Battery diaphragm, preparation method and application thereof, and lithium battery

Abstract

The invention belongs to the technical field of lithium batteries, and provides a battery diaphragm, a preparation method and application thereof, and a lithium battery. The battery diaphragm sequentially comprises a first oil-based coating layer, a second oil-based ceramic coating layer, a base membrane, a third oil-based ceramic coating layer and a fourth oil-based coating layer, and the second oil-based ceramic coating layer and the third oil-based ceramic coating layer comprise the following raw material components: inorganic nano-particles, inorganic nano-particles, inorganic nano-particles, inorganic nano-particles, inorganic nano-particles, inorganic nano-particles and inorganic nano-particles. The first oil system coating layer and the fourth oil system coating layer are prepared from the following raw material components: aramid fibers and polyvinylidene fluoride. The battery diaphragm has the characteristics of low closed pore and high diaphragm rupture, can be further used for preparing a high-safety lithium battery, can meet the requirements of safety test and cycle performance, and can be used as a digital wearable battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and more specifically, to a battery separator, a preparation method and application thereof, and a lithium battery. Background Art

[0002] Currently, wearable batteries have increasingly stringent safety requirements, such as the need to pass high-safety needle penetration tests. However, conventional product systems, such as conventional battery separator base films containing a ceramic layer (boehmite) and PVDF adhesive, cannot meet all of the needle penetration test requirements. For digital wearable batteries of a certain capacity, after being fully charged at room temperature, the pass rate for the room-temperature needle penetration test is only around 30-50%, which cannot meet the high safety requirements for daily use.

[0003] Therefore, there is an urgent need to develop a battery separator that can pass all needle puncture tests and has high safety in order to prepare high-safety lithium batteries. Summary of the Invention

[0004] The present invention aims to address at least one of the technical problems existing in the aforementioned prior art. To this end, the present invention provides a battery separator, a method for preparing the separator, and its application, as well as a lithium battery. The battery separator provided by the present invention is highly safe, with a 100% pass rate in safety testing (100% pass rate in the needle penetration test), and can be further used to prepare high-safety lithium batteries with excellent cycle performance and rate capability.

[0005] A first aspect of the present invention provides a battery separator.

[0006] Specifically, a battery separator includes, in sequence, a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer, and a fourth oil-based coating layer;

[0007] The first oil-based coating layer and the fourth oil-based coating layer are made of the same material;

[0008] The second oil-based ceramic coating layer and the third oil-based ceramic coating layer are made of the same material;

[0009] The raw material components of the second oil-based ceramic coating layer and the third oil-based ceramic coating layer include: inorganic nanoparticles;

[0010] The raw material components of the first oil-based coating layer and the fourth oil-based coating layer include: aramid and polyvinylidene fluoride (PVDF).

[0011] The present invention applies two different oil-based coatings to both sides of the base membrane. The second and third oil-based ceramic coating layers are made of inorganic nanoparticles, which exhibit high hardness, high wear resistance, and excellent chemical stability. They act as dense protective layers and can improve the thermal stability and mechanical strength of the battery separator. The present invention also utilizes aramid and polyvinylidene fluoride to make the first and fourth oil-based coating layers. Aramid is a high-performance fiber material with high strength, high modulus, excellent heat resistance, and strong flame retardancy, while PVDF has excellent adhesion and chemical stability. The combination of aramid and PVDF can fully leverage the advantages of both, further improving the mechanical strength, thermal stability, and electrochemical performance of the battery separator. In a needle puncture test, when a steel needle penetrates the battery separator, the high-strength aramid and the excellent adhesion of PVDF work together to ensure that the battery separator maintains structural integrity when subjected to external force. The oil-based ceramic coatings on both sides effectively resist the impact of the needle puncture, preventing the battery separator from being punctured or torn. In addition, ceramic materials, namely alumina, boehmite (γ-alumina hydrate, molecular formula is γ-AlOOH), and aramid (aromatic polyamide fiber) all have good thermal stability and can maintain structural stability at high temperatures. Therefore, in the needle penetration test, when a short circuit or overheating occurs inside the lithium battery, the coating layers on both sides can effectively prevent the rapid transfer and diffusion of heat, thereby improving thermal stability. Even if the battery separator undergoes thermal shrinkage of a certain size, it can still maintain the integrity of the battery separator and prevent short circuits between the positive and negative electrodes, thereby ensuring the safety of the battery and reducing the risk of thermal runaway. Although PVDF is not as high-temperature resistant as ceramic materials and aramid, it can maintain stable performance in the temperature range of 100-280 ° C and enhance electrochemical performance. Moreover, PVDF has good liquid retention and wettability, can absorb and retain more electrolyte, and rationally allocate the electrolyte components of the battery, add some heat-stable additives, and simultaneously inhibit the decomposition of the electrolyte, which plays a role in auxiliary protection and synchronous safety improvement to a certain extent. In the needle puncture test, when the steel needle penetrates the battery separator, the coating layer containing PVDF (the first oil-based coating layer and the fourth oil-based coating layer) can quickly absorb the electrolyte and prevent it from leaking. At the same time, the addition of aramid also helps to improve the ion conductivity of the battery separator, so that the battery can still maintain stable electrochemical performance when subjected to external force impact. The battery separator with a multi-layer coating structure provided by the present invention achieves a comprehensive improvement in mechanical strength, thermal stability and electrochemical performance through the synergistic effect between the layers. Each layer can jointly resist the impact of the needle, prevent the rapid transfer and diffusion of heat, absorb the electrolyte and promote lithium ion transmission, thereby significantly improving the puncture resistance and safety of the battery cell.

[0012] The low closed-pore temperature mentioned in the present invention refers to the low temperature when the micropores are closed (such as reaching 105-120°C); the high membrane rupture temperature refers to the high temperature when the battery separator ruptures and melts (such as reaching 205-230°C).

[0013] Preferably, the inorganic nanoparticles are alumina and / or boehmite.

[0014] Preferably, the aramid is poly(m-phenylene isophthalamide) fiber (abbreviated as meta-aramid).

[0015] Preferably, the base film is a polyethylene (PE) base film and / or a polypropylene (PP) base film.

[0016] Preferably, the base film has a closed-cell temperature of 105-120° C., and / or a film-breaking temperature of 205-230° C. The base film used in the present invention has low closed-cell and high film-breaking properties.

[0017] Preferably, the base film has a thickness of 4-9 μm.

[0018] Preferably, the first oil-based ceramic coating layer and the fourth oil-based ceramic coating layer have the same thickness.

[0019] Preferably, the second oil-based ceramic coating layer and the third oil-based ceramic coating layer have the same thickness.

[0020] Preferably, the thickness of the first oil-based ceramic coating layer and the fourth oil-based ceramic coating layer are both 1-3 μm.

[0021] More preferably, the thickness of the first oil-based ceramic coating layer and the fourth oil-based ceramic coating layer are both 2-3 μm.

[0022] Preferably, the thickness of the second oil-based ceramic coating layer and the third oil-based ceramic coating layer are both 1-3 μm.

[0023] More preferably, the thickness of the second oil-based ceramic coating layer and the third oil-based ceramic coating layer are both 2-3 μm.

[0024] A second aspect of the present invention provides a method for preparing a battery separator.

[0025] A method for preparing a battery separator comprises the following steps:

[0026] Inorganic nanoparticles and organic solvent I are mixed to obtain coating I; aramid, polyvinylidene fluoride and organic solvent II are mixed to obtain coating II; coating I is then applied to the front and back sides of the base film respectively to obtain the second and third oil-based ceramic coating layers, respectively; coating II is then applied to the surface of the second and third oil-based ceramic coating layers respectively to obtain the first and fourth oil-based coating layers, respectively, to prepare the battery separator.

[0027] Preferably, the organic solvent I and the organic solvent II are independently selected from at least one of dimethylacetamide (DMAC), ethylene glycol butyl ether, propylene glycol methyl ether, and isobutyl acetate.

[0028] Preferably, the coating process is one of dip coating, roller coating, spray coating and extrusion coating.

[0029] A third aspect of the present invention provides a use of a battery separator.

[0030] The invention discloses an application of a battery separator in preparing a battery.

[0031] Preferably, the battery is a lithium battery.

[0032] A fourth aspect of the present invention provides a lithium battery.

[0033] A lithium battery comprises a positive electrode, a negative electrode, the battery separator and an electrolyte.

[0034] Preferably, the positive electrode includes a positive electrode current collector and a first coating layer coated on the surface of the positive electrode current collector, the first coating layer is obtained by coating a first slurry, and the raw material components of the first slurry include a positive electrode active material, a first binder, a first solvent and a first conductive agent.

[0035] Preferably, in the first coating layer, the mass content of the positive electrode active material is 96.5-98.5%.

[0036] Preferably, the positive electrode active material is lithium cobalt oxide (LiCoO2) and / or lithium nickel cobalt manganese oxide ternary material.

[0037] Preferably, the first binder is polyvinylidene fluoride (PVDF) and / or polyacrylonitrile (PAN). The positive electrode main material can improve the capacity and endurance of the battery.

[0038] Preferably, the first solvent is N-methylpyrrolidone (NMP).

[0039] Preferably, the first conductive agent is conductive carbon black (SP) and / or carbon nanotubes (CNT).

[0040] Preferably, the positive electrode current collector is aluminum foil.

[0041] Preferably, the thickness of the positive electrode current collector is 9-12 μm.

[0042] Preferably, the thickness of the negative electrode current collector is 4.5-6 μm.

[0043] Preferably, the negative electrode current collector is copper foil.

[0044] Preferably, the negative electrode includes a negative electrode current collector and a second slurry coated on the surface of the negative electrode current collector, the second coating layer is obtained by coating the second slurry, and the raw material components of the second slurry include a negative electrode active material, a second conductive agent, a thickener, a second binder and a second solvent.

[0045] Preferably, in the second coating layer, the mass content of the negative electrode active material is 96.5-98.5%.

[0046] Preferably, the thickener is sodium carboxymethyl cellulose (CMC) and / or sodium carboxymethyl starch (CMS).

[0047] Preferably, the negative electrode active material is artificial graphite and / or natural graphite.

[0048] Preferably, the second conductive agent is conductive carbon black (SuperP) and / or conductive graphite.

[0049] Preferably, the second binder is styrene-butadiene rubber (SBR) and / or polyacrylic acid (PAA).

[0050] Preferably, the second solvent is deionized water.

[0051] Preferably, the components of the electrolyte include lithium salt, organic solvent and flame retardant additive.

[0052] Preferably, the lithium salt is an inorganic lithium salt and / or an organic lithium salt.

[0053] Preferably, the inorganic lithium salt is lithium hexafluorophosphate (LiPF6) and / or lithium tetrafluoroborate (LiBF4).

[0054] Preferably, the organic lithium salt is lithium bis(oxalatoborate) (LiBOB) and / or lithium bis(fluorosulfonyl)imide (LiFSI).

[0055] Preferably, the organic solvent is at least one of a carbonate organic solvent, an organic ether organic solvent, a carboxylate organic solvent, and a sulfite organic solvent.

[0056] Preferably, the carbonate organic solvent is a cyclic carbonate organic solvent and / or a chain carbonate organic solvent.

[0057] Preferably, the cyclic carbonate is ethylene carbonate and / or propylene carbonate.

[0058] Preferably, the chain carbonate is at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0059] Preferably, the organic ether organic solvent is a chain ether organic solvent and / or a cyclic ether organic solvent.

[0060] Preferably, the chain ether organic solvent is dimethoxymethane and / or ethylene glycol dimethyl ether.

[0061] Preferably, the cyclic ether organic solvent is tetrahydrofuran (THF) and / or 2-methyltetrahydrofuran.

[0062] Preferably, the carboxylate organic solvent is at least one of ethyl acetate, methyl propionate and ethyl propionate.

[0063] Preferably, the sulfite organic solvent is at least one of dimethyl sulfite (DMS), diethyl sulfite (EMS), ethylene sulfite (ES), and propylene sulfite (PS).

[0064] Preferably, the flame retardant additive is trimethyl phosphate (TMP) and / or trimethyl borate (TMB).

[0065] More preferably, the flame retardant additives are trimethyl phosphate (TMP) and trimethyl borate (TMB).

[0066] The present invention uses a combination of trimethyl phosphate (TMP) and trimethyl borate (TMB) as flame retardant additives. This flame retardant effect is achieved through different mechanisms, including preventing internal reactions in the battery when the temperature rises, effectively reducing the risk of thermal runaway. This combination of trimethyl phosphate (TMP) and trimethyl borate (TMB) improves the battery's thermal stability while maintaining its electrochemical performance.

[0067] A fifth aspect of the present invention provides a method for preparing a lithium battery.

[0068] A method for preparing a lithium battery comprises the following steps:

[0069] (1) mixing the raw material components of the first slurry to prepare the first slurry, then coating the first slurry on the surface of the positive electrode current collector, performing a first vacuum drying and a first pressing to obtain the positive electrode;

[0070] (2) mixing a thickener with a second solvent, then adding a negative electrode active material, a second conductive agent, and a second binder to mix to prepare a second slurry, then coating the second slurry on the surface of the negative electrode current collector, performing a second vacuum drying and a second pressing to obtain a negative electrode;

[0071] (3) The battery separator is wound to form a battery core, which is then assembled with a positive electrode, a negative electrode, and an electrolyte to form the lithium battery.

[0072] Preferably, the temperature of the first vacuum drying is -0.085 to -0.1 MPa, and / or the time of the first vacuum drying is 6 to 9 hours.

[0073] Preferably, the first pressing pressure is 0.2-0.6 MPa, and / or the first pressing time is 2-5 seconds.

[0074] Preferably, the temperature of the second vacuum drying is -0.085 to -0.1 MPa, and / or the time of the second vacuum drying is 6 to 8 hours.

[0075] Preferably, the second pressing pressure is 0.2-0.6 MPa, and / or the second pressing time is 2-5 seconds.

[0076] Compared with the prior art, the present invention has the following beneficial effects:

[0077] The battery separator of the present invention sequentially comprises a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer, and a fourth oil-based coating layer. The first and fourth oil-based coating layers are made of the same material; the second and third oil-based ceramic coating layers are made of the same material; the raw materials of the second and third oil-based ceramic coating layers include inorganic nanoparticles; and the raw materials of the first and fourth oil-based coating layers include aramid and polyvinylidene fluoride. The battery separator provided by the present invention can provide the additional function of thermal shutdown. The battery separator of the present invention can be further used to prepare lithium batteries, thereby improving the safety of lithium batteries. The prepared lithium batteries can meet test safety regulations. Wearable digital batteries below 1.0Ah can meet needle puncture safety regulations. At the same time, the battery functional performance tests can meet the requirements, while also maintaining the performance of the lithium battery and meeting the cycle requirements, making it suitable for use as wearable digital batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 This is a schematic diagram of the operation process for preparing a lithium battery in Application Example 1;

[0079] Figure 2 Graph showing the capacity retention rate test results of the lithium batteries of Application Example 1 and Comparative Application Example 1 during normal temperature cycle testing;

[0080] Figure 3 Graph showing thickness expansion rate test results of the lithium batteries of Application Example 1 and Comparative Application Example 1 during normal temperature cycle testing;

[0081] Figure 4 This is the discharge test result diagram of the lithium battery at different temperatures in Application Example 1;

[0082] Figure 5 This is the rate performance test result diagram of the lithium battery in Application Example 1. DETAILED DESCRIPTION

[0083] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are given for illustration. It should be noted that the following examples do not limit the scope of protection claimed by the present invention.

[0084] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods.

[0085] The normal temperature mentioned in the present invention refers to 25±3°C.

[0086] Example 1

[0087] A battery separator comprises a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer and a fourth oil-based coating layer in sequence.

[0088] The method for preparing the battery separator comprises the following steps:

[0089] (1) Base film: A base film with a thickness of 5 μm is selected (the base film material is polyethylene, the closed cell temperature of the base film is 120°C, and the film breaking temperature is 225°C);

[0090] (2) Preparation of coating I and coating II:

[0091] Alumina and DMAC were mixed to prepare coating I.

[0092] Coating II was prepared by mixing meta-aramid, PVDF (the weight ratio of meta-aramid to PVDF was 4:6), and DMAC.

[0093] (3) First, coating I is sprayed on the front and back sides of the base film to obtain the second and third oil-based ceramic coating layers, each of which has a thickness of 2 μm; then, coating II is sprayed on the ceramic coating layer to obtain the first and fourth oil-based coating layers, each of which has a thickness of 2 μm.

[0094] The closed-cell temperature of the battery separator prepared by the present invention reaches 220°C, which reduces the thermal shrinkage near the puncture point. The mixed coating has better ductility, can effectively fill the pinholes, and delay the direct contact between the positive and negative electrodes. The outermost layer is coated with a high-temperature resistant, highly flexible, and excellently bonded coating II (a co-solution of oil-based aramid and PVDF), which can improve safety under extreme conditions, improve the wettability of the electrolyte, and have higher adhesion with the electrode, thereby improving the cycle performance and high-temperature and high-voltage resistance of the battery cell, and the characteristics of capacity retention.

[0095] Example 2

[0096] A battery separator comprises a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer and a fourth oil-based coating layer in sequence.

[0097] The difference between the above-mentioned preparation method of the battery separator and Example 1 is that the thickness of the base film in step (1) is replaced by 7 μm

[0098] (The base film is made of polyethylene, the closed-cell temperature of the base film is 120°C, and the film-breaking temperature is 225°C); the alumina in step (2) is replaced with an equal weight of boehmite; and in step (3), coating II is applied on the ceramic coating layer using an extrusion coating process.

[0099] The present invention produces the first and fourth oil-based coating layers to improve comprehensive performance. Specifically, aramid can improve the high temperature resistance, puncture resistance, and oxidation resistance of the battery separator, and PVDF can enhance the hardness of the battery core and improve the ionic conductivity and cycle life of the battery. The combination of the two can achieve a better balance in the battery separator in terms of high temperature resistance, puncture resistance, oxidation resistance, adhesion, and liquid absorption and retention, thereby improving the overall performance of the battery. In addition, in terms of safety: the high temperature resistance and puncture resistance of aramid can effectively prevent the battery from short circuiting, fire and other safety accidents when it is exposed to high temperature or subjected to external force impact. The adhesion and chemical stability of PVDF can also ensure the safe operation of the battery to a certain extent. The synergistic effect of the two can further improve the safety of the battery. The oil-based ceramic coating containing alumina is coated on both sides to improve the high temperature resistance of the battery separator, and the battery separator can maintain its intact shape below 250°C; it can neutralize the free HF in the electrolyte and improve the acid resistance and safety performance of the battery; nano-alumina can form a solid solution in lithium batteries, improve the rate and cycle performance; good wettability improves the battery's liquid absorption and liquid retention capabilities.

[0100] Comparative Example 1

[0101] A battery separator, which differs from Example 1 in that the base membrane used is a conventional pure base membrane, which does not have the low closed-pore and high rupture properties, has a closed-pore temperature of 135°C and a rupture temperature of 155°C, and only contains a first oil-based coating layer and a second oil-based ceramic coating layer.

[0102] Comparative Example 2

[0103] A battery separator, different from Example 1, in that the base film is a conventional pure base film, which does not have the low closed-pore and high rupture properties, has a closed-pore temperature of 135°C and a rupture temperature of 155°C. Safety test needle penetration test items cannot be 100% passed.

[0104] Comparative Example 3

[0105] A battery separator, which differs from Example 1 in that it only includes a first oil-based coating layer and a second oil-based ceramic coating layer. Safety test needle penetration test items cannot achieve 100% passing.

[0106] Comparative Example 4

[0107] A battery separator, which differs from Example 1 in that aramid is not added to coating II. Safety test needle puncture items cannot achieve 100% passing.

[0108] Application Example 1

[0109] A preparation method of a lithium battery (642833-800mAh) Figure 1 ), including the following steps:

[0110] (1) Positive electrode: Take a 9 μm thick positive electrode current collector aluminum foil, coat the first slurry on the surface of the positive electrode current collector, vacuum dry it at 60 ° C for 8 h, and then roll it into a 100 μm thick electrode sheet at a pressure of 0.35 MPa, matching the corresponding compaction density (compaction density is 4.05 g / cm 3 ) to produce a positive electrode sheet with an active material content of 98.5% by mass. The first slurry is prepared by dissolving the binder polyvinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) to produce a positive electrode paste. The main positive electrode material of lithium cobalt oxide (LiCoO2), conductive carbon black (SP) with a mass content of 0.5%, and carbon nanotubes (CNT) with a mass content of 1.0% are then thoroughly mixed and stirred to produce a first slurry (positive electrode slurry) with a solid content of 75% and a viscosity of 6000 mPa·s.

[0111] (2) Negative electrode: Take a 5 μm thick copper foil as the negative electrode current collector, coat the second slurry on the surface of the negative electrode current collector, vacuum dry it at 55 ° C for 8 h, and then roll it into a 135 μm thick electrode sheet at a pressure of 0.4 MPa. The matching compaction density is 1.73 g / cm 3 The second slurry is prepared by dissolving the thickener CMC (sodium carboxymethyl cellulose) in deionized water to form a glue solution, which is then mixed with 96.5% artificial graphite, 0.8% conductive carbon black (SP), and 1.0% SBR binder. The mixture is then slurried with deionized water to form a second slurry (negative electrode slurry) with a solid content of 46% and a viscosity of 7000 mPa·s.

[0112] The above positive and negative electrode sheets are welded to the tabs, and after welding, the tabs are taped and inspected by a CCD vision system (charge coupled device vision system).

[0113] (3) Battery electrolyte: It is composed of 75% by mass of organic solvent (ethylene carbonate EC), 15% by mass of lithium hexafluorophosphate (LiPF6), and 10% by mass of flame retardant additives (trimethyl phosphate (TMP) and trimethyl borate (TMB) in a weight ratio of 5:5). The electrolyte has thermally stable additives and has the effect of a composite flame retardant additive. The composition combines additives with multiple flame retardant mechanisms to simultaneously inhibit the decomposition of the electrolyte, playing a role in auxiliary protection and improving safety to a certain extent. Ultimately, it effectively improves the safety performance of the battery while maintaining the functional performance of the lithium-ion battery, and the number of cycles can meet the requirements.

[0114] (4) The positive electrode, negative electrode and battery separator prepared in Example 1 are wound into a battery cell through a fully automatic winding machine, a pole piece supply system, a battery separator unwinding system, a tension control system, a tail glue system, a material unloading transmission system, a foot switch and a human-machine interface system. The battery is sealed at a temperature of 185°C, a pressure of 0.35 MPa and a packaging time of 4 seconds. The top and bottom seals of the battery are completed. The airbag side injection port is filled with heat-stable additives. The specific types include film-forming additives: vinyl carbonate (VC), fluoroethylene carbonate (FEC), and flame retardant additives: trimethyl phosphate (TMP) and trimethyl borate (TMB). It has the effect of a composite flame retardant additive to inhibit the decomposition of the electrolyte. Then the pre-sealing is completed. The battery after injection is placed at a high temperature of 45°C for 16 hours to 24 hours. The battery is formed and degassed. The upper and lower seals of the two seals are heated and closed and air pressure is applied to complete the capacity separation and make the finished battery.

[0115] The battery model of Application Example 1 is "642833-800mAh", where 642833 indicates a thickness of 6.4 mm, a width of 28 mm, and a length of 33 mm.

[0116] Application Example 2

[0117] A lithium battery is prepared using the preparation method of Application Example 1. The only difference from Application Example 1 is that the battery separator of Example 1 is replaced by the battery separator of Example 2.

[0118] Comparative Application Example 1

[0119] A conventional material system is used to produce a battery. The difference from Application Example 1 is that the battery separator of Comparative Example 1 is used and there is no flame retardant additive in the electrolyte.

[0120] The results show that after the lithium battery of comparative application example 1 is fully charged at room temperature, the pass rate of the needle penetration test at room temperature is about 30%-50%, which cannot meet the daily use requirements of high battery safety.

[0121] Comparative Application Example 2

[0122] A conventional material system is used to produce a battery. The difference from Application Example 1 is that the battery separator of Comparative Example 2 is used and there is no flame retardant additive in the electrolyte.

[0123] Comparative Application Example 3

[0124] A conventional material system is used to produce a battery. The difference from Application Example 1 is that the battery separator of Comparative Example 3 is used and the electrolyte does not contain flame retardant additives.

[0125] Comparative Application Example 4

[0126] A conventional material system is used to produce a battery. The difference from Application Example 1 is that the battery separator of Comparative Example 4 is used and the electrolyte does not contain flame retardant additives.

[0127] Product effect testing

[0128] 1. Security

[0129] (1) Test method

[0130] First, fully charge the battery at the following temperature requirements and a current of 0.5C. Then, insert a steel nail with a diameter of 2.5mm-3.5mm vertically through the center of the fully charged battery cell at a speed of 2m / min and maintain this speed for more than 30 minutes. The following items are tested respectively: (1) fully charged acupuncture at room temperature, (2) fully charged acupuncture at low temperature of -5℃, (3) fully charged at low temperature of -5℃, and external short circuit test at room temperature. The test quantities are 20pcs, 10pcs, and 10pcs respectively.

[0131] Instrument: Battery Penetration Tester.

[0132] Acceptance requirements: Within the initial 10 minutes of the nail penetration test, the battery cell shall not smoke, catch fire or explode.

[0133] (2) Test results

[0134] Table 1 Safety regulation test results of various lithium batteries (referred to as safety regulation test)

[0135]

[0136] As can be seen from the table above, the lithium battery of Application Example 1 has a high safety test pass rate, with 100% passing rate in various needle penetration tests, indicating high safety.

[0137] 2. Cyclic performance capacity retention rate and thickness expansion rate

[0138] (1) Test method:

[0139] At room temperature (25±3°C), charge the battery to 4.4V at a constant current of 1.0C. Then charge at a constant voltage of 4.4V until the charge cut-off current is ≤0.02C. Discharge the battery to 3.0V at a current of 1.0C. Repeat these steps 500 times.

[0140] Acceptance requirements:

[0141] The capacity after 500 cycles is divided by the capacity of the initial first week, and the capacity retention rate is ≥80%;

[0142] The thickness after 500 cycles is divided by the initial thickness in the first cycle, and the thickness expansion rate is ≤8%.

[0143] (2) Test results

[0144] Table 2 Capacity retention test results of different lithium batteries under normal temperature cycle test

[0145]

[0146] In the table, 1#-3# represent three batteries produced in the same batch.

[0147] Depend on Figure 2 As can be seen from Table 2, the cycle performance capacity retention rate of the battery of Application Example 1 is high and the performance remains unchanged. Figure 2 It can be seen that the curves of batteries 1#-3# produced in the same batch overlap, indicating that the batch consistency of the batteries is very good, and there will be no deviation or difference in the quality of the battery products. The capacity retention rate and thickness expansion rate data after all cycles are basically consistent. In contrast, the capacity retention rate of application example 1 (conventional material battery) is lower, and the curve separation is more obvious, indicating that the battery batch consistency is poor. Figure 2 In the figure, Cycle Capacity Retention is the cycle capacity retention rate, and CycleNumber is the cycle number.

[0148] Table 3 Thickness expansion test results of different lithium batteries under normal temperature cycle test

[0149]

[0150] In the table, 1#-3# represent three batteries produced in the same batch.

[0151] Depend on Figure 3 As shown in Table 3, the thickness expansion rate of the battery in Application Example 1 is low in the room temperature cycle test, which meets the requirements, indicating that the battery has high stability and safety in use. Figure 3 In the figure, Swelling is the expansion rate and Cycle Number is the number of cycles.

[0152] 3. High and low temperature discharge performance test

[0153] (1) Test method: At room temperature (25±3°C), charge the battery to 4.4V at a constant current of 0.5C, then charge at a constant voltage of 4.4V until the charge cut-off current is ≤0.02C. Place the battery in the following temperature ranges: 25°C±2°C, 60°C±2°C, 0°C±2°C, -10°C±2°C, -20°C±2°C for 2 hours, and discharge the battery to a cut-off voltage of 3.0V at a discharge current equal to the capacity rating. Inspect the battery for appearance.

[0154] Instruments: high-precision battery performance testing system, high and low temperature chamber.

[0155] (2) Test results

[0156] Table 4 Discharge test results of lithium batteries in application example 1 at different temperatures

[0157]

[0158] Depend on Figure 4 As can be seen from Table 4, the discharge performance test results of the lithium battery of Application Example 1 at different temperatures are all qualified and meet the requirements. The battery has high reliability under extreme temperature conditions. Figure 4 Where Voltage is voltage and Capacityretention rate is capacity retention rate.

[0159] 4. Rate performance test

[0160] (1) Test method: At room temperature (25±3°C), charge the battery to 4.4V at a constant current of 0.5C, then charge at a constant voltage of 4.4V until the charge cut-off current is ≤0.02C. After stopping charging, place the battery aside for 0.5h to 1h, and discharge the battery to the cut-off voltage at a constant current of 0.2C, 0.5C, 1.0C, and 1.5C at 25±3°C (3 cycles, based on the highest discharge capacity).

[0161] (2) Test results

[0162] Table 5 Rate performance test results of lithium battery in Application Example 1

[0163]

[0164] Depend on Figure 5 As shown in Table 5, the lithium battery of Application Example 1 is qualified at different rate tests at 25°C, and all meet the requirements, with excellent rate performance. Figure 5 Where Voltage is voltage and Capacity retention rate is capacity retention rate.

[0165] In summary, the battery separator provided by the present invention includes a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer and a fourth oil-based coating layer in sequence. The different coating layers are matched with a base film with the characteristics of high safety, high strength, low closed pores, high rupture, high strength and thin film to improve the safety of lithium batteries. The battery separator can provide an additional function of thermal shutdown. The battery separator itself has a microporous structure (the present invention uses a base film with a microporous structure). When the battery generates a large amount of heat and the temperature is close to the melting point of the material polymer, the micropores close to form a thermal shutdown, which can block the continued transmission of ions to form a short circuit and play a role in protecting the battery. Therefore, a battery separator with a low closed pore temperature and a high rupture temperature can greatly improve the battery safety performance. However, ordinary battery separators generally rupture at around 130-150°C, and the rupture temperature is low. The battery separator of the present invention has a rupture temperature as high as 205-230°C, and the resulting battery is safer. In order to make the closed-pore and ruptured membranes of battery separators play a role in battery safety, the longer the temperature interval between closed-pore and ruptured membranes, the more obvious the difference between the low closed-pore and high ruptured membrane characteristics of the base membrane will be, ultimately achieving the goal of comprehensively improving safety, while simultaneously providing good mechanical support and chemical stability to maintain battery performance.

Claims

1. A battery separator, characterized in that: The invention sequentially comprises a first oil-based coating layer, a second oil-based ceramic coating layer, a base film, a third oil-based ceramic coating layer and a fourth oil-based coating layer; The first oil-based coating layer and the fourth oil-based coating layer are made of the same material; The second oil-based ceramic coating layer and the third oil-based ceramic coating layer are made of the same material; The raw material components of the second oil-based ceramic coating layer and the third oil-based ceramic coating layer include: inorganic nanoparticles; The raw material components of the first oil-based coating layer and the fourth oil-based coating layer include: aramid and polyvinylidene fluoride.

2. The battery separator according to claim 1, characterized in that The inorganic nanoparticles are aluminum oxide and / or boehmite.

3. The battery separator according to claim 1, characterized in that The base film is a polyethylene base film or a polypropylene base film.

4. The method for preparing a battery separator according to any one of claims 1 to 3, characterized in that: The steps include: Inorganic nanoparticles and organic solvent I are mixed to obtain coating I; aramid, polyvinylidene fluoride and organic solvent II are mixed to obtain coating II; coating I is then applied to the front and back sides of the base film respectively to obtain the second and third oil-based ceramic coating layers, respectively; coating II is then applied to the surface of the second and third oil-based ceramic coating layers respectively to obtain the first and fourth oil-based coating layers, respectively, to prepare the battery separator.

5. Use of the battery separator according to any one of claims 1 to 3 in the preparation of batteries.

6. A lithium battery, characterized in that: The battery comprises a positive electrode, a negative electrode, the battery separator according to any one of claims 1 to 3, and an electrolyte.

7. The lithium battery according to claim 6, characterized in that The positive electrode includes a positive electrode current collector and a first coating layer coated on the surface of the positive electrode current collector. The first coating layer is obtained by coating a first slurry. The raw material components of the first slurry include a positive electrode active material, a first binder, a first solvent and a first conductive agent.

8. The lithium battery according to claim 6, characterized in that The negative electrode includes a negative electrode current collector and a second slurry coated on the surface of the negative electrode current collector. The second coating layer is obtained by coating the second slurry. The raw material components of the second slurry include a negative electrode active material, a second conductive agent, a thickener, a second binder and a second solvent.

9. The lithium battery according to claim 6, characterized in that The components of the electrolyte include lithium salt, organic solvent and flame retardant additive.

10. The method for preparing a lithium battery according to any one of claims 6 to 9, characterized in that: The steps include: (1) mixing the raw material components of the first slurry to prepare the first slurry, then coating the first slurry on the surface of the positive electrode current collector, performing a first vacuum drying and a first pressing to obtain the positive electrode; (2) mixing a thickener with a second solvent, then adding a negative electrode active material, a second conductive agent, and a second binder to mix to prepare a second slurry, then coating the second slurry on the surface of the negative electrode current collector, performing a second vacuum drying and a second pressing to obtain a negative electrode; (3) The battery separator is wound to form a battery core, which is then assembled with a positive electrode, a negative electrode, and an electrolyte to form the lithium battery.

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