Dmytro Lisovytskiy
"Phase transitions in nanocrystalline Li-Mn-O systems
- application of Rietveld method"

Abstract

Lithium Manganese spinel oxides are one of the most promising materials for producing of the electrodes for modern Li-ion rechargeable batteries because of their low cost and low toxicity in comparison with actually used compounds. Considering the wide interest in nanocrystalline systems which in many instances exhibit unusual properties as compared to the bulk materials it is generally anticipated that batteries constructed on the base of nanocrystalline spinel electrodes will have much better properties and will supply electrical energy not only for portable electronic application but also for electric vehicles. It is difficult to fabricate Li-Mn spinel electrode material with exact predetermined composition. Hence, there is the necessity of carrying out phase analysis of the synthesized spinel compounds. Another difficulty in application is that Li-Mn spinel at lowered temperature undergoes structural phase transition. This could affect the performance of the battery.

In the present work, systematic X-Ray diffraction (XRD) (including Rietveld) analysis and impedance spectroscopy studies of laboratory synthesized by sol-gel method and commercial Li-Mn spinel nanooxides have been undertaken in order to determine structural and electrical parameters and to estimate changes in phase composition caused by course of temperature phase transition and to study phenomena which accompany this phase transformation.

Special emphasis was put on the possibility, feasibility and usefulness of the use of Rietveld method for this particular case of nanomaterials.

As a special interest in materials studies are simultaneous measurements of different parameters, for the main group of samples which undergo the phase transition, in addition to standard XRD in Bragg-Brentano geometry measurements, simultaneous measurements of impedance spectra in the frequency range from 1 MHz to 0.1 Hz and X-ray pattern in non focusing geometry were performed, in the temperature range from 25^oC to +100^oC. For this purpose dedicated equipment was constructed and used. This way a correlation between the XRD profile parameters and conductivity calculated from impedance spectra was evidenced. LiMn₂O₄ and Li_1.005Mn_1.995O₄ samples obtained by high temperature sol gel method (800°C) exhibited phase transition from cubic to orthorhombic structure upon cooling below room temperature. Structural transition was clearly visible in the X-ray pattern as splitting of structure-sensitive reflections, e.g. 400, of cubic structure, and was accompanied by a decrease of conductivity by a factor of about 10 in the case of samples prepared from powders obtained by sol-gel method or supplied by Alfa Aesar Co. and heat treated up to 800°C. The phase transition was not complete even at -25°C in the samples prepared from powder supplied by Sigma-Aldrich Co. and heat treated also up to 800°C and no stepwise change of conductivity was observed for this sample. Additional phases, Mn₃O₄ or Li₂MnO₃, were found in commercial samples, while the spinels prepared by high temperature sol-gel method were single phase. It was evidenced that the appropriate application of the Rietveld method allowed not only precise phase identification but also quantitative analysis of the phase composition.