Double-sided hetero-coated separator for silicon negative electrode battery, preparation method and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a double-face heterogeneous coating diaphragm for a silicon negative electrode battery, a preparation method and a lithium ion battery, which comprises a porous polymer film, a first coating facing an anode and a second coating facing a cathode, the first coating is a spandex layer, the spandex layer comprises a composite of a thermoplastic polyurethane elastomer and a fluorine-based polymer, and the second coating is an aramid layer, the aramid layer comprises a sulfonated aramid nanofiber grid. The double-face heterogeneous coating diaphragm for the silicon negative electrode battery can systematically solve the coupling failure problem of silicon negative electrode interface stress concentration and thermal runaway.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a double-sided heterogeneous coating separator for silicon anode batteries, its preparation method, and lithium-ion batteries. Background Technology

[0002] Silicon-based anodes are a key material in lithium-ion batteries, possessing advantages such as high specific capacity and low operating potential. However, silicon-based anode systems suffer from multiple interface failure mechanisms due to the lithium intercalation expansion rate (>300%): First, drastic volume changes cause traditional separators to have high hardness (>5 GPa), exacerbating stress concentration at the electrode interface and leading to active material shedding and cycle failure. Second, the dynamic rupture / regeneration of the SEI film during cycling accelerates electrolyte decomposition. While PVDF double-sided coatings improve adhesion, their unreasonable porosity distribution (<30%) degrades electrolyte wettability (contact angle >65°), and coating cracking is likely to occur after long-term cycling. Third, highly active silicon surfaces are prone to local thermal runaway under thermal abuse scenarios. Although single-sided aramid coatings can raise the thermal pore-closing temperature to 250°C, their rigid structure cannot adapt to the dynamic deformation of the silicon anode, and the hydrophobicity of the unmodified aramid surface (contact angle >60°) further deteriorates interfacial ion transport.

[0003] In view of this, it is necessary to design a separator and a lithium-ion battery using the separator in order to systematically solve the problem of stress concentration and thermal runaway coupling failure at the silicon anode interface. Summary of the Invention

[0004] To overcome the shortcomings of the existing technology, the present invention provides a double-sided heterogeneous coating separator for silicon anode batteries, a preparation method thereof, and a lithium-ion battery.

[0005] One objective of this invention is to provide a double-sided heterogeneous coating separator for silicon anode batteries, comprising: a porous polymer film, which is a polymer film material with a thickness of 5-20 μm; a first coating facing the anode, which is an spandex layer comprising a composite of thermoplastic polyurethane elastomer and fluorinated polymer, with a thickness of 4-8 μm; and a second coating facing the cathode, which is an aramid layer comprising a sulfonated aramid nanofiber mesh, with a thickness of 3-6 μm. The thickness of the first coating is greater than that of the second coating, and the thickness ratio between the two is 1.2-1.5:1. The porosity of the first coating is greater than that of the second coating. Preferably, the difference in porosity between the first and second coatings is ≥15%, the porosity of the first coating is 45±3%, and the porosity of the second coating is 30±2%.

[0006] The second objective of this invention is to provide a method for preparing a double-sided heterogeneous coating separator for silicon anode batteries, comprising the following steps: firstly, coating an spandex layer on the side of a porous polymer film facing the anode, drying it and immediately flipping it over, and then coating an aramid layer on the side of the porous polymer film facing the cathode using a high-voltage electrospinning device.

[0007] Preferably, the first coating is prepared by mixing thermoplastic polyurethane elastomer and fluoropolymer (such as polytetrafluoroethylene PTFE or polyvinylidene fluoride PVDF) at a mass ratio of 95:5, dissolving them in a mixed solvent of N,N-dimethylformamide and acetone (volume ratio of N,N-dimethylformamide to acetone 3:1), and stirring at high speed (3000 rpm, 2 hours) to form a homogeneous viscous slurry. The solid content of this viscous slurry is 10-20%, and the viscosity is 5000-8000 mPa·s. This viscous slurry is then coated onto the surface of a porous polymer film facing the anode to form a composite of thermoplastic polyurethane elastomer and fluoropolymer with a thickness of approximately 4 μm, a porosity of 45%, and an elastic modulus of 1.0 GPa. The high elastic modulus can buffer the dynamic stress caused by high expansion during silicon anode cycling. After coating, acetone evaporates rapidly, and phase separation occurs first on the surface, forming small pores. DMF evaporates slowly, and phase separation gradually occurs in the bottom layer, forming large pores. The gradient phase separation induced by the difference in evaporation rate is used to achieve uniformity of lithium ion flow.

[0008] Preferably, the second coating is prepared by first dissolving meta-aramid in a mixed solution of N-methylpyrrolidone and N,N-dimethylformamide, with a volume ratio of N-methylpyrrolidone to N,N-dimethylformamide of 4:1. Then, 5 wt% chlorosulfonic acid is added, and the mixture is sulfonated at 60°C for 2 hours to obtain a sulfonated aramid precursor solution. The solid content of this sulfonated aramid precursor solution is 10-15%, and the viscosity is 6000-8000 mPa·s. Using a high-voltage electrospinning device (voltage 15 kV, receiving distance 15 cm, solution flow rate 0.5 mL / h), the sulfonated aramid solution is sprayed onto the cathode-facing surface of a porous polymer film, forming a three-dimensional nanofiber network with a diameter of 200-500 nm. After heating and curing, a sulfonated aramid nanofiber network is formed with a thickness of 3 μm, a porosity of 30%, and a sulfonation degree of 1.0 mmol / g on the aramid nanofiber surface. The aramid layer needs to undergo sulfonation treatment, which involves grafting sulfonic acid groups onto the aramid surface using reagents such as chlorosulfonic acid. This significantly increases the surface polarity, making it easier for the aramid to form hydrogen bonds with polar electrolytes (such as EC / DEC), thus accelerating electrolyte penetration.

[0009] The present invention has the following technical effects: (1) Silicon anodes have a high expansion rate, resulting in a short cycle life. Therefore, the separator needs to have better mechanical strength and thermal stability. In this solution, the spandex layer coated on the surface of the porous polymer film facing the anode provides flexibility and elasticity, which can alleviate the stress caused by the volume change of the silicon anode during cycling. The aramid layer facing the cathode provides high heat resistance and mechanical strength to prevent thermal runaway.

[0010] (2) By coating both sides of a porous polymer film with spandex and aramid layers, a rigid-flexible coupled double-sided heterogeneous coating separator is formed. On the negative electrode side, dynamic stress buffering and lithium-ion flow homogenization are achieved by controlling the controllable elastic modulus (0.5~1.2GPa) and gradient porous structure (porosity 45%) of the spandex layer. On the positive electrode side, a high-temperature safety barrier is formed based on a sulfonated aramid nanofiber network, combined with the thermal pore-closing characteristics at 280℃ and the oxidation resistance potential at 4.8V. The porosity of the double-sided coating is designed in a gradient manner (45% on the negative electrode side vs. 30% on the positive electrode side) to synergistically optimize electrolyte wettability and ion migration efficiency, thereby improving the overall performance of the cell. Through this design, the problem of stress concentration and thermal runaway coupling failure at the silicon negative electrode interface can be systematically solved.

[0011] (3) Coating aramid and spandex layers onto porous polymer films can overcome the shortcomings of single-layer coatings (coating only aramid or only spandex layers): If an unsulfonated aramid layer is coated only on the cathode-facing surface of the porous polymer base film of the separator, the electrolyte wettability will be insufficient. The sulfonated aramid used in this scheme has sulfonic acid groups (-SO3H) on its surface that significantly increase the surface energy (~50-60 mN / m), forming hydrogen bonds with the electrolyte, reducing the contact angle, and allowing the electrolyte to spread rapidly. Simultaneously coating the anode-facing surface of the substrate separator with spandex layers, which have an elastic modulus of 0.5-1.2 GPa, provides a very valuable dynamic deformation buffer space for silicon anodes. Furthermore, a bi-sided heterogeneous porosity gradient (45% on the anode side vs. 30% on the cathode side) is set for the double-sided coating to optimize the ion transport path, improve electrolyte compatibility, and facilitate electrolyte wettability. Because of the addition of the aramid layer, the physical isolation function of the separator is maintained in the temperature range of 200°C to 300°C or even higher, preventing the positive and negative electrodes from directly contacting each other and causing a large-area internal short circuit. This buys valuable reaction time for the battery management system and delays or prevents the occurrence of thermal runaway. Attached Figure Description

[0012] Figure 1 This is a schematic diagram showing the distribution of the diaphragm, positive electrode, and negative electrode.

[0013] In the figure, 1 is the porous polymer film; 2 is the first coating; 3 is the second coating; 4 is the anode; and 5 is the cathode. Detailed Implementation

[0014] The principles and features of the present invention are described below with reference to embodiments; the examples given are only for explaining the present invention and are not intended to limit the scope of the present invention. Example

[0015] This embodiment provides a double-sided heterogeneous coating separator for silicon anode batteries, comprising a porous polymer film 1, a first coating 2, and a second coating 3. The porous polymer film 1 is a polymer film material, which can be a common woven membrane, a polyolefin separator, or a nonwoven membrane such as nonwoven fabric, composite membrane, separator paper, or rolled membrane. In this embodiment, a 9μm PE wet-process separator with a porosity of 40% is used. The first coating 2 is an spandex layer, coated on the side of the porous polymer film 1 facing the anode 4. The spandex layer comprises a composite of thermoplastic polyurethane elastomer and fluorinated polymer. The second coating 3 is an aramid layer, coated on the side of the porous polymer film facing the cathode 5. The aramid layer comprises a sulfonated aramid nanofiber mesh. The thickness of the porous polymer base film is 5~20μm, the thickness of the first coating 2 is 4~8μm, and the thickness of the second coating 3 is 3~6μm. The thickness of the first coating 2 is greater than the thickness of the second coating 3, and the ratio of the thickness of the first coating 2 to the thickness of the second coating 3 is (1.2~1.5):1. Based on the above, the following embodiments can be derived (not exhaustive). In this embodiment, the porosity of the first coating 2 is 45%, and the elastic modulus is 1 GPa. Dynamic stress buffering and lithium-ion flow homogenization are achieved through its controllable elastic modulus (0.5-1.2 GPa) and gradient porous structure (porosity 45%). The porosity of the second coating 3 is 30%. Based on this, the first coating 2, the porous polymer film 1, and the second coating 3 form a gradient design, which can synergistically optimize electrolyte wettability and ion migration efficiency, and improve the overall performance of the battery cell.

[0016] Table 1. Film thickness (in μm) for each embodiment Porous polymeric film 1 5 9 20 First coating 2 6 4 8 Second coating 3 5 3 - Example

[0017] This embodiment provides a method for preparing a double-sided heterogeneous coating separator for silicon anode batteries. First, an spandex layer is coated on the side of a porous polymer film 1 facing the anode 4. After drying, the film is immediately flipped over, and an aramid layer is coated on the side of the porous polymer film 1 facing the cathode 5 using a high-voltage electrospinning device, preventing solvent penetration from affecting the other side. The parameters of the high-voltage electrospinning device used above are: voltage 15 kV, receiving distance 15 cm, and solution flow rate 0.5 mL / h.

[0018] The first coating 22 is prepared as follows: thermoplastic polyurethane elastomer (TPU) and fluoropolymer (such as polytetrafluoroethylene PTFE or polyvinylidene fluoride PVDF) are mixed at a mass ratio of 95:5 and dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and acetone (volume ratio 3:1). The mixture is stirred at high speed (3000 rpm, 2 hours) to form a homogeneous viscous slurry (solid content 10-20%, viscosity 5000-8000 mPa·s). This viscous slurry is coated onto the surface of the porous polymer base film 1 facing the anode 44, forming a composite 2 of thermoplastic polyurethane elastomer and fluoropolymer with a thickness of approximately 4 μm, a porosity of 45%, and an elastic modulus of 1.0 GPa. The high elastic modulus can buffer the dynamic stress caused by high expansion during silicon anode cycling. After coating, acetone evaporates rapidly, and phase separation occurs first on the surface, forming small pores. DMF evaporates slowly, and phase separation gradually occurs in the bottom layer, forming large pores. The gradient phase separation induced by the difference in evaporation rate is used to achieve uniformity of lithium ion flow.

[0019] The preparation method of the second coating 33 is as follows: First, meta-aramid is dissolved in NMP / DMF (volume ratio 4:1), and 5wt% chlorosulfonic acid is added and sulfonated at 60℃ for 2 hours to obtain a sulfonated aramid precursor solution (solid content 10-15%, viscosity 6000-8000 mPa·s); the sulfonated aramid solution is sprayed onto the surface of the porous polymer base film 1 facing the cathode 55 using a high-voltage electrospinning device (voltage 15 kV, receiving distance 15 cm, solution flow rate 0.5 mL / h) to form a three-dimensional nanofiber network with a diameter of 200-500 nm. After heating and curing, a sulfonated aramid nanofiber network 3 is formed with a thickness of 3 μm, a porosity of 30%, and a sulfonation degree of aramid nanofiber surface of 1.0 mmol / g. The aramid layer needs to undergo sulfonation treatment, which involves grafting sulfonic acid groups onto the aramid surface using reagents such as chlorosulfonic acid. This significantly increases the surface polarity, making it easier for the aramid to form hydrogen bonds with polar electrolytes (such as EC / DEC), thus accelerating electrolyte penetration. Example

[0020] This embodiment provides a lithium-ion battery that includes the separator described in Embodiment 1. Besides the separator, it also includes common battery components such as a positive electrode, a silicon negative electrode, electrolyte, and battery casing, which will not be elaborated further here. The use of this separator ensures close adhesion between the separator and the silicon negative electrode during cycling, resulting in more uniform lithium deposition on the negative electrode, suppressing dendrite formation, and improving the thermal stability and mechanical properties of the separator. This, in turn, enhances battery safety and extends battery life.

[0021] Comparative Example Using Example 3 as an experimental example, the difference between the lithium-ion battery in Example 3 and the lithium-ion battery in Example 4 is that the lithium-ion battery in Example 5 has a 3 μm aramid coating on both sides of the porous polymer film 1 of the separator. Three performance tests were performed on Example 3 and Example 4 respectively, namely capacity retention rate after 500 cycles, thermal shrinkage rate, and wetting time. The results are shown in Table 2.

[0022] The wettability of the membranes was investigated by dropping 20 μL of PC solution onto each membrane. The sulfonic acid groups (-SO3H) on the sulfonated aramid surface significantly increased the surface energy (~50-60 mN / m), forming hydrogen bonds with the electrolyte, resulting in a contact angle <15° and rapid electrolyte spread. The results showed that the membrane in Example 2, due to the strong hydrophilicity of the sulfonated aramid surface, achieved a 3-fold increase in wetting speed (5-10 seconds vs. 30-60 seconds). Although the spandex surface was hydrophobic (contact angle ~50°), the high permeability of the sulfonated aramid surface dominated overall wetting, allowing the electrolyte to diffuse rapidly through the PE substrate pores (porosity ~40%). In contrast, the comparative membrane with unsulfonated aramid on both sides (surface energy ~30-40 mN / m) resulted in a contact angle >60°, requiring the electrolyte to penetrate slowly via capillary action, leading to a longer wetting time.

[0023] The shrinkage rate of the diaphragm was tested by baking it at 150°C for 1 hour. In Example 2, the sulfonated aramid diaphragm's sulfonic acid groups are chemically bonded to the PE substrate (e.g., Si-OS bonds), and the high crystallinity (~30%) of the sulfonated layer inhibits high-temperature melt flow, resulting in a shrinkage rate ≤6%. The interfacial stability at high temperatures is also superior. In the comparative example, the pure aramid exhibits molecular chain slippage at 150°C, leading to shrinkage of the double-sided coating, with a shrinkage rate of 12.3%. The PE substrate also suffers from melt deformation due to a lack of constraint. Furthermore, the unsulfonated aramid only exhibits physical adsorption with the PE substrate, making it prone to delamination at high temperatures, which exacerbates uneven shrinkage.

[0024] Cells made with the two types of separators were cycled at 1C / 1C under RT conditions. After 500 cycles, Example 2 showed better capacity retention than the comparative example. The sulfonic acid groups (-SO3H) formed strong hydrogen bonds with the polar electrolyte solvent (such as EC / DEC), resulting in a contact angle <15°. This allowed for rapid electrolyte penetration, reducing polarization and maintaining electrode wettability, thus minimizing side reactions. Furthermore, the spandex surface provided an auxiliary effect. Although hydrophobic, the sulfonated surface dominated penetration. The PE pores (~40%) in the substrate absorbed liquid through capillary action, increasing the overall liquid absorption rate and maintaining long-term electrode wettability. Overall, Example 2 exhibited lower cycle decay and superior cycle performance due to its low polarization, high liquid retention capacity, reduced lithium dendrite growth and side reactions, and lower cycle decay rate. In contrast, the comparative example, with its double-sided unsulfonated aramid, had a low surface energy (~30-40 mN / m), a contact angle >60°, and slow electrolyte penetration, affecting the overall liquid absorption rate and ultimately leading to increased polarization. This made lithium dendrites more likely to puncture the separator, accelerating capacity decay.

[0025] Table 2 Comparison of performance parameters between experimental and comparative examples Example 2 89% 5.8% 5-10 seconds (rapid penetration of sulfonated surface) Comparative Example 68% 12.3% 30-60 seconds (slow penetration on both sides) The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A double-sided heterogeneous coating separator for silicon anode batteries, characterized in that, include: Porous polymeric thin films; A first coating facing the anode, the first coating being an spandex layer comprising a composite of a thermoplastic polyurethane elastomer and a fluoropolymer; A second coating facing the cathode, the second coating being an aramid layer comprising a sulfonated aramid nanofiber mesh; The porous polymer film has a thickness of 5~20μm, the first coating has a thickness of 4~8μm, the second coating has a thickness of 3~5μm, and the thickness of the first coating is greater than the thickness of the second coating. The porosity of the first coating is 45±3%, and the porosity of the second coating is 30±2%.

2. The double-sided heterogeneous coating separator for silicon anode batteries according to claim 1, characterized in that, The ratio of the thickness of the first coating to the thickness of the second coating is (1.2-1.5):

1.

3. A method for preparing a double-sided heterogeneous coating separator for a silicon anode battery as described in any one of claims 1 or 2, characterized in that, Includes the following steps: First, an spandex layer is coated on the side of the porous polymer film facing the anode. After drying, the film is immediately turned over, and an aramid layer is coated on the side of the porous polymer film facing the cathode using a high-voltage electrospinning device.

4. The method for preparing a double-sided heterogeneous coating separator for a silicon anode battery according to claim 3, characterized in that, The method for preparing the spandex layer is as follows: thermoplastic polyurethane elastomer and fluorinated polymer are mixed at a mass ratio of 95:5 and dissolved in a mixed solvent of N,N-dimethylformamide and acetone; the mixture is stirred at high speed until a homogeneous viscous slurry is formed.

5. The method for preparing a double-sided heterogeneous coating separator for a silicon anode battery according to claim 4, characterized in that, The fluorinated polymer is polytetrafluoroethylene or polyvinylidene fluoride.

6. The method for preparing a double-sided heterogeneous coating separator for a silicon anode battery according to claim 3, characterized in that, The aramid layer is prepared by dissolving meta-aramid in a mixed solution of N-methylpyrrolidone and N,N-dimethylformamide, wherein the volume ratio of N-methylpyrrolidone to N,N-dimethylformamide is 4:

1. 5 wt% chlorosulfonic acid is added and sulfonation is carried out at 60°C for 2 hours to obtain a sulfonated aramid precursor solution.

7. The method for preparing a double-sided heterogeneous coating separator for a silicon anode battery according to claim 3, characterized in that, The aramid layer is sulfonated, with the degree of sulfonation controlled at 0.8~1.2 mmol / g.

8. A lithium-ion battery comprising a double-sided heterogeneous coating separator for a silicon anode battery as described in claim 1.

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