Crystal Modulation And Electrochemical Performance Of Na₃Sc₂P₃O₁₂-based Solid Electrolyte

Compared with lithium-ion batteries,sodium-ion batteries possess the advantages of low cost and easy access to sources.The research and development of the positive,negative and electrolyte materials of sodium-ion batteries is of great significance to develop sodium-ion energy storage devices.The sodium superionic conductor（NASICON）with a stoichiometric ratio of Na₃Sc₂P₃O₁₂（NSP）has a variety of crystal types,namelyα,βandγphase.Different crystal structures exhibit different ion transport characteristics,resulting in the varied the ionic conductivity and the electrochemical performance of the material.This thesis takes Na₃Sc₂P₃O₁₂（NSP）-based solid electrolyte as the research object to optimize its ionic conductivity and electrochemical performance through the crystal structure adjustment by doping ions.Furthermore,we investigated the effect of using different crystal types of NSP as inorganic fillers on the gel polymer’s chemical properties.The main results obtained are as follows:1.Silicon ion doping is adopted to adjust the crystal types of Na₃Sc₂P₃O₁₂ solid electrolyte and its electrochemical performance.The silicon ion-doped Na_3+xSc₂P_3-xSi_xO₁₂（x=0,0.26,0.5）was prepared by high temperature solid phase method.XRD analysis shows that the Na₃Sc₂P₃O₁₂ sample exhibits inα-type in the absence of silicon ion doping,while the samples of Na_3.26Sc₂P_2.74Si_0.26O₁₂ and Na_3.5Sc₂P_2.5Si_0.5O₁₂ with silicon doping 0.26 and 0.5 exhibit a sameβ-typed crystal.The magnitude of electrical conductivity of these three samples at room temperature is in the order of Na_3.5Sc₂P_2.5Si_0.5O₁₂,Na_3.26Sc₂P_2.74Si_0.26O₁₂ and Na₃Sc₂P₃O₁₂,showing that the conductivity ofβ-Na₃Sc₂P₃O₁₂ is greater thanα-Na₃Sc₂P₃O₁₂.This is due to the conversion of the crystal structure from monoclinic to triclinic.The influence of different amounts of Na₃BO₃ sintering flux on the electrical conductivity of the material was also investigated.The experimental results show that the sample with 4.8 wt.percent Na₃BO₃ exhibited an best performance in each group of sintered bodies with the electrical conductivities values of 8.680×10^-5 S cm^-1,2.024×10^-4 S cm^-1,2.436×10^-4 S cm^-1 respectively.2.The effect of different crystalline-type Na₃Sc₂P₃O₁₂ fillers on gel polymer electrolytes was investigated by adding Na₃Sc₂P₃O₁₂ fillers to PVDF-HFP/PMMA gel polymer electrolyte.The relation of the influence of different crystalline Na₃Sc₂P₃O₁₂on the gel polymer electrolyte and battery performance was studied in detail.The experimental results show that whenα-Na₃Sc₂P₃O₁₂,β-Na_3.5Sc₂P_2.5Si_0.5O₁₂ andγ-Na₄Sc₂P₂Si O₁₂ are used as inorganic fillers,their optimal addition amounts are all the same as 7 wt.percent.In addition,in-depth research has been done on the gel electrolyte and battery performance through preparing three crystal types with the optimal addition amount.The results show that for the gel electrolyte withγ-Na₄Sc₂P₂Si O₁₂ as the inorganic filler gets,the optimal conductivity to 2.783×10^-3 S cm^-1 and the first discharge specific capacity is at 92.3 m Ah·g^-1,and the retention rate is as high as 98.18percent after 100 cycles.Following theγ-Na₄Sc₂P₂Si O_12,the filler electrolyte and battery performance ofβ-Na_3.5Sc₂P_2.5Si_0.5O₁₂ andα-Na₃Sc₂P₃O₁₂ behave in sequence.This is because the porosity variation of gel electrolytes prepared from inorganic fillers with different crystal types,the different capability to absorb electrolyte,and different conductivity of inorganic fillers themselves.3.The mechanochemical（MC）method was applied to modify the interface of Na₃Sc₂P₃O₁₂-PVDF-HFP/PMMA to improve the performance of the composite gel polymer electrolyte battery.Upon ball milling the inorganic filler and adding it to the polymer solution,it is found that the gel electrolyte prepared by Na₃Sc₂P₃O₁₂ ball milling for 4 h exhibits the optimal performance with electrical conductivity of3.078×10^-3 S cm^-1,the first discharge specific capacity of 93.6 m Ah·g^-1,and the retention rate as high as 97.97 percent after 100 cycles.This is due to the uniform of particle size for Na₃Sc₂P₃O₁₂ powder after ball milling for 4 h.The gel electrolyte prepared with this powder leads to the optimal porosity and capability to absorb electrolyte.