【作者】 施少君；

【导师】 涂江平；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 本文主要以三元氧化物正极材料及由其衍生出的富锂锰基正极材料为研究对象,针对材料体系本征电导率低、倍率性能差、循环稳定性差等缺点,分别通过合成方法的创新改进、表面修饰及电极材料微结构的控制等方法来提高材料的电化学性能。主要研究内容与结果如下：采用简化的共沉淀法制备了具有片状一次颗粒的LiMn0.4Ni0.4c0.2O2层状正极材料。研究了合成温度与材料电化学性能的关系,其中800℃煅烧合成的材料具有80-100nm的片层厚度和最好的电化学性能,其在5C(1400mA g-1)的大电流下放电容量达到160mAhg-1,并且在经过50次循环后,容量保持率高达80%。优良的电化学性能主要是由于其具有很好的结晶性和薄片状一次颗粒。经GITT测试计算得锂离子扩散系数为10-11-10-12cm2s-1。采用简单的湿化学法对LiNi1/3Co1/3Mn1/3O2正极材料进行了表面氟化物的修饰改性处理。所采用的氟化物为CaF2和LiF。高分辨TEM下观察所包覆CaF2层厚度为4-8nm,而LiF则发生了表面氟掺杂。CaF2包覆后材料在室温下的循环稳定性提高,0.1C下50循环后容量保持率为98.1%,即使是在5C大电流下,50循环后容量保持率也在85%以上。而LiF修饰后材料室温下lO C放电容量达137mAh g-1,在60℃温度下,1C循环50次后,容量保持率为93.5%。低温下性能也很突出,在-20℃时0.1C放电容量达85.6mAh g-1, EIS研究发现,经过氟化物修饰后,LiNi1/3Co1/3Mn1/3O2电极电荷转移阻抗得到了明显的抑制,这是材料电化学性能提高的重要原因。采用乙醇同时作为液相燃烧法中的溶剂和助燃剂,合成了富锂锰基正极材料Li[Li0.2Mn0.54Ni0.13Co0.13]O2。比较了不同温度下合成的材料的形貌和电化学性能。800℃煅烧得到的材料颗粒尺寸为50-150nm,倍率性能优异,再200和2000mAg-1电流密度下放电容量达到了238.6和165.0mAh g-,且具有较好的容量保持率。此外,以蔗糖为溶胶凝胶的络合剂,采用不同的金属源合成了富锂锰基Li[Li0.2Mn0.56Ni0.16Co0.08]O2正极材料。研究了不同金属源对所得材料的结构形貌及电化学性能的影响。采用硝酸盐为金属源所制备的Li[Li0.2Mn0.56Ni0.16Co0.08]O2正极材料具有三维多孔形貌,其表面积高达10.09m2g-1。在200mAg-1和2000mA g-1电流密度下放电容量达到了247.8mAh g-1和135.5mAh g-1.采用融熔注入法对Li[Li0.2Mn0.54Ni0.13Co0.13]O2富锂锰基正极材料进行了表面MgO修饰处理究。当MgO包覆量为2wt.%时,Li[Li0.2Mn0.54Ni0.13Co0.13]O2材料具有最好的电化学性能。在室温200mA g-1电流密度下,100次循环后容量保持率为96.4%。当工作温度提高到60℃时,在同样的电流下经过50次循环后,电池的容量保持率也高达94.3%。同时对Li[Li0.2Mn0.56Ni0.16Co0.08]O2富锂锰基正极材料进行了表面稀土氧化物Sm203修饰的研究。Sm203表面修饰同样主要起到了改善Li[Li0.2Mn0.56Ni0.16Co0.08]O2正极材料的循环稳定性的作用。在室温下2.0-4.8V电压区间200mA g-1的电流密度下,经过80次循环后仍具有214.6mAhg-1的容量,容量保持率高达91.5%。采用气凝胶模板法制备了多孔Li[Li0.2Mn0.54M0.13Co0.13]O2正极材料。800℃煅烧获得的材料在200mA g-1和2000mA g-1电流密度下放电容量达到了244.0mAh g-1和153.9mAh g-1。但是其循环性能欠佳。而900℃煅烧获得的材料虽然颗粒长大明显,比表面积有所下降,但是由于多孔结构的存在,也表现出良好的倍率性能,且循环稳定性提高。在200mAg-1电流密度下其放电容量容量达220.2mAh g-1。即使在2000mA g-1的大电流下,其容量也高达129.8mAh g-1,并且在120个循环后容量几乎没有衰减。对Li[Li0.2Mn0.54Ni0.13Co0.13]O2进行了融熔盐处理,发现经LiCl融熔盐和KCl融熔盐处理后颗粒分别呈类立方体状和薄片状。KCl融熔盐处理后的材料具有高达17.05m2g-1的比表面积及优异的大电流放电性能。在200mA g-1和2000mAg-1电流下放电容量可分别达到254.1mAh g-1和168.5mAh g-1。但是其容量衰减过快。而经LiCl融熔盐处理后的材料虽然放电容量略微下降,但是循环稳定性得到了大幅提高,在200mA g-1电流下经过80次循环其容量保持率高达94.9%。研究发现,这种现象与材料的锂离子扩散系数无关,主要由材料的组织调控。采用碳酸盐二元模板法制备了具有空心立方结构二次颗粒的新型Li[Li0.2Mn0.50Ni0.05Co0.25[O2正极材料。该空心结构的形成,有效的提高了材料的比表面积、缩短了锂离子扩散路径,从而提高了材料在大电流下的放电容量。在200mA g-1和2000mA g-1电流下放电容量可分别达到208mAh g-1和110mAhg-1。二元模板法是一种很好的获得不同形貌的富锂锰基锂离子电池正极材料的有效方法。更多还原

【Abstract】 The research work of this thesis is mainly based on the layered oxide and its derivation called Li-rich manganese based layered oxide cathode materials for Li-ion batteries. Due to the poor rate capability and cycle stability, new synthesis, surface modification and morphology adjustment are performed to improve the electrochemical performances. The main research contents and results are as follows:Ball-like LiMn0.4Ni0.4Co0.2O2particles composed of flakes are synthesized by a simplified co-precipitation method. The relationship between the sintered temperature and electrochemical performance of the layered oxides is investigated. The layered oxide with a flake thickness of80-100nm synthesized at800℃has the best electrochemical performance. An initial discharge capacity of160mAh g-1is obtained at5C (1400mA g-1) in the voltage range of2.5-4.5V, and the capacity retention is80%after50cycles. The excellent rate capability is attributed to the good crystallinity and the formation of flake primary particles. In addition, a detailed study of diffusion coefficient of Li+(DLi+) is carried out to further understand this cathode material. The value of DLi+calculated is in the range of10-11-10-12cm2s-1CaF2and LiF modified LiMn1/3Ni1/3Co1/3O2cathode materials are prepared via a wet chemical process. The well coated CaF2layer on the LiMn1/3Ni1/3Co1/3O2particle has a thickness of4-8nm determined by High-resolution transmission electron microscopy (TEM) analysis. However, surface F-doping is observed after LiF modification. Cycling stability of the CaF2-coated LiMn1/3Ni1/3Co1/3O2is improved distinctly. Discharge capacity retention of98.1%is obtained at0.1C after50cycles. Furthermore, even at a high charge-discharge rate of5C, the capacity retention maintains above85%. The LiF-modified oxide delivers a high discharge capacity of137mAh g-1at10C at room temperature and exhibits capacity retentions93.5%at1C at60℃after50cycles. Furthermore, it has reversible capacities of85.6mAh g-1at0.1C at-20℃. Electrochemical impedance spectroscopy (EIS) shows that the modified layer stabilizes the surface structure and reduces the charge transfer resistance, resulting in the improved electrochemical performances.Li-rich manganese based layered oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2are synthesized by combustion reaction using alcohol as both solvent and fuel. After comparing the morphology and electrochemical performances of the layered oxides synthesized at different temperatures, the oxide synthesized at800℃with particle size of50-150nm exhibits the best electrochemical performances. High discharge capacities of238.6and165.0mAh g-1are obtained at current densities of200and2000mA g-1in the voltage range of2.0-4.8V, respectively. In addition, Li[Li0.2Mn0.56Ni0.16Co0.08]O2cathode materials are synthesized by sol-gel process with sucrose as complexing agent. The effect of morphology on the electrochemical performances is studied under the using of different metal sources. Porosity with high specific surface area of10.09m2g-1is only observed for the oxide powder synthesized with nitrate. Simultaneously, high discharge capacity of247.8mAh g-1and135.5mAh g-1are obtained at current densities of200mA g-1and2000mA g-1, respectively.MgO-coated Li[Li0.2Mn0.54Ni0.13Co0.13]O2are synthesized via melting impregnation method followed by a solid state reaction. The2wt.%MgO coated cathode exhibits the excellent cycling stability with capacity retention of96.4%at a current density of200mA g-1after100cycles at room temperature, and94.3%after50cycles at60℃. Simultaneously, Sm2O3-modified Li[Li0.2Mn0.56Ni0.16Co0.08]O2is also synthesized via a simple wet chemical process. Similar effect is obtained as to improve the cycle stability of the cathode. After Sm2O3surface modification, high discharge capacity of214.6mAh g-1with retention of91.5%is obtained at a current density of200mA g-1between2.0V and4.8V after80cycles.Macroporous Li1.2Mn0.54Ni0.13Co0.13O2cathode materials with high crystallinity and hexagonal ordering are synthesized by aerogel template. The Li-rich layered oxide synthesized at800℃delivers high discharge capacities of244.0mAh g-1and153.9mAh g-1at current densities of200mA g-1and2000mA g-1between2.0V and4.8V, respectively. However, the cycle stability is unsatisfactory. Increasing the synthesis temperature to900℃, the particles grows with a decrease of surface area. However, the macroporous Li1.2Mn0.54Ni0.13Co0.13O2delivers a high discharge capacity of220.2mAh g-1at a current density of200mA g-1,129.8mAh g-1at a current density of2000mAg-1and almost no capacity fading after120cycles.Li[Li0.2Mn0.54Ni0.13Co0.13]O2cathode materials are treated in molten salts. Cube-like and plate-like particles are obtained after treated in LiCl and KCl molten salts at800℃, respectively. High discharge capacities of254.1mAh g-1and173.4mAh g-1are obtained at current densities of200mA g-1and2000mA g-1, respectively, for the oxide treated in KCl molten salt with large specific area of17.05 m2g1. However, the cycle stability is poor. In addition, enhanced cycle stability with capacity retention of98.7%after50cycles at1C is obtained for the oxide treated in LiCl molten salt with sacrifice of a little capacity. Such electrochemical performance change is proved to be independent of Li+diffusion coefficient through GITT.Hollow Li1.2Mn0.5Co0.25Ni0.05O2microcube is synethesized through a simple binary template method. Such special morphology efficiently increases the surface area, decreases the path of Li+diffusion, and then improves the electrochemical performances at high current density. High reversible discharge capacities of208mAh g-1and110mAh g-1are obtained at a current density of200mA g-1and2000mA g-1, respectively. It is remarkable that such binary template method is an effective way to obtain Li-rich manganese based layered cathode material with specific morphology.更多还原