Modification Of Iron-based Mixed Phosphates And Their Electrochemical Performanc

Potassium-ion batteries（PIBs）have attracted extensive attention in recent years due to their advantages such as abundant raw materials,high operating voltage,and relatively high energy density.However,the large ionic radius of potassium leads to irreversible structural deformation of the material during charging and discharging,making PIBs typically have a low reversible capacity and poor multiplicat i ve performance.Therefore,exploring a suitable cathode materials with excelle nt electrochemical performance and stable structure has become the key to developing PIBs.The iron-based mixed polyanion material Na₄Fe₃（PO₄）₂（P₂O₇）is a very promising cathode material for PIBs due to its low synthesis cost,environmental protection,high theoretical capacity(129 m Ah g^-1),and three-dimension open framework.However,the intrinsic low electrical conductivity of the material hinders its further developme nt in the field of electrochemical energy storage.Herein,this thesis intends to improve the electrochemical performance of Na₄Fe₃（PO₄）₂（P₂O₇）materials through differe nt modification methods,and explore the potassium storage mechanism and structural evolution process during charge and discharge.The specific content is as follows:（1）Design and synthesis of metal ion-doped iron-based mixed polyanion materia ls.Na₄Fe_2.7M_0.3（PO₄）₂（P₂O₇）（M=Zn,W,Ni）cathode materials doped with different metal ions were prepared by the sol-gel method combined with the subsequent high-temperature annealing process.The effects of different transition metal（Zn,W and Ni）doping on the crystal structure and microscopic morphology of the materials were investigated by XRD,Raman,SEM,and EDS characterization methods.The results show that transition metal doping does not change the crystal structure of the materia ls.However,W doping causes the material to undergo grain orientation growth while increasing the graphitization of the encapsulated carbon in the material.The influe nce of different transition metal doping on the electrochemical performance of the material was compared by CV,GCD,EIS and GITT test methods.The results show that Zn,W,and Ni doping increases the electrochemical polarization and reduces the reversibilit y of the electrochemical reaction.The increase of electrochemical impedance during cycling tests is the main reason for the electrode capacity decay.However,W doping improves the rate performance of the material,whose specific capacity is 71.6 m Ah g^-1 at 2C current density.The excel ent rate performance comes from the fact that W doping improves the material’s electrical conductivity.（2）Construction of reduced graphene oxide coated iron-based mixed polyanio n composites.Na₄Fe₃（PO₄）₂（P₂O₇）@r GOx%（x=1,2,4）composites coated with differe nt proportions of graphene were prepared by freeze-drying combined with subsequent high-temperature annealing process.In order to optimize electrode materials,XRD,Raman,TG,XPS,SEM,BET,TEM and EDS characterizations were carried out to explore the influence of different ratios of graphene coating on the crystal structure and microscopic morphology of composite materials.The results show that graphene coating does not change the crystal structure of the material.The degree of graphitization decreases with the increase of coating amount.The reaction mechanis m and structure evolution during the charging and discharging process were characterized by ex-situ XPS and ex-situ XRD,revealing the highly reversible potassium intercalation/deintercalation processes.The effects of different ratios of graphene coating on the electrochemical performance of composite materials were compared by CV,GCD,EIS and GITT test methods.The results show that the material after graphene coating has less polarization and excel ent electrochemical reaction kinetics,which achieves excel ent rate capability(80.3 m Ah g^-1 at 2 C)and cycling stability（82.1%capacity retention after 500 cycles at 2 C）.In addition,the increase of electrochemic a l impedance and the collapse of the structure during cycling are the main reasons for the capacity fading of the electrode.These results confirm the advantages of the reduced graphene oxide conductive network in enhancing the electrochemical performance.