Low temperature sintered oxide-based solid state electrolyte, method of making and use thereof

Abstract

The application belongs to the technical field of solid-state sodium ion batteries, and specifically discloses a low-temperature sintered oxide agent solid-state electrolyte as well as a preparation method and application thereof. 12 The solid-state electrolyte comprises a NASICON type Na3Zr2Si2PO 24 KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, Na8Al6Si6O 24 Cl2, NaAlSi2O6. The application improves the density and ionic conductivity of the solid-state electrolyte by introducing special sintering aids, and also reduces the sintering temperature of the solid-state electrolyte, which triggers a significant secondary recrystallization effect at a lower sintering temperature, so as to realize the synergistic improvement of the solid-state electrolyte in ionic conductivity, interface stability and long cycle performance.

Description

Technical Field

[0001] This invention relates to the field of solid-state sodium-ion battery technology, and in particular to a low-temperature sintered oxide solid electrolyte, its preparation method, and its application. Background Technology

[0002] To meet the ever-increasing demands for safety and cost in energy storage systems, upgrading traditional lithium-ion battery technology is crucial. In this context, solid-state sodium metal batteries combine the high capacity potential of sodium metal anodes with the inherent safety characteristics of solid electrolytes, exhibiting significant advantages in energy density, output power, operating temperature range, and safety redundancy, and are widely recognized as a key candidate for future energy storage technology.

[0003] Among various solid electrolytes, NASICON structural materials (chemical formula Na) are... 1+x Zr2Si x P 3-x O 12 Oxide electrolytes (0≤x≤3) have attracted considerable attention due to their excellent chemical and electrochemical stability and high ionic conductivity. However, these oxide electrolytes are inherently rigid, resulting in insufficient interparticle bonding after sintering, low material density, and numerous internal pores, thus limiting the overall improvement of ionic conductivity. Simultaneously, insufficient solid-state contact often forms between the electrolyte and the sodium metal anode, with poor interfacial adhesion and limited actual contact area. This easily leads to localized current accumulation, stimulating sodium dendrites to grow along grain boundaries or structural defects, ultimately threatening the battery's cycle durability and safe operation.

[0004] To address these shortcomings, the industry often uses low-temperature eutectic sintering aids to improve them. These aids can generate a liquid phase or additional phase in the grain boundary region during sintering, promoting particle reorganization and filling pores, thereby increasing the bulk density and providing some resistance to dendrite propagation. In the prior art, patent CN119994161A discloses a sodium-ion composite solid electrolyte and its preparation and application. By incorporating sintering aids, the sintering temperature of the sodium silicate electrolyte is significantly reduced, but the resulting material still suffers from insufficient density, and its electrochemical performance lacks sufficient verification. Patents CN115472901A and CN116169347B both disclose processes for low-temperature preparation of NASICON-type sodium-ion solid electrolytes; however, the ionic conductivity of the resulting electrolytes does not reach 1 mS / cm. -1 This makes it difficult to meet the practical application standards of solid-state batteries.

[0005] Currently, most additives or the grain boundary phases they induce exhibit poor ion conductivity, or even insulating properties. This severely hinders the migration of sodium ions near the grain boundaries, preventing a substantial improvement in the overall ionic conductivity of the electrolyte. Therefore, facing the industrialization challenge of NASICON-type sodium-ion solid electrolytes with unsatisfactory ionic conductivity due to excessively high sintering temperatures and large grain boundary resistance, developing a novel sintering aid that can effectively reduce sintering temperature, significantly improve material density, and maintain or optimize grain boundary ion transport efficiency has become a key technical problem that must be overcome. Summary of the Invention

[0006] In view of the above-mentioned problems, the present invention provides a low-temperature sintering oxide-based solid electrolyte, its preparation method, and its application. The present invention improves the density and ionic conductivity of the solid electrolyte by introducing a special sintering aid. Furthermore, the introduction of the special sintering aid reduces the sintering temperature of the solid electrolyte, inducing a significant secondary recrystallization effect at the lower sintering temperature; thereby achieving a synergistic improvement in ionic conductivity, interfacial stability, and long-cycle performance of the solid electrolyte.

[0007] To address the aforementioned problems, this invention provides a low-temperature sintered oxidant solid electrolyte, comprising NASICON-type Na3Zr2Si2PO4. 12 The matrix and sintering aids dispersed in the matrix; the sintering aids include KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, and Na8Al6Si6O. 24 At least one of Cl2 and NaAlSi2O6.

[0008] Preferably, the sintering aid is KAlSi3O8.

[0009] Preferably, the mass ratio of the matrix to the sintering aid is 30-99:1.

[0010] Preferably, the solid electrolyte further includes a binder; the binder includes at least one of polyvinyl alcohol and polyvinyl butyral.

[0011] Preferably, the mass ratio of the matrix to the adhesive is 5-15:1.

[0012] Based on the same inventive concept, the present invention also provides a method for preparing the warm-sintered oxidant solid electrolyte as described above, comprising the following steps: S1. After the Zr source, Si source, P source, Na source and solvent A are mixed by ball milling for the first time, the solvent A is removed to obtain a mixed precursor; S2. The mixed precursor is subjected to a first solid-state sintering to obtain a first sintered material; solvent B is added to the first sintered material and it is mixed by a second ball milling, and then the solvent B is removed and the mixture is pulverized to obtain an initial sintered powder. S3. The initial sintering powder, binder and sintering aid are mixed to form a solid electrolyte green embryo; the solid electrolyte green embryo is subjected to a second solid-phase sintering and then naturally cooled to room temperature to obtain the low-temperature sintered oxide solid electrolyte.

[0013] Preferably, the media used in the first / second ball milling is zirconia balls, and the ratio of the total mass of the zirconia balls to the total mass of the Zr source, Si source, P source, and Na source to the mass of the first sintering material is 10-30:1. The ball milling temperature is 20-30℃, the ball milling speed is 400-500 rpm, and the ball milling time is 10-15 h.

[0014] Preferably, in step S3, the preparation method of the sintering aid is as follows: weigh the raw materials according to the stoichiometric ratio of different elements in the sintering aid, mix them by ball milling, add solvent C and continue ball milling to mix, remove the solvent C to obtain precursor powder; heat the precursor powder to 1000-1500℃ at a heating rate of 2-5℃ / min and hold for 10-15h for solid-state sintering to obtain the sintering aid; The sintering aid comprises at least three of the following raw materials: Na source, K source, Ca source, Al source, Si source, and Cl source. The Na source is at least one of Na₂CO₃, NaHCO₃, NaOH, and Na₂O; the K source is at least one of K₂CO₃, KHCO₃, KOH, and K₂O; the Ca source is at least one of CaCO₃, Ca(HCO₃)₂, Ca(OH)₂, and CaO; the Al source is at least one of Al₂O₃ and Al(OH)₃; the Si source is at least one of SiO₂ and Si(OH)₄; and the Cl source is at least one of NaCl and CaCl₂. The solvent C is at least one of ethanol and isopropanol. Preferably, the mass of the solvent C is the same as the total mass of the raw materials.

[0015] It should be noted that the sintering aid in this application is prepared by the above-described preparation method, and it is different from commercially available KAlSi3O8 (potassium feldspar), NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6 (leucite), and Na8Al6Si6O4. 24Unlike ores such as Cl2 and NaAlSi2O6, the sintering aids prepared by the method in this application are pure KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, and Na8Al6Si6O. 24 Cl2 and NaAlSi2O6, while commercially available KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, and Na8Al6Si6O are also available. 24 Ores such as Cl2 and NaAlSi2O6 contain impurities, which affect the performance of the solid electrolyte of this application (the disadvantages far outweigh the advantages) and cannot be directly used for the preparation of solid electrolytes.

[0016] It should be noted that the above-mentioned weighing of raw materials according to the stoichiometric ratio of KAlSi3O8 specifically refers to the preparation of raw materials according to the elements (deoxygen-removing elements) of the target sintering aid. The O element will be automatically balanced according to the charge conservation.

[0017] Preferably, the solid-phase sintering of the sintering aid is carried out in a muffle furnace.

[0018] Preferably, in step S1, the Zr source is at least one of Na2CO3, NaHCO3, NaOH, and Na2O; the Si source is at least one of SiO2 and Si(OH)4; the P source is at least one of NH4H2PO4, (NH4)2HPO4, and P2O5; and the Na source is at least one of Na2CO3, NaHCO3, NaOH, and Na2O. In step S1, solvent A is at least one of ethanol and isopropanol. Preferably, the mass of solvent A is the same as the total mass of the Zr source, Si source, P source, and Na source. In step S2, solvent B is at least one of ethanol and isopropanol. Preferably, the mass of solvent B is the same as the mass of the first sintering material.

[0019] Preferably, in the preparation process of the sintering aid, the ball milling medium is zirconium dioxide balls, the ratio of the total mass of zirconium dioxide balls to the mass of raw materials is 10-30:1, the ball milling temperature is 20-30℃, the ball milling speed is 400-500 rpm, and the ball milling time is 10-15h.

[0020] Preferably, during the preparation of the sintering aid, the container for solid-phase sintering is a corundum crucible or a magnesium oxide crucible.

[0021] Preferably, the first solid-state sintering specifically involves heating to 900-1100℃ at a heating rate of 2-5℃ / min and holding for 10-15 hours in an air or oxygen atmosphere; and / or, the second solid-state sintering specifically involves heating to 600-700℃ at a heating rate of 2-5℃ / min and holding for 2-5 hours, and then heating to 1000-1200℃ at a heating rate of 2-5℃ / min and holding for 10-15 hours.

[0022] Preferably, in the preparation process of the solid electrolyte, the medium for the first ball milling / second ball milling is zirconium dioxide balls, the ratio of the total mass of zirconium dioxide balls to the mass of raw materials (Zr source, Si source, P source, Na source / first sintering material) is 10-30:1, the ball milling temperature is 20-30℃, the ball milling speed is 400-500 rpm, and the ball milling time is 10-15h.

[0023] Preferably, the container for the first solid-state sintering / second solid-state sintering is a corundum crucible or a magnesium oxide crucible.

[0024] Preferably, the first solid-state sintering / second solid-state sintering is carried out in a muffle furnace.

[0025] Preferably, in step S3, the adhesive is a polyvinyl alcohol solution (solvent is water) with a concentration of 3-10 wt% and / or a polyvinyl butyral solution (solvent is ethyl acetate) with a concentration of 3-10 wt%.

[0026] Preferably, in step S3, the diameter of the solid electrolyte embryo is 12-18 mm and the thickness is 1.0-1.2 mm.

[0027] Based on the same inventive concept, this invention also discloses the application of the low-temperature sintered oxide solid electrolyte or the low-temperature sintered oxide solid electrolyte prepared by any of the above-described methods in solid sodium-ion batteries.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention innovatively introduces sintering aids into NASICON-type sodium-ion solid electrolytes to reduce the required sintering temperature. These substances are essentially low-melting-point (or cryogenically eutectic) aluminosilicates. Their core principle as sintering aids is liquid-phase sintering. The conventional sintering temperature for NASICON-type solid electrolytes is typically as high as 1200-1400℃. The melting or softening point of these aluminosilicate aids is typically between 900-1150℃, much lower than the host material; their key role is that aluminum ions (Al... 3+Al can form a stable aluminosilicate liquid phase with a moderate viscosity in the silicon-oxygen network. This liquid phase not only effectively wets the NZSP grains but also achieves grain "bonding" and rearrangement through controlled chemical interactions with the NZSP surface (such as slight interfacial dissolution and ion exchange). In contrast, the liquid phase formed by simple silicates (such as potassium silicate) is chemically too reactive or has too low a viscosity, which can easily lead to excessive dissolution of NZSP grains, damage to the main structure, or insufficient capillary force to drive the densification process. Therefore, a specific Al / Si ratio is key to controlling the physicochemical properties of the liquid phase and achieving efficient and mild sintering. During sintering, the sintering aid melts before the NASICON particles to form the liquid phase. The formed liquid phase can significantly improve the density of solid electrolytes through the following mechanisms: (1) Particle rearrangement: The liquid phase encapsulates solid particles, reduces friction, and makes the particles easier to slide and rearrange under the action of capillary force, filling the pores; (2) Dissolution-precipitation: The liquid phase can slightly dissolve the high surface energy NASICON fine particles (or some of their components), and then reprecipitate on the surface of larger particles, promoting grain growth and pore elimination; (3) Mass transfer: The liquid phase becomes a fast channel for mass transfer, accelerating the sintering kinetics process.

[0029] (2) In various sintering aids (KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, Na8Al6Si6O) 24Among Cl2 and NaAlSi2O6, potassium salts exhibit the most significant effect, primarily due to the large radius of potassium ions. This characteristic results in a liquid phase with lower viscosity and better fluidity, enabling efficient wetting and bonding of NASICON grains. Specifically, regarding the differences between different potassium salts, KAlSi3O8 generally shows a superior additive effect compared to KAlSi2O6, mainly due to its higher SiO2 / Al2O3 ratio (i.e., a higher silicon-to-aluminum ratio). The higher silicon content results in an aluminosilicate liquid phase with lower viscosity and better thermal stability, providing more sustained capillary forces during sintering and promoting more complete particle rearrangement and pore filling. Simultaneously, its chemical composition is closer to the NASICON matrix, leading to milder interfacial reactions and facilitating the formation of clean grain boundaries with low impedance. In contrast, the liquid phase formed by KAlSi2O6 has a relatively higher viscosity and may crystallize more quickly, resulting in a slightly less effective densification-promoting effect and grain boundary optimization. More importantly, excessively large potassium ions are difficult to penetrate the NASICON lattice, thus avoiding disruption of sodium ion transport channels. Their effect is effectively confined to the grain boundary region, achieving excellent densification while maximizing the bulk conductivity of the grains. In contrast, sodium salt sintering aids have good compatibility due to their chemical homology with the matrix, but their liquid-phase sintering efficiency is generally lower than that of potassium salts. Furthermore, the introduced sodium ions may compete with intrinsic sodium ions for sites, interfering with ion transport within the lattice. The role of calcium salt sintering aids is more complex. Divalent calcium ions create sodium vacancies in the lattice through heterovalent doping, which theoretically helps improve bulk conductivity. However, the resulting grain boundary phases often have strong insulating properties, and excessive doping can severely distort the crystal structure, making it difficult to stably control the overall effect.

[0030] In summary, the potassium-based additive (KAlSi3O8) of this invention achieves the best balance among the three mutually restrictive objectives of reducing sintering temperature, increasing density, and maintaining high ionic conductivity due to its unique physicochemical properties, making it the best-performing NASICON sintering aid system to date. The application of sodium and calcium salts, however, requires more precise control over composition and process. Attached Figure Description

[0031] Figure 1 The XRD pattern of KAlSi3O8 prepared in Example 1 of this invention; Figure 2 This is a SEM image of the solid electrolyte sheet prepared in Example 1 of the present invention after polishing. Figure 3 The image shows the XRD pattern of the solid electrolyte sheet prepared in Example 1 of this invention. Figure 4 The AC impedance diagram of the Au|SSE|Au battery assembled from the solid electrolyte sheet prepared in Example 1 of this invention at room temperature; Figure 5 The critical current density test diagram of the Na|SSE|Na battery assembled from the solid electrolyte sheet prepared in Example 1 of the present invention at increasing current density. Figure 6 The Na|SSE|Na battery assembled from the solid electrolyte sheet prepared in Example 1 of this invention operates at 0.1 mA·cm⁻¹. -2 Constant current charge-discharge cycle performance diagram under current density; Figure 7 This is a SEM image of the solid electrolyte sheet prepared in Comparative Example 1 of this invention after polishing. Figure 8 The image shows the XRD pattern of the sintering aid NaAlSi3O8 prepared in Comparative Example 2 of this invention. Detailed Implementation

[0032] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention. Other embodiments obtained by those skilled in the art based on the implementation of the present invention without creative effort are all within the scope of protection of the present invention.

[0033] It should be noted that the reagents and raw materials involved in the embodiments of the present invention can all be purchased commercially.

[0034] To address the challenges in the industrialization of NASICON-type sodium-ion solid electrolytes, this invention provides a low-temperature sintering oxide-based solid electrolyte, its preparation method, and its applications. This invention improves the density and ionic conductivity of the solid electrolyte by introducing a special sintering aid. Furthermore, the introduction of this special sintering aid reduces the sintering temperature of the solid electrolyte, inducing a significant secondary recrystallization effect at the lower sintering temperature; thus achieving a synergistic improvement in ionic conductivity, interfacial stability, and long-cycle performance of the solid electrolyte.

[0035] The following examples and comparative models further illustrate this point.

[0036] Example 1 A low-temperature sintered oxide solid electrolyte, comprising NASICON-type Na3Zr2Si2PO4 12 The matrix and the sintering aid KAlSi3O8 dispersed in the matrix.

[0037] The preparation method of sintering aid KAlSi3O8 includes the following steps: First, 0.2 mol of K₂CO₃, 0.2 mol of Al₂O₃, and 1.2 mol of SiO₂ were weighed and placed in a zirconia ball mill jar at a ball milling media (zirconia balls) / powder mass ratio of 20:1. 103 ml of anhydrous ethanol was added, and the ball milling temperature was set to 25°C, the ball milling speed to 450 rpm, and the ball milling time to 12 hours. The mixture was then uniformly ball-milled. After ball milling, the solvent in the mixture was evaporated by heating, and the powder was pulverized and separated using a sieve analyzer to obtain the mixed precursor.

[0038] Subsequently, the mixed precursor powder was placed in a corundum crucible and sintered in a muffle furnace under an air atmosphere at a heating rate of 3°C / min, reaching 1200°C and holding for 12 hours. After natural cooling to room temperature, the final product was obtained. X-ray diffraction (XRD) analysis showed that the final product was KAlSi3O8 (… Figure 1 ).

[0039] A method for preparing a low-temperature sintered oxidant solid electrolyte includes the following steps: (1) Weigh out 0.33 mol of Na2CO3, 0.4 mol of ZrO2, 0.4 mol of SiO2 and 0.2 mol of NH4H2PO4 (structural formula Na3Zr2Si2PO4). 12 To compensate for sodium loss at high temperatures, an excess of 10 wt% Na₂CO₃ was used. The mixture was placed in a zirconia ball mill jar with a ball milling media (zirconia balls) / powder mass ratio of 20:1. 128 ml of anhydrous ethanol was added. The ball milling temperature was set to 25°C, the milling speed to 450 rpm, and the milling time to 12 hours. The mixture was then uniformly ball-milled. After milling, the solvent in the mixture was evaporated by heating, and the powder was pulverized and separated using a sieve to obtain the mixed precursor.

[0040] (2) The mixed precursor powder was placed in a corundum crucible and sintered in a muffle furnace with an air atmosphere at a heating rate of 3°C / min, reaching 1000°C and holding for 12 hours. After natural cooling to room temperature, the first sintered material was obtained. The first sintered material was placed in a zirconia ball mill jar with a ball milling media (zirconia balls) / powder mass ratio of 20:1, and 10 ml of anhydrous ethanol was added. The ball milling temperature was set to 25°C, the ball milling speed was 450 rpm, and the ball milling time was 12 hours. The powder was then uniformly ball milled. After ball milling, the solvent in the mixture was heated to dryness, and the powder was pulverized and separated using a sieve to obtain the initial sintered powder.

[0041] (3) The initial sintering powder, PVA binder, and sintering aid KAlSi3O8 in a mass ratio of 490:49:10 were uniformly mixed in a mortar, wherein the PVA binder was prepared as a 5% aqueous solution. After grinding until the water was fully evaporated, the mixed powder was pressed into a solid electrolyte green body with a diameter of 16 mm and a thickness of 1.1 mm, and then placed in a corundum crucible. Sintering was carried out in a muffle furnace with an air atmosphere at a heating rate of 3 °C / min. The temperature was first raised to 600 °C and held for 3 hours, and then raised to 1100 °C and held for 10 hours. After natural cooling to room temperature, solid electrolyte sheets were obtained. The relative density of the ceramic sheets was measured using Archimedes' displacement method.

[0042] Scanning electron microscopy (SEM) analysis of the solid electrolyte sheet prepared above, after a certain degree of polishing, revealed that its surface exhibited a dense state. Figure 2 X-ray diffraction (XRD) analysis of the prepared solid electrolyte sheet after polishing revealed that it exhibits a standard NASICON-type crystal structure with space group C2 / c. Furthermore, due to the low content of sintering aids, no obvious diffraction peaks of the KAlSi3O8 phase were observed, and the aids did not significantly affect the NASICON-type crystal structure.

[0043] The gold-plated electrodes on both sides of the NASICON-type sodium-ion solid electrolyte prepared above were fabricated into an Au|SSE|Au battery using magnetron sputtering technology. Conductivity tests showed that the room-temperature ionic conductivity of the NASICON-type sodium-ion solid electrolyte in this embodiment was 2.63 mS·cm. -1 ( Figure 4 This indicates that the sintering aid KAlSi3O8 can eliminate grain boundaries and increase density, thereby enabling the synthesis of solid electrolytes with high ionic conductivity at low sintering temperatures.

[0044] Using magnetron sputtering, sodium metal sheets were attached to both sides of the prepared NASICON-type sodium-ion solid electrolyte, which was then encapsulated in a CR-2032 coin cell to assemble a sodium metal symmetric cell (Na|SSE|Na). Constant current charge-discharge tests revealed that the critical current density of this sodium metal symmetric cell was 0.6 mA·cm⁻¹. -2 And at a current density of 0.1 mA·cm -2 Under these conditions, it can cycle stably for 920 hours without any short circuit. Figures 5-6 This result demonstrates that the solid electrolyte of this embodiment has excellent sodium dendrite suppression capabilities.

[0045] Comparative Example 1 The only difference between this comparative example and Example 1 is that no sintering aids were added; all other operations and parameters were the same as in Example 1.

[0046] Scanning electron microscopy (SEM) analysis of the solid electrolyte sheet prepared above, after a certain degree of polishing, revealed that it exhibited a highly dense, uniform surface without visible pores. Figure 7 This indicates that the material has reached a sintering state close to its theoretical density.

[0047] Comparative Example 2 The only difference between this comparative example and Example 1 is that the sintering aid is NaAlSi3O8, while all other operations and parameters are the same as in Example 1.

[0048] The preparation method of the above sintering aid is the same as in Example 1, except that K2CO3 is replaced with Na2CO3. All other operations and parameters are the same as in Example 1. X-ray diffraction (XRD) analysis of the prepared sintering aid shows that the prepared sintering aid is NaAlSi3O8 (… Figure 8 ).

[0049] Comparative Example 3 The only difference between this comparative example and Example 1 is that the sintering aid is CaAl2Si2O8, while all other operations and parameters are the same as in Example 1.

[0050] Comparative Example 4 The only difference between this comparative example and Example 1 is that the sintering aid is NaAlSiO4, while all other operations and parameters are the same as in Example 1.

[0051] Comparative Example 5 The only difference between this comparative example and Example 1 is that the sintering aid is KAlSi2O6, while all other operations and parameters are the same as in Example 1.

[0052] Comparative Example 6 The only difference between this comparative example and Example 1 is that the sintering aid is Na8Al6Si6O. 24 Cl2, other operations and parameters are the same as in Example 1.

[0053] Comparative Example 7 The only difference between this comparative example and Example 1 is that the sintering aid is NaAlSi2O6, while all other operations and parameters are the same as in Example 1.

[0054] Comparative Example 8 The only difference between this comparative example and Example 1 is that the mass ratio of the initial sintering powder, PVA binder, and sintering aid KAlSi3O8 is 990:99:10. All other operations and parameters are the same as in Example 1.

[0055] Comparative Example 9 The only difference between this comparative example and Example 1 is that the mass ratio of the initial sintering powder, PVA binder, and sintering aid KAlSi3O8 is 970:97:30. All other operations and parameters are the same as in Example 1.

[0056] Comparative Example 10 The only difference between this comparative example and Example 1 is that the temperature of the second solid-state sintering is 1250°C, while all other operations and parameters are the same as in Example 1.

[0057] Comparative Example 11 The only difference between this comparative example and Example 1 is that the sintering aid is KBSi3O8, while all other operations and parameters are the same as in Example 1.

[0058] The difference between the preparation method of sintering aid KBSi3O8 and the preparation method of sintering aid KAlSi3O8 in Example 1 is that Al2O3 is replaced with B2O3, while other operations and parameters are the same as in Example 1.

[0059] Comparative Example 12 The only difference between this comparative example and Example 1 is that the sintering aid KAlSi3O8 in the preparation process of the solid electrolyte is replaced with the raw materials for preparing the sintering aid KAlSi3O8 (0.2 mol K2CO3, 0.2 mol Al2O3 and 1.2 mol SiO2). All other operations and parameters are the same as in Example 1.

[0060] The specific preparation method of solid electrolyte is as follows: (1) Weigh 0.3 mol of Na2CO3, 0.4 mol of ZrO2, 0.4 mol of SiO2, and 0.2 mol of NH4H2PO4. To compensate for sodium loss at high temperatures, Na2CO3 is added in excess by 10 wt%. Place the mixture in a zirconium dioxide ball mill jar with a ball milling media (zirconia balls) / powder mass ratio of 20:1. Add 128 ml of anhydrous ethanol. Set the ball milling temperature to 25°C, the ball milling speed to 450 rpm, and the ball milling time to 12 hours. Then, uniformly ball mill the mixed powder. After ball milling, heat and evaporate the solvent in the mixture to dryness. After pulverizing, separate the ball powder using a sieve to obtain the mixed precursor.

[0061] (2) The mixed precursor powder was placed in a corundum crucible and sintered in a muffle furnace with an air atmosphere at a heating rate of 3°C / min, reaching 1000°C and holding for 12 hours. After natural cooling to room temperature, the first sintered material was obtained. The first sintered material was placed in a zirconia ball mill jar with a ball milling media (zirconia balls) / powder mass ratio of 20:1, and 10 ml of anhydrous ethanol was added. The ball milling temperature was set to 25°C, the ball milling speed was 450 rpm, and the ball milling time was 12 hours. The powder was then uniformly ball milled. After ball milling, the solvent in the mixture was heated to dryness, and the powder was pulverized and separated using a sieve to obtain the initial sintered powder.

[0062] (3) The initial sintering powder, PVA binder, and sintering aid KAlSi3O8 raw materials (K2CO3, Al2O3, and SiO2 molar ratio of 1:1:6) with a mass ratio of 490:49:10 were uniformly mixed in a mortar. The PVA binder was prepared as a 5% aqueous solution. After grinding until the water was fully evaporated, the mixed powder was pressed into a solid electrolyte green body with a diameter of 16 mm and a thickness of 1.1 mm, and then placed in a corundum crucible. Sintering was carried out in a muffle furnace with an air atmosphere at a heating rate of 3 °C / min. The temperature was first raised to 600 °C and held for 3 hours, and then raised to 1100 °C and held for 10 hours. After natural cooling to room temperature, a solid electrolyte sheet was obtained.

[0063] Performance testing and results analysis: The solid electrolytes prepared in Example 1 and Comparative Examples 1-12 were subjected to density testing, room temperature ionic conductivity of Au|SSE|Au batteries, critical current density and cycle time of Na|SSE|Na batteries, and the results are shown in Table 1 below.

[0064] Table 1:

[0065] As shown in Table 1, potassium salts, especially KAlSi3O8, exhibit the most outstanding performance in improving the overall performance of NASICON-type solid electrolytes. Example 1, using KAlSi3O8 as a sintering aid, achieved an electrolyte sheet with an extremely high density of 96.5%, corresponding to a room-temperature ionic conductivity of 2.63 mS / cm. -1 This far exceeds all comparative examples; its critical current density is 0.6 mA·cm⁻¹. -2 It also cycled stably for 920 hours in sodium metal symmetric cell tests.

[0066] As shown in Table 1, compared with Comparative Example 1 without sintering aids, all sintering aids (Example 1 and Comparative Examples 2-12) brought some improvement, but the improvement of potassium salt sintering aid (Example 1) was orders of magnitude.

[0067] As shown in Table 1, compared with NaAlSi3O8 (Comparative Example 2), which is also an alkali metal sodium salt sintering aid, KAlSi3O8 (Example 1) has an ionic conductivity that is more than 10 times higher and a cycle life that is more than 4 times higher.

[0068] As shown in Table 1, compared with Comparative Example 5 (KAlSi2O6), which is also a potassium salt but has a different structure, KAlSi3O8 still has a significant advantage (2.63 vs. 1.02 mS·cm). -1 This indicates that the exact phase composition is also crucial.

[0069] As shown in Table 1, Na8Al6Si6O 24 Neither Cl2 (Comparative Example 6) nor NaAlSi2O6 (Comparative Example 7) achieved a level comparable to KAlSi3O8 (Example 1). The superior efficacy of potassium salts can be attributed to the formation of a low-temperature, low-viscosity liquid phase with minimal interference to the bulk lattice under the influence of potassium ions with large ionic radii, thereby achieving optimal sintering densification and a relatively friendly grain boundary environment.

[0070] As shown in Table 1, Comparative Examples 8 and 9 further demonstrate that there is an optimal window for the amount of additives. Deviations from this optimal ratio (whether too little or too much) lead to a decrease in performance, indicating that KAlSi3O8 (Example 1) does not simply act as an inert filler, but rather participates in and optimizes the sintering process through precise physicochemical interactions.

[0071] As shown in Table 1, compared with Comparative Example 10, KAlSi3O8 as a sintering aid (Example 1) can significantly reduce the sintering temperature of the solid electrolyte.

[0072] As can be seen from the comparison between Example 1 and Comparative Example 11 in Table 1, KBSi3O8 completely fails as a sintering aid, and its performance is even far inferior to other sodium salt sintering aids and calcium salt sintering aids (Comparative Examples 2-4). This indicates that not all elements that can form compounds with potassium and silicon can work effectively. The B in the sintering aid KBSi3O8 of Comparative Example 11... 3+ In silicate networks, NZSP typically exists as a mixture of [BO3] triangles and [BO4] tetrahedra. Its coordination state is sensitive to temperature and composition, leading to excessively low viscosity and high chemical reactivity in the potassium borosilicate liquid phase. This results in excessive dissolution of the NZSP surface during sintering, disrupting its lattice integrity rather than promoting its ordered growth. In contrast, in the sintering aid KAlSi3O8 of Example 1, Al... 3+Al stably replaces a portion of the [SiO4] tetrahedra in the form of [AlO4] tetrahedra, forming a uniform and continuous aluminosilicate network with the silicon-oxygen network. The liquid phase formed after melting of this network exhibits moderate viscosity and chemical activity. It provides good flowability to wet particles and drive densification without excessively dissolving or eroding NASICON (NZSP) grains. Therefore, Al plays an irreplaceable core role in KAlSi3O8. It not only determines the formation of a low-temperature liquid phase with ideal viscosity and chemical stability, thus efficiently driving densification, but also ensures high chemical compatibility with the NASICON substrate, avoiding harmful side reactions, and ultimately forming an interfacial phase at the grain boundaries with both good mechanical strength and a certain degree of ion conductivity. Replacing Al with B disrupts the physicochemical balance of the entire system, leading to sintering failure, interfacial poisoning, and performance degradation. This comparison profoundly reveals that the choice of the central element for a highly efficient sintering aid in NASICON electrolytes is by no means arbitrary; Al... 3+ Its unique electronic structure and coordination properties are key to its success.

[0073] As can be seen from the comparison between Example 1 and Comparative Example 12 in Table 1, simply changing the sintering aid from a pre-synthesized single compound to a physically mixed mixture of multiple raw materials resulted in the near-complete loss of its sintering-promoting and performance-enhancing effects, with performance degrading to near-unaided levels. The root cause of this significant difference lies in the completely different high-temperature reaction pathways and liquid phase formation processes resulting from the two addition methods, profoundly affecting sintering kinetics and the final microstructure. In Comparative Example 12, the mixture of K2CO3, Al2O3, and SiO2 powders, even if macroscopically homogeneous, remained microscopically separated. During the heating process, K2CO3 decomposes at approximately 900℃ to generate highly reactive K2O, while the interdiffusion and reaction of Al2O3 and SiO2 require higher temperatures and are slower. This prevents the liquid phase from forming instantaneously and uniformly at a specific temperature. Therefore, the effectiveness of the sintering aid KAlSi3O8 in Example 1 lies not only in its K, Al, Si, and O elements, but also in the uniform, single-phase crystal structure formed by these elements in a specific ratio and chemical bonding. Only by adding it in this pre-synthesized single-phase form can a uniform liquid phase with ideal physicochemical properties be released during sintering according to the designed "program," thereby simultaneously achieving the triple goals of extreme densification, low-resistivity grain boundary formation, and lattice protection. Directly adding the raw material mixture cannot replicate this uniform initial state at the microscopic scale, causing the entire liquid-phase sintering mechanism to fail, and the performance of the final product naturally plummets. This profound understanding has important guiding significance for designing efficient sintering aids for other systems.

[0074] In summary, the performance results in Table 1 fully demonstrate the significant advantages of the potassium salt (KAlSi3O8) of Example 1 of this invention as a sintering aid for NASICON solid electrolytes. It not only significantly reduces the sintering temperature and achieves a nearly fully densified structure, but more importantly, the resulting solid electrolyte exhibits synergistic improvements in ionic conductivity, interfacial stability, and long-cycle performance, providing a key material solution for the development of high-performance solid sodium batteries.

[0075] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A low-temperature sintered oxidant solid electrolyte, characterized in that, Including NASICON type Na3Zr2Si2PO 12 The matrix and sintering aids dispersed in the matrix; the sintering aids include KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, NaAlSiO4, KAlSi2O6, and Na8Al6Si6O. 24 At least one of Cl2 and NaAlSi2O6.

2. The low-temperature sintered oxide solid electrolyte according to claim 1, characterized in that, The sintering aid is KAlSi3O8.

3. The low-temperature sintered oxide solid electrolyte according to claim 1 or 2, characterized in that, The mass ratio of the matrix to the sintering aid is 30-99:

1.

4. The low-temperature sintered oxide solid electrolyte according to claim 1 or 2, characterized in that, The solid electrolyte further includes a binder; the binder includes at least one of polyvinyl alcohol and polyvinyl butyral.

5. The low-temperature sintered oxide solid electrolyte according to claim 3, characterized in that, The mass ratio of the matrix to the adhesive is 5-15:

1.

6. The method for preparing a low-temperature sintered oxide solid electrolyte according to any one of claims 1-5, characterized in that, Includes the following steps: S1. After the Zr source, Si source, P source, Na source and solvent A are mixed by ball milling for the first time, the solvent A is removed to obtain a mixed precursor; S2. The mixed precursor is subjected to a first solid-state sintering to obtain a first sintered material; solvent B is added to the first sintered material and it is mixed by a second ball milling, and then the solvent B is removed and the mixture is pulverized to obtain an initial sintered powder. S3. The initial sintering powder, binder and sintering aid are mixed to form a solid electrolyte green embryo; the solid electrolyte green embryo is subjected to a second solid-phase sintering and then naturally cooled to room temperature to obtain the low-temperature sintered oxide solid electrolyte.

7. The method for preparing a low-temperature sintered oxidant solid electrolyte according to claim 6, characterized in that, In step S3, the preparation method of the sintering aid is as follows: weigh the raw materials according to the stoichiometric ratio of different elements in the sintering aid, mix them by ball milling, add solvent C and continue ball milling to mix, remove the solvent C to obtain precursor powder; heat the precursor powder to 1000-1500℃ at a heating rate of 2-5℃ / min and hold for 10-15h for solid-state sintering to obtain the sintering aid; The sintering aid comprises at least three of the following raw materials: Na source, K source, Ca source, Al source, Si source, and Cl source. The Na source is at least one of Na2CO3, NaHCO3, NaOH, and Na2O; the K source is at least one of K2CO3, KHCO3, KOH, and K2O; the Ca source is at least one of CaCO3, Ca(HCO3)2, Ca(OH)2, and CaO; the Al source is at least one of Al2O3 and Al(OH)3; the Si source is at least one of SiO2 and Si(OH)4; and the Cl source is at least one of NaCl and CaCl2. The solvent C is at least one of ethanol and isopropanol.

8. The method for preparing a low-temperature sintered oxide solid electrolyte according to claim 6, characterized in that, In step S1, the Zr source is at least one of Na2CO3, NaHCO3, NaOH, and Na2O; the Si source is at least one of SiO2 and Si(OH)4; the P source is at least one of NH4H2PO4, (NH4)2HPO4, and P2O5; and the Na source is at least one of Na2CO3, NaHCO3, NaOH, and Na2O. In steps S1 / S2, solvent A / B is at least one of ethanol and isopropanol.

9. The method for preparing a low-temperature sintered oxide solid electrolyte according to claim 6, characterized in that, The first solid-state sintering specifically involves heating to 900-1100℃ at a heating rate of 2-5℃ / min and holding for 10-15 hours in an air or oxygen atmosphere; and / or, the second solid-state sintering specifically involves heating to 600-700℃ at a heating rate of 2-5℃ / min and holding for 2-5 hours, and then heating to 1000-1200℃ at a heating rate of 2-5℃ / min and holding for 10-15 hours.

10. The application of the low-temperature sintered oxide solid electrolyte prepared by the method of any one of claims 1-6 or any one of claims 6-9 in a solid sodium-ion battery.

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