Nanostructure Design And Optimization Of LiFePO₄/C Composites

Lithium iron phosphate (LiFePO4) is a cathode material of lithium-ion battery, because it has relatively high theoretical specific capacity (170mAhg-1), good thermal stability and excellent cyclic performance. However, the intrinsically poor electronic and ionic conductivity greatly impedes its electrochemical performance. Based on the current research progress of LiFePO4, we studied the effects of different carbon sources on the electrochemical performance of LiFePO4/C, and the nanostructure optimization of LiFePO4/CA (carbon aerogel) composites to enhance the electrochemical performance.First, we prepared LiFePO4/C using different carbon sources including polyethylene glycol (PEG), glucose, sucrose, citric acid through the carbon thermal reduction technology. The resulting LiFePO4/C is of a grain size of100nm, and the particle size of LiFePO4/C is hundreds of nanometers to several microns. The final carbon content in LiFePO4/C is proportional to the weight of the carbon sources. LiFePO4/C with proper carbon content exhibites improved electrochemical performance. But too high carbon content will hinder lithium ion diffusion, increase the polarization of charge-discharge. and thus decrease the discharge capacity. The particle size of LiFePO4/C synthesized using divalent iron sources is apparently smaller than that using trivalent iron sources, resulting a better electrochemical performance. The LiFePO4/C prepared by divalent iron sources have the highest discharge capacity when the carbon contents are4.87%,9.13%,1.81%and5.31%using PEG, citric acid, glucose and sucrose as carbon sources, respectively. And the discharge capacities at0.1C,0.5C and1C are about140mAhg-1,120mAhg-1and110mAhg-1. There’s little difference in electrochemical performance of LiFePO4/C when using the four different carbon sources because the materials can reach the similarly highest capacity at their best carbon contents, but glucose and sucrose have the minimum addition amount.The prepared carbon aerogels (CA) with a large number of micropores and mesopores have varied pore sizes and distributions. In the synthesized LiFePO4/CA nanocomposites, LiFePO4nanoparticles smaller than50nm are confined to a three-dimensional CA network, forming an idea nanostructure. The discharge capacities of all the LiFePO4/CA samples at the rate of0.2C are close to the theoretical value. They also have excellent rate performance. For example, the discharge capacity of LiFePo4/CA500at the rate of80C is71.6mAhg-1. LiFePo4/ACA500prepared by KOH activated CA500exhibits an elevated rate performance, and its discharge capacities at the rate of IOC,40C and80C are141.3mAhg-1,112.4mAhg"1and71.9mAhg-1, respectively. The addition of glucose in the preparation of LiFePo4/CA1500can further improve its rate performance, its discharge capacity at the rate of IOC and40C are141.6mAhg-1and90.7mAhg-1, respectively. After1000cycles at the discharge rate of IOC, the synthesized nanocompsites have a capacity retention of about99%, which shows its superior cyclic stability.LiFePo4/C prepared with a small amount of CA has a carbon content of less than10%. The added CA optimizes the particle morphology, decreases the grain size and shortens the lithium ion transmission distance, resulting in better lithium ion conduction. The intrinsically porous structure of CA also improves the contact of active material with electrolyte, resulting in the enhanced electrochemical performance. LiFePO4/C synthesized by the sol-gel method with a small amount CA obviously shows improved rate performance, but the capacity at low discharge rates is slightly decreased. In comparison, LiFePO4/C prepared by the solid-state method has a comprehensively improved electrochemical performance.