Composite diaphragm, preparation method and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a composite diaphragm, a preparation method and a lithium ion battery. The composite diaphragm is formed by stacking four layers of structures, the first layer is a lithium supplement-flame retardant layer, the second layer and the fourth layer are both ion conductor-oxide layers, and the third layer is a base film; wherein the lithium supplement-flame retardant layer contains a lithium supplement agent and a phosphorus flame retardant. The composite diaphragm not only can effectively supplement lithium element in the charging and discharging process of the battery and reduce the attenuation of the battery capacity, but also has good flame retardant property and improves the safety of the battery, and meanwhile, collaborative optimization of all functional layers is realized through unique structural design.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a composite diaphragm, a preparation method of the composite diaphragm and a lithium ion battery. BACKGROUND

[0002] With the wide application of batteries in many fields, especially in electric vehicles and energy storage systems, higher requirements are put forward for the safety and long-life performance of batteries. During the operation of the battery, lithium loss leads to prominent battery capacity attenuation problems, and the safety hazards caused by battery thermal runaway cannot be ignored. Traditional diaphragms cannot meet the growing demand for high-performance batteries as they are difficult to balance multiple key performances such as lithium supplement and flame retardation. SUMMARY

[0003] The purpose of the present application is to overcome the above technical problems, provide a composite diaphragm and a preparation method, and a lithium ion battery. The composite diaphragm not only can effectively supplement lithium elements during the charging and discharging process of the battery, reduce the capacity attenuation of the battery, but also has good flame retardation performance, improves the safety of the battery, and realizes the synergistic optimization of each functional layer through a unique structural design.

[0004] In order to achieve the above purpose, the present application provides a composite diaphragm, which is composed of four layers of structure stacked together, the first layer is a lithium supplement-flame retardant layer, the second layer and the fourth layer are both ion conductor-oxide layers, and the third layer is a base film; wherein the lithium supplement-flame retardant layer contains a lithium supplement agent and a phosphorus-based flame retardant.

[0005] Preferably, the lithium supplement agent and the phosphorus-based flame retardant can form a Li-O-P bond.

[0006] In the present application, without special circumstances, the four layers of structure stacked together means that the lithium supplement-flame retardant layer, the ion conductor-oxide layer, the base film and the ion conductor-oxide layer are sequentially arranged from top to bottom or from bottom to top.

[0007] The composite diaphragm provided by the present application has the functions of lithium supplement and flame retardation, and has excellent ion conduction performance under the condition of good oxidation and reduction resistance, thereby improving the low temperature performance of the battery cell. Through careful design of the material selection and preparation process of the lithium supplement-flame retardant layer, lithium ions can be released at a suitable potential to provide continuous lithium source supplement for the battery; before lithium supplement, the phosphorus-based flame retardant and the lithium supplement agent are closely combined together, after lithium supplement, the lithium supplement agent is gasified and decomposed or phase-transformed into smaller particles to realize the function of "pore forming", thereby improving the drawbacks of decreased porosity and increased air permeability caused by the addition of the phosphorus-based flame retardant. That is, after lithium supplement, the original phosphorus-based flame retardant can continue to effectively play its flame retardation function.

[0008] The second aspect of the present application provides a preparation method of a composite separator, the preparation method comprising:

[0009] (1) coating a slurry containing an ion conductor and an oxide on both sides of a base film, and performing first drying to form ion conductor-oxide layers on both sides of the base film, respectively;

[0010] (2) coating a slurry containing a lithium supplement agent and a phosphorus-based flame retardant on any ion conductor-oxide layer, and performing second drying to form a lithium supplement-flame retardant layer, thereby obtaining the composite separator.

[0011] The third aspect of the present application provides a lithium ion battery, which contains a positive electrode, a negative electrode, and the composite separator provided by the first aspect or the composite separator prepared by the preparation method provided by the second aspect.

[0012] Among them, the first layer of the composite separator is close to the positive electrode side, and the fourth layer is close to the negative electrode side.

[0013] Through the above technical solution, the composite separator provided by the present application can supplement lithium elements in real time during the operation of the battery, effectively inhibit the attenuation of the battery capacity, and improve the cycle performance of the battery. At the same time, it also has good flame retardant performance, and improves the safety performance of the battery.

[0014] At the same time, in the composite separator provided by the present application, the specific lithium supplement-flame retardant layer can ensure the initial flame retardant performance, and as the lithium supplement process proceeds, the porosity is improved to promote the transmission of lithium ions, thereby realizing the optimization of lithium supplement and transmission performance. The ion conductor-oxide layer ensures the rapid transmission of lithium ions, improves the stability and reliability of the separator, and thus improves the charge-discharge performance and safety of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a structural schematic diagram of a composite separator provided by the present application. DETAILED DESCRIPTION

[0016] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges or values should be construed to be approximations that allow for significant variation. Various ranges of values that are explicitly delineated between the endpoints, between a endpoint and a single value, and between single values, can be combined to form one or more new ranges of values that are also expressly disclosed.

[0017] The first aspect of the present application provides a composite separator, which is composed of four layers of structure stacked together, the first layer is a lithium supplement-flame retardant layer, the second layer and the fourth layer are both ion conductor-oxide layers, and the third layer is a base film; wherein the lithium supplement-flame retardant layer contains a lithium supplement agent and a phosphorus-based flame retardant.

[0018] In the present application, the schematic diagram of the composite diaphragm structure is shown as Figure 1 The one side surface of the base film is an ion conductor-oxide layer, and the other side surface is sequentially an ion conductor-oxide layer and a lithium supplement-flame retardant layer from inside to outside. Preferably, only the side of the ion conductor-oxide layer faces the negative electrode, and the side containing the lithium supplement-flame retardant layer faces the positive electrode.

[0019] In the present application, preferably, the lithium supplement agent and the phosphorus-based flame retardant can form a Li-O-P bond. Specifically, the lithium supplement agent and the phosphorus-based flame retardant can form a Li-O-P bond during the lithium supplement process.

[0020] In the present application, in the lithium supplement-flame retardant layer, the lithium supplement agent can release lithium elements during the battery operation process, and the phosphorus-based flame retardant is closely combined with the lithium supplement agent in the normal state, and can change the structure to increase the gap between the phosphorus-based flame retardant molecules after the lithium supplement agent works, thereby improving the porosity of the layer.

[0021] At the same time, the phosphorus-based flame retardant containing a phosphorus-oxygen group not only ensures the flame retardant effect in the initial stage of the normal operation of the battery, but also provides a more unobstructed path for the transmission of lithium ions during the lithium supplement process, thereby reducing the resistance of lithium ion transmission.

[0022] In the present application, preferably, in the lithium supplement-flame retardant layer, the mass ratio of the lithium supplement agent to the phosphorus-based flame retardant is 5-50:5-40, for example, 5:5, 5:10, 5:25, 10:10, 10:15, 10:25, 15:10, 15:20, 15:25, 20:10, 20:20, 20:25, 25:10, 25:20, 25:25, 28:10, 28:15, 30:10, 30:15, 30:25, 40:20, 40:40, 50:5, 50:25, 50:40, and any value in the range formed by any two numerical values, and preferably 10-30:10-25.

[0023] In the present application, the above mass ratio can simultaneously optimize the lithium supplement effect and the flame retardant effect, thereby improving the lithium ion diffusion rate and the porosity of the lithium supplement-flame retardant layer.

[0024] In some embodiments of the present application, preferably, the lithium supplement agent is selected from Li2C4O4, Li2CO3, Li2NiO2, Li2C2O2, and derivatives Li 2-x M xat least one of C2O4, wherein 0 < x < 2, M is a metal element, and further preferably, M is selected from at least one of Ni, Mn and Fe. Through careful design of the lithium supplement layer material selection and preparation process, lithium ions can be released at a suitable potential to provide a continuous lithium source supplement for the battery.

[0025] In the present application, the lithium supplement-flame retardant layer is arranged on the positive electrode side, and the above-mentioned safe and stable lithium supplement is used for lithium supplement, which can not only adjust the potential as set, but also has the characteristics of safety and low environmental requirements compared with lithium supplement on the negative electrode side.

[0026] In some embodiments of the present application, preferably, the phosphorus-based flame retardant is selected from a flame retardant containing a phosphorus-oxygen bond; in the present application, the phosphorus-oxygen bond includes but is not limited to P=O, P-O-, etc.

[0027] In some embodiments of the present application, further preferably, the phosphorus-based flame retardant is selected from at least one of polyamine polyphosphate, phosphorus amine, trimethylphenyl phosphate and phosphazene compounds; in the present application, the phosphazene compound includes but is not limited to hexaphenoxycyclophosphazene, etc.

[0028] In the present application, the flame retardant mechanism of the phosphazene compound is: endothermic decomposition when heated, cooling effect, releasing CO2, NH3, H2O gas, diluting oxygen, forming non-volatile protective film on the surface of the polymer, and isolating air; in addition, PO· is formed during polymer combustion, which combines with H·, HO· free radicals in the flame to inhibit the flame.

[0029] In the present application, the second layer and the fourth layer are the same in composition, both being ion conductor-oxide layers, unless otherwise specified.

[0030] In the present application, preferably, the ion conductor-oxide layer contains an ion conductor and an oxide.

[0031] In the present application, the ion conductor can quickly conduct lithium ions to improve the charge and discharge efficiency of the battery; the oxide has the effect of enhancing the mechanical properties and chemical stability of the separator, and at the same time, screening and regulating the lithium ions released by the lithium supplement-flame retardant layer to ensure that the lithium ions can be uniformly and stably transmitted to other parts of the battery.

[0032] In the present application, further preferably, the mass ratio of the ion conductor and the oxide is 1-50:5-50, for example, 1:5, 1:10, 5:10, 5:20, 5:30, 20:10, 20:20, 20:30, 25:10, 25:15, 25:25, 25:30, 30:10, 30:30, 50:10, 50:20, 50:50, and any value in the range between any two numerical values, preferably 5-30:10-30.

[0033] In some embodiments of the present application, preferably, the ion conductivity of the ion conductor is ≥10 -4 S / cm, preferably selected from solid-state electrolytes, more preferably selected from at least one of PEO, LiPON, LATP and LLTO.

[0034] In the present application, polyethylene oxide is abbreviated as PEO, lithium phosphorus oxynitride is abbreviated as LiPON, lithium aluminum titanium phosphate is abbreviated as LATP, and lithium lanthanum titanium oxide is abbreviated as LLTO.

[0035] In the present application, the ion conductivity parameter is measured by the alternating current impedance method, including: clamping the sample to be measured between two stainless steel disc electrodes (SS), measuring the ion conductivity (σ) by electrochemical impedance spectroscopy (EIS), and calculating according to formula (1): wherein R b is the volume resistance (R b determined by impedance spectroscopy) of the sample to be measured, and L and S are the thickness and area of the sample to be measured.

[0036] In the present application, the ion conductivity parameter is measured at room temperature, which refers to 25±2℃.

[0037] In some embodiments of the present application, preferably, the oxide is selected from metal oxides, preferably selected from Al2O3 and / or TiO2.

[0038] In the present application, the base film serves as the supporting structure of the entire composite separator, providing good mechanical strength and flexibility. The base film uses conventional separator materials, including but not limited to polypropylene (PP), polyethylene (PE), etc.

[0039] In the present application, preferably, the thickness of the lithium supplementing-flame-retardant layer is 0.1-10 μm, for example, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 4 μm, 5 μm, 6 μm, 8 μm, 10 μm, and any value in the range between any two numerical values, preferably 0.5-8 μm.

[0040] In the present application, when the thickness of the lithium supplementing-flame-retardant layer is <0.1 μm or even none, the lithium supplementing amount may be insufficient, or the flame-retardant effect may be poor, so that the shrinkage of the composite separator is significantly increased, the surface layer has the risk of powder dropping and falling off, the safety is reduced, and the cycle performance is reduced; when the thickness of the lithium supplementing-flame-retardant layer is >10 μm, the overall impedance of the separator may be increased, the lithium ion transmission performance is reduced, the air permeability of the composite separator is increased, and the cycle performance is reduced.

[0041] In the present application, the thickness parameters are measured by a thickness gauge.

[0042] In the present application, preferably, the thicknesses of the second layer and the fourth layer are selected from the same or different.

[0043] In the present application, preferably, the thickness of the ion conductor-oxide layer is 0.1-7 μm, for example, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 4 μm, 5 μm, 7 μm, and any value in the range formed by any two numerical values, preferably 0.5-5 μm.

[0044] In the present application, when the thickness of the ion conductor-oxide layer is <0.1 μm or even none, the surface of the base film may not be completely covered, the risk of puncture is increased, the ion conductivity of the composite separator is reduced, the shrinkage is increased, the impedance is increased, and the cycle performance is reduced; when the thickness of the ion conductor-oxide layer is >7 μm, the cost may be increased, the air permeability is increased, the impedance is increased, and the cycle performance is reduced.

[0045] In the present application, preferably, the thickness of the base film is 3-15 μm, for example, 3 μm, 5 μm, 7 μm, 9 μm, 10 μm, 12 μm, 15 μm, and any value in the range formed by any two numerical values, preferably 3-10 μm.

[0046] In the present application, the preparation method of the composite separator has a wide selection range, as long as the composite separator meets the four-layer structure setting described above. Preferably, the four layers of materials are combined together by a forming treatment to form a whole which is tightly combined and functionally complementary, and the forming treatment mode includes but is not limited to coating, hot pressing, etc.

[0047] In the present application, preferably, the air permeability of the composite separator is ≤ 145 S / 100 mL, for example, 140 S / 100 mL, 135 S / 100 mL, 130 S / 100 mL, 125 S / 100 mL, 120 S / 100 mL, 118 S / 100 mL, 117 S / 100 mL, 116 S / 100 mL, 115 S / 100 mL, 113 S / 100 mL, 112 S / 100 mL, 110 S / 100 mL, 105 S / 100 mL, 100 S / 100 mL, 95 S / 100 mL, 90 S / 100 mL, and any value in the range consisting of any two numerical values, preferably ≤ 130 S / 100 mL, more preferably 110-120 S / 100 mL.

[0048] In the present application, the above-mentioned air permeability parameter is measured by using a Gurley 4340 air permeability instrument of the Gurley Company of the United States.

[0049] In the present application, preferably, the ionic conductivity of the composite separator is ≥ 0.8 ms / cm, for example, 0.85 ms / cm, 0.86 ms / cm, 0.88 ms / cm, 0.90 ms / cm, 0.91 ms / cm, 0.92 ms / cm, 0.93 ms / cm, 0.94 ms / cm, 0.95 ms / cm, 0.97 ms / cm, 0.98 ms / cm, 0.99 ms / cm, 1 ms / cm, and any value in the range consisting of any two numerical values, preferably ≥ 0.9 ms / cm.

[0050] In the present application, the above-mentioned ionic conductivity parameter is measured by using an electrochemical workstation to measure the composite separator to be tested at 25±2℃.

[0051] In the present application, preferably, the shrinkage of the composite separator is ≤ 1%, for example, 1%, 0.8%, 0.5%, 0.2%, 0.1%, 0.08%, 0.06%, 0.05%, 0.04%, 0.03%, 0.01%, and any value in the range consisting of any two numerical values, preferably ≤ 0.1%.

[0052] In the present application, the above-mentioned shrinkage parameter is tested as follows: the composite separator to be tested is cut into a size of 100×100 mm and placed in an oven at 150℃ for 2 h, and the maximum shrinkage of the area is calculated.

[0053] The second aspect of the present application provides a preparation method of a composite separator, which comprises:

[0054] (1) coating a slurry containing an ionic conductor and an oxide on both sides of the base film, and forming an ionic conductor-oxide layer on both sides of the base film through first drying;

[0055] (2) coating the slurry containing the lithium supplement agent and the phosphorus-based flame retardant on either side of the ion conductor-oxide layer, and performing second drying to form a lithium supplement-flame retardant layer, thereby obtaining a composite separator.

[0056] In the present application, the types and physical parameters of the ion conductor, the oxide, the base film, the lithium supplement agent and the phosphorus-based flame retardant are defined according to the above, and no further description is given herein.

[0057] In the present application, in step (1), coating the slurry containing the ion conductor and the oxide on both sides of the base film means coating the slurry on both sides of the base film respectively, and then forming the ion conductor-oxide layer on both sides respectively.

[0058] In the present application, in step (1), preferably, the mass ratio of the ion conductor to the oxide in the slurry containing the ion conductor and the oxide is 1-50:5-50, for example, 1:5, 1:10, 5:10, 5:20, 5:30, 20:10, 20:20, 20:30, 25:10, 25:15, 25:25, 25:30, 30:10, 30:30, 50:10, 50:20, 50:50, and any value in the range between any two numerical values, preferably 5-30:10-30.

[0059] In the present application, in step (1), the first drying is intended to remove the solvent in the slurry containing the ion conductor and the oxide. Preferably, the temperature of the first drying is 60-120°C, and the time is 0.1-15h.

[0060] In the present application, in step (2), the mass ratio of the lithium supplement agent to the phosphorus-based flame retardant in the slurry containing the lithium supplement agent and the phosphorus-based flame retardant is 5-50:5-40, for example, 5:5, 5:10, 5:25, 10:10, 10:15, 10:25, 15:10, 15:20, 15:25, 20:10, 20:20, 20:25, 25:10, 25:20, 25:25, 28:10, 28:15, 30:10, 30:15, 30:25, 40:20, 40:40, 50:5, 50:25, 50:40, and any value in the range between any two numerical values, preferably 10-30:10-25.

[0061] In the present application, in step (2), the second drying is intended to remove the solvent in the slurry containing the lithium supplement agent and the phosphorus-based flame retardant. Preferably, the temperature of the second drying is 60-120°C, and the time is 0.1-15h.

[0062] In the present application, the solid content of the slurry containing the ion conductor and the oxide and the solid content of the slurry containing the lithium supplement agent and the phosphorus-based flame retardant are each independently 40-60wt%.

[0063] In the present application, the solvent of the slurry containing ion conductor and oxide and the solvent of the slurry containing lithium supplement agent and phosphorus-based flame retardant are each independently selected from DMF and / or NMP.

[0064] In the present application, the slurry containing ion conductor and oxide contains a binder in addition to ion conductor and oxide. Similarly, the slurry containing lithium supplement agent and phosphorus-based flame retardant contains a binder in addition to lithium supplement agent and phosphorus-based flame retardant.

[0065] In the present application, the binder includes, but is not limited to, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF) and its modified materials, nitrile rubber (NBR) and its modified materials, polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), polyamide-imide (PAI), polyethyleneimine (PEI), polyimide (PI) and t-butyl acrylate triethoxyl vinyl silane (TBATEVS), etc.

[0066] In one specific embodiment of the present application, the preparation method comprises:

[0067] 1) Base film selection: selecting appropriate polypropylene or polyethylene;

[0068] 2) Preparation of ion conductor-oxide layer: mixing ion conductor and oxide respectively, then adding solvent and binder, stirring uniformly to form a slurry with solid content of 40-60wt%; specifically, the mass ratio of ion conductor, oxide and binder is 1-50:5-50:0-5, preferably 5-30:10-30:1-5; coating the above slurry on both sides of the base film by means of doctor blade coating or casting, then performing first drying and first curing to form ion conductor-oxide layer;

[0069] 3) Preparation of lithium supplement-flame retardant layer: mixing lithium supplement agent, flame retardant and binder in solvent, stirring uniformly to form a slurry with solid content of 40-60wt%; specifically, the mass ratio of lithium supplement agent, flame retardant and binder is 5-50:5-40:0-5, preferably 10-30:10-25:1-5; coating the above slurry on either ion conductor-oxide layer by means of doctor blade coating or casting, then performing second drying and second curing to form lithium supplement-flame retardant layer, obtaining a composite separator with four layers of structure arranged in layers.

[0070] The third aspect of the present application provides a lithium ion battery, which contains a positive electrode, a negative electrode, and the composite separator provided by the first aspect, or the composite separator prepared by the preparation method provided by the second aspect.

[0071] The first layer of the composite separator is close to the positive electrode side, and the fourth layer is close to the negative electrode side.

[0072] In the present application, preferably, the lithium ion battery is selected from a liquid battery, a semi-solid electrode, and a full-solid battery.

[0073] In the present application, preferably, the lithium ion battery has a working temperature of -40°C to 80°C.

[0074] According to a particularly preferred embodiment of the present application, a composite separator is composed of four layers of structure stacked together, the first layer is a lithium supplement-flame retardant layer, the second layer and the fourth layer are both ion conductor-oxide layers, and the third layer is a base film; the lithium supplement-flame retardant layer contains a lithium supplement agent and a phosphorus-based flame retardant;

[0075] The lithium supplement agent and the phosphorus-based flame retardant can form a Li-O-P bond.

[0076] The thickness of the lithium supplement-flame retardant layer is 0.5-8 μm; the thickness of the ion conductor-oxide layer is 0.5-5 μm; the thickness of the second layer and the fourth layer is selected from the same or different.

[0077] The present application will be described in detail below through examples.

[0078] The polyethylene is purchased from Hebei Jinli New Energy Technology Co., Ltd., model SU09.

[0079] The air permeability parameter is measured by a Gurley 4340 air permeability instrument of the Gurley Company, USA;

[0080] The ion conductivity parameter is measured by an electrochemical scheme to measure the ion conductivity of the composite separator to be tested at 25°C.

[0081] The shrinkage parameter: the composite separator to be tested is cut into a size of 100x100 mm and placed in an oven at 150°C for 2h, and the maximum shrinkage of the area is calculated.

[0082] Cycle performance test:

[0083] Assemble the battery: use the composite separator provided in the examples and comparative examples as the separator, use NCM811 as the positive electrode, use a 25wt% SiO+75wt% graphite system as the negative electrode, use EC+DMC / EMC (1:3, volume ratio)+1mol LiF6PO4 as the electrolyte, and assemble to obtain a lithium ion battery. Record the cycle number when the lithium ion battery assembled by the examples and comparative examples is cycled to 80% SOC at 45°C-1C / 1C, full SOC cycle.

[0084] Example 1

[0085] (1) 20 parts by mass of ion conductor LATP, 20 parts by mass of alumina, 2 parts by mass of binder PVDF and solvent NMP were mixed to obtain a slurry with a solid content of 50 wt%, which was coated on both sides of a base film (polyethylene, thickness 7 μm) by a doctor blade method, dried at 80°C for 2 h, and a second ion conductor-oxide layer with a thickness of 2 μm and a fourth ion conductor-oxide layer with a thickness of 2 μm were formed on both sides of the base film, respectively;

[0086] (2) 15 parts by mass of lithium supplement Li2C4O4, 20 parts by mass of flame retardant polyamine polyphosphate, 3 parts by mass of binder PVA and DMF were mixed to obtain a slurry with a solid content of 50 wt%, which was coated on the surface of the second ion conductor-oxide layer, dried at 80°C for 2 h, and a first lithium supplement-flame retardant layer with a thickness of 4 μm was formed, obtaining a composite separator S1.

[0087] Example 2

[0088] (1) 5 parts by mass of ion conductor LLTO, 10 parts by mass of alumina, 1 part by mass of binder PVDF and solvent NMP were mixed to obtain a slurry with a solid content of 50 wt%, which was coated on both sides of a base film (polyethylene, thickness 9 μm) by a doctor blade method, dried at 80°C for 2 h, and a second ion conductor-oxide layer with a thickness of 0.5 μm and a fourth ion conductor-oxide layer with a thickness of 1 μm were formed on both sides of the base film, respectively;

[0089] (2) 10 parts by mass of lithium supplement Li2CO3, 10 parts by mass of flame retardant phosphoramine, 1 part by mass of PAA and DMF were mixed to obtain a slurry with a solid content of 50 wt%, which was coated on the surface of the second ion conductor-oxide layer, dried at 80°C for 2 h, and a first lithium supplement-flame retardant layer with a thickness of 0.5 μm was formed, obtaining a composite separator S2.

[0090] Example 3

[0091] (1) 30 parts by mass of ion conductor LiPON, 30 parts by mass of alumina, 5 parts by mass of binder PVDF and solvent NMP were mixed to obtain a slurry with a solid content of 50 wt%, which was coated on both sides of a base film (polyethylene, thickness 5 μm) by a doctor blade method, dried at 80°C for 2 h, and a second ion conductor-oxide layer with a thickness of 5 μm and a fourth ion conductor-oxide layer with a thickness of 3 μm were formed on both sides of the base film, respectively;

[0092] (2) 30 parts by mass of a lithium supplementing agent Li2C2O2, 25 parts by mass of a flame retardant tricresyl phosphate, 4 parts by mass of PVDF and NMP were mixed to obtain a slurry with a solid content of 50 wt%, the slurry was coated on the surface of the second ion conductor-oxide layer, dried at 80°C for 2 h, and a first lithium supplementing-flame retardant layer with a thickness of 8 μm was formed to obtain a composite separator S3.

[0093] Example 4

[0094] (1) 25 parts by mass of an ion conductor PEO, 15 parts by mass of TiO2, 2 parts by mass of a binder PVDF and a solvent NMP were mixed to obtain a slurry with a solid content of 50 wt%, the slurry was coated on both sides of a base film (polyethylene, thickness of 3 μm) by doctor blade coating, dried at 80°C for 2 h, and a second ion conductor-oxide layer with a thickness of 1 μm and a fourth ion conductor-oxide layer with a thickness of 5 μm were formed on both sides of the base film, respectively;

[0095] (2) 28 parts by mass of a lithium supplementing agent Li2NiO2, 15 parts by mass of a flame retardant hexaphenoxycyclotriphosphazene, 3 parts by mass of PVDF and NMP were mixed to obtain a slurry with a solid content of 50 wt%, the slurry was coated on the surface of the second ion conductor-oxide layer, dried at 80°C for 2 h, and a first lithium supplementing-flame retardant layer with a thickness of 6 μm was formed to obtain a composite separator S4.

[0096] Example 5

[0097] According to the method of Example 1, except that,

[0098] In step (2), the thickness of the first lithium supplementing-flame retardant layer was controlled to be 0.1 μm;

[0099] The remaining conditions were the same, and a composite separator S5 was obtained.

[0100] Example 6

[0101] According to the method of Example 1, except that,

[0102] In step (1), the thickness of the second ion conductor-oxide layer and the fourth ion conductor-oxide layer was controlled to be 0.1 μm, respectively; the remaining conditions were the same, and a composite separator S6 was obtained.

[0103] Example 7

[0104] According to the method of Example 1, except that,

[0105] In step (2), the thickness of the first lithium supplementing-flame retardant layer was controlled to be 10 μm;

[0106] The remaining conditions were the same, and a composite separator S7 was obtained.

[0107] Example 8

[0108] The method of Example 1 was followed, except that,

[0109] In step (1), the thickness of the second ion conductor-oxide layer and the fourth ion conductor-oxide layer were both regulated to 7 μm; the rest of the conditions were the same, and a composite separator S8 was obtained.

[0110] Comparative Example 1

[0111] The method of Example 1 was followed, except that,

[0112] There was no step (2), i.e., the intermediate product of step (1) was directly used as the composite separator DS1, i.e., the first lithium supplement-flame retardant layer was not prepared.

[0113] Comparative Example 2

[0114] The method of Example 1 was followed, except that,

[0115] In step (1), coating was only performed on one side of the base film to form a fourth ion conductor-oxide layer with a thickness of 2 μm;

[0116] In step (2), a slurry containing a lithium supplement agent and a phosphorus-based flame retardant was coated on the other side of the base film;

[0117] The rest of the conditions were the same, and a composite separator DS2 was obtained.

[0118] Comparative Example 3

[0119] An alumina slurry was coated on both sides of a base film (polyethylene, thickness of 7 μm), dried at 80 °C for 2 h, and an alumina layer with a thickness of 2 μm was formed on both sides of the base film, and a composite separator DS3 was obtained.

[0120] Comparative Example 4

[0121] The method of Example 1 was followed, except that,

[0122] In step (1), 20 parts by mass of the ion conductor LATP was replaced with an equal amount of Al2O3, and the rest of the conditions were the same, i.e., 40 parts by mass of alumina was mixed with other components to form a second alumina ceramic layer with a thickness of 2 μm and a fourth alumina ceramic layer with a thickness of 2 μm;

[0123] In step (2), no flame retardant polyphosphoric amine was added, and the rest of the conditions were the same, and a lithium supplement layer (close to the positive electrode side) with a thickness of 4 μm was formed, and a composite separator DS4 was obtained.

[0124] Comparative Example 5

[0125] The method of Example 1 was followed, except that,

[0126] In step (2), the type of the flame retardant is replaced by antimony trioxide;

[0127] The remaining conditions are the same, and the composite separator DS5 is obtained.

[0128] Table 1

[0129]

[0130]

[0131] From the data in Table 1, compared with Comparative Examples 1-5, the composite separator provided by the application used in Examples 1-8 has excellent air permeability, ion conductivity and shrinkage under the premise of normal appearance of the composite separator, and the assembled lithium ion battery has excellent cycle performance.

[0132] Compared with Examples 5-8, Examples 1-4 have improved performance of the composite separator by adjusting the thickness of the lithium supplement-flame retardant layer and the ion conductor-oxide layer.

[0133] As can be seen from the comparison between Comparative Example 1 and Comparative Example 3, when the composite separator only contains an aluminum oxide layer, the ion conductivity is low, the shrinkage is high, the safety is reduced, and the cycle life is also poor.

[0134] As can be seen from the comparison between Comparative Example 1 and Comparative Example 4, when the composite separator does not contain an ion conductor and a phosphorus-based flame retardant, the ion conductivity is low, the yield is high, the safety is reduced, and the lithium supplement life is also poorer than the application.

[0135] As can be seen from the comparison between Comparative Example 1 and Comparative Example 5, when the flame retardant is a non-phosphorus-based flame retardant, the synergistic effect of the phosphorus-based flame retardant and the lithium supplement agent cannot be achieved; the ion conductivity difference is not large, the flame retardant performance is not obviously deteriorated, but the internal resistance of the cell after formation is slightly increased, which affects the cycle effect.

[0136] The preferred embodiments of the application are described in detail above, but the application is not limited thereto. Within the technical concept of the application, various simple modifications can be made to the technical solutions of the application, including the combination of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the application and fall within the protection scope of the application.

Claims

1. A composite diaphragm, characterized in that, The composite membrane is composed of four layers stacked together. The first layer is a lithium-supplementing and flame-retardant layer, the second and fourth layers are both ion conductor-oxide layers, and the third layer is a base film. The lithium-supplementing and flame-retardant layer contains a lithium-supplementing agent and a phosphorus-based flame retardant.

2. The composite diaphragm according to claim 1, wherein, The lithium supplement and the phosphorus-based flame retardant can form Li-OP bonds; And / or, in the lithium-replenishing flame retardant layer, the mass ratio of the lithium replenishing agent to the phosphorus-based flame retardant is 5-50:5-40, preferably 10-30:10-25.

3. The composite diaphragm according to claim 1 or 2, wherein, The lithium supplementing agent is selected from at least one of Li2C4O4, Li2CO3, Li2NiO2, Li2C2O2 and its derivative Li 2-x M x C2O4, where 0 < x < 2, M is a metal element, and preferably, M is selected from at least one of Ni, Mn and Fe; And / or, the phosphorus-based flame retardant is selected from flame retardants containing phosphorus-oxygen bonds; And / or, the phosphorus-based flame retardant is selected from at least one of ammonium polyphosphate, phosphatidyl phosphate, tricresyl phosphate, and phosphazene compounds.

4. The composite diaphragm according to any one of claims 1-3, wherein, The ion conductor-oxide layer contains ion conductors and oxides; And / or, the mass ratio of the ionic conductor to the oxide is 1-50:5-50, preferably 5-30:10-30; And / or, the ionic conductivity of the ionic conductor is ≥10. -4 S / cm, preferably selected from solid electrolytes, more preferably selected from at least one of PEO, LiPON, LATP and LLTO; And / or, the oxide is selected from metal oxides, preferably from Al2O3 and / or TiO2; And / or, the base film is selected from polypropylene and / or polyethylene.

5. The composite separator according to any one of claims 1-4, wherein, The thickness of the lithium-replenishing and flame-retardant layer is 0.1-10 μm, preferably 0.5-8 μm; And / or, the thicknesses of the second and fourth layers are selected to be the same or different; And / or, the thickness of the ion conductor-oxide layer is 0.1-7 μm, preferably 0.5-5 μm; And / or, the thickness of the base film is 3-15 μm, preferably 3-10 μm.

6. The composite separator according to any one of claims 1-5, wherein, The air permeability of the composite membrane is ≤145S / 100mL, preferably ≤130S / 100mL; And / or, the ionic conductivity of the composite membrane is ≥0.8 ms / cm, preferably ≥0.9 ms / cm; And / or, the shrinkage rate of the composite diaphragm is ≤1%, preferably ≤0.1%.

7. A method for preparing a composite diaphragm, characterized in that, The preparation method includes: (1) A slurry containing ionic conductors and oxides is coated on both sides of a base film, and after a first drying, ionic conductor-oxide layers are formed on both sides of the base film respectively; (2) A slurry containing lithium supplement and phosphorus flame retardant is coated on any ionic conductor-oxide layer, and after a second drying, a lithium supplement-flame retardant layer is formed to obtain a composite membrane.

8. The preparation method according to claim 7, wherein, In the slurry containing ionic conductors and oxides, the mass ratio of the ionic conductors and oxides is 1-50:5-50, preferably 5-30:10-30; And / or, in the slurry containing lithium supplementer and phosphorus flame retardant, the mass ratio of lithium supplementer to phosphorus flame retardant is 5-50:5-40, preferably 10-30:10-25; And / or, the solvents of the slurry containing ionic conductors and oxides and the slurry containing lithium supplementers and phosphorus-based flame retardants are each independently selected from DMF and / or NMP; And / or, the slurry containing ionic conductors and oxides and the slurry containing lithium supplementers and phosphorus-based flame retardants also each independently contain a binder.

9. A lithium-ion battery, characterized in that, The lithium-ion battery contains a positive electrode, a negative electrode, and a composite separator as described in any one of claims 1-6, or a composite separator prepared by the preparation method described in claim 7 or 8. In the composite membrane, the first layer is close to the positive electrode, and the fourth layer is close to the negative electrode.

10. The lithium-ion battery according to claim 9, wherein, The lithium-ion battery is selected from liquid batteries, semi-solid electrodes, and all-solid batteries; And / or, the lithium-ion battery operates at a temperature of -40°C to 80°C.

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