Composite diaphragm, preparation method thereof and sodium metal battery

Abstract

The invention relates to the technical field of battery materials, and provides a composite diaphragm, a preparation method thereof and a sodium metal battery. The composite diaphragm comprises a base membrane and a composite coating coated on the surface of the base membrane, the composite coating comprises a first coating and a second coating which are sequentially arranged close to the base film, and the first coating is arranged close to the base film; the first coating comprises the following components: 80-98% wt of a molecular sieve and 2-20% wt of a first adhesive; the second coating comprises the following components: 30-79% wt of an ionic conductor, 20-69% wt of a polymer solid electrolyte and 1-10% wt of a second adhesive; and the ionic conductor is selected from at least one of lithium titanium aluminum phosphate, sodium fluoride, sodium hexafluoroaluminate, sodium zirconium silicon phosphorus oxide, beta-Al2O3, sodium borohydride and sodium titanium phosphate. After the composite diaphragm is applied to the sodium metal battery, the charge-discharge efficiency of the negative electrode sodium metal can be improved, the dendritic crystal growth of the sodium metal is reduced, and the gas production is reduced, so that the electrochemical performance of the sodium metal battery is improved.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically, to composite separators and their preparation methods, and sodium metal batteries. Background Technology

[0002] With the ongoing energy and environmental crises, developing sustainable energy has become a crucial trend in current energy development. Sodium-ion batteries, due to their abundant raw material resources and low cost, have emerged as a significant candidate for electrochemical energy storage.

[0003] Among them, the anode-free sodium-ion battery directly uses copper or aluminum foil as the anode, utilizing sodium ion reduction electroplating and oxidation stripping as the anode reaction to construct a cathode-free sodium metal battery, which has extremely high energy density and potential low-cost feasibility. However, using traditional battery systems usually leads to uneven sodium metal deposition during charging, low battery coulombic efficiency, and severe gas generation.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a composite separator, a method for preparing the same, and a sodium metal battery, aiming to improve at least one of the problems mentioned in the background art.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a composite diaphragm, comprising a base membrane and a composite coating applied to the surface of the base membrane; The composite coating includes a first coating and a second coating disposed sequentially close to the base film, with the first coating disposed close to the base film; The first coating consists of 80-98%wt molecular sieve and 2-20%wt first binder; The second coating consists of 30-79%wt of ionic conductor, 20-69%wt of polymer solid electrolyte, and 1-10%wt of second adhesive; The ionic conductor is selected from at least one of lithium titanium aluminum phosphate, sodium fluoride, sodium hexafluoroaluminate, sodium zirconium silicon phosphorus oxide, β-Al2O3, sodium borohydride and sodium titanium phosphate.

[0007] In an optional embodiment, the molecular sieve is selected from at least one of 3A molecular sieve, 4A molecular sieve, 5A molecular sieve, 10Z molecular sieve, 13Z molecular sieve and Y-type molecular sieve.

[0008] In an optional embodiment, the first adhesive is selected from at least one of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

[0009] In an optional embodiment, the second adhesive is selected from at least one of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

[0010] In an optional embodiment, the polymer solid electrolyte is selected from at least one of polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate.

[0011] In an optional implementation, the base film is a PE film or a PP film.

[0012] In an optional embodiment, the thickness of the first coating is 0.5~5μm; And / or, the thickness of the second coating is 0.5~5μm.

[0013] Secondly, the present invention provides a method for preparing a composite separator as described in any of the foregoing embodiments, comprising: A first slurry containing molecular sieves and a first binder is applied to a base membrane, followed by a first drying process to form a first coating layer. A second slurry containing an ionic conductor, a polymer solid electrolyte, and a second binder is applied onto the first coating layer, followed by a second drying process to obtain the second coating layer. In an optional embodiment, the temperature during the first drying and / or the second drying is 60~150°C, and the drying time is 5~12h.

[0014] Thirdly, the present invention provides a sodium metal battery, comprising a composite separator as described in any of the foregoing embodiments, wherein a second coating of the composite separator is bonded to a sodium negative electrode.

[0015] The present invention has the following beneficial effects: The composite membrane provided in this invention has a first coating containing a large number of molecular sieves. On one hand, the molecular sieves contain numerous pores smaller than 1 nm, which can effectively adjust the solvation structure of sodium ions in the electrolyte. When solvated sodium ions pass through the molecular sieve layer, some solvent is adsorbed inside the molecular sieve pores, reducing solvent decomposition on the sodium metal surface. On the other hand, the large number of pores in the molecular sieves can effectively adsorb the large amount of gas generated by the reduction of the electrolyte by sodium metal, avoiding interface unevenness caused by gas accumulation at the negative electrode interface. The second coating is formed by coating the surface of the first coating with an ion conductor possessing high sodium ion conductivity and high reduction stability, and a polymer solid electrolyte with adhesive properties. The second coating further reduces the contact between the liquid electrolyte and sodium metal, lowering the risk of electrolyte decomposition. Furthermore, the coating composed of the ion conductor, polymer solid electrolyte, and binder has good adhesive properties and can be bonded to the negative electrode by hot pressing, forming a tightly adhered interface, which facilitates the reversible deposition and stripping of sodium metal.

[0016] Therefore, when the composite separator provided in this embodiment of the invention is applied to a sodium metal battery, it can improve the charge and discharge efficiency of the sodium metal anode, reduce the growth of sodium metal dendrites, reduce gas production, and thus improve the electrochemical performance of the sodium metal battery. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the structure when the composite diaphragm is bonded to the negative electrode according to an embodiment of the present invention; Figure 2 This is a SEM image of the first coating obtained during the preparation process of Example 1; Figure 3 This is an SEM image of the second coating of the composite membrane prepared in Example 1. Detailed Implementation

[0019] 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.

[0020] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0021] like Figure 1 As shown, the composite diaphragm provided in this embodiment of the invention includes a base membrane and a composite coating applied to the surface of the base membrane; The composite coating includes a first coating and a second coating disposed sequentially close to the base film, with the first coating disposed close to the base film; The first coating consists of 80-98%wt molecular sieve and 2-20%wt first binder; The second coating consists of 30-80%wt of ionic conductor, 20-70%wt of polymer solid electrolyte, and 1-10%wt of second adhesive; The ionic conductor is selected from at least one of lithium titanium aluminum phosphate, sodium fluoride, sodium hexafluoroaluminate, sodium zirconium silicon phosphorus oxide, β-Al2O3, sodium borohydride and sodium titanium phosphate.

[0022] The composite membrane provided in this invention has a first coating containing a large number of molecular sieves. On one hand, the molecular sieves contain numerous pores smaller than 1 nm, which can effectively adjust the solvation structure of sodium ions in the electrolyte. When solvated sodium ions pass through the molecular sieve layer, some solvent is adsorbed inside the molecular sieve pores, reducing solvent decomposition on the sodium metal surface. On the other hand, the large number of pores in the molecular sieves can effectively adsorb the large amount of gas generated by the reduction of the electrolyte by sodium metal, avoiding interface unevenness caused by gas accumulation at the negative electrode interface. The second coating is formed by coating the surface of the first coating with an ion conductor possessing high sodium ion conductivity and high reduction stability, and a polymer solid electrolyte with adhesive properties. The second coating further reduces the contact between the liquid electrolyte and sodium metal, lowering the risk of electrolyte decomposition. Furthermore, the coating composed of the ion conductor, polymer solid electrolyte, and binder has good adhesive properties and can be bonded to the negative electrode by hot pressing, forming a tightly adhered interface, which facilitates the reversible deposition and stripping of sodium metal.

[0023] Therefore, when the composite separator provided in this embodiment of the invention is applied to a sodium metal battery, it can improve the charge and discharge efficiency of the sodium metal anode, reduce the growth of sodium metal dendrites, reduce gas production, and thus improve the electrochemical performance of the sodium metal battery.

[0024] It should be noted that in the first coating, the proportion of molecular sieve should not be too low, otherwise it will lead to incomplete gas absorption, causing interface deterioration, battery bulging, and other adverse effects. Conversely, the proportion of molecular sieve should not be too high, as this reduces the proportion of binder, resulting in poor adhesion between the first coating and the base membrane. In the second coating, the content of ionic conductors should not be too high, as this affects interface adhesion and uniformity; nor should it be too low, as this results in low ionic conductivity and high impedance at the interface. Similarly, the proportion of polymer solid electrolyte should not be too high, as this reduces ionic conductivity and increases interface impedance; nor should it be too low, as this results in poor interface adhesion and uniformity. Therefore, the proportions of each component in each coating should be within the range required by this invention to ensure optimal performance of the composite separator.

[0025] Optionally, the chemical formula of sodium zirconium silicon phosphorus oxide is Na. 1+x Zr2Si x P 3-x O 12 (0 <x<3)。

[0026] Optionally, the molecular sieve is selected from at least one of 3A molecular sieve, 4A molecular sieve, 5A molecular sieve, 10Z molecular sieve, 13Z molecular sieve and Y-type molecular sieve.

[0027] Optionally, the first adhesive is selected from at least one of sodium carboxymethyl cellulose (CMC), polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

[0028] Optionally, the second adhesive is selected from at least one of sodium carboxymethyl cellulose (CMC), polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

[0029] Optionally, the polymer solid electrolyte is selected from at least one of polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate.

[0030] Optionally, the base film is a PE film or a PP film.

[0031] Optionally, to ensure better performance of the composite diaphragm, the thickness of the first coating is 0.5~5μm; and / or, the thickness of the second coating is 0.5~5μm.

[0032] The method for preparing the composite separator provided in this embodiment of the invention includes: A first slurry containing molecular sieves and a first binder is applied to a base membrane, followed by a first drying process to form a first coating layer. A second slurry containing an ionic conductor, a polymer solid electrolyte, and a second binder is applied onto the first coating layer, followed by a second drying process to obtain the second coating layer.

[0033] Specifically: S1, First application Molecular sieves and a first binder are dispersed in deionized water and mixed evenly by stirring to obtain a first slurry with a solid content of 8-30%. The first slurry is applied to the surface of the base film (it can be one side or both sides opposite). After applying the first slurry, it is dried at 60~150℃ for 5~12h to form the first coating.

[0034] S2, Second application The ion conductor, polymer solid electrolyte and second binder are dispersed in deionized water and mixed evenly by stirring to obtain a second slurry with a solid content of 8-30%. The second slurry is applied to the surface of the second coating layer, and then dried at 60~150℃ for 5~12h to form the second coating layer, thereby obtaining the composite diaphragm.

[0035] The preparation method provided in this embodiment of the invention can produce the composite separator provided in this embodiment of the invention.

[0036] The sodium metal battery provided in this embodiment of the invention includes a composite separator provided in this embodiment of the invention, wherein the second coating of the composite separator is bonded to the sodium negative electrode.

[0037] The sodium metal battery provided in this embodiment of the invention has better electrochemical performance because it includes the composite separator provided in this embodiment of the invention.

[0038] Specifically, the positive electrode active material of sodium metal batteries is, for example, sodium iron sulfate, nickel-iron-manganese layered oxide, or sodium iron pyrophosphate.

[0039] Example 1 3A molecular sieve and sodium carboxymethyl cellulose were added to deionized water at a mass ratio of 80:20 and mixed evenly to obtain a first slurry with a solid content of 17%. The first slurry was applied to one side of a PP film with a thickness of 18 μm, and then dried at 100℃ for 6 hours to form a first coating with a thickness of 2 μm. An SEM image of the first coating was taken, as shown below. Figure 2 As shown, the coating surface has a large number of pores. Lithium aluminum titanium phosphate, polyethylene oxide and sodium carboxymethyl cellulose were added to deionized water in a mass ratio of 30:60:10 and mixed evenly to obtain a second slurry with a solid content of 14%. The second slurry was applied to the surface of the first coating, and then dried at 100°C for 6 hours to form a second coating with a thickness of 2 μm, resulting in a composite diaphragm. An SEM image of the second coating was taken, as shown below. Figure 3 As shown.

[0040] Example 2 4A molecular sieve and polyacrylic acid were added to deionized water at a mass ratio of 90:10 and mixed evenly to obtain a first slurry with a solid content of 30%. The first slurry was applied to one side of a PP film with a thickness of 16 μm, and then dried at 60°C for 12 h to form a first coating with a thickness of 5 μm. Sodium fluoride, polyacrylonitrile, and polyacrylic acid were added to deionized water in a mass ratio of 79:20:1 and mixed evenly to obtain a second slurry with a solid content of 30%. The second slurry is applied to the surface of the first coating, and then dried at 60°C for 12 hours to form a second coating with a thickness of 5 μm, thus obtaining a composite diaphragm.

[0041] Example 3 10Z molecular sieve and styrene-butadiene rubber were added to deionized water at a mass ratio of 98:2 and mixed evenly to obtain a first slurry with a solid content of 8%. The first slurry was applied to one side of a PP film with a thickness of 20 μm, and then dried at 150°C for 5 h to form a first coating with a thickness of 0.5 μm. Sodium hexafluoroaluminate, polymethyl methacrylate and styrene-butadiene rubber were added to deionized water in a mass ratio of 50:40:1 and mixed evenly to obtain a second slurry with a solid content of 8%. The second slurry is applied to the surface of the first coating, and then dried at 150°C for 5 hours to form a second coating with a thickness of 0.5 μm, thus obtaining a composite diaphragm.

[0042] Example 4 5A molecular sieve and sodium polyacrylate were added to deionized water at a mass ratio of 85:15 and mixed evenly to obtain a first slurry with a solid content of 13%. The first slurry was applied to one side of a PE film with a thickness of 14 μm, and then dried at 80°C for 7 h to form a first coating with a thickness of 1 μm. Sodium borohydride, polymethyl methacrylate and sodium polyacrylate were added to deionized water in a mass ratio of 30:69:1 and mixed evenly to obtain a second slurry with a solid content of 16%. The second slurry is applied to the surface of the first coating, and then dried at 80°C for 7 hours to form a second coating with a thickness of 3 μm, thus obtaining a composite diaphragm.

[0043] Comparative Example 1 This comparative example is basically the same as Example 1, except that the first coating is not provided.

[0044] Comparative Example 2 This comparative example is basically the same as Example 1, except that a second coating is not provided.

[0045] Comparative Example 3 This comparative example is basically the same as Example 1, except that in the second coating, an equal amount of lithium titanium aluminum phosphate is used to replace an equal amount of polyethylene oxide.

[0046] Comparative Example 4 This comparative example is basically the same as Example 1, except that in the second coating, an equal amount of polyethylene oxide is used to replace an equal amount of lithium titanium aluminum phosphate.

[0047] Comparative Example 5 This comparative example provides a composite membrane, which is the PP membrane mentioned in Example 1, but without a first coating and a second coating.

[0048] Experimental Example Using the composite separators provided in the various embodiments and comparative examples as separators, copper foil as the negative electrode, and sodium iron pyrophosphate as the positive electrode material, a monolithic soft-pack sodium metal battery is assembled. A specific example of the preparation method for the soft-pack battery is as follows: S1. Sodium iron pyrophosphate, carbon black, and PVDF are homogenized in NMP solution at a mass ratio of 94:3:3, coated onto aluminum foil, and dried to form a positive electrode sheet.

[0049] S2. Using 1.0 mol / L NaPF6 dissolved in ethylene glycol dimethyl ether as the electrolyte, the composite separator prepared in the above examples or comparative examples is used as the separator, the positive electrode prepared in S1 and copper foil are used as the negative electrode, and a single soft-pack battery is assembled in an argon-filled glove box.

[0050] The electrochemical performance of the prepared sodium metal battery was tested to indirectly reflect the performance of the separator. The test method is as follows, and the test results are recorded in Table 1.

[0051] First Coulomb efficiency test of the battery: The assembled battery is charged to 3.4 V at 0.1C and then discharged to 2.0 V at 0.1C. The ratio of the battery's discharge capacity to its charge capacity is calculated as Q. Cycle life test: The installed battery was charged to 3.4 V at 0.5 C at 25℃, and then discharged to 2.0 V at 1 C for 100 cycles. The capacity retention rate was recorded. Battery rate performance test: Charge the installed battery to 3.4 V at 0.1C, then discharge it to 2.0 V at 0.1C. Calculate the ratio of battery discharge capacity to charge capacity as Q1. Then charge it to 3.4 V at 0.1C, then discharge it to 2.0 V at 2C. Calculate the ratio of battery discharge capacity to charge capacity as Q2. The capacity retention rate at 2C rate is Q2 / Q1.

[0052] Battery gas production and expansion rate test: Prepare a container filled with a fixed volume of water, place it on a balance, and zero the balance. The assembled, untested pouch battery from the above embodiment is suspended and immersed in the water container on the balance (the battery is suspended in the water). The mass value M1 is read from the balance. After 100 cycles of the battery's lifespan test, it is again suspended and immersed in a container filled with the same volume of water (the battery is suspended in the water), and its mass reading is M2. The battery expansion rate is (M2-M1) / M1.

[0053] Table 1. Electrochemical performance of batteries prepared with composite separators in each embodiment and comparative example.

[0054] As can be seen from Table 1, the composite separators prepared in the various embodiments of the present invention, when used to make sodium metal batteries, result in batteries with better electrochemical performance.

[0055] Comparing Example 1 with Comparative Example 1, Comparative Example 1 has a significantly lower cycle capacity retention rate and a significantly greater volume expansion, indicating that if the first coating is not provided, there will be more gas generation at the interface, resulting in greater battery swelling and deterioration of the battery's cycle performance. Comparing Example 1 with Comparative Example 2, Comparative Example 2 showed significantly lower initial coulombic efficiency and cycle capacity retention, and more gas production. This indicates that without a second coating, there are more side reactions at the interface, which will affect the reversibility of sodium metal deposition and dissolution reactions, resulting in faster battery cycle degradation. Comparing Example 1 with Comparative Example 3, Comparative Example 3 showed poorer initial coulombic efficiency, cycle capacity retention, and battery swelling, indicating that if the second coating does not contain a polymer solid electrolyte, the overall electrochemical performance is relatively worse. Comparing Example 1 with Comparative Example 4, Comparative Example 4 showed poor initial coulombic efficiency, cycle capacity retention, and battery swelling, and its rate discharge performance was significantly worse. This indicates that if the second coating does not contain ionic conductors, the rate performance will be significantly weakened.

[0056] Comparing Example 1 with Comparative Example 5, Comparative Example 5 showed poor initial coulombic efficiency and cycle capacity retention, and the battery swelled significantly, indicating that the overall battery performance was poor without a coating.

[0057] In summary, the composite diaphragm provided in the embodiments of the present invention has the following characteristics: 1. Improved charge / discharge efficiency and cycle capacity retention: The introduction of composite separators improves the reversibility of sodium ion deposition and dissolution, thereby improving the cycle capacity retention of sodium metal batteries without negative electrodes.

[0058] 2. Improved lifespan: The introduction of composite membranes can alleviate sodium dendrite growth, improve cycle life, and avoid battery short circuits caused by sodium dendrites.

[0059] 3. Reduced gas production: The use of composite membranes reduces the side reactions between sodium metal and electrolyte, significantly reducing gas production and battery volume expansion during cycling.

[0060] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite diaphragm, characterized in that, Includes a base film and a composite coating applied to the surface of the base film; The composite coating includes a first coating and a second coating disposed sequentially near the base film, wherein the first coating is disposed near the base film. The first coating consists of 80-98%wt molecular sieve and 2-20%wt first binder; The second coating comprises 30-79%wt of ionic conductor, 20-69%wt of polymer solid electrolyte, and 1-10%wt of second adhesive; The ionic conductor is selected from at least one of lithium titanium aluminum phosphate, sodium fluoride, sodium hexafluoroaluminate, sodium zirconium silicon phosphorus oxide, β-Al2O3, sodium borohydride, and sodium titanium phosphate.

2. The composite diaphragm according to claim 1, characterized in that, The molecular sieve is selected from at least one of 3A molecular sieve, 4A molecular sieve, 5A molecular sieve, 10Z molecular sieve, 13Z molecular sieve and Y-type molecular sieve.

3. The composite diaphragm according to claim 1, characterized in that, The first adhesive is selected from at least one of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

4. The composite diaphragm according to claim 1, characterized in that, The second adhesive is selected from at least one of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

5. The composite diaphragm according to claim 1, characterized in that, The polymer solid electrolyte is selected from at least one of polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate.

6. The composite diaphragm according to claim 1, characterized in that, The base film is a PE film or a PP film.

7. The composite diaphragm according to claim 1, characterized in that, The thickness of the first coating is 0.5~5μm; And / or, the thickness of the second coating is 0.5~5μm.

8. The method for preparing the composite separator according to any one of claims 1 to 7, characterized in that, include: A first slurry containing the molecular sieve and the first binder is applied to the base film, followed by a first drying process to form the first coating. A second slurry containing the ionic conductor, the polymer solid electrolyte, and the second binder is applied to the first coating, followed by a second drying process to obtain the second coating.

9. The preparation method according to claim 8, characterized in that, The temperature during the first drying and / or the second drying is 60~150℃, and the drying time is 5~12h.

10. A sodium metal battery, characterized in that, The composite membrane as described in any one of claims 1 to 7 is wherein the second coating of the composite membrane is bonded to the sodium negative electrode.

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