Study On The Preparation And Modification Of The Precursor Iron Phosphate For Lithium Ion Battery Cathode Materials

This paper focused on the preparation and modification of lithium iron phosphate for the lithium ion battery cathode material,in which iron phosphate used as the precursor.In order to improve the electrical properties of lithium iron phosphate,the nano-porous iron phosphate precursor and disk-like iron phosphate precursor were prepared by the liquid phase crystal method.Finally,the effect of different modification methods for the iron phosphate precursor on the electrochemical properties of the lithium iron phosphate were also discussed.Firstly,iron phosphate precursors were prepared using liquid phase crystal method,in which ferric chloride,phosphoric acid and nitric acid used as raw material,and the products were characterized by XRD,SEM,FT-IR techniques.The effects of reaction time,reaction temperature and nitric acid concentration on index and yield of samples were discussed.Secondly,at the beginning of the crystallization,the nano-porous iron phosphate precursor was obtained by adding nano-silica,then a certain concentration of hydrofluoric acid solution were used to remove nano silica.The effects of the amount of nanometer silicon dioxide,the concentration of the hydrofluoric acid solution and soaking time on the final morphology and the purity were discussed.The results showed that when the nitric acid concentration of 0.6mol/L,reaction temperature of 95?and the reaction time of 150 min the iron phosphate?FP?samples have the highest index and the largest yield.When the molar ratio of n_Fe:n_Si=1:0.2,hydrofluoric acid solution concentration of 2 _wt%and the soaking time of 60min,the iron phosphate?P-FP60?precursor not only have the nano-porous on the surface,but also have the crisscrossed nano-pores network structure inside.Carbon coated lithium iron phosphate?LFP/C?was synthesized by the high temperature solid phase reduction method with the precursors phosphate iron.By the electrochemical performance test.The results showed that the initial discharge capacity of the LiFePO₄ that prepared by nano-pore structure at 0.1 C increased by24.65%,reached to 159.25 mAh·g^-1,and the resistance reduced by 90%,just 48.32?.compared to the LiFePO₄ that prepared by non nano-pore structure of iron phosphate precursors.At the same time,it also had good reversible cycle stability and excellent rates performance.In the process of crystallization,surfactant CTAB and SDS was added respectively.Through the particle size analysis,it showed that when the surfactant was SDS,the preparation of iron phosphate precursor had fine particle size compared to CTAB,when the ferric nitrate was used as the ferric source,the particle size of iron phosphate precursors was just around 200nm.The effects of different SDS concentration on product index was discussed,the results showed that when the concentration of SDS was 7.5 _wt%,the products had the highest index.The lithium iron phosphate?D-LFP/C?cathode material was prepared by the high temperature solid phase reduction method with the nano-disked iron phosphate?D-FP?as precursor.When D-LFP/C was tested by electrochemical performance.The results showed that the initial discharge capacity of the D-LFP/C at 0.1 C increased by24.65%,reaching to 159.74 mAh·g^-1,and 66.67 mAh·g^-11 at 20 C.The capacity almost no decay after 200 cycles at 0.5 C.The electrode resistance was just 35.73?reduced by 92.4%,as well as the good rate performance and cycle stability were obtained compared with the LFP/C.