Synthesis And Electrochemical Characterization Of LiFePO₄ Cathode Material With Porous Carbon Layers

With the environmental pollution intensifies and the fog and haze appears, as well as the traditional energies(coal, oil and gas) exhausted, human care for the development and use of efficient and clean new energy. As a kind of representative energy storage device, the use of lithium-ion batteries has received much attention. The olivine structure Li Fe PO4(LFP) as a cathode material for lithium-ion batteries, with low price, safety and non-polluting, high theoretical capacity, good cycling stability and other advantages. However, the low er electronic conductivity and lithium ion diffusion rate is a serious impact on hindering the large-scale commercial applications. Considering this, the paper gives a facile method to synthesis the Li Fe PO4 to improve the problems. PAN and PAN-b-PMMA are taken as carbon sources, respectively. The carbon layer has the effects of controlling the particle size and morphology,dispersity and improving the aggregation. Using XRD, SEM, TEM, BET and Raman to characterize the structure and morphology, and the galvanostatic charge/discharge system are used to study the electrochemical properties of cathode materials. The main contents are as follows:Firstly, using carbon thermal reduction method, the low cost Fe SO4·7H2O as original materials and two steps to prepare Li Fe PO4. We study the initial concentration of the reactants, the kind of carbon sources and calcination temperature that impact on the structure and morphology of Li Fe PO4 and electrochemical properties. The results as follows: PAN as carbon source, the prepared Li Fe PO4/C particles dispersed more evenly, no significant agglomeration, the outside of Li Fe PO 4 particles coated with a layer thickness of the carbon about 4 nm. The charge-discharge tests show that the coated electrode material has the best electrochemical performance of 164.7 m A h g-1 at 0.2 C rate, superior to the primal Li Fe PO4, even the discharge specific capacity can reach 62.1 m A h g-1 when the current density reaches 30 C. Also, the charge transfer resistance Rct of Li Fe PO4/C is 58.62 Ω, inferior to the 66.73 Ω of unmodified Li Fe PO4. That is to say, the existence of carbon source enhances the electronic conductivity of Li Fe PO4 to a large extent.Secondly, an in situ carbon thermal reduction method has been synthesized shell-core structure Li Fe PO4/PC with mesoporous carbon layers. We study how the reaction temperature and content of carbon source affect the electrochemical performance of Li Fe PO4/PC. Research shows that: when the reaction temperature is 700 °C and the content is 10 %,Li Fe PO4/PC displays the good morphology and the superior electrochemical performance. The Li Fe PO4/PC has the mesoporous carbon layers with a size of about 15.06 nm, dispersed well, no agglomeration, and the outer of the sample coated with carbon layers. The sample has a good rate performance of 78.9 m A h g-1 at a current density of 30 C, it also have excellent cycle performance of above 90 % after 500 cycles.