LiFePO₄/C Cathode Material Synthesized By The High Temperature High Energy Ball Milling Method

Recently, as one of the most potential cathode materials for Lithium-ion batteries, LiFePO4 has attracted a great deal of attention. However, poor electronic conductivity and slow diffusion of lithium ions of LiFePO4 material prevent its practical applications in Lithium-ion batteries. In order to enhance the electronic conductivity and the diffusion of lithium ions of the material, as well as to gain the LiFePO4 cathode material with excellent properties, a novel preparation method of high temperature high energy ball milling (HTHEBM) was developed. LiH2PO4 and Fe2O3 were used as Li source and Fe source, glucose was used as reducing agent and carbon source. The influences of the operation parameters of HTHEBM method, the carbon sources, the addition amounts of carbon and the ion-doping on the properties of the LiFePO4 material were discussed in detail. The paper has an important reference value for the preparation and application of the LiFePO4 material.After detailed studied, the results showed that:(1) The LiFePO4/C sample with the best properties was synthesized at 600"C for 9h, the ball-to-powder ratio was12:1 and the rotating speed was 80r/min. The button cells with LiFePO4/C composites synthesized at the optimal conditions were assembled for testing the electrochemical performances of the sample. The cells showed the initial discharge capacities of 153mAh/g、144mAh/g、130mAh/g and 108.5mAh/g at 0.1C、1C、5C and 10C, respectively, no obvious capacity fading after the 30th cycle test. (2) The properties of the LiFePO4/C samples synthesized with organic carbon sources were better than which synthesized with the inorganic carbon sources. The sample synthesized with glucose showed the best properties and the initial discharge capacity was 153mAh/g at 0.1C. After the 20th cycle test, the retention ratio of the sample was 98.5%. (3) The addition amounts of glucose as carbon source had a significant impact on the performances of the LiFePO4/C samples. It was proved that the properties of the samples were enhanced with the increasing amount of carbon source. But, the Fe2P impurities were produced when the excessive carbon source was added. The sample showed the best properties with 155mAh/g discharge capacity at 0.1 C as the addition amount was 8%. (4) The LiFePO4/C samples were synthesized by Sn4+-doped or F’-doped. The results proved that the crystal structures of the LiFePO4 material weren’t changed by ion-doping. The LiFe0.97Sn0.03P04/C sample with the best properties delivered 143mAh/g discharge capacities at 5C as the doping content of Sn4+ was 0.03. After the 30th cycle test, the fade of the discharge capacity wasn’t obviously. As the doping content of F- was 0.03, the LiFePO3.97F0.03/C sample with the best properties delivered 147mAh/g discharge capacity at 5C and the discharge capacity fade slightly after the 30th cycle test. (4) The LiFe0.97Sn0.03P03.97F0.03/C sample was prepared via co-doping of Sn4+ and F- The co-doping had no influence on the crystal structure of LiFePO4, but reduced the lattice parameters and the volume of the material. The sample delivered 140.1mAh/g discharge capacity at 5C.