Study On Synthesis And Modification Of Cathode Materials For Lithium Ion Batteries

The paper adopted carbothermal reduction to synthesize the cathode materials of LiFePO₄/C,Li₃V₂?PO₄?₃/C and xLiFePO₄-yLi₃V₂?PO₄?₃/C,studying synthesis process and ion-doping on the effecs of the composite materials.With the methods of X ray diffraction?XRD?,scanning electron microscopy?SEM?,charge and discharge tests,AC impedance tests and cyclic voltammetry measurements,the crystal structure,surface morphology and electrochemical performance of the materials were investigated and analyzed.LiFePO₄/C materials were prepared by carbothermal reduction method,studying the different preparation process condition on the impact of properties.The results showed that on the condition of 5.0 wt% as the fixed carbon content,the way of coating as frist and second times with the amount of 2.5 wt% respectively,presintering at 350?for 4h and sintering at 750 ? for 8h,LiFePO₄ material was perfectly synthesized,obtaining the small particle size and uniform distribution,higher discharge capacity(135.348 mAh·g^-1),and the capacity retention rate of 98.25% after 20 cycles,which had the good cycling performance.Li₃V₂?PO₄?₃/C materials and xLiFePO₄-yLi₃V₂?PO₄?₃/C materials were synthesized by carbothermal reduction method and composite materials,studying the effects of composite patterns,composite ratio and sintering temperature on its performance.The results showed that the the optimum sintering temperature of synthesized Li₃V₂?PO₄?₃/C materials was 750?,on this condition,the materials had a monoclinic structure and had the initial discharge capacity of 104.573 mAh·g^-1 at 0.2C.With the composite pattern of sintering composite,the composite ratio of 9:1,sintering temperature of 750? for 8h,the prepared materials xLiFePO₄-yLi₃V₂?PO₄?₃/C had the olivine structure and the monoclinic structure,with the high first discharge capacity of 145.535 mAh·g^-1 at 0.1C,the capacity retention rate of 96.23% and good electrochemical performance.LiFe1-xMgxPO₄/C was synthesized by Mg²⁺ doped in Fe?x=0.00,0.02,0.04,0.06?.The results showed that the XRD fingerprint of doped materials was consistent with LiFePO₄ standard spectra picture with olivine structure.The LiFe1-xMgxPO₄/C(x=0.02,LiFe_0.98Mg_0.02PO₄/C)was of the best performance,fine particles and uniform discharge capacity of 143.493 mAh·g^-1at 0.1C rate ratio,increasing by about 6.65% compared with undoped materials(discharge capacity of 134.541 mAh·g^-1),after 20 cycles,the capacity retention ratio retention rate was 97.6%,materials still had a higher discharge capacity and good rate performance at 0.1C,0.5C,1C and 5C after 20 cycles.In the optimum doping amount of 0.02,Mg²⁺ and Mn²⁺ were respectively adopd in Fe for9LiFe_0.98M_0.02PO₄-Li₃V₂?PO₄?₃/C?M=no-doped,Mg,Mn?composite materials for the research on the modification.The results showed that the synthesis of9LiFe0.98Mn0.02PO₄-Li₃V₂?PO₄?₃/C doped Mn²⁺exhibited the best electrochemical performance,at 0.1C,0.5C,1C and 5C,the first discharge capacity was 159.033 m Ah·g^-1,148.947 mAh·g^-1,133.501 mAh·g^-1and 123.657 mAh·g^-1;corresponding discharge capacity after 20 cycles were 148.467 mAh·g^-1,138.307 mAh·g^-1,126.011 m Ah·g^-1and 118.154 mAh·g^-1,which had a better cycle the rate capability,equipped with the electrochemical properties advantages both the Li₃V₂?PO₄?₃ and the Li FePO₄ respectively.