Polyethylene diaphragm for high-wettability lithium battery and preparation method of polyethylene diaphragm

Abstract

The invention discloses a high-wettability polyethylene diaphragm for a lithium battery and a preparation method of the high-wettability polyethylene diaphragm, and relates to the technical field of polyethylene diaphragms for lithium batteries. The high-wettability polyethylene diaphragm for the lithium battery comprises the following components in parts by mass: 50-80 parts of a polyethylene base material, 5-20 parts of inorganic nanoparticles, 1-5 parts of a surfactant, 10-30 parts of a pore-forming agent, 0.5-3 parts of a coupling agent and 1-2 parts of a stabilizer. And through a dynamic coordination network of the stabilizer and a multi-component synergistic effect, the diaphragm-electrolyte interface characteristic is optimized, and more efficient lithium ion transmission is realized. The heteroatom bridging effect of the rigid stabilizer skeleton and the inorganic particles inhibits high-temperature shrinkage and stress deformation, and improves the reliability of the diaphragm under extreme working conditions. Uniform and intercommunicated micro-nano pores are formed through space cooperative regulation and control of the pore-forming agent and the stabilizer, mechanical strength and electrolyte infiltration consistency are considered, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of polyethylene diaphragms for lithium batteries, and in particular to a highly wettable polyethylene diaphragm for lithium batteries and a preparation method thereof. Background Art

[0002] With the booming development of the new energy industry, lithium batteries, as efficient and clean energy storage devices, are increasingly in demand in electric vehicles, energy storage systems, and consumer electronics. The performance of lithium batteries depends not only on the electrode materials but also on the performance of the separator. As a key component of lithium batteries, the separator primarily separates the positive and negative electrodes, prevents short circuits, and provides a pathway for lithium ions to travel.

[0003] Currently, polyethylene diaphragms are widely used due to their low cost, good mechanical properties and high chemical stability. However, the existing technology has some shortcomings. First, the wettability of polyethylene diaphragms is poor, which limits the transmission efficiency of lithium ions in the electrolyte, thereby affecting the charge and discharge speed and overall performance of the battery. Secondly, the thermal stability of the diaphragm needs to be improved. When the battery operates in a high temperature environment or an internal short circuit occurs, the diaphragm is prone to shrinkage, causing battery failure and even safety problems. In addition, the pore structure and size distribution of the diaphragm are not ideal, which not only affects the transmission consistency of lithium ions, but may also shorten the cycle life of the battery. In response to these problems, it is particularly urgent to develop a polyethylene diaphragm for lithium batteries with high wettability, high thermal stability and excellent pore structure. Summary of the Invention

[0004] The purpose of the present invention is to address the problems existing in the prior art and to provide a polyethylene separator for lithium batteries with high wettability, high thermal stability and excellent pore structure and a preparation method thereof.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a highly wettable polyethylene separator for lithium batteries, characterized by comprising the following components in parts by weight: 50-80 parts of a polyethylene substrate, 5-20 parts of inorganic nanoparticles, 1-5 parts of a surfactant, 10-30 parts of a pore former, 0.5-3 parts of a coupling agent, and 1-2 parts of a stabilizer; The stabilizer has a structure shown in Formula 1: Formula 1; The Z1 is selected from: -O-, -S-, -N(H)-, -C(CH3)(CH3)-.

[0006] Furthermore, the inorganic nanoparticles are at least one of silicon dioxide, aluminum oxide or barium titanate, and have a particle size of 20-100 nm.

[0007] Furthermore, the surfactant is fatty alcohol polyoxyethylene ether.

[0008] Furthermore, the polyethylene substrate is a mixture of polypropylene and high-density polyethylene; the density of the high-density polyethylene is: 0.941-0.960g / cm 3 .

[0009] Furthermore, the pore-forming agent is a mixture of polyethylene glycol and liquid paraffin, with a mass ratio of 1:(1-3) parts.

[0010] Furthermore, the coupling agent is trimethyl phosphate.

[0011] Furthermore, the stabilizer is a compound represented by the following structure: Stabilizer 1; Stabilizer 2; Stabilizer 3; Stabilizer 4.

[0012] Furthermore, the synthesis method of the stabilizer is: ; Step 1: Raw materials 1 and 2 are synthesized through substitution reaction to obtain intermediate 1; Step 2: Intermediate 1 is subjected to borylation to obtain intermediate 2; Step 3: intermediate 2 and raw material 3 are synthesized by Suzuki coupling reaction to obtain a stabilizer.

[0013] A method for preparing a highly wettable polyethylene separator for lithium batteries comprises the following steps: S1. The polyethylene substrate, inorganic nanoparticles, surfactant and pore-forming agent are melt-blended at 160-200 ° C to form a slurry; S2. The slurry is extruded and cast and then biaxially stretched, longitudinally stretched 3-5 times and transversely stretched 2-3 times to form a base film; S3. Soak the base film in a solvent at 60-80°C for 10-20 minutes, and then heat-set it at 90-100°C for 10-20 minutes to obtain a highly wettable polyethylene separator for lithium batteries.

[0014] Furthermore, the solvent in step S3 is at least one of xylene, acetone or isopropyl alcohol.

[0015] Furthermore, the heat setting treatment in step S3 adopts a step-by-step temperature increase method, first treating at 80-85° C. for 5-10 minutes, and then heating to 95-100° C. for 5-10 minutes.

[0016] The rigid planar structure of the stabilizer's parent core, Formula 1, is a polycyclic fused aromatic hydrocarbon skeleton. Its planar conjugated structure enhances thermal stability. The conjugated π-electron system of the fused aromatic hydrocarbon effectively disperses intramolecular thermal stress and blocks free radical chain reactions, ensuring dimensional stability of the separator during processing at 160-200°C and during high-temperature battery operation. The heteroatom bridging effect of the parent core forms a dynamic coordination network at the Z1 site of the parent core. Oxygen and sulfur atoms form hydrogen bonds with inorganic nanoparticles, enhancing nanoparticle dispersion uniformity. Nitrogen atoms generate dipole-dipole interactions with the trimethyl phosphate coupling agent, forming a three-dimensional crosslinked network that stabilizes porosity.

[0017] In a multi-component synergistic system, the functions of each component are highly complementary and dynamically coupled: the polyethylene substrate constructs a basic mechanical network through the intercalation of crystalline regions and the stabilizer's rigid backbone; inorganic nanoparticles form a nano-reinforced phase via heteroatom bridging in the stabilizer; surfactants and stabilizers synergistically reduce the polymer-electrolyte interfacial tension; the steric hindrance effects of the pore former and stabilizer jointly maintain the stability of the pore structure; and the coupling agent constructs thermally responsive crosslinking points through dynamic coordination with the stabilizer. This multi-layered synergy achieves simultaneous improvements in material strength, wettability, pore connectivity, and thermal stability, forming a functional enhancement pathway encompassing "structural stabilization, interface optimization, and pore regulation."

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. Significantly improved wettability and ion transfer efficiency: Through the dynamic coordination network of the stabilizer and the synergistic effect of multiple components, the diaphragm-electrolyte interface characteristics are optimized to achieve more efficient lithium ion transfer.

[0019] 2. Enhanced thermal and structural stability: The rigid stabilizer skeleton and the heteroatom bridging effect of inorganic particles inhibit high-temperature shrinkage and stress deformation, and improve the reliability of the diaphragm under extreme working conditions.

[0020] 3. Improved pore structure uniformity and connectivity: The spatial synergistic regulation of pore formers and stabilizers forms uniform and interconnected micro-nanopores, taking into account both mechanical strength and electrolyte infiltration consistency, thereby extending the battery cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The invention provides a method for synthesizing the stabilizer. DETAILED DESCRIPTION

[0022] The following will clearly and completely describe the technical solution of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0023] Synthesis example 1 Synthesis of stabilizer 1: ; In the first step, under a nitrogen atmosphere, 20 g of raw material 1 and 21.58 g of aluminum trichloride were added sequentially to the reaction system. 200 g of dichloromethane was then added, and 17.54 g of raw material 2 was slowly added dropwise at 0°C. The mixture was allowed to react at -20 to 10°C for 8 h. After completion of the reaction, the reaction solution was slowly poured into 500 ml of 0.1 mol / L HCl at 0°C and stirred for 30 min. The mixture was allowed to stand and separate, retaining the organic phase. The aqueous phase was washed three times with 50 ml of dichloromethane. The organic phases were combined, and 100 ml of 0.1 mol / L sodium bicarbonate solution was added. The mixture was stirred and shaken, and the pH was adjusted to neutral. The retained organic phase was dried over 20 g of anhydrous magnesium sulfate, filtered, rotary evaporated, and oven-dried to obtain 24.83 g of intermediate 1. MS [MS+1]: 390.

[0024] In the second step, under a nitrogen atmosphere, 24.83 g of intermediate 1 and 500 ml of ultra-dry tetrahydrofuran were added sequentially to the reaction system. The temperature was lowered to -70°C, and 4.31 g of n-butyl lithium was slowly added dropwise. After the addition was complete, the mixture was stirred for 1 hour. Then, 17.91 g of triisopropyl borate was added dropwise. After the addition was complete, the mixture was naturally warmed to room temperature and reacted overnight. The solvent was then evaporated to obtain 16.91 g of intermediate 2. MS [MS+1]: 357.

[0025] In the third step, under a nitrogen atmosphere, 16.91 g of intermediate 2, 17.93 g of raw material 3, and 19.69 g of anhydrous potassium carbonate were added to the reaction system in sequence. A mixed solution of toluene, ethanol, and water (volume ratio 2:1:1) was added, and the nitrogen atmosphere was replaced twice. Under nitrogen protection, 1.65 g of tetrakis(triphenylphosphine)palladium was added to the system, and the nitrogen atmosphere was replaced twice. The system was heated to 75°C and refluxed for 10 hours. The heat was turned off, the system was cooled to room temperature, and the liquid was allowed to stand for separation. The aqueous phase was extracted twice with ethyl acetate. The organic phases were combined, washed three times with water, and dried by spin drying. The system was then chromatographed using a mixture of petroleum ether and dichloromethane as the eluent to obtain 20.01 g of a stabilizer. MS [MS+1]: 589.

[0026] Synthesis Example 2-Synthesis Example 4 The compounds synthesized in Synthesis Examples 2 to 4 were prepared according to the method of Synthesis Example 1, except that the raw material 1 was replaced. The rest of the process remained the same as in Synthesis Example 1. The specific structure of raw material 1, stabilizer structure, and MS [MS+1] data are shown in the table below.

[0027]

[0028] Example 1 The preparation of a highly wettable polyethylene separator for lithium batteries comprises the following steps: Raw material ratio (mass parts): polyethylene substrate: 60 parts of high-density polyethylene and 20 parts of polypropylene, a total of 80 parts; inorganic nanoparticles: 15 parts of silicon dioxide; surfactant: 3 parts of fatty alcohol polyoxyethylene ether; pore-forming agent: 10 parts of polyethylene glycol and 20 parts of liquid paraffin; coupling agent: 2 parts of trimethyl phosphate; stabilizer: 1.5 parts of the stabilizer prepared in Synthesis Example 1.

[0029] The preparation method comprises the following steps: S1. Add polyethylene substrate, inorganic nanoparticles, surfactant, and pore-forming agent into a twin-screw extruder and melt-blend at 180°C for 30 minutes to form a uniform slurry.

[0030] S2. The slurry was extruded through a slot die to form a film at a casting temperature of 185° C. The base film was biaxially stretched: 4 times in the longitudinal direction and 2.5 times in the transverse direction to form a base film with a thickness of 20 μm.

[0031] S3. Immerse the substrate film in 70°C isopropyl alcohol for 15 minutes to remove any residual porogen. Heat setting treatment: First, heat at 83°C for 8 minutes, then heat to 98°C for 10 minutes to obtain a highly wettable polyethylene separator for lithium batteries.

[0032] Example 2-Example 4 Referring to the preparation method in Example 1, the stabilizers therein were replaced with the stabilizers synthesized in Synthesis Examples 2 to 4 in sequence, and the rest remained unchanged.

[0033] Comparative Example 1 Refer to the preparation method in Example 1, except that the stabilizer is replaced by comparative compound 1, and the rest remain unchanged.

[0034] Comparative compound 1: .

[0035] Comparative Example 2 Refer to the preparation method in Example 1, except that the stabilizer is replaced by comparative compound 2, and the rest remain unchanged.

[0036] Comparative compound 2: .

[0037] Comparative Example 3 Refer to the preparation method in Example 1, without adding the stabilizer, and keep the rest unchanged.

[0038] Comparative Example 4 Referring to the preparation method in Example 1, the raw material ratio (mass parts) was replaced as follows: polyethylene substrate: 65 parts of high-density polyethylene and 20 parts of polypropylene, a total of 85 parts; inorganic nanoparticles: 20 parts of silica; surfactant: 3 parts of fatty alcohol polyoxyethylene ether; pore-forming agent: 10 parts of polyethylene glycol and 20 parts of liquid paraffin; coupling agent: 2 parts of trimethyl phosphate; stabilizer: 1.5 parts of the stabilizer prepared in Synthesis Example 1, and the rest remained unchanged.

[0039] Performance testing: 1. Porosity Test: The diaphragm porosity is determined using the mercury intrusion method (GB / T21650.1-2008). The sample is placed in a mercury intrusion instrument at a pressure range of 0.1-400 MPa, and the percentage of pore volume to total volume is calculated.

[0040] 2. Liquid absorption rate test: Cut the diaphragm into 5cm×5cm samples and soak them in 1mol / LLiPF6 / EC:DMC (volume ratio 1:1) electrolyte for 30 minutes. After taking it out, wipe off the surface liquid and calculate the liquid absorption rate: Liquid absorption rate (%) = [(W2-W1) / W1]×100%, where W1 is the dry film mass and W2 is the wet film mass.

[0041] 3. Ionic conductivity test: The separators were assembled into a symmetrical cell (stainless steel electrodes) and the ionic conductivity was measured using electrochemical impedance spectroscopy (EIS) in the frequency range of 0.1 Hz to 100 kHz. The calculation formula is: σ = d / (R × A) (d is the separator thickness, R is the bulk resistance, and A is the electrode area).

[0042]

[0043] The examples as a whole were significantly superior to the comparative examples. The stabilizers containing specific heteroatom structures (Examples 1-4) showed simultaneous improvements in pore connectivity, electrolyte wettability, and ion transport efficiency, with performance varying gradually with heteroatom type and steric hindrance in the stabilizer molecules. In the comparative examples, the absence or structural changes of the stabilizer (Comparative Examples 1-3) led to significant decreases in porosity, liquid absorption, and conductivity, while an imbalance in the inorganic-substrate ratio (Comparative Example 4) further exacerbated performance degradation, demonstrating that the multi-component synergistic effect and the stabilizer's dynamic coordination network play a decisive role in the performance of the separator.

[0044] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A highly wettable polyethylene separator for lithium batteries, characterized in that: The invention comprises the following components in parts by weight: 50-80 parts of polyethylene substrate, 5-20 parts of inorganic nanoparticles, 1-5 parts of surfactant, 10-30 parts of pore-forming agent, 0.5-3 parts of coupling agent, and 1-2 parts of stabilizer; The stabilizer has a structure shown in Formula 1: Formula 1; The Z1 is selected from: -O-, -S-, -N(H)-, -C(CH3)(CH3)-.

2. A highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The inorganic nanoparticles are at least one of silicon dioxide, aluminum oxide or barium titanate, and have a particle size of 20-100 nm.

3. A highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The surfactant is fatty alcohol polyoxyethylene ether.

4. A highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The polyethylene substrate is a mixture of polypropylene and high-density polyethylene; the density of the high-density polyethylene is: 0.941-0.960g / cm 3 .

5. The highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The pore-forming agent is a mixture of polyethylene glycol and liquid paraffin, with a mass ratio of 1:(1-3) parts.

6. A highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The coupling agent is trimethyl phosphate.

7. A highly wettable polyethylene separator for lithium batteries according to claim 1, characterized in that: The stabilizer is a compound shown in the following structure: Stabilizer 1; Stabilizer 2; Stabilizer 3; Stabilizer 4.

8. A method for preparing a highly wettable polyethylene separator for lithium batteries according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. The polyethylene substrate, inorganic nanoparticles, surfactant and pore-forming agent are melt-blended at 160-200 ° C to form a slurry; S2. The slurry is extruded and cast and then biaxially stretched, longitudinally stretched 3-5 times and transversely stretched 2-3 times to form a base film; S3. Soak the base film in a solvent at 60-80°C for 10-20 minutes, and then heat-set it at 90-100°C for 10-20 minutes to obtain a highly wettable polyethylene separator for lithium batteries.

9. The method for preparing a highly wettable polyethylene separator for lithium batteries according to claim 8, characterized in that: The solvent in step S3 is at least one of xylene, acetone or isopropyl alcohol.

10. The method for preparing a highly wettable polyethylene separator for lithium batteries according to claim 8, characterized in that: The heat setting treatment in step S3 adopts a step-by-step temperature increase method, first treating at 80-85° C. for 5-10 minutes, and then heating to 95-100° C. for 5-10 minutes.

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