High-temperature-resistant composite diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention belongs to the technical field of batteries, and discloses a high-temperature-resistant composite diaphragm, a preparation method thereof and a lithium ion battery. The composite diaphragm comprises a diaphragm base material, a zinc borate coating and a polymer coating, the zinc borate coating is arranged on a single face or double faces of the diaphragm base material, the polymer coating is arranged on the surface of the zinc borate coating, and raw materials of the zinc borate coating comprise zinc borate, a binder and a solvent. The polymer coating is prepared from the following raw materials: methyl methacrylate, acrylic anhydride, succinonitrile, an initiator and lithium salt. The composite diaphragm disclosed by the invention does not generate obvious thermal shrinkage during high-temperature heating, and pores are closed during high-temperature heating, so that crosstalk between an oxidizing substance of a positive electrode and a reducing substance of a negative electrode is avoided, and a thermal runaway initiation temperature T2 is delayed or even avoided.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a high temperature resistant composite diaphragm and a preparation method thereof, and a lithium ion battery. Background Art

[0002] High nickel oxide positive electrode material LiNi x Co y Mn z O2 (NCM, where x+y+z=1) has a layered crystal structure and has attracted great interest due to its high theoretical specific capacity (180-250mAh / g), high operating voltage, and less use of expensive Co. x -graphite or Si-graphite-based composites) pairing is one of the most important technical routes for developing next-generation commercial lithium-ion batteries with energy density exceeding 300Wh / kg. However, the cycle life and safety performance of high-nickel (Ni>80%) lithium-ion batteries have always been poor, hindering their large-scale commercial application.

[0003] The main reasons for the poor cycling stability of high-nickel NCM include residual alkali on the surface, transition metal dissolution catalyzed by trace water in the battery, and the precipitation of oxygen free radicals; the main reason for its poor thermal stability is that the fully charged NCM positive electrode continues to release oxygen during heating, which reacts exothermically with the LiC6 at the negative electrode, thereby causing thermal runaway.

[0004] Therefore, how to avoid the positive and negative electrodes from contacting each other when thermal abuse occurs and how to prevent the oxygen released by the positive electrode from contacting the negative electrode is a technical problem that needs to be solved urgently in this field. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present invention provides a high temperature resistant composite diaphragm and a preparation method thereof and a lithium ion battery, wherein the composite diaphragm does not undergo obvious thermal shrinkage when heated at high temperature, and the pores are closed when heated at high temperature, thereby avoiding crosstalk between the oxidizing substances of the positive electrode and the reducing substances of the negative electrode, delaying or even avoiding the thermal runaway initiation temperature T2.

[0006] In a first aspect, the present invention provides a composite diaphragm, comprising a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is disposed on one side or both sides of the diaphragm substrate, and the polymer coating is disposed on the surface of the zinc borate coating, the raw materials of the zinc borate coating include zinc borate, a binder and a solvent, and the raw materials of the polymer coating include methyl methacrylate, acrylic anhydride, succinonitrile, an initiator and a lithium salt.

[0007] In some embodiments of the present invention, the thickness of the zinc borate coating is 0.1-50 μm, preferably 0.1-30 μm, more preferably 1-10 μm, and even more preferably 2-4 μm.

[0008] In some embodiments of the present invention, the thickness of the polymer coating is 0.1-50 μm, preferably 0.1-30 μm, more preferably 1-10 μm, and even more preferably 2-4 μm.

[0009] In some embodiments of the present invention, the mass percentage of the zinc borate coating in the composite membrane is 30%-90%; for example, it can be any point value among 30%, 40%, 50%, 60%, 70%, 80%, 90%, or a range value between any two point values.

[0010] In some embodiments of the present invention, the mass percentage of the polymer coating in the composite membrane is 10%-50%; for example, it can be any point value among 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range value between any two point values.

[0011] In some embodiments of the present invention, the mass ratio of the zinc borate, the binder and the solvent is 37:(0.7-1.5):(30-250).

[0012] In some embodiments of the present invention, the mass ratio of methyl methacrylate, acrylic anhydride, succinonitrile, initiator and lithium salt is 9:(0.5-1.5):(5-15):(0.05-0.15):(3.75-11.25).

[0013] In some embodiments of the present invention, the zinc borate comprises at least one of anhydrous zinc borate, 3.5 hydrate zinc borate, 7 hydrate zinc borate and 5 hydrate zinc borate.

[0014] In some embodiments of the present invention, the binder includes at least one of polyvinylidene fluoride, nitrile rubber, hydrogenated nitrile rubber, hydrogenated styrene-butadiene block copolymer, carboxymethyl cellulose, sodium carboxymethyl cellulose, polytetrafluoroethylene and styrene-butadiene rubber.

[0015] In some embodiments of the invention, the solvent comprises water.

[0016] In some embodiments of the present invention, the initiator includes at least one of azobisisobutyronitrile, benzoin diethyl ether, benzoyl oxide, azobisisoheptylnitrile, benzoin ethyl ether, benzoin isopropyl ether and benzoin butyl ether.

[0017] In some embodiments of the present invention, the lithium salt includes at least one of lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), lithium bis(difluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6) and lithium hexafluorophosphate (LiPF6).

[0018] In some embodiments of the present invention, the diaphragm substrate includes one of a polyethylene diaphragm, a polypropylene diaphragm, and a polyethylene film-polypropylene film composite diaphragm.

[0019] The second aspect of the present invention provides a method for preparing the composite diaphragm according to the first aspect of the present invention, comprising the following steps:

[0020] The zinc borate, binder and solvent are mixed, applied to one side or both sides of the diaphragm substrate, and dried to form the zinc borate coating;

[0021] The methyl methacrylate, acrylic anhydride, succinonitrile, initiator and lithium salt are mixed, coated on the surface of the zinc borate coating, and heated for polymerization to form the polymer coating to obtain the composite diaphragm.

[0022] In some embodiments of the present invention, the drying temperature is 60-90°C; for example, it can be any point value among 60°C, 70°C, 80°C, 90°C, or a range value between any two point values.

[0023] In some embodiments of the present invention, the drying time is 4-24 hours; for example, it can be any point value among 4 hours, 8 hours, 12 hours, 16 hours, 20 hours, 24 hours, or a range value between any two point values.

[0024] In some embodiments of the present invention, the temperature of the heating polymerization is 60-80°C; for example, it can be any point value among 60°C, 70°C, 80°C, or a range value between any two point values.

[0025] In some embodiments of the present invention, the heating polymerization time is 8-14 hours; for example, it can be any point value among 8 hours, 10 hours, 12 hours, 14 hours, or a range value between any two point values.

[0026] According to a third aspect of the present invention, a lithium ion battery is provided. The lithium ion battery comprises the composite diaphragm according to the first aspect of the present invention.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The present invention significantly reduces the thermal shrinkage rate of the composite diaphragm by combining a specific zinc borate coating and a polymer coating. At the same time, the pores of the composite diaphragm are closed when heated at high temperatures (above 200°C), thereby avoiding chemical crosstalk and further thermal runaway caused by oxygen release from the positive electrode during thermal runaway. The composite diaphragm of the present invention is used in lithium-ion batteries to improve the thermal stability of the battery.

[0029] (2) The preparation process of the composite diaphragm of the present invention is simple and suitable for industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a SEM image of the initial state of the composite diaphragm of Example 1;

[0031] Figure 2 This is a SEM image of the composite membrane of Example 1 after being treated at 200°C;

[0032] Figure 3 This is a test chart of the charge and discharge rate performance of the button battery assembled based on Example 1. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below by specific examples. The raw materials, reagents or devices used in the examples can be obtained from conventional commercial sources or by prior art methods unless otherwise specified. Unless otherwise specified, the experiments or test methods are conventional methods in the art.

[0034] Example 1

[0035] A composite diaphragm comprises a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is arranged on one side of the diaphragm substrate, and the polymer coating is arranged on the surface of the zinc borate coating, the zinc borate coating accounts for 31.9% of the mass percentage of the composite diaphragm, and the polymer coating accounts for 17.8% of the mass percentage of the composite diaphragm; wherein the diaphragm substrate is a polypropylene diaphragm, the raw materials of the zinc borate coating include 3.5 water zinc borate, carboxymethyl cellulose solution (the mass percentage of carboxymethyl cellulose is 1.5%), styrene butadiene rubber solution (the mass percentage of styrene butadiene rubber is 45.03%) and water, and the raw materials of the polymer coating include methyl methacrylate, acrylic anhydride, succinonitrile, azobisisobutyronitrile and lithium bistrifluoromethylsulfonyl imide.

[0036] The preparation method of the composite diaphragm comprises the following steps:

[0037] S1, 2.554 g of 3.5-hydrate zinc borate, 2.2933 g of carboxymethyl cellulose solution, 0.0751 g of styrene-butadiene rubber solution and 2.983 g of water were mixed, applied to one side of a polypropylene diaphragm, and dried at 80° C. for 12 h to form a zinc borate coating with a thickness of 2 μm;

[0038] S2. Mix 8 g of succinonitrile, 6 g of lithium bis(trifluoromethylsulfonyl)imide, 7.2 g of methyl methacrylate, 0.8 g of acrylic anhydride and 0.04 g of azobisisobutyronitrile, apply them on the surface of the zinc borate coating, polymerize them at 60°C for 12 hours to form a polymer coating with a thickness of 2 μm, and thus obtain a composite diaphragm.

[0039] Example 2

[0040] A composite diaphragm comprises a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is arranged on one side of the diaphragm substrate, and the polymer coating is arranged on the surface of the zinc borate coating; wherein the diaphragm substrate is a polypropylene diaphragm, the raw materials of the zinc borate coating include 3.5 water zinc borate, carboxymethyl cellulose solution (the mass percentage of carboxymethyl cellulose is 1.5%), styrene butadiene rubber solution (the mass percentage of styrene butadiene rubber is 45.03%) and water, and the raw materials of the polymer coating include methyl methacrylate, acrylic anhydride, succinonitrile, azobisisobutyronitrile and lithium bistrifluoromethylsulfonyl imide.

[0041] The preparation method of the composite diaphragm comprises the following steps:

[0042] S1, 2.554 g of 3.5-hydrate zinc borate, 2.2933 g of carboxymethyl cellulose solution, 0.0751 g of styrene-butadiene rubber solution and 2.983 g of water were mixed, applied to one side of a polypropylene diaphragm, and dried at 80° C. for 12 h to form a zinc borate coating with a thickness of 4 μm;

[0043] S2. Mix 8 g of succinonitrile, 6 g of lithium bis(trifluoromethylsulfonyl)imide, 7.2 g of methyl methacrylate, 0.8 g of acrylic anhydride and 0.04 g of azobisisobutyronitrile, apply them on the surface of the zinc borate coating, polymerize them at 60°C for 12 hours to form a polymer coating with a thickness of 2 μm, and thus obtain a composite diaphragm.

[0044] Example 3

[0045] A composite diaphragm comprises a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is arranged on one side of the diaphragm substrate, and the polymer coating is arranged on the surface of the zinc borate coating; wherein the diaphragm substrate is a polypropylene diaphragm, the raw materials of the zinc borate coating include 3.5 water zinc borate, carboxymethyl cellulose solution (the mass percentage of carboxymethyl cellulose is 1.5%), styrene butadiene rubber solution (the mass percentage of styrene butadiene rubber is 45.03%) and water, and the raw materials of the polymer coating include methyl methacrylate, acrylic anhydride, succinonitrile, azobisisobutyronitrile and lithium bistrifluoromethylsulfonyl imide.

[0046] The preparation method of the composite diaphragm comprises the following steps:

[0047] S1, 2.554 g of 3.5-hydrate zinc borate, 2.2933 g of carboxymethyl cellulose solution, 0.0751 g of styrene-butadiene rubber solution and 2.983 g of water were mixed, applied to one side of a polypropylene diaphragm, and dried at 80° C. for 12 h to form a zinc borate coating with a thickness of 4 μm;

[0048] S2. Mix 8 g of succinonitrile, 6 g of lithium bis(trifluoromethylsulfonyl)imide, 7.2 g of methyl methacrylate, 0.8 g of acrylic anhydride and 0.04 g of azobisisobutyronitrile, apply them on the surface of the zinc borate coating, polymerize them at 60°C for 12 hours to form a polymer coating with a thickness of 4 μm, and thus obtain a composite diaphragm.

[0049] Example 4

[0050] A composite diaphragm comprises a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is arranged on both sides of the diaphragm substrate, and the polymer coating is arranged on the surface of the zinc borate coating; wherein the diaphragm substrate is a polypropylene diaphragm, the raw materials of the zinc borate coating include 3.5 water zinc borate, carboxymethyl cellulose solution (the mass percentage of carboxymethyl cellulose is 1.5%), styrene butadiene rubber solution (the mass percentage of styrene butadiene rubber is 45.03%) and water, and the raw materials of the polymer coating include methyl methacrylate, acrylic anhydride, succinonitrile, azobisisobutyronitrile and lithium bistrifluoromethylsulfonyl imide.

[0051] The preparation method of the composite diaphragm comprises the following steps:

[0052] S1, 2.554 g of 3.5-hydrate zinc borate, 2.2933 g of carboxymethyl cellulose solution, 0.0751 g of styrene-butadiene rubber solution and 2.983 g of water were mixed, applied on both sides of a polypropylene diaphragm, and dried at 80° C. for 12 h to form a zinc borate coating with a thickness of 2 μm on each side;

[0053] S2. Mix 8 g of succinonitrile, 6 g of lithium bis(trifluoromethylsulfonyl)imide, 7.2 g of methyl methacrylate, 0.8 g of acrylic anhydride and 0.04 g of azobisisobutyronitrile, apply them on the surface of the zinc borate coating, and polymerize them at 60°C for 12 hours to form a polymer coating with a thickness of 2 μm on each side to obtain a composite diaphragm.

[0054] Comparative Example 1

[0055] The difference from Example 1 is that in Comparative Example 1, no zinc borate coating and polymer coating are provided, that is, only a polypropylene diaphragm is selected.

[0056] Comparative Example 2

[0057] The only difference from Example 2 is that no zinc borate coating is provided in Comparative Example 2, and the other conditions are the same as those in Example 2.

[0058] Comparative Example 3

[0059] The only difference from Example 2 is that Comparative Example 3 does not have a polymer coating, and the other conditions are the same as those of Example 2.

[0060] Comparative Example 4

[0061] The only difference from Example 3 is that the polymer coating is replaced by a polymethyl methacrylate coating in Comparative Example 4, and the other conditions are the same as those of Example 3. The polymethyl methacrylate coating is prepared by dissolving polymethyl methacrylate, dimethyl carbonate (DMC) and N-methylpyrrolidone (NMP) in a mass ratio of 1:1:2, coating the mixture on the surface of the zinc borate coating, and drying the mixture.

[0062] Comparative Example 5

[0063] The only difference from Example 3 is that the zinc borate coating is not provided in Comparative Example 5, and the polymer coating is replaced by a polyvinyl alcohol coating, and the other conditions are the same as those of Example 3. The polyvinyl alcohol coating is formed by mixing polyvinyl alcohol and water in a mass ratio of 1:3, heating to 70°C to dissolve, coating on one side of the polypropylene separator, and drying.

[0064] Comparative Example 6

[0065] The only difference from Example 3 is that no zinc borate coating is provided in Comparative Example 6, and the other conditions are the same as those in Example 3.

[0066] Comparative Example 7

[0067] The only difference from Example 1 is that in Comparative Example 7, 3.5 equivalent amounts of zinc borate water are replaced with aluminum hydroxide, and the other conditions are the same as those in Example 1.

[0068] Rate performance test

[0069] The membranes provided in Examples 1-3 and Comparative Examples 1-6 were used for rate performance test (2.75-4.25V) of lithium cobalt oxide vs. Li button cells, and thermal stability test of the membranes at 200°C; the test results are shown in Table 1.

[0070] Table 1

[0071]

[0072]

[0073] As can be seen from Table 1, compared with Comparative Example 1, Example 1 is provided with a zinc borate coating and a polymer coating, and its thermal shrinkage rate is significantly reduced; in Example 1, the thickness of the zinc borate coating and the polymer coating are both 2 microns, and the rate performance of the diaphragm assembled button battery is close to the rate performance in Comparative Example 1. Comparative Example 2 does not have a zinc borate coating, and the discharge specific capacity of the battery at 0.5-5C is significantly reduced. It may be that the polymer coating blocks part of the pores of the diaphragm substrate. It can be inferred that the zinc borate coating is crucial to maintaining the battery rate performance; in Comparative Example 6, the thickness of the polymer coating is further increased, and its rate performance is further reduced. Comparative Example 3 does not have a polymer coating, and the pores of the diaphragm are not closed at high temperatures, which is prone to thermal runaway. In Comparative Examples 4 and 5, the polymer coating is replaced with a polymethyl methacrylate coating and a polyvinyl alcohol coating, and the rate performance is severely reduced. It should be that the pores of the diaphragm are partially blocked after the coating is replaced. Comparative Example 7 replaces the zinc borate coating with an aluminum hydroxide coating, and the thermal shrinkage rate of the diaphragm increases. It is inferred that this is because the thermal decomposition temperature of aluminum hydroxide is higher than that of zinc borate, and the thermal shrinkage rate of the composite diaphragm is significantly increased. After double-sided coating was adopted in Example 4, the thermal shrinkage rate of the composite diaphragm was reduced, but the rate performance was also reduced to a certain extent.

[0074] In addition, the scanning electron microscope (SEM) image of the initial state of the composite diaphragm of Example 1 is as follows: Figure 1 As shown; the SEM image after treatment at 200℃ is shown Figure 2 As shown, the degree of polymerization of the polymer coating increases, resulting in the closure of surface pores.

[0075] The charge and discharge rate performance of the button cell assembled based on Example 1 is as follows Figure 3 As shown, 0.5 / 1 / 3 / 5C discharge and overcharge are all 0.5C constant current and constant voltage charging; 0.1C 1 cycle, 0.5 / 1 / 3 / 5C 3 cycles each. Figure 3 It can be seen that the 0.5C discharge specific capacity is 99.67% of that of 0.1C, the 1C discharge specific capacity is 98.46% of that of 0.1C, and the 3C discharge specific capacity is 94.31% of that of 0.1C. The battery rate performance is good.

[0076] The preferred embodiments of the present invention are specifically described above, but the invention is not limited to the embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A composite diaphragm, characterized in that: The composite diaphragm comprises a diaphragm substrate, a zinc borate coating and a polymer coating, wherein the zinc borate coating is disposed on one side or both sides of the diaphragm substrate, and the polymer coating is disposed on the surface of the zinc borate coating. The raw materials of the zinc borate coating comprise zinc borate, a binder and a solvent, and the raw materials of the polymer coating comprise methyl methacrylate, acrylic anhydride, succinonitrile, an initiator and a lithium salt.

2. The composite diaphragm according to claim 1, characterized in that: The zinc borate coating has a thickness of 0.1-50 μm; and / or the polymer coating has a thickness of 0.1-50 μm.

3. The composite diaphragm according to claim 1, characterized in that: The zinc borate coating accounts for 30%-90% of the composite membrane by mass; and / or the polymer coating accounts for 10%-50% of the composite membrane by mass.

4. The composite diaphragm according to claim 1, characterized in that: The mass ratio of the zinc borate, the binder and the solvent is 37:(0.7-1.5):(30-250).

5. The composite diaphragm according to claim 1, characterized in that: The mass ratio of the methyl methacrylate, acrylic anhydride, succinonitrile, initiator and lithium salt is 9:(0.5-1.5):(5-15):(0.05-0.15):(3.75-11.25).

6. The composite diaphragm according to claim 1, characterized in that: The binder includes at least one of polyvinylidene fluoride, nitrile rubber, hydrogenated nitrile rubber, hydrogenated styrene-butadiene block copolymer, carboxymethyl cellulose, sodium carboxymethyl cellulose, polytetrafluoroethylene and styrene-butadiene rubber; and / or, the solvent includes water; and / or, the initiator includes at least one of azobisisobutyronitrile, benzoin diethyl ether, benzoyl oxide, azobisisoheptylnitrile, benzoin ethyl ether, benzoin isopropyl ether and benzoin butyl ether; and / or, the lithium salt includes at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide), lithium bis(trifluoromethylsulfonylimide), lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate and lithium hexafluorophosphate.

7. The method for preparing the composite diaphragm according to any one of claims 1 to 6, characterized in that: The following steps are involved: The zinc borate, binder and solvent are mixed, applied to one side or both sides of the diaphragm substrate, and dried to form the zinc borate coating; The methyl methacrylate, acrylic anhydride, succinonitrile, initiator and lithium salt are mixed, coated on the surface of the zinc borate coating, and heated for polymerization to form the polymer coating to obtain the composite diaphragm.

8. The preparation method according to claim 7, characterized in that: The drying temperature is 60-90° C.; and / or the drying time is 4-24 hours.

9. The preparation method according to claim 7, characterized in that: The temperature of the heating polymerization is 60-80° C.; and / or the time of the heating polymerization is 8-14 hours.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the composite separator according to any one of claims 1 to 6.

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