Preparation And Property Of NASICON Solid State Electrolytes For Sodium Ion Batteries

At present,most of the commercial sodium ion electrolytes are carbonate-based liquid organic electrolytes,which are easily leaked,flammable,and polluted,even caused explosion when the battery is out of thermal control.Sodium super ion conductor（NASICON）inorganic solid electrolytes have good conductivity due to their excellent three-dimensional sodium ion channel.And NASICON materials poessess good air stability,high thermal stability,wide electrochemical window and good compatibility between sodium metal which can solve the safety problems of battery and hope to developing batteries with high energy density.The ionic conductivity of pure Na₃Zr₂Si₂PO₁₂ solid electrolyte is 6.4×10^-4S/cm at room temperature.However,the ionic conductivity of electrolytes are not enough for the actual application,and the sintering performance is poor which need be prepared at high temperature（>1200 ^oC）.In this paper,the Na₃Zr₂Si₂PO₁₂ solid electrolyte is mainly synthesized by the solid state reaction method.The sintering performance and ionic conductivity of the samples are improved by single/dual doping of the Zr site or by adding additives.Mainly include the following:1.For the first time,Sm³⁺was used as a single doping element to replace the Zr⁴⁺of Na₃Zr₂Si₂PO₁₂ solid electrolyte.In order to obtain a solid electrolyte with high density and high ionic conductivity,we studied the amount of Sm³⁺for the microstructure and performance of solid electrolyte.The results show that the structure of sample becomes denser and the ionic conductivity becomes higher with the dopoing of Sm³⁺.When the amount of Sm³⁺is up to 0.09,the ionic conductivity is the best at room temperature,which is 1.14×10^-3 S/cm.And by studying the Arrhenius plot of total ionic conductivity of Na_3.09Zr_1.91Sm_0.09Si₂PO₁₂,it was found that the ionic conductivity of the electrolyte increases with the temperature and they are formed a good linear relationship.What’s more,the activation energy of Na_3.09Zr_1.91Sm_0.09Si₂PO₁₂ is 0.32 e V.2.The microstructure and performance of co-doping Mg²⁺and different elements(Zn²⁺,Al³⁺,Sn⁴⁺or Nb⁵⁺)substituted Zr⁴⁺on the solid electrolyte of Na₃Zr₂Si₂PO₁₂ was studied.The NMZZSP,NMAZSP,NMSZSP,and NMNZSP materials synthesized by different co-doped element components have a tendency to increase in grain size relative to the undoped electrolyte,and the grains size is more uniform.Among them,NMAZSP and NMSZSP materials exhibited glass phase on the surface mi crostructure,which promotes the densification of the material structure and at the same time improves the ionic conductivity of the material.And Na_3.3Mg_0.1Al_0.1Zr_1.8Si₂PO₁₂ sample showed the optimum ionic conductivity,which is 1.02×10^-3 S/cm.The influence of the amount of co-doped components on the performance of the electrolyte was researched by synthesizing Na_3.2+2xMg_xZn_0.1Zr_2-xSi₂PO₁₂（x=0.02,0.04,0.08,0.1）and Na_3.28+2yMg_0.04Zn_yZr_ySi₂PO₁₂（x=0.05,0.08,0.1,0.12,0.15）by solid state reaction method.The research shows that the Zr O₂ impurity peak is enhanced with the Mg-doping content increases and the ionic conductivity of the samples increases firstly and then decreases.When x=0.04,y=0.1,Na_3.28Mg_0.04Zn_0.1Zr_1.96Si₂PO₁₂ exhibit the optimum ionic conductivity of 1.13×10^-3 S/cm.3.Research of the effect of the additive tin antimony oxide（ATO）on the ionic conductivity and sintering temperature of the Na₃Zr₂Si₂PO₁₂ solid electrolyte.The results show that the addition of ATO can significantly enhance the sintering performance of the electrolyte,strengthen the densification process,and improve the conductivity of the Na₃Zr₂Si₂PO₁₂ solid electrolyte.The Na₃Zr₂Si₂PO₁₂ solid electrolyte sintered at 1100 ^oC with 5 wt.%ATO additive shows a relatively high ionic conductivity of 1.43×10^-3 S/cm at room temperature and a wide electrochemical window up to 4.6 V（vs.Na/Na⁺）.In addition,the Na₃V₂（PO₄）₃/NASICON/Na all-solid sodium battery was assembled and showed good cycle performance.The specific discharge capacity of the battery a t 0.1C is112.7 m Ah/g.As the cycle progresses,the polarization voltage increases and the specific discharge capacity decreases.The performance of the battery is also affected by the current density.As the current density increases,the capacity decay becomes more obvious.