Design And Preparation Of Battery Grade Iron Phosphate Doped With Multi-ionic Species

In recent years,demand for lithium ion batteries in energy storage and power field grows rapidly.Due to the high stability,the good cycle performance,safety,environmental protection,low cost and high cost performance advantages,Lithium iron phosphate(LiFePO4)has become a typical representative of the new electrode materials,the performance of the lithium iron phosphate can be improved by doping,but doping uniformity can not be effectively improved.The iron phosphate mixed evenly was propared by putting the ions doping into iron phosphate from the iron phosphate lithium synthetic stage.With ferrous sulfate,by-product of titanium dioxide as source of iron,the technology was industrializad,and this realized the comprehensive utilization of by-products and circular economy,and the performance of the lithium iron phosphate was effectively improved with more ions doped iron phosphate as the raw material preparation.In recent years,with the rapid growth of lithium-ion batteries,and the continuous expansion of application fields,the lithium-ion battery has become significant high-tech product in 21st century for national economy and our life.Lithium iron phosphate(LiFePO4)was used as the cathode material in lithium-ion batteries trace back from 1997.Because of its special superiority,such as high stability,fine cycling performance,environmentally friendly,and high cost performance,it has become a typical representative of new electrode material.The iron phosphate(FePO4),as the main raw material of LiFePO4,has attracted the wide attention for the characteristics of non-toxicity,low cost and stable structure.In this paper,the iron phosphate was originally synthesized in ethanol-water mixture system,the features of the iron phosphate were porous,difficult SSA and low impurity content.When ethanol was replaced by NaOH,Na+ in the iron phosphate was avoided.The optimum conditions of different phosphate particle preparation were obtaind by orthogonal experiment.Which showed that the iron phosphate was monoclinic polymorph,containing two crystal water molecules.The dehydration temperature of crystal water was between 150 and 160?.The mole ratios of phosphorus to iron in two kinds FePO4 were both approximately 1:1.The finer granularity of iron phosphate would cause more sulfur impurities.In the ethanol-water system,multi-doped iron phosphate was synthesized with Ti,Mn and Mg.The Ti,Mg and Mn ions successfully replaced the iron in the crystal lattice of iron phosphate,which changed the crystal lattice parameters,enhanced the inter-planar crystal spacing,favored the disembedding of Li~+ and enhanced the electrical property of LiFePO4.The mole ratio of phosphorus to iron and the sulfur content increased gradually,too.It can be explaid that the doping ions replace Fe atom and combine more P atom,which causes the increase of P content in the product.The lithium iron phosphate,prepared with doped iron phosphate through dry grinding,can not only enhance its electro-chemical property and utilization ratio but also avoid the difficulty of mixing.Comparation of doped and non-doped lithium iron phosphate suggests:The discharge capacity and cycling performance of the 0.5%doped Ti,Mg and Mn were the best.The discharge capacity reached 169mA·h/g under 0.2C and retention rate of capacity was 97.42%after 150 circles.This might be that the doped Ti,Mn,Mg ion into the lattice iron phosphate makes crystal spacing increase,which favored the disembedding of Li+.The electrochemical performance of lithium iron phosphate prepared by such doping iron phosphate was undoubtedly improved.The project of 5000 tons/year iron phosphate was designed and fulfilled.Two kinds of products were produced in production line,multi ions doped iron phosphate and non-doped iron phosphate.The characteristics of the doped iron phosphate were as follows:The primary product was spherical nano powder with the diameter of 30?50 nm,monoclinic crystal type,the transformation crystal type point exists at 600?.The P:Fe(mol)in the multi ion doped iron phosphate was about 1.04.The content of Ti,Mn and Mg were 0.5%,0.05%and 0.02%.Respectively the D50 was about 3?m and Dmax was below 15?m.Its BET was more than 80m2/g.While the primary product characteristics was also spherical with the diameter less than 100nm and monoclinic crystal type;There was crystal transformation in the process of higher temperature roasting,but no mutation point.The P:Fe(mol)in the non-doped iron phosphate was about 1.02.Other impurities were not detected.The D50was more than 10?m,and Dmax was more than 40?m.BET was less than 70m2/g.After four tests,the discharge capacity of button cell using doped lithium iron phosphate was above 155mAh/g under 0.2C.The first charge and discharge ratio was more than 97%,while the discharge capacity was more than 145mAh/g for 1C.As to the non-doped iron phosphate,all the performance were lower than those doped.It shows that the multi-ion doping has a positive effect on improving the electrochemical performance of lithium iron phosphate.