A polymer-based solid-state electrolyte membrane for a sodium metal battery and a preparation method and applications thereof

Abstract

The application discloses a preparation method of a polymer-based solid electrolyte film for a high-voltage sodium metal battery, which comprises the following steps: (1) dispersing double-vacancy titanium oxide nanosheets in a solvent to obtain a dispersion liquid; (2) mixing a polymer, a sodium salt and the dispersion liquid in step (1) by ball milling to obtain an electrolyte slurry, and drying and film-forming the electrolyte slurry to obtain the polymer-based solid electrolyte film. The application discloses a preparation method of a polymer-based solid electrolyte film for a high-voltage sodium metal battery, which utilizes Lewis acidic sites (oxygen vacancies) and Lewis basic sites (titanium vacancies) on the surface of double-vacancy titanium oxide nanosheets to adjust the coordination environment of sodium ions, promote the migration of sodium ions, inhibit side reactions, improve the cycle and electrochemical stability of the sodium metal battery, and the sodium metal battery prepared by the method has excellent cycle stability under normal pressure and high pressure.

Description

Technical Field

[0001] This invention relates to the technical field of solid electrolytes, and more particularly to a polymer-based solid electrolyte membrane for sodium metal batteries, its preparation method, and its application. Background Technology

[0002] With the growing demand for sustainable energy storage solutions to support renewable energy development, the development of safe and efficient battery technologies is becoming increasingly important. Sodium-ion batteries (SIBs), due to the abundant and widely distributed nature of sodium resources, have been considered a highly attractive candidate technology for large-scale electrochemical energy storage. Compared to currently commercialized hard carbon materials, sodium metal anodes offer advantages due to their high theoretical specific capacity (1165 mAh g⁻¹). −1 The sodium metal anode and its low redox potential (−2.714 V vs. SHE) can further enhance the overall battery capacity, thereby driving the development of next-generation low-cost and high-energy-density battery technologies. However, when sodium metal anodes are used in conjunction with traditional liquid organic electrolytes, serious safety challenges arise due to the highly flammable nature of the solvents in these electrolytes.

[0003] Solid-state sodium metal batteries (SSMBs) utilize solid-state electrolytes (SSEs) to replace traditional liquid electrolytes, making them a promising energy storage technology due to their high safety and high energy density. Among various solid-state electrolytes, polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) have attracted widespread attention due to their excellent flexibility and ease of processing. To further improve their performance, inorganic fillers (such as SiO2, Al2O3, TiO2, etc.) are often introduced to construct composite polymer electrolytes (CPEs). These fillers can effectively reduce the crystallinity and glass transition temperature of the polymer, increase the free Na⁺ concentration, thereby significantly improving ionic conductivity, Na⁺ transport number, and electrochemical stability. Among numerous fillers, metal oxides are preferred materials due to their excellent thermal stability and chemical compatibility.

[0004] However, PEO-based electrolytes themselves have poor high-voltage tolerance and unsatisfactory interfacial compatibility with high-voltage cathodes, severely limiting their practical application in high-voltage systems. To address this issue, researchers have generally attempted to introduce functional additives to enhance high-voltage performance by forming a stable interfacial layer or increasing the bulk oxidation potential. However, the addition of additives also complicates the electrolyte system, increasing not only costs and processing difficulty but also potentially causing problems such as filler agglomeration and uneven ionic conductivity distribution. More seriously, some additives may trigger side reactions at the sodium metal anode interface, forming an unstable, high-impedance solid electrolyte interphase (SEI) film, which accelerates capacity decay and impairs cycle stability.

[0005] If the inorganic filler can be structurally designed and functionally modified to enhance its conductivity while improving interfacial compatibility and high-voltage stability, high-performance, high-reliability solid-state sodium batteries may be realized. However, due to the limited chemical activity of the inorganic filler itself, endowing it with multiple functions remains a significant challenge. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention discloses a method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries. This method utilizes Lewis acidic sites (oxygen vacancies) and Lewis basic sites (titanium vacancies) on the surface of dual-vacancy titanium oxide nanosheets to regulate the coordination environment of sodium ions, simultaneously promoting sodium ion migration, suppressing side reactions, and improving the cycling and electrochemical stability of the sodium metal battery. The sodium metal battery prepared by this invention exhibits excellent cycling stability under both ambient and high pressure.

[0007] The specific technical solution is as follows:

[0008] A method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries includes the following steps:

[0009] (1) Disperse the double-vacancy titanium dioxide nanosheets in a solvent to obtain a dispersion;

[0010] (2) The polymer, sodium salt and dispersion from step (1) are mixed with spherical ink to obtain an electrolyte slurry, which is then dried and film-formed to obtain a polymer-based solid electrolyte membrane.

[0011] The preparation method disclosed in this invention uses titanium dioxide nanosheets with both oxygen and titanium vacancies as inorganic fillers. These nanosheets are ball-milled with a polymer and sodium salt in a solvent, then dried to form a film, thus preparing a polymer-based solid electrolyte. Detailed characterization revealed that the interaction between the dual-vacancy titanium dioxide nanosheets and the polymer is a key factor in achieving cycle stability.

[0012] In step (1) of this invention, the preparation of the double-vacancy titanium oxide nanosheets specifically includes:

[0013] S1. Titanium oxide nanosheets are used as raw materials. After peeling, a titanium oxide nanosheet suspension is obtained and then dried to obtain titanium oxide nanosheets.

[0014] The stripping time shall not be less than 10 days;

[0015] The thickness of the titanium dioxide nanosheets is ≤10 nm;

[0016] S2. The titanium oxide nanosheets are annealed in air, cooled to room temperature, and then reduced in a reducing atmosphere to obtain the double-vacancy titanium oxide nanosheets.

[0017] The reduction treatment temperature is 500~750℃.

[0018] In this invention, the double-vacancy titanium oxide nanosheets are prepared by the above-mentioned specific process. Experiments have shown that the thickness of the titanium oxide nanosheets obtained by exfoliation in step S1 and the temperature of the reduction treatment in step S2 significantly affect the performance of the prepared polymer-based solid electrolyte membrane and the electrochemical performance of the final assembled battery.

[0019] Experiments revealed that the exfoliation time directly affects the thickness of the prepared titanium dioxide nanosheets, and also influences the final thickness of the double-vacancy titanium dioxide nanosheets. When the exfoliation time is not less than 10 days, the thickness of the prepared titanium dioxide nanosheets is ≤10 nm, and the thickness of the final double-vacancy titanium dioxide nanosheets is ≤2 nm.

[0020] Preferably, the exfoliation time is not less than 15 days, and the thickness of the prepared titanium dioxide nanosheets is ≤5 nm.

[0021] Further electrochemical performance testing showed that the cycle stability of the assembled battery was continuously improved as the above parameters were continuously optimized; however, when the peeling time was less than 10 days, the prepared titanium dioxide nanosheets became too thick, and the cycle stability of the final assembled battery decreased significantly.

[0022] Experiments revealed that the temperature drop during reduction treatment significantly affected the total vacancy content in the prepared double-vacancy titanium dioxide nanosheets. When the reduction treatment temperature was too high, such as 800℃ or 900℃, no vacancies were observed.

[0023] Preferably, the reduction treatment temperature is 600~700℃; more preferably 700℃.

[0024] Further electrochemical performance testing revealed that, with continuous optimization of the aforementioned parameters, the cycle stability of both the assembled atmospheric pressure full cell and the high pressure full cell continued to improve.

[0025] In step S1 of the present invention, the drying is selected from freeze drying, which involves pre-cooling with liquid nitrogen and then placing the product in a freeze dryer at -40~50 ℃ for drying.

[0026] In step S2 of the present invention, the annealing temperature is 400~750℃ and the annealing time is 1~8 h.

[0027] The reducing atmosphere is selected from a mixture of hydrogen and carrier gas;

[0028] The carrier gas is selected from common gas types such as nitrogen and helium.

[0029] Preferably, the hydrogen volume ratio in the mixture is 10-90%; more preferably, the hydrogen volume ratio is 50%.

[0030] Tests showed that the thickness of the prepared double-vacancy titanium oxide nanosheets was no more than 2 nm.

[0031] In step (1) of this invention:

[0032] The solvent is selected from one or more of acetonitrile, butyronitrile, and acetone;

[0033] The concentration of the double-vacancy titanium dioxide nanosheets in the dispersion is 1-15 wt%; preferably 5-9 wt%.

[0034] The dispersion is selected from ultrasonic dispersion, and the time is not less than 30 minutes.

[0035] In step (2) of this invention:

[0036] The polymer is selected from one or more of polyethylene oxide (PEO), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and polyacrylonitrile (PAN);

[0037] Preferably, the polymer is selected from PEO.

[0038] The sodium salt is selected from one or more of sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium perchlorate, and sodium hexafluorophosphate.

[0039] In step (1), the mass ratio of the double-vacancy titanium dioxide nanosheets, polymer and sodium salt is 1:(5~20):(1~10).

[0040] In step (2) of this invention:

[0041] The ductile ink is rotated at a speed of 500~700 rpm for a time of not less than 20 hours.

[0042] Experiments have shown that if ball milling is replaced with ultrasonic mixing, a method commonly used in the field, the cycle stability of the final assembled battery will decrease significantly.

[0043] The present invention also discloses a polymer-based solid electrolyte membrane obtained according to the preparation method described above, wherein the thickness of the polymer-based solid electrolyte membrane is 50~70 μm.

[0044] The present invention also discloses a solid sodium metal battery, comprising a positive electrode, a negative electrode, a separator, and the polymer-based solid electrolyte membrane.

[0045] Preferably, the positive electrode is selected from sodium vanadium phosphate and / or sodium fluorophosphate.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] This invention discloses a method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries. The method utilizes Lewis acidic sites (oxygen vacancies) and Lewis basic sites (titanium vacancies) on the surface of double-vacancy titanium oxide nanosheets to regulate the coordination environment of sodium ions, thereby promoting sodium ion migration, suppressing side reactions, and improving the cycle life and electrochemical stability of sodium metal batteries.

[0048] Detailed characterization revealed that the interaction between titanium dioxide nanosheets and the polymer is a key factor in achieving high-voltage cycling stability. Experiments showed that the thickness of the double-vacancy titanium dioxide nanosheets, their vacancy concentration, and the mixing method with the polymer play a crucial role in obtaining the target product. If the double-vacancy titanium dioxide nanosheets are too thick (greater than 2 nm), they cannot form a good mechanical interface with PEO, and the active sites on the titanium dioxide surface cannot be fully utilized, promoting dendrite growth and reducing the ion transport capacity of the electrolyte. Only when oxygen and titanium vacancies are present simultaneously in titanium dioxide can the dual vacancies work synergistically, enabling the solid-state electrolyte to achieve cycling stability. Furthermore, ball milling can achieve thorough and uniform mixing of titanium dioxide nanosheets and PEO, improving interfacial contact and achieving stable atmospheric and high-voltage cycling performance in sodium metal solid-state batteries. Attached Figure Description

[0049] Figure 1 A scanning electron microscope (SEM) image of the titanium oxide nanosheets prepared in step S1 of Example 1;

[0050] Figure 2 An atomic force microscope (AFM) image of the titanium oxide nanosheets prepared in step S1 of Example 1;

[0051] Figure 3 An atomic force microscope (AFM) image of the oxygen- and titanium-vacancy-rich ultrathin titanium oxide nanosheets prepared in step S3 of Example 1;

[0052] Figure 4 The transmission electron microscope high-angle annular dark field image (STEM, a) of the oxygen- and titanium-vacancy-rich ultrathin titanium oxide nanosheets prepared in step S3 of Example 1, and the intensity change of the atomic columns shown by the corresponding contour lines (b).

[0053] Figure 5 Scanning electron microscope (SEM) images of the surface and cross-section (inset) of the PEO-based solid electrolyte membrane prepared for Example 1;

[0054] Figure 6 Scanning electron microscope (SEM) images of the surface and cross-section (inset) of the PEO-based solid electrolyte membrane prepared for Comparative Example 1;

[0055] Figure 7 The stress-strain curves of the PEO-based solid electrolyte membrane prepared in Example 1 are shown, and the stress-strain curves of the electrolyte membrane prepared in Comparative Example 1 without filler are given for comparison.

[0056] Figure 8 The image is a scanning electron microscope (SEM) image of the titanium oxide nanosheets prepared in step S1 of Comparative Example 1.

[0057] Figure 9 Raman spectra of ultrathin titanium oxide nanosheets rich in oxygen vacancies and titanium vacancies prepared in Examples 1-3 and Comparative Examples 3 and 4 are provided for comparison with the Raman spectra of the unreduced sample.

[0058] Figure 10 Electron paramagnetic resonance (EPR) spectra of ultrathin titanium oxide nanosheets rich in oxygen and titanium vacancies prepared in Examples 1, 2 and Comparative Example 3 are presented for comparison, and the EPR spectra of the unreduced samples are given for comparison.

[0059] Figure 11 Examples 1-3 and Comparative Examples 3 and 4 show the X-ray diffraction (XRD) spectra of ultrathin titanium oxide nanosheets rich in oxygen vacancies and titanium vacancies, respectively, and the XRD spectra of the unreduced samples are given for comparison.

[0060] Figure 12 The X-ray diffraction (XRD) spectra of the PEO-based solid electrolyte membranes prepared in Examples 1 and 2 and Comparative Examples 1 and 3, respectively.

[0061] Figure 13 The X-ray photoelectron spectroscopy (XPS) of the PEO-based solid electrolyte membranes prepared in Example 1 and Comparative Example 1 are shown, and the XPS of A700-PEO (the PEO-based solid electrolyte membrane was prepared by replacing H700 with unreduced intermediate product A700, and the preparation process was the same as in Example 1) is given as a comparison.

[0062] Figure 14 The ionic conductivity curves of PEO-based solid electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, respectively, in the range of 30-80°C;

[0063] Figure 15 The DC polarization curves of the symmetric battery assembled with the PEO-based solid electrolyte membrane prepared in Example 1 under a 10 mV perturbation (inset: electrochemical impedance spectroscopy (EIS) before and after polarization) and the sodium ion transference number were calculated.

[0064] Figure 16The DC polarization curves of the symmetric battery assembled with the PEO-based solid electrolyte membrane prepared in Example 2 under a 10 mV perturbation (inset: electrochemical impedance spectroscopy (EIS) before and after polarization) and the sodium ion transference number were calculated.

[0065] Figure 17 The DC polarization curves of the symmetric battery assembled with the PEO-based solid electrolyte membrane prepared in Example 3 under a 10 mV perturbation (inset: electrochemical impedance spectroscopy (EIS) before and after polarization) and the sodium ion transference number were calculated.

[0066] Figure 18 The DC polarization curves of the symmetric cell assembled from the PEO-based solid electrolyte membrane prepared for Comparative Example 3 under a 10 mV perturbation (Inset: Electrochemical impedance spectroscopy (EIS) before and after polarization) and the sodium ion transference number were calculated.

[0067] Figure 19 The cycle performance of symmetric batteries assembled from PEO-based solid electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, respectively;

[0068] Figure 20 The sodium metal full cells assembled with PEO-based solid electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 to 3, respectively, exhibited cycling performance at 0.5C.

[0069] Figure 21 Cyclic performance of sodium metal full cells assembled from PEO-based solid electrolyte membranes prepared in Comparative Examples 5-7;

[0070] Figure 22 The cycling performance of sodium metal full cells assembled with PEO-based solid electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, under high voltage (4.2 V 0.1 C) conditions. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially. The features and performance of the present invention will be further described in detail below with reference to the embodiments.

[0072] Example 1

[0073] S1: TiO2 (rutile), Li2CO3, K2CO3, and MoO3 were weighed according to the molar ratio of 1.73:0.13:1.67:1.27, ground evenly, and then placed in a platinum crucible. The mixture was calcined at 1473 K for 10 h. The product was then washed several times with water and dried to obtain K. 0.8[Ti 1.73 Li 0.27 O4; K 0.8 [Ti 1.73 Li 0.27 O4 was placed in a 1 M HCl aqueous solution and stirred for 5 days, with the HCl aqueous solution being replaced daily to prepare H. 1.07 Ti 1.73 O4·H2O; 0.005 mol H 1.07 Ti 1.73 O4·H2O was placed in a 0.005 M, 1 L tetrabutylammonium hydroxide (TBAOH) aqueous solution and allowed to stand for half a month to peel off, resulting in a titanium dioxide nanosheet suspension. The prepared titanium dioxide nanosheet suspension was first pre-frozen with liquid nitrogen and then freeze-dried in a freeze dryer to obtain titanium dioxide nanosheets.

[0074] S2: Place titanium oxide nanosheets in a tube furnace, heat to 700°C in air and hold for 4 hours for annealing, then cool to room temperature. Annealing can remove tetrabutylammonium hydroxide (TBAOH) inside the titanium oxide nanosheets and promote the recrystallization of freeze-dried ferriferrite titanium oxide into anatase titanium oxide rich in titanium vacancies, denoted as A700.

[0075] S3: Reheat the tube furnace. When the temperature at the center of the tube furnace reaches 700 °C, use a push-pull rod to push the sample into the center of the furnace tube. Introduce a mixture of 150 sccm hydrogen and 150 sccm argon. After holding for 120 min, turn off the hydrogen and continue to introduce argon. Pull the sample out of the central area (away from the center of the tube furnace, equivalent to a sudden drop to room temperature). At this time, argon acts as a protective gas to prevent the sample rich in oxygen and titanium vacancies from being oxidized. After the temperature drops to room temperature, it can be taken out of the furnace tube. The product is a double-vacancy ultrathin titanium oxide nanosheet rich in oxygen and titanium vacancies, denoted as H700.

[0076] S5: Weigh out double-vacancy ultrathin titanium dioxide nanosheets in a glove box, and disperse them in anhydrous acetonitrile using an ultrasonic method to obtain a 7% mass fraction double-vacancy titanium dioxide nanosheet dispersion. The ultrasonic time is greater than 30 min.

[0077] S6: PEO and NaTFSI were added to the dispersion of double-vacancy titanium dioxide nanosheets. The mass ratio of double-vacancy titanium dioxide nanosheets, PEO and NaTFSI added in step S5 was 1:10:4. The mixture was then continuously ball-milled at 600 rpm and 30°C for 20 h. The uniform electrolyte slurry was poured onto a polytetrafluoroethylene (PTFE) plate and dried in an inert atmosphere at 60°C for 24 h to remove residual anhydrous acetonitrile. A film was formed to obtain a PEO-based solid electrolyte membrane, designated as H700-PEO.

[0078] Figure 1 The image shown is a scanning electron microscope (SEM) image of the titanium oxide prepared in step S1 of this embodiment. It is observed that the titanium oxide exhibits a cross-linked nanosheet morphology.

[0079] Figure 2 The image shown is an atomic force microscope (AFM) image of the titanium oxide nanosheets prepared in step S1 of this embodiment. The thickness curve of the prepared titanium oxide nanosheets shows that the thickness is 0.5~5.0 nm.

[0080] Figure 3 The image shown is an atomic force microscope (AFM) image of the oxygen- and titanium-vacancy-rich ultrathin titanium oxide nanosheets prepared in step S3 of this embodiment. The AFM thickness curve shows that the prepared oxygen- and titanium-vacancy-rich ultrathin titanium oxide nanosheets have a smooth surface and a thickness of about 1.46 nm.

[0081] Figure 4 The image shows a high-angle annular dark-field transmission electron microscope (STEM, Figure a) image of the oxygen- and titanium-vacancy-rich ultrathin titanium oxide nanosheets prepared in step S3 of this embodiment, and the corresponding contour lines showing the intensity changes of their atomic rows (Figure b). Observation clearly reveals that in selected areas, there are atomic rows with decreased atomic intensity, indicating the presence of titanium vacancies.

[0082] Figure 5 The images shown are scanning electron microscope (SEM) images of the surface and cross-section (inset) of the PEO-based solid electrolyte membrane (H700-PEO) finally prepared in this embodiment. Observation reveals that the electrolyte membrane exhibits a flat surface, which helps improve interfacial contact. The cross-sectional images show that the thickness of H700-PEO is controlled at approximately 50–70 micrometers, close to that of acceptable commercially available separators.

[0083] Comparative Example 1

[0084] PEO and NaTFSI in a mass ratio of 10:4 were continuously ball-milled at 600 rpm and 30°C for 20 h. The uniform electrolyte slurry was poured onto a polytetrafluoroethylene (PTFE) plate and dried in an inert atmosphere at 60°C for 24 h to remove residual anhydrous acetonitrile. The resulting film was a PEO-based solid electrolyte membrane, denoted as Filler-free.

[0085] Figure 6 The image shows a scanning electron microscope (SEM) image of the surface and cross-section (inset) of the PEO-based solid electrolyte membrane prepared for Comparative Example 1. It can be observed that the surface is uneven and has large undulations, with a thickness of 76 micrometers.

[0086] contrast Figure 5 and Figure 6It was found that the addition of double-vacancy titanium dioxide nanosheets had a perturbation effect on PEO, reducing the crystallinity of PEO, making PEO more uniformly dispersed, and achieving a commercially viable thickness for the electrolyte membrane.

[0087] Figure 7 The stress-strain curve of the PEO-based solid electrolyte membrane prepared in Example 1 is shown, and the stress-strain curve of the PEO-based solid electrolyte membrane prepared in Comparative Example 1 without filler is also given for comparison. The actual test object is also shown at the bottom of the figure. Compared with Comparative Example 1, the H700-PEO prepared in this example exhibits excellent tensile properties, reaching 400% elongation at break and a maximum tensile stress of 0.25 MPa. It can completely recover its shape after repeated 180° folding. This may be due to the inhibition of PEO crystallinity by the double-vacancy ultrathin titanium oxide nanosheets, which is consistent with the XRD results, that is, the addition of double-vacancy titanium oxide nanosheets enhances the mechanical properties of the electrolyte membrane.

[0088] Comparative Example 2

[0089] The preparation process is basically the same as in Example 1, except that in step S1, the static peeling time is replaced with 5 days. The product is designated as H700-T-PEO.

[0090] Figure 8 The image shows a scanning electron microscope (SEM) image of the titanium oxide nanosheets prepared in step S1 of this comparative example. It was observed that the thickness of the nanosheets was about 55 nm.

[0091] Example 2

[0092] The preparation process is basically the same as in Example 1, except that in step S3, the center temperature of the tube furnace is replaced with 600℃, the sample number is recorded as H600, and the corresponding electrolyte membrane number is recorded as H600-PEO.

[0093] Example 3

[0094] The preparation process is basically the same as in Example 1, except that in step S3, the center temperature of the tube furnace is replaced with 500℃, the sample number is recorded as H500, and the corresponding electrolyte membrane number is recorded as H500-PEO.

[0095] Comparative Example 3

[0096] The preparation process is basically the same as in Example 1, except that in step S3, the center temperature of the tube furnace is replaced with 800℃, the sample number is recorded as H800, and the corresponding electrolyte membrane number is recorded as H800-PEO.

[0097] Comparative Example 4

[0098] The preparation process is basically the same as in Example 1, except that in step S3, the center temperature of the tube furnace is replaced with 900℃, the sample number is recorded as H900, and the corresponding electrolyte membrane number is recorded as H900-PEO.

[0099] Comparative Examples 5-7

[0100] The preparation process is basically the same as in Example 1, except for step S6:

[0101] The ball milling method was replaced with ultrasound, with an ultrasound frequency of 20 kHz and a power of 150 W. The ultrasound mixing times were 5 min, 7 min, and 10 min, respectively.

[0102] Figure 9 Raman spectra of ultrathin titanium oxide nanosheets rich in oxygen and titanium vacancies prepared in Examples 1-3 and Comparative Examples 3 and 4, respectively, are provided for comparison. The Raman spectrum of sample A700 without reduction treatment is also given. Observation of these spectra reveals that after reduction, the main Raman peak positions in the samples of Examples 1-3 exhibit a blue shift accompanied by peak broadening, indicating that the original symmetry of the samples is disrupted and disorder increases with the formation of oxygen vacancies. In Comparative Examples 3-4, the Raman peak positions and shapes are completely altered after reduction due to the transformation of the crystal form.

[0103] Figure 10 Electron paramagnetic resonance (EPR) images of ultrathin titanium oxide nanosheets rich in oxygen and titanium vacancies prepared in Examples 1, 2, and Comparative Example 3 are shown, with the EPR image of sample A700 without reduction treatment provided for comparison. Observing the EPR of the intermediate product (A700) before reduction, it can be seen that after air annealing, its peak position is at g=1.998, indicating the presence of titanium vacancies. After hydrogen reduction treatment, the number of oxygen vacancies gradually increases, and the EPR peak position shifts towards the direction of oxygen vacancies. Furthermore, it can be observed that the sample of Example 1 has oxygen vacancies. The sample with the highest number of vacancies also contains both oxygen and titanium vacancies. The peak position of the sample in Example 2 is between titanium vacancies (g=1.998) and oxygen vacancies (g=2.003), indicating that this sample contains both titanium and oxygen vacancies. Further comparison between the A700 sample and the sample prepared in Example 1 shows that the number of oxygen and titanium vacancies in the product prepared in Example 2 is increased compared to the A700 sample, but slightly less than that in the product of Example 1. The reason for this may be due to the decrease in reduction temperature. In contrast, no vacancies appeared in the sample prepared in Comparative Example 3.

[0104] Figure 11The XRD patterns of ultrathin titanium oxide nanosheets rich in oxygen and titanium vacancies prepared in Examples 1-3 and Comparative Examples 3 and 4 are shown. The XRD pattern of the unreduced intermediate product (A700) is also provided for comparison. The comparison reveals that after hydrogen reduction treatment, the anatase titanium oxide was still maintained, but the peak positions of the XRD pattern broadened, indicating that the periodic symmetry of the sample decreased with the increase of oxygen vacancies. Specifically, the sample prepared in Example 3 (H500) showed less broadening than that in Example 1 (H700), possibly due to the lower reduction temperature. The samples prepared in Comparative Examples 3 and 4 both underwent phase transitions, with oxygen and titanium vacancies disappearing in the lattice.

[0105] Figure 12 The X-ray diffraction (XRD) spectra of the PEO-based solid electrolyte membranes prepared in Examples 1 and 2 and Comparative Examples 1 and 3 are shown. After the addition of titanium oxide, the PEO peaks broadened accordingly, and the intensity of the (120) and (112) diffraction peaks of PEO decreased. The lowest crystallinity peak (at 23.6°, in the green area) in the H700-PEO sample indicates that the crystallinity of the PEO segments decreased. The H800-PEO sample showed a stronger diffraction peak, indicating that its perturbation effect was weak and did not significantly reduce the crystallinity of the PEO segments.

[0106] Figure 13 X-ray photoelectron spectroscopy (XPS) spectra of the PEO-based solid electrolyte membranes prepared in Example 1 and Comparative Example 1 are shown. XPS of A700-PEO (the PEO-based solid electrolyte membrane was prepared by replacing H700 with unreduced intermediate product A700, using the same preparation process as in Example 1) is also provided for comparison. It can be observed that the addition of H700 increases the area of ​​the CO-Na bonds, resulting in more NaTFSI dissociation. Free Na⁺ readily interacts with the EO groups in the PEO chain through Lewis acid-base coordination. Furthermore, the Na-O bond energy in H700-PEO shifts to a higher value (529.28 eV). This further confirms that the addition of double-vacancy titanium dioxide nanosheets significantly promotes the decomposition of NaTFSI and the dissociation of Na⁺. + Coordination with EO groups, thereby forming continuous Na + Transmission channels are beneficial for promoting Na + Fast transmission.

[0107] Electrochemical performance testing:

[0108] 1. Assemble symmetrical batteries

[0109] The electrolyte membranes prepared in each embodiment or comparative example were cut into 18 mm diameter discs as electrolyte membranes, and stainless steel was cut into 14 mm diameter discs as positive and negative electrodes. Symmetrical button cells (SS / electrolyte membrane / SS) with stainless steel (SS) electrodes were assembled in a glove box for testing ionic conductivity.

[0110] Within a temperature range of 30~80°C (frequency range: 0.01 Hz to 10 Hz), 6 The ionic conductivity of the solid electrolyte membrane was determined by performing AC impedance spectroscopy (Hz). The ionic conductivity (σ) was calculated using the following formula:

[0111]

[0112] Where L is the thickness of the solid electrolyte membrane, R is the electrolyte impedance measured by AC impedance spectroscopy at different temperatures, and S is the contact area of ​​the SS electrode with a diameter of 15.6 mm.

[0113] To determine the sodium ion transport number (t) Na+ Sodium sheets were cut into 14 mm diameter discs to serve as the positive and negative electrodes, and a symmetrical sodium cell was assembled in a glove box. A constant voltage of 10 mV was applied for 6000 seconds, and the EIS spectra of the symmetrical sodium cell were recorded before and after polarization. t was calculated. Na+ The equation is as follows:

[0114]

[0115] Among them, I SS Let I0 be the steady-state current, I0 be the initial current, ΔV be the applied polarization voltage (ΔV = 10 mV), and R0 and R... SS These are the interface resistances before and after polarization, respectively.

[0116] Figure 14 Table 1 below shows the ionic conductivity curves of the electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, respectively, between 30 and 80°C, along with the ionic conductivity values ​​calculated using the above formula. It can be observed that H700-PEO exhibits the highest conductivity (0.503 mS cm⁻¹) at 60°C, which is consistent with the conclusion of decreased crystallinity observed in the XRD characterization results. The electrolyte membrane prepared in Example 2 shows a corresponding increase in crystallinity compared to Example 1, with a conductivity decrease to 3.98 × 10⁻¹ at 60°C. −4 In contrast, the sample in Comparative Example 1, lacking the addition of dual-vacancy titanium dioxide nanosheets, exhibited higher crystallinity, resulting in a conductivity decrease to 3.17 × 10⁻⁶ at 60 °C. −4 mS cm⁻¹. In Comparative Example 3, due to the change in the crystal structure of titanium oxide, the electron transport within the crystal changed accordingly, and the conductivity decreased to 3.32 × 10⁻¹ at 60 °C.−4 mS cm⁻¹.

[0117] Table 1. Ionic conductivity (σ) of electrolytes at different temperatures

[0118]

[0119] Figure 15 The DC polarization curves of the symmetric cell assembled from the composite electrolyte membrane (H700-PEO) prepared in Example 1 under a 10 mV perturbation are shown in the inset (EIS spectra before and after polarization), and the sodium ion transference number is calculated. Benefiting from the coordination-induced effect of the double vacancies in H700-PEO, H700-PEO exhibits considerable t Na+ The value reached 0.443, which is significantly higher than the 0.181 of the unfilled electrolyte in Comparative Example 1.

[0120] Figure 16 The DC polarization curve of the symmetric cell assembled from the PEO-based solid electrolyte membrane (H600-PEO) prepared in Example 2 under a 10 mV perturbation is shown in the figure (inset: electrochemical impedance spectroscopy (EIS) before and after polarization). The sodium ion transference number was calculated to be 0.342, which is significantly higher than 0.181 of the unfilled electrolyte in Comparative Example 1.

[0121] Figure 17 The DC polarization curve of the symmetric cell assembled from the PEO-based solid electrolyte membrane (H500-PEO) prepared in Example 3 under a 10 mV perturbation is shown in the figure (inset: electrochemical impedance spectroscopy (EIS) before and after polarization). The sodium ion transference number was calculated to be 0.281, which is significantly higher than 0.181 of the unfilled electrolyte in Comparative Example 1.

[0122] Figure 18 The DC polarization curve of the symmetric cell assembled from the PEO-based solid electrolyte membrane (H800-PEO) prepared for Comparative Example 3 under a 10 mV perturbation is shown in the figure (inset: electrochemical impedance spectroscopy (EIS) before and after polarization). The sodium ion transference number was calculated to be 0.235, which is significantly lower than 0.443 in Example 1.

[0123] Figure 19 The cycling performance of symmetric batteries assembled with PEO-based solid electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, respectively, shows that the sodium symmetric battery equipped with H700-PEO can maintain stable cycling for more than 1120 hours, which is significantly better than other electrolyte membranes, and the polarization voltage remains basically stable. The performance of H600-PEO is slightly worse than that of H700-PEO, but significantly better than that of the electrolyte without filler. The cycling performance of H800-PEO is greatly reduced due to the change in the titanium oxide crystal form and the disappearance of oxygen vacancies and titanium vacancies in the crystal structure.

[0124] 2. Assemble an atmospheric pressure all-solid-state sodium battery

[0125] The electrolyte membranes prepared in each embodiment or comparative example were cut into 18 mm diameter discs as electrolyte membranes. Sodium vanadium phosphate (NVP), Super P conductive additive, and polyvinylidene fluoride (PVDF) binder were mixed in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 8:1:1. This mixture was then coated onto aluminum foil and subsequently vacuum dried at 60 °C for 12 hours. After drying, the aluminum foil was cut into 14 mm diameter discs as positive electrodes, and sodium discs were used as negative electrodes. All-solid-state sodium batteries (SMBs) were assembled in a glove box, and electrochemical performance was tested.

[0126] Figure 20 The cycling performance of sodium metal full cells assembled with electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1-3, at 0.5C was compared. It was found that the sodium metal full cell assembled with H700-PEO achieved an initial capacity of 109.2 mAh g⁻¹ at 0.5C, and retained 82.6% of its capacity after 350 cycles (91.2% after 150 cycles), while also achieving an average coulombic efficiency of 99.68%. Example 2 showed a slight decrease in cycle performance due to the reduced number of vacancies. Comparative Example 1, lacking the addition of dual-vacancy titanium dioxide nanosheets, experienced rapid cycle decay and broke circuit within 200 cycles. The addition of thicker dual-vacancy titanium dioxide nanosheets in Comparative Example 2 negatively impacted cycle performance, with the specific capacity decreasing to 37 mAh g⁻¹ after 350 cycles. The addition of titanium dioxide nanosheets in Comparative Example 3 altered the crystal structure of the added nanosheets, resulting in significant cycle performance degradation.

[0127] Figure 21 To evaluate the cycling performance of sodium metal full cells assembled with electrolyte membranes prepared in Comparative Examples 5-7, it was observed that the full cell exhibited better cycling performance after 7 minutes of sonication. This may be because the titanium oxide nanosheets and PEO were not sufficiently mixed at 5 minutes, and the nanosheet structure was damaged due to excessive sonication at 10 minutes. However, compared to the full cells prepared by ball milling, the performance was still inferior. Figure 20 The performance of the ultrasonic method was significantly reduced, indicating that the ultrasonic method cannot be applied to the actual production of this system.

[0128] 3. Assemble a high-voltage all-solid-state sodium battery

[0129] The electrolyte membranes prepared in each embodiment or comparative example were cut into 18 mm diameter discs as electrolyte membranes. Sodium vanadium fluorophosphate (NVOPF), Super P conductive additive, and polyvinylidene fluoride (PVDF) binder were mixed in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 8:1:1. This mixture was then coated onto aluminum foil and subsequently vacuum dried at 60 °C for 12 hours. After drying, the aluminum foil was cut into 14 mm diameter discs as positive electrodes, and sodium discs were used as negative electrodes. All-solid-state sodium batteries (SMBs) were assembled in a glove box, and electrochemical performance was tested.

[0130] Figure 22 The cycling performance of sodium metal full batteries assembled with electrolyte membranes prepared in Examples 1 and 2, and Comparative Examples 1 and 3, under high voltage (4.2 V 0.1 C) was compared. It was found that under high voltage, the sodium metal battery of Example 1 achieved an initial capacity of 125.8 mAh g⁻¹, with a significantly improved initial charge-discharge efficiency of 92.33%, and a capacity retention rate of 87.22% after 100 cycles, realizing the preparation of an additive-free high-voltage sodium metal all-solid-state battery. Example 2 also operated under high voltage, maintaining a high capacity retention rate after 100 cycles. In Comparative Example 1, due to the lack of disturbance from the double-vacancy titanium dioxide nanosheet filler and the interaction of chemical bonds, the PEO segments easily decomposed under high voltage, leading to a severe decline in cycling performance. In Comparative Example 3, because the added titanium dioxide lacked oxygen and titanium vacancies, it could not disturb or adsorb the PEO segments, thus its cycling performance also decreased rapidly.

[0131] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. The specific examples used above to illustrate the present invention are only for the purpose of helping to understand the present invention and are not intended to limit the present invention. Those skilled in the art to which this invention pertains can make several simple deductions, modifications, substitutions, or combinations based on the concept of the present invention. These deductions, modifications, substitutions, or combinations also fall within the scope of the claims of the present invention.

Claims

1. A method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries, characterized in that, Includes the following steps: (1) Disperse the double-vacancy titanium dioxide nanosheets in a solvent to obtain a dispersion; (2) The polymer, sodium salt and dispersion from step (1) are mixed with spherical ink to obtain an electrolyte slurry, which is then dried and film-formed to obtain a polymer-based solid electrolyte membrane.

2. The method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries according to claim 1, characterized in that, In step (1), the preparation of the double-vacancy titanium oxide nanosheets specifically includes: S1. Titanium oxide nanosheets are used as raw materials. After peeling, a titanium oxide nanosheet suspension is obtained and then dried to obtain titanium oxide nanosheets. The stripping time shall not be less than 10 days; The thickness of the titanium dioxide nanosheets is ≤10 nm; S2. The titanium oxide nanosheets are annealed in air, cooled to room temperature, and then reduced in a reducing atmosphere to obtain the double-vacancy titanium oxide nanosheets. The reduction treatment temperature is 500~750℃.

3. The method for preparing the polymer-based solid electrolyte for sodium metal batteries according to claim 2, characterized in that: In step S1, the peeling time is no less than 15 days, and the thickness of the titanium dioxide nanosheets is ≤5 nm. In step S2, the temperature of the reduction treatment is 600~700℃.

4. The method for preparing the polymer-based solid electrolyte for sodium metal batteries according to claim 2, characterized in that: In step S2, the annealing temperature is 400~750℃.

5. The method for preparing the polymer-based solid electrolyte for sodium metal batteries according to claim 1, characterized in that, In step (1): The solvent is selected from one or more of acetonitrile, butyronitrile, and acetone; The concentration of the double-vacancy titanium dioxide nanosheets in the dispersion is 1~15 wt%.

6. The method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries according to claim 1, characterized in that, In step (2): The polymer is selected from one or more of polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene, and polyacrylonitrile; The sodium salt is selected from one or more of sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium perchlorate, and sodium hexafluorophosphate; In step (1), the mass ratio of the double-vacancy titanium dioxide nanosheets, polymer and sodium salt is 1:(5~20):(1~10).

7. The method for preparing a polymer-based solid electrolyte membrane for sodium metal batteries according to claim 1, characterized in that, In step (2): The ductile ink is rotated at a speed of 500~700 rpm for a time of not less than 20 hours.

8. A polymer-based solid electrolyte membrane obtained by the preparation method according to any one of claims 1 to 7, characterized in that, The thickness of the polymer-based solid electrolyte membrane is 50~70 μm.

9. A solid-state sodium metal battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and a polymer-based solid electrolyte membrane according to claim 8.

10. The solid-state sodium metal battery according to claim 9, characterized in that, The positive electrode is selected from sodium vanadium phosphate and / or sodium fluorophosphate.

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