Three-layer sodium-ion battery diaphragm and preparation method thereof

Abstract

The invention discloses a three-layer sodium-ion battery diaphragm and a preparation method thereof, and belongs to the technical field of battery diaphragms.The diaphragm is of a three-layer A / B / A structure, the A layer is made of sodium-based montmorillonite modified cellulose-based diaphragm, montmorillonite has a high specific surface area and a pore structure, battery permeability and ion transmission efficiency are improved, and energy density and cycling stability are improved. The montmorillonite is subjected to sodium modification, and the sodium montmorillonite is superior to common montmorillonite and is high in dispersity, thermal stability and cation exchange capacity. According to the modified montmorillonite cellulose diaphragm, montmorillonite is hydrated to form a sheet structure to adsorb metal particles with positive charges, and montmorillonite-cellulose combination of the diaphragm is enhanced by combining with fibers with negative charges. The wettability and the liquid absorption rate are improved by the cellulose-based diaphragm, the degree of freedom of sodium ions is enhanced by the sodium montmorillonite, the ion migration speed is increased, and the battery performance is improved. And the layer B adopts a polyolefin diaphragm, so that the temperature requirements under different conditions are met, and the self-turn-off performance is provided.

Description

Technical Field

[0001] The invention belongs to the technical field of battery separators, and in particular relates to a three-layer sodium ion battery separator and a preparation method thereof. Background Art

[0002] As the global demand for sustainable energy solutions grows, lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, large-scale energy storage and other fields. However, the shortage of lithium resources and the cost of lithium carbonate are becoming increasingly serious, and the development of new energy storage systems is urgent. In recent years, low-cost sodium-ion batteries have attracted widespread attention in academia and the business community. Their abundant reserves and similarities with lithium batteries make them expected to fill the gap in the lithium battery industry in the field of large-scale energy storage.

[0003] Sodium-ion batteries have gradually become a research hotspot due to their abundant sodium resources, low cost, and electrochemical mechanisms similar to lithium-ion batteries. However, due to the difference in solvents in the sodium-ion battery system, the wettability of the polyolefin separator used in current lithium-ion batteries is relatively poor, which reduces the mobility of sodium ions and makes its cycle efficiency poor. Therefore, according to the characteristics of sodium-ion battery electrolytes, scientific researchers have developed cellulose separators, which have the characteristics of good wettability, low internal resistance, and excellent thermal dimensions. However, there are still safety issues. In the event of thermal runaway, the cellulose separator does not have a self-closing feature, which can easily cause a fire and threaten life and property safety. In addition, energy density is also an issue that needs to be optimized. As a technician in this field, it is urgent to develop a new production method for a three-layer sodium-ion battery separator. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a three-layer sodium ion battery separator. The separator is divided into three layers of A / B / A structure. The A layer is a modified cellulose membrane. The cellulose-based separator is modified by sodium-montmorillonite and ceramic particles. The sodium-montmorillonite can improve the electrolyte permeability and ion transfer rate of the separator. The ceramic particles provide a certain temperature resistance, thereby improving the energy density and stability of the battery. The reason for using a cellulose-based separator is that cellulose can improve the wetting performance and improve the liquid absorption rate. The B layer uses a polyolefin separator to meet the temperature requirements in different situations and provide self-shutdown performance. Then, the A and B layers are compounded with an adhesive.

[0005] A three-layer sodium ion battery separator, the separator sequentially comprises a three-layer A / B / A structure consisting of a modified cellulose separator A layer, a polyolefin separator B layer and a modified cellulose separator A layer, the modified cellulose separator A layer is a sodium-based montmorillonite modified cellulose separator, and the B layer is a polypropylene separator.

[0006] The preparation method of the above-mentioned three-layer sodium ion battery separator comprises the following steps: (1) polypropylene is put into a screw extruder, the extruder temperature is 180°C, cooled at 80°C, and heat-treated at 100°C for 8 hours; the heat-treated membrane is longitudinally cold-stretched at 60°C with a stretch ratio of 1.8, and then longitudinally hot-stretched at 110°C with a stretch ratio of 3.0; the stretched membrane is heat-set at 105°C for 3 minutes, and then cooled and rolled to obtain a sodium ion battery B-layer polyolefin separator for standby use; (2) montmorillonite is crushed to pass through a 100-140 mesh filter, dispersed in deionized water, and configured into a slurry with a solid content of 8% to 12%, preferably a solid content of 10%, stirred for 2 to 4 hours, preferably 2 hours, and then allowed to stand for 0.5 to 1 hour, preferably 0.5 hours, allowed to stand for sedimentation, and the upper suspension is taken and filtered. Washing, washing with deionized water for 3 times, and drying to obtain refined montmorillonite; (3) preparing the refined montmorillonite into 8% to 12% slurry, preferably with a solid content of 10%, adding a sodium agent, and after ultrasonic treatment, filtering, washing with 5% mass concentration of dilute hydrochloric acid until neutral, drying, and grinding to obtain sodium-based montmorillonite; (4) then mixing sodium-based montmorillonite, ceramic particles, dispersant, thickener and water in a mass ratio of 3% to 5%: 3% to 5%: 0.3% to 0.5%: 0.5% to 1%: 88.5% to 93.2%; soaking the cellulose membrane in the solution for 30 to 60 minutes, preferably for 30 minutes, then taking it out and drying it to obtain a sodium ion battery membrane layer A; (5) using a binder to bond the sodium ion battery membrane layer A and the sodium ion battery membrane layer B to form a sodium ion battery membrane with an A / B / A three-layer structure.

[0007] The refined montmorillonite is configured into a slurry, a sodiumizing agent is added, stirred, and after ultrasonic treatment, filtered, washed with dilute hydrochloric acid until neutral, dried, and ground to obtain sodium-based montmorillonite. The sodium-based montmorillonite and ceramic particles are configured into a solution, a dispersant and a thickener are added, and the solution is uniformly mixed. The cellulose diaphragm is immersed in the solution, taken out and dried to obtain the sodium ion battery diaphragm A layer. The B layer is a polyolefin diaphragm, which mainly provides a self-shutoff function for the diaphragm to ensure the safety of the battery. Then the three layers of A / B / A are bonded to obtain the sodium ion battery diaphragm.

[0008] Furthermore, the ceramic particles in step (4) are aluminum oxide. The ceramic particles may also be selected from one or more of magnesium oxide, titanium dioxide, silicon dioxide, and antimony trioxide.

[0009] Furthermore, the sodiumizing agent in step (3) is composed of 5-10% trisodium phosphate and 90-95% sodium carbonate in mass percentage.

[0010] Furthermore, the ultrasonic treatment conditions in step (3) are 70-90° C., 15-30 min, 20-20 kHz, and 60 W.

[0011] Furthermore, in step (2), the drying temperature is 90-110° C. and the drying time is 30 minutes.

[0012] Furthermore, in step (4), the dispersant is selected from sodium lauryl sulfate or sodium polyacrylate, and the thickener is selected from sodium carboxymethyl cellulose, hydroxypropyl methylcellulose or polyvinyl alcohol.

[0013] Furthermore, in step (4), the drying temperature is 120° C. and the drying time is 10 min.

[0014] Furthermore, the adhesive in step (5) is PVDF.

[0015] Furthermore, the cellulose separator is made by folding a cellulose matrix extracted from one or more natural raw materials such as cotton, bamboo or wood.

[0016] Beneficial effects of the present invention: The present invention provides a three-layer sodium ion battery separator, which is divided into three layers of A / B / A structure, and the A layer is a modified cellulose membrane. The sodium-based montmorillonite is used to modify the cellulose-based separator. The montmorillonite has a high specific surface area and a pore structure, which can improve the permeability of the battery electrolyte and the ion transmission efficiency, thereby improving the energy density and cycle stability of the battery. In addition, the sodium modification of montmorillonite is because the sodium-based montmorillonite has the characteristics of large dispersibility, thermal stability, and cation exchange capacity compared with ordinary montmorillonite. Secondly, the montmorillonite-modified cellulose-based separator has the advantage that montmorillonite has a strong hydration effect, can form a fine sheet structure, present an alternating state, can adsorb a large amount of positively charged metal particles, and can be combined with negatively charged fibers to enhance the combination between montmorillonite and fibers. It is beneficial to enhance the ionic conductivity of the battery and the liquid absorption rate of the separator to the electrolyte. At the same time, because the cellulose-based separator used in the present invention, the montmorillonite also has a certain anti-corrosion function, which can extend the shelf life of the separator. The reason for the cellulose-based diaphragm is that cellulose can improve the wettability and liquid absorption rate. Secondly, it can form hydrogen bonds with the solvent, which reduces the binding energy between the solvent molecules and the sodium ions, thereby increasing the freedom of the sodium ions and accelerating the ion migration speed, thereby improving the battery performance. The B layer uses a polyolefin diaphragm to meet the temperature requirements under different conditions and provide self-shutdown performance. Then, the A and B layers are compounded with an adhesive. The adhesive uses PVDF, which can resist solvent and acid and alkali corrosion. DETAILED DESCRIPTION

[0017] The present invention is described below with specific examples, but is not intended to be limiting of the present invention. Example 1

[0018] Raw materials: XQ-5545 sodium polyacrylate from Xinqi Chemical; FH-FNP montmorillonite powder from Fenghong New Materials; R05SDP alumina particles from Zide New Materials; LK50 sodium carboxymethyl cellulose from Hongbo; FL2300 PVDF resin mixed with N-methyl-2-pyrrolidone and stirred evenly to obtain a 3% binder; 20g / m3 cellulose diaphragm 2 , 40μm thick nanocellulose lithium battery separator Longyou Jinchang 20G, wherein the nanocellulose is 90% α-cellulose, 70% crystallinity and an average diameter of 600nm; S802 polypropylene resin is put into a screw extruder, the extruder temperature is 180℃, cooled at 80℃, and heat treated at 100℃ for 8h. After heat treatment, the membrane is longitudinally cold stretched at 60℃ with a stretch ratio of 1.8, and then longitudinally hot stretched at 110℃ with a stretch ratio of 3.0. The stretched membrane is heat-set at 105℃ for 3min, and then the polypropylene separator is cooled and rolled for use.

[0019] The montmorillonite was crushed and passed through a 100-mesh sieve, dispersed in deionized water, and prepared into a 10% slurry, stirred for 2 hours, and then allowed to stand for 0.5 hours. The upper suspension was taken, filtered, washed, washed with deionized water for 3 times, and dried to obtain refined montmorillonite; The refined montmorillonite was prepared into a 10% slurry, and a sodiumizing agent was added, the sodiumizing agent was composed of 5% trisodium phosphate and 95% sodium carbonate, and after ultrasonic treatment at 20kHz, power of 60W, 90℃, 15min, it was filtered, washed with 5% dilute hydrochloric acid until neutral, dried at 110℃ for 30min, and ground to obtain sodium-based montmorillonite, and then sodium-based montmorillonite, alumina, sodium polyacrylate, sodium carboxymethyl cellulose: water were mixed uniformly in a ratio of 3:3:0.3:0.5:93.2 mass%, and the cellulose diaphragm was soaked in the solution for 60min, and then it was taken out and dried at 120℃ for 10min. The sodium ion battery diaphragm A layer was obtained. The A layer and the B layer were bonded with a PVDF adhesive to form a sodium ion battery diaphragm with an A / B / A three-layer structure.

[0020] Specifically, a three-layer A / B / A structure consisting of a modified cellulose membrane layer A, a polyolefin membrane layer B and a modified cellulose membrane layer A is stacked in sequence, and PVDF adhesive is added to the gap for coating. The modified cellulose membrane layer A is a sodium-montmorillonite modified cellulose membrane, and the B layer is a polypropylene membrane. Put them into a plate-and-frame mold, and put the mold on the lower plate of a flat hydraulic press that has been heated to a constant temperature of 70°C. Raise the lower plate of the hydraulic press until the mold contacts the upper plate, and keep it without pressure for 10 minutes. Slowly increase the pressure to 3MPa, keep it for 3 minutes, and then take out the mold. Example 2

[0021] The method is the same as that of Example 1, except that the mass % ratio of sodium montmorillonite, alumina, sodium polyacrylate and sodium carboxymethyl cellulose is 5:3:0.4:0.5:91.1. Example 3

[0022] The method is the same as that of Example 1, except that the mass % ratio of sodium montmorillonite, alumina, sodium polyacrylate and sodium carboxymethyl cellulose is 5:5:0.5:1:88.5.

[0023] Comparative Example 1 The method is the same as that of Example 1, except that sodium montmorillonite is not added to the impregnation solution.

[0024] Comparative Example 2 The method is the same as that of Example 1, except that ceramic alumina is not added to the infiltration solution.

[0025] Comparative Example 3 The method is the same as that of Example 1, except that when the sodium-montmorillonite is modified, the mesh number of the sieve is 140 mesh, and the dispersant content is 1.5%.

[0026] Comparative Example 4 It is the polypropylene diaphragm of Example 1.

[0027] Note: The performance test is carried out according to the following method, the test environment temperature is 23℃, the relative humidity is 50%, and the state is adjusted for 6 hours: (1) Liquid absorption rate A 1 mol / L lithium hexafluorophosphate dimethyl carbonate solution and ethyl methyl carbonate were mixed in a volume ratio of 1:1 to prepare an electrolyte, and then the composite diaphragm sample was immersed in the electrolyte for 12 hours. After completion, the excess electrolyte on the surface was wiped off and the liquid absorption rate was calculated.

[0028] (2) Membrane rupture temperature The membrane rupture temperature is measured by the resistance mutation method, and the point where the resistance suddenly increases is the membrane rupture temperature.

[0029] (3) Ionic conductivity: The bulk resistance was tested using the steel sheet / diaphragm / steel sheet assembly method and the electrochemical impedance spectroscopy (EIS) mode of a VMP3B-10 electrochemical workstation (Bio-LogicScience Instruments), with a perturbation voltage amplitude of 5 mV and a frequency of 10 mHz to 1 MHz. The calculated relationship between bulk resistance and ionic conductivity is: ionic conductivity is equal to the ratio of the diaphragm thickness to the product of the diaphragm resistance and the effective contact area.

[0030] The sodium ion battery separators prepared in Examples 1 to 3 and Comparative Examples 1 to 4 above have liquid absorption, membrane rupture temperature and porosity results shown in Table 1 below.

[0031] Table 1 Test results of sodium ion battery separators prepared in Examples 1 to 3 and Comparative Examples 1 to 4

[0032] It can be seen from the above table that the three-layer sodium ion battery separator provided by the present invention can greatly improve the membrane breaking temperature, liquid absorption rate and ionic conductivity. Compared with Example 1, the ionic conductivity of Example 2 is improved to a certain extent. The reason is that sodium-based montmorillonite has a higher specific surface area and a porous structure, which can improve the adsorption capacity and ionic conductivity of ions. Compared with Example 2, Example 3 increases the content of ceramic particles, so that the membrane breaking temperature is slightly increased, but the increase is not large. Compared with Example 1, Comparative Example 1 does not add sodium-based montmorillonite infiltration, resulting in a significant decrease in its ionic conductivity. It can be seen that sodium-based montmorillonite improves ionic conductivity. Compared with Example 1, the membrane breaking temperature of Comparative Example 2 decreases, mainly due to the lack of ceramic particles. Compared with Example 1, under the same conditions, Comparative Example 3 reduces the particle size of sodium-based montmorillonite, improves its own sodium degree, and has almost no difference in performance. However, when the sodium degree of montmorillonite is too high, the charge repulsion between the montmorillonite particles is enhanced, which leads to a decrease in its dispersibility. When preparing the solution, more dispersants need to be added to make it dispersed completely. At the same time, when the sodium degree is high, its adsorption performance is also reduced. Therefore, considering various factors such as processing performance and cost, Example 3 is the best choice of the present invention.

[0033] The above is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. All equivalent changes and modifications made according to the content of the present invention are included in the patent scope of the present invention.

Claims

1. A three-layer sodium ion battery separator, characterized in that: The membrane comprises a three-layer A / B / A structure consisting of a modified cellulose membrane layer A, a polyolefin membrane layer B and a modified cellulose membrane layer A, wherein the modified cellulose membrane layer A is a sodium-montmorillonite modified cellulose membrane, and the B layer is a polypropylene membrane.

2. The method for preparing a three-layer sodium ion battery separator according to claim 1, characterized in that: The method comprises the following steps: (1) feeding polypropylene into a screw extruder at an extruder temperature of 180°C, cooling at 80°C, and heat treating at 100°C for 8 hours; the heat-treated membrane is longitudinally cold stretched at 60°C with a stretch ratio of 1.8, and then longitudinally hot stretched at 110°C with a stretch ratio of 3.0; the stretched membrane is heat-set at 105°C for 3 minutes, and then cooled and rolled to obtain a sodium ion battery B layer polyolefin diaphragm for standby use; (2) pulverizing montmorillonite until it passes through a 100-140 mesh filter, dispersing it in deionized water, and preparing a slurry with a solid content of 10%, stirring it for 2 hours, and then standing it for 0.5 hours, standing it for sedimentation, taking the upper suspension, filtering it, and washing it with deionized water. 3 times, and dried to obtain refined montmorillonite; (3) the refined montmorillonite is prepared into a 10% slurry, a sodiumizing agent is added, and after ultrasonic treatment, it is filtered, washed with 5% diluted hydrochloric acid until neutral, and then dried, and ground to obtain sodium-based montmorillonite; (4) sodium-based montmorillonite, ceramic particles, dispersant, thickener and water are then mixed evenly in a mass ratio of 3%-5%: 3%-5%: 0.3%-0.5%: 0.5%-1%: 88.5%-93.2%; a cellulose diaphragm is immersed in the solution for 60 minutes, and then taken out and dried to obtain a sodium ion battery diaphragm layer A; (5) the sodium ion battery diaphragm layer A and the sodium ion battery layer B are bonded by a binder to form a sodium ion battery diaphragm with an A / B / A three-layer structure.

3. The preparation method according to claim 2, characterized in that: The ceramic particles in step (4) are aluminum oxide.

4. The preparation method according to claim 2, characterized in that: The sodiumizing agent in step (3) is composed of 5% trisodium phosphate and 95% sodium carbonate by mass percentage.

5. The preparation method according to claim 2, characterized in that: The ultrasonic treatment conditions in step (3) are 90° C., reaction time 15 min, frequency 20 kHz, and power 60 W.

6. The preparation method according to claim 2, characterized in that: The drying temperature in step (2) is 10° C. and the drying time is 30 min.

7. The preparation method according to claim 2, characterized in that: In the step (4), the dispersant is sodium polyacrylate and the thickener is sodium carboxymethyl cellulose.

8. The preparation method according to claim 2, characterized in that: The drying temperature in step (4) is 120° C. and the drying time is 10 min.

9. The preparation method according to claim 2, characterized in that: The adhesive in step (5) is PVDF.

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