Study On Preparation And Electrochemical Performance Of NASICON-type Solid Electrolyte

In recent years,the application of lithium-ion batteries in portable electronic products,electric vehicles and even aerospace has become more and more common.However,organic liquid electrolytes were applied in most commercial lithium-ion batteries,which have potential safety risk of leakage and inflammability.Solid electrolyte instead of traditional organic liquid electrolyte can effectively improve the safety property of battery due to its excellent mechanical strength and thermal stability.At the same time,the storage of lithium carbonate is very limited,which makes the development cost of lithium-ion batteries more and more expensive.Sodium is rich in resources and cheap on the earth.Na₃Zr₂Si₂PO₁₂ solid electrolyte based on NASICON has become the most potential inorganic solid electrolyte for sodium ions due to its low price,high safety and stable chemical properties.The room temperature conductivity of Na₃Zr₂Si₂PO₁₂ solid electrolyte is only 10^-4 S cm^-1,which cannot meet the application requirements of all-solid-state batteries.The preparation of Na₃Zr₂Si₂PO₁₂ materials requires high temperature and is prone to produce impurity phases.In this paper,by adding sintering additives and co-doping method,the sintering temperature of the electrolyte is reduced,and its density and electrochemical performance are improved.In addition,the effects of ionic radius and valence on the structure and properties of the electrolyte in the co-doped system were investigated.The specific studies are as follows:1.On the basis of Na₃Zr₂Si₂PO₁₂,Y element was used to doped Na₃Zr₂Si₂PO₁₂ solid electrolyte,and the effects of Bi₂O₃ additive on the phase,microstructure and ionic conductivity of Na_3.1Y_0.1Zr_1.9Si₂PO₁₂ solid electrolyte were investigated.By adding 1 wt.%Bi₂O₃ to Na_3.1Y_0.1Zr_1.9Si₂PO₁₂ and sintering at 1100℃ for 6 h,excellent electrical conductivity and dense microstructure of 1.21×10^-3 S cm^-1 can be obtained at room temperature.At the same time,the electrochemical performance test showed that the electrochemical stability window was up to 4.8 V,and had a good cycle stability.2.The effects of different doping amounts of Y and La on Na₃Zr₂Si₂PO₁₂ solid electrolyte were investigated by co-doping Na₃Zr₂Si₂PO₁₂ solid electrolyte with rare earth elements Y and La.In Na_3.1+xLa_xY_0.1Zr_1.9-xSi₂PO₁₂（NL_xYZSP,x=0,0.025,0.05,0.075,0.1）system,the most uniform and dense structure was obtained when x=0.075,and the high ionic conductivity of 1.81×10^-3 S cm^-1 was achieved,which was much higher than the conductivity of Y doped sample.In Na_3.075+xY_xLa_0.075Zr_1.925-xSi₂PO₁₂（NY_xLZSP,x=0、0.05、0.1、0.15、0.2）system,with the increase of Y doping content,Na₃La（PO₄）₂ and LaPO₄of the system increase gradually,which has a positive effect on the improvement of conductivity.However,the increase of proosity limits the further improvement of ionic conductivity.The most excellent Y0.1La0.075 solid electrolyte was selected to assemble the Na symmetric battery,which showed a wide electrochemical window of 5 V and excellent cycle performance at room temperature.3.Through the combination of Y with different valence elements(Zn²⁺,La³⁺,Sn⁴⁺)and Y with different radius elements(Sc³⁺,Yb³⁺,Sm³⁺,La³⁺),The influence of valence state and radius of doped ions on NASICON solid electrolyte in the co-doped system was studied by using the control variable method.In the co-doped system,the valence and ionic radius of the co-doped elements have little effect on the phase,and NASICON solid electrolytes without secondary phases such as ZrO₂ can be obtained.For the microstructure of the solid electrolyte,the larger the average ionic radius of the co-doping element,the higher the density of the electrolyte,and the greater the conductivity.The valence has little effect on the microscopic morphology.However,the conductivity gradually decreases with the increase of the valence.Further constant current charge and discharge tests show that the polarization voltage of the solid electrolyte is positively correlated with its own conductivity.