The Study On Preparation Of Battery Level Iron Phosphate

Olivine lithium iron phosphate is one of the cathode materials which used inpower lithium-ion batteries. We often produced the lithium iron phosphate by ironphosphate and lithium compound as the raw material and adding other ingredients atthe same time, which exhibit large electrochemical activity and high tap density, sothe industry pay more attention to it. There are a variety of technology procedures toprepare the iron phosphate, the methods mostly are as follows: one is we can obtainthe product by transferring metal iron which is prepared by metal into iron hydroxideand then react with phosphoric acid,we can obtain the product.; in addition, we canuse the iron salt to get it by replacement reaction, using the ammonium hydroxide toadjust the reaction system. We can obtain the ultimately product isFe(OH)x(PO4)y nH2O by the above method which the number of hydroxyl andhydrated crystalline water is uncertain; or the product is Fe(NH4)x(OH)y(PO4)z nH2Owhich the number of hydroxyl, ammonium and basic hydration water is uncertain. Ifwe use such a crystal structure which is made of uncertain iron phosphate ratio,hydroxyl, ammonium and crystallization water as raw material to prepare the lithiumiron phosphate, the final mixture is difficult to control the ratio of lithium, iron andphosphate as we required. The paper prepared the iron phosphate, which have stablecrystalline structure, the ratio of iron and phosphate and crystalline water byoptimizing process and electrolysis. With homemade iron phosphate as raw materialget carbon-coated lithium iron phosphate by high temperature solid phase. Wecharacterized the physical and chemical properties of iron phosphate and lithium ironphosphate by SEM, TG-DSC, XRD, CV, EIS and other analytical techniques. Theconclusion as follows:(1) Precipitation optimum process parameters: we can obtain the monoclinicsystem yellow-white iron phosphate products in the ratio of iron and phosphate is0.96-1.0, the number of crystalline water is2.0-2.2under the following conditions:control the molar ratio of ferric nitrate and ammonium di-hydrogen phosphate is1:2 during the reaction system; the CTAB or SDBS what the mass of it is the ferricnitrate’s1.5%as additives and control the pH of the reaction between2.0-2.5, thetemperature is65℃-75℃.(2)The ammonium di-hydrogen phosphate as electrolyte at a concentration of1.0mol/L, using the phosphoric and sodium hydroxide to adjust the pH=2, using themembrane-type electrolytic and the metal iron as the anode and nickel as the cathode,using the electrolysis methods of constant current or constant voltage, during theprocess we adding20%hydrogen peroxide solution constantly into electrolyte, theratio of hydrogen peroxide and electrolysis current is1:1, then we can obtain theamorphous FePO42H2O. When we using the constant voltage electrolysis obtainedthe product,the particles of FePO42H2O is more uniform.(3) Prepared FePO4by co-precipitation and electrolytic method, and the lithiumhydroxide as materials, the glucose as a reducing agent, then we prepared the goodcrystalline of LiFePO4/C composites by carbon thermal reduction. The morphologyof LiFePO4/C composite don’t change a lot at different surfactant (CTAB and SDBS),which is hexagonal and spherical respectively. The XRD analysis exhibit that theproduct is well match with olivine LiFePO4(NO.40-1499), and there are no impurityphase in product. The morphologies of LiFePO4/C composite synthesized by differentelectrolysis are spherical, which are well match with the orthogonal crystal system(NO.19-0712). However, the peak of the material which obtained under constantcurrent is shape and has high crystalline.(4)We discuss the performance of LiFePO4/C composite which synthesized byCTAB, SDBS and electrolysis process. The electrochemical tests showed that due tothe precursor synthesized by SDBS have spherical structure, so it has a stabledischarge voltage in the following working, the spherical structure improves the tapdensity of LiFePO4/C and increase the surface area of the product. Precursors wereprepared by constant current and constant voltage to obtain the composite materialswhich have good electrochemical performance,the first charge-discharge efficiency is95.3%and96.6%respectively, and the initial capacity is148.1mAh/g and150.2mAh/g.