Conductivity Manipulation And Conduction Behavior Of Na_3.4Zr₂Si_2.4P_0.6O₁₂Based Electrolyte For All-Solid-State Na Battery

Compared with lithium-ion batteries,sodium ion secondary batteries are expected to be applied in large-scale energy storage due to the advantages of abundant sodium resources and low cost.The rapid development in recent years of sodium ion secondary batteries will undoubtedly accelerate its commercialization process.Traditional sodium ion batteries have serious safety risks due to the use of organic liquid electrolytes with poor thermal stability,flammability,and leakage.Compared with organic liquid electrolytes,solid electrolytes have the advantages of high thermal stability,low flammability,no leakage and volatility,and no risk of fire and explosion.Using solid electrolytes to replace traditional organic liquid electrolytes can greatly improve the battery security,therefore has a broader development prospect.Among the solid electrolytes,NASICON solid electrolyte Na_3.4Zr₂Si_2.4P_0.6O₁₂?NZSP?has attracted attention because of its advantages such as high ionic conductivity,good stability to sodium anode,and excellent mechanical properties.This paper mainly explored the preparation process of NZSP solid electrolyte,and further improves the electrochemical performance of NZSP by introducing NaF as a sintering aid.In addition,by doping and modifying its Zr site in this paper,the relationship between the concentration of sodium ions in the crystal structure after doping and the conductivity of the electrolyte was explored as well.In the research of this paper,high-performance NZSP electrolyte was prepared by a new solution-assisted solid state reaction.The effects of sintering temperature and sintering time on the evolution of NZSP phase were investigated.High-purity NZSP electrolyte were prepared by being sintered at 1260 ? for 5hours.It was found that the ion conductivity at room temperature could reach 2.77mScm^-1,the density reached more than 90%,and the electrolyte sodium conduction activation energy was 0.28eV.On the basis of the previous work,by introducing NaF sintering aids,the density of NZSP solid electrolyte was improved and its electrochemical performance was also enhanced.XRD analysis showed that the phase structure was stable after the introduction of NaF,and only a small amount of ZrO₂ heterophase existed.After the introduction of NaF sintering aid,the electrolyte sheet was denser,the transmission of sodium ions in the grain boundary was significantly improved,and the ionic conductivity was increased to 4.21mScm^-1.The results of the study indicated that the introduction of NaF sintering aid was beneficial to sodium conduction.By using heterovalent metal cations(Sc³⁺,In³⁺,Nb⁵⁺)to dope-modify the NZSP solid electrolyte Zr site,introducing more sodium ions or sodium vacancies into the crystal structure and adjusting the doping amount,the research found that Na_3.3Zr_1.9Nb_0.1Si_2.4P_0.6O₁₂?NNZSP0.1?had the best performance,and the ion conductivity was as high as 5.02mScm^-1,indicating that the optimal sodium ion concentration was 3.3 mol per unit formula.Na|NNZSP0.1|Na symmetrical battery was assembled,which can cycle stably for 60 h at a current density of 0.05mAcm^-2.The verification result of Na_3.3Zr₂Si_2.3P_0.7O₁₂ with the same sodium ion concentration of 3.3 mol showed that the increase in conductivity after Nb doping was not only the effect of sodium ion concentration in the crystal structure.The analysis of the grain and grain boundary impedance showed that the grain impedance of the NNZSP0.1 sample was slightly reduced,and the grain boundary impedance was significantly reduced,indicating that the Nb doping had a sintering assisting effect,and the phase purity was higher after the Nb doping,no ZrO₂ heterophase was generated to reduce ionic conductivity,and therefore,the grain boundary conductivity was significantly improved.