Diaphragm, secondary battery and electric device

Abstract

The invention relates to a diaphragm, a secondary battery and an electric device, and belongs to the technical field of batteries. The diaphragm provided by the invention comprises a base membrane, a first fiber layer arranged on the surface of one side of the base membrane and a second fiber layer arranged on the surface of the other side of the base membrane, the first fiber layer comprises aramid fibers; the second fiber layer comprises modified cellulose fibers, the modified cellulose fibers comprise modified groups, and the modified groups comprise at least one of quaternary amino groups, amino groups and hydroxyl groups. According to the asymmetric fiber coating functionalized diaphragm, on the basis of providing mechanical support, solvent oxidation gas production paths in an electrolyte are reduced, solvent decomposition gas production is reduced, HF is neutralized, SEI corrosion caused by HF is reduced, lithium ion solvation can be enhanced, the ionic conductivity can be improved, and the service life of the diaphragm is prolonged. Therefore, the cycle performance and the energy density of the secondary battery are effectively improved, the gas production in the cycle process is reduced, and the safety performance of the secondary battery is improved.

Description

Technical Field

[0001] This application belongs to the field of battery technology, specifically relating to a separator, a secondary battery, and an electrical device. Background Technology

[0002] Currently, commercial lithium iron phosphate (LFP) batteries are widely used in power and energy storage fields. However, due to the formation of the solid electrolyte interphase (SEI) and partial irreversible lithiation on the graphite anode, the initial coulombic efficiency during the first SEI film formation is less than 100%, resulting in a corresponding decrease in energy density and cycle life. Cathode lithium replenishment additives compensate for lithium losses during the first charge-discharge and cycling processes by adding lithium-rich compounds to the cathode, thereby improving initial capacity and cycle life and enhancing product competitiveness. However, lithium replenishment formation aging generates a large amount of gas, increases process time, and the high formation pressure leads to interface lithium plating, which also generates a large amount of gas during subsequent cycling, causing serious negative impacts on battery manufacturing and subsequent battery use. Summary of the Invention

[0003] The purpose of this application is to solve the technical problems of prominent gas production and poor cycle performance in the prior art, and to provide a diaphragm, secondary battery and power device that can effectively reduce gas production and improve cycle performance.

[0004] To achieve the above objectives, a first aspect of this application provides a diaphragm comprising a base membrane, a first fiber layer disposed on one side surface of the base membrane, and a second fiber layer disposed on the other side surface of the base membrane; the first fiber layer comprises aramid fibers; the second fiber layer comprises modified cellulose fibers, wherein the modified cellulose fibers comprise modified groups, and the modified groups comprise at least one of quaternary amino, amino, and hydroxyl groups.

[0005] As an embodiment of this application, the modified cellulose fiber is grafted with a modified compound, which includes at least one of quaternized poly(dimethylamino)ethyl methacrylate, epichlorohydrin-dimethylamine, layered double hydroxide, and cross-linked chitosan.

[0006] As an embodiment of this application, the grafting rate of the modified compound is 20% to 30%.

[0007] As an embodiment of this application, the thickness ratio of the first fiber layer to the thickness of the second fiber layer is (1~2):1.

[0008] As an embodiment of this application, the thickness of the first fiber layer is 3μm~9μm.

[0009] As an embodiment of this application, the thickness of the second fiber layer is 2μm~6μm.

[0010] As an embodiment of this application, the aramid fiber surface has a first polymer coating layer.

[0011] As an embodiment of this application, the modified cellulose fiber surface has a second polymer coating layer.

[0012] As an embodiment of this application, the first polymer coating layer and the second polymer coating layer each independently include a pH-responsive polymer, wherein the pH-responsive polymer has a solubility ≤0.5 mg / mL at 25°C and pH>5, and a solubility >0.5 mg / mL at 25°C and pH≤5.

[0013] As an embodiment of this application, the thickness of the first polymer coating layer and the thickness of the second polymer coating layer are each independently 30nm~200nm.

[0014] As an embodiment of this application, the pH-responsive polymer includes at least one of sodium polystyrene sulfonate, polymethyl methacrylate, and ethyl cellulose.

[0015] As an embodiment of this application, the tortuosity τ of the diaphragm is ≥8.

[0016] As an embodiment of this application, the diameter of the aramid fiber is 500 nm to 1000 nm.

[0017] As an embodiment of this application, the diameter of the modified cellulose fiber is 300 nm to 500 nm.

[0018] As an embodiment of this application, the aramid fiber includes at least one of para-aramid and meta-aramid.

[0019] As an embodiment of this application, the base film includes a PET film.

[0020] As an embodiment of this application, the thickness of the diaphragm is 14μm~30μm.

[0021] In a second aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and the separator described in this application, wherein the positive electrode contains a lithium replenishing agent, the lithium replenishing agent comprising Li5FeO4.

[0022] As an embodiment of this application, the positive electrode sheet includes a positive current collector and a positive electrode coating disposed on at least one side of the positive current collector, and the lithium replenishing agent is disposed on the surface of the positive electrode coating.

[0023] As an embodiment of this application, the first fiber layer in the separator faces the positive electrode sheet, and the second fiber layer faces the negative electrode sheet.

[0024] In a third aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0025] The beneficial effects of this application are as follows: The separator provided in this application has a first fiber layer on one side of the base membrane and a second fiber layer on the other side. The first fiber layer includes aramid fibers, and the second fiber layer includes modified cellulose fibers. The modified groups of the modified cellulose fibers include at least one of quaternary amino, amino, and hydroxyl groups. Through this asymmetric fiber coating functionalized separator, it can reduce the solvent oxidation and gas generation pathway in the electrolyte, reduce solvent decomposition and gas generation, neutralize HF to reduce its corrosion of SEI, and enhance lithium-ion solvation and improve ionic conductivity, thereby effectively improving the cycle performance and energy density of the secondary battery, reducing gas generation during the cycle, and improving the safety performance of the secondary battery. Detailed Implementation

[0026] To better illustrate the purpose, technical solution, and advantages of this application, the following will provide further explanation of this application in conjunction with specific embodiments.

[0027] Unless otherwise specified, the reagents, methods and equipment used in this application are all conventional reagents, methods and equipment in the field.

[0028] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0030] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0031] In one embodiment of this application, a diaphragm is provided, comprising a base membrane, a first fiber layer disposed on one side surface of the base membrane, and a second fiber layer disposed on the other side surface of the base membrane; the first fiber layer comprises aramid fibers; the second fiber layer comprises modified cellulose fibers, wherein the modified cellulose fibers comprise modified groups, and the modified groups comprise at least one of quaternary amino, amino, and hydroxyl groups.

[0032] The separator provided in this application has a first fiber layer on one side of the base membrane and a second fiber layer on the other side. The first fiber layer includes aramid fibers, and the second fiber layer includes modified cellulose fibers. The modified groups of the modified cellulose fibers include at least one of quaternary amino, amino, and hydroxyl groups. Through this asymmetric fiber coating functionalized separator, it can reduce the solvent oxidation and gas generation pathway in the electrolyte, reduce solvent decomposition and gas generation, neutralize HF to reduce its corrosion of SEI, and enhance lithium-ion solvation and improve ionic conductivity, thereby effectively improving the cycle performance and energy density of the secondary battery, reducing gas generation during the cycle, and improving the safety performance of the secondary battery.

[0033] Specifically, the diaphragm of this application has a first fiber layer containing aramid fibers on one side of the base membrane. When the benzene ring conjugated structure in the aramid fibers encounters active free radicals (e.g., ·O, ·OH), it undergoes single-electron transfer, absorbing unpaired electrons from the free radicals. The generated aromatic free radicals (Ar·) are stabilized by the conjugated system, terminating the oxygen free radicals (·O) released by the lithium supplement, thus blocking the free radical chain reaction and reducing the solvent oxidation gas generation pathway from the source. Furthermore, the N-H bond dissociation energy in the amide bond (—NH—CO—) of the aramid fibers is low (approximately 85 kcal / mol), making it easy for free radicals to capture hydrogen atoms. The generated nitrogen free radicals are stabilized through resonance, further reducing the further reaction of the aramid fibers. Simultaneously, a second fiber layer including modified cellulose fibers is provided on the other side of the base membrane. The modified groups include at least one of quaternary amino, amino, and hydroxyl groups. These modified groups can neutralize HF, reducing its corrosion of the SEI and reducing solvent decomposition gas generation; while the hydroxyl groups of the cellulose fibers enhance the Li… + Solvation improves ionic conductivity.

[0034] It should be noted that the testing method for the modified groups in the modified cellulose fibers is as follows: Qualitative analysis using Fourier transform infrared spectroscopy (FT-IR) is used to determine whether characteristic functional groups (such as quaternary ammonium groups and amino groups) have been successfully introduced, resulting in new characteristic absorption peaks attributed to the modified groups. X-ray photoelectron spectroscopy (XPS) is used to accurately determine the surface elemental composition (especially nitrogen) and analyze the chemical state of nitrogen to confirm quaternization. The modified sample should be detectable with nitrogen (N) signals in the full spectrum. Characteristic peaks at the corresponding binding energies are fitted in the high-resolution N1s spectrum.

[0035] In one embodiment, the modified cellulose fibers are grafted with a modifying compound, the modifying compound including at least one of quaternized poly(dimethylamino)ethyl methacrylate (quaternized pDMAEMA), epichlorohydrin-dimethylamine (EPI-DMA), layered double hydroxide (LDH), and cross-linked chitosan.

[0036] It should be noted that the testing method for the modified compounds grafted onto the modified cellulose fibers is as follows: depending on the type of modified compound, one or more of the following methods are selected for characterization: (1) For polymer-based modified compounds (such as quaternized pDMAEMA), thermogravimetric analysis (TGA) can be used to calculate the grafting amount, and the chemical structure can be confirmed by combining FT-IR and XPS; (2) For inorganic nanomaterials (such as LDH), X-ray diffraction (XRD) can be used to identify its crystal structure, and its morphology and elemental distribution can be observed by combining scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS); (3) For biopolymers (such as cross-linked chitosan), characteristic functional groups can be confirmed by FT-IR, and its cross-linked structure can be verified by acid solubility test.

[0037] This application modifies cellulose fibers with a modifying compound, which can neutralize HF to reduce its corrosion of SEI and reduce solvent decomposition gas production.

[0038] In one embodiment, the grafting rate of the modified compound is 20% to 30%.

[0039] For example, the grafting rate of the modified compound can be any point value or any two points between 20% and 30%, such as one or any two of 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%.

[0040] It should be noted that the testing and calculation method for the grafting rate of the modified compound is as follows: the modified cellulose fiber layer is scraped from the composite membrane sample, and then, according to the type of modified compound, the corresponding physical or chemical quantitative method is used for determination and calculation. For polymer-based modified compounds (such as quaternized pDMAEMA), thermogravimetric analysis can be used: by comparing the difference in weight loss rate (ΔW) of cellulose fibers before and after modification in the characteristic decomposition temperature range of the polymer, the grafting rate is calculated according to the formula ≈ (ΔW / (1 - ΔW)) × 100%. For inorganic nanomaterials (such as LDH), the ash method can be used: the initial mass (M1) of the modified cellulose fiber layer is weighed, and after high-temperature calcination to remove organic matter, the mass of the residual inorganic matter (M2) is weighed, and the grafting rate is calculated according to the formula ≈ (M2 / M1) × 100%. For biopolymers (such as cross-linked chitosan), elemental analysis can be used: by measuring the content of characteristic elements (such as nitrogen) in the modified fiber, the grafting rate can be calculated based on the theoretical content of the element in the modified compound.

[0041] In one embodiment, the thickness ratio of the first fiber layer to the thickness of the second fiber layer is (1~2):1.

[0042] For example, the ratio of the thickness of the first fiber layer to the thickness of the second fiber layer can be any point value or any two points range between (1~2):1, such as one or any two of the following: 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1.

[0043] This application research found that when the thickness ratio of the first fiber layer to the second fiber layer is within the above-mentioned range, the free radical quenching ability on the cathode side and the acid neutralization ability on the anode side can be matched, thereby synergistically and efficiently reducing battery gas production, while maintaining the low impedance of the separator and the overall electrochemical performance of the battery.

[0044] In one embodiment, the thickness of the first fiber layer is 3 μm to 9 μm.

[0045] For example, the thickness of the first fiber layer can be any point value or any two-point range value between 3μm and 9μm, such as one or any two of 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, and 9μm.

[0046] In one embodiment, the thickness of the second fiber layer is 2 μm to 6 μm.

[0047] For example, the thickness of the second fiber layer can be any point value or any two-point range value between 2μm and 6μm, such as one or any two of 2μm, 3μm, 4μm, 5μm, and 6μm.

[0048] This application study found that the thickness of the first fiber layer and the second fiber layer within the above range can match the short diffusion distance of ·O radicals and the long-range migration characteristics of HF, thereby effectively quenching ·O radicals and neutralizing HF, reducing SEI corrosion, and inhibiting solvent decomposition and gas generation.

[0049] It should be noted that the thicknesses of the first and second fiber layers can be observed and measured using a scanning electron microscope (SEM) to examine the membrane cross-section. Higher resolution measurements and verification can also be performed using a transmission electron microscope (TEM) on ultrathin slice samples.

[0050] In one embodiment, the aramid fiber surface has a first polymer coating layer.

[0051] In one embodiment, the modified cellulose fiber surface has a second polymer coating layer.

[0052] In one embodiment, the first polymer coating layer and the second polymer coating layer each independently comprise a pH-responsive polymer, wherein the pH-responsive polymer has a solubility ≤0.5 mg / mL at 25°C and pH > 5, and a solubility >0.5 mg / mL at 25°C and pH ≤ 5.

[0053] This study found that applying a polymer coating layer including a pH-responsive polymer to the surface of aramid fibers or modified cellulose fibers can reduce the ineffective consumption of the separator during the formation and aging stage. This allows the separator to initiate a chain reaction that blocks the release of oxygen free radicals and neutralizes HF after degassing during formation and aging, thus reducing SEI corrosion and solvent decomposition gas generation. Specifically, the pH-responsive polymer has a solubility ≤0.5 mg / mL at 25°C and pH > 5, and a solubility >0.5 mg / mL at pH ≤ 5. During the initial formation stage, the pH value is high, and the pH-responsive polymer shell is stable, effectively isolating the first and second cellulose layers. During subsequent long-cycle or high-temperature storage, the pH value decreases due to the accumulation of byproduct acid from the lithium replenishment agent, causing the pH-responsive polymer to dissolve and exposing the active groups of the fibers to inhibit gas generation. Applying a polymer coating layer including a pH-responsive polymer to the surface of aramid fibers or modified cellulose fibers can achieve delayed response, improving battery safety and cycle life.

[0054] It should be noted that the solubility of the pH-responsive polymer described in this application at different pH values ​​refers not only to the physical dissolution process, but also to the process by which the pH-responsive polymer is dispersed from the fiber surface into the electrolyte due to chemical reactions (such as hydrolysis and erosion).

[0055] In one embodiment, the thickness of the first polymer coating layer and the thickness of the second polymer coating layer are each independently 30 nm to 200 nm.

[0056] For example, the thickness of the first polymer coating layer and the thickness of the second polymer coating layer can each be independently any point value or any two points within the range of 30nm to 200nm, such as one or any two of the following ranges: 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, and 200nm.

[0057] It should be noted that the thickness of the first polymer coating layer and the thickness of the second polymer coating layer can be obtained by SEM image measurement.

[0058] This study found that the thickness of the first and second polymer coating layers directly affects their dissolution duration in the acidic environment of the electrolyte. By controlling the thickness of both layers within the aforementioned range, it is possible to ensure that the coating layers dissolve moderately slowly and continuously during the main gas generation phase of battery cycling. This allows for the gradual release of the active sites of the internal fibers as needed, reducing premature depletion or delayed activation, and contributing to a sustained improvement in the overall gas generation suppression effect of the separator.

[0059] In one embodiment, the pH-responsive polymer includes at least one of sodium polystyrene sulfonate (PSS), polymethyl methacrylate (PMMA), and ethyl cellulose.

[0060] Specifically, sodium polystyrene sulfonate, polymethyl methacrylate, and ethyl cellulose are initially stable in the electrolyte. However, as the electrolyte matures and cycling progresses, the accumulation of HF leads to a decrease in the electrolyte pH. At lower pH values, sodium polystyrene sulfonate undergoes a sudden change in solubility, dissolving into the electrolyte and exposing the active sites on the fibers, thus suppressing gas generation and achieving a delayed response. Polymethyl methacrylate hydrolyzes in acidic environments with a pH less than 4.0, while ethyl cellulose undergoes gradual dissolution of its ether bonds in strongly acidic environments, becoming porous and peeling off from the surface, thereby exposing the fibers within the fiber layer, suppressing solvent gas generation, achieving a delayed response of the separator, and improving the battery's safety and cycle performance.

[0061] In one embodiment, the sodium polystyrene sulfonate has a sulfonation degree of 75% to 85% and a weight-average molecular weight of 70 kDa to 100 kDa.

[0062] The degree of sulfonation of sodium polystyrene sulfonate determines the density of charged sulfonic acid groups (-SO3Na) on its molecular chain, which controls the dissolution pH threshold and hydrophilicity. Within the above-mentioned degree of sulfonation, the dissolution threshold is ideally located in the pH range of ≈ 4.3 - 4.8, with suitable dissolution kinetics. Within the above-mentioned weight-average molecular weight range, a moderate dissolution rate is provided, achieving a good delayed release effect.

[0063] It should be noted that the test method for the degree of sulfonation of the sodium polystyrene sulfonate is as follows: elemental analysis is used to determine the content of sulfur (S) in the sample and calculate the proportion of sulfonic acid groups; the test method for the weight-average molecular weight is as follows: gel permeation chromatography is used to separate polymer molecules based on their different hydrodynamic volumes, and Mw is obtained by comparing with a known molecular weight standard.

[0064] In one embodiment, the tortuosity τ of the diaphragm is ≥8.

[0065] This study found that a membrane with a tortuosity τ≥8 can block the penetration of solvent in the electrolyte and reduce oxidation side reactions.

[0066] In one embodiment, the diameter of the aramid fiber is 500 nm to 1000 nm.

[0067] For example, the diameter of the aramid fiber can be any point value or any two points between 500nm and 1000nm, such as one or any two of 500nm, 600nm, 700nm, 800nm, 900nm, and 1000nm.

[0068] It should be noted that the diameter of the aramid fiber is measured by taking a small amount of the diluted aramid slurry after drying, spreading it evenly on a conductive adhesive, plating it with gold, taking a SEM image to obtain a nanometer-resolution image, and then measuring it.

[0069] In one embodiment, the modified cellulose fiber has a diameter of 300 nm to 500 nm.

[0070] For example, the diameter of the modified cellulose fiber can be any point value or any two points between 300nm and 500nm, such as one or any two of 300nm, 350nm, 400nm, 450nm, and 500nm.

[0071] It should be noted that the diameter of the modified cellulose fiber was measured using SEM.

[0072] In one embodiment, the aramid fiber includes at least one of para-aramid and meta-aramid.

[0073] In one embodiment, the aramid fiber is para-aramid.

[0074] This study found that para-aramid has higher crystallinity and a higher free radical capture rate than meta-aramid, and that using para-aramid has better results in suppressing gas production.

[0075] In one embodiment, the base film comprises a PET film.

[0076] In one embodiment, the thickness of the diaphragm is 14 μm to 30 μm.

[0077] For example, the thickness of the diaphragm can be any point value or any two-point range value between 14μm and 30μm, such as one or any two of the following: 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, and 30μm.

[0078] In one embodiment of this application, a secondary battery is provided, including a positive electrode, a negative electrode, and the separator described in this application. The positive electrode contains a lithium replenishing agent, which includes Li5FeO4.

[0079] Li5FeO4 (LFO) has a high theoretical specific capacity for lithium supply and a high cost-performance ratio. Adding LFO can effectively improve the full battery's charge capacity, discharge capacity, cycle capacity retention, and high-temperature storage life.

[0080] In one embodiment, the positive electrode includes a positive current collector and a positive electrode coating disposed on at least one side of the positive current collector, wherein the lithium supplement is disposed on the surface of the positive electrode coating.

[0081] This application places the lithium replenishing agent on the surface of the positive electrode coating, which allows for sufficient contact with the electrolyte and results in high lithium replenishment efficiency.

[0082] In one embodiment, the lithium replenishing agent accounts for 0.5%-2% of the mass of the positive electrode coating.

[0083] For example, the mass of the lithium replenishing agent as a percentage of the mass of the positive electrode coating can be any point value or any two points within the range of 0.5% to 2%, such as one or any two of the following: 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2%.

[0084] In one embodiment, the first fiber layer in the separator faces the positive electrode and the second fiber layer faces the negative electrode.

[0085] The positive electrode includes a lithium replenishing agent. During the aging and formation stages, i.e., the first charge, the lithium replenishing agent generates oxygen free radicals on the positive electrode side. The oxygen free radicals have an extremely short half-life and a short diffusion distance. The first fiber layer facing the positive electrode enables the aramid fibers to effectively capture oxygen free radicals and inhibit their release.

[0086] In one embodiment, the positive electrode coating comprises a positive electrode active material. This application does not limit the positive electrode active material; any known positive electrode active material can be used. As an example, the positive electrode active material can be at least one of lithium iron phosphate, lithium manganese iron phosphate, and ternary positive electrode materials.

[0087] In one embodiment, the positive current collector can be made of a material with good conductivity and mechanical strength, serving both as a conductor and a current collector.

[0088] In one embodiment, the positive electrode current collector may be a metal foil or a composite current collector (a composite current collector may be formed by depositing metal material on a polymer substrate). As an example, the positive electrode current collector may be aluminum foil or carbon-coated aluminum foil.

[0089] In one embodiment, the positive electrode coating further includes a positive electrode binder and a positive electrode conductive agent.

[0090] This application does not limit the positive electrode binder and positive electrode conductive agent; any known positive electrode binder and positive electrode conductive agent may be used. For example, the positive electrode binder may be at least one of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); the positive electrode conductive agent may be at least one of Super P, carbon nanotubes, and graphene.

[0091] Since the positive electrode of this application contains lithium ferrite (Li5FeO4) lithium replenishing agent, it will generate high potential and active oxygen substances during charging. Therefore, in some embodiments, a binder with high electrochemical oxidation stability, such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), can be selected to ensure the structural integrity of the electrode during long-term cycling.

[0092] In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material. This application does not limit the negative electrode active material, and any known negative electrode active material can be used. As an example, the negative electrode active material can be at least one of artificial graphite, natural graphite, silicon-carbon composite material, elemental silicon, silicon suboxide, and hard carbon.

[0093] In one embodiment, the negative electrode current collector can be made of a material with good conductivity and mechanical strength, serving both to conduct electricity and collect current. In another embodiment, the negative electrode current collector can be a metal foil or a composite current collector (a composite current collector can be formed by depositing metal material on a polymer substrate). As an example, copper foil is used as the negative electrode current collector.

[0094] In one embodiment, the secondary battery further includes an electrolyte.

[0095] In one embodiment, the electrolyte comprises an organic solvent and a lithium salt.

[0096] This application does not limit the use of organic solvents and lithium salts; any known organic solvents and lithium salts may be used.

[0097] For example, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0098] It should be noted that, in order to achieve the pH-responsive function of the separator in this application and suppress side reactions, the electrolyte should be strictly controlled for its water content (≤20 ppm) and free acid (HF) content (≤100 ppm). This is both a condition for triggering the dissolution of the coating layer and a key factor in reducing additional gas sources and ensuring long-term cycle stability of the battery.

[0099] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0100] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0101] Example 1 This application provides a separator and a secondary battery. The preparation method of the separator and the secondary battery includes the following steps: (1) Preparation of diaphragm S1. Para-aramid nanofibers (500 nm in diameter) were immersed in a 1.0 wt% PSS aqueous emulsion (sulfonation degree of 80%, Mw=70 kDa), ultrasonically dispersed (300 W, 30 min, ice bath) and centrifuged (10000 rpm × 20 min), and then freeze-dried to obtain aramid fibers containing a first polymer (PSS) coating layer with a thickness of 50 nm. Cellulose fibers grafted with quaternized pDMAEMA (grafting rate of quaternized pDMAEMA 25%) were immersed in a 1.0 wt% PSS aqueous emulsion (sulfonation degree of 80%, Mw=70kDa), ultrasonically dispersed (300W, 30min, ice bath) and centrifuged (10000rpm×20min), and then freeze-dried to obtain modified cellulose fibers containing a second polymer (PSS) coating layer with a thickness of 50 nm. S2. The obtained aramid fibers containing the first polymer (PSS) coating layer are prepared into a first fiber layer slurry with a solid content of 10%, ultrasonically dispersed, and the viscosity of the first fiber layer slurry is 3500 mPa·s. The first fiber layer slurry is deposited on a PET base film using a vacuum filtration device, and hot-pressed at a temperature of 80 ℃ and a pressure of 10 MPa to obtain a first fiber layer on one side of the PET base film. Subsequently, a solution of modified cellulose fibers containing the second polymer (PSS) coating layer is sprayed onto the other side of the PET base film, with a spraying basis weight of 2 g / m³. 2 The membrane is then vacuum dried at 60°C to obtain a second fiber layer on the other side of the PET base film, thus obtaining a separator. The thickness of the first fiber layer in the separator is 6 μm, the thickness of the second fiber layer is 4 μm, and the thickness of the PET base film is 8 μm.

[0102] (2) Preparation of positive electrode sheet Lithium iron phosphate (LFP), a conductive agent (Super P), and a binder (PVDF) were mixed at a mass ratio of 92:4:4. Then, NMP solvent was added, and the mixture was stirred to obtain a positive electrode slurry. The positive electrode coating slurry with a viscosity of 4000 mPa·s was uniformly coated onto both surfaces of the fluid-coated carbon aluminum foil of the positive electrode current collector, and then transferred to an oven for drying to obtain the positive electrode coating. LFO was dispersed in NMP at a mass ratio of 95:3:2 to prepare an LFO slurry. The LFO dry mass was 1.5% of the dry mass of the positive electrode coating and was coated onto the surface of the positive electrode coating. The mixture was then dried again at 85°C for 2 hours and rolled using a roller mill at 25°C and 120 MPa pressure until the compacted density was approximately 2.4 g / cm³. 3 This yields the positive electrode sheet.

[0103] (3) Preparation of negative electrode sheet Artificial graphite, conductive carbon black (SP), sodium carboxymethyl cellulose, and styrene-butadiene rubber were mixed evenly at a mass ratio of 97:1.0:0.8:1.2. Then, deionized water (total water volume being 50% of the total solid mass) was added in three portions, and the mixture was stirred at 1500 rpm for 3 hours in a vacuum planetary mixer to prepare a homogeneous negative electrode slurry with a solid content of approximately 45%. The homogeneous negative electrode slurry was then further mixed with 50 mg / cm³ of water. 2The areal density is uniformly coated on both sides of a 6μm thick copper foil, dried at 110℃, and then rolled at 25℃ and 80 MPa pressure until the compacted density is approximately 1.6 g / cm³. 3 After slitting and cutting, the negative electrode sheet is obtained.

[0104] Preparation of electrolyte Ethylene carbonate (EC), ethyl methyl carbonate (EMC), ethyl propionate (EP), vinylene carbonate (VC), 1,3-propanesulfonyl lactone (PS), ethylene sulfate (DTD), and tris(trimethylsilane)phosphate (TMSP) were mixed in a mass ratio of 20:30:45:3:1:0.5:0.5, followed by the addition of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI). The molar concentration of LiPF6 in the electrolyte was 0.5 M, and the molar concentration of LiFSI was 0.7 M.

[0105] (5) Preparation of secondary batteries The prepared positive electrode, negative electrode, separator and other battery components are assembled and then subjected to processes such as shaping, baking, packaging, liquid injection, formation and capacity testing to obtain a soft-pack secondary battery.

[0106] Examples 2-4 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in Example 1 is that the parameters in Table 1 are achieved by adjusting the type of modified compound.

[0107] Examples 5-6 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in Example 1 is that the parameters in Table 1 are achieved by adjusting the type of pH-responsive polymer.

[0108] Examples 7-12 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in embodiment 1 is that the thickness of the first fiber layer and the second fiber layer is adjusted by adjusting the deposition amount and the spraying amount to achieve the parameters in Table 1.

[0109] Examples 13-15 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in this application and that in Example 1 is that the thickness of the first polymer coating layer and the second polymer coating layer is adjusted by adjusting the concentration of the PSS aqueous emulsion to achieve the parameters in Table 1.

[0110] Examples 16-19 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery and those in Example 1 is that the grafting rate of the quaternized pDMAEMA-modified cellulose fibers is adjusted by adjusting the amount of quaternized pDMAEMA added to achieve the parameters in Table 1.

[0111] Example 20 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in embodiment 1 is that the parameters in Table 1 are achieved by adjusting the type of aramid fiber.

[0112] Example 21 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in embodiment 1 is that the separator and the secondary battery do not contain a second polymer coating layer to achieve the parameters in Table 1.

[0113] Example 22 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in embodiment 1 is that the separator and the secondary battery do not contain a first polymer coating layer and a second polymer coating layer to achieve the parameters in Table 1.

[0114] Examples 23-24 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in embodiment 1 is that the mass of the LFO is adjusted to achieve the parameters in Table 1.

[0115] Example 25 This application provides a separator and a secondary battery. The difference between the separator and the secondary battery in this application and that in Example 1 is that LFO is added to the positive electrode coating slurry during the preparation of the positive electrode sheet to obtain the secondary battery.

[0116] Comparative Example 1 This application provides a diaphragm and a secondary battery in a comparative example. The difference between the diaphragm and the secondary battery in Example 1 is that the diaphragm and the secondary battery do not contain a first fiber layer and a second fiber layer to achieve the parameters in Table 1.

[0117] Comparative Example 2 This application provides a diaphragm and a secondary battery in a comparative example. The difference between the diaphragm and the secondary battery in Example 1 is that the diaphragm and the secondary battery do not contain a first fiber layer to achieve the parameters in Table 1.

[0118] Comparative Example 3 This application provides a diaphragm and a secondary battery in a comparative example. The difference between the diaphragm and the secondary battery in Example 1 is that the diaphragm and the secondary battery do not contain a second fiber layer to achieve the parameters in Table 1.

[0119] Comparative Example 4 This application provides a diaphragm and a secondary battery in a comparative example. The difference between the diaphragm and the secondary battery in Example 1 is that the cellulose fibers included in the second fiber layer have not been modified by quaternized pDMAEMA grafting to achieve the parameters in Table 1.

[0120] The types of aramid fibers, the thickness H1 of the first fiber layer, the types of modified compounds, the thickness H2 of the first fiber layer, the ratio of the thickness of the first fiber layer to the thickness of the second fiber layer H1:H2, the types of pH-responsive polymers, the thickness h1 of the first polymer coating layer, the thickness h2 of the second polymer coating layer, and the percentage of LFO mass to the positive electrode coating mass R in the examples and comparative examples are shown in Table 1.

[0121] Table 1. Parameters of the separator and secondary battery The separators and secondary batteries of the examples and comparative examples were subjected to performance tests. The test results are shown in Table 2. The performance test methods are as follows: 1. Wettability test: Using a German DataPhysics OCA20 contact angle measuring instrument, 5 μL of electrolyte was dropped onto the diaphragm surface using a micro-syringe. A high-speed camera (1000 frames / second) captured the droplet morphology, and the software automatically fitted the droplet profile to calculate the contact angle θ.

[0122] 2. Tortuousness test: Assemble a coin cell (stainless steel | separator | stainless steel), and measure the ionic conductivity σ using EIS (EIS test conditions: 0.1 Hz-1 MHz, amplitude 10 mV). Then, use the formula... Calculate tortuosity .in The conductivity of the bulk electrolyte (mS / cm) The thickness of the diaphragm is (μm). The porosity of the diaphragm was measured using a gas-liquid method pore size analyzer, which was manufactured by PMI Corporation in the United States.

[0123] 3. Air permeability test: Using a Gurley Densometer 4110N, a constant pressure difference (1.22 kPa) was applied to the diaphragm, and the permeation time of 100 mL of air was recorded. This was repeated 5 times, and the average value was taken to calculate the Gurley value (s / 100 mL). The Gurley value can be used to characterize the air permeability, which refers to the time required for a specific amount of air to pass through a diaphragm of a specific area under a specific pressure.

[0124] 4. Expansion Rate Test Measure the initial thickness of the battery The batteries were stored in a 60°C oven at 100% SOC for 28 days. After being removed and cooled to 25°C, the thickness was measured. .

[0125] Expansion rate = .

[0126] 5.500-cycle retention rate test Using the Xinwei CT-4008 battery testing system, the battery was tested at 25℃ ± 1℃ with 1C constant current and constant voltage charging up to 3.65V and a cutoff current of 0.05C. It was then tested at 1C constant current discharge up to 2.5V. This is the setup for one complete cycle. After 500 cycles, the battery capacity retention rate was tested.

[0127] 6. Circulating gas production Using a gas chromatograph (GC, Agilent 8890) and a water displacement and gas collection device, the fully discharged battery after 500 cycles was punctured and connected to a gas collection bag in an inert atmosphere. The volume of water displacement was collected using a graduated cylinder, and the total gas volume (mL) was calculated. The cycle gas production = total gas production / total cumulative cycle discharge capacity.

[0128] 7. Test of free acid (HF) content in electrolyte after cycling In an argon-protected glove box, the battery after 500 cycles was disassembled, and the electrolyte was collected. The free acid (HF) content was determined by acid-base titration: approximately 0.5 g of electrolyte sample was accurately weighed, dissolved in 20 mL of ethanol-water mixed solvent, and titrated to the endpoint with 0.01 mol / L potassium hydroxide (KOH) standard solution using bromothymol blue as an indicator. The HF content (ppm) was calculated based on the volume of standard solution consumed.

[0129] Table 2. Performance test results of separator and secondary battery As can be seen from the table above, when the technical solution of this application is adopted, the obtained separator has a good effect on suppressing gas production, and the secondary battery has excellent cycle performance.

[0130] As can be seen from Examples 1-25 and Comparative Examples 1-3, when the first fiber coating or the second fiber coating is not introduced, or neither is introduced, the gas production of the secondary battery increases and the battery performance decreases.

[0131] As can be seen from Examples 1-25 and Comparative Example 4, the cellulose fibers in the second fiber layer, without quaternized pDMAEMA grafting modification, resulted in a decrease in the cycle performance and an increase in gas production of the secondary battery. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the essence and scope of the technical solutions of this application.

Claims

1. A diaphragm, characterized in that, The device includes a base membrane, a first fiber layer disposed on one side surface of the base membrane, and a second fiber layer disposed on the other side surface of the base membrane; the first fiber layer includes aramid fibers; the second fiber layer includes modified cellulose fibers, wherein the modified cellulose fibers include modifying groups, and the modifying groups include at least one of quaternary amino, amino, and hydroxyl groups.

2. The diaphragm according to claim 1, characterized in that, The modified cellulose fibers are grafted with a modified compound, which includes at least one of quaternized poly(dimethylamino)ethyl methacrylate, epichlorohydrin-dimethylamine, layered double hydroxide, and cross-linked chitosan.

3. The diaphragm according to claim 2, characterized in that, The grafting rate of the modified compound is 20% to 30%.

4. The diaphragm according to claim 1, characterized in that, The thickness ratio of the first fiber layer to the thickness of the second fiber layer is (1~2):

1.

5. The diaphragm according to claim 1 or 4, characterized in that, The thickness of the first fiber layer is 3μm to 9μm; and / or the thickness of the second fiber layer is 2μm to 6μm.

6. The diaphragm according to claim 1, characterized in that, The aramid fiber has a first polymer coating layer on its surface; and / or, the modified cellulose fiber has a second polymer coating layer on its surface; The first polymer coating layer and the second polymer coating layer each independently include a pH-responsive polymer, wherein the pH-responsive polymer has a solubility ≤0.5 mg / mL at 25°C and pH > 5, and a solubility >0.5 mg / mL at 25°C and pH ≤ 5.

7. The diaphragm according to claim 6, characterized in that, The thickness of the first polymer coating layer and the thickness of the second polymer coating layer are each independently 30 nm to 200 nm; and / or, the pH-responsive polymer includes at least one of sodium polystyrene sulfonate, polymethyl methacrylate, and ethyl cellulose.

8. The diaphragm according to claim 1, characterized in that, At least one of the following conditions must be met: A. The tortuosity τ of the diaphragm is ≥8; B. The diameter of the aramid fiber is 500 nm to 1000 nm; C. The diameter of the modified cellulose fiber is 300 nm to 500 nm; D. The aramid fiber includes at least one of para-aramid and meta-aramid; E. The base film includes a PET film; F. The thickness of the diaphragm is 14μm~30μm.

9. A secondary battery comprising a positive electrode, a negative electrode, and a separator according to any one of claims 1 to 8, wherein the positive electrode contains a lithium replenishing agent, the lithium replenishing agent comprising Li5FeO4.

10. The secondary battery according to claim 9, characterized in that, The positive electrode includes a positive current collector and a positive electrode coating disposed on at least one side of the positive current collector, and the lithium supplement is disposed on the surface of the positive electrode coating.

11. The secondary battery according to claim 9, characterized in that, In the separator, the first fiber layer faces the positive electrode, and the second fiber layer faces the negative electrode.

12. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 9 to 11.

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