A novel mechanochemical synthesis of LiMnPO4 with manganese violet and ammonium manganese phosphate as precursors

Abstract

Olivine lithium manganese phosphate was synthesized by a mechanochemical method. Inorganic cathode materials can be divided into three kinds of lithium metal oxide layered, spinel oxide and olivine metal phosphate. One important of the cathode materials used in Li-ion cells is LiMnPO4. In this paper two different precursors, manganese violet and ammonium manganese phosphate proposed as reactant in the synthesis of LiMnPO4 nanostructures. Reaction between NH4MnP2O7 and Li2CO3 takes placed in solid state at room temperature. In this mechanochemical process, a solid acid and base reaction occurred between ammonium and carbonate ions. In addition, MnIII is unstable oxidation state and convert to crystalline β-MnO2, Li2MnP2O7 and NH4H2PO4 in the disproportionation reaction. Results showed that mechanochemical reaction between NH4MnP2O7 with Li2CO3 as precursors lead to concurrent redox and acid-base reaction. As a result, we observed there is a tendency for manganese violet to disproportionation into MnO2 and manganese pyrophosphate salt in a solid-state reaction. Finally, a redox reaction between β-MnO2 and Li2MnP2O7 in pyrolysis reaction under inert atmosphere give LiMnPO4. In the other hand in the mechanochemical reaction between NH4MnPO4 and Li2CO3, only acid and base reaction occurred and the NH4+ ions were replaced with Li+ ions and amorphous LiMnPO4 was prepared. The amorphous LiMnPO4 convert to crystalline LiMnPO4 by annealing the powder under inert atmosphere. The chemical changes in formation of LiMnPO4 compound was characterized by X-Ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and energy dispersion x-ray analyses (EDX). FTIR spectra showed that the NH4+ ions were replaced with Li+ ions. X-ray diffraction show that by variation of precursor from manganese violet to ammonium manganese phosphate in the mechanochemical reaction the crystalline pattern changes to amorphous and also the reaction pathway was changed. The SEM images of LiMnPO4 nanoparticles that obtained by two precursors exhibit squamous morphologies with well-defined grain boundaries and nonuniform particle size distributions. The particle size distributions of crystalline LiMnPO4 obtained by NH4MnP2O7 and NH4MnPO4 were found in the region 60–260 and 20–130 nm respectively.