Polymer electrolyte, preparation method thereof and battery

Abstract

The invention discloses a polymer electrolyte, a preparation method thereof and a battery, and the preparation method of the polymer electrolyte comprises the following steps: dissolving a polymer containing a nucleophilic functional group in a polar solvent to obtain a first polymer solution; dissolving a natural high-molecular polymer in a polar solvent to obtain a second polymer solution; mixing the first polymer solution and the second polymer solution to form a double-polymer solution; adding a cross-linking agent into the double-polymer solution, casting, standing and curing to obtain a polymer film; immersing the polymer membrane in a polar solvent for removing the residual cross-linking agent and the non-cross-linked polymer and carrying out soaking swelling absorption, and then freeze-drying at a temperature of-55 DEG C to-35 DEG C, so that pores are formed in the polymer membrane to form a porous polymer membrane; and soaking the porous polymer membrane to absorb electrolyte containing lithium salt, and standing to obtain the dual-network polymer electrolyte. The ionic conductivity of the polymer electrolyte is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and more particularly, to a polymer electrolyte, a preparation method thereof, and a battery. Background Art

[0002] With the rapid development of the new energy vehicle and large-scale energy storage fields, there are increasingly high demands for the energy density and safety of secondary batteries. Lithium metal anodes have an extremely high theoretical capacity (3860 mAh g -1 ), so lithium metal batteries are considered to be the next generation of high energy density batteries with great promise. However, lithium metal is prone to generate sharp lithium dendrites during electrochemical cycling, posing a short-circuit risk of diaphragm puncture. Coupled with the characteristics of organic electrolytes being flammable, explosive, and prone to leakage, it brings great potential safety hazards and limits the practical application of lithium metal batteries.

[0003] Solid / semi-solid polymer electrolytes have characteristics such as high flexibility, high lithium compatibility, and light weight, making them effective alternatives to liquid electrolytes. However, the room-temperature ionic conductivity of polymer electrolytes is relatively low. The conductivity of traditional PEO (polyethylene oxide)-based polymer electrolytes is two orders of magnitude lower than that of liquid electrolytes. Existing literature and patents have reported some modification techniques for polymer electrolytes. For example, Prior Art 1 (Publication No.: CN115882061A, Application Date: March 31, 2023) discloses the preparation and application of a polyrotaxane-based polymer electrolyte, designing a polyrotaxane polymer system composed of linear polymers and cyclic molecules, and promoting ion transport through the synergistic effect of different structural molecules, thus significantly improving the room-temperature ionic conductivity and ion transference number of the electrolyte. However, a firm and solid network has not been formed between polymer molecules, and its mechanical strength is difficult to effectively resist the growth of lithium dendrites. Therefore, an additional porous support separator is required for assistance. Another example is Prior Art 2 (Publication No.: CN114539451A, Application Date: May 27, 2022) which discloses a hydroxy-rich single-ion conductor polymer SPVA-Li and its preparation method and application. Lithiation modification is carried out on PVA (polyvinyl alcohol) to obtain a hydroxy-rich single-ion conductor polymer, and then it is blended with PEO and applied to lithium metal batteries, showing good ionic conductivity and ion transference number. However, the polymer matrix is only combined by the hydrogen bond between the hydroxyl groups on the modified PVA chain segments and the ether bonds in PEO, and the physical cross-linking strength of the blend is weak, and there is still room for improvement in the mechanical stability of the polymer. Another example is Prior Art 3 (Publication No.: CN114204113A, Application Date: March 18, 2022) which discloses a preparation method of a grid-like gel-type polymer electrolyte semi-solid battery. The synthesized grid-like gel polymer electrolyte has rich pores, effectively enhancing the liquid retention capacity and lithium storage capacity, showing a relatively high ionic conductivity. However, it is necessary to perform surface pore formation on the basis of the synthesized base film and then use magnetron sputtering technology for lithium supplementation, and the preparation process is relatively complex, which is not conducive to large-scale promotion and production in industrial lithium metal batteries.

[0004] Therefore, there is an urgent need to provide a polymer electrolyte applicable to lithium metal batteries with balanced performance in terms of ionic conductivity, mechanical properties, liquid retention properties, etc., and a preparation method of the polymer electrolyte that is simple to manufacture, low-cost, and easy to promote. Summary of the Invention

[0005] In view of this, the present invention provides a polymer electrolyte, its preparation method, and a battery to solve the following problems:

[0006] By selecting polymers and natural polymers rich in nucleophilic polar functional groups as the matrix and chemically crosslinking the polymer electrolyte, the mechanical strength of the polymer electrolyte is improved through the construction of a double-network structure to better resist dendrite growth and deformation failure in lithium metal batteries;

[0007] The electrolyte membrane is pore-formed by a freeze-drying process, and the manufacturing process is optimized. It is fully soaked and swollen before freeze-drying, so that more solvent molecules are removed in the same volume of polymer during the freeze-drying process, leaving more pores, obtaining an increase in porosity and liquid absorption rate to overcome the problem of low ionic conductivity of traditional polymer electrolytes;

[0008] The combined action of the chemical crosslinking agent and the hydrogen bond crosslinking generated during the freeze-drying process makes the polymer skeleton more stable, which is beneficial to improving the thermal stability and promoting the increase of the cycle life of lithium metal batteries;

[0009] Develop a preparation process with lower cost and simpler procedures, without an additional support membrane, reduce costs, simplify the battery structure, and facilitate large-scale production and promotion.

[0010] On the one hand, the present invention provides a method for preparing a polymer electrolyte, including:

[0011] Dissolve the polymer containing nucleophilic functional groups in a polar solvent to obtain a first polymer solution;

[0012] Dissolve the natural polymer in a polar solvent to obtain a second polymer solution;

[0013] Mix the first polymer solution and the second polymer solution to form a double-polymer solution;

[0014] Add a crosslinking agent to the double-polymer solution, pour it into a mold, and let it stand and cure to obtain a polymer membrane;

[0015] Immerse the polymer membrane in a polar solvent to remove residual crosslinking agent and uncrosslinked polymer and perform soaking and swelling absorption, and then freeze-dry at a temperature of -55°C to -35°C for 6h to 24h to form pores in the polymer membrane to form a porous polymer membrane;

[0016] Soak and absorb the lithium salt-containing electrolyte in the porous polymer membrane and let it stand to obtain a double-network polymer electrolyte.

[0017] Optionally, based on the mass of the double-polymer solution being 100%, the sum of the mass ratios of the polymer containing nucleophilic functional groups and the natural polymer is 5wt.% to 25wt.%.

[0018] Optionally, the crosslinking agent includes at least one of glutaraldehyde, boric acid, borax, suberoyl chloride, epichlorohydrin, and diisocyanate.

[0019] Optionally, the polymer containing a nucleophilic functional group includes a polymer containing a hydroxyl group, a carboxyl group, and / or an amino group.

[0020] Optionally, the polymer containing a nucleophilic functional group includes at least one of polyvinyl alcohol, polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polymethacrylic acid, and polyethoxyethyleneimine.

[0021] Optionally, the natural polymer includes at least one of sodium alginate, gelatin, cellulose, carrageenan, guar gum, chitin, and agar.

[0022] Optionally, the polar solvent includes at least one of water, dimethyl sulfoxide, N,N-dimethylformamide, ethanol, ethylene glycol, glycerol, isopropanol, and acetonitrile.

[0023] Optionally, the electrolyte includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,3-dioxolane, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether.

[0024] On the other hand, the present invention provides a polymer electrolyte, which is a double-network polymer electrolyte obtained by the preparation method described above. The ionic conductivity of the polymer electrolyte is greater than 1.5 mS cm -1 , the porosity of the polymer electrolyte is greater than 80%, the liquid retention rate of the polymer electrolyte is greater than 70%, the tensile strength of the polymer electrolyte is greater than 5 Mpa, and the elongation at break is greater than 150%.

[0025] On the other hand, the present invention also provides a battery, including a stacked positive electrode sheet, a polymer electrolyte membrane, and a negative electrode, wherein the polymer electrolyte is the above polymer electrolyte.

[0026] Compared with the prior art, the polymer electrolyte, its preparation method, and the battery provided by the present invention at least achieve the following beneficial effects:

[0027] 1. In the present invention, a polymer containing a nucleophilic functional group is used. The nucleophilic functional group can combine with Li + , has good compatibility with lithium metal, and can reduce the solvation of Li + , facilitating the rapid migration of lithium ions and significantly improving the ionic conductivity. The ionic conductivity is greater than 1.5 mS cm -1 ;

[0028] 2. During the production process of the present invention, the polymer membrane is freeze-dried. The freeze-drying process can cause physical hydrogen bond cross-linking of the polymer with nucleophilic functional groups. Combining with chemical cross-linking can further increase the stability of the polymer electrolyte, form a stable double cross-linked network, which is beneficial to resisting lithium dendrites. Without an additional support membrane, it has good application space in lithium metal batteries;

[0029] 3. During the production process of the present invention, a process combining freeze-drying and immersion swelling is used, which enables more solvent molecules to be removed in the same volume of polymer during the freeze-drying process, effectively creating pores. Coupled with the three-dimensional ion transport channels of the double cross-linked network, it effectively improves the ionic conductivity of the polymer electrolyte. Of course, adopting the freeze-drying process can strongly create pores, providing richer pores for the electrolyte membrane. The porosity of the polymer electrolyte is greater than 80%, thereby absorbing more electrolyte. The cross-linked double network structure is beneficial to binding the electrolyte, providing good thermal stability and liquid retention ability for the polymer electrolyte. The liquid retention rate of the polymer electrolyte is greater than 70%. Moreover, due to the formation of the cross-linked network, the mechanical strength of the polymer electrolyte is significantly improved. The tensile strength of the polymer electrolyte is greater than 5 Mpa, and the elongation at break is greater than 150%;

[0030] 4. The raw materials of the present invention are safe, low-cost, have good compatibility with natural polymer polymers, and do not require additional modification. The production process has low requirements for equipment, a simple process, and is easy to promote on a large industrial scale.

[0031] Of course, it is not necessarily required that any product implementing the present invention simultaneously achieves all the above-mentioned technical effects.

[0032] Through the following detailed description of the exemplary embodiments of the present invention with reference to the accompanying drawings, other features and advantages of the present invention will become clear. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings incorporated in the specification and constituting a part of the specification illustrate embodiments of the present invention and, together with the description, are used to explain the principles of the present invention.

[0034] Figure 1 is a flowchart of a method for preparing a polymer electrolyte provided by the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] Now, various exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present invention.

[0036] The following description of at least one exemplary embodiment is merely illustrative in nature and in no way limits the present invention or its application or use.

[0037] Techniques, methods, and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be regarded as part of the specification.

[0038] In all examples shown and discussed herein, any specific values should be construed as merely exemplary and not as a limitation. Thus, other examples of the exemplary embodiments may have different values.

[0039] It should be noted that like reference numerals and letters refer to like items in the following figures, and thus, once an item is defined in one figure, further discussion thereof is not required in subsequent figures.

[0040] Combined with Figure 1 , the present invention provides a method for preparing a polymer electrolyte, and the specific steps are as follows:

[0041] S11, dissolve the polymer containing nucleophilic functional groups in a polar solvent to obtain a first polymer solution;

[0042] S12, dissolve the natural polymer in a polar solvent to obtain a second polymer solution;

[0043] S2, mix the first polymer solution and the second polymer solution to form a dual-polymer solution;

[0044] S3, add a crosslinking agent to the dual-polymer solution, cast a mold, and stand still for curing to obtain a polymer film;

[0045] S4, immerse the polymer film in a polar solvent to remove residual crosslinking agent and uncrosslinked polymer and perform soaking and swelling absorption, and then freeze-dry at a temperature of -55°C to -35°C for 6 h to 24 h to form pores in the polymer film to obtain a porous polymer film;

[0046] S5, soak and absorb the electrolyte containing lithium salt in the porous polymer film, and stand still to obtain a dual-network polymer electrolyte.

[0047] Specifically, no specific limitation is imposed on the sequence of steps S11 and S12 above. Step S11 can be implemented first and then step S12, or step S12 can be implemented first and then step S11, or steps S11 and S12 can be implemented simultaneously.

[0048] For step S11, when dissolving the polymer containing nucleophilic functional groups in a polar solvent, optionally, the dissolution temperature is 25°C to 90°C, and the dissolution process can use magnetic stirring for 1 h to 3 h. For example, the dissolution temperature is 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 75°C, 85°C, 90°C, and of course it can also be any value between 25°C and 90°C. It should be noted that the higher the dissolution temperature, the stronger the dissolution ability of the polar solvent. However, too high a dissolution temperature will increase energy consumption. In the embodiments of the present invention, the dissolution temperature is 25°C to 90°C, which can not only improve the dissolution ability of the polar solvent, but also facilitate low energy consumption. Magnetic stirring promotes the contact between the polar solvent and the polymer containing nucleophilic functional groups through mechanical shear force. The magnetic stirring time is 1 h, 1.5 h, 2 h, 2.5 h, 3 h, and of course it can also be any value between 1 h and 3 h. It should be noted that the higher the degree of polymerization of the polymer containing nucleophilic functional groups, the longer the magnetic stirring time, that is, the magnetic stirring time is positively correlated with the degree of polymerization of the polymer containing nucleophilic functional groups.

[0049] For step S12, when dissolving the natural polymer in a polar solvent, optionally, the dissolution temperature is 25°C to 90°C, and the dissolution process can use magnetic stirring for 1 h to 3 h. For example, the dissolution temperature is 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 75°C, 85°C, 90°C, and of course it can also be any value between 25°C and 90°C. It should be noted that the higher the dissolution temperature, the stronger the dissolution ability of the polar solvent. However, too high a dissolution temperature will increase energy consumption. In the embodiments of the present invention, the dissolution temperature is 25°C to 90°C, which can not only improve the dissolution ability of the polar solvent, but also facilitate low energy consumption. Magnetic stirring promotes the contact between the polar solvent and the natural polymer through mechanical shear force. The magnetic stirring time is 1 h, 1.5 h, 2 h, 2.5 h, 3 h, and of course it can also be any value between 1 h and 3 h. It should be noted that the higher the degree of polymerization of the natural polymer, the longer the magnetic stirring time, that is, the magnetic stirring time is positively correlated with the degree of polymerization of the natural polymer.

[0050] For steps S11 and S12, optionally, the polar solvent includes at least one of water, dimethyl sulfoxide, N,N-dimethylformamide, ethanol, ethylene glycol, glycerol, isopropanol, and acetonitrile.

[0051] It should be noted that the polarity of the solvent will affect the solubility of the polymer in it, the stirring time, the stirring temperature, etc. Since the selected polymer itself has nucleophilic functional groups and has a high polarity, it will have a higher solubility in a polar solvent. If a non-polar solvent is selected, the dissolution will be difficult and the dissolution concentration for casting into a film cannot be achieved.

[0052] Optionally, based on the mass of the double-polymer solution being 100%, the sum of the mass ratios of the polymer containing nucleophilic functional groups and the natural polymer is 5 wt.% to 25 wt.%.

[0053] Specifically, the mass ratio ranges of the polymer containing nucleophilic groups or the natural polymer (relative to the total mass of the polymer and the solvent) are respectively 0 to 20 wt.%, and specifically can be 0, 1, 2.5, 5 wt.%, 7.5 wt.%, 10 wt.%, 12.5 wt.%, 15 wt.%, 17.5 wt.%, 20 wt.%, 22.5 wt.%, 25 wt.%. However, it should be ensured that the sum of the mass ratios of the polymer containing nucleophilic groups and the natural polymer in the total solution is between 5 wt.% and 25 wt.%. Too low a polymer ratio cannot smoothly form a stable and shaped polymer electrolyte membrane during subsequent preparation, while too high a polymer ratio makes the solution viscosity too high, affecting the casting effect, resulting in a too thick polymer electrolyte membrane, low porosity, and poor ion transport effect.

[0054] For step S2, to mix the first polymer solution and the second polymer solution to form a double-polymer solution, it is necessary to cool the first polymer solution and the second polymer solution to room temperature and then mix them together to form a homogeneous solution to obtain the double-polymer solution.

[0055] Natural polymers can effectively improve the shortcomings of the ionic conductivity of conventional polymers. The rich nucleophilic polar functional groups (such as hydroxyl, carboxyl, amino, etc.) on the polymer side chains have high lithium affinity and can promote the effective transport of lithium ions.

[0056] For step S3, adding a cross-linking agent to the double-polymer solution, casting into a mold, and standing for curing to obtain a polymer membrane. Optionally, adding a certain amount of cross-linking agent to the double-polymer solution, quickly stirring and then casting into a polytetrafluoroethylene mold, and standing for 2 - 12 h to complete curing, thereby obtaining the polymer membrane.

[0057] Optionally, the cross-linking agent includes at least one of glutaraldehyde, boric acid, borax, suberoyl chloride, epichlorohydrin, and diisocyanate.

[0058] In the present invention, adding a cross-linking agent acidified with dilute sulfuric acid requires a acidic environment to cross-link smoothly. This should not be considered an initiator because sulfuric acid itself does not participate in the induction and initiation reaction, but provides an acidic environment and can be regarded as a catalyst. After adding the cross-linking agent, stirring is also required to ensure sufficient mixing, but the stirring time cannot be too long, otherwise it may not be possible to smoothly cast into the mold due to the increased viscosity caused by cross-linking. A quick stirring and mixing is sufficient.

[0059] For step S4, the cast polymer film is immersed in a vessel containing a polar solvent and kept for 4 - 18 h to remove the residual cross-linking agent and uncross-linked polymer and to allow for sufficient swelling and liquid absorption, and then freeze-dried at -55 to -30 °C for 6 - 24 h to form a polymer electrolyte membrane.

[0060] Optionally, the polar solvent includes at least one of water, dimethyl sulfoxide, N,N-dimethylformamide, ethanol, ethylene glycol, glycerol, isopropanol, and acetonitrile. The polar solvent here has good solubility for the uncross-linked raw material monomers, while the cross-linked polymer has a stable structure and is not easily soluble, so the polar solvent can be used to remove the residual reaction monomers.

[0061] In this step S4, swelling and liquid absorption are also combined to allow the polymer film to fully absorb the solvent, producing a swelling effect and increasing the liquid content. In this way, more solvent molecules will be removed during the subsequent freeze-drying process, leaving more pores.

[0062] In the prior art, repeated freezing and thawing are carried out at low temperatures (-30 °C to -10 °C) to allow hydrogen bond cross-linking of the polymer during the freezing-thawing process to form amorphous crystalline regions, achieving a physical cross-linking. The freeze-drying method of the present invention needs to be carried out at a relatively low temperature without a repeated freezing and thawing process. The purpose is not only to achieve physical cross-linking, but more importantly, to allow the solvent molecules in the polymer film to crystallize and expand during the low-temperature freezing process, and then directly sublimate the solid solvent into a gas under vacuum conditions, leaving a large number of pores in the polymer film, thus forming a pore-forming effect. The freeze-drying temperature of the present invention can be between -55 and -35 °C, specifically -55, -50, -45, -40, -35. If the temperature is too low, the equipment burden requirement is large and the freezing speed is too fast, resulting in large ice crystal particles that are not easily sublimated away; if the temperature is too high, the sample is not completely frozen, the solvent removal is not thorough, and the pore structure of the sample may collapse.

[0063] The present invention uses a process combining freeze-drying and immersion swelling during the production process. The process of combining immersion swelling and freeze-drying can effectively increase the porosity and liquid absorption rate of the polymer electrolyte membrane and ensure the ionic conductivity. The combination of freeze-drying and immersion swelling allows more solvent molecules to be removed in the same volume of polymer during the freeze-drying process, which can effectively form pores. Coupled with the three-dimensional ion transport channels of the double cross-linked network, the ionic conductivity of the polymer electrolyte is effectively improved;

[0064] The freezing process promotes physical cross-linking of polymer molecules, further enhancing the stability of the polymer matrix. Chemical cross-linking + hydrogen bond binding can construct a stable double-network polymer, showing good mechanical strength and thermal stability;

[0065] Adopting the freeze-drying process can effectively create pores, providing the electrolyte membrane with richer pores, thereby absorbing more electrolyte. The cross-linked double-network structure is conducive to binding the electrolyte, providing good thermal stability and liquid retention capacity for the polymer electrolyte.

[0066] For step S5, the porous polymer membrane is soaked in and absorbs the electrolyte containing lithium salt, and then left standing to obtain the double-network polymer electrolyte.

[0067] Optionally, the porous polymer membrane is soaked in and absorbs the electrolyte containing lithium salt and left standing for 12h to 48h to obtain the double-network polymer electrolyte. Specifically, the standing time can be 12h, 16h, 18h, 20h, 22h, 24h, 26h, 28h, 30h, 32h, 34h, 36h, 40h, 42h, 44h, 46h or 48h, and of course it can be any value between 12h and 48h. It should be noted that soaking helps to form a more stable interfacial layer and improve the overall performance of the battery. The longer the standing time, the more lithium ions are absorbed, which is more conducive to improving the ionic conductivity of the polymer electrolyte. However, if the standing time is too long, even if the standing time is extended after the absorbed lithium ions are saturated, the amount of absorbed lithium ions will not increase, increasing the cost. In the present invention, the first polymer electrolyte is soaked in and absorbs the electrolyte containing lithium salt and left standing for 12h to 48h, which can not only form a stable interfacial layer, but also improve the ionic conductivity without increasing the manufacturing cost.

[0068] Optionally, the polymer containing nucleophilic functional groups includes polymers containing hydroxyl, carboxyl and / or amino groups.

[0069] Hydroxyl (-OH) has strong polarity and is easy to form hydrogen bonds with water or polar solvents, promoting dissolution (such as PVA, PEG). Carboxyl (-COOH) can be dissociated into -COO-, which can enhance the solvation effect (such as PAA). Amino (-NH2) is a basic group and is easy to interact with acidic solvents (such as DMSO) or oxygen-containing solvents (such as NMP) (such as PEIE).

[0070] Optionally, the polymer containing nucleophilic functional groups includes at least one of polyvinyl alcohol (PVA), polyethylene glycol (PEG), polypropylene glycol (PPG), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), polyacrylamide (PAM), polymethacrylic acid (PMAA), polyethoxyethyleneimine (PEIE).

[0071] Optionally, the natural polymer includes at least one of sodium alginate, gelatin, cellulose, carrageenan, guar gum, chitin and agar.

[0072] The dissolution of natural polymer requires selecting a matching solvent according to its chemical structure and optimizing the dissolution efficiency through conditions such as temperature and stirring. Natural polymer has good compatibility and does not require additional modification.

[0073] Optionally, the solvent component of the electrolyte is at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (DME), and diethylene glycol dimethyl ether (DG).

[0074] The electrolyte can be carbonate-based, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the electrolyte can also be ether-based, such as 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (DME), and diethylene glycol dimethyl ether (DG). Specifically, carbonate-based electrolytes have a high dielectric constant, which helps with the dissociation of lithium salts. Ether-based electrolytes have a low viscosity, which helps improve the fluidity of the electrolyte.

[0075] On the other hand, the present invention provides a polymer electrolyte, which is a double-network polymer electrolyte obtained by the preparation method of any one of the above. The ionic conductivity of the polymer electrolyte is greater than 1.5 mS cm -1 , the porosity of the polymer electrolyte is greater than 80%, the liquid retention rate of the polymer electrolyte is greater than 70%, the tensile strength of the polymer electrolyte is greater than 5 Mpa, and the elongation at break is greater than 150%.

[0076] On the other hand, the present invention also provides a battery, which includes a stacked positive electrode sheet, a polymer electrolyte membrane, and a negative electrode. The polymer electrolyte is the above polymer electrolyte.

[0077] Example 1

[0078] This example is a preparation method of a polymer electrolyte, and the process is as follows:

[0079] Add 7.5 g of 1799-type PVA (degree of alcoholysis 99%) to 50 mL of deionized water, stir magnetically at 90 °C for 2 h until completely dissolved, and then cool to room temperature to obtain the first polymer solution.

[0080] Add 7.5 g of gelatin to 50 mL of deionized water, stir magnetically at 25 °C for 1 h until completely dissolved to obtain the second polymer solution.

[0081] Mix the above two polymer solutions and stir to form a homogeneous solution, which is the double-polymer solution;

[0082] After adding 5 mL of glutaraldehyde aqueous solution (5 wt.%) acidified with dilute sulfuric acid, it was transferred to a polytetrafluoroethylene mold and allowed to stand for 4 h to obtain a preliminarily cured polymer film through initial curing;

[0083] Then, the polymer film was immersed in a glassware containing deionized water for 12 h to remove residual crosslinking agents and uncrosslinked polymer molecules and to allow for sufficient swelling and liquid absorption. Then, the polytetrafluoroethylene mold was placed in a freeze dryer and dried at -55 °C for 6 h, and the obtained porous polymer film was removed;

[0084] After the porous polymer film was soaked and absorbed with 1 M LiPF6 EC / DEC electrolyte for 24 h, a double-network polymer electrolyte was obtained.

[0085] It should be noted that the crosslinking reaction between PVA and glutaraldehyde is an aldol reaction between hydroxyl groups and aldehyde groups. The crosslinking between PVA and boric acid is a reaction between hydroxyl groups and boron-hydrogen bonds to form borate esters.

[0086] In the electrolyte, LiPF6 is lithium hexafluorophosphate, EC is ethylene carbonate, and DEC is diethyl carbonate.

[0087] Example 2

[0088] This example is a preparation method of a polymer electrolyte, and the process is as follows:

[0089] 10 g of PVA type 1799 (degree of alcoholysis 99%) was added to 50 mL of deionized water, and it was magnetically stirred at 90 °C for 2 h until completely dissolved and then cooled to room temperature to obtain a first polymer solution.

[0090] 5 g of gelatin was added to 50 mL of deionized water, and it was magnetically stirred at 25 °C for 1 h until completely dissolved to obtain a second polymer solution.

[0091] After mixing the above two polymer solutions and stirring to form a homogeneous solution, it was a double-polymer solution;

[0092] After adding 5 mL of glutaraldehyde aqueous solution (mass fraction 5 wt.%) acidified with dilute sulfuric acid, it was transferred to a polytetrafluoroethylene mold and allowed to stand for 4 h to obtain a preliminarily cured polymer film through initial curing;

[0093] Then, the polymer film was immersed in a glassware containing deionized water for 12 h to remove residual crosslinking agents and uncrosslinked polymer molecules and to allow for sufficient swelling and liquid absorption. Then, the polytetrafluoroethylene mold was placed in a freeze dryer and dried at -33 °C for 24 h, and the obtained porous polymer film was removed;

[0094] After the porous polymer film was soaked and absorbed with 1 M LiPF6 EC / DEC electrolyte for 24 h, a double-network polymer electrolyte was obtained.

[0095] Example 3

[0096] This example is a preparation method of a polymer electrolyte, and the process is as follows:

[0097] Add 5 g of type 1799 PVA (degree of alcoholysis 99%) to 50 mL of deionized water, stir magnetically at 90 °C for 2 h until completely dissolved, and then cool to room temperature to obtain the first polymer solution.

[0098] Add 10 g of gelatin to 50 mL of deionized water, stir magnetically at 25 °C for 1 h until completely dissolved to obtain the second polymer solution.

[0099] Mix the above two polymer solutions and stir to form a homogeneous solution, which is the double-polymer solution;

[0100] Add 5 mL of an aqueous glutaraldehyde solution acidified with dilute sulfuric acid (mass fraction 5 wt.%), and then transfer it to a polytetrafluoroethylene mold. Let it stand for 4 h for preliminary curing to obtain a polymer membrane;

[0101] Then immerse the polymer membrane in a glassware filled with deionized water for 12 h to remove residual cross-linking agents and uncrosslinked polymer molecules and to perform sufficient swelling and liquid absorption. Then place the polytetrafluoroethylene mold in a freeze dryer and dry at -40 °C for 18 h, and remove to obtain the porous polymer membrane;

[0102] After soaking and absorbing the 1 M LiPF6 EC / DEC electrolyte for 24 h, the double-network polymer electrolyte is obtained.

[0103] Example 4:

[0104] This example is a preparation method of a polymer electrolyte, and the process is as follows:

[0105] Add 7.5 g of type 1799 PVA (degree of alcoholysis 99%) to 50 mL of deionized water, stir magnetically at 90 °C for 2 h until completely dissolved, and then cool to room temperature to obtain the first polymer solution.

[0106] Add 7.5 g of gelatin to 50 mL of deionized water, stir magnetically at 25 °C for 1 h until completely dissolved to obtain the second polymer solution.

[0107] Mix the above two polymer solutions and stir to form a homogeneous solution, which is the double-polymer solution;

[0108] Add 5 mL of an aqueous boric acid solution (mass fraction 5 wt.%), and then transfer it to a polytetrafluoroethylene mold. Let it stand for 4 h for preliminary curing to obtain a polymer membrane;

[0109] Then immerse the polymer membrane in a glassware filled with deionized water for 12 h to remove the residual crosslinking agent and uncrosslinked polymer molecules and to fully swell and absorb liquid. Then place the polytetrafluoroethylene mold in a freeze dryer and dry it at -40 °C for 18 h, and remove the obtained porous polymer membrane;

[0110] After soaking and absorbing 1 M LiPF6 EC / DEC electrolyte in the porous polymer membrane for 24 h, a double-network polymer electrolyte is obtained.

[0111] Example 5:

[0112] This example is a preparation method of a polymer electrolyte, and the process is as follows:

[0113] Add 7.5 g of polyacrylic acid to 50 mL of deionized water, and stir magnetically at 25 °C for 2 h until completely dissolved, and then cool to obtain a first polymer solution.

[0114] Add 7.5 g of gelatin to 50 mL of deionized water, and stir magnetically at 25 °C for 1 h until completely dissolved to obtain a second polymer solution.

[0115] Mix the above two polymer solutions and stir to form a homogeneous solution, which is a double-polymer solution;

[0116] Add 5 mL of an aqueous solution of glutaraldehyde acidified with dilute sulfuric acid (mass fraction 5 wt.%) and transfer it to a polytetrafluoroethylene mold, and let it stand for 4 h to preliminarily cure to obtain a polymer membrane;

[0117] Then immerse the polymer membrane in a glassware filled with deionized water for 12 h to remove the residual crosslinking agent and uncrosslinked polymer molecules and to fully swell and absorb liquid. Then place the polytetrafluoroethylene mold in a freeze dryer and dry it at -40 °C for 18 h, and remove the obtained porous polymer membrane;

[0118] After soaking and absorbing 1 M LiPF6 EC / DEC electrolyte in the porous polymer membrane for 24 h, a double-network polymer electrolyte is obtained.

[0119] Comparative Example 1

[0120] This comparative example is a preparation method of a single-crosslinked polymer electrolyte, and the process is as follows:

[0121] 15 g of PVA type 1799 (degree of alcoholysis 99%) was added to 100 mL of deionized water, and magnetically stirred at 90 °C for 2 h until completely dissolved, then cooled to room temperature. After adding 5 mL of glutaraldehyde aqueous solution acidified with dilute sulfuric acid (mass fraction 5 wt.%), it was transferred to a polytetrafluoroethylene mold, allowed to stand for 4 h for preliminary curing, and then immersed in a glassware filled with deionized water for 12 h to remove residual cross-linking agent and uncross-linked polymer molecules. Then, the polytetrafluoroethylene mold was placed in a vacuum drying oven and dried at 60 °C for 12 h. The obtained polymer dry film was taken off, soaked and sucked with 1 M LiPF6 EC / DEC electrolyte for 24 h to obtain a single-network polymer electrolyte.

[0122] Comparative Example 2

[0123] This comparative example is a preparation method of a blended polymer electrolyte, and the process is as follows:

[0124] 7.5 g of PVA type 1799 (degree of alcoholysis 99%) was added to 50 mL of deionized water, and magnetically stirred at 90 °C for 2 h until completely dissolved, then cooled to room temperature. 7.5 g of gelatin was added to 50 mL of deionized water, and magnetically stirred at 25 °C for 1 h until completely dissolved. The above two polymer solutions were mixed and stirred to form a homogeneous solution, then transferred to a polytetrafluoroethylene mold, allowed to stand for 4 h for preliminary curing, and then immersed in a glassware filled with deionized water for 12 h. Then, the polytetrafluoroethylene mold was placed in a freeze dryer and dried at -40 °C for 18 h. The obtained polymer dry film was taken off, soaked and sucked with 1 M LiPF6 EC / DEC electrolyte for 24 h to obtain a blended polymer electrolyte.

[0125] The following is the process of assembling a lithium metal battery:

[0126] The positive electrode material NCM811, binder PVDF and conductive agent carbon black were dispersed in N-methylpyrrolidone and mixed evenly to obtain a slurry, which was double-sided coated on an aluminum foil current collector, vacuum dried and then die-cut to obtain a positive electrode sheet of a certain size. The lithium copper composite tape was die-cut to obtain a lithium metal negative electrode sheet of a certain size. According to the requirements of the test items, the positive electrode sheet, polymer electrolyte membrane and negative electrode sheet were assembled into 2032-type button batteries or soft-pack batteries encapsulated with aluminum plastic film respectively.

[0127] Perform performance tests on the above Examples 1 - 5 and Comparative Example 1, Comparative Example 2

[0128] (1) Use a universal tensile testing machine to perform tensile operations on the polymer electrolyte membranes of each example, and record the tensile strength and elongation at break of the polymer electrolyte. The results are shown in Table 1.

[0129] Table 1 Comparison of tensile test results of polymer electrolytes

[0130] Tensile strength (MPa) Elongation at break (%) Example 1 6.13 213.2 Example 2 6.36 162.5 Example 3 5.49 234.6 Example 4 5.02 170.1 Example 5 5.76 226.2 Comparative Example 1 4.17 125.7 Comparative Example 2 1.31 85.4

[0131] It can be seen from the comparison of the test results that the polymer electrolyte with a crosslinked network structure has significantly improved mechanical strength compared to the uncrosslinked blend polymer electrolyte. Among them, the tensile strength and elongation at break of the single-network polymer electrolyte in Comparative Example 1 are significantly inferior to those of the double-network polymer electrolyte in Examples 1-5, showing poor flexibility. Therefore, comprehensively evaluating the mechanical properties, the double-network polymer electrolyte is better.

[0132] (2) A 2032-type button battery was assembled with a stainless steel sheet as the blocking electrode for testing the ionic conductivity, and the results are shown in Table 2.

[0133] Table 2 Comparison of Ionic Conductivities of Polymer Electrolytes

[0134] <![CDATA[Ionic conductivity (mScm -1 , 25 °C)]]> Example 1 2.43 Example 2 1.64 Example 3 3.33 Example 4 2.67 Example 5 2.83 Comparative Example 1 0.55 Comparative Example 2 0.27

[0135] Benefiting from the interaction between the abundant functional groups on the natural polymer and lithium ions, the conductivity of the double-network polymer electrolyte is higher than that of the single-network polymer electrolyte in Comparative Example 1 using a single polymer matrix. Comparative Example 2 is just a simple blend of two polymers, without forming an effective crosslinked network structure and lacking the three-dimensional ion channels provided by the double-network structure, so the ionic conductivity shown is low.

[0136] (3) The porosity of the polymer electrolyte membranes in each group was tested. Then, after assembling the polymer electrolytes into lithium metal soft-pack batteries, they were placed in an environment of 50 °C in a non-fully sealed state, and the difference in the weight change of the batteries within 7 days was recorded and compared with the weight of the initial dry battery core to obtain the liquid retention rate data of the polymer electrolyte inside the battery, and the results are shown in Table 3.

[0137] Table 3 Comparison of Porosity and Liquid Retention Rate Results of Polymer Electrolytes

[0138] Porosity (%) Liquid retention rate (%) Example 1 93.2％ 81.5 Example 2 91.8％ 78.6 Example 3 90.6％ 76.2 Example 4 84.5％ 72.3 Example 5 87.1％ 73.9 Comparative Example 1 28.4％ 64.6 Comparative Example 2 80.8％ 60.2

[0139] It can be seen that Examples 1-5 have higher porosity and liquid retention rate compared to Comparative Examples 1-2. Freeze-drying, as a powerful pore-forming and drying process, can provide more abundant pores for the electrolyte membrane, absorb more electrolyte, and the crosslinked double-network structure is conducive to binding the electrolyte, providing good thermal stability and liquid retention ability for the polymer electrolyte.

[0140] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present invention. Those skilled in the art should understand that the above embodiments can be modified without departing from the scope and spirit of the present invention. The scope of the present invention is defined by the appended claims.

Claims

1. A method for preparing a polymer electrolyte, characterized in that, Comprising: Dissolving a polymer containing a nucleophilic functional group in a polar solvent to obtain a first polymer solution; Dissolving a natural polymer in a polar solvent to obtain a second polymer solution; Mixing the first polymer solution and the second polymer solution to form a dual-polymer solution; Adding a crosslinking agent to the dual-polymer solution, casting, and allowing to stand and cure to obtain a polymer film; Immersing the polymer film in a polar solvent to remove residual crosslinking agent and uncrosslinked polymer and performing immersion swelling absorption, and then freeze-drying at a temperature of -55°C to -35°C for 6 h to 24 h to form pores in the polymer film to obtain a porous polymer film; Immersing and absorbing an electrolyte containing a lithium salt in the porous polymer film and allowing to stand to obtain a dual-network polymer electrolyte.

2. The preparation method of the polymer electrolyte according to claim 1, characterized in that, Based on the mass of the dual-polymer solution being 100%, the sum of the mass ratios of the polymer containing a nucleophilic functional group and the natural polymer is 5 wt.% to 25 wt.%.

3. The preparation method of the polymer electrolyte according to claim 1, characterized in that, The crosslinking agent includes at least one of glutaraldehyde, boric acid, borax, suberoyl chloride, epichlorohydrin, and diisocyanate.

4. The preparation method of the polymer electrolyte according to claim 1, wherein, The polymer containing a nucleophilic functional group includes a polymer containing a hydroxyl group, a carboxyl group, and / or an amine group.

5. The method for preparing a polymer electrolyte according to claim 1 or 4, characterized in that, The polymer containing a nucleophilic functional group includes at least one of polyvinyl alcohol, polyethylene glycol, polypropylene glycol, polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polymethacrylic acid, and polyethoxyethyleneimine.

6. The preparation method of the polymer electrolyte according to claim 1, characterized in that, The natural polymer includes at least one of sodium alginate, gelatin, cellulose, carrageenan, guar gum, chitin, and agar.

7. The preparation method of the polymer electrolyte according to claim 1, characterized in that The polar solvent includes at least one of water, dimethyl sulfoxide, N,N-dimethylformamide, ethanol, ethylene glycol, glycerol, isopropanol, and acetonitrile.

8. The preparation method of the polymer electrolyte according to claim 1, characterized in that, The electrolyte includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,3-dioxolane, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether.

9. A polymer electrolyte, characterized in that, The polymer electrolyte is a double-network polymer electrolyte obtained by the preparation method according to any one of claims 1 to 8, and the ionic conductivity of the polymer electrolyte is greater than 1.5 mS cm -1 , the porosity of the polymer electrolyte is greater than 80%, the liquid retention rate of the polymer electrolyte is greater than 70%, the tensile strength of the polymer electrolyte is greater than 5 Mpa, and the elongation at break is greater than 150%.

10. A battery, characterized in that, Comprising a stacked positive electrode sheet, a polymer electrolyte membrane, and a negative electrode, and the polymer electrolyte is the polymer electrolyte according to claim 9.

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