Study On Synthesis And Modification Of Lithium Iron Phosphate Cathode Materials For Lithium-ion Battery

LiFePO₄ cathode materials has advantages in the field of high theoretical capacity（170 mAh·g-1）, stable structure, environmental friendliness, and low cost. Meanwhile, they have great practical application in power battery. However, their large-scale applications are restricted by low electronic conductivity, lithium ion diffusivity rate, operating voltage and energy density. For these shortcomings, in this article, nano-LiFePO₄ materials were prepared by three different synthesis methods, and the electrochemical performance of LiFePO₄ were improved by carbon coating, reducing crystal size and multi-composite through sol-gel method.In the synthesis of LiFePO₄: LiFePO₄ was synthesized by ion exchange method, two-step method, and sol-gel method, whose particle size were about 100-200 nm,50-100 nm, 30-80 nm, respectively. And the discharge capacities were 144.6, 146.4, and 160.5 mAh·g-1 at 0.5 C after 50 charge-discharge cycles, respectively. So the modification of reducing crystal size makes a great difference to electrochemical performance of LiFePO₄ and the sol-gel method is the best synthesis method.In the design, preparation, and electrochemical performance of phosphate-based（1-x）LiFePO₄·x LiMnPO₄/C and（1-x）LiFePO₄·xLi3V2（PO₄）3 binary composite materials: A series of binary composite materials of Fe-Mn and Fe-V were prepared by sol-gel method. The test showed that the electrochemical performance of the material is the best when the ratio of Fe、Mn is 9:1, and the discharge capacity reached 161.4 mAh·g-1 after 50 charge-discharge cycles at 0.05 C. That the electrochemical performance of the material is the best when the ratio of Fe、V is 5:3, and the discharge capacity reached 153.0 mAh·g-1 after 50 charge-discharge cycles at 0.5 C. The discharge capacity of low rate of composite materials is slightly lower than pure LiFePO₄, but the discharge capacity of composite materials is well above pure LiFePO₄. So high-rate electrochemical performance of LiFePO₄ added by Mn or V was improved.In the design,preparation,and electrochemical performance of phosphate-based ternary composite materials zLiFePO₄·xLiMnPO₄·yLi3V2（PO₄）3/C: A series of phosphate-based zLiFePO₄·xLiMnPO₄·yLi3V2（PO₄）3/C ternary composite materials were prepared by sol-gel method. The test showed that the crystallinity, particle size, and electrochemical performance of the composite materials is good when the ratio of Fe、Mn、V is 8:5:3. The initial discharge capacity was 154.6 mAh·g-1 at 0.05 C and the discharge capacity was 151.2 mAh·g-1 after 50 charge-discharge cycles, the capacity retention was 97.8 %. The electrochemical performance of the composite materials is good when the ratio of Fe、V、Mn is 9:7:2. The initial discharge capacity was 151.3 mAh·g-1 at 0.5C and the discharge capacity was 150.6 mAh·g-1 after 50 charge-discharge cycles, the capacity retention was 99.5%. So the operating voltage, electronic conductivity, lithium ion diffusivity rate, discharge specific energy and high rate electrochemical performance of Li FePO₄ were improved by being added by LiMnPO₄ of high operating voltage and Li3V2（PO₄）3 of high lithium ion diffusivity rate, which shows great meanings of practical application.