Preparation And Performance Of Lithium Ion Battery Cathode Material LiMn_yFe_1-yPO₄/C

Olivine-type LiMPO4(M=Fe、 Mn) are a potential cathode material for the next generation of Li-ion battery due to its lower cost, environmental compatibility, high safety and thermal stability. LiMnyFe1-yPO4has excellent electrochemical performance and a Voltage platform of4.1V versus Li+/Li, which makes LiMnyFe1-yPO4have higher energy density than LiFePO4, and better electrochemical performance than LiMnPO4. Therefore, LiMnyFe1-yP04have received greater and greater attention in recent years.In this paper, LiMnyFe1-yPO4/C was synthesized via traditional solid-state reaction and the novel "low temperature solid-state reaction/hydrothermal synthesis" method. TGA/DTA、 XRD、SEM、TEM、 galvanostic charge/discharge、 CV and EIS were used to characterize it. The influences on LiMnyFe1-yPO4/C were studyed, of presintering temperature in traditional solid-state reaction, various parameters in "low temperature solid-state reaction/hydrothermal synthesis" method and the content of Mn in LiMnyFe1-yPO4/C. Main conclusions are listed as follows:(1) Pure olivine-type LiMn0.4Fe0.6PO4was obtained via traditional solid-state reaction from FeC2O4·2H2O, NH4H2PO4, Li2CO3, MnCO3and C12H22O11. The raw materials decompose and react with each other before425℃during forming LiMn0.4Fe0.6PO4/C. When the process of low temperature solid-state reaction is500℃for4h, the presintering products are pure olivine-type LiMPO4. The initial discharge capacity of final LiMn0.4Fe0.6PO4/C products by presintering500℃at0.1C is higher compared with those by presintering300℃and400℃. However, the reversible capacities at0.5C change little during cycling. The electrochemical performance of all LiMn0.4Feo.6P04/C products is more excellent than LiMno.4Feo.6PO4.(2) Pure olivine-type LiMn0.4Feo.6P04/C was obtained via "low temperature solid-state reaction/hydrothermal synthesis" method. LiMno.4Fe0.6P04/C products prepared via the process,"adding sugar+solid-state reaction at400℃for4h+hydrothermal synthesis at180℃for18h+the carbon coating treatment at700℃for3h", havehomogeneous particles of size250nm with about5nm carbon coated layer, an initial discharge capacity of160mAhg-1at0.1C,143mAhg-1at0.5C rate and a specific capacity of about133mAhg-1after50times cycling at0.5C-rate. The temperatures of solid-state reaction and hydrothermal synthesis, as well as the adding sugar process, had great influence on the morphology and the electrochemical performance of LiMn0.4Feo.6P04/C. The best process of low temperature solid-state reaction combined with hydrothermal synthesis is "solid-state reaction at400℃for4h+hydrothermal synthesis at180℃for18h+adding sugar+the carbon coating treatment at700℃for3h". LiMno.4Fe0.6P04/C via the best process has homogeneous particles of size100nm, an initial discharge capacity of162mAhg"’at0.1C, a specific capacity of about137mAhg-1after50times cycling at0.5C-rate.(3) A series of LiMnyFe1-yPO4/C materials were prepared via low temperature solid-state reaction combined with hydrothermal synthesis. The lattice parameters of LiMnyFe1-yPO4crystals change nearly depending on the Vegard’s law between LiFePO4and LiMnPO4crystals. With the increase of Mn content, the area of the manganese peaks increases and that of the iron peaks decreases, the observed voltage of the iron oxidation/reduction processincreases slightly and that of the manganes eoxidation/reduction process is equal to4.05V. The initial discharge capacity of LiMno.2Feo.8PO4, LiMn0.4Feo.6PO4, LiMno.6Fe0.4PO4and LiMn0.8Feo.2PO4is150mAhg-1、153mAhg-1、135mAhg-1and115mAhg-1, respectively。 The diffusion coefficient of Li+in LiMnyFe1-yPO4crystals become different with various Mn content. There existed a SEI film on LiMnyFe1-yPO4(y≥0.5).The above research shows that LiMnyFel-yPO4/C materials have excellent electrochemical performance, prepared via "low temperature solid-state reaction/hydrothermal synthesis", which may be used for the industialization in the near future.