Sodium ion composite solid electrolyte, preparation method thereof and sodium solid-state battery

Abstract

The invention discloses a sodium ion composite solid electrolyte, a preparation method thereof and a sodium solid-state battery. The electrolyte comprises a raw material mixture formed by mixing a polymer matrix, a basic sodium salt and a sodium salt additive, and a nanoparticle filler uniformly dispersed in the raw material mixture, the molar ratio of the polymer matrix to the basic sodium salt to the sodium salt additive is (12-18): 1: (0.05-0.5); the mass of the nano-particle filler accounts for 5-20wt% of the mass of the raw material mixture; the sodium salt additive is introduced into the polymer matrix, so that sodium ions in the sodium solid-state battery are uniformly deposited, the growth of sodium metal dendrites is inhibited, and the room-temperature ionic conductivity and cycling stability of the electrolyte to sodium metal are improved; the sodium ion polymer solid electrolyte prepared by the invention has the characteristics of lower glass transition temperature, excellent ion conduction capability and greatly enhanced stability to sodium metal.

Description

Technical Field

[0001] The present invention relates to the technical field of solid electrolyte membrane preparation and solid-state battery technology, and in particular to a sodium ion composite solid electrolyte and a preparation method thereof, and a sodium solid-state battery. Background Art

[0002] As a key component of sodium solid-state batteries, the intrinsic properties of sodium ion solid electrolytes largely determine the overall performance of the battery. Among them, ionic conductivity, mechanical strength and interfacial stability to electrode materials are the three core indicators. However, the existing technical solutions for preparing polymer-based sodium solid electrolytes have significant defects. On the one hand, the polymer matrix has high crystallinity at room temperature, which hinders the transmission of sodium ions, resulting in low ionic conductivity and cannot meet the requirements of practical applications; on the other hand, polymer-based electrolytes are unstable to positive and negative electrode materials, which is specifically reflected in poor solid-solid interface compatibility, large initial interface impedance, and continuous interfacial side reactions during the cycle, exacerbating the growth of interface impedance, seriously affecting the cycle life of the battery.

[0003] Chinese invention patent application publication number CN118919830A discloses a sodium ion composite all-solid-state electrolyte and its preparation method. This method disperses sodium salt and sulfide electrolyte materials in a polymer matrix to improve the electrolyte's ionic conductivity, achieve a more stable interface, and lower impedance. However, the use of sulfide as an additive poses serious environmental pollution, hindering practical engineering applications and large-scale production. Chinese invention patent application publication number CN115000499A discloses a solid electrolyte membrane, its preparation method, and a solid-state battery. This method introduces fluoride into the polymer electrolyte to improve solid-state electrolyte performance and interface stability. However, this method only significantly improves sodium metal stability at high temperatures and does not completely resolve the instability of the sodium metal negative electrode in the polymer solid electrolyte, making it difficult to achieve long-term stable cycling. Summary of the Invention

[0004] In order to overcome the defects of the above-mentioned prior art, the object of the present invention is to provide a sodium ion composite solid electrolyte, a preparation method thereof, and a sodium solid-state battery. By adding a sodium salt additive for interface chemistry regulation to the polymer matrix, a negative electrode interface protection layer (SEI) and a positive electrode interface protection layer (CEI) are formed in situ during the electrochemical process, thereby improving the cyclic stability of the sodium ion polymer solid electrolyte to sodium metal, achieving uniform deposition of sodium ions to inhibit the growth of sodium metal dendrites. The sodium ion polymer solid electrolyte prepared by the present invention has a lower glass transition temperature, excellent ion conductivity, and greatly enhanced stability to sodium metal, which can promote the long-term stable operation of the sodium solid-state battery.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is as follows:

[0006] A sodium ion composite solid electrolyte comprises a raw material mixture formed by mixing a polymer matrix, a basic sodium salt, and a sodium salt additive, and a nanoparticle filler uniformly dispersed in the raw material mixture; the molar ratio of polymer matrix:basic sodium salt:sodium salt additive is (12-18):1:(0.05-0.5); the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture;

[0007] The polymer matrix is polyethylene oxide (PEO), the basic sodium salt is sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) or sodium bis(fluorosulfonyl)imide (NaFSI), the sodium salt additive is one of sodium difluorooxalatoborate (NaDFOB) and sodium bis(oxalatoborate) (NaBOB) or a mixture of the two in any ratio; the nanoparticle filler is an inert filler, and the inert filler is one of Al2O3, SiO2, TiO2, and ZrO2.

[0008] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0009] Step 1: After mixing the nanoparticle filler and the dispersant, ultrasonically disperse them to obtain a particle dispersion;

[0010] Step 2: Weighing a raw polymer matrix, a basic sodium salt, and a sodium salt additive in a molar ratio of (12-18):1:(0.05-0.5) to obtain a raw material mixture; adding a particle dispersion to the raw material mixture, wherein the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture; then adding a dispersant and stirring to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%;

[0011] The polymer matrix is polyethylene oxide (PEO), the basic sodium salt is sodium bistrifluoromethylsulfonyl imide (NaTFSI) or sodium bisfluorosulfonyl imide (NaFSI), and the sodium salt additive is one of sodium difluorooxalatoborate (NaDFOB) and sodium bisoxalatoborate (NaBOB) or a mixture of the two in any ratio;

[0012] Step 3: coating the sodium ion composite solid electrolyte slurry on a polytetrafluoroethylene membrane, drying the membrane, and then standing the membrane at room temperature in a dry environment for at least 12 hours to obtain a sodium ion composite solid electrolyte film.

[0013] In step 1, based on 100 parts by mass, the ratio of nanoparticle filler to dispersant is (5-10): (90-95).

[0014] In step 1, the nanoparticle filler is an inert filler, and the inert filler is one of Al2O3, SiO2, TiO2, and ZrO2.

[0015] The dispersants in step 1 and step 2 are both anhydrous acetonitrile (ACN) or N-methylpyrrolidone (NMP).

[0016] In step 3, the drying process is: first drying in a forced air drying oven at 50-60°C for 0.5-1h, then drying in a vacuum drying oven at 50-60°C for 24-36h; the drying environment is: a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm.

[0017] In step 3, the coating thickness of the sodium ion composite solid electrolyte slurry is 800-2000 μm.

[0018] An application of the above-mentioned sodium ion composite solid electrolyte or the sodium ion composite solid electrolyte prepared by the above-mentioned preparation method in a sodium solid-state battery.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. By adding sodium salt additives, the present invention can effectively reduce the crystallinity of polymer chain segments and achieve an improvement in ionic conductivity; based on the "sodium salt additive-basic sodium salt-polymer matrix" multi-salt synergistic system, the synergistic optimization of material properties is achieved.

[0021] 2. The present invention introduces a functional sodium salt additive into the polymer matrix to achieve efficient ion transport, thereby reducing the internal resistance of the battery. Under the action of the sodium salt additive, the deposition and stripping of sodium metal are uniform. After cycling, the surface of the sodium metal negative electrode is smooth and free of dendrites, forming a stable interface layer, improving the interface contact between the electrolyte and the electrode, effectively reducing the interfacial impedance, and promoting the rapid migration of sodium ions, enabling the battery to operate stably over a long period of time. The present invention simultaneously solves the two key technical problems of interface stability and ion conduction, significantly improving the performance of sodium solid-state batteries.

[0022] 3. The present invention sets the addition range of the sodium salt additive to 0.05-0.5 of the basic sodium salt ratio, which can reduce the crystallinity of the polymer electrolyte and form a negative electrode interface protection layer (SEI) with the sodium metal negative electrode during the electrochemical test, effectively inhibiting the interfacial side reactions during the cycle, thereby improving the ionic conductivity and cycle stability of the polymer solid electrolyte.

[0023] In summary, the present invention improves interfacial stability while maintaining high ionic conductivity by adding a sodium salt additive. This will significantly advance the commercialization of safe, long-life sodium solid-state batteries, with broad application prospects in new energy vehicles, large-scale energy storage, and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1Electrochemical impedance spectroscopy (EIS) diagrams of the electrolytes prepared in all examples and comparative examples at room temperature.

[0025] Figure 2 These are DSC test graphs of the electrolytes prepared in Example 1 and Comparative Example 1.

[0026] Figure 3 This is the SEM image of the sodium metal negative electrode after the sodium solid-state battery reaction prepared in Comparative Example 1.

[0027] Figure 4 This is an SEM image of the sodium metal negative electrode after the sodium solid-state battery reaction prepared in Example 1.

[0028] Figure 5 Optical photos of the electrolytes after the sodium solid-state battery reaction prepared in Example 1 and Comparative Example 1, wherein Figure 5 (a) is comparative example 1, Figure 5 (b) is Example 1.

[0029] Figure 6 The SEM images of the electrolytes after the sodium solid-state battery reaction prepared in Example 1 and Comparative Example 1 are shown in FIG. Figure 6 (a) is comparative example 1, Figure 6 (b) is Example 1.

[0030] Figure 7 This is a full battery cycle diagram of the sodium solid-state battery prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0032] A sodium ion composite solid electrolyte comprises a raw material mixture formed by mixing a polymer matrix, a basic sodium salt, and a sodium salt additive, and a nanoparticle filler uniformly dispersed in the raw material mixture; the molar ratio of polymer matrix:basic sodium salt:sodium salt additive is (12-18):1:(0.05-0.5); the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture;

[0033] The polymer matrix is polyethylene oxide (PEO), the basic sodium salt is sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) or sodium bis(fluorosulfonyl)imide (NaFSI), and the sodium salt additive is one or a mixture of two of sodium difluorooxalatoborate (NaDFOB) and sodium bis(oxalatoborate) (NaBOB) in any ratio; the nanoparticle filler is an inert filler, and the inert filler is nanoparticles such as Al2O3, SiO2, TiO2, and ZrO2.

[0034] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0035] Step 1: Based on 100 parts by mass, 5-10 parts of nanoparticle filler and 90-95 parts of dispersant are mixed, and then dispersed in an ultrasonic disperser for 2 hours to obtain a particle dispersion;

[0036] The nanoparticle filler is an inert filler, and the inert filler is nanoparticles such as Al2O3, SiO2, TiO2, ZrO2, etc.; the dispersant is anhydrous acetonitrile (ACN) or N-methylpyrrolidone (NMP);

[0037] Step 2: Weighing a raw polymer matrix, a basic sodium salt, and a sodium salt additive in a molar ratio of (12-18):1:(0.05-0.5) to obtain a raw material mixture; adding a particle dispersion to the raw material mixture, wherein the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture; then adding an appropriate amount of dispersant, stirring at 50-60°C and 400-600r / min for 18-30h to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%;

[0038] The polymer matrix is polyethylene oxide (PEO), whose chemical formula is (C2H4O)n; the basic sodium salt is sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) or sodium bis(fluorosulfonyl)imide (NaFSI); the sodium salt additive is one or a mixture of sodium difluorooxalatoborate (NaDFOB) and sodium bis(oxalatoborate) (NaBOB); the dispersant is anhydrous acetonitrile (ACN) or N-methylpyrrolidone (NMP);

[0039] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on a polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 50-60°C for 0.5-1h, then dry it in a vacuum drying oven at 50-60°C for 24-36h, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for at least 12h to obtain a sodium ion composite solid electrolyte film; the coating thickness of the sodium ion composite solid electrolyte slurry is 800-2000μm.

[0040] Example 1

[0041] A sodium ion composite solid electrolyte comprises a raw material mixture of PEO, NaTFSI and NaDFOB, and Al2O3 nanoparticles uniformly dispersed in the raw material mixture; the molar ratio of PEO:NaTFSI:NaDFOB is 12:1:0.5; the mass of the Al2O3 nanoparticles accounts for 10wt% of the mass of the raw material mixture.

[0042] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0043] Step 1: Based on 100 parts by mass, 5 parts of Al2O3 nanoparticles were mixed with 95 parts of ACN, and then dispersed under an ultrasonic disperser for 2 hours to obtain an Al2O3 particle dispersion;

[0044] Step 2: PEO, NaTFSI, and NaDFOB are weighed in a molar ratio of 12:1:0.5 to obtain a raw material mixture; an Al2O3 particle dispersion is added to the raw material mixture, wherein the mass of the Al2O3 nanoparticles accounts for 10 wt% of the mass of the raw material mixture; an appropriate amount of ACN is then added to maintain the total solid content of the electrolyte slurry at 20-30 wt%; and the mixture is stirred at 60°C and 400 rpm for 24 hours to obtain a sodium ion composite solid electrolyte slurry;

[0045] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on the polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 60°C for 1 hour, then dry it in a vacuum drying oven at 60°C for 24 hours, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for 24 hours to obtain a sodium ion composite solid electrolyte film. The coating thickness of the sodium ion composite solid electrolyte slurry is 1600μm.

[0046] Comparative Example 1

[0047] The difference from Example 1 is that in step 2, NaDFOB as a sodium salt additive is not added.

[0048] Example 2

[0049] A sodium ion composite solid electrolyte comprises a raw material mixture of PEO, NaFSI and NaDFOB, and Al2O3 nanoparticles uniformly dispersed in the raw material mixture; the molar ratio of PEO:NaFSI:NaDFOB is 12:1:0.05; the mass of the Al2O3 nanoparticles accounts for 5wt% of the mass of the raw material mixture.

[0050] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0051] Step 1: Based on 100 parts by mass, 10 parts of Al2O3 nanoparticles were mixed with 90 parts of ACN, and then dispersed in an ultrasonic disperser for 2 hours to obtain an Al2O3 particle dispersion;

[0052] Step 2: weighing raw materials PEO, NaFSI, and NaDFOB in a molar ratio of 12:1:0.05 to obtain a raw material mixture; adding an Al2O3 particle dispersion to the raw material mixture, wherein the mass of the Al2O3 nanoparticles accounts for 5wt% of the mass of the raw material mixture; then adding an appropriate amount of ACN, stirring at 55°C and 500r / min for 30h to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%;

[0053] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on the polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 55°C for 1 hour, then dry it in a vacuum drying oven at 55°C for 30 hours, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for 24 hours to obtain a sodium ion composite solid electrolyte film; the coating thickness of the sodium ion composite solid electrolyte slurry is 800μm.

[0054] Comparative Example 2

[0055] The difference from Example 2 is that in step 2, NaDFOB as a sodium salt additive is not added.

[0056] Example 3

[0057] A sodium ion composite solid electrolyte comprises a raw material mixture of PEO, NaTFSI and NaDFOB, and SiO2 nanoparticles uniformly dispersed in the raw material mixture; the molar ratio of PEO:NaTFSI:NaDFOB is 18:1:0.05; the mass of the SiO2 nanoparticles accounts for 20wt% of the mass of the raw material mixture.

[0058] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0059] Step 1: Based on 100 parts by mass, 7 parts of SiO2 nanoparticles and 93 parts of ACN were mixed, and then dispersed in an ultrasonic disperser for 2 hours to obtain a SiO2 particle dispersion;

[0060] Step 2: weighing raw materials PEO, NaTFSI and NaDFOB in a molar ratio of 18:1:0.05 to obtain a raw material mixture; adding a SiO2 particle dispersion to the raw material mixture, wherein the mass of the SiO2 nanoparticles accounts for 20wt% of the mass of the raw material mixture; then adding an appropriate amount of ACN, stirring at 60°C and 500r / min for 24h to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%;

[0061] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on the polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 50°C for 1 hour, then dry it in a vacuum drying oven at 50°C for 36 hours, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for 24 hours to obtain a sodium ion composite solid electrolyte film; the coating thickness of the sodium ion composite solid electrolyte slurry is 1200μm.

[0062] Comparative Example 3

[0063] The difference from Example 3 is that in step 2, NaDFOB as a sodium salt additive is not added.

[0064] Example 4

[0065] A sodium ion composite solid electrolyte comprises a raw material mixture of PEO, NaTFSI and NaBOB, and ZrO2 nanoparticles uniformly dispersed in the raw material mixture; the molar ratio of PEO:NaTFSI:NaBOB is 14:1:0.3; the mass of the ZrO2 nanoparticles accounts for 10wt% of the mass of the raw material mixture.

[0066] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0067] Step 1: Based on 100 parts by mass, 5 parts of ZrO2 nanoparticles were mixed with 95 parts of NMP, and then dispersed under an ultrasonic disperser for 2 hours to obtain a ZrO2 particle dispersion;

[0068] Step 2: weighing raw materials PEO, NaTFSI and NaBOB in a molar ratio of 14:1:0.3 to obtain a raw material mixture; adding a ZrO2 particle dispersion to the raw material mixture, wherein the mass of the ZrO2 nanoparticles accounts for 10wt% of the mass of the raw material mixture; then adding an appropriate amount of NMP, stirring at 50°C and 600r / min for 18h to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%;

[0069] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on the polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 60°C for 0.5h, then dry it in a vacuum drying oven at 60°C for 24h, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for 24h to obtain a sodium ion composite solid electrolyte film; the coating thickness of the sodium ion composite solid electrolyte slurry is 1800μm.

[0070] Comparative Example 4

[0071] The difference from Example 4 is that NaBOB as a sodium salt additive is not added in step 2.

[0072] Example 5

[0073] A sodium ion composite solid electrolyte comprises a raw material mixture of PEO, NaTFSI, and a sodium salt additive, and TiO2 nanoparticles uniformly dispersed in the raw material mixture; the molar ratio of PEO:NaTFSI:sodium salt additive is 14:1:0.5; the sodium salt additive is a mixture of NaBOB and NaDFOB in a mass ratio of 3:7; the mass of the TiO2 nanoparticles accounts for 10 wt% of the mass of the raw material mixture.

[0074] A method for preparing a sodium ion composite solid electrolyte comprises the following steps:

[0075] Step 1: Based on 100 parts by mass, 5 parts of TiO2 nanoparticles were mixed with 95 parts of ACN, and then dispersed in an ultrasonic disperser for 2 hours to obtain a TiO2 particle dispersion;

[0076] Step 2: weighing raw materials PEO, NaTFSI, and a sodium salt additive in a molar ratio of 14:1:0.3 to obtain a raw material mixture; adding a TiO2 particle dispersion to the raw material mixture, wherein the mass of the TiO2 nanoparticles accounts for 10wt% of the mass of the raw material mixture; then adding an appropriate amount of ACN, stirring at 60°C and 400r / min for 24h to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%; the sodium salt additive is a mixture of NaBOB and NaDFOB in a mass ratio of 3:7;

[0077] Step 3: Use a fully automatic coating machine to coat the sodium ion composite solid electrolyte slurry on the polytetrafluoroethylene membrane; after coating, first dry it in a blast drying oven at 60°C for 1 hour, then dry it in a vacuum drying oven at 60°C for 24 hours, and let it stand at room temperature in a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm for 24 hours to obtain a sodium ion composite solid electrolyte film; the coating thickness of the sodium ion composite solid electrolyte slurry is 2000μm.

[0078] Comparative Example 5

[0079] The difference from Example 5 is that NaBOB and NaDFOB as sodium salt additives are not added.

[0080] The sodium ion composite solid electrolyte films prepared in the above examples and comparative examples were assembled into sodium solid-state batteries in the following manner, and the battery performance was tested, as shown in Table 1.

[0081] 1. First, prepare the positive electrode slurry: according to the mass ratio of 8:1:1:30, mix the positive electrode active material with the binder, the conductive additive, and the dispersion medium, and stir at a stirring speed of 400r / min at room temperature for 5 hours to prepare a uniform positive electrode slurry; wherein the sodium ion positive electrode active material is sodium nickel iron manganese oxide; the dispersion medium is N-methylpyrrolidone (NMP), the binder is polyvinylidene fluoride (PVDF), and the conductive additive is a superconducting carbon black material; the positive electrode slurry is coated on the current collector with a scraper; first dry it in a blast drying oven at 80°C for 1 hour, and then dry it in a vacuum drying oven at 60°C for 24 hours to obtain a positive electrode sheet; the current collector is carbon-coated aluminum foil, and the thickness of the positive electrode slurry is 100μm;

[0082] 2. Sodium metal is used as the negative electrode. Then, in an argon glove box, a positive electrode with a diameter of 10 mm, a sodium ion composite solid electrolyte film with a diameter of 16 mm, and a sodium metal with a diameter of 12 mm are placed into a mold in that order. A sealing machine is used to package the battery into a CR2025 button cell and store it in the glove box for testing.

[0083] Table 1 Performance comparison of sodium ion composite solid electrolytes in different embodiments and comparative examples

[0084] Group Body impedance (Ω) Thickness (μm) Room temperature ionic conductivity (mS / cm) Example 1 410 32.1 0.039 Example 2 461 29.7 0.032 Example 3 481 24.2 0.025 Example 4 433 37.4 0.043 Example 5 262 24.2 0.046 Comparative Example 1 2013 93 0.0023 Comparative Example 2 2962 65 0.0011 Comparative Example 3 2740 83 0.0015 Comparative Example 4 2575 98 0.0019 Comparative Example 5 2337 80 0.0017

[0085] From Table 1 and Figure 1 It can be seen that the electrochemical impedance of the sodium ion composite solid electrolyte prepared in Example at room temperature is significantly lower than that of the comparative example, while the room temperature ionic conductivity of the sodium ion composite solid electrolyte prepared in Example is significantly higher than that of the comparative example, indicating that the room temperature ionic conductivity of the sodium ion composite solid electrolyte can be greatly improved by introducing the sodium salt additive.

[0086] like Figure 2 As shown, differential scanning calorimetry (DSC) tests show that the glass transition temperature (Tg) and melting temperature (Tm) of Example 1 are reduced from -33.35°C and 50.93°C to -34.95°C and 44.91°C, respectively, compared to Comparative Example 1. The reduction in glass transition temperature and melting temperature indicates that the crystallinity of the polymer is reduced. The flexibility of the polymer molecular chain increases, resulting in enhanced mobility between chain segments, thereby affecting the orderly arrangement of the molecular chains and reducing the crystallization ability. At the same time, the lower melting temperature further illustrates that the crystallinity of the polymer is weakened. This change may be related to the adjustment of the polymer chain structure and the influence of the sodium salt additive. All these factors work together to inhibit the crystallization process, resulting in a decrease in crystallinity.

[0087] like Figure 3As shown, the sodium metal surface after the cycle of Comparative Example 1: During the cycle of the sodium ion composite solid electrolyte without the addition of sodium salt additives, the deposition and stripping of sodium metal are uneven, which is specifically reflected in the presence of many spherical dendrites on the surface of the sodium metal negative electrode. The formation of these dendrites is caused by the local overpotential during the deposition of sodium metal, resulting in irregular deposition of sodium metal on the negative electrode surface. The presence of dendrites not only affects the cycle stability of the battery, but may also cause a short circuit in the battery, ultimately leading to battery failure. Therefore, controlling the uniformity of sodium metal deposition is the key to improving battery performance and safety.

[0088] like Figure 4 As shown, the surface of sodium metal after the cycle of Example 1: under the action of the sodium salt additive, the deposition and stripping of sodium metal are uniform, the surface of the sodium metal negative electrode after the cycle is flat, no dendrites are formed, and a negative electrode interface protection layer (SEI) with a smooth surface is produced, which effectively inhibits the interface side reactions during the cycle and greatly improves the cycle stability of the sodium solid-state battery. The sodium salt additive reduces the local overpotential during the deposition process and inhibits irregular deposition by optimizing the migration behavior of sodium ions in the sodium ion composite solid electrolyte. This uniform deposition method helps to improve the cycle stability of the sodium solid-state battery, prevent dendrites from causing short circuits, and thus promote the long-term stable operation of the sodium solid-state battery.

[0089] like Figure 5 As shown, after the cycles, the symmetrical batteries of Example 1 and Comparative Example 1 were disassembled, and it was found that the sodium ion composite solid electrolyte of Comparative Example 1 turned black after the cycles, while the Example 1 maintained its original morphology after the cycles. Figure 6 As shown, under a scanning electron microscope, there is a gray-black reaction area between the sodium metal and the electrolyte in Comparative Example 1, while this phenomenon does not occur in Example 1. Figure 5 and Figure 6 It was revealed that the stability of the sodium ion composite solid electrolyte to the sodium metal negative electrode was improved by introducing a sodium salt additive into the sodium ion composite solid electrolyte.

[0090] like Figure 7 As shown, the stability of the full battery cycle of Example 1 is significantly better than that of Comparative Example 1.

Claims

1. A sodium ion composite solid electrolyte, characterized in that: The invention comprises a raw material mixture formed by mixing a polymer matrix, a basic sodium salt and a sodium salt additive, and a nanoparticle filler uniformly dispersed in the raw material mixture; the molar ratio of the polymer matrix: the basic sodium salt: the sodium salt additive is (12-18): 1: (0.05-0.5); the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture.

2. A sodium ion composite solid electrolyte according to claim 1, characterized in that: The polymer matrix is polyethylene oxide (PEO), the basic sodium salt is sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) or sodium bis(fluorosulfonyl)imide (NaFSI), the sodium salt additive is one of sodium difluorooxalatoborate (NaDFOB) and sodium bis(oxalatoborate) (NaBOB) or a mixture of the two in any ratio; the nanoparticle filler is an inert filler, and the inert filler is one of Al2O3, SiO2, TiO2, and ZrO2.

3. A method for preparing a sodium ion composite solid electrolyte, characterized in that: The following steps are involved: Step 1: After mixing the nanoparticle filler and the dispersant, ultrasonically disperse them to obtain a particle dispersion; Step 2: Weighing a raw polymer matrix, a basic sodium salt, and a sodium salt additive in a molar ratio of (12-18):1:(0.05-0.5) to obtain a raw material mixture; adding a particle dispersion to the raw material mixture, wherein the mass of the nanoparticle filler accounts for 5-20wt% of the mass of the raw material mixture; then adding a dispersant and stirring to obtain a sodium ion composite solid electrolyte slurry with a total solid content of 20-30wt%; The polymer matrix is polyethylene oxide (PEO), the basic sodium salt is sodium bistrifluoromethylsulfonyl imide (NaTFSI) or sodium bisfluorosulfonyl imide (NaFSI), and the sodium salt additive is one of sodium difluorooxalatoborate (NaDFOB) and sodium bisoxalatoborate (NaBOB) or a mixture of the two in any ratio; Step 3: coating the sodium ion composite solid electrolyte slurry on a polytetrafluoroethylene membrane, drying the membrane, and then standing the membrane at room temperature in a dry environment for at least 12 hours to obtain a sodium ion composite solid electrolyte film.

4. The method for preparing a sodium ion composite solid electrolyte according to claim 3, wherein: In step 1, based on 100 parts by mass, the ratio of nanoparticle filler to dispersant is (5-10): (90-95).

5. The method for preparing a sodium ion composite solid electrolyte according to claim 3, wherein: In step 1, the nanoparticle filler is an inert filler, and the inert filler is one of Al2O3, SiO2, TiO2, and ZrO2.

6. The method for preparing a sodium ion composite solid electrolyte according to claim 3, wherein: The dispersants in step 1 and step 2 are both anhydrous acetonitrile (ACN) or N-methylpyrrolidone (NMP).

7. The method for preparing a sodium ion composite solid electrolyte according to claim 3, wherein: In step 3, the drying process is: first drying in a forced air drying oven at 50-60°C for 0.5-1h, then drying in a vacuum drying oven at 50-60°C for 24-36h; the drying environment is: a drying room with a water content of less than 0.01ppm and an oxygen content of less than 0.5ppm.

8. The method for preparing a sodium ion composite solid electrolyte according to claim 3, wherein: In step 3, the coating thickness of the sodium ion composite solid electrolyte slurry is 800-2000 μm.

9. Use of the sodium ion composite solid electrolyte according to any one of claims 1 to 2 or the sodium ion composite solid electrolyte prepared according to the preparation method according to claims 3 to 8 in a sodium solid-state battery.

Images