| In recent years,sodium-ion batteries（SIBs）have gained much attention due to the high abundance and low cost of sodium resources.However,SIBs using organic liquid electrolytes encounter severe safety problems,caused by the poor thermal stability,and flammability of organic solvents.In contrast,solid-state sodium batteries（SSBs）using solid electrolytes（SEs）demonstrate the advantages of high safety,wider operating temperature range and feasibility of stacking.Besides,SEs can be directly paired with metallic sodium anode,enhancing the energy density of the battery system.As one of the key components of SSBs,SEs play a critical role in determining the cycling life,energy density and power density of the batteries.Therefore,the exploration of suitable SE materials to satisfy the demand for room-temperature sodium-based SSBs is of great significance.Among different types of SEs,oxide SEs hold great promise owing to the high ion transference number,good mechanical properties,and excellent thermal stability.Nevertheless,challenges still exist such as high sintering temperature,poor interfacial contact with electrodes and large grain boundary resistance leading to insufficient total room-temperature ionic conductivity(10-4～10-3 S cm-1).To solve these issues,in this thesis,a series of high-performance Na5MSi4O12（M=Y,Sm）type SEs are designed and studied for sodium-based SSBs.The ionic transport characteristics and microstructure of them were studied in detail.Additionally,the evolution of the interface between SEs and sodium metal,as well as the effect of interface properties on dendrite growth were revealed by multi-scale characterizations.Finally,a series of SSBs with different SE materials were constructed which demonstrate good electrochemical performance.The presented results could shed light on the ongoing research for the advanced sodium-based SSBs.The main conclusions are summarized as follows:Firstly,we proposed a hexagonal-prismatic-structured fast Na+ ion conductor Na5YSi4O12.Owing to the inherent three-dimensional diffusion pathway and optimized synthesis procedure,the Na5YSi4O12 SE achieves a high room-temperature ionic conductivity of 1.59 × 10-3 S cm-1 and low activation energy of 0.2 eV,which is competitive in the state-of-the-art Li-and Na-based oxide fast ionic conductors.Furthermore,the SE demonstrates an excellent electrochemical stability up to 8.0 V,a stable sodium stripping/plating at a 2.2 mA cm-2 and a long-lived cycling performance for 1200 h without short circuiting.Impressively,using the prepared SE,a Na3V2（PO4）3‖Na5YSi4O12‖Na SSB full cell delivers comparable electrochemical performance as that using organic liquid electrolyte:an initial charge and discharge capacities of 119 and 115 mAh g-1,respectively.Under a current rate of 1 C,a reversible capacity of 98 mAh g-1 can be delivered,with almost no capacity decay after 500 cycles.Secondly,a new hexagonal-prismatic Na5SmSi4O12 was successfully synthesized with higher room-temperature ionic conductivity of 2.90 × 10-3 S cm-1 and lower activation energy of 0.15 eV.Interestingly,an electrochemically induced crystalline-toamorphous（CTA）transformation of Na5SmSi4O12 SE is observed during repetitive deposition and stripping of Na.Computational simulation combined with experimental characterization revealed that such CTA transformation is driven by the large lattice strain during ion intercalation,which,on the other hand,can be speeded up in the Li symmetric cell because of the mismatch between Li+and Na+ ionic radius.Owing to the decreased Na+ migration activation energy,stabilized interfacial properties,and suppressed dendrite formation after SE amorphization,an all-solid-state symmetric cell delivers an improved critical current density of 1.4 mA cm-2 and excellent cycling life of over 800 h at 0.15 mAh cm-2.Moreover,the Na3V2（PO4）3‖Na5SmSi4O12‖Na SSB full cell achieves a remarkable cycling performance over 4000 cycles（6 months）with～100%capacity retention,which further suggests the feasibility of Na5SmSi4O12 towards practical application.Thirdly,to synergistically reduce the synthesis temperature and grain boundary resistance of oxide SEs,we performed grain-boundary engineering strategy on Na5SmSi4O12.By adjusting the amount of starting materials,the Na5SmxSi4O12（x=1,0.9,0.8,0.7,0.6）self-forming composite SEs consisting of Na5SmSi4O12 and Na9SmSi6O18 were obtained at a relatively low sintering temperature of 900℃.The highest ionic conductivity of 3.48 × 10-3 S cm-1 is obtained when x=0.3,which is one order of magnitude higher than the total conductivity of Na5SmSi4O12(5.76 × 10-4 S cm-1).Even at -40℃,it can still deliver a high ionic conductivity of 1.28 × 10-4 S cm-1.Upon further reducing the sintering temperature to 850℃,Na5Sm0.7Si4O12 can still deliver a high ionic conductivity of 3.31 × 10-3 S cm-1 at room temperature.Interestingly,the grain boundary of Na5SmxSi4O12 composite SEs turns out to be blurred and covered with binder like amorphous glassy materials,which boosts the ion migration across the grain boundary.Moreover,benefiting from the increased density,Na5SmxSi4O12 can suppress the growth of dendrite,resulting in improved critical current density and longer cycling life.As a result,a long-term cycling over 2800 h is realized at the current density of 0.1 mA cm-2 at room temperature without overpotential increase or short circuit.Lastly,inspired by the idea of high entropy material design,a high-entropy hexagonal-prismatic Na4.9Sm0.3Y0.2Gd0.2La0.1Al0.1Zr0.1Si4O12 was successfully synthesized after carefully choosing adaptable elements and optimizing synthesis condition.The material can be obtained at a relatively low sintering temperature of 950℃,with a high room-temperature ionic conductivity of 6.7 × 10-4 S cm-1 and low activation energy of 0.22 eV.Remarkably,the ionic conductivity is much higher than that of the low entropy Na5YSi4O12 SE sintered at same temperature.Furthermore,it demonstrates an excellent electrochemical stability up to 6.0 V,a stable sodium stripping/plating at current density up to 0.6 mA cm-2,a superior rate performance and a long-lived cycling over 700 h without short circuiting.As a result,Na3V2（PO4）3‖highentropy SE‖Na SSB delivers a desirable cycling stability with almost no capacity decay after 600 cycles and high Columbic efficiency over 99.9%at room temperature.All these results not only suggest that Na4.9Sm0.3Y0.2Gd0.2La0.1Al0.1Zr0.1Si4O12 is a highly promising SE candidate,but also provide a promising SE designing principle for roomtemperature sodium-ion conductor.In summary,a series of Na5MSi4O12-type materials are designed and studied as SEs for sodium-based SSBs.The ion transport characteristics of them were investigated in detail through the combination of molecular dynamics simulation and experiment characterizations.To tackle the challenges of the high sintering temperature and high grain boundary resistance of oxide electrolytes,we performed grain boundary modification and multi-element cooperative regulation strategy to synchronously decrease sintering temperature and increase total ionic conductivity.For unclear interfacial problems of Na5MSi4O12-type SEs,the evolution of interface between SEs and sodium metal was revealed by collaborative multi-scale characterization techniques and theoretical simulation,which was also extended to lithium-based system.The sodium-based SSBs in this thesis show great possibility in the applications of future large-scale energy storage.Our work can also provide a good starting point and shed light on the subsequent research on novel SSBs and key SE materials. |
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