deionized water could effectively eliminate the lithium impurity (LiOH and Li2CO3) on the surface of LiNi0.8Co0.15Al0.05O2powders, and improve slightly its storage capability without changing structure and discharge capacity. After storage in air for three months, the initial discharge specific capacity of the fresh LiNi0.8Co0.15Al0.05O2material was127.5mAh·g-1, and the capacity retention was66.5%after30cycles. While the corresponding values of the washed sample were160mAh·g-1and82%, respectively. The heat-treatment temperature after washing had obvious influence on the structure and storage capability of washed LiNi0.8Co0.15Al0.05O2. After heat treatment under450-600℃, the storage performance of the material was enhanced due to the formation of NiO on the material surface, but the discharge capacity was decreased. However, the cathode material treated under higher than600℃or lower than450℃showed the similar storage performance with the fresh sample. In addition, the air humidity during storage had remarkable effects on the fresh LiNi0.8Co0.15Al0.05O2material. The higher the humidity was, the more the discharge capacity and capacity retention decreased. However, the moisture had little influence on the storage capability of the material treated with washing and heat treatment under450℃. More meaningfully, washing twice the fresh LiNi0.8Co0.15Al0.05O2material with ethonal showed the same effect as the said washing with low-temperature water.The storage performance of the3.0wt.%LiCoO2-coated LiNi0.8Co0.15Al0.05O2cathode material prepared by the molten salt method was substantially enhanced. After storage in air with80%relative humidity for three months, the bare LiNi0.8Co0.15Al0.05O2material delivered an initial discharge capacity of127.5mAh·g-1and the capacity retention was66.5%after30cycles. Contrastively, the initial discharge capacity of the LiCoO2-coated LiNi0.8Co0.15Al0.05O2material was186.6mAh·g-1, and the capacity retention was95.3%after30cycles,90%after50cycles, respectively. XPS analyses demonstrated that Co3+in the LiCoO2coating layer has excellent chemical stability and can resist the erosion of CO2and H2O in the air. Thus, the reduction of Ni3+to Ni2+on the surface of LiNi0.8Co0.15Al0.05O2was effectively suppressed by the LiCoO2coating, and the formation amounts of NiO and Li2CO3were decreased greatly, which was responsible for the improvement of storage property of LiNi0.8Co0.15Al0.05O2.更多还原