高性能LiFePO4/碳纳米笼锂离子电池正极材料

Carbon Nanocages Supported LiFePO_4 Nanoparticles as High-Performance Cathode for Lithium Ion Batteries

以具有高比表面积、分级孔结构和优良导电性的碳纳米笼(CNCs)为载体,制得了粒子尺寸为10~25 nm且高度分散的LiFePO4/CNCs复合物.以LiFePO4/CNCs复合物作为锂离子电池的正极材料,在0.1 C倍率下首次放电比容量达到163 mAh?g-1,15 C和30 C倍率下的放电比容量可达96和75 mAh?g-1;在15 C倍率下循环200圈后,其放电比容量仍保持在92 mAh?g-1,显著优于LiFePO4/CNTs复合物.这些结果表明,LiFePO4/CNCs复合物具有优异的倍率性能和循环稳定性,是一种性能优良的锂离子电池正极材料,其性能源自CNCs载体的高比表面积、分级孔结构和优异导电性以及LiFePO4颗粒的纳米化和高结晶度.

Olivine-type LiFePO4, as an important cathode material of lithium-ion batteries, has been paying numerous attentions owing to the high theoretical capacity, good cycle stability, environmental friendliness, safety, etc. But, for bulk LiFePO4 materials, there are some shortcomings such as the low conductivity of ca. 10-9 S·cm-1 and low diffusion coefficient of lithium ions. Some methods have been developed to improve its performance, including supporting, packaging, doping, and/or nanocrystallization. Carbon materials have been widely used in lithium-ion batteries as the conductive agent and modifier owing to the good conductivity, excellent electrochemical/mechanical stability, special pore structures favoring the mass transfer, low cost, and environmental friendliness. Among these, the conductivity and pore structure have a significant impact on the electrochemical performance of the cathode materials such as LiFePO4. The carbon nanomaterials with three-dimensional hierarchical structure are paying more and more attentions because of the special pore structure. Herein, we reported the high performance of the cathode materials, LiFePO4 supported on carbon nanocages(CNCs). CNCs are prepared using benzene as precursors and magnesium oxide as template, which have high specific surface area up to 1274 m2· g-1, excellent electrical conductivity of 1.44 S·cm-1, multiscale pore structure, etc. The LiFePO4/CNCs nanocomposite was prepared following the procedure. CNCs were impregnated with the Li+, Fe2+ and 34PO--containing acid solutions, the final products were obtained by sintering at 700 ℃ under the gaseous mixture of Ar and 5 vol.% H2. The component, structure, and electrochemical performances were investigated by X-ray diffraction, transmission electron microscope, thermogravimetry, and electrochemical tests. The LiFePO4 nanoparticles with the size of 10~25 nm were homogeneously dispersed on CNCs. The bulk conductivity of LiFePO4/CNCs was 0.529 S·cm-1, far higher than 2.27×10-9 S·cm-1 of pure LiFePO4. The LiFePO4/CNCs composite delivered a large discharge capacity of 163 mAh·g-1 at a rate of 0.1 C, close to the theoretical capacity of LiFePO4. Even at the high rates of 15 C and 30 C, the discharge capacities still reach 96 and 75 mAh·g-1, respectively. After 200 cycles at the high rate of 15 C, the tested cell still remains a specific capacity of 92 mAh·g-1. These results indicated that the LiFePO4/CNCs composite as cathode material of lithium-ion batteries possessed excellent high-rate performance and cycling stability, superior to the LiFePO4/CNTs composite. It could be ascribed to high surface area, hierarchical porous structure, and excellent electrical conductivity of CNCs support as well as the nanoscale size and high crystallinity of LiFePO4 particles.