Modified polyvinyl alcohol electrolyte membrane and preparation method and application thereof

Abstract

The application discloses a modified polyvinyl alcohol electrolyte membrane and a preparation method and application thereof, and belongs to the technical field of lithium batteries. The polyvinyl alcohol containing a large number of hydroxyl structures is used as a base body, a cyano structure with polarity is introduced, lithium ions can form effective coordination, the migration of the lithium ions is promoted, and the ionic conductivity of a solid electrolyte membrane is improved; meanwhile, a long-chain alkyl structure of an acetal ring is further introduced, the water absorption of the polyvinyl alcohol is effectively reduced, the stability of the polyvinyl alcohol is improved, the crystallinity of the polyvinyl alcohol is significantly reduced, the flexibility of the polyvinyl alcohol is improved, and the migration of the lithium ions is further promoted; in addition, the introduced acetal ring structure can improve the toughness of the polyvinyl alcohol, improve the mechanical properties, prevent lithium dendrites, and finally realize the comprehensive improvement of the safety performance and the electrical performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, specifically to a modified polyvinyl alcohol electrolyte membrane, its preparation method, and its application. Background Technology

[0002] With the continued consumption of fossil fuels and increasing environmental awareness, new energy storage technologies are attracting widespread attention. Among various energy storage technologies, lithium-ion batteries have become the core power source for portable electronic devices, electric vehicles, and large-scale energy storage power stations due to their advantages such as high operating voltage and long cycle life. However, the organic electrolytes used in traditional lithium-ion batteries pose safety hazards such as flammability, explosiveness, and leakage. This problem is particularly prominent in future battery systems with high energy density and high power output, severely restricting their further development. To overcome this bottleneck, solid polymer electrolytes, due to their excellent safety, are gradually becoming a research hotspot as a potential alternative material to organic electrolytes.

[0003] Polyvinyl alcohol (PVA) has attracted widespread attention in the field of solid-state polymer electrolytes due to its excellent film-forming properties, good mechanical strength, and environmental friendliness. Its hydroxyl polar groups possess excellent hydrogen bond donor characteristics, enabling strong interactions with lithium ions and providing pathways for lithium ion migration. However, PVA itself has relatively low room-temperature ionic conductivity, and the large number of hydroxyl groups can lead to a narrow electrochemical window, making it difficult to match with high-voltage cathode materials and limiting its practical application in all-solid-state lithium batteries. How to achieve a balance of its comprehensive performance through PVA modification has become a current research hotspot.

[0004] The interaction between the cyano group (-C≡N) and lithium ions is considered beneficial for improving the ionic conductivity of electrolytes. Patent CN118146410A provides a method for preparing cyanoethylated polyvinyl alcohol, which includes carrying out an addition reaction of a mixture containing acrylonitrile, polyvinyl alcohol and water in the presence of a quaternary ammonium base. However, acrylonitrile is a highly toxic, flammable and potentially carcinogenic substance. If the stirring or heating is not done properly during the reaction, acrylonitrile can easily volatilize and escape, causing a serious risk of poisoning and hindering large-scale production.

[0005] Therefore, developing a cyano-substituted polyvinyl alcohol electrolyte is of great significance for improving the overall performance of solid-state electrolytes, such as ionic conductivity, mechanical properties, and electrochemical stability window, as well as for promoting the practical application of solid-state lithium batteries. Summary of the Invention

[0006] The purpose of this invention is to propose a modified polyvinyl alcohol electrolyte membrane, its preparation method and application, in order to solve the problems of low lithium-ion transference number and insufficient electrolyte mechanical properties in the prior art.

[0007] To achieve its objectives, the present invention employs the following technical solution: This invention first discloses a method for preparing a modified polyvinyl alcohol electrolyte membrane, comprising the following steps: 1) Add polyvinyl alcohol resin to distilled water and dissolve it completely at 90~95℃ to obtain a polyvinyl alcohol solution; cool the polyvinyl alcohol solution to 40~60℃ and keep it at that temperature, add hydrochloric acid solution and stir thoroughly until the mixture is homogeneous; lower the system temperature to 0~10℃, add modifier one and modifier two, and keep the reaction at that temperature for 0.5~2h. 2) Heat the reaction system to 20~45℃ and keep it at that temperature for 1h~3h to obtain a modified polyvinyl alcohol mixture; 3) Cool the modified polyvinyl alcohol mixture to 15~18℃, then add alkali solution to the system, keep warm and stir for 5~30 min, filter to obtain modified polyvinyl alcohol solid product; wash the solid product repeatedly with distilled water and dry at 40~45℃ to obtain modified polyvinyl alcohol resin. 4) The modified polyvinyl alcohol resin, plasticizer, and lithium salt solution are uniformly mixed, plasticized by an extruder, formed by a co-extrusion die, and dried to obtain a modified polyvinyl alcohol electrolyte membrane.

[0008] Preferably, in step 1), the first modifier is an aldehyde with a cyano group, selected from at least one of cyanobenzaldehyde, 3-cyanopropionaldehyde, 2,2-dimethyl-4-cyanobutyraldehyde, 6-aldehyde-2-(methimero)nicotinic acid cyano, and 2-formaldehyde-4-cyanopyridine; the molar ratio of the first modifier to polyvinyl alcohol resin is 0.04~0.40:1.

[0009] Preferably, in step 1), the second modified compound is a long-chain aliphatic aldehyde with 4 or more carbon atoms, selected from at least one of butyraldehyde, pentanaldehyde, hexanal, heptaldehyde, octanaldehyde, nonanaldehyde, decanaldehyde, cinnamaldehyde, and undecaldehyde; the molar ratio of the second modified compound to polyvinyl alcohol resin is 0.03~0.25:1.

[0010] Preferably, in step 1), the modification one and modification two are added by first mixing and dissolving modification one and modification two in a strong polar solvent, and then adding them to a polyvinyl alcohol solution; the strong polar solvent is selected from at least one of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) and tetrahydrofuran (THF).

[0011] Preferably, in step 1), the degree of polymerization of the polyvinyl alcohol resin is not less than 1700, the degree of alcoholysis is 88%~99%, and the solid content of the polyvinyl alcohol solution is 4%~9%.

[0012] Preferably, in step 1), the concentration of the hydrochloric acid solution is 33 wt%, and the volume of the hydrochloric acid solution added accounts for 1% to 8% of the total volume of the polyvinyl alcohol solution.

[0013] Preferably, in step 4), the plasticizer has polar groups and good compatibility with the modified polyvinyl alcohol resin. The plasticizer is selected from at least one of succinic anhydride, acrylamide, polypropylene adipate, diphenyl isooctyl phosphate, tri(2-ethylhexyl) phosphate, and triethylene glycol diisooctyl ester. The amount of plasticizer added is 0 to 30% of the mass of the modified polyvinyl alcohol resin.

[0014] Preferably, in step 4), the lithium salt is selected from at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium nitrate, lithium dioxoborate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium hexafluorophosphate, and the mass ratio of the lithium salt to the modified polyvinyl alcohol resin is 0.2 to 0.8:1.

[0015] The present invention also discloses a lithium-ion battery, comprising a positive electrode shell, a positive electrode, an electrolyte membrane, a negative electrode, and a negative electrode shell stacked in sequence, wherein the electrolyte membrane is the modified polyvinyl alcohol electrolyte membrane described in the present invention.

[0016] The modified polyvinyl alcohol electrolyte membrane prepared by this invention has the following characteristics: ionic conductivity ≥ 2.5 × 10⁻⁶. -4 S•cm -1 The lithium-ion transference number is ≥0.700, the electrochemical stability window is ≥4.45V, and the tensile strength is ≥20.8MPa. The initial discharge specific capacity of the battery assembled from it is ≥125mAh / g, and the capacity retention rate of the battery after 800 cycles is ≥95%.

[0017] The beneficial effects of this invention are reflected in: 1. This invention provides a modified polyvinyl alcohol (PVA) electrolyte membrane. It introduces a cyano group (-C≡N) into the PVA chain via an acetalization reaction, resulting in a cyano group with enhanced polarity, thermal stability, and resistance to electrochemical oxidation. This cyano group effectively complexes with lithium ions, promoting lithium salt dissociation and significantly improving the ionic conductivity of the electrolyte membrane. Simultaneously, the introduction of the long-chain aliphatic aldehyde enhances the flexibility of the polymer chain, reduces crystallinity, and provides more channels for ion migration, further improving ion conduction efficiency. Furthermore, the acetal ring structure formed by the aldehyde and two adjacent hydroxyl groups in PVA significantly enhances the toughness of PVA, improves the mechanical properties of the electrolyte membrane, effectively inhibits lithium dendrite growth, and improves battery safety. In addition, the formation of the hydrophobic acetal ring structure effectively suppresses the hydrophilicity of PVA, contributing to improved chemical stability of the PVA electrolyte.

[0018] 2. Through systematic research on formulation and process parameters, this invention clarifies the key influencing factors and performance regulation rules of cyano- and long-chain alkyl-modified polyvinyl alcohol. By controlling process parameters, it has mastered the coordination rules of the proportion of cyano, hydroxyl and long-chain structures in the modified polyvinyl alcohol structure, and achieved synergistic regulation of multiple properties of electrolyte membrane.

[0019] 3. The method for preparing the modified polyvinyl alcohol electrolyte membrane provided by this invention utilizes widely available raw materials and involves a simple process. The modification of polyvinyl alcohol does not involve complex process parameters such as high temperature and high pressure, making it easy for industrial production. Furthermore, the processing of the polyvinyl alcohol electrolyte membrane is a mature thin-film processing technology. This invention achieves improvements in key indicators of the electrolyte membrane, such as electronic conductivity and resistance to lithium dendrite formation, through simple technical means, providing a solid guarantee for the long-term stable operation of solid-state batteries. Attached Figure Description

[0020] The invention will now be further described with reference to the accompanying drawings.

[0021] Figure 1 The reaction equation and chemical structural formula of the modified product are shown in Example 1. Figure 2 The infrared spectra of the modified polyvinyl alcohol resin and the raw materials 4-cyanobenzaldehyde and polyvinyl alcohol resin in Example 1 are shown. Figure 3 The 1H NMR spectra of the modified polyvinyl alcohol resin, 4-cyanobenzaldehyde raw material, and polyvinyl alcohol resin raw material in Example 1 are shown. Figure 4 The image shows the electrochemical window test spectrum of the modified polyvinyl alcohol electrolyte membrane obtained in Example 1. Detailed Implementation

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane, including the following steps: 1) Dissolve 1000g of PVA resin with an average degree of polymerization of 1700 and a degree of alcoholysis of 99% in 15.7L of pure water and stir at 90℃ for 4h to fully dissolve it to obtain a PVA solution; cool the PVA solution to 50℃ and keep it at that temperature, add 315mL of 33% hydrochloric acid solution and stir thoroughly until the mixture is homogeneous; when the system temperature drops to 5℃ (reaction temperature one), add a mixture of 298g of modifier one (4-cyanobenzaldehyde, molar ratio to PVA of 0.1:1) and 98.33g of modifier two (butyraldehyde, molar ratio to PVA of 0.06:1) that has been dissolved in 500mL of dimethyl sulfoxide solution, controlling the addition time to be 15~30min; then react at 5℃ for 1h.

[0024] 2) After the low-temperature reaction is completed, the reaction temperature is raised from 5℃ to 20℃ (reaction temperature two) within 1 hour, and the reaction is carried out at 20℃ for 3 hours to obtain a modified polyvinyl alcohol mixture.

[0025] 3) Cool the modified polyvinyl alcohol mixture to 15°C, then add 315g of 33% NaOH solution and stir for 25 minutes; filter to obtain the modified polyvinyl alcohol solid product; wash the solid product repeatedly with distilled water and dry at 42°C for 24 hours to obtain the modified polyvinyl alcohol resin. 4) 1000g of modified polyvinyl alcohol resin, a solution containing 300g of lithium bis(trifluoromethanesulfonylimide) (solvent is DMSO, total solution mass is 400g) and 100g of triethylene glycol diisooctanoate are thoroughly mixed in a high-speed mixer, then passed into an extruder at a temperature of 170℃ for plasticization and extrusion, and then vacuum dried at 80℃ to obtain the improved polyvinyl alcohol electrolyte membrane.

[0026] Figure 2 The Fourier transform infrared (FTIR) spectra of the modified polyvinyl alcohol resin obtained in Example 1, along with the raw materials 4-cyanobenzaldehyde and polyvinyl alcohol resin, are shown. The testing method is as follows: 1-2 mg of dry modified polyvinyl alcohol resin and approximately 200 mg of dry KBr powder are thoroughly ground until homogeneous and pressed into a thin sheet. The sample is then tested using a Fourier transform infrared (FTIR) spectrometer. As can be seen from the figure, the 4-cyanobenzaldehyde raw material has a wavelength of approximately 2232 cm⁻¹. -1 The characteristic stretching vibration peak of the cyano group (-C≡N) is present at the position; the infrared spectrum of the modified polyvinyl alcohol resin also shows this characteristic peak at the same wavenumber position, while the pure polyvinyl alcohol resin does not have this absorption peak, proving that 4-cyanobenzaldehyde has been successfully grafted onto the polyvinyl alcohol backbone.

[0027] Figure 3The above are comparative NMR spectra of the modified polyvinyl alcohol resin obtained in Example 1, the raw material polyvinyl alcohol resin (PVA), and 4-cyanobenzaldehyde. The test method is as follows: take an appropriate amount of the analyte into an NMR tube, add deuterated dimethyl sulfoxide solution, dissolve it completely at 95°C for 4 hours, and then scan it 256 times at 600 MHz using an NMR spectrometer.

[0028] Depend on Figure 3 As shown in (a), the 4-cyanobenzaldehyde monomer exhibits a characteristic signal peak of aldehyde hydrogen (-CHO) at a chemical shift of approximately 10.0 ppm, and a characteristic peak of benzene ring hydrogen at 7.5-8.5 ppm. In the spectrum of the modified polyvinyl alcohol resin, the characteristic peak of -CHO at a chemical shift of 10.0 ppm completely disappears, and the signal peak of benzene ring hydrogen changes from a sharp peak in the monomer environment to a broad peak in the polymer environment, proving that the aldehyde group of 4-cyanobenzaldehyde has fully participated in the reaction, and its benzene ring structure has been successfully grafted onto the PVA backbone.

[0029] Depend on Figure 3 As shown in (b), pure PVA exhibits a strong broad peak at a chemical shift of 4.0-4.7 ppm, generated by the side hydroxyl group (-OH). In the spectrum of modified polyvinyl alcohol resin, the intensity of this -OH peak is significantly reduced, indicating that a large number of PVA hydroxyl groups participated in the reaction; at the same time, a new broad signal peak appears at a chemical shift of 4.3-4.7 ppm, which corresponds to the characteristic hydrogen of the acetal structure (-OCO-), directly proving the formation of the acetal bond between the aldehyde group and the hydroxyl group.

[0030] Depend on Figure 3 As shown in (c), the methylene (-CH2-) signal peak of the main chain of pure PVA is located at 1.0-1.8 ppm. In the spectrum of modified polyvinyl alcohol resin, in addition to retaining the characteristic peaks of the PVA main chain, a significant new signal peak appears at a chemical shift of 0.8-1.0 ppm. This peak is the characteristic peak of the terminal methyl (-CH3) of the n-butyl side chain introduced after butyraldehyde acetalization. At the same time, the peak shape and distribution of the -CH2- peak at a chemical shift of 1.0-1.8 ppm have changed significantly, indicating that the alkyl side chain introduced by butyraldehyde has been successfully grafted onto PVA.

[0031] In conclusion, Figure 3 The 1H NMR results clearly demonstrate that PVA has undergone the expected acetalization reaction with 4-cyanobenzaldehyde and butyraldehyde.

[0032] Example 2 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the modifier is changed from 298g of 4-cyanobenzaldehyde to 188.84g of 3-cyanopropionaldehyde (molar ratio with PVA is 0.1:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0033] Example 3 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the amount of the modified 4-cyanobenzaldehyde added is changed from 298g to 596.05g (the molar ratio with PVA is 0.2:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0034] Example 4 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the amount of the modifier 4-cyanobenzaldehyde added is changed from 298g to 894.08g (the molar ratio with PVA is 0.3:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0035] Example 5 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the modified material 2 is changed from 98.33g of butyraldehyde to 117.45g of pentanal (molar ratio with PVA is 0.06:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0036] Example 6 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the modified material 2 is changed from 98.33g of butyraldehyde to 174.83g of octanaldehyde (molar ratio with PVA is 0.06:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0037] Example 7 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the degree of alcoholysis of the PVA raw material is changed from 99% to 88%, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0038] Example 8 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the degree of polymerization of the PVA raw material is changed from 1700 to 2400, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0039] Example 9 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 4), the amount of triethylene glycol diisooctanoate added is changed from 100g to 200g, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0040] Example 10 This embodiment provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 4), triethylene glycol diisooctanoate is replaced with tri(2-ethylhexyl) phosphate, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0041] Comparative Example 1 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. The difference from Example 1 is that the modifier 4-cyanobenzaldehyde was not added in step 1), while the other raw materials, parameters and preparation process are the same as in Example 1.

[0042] Comparative Example 2 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 1), the amount of the modified 4-cyanobenzaldehyde added is changed from 298g to 89.4g (the molar ratio with PVA is 0.03:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0043] Comparative Example 3 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 1), the amount of the modified 4-cyanobenzaldehyde added is changed from 298g to 1221.8g (the molar ratio with PVA is 0.41:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0044] Comparative Example 4 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. The difference from Example 1 is that the modifier di-n-butyraldehyde was not added in step 1), while the other raw materials, parameters and preparation process are the same as in Example 1.

[0045] Comparative Example 5 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 1), the amount of the modified di-n-butyraldehyde added is changed from 98.33g to 41g (the molar ratio with PVA is 0.025:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0046] Comparative Example 6 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 1), the amount of the modified di-n-butyraldehyde added is changed from 98.33g to 426.1g (the molar ratio with PVA is 0.26:1). The other raw materials, parameters and preparation process are the same as in Example 1.

[0047] Comparative Example 7 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that in step 1), 4-cyanobenzaldehyde and butyraldehyde are not dissolved in dimethyl sulfoxide solution, but are directly added to the reaction system. The other raw materials, parameters and preparation process are the same as in Example 1.

[0048] Comparative Example 8 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the reaction temperature in step 1) is changed from 5°C to 12°C, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0049] Comparative Example 9 This comparative example provides a method for preparing a modified polyvinyl alcohol electrolyte membrane. Compared with Example 1, the difference is that the reaction temperature in step 2) is changed from 20°C to 48°C, while the other raw materials, parameters and preparation process are the same as in Example 1.

[0050] The modified polyvinyl alcohol and electrolyte membranes prepared in Examples 1-10 and Comparative Examples 1-9 were subjected to measurements of ionic conductivity, lithium-ion transport number, electrochemical window, mechanical strength, discharge capacity, and cycle performance. The measurement methods are as follows: 1) Ionic conductivity: At room temperature, the resistance of the electrolyte membrane was measured by electrochemical impedance spectroscopy with a frequency range of 0.01~1000kHz and an amplitude of 10mV. 2) Lithium-ion transference number: A symmetrical Li|electrolyte|Li battery was assembled and tested using a combination of chronoamperometry and AC impedance spectroscopy. The frequency range of AC impedance spectroscopy was 0.01~1000kHz with an amplitude of 10mV. The bias voltage applied for chronoamperometry was 10mV and the sampling time was 1h. 3) Electrochemical window: The electrochemical stability window of the electrolyte membrane was evaluated by linear sweep voltammetry (LSV); the scanning voltage range was 2–6 V, and the scanning rate was 5 mV s. -1 ; 4) Mechanical Strength Test: The mechanical strength of the electrolyte membrane was tested using a universal testing machine. The sample was prepared as a film approximately 30mm × 20mm in size, clamped onto the instrument, and the tensile speed was controlled at 10mm·min. -1 .

[0051] 5) Discharge capacity and cycle performance test: Assemble lithium iron phosphate LFP|electrolyte|Li batteries and test the initial discharge capacity of the batteries at 25℃ and 0.3C discharge conditions; after 800 cycles, the capacity retention rate is obtained by comparing the initial capacity and the capacity after the 800th cycle.

[0052] The test results are shown in Table 1.

[0053] Table 1 Performance tests of modified polyvinyl alcohol electrolyte membranes and their batteries in Examples 1-10 and Comparative Examples 1-9

[0054]

[0055] Note: Ionic conductivity is a core indicator for evaluating the ion migration ability of a material or system, directly determining the rate performance (whether it can charge and discharge quickly), operating temperature range (whether it can be used normally at room temperature), internal resistance, and energy efficiency of solid-state batteries. Therefore, the higher the ionic conductivity, the easier it is for the surface to charge and discharge quickly, and the lower the internal resistance of the system.

[0056] The magnitude of the lithium-ion transference number directly reflects the independent transport efficiency of lithium ions in the electrolyte: the higher the transference number (closer to 1), the less anion or other ions migrate, which can significantly reduce the additional internal resistance caused by anion concentration polarization during charging and discharging, effectively suppress lithium dendrite growth, and improve the rate performance and energy efficiency of the battery.

[0057] The size of the electrochemical stability window determines the voltage range that the electrolyte can withstand, that is, the maximum potential difference allowed between the positive and negative electrodes of the battery without redox decomposition. The wider the window, the better it can match the high-voltage positive electrode and the low-voltage negative electrode, thereby significantly improving the energy density and safety of the whole battery.

[0058] The tensile strength of the electrolyte membrane determines its ability to resist mechanical deformation, external force damage, and suppress lithium dendrite puncture: the higher the tensile strength, the less likely the membrane is to break or short-circuit when the battery is assembled, bent, or subjected to volume changes, which is particularly important for improving the safety of all-solid-state batteries.

[0059] The initial discharge specific capacity of a battery directly reflects the amount of electricity that the active material can release during the first discharge. It is a core indicator for measuring the lithium storage capacity of electrode materials and the energy density of the entire battery. The higher the initial discharge specific capacity, the more electrical energy a unit mass of material can provide, which is more beneficial for improving battery range and weight reduction.

[0060] Capacity retention directly reflects the battery's interfacial stability, side reaction suppression capability, and structural resistance to degradation during long-term cycling. Therefore, the higher the capacity retention, the better.

[0061] As can be seen from the data recorded in Table 1, Examples 1-10, through precise control of synthesis parameters, introduced cyano groups with stronger polarity, better thermal stability, and better electrochemical oxidation resistance, as well as long-chain aliphatic aldehydes that can improve the flexibility of polymer chains, reduce crystallinity, and provide more channels for ion migration into polyvinyl alcohol. Furthermore, by optimizing the film-forming formulation, the electrolyte membrane and the corresponding battery ultimately maintained good performance: the ionic conductivity of the electrolyte was ≥2.5 × 10⁻⁶. -4 S•cm -1 Lithium-ion transference number ≥ 0.700, electrochemical stability window ≥ 4.45V, tensile strength ≥ 20.8MPa; initial discharge specific capacity ≥ 125mAh / g, capacity retention rate after 800 cycles ≥ 95%.

[0062] Comparative Example 1 showed no polyvinyl alcohol cyano modification, Comparative Example 2 showed insufficient modification, and Comparative Example 3 showed excessive modification. The results showed that the ionic conductivity, lithium-ion transference number, electrochemical stability window, initial discharge specific capacity, and capacity retention of the corresponding electrolyte membrane all decreased to some extent. This indicates that the cyano group of polyvinyl alcohol can improve the performance of the electrolyte membrane and battery. However, the degree of modification of polyvinyl alcohol cyano group needs to be precisely controlled, otherwise it will significantly affect the various performance characteristics of the product.

[0063] In Comparative Example 4 (without Modifier II), and in Comparative Examples 5 and 6 (insufficient or excessive Modifier II), the performance of the electrolyte membrane and battery were also affected. Insufficient modification with long-chain aliphatic aldehydes resulted in insufficient flexibility of the electrolyte membrane, affecting ionic conductivity; conversely, excessive modification with long-chain aliphatic aldehydes reduced the hydroxyl content in polyvinyl alcohol, which also affected ionic conductivity to some extent. Therefore, precise control of the ratio of nitrile groups, hydroxyl groups, and long-chain structures is necessary to achieve synergistic regulation of multiple electrolyte membrane properties.

[0064] Comparative Example 7 failed to achieve uniformity in the addition of the aldehyde feedstock, resulting in uneven reaction and affecting the performance of the electrolyte membrane. The unsuitable reaction temperatures in Comparative Examples 8 and 9 also affected the structural and performance differences of the nitrile-modified polyvinyl alcohol. Therefore, the modification of polyvinyl alcohol not only depends on the selection of the aldehyde feedstock but also places high demands on its synthesis process parameters.

[0065] As the above analysis shows, Examples 1-10, through precise control of process formulations and parameters, yielded products with suitable ionic conductivity, lithium-ion transference number, electrochemical stability window, mechanical properties, initial discharge specific capacity, and capacity retention. Therefore, this invention enables the preparation of high-performance modified polyvinyl alcohol electrolyte membranes.

[0066] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a modified polyvinyl alcohol electrolyte membrane, characterized in that, Includes the following steps: 1) Polyvinyl alcohol resin is added to distilled water and dissolved completely at 90-95℃ to obtain a polyvinyl alcohol solution; the polyvinyl alcohol solution is cooled to 40-60℃ and kept at this temperature, hydrochloric acid solution is added and stirred thoroughly until homogeneous; the system temperature is lowered to 0-10℃, modifier one and modifier two are added, and the reaction is carried out at this temperature for 0.5-2 hours; wherein modifier one is an aldehyde with a cyano group, and modifier two is a long-chain aliphatic aldehyde with 4 or more carbon atoms; 2) Heat the reaction system to 20~45℃ and keep it at that temperature for 1~3 hours to obtain a modified polyvinyl alcohol mixture; 3) Cool the modified polyvinyl alcohol mixture to 15~18℃, then add alkali solution to the system, keep warm and stir for 5~30 min, filter, and obtain the modified polyvinyl alcohol solid product. The solid product was repeatedly washed with distilled water and dried at 40-45℃ to obtain modified polyvinyl alcohol resin. 4) The modified polyvinyl alcohol resin, plasticizer, and lithium salt solution are uniformly mixed, plasticized by an extruder, formed by a co-extrusion die, and dried to obtain a modified polyvinyl alcohol electrolyte membrane.

2. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 1): the first modifier is at least one of cyanobenzaldehyde, 3-cyanopropionaldehyde, 2,2-dimethyl-4-cyanobutyraldehyde, 6-aldehyde-2-(methimeryl)nicotinic cyano, and 2-formaldehyde-4-cyanopyridine; the molar ratio of the first modifier to polyvinyl alcohol resin is 0.04~0.40:

1.

3. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 1), the modified compound 2 is at least one of butyraldehyde, pentanaldehyde, hexanal, heptaldehyde, octanaldehyde, nonanaldehyde, decanaldehyde, cinnamaldehyde, and undecaldehyde; the molar ratio of the modified compound 2 to polyvinyl alcohol resin is 0.03~0.25:

1.

4. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 1): Modifier 1 and Modifier 2 are added by first mixing and dissolving Modifier 1 and Modifier 2 in a strong polar solvent, and then adding them to the polyvinyl alcohol solution; the strong polar solvent is at least one of dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone and tetrahydrofuran.

5. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 1): the degree of polymerization of the polyvinyl alcohol resin is not less than 1700, the degree of alcoholysis is 88%~99%, and the solid content of the polyvinyl alcohol solution is 4%~9%.

6. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 1), the concentration of the hydrochloric acid solution is 33 wt%, and the volume of the hydrochloric acid solution added accounts for 1% to 8% of the total volume of the polyvinyl alcohol solution.

7. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 4), the plasticizer is at least one of succinic anhydride, acrylamide, polypropylene adipate, diphenyl isooctyl phosphate, tri(2-ethylhexyl) phosphate, and triethylene glycol diisooctyl ester, and the amount of plasticizer added is 0 to 30% of the mass of the modified polyvinyl alcohol resin.

8. The method for preparing a modified polyvinyl alcohol electrolyte membrane as described in claim 1, characterized in that, In step 4), the lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium nitrate, lithium dioxoborate, lithium difluorooxalate borate, lithium tetrafluoroborate, and lithium hexafluorophosphate, and the mass ratio of the lithium salt to the modified polyvinyl alcohol resin is 0.2 to 0.8:

1.

9. A modified polyvinyl alcohol electrolyte membrane, characterized in that, Prepared according to the preparation method described in any one of claims 1 to 8.

10. A lithium-ion battery, characterized in that, The modified polyvinyl alcohol electrolyte membrane according to claim 9 is used.

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