Synthesis and performance of LiMn0.7Fe0.3PO4 cathode material for lithium ion batteries

Pure and carbon-containing olivine LiMn_0.7Fe_0.3O₄ were synthesized at 600 °C by the method of solid-state reaction. Structure, surface morphology and charge/discharge performance of LiMn_0.7Fe_0.3O₄ were characterized by X-ray diffraction, scanning electron microscopy, and electrochemical measurement, respectively. The prepared materials with and without carbon both show the single olivine structure. The morphologies of primary particles are greatly affected by the addition of carbon. Large particles (500-1000 nm) and densely sintered blocks were observed in pure LiMn_0.7Fe_0.3PO₄, which made the insertion and extraction of lithium ions difficult. Battery made from this sample can not charge and discharge effectively. The carbon-containing LiMn_0.7Fe_0.3PO₄ has a small particle size (100-200 nm) and a regular appearance. This material demonstrates high reversible capacity of about 120 mAh g⁻¹, perfect cycling performance, and excellent rate capability. It is obvious that the addition of carbon plays an important role in restricting the particle size of the material, which helps to prepare LiMn_0.7Fe_0.3PO₄ with excellent electrochemical performance. The electrochemical reaction resistance is much lower in the partly discharged state than in the fully charged or fully discharged state by the measurement of ac impedance for carbon-containing LiMn_0.7Fe_0.3PO₄. It is indicated that the mixed-valence of Fe³⁺/Fe²⁺ or Mn³⁺/Mn²⁺ is beneficial to the transfer of electron which happens between the interface.