Hydrothermal synthesis, evolution, and electrochemical performance of LiMn0.5Fe0.5PO4 nanostructures

Abstract

LiMn_0.5Fe_0.5PO₄ (LMFP) materials are synthesized by the hydrothermal approach in an organic-free and surfactant-free aqueous solution. The phase and morphological evolution of the material intermediates at different reaction temperatures and times are characterized by XRD, SEM and TEM, respectively. The results show that during temperature increase, the solubility product (K_sp) of the precursors (Li₃PO₄, Fe₃(PO₄)₂ and (Mn,Fe)₃(PO₄)₂) is the decisive parameter for the precipitation processes. Once the temperature locates at the range of 100–110 °C, the unstable precursors dissolve quickly and then LMFP nuclei are formed, followed by a dissolution-reprecipitation process. As the reaction progresses, the primary particles self-aggregate to form rod or plate particles to reduce the overall surface energy through oriented attachment (OA) and the Ostwald ripening (OR) mechanism. Moreover, the resultant concentration of the precursor significantly affects the crystal size of LMFP by altering the supersaturation degree of solution at the nucleation stage. The carbon coated LMFP nanostructure synthesized at 0.6 mol L⁻¹ (resultant concentration of PO₄³⁻) delivers discharge capacities of 155, 100 and 81 mA h g⁻¹ at 0.1, 5 and 20 C rate, respectively. The understanding of nanostructural evolution and its influence on the electrochemical performance will pave a way for a high-performance LMFP cathode.