Lithium ion battery

Abstract

The invention provides a lithium ion battery, belongs to the technical field of lithium ion batteries, and solves the problem that high and low temperature performances of a lithium ion battery in the prior art are difficult to consider. The lithium ion battery comprises a positive plate, a negative plate and a diaphragm located between the positive plate and the negative plate, the negative plate comprises a negative current collector and a negative active layer arranged on at least one side of the negative current collector, and the diaphragm comprises a base film and a first adhesive layer coated on the surface of one side of the base film; the surface of the side, provided with the first adhesive layer, of the diaphragm faces the negative plate, the surface, provided with the first adhesive layer, of the diaphragm comprises a first granular fluorine-containing binder, and the average particle size of secondary particles of the first granular fluorine-containing binder is D1; the negative electrode active layer comprises a second granular fluorine-containing binder, and the average particle size of primary particles of the second granular fluorine-containing binder is D2; d1 and D2 meet the condition that D1 / D2 is greater than or equal to 40 and less than or equal to 2000. The battery has good high-temperature performance and low-temperature performance at the same time.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a lithium-ion battery. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in fields such as portable electronic devices, electric vehicles, and energy storage power stations due to their advantages such as high energy density and long cycle life. However, the performance of existing lithium-ion batteries in extreme temperature environments is still not satisfactory. Especially under high-temperature and low-temperature conditions, key indicators such as capacity and cycle life will show a significant decline, severely limiting the application range of lithium-ion batteries.

[0003] In the prior art, improvement solutions for the low-temperature or high-temperature performance of lithium-ion batteries often can only solve a certain problem singly and are difficult to achieve the balance of both. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the problem that it is difficult to balance the high-temperature and low-temperature performance of lithium-ion batteries in the prior art, so as to provide a lithium-ion battery.

[0005] For this purpose, the present invention provides the following technical solutions.

[0006] A lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The separator includes a base film and a first adhesive layer coated on one surface of the base film. The surface of the separator provided with the first adhesive layer faces the negative electrode sheet. The surface of the separator provided with the first adhesive layer includes first granular fluorine-containing binders, and the average particle size of the secondary particles of the first granular fluorine-containing binders is D1;

[0007] The negative electrode active layer includes second granular fluorine-containing binders, and the average particle size of the primary particles of the second granular fluorine-containing binders is D2;

[0008] The relationship between D1 and D2 satisfies: 40 ≤ D1 / D2 ≤ 2000;

[0009] In a possible implementation manner, D1 satisfies: 10 μm ≤ D1 ≤ 100 μm;

[0010] In a possible implementation manner, D2 satisfies: 50 nm ≤ D2 ≤ 500 nm.

[0011] In a possible implementation manner, the negative electrode active layer further includes a negative electrode active material, and the ratio of D1 to the median particle size D50 of the negative electrode active material satisfies 1.3 ≤ D1 / D50 ≤ 13;

[0012] In a possible implementation, 2 ≤ D1 / D50 ≤ 7;

[0013] In a possible implementation, 2 μm ≤ D50 ≤ 15 μm, preferably 4 μm ≤ D50 ≤ 10 μm;

[0014] In a possible implementation, the negative electrode active material includes a carbon-based active material.

[0015] In a possible implementation, the average particle size D1 of the secondary particles of the first particulate fluorine-containing binder in the first adhesive layer and the thickness H of the first adhesive layer satisfy the relationship: 5 ≤ D1 / H ≤ 100;

[0016] In a possible implementation, 0.5 μm ≤ H ≤ 4 μm, preferably 1 μm ≤ H ≤ 2 μm.

[0017] In a possible implementation, the average particle size D1 of the secondary particles of the first particulate fluorine-containing binder in the first adhesive layer and the coverage rate X of the first adhesive layer on the separator satisfy: 15 ≤ D1 / X ≤ 400;

[0018] In a possible implementation, 34 ≤ D1 / X ≤ 400;

[0019] In a possible implementation, the coverage rate X of the first adhesive layer on the separator satisfies 15% ≤ X ≤ 80%, preferably, 30% ≤ X ≤ 60%.

[0020] In a possible implementation, 60 ≤ D1 / D2 ≤ 1000;

[0021] In a possible implementation, the first particulate fluorine-containing binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin;

[0022] In a possible implementation, the second particulate fluorine-containing binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0023] In a possible implementation, the thermal shrinkage rate Y of the width direction of the separator when heated at 90 °C for 2 h is ≤ 3%;

[0024] In a possible implementation, the thickness of the base film is 5 μm to 16 μm.

[0025] In a possible implementation, the compaction density PD of the negative electrode active layer satisfies 1.35 g / cm 3 ≤PD≤1.75 g / cm 3 ;

[0026] In a possible implementation, the porosity Z of the negative electrode sheet satisfies 5% ≤ Z ≤ 40%, and optionally, 28% ≤ Z ≤ 35%;

[0027] In a possible implementation, the negative electrode active layer further includes a thickening agent. Optionally, the thickening agent includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyacrylic acid, branched sodium polyacrylate, and branched lithium polyacrylate.

[0028] In a possible implementation, the negative electrode active layer further includes a conductive agent, and the conductive agent satisfies at least one of the following conditions:

[0029] (1) The particle size of the secondary particles of the conductive agent is D3, and 0.2 μm ≤ D3 ≤ 5 μm;

[0030] Optionally, 0.5 μm ≤ D3 ≤ 3 μm.

[0031] (2) The mass content of the conductive agent in the negative electrode active layer is B1, and 0.1% ≤ B1 ≤ 5%;

[0032] Optionally, 0.5% ≤ B1 ≤ 3%;

[0033] (3) The conductive agent includes one or more of carbon black, carbon spheres, carbon fibers, graphite, and graphene;

[0034] Optionally, the carbon black includes one or more of acetylene black and furnace black.

[0035] The technical solution of the present invention has the following advantages:

[0036] A lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The separator includes a base film and a first adhesive layer coated on one surface of the base film. The side surface of the separator provided with the first adhesive layer faces the negative electrode sheet. The surface of the separator provided with the first adhesive layer includes a first granular fluorine-containing binder, and the average particle size of the secondary particles of the first granular fluorine-containing binder is D1; the negative electrode active layer includes a second granular fluorine-containing binder, and the average particle size of the primary particles of the second granular fluorine-containing binder is D2; the following is satisfied between D1 and D2: 40 ≤ D1 / D2 ≤ 2000. When the value of D1 / D2 is too small, that is, when the average particle size of the secondary particles of the first granular fluorine-containing binder in the separator is small and the primary particles of the second granular fluorine-containing binder in the negative electrode sheet are large, the small average particle size of the secondary particles means that the average bonding area at the interface between the separator and the negative electrode sheet is small, and the large primary particles of the second granular fluorine-containing binder in the negative electrode sheet mean that the effective bonding sites inside the negative electrode active layer become fewer. In this way, both the bonding at the interface between the separator and the negative electrode sheet and the bonding inside the negative electrode sheet are poor, and the improvement effect at high temperature is very small. When the value of D1 / D2 is too large, although the bonding at the interface between the separator and the electrode sheet and the bonding inside the electrode sheet are both good, the too large average particle size of the secondary particles may cause the pores of the separator to be blocked and the interface impedance to increase, which will weaken the low-temperature performance and cause lithium plating. Therefore, in this application, by providing binders with the same chemical composition on the surface where the first adhesive layer and the negative electrode active layer are in contact and controlling D1 / D2 within a suitable range, good interfacial bonding force and liquid retention property at the interface can be achieved, and while not sacrificing the low-temperature performance, the high-temperature performance is improved, achieving the balance between the low-temperature performance and the high-temperature performance of the battery. Description of the Drawings

[0037] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0038] Figure 1 SEM image of the surface of the separator with a granular PVDF first adhesive layer provided in Example 1;

[0039] Figure 2 SEM image of the negative electrode sheet in Example 1. Detailed Embodiments

[0040] The following embodiments are provided to better understand the present invention further. It is not limited to the best mode described, and does not limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts falls within the protection scope of the present invention.

[0041] For those embodiments in which specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained commercially.

[0042] The present application provides a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector. The separator includes a base film and a first adhesive layer coated on one surface of the base film. The side surface of the separator provided with the first adhesive layer faces the negative electrode sheet. The surface of the separator provided with the first adhesive layer includes a first granular fluorine-containing binder, and the average particle size of the secondary particles of the first granular fluorine-containing binder is D1;

[0043] The negative electrode active layer includes a second granular fluorine-containing binder, and the average particle size of the primary particles of the second granular fluorine-containing binder is D2;

[0044] The relationship between D1 and D2 satisfies: 40 ≤ D1 / D2 ≤ 2000.

[0045] The negative electrode active layer of this application uses a granular fluorinated binder, which can significantly improve the low-temperature performance of the battery. In lithium-ion batteries, the viscosity of the electrolyte increases under low-temperature conditions, and the diffusion rate of ions will decrease sharply, resulting in the inability to provide sufficient power when the battery needs to output a large current, such as when quickly starting a vehicle, seriously affecting the normal use of the device. Traditional negative electrode sheets mostly use styrene-butadiene rubber-based binders (SBR). Such binders will harden at low temperatures, causing the pore structure of the negative electrode sheet to shrink, weakening the bonding force between the negative electrode active material and the current collector, resulting in the shedding of the active layer, and even causing the electrode sheet to crack due to volume expansion / contraction during the charge and discharge process. The negative electrode active layer of this application uses a granular fluorinated binder. The granular fluorinated binder can still maintain a high elastic state at low temperatures and has better low-temperature toughness, making the negative electrode sheet not prone to cracking or shedding of the active layer due to reduced adhesion at low temperatures. The granular fluorinated binder can still maintain a high elastic state at low temperatures, so its liquid absorption capacity is good and it can combine more electrolyte. On the other hand, the granular fluorinated binder contains more C-F bonds. The electronegativity of F is very high, making C-F highly polar. The electron cloud density of the C-F bond is biased towards the F atom. And because F has lone pairs of electrons, it jointly causes F to weaken the solvation effect of the solvent on lithium ions during the lithium ion transport process, that is, it shows a faster ion transport rate and SEI film permeability. The combined effect of the two makes the electrolyte still able to penetrate more easily into the interior of the negative electrode sheet even at low temperatures, reducing the lithium ion diffusion resistance and increasing the lithium ion transport rate, thereby improving the low-temperature performance of the battery including this negative electrode sheet.

[0046] However, the granular fluorinated binder has an uncrosslinked linear structure, and its good affinity with the electrolyte at high temperatures will cause the granular fluorinated binder to swell greatly; the negative electrode sheet will expand and contract during the charge and discharge process. The large swelling of the granular fluorinated binder at high temperatures will make the expansion and contraction of the lithium-ion battery under high-temperature conditions more prominent, resulting in gaps at the interface between the separator and the negative electrode sheet. In addition, the large swelling of the fluorinated binder in the negative electrode sheet at high temperatures will also weaken the bonding force between the negative electrode active layer and the negative electrode current collector, leading to problems such as insufficient stability of the negative electrode sheet and increased side reactions during high-temperature storage / cycling of the battery containing this negative electrode sheet, manifested as increased loss of high-temperature storage capacity, low cycle retention rate, and easy gas generation.

[0047] Based on this, the present application provides a second particulate fluorine-containing binder in the first adhesive layer, and simultaneously controls the particle size ratio of the first particulate fluorine-containing binder to the second particulate fluorine-containing binder within the range of 40 to 2000, so that the lithium-ion battery can improve the high-temperature performance while not sacrificing the low-temperature performance, achieving a balance between the low-temperature performance and the high-temperature performance of the battery. There is the same composition of binder on the surface where the first adhesive layer contacts the negative electrode active layer, so that the separator and the negative electrode sheet have good compatibility. The entanglement force between the binder molecules in the separator and the negative electrode sheet is stronger, the interfacial adhesion force is strong, and the interface is more firm, thus avoiding the generation of gaps at the interface between the separator and the negative electrode sheet. The first particulate fluorine-containing binder in the first adhesive layer of the separator can generate a stronger binding force on the negative electrode sheet during the expansion of the battery, making it difficult for the negative electrode active layer to separate from the negative electrode current collector, better maintaining the interface integrity between the negative electrode sheet and the separator and between the negative electrode active layer and the negative electrode current collector at high temperature, improving the stability of the negative electrode sheet, and avoiding the problem of aggravated side reactions during high-temperature storage / cycling, thereby improving the high-temperature performance. However, when the value of D1 / D2 is too small, that is, when the average secondary particle size of the first particulate fluorine-containing binder in the separator is small and the primary particle size of the second particulate fluorine-containing binder in the negative electrode sheet is large, the small average secondary particle size means a small average adhesion area at the interface between the separator and the negative electrode sheet, and the large primary particle size of the second particulate fluorine-containing binder in the negative electrode sheet means fewer effective binding sites inside the negative electrode active layer. In this way, both the adhesion at the interface between the separator and the negative electrode sheet and the adhesion inside the negative electrode sheet are poor, and the high-temperature improvement effect is very small. When the value of D1 / D2 is too large, although the adhesion at the interface between the separator and the electrode sheet and the adhesion inside the electrode sheet are both good, the too large average secondary particle size may cause blockage of the separator pores and an increase in the interfacial impedance, which will weaken the low-temperature performance and cause lithium deposition. Therefore, by providing a binder with the same chemical composition on the surface where the first adhesive layer contacts the negative electrode active layer and controlling D1 / D2 within a suitable range, the present application can make the interfacial adhesion force and the liquid retention property of the interface good, improve the high-temperature performance while not sacrificing the low-temperature performance, and achieve a balance between the low-temperature performance and the high-temperature performance of the battery. Exemplarily, D1 / D2 can be 50, 100, 200, 300, 500, 700, 1000, 1200, 1500, 1800, 2000, etc. or values within the range formed by any two of the above-mentioned values.

[0048] In a possible implementation manner, 60 ≤ D1 / D2 ≤ 1000.

[0049] The average particle size D1 of the secondary particles of the first particulate fluorine-containing binder refers to the average value of the diameters of the aggregates of the first particulate fluorine-containing binder on the surface of the first adhesive layer. D1 is obtained by observing and measuring through SEM scanning electron microscope: Measure 10 regions, respectively obtain the average particle size of the first particulate fluorine-containing binder in the 10 regions, and then take the average value of the 10 average particle sizes, which is D1.

[0050] The average particle size D1 of the primary particles of the second particulate fluorine-containing binder refers to the average value of the diameters of the second particulate fluorine-containing binder particles in the negative electrode active layer. D1 is obtained by observing and measuring through SEM (scanning electron microscope): different regions are selected, 10 regions are measured, and the average particle sizes of the second binder in the 10 regions are obtained respectively. Then, the average value of the 10 average particle sizes is taken to obtain D1.

[0051] In a possible implementation manner, 10 μm ≤ D1 ≤ 100 μm. Exemplarily, D1 can be 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, etc., or values within the range composed of any two of the above values.

[0052] In a possible implementation manner, 50 nm ≤ D2 ≤ 500 nm. Exemplarily, D2 can be 50 nm, 100 nm, 150 nm, 0.2 μm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, etc., or values within the range composed of any two of the above values.

[0053] In a possible implementation manner, the first particulate fluorine-containing binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin;

[0054] In a possible implementation manner, the second particulate fluorine-containing binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0055] In a possible implementation manner, the negative electrode active layer further includes a negative electrode active material, and the ratio of D1 to the median particle size D50 of the negative electrode active material satisfies 1 ≤ D1 / D50 ≤ 13.

[0056] By controlling the ratio of the average particle size D1 of the first particulate fluorine-containing binder secondary particles in the separator to the median particle size D50 of the negative electrode active material within an appropriate range, the lithium ion transport rate in the first adhesive layer is matched with the lithium deintercalation / insertion rate on the surface layer of the negative electrode active material, thereby avoiding lithium deposition and improving the low-temperature performance. If D1 / D50 is too large, that is, D1 is relatively large while D50 is relatively small, the ion transport path becomes longer due to the relatively large D1, and there is a risk of pore blockage in the separator, resulting in a slow lithium ion transport rate on the separator side. However, since D50 is relatively small, the lithium ion transport rate in the negative electrode active material is fast, and the transport rates at the two locations do not match, reducing the charge and discharge rate of the battery and deteriorating the low-temperature performance. If D1 / D50 is too small, that is, D1 is relatively small while D50 is relatively large, the lithium ion transport rate on the separator side is fast, while the transport rate at the negative electrode active material is slow, and the transport rates at the two locations also do not match, which will cause lithium deposition and lead to the deterioration of the battery's low-temperature performance. Exemplarily, D1 / D50 can be 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 9.9, etc. or a value within the range formed by any two of the above values. Optionally, 2≤D1 / D50≤7.

[0057] In one possible implementation, 2μm≤D50≤15μm.

[0058] By controlling the median particle size D50 of the negative electrode active material within the range of 2μm to 15μm and acting together with the first particulate fluorine-containing binder in the first adhesive layer of the separator, the battery can achieve both high-temperature and low-temperature performance. If D50 is too small, although smaller negative electrode active material particles can provide better ion transport and active sites at low temperatures, at high temperatures, due to the smaller particle size of the negative electrode active material, the larger contact area with the electrolyte will trigger side reactions, resulting in problems such as the repeated decomposition and reconstruction of the SEI film at high temperatures, leading to a reduction in the high-temperature performance of the battery; conversely, if D50 is too large, it will cause the negative electrode active material to be crushed and cracked during the rolling of the negative electrode sheet, thereby deteriorating the high-temperature and low-temperature performance of the battery. Exemplarily, D50 can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, etc. or a value within the range formed by any two of the above values. Optionally, 4μm≤D50≤10μm.

[0059] The test method for the particle size D50 of the negative electrode active material includes: after discharging the lithium-ion battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking it in dimethyl carbonate (DMC) solvent at 60 °C for 5 h, and then rinsing it with DMC to remove the lithium salts attached to the negative electrode sheet. After calcining in a tube furnace at 500 °C for 3 h under a nitrogen atmosphere, the residual material of the negative electrode active layer is gently scraped off from the surface of the negative electrode current collector to obtain a composite material, and measured using a Malvern particle size analyzer. The test steps are as follows: Disperse the composite material in deionized water containing a dispersant (such as nonylphenol polyoxyethylene ether, with a content of 0.02 - 0.03 wt%) to form a mixture, ultrasonicate the mixture for 2 minutes, and then put it into the Malvern particle size analyzer for testing.

[0060] In a possible implementation, the negative electrode active material includes a carbon-based active material, and the carbon-based active material in the negative electrode active material layer includes at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon. Optionally, the raw material of the graphite is selected from one or more of petroleum coke, needle coke, and pitch coke single particles.

[0061] In a possible implementation, the average particle size D1 of the secondary particles of the first particulate fluorine-containing binder in the first adhesive layer and the thickness H of the first adhesive layer satisfy the relationship: 5 ≤ D1 / H ≤ 100.

[0062] Reasonable control of D1 / H can control the distribution uniformity of fluoropolymer particles, optimize the adhesion between the adhesive layer and the base film, so as to form good interfacial contact between the separator and the negative electrode, and improve the cycle performance of the battery. When D1 is large, that is, the secondary particles of the first particulate fluorine-containing binder in the first adhesive layer are larger, at this time, the coating thickness of the first adhesive layer also needs to change accordingly. This is because, at the same adhesive layer coverage rate, the larger D1 is, the fewer the number of the first particulate fluorine-containing binders is, and the bonding sites are sparser. At this time, it is necessary to appropriately increase the coating thickness of the first adhesive layer to meet the interfacial bonding. Similarly, when D1 is small, the coating thickness of the adhesive layer also needs to change accordingly, while avoiding D1 being too small to block the pores of the separator while satisfying the bonding between the separator and the adhesive. However, D1 / H cannot be too large. If it is too large, it will increase the risk of separator pore blockage, and at the same time, it may also cause an increase in the contact area between the particulate fluorine-containing binder and the electrolyte, resulting in risks such as swelling and gas generation of fluoropolymer particles at high temperatures, deteriorating the cycle performance. If D1 / H is too small, it will lead to insufficient bonding between the separator and the negative electrode, and also cause the battery to expand more severely, increasing the lithium ion transmission path and deteriorating the cycle performance. Exemplarily, D1 / H can be 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, etc. or values within the range composed of any two of the above values.

[0063] In a possible implementation, 0.5μm ≤ H ≤ 4μm.

[0064] The thickness of the first adhesive layer has a certain influence on the adhesion between the separator and the negative electrode sheet. In this application, the thickness H of the first adhesive layer is controlled within the range of 0.5μm to 4μm, which can well balance the relationship between interface adhesion and impedance, thereby improving the high-temperature performance of the battery while having good low-temperature performance. If H is too small, the interface adhesion between the separator and the negative electrode sheet is poor, and the improvement effect of the high-temperature performance is not obvious. If H is too large, the interface adhesion between the separator and the electrode sheet is good, but the impedance will increase, the battery expansion will increase, resulting in a decrease in the low-temperature performance of the battery. When the thickness of the first adhesive layer is within the range of 0.5μm to 4μm, the adhesion effect between the first granular fluorinated binder in the first adhesive layer and the second granular fluorinated binder in the negative electrode sheet is better while avoiding an increase in impedance. Exemplarily, H can be 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, etc. or a value within the range composed of any two of the above values. Optionally, 1μm ≤ H ≤ 2μm.

[0065] The test methods for the thickness H of the first adhesive layer and the base film thickness include: after discharging the lithium-ion battery to 0% SOC, disassembling and taking out the separator, observing the cross-section through CP slicing, and the thickness H of the first adhesive layer and the thickness of the base film can be measured.

[0066] In a possible implementation, the average particle size D1 of the secondary particles of the first granular fluorinated binder in the first adhesive layer and the coverage rate X of the first adhesive layer on the separator satisfy: 15 ≤ D1 / X ≤ 400.

[0067] Controlling D1 / X can avoid local electrolyte infiltration differences caused by uneven coverage, improve battery consistency, and at the same time, while ensuring sufficient electrolyte infiltration, avoid side reactions caused by excessive pores; if D1 / X is too small, the first granular fluorinated binder particles are closely packed, and the porosity is relatively low, which may limit the uniformity of electrolyte diffusion, resulting in local liquid deficiency and lithium plating in the battery, deteriorating the battery cycle performance, and even increasing the risk of battery short circuit; if D1 / X is too large, the first granular fluorinated binder particles are sparsely distributed, which may lead to insufficient local coverage, resulting in poor adhesion between the local separator and the negative electrode sheet, easily causing the negative electrode and the separator to lose contact during cycling, and the lithium ion transmission rate decreases, reducing the low-temperature power and cycle performance of the battery (because the lithium ion electrolyte becomes viscous at low temperatures, and the lithium ion transmission rate is slower than at room temperature, and the negative electrode and the separator lose contact, making the lithium ion transmission even slower). Exemplarily, D1 / X can be 15, 20, 30, 35, 50, 100, 200, 300, 400, etc. or a value within the range composed of any two of the above values.

[0068] In a possible implementation manner, the coverage rate X of the first adhesive layer on the diaphragm satisfies 15%≤X≤80%.

[0069] Coverage refers to the ratio of the area of ​​the first adhesive layer on the diaphragm per unit area to the area of ​​the base film. The coverage X of the first adhesive layer is in the range of 15% to 80%. Combined with the first granular fluorine-containing binder and the second granular fluorine-containing binder, the interfacial adhesion between the diaphragm and the negative electrode can be ensured while avoiding excessive impedance, so that the battery can take into account both high temperature performance and low temperature performance. If X is too small, the interfacial adhesion is poor, and the effect of improving high temperature performance is not obvious. If X is too large, the interfacial adhesion is strong, but the interfacial impedance will increase, resulting in the degradation of low temperature performance. Exemplarily, X can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, 80%, etc. or a value within the range of any two of the above values. Optionally, 30%≤X≤60%.

[0070] The coverage rate X of the first adhesive layer: observe through SEM scanning electron microscope, select one area to take a picture, and then use the image processing software ZEN Analyzer that comes with the SEM to divide the adhesive layer area of ​​the selected area (due to the large difference in the morphology of the adhesive layer and the base film, it is easy to identify) by the total area of ​​the selected area to obtain the coverage rate X1. Test 10 areas and calculate the average value X=(X1+X2+...X10) / 10.

[0071] In a possible implementation manner, the thermal shrinkage rate Y of the separator in the width direction when heated at 90° C. for 2 hours is ≤3%.

[0072] The diaphragm will shrink at high temperatures, that is, the diaphragm shrinks during high-temperature storage, which leads to the risk of failure of the interface bonding between the diaphragm and the negative electrode, and excessive thermal shrinkage will cause short circuit risk. Lithium-ion batteries with Y≤3% have excellent high-temperature storage performance.

[0073] In a possible implementation, the base film has a thickness of 5 μm to 16 μm. For example, the base film may have a thickness of 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, etc., or a value within a range formed by any two of the above values.

[0074] In a possible implementation, the compaction density PD of the negative electrode active layer satisfies 1.35≤PD≤1.75; if the compaction density PD of the negative electrode active layer is too low, the contact between the negative electrode active material particles is insufficient, the electron conductivity is poor, and if the compaction density is too high, the porosity is low, and the electrolyte wettability is poor. In the range of 1.35≤PD≤1.75, the high temperature cycle and storage performance are excellent.

[0075] The compaction density PD of the negative electrode active layer: Take the negative electrode sheet, punch out circular sheets of a fixed size with a punching machine, weigh them, and subtract the weight of the copper foil (which can be calculated based on the density, thickness, and area of copper) to obtain the weight of the active layer. Finally, divide by the area and then divide by two to obtain the areal density of the active layer on one side of the electrode sheet. Divide the areal density of the negative electrode active layer by the thickness of the negative electrode active layer to obtain the compaction density.

[0076] In a possible implementation, the porosity Z of the negative electrode sheet satisfies 5% ≤ Z ≤ 40%, optionally, 28% ≤ Z ≤ 35%; exemplarily, Z can be 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40%. When the value of Z is too small, the liquid retention capacity of the negative electrode sheet is low, and there is a risk of electrolyte drying under high-temperature conditions, that is, there is a risk of failure at high temperature. When the value of Z is too large, although the liquid retention capacity is high, the contact points between the negative electrode active material particles in the negative electrode sheet become smaller, and the contact resistance becomes larger, which will also lead to poor high-temperature performance. Satisfying 5% ≤ Z ≤ 40% can ensure that the battery has good high-temperature performance.

[0077] The test method for the porosity of the negative electrode sheet: Utilize the phenomenon that a gas (such as nitrogen or carbon dioxide) is adsorbed on the surface of a porous material under different pressures, and calculate the porosity by analyzing the gas adsorption amount. Steps: Place the negative electrode sheet sample in a vacuum environment; introduce the gas under different pressures and measure the gas adsorption amount; calculate the pore volume and porosity according to the adsorption curve.

[0078] In a possible implementation, the negative electrode active layer further includes a thickening agent. Optionally, the thickening agent includes at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, branched sodium polyacrylate, and branched lithium polyacrylate.

[0079] In a possible implementation, the negative electrode active layer further includes a conductive agent, and the particle size of the secondary particles of the conductive agent is D3, where 0.2 μm ≤ D3 ≤ 5 μm.

[0080] If D3 is too small, the conductive agent is in an over-dispersed state, making it difficult to build an effective conductive network. The liquid absorption efficiency of the conductive agent is low, resulting in a low liquid retention capacity of the negative electrode sheet, poor ion and electron transport capabilities, and easy electrolyte dry-out at high temperatures, weakening the high-temperature performance. If D3 is too large, the dispersion distribution of the conductive agent is uneven, which will lead to an uneven construction of the conductive network and uneven current density, deteriorating the high-temperature performance of the battery. Satisfying 0.2 μm ≤ D3 ≤ 5 μm can construct an effective and uniform conductive network, facilitating electron and ion transport, thereby ensuring that the battery has good high-temperature performance. Combining the first particulate fluorine-containing binder in the first adhesive layer and the second particulate fluorine-containing binder in the negative electrode active layer and controlling the particle sizes of both enables the battery to simultaneously take into account high-temperature and low-temperature performance. Exemplarily, D3 can be 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, etc. or a value within the range composed of any two of the above values. Optionally, 0.5 μm ≤ D3 ≤ 3 μm.

[0081] The particle size D3 of the secondary particles of the conductive agent is observed and measured by SEM (scanning electron microscope): Different regions are selected, 10 regions are measured, and the average particle sizes are obtained respectively, and then the final average value is taken to obtain D3.

[0082] In a possible implementation manner, the mass content of the conductive agent in the negative electrode active layer is B1, and 0.1% ≤ B1 ≤ 5%.

[0083] If the content B1 of the conductive agent is too small, it is difficult to build an effective electron and ion transport network, affecting the performance of the battery under various temperature conditions; if the content B1 of the conductive agent is too large, it will affect the initial efficiency of the battery, increase the irreversible loss of active lithium, and in a high-temperature environment, the excessive conductive agent may catalyze more electrolyte decomposition, generating harmful gases such as HF, increasing the internal resistance of the battery, and deteriorating the high-temperature performance of the battery. The content of the conductive agent in this application satisfies 0.1% ≤ B1 ≤ 5%, enabling the lithium-ion battery to simultaneously take into account high and low temperature performance. Optionally, 0.5% ≤ B1 ≤ 3%; Exemplarily, B1 can be 0.1%, 0.2%, 0.5, 0.8%, 1%, 2%, 3%, 4%, 5%, etc. or a value within the range composed of any two of the above values.

[0084] In a possible implementation manner, the conductive agent includes one or more of carbon black, carbon spheres, carbon fibers, graphite, and graphene;

[0085] In a possible implementation manner, the conductive agent includes carbon black;

[0086] In a possible implementation manner, the carbon black includes one or more of acetylene black and furnace black.

[0087] In a possible implementation, the first granular PVDF is prepared by emulsion polymerization;

[0088] Optionally, the raw material monomers for preparing the first granular PVDF by emulsion polymerization include vinylidene fluoride;

[0089] Optionally, the raw material monomers for preparing the first granular PVDF by emulsion polymerization further include at least one of tetrafluoroethylene, hexafluoropropylene, propylene, and fluorinated acrylate;

[0090] In a possible implementation, the melting point of the first granular PVDF is 158 - 168 °C;

[0091] In a possible implementation, the second granular PVDF is prepared by emulsion polymerization;

[0092] Optionally, the raw material monomers for preparing the second granular PVDF by emulsion polymerization include vinylidene fluoride;

[0093] Optionally, the raw material monomers for preparing the second granular PVDF by emulsion polymerization further include at least one of tetrafluoroethylene, hexafluoropropylene, propylene, and fluorinated acrylate;

[0094] In a possible implementation, the melting point of the second granular PVDF is 160 - 175 °C.

[0095] This application also provides a method for preparing a secondary battery, including:

[0096] Preparing a negative electrode sheet:

[0097] (1) Mixing a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, a thickening agent, and a first solvent uniformly to obtain a negative electrode slurry;

[0098] (2) Coating the negative electrode slurry on the surface of a negative electrode current collector and baking to obtain the negative electrode sheet.

[0099] Preparing a positive electrode sheet: Mixing a positive electrode active material, a positive electrode conductive agent, a positive electrode binder, and a second solvent to prepare a positive electrode slurry, and coating the positive electrode slurry on a positive electrode current collector and drying to obtain the positive electrode sheet.

[0100] Assembling the positive electrode sheet, the negative electrode sheet, and a separator into an electric core by the commonly used winding or stacking method in the industry, then encapsulating with an aluminum-plastic film, and successively going through baking, injecting electrolyte, forming, and second sealing processes to obtain a lithium-ion battery.

[0101] In a possible implementation, the mass ratio of the negative electrode active material: the negative electrode binder PVDF: the conductive agent: the thickening agent = 90 to 98: 1 to 5: 1 to 5: 1 to 5. Exemplarily, the mass ratio of the negative electrode active material, the negative electrode binder PVDF, carbon black, and the thickening agent is 95: 1.8: 2: 1.2.

[0102] Optionally, the thickening agent includes one or more of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose.

[0103] Optionally, the negative electrode current collector is a single-sided polished copper foil, a double-sided polished copper foil, or a porous copper foil.

[0104] Optionally, the first solvent includes water.

[0105] Optionally, the positive electrode active material includes at least one of lithium iron phosphate LFP, ternary nickel cobalt manganese oxide NCM, lithium cobalt oxide LCO, and lithium iron manganese phosphate LFMP.

[0106] Optionally, the positive electrode conductive agent includes at least one of conductive carbon black, conductive graphite, multi-walled carbon nanotubes, single-walled carbon nanotubes, conductive carbon fibers, and graphene.

[0107] Optionally, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF) and polyimide.

[0108] Optionally, the positive electrode current collector is an aluminum foil or a carbon-coated aluminum foil.

[0109] Optionally, the second solvent includes one or more of N-methylpyrrolidone (NMP) and N-ethylpyrrolidone.

[0110] Example 1

[0111] This example provides a lithium-ion battery, and its preparation method includes:

[0112] Preparing a positive electrode sheet: adding the binder PVDF and conductive carbon black to NMP, after stirring evenly, adding the positive electrode active material lithium iron phosphate, and stirring to obtain a uniformly dispersed positive electrode slurry, wherein the solid components include 96.5 wt% of lithium iron phosphate, 1.5 wt% of conductive carbon black, and 2 wt% of PVDF. The solid content of the positive electrode slurry is 60 wt%, and the viscosity is 10300 mPa·s. Coating the positive electrode slurry evenly on both sides of the aluminum foil, drying at 100 to 130 °C for 4 h, and compacting it with a roll press, the compacting density is 2.4 g / cm 3 , obtaining a positive electrode sheet, and the resistance of the positive electrode sheet is 210 mΩ.

[0113] Preparation of the negative electrode sheet: An aqueous PVDF emulsion, graphite, a thickening agent sodium carboxymethyl cellulose (CMC), and conductive carbon black are mixed and dispersed in deionized water to obtain a negative electrode slurry, where the solid components include 95.2 wt% graphite, 1.5 wt% CMC, 1.8 wt% conductive carbon black, and 1.5 wt% PVDF. The solid content of the negative electrode slurry is 48.6 wt%, and the viscosity is 6780 mPa·s. The negative electrode slurry is uniformly coated on both sides of a copper foil, dried at 70 - 100 °C for 5 h, and compacted by a roll press, where the compaction density is 1.55 g / cm 3 , and the average value of the secondary particle size of the conductive carbon black in the obtained negative electrode sheet is 2 μm, and the porosity of the negative electrode sheet is 31%.

[0114] After welding the electrode tabs of the positive electrode sheet and the negative electrode sheet, together with an aqueous PVDF separator (including a PP-based film with a thickness of 9 μm; a ceramic layer is coated on one side of the base film with a thickness of 2 μm; adhesive layers are provided on both surface sides of the base film coated with the ceramic layer, and the average particle size of PVDF particles in the adhesive layer is 40 μm, the thermal shrinkage rate in the width direction at 90 °C for 2 h is 2.5%, the adhesive layer coverage rate is 30%, and the single-sided thickness of the adhesive layer is 1 μm), they are wound into an electric core, encapsulated, and then an electrolyte (including 1 mol / L lithium hexafluorophosphate in a binary solvent with a volume ratio of 1:1 of dimethyl carbonate and ethylene carbonate, and adding 10 wt% fluoroethylene carbonate as an additive) is injected, and after hot pressing and secondary sealing, a lithium-ion battery is obtained.

[0115] The SEM image of the aqueous PVDF separator in this example is as Figure 1 shown. Figure 1 When a granular PVDF first adhesive layer is provided on the surface of the separator, the granular PVDF is unevenly spaced on the surface of the separator. In another embodiment, the granular PVDF can also be arranged in an orderly and uniform manner on the surface of the separator. The SEM image of the negative electrode sheet is as Figure 2 shown.

[0116] The parameters of the lithium-ion batteries in each example and comparative example are shown in Table 1.

[0117] Table 1 Lithium-ion battery parameters

[0118]

[0119]

[0120] Lithium-ion battery performance test method:

[0121] (1) High-temperature performance:

[0122] At 45 °C, charge and discharge at 1C / 1C for 1000 cycles, and calculate the capacity retention rate and the expansion rate of the battery after 1000 cycles.

[0123] Measure the residual capacity and recovery capacity of the battery for 60 days at 60°C and 100% SOC, and calculate the percentage of residual capacity and the percentage of recovery capacity.

[0124] The residual capacity refers to the amount of electricity obtained by storing the battery at a fully charged state (i.e., 100% SOC) for 60 days and then discharging it to an empty state. The percentage obtained by dividing the residual capacity by the initial fully charged electricity is the percentage of residual capacity; the ratio of the electricity obtained by recharging the discharged-to-empty battery to the initial fully charged electricity is the recovery capacity retention rate.

[0125] (2) Low-temperature performance:

[0126] Power discharge at -30°C: 1) Under an environment of (25 ± 2)°C, discharge at a standard constant current until the discharge termination voltage, and set aside for 30 min; then charge at a standard constant current and constant voltage until the charge limit voltage, with a cut-off current of 0.05C, and set aside for 30 min; discharge at a standard constant current until the discharge termination voltage to obtain the actual capacity C0 of the battery cell; set aside for 30 min; 2) Charge at a standard constant current and constant voltage until the charge limit voltage, with a cut-off current of 0.05C, and discharge at 1C0 for 30 min., which is 50% SOC; 3) Let it stand at -30°C for 4 h; 4) Test the 10C0 discharge for 30 s, and then discharge at 10C0 for 30 s again after standing for 10 min., with a sampling accuracy of 10 ms. Record the voltage value at 2 s of the discharge time.

[0127] The test results are shown in Table 2.

[0128] Table 2 Performance of Lithium-ion Batteries

[0129]

[0130] As can be seen from Table 2, compared with the comparative example battery, the lithium-ion battery of the embodiment of the present application has a higher capacity retention rate and a lower battery swelling rate at 45°C, a higher percentage of residual capacity and a higher recovery capacity at 60°C; at the same time, the voltage value at 2 s of the discharge at -30°C is higher, that is, the low-temperature discharge power is improved, and the low-temperature discharge performance of the battery is enhanced. The battery of the present application has good high-temperature performance and low-temperature performance at the same time.

[0131] Obviously, the above embodiments are only examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector, the separator comprises a base film and a first glue layer coated on one side of the base film, and the side surface of the separator provided with the first glue layer is arranged toward the negative electrode sheet, characterized in that: The surface of the diaphragm provided with the first adhesive layer comprises a first granular fluorine-containing binder, and the average particle size of the secondary particles of the first granular fluorine-containing binder is D1; The negative electrode active layer includes a second particulate fluorine-containing binder, and the average particle size of primary particles of the second particulate fluorine-containing binder is D2; The relationship between D1 and D2 is as follows: 40≤D1 / D2≤2000; Optionally, D1 satisfies: 10 μm≤D1≤100 μm; Optionally, D2 satisfies: 50nm≤D2≤500nm.

2. The lithium-ion battery according to claim 1, characterized in that The negative electrode active layer further includes a negative electrode active material, and the ratio of D1 to the median particle size D50 of the negative electrode active material satisfies 1.3≤D1 / D50≤13; Optional, 2≤D1 / D50≤7; Optionally, 2 μm ≤ D50 ≤ 15 μm, preferably 4 μm ≤ D50 ≤ 10 μm; Optionally, the negative electrode active material includes a carbon-based active material.

3. The lithium-ion battery according to claim 1, characterized in that The average particle size D1 of the secondary particles of the first granular fluorine-containing binder in the first adhesive layer and the thickness H of the first adhesive layer satisfy the relationship: 5≤D1 / H≤100; Optionally, 0.5 μm≤H≤4 μm, preferably 1 μm≤H≤2 μm.

4. The lithium-ion battery according to claim 1, characterized in that The average particle size D1 of the secondary particles of the first granular fluorine-containing binder in the first adhesive layer and the coverage X of the first adhesive layer on the diaphragm satisfy: 15≤D1 / X≤400; Optional, 34≤D1 / X≤400; Optionally, the coverage rate X of the first adhesive layer on the diaphragm satisfies 15%≤X≤80%, preferably, 30%≤X≤60%.

5. The lithium ion battery according to any one of claims 1 to 4, characterized in that: At least one of the following conditions is met: (1)60≤D1 / D2≤1000; (2) the first particulate fluorine-containing binder comprises at least one of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin; (3) The second particulate fluorine-containing binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, hexafluoropropylene, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin.

6. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The thermal shrinkage rate Y of the diaphragm in the width direction when heated at 90° C. for 2 hours is ≤3%; and / or The base film has a thickness of 5 μm to 16 μm.

7. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The compaction density PD of the negative electrode active layer satisfies 1.35 g / cm 3 ≤PD≤1.75g / cm 3 .

8. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The porosity Z of the negative electrode sheet satisfies 5%≤Z≤40%, and optionally, 28%≤Z≤35%.

9. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The negative electrode active layer further includes a thickener. Optionally, the thickener includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyacrylic acid, branched sodium polyacrylate, and branched lithium polyacrylate.

10. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The negative electrode active layer further includes a conductive agent, and the conductive agent satisfies at least one of the following conditions: (1) The particle size of the secondary particles of the conductive agent is D3, 0.2 μm≤D3≤5 μm; Optionally, 0.5μm≤D3≤3μm; (2) The mass content of the conductive agent in the negative electrode active layer is B1, 0.1%≤B1≤5%; Optional, 0.5% ≤ B1 ≤ 3%; (3) The conductive agent includes one or more of carbon black, carbon spheres, carbon fibers, graphite, and graphene; Optionally, the carbon black includes one or more of acetylene black and furnace black.

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