Study On Synthesis And Electrochemical Performances Of Olivine Phosphate Cathode Materials For Lithium Ion Batteries

The electrode materials,especially the cathode materials,determine the electrochemical performances of one lithium ion battery（LIB）.Moreover,the proportion of cost for cathode material can achieves 40%of one battery.Considering the performances and cost as two crucial factors,the cathode materials with excellent properties and low cost have always dominated the developing trend of the research and commociallization for LIBs.The olvine LiFePO₄（LFP）and LiMnPO₄（LMP）materials have many advantages,such as better structure and thermal stability,rich resource and evironmentally frendly.LFP has been applied as cathode material in power batteries and other high-end fields,while LMP with higer valtage vs.Li+/Li will become an advanced highspot of the cathode materials for the next-generation high energy density LIBs.This thesis takes the synthesis of high-performanced olivine cathode materials by low-cost technology as the starting point.LFP was prepared by a wet milling route which is suitable for industrialized production;the composite surfactants were introduced as coating carbon source to enhance the electrical performances of the as-prepared LiFePO₄/C materials.The spherial carbon-coated nano-LiFePO₄/C particles with core-shell structure were prepared by a simplified one-step process using FeOOH as iron source.The nano-sized LMP particles were synthesized by a liquid phase method;the N-doped carobn layer from dopemine was coated on the particles.At the same time,we conducted the research on the pilot-scale prodution and commecial process.The carbon-coated LiFePO₄/C was prepared by a wet milling method with the compounding use of two kinds of surfactants instead of traditional carbon source.The particles disperse uniformly and the residual carbon has higher graphitic degree.The synergistic effect of the composite surfactant was studied as well as its effect on the electrochemical performances.The as-prepared LiFePO₄/C material exhibits superior electrochemical properties.The specific discharge capacity is 168.7 and 111.8 mAh/g at the rates of 0.1 and 10C,respectively.The capacity fading is less than 1%after 200 cycling times,which shows good rate and cyclic performances.The nano-sized spherical LiFePO₄/C particles with core-shell structure were prepared by a facile one-step route using FeOOH as iron source.The sample shows superior electrochemical performances.The specific discharge capacity is 163.6 and 121.6 mAh/g at the rates of 0.1 and 5C,respectively.There is no obvious fading of the capacity after 200 cycling time.In addition,the prepared material also delivers good capacity at low temperature.The formation mechamism was studied.With the help of surfactant,the crytal growth of the interphase can be controlled in a solid state systerm,as a result,a kind of porous LiFePO₄/C particles with three-dimentional structure was designed and successfully synthesized.This research offers a new thought for the commercial preparation of LiFePO₄/C production.The rod-like LiMnPO₄/C particles were synthesiezed by a coprecipitation in liquid.The particles disperse uniformly and the size is about 100 nm.The dopemine was selected containing N element as coating carbon source to further improve the electronic conductivity.The XRD,XPS and TEM were used to characterize the structure,morphology and carbon structure.The effect of surface modification by N-doped carbon on the enhancement of the electrochemical performances was studied.On the basis of the pilot production,the three generations of LiFePO₄/C productions prepared using different coating carbon source were selected as the research objects.The effect of different carbon source on the electrochemidal performances was explored.The pilot technology was successfully applied in enterprise for large-scale production of LiFePO₄/C by inductrial equipments.The product material exhibits spherial and ellipsoid shape and excellent electrochemical performances in soft-packed cell using graphite as anode material.The ratio of the capacity at 5C to 0.1 C is 92.1%,and the retention is more than 90%after 500 cycles.The achievement realizes the innovation cooperation between laboratory and enterprise,which gathers theory experiences and reflects important application value.