Composite positive pole piece as well as preparation method and application thereof

Abstract

The invention discloses a composite positive pole piece as well as a preparation method and application thereof. The composite positive pole piece provided by the invention has a sandwich structure and comprises a ceramic layer, a positive active material layer, a current collector, a positive active material layer and a ceramic layer from bottom to top, the ceramic layer contains an active lithium material; the average particle size D10 of the active material in the positive electrode active material layer is 2.1 + / -1.0 [mu] m, the average particle size D50 is 3.9 + / -1.0 [mu] m, and the average particle size D90 is 6.8 + / -3.0 [mu] m; the average particle size D10 of the active material in the ceramic layer is 0.1-0.5 [mu] m, the average particle size D50 is 0.5-1.2 [mu] m, and the average particle size D90 is less than or equal to 2.5 [mu] m. The positive pole piece and the negative pole piece disclosed by the invention have relatively high conductivity, mechanical strength and safety.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium - ion batteries, and particularly relates to a composite positive electrode sheet, a preparation method thereof and an application thereof. Background Art

[0002] With the rapid development of lithium - ion battery technology and its extensive application in electric vehicles and electronic consumer products, higher requirements for the safety of lithium - ion batteries have been put forward. Generally, the main components of a lithium - ion battery are a positive electrode, a negative electrode, a separator, an electrolyte and a current collector. Among them, the main components of the electrolyte include a solvent, an additive and a lithium salt, which play the role of transferring lithium ions and providing some active lithium inside the battery.

[0003] At present, to improve the safety of lithium batteries, flame - retardant electrolytes are usually used, but flame retardants usually contain toxicity and are not conducive to environmental protection; in addition, at high temperatures, the PP or PE separator will shrink, making it difficult to play the role of isolating the positive and negative electrodes. Therefore, relying solely on the separator during the operation of a lithium - ion battery cannot effectively reduce the occurrence of lithium - ion battery safety accidents.

[0004] CN115995527A discloses a high - safety positive electrode sheet, a preparation method thereof and an application thereof. The positive electrode sheet includes a bottom - coating layer on the surface of the current collector and a cathode active coating on the surface of the bottom - coating layer; wherein, the raw materials of the bottom - coating layer include a binder, a conductive agent, lithium iron phosphate and ceramic particles. The bottom - coating layer acts as a safety coating to play a wrapping role during the needle - punching test, avoiding direct contact between the fully - charged anode and the aluminum foil, and at the same time forming a large resistance to slow down the thermal runaway of the battery cell. However, there is still 10 - 20% of the lithium iron phosphate active material in the bottom layer of this positive electrode sheet in direct contact with the aluminum foil current collector, and the upper layer is a normal positive electrode active material. There is no substantial isolation between the upper and lower layers, and the wrapping effect during the needle - punching test is limited. Summary of the Invention

[0005] The purpose of the present invention is to provide a composite positive electrode sheet, a preparation method thereof and an application thereof. The positive composite electrode can endow the lithium - ion battery with high conductivity, mechanical strength and safety performance.

[0006] To achieve the above - mentioned purpose, the technical solution adopted by the present invention is as follows:

[0007] In the first aspect, the present invention provides a composite positive electrode sheet, comprising: a ceramic layer, a positive electrode active material layer, a current collector, a positive electrode active material layer and a ceramic layer;

[0008] The ceramic layer contains an active lithium material;

[0009] The average particle sizes of the active material in the positive electrode active material layer are D10 = 2.1 ± 1.0 μm, D50 = 3.9 ± 1.0 μm, and D90 = 6.8 ± 3.0 μm;

[0010] The average particle sizes of the active material in the ceramic layer are D10 = 0.1 - 0.5 μm, D50 = 0.5 - 1.2 μm, and D90 ≤ 2.5 μm.

[0011] In the present invention, a positive electrode material layer and a ceramic layer are sequentially formed on both sides of the current collector, adding a safety barrier between the positive and negative electrodes, allowing lithium ions to migrate through while isolating electrons. During a pinprick or extrusion test, even if the temperature rises sharply, it can effectively prevent direct contact between the fully charged positive electrode and the negative electrode, and it does not shrink and is non-combustible at high temperatures, significantly improving the safety of the lithium-ion battery; at the same time, the ceramic layer contains active lithium, which can supplement a certain amount of active lithium and play a role in lithium supplementation; in addition, the particle sizes of the positive electrode active material and the ceramic material are reasonably graded, making the obtained composite positive electrode sheet have both high conductivity and mechanical strength.

[0012] The thickness ratio of the positive electrode active material layer to the ceramic layer is 15 - 20:1. This thickness ratio can not only meet the safety requirements but also meet the energy density requirements, improving the reliability of processing.

[0013] According to a specific embodiment of the present invention, the thickness of the positive electrode active material layer is 100 - 250 μm; the thickness of the ceramic layer is 7 - 13 μm.

[0014] The preparation raw materials of the positive electrode active material layer include: active material, conductive agent, and binder; in the preparation raw materials, the mass fraction of the active material is 95% - 98%, the mass fraction of the conductive agent is 0.5% - 2.5%, and the mass fraction of the binder is 0.5% - 2.5%.

[0015] The active material is any one or a combination of two or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, and lithium nickel cobalt manganese oxide.

[0016] The conductive agent is any one or a combination of two or more of powdered carbon black, single-walled / multi-walled carbon nanotubes, carbon fibers, and graphene.

[0017] The binder is polyvinylidene fluoride.

[0018] The preparation raw materials of the ceramic layer include ceramic material, binder, and active lithium material; in the preparation raw materials, the mass fraction of the ceramic material is 70% - 75%, the mass fraction of the binder is 15% - 20%, and the mass fraction of the active lithium material is 1% - 5%.

[0019] The ceramic material is alumina and / or boehmite.

[0020] The binder is polyvinylidene fluoride.

[0021] The active lithium material is lithium titanium aluminum phosphate.

[0022] The present invention uses the above materials to prepare a ceramic slurry, which can play a protective role for the positive electrode material layer and improve the conductivity.

[0023] The current collector is aluminum foil; the thickness of the current collector is 11-15 μm. Selecting 11-15 μm copper foil can improve the energy density per unit volume of the battery cell.

[0024] In a second aspect, the present invention provides a method for preparing the above composite positive electrode sheet, including the following steps:

[0025] S1. Prepare a positive electrode slurry;

[0026] S2. Prepare a ceramic slurry;

[0027] S3. Use a double-layer coating technique to perform double-layer coating on both sides of the current collector, dry, roll, and cut to obtain a composite positive electrode sheet.

[0028] The viscosity of the positive electrode slurry is 2000-7000 mPas, and the solid content is 60%-75%, preferably 74%.

[0029] The viscosity of the ceramic slurry is 800-1500 mPas, and the solid content is 20-30%, preferably 25%.

[0030] Research has found that when the solid content of the positive electrode slurry is as large as possible under the condition that the viscosity meets the production conditions, it is more conducive to coating and drying. The solid content of the ceramic slurry is small because the coating thickness is small, and more liquid slurry is required per unit volume so as not to cause virtual coating on the surface of the electrode sheet. Preferably, the viscosity of the slurry used for coating the positive electrode active material layer is 1000-2000 mPas higher than that of the slurry used for coating the ceramic layer, which can avoid or reduce the problem of mutual dissolution of the positive electrode slurry and the ceramic slurry. By changing the solid content of each layer of slurry, the appearance defects of the multi-layer coated electrode sheet can be improved, and the yield of the electrode roll can be increased.

[0031] The bottom coating surface density of the positive electrode slurry is 400-500 g / m 2 ; the compaction density of the composite positive electrode sheet is 2.5-3.5 g / cm 3 , ensuring that the electrolyte is easily infiltrated, the active material particles are not easily broken, increasing the energy density per unit volume of the battery cell, and reducing the contact resistance between the active material particles; preferably 3.4 g / cm 3 .

[0032] Among them, the calculation formula for the compaction density is: Compaction density = Coating areal density / (Electrode thickness - Foil thickness).

[0033] In a third aspect, the present invention also provides a lithium-ion battery, comprising: a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet is the above-mentioned composite positive electrode sheet.

[0034] Preferably, the positive electrode sheet is a lithium iron phosphate / graphite system, a ternary / graphite system electrode sheet, or a ternary / graphite silicon carbon system.

[0035] The specification size of the negative electrode sheet is 99.7 * 237.5 mm, and the specification size of the positive electrode sheet is 97.7 * 233.5 mm.

[0036] The separator is a double-sided coated oil-based separator with a thickness of 15 μm.

[0037] The electrolyte is a lithium salt electrolyte with lithium hexafluorophosphate as the main salt, and its solvents are EC (ethylene carbonate), DMC (diethyl carbonate), EMC (ethyl methyl carbonate), etc.

[0038] The lithium-ion battery is a fast-charging lithium-ion battery. Its preparation process adopts the Z-stack hot pressing process and the double-sided tab-out method. The battery body is encapsulated with an aluminum-plastic film, and after processes such as baking, liquid injection, formation, and grading, a fast-charging lithium-ion battery is obtained.

[0039] It should be noted that the present invention is not only applicable to soft-pack lithium-ion batteries, but also applicable to square and cylindrical lithium-ion batteries; the cell process can adopt the Z-stack, winding, or thermal composite process path.

[0040] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0041] 1. The present invention coats a ceramic layer containing active lithium on the surface of a conventional positive electrode sheet. On the one hand, the inorganic lithium salt ceramic layer has a lithium ion conduction function and high strength, and is not afraid of being pierced by foreign objects such as lithium dendrites and burrs, improving the conductivity and mechanical strength of the lithium-ion battery; on the other hand, the organic separator polymer layer has a thermal shut-off coating function. At room temperature, the pores between particles can ensure the smooth shuttling of lithium ions back and forth. At a higher temperature, the coating particles melt and collapse to isolate the passage of lithium ions, enhancing the safety performance of the lithium-ion battery.

[0042] 2. The present invention uses the upper coating technology to form a positive electrode active material layer and a ceramic layer on both sides of the current collector, realizing the preparation of a positive electrode composite electrode; the obtained coating has high quality, and the process operation is simple, which is suitable for large-scale industrial production.

[0043] 3. The cell capacity of the solid-state lithium-ion battery prepared from the positive composite electrode provided by the present invention reaches 140 Ah, and the cell passes the tests such as short circuit, pinprick, and extrusion safely, showing excellent safety performance compared with traditional cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic structural diagram of the positive composite electrode provided by an embodiment of the present invention; in the figure: 1. Current collector aluminum foil; 2. Positive active material layer; 3. Ceramic layer; 4. Ceramic particles; 5. Positive active material.

[0045] Figure 2 It is a schematic diagram of the coating feeding principle and the cross-section of the coating die used in the preparation process of the positive composite electrode according to an embodiment of the present invention; in the figure: 1. Positive electrode slurry; 2. Ceramic slurry. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0047] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified.

[0048] The reagents, materials, instruments, etc. used in the following embodiments can be obtained from commercial channels unless otherwise specified.

[0049] Embodiment 1

[0050] As Figure 1 shown, the composite positive electrode provided in this embodiment has a sandwich structure, including from bottom to top: ceramic layer 3, positive active material layer 2, current collector 1, positive active material layer 2, and ceramic layer 3;

[0051] The preparation of the lithium-ion battery is carried out according to the following specific operation steps:

[0052] (1) Slurry mixing

[0053] a. Prepare the positive electrode slurry, and the specific proportions of each component are as follows: Lithium nickel cobalt manganate (D10 is 2.2 μm, D50 is 3.8 μm, and D90 is 6.5 μm): Carbon black (SP): Multi-walled carbon nanotubes (CNT): Polyvinylidene fluoride (PVDF) = 97.1%: 0.5%:

[0054] 1.2%: 1.2% (weight ratio). After homogenization, the slurry is obtained, with a viscosity of 5500 mPas and a solid content of 74%;

[0055] b. Prepare the ceramic slurry. The specific proportions of each component are as follows: ceramic powder (D10 is 0.4 μm, D50 is 0.8 μm, and D90 is 2.2 μm): lithium titanium aluminum phosphate: polyvinylidene fluoride (PVDF) = 28.2%: 61.5%: 10.3%. After homogenization, the slurry is obtained, with a viscosity of 1100 mPas and a solid content of 22.5%.

[0056] c. Prepare the negative electrode slurry. The specific proportions of each component are as follows: artificial graphite (C): conductive agent carbon black (SP): thickener sodium carboxymethyl cellulose (CMC) and binder styrene-butadiene rubber (SBR) = 96.5%: 0.6%: 1.1%: 1.8%. After homogenization, the slurry is obtained, with a viscosity of 4200 mPas and a solid content of 54.2%.

[0057] (2) Coating

[0058] Positive electrode double-layer coating process: Use the positive electrode slurry as the bottom coating (i.e., close to the current collector) and the ceramic slurry as the top coating (i.e., far from the current collector) to perform double-layer coating on the upper and lower surfaces of the current collector. The surface density of the positive electrode slurry bottom coating is 450 g / m 2 , the thickness of the ceramic layer coating is 8 μm, and the current collector uses an aluminum foil with a thickness of 13 μm.

[0059] Negative electrode coating process: Use the conventional coating process, with a surface density of 210 g / m 2 , and the current collector uses a copper foil with a thickness of 6 μm.

[0060] The coating feeding principle and the cross-sectional schematic diagram of the coating die are as Figure 2 shown. The positive electrode slurry flows out from the lower die head to the aluminum foil current collector, and the ceramic slurry flows out from the upper die head to the surface layer of the positive electrode slurry. Strictly speaking, the positive electrode slurry is first coated on the foil material, and then the ceramic slurry is coated on the surface of the positive electrode material. Almost simultaneously in time, the coating is carried out.

[0061] (3) Rolling and slitting

[0062] The negative electrode sheet is compacted at 1.65 g / cm 3 . After rolling, the thickness of the negative electrode sheet is 133.3 μm, and the slitting size is 237.5 mm;

[0063] The positive electrode sheet is compacted at 3.4 g / cm 3 . After rolling, the thickness of the positive electrode sheet is 118.3 μm, and the slitting size is 233.5 mm.

[0064] (4) Cell manufacturing

[0065] The cutting width dimension of the positive electrode sheet is 97.5 mm, the cutting width dimension of the negative electrode sheet is 99.5 mm, the number of stacked sheets is 47 positive electrode sheets and 48 negative electrode sheets. A dry battery cell is obtained through assembly processes such as ultrasonic welding and laser cutting, and is filled with electrolyte after baking. The amount of electrolyte filled is 308 g ± 5 g. Then, a fast-charging lithium-ion battery is obtained through formation and grading. The capacity of the battery cell is 140 Ah.

[0066] Example 2

[0067] The preparation method of this example is the same as that of Example 1, except that the solid content of the positive electrode slurry is 72% and the solid content of the ceramic slurry is 20%.

[0068] Example 3

[0069] The preparation method of this example is the same as that of Example 1, except that the coating thickness of the ceramic layer is 13 μm.

[0070] Comparative Example 1

[0071] Compared with Example 1, the difference is only that: the ceramic layer is not coated on the surface of the positive electrode active material layer.

[0072] Comparative Example 2

[0073] Compared with Example 1, the difference is only that: the coating thickness of the ceramic layer is 5 μm.

[0074] Test Example 1, Pinprick and Extrusion Tests of Lithium Batteries

[0075] The passing rates of the pinprick and extrusion tests of the lithium batteries prepared in each example and comparative example were tested respectively, and the results are shown in Table 1.

[0076] Test method and parameters for pinprick: The voltage of the lithium-ion battery after being fully charged, 4.2 V, is fixed on the fixture of the pinprick device. The pinprick speed is usually set to 20 mm / s, and the pinprick time is set to 1 minute. Press the start button. After about 3 seconds, the pinprick test is automatically carried out. After reaching the preset time, the oil cylinder automatically rises, and the steel needle returns to its original position. Wait for 1 minute and observe whether the battery explodes or catches fire to determine the result.

[0077] Test method and parameters for extrusion test: The lithium-ion battery with a voltage of 4.2 V after being fully charged is placed on the extrusion plane, and the battery is extruded perpendicular to the plate direction with an extrusion force of 13 kN. When the pressure reaches the maximum value, the extrusion test stops. During and after the extrusion process, observe whether the battery will have an external short circuit, catch fire or explode.

[0078] Table 1 Passing Rates of Pinprick and Extrusion Tests of Lithium Batteries

[0079] Serial number Acupuncture passing rate Compression test passing rate Example 1 100％(3 / 3) 100％(3 / 3) Example 2 100％(3 / 3) 100％(3 / 3) Example 3 100％(3 / 3) 100％(3 / 3) Comparative example 1 0％(0 / 3) 33％(1 / 3) Comparative example 2 33％(1 / 3) 67％(2 / 3)

[0080] Note: 100% (3 / 3) means that 3 pieces were tested and all 3 passed, with a pass rate of 100%.

[0081] As shown in Table 1, the lithium batteries prepared in Examples 1-3 passed 100% of the puncture and extrusion tests. The surface of the positive electrode sheet is coated with a ceramic layer. During puncture, the diaphragm may shrink under short-circuit heating conditions, but the presence of the ceramic layer avoids direct contact between the positive and negative electrode sheets.

[0082] The battery produced in Comparative Example 1 had a needle puncture pass rate of 0%, primarily because the positive electrode sheet was not coated with a ceramic layer, failing to isolate the lithium battery from further thermal runaway. The battery produced in Comparative Example 2 had a needle puncture pass rate of 33%, primarily because the ceramic layer was too thin, failing to effectively isolate the positive and negative electrodes after the separator contracted. Furthermore, the extrusion test results for Comparative Examples 1 and 2 showed that only the pass rate was significantly lower than that of the battery produced in Example 1.

[0083] Test Example 2: Film resistivity and peel strength test of positive electrode sheet

[0084] The film resistivity and peel strength of the positive electrode sheets prepared in each embodiment and comparative example were tested respectively. Detailed data are shown in Table 2.

[0085] The test method for diaphragm resistivity is the 4-probe method. The operating steps are: use a screw micrometer to test the thickness of the electrode before the test, then input the current and voltage values, place the electrode on the test table, and slowly lower the test probe for measurement. Measure each electrode 3 times and take the average value.

[0086] The peel strength test method adopts 180° peel test, and the test conditions are: tensile speed 150mm / min, sample width 19mm, and sample length 60mm.

[0087] The A side and B side represent the upper and lower surfaces of the current collector.

[0088] Table 2 Film resistivity and peel strength

[0089]

[0090] As can be seen from Table 2, the film resistivity and peel strength levels of the composite positive electrode sheets prepared in Examples 1-3 are still significantly higher than the industry's peel strength process standard (≥150N / m) and film resistivity process standard (≤1.0Ω.m).

[0091] Comparative Example 3

[0092] The only difference from Example 1 is that the average particle size of the positive electrode active material is different, and the specific gradation is: lithium nickel cobalt manganese oxide (D10 is 3.5 μm, D50 is 5.8 μm and D90 is 10.5 μm).

[0093] The results show that due to the unsuitable particle size distribution of the positive electrode active material, the passing rates of the obtained lithium-ion batteries in the needle penetration and extrusion tests are both 100%, but the resistivity of the diaphragm is 1.133 Ωm, which is unqualified; the peel strength is: 178 N / m for side A and 132 N / m for side B, which is unqualified.

[0094] Comparative Example 4

[0095] The difference from Example 1 is only that the average particle size of the ceramic active material is different, and the specific particle size distribution is: D10 = 0.6 μm, D50 = 1.3 μm, and D90 ≤ 2.8 μm.

[0096] The results show that due to the unsuitable particle size distribution of the ceramic active material, the passing rates of the obtained lithium-ion batteries in the needle penetration and extrusion tests are both 100%, but the resistivity of the diaphragm is 1.258 Ωm, which is unqualified, and the peel strength is: 183 N / m for side A and 164 N / m for side B.

[0097] Comparative Example 5

[0098] The difference from Example 1 is only that the thickness of the ceramic layer is 14 μm.

[0099] The results show that due to the too thick ceramic layer, the passing rates of the obtained lithium-ion batteries in the needle penetration and extrusion tests are both 100%, the resistivity of the diaphragm is 1.436 Ωm, and the peel strengths are: 218 N / m for side A and 179 N / m for side B, respectively.

[0100] Although the present invention has been described in detail above with general descriptions and specific embodiments, based on the present invention, some modifications or improvements can be made, which are obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

Claims

1. A composite positive electrode plate, comprising: A ceramic layer, a positive electrode active material layer, a current collector, a positive electrode active material layer, and a ceramic layer; The ceramic layer contains an active lithium material; The average particle sizes of the active materials in the positive electrode active material layer are D10 = 2.1 ± 1.0 μm, D50 = 3.9 ± 1.0 μm, and D90 = 6.8 ± 3.0 μm; The average particle sizes of the active materials in the ceramic layer are D10 = 0.1 - 0.5 μm, D50 = 0.5 - 1.2 μm, and D90 ≤ 2.5 μm.

2. The composite positive electrode sheet according to claim 1, wherein The thickness ratio of the positive electrode active material layer to the ceramic layer is 15 - 20:

1.

3. The composite positive electrode sheet according to claim 2, characterized in that, The thickness of the positive electrode active material layer is 100 - 250 μm; the thickness of the ceramic layer is 7 - 13 μm.

4. The composite positive electrode sheet according to any one of claims 1-3, characterized in that, The preparation raw materials of the positive electrode active material layer include: active materials, conductive agents, and binders; In the preparation raw materials, the mass fraction of the active materials is 95% - 98%, the mass fraction of the conductive agents is 0.5% - 2.5%, and the mass fraction of the binders is 0.5% - 2.5%; The active materials are any one or a combination of two or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium iron phosphate, lithium iron manganese phosphate, and lithium nickel cobalt manganeseate; The conductive agents are any one or a combination of two or more of powdered carbon black, single-walled / multi-walled carbon nanotubes, carbon fibers, and graphene; The binder is polyvinylidene fluoride.

5. The composite positive electrode sheet according to any one of claims 1-4, characterized in that, The preparation raw materials of the ceramic layer include ceramic materials, binders, and active lithium materials; In the preparation raw materials, the mass fraction of the ceramic materials is 70% - 75%, the mass fraction of the binders is 15% - 20%, and the mass fraction of the active lithium materials is 1% - 5%; The ceramic materials are alumina and / or boehmite; The binder is polyvinylidene fluoride; The active lithium material is lithium titanium aluminum phosphate.

6. The composite positive electrode sheet according to any one of claims 1-5, characterized in that, The current collector is aluminum foil; the thickness of the current collector is 11 - 15 μm.

7. The method for preparing the composite positive electrode sheet according to any one of claims 1 - 6, comprising the following steps: S1. Prepare a positive electrode slurry; S2. Prepare a ceramic slurry; S3. Use a double-layer coating technique to perform double-layer coating on both sides of the current collector, dry, and roll to obtain a composite positive electrode sheet.

8. The preparation method according to claim 7, characterized in that, The viscosity of the positive electrode slurry is 2000 - 7000 mPas, and the solid content is 60% - 75%; The viscosity of the ceramic slurry is 800 - 1500 mPas, and the solid content is 20 - 30%.

9. The preparation method according to claim 7 or 8, characterized in that: The base coating surface density of the positive electrode paste is 400-500 g / m 2 ; The compaction density of the composite positive electrode sheet is 2.5-3.5 g / cm 3 .

10. A lithium-ion battery comprising: A positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; The positive electrode sheet is the composite positive electrode sheet according to any one of claims 1 - 6.

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