A pre-sodium ionization diaphragm and a preparation method thereof

Abstract

The application belongs to the field of sodium ion battery separators, and particularly relates to a pre-sodiumized separator and a preparation method thereof. The pre-sodiumized separator takes pre-sodiumized active material as a surface layer and takes a separator base film as a covering carrier, and the pre-sodiumized separator is obtained through drying. The pre-sodiumized separator is used as a sodium ion battery separator, and the method is to prepare the pre-sodiumized active material into a coating liquid, then cover the coating liquid on the separator base film, and obtain the pre-sodiumized separator through drying. The pre-sodiumized separator can effectively improve the initial efficiency of the sodium ion battery, thereby improving the energy density of the battery, and does not affect the processing performance and structure of the positive electrode plate. The preparation process is simple, the environmental requirement is low, and the production cost is low.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery separators, specifically relating to a pre-sodiumized separator and its preparation method. Background Technology

[0002] Sodium-ion batteries have broad application prospects in energy storage, low-speed vehicles, and other fields due to the abundance of sodium resources and low cost. However, during the initial charge and discharge process of sodium-ion batteries, the initial charge and discharge efficiency is low due to issues such as the formation of an SEI film on the negative electrode surface and the presence of side reactions from other impurities in the positive electrode material, thus reducing the battery's energy density.

[0003] To improve the energy density of sodium-ion batteries, it is necessary to replenish sodium to compensate for the irreversible sodium loss during SEI film formation. Currently, the commonly used method for sodium replenishment is the additive method. This method does not require changes to existing production processes and is simple to operate. Currently, common cathode sodium replenishment additives are mainly inorganic sodium salts; however, these additives generally leave solid or gaseous residues after the reaction, affecting the cathode structure.

[0004] For example, patent CN111834622A provides a multilayer sodium iron sulfate positive electrode sheet with lithium / sodium replenishment function. It has a positive electrode material layer and a lithium / sodium replenishment material layer on both sides of the positive electrode current collector. However, this leads to the following problems: 1) the positive electrode processing performance deteriorates, resulting in material shedding; 2) the sodium replenishment additive leaves vacancies in the electrode sheet after the first charge cycle, causing unevenness between the lithium / sodium replenishment material layer and the positive electrode material layer. Patent CN115117558A provides a method for replenishing sodium in the separator by transferring the sodium replenishing agent, binder, and conductive agent to the base film surface through a slurry coating. While this method can improve the initial efficiency of sodium-ion batteries, it requires dispersing the conductive agent and binder, increasing the cost of cell manufacturing. It may also use organic solvents, posing a certain environmental hazard. Furthermore, after the sodium replenishing additive decomposes and is consumed during the reaction, the residual conductive agent and binder on the separator affect the separator's permeability, reducing the cell's energy density.

[0005] Therefore, it is essential to develop a sodium replenishment method to solve the above-mentioned problems with additives, thereby improving the energy density and performance of sodium-ion batteries. Summary of the Invention

[0006] To address the problems in the prior art, this invention provides a pre-sodium-modified separator, which solves the defects of existing separators. By pre-sodium-modified active materials, it compensates for the irreversible capacity loss in the first cycle of sodium-ion batteries, increasing cell capacity by 3-10% and energy density by 3-15%.

[0007] To achieve the above technical objectives, the technical solution of the present invention is as follows:

[0008] A pre-sodium-treated separator is obtained by drying a pre-sodium-treated active material as the surface layer and a separator base membrane as the covering carrier. The pre-sodium-treated separator is used as a separator for sodium-ion batteries.

[0009] The pre-sodium-treated active material is one or more of inorganic sodium salts, organic sodium salts, and sodium oxides.

[0010] Furthermore, the pre-sodiumized active material is one or more of Na2C2O4, Na2C4O4, Na2C4O6, and Na2CO3.

[0011] The diaphragm base membrane is made of one or more of polyethylene, polypropylene, and cellulose.

[0012] The method for preparing the pre-sodium-modified diaphragm includes: preparing a coating liquid from a pre-sodium-modified active material, then covering the coating liquid onto the diaphragm base membrane, and drying to obtain the pre-sodium-modified diaphragm.

[0013] The coating solution uses deionized water as a solvent.

[0014] The coating solution is a solution or dispersion of the pre-sodiumized active substance.

[0015] The mass ratio of the pre-sodiumized active material to deionized water in the coating solution is 1:5-100.

[0016] The coating solution contains sodium alginate, and the amount of sodium alginate added is 5-10% of the mass of the pre-sodiumized active material.

[0017] The thickness of the pre-sodium-treated active material is 0.2-2 μm, and the density of the coating surface is 0.05-4 mg / cm³. 2 .

[0018] The drying temperature is 45-85℃, and the drying time is 5-20 minutes.

[0019] As can be seen from the above description, the present invention has the following advantages:

[0020] 1. This invention solves the defects of existing separators. By pre-sodiumizing the active material, it compensates for the irreversible capacity loss in the first cycle of sodium-ion batteries, increasing the cell capacity by 3-10% and the energy density by 3-15%.

[0021] 2. This invention utilizes sodium alginate as an adhesive, effectively improving the connectivity between the pre-sodiumized active material and the membrane base, eliminating the problem of poor connectivity stability of the pre-sodiumized active material. At the same time, sodium alginate has its own sodium ion structure, which plays a certain role in sodium ion replenishment.

[0022] 3. This invention utilizes the -COO- group present in sodium alginate. -The group exhibits polyanionic properties in aqueous solution and has a certain viscosity, which can prevent the agglomeration and sedimentation of sodium supplementation agent, improve the stability of sodium supplementation slurry, and thus increase the coating effect.

[0023] 4. This invention utilizes the gel structure of sodium alginate, which forms a porous structure after being coated onto the diaphragm and dried. This does not affect the air permeability of the diaphragm and has little impact on sodium ion conduction and the rate of the battery cell.

[0024] 4. This invention can effectively avoid the damage to the positive electrode structure caused by directly adding sodium additives to the positive electrode material, without affecting the positive electrode processing performance and improving the stability of the battery cell.

[0025] 5. The pre-sodium active material provided in this invention is completely consumed during the formation process, without increasing the mass of the separator and the cell, thus improving the energy density of the cell.

[0026] 6. The pre-sodium-treated diaphragm of the present invention uses water as a solvent, which is environmentally friendly and low in cost. The pre-sodium-treated diaphragm preparation process is simple, the drying temperature is low, and the production efficiency is high. Attached Figure Description

[0027] Figure 1 This is a comparison chart of the charge-discharge capacity of half-cells in Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0028] Combination Figure 1 This article describes a specific embodiment of the present invention in detail, but does not limit the scope of the claims of the present invention in any way.

[0029] Example 1

[0030] Pre-sodium-modified active material Na₂C₂O₄ and deionized water were mixed and dissolved at a mass ratio of 1:40. 8% sodium alginate (by mass of the pre-sodium-modified active material) was then added and stirred until a homogeneous solution was obtained. This solution was then sprayed onto the surface of a 12 μm thick polyethylene film to form a coating with a thickness of 1.5 μm and an areal density of 0.35 mg / cm³. 2 The pre-sodiumized diaphragm was obtained by baking in an oven at 85°C for 20 minutes.

[0031] A coin cell was fabricated by sequentially preparing a sodium ferric sulfate positive electrode, a pre-sodiumized separator, and a sodium metal sheet, with the pre-sodiumized layer on the separator facing the positive electrode side. 20 μL of electrolyte was then injected to obtain the coin cell. The electrolyte solvent was a 1:1 volume ratio EC-PC mixture, with a solute of 1 mol / L sodium hexafluorophosphate.

[0032] A bare cell was fabricated using a sodium ferric sulfate positive electrode, a pre-sodiumized separator, and a hard carbon negative electrode, according to specified data. The pre-sodiumized layer on the separator faced the positive electrode side. The bare cell was placed in an aluminum-plastic film, and electrolyte was injected. After encapsulation and formation, a sodium-ion full cell was fabricated. The electrolyte solvent was a 1:1 volume ratio EC-PC mixture, and the solute was 1 mol / L sodium hexafluorophosphate.

[0033] Comparative Example 1

[0034] A coin cell was fabricated by sequentially preparing a sodium ferric sulfate positive electrode, a 12 μm polyethylene film, and a sodium metal sheet, and then injecting an electrolyte to obtain the coin cell. The electrolyte solvent was an EC-PC mixture with a volume ratio of 1:1, and the solute was 1 mol / L sodium hexafluorophosphate.

[0035] A bare cell was fabricated using sodium ferric sulfate positive electrode, a 12μm polyethylene film, and a hard carbon negative electrode according to specifications. The bare cell was then placed in an aluminum-plastic film, injected with electrolyte, and after encapsulation and formation, a sodium-ion full cell was fabricated. The electrolyte was a 1:1 volume ratio EC-PC mixture, with a solute of 1 mol / L sodium hexafluorophosphate.

[0036] The first-cycle charging data of the sodium-ion coin cells prepared in Example 1 and Comparative Example 1 are as follows: Figure 1 As shown, from Figure 1 It can be seen that the specific capacity of the button cell in Example 1 during the first charge is significantly better than that in Comparative Example 1, indicating that the pre-sodium-treated separator provided by the present invention can contribute a certain amount of specific capacity during the battery formation process.

[0037] The first-cycle charge-discharge data and cycle data of the sodium-ion full batteries prepared in Example 1 and Comparative Example 1 are shown in the table below:

[0038]

[0039] As can be seen from Table 1, the specific capacity and initial efficiency of the battery in Example 1 are higher than those in Comparative Example 1. At the same time, the capacity retention rate after 2000 cycles is also better than that in Comparative Example 1, indicating that the pre-sodiumized separator provided by the present invention has good sodium replenishment ability.

[0040] Example 2

[0041] Pre-sodium-treated active material Na₂C₄O₄ and deionized water were mixed and dissolved at a mass ratio of 1:5. Then, 5% sodium alginate (by mass of the pre-sodium-treated active material) was added and stirred until homogeneous to obtain a solution. This solution was then sprayed onto the surface of a 12μm thick polypropylene base film to form a coating with a thickness of 2μm and an areal density of 4mg / cm³. 2 The pre-sodiumized diaphragm was obtained by baking in an oven at 45°C for 5 minutes.

[0042] A bare cell was fabricated using a sodium ferric sulfate positive electrode, a pre-sodiumized separator, and a hard carbon negative electrode, according to specified data. The pre-sodiumized layer on the separator faced the positive electrode side. The bare cell was placed in an aluminum-plastic film, and electrolyte was injected. After encapsulation and formation, a sodium-ion full cell was fabricated. The electrolyte solvent was a 1:1 volume ratio EC-PC mixture, and the solute was 1 mol / L sodium hexafluorophosphate.

[0043] Example 3

[0044] Pre-sodiumized active material Na2C4O6 and deionized water were mixed and dissolved at a mass ratio of 1:100. Sodium alginate (10% of the mass of the pre-sodiumized active material) was then added and stirred until a homogeneous solution was obtained. This solution was then sprayed onto the surface of a 12μm thick cellulose-based membrane to form a coating with a thickness of 2μm and an areal density of 4mg / cm³. 2 The pre-sodiumized diaphragm was obtained by baking in an oven at 85°C for 20 minutes.

[0045] A bare cell was fabricated using a sodium ferric sulfate positive electrode, a pre-sodiumized separator, and a hard carbon negative electrode, according to specified data. The pre-sodiumized layer on the separator faced the positive electrode side. The bare cell was placed in an aluminum-plastic film, and electrolyte was injected. After encapsulation and formation, a sodium-ion full cell was fabricated. The electrolyte solvent was a 1:1 volume ratio EC-PC mixture, and the solute was 1 mol / L sodium hexafluorophosphate.

[0046] Example 4

[0047] Pre-sodium-treated active material Na₂CO₃ and deionized water were mixed and dissolved at a mass ratio of 1:50. Then, sodium alginate (8% of the pre-sodium-treated active material) was added and stirred until homogeneous to obtain a solution. This solution was then sprayed onto the surface of a 12 μm thick polyethylene film to form a coating with a thickness of 1 μm and an areal density of 2 mg / cm³. 2 The pre-sodiumized diaphragm was obtained by baking in an oven at 65°C for 10 minutes.

[0048] A bare cell was fabricated using a sodium ferric sulfate positive electrode, a pre-sodiumized separator, and a hard carbon negative electrode, according to specified data. The pre-sodiumized layer on the separator faced the positive electrode side. The bare cell was placed in an aluminum-plastic film, and electrolyte was injected. After encapsulation and formation, a sodium-ion full cell was fabricated. The electrolyte solvent was a 1:1 volume ratio EC-PC mixture, and the solute was 1 mol / L sodium hexafluorophosphate.

[0049] The first-cycle charge-discharge data and cycle data of the sodium-ion full batteries prepared in Examples 1-4 are shown in the table below:

[0050]

[0051] It is understood that the above detailed description of the present invention is for illustrative purposes only and is not intended to limit the technical solutions described in the embodiments of the present invention. Those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention to achieve the same technical effects; as long as the usage requirements are met, they are all within the protection scope of the present invention.

Claims

1. A pre-sodium-treated diaphragm, characterized in that: A pre-sodium-treated separator is obtained by drying a pre-sodium-treated active material as the surface layer and a separator base membrane as the covering carrier. The pre-sodium-treated separator is used as a separator for sodium-ion batteries. The method for preparing the pre-sodium-modified diaphragm includes: preparing a coating solution from the pre-sodium-modified active material, then covering the diaphragm base membrane with the coating solution, and drying it to obtain the pre-sodium-modified diaphragm. The drying temperature is 45-85℃ and the drying time is 5-20 min. The coating solution also contains sodium alginate, and the amount of sodium alginate added is 5-10% of the mass of the pre-sodium-modified active material.

2. The pre-sodium-treated membrane according to claim 1, characterized in that: The pre-sodium-treated active material is one or more of inorganic sodium salts, organic sodium salts, and sodium oxides.

3. The pre-sodium-treated membrane according to claim 2, characterized in that: The pre-sodium-treated active material is one or more of Na2C2O4, Na2C4O4, Na2C4O6, and Na2CO3.

4. The pre-sodium-treated membrane according to claim 1, characterized in that: The diaphragm base membrane is made of one or more of polyethylene, polypropylene, and cellulose.

5. The pre-sodium-treated membrane according to claim 1, characterized in that: The coating solution uses deionized water as a solvent.

6. The pre-sodium-treated membrane according to claim 1, characterized in that: The coating solution is a solution or dispersion of the pre-sodiumized active substance.

7. The pre-sodium-treated membrane according to claim 1, characterized in that: The mass ratio of the pre-sodiumized active material to deionized water in the coating solution is 1:5-10.

8. The pre-sodium-treated membrane according to claim 1, characterized in that: The thickness of the pre-sodium-treated active material is 0.2-2 μm, and the density of the coating surface is 0.05-4 mg / cm³. 2 .

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