| LiCoO2 is the popular cathode material for rechargeablelithium-ion batteries currently. But it also has many shortcomingssuch as it is unsuitable for power supplies and Co is expensive andtoxic, so it is desirable to exploit new energy-storage material orimprove the total behaviors of LiCoO2 by means of doping. Thispaper has studied systemically the synthesize, crystal structures andelectronic structures of LiCo1-xMnxO2. A series of valuableproductions have obtained.LiNO3, Co(NO3)2 and (CH3COO)3Mn2 are used as rawmaterials and LiCo1-xMnxO2 is synthesized by sol-gel method.XRD studies show that when x≤0.3, LiCo1-xMnxO2 is crystallizedin a layered hexagonal phase of α-NaFeO2 structure under thesintering temperatures of 8000C-9000C. As the increase of Mndoping, the cell parameters a and c along with the cell volumes allenlarge. It is attributed to the size effect of doped element and theescape of some lithium ions as the increase of sintering temperature,which enhances the repulsion between the layers of oxygen ionsthen leading to the expanse of crystal lattices. XRD studies alsoindicate that increasing the sintering temperature can improve thesolid solubility of Mn. The best sintering temperature is found to be8000C and the solid solubility limit of Mn can reach 30%.Whenx=1.0, the samples are the mixtures of LiMn2O4 and Li2MnO3. Theanalysis of XRD patterns shows the samples crystallize wellenough after they have been annealed for 3 hours. More sinteringtime doesn't affect the crystal structures of the samples remarkably.So the sintering temperature is the key factor to control thecrystalline states of the materials.Transmission Electron Microscope (TEM) is used to study theappearance of the samples which are synthesized under differentsintering temperatures. The results show that the samplessynthesized under 800℃are the best, where the sample grains havethe regular shapes of polyhedrons and the grain size distribution isvery homogeneous with an average grain size of about 0.4μm, andthe crystalline grains are connected very well.So, Raman, electron diffraction and EELS measurements areperformed on LiCo1-xMnxO2 sintered under 8000C, and then thechanges of the samples'microstructures and their electronicstructures are presented.When x=0 and 0.3, the FTRaman spectra of LiCo1-xMnxO2have A1g and Eg peaks only,which reveals that the hexagonallayered structure is maintained very well till the doping of Mn inLiCoO2 reaches 30%. When x=0.4, a small peak appears betweenthe two peaks, which reveals mixed phase have been introduced tothe system. The doping of Mn increases the force constant K anddecreases the reduced mass μ, which makes the Raman vibrationfrequencies shift to the direction of high wave numbers.Simultaneously, the Full Width Half Maximum (FWHM) of eachspectral lines changes remarkably with the changes of Mn contents,which reveals that the Mn-doping has made some effects on thepoint-group symmetry.When x=1.0, the Raman spectra of LiCo1-xMnxO2 appearmany peaks. After dividing the peaks and making comparison, it isproved that the sample is a mixed phase of LiMn2O4 and Li2MnO3.According to the diffraction rings which have strong diffractionintensities in the polycrystal electron diffraction spectra of LiCo1-XMnXO2(x=1.0), the interplanar crystal spacing is calculated, and itis found that the results consist with the polycrystal electrondiffraction analog computation results of LiMn2O4 by means ofEMS simulation program with a condition of 300kV acceleratingvoltage. So it is concluded that the crystal structure correspondingto the strong diffraction rings is cubic spinel LiMn2O4. The precincthigh-resolution electron microscope shows a layered structure of0.28 nm. The structure of cubic spinel LiMn2O4 is simulated alsoby a first-principles software and it is found that in (111) latticeplane the spacing of Mn along [011] direction is 0.28nm,whichaccords to the results of high-resolution electron microscope. Itproves again that this phase is cubic spinel LiMn2O4. The Ramanand XRD spectra also reveal this result. In the diffraction rings of polycrystal precinct high-resolutionelectron microscopy for LiMn0.2Co0.8O2, no mixed diffraction dotsare detected, which indicates there aren't any mixed phases in thesample. The calculated interplanar crystal spacing according to thestrong diffraction rings in the polycrystal electron diffractionspectra of LiCo0.8Mn0.2O2 agrees well with the polycrystal electrondiffraction analog computation results of LiCoO2 by means of EMSsimulation program with a condition of 300kV accelerating voltage,which proves the sample maintains a good hexagonal structure ofα-NaFeO2 type。 The polycrystal electron diffraction spectra of Li Co0.7Mn0.3O2are as same as that of LiCo0.8Mn0.2O2, which indicates that thechange of Mn-doping quantities doesn't change the crystalstructure. Because electron diffraction has a error of 3%, largerthan XRD,the polycrystal electron diffraction spectra can notdetect the change in the cell parameters arised from the change ofMn-doping quantities. The EELS spectra of the Co and Mn L2, 3 edges for LiCo1-xMnxO2 (x=0.2,0.3) show that the intensities of both L2 and L3edges don't change, which indicate that the valence state of Mnions as the Mn dopant increases. When x=1.0, the different valence state of Mn ions leads tosome changes in the energies and shapes of white lines, and asatellite peak appear nearby the L2 edge in the sample's EELSspectra. When x=0.2 and 0.3, the EELS results of O K edge show adecrease of the intensities in the prepeak, which indicates the Mn-doping enhances somewhat the ionicity of O ions, that is to say theionicity of O ions in LiCo0.7Mn0.3O2 is somewhat higher than thatin LiCo0.8Mn0.2O2. This relates to the difference of electronegativitybetween Mn and Co ions and proves the validity of conclusionmade by Ceder et al.. In summary,This paper has synthesized successfully a seriesof LiCo1 MnxO2 by means of sol-gel method. Many characterizing -xinstruments such as XRD, Raman, TEM, EELS and electrondiffraction have employed to study the materials'electronicstructures and crystal structures systematacially. Some innovative...
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