Effect Of Preparation Process Of Ferric Phosphate On Properties Of Lithium Iron Phosphate

As a kind of renewable energy,LiFePO₄ battery material has high electrochemical activity and high tap density,and is widely used in electric vehicle and energy storage materials.However,the energy density and charge-discharge performance stability of LiFePO₄ has been a major problem.The precursor FePO₄ has an important effect on the electrochemical properties of LiFePO₄.Therefore,it is urgent to study the effect of the FePO₄ on the electrochemical performance of LiFePO₄ and its mechanism.In this thesis,the effect of preparation technology of FePO₄ on its physical and chemical properties was studied,and the energy density of LiFePO₄ was increased by optimizing the calcination temperature of FePO₄,the performance stability of LiFePO₄ was improved by optimizing the P/Fe ratio.In this thesis,different P/Fe,calcination temperature,aging time and feeding time were used to synthesize battery grade FePO₄,the iron-phosphorus ratio,specific surface area,surface morphology,phase composition and particle size of the precursor FePO₄ were investigated.The results show that when P/Fe=1.05-1.2,the raw material of FePO₄ can meet the requirement of iron-phosphorus ratio of battery grade,the ratio of iron to phosphorus in FePO₄ was decreased due to the entry of phosphoric acid into the product.When the calcination temperature was increased from 550℃to 650℃,the specific surface area of cell-grade FePO₄ was 4-10 m²/g,but it decreased with the increase of calcination temperature.In addition,the specific surface area of iron phosphate can be increased by increasing aging time.The content of S impurity in FePO₄ is affected by reaction temperature,water washing and aging time.With the increase of reaction temperature,the content of S impurity in FePO₄ decreases,and the content of S decreases with the increase of water washing amount and aging time.The sulfur impurity can be reduced to 14 ppm by three times water washing,and to 10 ppm by 150 min aging time.With the increase of aging and feeding time,the morphology of iron phosphate changed from inhomogeneous“Starfish”to disc-like,and then the flake-like structure was assembled into loose and porous spherical“Agate-like”structure,this structure can improve the compaction density of FePO₄.LiFePO₄/C was synthesized from FePO₄,Li₂CO₃,Ti O₂ and C₆H₁₂O₆（Glucose）at different calcination temperatures and from different raw materials,it was characterized by XRD,SEM,particle size and electrochemical properties.The results show that the electrochemical properties of LiFePO₄/C first increase and then decrease with the increase of calcination temperature,and when the calcination temperature is 550℃(the specific surface area is 7.08m²·g^-1),the electrochemical performance of the synthesized LiFePO₄/C was the best,the first charge was 161.72 mAh·g^-1,the first discharge was 161.33 mAh·g^-1,the theoretical capacity was 95%,the first charge and discharge efficiency was 99.76%,the highest average discharge specific capacity of LiFePO₄/C was 161.07 mAh·g^-1 at 0.1C,and the electrochemical performance of LiFePO₄/C first increased and then decreased with the increase of P/Fe ratio,when the P/Fe ratio was 1.1（the Fe/P ratio was 0.973）,the electrochemical performance of LiFePO₄/C was the best,the first charge was 158.33 mAh·g^-1 at 0.1C,the first discharge was156.38 mAh·g^-1,the first charge-discharge efficiency was 98.77%,the average discharge specific capacity for 20 cycles was 159.52 mAh·g^-1 at 0.1C,and the capacity retention rate was100%at short cycle.