Synthetic Optimization Of Iron Phosphate To Lithium Iron Phosphate And Sodium Iron Fluorophosphate Cathodes

In recent years,the rapid development of portable electronics and electric vehicles has promoted the research and application of lithium-ion batteries（LIBs）.As a cathode material for commercial lithium-ion batteries,lithium iron phosphate（Li Fe PO₄）stands out for its unique advantages of high specific capacity,high thermal stability,high safety and low cost among the cathode materials for lithium-ion batteries.Numerous studies have been conducted to improve the kinetic performance of Li Fe PO₄ based on its own low electrical conductivity and lithium ion diffusion rate.The study of iron phosphate as a precursor for the preparation of Li Fe PO₄ cathode materials and the diversity of iron phosphate processes has led to changes in the structure and morphology of iron phosphate having a dramatic impact on the electrochemical performance of Li Fe PO₄ cathode materials.To address these issues,this study uses electrolysis to synthesize iron phosphate with different morphologies and structures by adjusting the p H of the solution as the object of study,which is prepared into carbon coated Li Fe PO₄ cathode material and Na₂Fe PO₄F cathode material by carbon thermal reduction.The effect of different morphologies and structures of iron phosphate precursors on the performance of LIB and NIB was analyses.The content of this thesis is divided into three systems as follows:（1）In the synthesis of precursor hydrated iron phosphate by electrolytic anodic flocculation waste iron scrap is fully utilized as the iron source for the preparation of precursor iron phosphate,phosphoric acid as the phosphorus source,hydrogen peroxide as the oxidising agent and ammonia as the p H adjuster.The study was carried out to determine the optimum p H,reaction temperature,reaction time and process parameters such as dehydration temperature and dehydration time for optimisation.Different crystalline structures and morphologies of iron phosphate were synthesized under the optimum process conditions and characterisation such as XRD,SEM,ICP,FT-IR,TG-DSC,BET,XPS and particle size distribution were used to analyses the influences on the synthesis of iron phosphate precursors.（2）Different shaped and structured precursors of Fe PO₄ were used to synthesise homogeneous carbon-coated Li Fe PO₄ particles by carbon thermal reduction.The three-dimensional morphology and structure of the Fe PO₄ precursors were prepared into Li Fe PO₄ particles with the advantages of promoting electrolyte penetration and enhancing electron migration,thus comparing the enhanced electrochemical performance.Self-assembled spherical iron phosphate particles from nanorods and nanoparticles were prepared as Li Fe PO₄ cathode materials,which belonged to the orthorhombic crystal system by XRD analysis（JCPDS Card.No:81-1173）.The initial discharge specific capacities were 162.8,151.4,134.7,116.2,85.5 and 47.7m Ah·g^-1 at 0.2,0.5,1,2,5 and 10 C magnifications,respectively,and the discharge specific capacities were 104.1 and 99 m Ah·g^-1 after 300 cycles at 1 C current density with 85.5%and 84.7%capacity retention.The kinetic analysis showed a small charge transfer impedance and a high lithium ion diffusion coefficient in the electrochemical impedance.（3）The Na₂Fe PO₄F cathode materials were prepared by coating different structures and morphologies of iron phosphate with graphene oxide,combined with sodium fluoride and sodium acetate,and tested for initial charge/discharge,multiplicative performance and cycling performance based on comparing the effects of different precursor structures and morphologies on the electrochemical performance of the cathode materials.The results showed that the Na₂Fe PO₄F/C/r GO prepared from primary nanorods and nanoparticles self-assembled into sub-micron spherical structure precursors exhibited the best electrochemical performance with initial discharge specific capacities of 95.57 and 92.41 m Ah·g^-1 at 0.1C,while the Na₂Fe PO₄F/C/r GO materials prepared from mesoporous spherical particles and Nano spherical particles exhibited initial discharge specific capacities of 89.00 and 73.00 at0.1C.The capacity of the Na₂Fe PO₄F/C/r GO materials prepared from mesoporous spherical particles and Nano spherical particles was 89.00 and 73.25 m Ah·g^-1 at 0.1C,while the capacity of the Na₂Fe PO₄F/C/r GO materials tested at different multiplicities and at 1C for 100 cycles showed almost no degradation and a low charge transfer impedance by kinetic analysis.