Synthesis Of LiMn_yFe_1-yPO₄/C By Co-precipitation Method And Its Electrochemical Performances As The Cathode Materials

LiMnyFe1-yPO4/C (y≥0.6) composite cathode materials for lithium-ion batteries have the potential advantage of high energy density. However, the materials suffer of poor reversibility of Mn3+/Mn2+redox and Jahn-Teller during the charge-discharge process, which result in poor rate and cycle performances. In order to ensure the homogeneous distribution of transition metals, a precursor MnyFe1-yC2O4·2H2O is prepared by using an ammonium oxalate solution as the precipitator. Then the precursor reacts with other raw materials and the final phosphate solid solution LiMnyFe1-yPO4/C is obtained.According to the law of conservation of mass and law of simultaneous equilibibrim, the thermodynamics calculative model Mn2+-C2O42-H2O and Fe2+-C2O42--H2O is established and discover that with the pH value increasing the total remmant metal ions in the solution first decrease and then increase. The corresponding logarithms concentration of metal ions-pH value diagrams are also drawn. Mno.8Fe0.2C204·2H20is prepared by cocurrent flow feed at pH value3.0.The obtained precursor has narrow particle sizes distribution and stable component proportion.LiMn0.8Fe0.2P04/C composite is prepared using Mn0.8Fe0.2C2O4·2H2O as raw material. The synthetic conditions of LiMn0.8Fe0.2PO4/C are optimized by discussing roast temperature and carbon content. The optimum sample delivers a specific capcity of134.3、132.9、130.4、127.2and119.9mAh/g at0.1C、0.2C、0.5C、1C and2C, respectively. LiMnyFe1-yPO4/C(y=0.6, 0.7,0.8,0.9) are prepared under optimum conditions.To LiMn0.6Fe0.4PO4/C and LiMn0.8Fe0.2PO4/C, further research is necessary.LiMn0.6-yFeo.4NiyP04/C(y=0,0.05,0.1and0.2) are prepared using Mn0.6-yFeo.4Niy(C204)·2H20as raw material. In the case of an undoped sample, the cell delivers a high specific capacity of146.6mAh g-1at0.1C, but only88.9mAh/g at2C. The LiMno.55Fe0.4Nio.o5PO4/C delivers a lower specific capacity of142mAh g-1at0.1C, but maintains the discharge capacity of139、135、127and110mAh/g at0.2、0.5、1and2C, respectively. Samples with even higher Ni doping content, LiMno.5Fe0.4Nio.1PO4/C and LiMn0.4Feo.4Ni0.2P04/C, have relatively low capacities at low rates,129and124mAh/g at0.1C.LiMn0.75Feo.2Nio.o5PO4/C is prepared using Mn0.75Feo.2Nio.o5C204·2H20as raw material by liquid phase doping method. The sample delivers a specific capcity of139.6、137.5、134.5、130.1、122.2mAh/g at0.1、0.2、0.5、1and2C, respectively. The electrochemical performance is better than as-prepared LiMno.8Fe0.2PO4/C sample. The solid phase doping composite materials Li(Mn0.75Fe0.2Ni0.05)0.99M0.01PO4/C (M=V, Mg, Nb, Zn, Co, Ca) are prepared by high temperature solid state method, which have reversible discharge specific capacity of144.3、140.5、140.3、137.1mAh/g at0.1C, respectively. Li(Mn0.75Feo.2Nio.o5)o.99Mg0.01P04/C has the best rate performance. The discharge capacity of the sample delivers144.8、142.7、138.9and129.1mAh/g at0.2、0.5、1and2C rates.