A thermally safe coated separator and its preparation method and application

Abstract

The present invention relates to a thermally safe coated separator and its preparation method and application, belonging to the technical field of methods or devices for directly converting chemical energy into electrical energy. The thermally safe coated separator of the present invention includes a separator substrate and a coating provided on at least one surface of the separator substrate; the coating includes a bottom layer in contact with the separator substrate and a surface layer covering the bottom layer; both the bottom layer and the surface layer are mainly composed of an inorganic solid electrolyte and a phase change material, and the mass ratio of the phase change material in the bottom layer is less than its mass ratio in the surface layer. This thermally safe coated separator can not only maintain a high ionic conductivity, but also effectively delay the time of thermal runaway and significantly improve the safety of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of methods or devices for directly converting chemical energy into electrical energy, and particularly relates to a thermally safe coated separator and its preparation method and application. Background Art

[0002] As one of the key components of lithium-ion batteries, the separator has the functions of isolating the positive and negative electrodes to prevent short circuits and providing a transmission channel for lithium ions in the battery; the performance of the separator plays a key role in the interfacial structure, battery capacity, and cycling performance of the battery, especially having an important impact on the safety of the battery. Currently, lithium-ion battery separators are mainly polyolefin separators (such as polyethylene separators or polypropylene separators), but the heat resistance of polyolefin separators is generally average. The polyolefin microporous separator will shrink at about 100 °C; when the temperature rises to 130 - 150 °C, it will also cause the polyolefin separator to close pores, thereby blocking the transmission path of lithium ions and resulting in battery failure. At the same time, due to the small temperature difference between the melting destruction temperature and the pore closing temperature of the polyolefin separator (for example, the pore closing temperature of a PE separator is about 130 °C, and its melting destruction temperature is about 140 °C), and the remaining heat generated after pore closing will still cause the temperature of the separator to continue to rise, easily leading to the melting and destruction of the separator and causing accidents. To address this problem, existing polyolefin separators mainly use the method of surface coating with inorganic materials (such as alumina, boehmite, etc.) for surface modification, so that the separator can still maintain its original shape to prevent short circuits after reaching the softening temperature of the polyolefin, thereby improving the safety of the battery. However, in the actual application process, it is found that the method of simply coating inorganic materials on the surface of the polyolefin separator, although it can improve the thermal stability of the separator to prevent thermal runaway, it is difficult to effectively reduce the rate of temperature rise when thermal runaway occurs, resulting in the temperature of the battery core rising sharply in a very short time and getting out of control, and even catching fire and exploding. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a thermally safe coated separator and its preparation method and application.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0005] In the first aspect, the present invention provides a thermally safe coated separator, which includes a separator substrate and a coating provided on at least one surface of the separator substrate; the coating includes a bottom layer in contact with the separator substrate and a surface layer covering the bottom layer; both the bottom layer and the surface layer are mainly composed of an inorganic solid electrolyte and a phase change material, and the mass ratio of the phase change material in the bottom layer is less than its mass ratio in the surface layer.

[0006] The present invention uses a bottom layer and a surface layer mainly composed of an inorganic solid electrolyte and a phase change material to form a thermally safe coated separator as a coating of the separator, and regulates the mass ratio of the phase change material in the bottom layer and the surface layer. When the battery core undergoes thermal runaway, the surface layer close to the electrode side and containing a large amount of phase change material quickly absorbs the heat diffused from the runaway point (heat source), so as to reduce and slow down the diffusion of heat to the separator matrix, and effectively alleviate the side reactions and decomposition of the electrolyte and auxiliary materials; while the bottom layer in contact with the separator matrix and containing a small amount of phase change material can make full use of the inorganic solid electrolyte with good heat resistance to fully inhibit the heat shrinkage of the separator matrix, and use a small amount of phase change material to further absorb the heat transferred from the surface layer, so that the bottom layer and the surface layer work together to reduce the temperature rise rate from T1 (thermal instability temperature, the starting point of self-heating) to T2 (thermal runaway trigger temperature), extend the time of thermal runaway, and improve the safety performance of the battery; at the same time, the inorganic solid electrolyte uniformly distributed throughout the coating can endow the thermally safe coated separator with good ionic conductivity.

[0007] Optionally, the above-mentioned phase change material can specifically be at least one of an inorganic phase change material and an organic phase change material; among them, the inorganic phase change materials mainly include crystalline hydrated salts, molten salts, metals or alloys, etc., and the organic phase change materials mainly include paraffin, carboxylic acids, fatty alcohols, polyols, and polymer phase change materials, etc.

[0008] Optionally, the above-mentioned separator matrix can specifically be one of a PE microporous membrane, a PP microporous membrane, a PP / PE / PP three-layer composite microporous membrane, or a non-woven fabric separator. In addition, the total mass ratio of the inorganic solid electrolyte and the phase change material in the bottom layer is preferably greater than or equal to 60%, more preferably 65% - 75%; the total mass ratio of the inorganic solid electrolyte and the phase change material in the surface layer is preferably greater than or equal to 70%, more preferably 75% - 80%.

[0009] As a preferred embodiment of the thermally safe coated separator of the present invention, the surface layer has a multi-layer structure and the mass content of the phase change material in the surface layer decreases in a gradient along the vertical direction close to the bottom layer of the surface layer. When the surface layer includes multiple layers and the mass content of the phase change material in each layer is distributed in a gradient, the distribution of the phase change material in the surface layer can be made more uniform, thereby better absorbing the heat diffused from the thermal control point to the surroundings, so as to reduce the heat transfer to the separator matrix; at the same time, the inorganic solid electrolyte with a gradient distribution in the surface layer is also beneficial to improving the ionic conductivity of the thermally safe coated separator.

[0010] As a preferred embodiment of the thermally safe coated separator of the present invention, the mass content of the phase change material in the bottom layer is less than the mass content of the phase change material in the layer in the surface layer that contacts the bottom layer. When the surface layer has a multi-layer structure and the mass content of the phase change material in the layer in the surface layer that contacts the bottom layer is greater than the mass content of the phase change material in the bottom layer, the phase change material in the entire coating can be distributed in a gradient manner, thereby better absorbing and delaying the transfer and diffusion of heat, thus significantly prolonging the time of thermal runaway and significantly improving the safety performance of the battery.

[0011] As a preferred embodiment of the thermally safe coated separator of the present invention, the average thickness of the bottom layer is 0.1 - 5 μm; and / or, the average thickness of the surface layer is 0.1 - 5 μm. Optionally, the average thickness of the bottom layer in the coating can specifically be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, and the average thickness of the surface layer can specifically be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm.

[0012] As a preferred embodiment of the thermally safe coated separator of the present invention, the mass ratio of the phase change material in the bottom layer ≤ 35%; and / or, the mass ratio of the phase change material in the surface layer ≥ 45%. Optionally, the mass ratio of the phase change material in the bottom layer can specifically be 5%, 10%, 15%, 20%, 25%, 30%, preferably 10% - 30%; the mass ratio of the phase change material in the surface layer can specifically be 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%.

[0013] As a preferred embodiment of the thermally safe coated separator of the present invention, the inorganic solid electrolytes in the bottom layer and the surface layer are independently selected from at least one of garnet-type solid electrolytes, perovskite-type solid electrolytes, NASICON-type solid electrolytes, halide electrolytes, and sulfide electrolytes.

[0014] Optionally, the chemical general formula of the garnet-type solid electrolyte is Li 7-x La3Zr 2-x M x O 12 , where M is one of Ta, Nb, Hf, Al, Si, Ga, Ge, Sc, Ti, V, Y, and Sn; the chemical general formula of the perovskite-type solid electrolyte is Li 3x La 2 / 3-x TiO3, where x = 0.1 - 0.15; the chemical general formula of the NASICON-type solid electrolyte is LiM(PO4)3, where M is selected from Na + 、Nb 5+ 、Ta5 + , Ti 4+ , Ge 4+ , Zr 4+ , Sn 4+ , Nb 4+ , Hf 4+ , Al 3+ , Cr 3+ , Ga 3+ , Fe 3+ , Sc 3+ , In + , Y 3+ , La 3+ , Mg 2+ , Zn 2+ , Cu 2+ , Co 2 + , Mn 2+ , Fe 2+ at least one of; the general chemical formula of the halide electrolyte is Li a MX b , where M represents a metal element and X represents a halogen element; the general chemical formula of the sulfide electrolyte is Li 4-x A 1-y B y S4, where A can be Si or Ge, and B can be at least one of Zn, Al or P.

[0015] As a preferred embodiment of the thermally safe coated separator described in the present invention, the phase change materials in the bottom layer and the surface layer are independently selected from at least one of stearic acid, paraffin wax, n-dodecanol, n-octadecane, strontium titanate, and manganese dioxide.

[0016] As a preferred embodiment of the thermally safe coated separator described in the present invention, the components of the bottom layer and the surface layer further include a binder, a dispersant, a wetting agent, and a thickening agent.

[0017] Optionally, the above-mentioned binder includes one or more of styrene-butadiene latex, styrene-acrylic latex, polybutyl methacrylate, polyacrylate, and polymethyl methacrylate; the dispersant includes one or more of ammonium polyacrylate, sodium polyacrylate, polyacrylamide, polycarboxylate, polyvinyl alcohol, silicate colloid, and propylene glycol block polyether; the wetting agent includes one or several of sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, sodium butylnaphthalenesulfonate, and sodium isopropylnaphthalenesulfonate; the thickening agent includes one or more of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, hydroxyethyl cellulose, and polyvinyl alcohol-polyacrylic acid copolymer.

[0018] In a second aspect, the present invention provides a method for preparing the above-mentioned thermally safe coated separator, including the following steps:

[0019] S1, uniformly mixing the components in the bottom layer and the solvent (water, NMP or DMAC) to obtain coating slurry I, and uniformly mixing the components in the surface layer and the solvent (water, NMP or DMAC) to obtain coating slurry II;

[0020] S2, uniformly coating the coating slurry I described in S1 on the surface of the diaphragm substrate, and forming a bottom layer on the surface of the diaphragm substrate after drying;

[0021] S3. Evenly apply the coating slurry II described in S1 to the surface of the bottom layer described in S2, and obtain a heat-safe coated diaphragm after drying.

[0022] In a third aspect, the present invention provides a use of the above-mentioned thermal safety coated separator in a battery or a capacitor.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention uses a bottom layer and a surface layer mainly composed of an inorganic solid electrolyte and a phase change material as the coating of the diaphragm to form a thermally safe coated diaphragm, and regulates the content of the phase change material in the bottom layer and the surface layer, so that when the battery cell has thermal runaway, the surface layer close to the electrode side and containing a large amount of phase change material quickly absorbs the heat diffused from the runaway point (heat source), so as to reduce and slow down the diffusion of heat to the diaphragm matrix, and effectively alleviate the side reactions and decomposition of the electrolyte and auxiliary materials; while the bottom layer in contact with the diaphragm matrix and containing a small amount of phase change material can use the inorganic solid electrolyte with good heat resistance to fully inhibit the thermal shrinkage of the diaphragm matrix, and use a small amount of phase change material to further absorb the heat transferred from the surface layer, so that the bottom layer and the surface layer work together to reduce the temperature rise rate from T1 (thermal instability temperature, starting point of heat generation) to T2 (thermal runaway trigger temperature), prolong the thermal runaway time, and improve the safety performance of the battery; at the same time, the inorganic solid electrolyte evenly distributed in the entire coating can also give the thermally safe coated diaphragm good ionic conductivity. DETAILED DESCRIPTION

[0025] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0026] Unless otherwise specified, other materials, reagents, etc. used in the examples can be obtained from commercial sources.

[0027] Example 1

[0028] An embodiment of the thermal safety coated diaphragm of the present invention, a method for preparing the thermal safety coated diaphragm of this embodiment, comprises the following steps:

[0029] S1, the mixed material A (composed of inorganic solid electrolyte LATP (Li1.3 Al 0.3 Ti 1.7 (PO4)3) and phase change material (stearic acid) are composed in a mass ratio of 9:1), deionized water, dispersant (polyvinyl alcohol), thickener (sodium carboxymethyl cellulose), binder (polyacrylate) and wetting agent (sodium dodecylbenzenesulfonate) are mixed evenly in a mass ratio of 45:40:6:3:3:3 to obtain coating slurry I;

[0030] S2. Mixing material B (composed of inorganic solid electrolyte LATP and phase change material stearic acid in a mass ratio of 5:5), deionized water, dispersant (polyvinyl alcohol), thickener (sodium carboxymethyl cellulose), binder (polyacrylate) and wetting agent (sodium dodecylbenzenesulfonate) evenly in a mass ratio of 45:40:6:3:3:3 to obtain coating slurry II;

[0031] S3. Evenly coat the coating slurry I described in S1 on the surface of the separator matrix (PE separator) (double-sided coating), and after drying, form a bottom layer on the surface of the separator matrix (the average thickness of the bottom layer is 1 μm);

[0032] S4. Evenly coat the coating slurry II described in S2 on the surface of the bottom layer described in S3, and after drying, obtain a thermally safe coated separator (where the average thickness of the surface layer is 1 μm).

[0033] Example 2

[0034] An example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this example is basically the same as that in Example 1, except that the mixing material B in step S2 is composed of inorganic solid electrolyte (LATP) and phase change material (stearic acid) in a mass ratio of 3:7.

[0035] Example 3

[0036] An example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this example is basically the same as that in Example 1, except that the mixing material B in step S2 is composed of inorganic solid electrolyte (LATP) and phase change material (stearic acid) in a mass ratio of 1:9.

[0037] Example 4

[0038] An example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this example is basically the same as that in Example 1, except that the mixing material A in step S1 is composed of inorganic solid electrolyte (LATP) and phase change material (stearic acid) in a mass ratio of 7:3.

[0039] Example 5

[0040] An embodiment of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this embodiment is basically the same as that in Embodiment 1, except that the mixed material A in step S1 is composed of inorganic solid electrolyte LATP and phase change material stearic acid in a mass ratio of 9:1; the mixed material B in step S2 is composed of inorganic solid electrolyte LAGP and phase change material paraffin in a mass ratio of 5:5.

[0041] Comparative Example 1

[0042] A comparative example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this comparative example includes the following steps:

[0043] S1. Mix inorganic solid electrolyte (LATP), deionized water, dispersant (polyvinyl alcohol), thickener (sodium carboxymethyl cellulose), binder (polyacrylate), and wetting agent (sodium dodecylbenzenesulfonate) evenly in a mass ratio of 45:40:6:3:3:3 to obtain a coating slurry.

[0044] S2. Uniformly coat the coating slurry described in S1 on the surface of the separator substrate (PE separator) (double-sided coating), and after drying, obtain a thermally safe coated separator (where the average thickness of the coating is 2 μm).

[0045] Comparative Example 2

[0046] A comparative example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this comparative example includes the following steps:

[0047] S1. Mix phase change material (stearic acid), deionized water, dispersant (polyvinyl alcohol), thickener (sodium carboxymethyl cellulose), binder (polyacrylate), and wetting agent (sodium dodecylbenzenesulfonate) evenly in a mass ratio of 45:40:6:3:3:3 to obtain a coating slurry.

[0048] S2. Uniformly coat the coating slurry described in S1 on the surface of the separator substrate (PE separator) (double-sided coating), and after drying, obtain a thermally safe coated separator (where the average thickness of the coating is 2 μm).

[0049] Comparative Example 3

[0050] A comparative example of the thermally safe coated separator of the present invention. The preparation method of the thermally safe coated separator in this comparative example includes the following steps:

[0051] S1. Mix the mixed materials (composed of inorganic solid electrolyte (LATP) and phase change material (stearic acid) in a mass ratio of 5:5), deionized water, dispersant (polyvinyl alcohol), thickener (sodium carboxymethyl cellulose), binder (polyacrylate), and wetting agent (sodium dodecylbenzenesulfonate) evenly in a mass ratio of 45:40:6:3:3:3 to obtain a coating slurry;

[0052] S2. Uniformly coat the coating slurry described in S1 on the surface of the separator substrate (PE separator) (double-sided coating), and after drying, obtain a thermally safe coated separator (where the average thickness of the coating is 2 μm).

[0053] Performance Test

[0054] Stack the thermally safe coated separators in the examples and comparative examples with the positive electrode plate and the negative electrode plate in sequence, with the thermally safe coated separator in the middle of the positive and negative electrode plates. After winding, shaping, and tab welding, obtain a bare battery cell; then place the bare battery cell into a die-punched outer shell, inject electrolyte (1M LiPF6 solution, and the solvent is a compound of ethylene methyl carbonate (EMC) and ethylene carbonate (EC) in a volume ratio of 1:1), and after encapsulation, standing, formation, and grading, obtain the battery cell to be tested;

[0055] Among them, the preparation methods of the above positive electrode plate and negative electrode plate are as follows:

[0056] 1) Positive electrode plate

[0057] S1. Weigh the positive electrode active material (NCM532), conductive agent (conductive carbon black), dispersant (polyethylene glycol), and binder (polyvinylidene fluoride) in a mass ratio of 7:1:1:1, and then add NMP and mix evenly to obtain a positive electrode slurry;

[0058] S2. Uniformly coat the positive electrode slurry described in S1 on the surface of the positive electrode current collector (aluminum foil) (double-sided coating), and after drying, through cold pressing and slitting, obtain the positive electrode plate.

[0059] 2) Negative electrode plate

[0060] S1. Mix the negative electrode active material (silicon-carbon), conductive agent (conductive carbon black), binder (styrene-butadiene rubber (SBR)), and thickener (sodium carboxymethyl cellulose) in a mass ratio of 7:1:1.5:0.5, and then add deionized water and mix evenly to obtain a negative electrode slurry;

[0061] S2. Uniformly coat the negative electrode slurry described in S1 on the surface of the negative electrode current collector (copper foil) (double-sided coating), and after drying, through cold pressing and slitting, obtain the negative electrode plate.

[0062] The above-mentioned battery cells to be tested were subjected to a needle penetration test, and the ionic conductivity of the thermally safe coating separator was measured. The specific test method is as follows, and the test results are shown in Table 1.

[0063] 1). Needle penetration test: Place the battery cell to be tested in the needle penetration test equipment, and use a steel needle with a diameter of 5 mm (the tip shape is conical, and the angle is 30°) to pierce the battery cell along the direction perpendicular to the electrode plate at a needle penetration speed of 25 mm / s, and observe the state of the battery cell (whether it smokes, catches fire, or explodes).

[0064] 2). Ionic conductivity: Clamp the thermally safe coating separators in the examples and comparative examples between two circular steel sheets to assemble a button cell, and use an electrochemical workstation to measure its impedance value, and calculate the ionic conductivity (σ) through the following formula;

[0065] σ = d / (R × S), where d is the thickness of the thermally safe coating separator, in cm; R is the bulk resistance of the thermally safe coating separator, in Ω; S is the contact area between the thermally safe coating separator and the steel sheet, in cm 2 .

[0066] Table 1 Performance of the thermally safe coating separators in each example and comparative example

[0067]

[0068]

[0069] According to the data in Table 1, it can be seen that the fire ignition times of the battery cells in Examples 1 to 5 are all greater than or equal to 4 s, and the ionic conductivities all reach 2.39×10 -4 S / cm or more, indicating that the thermally safe coating separator of the present invention has good ionic conductivity while effectively extending the time of thermal runaway and improving the safety performance of the battery. At the same time, according to Comparative Examples 1 and 2, it can be seen that the coating with an inorganic solid electrolyte layer as the separator cannot effectively extend the time of thermal runaway; while the coating with a phase change material layer as the separator can significantly extend the time of thermal runaway, but at the same time it will also cause a decrease in ionic conductivity. In addition, according to Comparative Example 3, it can be seen that directly using a mixture of an inorganic solid electrolyte and a phase change material as the coating of the separator can also improve the time of thermal runaway, but the improvement effect is limited.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A thermally safe coated diaphragm, characterized in that: It includes a diaphragm substrate and a coating arranged on at least one side of the diaphragm substrate; the coating includes a bottom layer in contact with the diaphragm substrate and a surface layer covering the bottom layer; the bottom layer and the surface layer are mainly composed of an inorganic solid electrolyte and a phase change material, and the mass proportion of the phase change material in the bottom layer is less than its mass proportion in the surface layer.

2. The thermally safe coated diaphragm according to claim 1, characterized in that The surface layer has a multi-layer structure and the mass content of the phase change material in the surface layer is distributed in a gradient decreasing manner along the vertical direction of the surface layer close to the bottom layer.

3. The thermally safe coated diaphragm according to claim 2, characterized in that The mass content of the phase change material in the bottom layer is less than the mass content of the phase change material in the layer in contact with the bottom layer in the surface layer.

4. The thermally safe coated diaphragm according to claim 1, characterized in that The average thickness of the bottom layer is 0.1 to 5 μm; And / or, the average thickness of the surface layer is 0.1-5 μm.

5. The thermally safe coated diaphragm according to claim 1, characterized in that The mass proportion of the phase change material in the bottom layer is ≤35%; And / or, the mass proportion of the phase change material in the surface layer is ≥45%.

6. The thermally safe coated diaphragm according to claim 1, characterized in that The inorganic solid electrolyte in the bottom layer and the surface layer is independently selected from at least one of a garnet-type solid electrolyte, a perovskite-type solid electrolyte, a NASICON-type solid electrolyte, a halide electrolyte, and a sulfide electrolyte.

7. The thermally safe coated diaphragm according to claim 1, characterized in that The phase change materials in the bottom layer and the surface layer are independently selected from at least one of stearic acid, paraffin, n-dodecanol, n-octadecane, strontium titanate and manganese dioxide.

8. The thermally safe coated diaphragm according to claim 1, characterized in that The components of the bottom layer and the surface layer also include a binder, a dispersant, a wetting agent and a thickener.

9. The method for preparing the thermally safe coated diaphragm according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1, uniformly mixing the components in the bottom layer and the solvent to obtain coating slurry I, and uniformly mixing the components in the surface layer and the solvent to obtain coating slurry II; S2, uniformly coating the coating slurry I described in S1 on the surface of the diaphragm substrate, and forming a bottom layer on the surface of the diaphragm substrate after drying; S3. Evenly apply the coating slurry II described in S1 to the surface of the bottom layer described in S2, and obtain a heat-safe coated diaphragm after drying.

10. Use of the thermally safe coated separator according to any one of claims 1 to 8 in batteries or capacitors.

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