快离子导体表面修饰Cr掺杂型层状-尖晶石复合固溶体正极材料研究

Layer-spinel composite cathodes Li0.5+xNi0.25Mn0.75O2+x/2 possess high capacity, higher first colomb effiency and better cyclic performance. However, the layered structure transformation, higher cation disorder, low electronic conductivity and lithium diffusivity hinder the practical application of layer-spinel composite cathodes. In this project, nano-size Li4Ti5O12 surface modification combined with Cr doping have been used to improve the electrochemical performance of Li0.5+xNi0.25Mn0.75O2+x/2. Firstly, the conductivity on the material surface can be enhanced with nano-Li4Ti5O12 modificaition and core/shell mutual doped. Secondly, the cation disorder,content of Mn3+ and conductivity of bulk material can be improved with Cr doping. Thirdly, the mutual doping of Ti、Ni、Mn can also improve the electrochemical performance of Li0.5Ni0.25-yMn0.75-yCr2yO2+x/2 and Li4Ti5O12. Moreover, the layered structure transformantion can be restrained by forming steady Li2TiO3 in the surface layer with inleakaged Ti and controlled Mn3+ and cation disorder with doped Cr. This study shows the law and mechanism how the Li4Ti5O12 surface modification and Cr doping influence the lithium ion transport and electronic conductivity, how the Ti、Ni、Mn(Cr) enter into the lattices of core and shell, how the layered structure transformation is restrained. These all can provide new ideas and theoretial guidance for development and applicaion of new cathode materials for lithium ion battery.

层状-尖晶石复合固溶体正极材料Li0.5+xNi0.25Mn0.75O2+x/2具有容量高，首次库仑效率高，循环性能好等优点，但是较低的电子和离子电导率、较高的阳离子混排及Mn3+含量、循环过程中层状结构的转变等问题限制了该材料的应用。本项目拟采用快离子导体纳米钛酸锂表面修饰结合体相Cr掺杂对其进行综合改性。利用纳米钛酸锂表面修饰并结合核壳掺杂提升表面电导率，Cr掺杂抑制阳离子混排及Mn3+含量并提高体相电导率，利用包覆层和内核中的Ti、Ni、Mn的互掺杂现象提升基体材料电化学性能，通过Ti进入内核表层晶格形成稳定性更好的Li2TiO3成分并结合Cr掺杂抑制Mn3+和阳离子混排，抑制正极材料层状结构的转变。通过本项目研究，揭示钛酸锂包覆和Cr掺杂对锂离子传输特性及电导率的影响规律，Ti、Ni、Mn相互掺杂及层状结构转变的抑制机理，为新型锂离子电池正极材料的开发应用提供新思路和理论。

层状-尖晶石复合固溶体正极材料Li0.5+xNi0.25Mn0.75O2+x/2 具有容量高，首次库仑效率高，循环性能好等优点，但是较低的电子和离子电导率、较高的阳离子混排、循环过程中层状结构的转变等问题限制了该材料的应用。本项目采用了快离子导体纳米钛酸锂表面修饰结合体相Cr 掺杂对其进行综合改性。首先通过液氮冷却法优化制备工艺和锂含量，制备出性能优异的层状-尖晶石复合基体材料Li1.1Ni0.25Mn0.75O2.3。通过对Li1.1Ni0.25Mn0.75O2.3进行体相Cr掺杂，发现Li1.1Ni0.235Mn0.735Cr0.03O2.3具有最佳的电化学性能，研究表明Cr掺杂明显抑制阳离子混排和尖晶石结构的生长，改善了正极材料的倍率性能和结构稳定性。最后对Li1.1Ni0.235Mn0.735Cr0.03O2.3进行表面纳米Li4Ti5O12的包覆，利用纳米钛酸锂表面修饰并结合核壳掺杂提升表面电导率，通过溶胶-凝胶法得到Li4Ti5O12/Li1.1Ni0.235Mn0.735Cr0.03O2.3复合材料，利用包覆层和内核中的Ti、Ni、Mn 的互掺杂现象提升基体材料电化学性能，通过Ti 进入内核表层晶格形成稳定性更好的Li2TiO3 成分并结合Cr 掺杂抑制Mn3+和阳离子混排，抑制正极材料层状结构的转变。研究发现3% Li4Ti5O12包覆的复合材料具有最佳的电化学性能。首次放电容量达到271mAh/g，首次库伦效率达到94.1%，5C放电达到172mAh/g，100次循环容量保持率为99.4%。包覆后的锂离子扩散系数从4.256×10-12提升到1.432×10-11(cm2•s-1)。通过本项目研究，揭示钛酸锂包覆和Cr 掺杂对锂离子扩散系数及电导率的影响规律，Ti、Ni、Mn 相互掺杂及层状结构转变的抑制机理，为新型锂离子电池正极材料的开发应用提供新思路和理论。