Battery cell and preparation method thereof, battery, battery pack and electric equipment

Abstract

The invention provides a battery cell and a preparation method thereof, a battery, a battery pack and electric equipment, the battery cell comprises an electrode plate and a diaphragm which are laminated, the electrode plate comprises a current collector and an active layer arranged on at least one surface of the current collector, and the active layer comprises m sub-active layers; each active sub-layer comprises a plurality of pores, at least part of the pores are generated by liquidation and outflow of a hot melt adhesive, and the porosity of the active sub-layer is gradually reduced in the direction from the surface close to the current collector to the surface far away from the current collector. According to the electrode plate provided by the invention, the porosity of the sub-active layer is gradually reduced, so that the problem that the SOC of the electrode plate in the thickness direction is inconsistent can be effectively improved, and the quick charge cycle life of the battery can be prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a battery cell and a preparation method thereof, a battery, a battery pack and electrical equipment. Background Art

[0002] With the continuous development of miniaturization and long standby of portable electronic devices and the activation of high-power and high-energy devices, higher and higher requirements are placed on the energy density of energy storage batteries.

[0003] Increasing the surface density and compaction density of the electrode sheet is an effective means to improve the energy density of the battery. However, with the increase in the surface density and compaction density of the electrode sheet, the problem of inconsistent state of charge (SOC) in the thickness direction of the electrode sheet during high-rate charge and discharge occurs, resulting in a reduction in the battery cycle life. Therefore, for cells with high-rate charge and discharge requirements, it is necessary to reduce the surface density and compaction of the electrode sheet. However, this not only limits the energy density of the battery, but also has limited improvement effects. When the battery undergoes a fast charge cycle, it will still be affected by the problem of inconsistent SOC in the thickness direction of the electrode sheet. Summary of the invention

[0004] The present invention provides a battery core, which can effectively improve the problem of inconsistent SOC in the thickness direction of the electrode piece.

[0005] The present invention also provides a method for preparing a battery core, which can prepare the battery core and is easy to operate.

[0006] The present invention also provides a battery comprising the battery core, which has a long fast-charging cycle life.

[0007] The present invention also provides a battery pack comprising the battery, which has excellent rate capability and stability.

[0008] The present invention also provides an electrical device. Since the electronic device includes the battery, the electrical performance is relatively excellent and the service life is relatively long.

[0009] In a first aspect, the present invention provides a battery cell comprising stacked electrode sheets and a separator, wherein the electrode sheets comprise a current collector and an active layer disposed on at least one surface of the current collector, wherein the active layer comprises m layers of sub-active layers; each of the sub-active layers comprises a plurality of pores, at least some of the pores being generated by the liquefied outflow of hot melt adhesive, and the porosity of the sub-active layer gradually decreases from the surface close to the current collector to the surface away from the current collector, wherein m is an integer greater than or equal to 2.

[0010] Furthermore, a hot melt adhesive layer is further included on the upper surface of the active layer away from the current collector, and the electrode sheet close to the diaphragm is adhesively connected to the diaphragm through the hot melt adhesive layer.

[0011] Furthermore, the mass of the hot melt adhesive layer is 0.2-4wt% of the total mass of the active layer.

[0012] Furthermore, the electrode sheet is a positive electrode sheet and / or a negative electrode sheet; and / or the separator is a polypropylene film.

[0013] Furthermore, in the direction from the surface close to the current collector to the surface away from the current collector, the porosity of the first sub-active layer is 22%-36%, and the porosity of the n-1th sub-active layer and the porosity of the nth sub-active layer are in a ratio of 22-36:20-34; n is any integer from 2 to m.

[0014] Furthermore, the average pore diameter of the (n-1)th sub-active layer is greater than the average pore diameter of the nth sub-active layer.

[0015] Furthermore, each of the sub-active layers includes an active substance; and the active layer satisfies the following formula 1:

[0016] 1.2≤a / c≤4 Formula 1;

[0017] Wherein, c is the D50 of the active material in the n-th sub-active layer, and a is the D50 of the active material in the n-1-th sub-active layer.

[0018] Furthermore, each of the sub-active layers includes a conductive agent; wherein the mass ratio of the conductive agent in the nth sub-active layer to the conductive agent in the n-1th sub-active layer is 0.3-1:0.5-2.

[0019] In a second aspect, the present invention provides a method for preparing a battery cell as described in the first aspect, comprising the following steps:

[0020] Sequentially laminating and coating m layers of sub-active layer slurry comprising active material, binder, hot melt adhesive, solvent and conductive agent on at least one surface of a current collector; drying and rolling at a temperature lower than the melting point of the hot melt adhesive to obtain the electrode sheet;

[0021] The electrode sheets and the diaphragm are stacked and assembled, and hot-pressed at a temperature greater than the melting point of the hot melt adhesive, followed by casing, baking, liquid injection, chemical formation, and volume separation to obtain the battery cell;

[0022] Among them, in the slurry of the first sub-active layer, the mass proportion of hot melt adhesive is 0.25-2%, and the hot melt adhesive content ratio of the slurry of the n-1th sub-active layer and the slurry of the nth sub-active layer is 0.25-2:0-1.

[0023] Furthermore, the melting point of the hot melt adhesive is 60-120°C.

[0024] In a third aspect, the present invention provides a battery, comprising the battery cell described in the first aspect.

[0025] In a fourth aspect, the present invention provides a battery pack comprising the battery described in the third aspect.

[0026] In a fifth aspect, the present invention provides an electrical device, comprising the battery described in the third aspect or the battery pack described in the fourth aspect.

[0027] The battery cell provided by the present invention can gradually increase the electrolyte infiltration rate of the electrode sheet in the direction from the surface away from the current collector to the surface close to the current collector due to the gradual decrease in the porosity of the active layer of the electrode sheet. This helps to reduce the difference in ionic and electronic conductivity between the inside and outside of the electrode sheet, thereby effectively improving the problem of inconsistent SOC in the thickness direction of the electrode sheet, and at the same time helps to improve the fast charging cycle life of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for use in the embodiments of the present invention or related technologies. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A schematic diagram of pores generated by liquefaction and outflow of hot melt adhesive according to a specific embodiment of the present invention;

[0030] Figure 2 This is a microscopic representation of the first sub-active layer of the positive electrode sheet of Example 1 of the present invention;

[0031] Figure 3 This is a microscopic representation of the second sub-active layer of the positive electrode sheet of Example 1 of the present invention;

[0032] Figure 4 This is a microscopic characterization diagram of the first sub-active layer of the negative electrode sheet of Example 1 of the present invention;

[0033] Figure 5 This is a microscopic characterization diagram of the first sub-active layer of the negative electrode sheet of Example 1 of the present invention. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] With the increase of the surface density and compaction density of the electrode sheet, the problem of inconsistent state of charge (SOC) in the thickness direction of the electrode sheet during high-rate charge and discharge occurs. In detail, the part of the electrode sheet close to the current collector side has poor electrolyte infiltration, resulting in the inability of lithium ions to reach the bottom of the electrode sheet smoothly and quickly during the charge and discharge process, forming a large concentration polarization, while the part far away from the current collector side has a lower electronic conductivity on the outside of the electrode sheet due to its distance from the current collector. The difference in ion and electronic conductivity inside and outside the electrode sheet aggravates the electrochemical polarization, which in turn causes the problem of serious inconsistency in the thickness direction of the electrode sheet during high-rate charge and discharge, and rapid decay of the battery fast charge cycle capacity. In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0036] In a first aspect, the present invention provides a battery cell comprising stacked electrode sheets and a separator, wherein the electrode sheets comprise a current collector and an active layer disposed on at least one surface of the current collector, wherein the active layer comprises m layers of sub-active layers; each of the sub-active layers comprises a plurality of pores, at least some of the pores being generated by the liquefied outflow of hot melt adhesive, and the porosity of the sub-active layer gradually decreases from the surface close to the current collector to the surface away from the current collector, wherein m is an integer greater than or equal to 2.

[0037] It should be noted that, among the m layers of sub-active layers, the mth sub-active layer is the sub-active layer farthest from the current collector, and the corresponding first sub-active layer is the sub-active layer closest to the current collector.

[0038] In the present invention, the porosity of the sub-active layer gradually decreases from the surface close to the current collector to the surface far away from the current collector, that is, the porosity of the sub-active layer close to the current collector is higher than that of the sub-active layer relatively far away from the current collector, so that the sub-active layer close to the current collector has a higher liquid retention capacity, reducing the diffusion barrier of lithium ions. During high-rate charge and discharge, lithium ions can quickly reach the bottom of the electrode sheet, reducing the concentration polarization in the thickness direction of the electrode sheet, and reducing the difference in ionic and electronic conductivity inside and outside the electrode sheet, thereby improving the SOC consistency between the sub-active layers to a certain extent; in addition, compared with traditional pores formed by solid accumulation, at least part of the pores of the present invention are generated by liquefaction of hot melt adhesive, and the pore size of the pores is adjustable, which helps to further reduce the difference in ionic and electronic conductivity inside and outside the electrode sheet, thereby greatly improving the SOC consistency between the sub-active layers.

[0039] In an optional embodiment, a hot melt adhesive layer is further provided on the upper surface of the active layer away from the current collector, and the electrode sheet close to the diaphragm is adhesively connected to the diaphragm through the hot melt adhesive layer.

[0040] There is a certain space between the traditional diaphragm and the electrode sheet. During the diaphragm coating or winding process, the diaphragm may be wrinkled, resulting in increased battery cell impedance, lithium deposition and other problems. However, in the above-mentioned embodiment, since the electrode sheet and the diaphragm are bonded together by a hot-melt adhesive layer, the fit between the electrode sheet and the diaphragm is better, which can effectively improve the diaphragm wrinkles and the resulting increased battery cell impedance, lithium deposition and other problems.

[0041] In an optional embodiment, the mass of the hot melt adhesive layer is 0.2-4wt% of the total mass of the active layer.

[0042] Although the hot melt adhesive layer helps to bond between the electrode and the diaphragm, too much total content of the hot melt adhesive layer will also affect the ion and electron transmission of the electrode. Therefore, the above embodiment can avoid the adverse effects of the hot melt adhesive on the capacity and energy density of the battery by limiting the relationship between the content of the hot melt adhesive layer and the sub-active layer.

[0043] The electrode sheet of the present invention is applicable to both positive and negative electrode sheets. In order to further improve the fast charging cycle life of the battery, in a preferred embodiment, the electrode sheets are positive and negative electrode sheets; and / or the separator is a polypropylene film.

[0044] In an optional embodiment, in the direction from the surface close to the current collector to the surface away from the current collector, the porosity of the first sub-active layer is 22%-36%, and the porosity of the n-1th sub-active layer and the porosity of the nth sub-active layer are in a ratio of 22-36:20-34; n is any integer from 2 to m.

[0045] Since the electrolyte infiltration rate decays gradually rather than abruptly from the surface of the electrode away from the current collector to the surface close to the current collector, the above-mentioned embodiment can gradually increase the electrolyte infiltration rate from the surface of the electrode away from the current collector to the surface close to the current collector by limiting the porosity change law of two adjacent sub-active layers, which helps to further reduce the difference in ionic and electronic conductivity between the inside and outside of the electrode, thereby greatly improving the SOC consistency between the sub-active layers.

[0046] The present invention does not specifically limit the specific type of the current collector. For example, any one or more of copper foil, titanium foil, tin foil, chromium foil and composite foils of the above metals can be selected.

[0047] In an optional implementation manner, the average diameter of the pores in the (n-1)th sub-active layer is greater than the average diameter of the pores in the nth sub-active layer.

[0048] In a specific embodiment, when m is 2, the average pore diameter of the second sub-active layer is greater than the average pore diameter of the first sub-active layer, wherein the average pore diameter of the second sub-active layer is 0.05-1um; the average pore diameter of the first sub-active layer is 0.1-2um.

[0049] The average pore diameter can be measured, for example, by a scanning electron microscope (SEM): use SEM to obtain a surface morphology image of a cross section in the thickness direction, with the magnification of the SEM image being 2000 times; take multiple test points in the image, measure the sizes of multiple pores in the mth sub-active layer in each test point, and calculate the average to obtain the average pore diameter of the mth sub-active layer; the method for measuring the average pore diameters of other layers is similar.

[0050] In an optional embodiment, each of the sub-active layers includes an active material; the active layer satisfies the following formula 1:

[0051] 1.2≤a / c≤4 Formula 1;

[0052] Wherein, c is the D50 of the active material in the n-th sub-active layer, and a is the D50 of the active material in the n-1-th sub-active layer.

[0053] Among them, the above-mentioned embodiment limits the D50 distribution of the active material in the nth sub-active layer and the n-1th sub-active layer, so that the particle size of the active layer gradually increases from the side adjacent to the current collector to the side away from the current collector, so that the lithium ion extraction / insertion speed in the entire active layer is uniform, thereby making the charge distribution between adjacent particles in the thickness direction of the electrode uniform, and can further reduce the polarization caused by different SOC in the thickness direction of the electrode.

[0054] In an optional embodiment, each of the sub-active layers includes a conductive agent; wherein a mass ratio of the conductive agent in the nth sub-active layer to the conductive agent in the n-1th sub-active layer is 0.3-1:0.5-2.

[0055] Furthermore, the porosity of the conductive agent in the nth sub-active layer is greater than the porosity of the conductive agent in the n-1th sub-active layer. This embodiment can assist in regulating the porosity variation of the sub-active layer based on the porosity of the conductive agent itself.

[0056] When the electrode sheet of the present invention is a positive electrode sheet, each sub-active layer includes a positive electrode active material, a conductive agent, and a binder. For example, the positive electrode active material is one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium phosphate, and lithium nickel cobalt aluminum oxide. Among them, lithium manganese oxide can be selected from LiMnO2 and LiMn2O4; and lithium nickel cobalt manganese oxide material can be selected from LiNi 1 / 3Co 1 / 3 Mn 1 / 3 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.8 Co 0.1 Mn 0.1 O2 one or more; the binder is selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; preferably at least one of polyurethane, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, polyacrylic acid, polystyrene sulfonate, sodium carboxymethylate, epoxy resin and vinyl acetate; the conductive agent can be any known carbon material with conductive properties, for example: one or more of conductive carbon black, conductive graphite, carbon tubes, and graphene.

[0057] When the electrode sheet of the present invention is a negative electrode sheet, each sub-active layer includes a negative electrode active material, a conductive agent, and a binder. Exemplarily, the negative electrode active material is at least one of graphite, silicon-doped graphite, and lithium titanate. The above-mentioned silicon-doped graphite includes graphite and silicon, and the silicon can be selected from at least one of pure silicon, silicon carbon, silicon oxygen, and silicon alloy; the conductive agent can be selected from any well-known carbon material with conductive properties, for example: one or more of conductive carbon black, conductive graphite, carbon tube, and graphene; the binder is selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; preferably, it is at least one of polyurethane, polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, polyacrylic acid, polystyrene sulfonate, sodium carboxymethylate, epoxy resin, and vinyl acetate.

[0058] The thickness, surface density and thickness of the active layer of the electrode sheet are not specifically limited in the present invention. However, in order to balance the battery capacity, cycle life and energy density, in a specific embodiment, the thickness of the electrode sheet is 40-120 μm, including but not limited to: 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, etc.; the surface density of the electrode sheet is 3-10 mg / cm 2 , including but not limited to: 3.5mg / cm 2 , 4mg / cm 2 , 4.5mg / cm2 , 5mg / cm 2 , 5.5mg / cm 2 , 6mg / cm 2 、6.5mg / cm 2 , 7mg / cm 2 、7.5mg / cm 2 , 8mg / cm 2 、8.5mg / cm 2 , 9mg / cm 2 、9.5mg / cm 2 etc.; the thickness of the active layer is 20-60 μm, specifically including but not limited to: 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, etc.

[0059] In a specific embodiment, the electrode sheet is prepared by a method comprising the following steps:

[0060] Sequentially laminating and coating m layers of sub-active layer slurry comprising active material, binder, hot melt adhesive, solvent and conductive agent on at least one surface of a current collector; drying and rolling at a temperature lower than the melting point of the hot melt adhesive to obtain the electrode sheet;

[0061] The electrode sheets and the diaphragm are stacked and assembled, and hot-pressed at a temperature greater than the melting point of the hot melt adhesive, followed by casing, baking, liquid injection, chemical formation, and volume separation to obtain the battery cell;

[0062] Among them, in the slurry of the first sub-active layer, the mass proportion of hot melt adhesive is 0.25-2%, and the hot melt adhesive content ratio of the slurry of the n-1th sub-active layer and the slurry of the nth sub-active layer is 0.25-2:0-1.

[0063] In the above preparation method, a certain amount of hot melt adhesive is first added to each layer of slurry. After coating and drying, the hot melt adhesive is filled in the pores of each sub-active layer constructed by the active material and the conductive agent, which can play a certain space-occupying role. Then, the laminated electrode core is hot-pressed at a certain temperature. As the hot melt adhesive reaches the melting point, the hot melt adhesive liquefies and almost all flows out of the pores to the surface under pressure and solidifies to form a hot melt adhesive layer. The original space where the hot melt adhesive is lost in the sub-active layer forms relatively stable pores. Part of the principle is shown in the schematic diagram. Figure 1 .

[0064] In a second aspect, the present invention provides a method for preparing a battery cell, comprising the following steps:

[0065] Sequentially laminating and coating m layers of sub-active layer slurry comprising active material, binder, hot melt adhesive, solvent and conductive agent on at least one surface of a current collector; drying and rolling at a temperature lower than the melting point of the hot melt adhesive to obtain the electrode sheet;

[0066] The electrode sheets and the diaphragm are stacked and assembled, and hot-pressed at a temperature greater than the melting point of the hot melt adhesive, followed by casing, baking, liquid injection, chemical formation, and volume separation to obtain the battery cell;

[0067] Among them, in the slurry of the first sub-active layer, the mass proportion of hot melt adhesive is 0.25-2%, and the hot melt adhesive content ratio of the slurry of the n-1th sub-active layer and the slurry of the nth sub-active layer is 0.25-2:0-1.

[0068] In an optional embodiment, the melting point of the hot melt adhesive is 60-120°C.

[0069] It can be understood that as long as the melting point of the hot melt adhesive is 60-120°C and the hot melt adhesive does not react with other components of the electrode, it is within the protection scope of the present invention. As for the specific type of hot melt adhesive, the present invention does not specifically limit it. For example, it includes but is not limited to EVA hot melt adhesive (ethylene-vinyl acetate copolymer), polyethylene wax hot melt adhesive, acrylic resin hot melt adhesive, etc.

[0070] In a third aspect, the present invention provides a battery, comprising the battery cell described in the first aspect.

[0071] It should be noted that the above-mentioned batteries may include but are not limited to single cells, battery modules, battery packs, etc., that is, the actual application form of the battery provided by the present invention can be but is not limited to the listed products, and can also be other application forms. When the battery is a single cell, it includes at least one of a cylindrical battery, a square battery, etc.

[0072] Generally speaking, the battery also includes an electrolyte. The present invention does not specifically limit the separator, and any known porous structure separator with electrochemical stability and chemical stability can be selected, for example, it can be at least one of glass fiber, non-woven fabric, polyethylene, polypropylene or polyvinylidene fluoride. The separator can be a single layer or a multilayer.

[0073] The present invention does not specifically limit the above-mentioned electrolyte, for example, an electrolyte including an organic solvent and an electrolyte salt is selected. Among them, the organic solvent is used as a medium for transmitting ions in the electrochemical reaction, and can be an organic solvent for battery electrolyte known in the art, such as: one or more of fluorinated carbonates, fluorinated carboxylates, non-fluorinated carbonates, fluorinated carbonates, non-fluorinated carboxylates, and fluorinated carboxylates; the electrolyte salt is used as a source of ions, and can be used as an electrolyte salt for battery electrolyte known in the art, such as: one or more of lithium hexafluorophosphate, bistrifluoromethylsulfonyl imide, and bis(fluorosulfonyl)imide lithium.

[0074] In an optional embodiment, the diaphragm is a polypropylene film. When the active layer of the electrode sheet contains the hot melt adhesive, the hot melt adhesive has a good bonding effect, and the thermopiezoelectric electrode sheet and the diaphragm are better bonded, and the use of a polypropylene film (pure PP film) can avoid wrinkling, impedance increase, lithium precipitation and other problems.

[0075] In a fourth aspect, the present invention provides a battery pack comprising the battery described in the third aspect.

[0076] In a fifth aspect, the present invention provides an electrical device, comprising the battery described in the third aspect or the battery pack described in the fourth aspect.

[0077] It should be noted that the above-mentioned electronic equipment can be any conventional equipment that requires electricity, for example, including but not limited to computers, electric cars, air conditioners, refrigerators, washing machines, microwave ovens, printers, fax machines, etc.

[0078] The technical scheme of the present invention is further illustrated below in conjunction with specific examples. All parts, percentages and ratios described in the following examples are based on weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the instruments used in the examples are commercially available.

[0079] The porosity characterization method of the positive and negative / electrode sheets in the following embodiments: The porosity of the upper and lower layers can be compared through the CP-SEM of the electrode sheet.

[0080] Characterization method of hot melt adhesive content: After the battery is disassembled, the positive and negative electrodes and separators are immersed in DMC solution for 6 hours. After natural drying, they are cut and weighed to calculate the surface density of the positive electrode and separator to obtain A1 and B1. The hot melt adhesive on both sides of the positive electrode and the separator on the positive side are wiped off with a dust-free cloth moistened with NMP, and then weighed to calculate the surface density of the positive electrode and separator A2 and B2. (A1-A2+B1-B2) divided by the positive electrode surface density can be used to obtain the hot melt adhesive content in the positive electrode, and the negative electrode can be measured using the same method.

[0081] EVA hot melt adhesive CAS number: 24937-78-8;

[0082] Polyethylene wax CAS number: 9002-88-4;

[0083] Acrylic resin CAS number: 9007-20-9.

[0084] Example 1

[0085] This example provides a battery cell, including a positive electrode sheet, a negative electrode sheet and a polypropylene film;

[0086] The positive electrode sheet includes a copper foil and an active layer disposed on at least two surfaces of the copper foil, wherein the active layer includes two sub-active layers, which are the first sub-active layer, the second sub-active layer and the hot melt adhesive layer in order from the direction close to the copper foil to the direction away from the copper foil; the first sub-active layer includes 94.5 parts of lithium iron phosphate, 2 parts of PVDF, and 2 parts of conductive carbon black, and the D50 of lithium iron phosphate is 1 μm; the second sub-active layer includes 96 parts of lithium iron phosphate, 2 parts of polyvinylidene fluoride (PVDF), and 1.5 parts of conductive carbon black, and the D50 of lithium iron phosphate is 0.7 μm; the hot melt adhesive layer accounts for about 2.5% of the total mass of the active layer; the porosity of the sub-active layer gradually decreases from the surface close to the current collector to the surface away from the current collector; the total porosity of the active layer is 31.5%, the porosity of the first sub-active layer is 32.5%, and the porosity of the second sub-active layer is 29.5%. Among them, the microscopic characterization results of the first sub-active layer of the positive electrode are shown in Figure 2 The microscopic characterization results of the second sub-active layer are shown in Figure 3 As shown, by comparison Figure 2 and Figure 3 It can be seen that the first sub-active layer has more porosity and larger pore size than the second sub-active layer.

[0087] A negative electrode sheet, comprising an aluminum foil and an active layer disposed on at least two surfaces of the aluminum foil, wherein the active layer comprises two sub-active layers, which are, from close to the copper foil to far away from the copper foil, a first sub-active layer, a second sub-active layer and a hot melt adhesive layer; by weight, the first sub-active layer comprises 93 parts of artificial graphite, 2 parts of styrene-butadiene rubber (SBR), 2 parts of carboxymethyl cellulose (CMC), and 1.5 parts of conductive carbon black, and the artificial graphite has a D50 of 14.0 μm, a D10 of 8.6 μm, and a D90 of 23 μm; the second sub-active layer comprises 94.5 parts of artificial graphite, 2 parts of SBR, 2 parts of CMC, and 1 part of conductive carbon black, and the artificial graphite has a D50 of 11.8 μm, a D10 of 5.8 μm, and a D90 of 22.3 μm; the hot melt adhesive layer accounts for approximately 1.6% of the total weight of the active layer; the microscopic characterization results of the first sub-active layer of the negative electrode are shown in Figure 4 The microscopic characterization results of the second sub-active layer are shown in Figure 5 As shown, by comparison Figure 4 and Figure 5It can be seen that the first sub-active layer has more porosity and larger pore size than the second sub-active layer.

[0088] Preparation method of battery cell:

[0089] 1) Preparation of the positive electrode sheet, including the following steps:

[0090] The first sub-active layer slurry is prepared by mixing lithium iron phosphate, PVDF, conductive carbon black, EVA hot melt adhesive and solvent NMP in a mass ratio of 94.5:2:2:1.5:67;

[0091] The second sub-active layer slurry is prepared by mixing lithium iron phosphate, PVDF, conductive carbon black, EVA hot melt adhesive and solvent NMP in a mass ratio of 96:2:1.5:0.5:67;

[0092] The first sub-active layer slurry is coated on both sides of the current collector, and the second sub-active layer slurry is coated on the surface of the first sub-active layer, dried at 60°C, and rolled to obtain the positive electrode sheet (surface density = 400g / m 2 , compacted density = 2.46g / cm 3 ).

[0093] 2) Preparation of the negative electrode sheet, including the following steps:

[0094] Artificial graphite, SBR, CMC, conductive carbon black, EVA hot melt adhesive and solvent pure water are prepared into the first sub-active layer slurry in a mass ratio of 93:2:2:1.5:1.5:110;

[0095] The second sub-active layer slurry is prepared by mixing artificial graphite, SBR, CMC, conductive carbon black, EVA hot melt adhesive and solvent pure water in a mass ratio of 94.5:2:2:1:0.5:110;

[0096] The first sub-active layer slurry is coated on both sides of the current collector, and the second sub-active layer slurry is coated on the surface of the first sub-active layer. After drying at 60° C. and rolling, the negative electrode sheet (surface density = 183 g / m 2 , compacted density = 1.54g / cm 3 ).

[0097] 3) Preparation of electrolyte, comprising the following steps:

[0098] After mixing ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC) in a mass ratio of 2:5:3, 5% of the total mass of the electrolyte was added with fluoroethylene carbonate (FEC) and 13% of the total mass of the electrolyte was added with lithium hexafluorophosphate (LiPF6), and the mixture was stirred to obtain an electrolyte.

[0099] 4) Assembling battery cells: The positive and negative electrodes and separators are assembled through stacking and hot pressing at 100°C, followed by shelling, baking, liquid injection, formation and capacity separation to obtain qualified battery cells.

[0100] Example 2-13

[0101] The provided battery cell is substantially the same as that in Example 1, except that the parameters shown in Table 1 are changed.

[0102] Comparative Example 1

[0103] This example provides a battery cell, including a positive electrode sheet, a negative electrode sheet and a polypropylene film; the positive electrode sheet includes a copper foil and an active layer arranged on both surfaces of the copper foil, and the active layer includes 96% lithium iron phosphate, 2% PVDF, and 2% conductive carbon black by mass percentage, and the D50 of the lithium iron phosphate is 0.8μm, D10 is 0.35μm, and D90 is 4.5μm.

[0104] The negative electrode sheet includes an aluminum foil and an active layer arranged on two surfaces of the aluminum foil. By mass percentage, the active layer includes 95% graphite, 2% SBR, 2% CMC, and 1% conductive carbon black. The graphite has D50=14.0μm, D10=8.6μm, and D90=23.4μm.

[0105] Preparation method of battery cell:

[0106] 1) Preparation of the positive electrode sheet, including the following steps:

[0107] Active layer slurry is prepared by mixing lithium iron phosphate, PVDF, conductive agent and solvent NMP in a mass ratio of 96:2:2:67;

[0108] The active layer slurry was coated on both sides of the current collector, dried at 110°C, and rolled to obtain the positive electrode sheet (surface density = 400 g / m 2 , compacted density = 2.46g / cm 3 );

[0109] 2) Preparation of the negative electrode sheet, including the following steps:

[0110] The active layer slurry is prepared by mixing graphite, SBR, CMC, conductive agent and solvent pure water in a mass ratio of 95:2:2:1:100;

[0111] The active layer slurry was coated on both sides of the current collector, dried at 100°C, and rolled to obtain the positive electrode sheet (surface density = 182 g / m 2 , compacted density = 1.54g / cm 3 ).

[0112] 3) Preparation of electrolyte, comprising the following steps:

[0113] After mixing ethylene carbonate (EC): diethyl carbonate (DEC): propylene carbonate (PC) in a mass ratio of 2:5:3, 5% of the total mass of the electrolyte was added with fluoroethylene carbonate (FEC) and 13% of the total mass of the electrolyte was added with lithium hexafluorophosphate (LiPF6), and the mixture was stirred to obtain an electrolyte.

[0114] 4) Assembling batteries: The positive and negative electrodes and separators are stacked, hot pressed at 100°C, then shelled, baked, injected, formed, and capacity divided to obtain qualified battery cells.

[0115] Comparative Example 2

[0116] The provided battery cell is substantially the same as that in Example 1, except that no hot melt adhesive is included, and the first sub-active layer of the positive electrode sheet includes 96% lithium iron phosphate, 2% PVDF, 2% conductive carbon black, and the D50 of the lithium iron phosphate is 1.0 μm, D10 is 0.5 μm, and D90 is 6.5 μm; the second sub-active layer includes 96% lithium iron phosphate, 2% PVDF, 2% conductive carbon black, and the D50 of the lithium iron phosphate is 0.8 μm, D10 is 0.35 μm m, D90 = 4.5μm; the first sub-active layer of the negative electrode sheet includes 94.5% graphite, 2% SBR, 2% CMC, 1.5% conductive agent, D50 of graphite = 14.0μm, D10 = 8.6μm, D90 = 23μm; the second sub-active layer includes 95% graphite, 2% SBR, 2% CMC, 1% conductive agent, D50 of graphite = 11.8μm, D10 = 5.8μm, D90 = 22.3μm. The porosity of the second sub-active layer is lower than that of the first sub-active layer.

[0117] Test Case

[0118] The battery cells of each example were subjected to the following tests, and the results are recorded in Table 2.

[0119] SOC deviation test: The battery cell is charged at a constant current of 2C to 60% SOC, then the battery cell is disassembled, the negative electrode sheet is cut, stacked with the diaphragm and the lithium metal sheet, assembled in a glove box, and the electrolyte is injected to prepare a button battery. Among them, the electrolyte is an organic solvent containing lithium salt, wherein the lithium salt concentration is 1 mol / L, and the mass ratio of the organic solvent is 3:6:1 of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate and 1% mass fraction of vinylene carbonate. Each test battery prepared in the above steps is connected to the battery cabinet, left for 0.5h, and charged to 1.5V at a constant current of 0.1C. The voltage and capacity data are derived to make dV / dQ and capacity curves. The upper SOC state can be calculated based on the third-order capacity, and the lower SOC state can be calculated based on the average value of 60% SOC. The upper SOC minus the lower SOC can obtain the SOC deviation.

[0120] Cycle capacity retention rate: Under constant temperature of 25℃, charge at 1C constant rate to 3.6V, leave for 0.5h, discharge at 1C constant rate to 2V, leave for 0.5C, repeat the above steps 1000 times for cycle test.

[0121] Table 1:

[0122]

[0123]

[0124] Table 2:

[0125]

[0126] It can be seen from Table 2 that, compared with the comparative example, the SOC deviation of the negative electrode sheet of the embodiment is smaller in the thickness direction, and the cycle stability of the battery cell is more excellent.

[0127] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A battery cell comprising a stacked electrode sheet and a separator, wherein the electrode sheet comprises a current collector and an active layer disposed on at least one surface of the current collector, characterized in that: The active layer includes m sub-active layers; each sub-active layer includes a plurality of pores, at least some of which are generated by the liquefied outflow of hot melt adhesive, and the porosity of the sub-active layer gradually decreases from the surface close to the current collector to the surface away from the current collector, wherein m is an integer greater than or equal to 2.

2. The battery cell according to claim 1, characterized in that: A hot melt adhesive layer is further provided on the upper surface of the sub-active layer away from the current collector, and the electrode sheet close to the diaphragm is adhesively connected to the diaphragm through the hot melt adhesive layer.

3. The battery cell according to claim 2, characterized in that: The mass of the hot melt adhesive layer is 0.2-4wt% of the total mass of the active layer.

4. The battery cell according to any one of claims 1 to 3, characterized in that: The electrode sheet is a positive electrode sheet and / or a negative electrode sheet and / or, and the separator is a polypropylene film.

5. The battery cell according to any one of claims 1 to 4, characterized in that: In the direction from the surface away from the current collector to the surface close to the current collector, the porosity of the first sub-active layer is 22%-36%, and the porosity of the n-1th sub-active layer and the porosity of the nth sub-active layer are in a ratio of 22-36:20-34; n is any integer from 2 to m.

6. The battery cell according to claim 5, characterized in that: The average pore diameter of the (n-1)th sub-active layer is greater than the average pore diameter of the nth sub-active layer.

7. The battery cell according to claim 5 or 6, characterized in that: Each sub-active layer includes an active material; the active layer satisfies the following formula 1: 1.2≤a / c≤4 Formula 1; Wherein, c is the D50 of the active material in the n-th sub-active layer, and a is the D50 of the active material in the n-1-th sub-active layer.

8. The battery cell according to claim 5 or 6, characterized in that: Each of the sub-active layers includes a conductive agent; wherein the mass ratio of the conductive agent in the nth sub-active layer to the conductive agent in the n-1th sub-active layer is 0.3-1:0.5-2.

9. A method for preparing a battery cell according to any one of claims 1 to 8, characterized in that: The following steps are involved: Sequentially laminating and coating m layers of sub-active layer slurry comprising active material, binder, hot melt adhesive, solvent and conductive agent on at least one surface of a current collector; drying and rolling at a temperature lower than the melting point of the hot melt adhesive to obtain the electrode sheet; The electrode sheets and the diaphragm are stacked and assembled, and hot-pressed at a temperature greater than the melting point of the hot melt adhesive, followed by casing, baking, liquid injection, chemical formation, and volume separation to obtain the battery cell; Among them, in the slurry of the first sub-active layer, the mass proportion of hot melt adhesive is 0.25-2%, and the hot melt adhesive content ratio of the slurry of the n-1th sub-active layer and the slurry of the nth sub-active layer is 0.25-2:0-1.

10. The preparation method according to claim 9, characterized in that: The melting point of the hot melt adhesive is 60-120°C.

11. A battery comprising the battery cell according to any one of claims 1 to 8.

12. A battery pack, characterized in that: Comprising the battery of claim 11.

13. An electrical equipment, characterized in that: Comprising the battery according to claim 11 or the battery pack according to claim 12.

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