Preparation And Study Of PPy/LiMPO₄（M=Fe、Mn）Composite As Cathode Materials

For the shortcomings of low electronic and ionic conductivity of LiFePO₄as cathodematerial, the control of system reducibility and selection of carbon source have beenexplored in solvothermal reaction and solid-state reaction, electrochemically active PPywas also explored to composite with LiFePO₄, expecting excellent electrochemicalperformance of LiFePO₄cathode material. Preliminary exploration was done on LiMnPO₄.The LiFePO₄were first synthesized by solvothermal reaction with ethanol and water assolvent, the synthesis conditions were optimized such as ratio of solvent, reactantconcentration, ascorbic acid content and reaction temperature etc. In order to guarantee thepH values of thermal systems were not affected too much and the reducibility of the systemwas enhanced at the same time, glucose was chosen as an auxiliary reductant in thisreaction, the results showed that spindle olivine LiFePO₄were prepared without anyimpurity phases, and it have excellent capacity retention and cycling stability, and thehighest discharge capacity was161.3mAh/g at0.1C and108.6mAh/g at5C, respectively.This result suggests that glucose is the key factor for the well-distribution and neat crystalstructure of nanoparticles, thus the electrochemical performances of materials improved.The LiFePO₄were prepared by solid-state reaction, the synthesis conditions wereoptimized such as reaction temperature, reaction time, sintering status and the amount ofcarbon-coated. In order to eliminate uneven diffusion of organic carbon of the traditionalmethod, ferrous gluconate was choosen as both carbon source and partial iron source.LiFePO₄/C composites were prepared by high temperature solid phase in situ carbonationand the materials were characterized. The results showed that due to the strong chemicalbonds between gluconate and iron ions, the uneven diffusion of organic carbon wasprevented a uniform layer of carbon film was formed on the surface of particles, the graingrowth was inhibted, a complete conductive network was formed, thus the electrochemicalproperties were enhanced. The highest discharge capacity of LiFePO₄/C composite was161.6,147.2and123.3mAh/g at0.1C,0.2C and1C rate, respectively. There is no fade ofdischarge capacity after50cycles at1C rate.The favorable electrochemical LiFePO₄/C-PPy composites were synthesized by in situchemical oxidation polymerization. It showed the coating of PPy on LiFePO₄did notincrease its specific capacity but the stability of the materials increased under high rate.When the content of PPy was17%, the LiFePO₄/C-PPy has a good rate capability with noobvious decay of specific capacity after10cycles at1C rate.