Study Of Lithium Iron Phosphates As Cathode Materials For Lithium-ion Batteries

Olivine structure lithium iron phosphate,LiFePO₄,has recently attracted significant interest due to its high-safety,long cycle life,low cost and environmental benignity components.With aim to synthesize LiFePO₄/C composite with good electrochemical properties,many studies have been carried out including:synthesis and modification method,conductive webs build,structure properties, electrochemical properties and structure-properties relationship.In this study,the CTR method was employed to synthesize the carbon-coated LiFePO₄ using three different surface area carbon including acetylene black,VXC and BP2000 carbon black.The purpose of current investigation was to find the effects of different specific surface area carbon on the structure and electrochemical properties of LiFePO₄.According to active energy calculation of crystalline growth, the relationship between specific and particle size has been investigated.The results showed that the carbon with higher specific surface area could hinder the growth of LiFePO₄ crystalline clearly,improve the electrochemical kinetic of LiFePO₄ electrode.Based on the material design concept of "nano-micro",Using 2-methoxyethanol-water solution as the media,nano-LiFePO₄/carbon composite cathode material has been synthesized via a simple and new sol-gel route from iron nitrate,lithium dihydrogen phosphate.X-ray diffraction analysis,field emission scanning electron microscopy and transmission electron microscope observations showed that LiFePO₄ with a well-crystallized olivine structure appeared in the heat-treated powder,and primary particles were nanocrystalline,secondary particles were sphere-liked micrometer nano-cluster.Through pyrolysis of the sucrose dispersed in aqueous gelatin,carbon can be in situ coated on the surface of LiFePO₄ crystalline to form thin carbon film(5～8nm).To connecting between LiFePO₄ crystalline forms conductive webs inter secondary particle.Specific surface area measurement showed that coated carbon film with porous structure on the surface of LiFePO₄ particles did not block the direct contact between the active particles and penetrated electrolyte.According to adjust the secondary particle size using different milling time for heated products,tap density of final products could be optimized. The favorable physical characteristics of the nano-LiFePO₄/carbon composite materials exhibited excellent rate performance and cyclability,delivering a discharge capacity of 103 mAh·g^-1(10 C charge/discharge rate) after 50 cycles.LiFePO₄ materials with in situ formed conductive carbon and Fe₂P phase were synthesized by gel precursor assistant carbothermal reduction method.XRD and SEM-EDX analysis identified the existence of Fe₂P phase,which was produced by the reduction reaction of phosphate and iron oxide in excess of carbon.The results indicated that Fe₂P could optimize the conductive webs by increasing the electronic conductivity so as to promote the electrochemical kinetics.The electrochemical performances of the LiFePO₄/C+Fe₂P powder,synthesized at 700℃,containing 3.8 %of Fe₂P were evaluated using an electrochemical model cell by galvanostatic charge and discharge at different charge/discharge rates.The material achieved capacities of ca.160 mAh·g^-1 at 0.1 C rate and ca.102 mAh·g^-1 at 5 C rate, exhibiting good discharge capacity and rate capability.In addition,the effects of Fe₂P on charge/discharge and electrochemical properties at high temperature(55℃) were investigated.The results showed that Fe₂P was non-active material without structure change during charge and discharge,but the capacity at 55℃faded seriously.Finally,corresponding synthesis mechanism of Fe₂P at relatively low heat-treatment temperatures was obtained by TGA,XRD and FE-SEM.Recently there is increasing interest in spinel Li₄Ti₅O₁₂ as a potential anode material for Li-ion batteries.In order to improve the conductivity and tap density, coating carbon on the surface of Li₄Ti₅O₁₂ was synthesized by a rheological phase method.Its average particle size is about 2.1μm with a narrow size distribution as a result of homogeneous mixing of the precursors.The in situ carbon coating produced by decomposition of PVB played an important role in improving electrical conductivity,thereby enhancing the rate capacity of Li₄Ti₅O₁₂ as anode material in Li-ion batteries.The Li₄Ti₅O₁₂/C composite,synthesized at 800℃for 15 h under argon,containing 0.98 wt.%of carbon,exhibited better electrochemical properties in comparison with the pristine Li₄Ti₅O₁₂,which could be attributed to the enhanced electrical conductive network of the carbon coating on the particle surface.Finally, the electrochemical properties of Li₄Ti₅O₁₂/LiFePO₄ cell initially assessed by CV and charge/discharge measurements showed that it had a flat charge/discharge platform and no SEI film during Li⁺ insertion.