Synthesis And Modification Of Lithium Ion Secondary Battery Cathode Materials For Lithium Iron Phosphate

Recently,the olivine-type LiFePO₄ which offers advantages such as safety, environmental protection,lower cost,higher capacity and steady cycling performance is being actively investigated as a new cathode material for rechargeabal lithium-ion batteries.But the materials' poor conductivity and poor high rate charge-discharge capability limit its commercialization.In order to enhance the electrochemistry capability of material,this paper studied the effects of solid phase synthesis conditions, coating with metal powder or carbon,doping with metal-ion,complex coating and doping on the frame and properties of LiFePO₄.The" structure,morphology,and electrochemical performance of materials were also studied by means of XRD,SEM, AC impedance and electrochemical techniques.LiFePO₄ materials were synthesized by two-step solid phase route using stoichiometric amounts of Li₂CO₃,FeC₂O₄·2H₂O and(NH₄)₂ HPO₄ as raw materials.At first,this paper discussed the characteristics of materials synthesized at different synthesis conditions including different ratios of Li to Fe,different synthesis times and different synthesis temperatures.It indicated:the material possess of improved high rate charge-discharge capability when the ratio of Li to Fe was 1.05,but low rate charge-discharge capability is poorer.The material synthesized at 750℃for 24 hours with 1.00 ratio had pure phase frame,smaller particle size and a first discharge capacity of about 138.62 mAh/g.Therefore,we confirmed this condition is the best.The paper also reseached the effects of coating metal with 1%mass ratio on the properties of LiFePO₄ using AgNO₃,nano copper powder and nano aluminium powder as raw materials.The results of detection results showed that those metal powers existed in the samples,AC impedance value was minished.But charge-discharge capability and cycling performance of the samples were badly decreased.The reason requires further study.We discussed those effects of carbon coating on the properties of materials with various carbon sources including acetylene black,sucrose,graphite and citric acid.At last,the sample that coating with sucrose solution presents better electrochemistry property.Its particle size minished about 200 nm and its electrochemistry AC impedance reduced because of high conductivity of Fe₂P impurity.The mass ratio capacity of sample increased about 40 mAh at 2.0 mA.We found that 10%is the most appropriate ratio among different carbon coating quantities,because its ratio capacity increaed and particle size minished.Doping with Mn²⁺,Ni²⁺and Co²⁺substitute Fe site of LiFePO₄ materials were studied.The results showed that a small amount of metal-ion advanced the high rate charge-discharge capability of materials,and 0.2,0.02,0.15 were the best mole ratio respectively.The experiment also researched on metal-ion doping base on coating 5% carbon,then the mass ratio capacity of samples corresponding increased and the electrochemistry capability of materials improved.