Preparation And Characterization Of Novel Iron Phosphate Precursors And Lithium Iron Phosphate Cathode Materials

LiFePO₄ is an ideal cathode material of lithium ion batteries for electric vehicle and energy storage,for its outstanding advantages in cost,safety,cycling and high temperature performance,etc.Iron phosphate is the important precursor to synthesize LiFePO₄.At present,the new technologies to prepare high-quality,low-cost iron phosphates are being received increasing attention.In this paper,we studied three new methods,including controlled crystallization-aging crystallization method,oxidizing controlled crystallization-aging crystallization method and urea pyrolysis precipitation method,to prepare several new iron phosphates.LiFePO₄ cathode materials were prepared,using the iron phosphates as precursors.The electrochemical performance of the LiFePO₄ was systematically tested.The composition,structure,morphology and physicochemical properties of the iron phosphate precursors and LiFePO₄were characterized in detail by visible spectrophotometry,ICP analysis,chemical analysis,DTA/TGA,XRD,laser particle size analysis,and SEM,etc.It is shown the controlled crystallization-aging crystallization method is suitable for large scale production of FePO₄·2H₂O precursors with low cost and high quality continuously,stably and simply.The precursors show spherical shell shape and good fluidity.The oxidizing controlled crystallization-aging crystallization method has been further improved,which not only can achieve continuous and stable production of iron phosphate precursors,but also can eliminate the pre-oxidation step of Fe²⁺.The new method can simplify the process,significantly save the consumption of acid and alkali,and obviously reduced the costs.Through the research work,we proposed the aging crystallization method,which can convert amorphous FePO₄·2H₂O into crystalline FePO₄·2H₂O.Using this new method,we can effectively regulate and control the composition,structure and morphology of the precursors.This method is especially important to regulate and control the Fe/P molar ratio of FePO₄·2H₂O.Using the above two new technologies to synthesize FePO₄·2H₂O precursors,we can select FeSO₄·7H₂O or Fe?NO₃?₃·9H₂O as the Fe resources,and select NH₃·H₂O or NaOH as the alkali.The synthesized FePO₄·2H₂O precursors have high crystallinity and“micro-nano-structure”.The prepared LiFePO₄ cathode materials show excellent electrochemical performance.The urea pyrolysis precipitation method can obtain the crystalline FePO₄·2H₂O only in one step,and it can eliminate the aging crystallization process,further simplify the technology and reduce the costs.The graphene composited iron phosphate precursors were synthesized via the above three new technologies,and the graphene composited LiFePO₄ cathode materials were further prepared.The rate capability and electrochemical performance at high current density of the graphene composited LiFePO₄ were greatly improved.The pilot-scale production of oxidizing controlled crystallization-aging crystallization technology shows that the new technology is particularly suitable for large scale production of FePO₄·2H₂O precursors continuously and economically.The obtained LiFePO₄ cathode materials have excellent comprehensive properties,especially the outstanding advantage of high density.