Birgisson, Steinar9; Jensen, Kirsten Marie Ørnsbjerg9; Christiansen, Troels Lindahl10; Tyrsted, Christoffer7; Bøjesen, Espen Drath9; Christensen, Mogens9; von Bülow, Jon Fold8; Iversen, Bo Brummerstedt9
1 Department of Chemistry, Science and Technology, Aarhus University2 Department of Engineering - Membrane and Sensor Technology, Department of Engineering, Science and Technology, Aarhus University3 Department of Chemistry - Centre for Materials Crystallography (CMC), Department of Chemistry, Science and Technology, Aarhus University4 Interdisciplinary Nanoscience Center, Science and Technology, Aarhus University5 Interdisciplinary Nanoscience Center - INANO-Kemi, Langelandsgade, Interdisciplinary Nanoscience Center, Science and Technology, Aarhus University6 Department of Chemistry - Centre for Energy Materials (CEM), Department of Chemistry, Science and Technology, Aarhus University7 Interdisciplinary Nanoscience Center, Faculty of Science, Aarhus University, Aarhus University8 Haldor Topsøe A/S9 Department of Chemistry, Science and Technology, Aarhus University10 Department of Chemistry - Centre for Materials Crystallography (CMC), Department of Chemistry, Science and Technology, Aarhus University
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. Proposed reaction route for this system is shown 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. The in-situ setup can also be used for studying solvothermal reaction using other types of measurements, such as SAXS and total scattering. 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 reaction mechanism using and Johnson-Mehl-Avrami kinetic theory. Activation energies for each of the reactions can be calculated using Arrhenius equation. These information give us better fundamental understanding of the hydrothermal reaction system.