Reactive Milling Induced Structure Changes in Phenylphosphonic Acid Functionalized LiMn2O4 Nanocrystals – Synthesis, Rietveld Refinement, and Thermal Stability

Abstract

Spinel LiMn2O4 formed in a solid‐state reaction was treated with a high energy planetary ball mill. A mechanochemical in situ surface functionalization of the nanocrystallites with a size smaller than 10 nm was achieved by addition of phenylphosphonic acid. The functionalization was proven by infrared spectroscopy and it can be shown that it prevents the formation of Mn2O3 during the milling process. Structural changes of the samples were investigated via XRD and Rietveld refinement. Mild milling conditions induce an anisotropic broadening of the reflections caused by a distribution of lattice parameters. In this first stage, we propose a structure model based on orthorhombic LiMn2O4. DSC and in situ XRD measurements also verify the presence of the orthorhombic low temperature phase. Medium milling conditions induce a change of intensities, correlated to a cation disorder. Harsher milling conditions induce the transformation to a tetragonal phase. The thermal stability of the formed phases was investigated via in situ high temperature XRD. The reformation of cubic spinel is observed in all samples; however, the transition point depends on the previous milling parameters.