Molecular dynamics study on evaporation of metal nitrate-containing nanodroplets in flame spray pyrolysis.
Molecular dynamics study on evaporation of metal nitrate-containing nanodroplets in flame spray pyrolysis.
复制标题
火焰喷雾热解中含金属硝酸盐纳米液滴蒸发的分子动力学研究。
作者：
Hou D
火焰喷雾热解技术为制备锂离子电池正极材料LiNi1−x−yCoxMnyO2(NCM)提供了一条有利的途径。然而，目前还缺乏对纳米粒子通过FSP形成机理的详细了解。为了阐明NCM前驱体液滴在FSP中的蒸发过程，本工作采用经典分子动力学(MD)模拟方法，从微观角度研究了金属硝酸盐(包括LiNO3、Ni(NO3)2、Co(NO3)2和Mn(NO3)2)和水(作为溶剂)组成的纳米液滴的动态蒸发过程。通过跟踪质量密度的径向分布、金属离子数密度的径向分布、液滴直径和金属离子与氧原子的配位数(Cn)等关键特征的时间演化，对蒸发过程进行了定量分析。分子动力学模拟结果表明，在含MnO_3(M=Li、Ni、Co或Mn)纳米液滴的蒸发过程中，Ni2+、Co2+和Mn2+会在液滴表面析出，形成溶剂-核-溶质-壳结构，而Li+在蒸发的含LiNO_3液滴中的分布更加均匀，这是由于Li+相对于其他金属离子具有较高的扩散系数。对于含Ni(NO3)2或Co(NO3)2的纳米液滴的蒸发，M-OW(M=Ni或Co；OW代表水中的O原子)的CN随时间的演化是一个“自由H2O”蒸发阶段，在此阶段M-OW的CN和M-ON的CN都不随时间变化。模拟液滴蒸发的经典D2定律，求出了不同条件下的蒸发速率常数。与Ni或Co不同，Mn-OW的Cn随时间变化，但液滴直径的平方随时间的变化表明，含Ni(NO3)2-、Co(NO3)2-或Mn(NO3)2的液滴的蒸发速率几乎不受不同类型金属离子的影响。
Flame spray pyrolysis (FSP) provides an advantageous synthetic route for LiNi1−x−yCoxMnyO2 (NCM) materials, which are one of the most practical and promising cathode materials for Li-ion batteries. However, a detailed understanding of the NCM nanoparticle formation mechanisms through FSP is lacking. To shed light on the evaporation of NCM precursor droplets in FSP, in this work, we employ classical molecular dynamics (MD) simulations to explore the dynamic evaporation process of nanodroplets composed of metal nitrates (including LiNO3, Ni(NO3)2, Co(NO3)2, and Mn(NO3)2 as solutes) and water (as solvent) from a microscopic point of view. Quantitative analysis on the evaporation process has been performed by tracking the temporal evolution of key features including the radial distribution of mass density, the radial distribution of number density of metal ions, droplet diameter, and coordination number (CN) of metal ions with oxygen atoms. Our MD simulation results show that during the evaporation of an MNO3-containing (M = Li, Ni, Co, or Mn) nanodroplet, Ni2+, Co2+, and Mn2+ will precipitate on the droplet surface, forming a solvent–core–solute–shell structure; whereas the distribution of Li+ within the evaporating LiNO3-containing droplet is more even due to the high diffusivity of Li+ compared with other metal ions. For the evaporation of a Ni(NO3)2- or Co(NO3)2-containing nanodroplet, the temporal evolution of the CN of M–OW (M = Ni or Co; OW represents O atoms from water) suggests a “free H2O” evaporation stage, during which both CN of M–OW and CN of M–ON are unchanged with time. Evaporation rate constants at various conditions are extracted by making analogy to the classical D2 law for droplet evaporation. Unlike Ni or Co, CN of Mn–OW keeps changing with time, yet the temporal evolution of the squared droplet diameter indicates the evaporation rate for a Ni(NO3)2-, Co(NO3)2-, or Mn(NO3)2-containing droplet is hardly affected by the different types of the metal ions.