Battery cell suitable for secondary battery, and secondary battery using same

Abstract

A battery cell suitable for a secondary battery, comprising at least one composite electrode unit. The composite electrode unit comprises a positive electrode sheet and a negative electrode sheet; the positive electrode sheet comprises a first positive electrode active coating (4), the negative electrode sheet comprises a first negative electrode active coating (2), and the first positive electrode active coating (4) and the first negative electrode active coating (2) are stacked with an isolating coating (3) interposed therebetween; the length and width of the first negative electrode active coating (2) are respectively greater than the length and width of the first positive electrode active coating (4); and the isolating coating (3) comprises an inorganic material and a polymer, wherein the proportion of the inorganic material in the isolating coating (3) is not less than 70 wt%, the dielectric constant of the inorganic material is not less than 8, and the average particle diameter D50 of the inorganic material ranges from 0.2 to 5 μm.

Description

Battery cell suitable for secondary battery and secondary battery applying same

[0001] The present application claims priority to the Chinese patent application No. 202410613221X filed on May 16, 2024 with the Chinese Patent Office, the whole content of the above application is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of battery, in particular relates to a battery cell suitable for secondary battery and secondary battery applying same. BACKGROUND

[0003] Lithium ion battery has the advantages of high specific energy, low discharge, good cycle performance, etc., and has been widely used in consumer electronics, new energy vehicles and energy storage power stations. In order to meet market demand, developing lithium ion batteries with high energy density, high fast charging efficiency, cycle stability and high safety performance has become the future development trend. Lithium battery includes a battery cell made of positive and negative electrode sheets and a separator separating the positive and negative electrode sheets, wherein the role of the separator is to isolate the positive and negative electrodes and make the electrons in the battery cell not free to pass through, while the ions in the electrolyte can pass freely between the positive and negative electrodes to ensure the safety of the battery. TECHNICAL PROBLEM

[0004] The traditional battery separator is generally made of resin materials such as polyethylene or polypropylene, which has the disadvantages of poor mechanical strength, easy to be pierced, poor thermal stability, causing the battery to bulge or explode, etc. TECHNICAL SOLUTION

[0005] In a first aspect, the present application provides a battery cell suitable for a secondary battery, the battery cell comprising at least one composite electrode unit, the composite electrode unit comprising a positive electrode sheet and a negative electrode sheet; in the composite electrode unit, the positive electrode sheet comprises a first positive electrode active coating layer arranged towards the negative electrode sheet, the negative electrode sheet comprises a first negative electrode active coating layer arranged towards the positive electrode sheet, and the first positive electrode active coating layer and the first negative electrode active coating layer are stacked by a separation coating layer; the length and width of the first negative electrode active coating layer are greater than the length and width of the first positive electrode active coating layer; the separation coating layer comprises inorganic material and polymer, the proportion of inorganic material in the separation coating layer is not less than 70 wt%, the dielectric constant of the inorganic material is not less than 8, and the average particle size D50 of the inorganic material is 0.2-5 μm.

[0006] In a second aspect, the present application provides a secondary battery, wherein the secondary battery comprises the above battery cell. ADVANTAGEOUS EFFECTS

[0007] The application adopts the isolation coating layer located on the surface of the positive electrode sheet and / or the negative electrode sheet to replace the separator in the secondary battery, which can avoid the direct contact of the positive and negative electrodes caused by the thermal shrinkage of the separator, reduce the probability of short circuit of the secondary battery, and improve the safety performance of the secondary battery. The proportion of inorganic materials in the isolation coating layer, the particle size of the inorganic materials, and the dielectric constant are comprehensively controlled, so that the isolation coating layer has good thermal stability and structural stability under the condition of electronic insulation and good ion transmission efficiency. On the one hand, by effectively controlling the proportion, particle size and dielectric constant of the inorganic materials, the ion migration number, mechanical properties, insulation properties and chemical stability of the isolation coating layer can be improved, the regional difference of lithium ion transmission is reduced, and the secondary battery has excellent cycle stability.

[0008] On the other hand, with the help of the viscosity of the polymer, the inorganic materials can form an isolation coating layer with good structural stability and insulation performance. The insulation coating layer is not easy to shrink at high temperature and is not easy to fall off from the surface of the electrode sheet. By using the isolation coating layer with good insulation performance to replace the separator in the secondary battery, the thermal stability of the secondary battery can be improved, the normal transmission of lithium ions in the battery cycle process can be maintained, the direct contact between the conductive components of the positive and negative electrodes can be effectively prevented, the probability of self-discharge of the secondary battery can be reduced, and internal circuit of the battery can be avoided.

[0009] In addition, by designing the length and width of the first negative active coating layer and the first positive active coating layer, the first negative active coating layer can completely cover the first positive active coating layer during the process of stacking or winding the negative electrode sheet and the positive electrode sheet, that is, the projection of the first positive active coating layer on the isolation coating layer completely falls within the projection of the first negative active coating layer on the isolation coating layer, thereby reducing the possibility of lithium dendrite formation. The secondary battery provided by the application is not easy to cause thermal runaway at a high temperature of 200 DEG C or below, and can maintain normal operation for a long time. BRIEF DESCRIPTION OF DRAWINGS

[0010] FIG. 1 is a schematic structural diagram of a composite electrode unit in a secondary battery provided by embodiments 1-10;

[0011] FIG. 2 is a schematic structural diagram of a composite electrode unit in a secondary battery provided by embodiment 11;

[0012] FIG. 3 is a schematic structural diagram of a composite electrode unit in a secondary battery provided by embodiment 12;

[0013] The corresponding relationship of the above-mentioned reference numerals is as follows: 1. negative current collector, 2. first negative active coating layer, 3. isolation coating layer, 4. first positive active coating layer, 5. positive current collector. Embodiments of the application

[0014] In some embodiments, the polymer includes at least one of a high molecular binder, a polymer binder.

[0015] In some embodiments, the inorganic material accounts for 70-98.5 wt% in the isolation coating.

[0016] In some embodiments, the inorganic material has a specific surface area of 1-20 g / m 2 The use of the inorganic material with a specific surface area within the above range in combination with the polymer as the binder to form the isolation coating can improve the insulation performance, ion conduction efficiency and electrolyte wettability of the insulation coating, thereby reducing the self-discharge probability of the secondary battery, avoiding short circuit of the positive and negative electrodes, improving the electrolyte wettability of the secondary battery, and shortening the lithium ion transmission path.

[0017] In some embodiments, the inorganic material has a dielectric constant of 8-7000.

[0018] In some embodiments, the inorganic material includes at least one of boehmite, aluminum oxide (Al2O3), barium titanate (BaTiO3), tin oxide (SnO2), cerium oxide (CeO2), magnesium oxide (MgO), nickel oxide (NiO), calcium oxide (CaO), zinc oxide (ZnO), zirconium dioxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), titanium dioxide (TiO2).

[0019] In some embodiments, the thickness of the isolation coating is 2-50 μm. When the thickness of the isolation coating is within the above range, the insulation performance and lithium ion transmission efficiency of the isolation coating can be improved, not only shortening the lithium ion transmission path, improving the lithium ion transmission efficiency, and further improving the battery charge and discharge performance, but also reducing the back and forth shuttling of small particles such as conductive agents, binders and the like in the first positive active coating and / or the first negative active coating from the isolation coating, avoiding the internal short circuit of the battery. If the thickness of the isolation coating is too thin, the lithium ions and small particle components (such as binders, conductive agents, etc.) in the positive and negative electrode sheets are easy to shuttle back and forth in the isolation coating, which can reduce the insulation performance of the isolation coating, easily cause the battery to have self-discharge, and the gas generated during the battery cycle process can easily impact the too-thin isolation coating to damage and fall off, causing the positive and negative electrodes to directly contact and cause short circuit. If the thickness of the isolation coating is too thick, the lithium ion transmission path is long, the charge and discharge performance efficiency of the secondary battery is reduced, and the energy density of the secondary battery is reduced.

[0020] In some embodiments, the inorganic material: polymer in the isolation coating layer is (5.0-29.5):(0.5-2.0) by weight ratio. When the mass ratio of the inorganic material and the polymer in the isolation coating layer falls within the above range, the lithium ion transmission rate can be improved, the charge-discharge cycle performance of the secondary battery can be improved, and the adhesion between the isolation coating layer and the electrode sheet can also be improved, so that the isolation coating layer can keep the positive electrode sheet and the negative electrode sheet isolated during the battery cycle, reduce the probability of short circuit of the battery, and improve the safety performance of the secondary battery.

[0021] In some embodiments, the polymer includes at least one of polyvinylidene fluoride, polyimide, polyacrylic acid, polyacrylate, styrene butadiene rubber, carboxymethyl cellulose, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate. Using the above polymer as the binder of the isolation coating layer can provide good flexibility and rigidity for the isolation coating layer, and can improve the adhesion between the isolation coating layer and the first positive active coating layer and / or the first negative active coating layer, thereby improving the structural stability of the secondary battery during the cycle, and improving the cycle capacity retention rate and cycle stability of the secondary battery.

[0022] In some embodiments, the chemical structure of the polymer includes at least one of phenylene group, amide group, propionic acid group, and vinylidene fluoride group. The polymer with the above structure has good lithium salt solubility, chain segment flexibility, and heat resistance, which can promote the dissociation of lithium salt in the electrolyte into lithium ions, thereby improving the migration number of lithium ions in the secondary battery, and can also improve the adhesion between the polymer and the inorganic material, so that the isolation coating layer formed thereby has better structural stability and ion transmission efficiency, thereby improving the thermal stability and cycle performance of the secondary battery.

[0023] In some embodiments, the composite electrode unit satisfies a and / or b: a. the positive electrode sheet includes a positive electrode current collector, a first positive active coating layer, and an isolation coating layer which are sequentially compounded in order; b. the negative electrode sheet includes a negative electrode current collector, a first negative active coating layer, and an isolation coating layer which are sequentially compounded in order.

[0024] In some embodiments, the isolation coating layer includes a first isolation coating layer and a second isolation coating layer.

[0025] In some embodiments, the positive electrode sheet further includes a second positive active coating layer.

[0026] In some embodiments, the negative electrode sheet further includes a second negative active coating layer.

[0027] The composite electrode unit satisfies a and / or b: a. the positive electrode sheet comprises, in order, a second separator coating, a second positive electrode active coating, a positive electrode current collector, a first positive electrode active coating, and a first separator coating; b. the negative electrode sheet comprises, in order, a second separator coating, a second negative electrode active coating, a negative electrode current collector, a first negative electrode active coating, and a first separator coating.

[0028] In some embodiments, the preparation of the separator coating comprises the following operations: S1. mixing the inorganic material, the polymer, and the solvent in a mass ratio of inorganic material: polymer: solvent = (5.0-29.5): (0.5-2.0): (70-95) to prepare a coating slurry; S2. coating the coating slurry on the surface of the first positive electrode active coating and / or the first negative electrode active coating to form the separator coating. By comprehensively controlling the average particle size D50 of the inorganic material and the ratio of the separator slurry, the processing performance, the coating uniformity, and the storage stability of the separator slurry are improved, the separator coating prepared has higher uniformity and better structural stability, and the surface flatness of the separator coating formed by the separator slurry and the uniformity of the distribution of the inorganic material inside the separator coating are improved, thereby avoiding regional differences in lithium ion transmission and reducing the polarization growth rate of the secondary battery. In addition, during the solidification of the separator slurry, as the solvent gradually volatilizes, the inorganic material in the above particle size range can settle on the surface of the first positive electrode active coating and / or the first negative electrode active coating, so that the separator coating is not easy to fall off, the interface impedance can be reduced, and the cycle performance of the secondary battery can be improved.

[0029] In some embodiments, the solvent comprises at least one of N-methyl pyrrolidone, deionized water, ethanol, acetone, and dimethylformamide. The above-mentioned solvents have better compatibility with the inorganic material and the polymer, and the prepared separator slurry has moderate leveling property and viscosity, which is conducive to improving the storage stability of the separator slurry and reducing the difficulty of preparing the separator coating. In particular, the separator slurry can be stored for a long time, leaving enough coating time for the preparation process.

[0030] In some embodiments, when the composite electrode unit satisfies a, the single-sided coating area density of the first positive electrode active coating is 50-500 g / m2; and when the composite electrode unit satisfies b, the single-sided coating area density of the first negative electrode active coating is 20-300 g / m2. When the above conditions are satisfied, it is easier to match the active sites of the first positive electrode active coating and / or the first negative electrode active coating with the lithium ion transmission sites of the separator coating, thereby improving the ion transmission efficiency of the secondary battery and the cycle characteristics of the battery.

[0031] In some embodiments, the secondary battery comprises an electrolyte, and the electrolyte injection coefficient is 0.5-8.0 g / Ah.

[0032] In some embodiments, the secondary battery is prepared in a cutting and / or winding manner.

[0033] Embodiment 1

[0034] The present embodiment provides a secondary battery, and the specific structure is shown in FIG. 1. In the present embodiment, the isolation coating layer 3 includes a first isolation coating layer and a second isolation coating layer.

[0035] (1) Positive electrode sheet:

[0036] The positive electrode sheet provided by the present embodiment includes a first isolation coating layer, a first positive electrode active coating layer 4, a positive electrode current collector 5, a second positive electrode active coating layer, and a second positive electrode isolation coating layer arranged in sequence. The first positive electrode active coating layer 4 and the second positive electrode active coating layer are prepared from a positive electrode active slurry, and the first isolation coating layer and the second isolation coating layer are prepared from an isolation slurry. The positive electrode current collector 5 is an aluminum foil.

[0037] The isolation slurry includes inorganic material, polymer, and solvent, and the mass ratio of the inorganic material, the polymer, and the solvent is 13.5:1.5:85. Thus, the proportion of the inorganic material in the isolation coating layer is 90wt%, and the inorganic material is selected from aluminum oxide with a D50 of 1.5μm, a dielectric constant of 10.5, and a specific surface area of 6 g / m2. The polymer is selected from polyvinylidene fluoride PVDF, and the solvent is selected from N-methyl pyrrolidone (NMP).

[0038] The positive electrode active slurry includes positive electrode active material, binder, and conductive agent. The positive electrode active material is selected from lithium iron phosphate, the conductive agent is selected from conductive carbon black and carbon nanotubes, and the binder is selected from polyvinylidene fluoride PVDF. According to the weight ratio, the positive electrode active material: conductive carbon black: carbon nanotube: binder = 96:1.0:1.0:2.

[0039] The specific preparation method of the positive electrode sheet includes:

[0040] S0. Mix the raw materials for preparing the positive electrode active slurry uniformly, and coat them on the two oppositely arranged surfaces of the positive electrode current collector 5. After drying, the first positive electrode active coating layer 4, the positive electrode current collector 5, and the second positive electrode active coating layer are prepared in sequence.

[0041] S1. Mix the inorganic material, the polymer, and the solvent according to the mass ratio of inorganic material: polymer: solvent = 13.5:1.5:85 to prepare the coating slurry.

[0042] S2. Coat the coating slurry on the surfaces of the first positive electrode active coating layer 4 and the second positive electrode active coating layer to obtain a slurry coating layer. Dry the solvent in the slurry coating layer to form the first isolation coating layer and the second isolation coating layer. Thus, the positive electrode sheet is prepared. In the positive electrode sheet, the thickness of the first isolation coating layer is 5μm, and the thickness of the second isolation coating layer is 5μm.

[0043] (2) The negative electrode sheet:

[0044] The negative electrode sheet comprises a first negative electrode active coating layer 2, a negative electrode current collector 1, and a second negative electrode active coating layer arranged in sequence. The negative electrode current collector 1 is a copper foil. The first negative electrode active coating layer 2 and the second negative electrode active coating layer are prepared from a negative electrode active paste. The negative electrode active paste comprises a negative electrode active material, a binder, a thickening agent, and a conductive agent. The weight ratio of the negative electrode active material, the binder, the thickening agent, and the conductive agent is 96:1.0:1.5:1.5. The negative electrode active material is graphite. The binder is styrene-butadiene rubber. The thickening agent is carboxymethyl cellulose. The conductive agent is conductive carbon black.

[0045] The specific preparation method comprises the following steps:

[0046] The raw materials for preparing the negative electrode active paste are uniformly mixed and coated on the two oppositely arranged surfaces of the negative electrode current collector 1. After drying, the first negative electrode active coating layer 2, the negative electrode current collector 1, and the second negative electrode active coating layer are sequentially prepared, thereby forming the negative electrode sheet.

[0047] (3) The secondary battery:

[0048] The surface of the first separator coating layer of the positive electrode sheet is stacked towards the surface of the first negative electrode active coating layer 2 of the negative electrode sheet, so that the first positive electrode active coating layer 4 and the first negative electrode active coating layer 2 are stacked through the separator coating layer 3. The length and width of the first negative electrode active coating layer 2 are greater than those of the first positive electrode active coating layer 4. The negative electrode sheet is wrapped around the positive electrode sheet in the above stacking order to form a composite electrode unit. The composite electrode unit is subjected to a stacking process to obtain a battery cell suitable for a secondary battery. The battery cell is placed in an outer packaging shell, dried, injected with electrolyte at an electrolyte injection coefficient of 6.8 g / Ah, and subjected to vacuum packaging, standing, formation, and capacity grading processes to obtain the secondary battery.

[0049] Example 2

[0050] This example refers to the preparation method provided in Example 1 to prepare a secondary battery. The difference between this example and Example 1 is that, in the process of preparing the separator coating layer 3, aluminum oxide with a particle size D50 of 0.2 μm is selected instead of the inorganic material in Example 1. The remaining raw material ratios and preparation methods are strictly consistent with those of Example 1.

[0051] Example 3

[0052] This example refers to the preparation method provided in Example 1 to prepare a secondary battery. The difference between this example and Example 1 is that, in the process of preparing the separator coating layer 3, aluminum oxide with a particle size D50 of 5 μm is selected instead of the inorganic material in Example 1. The remaining raw material ratios and preparation methods are strictly consistent with those of Example 1.

[0053] Example 4

[0054] This example is prepared by the method provided in Example 1, and a secondary battery is prepared. The difference between this example and Example 1 is that, in the process of preparing the isolation slurry, the mass ratio of inorganic material, polymer and solvent is 29.5:0.5:70, so that the proportion of inorganic material in the isolation coating 3 is 98.3wt%. The rest of the raw material ratio, preparation method and Example 1 are strictly the same.

[0055] Example 5

[0056] This example is prepared by the method provided in Example 1, and a secondary battery is prepared. The difference between this example and Example 1 is that, in the process of preparing the isolation coating 3, the mass ratio of inorganic material and polymer is 5:2:93, so that the proportion of inorganic material in the isolation coating 3 is 71.5wt%. The rest of the raw material ratio, preparation method and Example 1 are strictly the same.

[0057] Example 6

[0058] This example is prepared by the method provided in Example 1, and a secondary battery is prepared. The difference between this example and Example 1 is that, in the process of preparing the positive electrode sheet, the thickness of the first isolation coating and the second isolation coating is 2μm by adjusting the coating amount of the isolation slurry. The rest of the raw material ratio, preparation method and Example 1 are strictly the same.

[0059] Example 7

[0060] This example is prepared by the method provided in Example 1, and a secondary battery is prepared. The difference between this example and Example 1 is that, in the process of preparing the positive electrode sheet, the thickness of the first isolation coating and the second isolation coating is 10μm by adjusting the coating amount of the isolation slurry. The rest of the raw material ratio, preparation method and Example 1 are strictly the same.

[0061] Example 8

[0062] This example is prepared by the method provided in Example 1, and a secondary battery is prepared. The difference between this example and Example 1 is that, in the process of preparing the positive electrode sheet, the thickness of the first isolation coating and the second isolation coating is 50μm by adjusting the coating amount of the isolation slurry. The rest of the raw material ratio, preparation method and Example 1 are strictly the same.

[0063] Example 9

[0064] This embodiment refers to the preparation method provided in embodiment 1 to prepare a secondary battery, the difference between this embodiment and embodiment 1 is that: in the process of preparing the isolation coating 3, the same mass of deionized water is selected instead of the solvent in embodiment 1. The rest of the raw material ratio, preparation method and embodiment 1 are strictly kept consistent.

[0065] Embodiment 10

[0066] This embodiment refers to the preparation method provided in embodiment 1 to prepare a secondary battery, the difference between this embodiment and embodiment 1 is that: in the process of preparing the isolation coating, the same mass of dimethylacetamide is selected instead of the solvent in embodiment 1. The rest of the raw material ratio, preparation method and embodiment 1 are strictly kept consistent.

[0067] Embodiment 11

[0068] This embodiment provides a kind of secondary battery, specific structure is as shown in Figure 2, in this embodiment, isolation coating includes third isolation coating and fourth isolation coating.

[0069] (1) positive sheet:

[0070] The positive sheet includes first positive active coating 4, positive current collector 5, second positive active coating arranged in sequence. Among them, the positive current collector 5 is aluminum foil, and the first positive active coating 4 and the second positive active coating are prepared from positive active paste. The positive active paste includes positive active material, binder and conductive agent, the positive active material selects lithium iron phosphate, the conductive agent selects conductive carbon black and carbon nanotube, and the binder selects polyvinylidene fluoride PVDF, and according to weight ratio, positive active material: conductive carbon black: carbon nanotube: binder = 96: 1.0: 1.0: 2.

[0071] The specific preparation method includes:

[0072] Mixing the raw materials for preparing the positive active paste uniformly, and coating on the two oppositely arranged surfaces of the positive current collector 5, and drying to prepare the first positive active coating 4, the positive current collector 5 and the second positive active coating in sequence, thereby forming the positive sheet.

[0073] (2) negative sheet:

[0074] The negative sheet provided in this embodiment includes first isolation coating, first negative active coating 2, negative current collector 1, second negative active coating and second isolation coating arranged in sequence. Among them, the first active coating and the second active coating are prepared from negative active paste, and the third isolation coating and the fourth isolation coating are prepared from isolation paste. The negative current collector 1 is copper foil.

[0075] The isolation slurry includes inorganic material, polymer, solvent, and the mass ratio of the inorganic material, the polymer and the solvent is 5:2:93. The inorganic material is selected from aluminum oxide with D50 of 1.5 μm, dielectric constant of 10.5 and specific surface area of 6 g / m2. The polymer is selected from polyvinylidene fluoride PVDF, and the solvent is selected from N-methyl pyrrolidone.

[0076] The negative active slurry includes negative active material, binder, thickening agent and conductive agent, and according to the weight ratio, the negative active material: binder: thickening agent: conductive agent = 96:1.0:1.5:1.5. The negative active material is graphite, the binder is butadiene styrene rubber, the thickening agent is carboxymethyl cellulose, and the conductive agent is conductive carbon black.

[0077] The specific preparation method of the negative plate includes:

[0078] S0. The raw materials for preparing the negative active slurry are uniformly mixed and coated on the two oppositely arranged surfaces of the negative current collector 1, and after drying, the first negative active coating layer 2, the negative current collector 1 and the second negative active coating layer are sequentially prepared;

[0079] S1. The inorganic material, the polymer and the solvent are mixed according to the mass ratio of inorganic material: polymer: solvent = 13.5:1.5:85 to prepare the coating slurry;

[0080] S2. The coating slurry is coated on the surfaces of the first negative active coating layer 2 and the second negative active coating layer to obtain the slurry coating, and the solvent in the slurry coating is dried to form the third isolation coating and the fourth isolation coating. Thus, the negative plate is prepared. In the negative plate, the thickness of the third isolation coating is 5 μm, and the thickness of the fourth isolation coating is 5 μm.

[0081] (3) Secondary battery:

[0082] The surface of the third isolation coating of the negative plate is stacked towards the surface of the first positive active coating layer 4 of the positive plate, so that the first positive active coating layer 4 and the first negative active coating layer 2 are stacked through the isolation coating 3; the length and width of the first negative active coating layer 2 are greater than the length and width of the first positive active coating layer 4. The negative plate is wrapped around the positive plate in the above stacking order to form a composite electrode unit. The composite electrode unit is obtained by the lamination process to obtain a battery cell suitable for a secondary battery. The battery cell is placed in an outer packaging shell, dried, and electrolyte is injected at an electrolyte injection coefficient of 6.8 g / Ah, and then subjected to vacuum packaging, standing, formation and capacity distribution processes to obtain a secondary battery. The separator used is EJ of 9+2+2 type.

[0083] Example 12

[0084] The embodiment provides a secondary battery, and the specific structure is shown in Figure 3. In the embodiment, the isolation coating layer comprises a first isolation coating layer, a second isolation coating layer, a third isolation coating layer and a fourth isolation coating layer.

[0085] (1) Positive electrode sheet:

[0086] The positive electrode sheet provided by the embodiment comprises a first isolation coating layer, a first positive electrode active coating layer 4, a positive electrode current collector 5, a second positive electrode active coating layer and a second positive electrode isolation coating layer arranged in sequence. The first positive electrode active coating layer 4 and the second positive electrode active coating layer are prepared from a positive electrode active slurry, and the first isolation coating layer and the second isolation coating layer are prepared from an isolation slurry. The positive electrode current collector 5 is an aluminum foil.

[0087] The isolation slurry comprises inorganic material, polymer and solvent, and the mass ratio of the inorganic material, the polymer and the solvent is 13.5:1.5:85. Thus, the proportion of the inorganic material in the isolation coating layer is 90wt%, and the inorganic material is selected from aluminum oxide with a D50 of 1.5μm, a dielectric constant of 10.5 and a specific surface area of 6 g / m2. The polymer is selected from polyvinylidene fluoride PVDF, and the solvent is selected from N-methyl pyrrolidone.

[0088] The positive electrode active slurry comprises a positive electrode active material, a binder and a conductive agent. The positive electrode active material is selected from lithium iron phosphate, the conductive agent is selected from conductive carbon black and carbon nanotubes, and the binder is selected from polyvinylidene fluoride PVDF. According to the weight ratio, the positive electrode active material: conductive carbon black: carbon nanotube: binder = 96:1.0:1.0:2.

[0089] The specific preparation method of the positive electrode sheet comprises:

[0090] S0. The raw materials for preparing the positive electrode active slurry are uniformly mixed and coated on the two oppositely arranged surfaces of the positive electrode current collector 5. After drying, the first positive electrode active coating layer 4, the positive electrode current collector 5 and the second positive electrode active coating layer are prepared in sequence.

[0091] S1. The inorganic material, the polymer and the solvent are mixed according to the mass ratio of inorganic material: polymer: solvent = 13.5:1.5:85 to prepare the coating slurry.

[0092] S2. The coating slurry is coated on the surfaces of the first positive electrode active coating layer 4 and the second positive electrode active coating layer to obtain a slurry coating layer. The solvent in the slurry coating layer is dried to form the first isolation coating layer and the second isolation coating layer. Thus, the positive electrode sheet is prepared. In the positive electrode sheet, the thickness of the first isolation coating layer is 5μm, and the thickness of the second isolation coating layer is 5μm.

[0093] (2) Negative electrode sheet:

[0094] The negative electrode sheet provided by the embodiment comprises a third isolation coating layer, a first negative electrode active coating layer 2, a negative electrode current collector 1, a second negative electrode active coating layer, and a fourth isolation coating layer arranged in sequence. The first active coating layer and the second active coating layer are prepared from a negative electrode active slurry, and the third isolation coating layer and the fourth isolation coating layer are prepared from an isolation slurry. The negative electrode current collector 1 is a copper foil.

[0095] The isolation slurry comprises inorganic material, polymer, and solvent, and the mass ratio of the inorganic material, the polymer, and the solvent is 5:2:93. The inorganic material is selected from aluminum oxide with a D50 of 1.5 μm, a dielectric constant of 10.5, and a specific surface area of 6 g / m2. The polymer is selected from polyvinylidene fluoride (PVDF), and the solvent is selected from N-methyl pyrrolidone.

[0096] The negative electrode active slurry comprises negative electrode active material, binder, thickening agent, and conductive agent, and the weight ratio of the negative electrode active material, the binder, the thickening agent, and the conductive agent is 96:1.0:1.5:1.5. The negative electrode active material is graphite, the binder is butadiene styrene rubber, the thickening agent is carboxymethyl cellulose, and the conductive agent is conductive carbon black.

[0097] The specific preparation method of the negative electrode sheet comprises:

[0098] S0. The raw materials for preparing the negative electrode active slurry are uniformly mixed and coated on the two oppositely arranged surfaces of the negative electrode current collector 1, and after drying, the first negative electrode active coating layer 2, the negative electrode current collector 1, and the second negative electrode active coating layer are prepared in sequence.

[0099] S1. The inorganic material, the polymer, and the solvent are mixed in a mass ratio of inorganic material: polymer: solvent = 13.5:1.5:85 to prepare a coating slurry;

[0100] S2. The coating slurry is coated on the surfaces of the first negative electrode active coating layer 2 and the second negative electrode active coating layer to obtain a slurry coating, and the solvent in the slurry coating is dried to form the third isolation coating layer and the fourth isolation coating layer. Thus, the negative electrode sheet is prepared. In the negative electrode sheet, the thickness of the third isolation coating layer is 5 μm, and the thickness of the fourth isolation coating layer is 5 μm.

[0101] (3) Secondary battery:

[0102] The surface of the third isolation coating of the negative electrode sheet is stacked towards the surface of the first isolation coating of the positive electrode sheet, and the first positive electrode active coating 4 and the first negative electrode active coating 2 are stacked by the isolation coating; the length and width of the first negative electrode active coating 2 are greater than the length and width of the first positive electrode active coating 4. The negative electrode sheet is wrapped around the positive electrode sheet in the above stacking order to form a composite electrode unit. The composite electrode unit is subjected to a stacking process to obtain a battery cell suitable for a secondary battery. The battery cell is placed in an outer packaging shell, dried, and injected with electrolyte at an electrolyte injection coefficient of 6.8 g / Ah, and then subjected to vacuum packaging, standing, formation, and capacity distribution processes to obtain a secondary battery.

[0103] Comparative Example 1

[0104] A secondary battery is prepared according to the following specific preparation method:

[0105] (1) Positive electrode sheet:

[0106] The positive electrode sheet provided by the present embodiment includes a first positive electrode active coating 4, a positive electrode current collector 5, and a second positive electrode active coating, which are sequentially arranged. The first positive electrode active coating 4 and the second positive electrode active coating are prepared from a positive electrode active slurry. The positive electrode current collector 5 is an aluminum foil.

[0107] The positive electrode active slurry includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material is lithium iron phosphate, the conductive agent is conductive carbon black and carbon nanotubes, and the binder is polyvinylidene fluoride (PVDF). According to the weight ratio, the positive electrode active material: conductive carbon black: carbon nanotube: binder = 96: 1.0: 1.0: 2.

[0108] The specific preparation method includes:

[0109] The raw materials for preparing the positive electrode active slurry are mixed uniformly and coated on the two oppositely arranged surfaces of the positive electrode current collector 5. After drying, the positive electrode sheet is obtained.

[0110] (2) Negative electrode sheet:

[0111] The negative electrode sheet includes a first negative electrode active coating 2, a negative electrode current collector 1, and a second negative electrode active coating, which are sequentially arranged. The negative electrode current collector 1 is a copper foil, and the first negative electrode active coating 2 and the second negative electrode active coating are prepared from a negative electrode active slurry. The negative electrode active slurry includes a negative electrode active material, a binder, a thickening agent, and a conductive agent. According to the weight ratio, the negative electrode active material: binder: thickening agent: conductive agent = 96: 1.0: 1.5: 1.5. The negative electrode active material is graphite, the binder is butadiene rubber, the thickening agent is carboxymethyl cellulose, and the conductive agent is conductive carbon black.

[0112] The specific preparation method includes:

[0113] The raw materials for preparing the negative active paste are mixed uniformly and coated on two oppositely arranged surfaces of the negative current collector 1, and after drying, the negative electrode sheet is prepared.

[0114] (3) Secondary battery:

[0115] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, and the cell is obtained through the stacking process, wherein the length and width of the first negative active coating layer 2 are greater than those of the first positive active coating layer 4; the cell is placed in an outer packaging shell, dried, and injected with electrolyte at an electrolyte injection coefficient of 6.8 g / Ah, and then subjected to vacuum packaging, standing, formation, and capacity grading processes to obtain a secondary battery. The separator used is a 9+2+2 type of EJ.

[0116] Comparative Example 2

[0117] This comparative example refers to the preparation method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that, in the process of preparing the isolation coating layer, aluminum oxide with a particle size D50 of 7 μm is selected instead of the inorganic material in Example 1. The remaining raw material ratios and preparation methods are strictly consistent with those of Example 1.

[0118] Comparative Example 3

[0119] This comparative example refers to the preparation method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that, in the process of preparing the isolation coating layer, aluminum oxide with a particle size D50 of 0.1 μm is selected instead of the inorganic material in Example 1. The remaining raw material ratios and preparation methods are strictly consistent with those of Example 1.

[0120] Comparative Example 4

[0121] This comparative example refers to the preparation method provided in Example 11 to prepare a secondary battery.

[0122] (1) Positive electrode sheet

[0123] The operation method and raw material ratio for preparing the positive electrode composite sheet in this example are strictly consistent with those for preparing the positive electrode sheet in Example 11. The positive electrode sheet includes a first positive active coating layer 4, a positive current collector 5, and a second positive active coating layer arranged in order.

[0124] (2) Negative electrode sheet

[0125] The operation method and raw material ratio for preparing the positive electrode composite sheet in this example are strictly consistent with those for preparing the positive electrode sheet in Example 11. The negative electrode sheet includes a first isolation coating layer, a first negative active coating layer 2, a negative current collector 1, a second negative active coating layer, and a second isolation coating layer arranged in order.

[0126] (3) Secondary battery:

[0127] The positive electrode sheet, the separator, and the negative electrode sheet were sequentially stacked with the surface of the first isolation coating of the positive electrode sheet facing the separator to obtain a battery cell by a stacking process, wherein the length and width of the first negative electrode active coating 2 were greater than the length and width of the first positive electrode active coating 4. The battery cell was placed in an outer packaging shell, dried, and injected with electrolyte at an electrolyte injection coefficient of 6.8 g / Ah, and then subjected to vacuum packaging, standing, formation, and capacity grading processes to obtain a secondary battery. The separator used was EJ of the 9+2+2 type.

[0128] The secondary batteries provided in Examples 1-12 and Comparative Examples 1-4 were subjected to performance tests.

[0129] Test items and test methods:

[0130] (1) Discharge capacity retention rate

[0131] Each secondary battery was subjected to standing for 24 hours after formation in a constant temperature environment at 25°C, constant current constant voltage charging to 3.65V at a current of 0.33C, and a cutoff current of 0.05C, then constant current discharging to 2.5V at a current of 0.33C, and the discharge capacity C1 was recorded. Then, constant current constant voltage charging to 3.65V at a current of 0.33C, and a cutoff current of 0.05C, standing at 60°C for 7D, then constant current discharging to 2.5V at a current of 0.33C, and the discharge capacity C2 was recorded. The discharge capacity retention rate R = C2 / C1 x 100%.

[0132] (2) IMP internal resistance test

[0133] Each secondary battery was subjected to standing for 24 hours after formation in a constant temperature environment at 25°C, and then tested using a HIOKI voltage resistance meter.

[0134] (3) Safety performance test

[0135] The safety performance of the secondary battery was verified using a hot box test. A thermocouple was attached to the surface of the battery, and the battery was placed in a hot box. The battery was heated to a higher temperature at a heating rate of 5±1°C / min from a starting temperature (room temperature). The temperature of the hot box was required to be 200°C, and then the temperature of the battery was recorded as a function of time at a constant temperature. The time for the battery to reach thermal runaway (smoke and combustion caused by internal heat reaction) in the 200°C hot box was determined.

[0136] Test results: The raw material composition of the isolation coating and the structure of the secondary battery of each secondary battery are shown in Table 1, and the test results of each secondary battery are shown in Table 2.

[0137] Table 1. Raw material composition of the isolation coating and structure of the secondary battery in each secondary battery

[0138]

[0139] Table 2. Test results of each secondary battery in the present test example

[0140]

[0141] Result analysis:

[0142] Comparing the test results of Examples 1-12 and Comparative Examples 1-4 in Table 2, it can be found that the comprehensive performance of the batteries provided by Examples 1-12 is better than that of the batteries provided by Comparative Examples 1-4.

[0143] The batteries provided by Example 1 and Comparative Example 1 are different in that the battery of Example 1 contains the isolation coating 3 and does not contain the separator, while the battery of Comparative Example 1 does not contain the isolation coating 3 and contains the separator. Comparing the comprehensive performance of Example 1 and Comparative Example 1, it can be found that compared with the battery containing the separator in Comparative Example 1, the battery provided by Example 1 has a longer thermal runaway time and has higher thermal stability. Among them, the separator in Comparative Example 1 is prone to thermal contraction in a high-temperature environment, causing a short circuit between the positive and negative electrodes, and then causing thermal runaway.

[0144] Comparing the performance of Examples 1-3 and Comparative Examples 2-3 in Table 2, it can be found that as the average particle size D50 of the inorganic material increases, the IMP internal resistance of the secondary battery shows a downward trend, the thermal runaway time of the secondary battery shows a downward trend, that is, the ion transport efficiency shows an upward trend, and the insulation performance of the isolation coating 3 decreases. When the average particle size D50 of the inorganic material is 0.2-5 μm, the insulation performance of the isolation coating 3 is good, the ion transport efficiency is high, the IMP internal resistance of the secondary battery using the isolation coating 3 is small, and the thermal runaway time is long. Among them, the battery provided by Comparative Example 2 can normally operate at room temperature, but is prone to failure at high temperature and has a low discharge capacity retention rate. This is because the average particle size D50 of the inorganic material used in Comparative Example 2 is too large, the voids in the isolation coating 3 are large, causing a self-discharge phenomenon during the standing process of the battery, and the small particle components in the positive and negative electrode sheets can shuttle in the voids of the insulation coating 3; and as the temperature rises, the shuttle speed and amount of small particle components increase, reducing the insulation performance of the isolation coating 3, so the battery provided by Comparative Example 2 can still operate normally at room temperature, but when in a high-temperature environment, the insulation of the isolation coating 3 decreases, and the battery provided by Comparative Example 2 is more prone to thermal runaway. The IMP internal resistance of the battery provided by Comparative Example 3 is high, which is because the average particle size D50 of the inorganic material is too small, and the inorganic material in the isolation coating 3 is stacked too tightly, causing the voids in the isolation coating 3 to be too small, and the lithium ion transport is hindered.

[0145] Comparing the secondary batteries provided by Example 11 and Comparative Example 4, it can be found that the batteries provided by Example 11 and Comparative Example 4 are different in that the battery of Example 11 contains the isolation coating 3 and does not contain the separator, and the battery of Comparative Example 4 contains the isolation coating 3 and also contains the separator. Compared with the battery provided by Comparative Example 4, the battery provided by Example 10 has a lower IMP internal resistance, because the isolation coating in Example 11 has good ion transmission efficiency, and the interface internal resistance between the separator and the isolation coating 3 in Comparative Example 4 is large, and the transmission of lithium ions is hindered.

[0146] Comparing the performances of Example 1, Example 4~5 in Table 2, it can be found that when the weight of inorganic material, polymer and solvent in the isolation slurry is in the range of 5.0~29.5:0.5~2.0:70~95, the adhesion between the isolation coating 3 and the first positive active coating 2 prepared from the isolation slurry is firm, when the proportion of polymer increases, the voids of the isolation coating decrease due to the increase of polymer, resulting in a decrease in lithium ion transmission channel, and the IMP internal resistance of the secondary battery is high. Comparing the performances of Example 1, Example 6~8 in Table 2, it can be found that when the thickness of the isolation coating is 2~50 μm, the isolation coating has good insulation performance and good lithium ion transmission efficiency, which is manifested by the small IMP internal resistance and long thermal runaway time of the batteries provided by Example 1, Example 6~8. Comparing the performances of Example 1, Example 9~10 in Table 2, it can be found that the comprehensive performance of the battery provided by Example 1, Example 9 is lower than that of the battery provided by Example 10, which shows that using different kinds of solvents to prepare the isolation slurry will result in different processing performance of the isolation slurry, and then affect the battery performance. When the solvent includes at least one of N-methyl pyrrolidone, deionized water, ethanol, acetone, dimethylformamide, the isolation slurry shows good processing performance. Comparing the comprehensive performance of the batteries provided by Example 1, Example 11~12, it can be found that when the isolation coating 3 is coated on the first positive active coating 4 and / or the isolation coating 3 is coated on the first negative active coating 2, the secondary batteries prepared can normally operate and have good thermal stability.

Claims

1. A battery cell suitable for a secondary battery, The battery cell comprises at least one composite electrode unit, the composite electrode unit comprising a positive electrode sheet and a negative electrode sheet; In the composite electrode unit, the positive electrode sheet comprises a first positive electrode active coating (4) disposed towards the negative electrode sheet, the negative electrode sheet comprises a first negative electrode active coating (2) disposed towards the positive electrode sheet, the first positive electrode active coating (4) and the first negative electrode active coating (2) are stacked by a separator coating (3) therebetween; the length and width of the first negative electrode active coating (2) are greater than the length and width of the first positive electrode active coating (4); The separator coating (3) comprises an inorganic material and a polymer, the proportion of the inorganic material in the separator coating (3) is not less than 70 wt%, the dielectric constant of the inorganic material is not less than 8, and the average particle size D50 of the inorganic material is 0.2-5 μm.

2. The cell of claim 1, wherein, The specific surface area of the inorganic material is 1 to 20 g / m 2 .

3. The cell of claim 1, wherein, The thickness of the separator coating (3) is 2-50 μm.

4. The cell of claim 1, wherein, In terms of weight ratio, in the separator coating (3), the inorganic material: the polymer = (5.0-29.5):(0.5-2.0).

5. The cell of claim 1, wherein, The polymer comprises at least one of polyvinylidene fluoride, polyimide, polyacrylic acid, polyacrylate, styrene butadiene rubber, carboxymethyl cellulose, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate.

6. The cell of any one of claims 1 to 5, wherein, The composite electrode unit satisfies a and / or b: a. The positive electrode sheet comprises a positive electrode current collector (5), the first positive electrode active coating (4), and the separator coating (3) sequentially compounded in order; b. The negative electrode sheet comprises a negative electrode current collector (1), the first negative electrode active coating (2), and the separator coating (3) sequentially compounded in order.

7. The cell of claim 6 wherein, The preparation of the separator coating (3) comprises the following operations: S1. Mixing the inorganic material, the polymer, and the solvent according to the mass ratio of the inorganic material: the polymer: the solvent = (5.0-29.5):(0.5-2.0):(70-95) to prepare a coating slurry; S2. Coating the coating slurry on the surface of the first positive electrode active coating (4) and / or the first negative electrode active coating (2) to form the separator coating (3).

8. The cell of claim 7 wherein, The solvent comprises at least one of N-methyl pyrrolidone, deionized water, ethanol, acetone, and dimethylformamide.

9. The battery cell of claim 6, wherein: When the composite electrode unit satisfies a, the single-sided coating area density of the first positive electrode active coating (4) is 50-500 g / m2; When the composite electrode unit satisfies b, the single-sided coating area density of the first negative electrode active coating (2) is 20-300 g / m2.

10. A secondary battery comprising the battery cell of any one of claims 1-9.

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