Structural Modification Of Nasicon-type Na₃Zr₂Si₂PO₁₂ Solid-state Electrolytes And Performances Study Of All-solid-state Batteries

With the advantages of high energy density,long cycle life and wide operating temperature range,lithium-ion batteries dominate the energy storage market.With the rapid growth of new energy vehicle sales in recent years and the future development of energy storage grids,the competition in the market for lithium resources will become increasingly intense.However,the global distribution of lithium resources is extremely uneven,and the contradiction between the growing demand for lithium and the unbalanced and insufficient lithium resources is deepening.Sodium-ion batteries have similar working principles to lithium-ion batteries,and due to the advantage of resources,sodium-ion batteries will become a perfect complement to lithium-ion batteries.In addition,most of the new energy vehicle batteries on sale today use liquid electrolytes,which have safety issues associated with thermal runaway,such as the frequent spontaneous combustion of electric vehicles like Tesla.The use of solid-state electrolytes instead of traditional liquid electrolyte can effectively solve the problem of spontaneous combustion of traditional liquid batteries.As the core of solid-state sodium batteries,the solid-state electrolyte determines the performance parameters of solid-state batteries greatly,including safety performance,cycling stability,temperature adaptation,and lifetime.Among the sodium solid-state electrolytes studied so far,the Nasicon electrolyte has the advantages of wide electrochemical window,excellent chemical and electrochemical stability,however,the bottleneck problem of lower ionic conductivity at room temperature must be solved to realize the large-scale application of Nasicon electrolyte materials.Therefore,it is of great importance to promote the development of Nasicon materials in the field of all-solid-state sodium ion batteries by microscopically modulating the materials from a structural perspective to achieve optimization of ionic conduction performance as well as all-solid-state battery performance,and also to elucidate the mechanisms of performance improvement.Based on this,this paper takes Nasicon type Na₃Zr₂Si₂PO₁₂ as the object of study and uses ion doping to regulate the phase structure,the concentration of carriers,the cell volume and chemical bonding of Nasicon type compounds to improve their ionic conductivity,while revealing the intrinsic mechanism by which ion doping affects the structure and ionic conduction.On this basis,the performance of all-solid-state batteries with Na₃Zr₂Si₂PO₁₂ as the solid-state electrolyte is further optimized,providing a basis for realizing all-solid-state batteries operating over a wide temperature range.The main research results obtained are as follows:1.IIIB paralog ion-doped Na₃Zr₂Si₂PO₁₂.Sc³⁺,Y³⁺,Gd³⁺and Lu³⁺ion doped Na₃Zr₂Si₂PO₁₂ materials were designed and synthesized by solid phase reaction method.The effects of the doping ion types and concentrations on the structure,morphology and ionic conductivity properties of Na₃Zr₂Si₂PO₁₂ were illustrated in detail by XRD,SEM and EIS tests.It was determined that the sample with a composition of Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂ possessed the highest ionic conductivity of 1.77×10^-3 S cm^-1 at room temperature.Structure analysis further shows that trivalent ion doping of Zr⁴⁺leads to more sodium ions entering the lattice,which increases the concentration of carriers in the crystal cell.More importantly,Sc³⁺doping not only reduces the temperature at which the Nasicon structure transitions from monoclinic to rhombic phase,increasing the mass fraction of the rhombic phase in the sample,but also changes the length of the Na-O bond and the size of the bottleneck that needs to be crossed for ion transport.Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂ samples possess the longest Na1-O and Na2-O bond lengths,and almost equal T1 and T2 dimensions.This study shows that ion doping can improve the ionic conductivity of Nasicon in a way that Sc³⁺doped Na₃Zr₂Si₂PO₁₂solid state electrolytes find application in future all solid-state sodium ion batteries.2.Performance of Sc³⁺-doped Na₃Zr₂Si₂PO₁₂ solid-state electrolytes for all-solid-state batteries.Based on the enhancement of the ionic conductivity of Nasicon materials by ion doping,the obtained optimal component Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂ electrolyte was further assembled into an all-solid-state battery.systematic electrochemical studies revealed that the constructed Na₃V₂（PO₄）₃/Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂/Na solid-state sodium ion battery has excellent electrochemical performances over a wide temperature range of 0 to 80°C.After 300 cycles at 30°C,the discharge specific capacity remains at 98.7m Ah g^-1,with a capacity retention rate of 90.9%.Even under freezing temperature,the battery still delivers a high discharge capacity of 99.2 m Ah g^-1 after 100 cycles at 0.1C,with excellent cycling performance and a high-capacity retention（89.4%）.In-situ EIS analysis showed that that the good cycling performance of Na₃V₂（PO₄）₃/Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂/Na all-solid-state batteries is closely related to the high repeatability of the Na_3.4Zr_1.6Sc_0.4Si₂PO₁₂/Na₃V₂（PO₄）₃ interfacial resistance R_C/SEduring charge and discharge.This study provides important information for the operation of Nasicon solid state electrolytes over a wide temperature range.3.Sc/In co-doped Na₃Zr₂Si₂PO₁₂.In this chapter,the Sc/In co-doping strategy is designed to modulate the phase content,structure and electrochemical properties of the Na₃Zr₂Si₂PO₁₂ sample by using Sc³⁺doping to induce the transition from monoclinic to rhombic phase and broadening the ion transport channels through In³⁺doping.Detailed studies determined that the Na_3.35Zr_1.65Sc_0.2In_0.15Si₂PO₁₂ sample possessed the highest room temperature ionic conductivity of 8.9×10^-4 S cm^-1.The assembled solid-state cells exhibited good cycling stability when the sample was used as a solid-state electrolyte.This study shows that the Sc/In co-doped Na₃Zr₂Si₂PO₁₂ material has potential as a solid-state electrolyte for all-solid-state sodium ion batteries,and the proposed co-doping strategy may also provide insight into the synthesis and preparation of more versatile materials.4.Sc/Mg co-doped Na₃Zr₂Si₂PO₁₂.Based on the+3-valent metal ion co-doping in the previous chapter,Sc/Mg co-doped Na₃Zr₂Si₂PO₁₂ was further synthesized to achieve the modulation of the crystal structure by lower concentration of ion doping through Mg²⁺regulation of the carrier concentration in the cell.Of the samples synthesized in the series,the Na_3.4Zr_1.7Sc_0.2Mg_0.1Si₂PO₁₂ sample had the highest room temperature ionic conductivity of 1.26×10^-3 S cm^-1 and an activation energy of 0.25e V.Electrochemical performance tests showed that the sample exhibited high discharge specific capacity and excellent cycling stability over the temperature range of 0-80°C when used as a solid-state electrolyte.The battery has been shown to retain 90%of its capacity after 440 cycles at 30°C and 1 C,and 105.5 m Ah g^-1 after 100 cycles at 50°C and 0.2 C,which is higher than that reported in the literature so far.Combined with structural and other analyses,the relationship between dopant ion concentration,cell volume,Na-O bond length,Na ion transport channel size and cell performance is revealed.This study shows that the introduction of Mg²⁺can bring down the price of the electrolyte material without sacrificing the battery performance.Modulation of the crystal structure of electrolyte materials and improvement of ionic conductivity through co-doping of low valence Mg²⁺and Sc³⁺could be an effective way to enhance the electrochemical performance of electrolyte materials.