Preparation And Electrochemical Study On Nano Lithium Iron Phosphate

Among all of the cathode materials in lithium-ion batteries, the olivine structure LiFePO₄ has developed into one of the most potential cathode materials because of its safety, environment-friendly and long cycle life. And it can be expected to be used in the development and application of power lithium-ion batteries. But due to its inherent defects of low conductivity and tap density, it was be limited in the application for lithium-ion batteries. In this article, we conducted the following studies.?1? Preparation of LiFePO₄/C materials by hydrothermal method and its electrochemical property study.LiFePO₄/C materials were synthesized by hydrothermal method and properties of the as-prepared materials were compared in the condition of different p H values,hydrothermal time and hydrothermal temperature. When the value of p H equals to 6and 8, parts of Fe2+ were oxidized to Fe3+, while p H>8, the materials had well crystal structure. At low p H values, we found serious material agglomeration. When p H=10,the size of particles was the most uniform and the size distribution of particles was the narrowest. In the condition of hydrothermal time of 12 h and and the hydrothermal temperature of 200 ?, XRD results showed that the materials had sharp peak, the strongest diffraction intensity, and the highest degree of crystallinity. In this condition,particles could get enough time and enough energy to nucleation and growth process to meet free growth of the different plane direction. Electrochemical test results showed that the samples prepared in the optimum conditions had the highest charge and discharge capacity and the most smooth curve platform, it had the best electrochemical properties, and its reversible specific capacity reached 145.2, 138.2,132.6, 123.2, 113.0 and 101.9 m Ah g-1 at 0.1C, 0.2C, 0.5C, 1C, 2C and 5C?1C=170m A g-1?, respectively.?2? Preparation of LiFePO₄/C materials by solid-phase method and its electrochemical property study.LiFePO₄/C materials was prepared by solid-phase method. Materials was synthesized after ball-milling 4 hours, 6 hours and 8 hours, respectively, named LFP-1,LFP-2 and LFP-3. In order to improve conductivity of the LiFePO₄, sucrose was added as carbon source. XRD test results showed that pure olivine lithium iron phosphate was synthesized. SEM test results showed that the morphology of LFP-2was regular, and the size of particles were between 200 nm and 400 nm. The firstcharge and discharge capacity of the three samples were 138.7 and 129.9 m Ah g-1,147.2 and 144.1 m Ah g-1, 144.7 and 138.7 m Ah g-1 at the rate of 0.1C, and the coulomb efficiency were 93.7%, 97.9% and 95.9%, respectively. After 100 cycles, the capacity retention rate were 95.4%, 96.9% and 94.5%, respectively. At the rate of0.1C, 0.2C, 0.5C, 1C, 2C and 5C, the discharged specific capacity were 144.1, 138.7,126.4, 118.6, 102.5 and 82.3 m Ah g-1. We proved that the prepared materials afer being ball-milled 6 hours display the best performance.?3? Synthesized of LiFePO₄/C nano-fibers by electrospinning method and its electrochemical property study.LiFePO₄/C nano-fibers were synthesized by electrospinning method. Materials prepared by electrospinning had good regularity and a diameter in the range of150-250 nm. After calcined at 300? in air, the fibers became smooth and maintain a good shape. Then calcined at 700? in N2 atmosphere,the materials remained fibrous structure and had a high aspect radio, the diameter of the fibers had a slight shrinkage than precursor materials. XRD analysis results showed that the prepared materials matched the standard LiFePO₄ well. The charge/discharge capacities in the first cycle were 153.3/148.9 m Ah g-1, 143.0/138.5 m Ah g-1, 124.4/120.5 m Ah g-1, 101.2/94.3m Ah g-1 and 83.9/ 81.6 m Ah g-1 at the rate of 0.1C, 0.5C, 1C, 2C and 5C, respectively,the oulomb efficiency were 97.1%, 96.9%, 96.9%, 93.4% and 97.3%. And the LiFePO₄/C nano-fibers delivered the initial capacity of 139.1 m Ah g-1 after 300 cycles at the rate of 0.1C, the capacity ratio was 93.4%. These results above suggested that the as prepared LiFePO₄/C nano-fibers have excellent electrochemical properties.