Thermal composite diaphragm and secondary battery

Abstract

The utility model provides a thermal composite diaphragm and a secondary battery. The thermal composite diaphragm comprises a base membrane, a bonding coating arranged on one side of the base membrane and a mixed coating arranged on the other side of the base membrane. Aiming at the technical problems of high short circuit rate and large self-discharge risk of the thermal composite diaphragm in the prior art, the utility model provides the thermal composite diaphragm with strong foreign matter resistance.

Description

Technical Field

[0001] The utility model belongs to the technical field of diaphragm materials, and in particular relates to a thermal composite diaphragm and a secondary battery. Background Art

[0002] The thermal composite process of power batteries refers to the technology of one-time composite of positive and negative electrode materials and separators, in which nanowire electrode materials are used to replace traditional materials to improve battery performance. This technology uses high temperature and high pressure conditions to completely fuse the positive and negative electrode materials together to form a complete battery unit, while also avoiding the reduction of battery life due to poor material bonding.

[0003] The thermal composite process is different from the traditional winding and lamination process. It has higher requirements for the diaphragm during use. Improper structural design of the diaphragm is prone to abnormal conditions such as high short circuit rate and large self-discharge during use. The thermal composite process requires the diaphragm and the negative electrode plate to be bonded by rolling with a composite roller. The temperature of the composite roller is generally 70-110°C and the pressure is generally 0.5-8MPa. The diaphragms disclosed in the prior art are generally not coated with a ceramic coating on the negative electrode side or coated with a thin ceramic coating. Therefore, it is very easy for foreign objects to puncture the diaphragm during use, which in turn causes a direct short circuit or a micro short circuit to cause high battery self-discharge.

[0004] Therefore, in the art, there is an urgent need to develop a diaphragm material suitable for thermal composite process to solve the above problems. Utility Model Content

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a thermal composite diaphragm and a secondary battery. In view of the technical problems of high short circuit rate and high self-discharge risk in the thermal composite diaphragm in the prior art, the present invention provides a thermal composite diaphragm with strong foreign matter resistance.

[0006] In order to achieve the purpose of this utility model, the utility model adopts the following technical solutions:

[0007] In a first aspect, the utility model provides a thermal composite diaphragm, which includes a base film, an adhesive coating disposed on one side of the base film, and a mixed coating disposed on the other side of the base film.

[0008] The utility model is based on the characteristics of the thermal composite process. By designing a thermal composite diaphragm with a specific structure, the utility model has the advantages of high base film puncture strength and not easy to be punctured, high bonding coating coverage, and the ability to bond at low temperature and low pressure, and the mixed coating has good resistance to foreign matter, thereby solving the problems of high short circuit rate and large self-discharge of the battery in the process.

[0009] Preferably, the base film has a thickness of 5-12 microns, for example, 5 microns, 6 microns, 7 microns, 8 microns, 9 microns, 10 microns, 11 microns, 12 microns, etc.

[0010] Preferably, the puncture strength of the base film is 350-800 gf, for example, 350 gf, 400 gf, 500 gf, 600 gf, 700 gf, 800 gf, etc.

[0011] Preferably, the adhesive coating is a polyvinylidene fluoride adhesive coating.

[0012] Preferably, the coverage of the bonding coating on one side of the base film is 30%-95%, for example, it can be 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, etc.

[0013] It should be noted that the coverage is calculated by selecting the colloid particles for area statistics and calculation when photographing the sample using a scanning electron microscope.

[0014] In the utility model, by regulating the coverage rate of the bonding coating on one side of the base film, the diaphragm can be well bonded to the positive electrode using a relatively low temperature, thereby avoiding damage to the diaphragm caused by high temperature and high pressure during hot pressing. Too low coverage rate will cause the bonding force between the diaphragm and the positive electrode to weaken, and the required hot pressing temperature and pressure will increase, and the short circuit rate and self-discharge risk of the battery will be higher. Conversely, the coverage rate of the bonding coating will be too high, which will increase the pore blocking effect of the diaphragm and increase the internal resistance of the battery.

[0015] Preferably, the coverage of the bonding coating on one side of the base film is 40%-70%, for example, it may be 40%, 45%, 50%, 55%, 60%, 65%, 70%, etc.

[0016] Preferably, the mixed coating is a mixed coating composed of ceramic particles and spherical binder.

[0017] In the present invention, the ceramic particles include but are not limited to aluminum oxide, calcium oxide, titanium oxide, etc.; the material of the spherical binder includes but is not limited to acrylic monomers or styrene acrylic monomers synthesized alone or by copolymerization.

[0018] Preferably, the diameter of the spherical binder is 1.5-3.9 microns, for example, it can be 1.5 microns, 1.8 microns, 2 microns, 2.2 microns, 2.5 microns, 2.8 microns, 3 microns, 3.2 microns, 3.5 microns, 3.9 microns, etc.

[0019] Preferably, the thickness ratio of the mixed coating to the base film is 1:(2.5-5), for example, it can be 1:2.5, 1:2.8, 1:3, 1:3.2, 1:3.5, 1:3.8, 1:4, 1:4.2, 1:4.5, 1:4.8, 1:5, etc.

[0020] In the utility model, by adjusting the thickness ratio of the mixed coating and the base film, the diaphragm and the negative electrode are not stabbed or punctured by the protruding particles on the surface of the negative electrode coating when being rolled by the composite roller, which is beneficial to reducing the short circuit rate in the process and reducing the technical problems of battery self-discharge. A too low thickness ratio will cause the proportion of ceramic particles in the mixed coating to be high, and the prepared diaphragm will easily absorb moisture and have a large K value. Otherwise, the mixed coating will be thinner, making the prepared diaphragm poorly resistant to foreign matter and having a large K value.

[0021] In a second aspect, the utility model provides a method for preparing the thermal composite diaphragm according to the first aspect, the method comprising the following steps:

[0022] A bonding coating is formed on one side of the base film, and then a mixed coating is formed on the other side of the base film to obtain the thermal composite separator.

[0023] In a third aspect, the utility model provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, the separator comprising the thermal composite separator according to the first aspect.

[0024] After hot pressing, the battery assembled by the heat composite diaphragm provided by the utility model has a short circuit rate of the stage group within 0.5%, a battery K value within 0.028mv / h, and a small self-discharge rate.

[0025] Compared with the prior art, the utility model has the following beneficial effects:

[0026] The utility model provides a thermal composite diaphragm. Based on the characteristics of the thermal composite process, a thermal composite diaphragm with a specific structure is designed. The diaphragm has the advantages of high base film puncture strength and not easy to be punctured, high bonding coating coverage, and the ability to bond at low temperature and low pressure, and the mixed coating has good resistance to foreign matter, thereby solving the problems of high short circuit rate and large self-discharge of the battery in the manufacturing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a coverage diagram of the mixed coating of the medium-heat composite diaphragm of the utility model, and the scale is 1μm;

[0028] Figure 2 This is a cross-sectional view of the medium-heat composite diaphragm of the utility model, and the scale is 5μm. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below by combining the accompanying drawings and specific implementation methods. Those skilled in the art should understand that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0030] Example 1

[0031] This embodiment provides a thermal composite diaphragm, which includes a polyethylene base film, a polyvinylidene fluoride bonding coating disposed on one side of the polyethylene base film, and a mixed coating disposed on the other side of the base film. The mixed coating is a mixed coating composed of aluminum oxide particles and polyacrylic acid spherical binder, wherein the mass ratio of the aluminum oxide particles to the polyacrylic acid spherical binder is 95:5.

[0032] The thickness of the polyethylene base film is 8 microns and the puncture strength is 600 gf; the coverage of the polyvinylidene fluoride adhesive coating on one side of the polyethylene base film is 70%. Figure 1 As shown; the diameter of the polyacrylic acid spherical binder in the mixed coating is 2.7 microns, and the thickness ratio of the mixed coating to the polyethylene base film is 1:3.5.

[0033] Example 2

[0034] This embodiment provides a thermal composite diaphragm, which includes a polyethylene base film, a polyvinylidene fluoride bonding coating disposed on one side of the polyethylene base film, and a mixed coating disposed on the other side of the base film. The mixed coating is a mixed coating composed of aluminum oxide particles and polyacrylic acid spherical binder, wherein the mass ratio of the aluminum oxide particles to the polyacrylic acid spherical binder is 94:6.

[0035] Among them, the thickness of the polyethylene base film is 5 microns and the puncture strength is 350gf; the coverage rate of the polyvinylidene fluoride adhesive coating on one side of the polyethylene base film is 30%; the diameter of the polyacrylic acid spherical adhesive in the mixed coating is 1.5 microns, and the thickness ratio of the mixed coating to the polyethylene base film is 1:2.5.

[0036] Example 3

[0037] This embodiment provides a thermal composite diaphragm, which includes a polyethylene base film, a polyvinylidene fluoride bonding coating disposed on one side of the polyethylene base film, and a mixed coating disposed on the other side of the base film. The mixed coating is a mixed coating composed of aluminum oxide particles and polyacrylic acid spherical binder, wherein the mass ratio of the aluminum oxide particles to the polyacrylic acid spherical binder is 96:4.

[0038] Among them, the thickness of the polyethylene base film is 12 microns and the puncture strength is 800gf; the coverage rate of the polyvinylidene fluoride adhesive coating on one side of the polyethylene base film is 95%; the diameter of the polyacrylic acid spherical adhesive in the mixed coating is 3.9 microns, and the thickness ratio of the mixed coating to the polyethylene base film is 1:5.

[0039] Example 4

[0040] The difference between this embodiment and embodiment 1 is that the coverage of the polyvinylidene fluoride adhesive coating on one side of the polyethylene base film is 20%, and the rest is the same as embodiment 1.

[0041] Example 5

[0042] The difference between this embodiment and embodiment 1 is that the thickness ratio of the mixed coating layer to the polyethylene-based film is 1:1.5, and the rest is the same as embodiment 1.

[0043] Comparative Example 1

[0044] The difference between this comparative example and Example 1 is that the mixed coating is replaced with an alumina ceramic coating of equal thickness, and the rest is the same as Example 1.

[0045] Test conditions

[0046] The examples 1 to 5 and the comparative example 1 provided Figure 2 The thermal composite separator shown is assembled to obtain a lithium-ion battery and tested. The test method is as follows:

[0047] First, preparation of positive electrode sheets: lithium iron phosphate material is used to prepare slurry, and the slurry is coated on carbon-coated aluminum foil and processed to obtain positive electrode sheets. The positive electrode accounts for 96.5% of the coating, and the rest is polyvinylidene fluoride adhesive and conductive carbon black.

[0048] Second, preparation of negative electrode sheets: artificial graphite is used to prepare slurry, and the slurry is coated on copper foil to make treated negative electrode sheets. The negative electrode accounts for 95.5% of the coating, and the rest is sodium carboxymethyl cellulose adhesive and conductive carbon black.

[0049] Third, make the pole group: use the hot composite process to bond the hot composite diaphragm and the negative electrode sheet in the form of hot roller hot pressing, and the hot roller temperature is 80°C. Cut the unit formed after hot pressing into single pieces with a hot cutter to form a sandwich structure of diaphragm-negative electrode-diaphragm. Stack the sandwich structure and the positive electrode sheet into a pole group in the form of a stack, and use a flat hot press to hot press at 90°C to complete the production of the pole group. Using a 50V voltage test, the short-circuit rate of the stage group after hot pressing is within 0.5%.

[0050] Fourth, packaging and liquid injection: the prepared electrode group is packaged in a square shell form, and then the electrolyte is injected.

[0051] Fifth, pre-charging and formation: pre-charging and forming the battery cell to obtain a lithium-ion battery.

[0052] Sixth, battery testing: The prepared lithium battery is subjected to formation, capacity division and self-discharge tests, and the battery K value is within 0.028mv / h.

[0053] The test results are shown in Table 1:

[0054] Table 1

[0055]

[0056]

[0057] As can be seen from Table 1, the utility model provides a thermal composite diaphragm. Based on the characteristics of the thermal composite process, a thermal composite diaphragm with a specific structure is designed, which has the advantages of high base film puncture strength and not easy to puncture, high bonding coating coverage, and the ability to bond at low temperature and low pressure, and the mixed coating has good resistance to foreign matter, thereby solving the problems of high battery short circuit rate and large self-discharge in the process.

[0058] The applicant declares that the utility model illustrates the process method of the utility model through the above-mentioned embodiments, but the utility model is not limited to the above-mentioned process steps, that is, it does not mean that the utility model must rely on the above-mentioned process steps to be implemented. The technicians in the relevant technical field should understand that any improvement of the utility model, the equivalent replacement of the raw materials selected by the utility model, the addition of auxiliary components, the selection of specific methods, etc., all fall within the protection scope and disclosure scope of the utility model.

Claims

1. A thermal composite diaphragm, characterized in that: The thermal composite diaphragm includes a base film, a bonding coating disposed on one side of the base film, and a mixed coating disposed on the other side of the base film; The coverage of the bonding coating on one side of the base film is 30%-95%; The thickness ratio of the mixed coating to the base film is 1:(2.5-5).

2. The thermal composite diaphragm according to claim 1, characterized in that: The thickness of the base film is 5-12 microns.

3. The thermal composite diaphragm according to claim 1 or 2, characterized in that: The puncture strength of the base film is 350-800 gf.

4. The thermal composite diaphragm according to claim 1, characterized in that: The adhesive coating is a polyvinylidene fluoride adhesive coating.

5. The thermal composite diaphragm according to claim 1, characterized in that: The coverage of the bonding coating on one side of the base film is 40%-70%.

6. The thermal composite diaphragm according to claim 1, characterized in that: The mixed coating is a mixed coating composed of ceramic particles and spherical binder.

7. The thermal composite diaphragm according to claim 6, characterized in that: The diameter of the spherical binder is 1.5-3.9 microns.

8. A secondary battery, characterized in that: The secondary battery comprises a positive electrode, a negative electrode, an electrolyte and a separator, wherein the separator comprises the thermal composite separator according to any one of claims 1 to 7.

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