Study On The Modification And Storage Performance Of Olivine LiMnPO₄ Cathode Material

Low cost, environmentally benign and excellent stability make the olivine structured lithium transition metal phosphates（LiMPO₄） be the promising cathode materials for application in lithium-ion batteries（LIBs） and LiFePO₄ has been commercialized. Now, LiMnPO₄ is considered as the next generation cathode material for LIBs because of its higher potential（4.1V vs Li+/Li） and higher theoretical energy density（697 Wh/kg）. However, LiMnPO₄ has an intrinsic defect, such as extremely low electrical conductivity involving its low electronic conductivity（<10-10S/cm）and sluggish Li ion diffusion in a curved trajectory down a [010] channel, the Jahn-Teller effect of the Mn3+ ion. Moreover, the electrochemical performance,especially the rate performance was restricted by these defects.In an effort to improve the rate and cycle performances of LiMnPO₄, the employed methods include reducing Li MnPO₄ crystal size to nano-scale, coating carbon on Li MnPO₄ crystallites and substituting Mn2+with Fe2+. The effect of the different preparation method and technological conditions on the structure,morphology and electrochemical performance was extensively explored. The main work and research findings of this paper are as follows:（1）: The cathode materials of LiMnPO₄ and LiMn1-xFexPO₄（x = 0.2, 0.3, 0.4, 0.5）solid solutions were synthesized by the modified solid state reactions after FePO₄ and MnPO₄.H2 O being pre-prepared via precipitation methods. We extensively research the effect of the carbon source, the content of carbon, the Mn/Fe ratio on the physical and electrochemical properties of the final products. And the LiMn0.6Fe0.4PO₄/C with the highest electronic conductivities, the highly ordered crystal structure and the significantly improved rate capabilities was prepared under the optimal condition. The+LiD of Li-Mn for LiMn0.6Fe0.4PO₄/C was 5.49 × 10-10 cm2/s based on the Randles-Sevcik equation. Thus, the Fe-doping in partial Mn sites is beneficial for the kinetic behaviors of LiMnPO₄.（2）: The LiMn1-x FexPO₄（x=0.2, 0.3, 0.4, 0.5） precursors were synthesized by employing a facile hydrothermal method with PEG400/water as solvent. Then, the precursors were mixed with ACM（15 wt.% of the theoretical mass of the precursors）and carbonized under the N2 atmosphere to yield carbon-coated LiMn1-xFexPO₄ nanorods. The addition of PEG400 facilitated the crystal growing parallel to the ac facet and maked b axis the shortest. The effect of the calcination temperature andMn/Fe ratio on the products was extensively studied. The LiMn0.5Fe0.5PO₄/C nanorods with the thickness of 30 nm and highly ordered lattice structure were synthesized under 700 oC. Moreover, the sample with the dramatically increased apparent Li ion diffusion coefficients demonstrated significantly improved rate and cycle performance.（3）: There are great difference on the structure, the optimal ratio of Mn/Fe and electrochemical performance between the LiMn1-xFexPO₄/C（x = 0.2, 0.3, 0.4, 0.5）synthesized by the different preparation method. And the difference on the electrochemical performance of LiMn0.6Fe0.4PO₄ is the most obvious. This is because the different synthesis methods and technology synthesis conditions have an influence on the crystallinity, particle morphology, Mn and Fe atomic distribution within the crystal and electrode kinetics behavior, which will further influence on the electrochemical performance of the cathode materials.