Lithium ion battery and electric device comprising same

Abstract

The invention provides a lithium ion battery and an electric device comprising the same, and relates to the technical field of lithium ion batteries. The diaphragm of the lithium ion battery is formed by laminating a polar functional layer, a supporting layer and a thermal closing layer, wherein the polar functional layer is prepared from aromatic polyamide nanofibers, and the aromatic polyamide nanofibers have high polarity and a nanopore structure, so that the wettability and ion transmission capability of the diaphragm to electrolyte can be remarkably improved; meanwhile, the lithium ion battery disclosed by the invention selects a non-aqueous organic solution which is low in surface tension and contains bis (trifluoromethanesulfonyl) imide lithium as an electrolyte, and the wettability and the ionic conductivity of the electrolyte in the aramid nanofiber polar functional layer disclosed by the invention can be remarkably improved through verification, so that a diaphragm-electrolyte interface synergistic effect is formed. Therefore, by optimizing the diaphragm structure and the electrolyte system, the safety performance, the rate capability and the cycle life of the lithium battery can be remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a lithium-ion battery and an electrical device comprising the same. Background Technology

[0002] Power tools and other electrical devices have strict requirements for the high-rate discharge performance and long cycle life of lithium-ion batteries, while also requiring excellent thermal safety to avoid overheating and thermal runaway accidents.

[0003] Traditional polyolefin (PE / PP) microporous membranes, while having low manufacturing costs and good chemical stability, suffer from poor electrolyte wettability and low thermal stability. They are prone to pore melting and closure or thermal shrinkage at high temperatures, leading to internal short circuits and thermal runaway.

[0004] Therefore, researching and developing a new type of lithium-ion battery to improve the hydrophilicity and high-temperature stability of the separator is key to enhancing battery safety and rate performance.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The primary objective of this invention is to provide a lithium-ion battery that, through the selection of the separator structure and the synergistic optimization of the electrolyte system, effectively alleviates the problems of poor electrolyte wettability, low thermal stability, and easy pore melting and closure or thermal shrinkage at high temperatures in existing traditional polyolefin (PE / PP) microporous separators, which can lead to internal short circuits and thermal runaway.

[0007] A second objective of the present invention is to provide an electrical device comprising the aforementioned lithium-ion battery.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0009] This invention provides a lithium-ion battery, comprising a casing, a core located within the casing, and an electrolyte. The core is formed by winding a positive electrode, a negative electrode, and a separator between them.

[0010] The diaphragm is composed of a polar functional layer, a support layer and a heat-closing layer, wherein the polar functional layer is made of aramid nanofibers (ANF).

[0011] The electrolyte is a non-aqueous organic solution containing lithium bis(trifluoromethanesulfonyl)imide.

[0012] Note: The abbreviation for lithium bis(trifluoromethanesulfonyl)imide is LiTFSI.

[0013] Furthermore, the thickness of the polar functional layer is 2–7 μm, the radial average diameter of the aromatic polyamide nanofibers is 20–200 nm, and the aromatic polyamide nanofibers contain polar functional groups such as amino or amide groups.

[0014] Furthermore, the thickness of the support layer of the diaphragm is 3–8 μm;

[0015] The support layer is polyethylene terephthalate (PET) nonwoven fabric or polyethylene terephthalate (PET) film.

[0016] Furthermore, the thickness of the heat-sealing layer of the diaphragm is 2–9 μm;

[0017] The heat-sealing layer is composed of low-density polyethylene (LDPE) or a blend containing low-density polyethylene (LDPE);

[0018] The melting point of the low-density polyethylene is 110–125°C.

[0019] Furthermore, the solvent in the electrolyte is a mixed solvent of dimethyl ether and 1,3-dioxolane;

[0020] Preferably, the volume ratio of the dimethyl ether to 1,3-dioxolane is 1-3:1-3;

[0021] Preferably, the concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 0.5–2.0 mol / L.

[0022] Furthermore, the method for preparing the diaphragm includes the following steps:

[0023] (A) Provide polyethylene terephthalate (PET) nonwoven fabric or film as a support layer, and then use electrospinning to deposit aromatic polyamide nanofibers on the support layer, and dry to obtain membrane intermediate A;

[0024] (B) On the other side of the membrane on which aromatic polyamide nanofibers are deposited, a low-density polyethylene emulsion is coated, pressed, and dried to obtain membrane intermediate B.

[0025] (C) The membrane intermediate B is hot-pressed to promote the bonding of the multilayer structure, and then vacuum dried to obtain the membrane.

[0026] Furthermore, in step (B), the low-density polyethylene emulsion contains 5 to 20 wt% of the total weight of the coating.

[0027] The present invention provides an electrical device comprising the aforementioned lithium-ion battery.

[0028] The present invention provides an electrical device comprising the aforementioned lithium-ion battery, which can be used to supply power to the electrical device.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] The lithium-ion battery provided by this invention has a separator composed of a polar functional layer, a support layer, and a thermal shut-off layer stacked together. The polar functional layer is made of aromatic polyamide nanofibers, which have high polarity and nanoporous structure, and can significantly improve the wettability of the separator to the electrolyte and the ion transport capability. This effectively alleviates the problems of poor electrolyte wettability and low thermal stability of existing traditional polyolefin (PE / PP) microporous separators, which are prone to pore melting and closure or thermal shrinkage at high temperatures, leading to internal short circuits and thermal runaway.

[0031] Meanwhile, this application uses a low-surface-tension non-aqueous organic solution containing lithium bis(trifluoromethanesulfonyl)imide as the electrolyte for its lithium-ion battery. Verification has shown that this significantly improves the wettability and ionic conductivity of the electrolyte within the polar functional layer composed of aromatic polyamide nanofibers, creating a synergistic effect at the separator-electrolyte interface. Through the optimization of the separator structure and electrolyte system described above, this invention can significantly improve the safety performance, rate performance, and cycle life of lithium batteries.

[0032] The present invention provides an electrical device comprising the aforementioned lithium-ion battery, which can be used to supply power to the electrical device. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] According to one aspect of the present invention, a lithium-ion battery includes a housing, a core located within the housing, and an electrolyte, said core being formed by winding a positive electrode, a negative electrode, and a separator between the two, wherein:

[0035] The diaphragm is composed of a polar functional layer, a support layer and a heat-closing layer, wherein the polar functional layer is made of aramid nanofibers (ANF).

[0036] The electrolyte is a non-aqueous organic solution containing lithium bis(trifluoromethanesulfonyl)imide.

[0037] The lithium-ion battery provided by this invention has a separator composed of a polar functional layer, a support layer, and a thermal shut-off layer stacked together. The polar functional layer is made of aromatic polyamide nanofibers, which have high polarity and nanoporous structure, and can significantly improve the wettability of the separator to the electrolyte and the ion transport capability. This effectively alleviates the problems of poor electrolyte wettability and low thermal stability of existing traditional polyolefin (PE / PP) microporous separators, which are prone to pore melting and closure or thermal shrinkage at high temperatures, leading to internal short circuits and thermal runaway.

[0038] Meanwhile, this application uses a low-surface-tension non-aqueous organic solution containing lithium bis(trifluoromethanesulfonyl)imide as the electrolyte for its lithium-ion battery. Verification has shown that this significantly improves the wettability and ionic conductivity of the electrolyte within the polar functional layer composed of aromatic polyamide nanofibers, forming a synergistic effect at the separator-electrolyte interface. Through the above structural and system optimizations, this invention can significantly improve the safety performance, rate performance, and cycle life of lithium batteries.

[0039] In a preferred embodiment of the present invention, the thickness of the polar functional layer is 2-7 μm, the radial average diameter of the aromatic polyamide nanofiber is 20-200 nm, and the aromatic polyamide nanofiber contains polar functional groups such as amino or amide groups.

[0040] In a preferred embodiment of the present invention, the thickness of the support layer of the diaphragm is 3-8 μm; the support layer is polyethylene terephthalate (PET) nonwoven fabric or polyethylene terephthalate (PET) film.

[0041] As a preferred embodiment, the polyethylene terephthalate (PET) support layer provides mechanical strength and morphological support.

[0042] In a preferred embodiment of the present invention, the thickness of the heat-sealing layer of the diaphragm is 2 to 9 μm;

[0043] The heat-sealing layer is composed of low-density polyethylene (LDPE) or a blend containing low-density polyethylene (LDPE), wherein the low-density polyethylene has a melting point of 110–125°C.

[0044] In a preferred embodiment, the melting point of the low-density polyethylene (LDPE) heat-closing layer is approximately between 110 and 125°C. When the temperature reaches the melting point, it melts and closes the pores, thereby automatically cutting off the ion pathway. This allows the membrane closure temperature range of the diaphragm in this application to be 110–120°C; the membrane rupture temperature of the diaphragm is ≥195°C.

[0045] In a preferred embodiment of the present invention, the solvent in the electrolyte is a mixed solvent of dimethyl ether and 1,3-dioxolane;

[0046] Preferably, the volume ratio of the dimethyl ether to 1,3-dioxolane is 1-3:1-3;

[0047] In the preferred embodiment described above, the concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 0.5–2.0 mol / L.

[0048] In a preferred embodiment of the present invention, the method for preparing the diaphragm includes the following steps:

[0049] (A) Provide polyethylene terephthalate (PET) nonwoven fabric or film as a support layer, and then use electrospinning to deposit aromatic polyamide nanofibers on the support layer, and dry to obtain membrane intermediate A;

[0050] (B) On the other side of the membrane on which aromatic polyamide nanofibers are deposited, a low-density polyethylene emulsion is coated, pressed, and dried to obtain membrane intermediate B.

[0051] (C) The membrane intermediate B is hot-pressed to promote the bonding of the multilayer structure, and then vacuum dried to obtain the membrane.

[0052] The method for preparing the separator provided by this invention firstly involves electrospinning to deposit aromatic polyamide nanofibers onto a PET support layer; subsequently, a low-density polyethylene emulsion is coated onto the other side of the separator with deposited aromatic polyamide nanofibers, and then pressed and dried; finally, the pressed separator is immersed in a non-aqueous organic electrolyte containing lithium bis(trifluoromethanesulfonyl)imide to obtain the separator. In this method, the TFSI- anion exhibits surfactant-like properties in the electrolyte, reducing the surface tension of the electrolyte. This helps improve the wettability and ionic conductivity of the electrolyte in the polar functional layer, forming a synergistic effect at the separator-electrolyte interface.

[0053] In the preferred embodiment described above, the low-density polyethylene content in the low-density polyethylene emulsion in step (B) accounts for 5 to 20 wt% of the total weight of the coating.

[0054] In a preferred embodiment, the contact angle of the separator in the electrolyte during battery assembly is ≤22.5°. Excellent wettability ensures rapid penetration and full wetting of the electrolyte at the separator-electrode interface, avoiding localized electrolyte shortages caused by poor wetting. The contact angle setting described in this application directly reduces the separator-electrode interface impedance, increases the migration rate of lithium ions at the interface, and is beneficial for improving rate performance and reducing polarization during high-power discharge.

[0055] Preferably, the positive electrode active material in the lithium-ion battery includes lithium nickel cobalt manganese oxide (Li1Ni). x Co y Mnz M b O2) where 0.70≤x≤0.95, 0.15≤y<0.45, 0.05≤z<0.45, 0.0≤b≤0.25, x+y+z+b=1, and element M includes one or more of zirconium (Zr), tungsten (W), titanium (Ti), aluminum (Al), strontium (Sr), boron (B) and neodymium (Nd).

[0056] Preferably, the active material of the negative electrode sheet includes at least one or more of artificial graphite, natural graphite, soft carbon, or hard carbon.

[0057] According to one aspect of the present invention, an electrical device includes the aforementioned lithium-ion battery.

[0058] The present invention provides an electrical device comprising the aforementioned lithium-ion battery, which can be used to supply power to the electrical device.

[0059] The technical solution of the present invention will be further described below with reference to the embodiments.

[0060] Example 1

[0061] A lithium-ion battery, wherein the method for preparing the lithium-ion battery includes:

[0062] (1) Method for manufacturing positive electrode plates:

[0063] Take positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2 (NMC811), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an N-methylpyrrolidone solvent system at a mass ratio of 96:1:1:2. Next, the positive electrode coating material was uniformly coated onto a 12.0 μm thick aluminum foil. After the electrode was dried, cold-pressed, slit, and cut, the positive electrode sheet was obtained with a compaction density of 3.5 g / cm³. 3 .

[0064] (2) Method for manufacturing the negative electrode:

[0065] A negative electrode coating material was formed by mixing graphite (the negative electrode active material), carbon nanotubes, sodium carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene rubber (SBR) as a binder in deionized water at a mass percentage of 96:1.5:1.0:1.5. The coating material was then coated onto a 15 μm thick copper foil. After drying, cold pressing, slitting, and cutting, the negative electrode sheet was obtained with a compaction density of 1.55 g / cm³. 3 .

[0066] (3) Preparation of electrolyte:

[0067] Electrode solution was prepared by dissolving LiTFSI lithium salt in a solvent containing a mixture of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) at a volume ratio of 0.5:0.5, with a lithium salt concentration of 1.0 mol / L. 2 wt% of fluoroethylene carbonate (FEC) and 2 wt% of vinylene carbonate (VC) were added to enhance the stability of the SEI film.

[0068] (4) Preparation of composite membrane:

[0069] 1. Aramid fibers (diameter 20.5 nm) were dissolved in a DMSO / water (v / v = 8:2) system at a concentration of 5 wt%. Potassium hydroxide and chloroform were added to obtain an ANF dispersion. The dispersion was stirred at 80 °C for 24 hours to allow it to swell and decompose. After removing impurities through a 0.5 μm filter membrane, the dispersion was ready for use.

[0070] 2. The first layer of ANF fiber membrane was deposited on a 5μm thick PET membrane by electrospinning (voltage: 15kV, collection distance: 15cm, flow rate: 0.5mL / h), with the thickness controlled to be about 5μm.

[0071] After completion, place in an 80℃ vacuum oven to dry for 12 hours.

[0072] 3. Apply LDPE emulsion to the other side of the PET separator by scraping, and control the coating amount to make the LDPE mass ratio 10wt% to obtain the heat-sealing layer;

[0073] Finally, a hot-pressing treatment at 100℃ is applied for 5 minutes at a pressure of 2MPa to promote the bonding of the multilayer structure. Subsequently, the residue is removed by vacuum drying at 60℃ for 24 hours.

[0074] (5) Assembly of lithium-ion batteries:

[0075] The positive and negative electrode sheets are rolled and slit respectively, and then wound together with the separator to obtain a 21700 cylindrical battery core. The battery core is then welded to the connecting piece and installed into the battery casing. After completing the liquid injection, sealing and formation processes, the lithium-ion battery of Example 1 is obtained. The casing of the lithium-ion battery is cylindrical with the following dimensions: diameter: 21.0 mm, length: 70.0 mm.

[0076] Examples 2-5

[0077] The difference between this embodiment and Embodiment 1 is that the thickness of the polar functional layer and the support layer are different from those in Embodiment 1. See Table 1 for details.

[0078] Table 1:

[0079] Group Polar functional layer thickness (μm) Support layer thickness (μm) Example 2 2 8 Example 3 3 7 Example 4 6 4 Example 5 7 3

[0080] Examples 6-9

[0081] The difference between this embodiment and Embodiment 1 is that the diameter of the aromatic polyamide nanofibers (ANF) added to the polar functional layer (ANF layer) is different from that in Embodiment 1, as detailed in Table 2.

[0082] Table 2:

[0083] Group ANF ​​diameter (nm) Example 6 5.8 Example 7 10.2 Example 8 56.2 Example 9 108.5

[0084] Examples 10-13

[0085] The difference between this embodiment and embodiment 1 is that the mass ratio of low-density polyethylene (LDPE) coated on the PET diaphragm in steps (4) and 3 is different from that in embodiment 1, but the rest is the same as in embodiment 1. See Table 3 for details.

[0086] Table 3:

[0087]

[0088] Examples 14-17

[0089] The difference between this embodiment and embodiment 1 is that, except for the volume ratio of DME and DOL in the electrolyte in step (3) being different from that in embodiment 1, the rest is the same as in embodiment 1. See Table 4 for details.

[0090] Table 4:

[0091] Group Volume ratio of DME to DOL in the electrolyte Example 14 1：0 Example 15 0.75：0.25 Example 16 0.25：0.75 Example 17 0：1

[0092] Example 18

[0093] The difference between this embodiment and Example 1 is that the concentration of LiTFSI in the electrolyte is 0.5 mol / L, while all other aspects are the same as in Example 1.

[0094] Example 19

[0095] The difference between this embodiment and Example 1 is that the concentration of LiTFSI in the electrolyte is 1.5 mol / L, while all other aspects are the same as in Example 1.

[0096] Comparative Example 1

[0097] The difference between this embodiment and Embodiment 1 is that the ANF layer is not composited; otherwise, they are the same as in Embodiment 1.

[0098] Comparative Example 2

[0099] The difference between this embodiment and Embodiment 1 is that the LDPE layer is not coated; otherwise, they are the same as in Embodiment 1.

[0100] Experimental Example 1

[0101] To verify the technical effects of the lithium-ion battery after the selection of the separator structure and the synergistic optimization of the electrolyte system in this application, this application conducted experimental verification on the lithium-ion batteries prepared in Examples 1 to 19 and Comparative Examples 1 and 2.

[0102] (I) The specific testing methods are as follows:

[0103] (1) Methods for treating the diaphragm:

[0104] First, discharge the lithium-ion battery to a constant current of 2.5V to ensure it is in a safe state, reducing the risk of short circuits or thermal runaway during disassembly. Carefully disassemble the battery within a glove box (protected by argon or other inert atmosphere) and remove the separator from the cylindrical cell. Immerse the removed separator in anhydrous dimethyl carbonate (DMC) solution for 30 minutes to dissolve and remove residual electrolyte and possible byproducts. Then, gently wipe the surface with lint-free paper, replacing the DMC solution and repeating the immersion-wiping process three times to ensure no residual contaminants remain on the separator surface. Finally, rinse the separator with anhydrous ethanol, wipe it again, and place it in the glove box for 48 hours to ensure it is completely dry, preventing interference from solvent residues in subsequent tests.

[0105] (2) Method for determining the average diameter of ANF:

[0106] After drying, the polar functional layer, support layer, and heat-sealing layer of the membrane are gently peeled off using a plastic scraper or blade (or by solvent-selective peeling). The polar functional layer ANF is then cut into small pieces, placed in a test tube, and sonicated with ethanol for 30 minutes. The middle membrane layer is then removed, and the remaining solution is centrifuged (10,000 rpm for 10 minutes). The supernatant is discarded, and the powder is redispersed with anhydrous ethanol, sonicated again for 10 minutes, and then centrifuged again. This process is repeated three times to ensure the purity of the powder sample. High-resolution images of the ANF (elongated linear nanowires) are then obtained using transmission electron microscopy (TEM). The diameters of the elongated linear nanowires are counted using image analysis software (≥100), and the average diameter of the ANF is calculated.

[0107] (3) Method for measuring diaphragm contact angle:

[0108] A micro-electrolyte solution (DME:DOL = 1:1) was dropped onto the treated and cleaned diaphragm surface using a micro-syringe. A side view image of the droplet was taken, and the droplet profile was fitted using software (such as the Young-Laplace equation or ellipse fitting method). The contact angle was calculated after the baseline was automatically detected.

[0109] Methods for determining ionic conductivity:

[0110] 1. Sample preparation: Immerse the prepared three-layer composite membrane in an electrolyte (such as 1M LiPF6, EC / DEC = 1:1, v / v) for at least 12 hours to ensure adequate wetting.

[0111] 2. Assemble a symmetrical cell: In an inert atmosphere (such as a glove box), two stainless steel electrodes (SS) are sandwiched with a separator that has been wetted with electrolyte to form an SS|separator|SS symmetrical structure, which is an impedance testing device with "no active electrode".

[0112] 3. Test equipment and parameters: Use an electrochemical workstation to perform AC impedance testing (EIS). The test conditions are as follows: frequency range: 1MHz~0.1Hz, AC disturbance voltage: 5~10mV, test temperature: 25℃ (temperature can be controlled by a constant temperature chamber).

[0113] Formula for calculating electrical conductivity:

[0114] Where: σ is the ionic conductivity (S / cm), L is the membrane thickness (cm), R is the high-frequency semicircular intercept in the Nyquist plot (Ω), and A is the effective area of ​​the peripheral electrode (cm²). 2 ).

[0115] (4) Method for determining the thermal shrinkage rate of the diaphragm:

[0116] Cut the diaphragm into rectangular samples of 10mm × 100mm. Lay the samples flat between A4 sheets of paper to prevent high-temperature curling. Place them in an oven with a temperature control accuracy of ±1℃, set the temperature to 150℃, heat for 1 hour without external force, and then cool to room temperature. Use a vernier caliper (accuracy 0.01mm) or a laser rangefinder to measure the initial length L0 (length direction) and the length L1 after heating, respectively. Calculate the shrinkage rate according to the formula (L0-L1) / L0×100%. Three samples should be tested in each direction and the average value should be taken. If the deviation exceeds 5%, the test should be repeated.

[0117] (5) Method for determining the diaphragm closure temperature:

[0118] A batch of membrane samples to be tested were placed in a constant temperature programmable oven with a temperature control accuracy of ±1℃. The initial temperature was set to 100℃ and the target temperature to 125℃, and the temperature was linearly programmed to rise at a rate of 1℃ / min. After each 1℃ increase, the temperature was maintained for 15 minutes to allow the membrane to reach thermal equilibrium. The membrane was then quickly removed and cooled to room temperature. Under an inert atmosphere, the ionic conductivity facing the electrolyte was measured using a symmetrical electrolyte fixture without electrodes. The conductivity trends at different temperatures were recorded. When a significant drop in ionic conductivity was observed (e.g., a drop greater than 50%), it was considered an "ion blocking effect" caused by pore closure or structural collapse of the membrane, and the corresponding temperature was defined as the pore closure temperature of the membrane.

[0119] (6) Method for determining the failure temperature (film rupture temperature) of the hot box:

[0120] The lithium-ion battery was placed in a 25°C constant temperature chamber for 4 hours and charged to 4.2V at a constant current and constant voltage of 1C, with a cutoff current of 0.01C. It was then allowed to stand for 10 minutes, followed by a temperature increase of 5°C / min, with a holding time of 10 minutes after each 5°C increase. The surface temperature of the lithium battery was monitored during the heating process. The oven temperature at which the temperature began to rise sharply was the oven failure temperature of the secondary battery.

[0121] (7) 10C discharge capacity retention test method:

[0122] Place the battery in a 25°C constant temperature chamber for 4 hours and test it according to the following steps:

[0123] 1. Charge to 4.2V under constant current and constant voltage conditions at 0.2C, cut off current at 0.1C, and let stand for 30 minutes;

[0124] 2. Discharge under constant current at 1C until 2.5V cutoff, with the capacitance value as Q1, and let stand for 30 minutes;

[0125] 3. Charge to 4.2V under constant current and constant voltage conditions at 0.2C, cut off current at 0.1C, and let stand for 30 minutes;

[0126] 4. Discharge at a constant current of 10C until the cutoff voltage is 2.5V, with the capacitance value being Q2, and let stand for 30 minutes; the capacitance retention rate at 10C is calculated as: Q2 / Q1×100.

[0127] (II) The specific test results are as follows:

[0128] (1) Examples 1 to 5 are the results of this application on the thickness of the ANF layer and the PET layer. See Table 5 for the specific results.

[0129] Table 5:

[0130]

[0131] As shown in Table 5, comparing Examples 1 to 5, it can be seen that when the total thickness is kept at 10 μm, adjusting the thickness ratio of ANF to PET layers has a significant impact on the membrane performance.

[0132] As the ANF ratio increases from 2:8 to 5:5, the contact angle decreases, indicating that the increased polar layer thickness enhances wettability; thermal stability is improved, and the film breakage temperature rises; ionic conductivity and 10C rate capacity retention are improved, demonstrating better high-rate adaptability, but with little impact on the film breakage temperature. However, further increasing the ANF ratio to 7:3 and 8:2 actually makes the structure more fragile. Insufficient PET support leads to a decrease in mechanical and thermal stability. At the same time, excessive ANF thickness also increases internal resistance, slightly reduces ionic conductivity, and decreases rate performance, exhibiting a significant performance peak effect.

[0133] (2) Examples 1, 6 to 9 are the results of this application on the diameter of ANF. See Table 6 for the specific results.

[0134] Table 6:

[0135]

[0136]

[0137] As shown in Table 6, comparing Examples 1, 6 to 9, it can be seen that as the diameter of the ANF nanofibers increases from 5.8 nm to 20.5 nm, the absorbency, ionic conductivity, and thermal stability of the membrane continuously improve, and the 10C discharge capacity retention rate increases from 67% to a maximum of 84%. This is because finer ANF fibers can construct a more uniform and dense three-dimensional nanoporous structure, enhancing the interaction between polar surface groups and the electrolyte, improving wettability and ion migration efficiency, while also enhancing mechanical support and thermal dimensional stability. However, when the ANF diameter further increases to above 52.6 nm, the excessively thick fibers lead to uneven pore size distribution and discontinuous connecting channels, limiting the effective penetration of electrolyte and ion conduction. Simultaneously, insufficient structural rigidity at high temperatures makes the membrane prone to thermal shrinkage and rupture, resulting in a significant decrease in the membrane's rate performance and thermal safety.

[0138] (3) Examples 1, 10 to 13 are the results of this application on the percentage of LDPE mass. See Table 7 for the specific results.

[0139] Table 7:

[0140]

[0141] As shown in Table 7, comparing Examples 10 to 13, it can be seen that as the LDPE coating ratio increases from 5% to 10%, the structural integrity and thermal response accuracy of the membrane under high-temperature conditions are significantly optimized. At this stage, the LDPE layer thickness is moderate, enabling it to rapidly seal pores near its melting point (~130°C), effectively cutting off ion pathways and achieving thermal shutdown, while maintaining low electrolyte resistance and good pore connectivity. The PET and ANF main structure achieve optimal coverage and fit at this ratio, exhibiting a high hot box rupture temperature, minimal thermal shrinkage, and maximized ionic conductivity and 10C rate capacity retention (up to 84%). However, when the LDPE ratio further increases to above 15%, the excessively thick coating overly masks the polar ANF layer surface, significantly inhibiting electrolyte wetting and penetration, leading to increased contact angle, contraction of ion migration channels, decreased conductivity, and intensified interfacial polarization. Meanwhile, due to the poor thermomechanical stability of LDPE, it is more prone to structural inhomogeneity and stress concentration after melting, resulting in an increase in the closed-cell temperature and a decrease in the hot box rupture temperature. The dual degradation of electrochemical kinetics and thermal stability leads to a significant decline in 10C rate discharge performance, verifying the existence of an optimal range for LDPE content.

[0142] (4) Examples 1, 14 to 17 are the results of this application on the volume ratio of electrode liquid DME and DOL. The specific results are shown in Table 8.

[0143] Table 8:

[0144] Group Ionic conductivity (mS / cm) 10C discharge capacity retention Example 1 0.62 84 Example 14 0.55 75.5 Example 15 0.59 80.9 Example 16 0.53 72.8 Example 17 0.5 68.7

[0145] As shown in Table 8, comparing Examples 14 to 17 of Example 1, it can be seen that the ratio of DME to DOL directly affects the physicochemical properties and interfacial kinetics of the electrolyte. As the DME content gradually transitions from 100% (1:0) to an equal ratio (0.5:0.5), the wettability of the electrolyte to the polar ANF layer increases, which is beneficial for the rapid migration of lithium ions. Therefore, the ionic conductivity and 10C discharge capacity retention gradually improve. However, when the DOL ratio continues to increase and reaches 0:1, the electrolyte viscosity increases and the polarity decreases, leading to a decrease in diffusion efficiency, an increase in interfacial resistance, and a significant decrease in ionic conductivity and 10C discharge capacity retention.

[0146] (5) Examples 1, 18-19 are the results of this application on the concentration of LiTFSI lithium salt. The specific results are shown in Table 9.

[0147] Table 9:

[0148]

[0149] As shown in Table 9, comparing Examples 18 and 19 of Example 1, it is evident that the LiTFSI concentration has a significant impact on the ion transport capacity and interfacial kinetics of the electrolyte. As the concentration increases from 0.5 mol / L to 1.0 mol / L, the number of migratable lithium ions in the solution increases, and the ion transport number and carrier concentration per unit volume increase simultaneously. This leads to increased electrolyte conductivity and decreased polarization, thereby improving the battery's high-rate response capability and significantly increasing the 10C discharge capacity retention to 84%. However, upon further increasing the concentration to 1.5 mol / L, despite the continued increase in lithium ion concentration, the ion mobility decreases due to the enhanced tendency of strong ion pair formation by LiTFSI (e.g., Li+···TFSI- association), increased electrolyte viscosity, and limited solvation kinetics. Consequently, ion conductivity and high-rate performance show a slight decline. This trend indicates that conductivity and rate capability are not linearly related to concentration, but rather that optimal ion transport and interfacial kinetic matching is achieved around 1.0 mol / L. Beyond this point, migration bottlenecks and polarization losses become performance limiting factors.

[0150] (6) Comparative Examples 1 and 2 are technical solutions in which the composite membrane does not contain an ANF layer or is not coated with an LDPE layer. The technical effects of these solutions compared with those of this application are shown in Table 10.

[0151] Table 10:

[0152]

[0153]

[0154] As shown in Table 10, compared with Example 1, Comparative Example 1 and Comparative Example 2 reveal the key roles of the polar functional layer and the thermal shut-off layer in the composite membrane system.

[0155] Specifically, in Comparative Example 1, which lacks an ANF polar nanofiber layer, the absence of a continuous electrolyte-loving network structure in the separator leads to a significant decrease in electrolyte wettability and an increase in interfacial resistance. This, in turn, causes lithium-ion diffusion lag and intensified electrode polarization, resulting in a substantial reduction in the battery's rate performance and kinetic response. In Comparative Example 2, although the ANF polar functional layer is retained, the lack of an LDPE thermal shutdown layer prevents the separator from achieving rapid pore closure and ion pathway blocking at high temperatures. This results in a significant increase in the membrane closure temperature and a delayed thermal runaway threshold, severely weakening the battery system's safety protection capabilities.

[0156] The above comparison verifies the dominant role of the ANF layer in optimizing kinetic performance and the irreplaceable role of the LDPE layer in the temperature-controlled circuit-breaking mechanism. Failure to incorporate the ANF layer leads to a severe decline in the kinetic performance of the lithium battery, while the absence of an LDPE layer results in a significant increase in the membrane's closing temperature, leading to a decrease in membrane safety performance.

[0157] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium-ion battery, comprising a casing, a core located within the casing, and an electrolyte, wherein the core is formed by winding a positive electrode, a negative electrode, and a separator between them, characterized in that, The diaphragm is composed of a polar functional layer, a support layer and a heat-sealing layer stacked together; The polar functional layer is made of aramid nanofibers; The electrolyte is a non-aqueous organic solution containing lithium bis(trifluoromethanesulfonyl)imide.

2. The lithium-ion battery according to claim 1, characterized in that, The thickness of the polar functional layer is 2-7 μm, the radial average diameter of the aromatic polyamide nanofiber is 20-200 nm, and the aromatic polyamide nanofiber contains amino or amide polar functional groups.

3. The lithium-ion battery according to claim 1, characterized in that, The thickness of the support layer of the diaphragm is 3–8 μm; The support layer is polyethylene terephthalate nonwoven fabric or polyethylene terephthalate film.

4. The lithium-ion battery according to claim 1, characterized in that, The thickness of the heat-sealing layer of the diaphragm is 2–9 μm; The heat-sealing layer is composed of low-density polyethylene or a blend containing low-density polyethylene, wherein the melting point of the low-density polyethylene is 110–125°C.

5. The lithium-ion battery according to claim 1, characterized in that, The solvent in the electrolyte is a mixture of dimethyl ether and 1,3-dioxolane. Preferably, the volume ratio of the dimethyl ether to 1,3-dioxolane is 1-3:1-3.

6. The lithium-ion battery according to claim 1, characterized in that, The concentration of lithium bis(trifluoromethanesulfonyl)imide in the electrolyte is 0.5–2.0 mol / L.

7. The lithium-ion battery according to claim 1, characterized in that, The method for preparing the diaphragm includes the following steps: (A) Provide polyethylene terephthalate nonwoven fabric or film as a support layer, then use electrospinning to deposit aromatic polyamide nanofibers on the support layer, and dry to obtain membrane intermediate A; (B) On the other side of the membrane on which aromatic polyamide nanofibers are deposited, a low-density polyethylene emulsion is coated, pressed, and dried to obtain membrane intermediate B. (C) The membrane intermediate B is hot-pressed to promote the bonding of the multilayer structure, and then vacuum dried to obtain the membrane.

8. The lithium-ion battery according to claim 7, characterized in that, In step (B), the low-density polyethylene emulsion contains 5 to 20 wt% of the total weight of the coating.

9. An electrical device, characterized in that, Including the lithium-ion battery according to any one of claims 1 to 8.

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