Composite coating diaphragm, preparation method thereof and lithium battery

Abstract

The invention relates to the technical field of battery diaphragms, in particular to a composite coating diaphragm, a preparation method thereof and a lithium battery. The composite coating diaphragm comprises a base membrane and a coating arranged on at least one side surface of the base membrane, the coating comprises a first binder and a second binder, the first binder comprises HNBR, and the second binder comprises PVDF and / or PAA. The preparation method comprises the following steps: mixing the first binder, the second binder, the first solvent and the second solvent to prepare slurry, coating the surface of at least one side of the base membrane with the slurry, and drying after coagulating bath treatment to obtain the composite coating diaphragm. The HNBR is introduced into the coating, so that the binding power of the diaphragm is remarkably improved; the porous bicontinuous structure is formed by utilizing the phase separation effect of the first binder and the second binder, the porosity of the diaphragm is improved, the air permeability of the diaphragm is improved, and the diaphragm is high in ionic conductivity, low in fluorine content, economical and environmentally friendly.

Description

TECHNICAL FIELD

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

[0002] As a key component of the battery, the lithium battery separator mainly plays a role in isolating the positive and negative materials, ensuring the safety of the battery and providing the electrolyte permeation channel. In recent years, with the progress of lithium battery technology and the growth of application demand, the performance requirements of the separator are continuously improved. In the prior art, polyvinylidene fluoride (PVDF) is widely used in commercial lithium batteries as a common coating material for lithium battery separators. PVDF as a coating can enhance the mechanical strength of the separator, improve the chemical corrosion resistance of the polyolefin separator and reduce the risk of high temperature short circuit.

[0003] However, the use of PVDF also has certain deficiencies: first, PVDF is a fluorinated polymer with a high content of fluorine, which has a potential impact on the environment; second, although the domestic PVDF production capacity has increased, high-end products still rely on imports, resulting in high costs. With the increasing demand for green environmental protection and cost control, reducing the use of PVDF in lithium battery separators has become a problem to be solved.

[0004] The current lithium battery separator still has obvious technical shortcomings: first, the adhesion between the separator and the electrode sheet is weak, especially under high temperature and high current working conditions, the separator is prone to insufficient bonding with the electrode material, resulting in a decrease in battery cycle performance; second, the existing lithium battery separator also has the problem of large air permeability increment, and the excessively high air permeability value may cause the pores of the separator to be blocked, affecting the ion transmission efficiency. Therefore, it is urgent to develop a new separator material or coating that can overcome the problems of excessive use of fluorine in the existing separator, poor adhesion between the separator and the electrode sheet, and large air permeability increment after coating, and meet the safety and stability requirements of lithium batteries under high performance requirements.

[0005] In view of this, the present application is proposed. SUMMARY

[0006] The purpose of the present application is to provide a composite coating separator, a preparation method thereof and a lithium battery, which significantly improves the adhesion of the separator by introducing HNBR into the coating; a porous double continuous structure is formed by the phase separation of the first and second adhesives, which improves the porosity of the separator and improves the air permeability of the separator. The ion conductivity of the separator is high, the fluorine content is low, and it is economical and environmentally friendly.

[0007] In order to achieve the above purpose of the present application, the following technical scheme is adopted: A composite coating separator, comprising a base film and a coating layer disposed on at least one side surface of the base film, the coating layer comprising a first binder and a second binder, the first binder comprising HNBR, and the second binder comprising PVDF and / or PAA.

[0008] Preferably, the mass ratio of the first binder and the second binder is 1-3:1-8.

[0009] Preferably, the combined acrylonitrile content of the HNBR is 20wt%-50wt%.

[0010] Preferably, the HNBR raw rubber Mooney viscosity is 55-65cps, volatile content is ≤0.8wt%, and residual unsaturation is ≤1.0%.

[0011] Preferably, the porosity of the coating layer is 15%-51%.

[0012] Preferably, the average pore size of the coating layer is 15-28nm.

[0013] Preferably, the Hansen distance Ra of the first binder and the second binder is 7-10.

[0014] The preparation method of the composite coating separator according to any one of the preceding embodiments, comprising the following steps: S1. mixing a first binder, a second binder, and a first solvent and a second solvent to obtain a slurry; wherein the first binder comprises HNBR, and the second binder comprises PVDF and / or PAA; S2. coating the slurry onto at least one side surface of a base film to obtain a separator precursor; S3. treating the separator precursor with a coagulation bath and drying to obtain the composite coating separator.

[0015] Preferably, the first solvent comprises at least one of toluene, xylene, and tetrahydrofuran.

[0016] Preferably, the second solvent comprises at least one of N,N-dimethylacetamide, N-methylpyrrolidone, and N,N-dimethylformamide.

[0017] Preferably, the ratio of the total mass of the first binder and the second binder to the total mass of the first solvent and the second solvent is 5-15:85-95.

[0018] Preferably, in step S3, the coagulation bath treatment comprises first coagulation bath treatment and second coagulation bath treatment performed in sequence. The first coagulation bath comprises an extractant and water, the extractant comprises at least one of N, N-dimethylacetamide, N-methylpyrrolidone and N, N-dimethylformamide; the first coagulation bath treatment is a multi-stage treatment, and the concentration of the extractant in the multi-stage first coagulation bath is sequentially reduced; The second coagulation bath is water, and the second coagulation bath treatment is a single-stage treatment or a multi-stage treatment.

[0019] A lithium battery comprising the composite coating diaphragm in any one of the preceding embodiments.

[0020] Compared with the prior art, the present application has the following advantages: (1) The present application introduces HNBR in the coating, effectively improving the adhesion of the diaphragm; the "phase separation" process between the first adhesive and the second adhesive in the coating creates a porous double-continuous structure, which not only improves the porosity of the diaphragm and improves the air permeability of the diaphragm, but also reduces the transmission resistance of lithium ions and improves the ionic conductivity.

[0021] (2) The present application uses HNBR to replace part of PVDF, or uses HNBR and PAA to completely replace traditional PVDF, effectively reducing the content of fluorine elements in the coating, which is economical and environmentally friendly.

[0022] (3) The method of the present application is simple, easy to operate, and has high repeatability. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0024] Figure 1 SEM surface image of the diaphragm prepared in Example 1 of the present application 10000 times; Figure 2 SEM surface image of the diaphragm prepared in Example 1 of the present application 10000 times; Figure 1 Image after imagej processing; Figure 3 SEM surface image of the diaphragm prepared in Example 8 of the present application 10000 times; Figure 4 SEM surface image of the diaphragm prepared in Example 8 of the present application 10000 times; Figure 3 Image after imagej processing; Figure 5 SEM surface image of the diaphragm prepared in Example 1 of the present application 10000 times; Figure 6 SEM surface image of the diaphragm prepared in Example 1 of the present application 10000 times;Figure 5 Image after imagej processing; Figure 7 SEM surface image of the separator prepared in Inventive Example 1 at 10000 times; Figure 8 SEM surface image of the separator prepared in Inventive Example 1 at 10000 times; Figure 7 Image after imagej processing; Figure 9 SEM surface image of the separator prepared in Inventive Example 1 at 10000 times; Figure 10 SEM surface image of the separator prepared in Inventive Example 1 at 10000 times; Figure 9 Image after imagej processing; Figure 11 EDS element distribution map of the coating surface in Inventive Example 1. DETAILED DESCRIPTION

[0025] The technical solutions of the present application will be described clearly and completely in the following combined with the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application. The specific conditions are not specified in the embodiments, which are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, which are all conventional products that can be purchased on the market.

[0026] The first aspect of the present application provides a composite coating separator, comprising a base film and a coating layer arranged on at least one side surface of the base film, the coating layer comprising a first binder and a second binder, the first binder comprising HNBR (hydrogenated nitrile rubber), and the second binder comprising PVDF (polyvinylidene fluoride) and / or PAA (polyamide acid).

[0027] HNBR is obtained by hydrogenation treatment of nitrile rubber (NBR), which retains the cyano group (-CN) in the acrylonitrile monomer; the cyano group is a strong polar group, which can react with the hydroxyl group (-OH), oxide or oxygen-containing functional group contained on the surface of the polar sheet, and the cyano group is combined with these polar surfaces through dipole-dipole interaction or hydrogen bond to form firm physical adsorption, thereby producing good adhesion; the introduction of HNBR in the coating layer can effectively improve the adhesion between the separator and the polar sheet; the HNBR in the first adhesive and the PVDF and / or PAA in the second adhesive are significantly different in molecular chain structure, polarity and solubility parameter, so they cannot be uniformly miscible at the molecular level, that is, they are incompatible; the phase separation caused by the incompatibility of the two forms a unique bicontinuous structure (two different components or phases form a continuous and mutually penetrating three-dimensional structure in space) in the coating at the micro level, and a large number of PVDF-HNBR phase interfaces and / or PAA-HNBR phase interfaces are formed in the membrane during the phase separation process, and the boundaries between these phases and phases exactly constitute efficient channels for lithium ion transmission, and the nano-pore channels formed by the phase separation are interconnected, forming an excellent ion conduction network; compared with the limited pores formed by PVDF itself, the interconnected nano-pore channels formed by phase separation have better continuity and interconnectivity, which not only improves the air permeability of the coated separator, but also significantly improves the ionic conductivity.

[0028] In some embodiments of the present application, the mass ratio of the first adhesive and the second adhesive is 1-3:1-8, for example, it can be any point value or a range value composed of any two point values in 3:1, 2:1, 1:1, 1:2, 1:4, 1:8; preferably 1-2:1-4.

[0029] In some embodiments of the present application, the combined acrylonitrile content of the HNBR used is 20wt%-50wt%, for example, it can be any point value or a range value composed of any two point values in 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%; preferably 35wt%-40wt%; if the acrylonitrile content is too low, the cyano group is insufficient, the adhesion is weak, and the phase separation is insufficient, the ionic conductivity is limited in the range of improvement; if the acrylonitrile content is too high, the pore channel will be coarsened due to excessive phase separation, the adhesion will be strong but the brittleness will be increased, the ionic conductivity will be decreased, and the air permeability value will be increased.

[0030] In some embodiments of the present application, the HNBR raw rubber used has a Mooney viscosity of 55-65cps, a volatile content of ≤0.8wt%, and a remaining unsaturation of ≤1.0%.

[0031] In some embodiments of the present application, the porosity of the coating layer is 15-51%, for example, can be any one value or a range value consisting of two point values selected from 15%, 20%, 30%, 40%, 51%.

[0032] In some embodiments of the present application, the average pore size of the coating layer is 15-28 nm, for example, can be any one value or a range value consisting of two point values selected from 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, 28 nm.

[0033] In some embodiments of the present application, the Hansen distance Ra of the first binder and the second binder is 7-10, for example, can be any one value or a range value consisting of two point values selected from 7, 7.12, 7.5, 8, 8.37, 9, 9.5, 10; controlling the Hansen distance Ra of the first binder and the second binder within the above range, the two can be phase separated to form a good bi-continuous structure. Wherein Ra = sqrt[4( δd) 2 + ( δp) 2 + ( δh) 2 ] according to the Hansen parameters (δd, δp, δh), δd is the difference between the δd of the first binder and the second binder, δp is the difference between the δp of the first binder and the second binder, δh is the difference between the δh of the first binder and the second binder; in the present application, the Hansen parameters (δd, δp, δh) of PVDF take δd = 17.2, δp = 12.5, δh = 9.2; the Hansen parameters (δd, δp, δh) of HNBR take δd = 18.4, δp = 6.0, δh = 4.5; the Hansen parameters (δd, δp, δh) of PAA take δd = 18.0, δp = 11.0, δh = 9.5; the Ra of PVDF and HNBR is about 8.37, indicating that the two are not compatible with each other and are prone to phase separation; the Ra of PAA and HNBR is about 7.12, indicating that the two are not compatible with each other and are prone to phase separation.

[0034] The second aspect of the present application provides a preparation method of the composite coating separator of any one of the preceding embodiments, comprising the following steps: S1. Mixing the first binder, the second binder and the first solvent and the second solvent to obtain a slurry; wherein the first binder comprises HNBR, and the second binder comprises PVDF and / or PAA; S2. coating the slurry on at least one side surface of the base film to obtain a separator precursor; S3. treating the separator precursor by a coagulation bath and drying to obtain a composite-coated separator.

[0035] The present application adopts a first binder containing HNBR and a second binder containing PVDF and / or PAA to prepare a composite coating. Compared with the traditional PVDF coating, the introduction of HNBR in the coating can significantly improve the adhesion between the separator and the pole piece on the one hand; on the other hand, the pore structure of the coating can be improved by the phase separation of the first binder and the second binder, and the gas permeability increment of the separator is reduced to improve the ion conductivity. The improvement of the gas permeability and the ion conductivity of the separator is mainly because the "phase separation" between the incompatible polymers creates a porous bicontinuous structure. Taking HNBR and PVDF as an example, the possible mechanism is as follows: HNBR is a saturated polar rubber containing strong polar cyanide (-CN), and PVDF is a semi-crystalline fluoropolymer with strong polarity, but the polarity source is different from that of HNBR; although both of them have polarity, there are significant differences in their molecular chain structure, polarity strength and solubility parameter, which leads to their inability to uniformly dissolve at the molecular level, i.e. "incompatible". When PVDF and HNBR are dissolved in a common solvent and coated into a film, the following key processes occur as the solvent evaporates: the initial solution is uniform, and the PVDF and HNBR molecular chains are uniformly dispersed in the solvent; as the solvent evaporates and the polymer concentration increases, the molecular chains begin to approach each other; since PVDF and HNBR are incompatible, they tend to "aggregate" by themselves rather than entangle with each other, which drives the occurrence of liquid-liquid phase separation, and the solution separates into individual PVDF-rich "microzones" and HNBR-rich "microzones", and the phase separation process forms a large number of PVDF-HNBR phase interfaces inside the film, and these phase-to-phase boundaries exactly constitute efficient channels for lithium ion transmission, and the nano-porous channels formed by the phase separation are interconnected, forming an excellent ion conduction network; when the solvent evaporates to a certain extent and the polymer concentration is high enough, the viscosity of the system increases sharply, and at the same time, if PVDF crystallizes, its crystal structure will act as a physical crosslinking point to "freeze" the entire structure, so the phase separation structure that has occurred before is fixed in the final dry film. Compared with the limited pores formed by PVDF itself, the interconnected nano-pores constructed by phase separation have better continuity and interconnectivity, which not only improves the gas permeability of the coated separator, but also significantly improves the ion conductivity.

[0036] In some specific embodiments of the present application, the first solvent in step S1 includes at least one of toluene (TOL), xylene (DMB) and tetrahydrofuran (THF).

[0037] In some embodiments of the present application, the second solvent in step S1 comprises at least one of N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF).

[0038] In some embodiments of the present application, the ratio of the total mass of the first binder and the second binder to the total mass of the first solvent and the second solvent is 5-15:85-95, for example, it can be any point value or a range value composed of any two point values in 5:95, 8:92, 10:90, 12:88, 15:85.

[0039] In some embodiments of the present application, in step S1, the step of preparing the slurry comprises: S101. Dissolve the first binder in the first solvent to obtain a first solution; dissolve the second binder in the second solvent to obtain a second solution; S102. Mix the first solution and the second solution, and stir at a stirring speed of 400-700 r / min for 0.5-2 h to obtain the slurry; for example, the stirring speed can be a range value composed of any point value in 400 r / min, 500 r / min, 600 r / min, 700 r / min, and the stirring time can be a range value composed of any point value or any two point values in 0.5 h, 1 h, 1.5 h, 2 h.

[0040] In some embodiments of the present application, in step S101, the step of dissolving the first binder comprises: mixing the first binder and the first solvent at 25-45°C at a stirring speed of 400-700 r / min for 12-24 h; for example, the dissolving temperature can be a range value composed of any point value or any two point values in 25°C, 30°C, 35°C, 40°C, 45°C; the stirring speed can be a range value composed of any point value or any two point values in 400 r / min, 500 r / min, 600 r / min, 700 r / min; and the mixing time can be a range value composed of any point value or any two point values in 12 h, 15 h, 18 h, 21 h, 24 h.

[0041] In some embodiments of the present application, in the first solution, the mass ratio of the first binder to the first solvent is 5-15:85-95, for example, it can be any point value or a range value composed of any two point values in 5:95, 8:92, 10:90, 12:88, 15:85.

[0042] In some embodiments of the present application, in step S101, the step of dissolving the second binder comprises: mixing the second binder and the second solvent at 25-45°C with stirring at 400-700 r / min for 3-5 h, for example, the dissolving temperature can be any one of 25°C, 30°C, 35°C, 40°C, 45°C or a range value formed by any two of them; the stirring speed can be any one of 400 r / min, 500 r / min, 600 r / min, 700 r / min or a range value formed by any two of them; the stirring mixing time can be any one of 3 h, 3.5 h, 4 h, 4.5 h, 5 h or a range value formed by any two of them.

[0043] In some embodiments of the present application, in the second solution, the mass ratio of the second binder to the second solvent is 5-15:85-95, for example, it can be any one of 5:95, 8:92, 10:90, 12:88, 15:85 or a range value formed by any two of them.

[0044] In some embodiments of the present application, in step S3, the coagulation bath treatment comprises first coagulation bath treatment and second coagulation bath treatment performed in sequence. The first coagulation bath comprises an extractant and water, and the extractant used comprises at least one of N,N-dimethylacetamide, N-methylpyrrolidone and N,N-dimethylformamide; the first coagulation bath treatment is a multi-stage treatment, and the concentration of the extractant in the multi-stage first coagulation bath decreases in sequence; The second coagulation bath is water, and the second coagulation bath treatment is a single-stage treatment or a multi-stage treatment.

[0045] In some embodiments of the present application, the first coagulation bath treatment is a two-stage treatment, comprising: immersing the separator precursor into a first-stage first coagulation bath and a second-stage first coagulation bath in sequence; wherein the concentration of the extractant in the first-stage first coagulation bath is 30wt%-60wt%, for example, it can be any one of 30wt%, 40wt%, 50wt%, 60wt% or a range value formed by any two of them; the concentration of the extractant in the second-stage first coagulation bath is 10wt%-35wt%, for example, it can be any one of 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt% or a range value formed by any two of them.

[0046] In some embodiments of the present application, the second coagulation bath treatment is also a two-stage treatment.

[0047] In some embodiments of the present application, the treatment time of the separator precursor in any stage of the coagulation bath is 1-5 min, for example, it can be any one of 1 min, 2 min, 3 min, 4 min, 5 min or a range value formed by any two of them.

[0048] In some embodiments of the present application, the temperature of the drying in step S3 is 50-60℃, for example, it can be any one of 50℃, 52℃, 55℃, 58℃, 60℃ or a range value composed of any two of them; the time of the drying is 5-20min, for example, it can be any one of 5min, 10min, 15min, 20min or a range value composed of any two of them.

[0049] The third aspect of the present application provides a lithium battery comprising the composite coated separator of any one of the preceding embodiments.

[0050] Some embodiments of the present application will be described in detail below with reference to specific application examples. The raw materials used in the examples, such as those not specifically described, can be purchased on the market.

[0051] Example 1 S1. Preparation of slurry S101. Mix the granular HNBR and toluene at a mass ratio of 5:95, stir at 600r / min at 25℃ for 24h to dissolve the HNBR in toluene, to obtain a HNBR solution with a solid content of 5%, as a first solution; wherein the combined acrylonitrile content of the HNBR is 35wt%; Mix the powdery PVDF and DMAc at a mass ratio of 5:95, stir at 600r / min at 25℃ for 4h to dissolve the PVDF in DMAc, to obtain a PVDF solution with a solid content of 5%, as a second solution; S102. Mix the first solution and the second solution at a mass ratio of 2:1, stir at a speed of 600r / min for 2h to be uniform, to obtain a slurry; S2. Coat the slurry prepared in step S1 on one side surface of a polyethylene-based film with a thickness of 7μm, to obtain a separator precursor; S3. Subject the separator precursor to a four-stage coagulation bath treatment in sequence, wherein the first-stage coagulation bath and the second-stage coagulation bath are both mixed solutions of DMAc and deionized water, with concentrations of 30wt% and 10wt% in sequence, and the third-stage coagulation bath and the fourth-stage coagulation bath are both deionized water, and the treatment time of each stage is 2min; then dehydrate by drying in an oven at 60℃ for 10min, to obtain the composite coated separator.

[0052] Example 2 Example 2 is similar to Example 1, the only difference is that the mass ratio of the first solution and the second solution is 1:1, i.e. the mass ratio of HNBR and PVDF is 1:1, and the rest of the conditions are the same as those of Example 1.

[0053] Example 3 Example 3 is similar to Example 1, except that the mass ratio of the first solution and the second solution is 1:2, i.e. the mass ratio of HNBR and PVDF is 1:2, and the rest of the conditions are the same as those of Example 1.

[0054] Example 4 Example 4 is similar to Example 1, except that the mass ratio of the first solution and the second solution is 1:4, i.e. the mass ratio of HNBR and PVDF is 1:4, and the rest of the conditions are the same as those of Example 1.

[0055] Example 5 Example 5 is similar to Example 1, except that the bound acrylonitrile content of the HNBR used is 20wt%, and the rest of the conditions are the same as those of Example 1.

[0056] Example 6 Example 6 is similar to Example 1, except that the bound acrylonitrile content of the HNBR used is 40wt%, and the rest of the conditions are the same as those of Example 1.

[0057] Example 7 Example 7 is similar to Example 1, except that the bound acrylonitrile content of the HNBR used is 50wt%, and the rest of the conditions are the same as those of Example 1.

[0058] Example 8 Example 8 is similar to Example 1, except that the PVDF in the second solution is replaced by PAA, and the mass ratio of HNBR and PAA is 1:1, and the rest of the conditions are the same as those of Example 1.

[0059] Example 9 Example 9 is similar to Example 8, except that the mass ratio of HNBR and PAA is 1:2, and the rest of the conditions are the same as those of Example 8.

[0060] Example 10 Example 10 is similar to Example 8, except that the mass ratio of HNBR and PAA is 1:8, and the rest of the conditions are the same as those of Example 8.

[0061] Comparative Example 1 Comparative Example 1 is similar to Example 1, except that a HNBR solution with a solid content of 5% is directly used as the slurry to coat a pure HNBR coating layer, and the rest of the conditions are the same as those of Example 1.

[0062] Comparative Example 2 Comparative Example 2 is similar to Example 1, except that a PVDF solution with a solid content of 5% is directly used as the slurry to coat a pure PVDF coating layer, and the rest of the conditions are the same as those of Example 1.

[0063] Comparative Example 3 Comparative Example 3 is similar to Example 1, except that a PAA solution with a solid content of 5% is directly used as the slurry to coat a pure PAA coating layer, and the rest of the conditions are the same as those in Example 1.

[0064] Test Example 1. Air permeability test The air permeability is tested by using a Kumagai air permeability tester according to GB / T 36363-2018, and the unit is s / 100cc.

[0065] 2. Air permeability increment test The air permeability of the composite coating separator and the base film is tested by using a Kumagai air permeability tester according to GB / T 36363-2018, respectively. The difference between the air permeability of the composite coating separator and the air permeability of the base film is the air permeability increment, and the unit is s / 100cc.

[0066] 3. Hot-pressing positive electrode adhesion test The hot-pressing positive electrode adhesion is tested by using a universal material testing machine according to GB / T 36877-2018, and the unit is N / m.

[0067] 4. Ionic conductivity test The ionic conductivity is tested by using a glove box and an electrochemical workstation according to GB / T 36363-2018, and the unit is mS / cm.

[0068] 5. The SEM images of each example and comparative example are processed by using ImageJ software to obtain the statistical data of the average pore size and porosity of the coating layer. The specific processing method is as follows: the image format is adjusted to 8bit by using ImageJ software, the picture threshold range of the surface image is adjusted after adding a ruler (0≤MinThr≤80, 150≤MaxThr≤255, for example, MinThr is 80 and MaxThr is 250), so that the hole structure edge in the picture is highlighted, the software is used to draw along the hole structure contour, specifically the part with higher gray value and appearing as white or close to white is drawn, and a drawing image is obtained. The smallest unit in the drawing image is recorded as one hole structure, and the smallest unit located at the edge of the drawing image that is not completely displayed is also recorded as one hole structure. The area S of the two-dimensional image fitted by the hole structure edge in the drawing image is measured by using ImageJ software, and the average pore size and porosity statistical data can be calculated by the software.

[0069] The test results of air permeability, air permeability increment, hot-pressing positive electrode adhesion, and ionic conductivity are shown in Table 1; and the test results of coating porosity and average pore size are shown in Table 2.

[0070] Table 1

[0071] From the data in Table 1, it can be seen that the air permeability increment of the separators of Examples 1-7 is lower than that of Comparative Example 1 (pure HNBR) and Comparative Example 2 (pure PVDF), which indicates that the use of mixed HNBR and PVDF coating can improve the hole blocking of the microporous membrane; the ionic conductivity of the separators of Examples 1-7 is higher than that of Comparative Example 1 and Comparative Example 2, which indicates that the mixed HNBR and PVDF coating can improve the ion transmission efficiency of the separator. The positive electrode adhesion of Example 1 is greater than that of Examples 2-4, which indicates that the mixed HNBR and PVDF coating and the higher proportion of HNBR can effectively improve the adhesion of the separator.

[0072] From the data in Examples 1 and 5-7, it can be seen that when the content of acrylonitrile combined in HNBR is 20wt%, the air permeability increment is significantly reduced, the adhesion and ionic conductivity are improved compared with Comparative Example 1 and Comparative Example 2, but relative to Example 1 and Example 6, the adhesion is reduced due to insufficient cyano groups, the phase separation is insufficient, and the ionic conductivity is reduced; when the content of acrylonitrile combined in HNBR is 50wt%, the air permeability increment is reduced, the adhesion and ionic conductivity are improved compared with Comparative Example 1 and Comparative Example 2, but relative to Example 1 and Example 6, the pore channel is coarsened due to excessive phase separation, the adhesion may be too strong but the brittleness increases, and the ionic conductivity is reduced; when the content of acrylonitrile combined in HNBR is 35%-40wt%, the phase separation forms an ideal bicontinuous structure, the adhesion and ionic conductivity are higher, and the air permeability increment is controllable.

[0073] From the data in Examples 8-10 and Comparative Example 1 and Comparative Example 3, it can be seen that the air permeability increment of the separators coated with mixed HNBR and PAA is comparable to that of the separators coated with pure PAA, the adhesion and ionic conductivity are improved compared with the separators coated with pure PAA, the air permeability increment is significantly reduced compared with the separators coated with pure PVDF, and the ionic conductivity is significantly improved; when the mass ratio of HNBR to PAA is 1:1, the hot-pressed positive electrode adhesion of the separator is increased by 16% compared with the separator coated with pure HNBR and by about 53 times compared with the separator coated with pure PAA.

[0074] Table 2

[0075] As shown in Figure 5 and Figure 6 , the coating layer of the separator coated with pure HNBR has almost no pores; as shown in Figure 7 and Figure 8 , the pore structure size uniformity of the coating layer of the separator coated with pure PVDF is poor; as shown in Figure 9 and Figure 10 , the pore structure size and pore distribution uniformity of the coating layer of the separator coated with pure PAA are poor; and as shown in Figure 1 , Figure 2 , Figure 3 andFigure 4 As shown, using HNBR and PVDF mixed coating or using HNBR and PAA mixed coating, uniform interpenetrating nanochannels can be obtained, and the coating hole structure size and hole distribution have good uniformity, and the gas permeability of the separator can be improved.

[0076] As shown, the F and N elements are uniformly distributed in the coating, i.e., HNBR and PVDF form a bicontinuous structure. Figure 11

[0077] Although the present application has been illustrated and described with reference to specific embodiments, it is to be understood that the above-described embodiments are merely illustrative of the principles of the present application and are not to be construed as limiting the scope of the present application. Those skilled in the art should understand that various modifications of the above-described embodiments, equivalents thereof, and / or integrations of some or all features of some or all embodiments can be made with respect to the technical solutions described in the above embodiments without departing from the spirit and scope of the present application. Such modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the embodiments of the present application. Therefore, it is meant that all such modifications and replacements are included in the scope of the appended claims.​

Claims

1. A composite-coated separator, characterized by, The composite coating separator comprises a base film and a coating layer arranged on at least one side surface of the base film, wherein the coating layer comprises a first binder and a second binder, the first binder comprises HNBR, and the second binder comprises PVDF and / or PAA.

2. The composite-coated separator of claim 1, wherein, The mass ratio of the first binder to the second binder is 1-3:1-8.

3. The composite coated separator of claim 1, wherein, The combined acrylonitrile content of the HNBR is 20wt%-50wt%. And / or, the HNBR raw rubber Mooney viscosity is 55-65cps, the volatile content is ≤0.8wt%, and the residual unsaturation is ≤1.0%.

4. The composite coating separator of claim 1, wherein, The porosity of the coating layer is 15%-51%.

5. The composite coating separator of claim 1, wherein, The average pore size of the coating layer is 15-28nm.

6. The composite-coated separator of claim 1, wherein The Hansen distance Ra of the first binder and the second binder is 7-10.

7. The method of producing a composite coating separator according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: S1. mixing a first binder, a second binder, and a first solvent and a second solvent to obtain a slurry; wherein the first binder comprises HNBR, and the second binder comprises PVDF and / or PAA; S2. coating the slurry on at least one side surface of a base film to obtain a separator precursor; S3. treating the separator precursor with a coagulation bath and drying to obtain the composite coating separator.

8. The method of claim 7, wherein the composite coating is prepared by a method comprising: At least one of the following characteristics is met: (1) the first solvent comprises at least one of toluene, xylene, and tetrahydrofuran; (2) the second solvent comprises at least one of N,N-dimethylacetamide, N-methylpyrrolidone, and N,N-dimethylformamide; (3) the ratio of the total mass of the first binder and the second binder to the total mass of the first solvent and the second solvent is 5-15:85-95.

9. The method of claim 7 or 8, wherein the composite coating separator is prepared by a method comprising: In step S3, the coagulation bath treatment comprises first coagulation bath treatment and second coagulation bath treatment performed in sequence; The first coagulation bath comprises an extractant and water, the extractant comprises at least one of N,N-dimethylacetamide, N-methylpyrrolidone, and N,N-dimethylformamide; the first coagulation bath treatment is a multi-stage treatment, and the extractant concentration in the multi-stage first coagulation bath decreases in sequence; The second coagulation bath is water, and the second coagulation bath treatment is a single-stage treatment or a multi-stage treatment.

10. A lithium battery, characterized by, The composite coating separator according to any one of claims 1-6.

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