Synthesis And Revisit Of C-LiFePO₄-PANI As Cathode Materials For Li-ion Battery

The olivine-structured orthophosphate LiFePO₄has been viewed as the most popularand promising lithium-ion batteries cathode material owing to its high operatingvoltage（³.5Vvs Li/Li+）, good cycle performance, low cost, environmental friendly andsafety. However, pure LiFePO₄shows poor reversible capacity and charging/dischargingperformance at high current density. In recent years, researchers have tried many methodssuch as controlling particle size,coating carbon and conductive polymer,doping metal ionand halogen ion to improve the performance of LiFePO₄,and have achieved greatprogress.In this paper, conductive polymer polyaniline（PANI） and C-LiFePO₄/PANIcomposite cathodes was successfully prepared by chemical oxidation.The synthesiscondition were systematically studied.The thermal stability of the as-prepared cathodeswere analysed by TG-DSC; The structure and morphology were characterized by XRD,FTIR spectra, SEM. Electrochemical performance was investigated by galvanosaticcharge/discharge cycling test and EIS.Experimental results show that the morphology and structure as well as theelectrochemical performance of PANI are greatly affected by the type and concentration ofthe protonic acid.PAIN shows good electrochemical performance when HCl with aconcentration of0mol/L is used as the doping acid. Its conductivity can be as high as2S cm^-1.The conductivity and electrochemical performance of PANI can also be influenced bythermal treatment.PANI shows good conductivity(1.9S cm^-1) and electrochemicalperformance when the temperature is about100℃.Its discharge capacity of the first cycleis107.3mAh g^-1at0.5C and it maintains40mAh g^-1when the current rises to5C.As thetemperature rises to200℃,the conductivity drops to10-7S cm^-1,which is as low as anonconductor.The discharge capacity of the first cycle is only45.6mAh g^-1and showsnearly no capacity at5C.PANI doped by HCl is very stable in the electrolyte.After being soaked in the electrolyte for about4weeks,the structure and morphology of PANI/HCl changes little.The same synthesis condition unsuits the preparation of well-performancedC-LiFePO₄/PANI.By coating C-LiFePO₄with PANI in this condition,the compositecathode shows poor electrochemical performance.The optimal synthesis condition of the C-LiFePO₄/PANI composite cathode isobatained through orthogonal experimental design.When the molar ratio of aniline andammuonium persulfate is1to0.8,the pH of the reaction system is4and the reaction timeis4hours,the C-LiFePO₄/PANI composite cathode shows the best performance.And pH ofthe reaction system affects greater than the other two factors.