Lithium ion battery composite separator, preparation method and lithium ion battery

Abstract

The application discloses a lithium ion battery composite diaphragm, a preparation method and a lithium ion battery, and relates to the technical field of lithium ion batteries. The lithium ion battery composite diaphragm comprises a base film and a functional coating arranged on the negative electrode side of the base film. The functional coating comprises, in sequence from the base film to the base film, an ion-conducting layer, a thermal barrier layer and a self-repairing layer. The base film is a polyimide and aramid nanofiber blended electrostatic spinning film. The ion-conducting layer comprises a modified fast ion conductor and a polyvinylidene fluoride-hexafluoropropylene composite. The thermal barrier layer comprises a cross-linked network of boron nitride nanosheets and polybenzimidazole. The self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds. The composite diaphragm provided by the application can significantly inhibit the purple area on the negative electrode surface, enhance the high-temperature self-protection and interface self-repairing of the lithium ion battery, and thus significantly improve the cycle and safety performance of the lithium ion battery.

Description

TECHNICAL FIELD

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

[0002] The lithium ion battery diaphragm is a physical barrier between the positive and negative electrodes, and its performance directly affects the cycle life and safety of the battery. The current commercial diaphragm (such as PE, PP) has two major problems:

[0003] 1. Negative electrode surface purple zone phenomenon: during the cycle process, uneven distribution of pores on the surface of the diaphragm leads to excessive deposition of lithium ions at the local negative electrode, forming a purple zone (a composite of electrolyte decomposition products and lithium dendrites), which causes capacity attenuation and short circuit risk;

[0004] 2. Thermal collapse failure: at high temperatures (> 130℃), the diaphragm melts and shrinks, causing the positive and negative electrodes to directly contact and accelerate thermal runaway.

[0005] To avoid the above defects, the prior art has made the following attempts:

[0006] 1) Ceramic coated diaphragm (such as Al2O3 coating): although the heat resistance is improved, the thickening of the coating leads to an increase in ion migration resistance (interface impedance increases by more than 30%), which exacerbates the formation of the purple zone;

[0007] 2) Polymer modified diaphragm (such as polyvinylidene fluoride (PVDF) coating): improves electrolyte wettability, but has insufficient high-temperature closure performance (thermal shutdown temperature < 150℃).

[0008] Therefore, there is an urgent need for a lithium ion battery composite diaphragm that improves lithium ion flux uniformity, high-temperature self-protection, and interface self-repair through material, structure, or preparation method. SUMMARY

[0009] The purpose of the present application is to provide a lithium ion battery composite diaphragm, a preparation method and a lithium ion battery, which solves the technical problems of the negative electrode surface purple zone phenomenon and thermal collapse failure of the lithium ion battery diaphragm in the prior art.

[0010] To achieve the above purpose, one embodiment of the present application provides a lithium ion battery composite diaphragm, comprising a base film and a functional coating arranged on the negative side of the base film; the functional coating comprises, in order from close to the base film to far from the base film: an ion-conducting layer, a thermal barrier layer, and a self-repairing layer.

[0011] Among them:

[0012] The base film is a polyimide and aramid nanofiber blended electrospun film;

[0013] The ion-conducting layer comprises a modified fast ion conductor and a polyvinylidene-hexafluoropropylene composite;

[0014] The thermal barrier layer comprises a cross-linked network of boron nitride nanosheets and polybenzimidazole.

[0015] The self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds.

[0016] In one preferred embodiment of the present application, the fast ion conductor is selected from LLP, LLZO or LATP.

[0017] In one preferred embodiment of the present application, the thickness of the ion-conducting layer is 3-5 microns.

[0018] In one preferred embodiment of the present application, the preparation of the ion-conducting layer comprises the following steps:

[0019] The fast ion conductor powder is immersed in a lithium salt ethanol solution for pre-doping treatment to obtain a modified fast ion conductor.

[0020] The modified fast ion conductor and polyvinylidene-hexafluoropropylene are mixed in a solvent to obtain a slurry.

[0021] The slurry is coated on the negative side of the base film, and then dried and sintered to form.

[0022] In one preferred embodiment of the present application, the particle size of the fast ion conductor powder is 200-500 nm, and the lithium salt ethanol solution is a 0.05-0.15 M LiNO3 ethanol solution.

[0023] In one preferred embodiment of the present application, the mass ratio of the modified fast ion conductor to polyvinylidene-hexafluoropropylene is 2-4:1.

[0024] In one preferred embodiment of the present application, the modified fast ion conductor and polyvinylidene-hexafluoropropylene are mixed in a solvent to obtain a slurry, which comprises: mixing the modified fast ion conductor, polyvinylidene-hexafluoropropylene and a dispersing agent in a solvent to obtain a slurry, the dispersing agent being carbon nanohorns, and the addition amount of the dispersing agent being 0.3-0.8 wt% of the total mass of the slurry.

[0025] In one preferred embodiment of the present application, the slurry is coated on the negative side of the base film, and then dried and sintered to form, which comprises: the coating thickness of the slurry being 8-12 microns, the drying condition being hot air drying at 60-70°C, and the sintering condition being sintering at a temperature increasing rate of 3-5°C / min to 120-140°C and holding for 10-20 min.

[0026] In one preferred embodiment of the present application, the thickness of the thermal barrier layer is 2-4 microns, the thermal conductivity is >8 W / m·K, and the temperature resistance is >300°C.

[0027] In one preferred embodiment of the present application, the preparation of the thermal barrier layer comprises the following steps:

[0028] The boron nitride nanosheet is immersed in a silane coupling agent ethanol solution for surface modification to obtain aminated boron nitride;

[0029] The aminated boron nitride and polybenzimidazole are dispersed in an epoxy resin, a photoinitiator is added and emulsified to obtain an emulsion;

[0030] The emulsion is coated on the surface of the ion-conducting layer, and a crosslinked network is formed by ultraviolet irradiation and heat treatment.

[0031] In one preferred embodiment of the present application, the silane coupling agent is 3-aminopropyl triethoxysilane, the concentration of the silane coupling agent ethanol solution is 2wt%-5wt%, and the surface modification conditions are refluxing at 70°C-80°C for 4h-5h.

[0032] In one preferred embodiment of the present application, the mass ratio of aminated BN to polybenzimidazole is 1:1-1.2, the photoinitiator is Irgacure 2959, the addition amount of the photoinitiator is 0.1wt%-0.3wt% of the mass of the epoxy resin, and the emulsification conditions are shearing at 10000rpm-12000rpm for 30min-40min.

[0033] In one preferred embodiment of the present application, the emulsion is coated on the surface of the ion-conducting layer, and a crosslinked network is formed by ultraviolet irradiation and heat treatment, which comprises: coating by electrostatic spraying, the spraying voltage is 30kV-40kV, the flow rate is 0.5mL / min-0.8mL / min, the wavelength of ultraviolet irradiation is 365nm, the intensity is 10mW / cm 2 -12mW / cm 2 , the irradiation time is 30s-50s, and the heat treatment conditions are heating at 70°C-80°C for 1h-2h.

[0034] In one preferred embodiment of the present application, the polyurethane microspheres are filled in the pores of the thermal barrier layer, and the diameter of the polyurethane microspheres is 0.5μm-2μm.

[0035] In one preferred embodiment of the present application, the preparation of the self-repairing layer comprises the following steps:

[0036] A dynamic polyurethane is synthesized by reacting a chain extender containing a disulfide bond, a polyol and a polyisocyanate;

[0037] The dynamic polyurethane is dissolved in an organic solvent, a nucleating agent is added, and microspheres are prepared by drying and molding;

[0038] The microspheres are dispersed in a dispersion solvent and permeated into the pores of the thermal barrier layer by negative pressure immersion.

[0039] In one preferred scheme of the present application, the chain extender containing disulfide bond comprises 4,4'-dithiodianiline, the polyol comprises polytetrahydrofuran, and the polyisocyanate comprises isophorone diisocyanate.

[0040] In one preferred scheme of the present application, the molar ratio of polytetrahydrofuran, isophorone diisocyanate and 4,4'-dithiodianiline is 2-3:2-3:1.

[0041] In one preferred scheme of the present application, the organic solvent comprises acetone, the concentration of the dynamic polyurethane is 8wt%-10wt%, and the nucleating agent comprises nano-SiO2, and the addition amount of the nucleating agent is 0.1wt%-0.3wt% of the mass of the polyurethane.

[0042] In one preferred scheme of the present application, the microspheres are dispersed in a dispersion solvent and permeated into the pores of the thermal barrier layer by negative pressure immersion, comprising: the dispersion solvent comprises acetone, the concentration of the microspheres in the dispersion solution is 3wt%-5wt%, and the negative pressure condition is a vacuum degree of-0.08MPa to-0.05MPa, and the immersion time is greater than or equal to 20min.

[0043] In one preferred scheme of the present application, the thickness of the base film is 15-25um, and the porosity is 65%-80%.

[0044] In one preferred scheme of the present application, the preparation of the base film comprises: dissolving polyimide and aramid nanofiber in N,N-dimethylacetamide at a mass ratio of 2-1:1, and obtaining the base film after electrospinning and heat treatment.

[0045] In one preferred scheme of the present application, the voltage of electrospinning is 15kV-25kV, the push-injection speed is 0.5mL / h-1.5mL / h, and the environmental humidity is 45%±5%; after spinning, heat treatment is carried out at a temperature rising rate of 3℃ / min-5℃ / min to 180℃-200℃.

[0046] In one preferred scheme of the present application, the base film has a pore size gradient distribution: the pore size on the negative electrode side is 50nm-100nm, and the pore size on the positive electrode side is 100nm-300nm.

[0047] In one preferred scheme of the present application, the base film is provided with a LiAlO2 coating layer on the positive electrode side, and the thickness of the LiAlO2 coating layer is 5nm-10nm.

[0048] In one preferred scheme of the present application, the LiAlO2 coating layer is prepared by an atomic layer deposition method, comprising:

[0049] The precursor combination comprises: a lithium source comprising lithium bis(trimethylsilyl)amide; an aluminum source comprising trimethylaluminum; and an oxygen source comprising ozone;

[0050] Deposition conditions: the base film is preheated to 150-180 DEG C, each deposition cycle includes: TMA pulse, N2 purge; O3 pulse, N2 purge; LiHMDS pulse, N2 purge;

[0051] After the LiAlO2 coating is deposited, it is fired in stages: first stage: 3-5 DEG C / min to 250-280 DEG C, 1-2 h at 250-280 DEG C in a N2 atmosphere; second stage: 8-10 DEG C / min to 400-450 DEG C, 2-3 h at 400-450 DEG C.

[0052] Based on the disclosed lithium ion battery composite separator, the application further discloses a preparation method of the lithium ion battery composite separator, which comprises the following steps:

[0053] providing a base film;

[0054] providing a functional coating on the negative side of the base film;

[0055] the functional coating comprises, in order from close to the base film to far from the base film, an ion-conducting layer, a thermal barrier layer and a self-repairing layer;

[0056] wherein:

[0057] the ion-conducting layer comprises a composite of modified fast ion conductor and polyvinylidene-hexafluoropropylene;

[0058] the thermal barrier layer comprises a crosslinked network of boron nitride nanosheets and polybenzimidazole;

[0059] the self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds.

[0060] In one of the preferred schemes of the application, the preparation method further comprises: depositing a LiAlO2 coating on the positive side of the base film.

[0061] The application further discloses a lithium ion battery, which comprises a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet; the separator is selected from the lithium ion composite separator described above or the lithium ion composite separator prepared by the preparation method described above.

[0062] To sum up, the application has the following beneficial effects:

[0063] 1. The lithium ion battery composite separator provided by the application comprises, on the negative side of the base film, an ion-conducting layer, a thermal barrier layer and a self-repairing layer arranged in order from close to the base film to far from the base film; through the synergistic effect of the three-layer composite structure, the lithium ion flux is homogenized, and the purple zone is inhibited from the source; materials with high thermal conductivity and high temperature resistance are selected to block the propagation of the thermal chain reaction; the disulfide bond microspheres repair the microcracks of the separator when local overheating occurs, and prevent dendrites from penetrating, thereby significantly improving the cycle and safety performance of the lithium ion battery.

[0064] 2. The ion-conducting layer ensures that the purple area on the negative electrode surface of the lithium-ion composite separator is ≤7% after 1000 cycles (compared to >30% for traditional separators).

[0065] 3. The thermal barrier layer allows the lithium-ion composite membrane to have a thermal shrinkage rate of <3% at 200℃ (compared to a shrinkage rate of >10% for traditional membranes).

[0066] 4. The LiAlO2 layer increases the oxygen generation start temperature on the positive electrode side of the lithium-ion composite separator from 280℃ to 350℃.

[0067] 5. The lithium-ion composite separator of this invention retains >88% capacity after 2000 cycles at 3C rate (compared to 60%-75% for traditional separators); gas production is reduced by 86% (Archimedes buoyancy test); and the critical current density for dendrite penetration is increased to ≥5 mA / cm². 2 (Traditional diaphragm is 3mA / cm) 2 This improves cycle and safety performance.

[0068] 6. The lithium-ion composite separator of the present invention can significantly suppress the purple region on the negative electrode surface, enhance the high-temperature self-protection and interface self-repair of lithium-ion batteries, thereby significantly improving their cycle and safety performance.

[0069] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention can be appreciated by way of the effects described in the description. Attached Figure Description

[0070] Figure 1 This is a schematic flowchart illustrating the preparation method of the lithium-ion battery composite separator in an embodiment of the present invention. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0072] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0073] The application discloses a lithium ion battery composite diaphragm, comprising a base film and a functional coating arranged on the negative side of the base film; the functional coating comprises, in sequence from the base film to the far side of the base film, an ion-conducting layer, a thermal barrier layer and a self-repairing layer.

[0074] The base film is a polyimide (PI) and aramid nanofiber (ANF) blended electrospun film; the ion-conducting layer comprises a modified fast ion conductor and a polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) composite; the thermal barrier layer comprises a cross-linked network of boron nitride (BN) nanosheets and polybenzimidazole (PBI); and the self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds (-S-S-).

[0075] The lithium ion battery composite diaphragm provided by the application has the following advantages: the base film is blended with polyimide (PI) and aramid nanofiber (ANF), the PI has good temperature resistance, the ANF brings better strength to the blended electrospun film, and the two jointly improve the thermal stability of the base film; the functional coating of the composite diaphragm is arranged on the negative side of the base film, and comprises, in sequence from the base film to the far side of the base film, an ion-conducting layer, a thermal barrier layer and a self-repairing layer; through the synergistic effect of the three-layer composite structure, the uniformization of lithium ion flux is realized, and the purple zone is inhibited from the source; materials with high thermal conductivity and high temperature resistance are selected to block the propagation of the thermal chain reaction; the disulfide bond microspheres repair the microcracks of the diaphragm when local overheating occurs, and the dendrites are prevented from penetrating, so that the cycle and safety performance of the lithium ion battery are significantly improved.

[0076] As an optional implementation, in the ion-conducting layer, the fast ion conductor is selected from LLP (Li3La2(PO4)3), LLZO (Li7La3Zr2O 12 ) or LATP (Li 1.3 Al 0.3 Ti 1.7 (PO4)3).

[0077] The ion-conducting layer of the application selects fast ion conductors such as LLP, LLZO or LATP to further improve the ion migration rate; the polyurethane microspheres are filled in the pores of the thermal barrier layer, and directional repair can be achieved, the microspheres in the damaged area are activated, and the undamaged part remains stable.

[0078] As an optional implementation, the thickness of the ion-conducting layer is 3-5 μm, for example, it can be 3 μm, 4 μm or 5 μm.

[0079] The ion-conducting layer is easy to break if it is too thin, and the impedance is increased if it is too thick, and the application further limits the thickness of the ion-conducting layer to 3-5 μm, so that the ion conduction efficiency and mechanical strength can be better balanced.

[0080] As an optional implementation, the preparation of the ion-conducting layer comprises the following steps:

[0081] (a) Fast ion conductor modification: the fast ion conductor powder is immersed in a lithium salt ethanol solution for pre-doping treatment to obtain a modified fast ion conductor;

[0082] (b) Slurry preparation: the modified fast ion conductor and PVDF-HFP are ball-milled and mixed in a solvent to obtain a slurry;

[0083] (c) Coating and curing: the slurry is coated on the negative side of the base film, and is dried and sintered to form.

[0084] The present application adopts lithium salt to pre-dope and modify the fast ion conductor powder, improves the interface ion migration rate of the fast ion conductor, and realizes the uniformization of lithium ion flux; and the curing method is helpful to eliminate internal stress and enhance the interlayer bonding force.

[0085] As an optional embodiment, in step (a), the particle size of the fast ion conductor powder is 200-500 nm, and the lithium salt ethanol solution is 0.05-0.15 M LiNO3 ethanol solution; the pre-doping treatment includes: ultrasonic dispersion for 30-60 min, for example, 30 min, 40 min, 50 min or 60 min, and then vacuum drying at 80-90℃, for example, 80℃, 85℃ or 90℃.

[0086] The present application further limits the particle size of the fast ion conductor powder to 200-500 nm to ensure its dispersibility, and a particle size greater than 500 nm will cause the slurry to settle and the coating to be uneven; and further limits the lithium salt concentration to form a stable lithium-rich interface layer, for example, if the lithium salt concentration is too large, the pre-doped layer is too thick and is easy to block the ion channel.

[0087] As an optional embodiment, in step (b), the mass ratio of the modified fast ion conductor to PVDF-HFP is (2-4):1, for example, 2:1, 2.3:1, 2.5:1, 3:1, 3.5:1 or 4:1; the solvent is NMP solvent; the ball-milling time is 5-8 h, for example, 5 h, 6 h, 7 h or 8 h, and the rotation speed is 400-500 rpm, for example, 400 rpm, 450 rpm or 500 rpm.

[0088] The present application further limits the mass ratio of the modified fast ion conductor to PVDF-HFP to balance the ion conduction and mechanical strength; and by limiting the ball-milling parameters, the fast ion conductor is further dispersed to avoid void blockage caused by its agglomeration.

[0089] As an optional implementation manner, in step (c), the wet coating film thickness is 8-12 μm, for example, can be 8 μm, 9 μm, 10 μm, 11 μm or 12 μm, the drying condition is hot air drying at 60-70 ℃, for example, can be 60 ℃, 65 ℃ or 70 ℃, the temperature is raised to 120-140 ℃, for example, can be 120 ℃, 130 ℃ or 140 ℃ at a rate of 3-5 ℃ / min, for example, can be 3 ℃ / min, 4 ℃ / min or 5 ℃ / min, and the sintering is performed for 10-20 min, for example, can be 10 min, 15 min or 20 min.

[0090] The application further limits the wet coating film thickness to accurately control the thickness of the dry film, and the obtained dry film is easy to be penetrated by dendrites when the thickness is too thin, and the impedance is increased when the thickness is too thick.

[0091] As an optional implementation manner, in step (b), a dispersant is further added.

[0092] As an optional implementation manner, the dispersant is carbon nanohorn (CNH), and the addition amount of the dispersant is 0.3-0.8 wt% of the total mass of the slurry, for example, can be 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt% or 0.8 wt%.

[0093] The application adds a dispersant to prevent the agglomeration of fast ion conductors and ensure the uniformity of the coating (electrolyte contact angle <5°).

[0094] As an optional implementation manner, the thickness of the thermal barrier layer is 2-4 μm, for example, can be 2 μm, 3 μm or 4 μm, the thermal conductivity is >8 W / m·K, and the temperature resistance is >300 ℃.

[0095] The application further limits the thickness of the thermal barrier layer, and the mechanical strength of the coating is insufficient when the thickness is too thin, and the coating is easy to be broken, and the interface impedance of the coating is increased when the thickness is too thick, so the thickness of the thermal barrier layer is set to 2-4 μm to avoid the sharp increase of the interface impedance of the traditional ceramic coating, and the thermal conductivity and temperature resistance of the thermal barrier layer are further improved to overcome the thermal dissipation failure of the coating in the prior art.

[0096] As an optional implementation manner, the preparation of the thermal barrier layer comprises the following steps:

[0097] (a) BN amination treatment: the BN nanosheet is immersed in a silane coupling agent ethanol solution for surface modification to obtain an aminated BN;

[0098] (b) emulsion preparation: the aminated BN and polybenzimidazole (PBI) are dispersed in an epoxy resin, a photoinitiator is added and emulsified to obtain an emulsion;

[0099] (c) Coating and curing: coating the emulsion on the surface of the ion-conducting layer, and forming a crosslinked network by ultraviolet irradiation and heat treatment.

[0100] In the present application, the silane coupling agent modifies and enhances the interface bonding between BN and resin, solving the problem of easy agglomeration of nanosheets in resin; the high thermal conductivity (thermal conductivity > 8 W / m K) of BN nanosheets and the high-temperature resistant skeleton (temperature resistance > 300℃) of PBI synergistically form a high-thermal-conductivity and high-temperature-resistant crosslinked network, which can effectively block the propagation of thermal chain reactions and reduce the risk of battery thermal runaway; the photo-thermal dual-curing method is used to quickly shape and complete the PBI polycondensation to form a dense network.

[0101] As an optional embodiment, in step (a), the silane coupling agent is 3-aminopropyl triethoxysilane (APTES); the concentration of the APTES ethanol solution is 2-5wt%, for example, it can be 2wt%, 3wt%, 4wt% or 5wt%; the surface modification condition is 70-80℃, for example, it can be 70℃, 75℃ or 80℃ reflux for 4-5h, for example, it can be 4h, 4.5h or 5h.

[0102] The mass ratio of the APTES ethanol solution to the BN nanosheet is (30-50):1.

[0103] The present application further limits the concentration of the APTES ethanol solution; a concentration <2wt% is not sufficient for modification, and a concentration >5wt% leads to multi-layer coating and reduces thermal conductivity.

[0104] In step (b), the mass ratio of the aminated BN to PBI is 1:(1-1.2), for example, it can be 1:1, 1:1.1 or 1:1.2; the mass ratio of the aminated BN to the epoxy resin is 1:(2-5); the photoinitiator is Irgacure 2959, and the addition amount of the photoinitiator is 0.1-0.3wt% of the mass of the epoxy resin, for example, it can be 0.1wt%, 0.2wt% or 0.3wt%; the emulsification condition is high-speed shearing at 10000-12000rpm, for example, it can be 10000rpm, 11000rpm or 12000rpm for 30-40min, for example, it can be 30min, 35min or 40min.

[0105] The present application further limits the mass ratio of the aminated BN to PBI; too little PBI (<1) leads to insufficient crosslinking, and too much PBI (>1.2) blocks the thermal conduction path; Irgacure 2959 is selected as the photoinitiator and an appropriate amount is selected, and the penetration depth of the photoinitiator ensures overall crosslinking.

[0106] In step (c), the coating method is electrostatic spraying, the spraying voltage is 30-40 kV, for example, it can be 30 kV, 35 kV or 40 kV, the flow rate is 0.5-0.8 mL / min, for example, it can be 0.5 mL / min, 0.6 mL / min, 0.7 mL / min or 0.8 mL / min; the wavelength of ultraviolet irradiation is 365 nm, the intensity is 10-12 mW / cm 2 , for example, it can be 10 mW / cm 2 , 11 mW / cm 2 or 12 mW / cm 2 , the irradiation time is 30-50 s, for example, it can be 30 s, 40 s or 50 s; the heat treatment condition is 70-80℃, for example, it can be 70℃, 75℃ or 80℃, heated for 1-2 h, for example, it can be 1 h, 1.5 h or 2 h.

[0107] As an optional embodiment, the polyurethane microspheres are filled in the pores of the crosslinked network of the thermal barrier layer.

[0108] As an optional embodiment, the diameter of the polyurethane microspheres of the self-repairing layer is 0.5-2 μm, for example, it can be 0.5 μm, 1 μm, 1.5 μm or 2 μm, and the self-repairing of the composite separator is triggered at 60-80℃, for example, it can be 60℃, 70℃ or 80℃, and the repair rate is > 85%.

[0109] As an optional embodiment, the preparation of the self-repairing layer comprises the following steps:

[0110] (a) Dynamic polyurethane synthesis: synthesizing dynamic polyurethane by reacting a chain extender containing disulfide bond, a polyol and a polyisocyanate;

[0111] (b) Microsphere preparation: dissolving the dynamic polyurethane in an organic solvent, adding a nucleating agent, and forming microspheres by drying;

[0112] (c) Pore filling: dispersing the microspheres in a dispersion solvent and penetrating into the pores of the thermal barrier layer by negative pressure impregnation.

[0113] The polyurethane microspheres are used as the self-repairing layer, the trigger temperature is perfectly matched with the battery thermal runaway early warning temperature, the repairing is performed when the temperature rises to the trigger temperature, the repairing is completed and the temperature is fixed, the recycling is avoided, the problem of the dendrite piercing the microcapsule in the traditional self-repairing process is avoided, and the active preventive repairing can be realized; the microspheres are used for isolating the disulfide bond and the electrolyte, and the side reaction between the repairing agent and the electrolyte in the traditional self-repairing process is avoided; the directional repairing can be realized by adopting the method, the microspheres of the damaged area are activated, and the undamaged part remains stable; the volume of the microspheres does not change after the repairing, the porosity retention rate of the diaphragm is greater than or equal to 87% (the repairing rate of the composite diaphragm is greater than or equal to 88.7%, the repairing rate of the composite diaphragm = (the ion conductivity after the damage / the initial ion conductivity) x 100%), the volume expansion of the microcapsule after the solidification in the traditional repairing process is avoided, and the ion conductivity is reduced.

[0114] As an optional implementation manner, in step (a), the disulfide bond-containing chain extender includes 4,4'-dithiodianiline (DTDA); the polyol includes polytetramethylene glycol (PTMG), and the polyisocyanate includes isophorone diisocyanate (IPDI); the reaction condition is 60-80 DEG C, for example, can be 60 DEG C, 70 DEG C or 80 DEG C, and the reaction is carried out under nitrogen protection for 6-8h, for example, can be 6h, 7h or 8h.

[0115] The application adopts 4,4'-dithiodianiline (DTDA) as a chain extender, which has an aniline structure and can reduce the disulfide bond energy barrier, and has double disulfide bonds (-S-S-) in the molecule, and the repairing rate is greater than 85%; nitrogen protection and limiting the chain extension reaction condition to 60-80 DEG C can prevent the oxidation of the disulfide bond and ensure the repairing activity.

[0116] As an optional implementation manner, the reaction molar ratio of PTMG:IPDI:DTDA is = (2-3): (2-3): 1, for example, can be 2:2:1, 2:3:1, 3:2:1 or 3:3:1.

[0117] In the application, PTMG provides flexibility for the self-repairing layer, too little PTMG and too strong segment rigidity, the microspheres cannot deform to fill the cracks; IPDI provides a rigid skeleton for the self-repairing layer, too much DTDA and too high crosslinking degree, the disulfide bond activity is limited; and DTDA provides dynamic bond density for the self-repairing layer.

[0118] As an optional implementation, in step (b), the organic solvent comprises acetone, the dynamic polyurethane concentration is 8-10wt%, for example, it can be 8wt%, 9wt% or 10wt%; the nucleating agent comprises nano-SiO2, the addition amount of the nucleating agent is 0.1-0.3wt% of the mass of the polyurethane, for example, it can be 0.1wt%, 0.2wt% or 0.3wt%; the drying method is spray drying, and the diameter of the obtained microspheres is 0.5-2μm, for example, it can be 0.5μm, 1μm, 1.5μm or 2μm.

[0119] In the preparation process of the microspheres of the application, acetone is selected as the organic solvent, which has a low boiling point and volatilizes rapidly after spray drying, thereby avoiding softening and adhesion of the microspheres; nano-SiO2 is used as the nucleating agent, and the microspheres prepared have uniform size, increased surface roughness, increased specific surface area and improved repair contact efficiency.

[0120] As an optional implementation, in step (c), the dispersion solvent comprises acetone, the concentration of the microspheres in the dispersion solution is 3-5wt%, for example, it can be 3wt%, 4wt% or 5wt%; the negative pressure condition is a vacuum degree of-0.08 to-0.05MPa, for example, it can be-0.08MPa, -0.07MPa, -0.06MPa or-0.05MPa, and the immersion time is ≥20min, for example, it can be 30min or 40min.

[0121] The application adopts the mode of negative pressure suction to break the air resistance, and the microsphere solution capillary penetrates into the pores to improve the filling rate of the thermal barrier layer; the immersion time is ≥20min to ensure the deep pore infiltration.

[0122] As an optional implementation, the thickness of the base film is 15-25μm, for example, it can be 15μm, 20μm or 25μm, and the porosity is 65-80%, for example, it can be 65%, 70%, 75% or 80%.

[0123] The base film of the application adopts a high porosity design to ensure ion conductivity, and at the same time, its thickness is limited to 15-25μm to balance the mechanical strength and ion transmission.

[0124] As an optional implementation, the preparation of the base film comprises: dissolving PI and ANF in N,N-dimethylacetamide (DMAC) at a mass ratio (2-1):1, for example, which can be 2:1, 1.5:1 or 1:1; the electrospinning voltage is 15-25kV, for example, which can be 15kV, 20kV or 25kV; the injection speed is 0.5-1.5mL / h, for example, which can be 0.5mL / h, 1mL / h or 1.5mL / h; the ambient humidity is 45%±5%, for example, which can be 40%, 42%, 45%, 48% or 50%; and after spinning, the temperature is raised to 180-200℃ at a rate of 3-5℃ / min, for example, which can be 3℃ / min, 4℃ / min or 5℃ / min, and the heat treatment is performed at a temperature of 180-200℃, for example, which can be 180℃, 190℃ or 200℃.

[0125] Rapid temperature rise can cause the micropores to collapse due to too rapid volatilization of DMAC, resulting in reduced porosity. The present application uses a temperature rise of 3-5℃ / min to avoid phase separation and stabilize the pore structure; and the heat treatment at 180-200℃ completely removes DMAC and promotes PI crystallization.

[0126] As an optional implementation, the base film has a pore size gradient distribution: the pore size on the negative electrode side is 50-100nm, for example, which can be 50nm, 60nm, 70nm, 80nm, 90nm or 100nm; and the pore size on the positive electrode side is 100-300nm, for example, which can be 100nm, 150nm, 200nm, 250nm or 300nm.

[0127] The base film of the present application has a pore size gradient distribution, with small pores of 50-100nm on the negative electrode side physically blocking dendrite growth, and together with the ion conductor layer, inhibiting the generation of purple areas; and large pores of 100-300nm on the positive electrode side improving the diffusion efficiency of electrolyte, with a contact angle of <5°. The use of a gradient transition structure avoids stress concentration at the interface, and the gradual change in pore size can resist delamination during the cycle process.

[0128] As an optional implementation, the base film has a LiAlO2 coating on the positive electrode side, with a thickness of 5-10nm, for example, which can be 5nm, 6nm, 7nm, 8nm, 9nm or 10nm.

[0129] As an optional implementation, the LiAlO2 coating is prepared by the following ALD method:

[0130] (a) Precursor combination: the lithium source includes lithium bis(trimethylsilyl)amide (LiHMDS); the aluminum source includes trimethylaluminum (TMA); and the oxygen source includes ozone (O3);

[0131] (b) Deposition conditions: TMA pulse, N2 purge; O3 pulse, N2 purge; LiHMDS pulse, N2 purge;

[0132] (c) LiAlO2 coating layer is deposited and then is fired in stages: first stage: 3℃ / min-5℃ / min temperature rise to 250℃-280℃, N2 atmosphere, 1h-2h holding; second stage: 8℃ / min-10℃ / min temperature rise to 400℃-450℃, 2h-3h holding.

[0133] The LiAlO2 coating layer formed by the ALD method increases the activation energy barrier of the oxygen production reaction of the positive electrode material from the thermodynamic level, and delays the thermal runaway process by increasing the initial temperature of the oxygen production reaction from 280℃ to 350℃; the oxygen ion conductivity of LiAlO2 is much lower than that of the positive electrode material, forming an ion transmission "barrier", when the positive electrode material produces oxygen ions at high temperature, the LiAlO2 layer inhibits the migration of oxygen ions to the electrolyte interface through the low conductivity characteristics, and blocks the chain propagation of the oxygen production reaction from the kinetic level, reducing the oxygen release amount (the gas production amount is reduced by 86% in the Archimedes buoyancy method test). Transition metal ions on the surface of the positive electrode material are easy to become catalytic sites for the oxidation and decomposition of the electrolyte. LiAlO2 forms a 5-10nm ultra-thin dense layer through the ALD process, accurately covers the positive electrode surface defects and active sites, inhibits the dissolution of transition metal and the side reaction of the electrolyte on the positive electrode surface, realizes electrochemical passivation, and reduces the oxygen production inducement from the root.

[0134] The application adopts the method of staged firing, the first stage is 250-280℃ holding: eliminating interface stress, preventing cracking; the second stage is 400-450℃ treatment: forming γ-LiAlO2 phase, improving ion conductivity, avoiding β phase distortion (β-LiAlO2 ion channel is easy to be blocked).

[0135] In a second aspect, the application provides a preparation method of the lithium ion battery composite separator as in the first aspect, as shown in the following steps: Figure 1

[0136] (1) providing a base film;

[0137] (2) setting a functional coating on the negative side of the base film; the functional coating comprises, in order from the base film to the far side of the base film: an ion-conducting layer, a thermal barrier layer, and a self-repairing layer.

[0138] The ion-conducting layer comprises a composite of a modified fast ion conductor and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP); the thermal barrier layer comprises a cross-linked network of boron nitride (BN) nanosheets and polybenzimidazole (PBI); and the self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds (-S-S-).

[0139] As an optional embodiment, the preparation method further comprises the following step: the positive side of the base film is provided with a LiAlO2 coating layer.

[0140] ​In a third aspect, the present application provides a lithium ion battery, the lithium ion battery comprising a positive electrode sheet, a separator, an electrolyte and a negative electrode sheet; the separator is selected from the composite separator of the first aspect or the composite separator prepared by the preparation method of the second aspect, and the composite separator comprises a base film, a functional coating on the negative electrode side of the base film, and a LiAlO2 coating on the positive electrode side of the base film.

[0141] The present application will be further described in detail below in combination with specific examples and comparative examples.

[0142] In the following examples and comparative examples:

[0143] Example 1

[0144] The preparation method of the composite separator comprises the following steps:

[0145] (1) providing a base film: dissolving PI and ANF (Xi'an Ziyue Biological Technology, Q-0330833) in N, N-dimethylacetamide (DMAC) at a mass ratio of 6:4; the voltage of electrospinning is 20 kV, the injection speed is 1.0 mL / h, and the environmental humidity is 45%; after spinning, heat treatment is performed at a temperature increasing rate of 3 ℃ / min to 180 ℃, thereby obtaining an electrospun film with a thickness of 20 μm and a porosity of 75%; the base film has a pore size gradient distribution: the pore size on the negative electrode side is 50-100 nm; and the pore size on the positive electrode side is 100-300 nm.

[0146] (2) setting a functional coating on the negative electrode side of the base film, the functional coating comprising, in order from close to the base film to far from the base film: an ion-conducting layer, a thermal barrier layer, and a self-repairing layer;

[0147] The preparation of the ion-conducting layer comprises the following steps:

[0148] (a) modifying LLP powder: immersing LLP powder (particle size 200-500 nm) in a 0.1 M LiNO3 ethanol solution, ultrasonic dispersion for 30 min, and then vacuum drying at 80 ℃;

[0149] (b) preparing a slurry: adding modified LLP and PVDF-HFP (Fulunolin Chemical Industry, FN-8020) (mass ratio 7:3) into NMP solvent, adding 0.5 wt% carbon nanohorn (CNH) as a dispersant, and ball milling for 6 h (rotation speed 400 rpm);

[0150] (c) coating and curing: coating the slurry on the negative electrode side of the base film, the wet film thickness is 10 μm, after hot air drying at 60 ℃, temperature sintering is performed at 120 ℃ (temperature increasing rate 5 ℃ / min to 120 ℃, holding for 10 min), thereby obtaining an ion-conducting layer comprising fast ion conductor LLP and PVDF-HFP with a thickness of 4 μm.

[0151] The preparation of the thermal barrier layer comprises the following steps:

[0152] (a) Amino-functionalization of BN: 40 g of boron nitride (BN) nanoplatelets (thickness 5-10 nm) were immersed in 2000 g of 3-aminopropyltriethoxysilane (APTES) in ethanol solution (2 wt% concentration) and refluxed at 70 °C for 4 h to obtain amino-functionalized BN (BN-NH2);

[0153] (b) Emulsion preparation: BN-NH2 and polybenzimidazole (PBI) were dispersed in 100 g of epoxy resin (EP828) at a mass ratio of 1:1, and 0.3 g of a photoinitiator (Irgacure 2959) was added. The mixture was emulsified at high speed (10000 rpm, 30 min);

[0154] (c) Coating and curing: the emulsion was uniformly sprayed onto the surface of the ion-conducting layer using electrostatic spraying at a voltage of 30 kV and a flow rate of 0.5 mL / min. Immediately after spraying, the sample was subjected to UV-thermal dual curing: first, UV irradiation (365 nm, 10 mW / cm2, 30 s) was used to initiate crosslinking of the epoxy resin, and then thermal treatment at 80 °C for 1 h was used to complete the polycondensation of PBI. Thus, a thermal barrier layer containing BN nanoplatelets and PBI was obtained, with a thickness of 3 μm, a thermal conductivity of >8 W / m•K, and a temperature resistance of >300 °C. 2

[0155] Preparation of BN nanoplatelets: 1.24 g of boric acid (H3BO3) and 36 g of urea (CO(NH2)2) were mixed and dissolved in 60 ml of deionized water, and stirred for 30 min until transparent. The mixed solution was transferred to a reaction kettle and hydrothermally treated at 180 °C for 16 h. The reaction kettle was naturally cooled to room temperature, and the product was washed with ethanol / water by centrifugation for 3 times and then washed with 0.1 M HCL once. The product was dried at 60 °C under vacuum to obtain BN nanoplatelets.

[0156] The preparation of the self-repairing layer includes the following steps:

[0157] (a) Dynamic polyurethane synthesis: 4,4'-dithiodianiline (DTDA) was used as a chain extender and reacted with polytetramethylene glycol (PTMG) and isophorone diisocyanate (IPDI) at a molar ratio of 1:2:3. The mixture was polymerized under nitrogen protection at 70 °C for 6 h to obtain a dynamic polyurethane containing disulfide bonds (PU-SS);

[0158] (b) Microsphere preparation: PU-SS was dissolved in acetone (8 wt% concentration), and 0.2 wt% of nano-SiO2 based on the mass of the polyurethane was added as a nucleating agent. Microspheres with a diameter of 0.5-2 μm were prepared by spray drying;

[0159] (c) Pore filling: The microspheres were dispersed in an acetone solution (5 wt% concentration), and vacuum impregnation was used to penetrate into the pores of the thermal barrier layer (vacuum degree -0.08 MPa, 20 min).​

[0160] (3) The base film positive electrode side is provided with a LiAlO2 coating layer, and the thickness is 8 nm;

[0161] The LiAlO2 coating layer is prepared by the following ALD method:

[0162] (a) Precursor combination: lithium source is bis(trimethylsilyl) amide lithium (LiHMDS); aluminum source is trimethylaluminum (TMA); oxygen source is ozone (O3);

[0163] (b) Deposition conditions: the base film is preheated to 150°C, and each deposition cycle includes: TMA pulse 0.1 s→N2purging 10 s; O3pulse 0.2 s→N2purging 15 s; LiHMDS pulse 0.3 s→N2purging 20 s;

[0164] (c) After the deposition of the LiAlO2 coating layer, it is subjected to staged firing: after the deposition is completed, it is subjected to staged firing in a tubular furnace N2atmosphere: first stage: heating to 250°C at 5°C / min, firing for 1 h; second stage: heating to 400°C at 10°C / min, firing for 2 h, crystallization treatment.

[0165] Example 2

[0166] The preparation method of the composite separator provided in this embodiment is basically the same as that in Example 1, and the difference lies in that, in the preparation method of the ion-conducting layer, the step of immersing the LLP powder in the lithium salt ethanol solution is not performed, and the unmodified LLP is mixed with PVDF-HFP.

[0167] Example 3

[0168] The preparation method of the composite separator provided in this embodiment is basically the same as that in Example 1, and the difference lies in that, the negative electrode side pore size of 50-100 nm; the positive electrode side pore size of 100-300 nm is changed to the same pore size of 100-200 nm on the positive electrode side and the negative electrode side.

[0169] Example 4

[0170] The preparation method of the composite separator provided in this embodiment is basically the same as that in Example 1, and the difference lies in that, in step (3-c), the staged firing is changed to heating to 400°C at 5°C / min, firing for 3 h, and crystallization treatment.

[0171] Example 5

[0172] The preparation method of the composite separator provided in this embodiment is basically the same as that in Example 1, and the difference lies in that, the base film positive electrode side is not provided with a LiAlO2 coating layer.

[0173] Comparative Example 1

[0174] The preparation method of the composite separator provided by the present comparative example is basically the same as that of Example 1, except that in step (2), no ion-conducting layer is provided on the negative electrode side of the base film.

[0175] Comparative Example 2

[0176] The preparation method of the composite separator provided by the present comparative example is basically the same as that of Example 1, except that in step (2), no thermal barrier layer is provided on the negative electrode side of the base film.

[0177] Comparative Example 3

[0178] The preparation method of the composite separator provided by the present comparative example is basically the same as that of Example 1, except that in step (2), no self-repairing layer is provided on the negative electrode side of the base film.

[0179] Comparative Example 4

[0180] The preparation method of the composite separator provided by the present comparative example is basically the same as that of Example 1, except that in step (2), no functional coating is provided on the negative electrode side of the base film.

[0181] Comparative Example 5

[0182] The preparation method of the composite separator provided by the present comparative example is basically the same as that of Example 1, except that in step (2), the preparation of the self-repairing layer includes the following steps: (a) synthesis of repair agent microcapsules: using interfacial polymerization, an oil phase composed of sebacoyl chloride and divinylbenzene (DVB) is mixed with a repair agent containing 4,4'-dithiodianiline; an aqueous phase composed of ethylenediamine and polyvinyl alcohol PVA (concentration 5 wt%) is prepared. The oil phase is slowly added to the aqueous phase under high-speed shearing ultrasonic action to form an oil / water emulsion, and interfacial polymerization is carried out at a constant temperature of 25-50°C under stirring to make the acyl chloride in the oil phase and the amine in the aqueous phase react on the droplet interface to form a polymer shell microcapsule encapsulating the liquid repair agent. After the reaction is completed, the microcapsules are centrifuged and washed repeatedly with deionized water to remove emulsifiers and unreacted monomers, and finally freeze-dried or vacuum-dried at low temperature to obtain solid microcapsule powder; (b) preparation of microcapsule dispersion: the polymer shell microcapsule powder obtained in step (a) is dispersed in acetone to prepare a microcapsule dispersion with a concentration of 5 wt%; (c) pore filling: the microcapsule dispersion prepared in step (b) is infiltrated into the pores of the thermal barrier layer by vacuum impregnation (vacuum degree -0.08 MPa, maintaining for 20 min).

[0183] Comparative Example 6

[0184] The present comparative example uses a traditional separator PE to replace the composite separator prepared in Example 1.

[0185] Performance detection

[0186] A lithium ion battery was prepared using the composite separator provided in Examples 1-5 and Comparative Examples 1-6, the lithium ion battery comprising: a positive electrode sheet with lithium iron phosphate as the active material, and a negative electrode sheet with graphite as the active material. The preparation method of the lithium ion battery comprises: assembling the positive electrode sheet, the negative electrode sheet and the composite separator into an electric core, and assembling the electric core into a battery according to the battery preparation process.

[0187] Test Example 1

[0188] The composite separators provided in the examples and comparative examples were subjected to heat shrinkage tests, and the batteries prepared in the examples and comparative examples were subjected to charge-discharge cycle tests, and the test results are shown in Table 1.

[0189] Heat shrinkage test: cut a sample of standard size (100 mm x 100 mm) from the separator material to be tested, ensuring that the sample is flat, free of folds, and undamaged. Initial size measurement: use a vernier caliper with sufficient precision to accurately measure the distance between marked points, record the initial length, and record the ambient temperature (usually room temperature). Heating treatment: place the prepared sample flat in an oven (120°C), and after the oven temperature stabilizes to the set value, place the sample in the oven. Record the start time. After reaching the specified time, quickly remove the sample (avoid exposing it to high temperature for a long time). Cooling and state adjustment: place the removed sample flat in a room temperature (e.g. 25±2°C), dry, and vibration-free environment to cool to room temperature. During cooling, the sample should be kept in a natural state, avoiding external force. The cooling time should be sufficient (usually at least 30 minutes) to ensure that the sample temperature is consistent with the ambient temperature and the size is stable. Final size measurement: use the same tools and methods as the initial measurement to accurately measure the distance between the same pair of marked points on the sample after cooling, and record the final length. Finally, the heat shrinkage rate can be calculated.

[0190] Charge-discharge cycle test: (1) 25℃±2℃ for 120min; (2) 1 / 3C CCCV (constant current constant voltage) charge to 3.65V, cut-off current 0.05C; (3) 1 / 3C CC (constant current) discharge to 2.5V; (4) 30min rest; (5) cycle 2-4 steps, two weeks; (6) 1 / 3C constant current constant voltage charge to 3.65V, cut-off current 0.05C; (7) 30min rest; (8) 1 / 3C constant current discharge to 2.5V; (9) 30min rest; (record the capacity as C0, the capacity is corrected every 100 weeks); (10) 3C0 constant current constant voltage charge to 3.65V, cut-off current 0.05C0, 30min rest; (11) 1C0 constant current discharge to 2.5V, 30min rest; (12) repeat steps 10-11 for 100 cycles; (13) 24h rest; (14) repeat steps 6-13 until 2000 cycles of cycle stop, the 2000th cycle charge capacity / the 1st cycle charge capacity is the 2000 cycle cycle capacity retention rate.

[0191] Critical current density test: constant current charge-discharge method was used: fixed single cycle plating lithium / peeling capacity (such as 0.5mAh / cm 2 ). 0.5mA / cm 2 cycle, charge / discharge time 1h each. Gradually increase the current density (0.1-0.5mA / cm 2 each time), repeat the test until short circuit. Termination condition: voltage drops to 0V or below the set threshold (such as-1V).

[0192] Gas production test: Archimedes buoyancy method was used, the battery was immersed in a constant temperature liquid, the buoyancy change (volume expansion) caused by the battery gas production was measured by a weighing sensor, and the gas volume was calculated according to Archimedes law.

[0193] Table 1: charge-discharge cycle test results of batteries prepared by examples and comparative examples

[0194]

[0195] From table 1, it can be seen that:

[0196] (1) The composite separator prepared by examples 1-5 of the application has obvious advantages in cycle capacity retention rate, purple area ratio, heat shrinkage rate, dendrite puncture critical current density and gas production.

[0197] (2) Compared with Example 1, in the preparation method of the ion-conducting layer of Example 2, since the fast-ion conductor modification is not performed, the purple area ratio increases, the thermal shrinkage increases, the capacity retention rate slightly decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases; compared with Example 1, in Example 3, the pore size gradient distribution is no longer used on the positive electrode side and the negative electrode side, the pore sizes on the positive electrode side and the negative electrode side are the same, the purple area ratio increases, the thermal shrinkage increases, the capacity retention rate decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases; compared with Example 1, in Example 4, after the LiAlO2 coating is deposited, the segmented firing is no longer performed, that is, only one segment firing process is performed, the purple area ratio increases, the thermal shrinkage increases, the capacity retention rate slightly decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases; compared with Example 1, in Example 5, the LiAlO2 coating deposition is not performed, the purple area ratio increases, the thermal shrinkage increases, the capacity retention rate slightly decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases.

[0198] (3) Compared with Example 1, in Comparative Example 1, the ion-conducting layer is not arranged on the negative electrode side of the base film, the purple area ratio significantly increases, the thermal shrinkage increases, the capacity retention rate significantly decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases. Compared with Example 1, in Comparative Example 2, the thermal barrier layer is not arranged on the negative electrode side of the base film, the purple area ratio significantly increases, the thermal shrinkage significantly increases, the capacity retention rate decreases, the critical current density significantly decreases and the gas production significantly increases, and the cycle and safety performance significantly decreases. Compared with Example 1, in Comparative Example 3, the self-repairing layer is not arranged on the negative electrode side of the base film, the purple area ratio significantly increases, the thermal shrinkage increases, the capacity retention rate decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases. Compared with Example 1, in Comparative Example 4, the functional coating is not arranged on the negative electrode side of the base film, the purple area ratio significantly increases, the thermal shrinkage significantly increases, the capacity retention rate significantly decreases, the critical current density significantly decreases and the gas production significantly increases, and the cycle and safety performance significantly decreases. Compared with Example 1, in Comparative Example 5, the microcapsules loaded with disulfide bond repair agents are used instead of the polyurethane microspheres loaded with dynamic disulfide bonds, the purple area ratio significantly increases, the thermal shrinkage increases, the capacity retention rate decreases, the critical current density decreases and the gas production increases, and the cycle and safety performance decreases. In Comparative Example 6, the traditional separator PE is used, the purple area ratio significantly increases, the thermal shrinkage significantly increases, the capacity retention rate significantly decreases, the critical current density significantly decreases and the gas production significantly increases, and the cycle and safety performance significantly decreases.

[0199] Test Example 2

[0200] The example 1 and the comparative example 5 of the present application sandwich the separator between two stainless steel electrodes, measure the alternating current impedance spectrum (EIS) to calculate the initial ionic conductivity and the conductivity after damage, to obtain the repair rate of the composite separator, and use the liquid immersion method to test the porosity of the separator, and calculate the porosity retention rate according to the initial porosity and the porosity after damage. The damage referred to in the present application refers to the lithium dendrite puncture of the separator, the pore closure and the like. The test results are shown in Table 2.

[0201] Table 2: Test results of example 1 and comparative example 2

[0202]

[0203] As can be seen from Table 2, compared with example 1, the comparative example 5 uses microcapsules containing disulfide bond repair agent instead of polyurethane microspheres loaded with dynamic disulfide bond. Since the microcapsules cannot be recycled, the porosity retention rate after damage is significantly reduced, and the repair rate of the composite separator after damage is also significantly reduced.

[0204] The above-described examples are merely to describe the preferred embodiments of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by ordinary engineering technicians in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. A lithium-ion battery composite separator, characterized by: The functional coating layer is sequentially composed of an ion-conducting layer, a thermal barrier layer, and a self-repairing layer from the base film to the negative electrode side. The base film is a polyimide and aramid nanofiber blended electrospun film. The ion-conducting layer comprises a modified fast ion conductor and a polyvinylidene-hexafluoropropylene composite. The thermal barrier layer comprises a cross-linked network of boron nitride nanosheets and polybenzimidazole. The self-repairing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds. The thickness of the ion-conducting layer is 3-5 microns. The preparation of the ion-conducting layer comprises the following steps: The fast ion conductor powder is immersed in a lithium salt ethanol solution for pre-doping treatment to obtain a modified fast ion conductor. The modified fast ion conductor and polyvinylidene-hexafluoropropylene are mixed in a solvent to obtain a slurry. The slurry is coated on the negative electrode side of the base film, dried, and sintered to form. The thickness of the thermal barrier layer is 2-4 microns, the thermal conductivity is >8 W / m·K, and the temperature resistance is >300℃. The preparation of the thermal barrier layer comprises the following steps: The boron nitride nanosheets are immersed in a silane coupling agent ethanol solution for surface modification to obtain amino boron nitride. The amino boron nitride and polybenzimidazole are dispersed in an epoxy resin, a photoinitiator is added, and emulsified to obtain an emulsion. The emulsion is coated on the surface of the ion-conducting layer, and a cross-linked network is formed by ultraviolet irradiation and heat treatment. The polyurethane microspheres are filled in the pores of the thermal barrier layer, and the diameter of the polyurethane microspheres is 0.5-2 microns. The preparation of the self-repairing layer comprises the following steps: A dynamic polyurethane is synthesized by reacting a disulfide bond-containing chain extender, a polyol, and a polyisocyanate. The dynamic polyurethane is dissolved in an organic solvent, a nucleating agent is added, and microspheres are obtained by drying and molding. The microspheres are dispersed in a dispersion solvent and permeated into the pores of the thermal barrier layer by negative pressure immersion. The fast ion conductor is selected from Li3La2(PO4)3, LLZO, or LATP.

2. The lithium-ion battery composite separator of claim 1, wherein: The particle size of the fast ion conductor powder is 200-500 nm, and the lithium salt ethanol solution is a 0.05-0.15 M LiNO3 ethanol solution.

3. The lithium-ion battery composite separator of claim 1, wherein: The mass ratio of the modified fast ion conductor to polyvinylidene-hexafluoropropylene is 2-4:

1.

4. The lithium-ion battery composite separator of claim 1, wherein: The modified fast ion conductor and polyvinylidene-hexafluoropropylene are mixed in a solvent to obtain a slurry, which comprises: the modified fast ion conductor, polyvinylidene-hexafluoropropylene, and a dispersant are mixed in a solvent to obtain a slurry, and the dispersant is carbon nanohorn, and the addition amount of the dispersant is 0.3-0.8 wt% of the total mass of the slurry.

5. The lithium-ion battery composite separator of claim 1, wherein: The slurry is coated on the negative electrode side of the base film, dried, and sintered to form, which comprises: the thickness of the slurry coating is 8-12 microns, the drying condition is hot air drying at 60-70℃, and the sintering condition is sintering at a temperature increasing rate of 3-5℃ / min to 120-140℃ and holding for 10-20 min.

6. The lithium-ion battery composite separator of claim 1, wherein: The silane coupling agent is 3-aminopropyl triethoxysilane, the concentration of the silane coupling agent ethanol solution is 2-5 wt%, and the surface modification condition is refluxing at 70-80℃ for 4-5 h.

7. The lithium-ion battery composite separator of claim 1, wherein: ​ 8. The lithium-ion battery composite separator of claim 1, wherein: The aminoated BN and the polybenzimidazole have a mass ratio of 1:1-1.2, the photoinitiator is Irgacure 2959, the addition amount of the photoinitiator is 0.1wt%-0.3wt% of the mass of the epoxy resin, and the emulsification conditions are shearing at 10000rpm-12000rpm for 30min-40min.

9. The lithium-ion battery composite separator of claim 1, wherein: The emulsion is coated on the surface of the ion-conducting layer, and a crosslinked network is formed by ultraviolet irradiation and heat treatment, including: electrostatic spraying, spraying voltage 30kV-40kV, flow rate 0.5mL / min-0.8mL / min, ultraviolet irradiation wavelength 365nm, intensity 10mW / cm 2 -12mW / cm 2 , irradiation time 30s-50s, heat treatment conditions: heating at 70℃-80℃ for 1h-2h.

10. The lithium-ion battery composite separator of claim 1, wherein: The disulfide bond-containing chain extender comprises 4,4'-dithiodianiline, the polyol comprises polytetrahydrofuran, and the polyisocyanate comprises isophorone diisocyanate.

11. The lithium-ion battery composite separator of claim 10, wherein: The reaction molar ratio of the polytetrahydrofuran, the isophorone diisocyanate and the 4,4'-dithiodianiline is 2-3:2-3:

1.

12. The lithium-ion battery composite separator of claim 1, wherein: The organic solvent comprises acetone, the concentration of the dynamic polyurethane is 8wt%-10wt%, and the nucleating agent comprises nano-SiO2, and the addition amount of the nucleating agent is 0.1wt%-0.3wt% of the mass of the polyurethane.

13. The lithium-ion battery composite separator of claim 1, wherein: The dispersion of the microspheres in the dispersion solvent and the penetration into the pores of the thermal barrier layer through negative pressure immersion comprise that the dispersion solvent comprises acetone, the concentration of the microspheres in the dispersion solution is 3wt%-5wt%, and the negative pressure condition is a vacuum degree of-0.08MPa to-0.05MPa, and the immersion time is ≥20min.

14. The lithium-ion battery composite separator of claim 1, wherein: The base film has a thickness of 15μm-25μm and a porosity of 65%-80%.

15. The lithium-ion battery composite separator of claim 1 or 14, wherein: The preparation of the base film comprises: dissolving polyimide and aramid nanofibers in N,N-dimethylacetamide at a mass ratio of 2-1:1, and obtaining the base film after electrospinning and heat treatment.

16. The lithium-ion battery composite separator of claim 15, wherein: The voltage of electrospinning is 15kV-25kV, the injection speed is 0.5mL / h-1.5mL / h, and the environmental humidity is 45%±5%; after spinning, heat treatment is carried out at a temperature increasing rate of 3℃ / min-5℃ / min to 180℃-200℃.

17. The lithium-ion battery composite separator of claim 1, wherein: The base film has a pore size gradient distribution: the pore size on the negative electrode side is 50nm-100nm, and the pore size on the positive electrode side is 100nm-300nm.

18. The lithium-ion battery composite separator of claim 1, wherein: The base film is provided with a LiAlO2 coating layer on the positive electrode side, and the thickness of the LiAlO2 coating layer is 5nm-10nm.

19. The lithium-ion battery composite separator of claim 18, wherein: The LiAlO2 coating layer is prepared by an atomic layer deposition method, comprising: The precursor combination: the lithium source comprises bis(trimethylsilyl)amino lithium; the aluminum source comprises trimethylaluminum; and the oxygen source comprises ozone; The deposition conditions: the base film is preheated to 150℃-180℃, and each deposition cycle comprises: TMA pulse, N2purging; O3pulse, N2purging; LiHMDS pulse, N2purging; After the deposition of the LiAlO2 coating layer, it is subjected to segmented firing: the first stage: increasing the temperature to 250℃-280℃ at a rate of 3℃ / min-5℃ / min, and keeping the temperature in a N2atmosphere for 1h-2h; the second stage: increasing the temperature to 400℃-450℃ at a rate of 8℃ / min-10℃ / min, and keeping the temperature for 2h-3h.

20. A method of producing the composite separator for lithium-ion batteries according to any one of claims 1 to 19, characterized in that, Comprising the following steps: Providing a base film; Providing a functional coating layer on the negative electrode side of the base film; The functional coating layer comprises, in order from close to the base film to far from the base film: an ion-conducting layer, a thermal barrier layer, and a self-repairing layer; Wherein: The ion-conducting layer comprises a composite of modified fast ion conductor and polyvinylidene-hexafluoropropylene. The thermal barrier layer comprises a cross-linked network of boron nitride nanoplatelets and polybenzimidazole; The self-healing layer comprises polyurethane microspheres loaded with dynamic disulfide bonds.

21. The method for preparing the lithium-ion battery composite separator as described in claim 20, characterized in that, The preparation method further comprises depositing a LiAlO2 coating on the cathode side of the base membrane.

22. A lithium-ion battery, characterized by: The lithium ion battery comprises a cathode sheet, a separator, an electrolyte, and an anode sheet; the separator is selected from the lithium ion battery composite separator according to any one of claims 1-19 or the lithium ion battery composite separator prepared by the preparation method according to any one of claims 20-21.

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