B. Dong, G. Li, X. Yang
University of Nottingham Ningbo China,
Keywords: micro/nano-structure, mesoporous, FePO4, lithium-ion batteries, two-step growth scheme
Summary:Olivine lithium iron phosphate (LiFePO4) has been considered as a promising cathode material in fabrication of lithium ion battery because of its features of low cost, lower toxicity, higher stability, excellent capacity retention, extremely flat discharge plateau (3.45 V) and good theoretical capacity of 170 mA h g-1. However, it also possesses some drawbacks, including poor intrinsic electronic conductivity (10−9 to 10−8 S cm−1), ionic conductivity (10−11 to 10−9 S cm−1) and ionic diffusivity (10−17 to 10−14 cm2 s−1), which have negtive impacts on the application of LiFePO4 to fabrication of high-rate lithium ion batteries. It has been recognised that reducing particle size to nano-scale and introducing porous structure can increase specific surface area and shorten the diffusion distance of Li+ ions, leading to an improvement on electrochemical performance of electrode materials but decrease in both the volumetric and the gravimetric energy density. Thus, it would be desirable for LiFePO4 to have micro-nano hierarchical structure with higher porosity to ensure maintainence of the features of high tap density and high rate capability. The present work has attempted to sythesise hierarchical micro/nano-structured mesoporous FePO4·2H2O composites by the adoption of two different two-step synthesis methods, i.e. two-step heterogeneous growth and two-step hydrothermal schemes. As many previous studies have demonstrated, the application of the impinging streams is able to significantly enhance the mass transfer rate of the reactant solutions through strong turbulent eddy interactions due to the impingement of two narrow reactant streams at high velocities. We thus employed the impinging stream reaction as primary process to synthesize nano-scale FePO4·2H2O seed crystals (30-50 nm). Two-step heterogeneous growth of nano-scale FePO4·2H2O seed crystals was realised by transferring the seed crystals to a stirring tank reactor to prepare hierarchical micro/nano-structured porous FePO4·2H2O spheres through a continuous co-precipitation process. The two-step hydrothermal technology allows nano-scale FePO4·2H2O seed crystals to mixing with glucose solution first. Afterwords, the mixture was transferred and sealed in Teflon-lined stainless steel autoclave, heated at 170 ℃ for several hours. The structural characterization of FePO4·2H2O and corresponding LiFePO4/C products were then carried out by X-ray diffraction, scanning electron microscopy, and Brunauer-Emmett-Teller surface Analyzer while the electrochemical performance was characterized by charge-discharge measurement. It has been clearly demonstarted that the hierarchical micro/nano-structured mesoporous LiFePO4/C products prepared by these two different methods can produce excellent rate capacities and superior cycling performance. In addition, it was found from the experiments that the tap density of LiFePO4/C products prepared using the two schemes can attain as high as 1.4 g cm-3. The improved electrochemical performance and tap density can be attributed to the formation of hierarchical and high mesoporous structures, large specific surface areas, nanoscale primary grains, secondary secondary particle size, and enhanced structural stability.