Studies On The Preparation,Modification And Na-storage Behaviors Of Na₂FeSiO₄ As Cathode Material For Sodium-ion Batteries

Sodium transition metal silicates,Na₂FeSiO₄,wich possess a three-dimensional(3D)open structure,good thermal stability,high theoretical reversible specific capacity and a very satisfying cost-effectiveness,have inspired the widespread attentions.However,the cathode material of Na₂FeSiO₄ usually suffers from the large irreversible capacity,unsatisfactory cycle-life and poor rate capability due to its inherently inferior electronic conductivity,which restrict their application in SIBs.In order to solve these defects of Na₂FeSiO₄,the synthetic methods for Na₂FeSiO₄material,the effect of crystal structure on electrochemical performance,kinetics of electrochemical reaction and modification routs for improving the electrochemical bahaviours have been discussed in detail.Finally,the cathode material of Na₂FeSiO₄with high discharge reversible capacity,good cycling performance and outstanding rate capability have been successfully prepared.This work offers a promising prospects for the commercial application of sodium iron-based orthosilicates materials in SIBs.(1)The cathode material of Na₂FeSiO₄ have been prepared via the sol-gel methode under different calcination temperature.Series of techniques such as FESEM,TEM,N₂ absorption/desorption curve,FITR and the electrochemical tests were explored to study the effect of alcination temperature on the microtopography,particle size distribution,crystal structure and electrochemical properties for Na₂FeSiO₄.We finally obtain the optimum synthesis routes for the cathode material.All the samples sintered at 550 ^oC?600 ^oC and 650 ^oC show the spherical morphology.The crystallinity is higher and higher as the temperature rises.Particularly,the sample sintered at 600 ^oC exhibits the better sphericity,maximum specific surface area(0.35cm³ g^-1)and high purity.Simultaneously,there is no impurities of Na₂Si O₃,sodium oxides or iron oxide in the sample.Meanwhile,the sample sintered at 600 ^oC shows the better reversible capacity(122.7 mAh g^-1)at 0.1C.After 100 cycles,the reversible capacity is 58.5 mAh g^-1 and the coulombic efficiency is 47.7%.(2)Based on the routs of electrochemical induction,the cathode material of Na₂FeSiO₄/C with the orthorhombic system and space group of Pb2₁a was successfully prepared via the crystal structure rearrangment of the C222₁-Na₂FeSiO₄/C during the first charging process.XRD and ex-situ XRD tests show that the C222₁ phase will fully translate into the Pb2₁a phase after the first charging,and the structure of Pb2₁a phase remains stable during the subsequent charging and discharging process.Simultaneously,after the structural transformation from C222₁ to Pb2₁a,the lattice volume of Na₂FeSiO₄ increases from 382.9214(3)?³to 62.32659(2)?³,forming the channel which is more conducive to the rapid transmission of Na⁺.So,the Pb2₁a-Na₂FeSiO₄/C material have the better cycling stability(the reversible capacity is 67 mAg^–1 after 100 cycles,accompanying with the capacity retation of 57.3%(3)The electrochemical kinetics of Na ions deintercalation in Na₂FeSiO₄/C composite electrode was studied in detail via the two electrochemical techniques(EIS and GITT).By introducing two theoretical EIS models,a series of process of Na⁺transmission in Na₂FeSiO₄/C electrode had been successfully simulated and obtained the kinetic parameters such as charge transfer impedance,SEI-film impedance and CPE capacitance,getting the variation tendency of these kinetic parameters at various desodiation states.After start charging,the R_ct of Na₂FeSiO₄/C electrode is very large(2810?),and the rate of electrode reaction is controlled by the R_ct;As the charging continues,the value of R_ct decrease rapidly,and the kinetics of electrode reaction is mainly controled by the rate of diffusion for Na⁺in the bulk material.Besides,the variation tendency for the diffusion coefficient of Na⁺(D_Na+)calculated from the EIS and GITT remains the same during the sodiation/desodiation process,and the value of D_Na+(order of magnitude in 10^-13-10^-12)for the second desodiation is larger than the first desodiation(order of magnitude in 10^-14-10^-13),demonstratting that the kinetics of Na⁺sodiation/desodiation in Na₂FeSiO₄ electrode is enhanced after electrochemical activation.(4)In order to overcome the defects of poor electronic conductivity and low discharge voltage,The superior Na-storage of Ni-substituted Na₂Fe_1-xNi_xSi O₄@C microspheres encapsulated with the special alveolation-like configuration had been synthesized successfully via the micromorphology control,Ni²⁺dopping in the crystal structure of Na₂FeSiO₄ and construction of a 3D porous carbon skeleton with high conductivity.Based on the effects of the synergism between Ni substitution and porous carbon skeleton with high conductivity,the cathode material of Na₂FeSiO₄used in SIB have the fast ion transfer channel,high electonic/ionic conductivity and the ideal Na⁺diffusion rate.Meanwhile,the first principles calculations showed that the band-gap energy and energy barrier of Na⁺migration for the Ni-substitued Na₂FeSiO₄ were lower that the Ni-free Na₂FeSiO₄ material.So,the rate capability of Ni-substituted Na₂FeSiO₄ material is greatly improved.The reversible capacity of 3%Ni-substitued Na₂FeSiO₄ is 197.5 mAh g^-1 at 0.1 C.After 100 cycles,the reversible capacity is 166.78 mAh g^-1,accompanying by the capacity retation of 84.4%.(5)In order to improve the rate capability and discharge voltage of Na₂FeSiO₄material,the Co-substituted Na₂Fe_1-xCo_xSi O₄@C material have been prepared via the sol-gel method.Based on the various of characterization such as XRD,FESEM,TEM,XPS,ICP,FTIR,EIS and so on,the effects of doping amount for Co in Na₂FeSiO₄on microstructure,the size of nanaoparticles,the crystal structure and electrochemical behaviours had been studied systematically.When the 4%Co was incoparated in the crystal structure of Na₂FeSiO₄,the sample had the good spherical morphology,and the active nano-particles were evenly dispersed,which possessed a hierarchical porous structure.The first discharge reversible specific capacity for the 4%Co-substituted Na₂FeSiO₄ is 173.8 mAh g^-1 at 0.1 C,and the discharge voltage of Co-substituted sample is higher(?0.26 V)than the Co-free sample.Meanwhile,the reversible capacity at a higher discharge rate(2 C)is 135.8 mAh g^-1.