Assat Gaurav, Manthiram Arumugam
Department of Mechanical Engineering, The University of Texas at Austin , Austin, Texas 78712, United States.
Inorg Chem. 2015 Oct 19;54(20):10015-22. doi: 10.1021/acs.inorgchem.5b01787. Epub 2015 Oct 2.
Lithium transition-metal phosphates, LiMPO4 (M = Mn, Fe, Co, and Ni), have attracted significant research interest over the past two decades as an important class of lithium ion battery cathode materials. However, almost all of the investigations thus far have focused on the olivine polymorph that exists in the orthorhombic Pnma space group. In this study, a distinct orthorhombic but non-olivine polymorph of LiMPO4, described by a Cmcm space group symmetry, has been synthesized with M = Mn, Fe, Co, and Ni. Of these, LiMnPO4 in the Cmcm space group is reported for the first time. A rapid microwave-assisted solvothermal (MW-ST) heating process with tetraethylene glycol (TEG) as the solvent and transition-metal oxalates as precursors facilitates the synthesis of these materials. The peak reaction temperatures and pressures were below 300 °C and 30 bar, respectively, which are several orders of magnitude lower than those of the previously reported high-pressure (gigapascals) method. X-ray diffraction (XRD) confirms the crystal structure with the Cmcm space group, and scanning electron micrographs indicate a submicrometer thin platelet-like morphology. The synthesis process conditions have been optimized to obtain impurity-free samples with the correct stoichiometry, as characterized by XRD and inductively coupled plasma-optical emission spectroscopy (ICP-OES). Upon heat treatment to higher temperatures, an irreversible transformation of the metastable Cmcm polymorphs into olivine is observed by XRD and Fourier transform infrared spectroscopy. Although the electrochemical activity of these polymorphs as lithium ion cathodes turns out to be poor, the facile synthesis under mild conditions has permitted easy access to these materials in a nanomorphology, some of which were not even possible before.
锂过渡金属磷酸盐LiMPO₄（M = Mn、Fe、Co和Ni）在过去二十年中作为一类重要的锂离子电池正极材料引起了广泛的研究兴趣。然而，迄今为止几乎所有的研究都集中在正交Pnma空间群中存在的橄榄石多晶型物上。在本研究中，已经合成了具有Cmcm空间群对称性的LiMPO₄的一种独特的正交但非橄榄石多晶型物，其中M = Mn、Fe、Co和Ni。其中，首次报道了Cmcm空间群中的LiMnPO₄。以四甘醇（TEG）为溶剂、过渡金属草酸盐为前驱体的快速微波辅助溶剂热（MW-ST）加热过程促进了这些材料的合成。峰值反应温度和压力分别低于300℃和30 bar，比先前报道的高压（吉帕斯卡）方法低几个数量级。X射线衍射（XRD）证实了具有Cmcm空间群的晶体结构，扫描电子显微镜照片表明其具有亚微米级的薄片状形态。通过XRD和电感耦合等离子体发射光谱（ICP-OES）表征，已优化合成工艺条件以获得具有正确化学计量比的无杂质样品。在高温热处理后，通过XRD和傅里叶变换红外光谱观察到亚稳的Cmcm多晶型物不可逆地转变为橄榄石。尽管这些多晶型物作为锂离子正极的电化学活性较差，但在温和条件下的简便合成使得能够容易地获得纳米形态的这些材料，其中一些材料甚至在此之前是不可能得到的。
