Study On One Step Preparation,Characterization And Application Of Crystalline Iron Phosphates

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 several new oxidizing precipitation methods by one step to synthesize crystalline FePO₄·2H₂O,using FeSO₄·7H₂O,Fe powders or Fe plates as Fe resources and H₂O₂ or O₂ as oxidants,respectively.Li FePO₄ cathode materials were prepared,using the FePO₄·2H₂O as precursors.The electrochemical performance of the Li FePO₄ was systematically tested.The composition,structure,morphology and physicochemical properties of the FePO₄·2H₂O and LiFePO₄ were characterized in detail by visible spectrophotometry,ICP analysis,chemical analysis,DTA/TGA,XRD,laser particle size analysis,and SEM,etc.Crystalline FePO₄·2H₂O precursors were prepared via oxidizing precipitation methods by one step at high temperature of 90-95℃,using FeSO₄·7H₂O,H₃PO₄ and H₂O₂ as raw materials.This new method can eliminate the aging crystallization process,further simplify the technology and reduce the costs.We further proposed the improved oxidizing precipitation methods.Using（NH₄）₂HPO₄,Na₃PO₄ or LiH₂PO₄ to partly replace H₃PO₄,or adding urea,the acidity of the reaction system can be effectively reduced and the yields of Fe PO₄·2H₂O precursors can be obviously raised.The above methods are suitable for large scale production of Fe PO₄·2H₂O precursors with high quality continuously,stably and simply.However,there are quite a few of residual S in FePO₄·2H₂O precursors as a result of the coprecipitation of SO₄^2-and PO₄^3-.The performance of the prepared Li Fe PO₄ cathode materials should be improved.The Fe powders or Fe plates can be dissolved in H₃PO₄ solution to obtain Fe（H₂PO₄）₂solution.Adding H₂O₂ into the Fe（H₂PO₄）₂ solution at suitable temperature,FePO₄·2H₂O precursors can be prepared via oxidizing precipitation method by one step.It is shown the temperatue,Fe resources,intermittent or continuous preparation can effect the composition,structure,morphology and physicochemical properties of the FePO₄·2H₂O.The amorphous and crystalline FePO₄·2H₂O can be obtained at low and high temperature,respectively.Using Fe powders as Fe resources at high temperature,the FePO₄·2H₂O synthesized via intermittent preparation had monoclinic structure,while the FePO₄·2H₂O synthesized via continuous preparation had orthorhombic structure.Using Fe plates as Fe resources at high temperature,the FePO₄·2H₂O precursors synthesized via intermittent preparation and continuous preparation had both orthorhombic structure.There are no residual S in the obtained FePO₄·2H₂O precursors.The prepared LiFePO₄ cathode materials showed excellent electrochemical performance.The above new technologies can use the Fe by-producs in automotive industry,recycle the mother liquid,avoid the sulphates by-products and further reduce the costs.Crystalline FePO₄·2H₂O precursors were also prepared via oxidizing precipitation methods by one step,using much cheaper O₂ to replace H₂O₂ as oxidants,and using FeSO₄·7H₂O,Fe powders or Fe plates as Fe resources,respectively.The prepared LiFePO₄ cathode materials showed good electrochemical performance.The new methods can obviously decrease the cost to prepare FePO₄·2H₂O precursors,have application foreground and are worthy of in-depth study.