Studies On Synthesis And Modification Of Doping LiFePO₄as Cathode Material For Lithium-ion Battery

The cathode material is one of the key factors to determine the performances oflithium-ion batteries, and it accounts for about40%of the entire cost of the battery.Therefore, it was very important to develop the high-performance cathode materialsfor the large-scale industrial application of lithium-ion battery. LiFePO₄has beenconsidered as one of the most promising cathode materials for Li-ion batteries due toits low cost, good security and excellent cycle stability. However the electronicconductivity and Li+diffusion rate of LiFePO₄were low, which limited the practicalapplication of LiFePO₄.In this paper, the olivine LiFePO₄was prepared by high temperature solid phasemethod using ferrous oxalate as the iron source, lithium carbonate as the lithiumsource and sugar as the carbon source. The impacts of the calcination temperature,holding time, milling media and carbon-coated amount on the structure, morphologyand electrochemical performances of LiFePO₄materials were investigated. Theresults showed that when ethanol/water （the volume ratio was4:1） was used as themilling media, and the amount of carbon-coated was4wt%, the materials synthesizedat700℃for15h showed the best electrochemical performance.In order to improve the electrochemical performance of LiFePO₄/C, Zn3（PO₄）2was coated on the surface of LiFePO₄/C. The results showed that the dischargespecific capacities of the materials were increases first and then decreases with theaugmentation of the amount of Zn3（PO₄）2. When the amount of the Zn3（PO₄）2coatingwas1wt.%, the initial discharge specific capacity of the material was158mAh g-1at0.1C.The Ni2+and Mn2+doping were also studied in this paper. The LiNiyFe1-yPO₄/Cand LiMnyFe1-yPO₄/C were synthesized by high temperature solid phase method usingnickel acetate as the nickel source and manganese acetate as the manganese source.The structure, morphology and electrochemical performance of the LiNiyFe1-yPO₄/Cand LiMnyFe1-yPO₄/C were characterized. The results showed that Ni2+or Mn2+doping did not change the crystal structure of LiFePO₄/C, but the discharge specificcapacity and cyclic stability of the materials were apparently enhanced. When theamount of Ni2+and Mn2+doped was2%and3%, the samples showed the bestelectrochemical performances among all the samples. The initial discharge capacitiesof LiNi0.02Fe0.98PO₄/C and LiMn0.03Fe0.97PO₄/C were160and162mAh g-1, respectively, and the capacity retentions of the samples were upto98%after50cyclesat a rate of1C. LiFe0.95Ni0.02Mn0.03PO₄/C sample prepared by high temperature solidphase method exhibits an enhanced electrochemical performances, the initialdischarge specific capacities of LiFe0.95Ni0.02Mn0.03PO₄/C at the rates of0.1C and1Cwere165and145mAh g-1, even at the high rate of10C, the capacity ofLiFe0.95Ni0.02Mn0.03PO₄/C was still as high as110mAh g-1.