【作者】 张进；

【导师】 曹高劭； 赵新兵；

【作者基本信息】 浙江大学 ， 材料科学与工程， 2008， 硕士

【摘要】 层状结构的含镍、钴、锰的新型三元金属嵌锂化合物具有比容量高、结构和热稳定性良好等特点,被认为是有望替代LiCoO₂的锂离子电池正极材料,成为近年来研究的热点。本文以提高锂离子电池正极材料LiNi_0.4Co_0.2Mn_0.4O₂的循环性能为主要目的,分别采用熔融浸渍法和固相反应法制备了表面有氧化物和LiAlO₂包覆的LiNi_0.4Co_0.2Mn_0.4O₂材料,并研究了其电化学性能。采用熔融浸渍法,以Al（NO₃）₃·6H₂O、Zn（NO₃）₂·6H₂O和Mg（NO₃）₂·6H₂O为包覆源,制备了表面有Al₂O₃、ZnO、MgO包覆的LiNi_0.4Co_0.2Mn_0.4O₂正极材料,研究了不同的包覆量（1 wt%、2 wt%、3 wt%、5 wt%）对材料电化学性能的影响。氧化物包覆后在LiNi_0.4Co_0.2Mn_0.4O₂表面形成了一层不均匀的絮状包覆物,同时材料的晶格常数发生了改变,包覆起到了表面修饰和部分掺杂的双重效果;3 wt%的Al₂O₃包覆后晶格常数改变最大,但包覆量为1 wt%的试样具有最佳的倍率性能和循环性能,表明材料内部晶格的稳定和包覆层的厚度都是影响材料性能的重要原因;包覆后虽然降低了初始容量,但却明显提高了LiNi_0.4Co_0.2Mn_0.4O₂的倍率和循环性能,1 wt%的Al₂O₃包覆试样在160 mA·g^-1的电流密度下循环300次后,还有104 mAh·g^-1的比容量,次衰减率为0.108%,明显低于试样RM的0.139%;通过对Al₂O₃、ZnO、MgO的循环特性进行对比,表明包覆材料本身的特性是影响LiNi_0.4Co_0.2Mn_0.4O₂电化学性能的重要原因。采用固相反应法,以LiNO₃和Al（OH）₃作为前驱反应物,成功地在LiNi_0.4Co_0.2Mn_0.4O₂表面包覆了一层不均匀的絮状包覆物α-LiAlO₂,用溶胶-凝胶法也获得了一层更加均匀的LiAlO₂,但是前者具有最佳的电化学性能;3 wt%的LiAlO₂包覆后明显提高了LiNi_0.4Co_0.2Mn_0.4O₂的循环稳定性和倍率性能,在1C和3C倍率分别获得了142 mAh·g^-1和108 mAh·g^-1的比容量,1C循环100次后次衰减率由包覆前的0.19%下降到0.14%,55℃循环150次的次衰减率由包覆前的0.19%下降到包覆后的0.15%。EIS分析表明,包覆层的存在导致SEI膜电阻略有增加,但电池循环过程中的电荷传递阻抗的增加却得到了有效抑制,因此减缓了总阻抗的增加,提高了循环稳定性。对影响LiNi_0.4Co_0.2Mn_0.4O₂材料电化学性能的因素进行了探讨,表明采用恒压步骤有利于抑制循环过程中的容量衰减,材料在3.0～4.3 V范围内具有最佳的稳定性,但容量较低,升高充电截止电压,容量上升但循环稳定性略有下降;表面包覆层改善材料循环性能是因为它可以吸收部分应力,缓和充放电过程的体积变化,从而抑制电池多次循环后的极化,提高循环稳定性。具体表现为多次循环后:CV图上表现为氧化/还原峰位差的减小;EIS图谱上表现为SEI膜电阻的增加,但多次循环后的总Z′值大大减小,充电过程中R_SEI和Q_SEI单调减小,SEI膜厚度随电极电位的升高而变薄,电荷转移电阻R_ct随着电位的升高先减小后增加,在4.0 V左右达到最小值,表明此时Li⁺脱出受到的阻碍最小;试样RM颗粒间脱离接触,电极片上出现许多空隙,致使材料导电性能变差,但经包覆后的试样多次循环后颗粒之间及颗粒与导电剂乙炔黑之间仍然接触良好,形貌变化较小。更多还原

【Abstract】 The layer-structured Nickel,Cobalt,Manganese as lithium insertion compounds with a high capacity,structure and thermo-stability have been extensively studied as possible alternatives to LiCoO₂.With the aim of improving its cycling properties,in the present work, melting impregnation method and solid section method were used to coat metal oxides and LiAlO₂ on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ cathode materials,respectively. Al₂O₃,ZnO and MgO coated LiNi_0.4Co_0.2Mn_0.4O₂ materials were synthesized by melting impregnation method using Al（NO₃）₃·6H₂O,Zn（NO₃）₂·6H₂O and Mg（NO₃）₂·6H₂O as coating resources.The effect of electrochemical properties with different coating amount of such materials has been studied.The results show that the surface of coated LiNi_0.4Co_0.2Mn_0.4O₂ particles are covered by a layer of asymmetric nano-oxides,the lattice parameters increased as well and this leads to a double effect of surface modification and partial doping;The 3 wt% Al₂O₃ coated samples have the largest lattice parameters but the 1 wt%Al₂O₃ coated ones have the best rate ability and cycling properties,which indicates that the stability of the crystal structure together with the thickness of the coating layer are important factors to the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂;Although decreasing the initial capacity for the coated ones,the rate ability and cycling stability has increased evidently,the 1 wt%Al₂O₃ coated ones have got a discharge capacity of 104 mAh·g^-1after 300 cycles at a current density of 160mA·g^-1,the capacity fading rate decreases from 0.139%per cycle to 0.108%per cycle for the coated ones;The characters of the coating material itself was the main factor to influence the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂ after comparing the cycling properties of the Al₂O₃,ZnO and MgO coated ones.Solid reaction method was used to coatα-LiAlO₂ on the surface of LiNi_0.4Co_0.2Mn_0.4O₂, a flocculent layer ofα-LiAlO₂ formed on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ particles after baking with the mixture of LiNO₃ and Al（OH）₃.The sol-gel method can also get a homogeneous on the surface of LiNi_0.4Co_0.2Mn_0.4O₂ particles,but the former has a better electrochemical properties.The 3 wt%coated ones show an effective improvement of the rate ability and cycling performance of LiNi_0.4Co_0.2Mn_0.4O₂,we have got the capacity of 142 mAh·g^-1and 108 mAh·g^-1when cycling at 1C and 3C rate.The room temperature test at 1 C after 100 cycles shows that the capacity fading rate decreases from 0.19%per cycle to 0.14%, high temperature tests at 55℃show that the capacity fading rate decreases from 0.19%to 0.15 after cycling at 1C rate for 150 times.Experimental impedance spectra（EIS）analysis indicates that the existence of the coating layer leads to the increase of the SEI resistance,but the increase of the charge transfer resistance have been restrained effectively thus slow down the increase of the total resistance.The factors that influence the electrochemical properties of LiNi_0.4Co_0.2Mn_0.4O₂ have been discussed,the results show that the constant voltage step during charge process could restrict the capacity loss in the cycles thus prolong the cycle life of the cells;The discharge capacity of LiNi_0.4Co_0.2Mn_0.4O₂ increase linearly with the raise of the upper cut-off voltage limit,but the cycling stability decreased.The existence of the coating layer increases the surface resistance,however,it can absorb the stress and relax the volume change during the charge and discharge process thus restricts the polarization during the cycles and enhances the cyclic stability.After cycles,with the effect of the coating layers,cyclic voltammogram（CV） indicates that the remove of the anodic and cathodic peaks is small,EIS results shows that the SEI film resistance increased a little,but the total Z’ decreased a lot;the SEI film resistance (R_SEI)and SEI film capacitance(Q_SEI)decrease linearly during the charge process which indicate the thickness of SEI film decreased;the charge transfer resistance(R_ct)first decreased and then increased,the lowest point at around 4 V indicates the smallest obstacle of delithiation.It could see from the composite electrodes that the contact of the LiNi_0.4Co_0.2Mn_0.4O₂ particles,the active particles and the conducting blinder are very well and the morphology change is small after cycles.更多还原