Preparation And Electrochemical Properties Of Iron-based Electrode Materials For Sodium-ion Batteries

Iron-based electrode materials are promising electrode candidates due to their low cost,environmental benignity and high theoretical capacity.In this study,iron-based cathode Na₂Fe PO₄F and anode FeS are investigated.Polyanion type cathode Na₂Fe PO₄F has stable structure,but suffers from low electron conductivity and ionic diffusion kinetics.In this study,morphology control and nitrogen doped carbon coating were employed to address these issues.Using sodium dodecyl sulfate（SDS）as pore-forming agent,a sol-gel synthesis route was employed to obtain Na₂Fe PO₄F/N-C with porous 3D network coated with nitrogen doped carbon,and the specific surface area and electric conductivity of the material were effectively improved.The composition and microstructure were characterized by XRD,SEM,BET,TGA and Raman spectra tests.The electrochemical properties were examined and demonstrated by galavnostatic charging and discharging,cyclic voltammetry and AC impedance methods.SDS can effectively anchor F^-ions in the Na₂Fe PO₄F precursor,and the self-assembly of SDS can form sufficient soft template for the porous Na₂Fe PO₄F network.The 3D porous network of Na₂Fe PO₄F can be effectively coated with polydopamine only when the polydopamine coating has rational molecular weight.When the molar ratio of SDS to raw materials is 1:5 and dopamine polymerization reaction time is 6 h,the obtained Na₂Fe PO₄F/N-C electrode exhibits excellent electrochemical performance.Under different current rates of 0.1,0.2,0.5,1,2 and 5 C,the porous Na₂Fe PO₄F/N-C material exhibited discharge capacity of 110.8,108.2,100.0,88.1,72.4 and 53.3 m A h g^-1.At the current rate of 1 C,the material delivered 84.1 m A h g^-1over 500 cycles,with93.6%capacity retained.Transition metal sulfide anode FeS has high specific capacity,but its application potentials are heavily restricted by the huge volume change during discharge/charge and the sluggish kinetics,which result in poor cycling and rate performance of the material.In this study,introducing another sulfide ZnS to form binary sulfide composites,morphology control and electrolyte optimization are employed to improve the electrochemical performance of the material.A co-precipitaion method was employed to obtain FeS/ZnS sulfide composites powder with nanoflower microstructure.Different electrolyte solvent was used in the half cells assembly and investigated on their effects on the electrochemical performance of FeS/ZnS.The results reveal that,with rational molar ratio of Fe/Zn,the sulfide composite can combine the merits from both individual sulfides,and different sulfides can act as mutual dispersant to prevent aggregation and buffer volume variations.The ether-based electrolyte solvent can form high quality SEI with the sulfide composite electrodes,leading to improved electrochemical performance of the battery.When the ratio of Fe/Zn is 1:1 and the electrolyte solvent is diglyme,the material can achieve high reversible capacity of 472.6 m A h g^-1and excellent rate performance of 203.9 m A h g^-1at a high current density of 2 A g^-1,demonstrating enhanced structure stability against volume variations and improved electrochemical kinetics of the material.