Synthesis And Electrochemical Characterization Of LiFePO₄Cathode Mateiral For Lithium-ion Battery

In this thesis, the newest development in research of electrode material oflithium-ion batteries has been reviewed. The LiFePO₄/C electrode material withcore-shell structure has been synthesized by carbonthermal reduction method fromFe²⁺and Fe³⁺salts, respectively. And the cube-like LiFePO₄has been prepared byhydrothermal method. The microstructures and morphologies of the electrode materialswere investigated by XRD, SEM and TEM. The electrochemical properties of theseelectrode materials were evaluated by galvanostatic charge-discharge. The maincontent of this thesis is shown as following:LiFePO₄/C electrode materials have been synthesized by carbonthermal reductionmethod using self-regulating FePO₄·2H₂O from iron source of FeSO₄·7H₂O. Mainfactors effecting on electrochemical performance of the product were discussed,including the way of adding hydrogen peroxide, different lithium salts, disparatephosphate source and various carbon resource. The results show that FePO₄precipitation is well dispersed with fine diameter and approximately sphericalmorphology when hydrogen peroxide is added by the way of injection. Then, the finalproducts exhibit superior electrochemical electrochemical property. LiFePO₄/Cparticles are sphere-like with an average size of100nm that reduce the diffusion timeof Li⁺. And a thin carbon film coats on the surface of LiFePO₄crystal, which facilitiesconduction of electrons. LiFePO₄/C material with the best property was prepared byusing LiOH·H₂O, H₃PO₄and soluble starch as lithium salt, phosphate source andcarbon resource, respectively. The sample delivered145,138,127,116and106mAh/gat0.5,1,2,5and10C, respectively, and it exhibited desirable cycling ability.FePO₄·3H₂O prepared from FeCl₃·6H₂O and H₃PO₄was used to synthesizeLiFePO₄/C electrode material by carbonthermal reduction method with soluble starchbeing used as carbon source and reducing agent. The properties of LiFePO₄/C samplessynthesized from different temperature, molar concentrations and pH during the liquidreaction were compared. Each resource has its own contribution to the elements thatconstitute the product, and the liquid process is easy control, facile operate and shorttime. Study reveals that the LiFePO₄/C composite has the best electrochemicalperformance when the liquid temperature, molar concentrations and pH are20C,0.1mol/L and2.14, respectively. From SEM images, it can be seen that the FePO₄·3H₂O particles are composed of smaller one, and the second particles are well dispersed withirregular spherical morphology. The partical size of LiFePO₄/C is in the range60-120nm, smaller than the size of FePO₄particals which is about80-200nm. Thisphenomenon may be attributed to the fracture of the second particles of FePO₄·3H₂Oprecipitation. In addition, high crystalline LiFePO₄/C has a core-shell structure withcarbon on the surface of LiFePO₄crystal, and, the measured tap density is1.2g/cm3.Discharge specific capacities of the LiFePO₄/C composite cathode were165,152,144,137,126,116,98, and89mAh/g when current rate were0.2,0.5,1,2,5,10,20, and30C, respectively. Remarkably, as long as the current rate reverses back low current rate,the specific capacity can recover to the original value. Furthermore, LiFePO₄/Ccomposite displayed superior electrochemical performance in long-term cycling at2Cand5C rates. Even at a high current density of20C, the material could still deliver aspecific capacity about71.4%of its initial capacity after500cycles.The LiFePO₄electrode material was prepared by hydrothemal method at230Cusing FeSO₄·7H₂O, LiOH·H₂O and H₃PO₄as start materials. The preliminarilyexperiments were carried out to analyze the electrochemical property of differentLiFePO₄samples from various organic acids and pH value. When the pH value is4.8,the sample has the best property using citric acid as additive. The particles of thematerial are high crystalline, well dispersed and cubic-like. The sample delivered144,123and99mAh/g at0.1,0.2, and0.5C, respectively, and it exhibited stablecirculation property.