Many solvothermal reactions have a great potential for environmentally friendly and easily scalable way for producing nanocrystalline materials on an industrial scale. Here we study hydrothermal formation of spinel LiMn2O4 which is a well-known cathode material for Li-ion batteries. The LiMn2O4 nanoparticles are formed by reducing KMnO4 in an aqueous solution containing Li-ions. The reducing agent is an alcohol (here ethanol) and the reaction takes place under high pressure and temperature. The LiMn2O4 nanocrystals are unstable towards further reduction to Mn3O4 nanocrystals. Possible reaction route for this system is presented in equations (1) and (2). (1) 4LiOH + 8KMnO4 + 7CH3CH2OH --> 4LiMn2O4 + 8KOH + 7CH3COOH + 5H2O (2) 12LiMn2O4 + 5CH3CH2OH + H2O --> 8Mn3O4 + 12LiOH + 5CH3COOH Our group has developed an experimental technique for in-situ measurements of solvothermal reactions under sub- and supercritical conditions [Becker et al, J. Appl. Crystallogr. (2010) 43]. The technique uses synchrotron X-ray radiation to measure time resolved powder x-ray diffraction patterns while the reaction is happening thereby giving real time information on crystalline phase formation, particle sizes and other structural properties for the reaction being studied. In-situ measurements at different reaction temperatures have been conducted to see how the formation rate and particle growth is affected by temperature while the precursor concentration is kept constant. The precursor solution is an aqueous solution with Li:Mn:EtOH molar ratio of approximately 1:2:7 and the reactions conditions are constant temperature at 220°C, 260°C, 300°C, 350°C and 400°C at 250 bar. First results show the formation of the LiMn2O4 and Mn3O4 phases, the growth of the nanocrystals of each phase and evolution of structural properties (such as unit cell constants) as a function of reaction time. Further analysis will involve estimation of reaction rate constants and activation energies for each of the reactions for better understanding of the hydrothermal reaction system.
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Conference on Electrochemical Science and Technology, 2013