Synthesis And Modification Of Na₂FePO₄F Cathode Material For Sodium Ion Batteries

Na₂FePO₄F is a promising cathode material for sodium ion batteries because of its high capacity,good cyclic stability and small volume expansion rate.However,it is limited in application due to low Na⁺migration rate and low electronic conductivity.In this dissertation,the materials were synthesized by hydrothermal method.The materials were improved mainly from the two aspects of morphology optimization and surface modification.In the process of hydrothermal synthesis of Na₂FePO₄F,the effects of solvent on the morphology and electrochemical properties of the materials were studied.The results show that the addition of organic solvent can prevent Fe²⁺from being oxidized and avoid the formation of impurity phase,increase the viscosity of the solution and produce steric effect,so as to obtain small particle size.The sample synthesized with30%ethylene glycol and 70%deionized water as mixed solvent has a spherical shape of about 800 nm,showing the best electrochemical performance and the most excellent Na⁺migration rate(D_Na+=1.121×10^-12cm²s^-1,112.3 m Ah g^-1at the rate of 0.1 C and 93.2%capacity retention rate after 200 cycles at 1 C).We selected DBS as the soft template and glucose as the carbon source to explore the effects of different hydrothermal temperatures,time and the amount of DBS on the phase,morphology and electrochemical properties of Na₂FePO₄F.When the amount of DBS was 5mmol and the hydrothermal reaction was carried out at 210℃for 15,20,25 and 30 h,the morphology of Na₂FePO₄F precursor changed from irregular to 4μm spherical,then disappeared,and finally changed to 1μm spherical particles.According to the changes of precursor morphology,we proposed the evolution mechanism of nucleation-dissolution-recrystallization.The specific capacity was 110.7 m Ah g^-1at the rate of0.1 C.The carbon source with lower decomposition temperature （sucrose,ascorbic acid and citric acid）was used to replace glucose.Under the same process conditions,the effects of carbon source on the phase,morphology and electrochemical properties of Na₂FePO₄F prepared by hydrothermal method were compared.The experimental results show that the reduction of carbon source decomposition temperature can make the primary particles more obvious and conducive to the formation of special morphology.The experimental results show that Na₂FePO₄F synthesized with citric acid and DBS has nano particle size,which helps to shorten the transmission distance between Na and electrons,so as to improve the conductivity of the material and make it have excellent electrochemical properties(113.2 m Ah g^-1at the rate of0.1 C and 73.5 m Ah g^-1at the rate of 5C).