| Orthorhombic LiMnO2 compound has little toxicity, low cost, high energy density and theoretical capacity (285mAh/g), etc. However, its electrochemistry properties are not stable. The capacity density and cycle life of orthorhombic LiMnO2 materials synthesized by different groups is different. It demonstrated the electrochemistry properties are relation with the preparation conditions. It discovered that the o-LiMnO2 prepared by different methods possesses some stacking faults, and the stacking faults are useful to the enhancement of capacity density and harmful to the cycle life. The materials with small crystal have high capacity and excellent cycleablity. So, o-LiMnO2 materials with small crystal size and proper quantity of stacking faults would be expected to improve the electrochemistry performance. Now, the preparation conditions are usually rigor for the reported different synthesis methods. Solid state reaction requires the materials to be calcined for several times under inert gases. The ionic exchange and wet chemical method need precursor and near chemistry composition, and so on. The quantity control of stacking faults is difficult. Finding the simple method is the important problem. Preparing small crystal and well controlling the stacking faults would advance the application of LiMnO2 materials.As positive materials of lithium batteries, o-LiMnO2 attracted much attention in recent years. Especially,manganese oxide is a strong correlative system, it reveal a lot of new properties. However, the research about o-LiMnO2 was limited on the materials synthesis and electrochemistry properties. On the other hand, the properties of the materials have a few reporting. Because the properties of the materials decide the possible function, the property researches are the application foundation. In order to enhance the electrochemistry performance of o-LiMnO2, the studies of physical and chemical properties are necessary. So, we investigated the relation between structure and properties of o-LiMnO2. This work is helpful to the understanding of the material physical properties, and ameliorates the material performance.This paper mainly investigated several aspects as below, and obtained valuable results.Firstly, we synthesized o-LiMnO2 samples with small crystal size and little stacking faults by hydrothermal method. XRD results showed that the samples have single phase, high-quality crystal, little stacking faults, integral layer structure. SEM and TEM images showed that the crystal size is within 100nm. High-resolution TEM revealed the uniform arraying MnO6 and LiO6 octahedron alternatively in the direction of b-axis. Fourier transform spectra showed four strong phonon vibration peaks revealing the orthorhombic MnO6 octahedral.Strong intensity and narrow bandwidth demonstrated o-LiMnO2 material have little structural defects and integral layer structure. Secondly, we investigated the dynamic and static state magnetism of o-LiMnO2 by quantum superconduction interferometer.Direct current (dc) magnetic susceptibility of low magnetic field (100Oe) and alternating current (ac) magnetic susceptibility showed re-entrant spin glass. There were three transformation from high temperature to low temperature for dc and ac magnetic susceptibility: (1) paramagnetic to antiferromagnetic state at Néel temperature TN≈261K; (2) ferrimagnetic stabilization zone at T275K. Contrasting with the magnetism transform range, the spin-phonon coupling was strong.Finally, we synthesized the Cr-doping LiMnO2 sample with hexagonal structure. The XRD patterns and Raman spectra showed the samples were single phase crystal with NaFeO2 structure. According to the ICP Plasma emission spectra, the composition is Li1.06Mn0.8Cr0.14O2. XPS spectra revealed the average value of Mn increased compared to the undoped LiMnO2, and Jahn-Teller distortion resulted from Mn3+ was restrained. Therefore, the structure is more stable.
|
PDF Full Text Request