A double-layer coated negative electrode sheet, a preparation method thereof, and a lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a double-layer coated negative electrode sheet, a preparation method thereof and a lithium ion battery. The double-layer coated negative electrode sheet comprises a lower coating layer and an upper coating layer arranged on at least one surface of a current collector, and the upper and lower coating layers respectively comprise upper and lower active materials and upper and lower binders; the lower active material comprises a lower negative electrode substrate and a pitch coating layer accounting for 0.2wt%-2.0wt% of the mass of the lower negative electrode substrate; and the upper active material comprises an upper negative electrode substrate and a lithium oxide, lithium nitride or lithium fluoride coating layer accounting for 0.1wt%-1.0wt% of the mass of the upper negative electrode substrate. Through targeted coating of the upper and lower active materials and differential selection of the upper and lower binders, the application improves the wettability of electrolyte to the lower active material, accelerates the desolvation process of the solvated lithium ions on the surface of the upper active material, and improves the energy density and cycle performance of the double-layer coated electrode sheet.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a double-coated negative electrode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] Energy density and fast-charging performance are two core indicators for evaluating the performance of lithium-ion batteries. Current technologies often improve battery energy density by increasing the amount of negative electrode active material coated on the negative electrode, i.e., increasing the coating thickness and compaction density. However, increasing the coating thickness and compaction density makes it more difficult for the electrolyte to penetrate the coating, especially closer to the current collector. This hinders lithium-ion transport within the coating, resulting in poor battery performance during high-rate charge and discharge.

[0003] To address the challenge of simultaneously achieving high energy density and high-rate charge / discharge performance in negative electrode sheets, current technologies employ a double-layer coating technique. However, batteries prepared using this technique currently suffer from issues such as high internal resistance and low energy efficiency. The primary reason is that after the rolling process, the porosity of the upper layer material in the double-coated negative electrode sheet is lower than that of the lower layer material. This hinders electrolyte wetting of the lower layer, increasing lithium-ion transport impedance. Furthermore, the lower layer material struggles to fully undergo lithium insertion / extraction reactions within a limited lithium-ion concentration, thus impacting battery capacity. Summary of the Invention

[0004] This invention provides a double-layer coated negative electrode sheet, its preparation method, and a lithium-ion battery. It addresses the technical problems of existing double-layer coated negative electrode sheets, such as difficulty in electrolyte wetting of the lower layer material and high impedance of the upper lithium-ion SEI film.

[0005] In a first aspect, the present invention provides a double-coated negative electrode sheet, comprising a current collector and a lower coating layer and an upper coating layer sequentially disposed on at least one surface of the current collector, wherein the lower coating layer comprises a lower active material and a lower binder, and the upper coating layer comprises an upper active material and an upper binder; The lower active material consists of a lower negative electrode substrate and a lower coating material coated on the surface of the lower negative electrode substrate. The lower coating material is selected from at least one of asphalt and metal oxides. Based on the mass of the lower negative electrode substrate, the mass fraction of the lower coating material is 0.2wt%-2.0wt%; The upper active material is composed of an upper negative electrode substrate and an upper coating material coated on the surface of the upper negative electrode substrate. The upper coating material is selected from at least one of lithium oxide, lithium nitride, and lithium fluoride. Based on the mass of the upper negative electrode substrate, the mass fraction of the upper coating material is 0.1wt%-1.0wt%; The upper and lower adhesives are each independently selected from at least one of modified carboxymethyl cellulose, polyacrylic acid, modified polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0006] In one optional embodiment, the lower negative electrode substrate and the upper negative electrode substrate are each independently selected from at least one of graphite, silicon carbide, and silicon oxide, and preferably both the lower negative electrode substrate and the upper negative electrode substrate are graphite. And / or, the underlying coating material is selected from at least one of bitumen and alumina; And / or, based on the mass of the lower negative electrode substrate, the mass fraction of the lower coating material is 0.5wt%-2.0wt%, more preferably 0.50wt%, 0.8wt%, or 2.0wt%; And / or, the upper coating material is selected from at least one of lithium oxide and lithium nitride; And / or, based on the mass of the upper negative electrode substrate, the mass fraction of the upper coating material is 0.1wt%-0.3wt%, more preferably 0.2wt%.

[0007] The mass ratio of the upper adhesive to the upper active material can be (1-5):(94-98), preferably 3:96.

[0008] The mass ratio of the lower layer adhesive to the lower layer active material can be (1-5):(94-98), preferably 3:96.

[0009] In one optional embodiment, the lower adhesive is selected from at least one of modified carboxymethyl cellulose and polyacrylic acid; And / or, the upper adhesive is selected from at least one of modified polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0010] In one optional embodiment, the lower coating material is selected from asphalt, preferably medium-temperature asphalt, such as modified medium-temperature asphalt XD200 from Liaoning Xinde New Material Technology Co., Ltd. And / or, the upper coating material is selected from lithium oxide; And / or, the lower adhesive is modified carboxymethyl cellulose (preferably CMC-2200 from Daicel Corporation of Japan, containing a small amount of short-chain hydrophobic groups, such as alkyl, ester, epoxy, etc.) and polyacrylic acid, preferably the mass ratio of the modified carboxymethyl cellulose and polyacrylic acid is 1:1; And / or, the upper adhesive is styrene-butadiene rubber and sodium carboxymethyl cellulose, preferably the mass ratio of styrene-butadiene rubber and sodium carboxymethyl cellulose is 1.5:1.

[0011] In one optional embodiment, the thickness of the lower coating layer is 60μm-70μm, preferably 65μm; And / or, the thickness of the upper coating layer is 60μm-70μm, preferably 65μm; And / or, the areal density of the lower coating layer is 25-75 g / m³. 2 Preferably 40-60g / m 2 More preferably 50g / m 2 ; And / or, the areal density of the upper coating layer is 25-75 g / m³. 2 Preferably 40-60g / m 2 More preferably 50g / m 2 .

[0012] Secondly, the present invention provides a method for preparing the above-mentioned double-layer coated negative electrode sheet, comprising the following steps: S1, the lower negative electrode substrate and the lower coating material are subjected to segmented heat treatment in the presence of organic solvent and dispersant in an inert atmosphere to obtain the lower active material; S2, the above-mentioned upper negative electrode substrate, the above-mentioned upper coating material, organic solvent and dispersant mixed solution are filtered and heat-treated to obtain the above-mentioned upper active material; S3, the lower active material described in S1 is mixed with the lower conductive agent, the lower binder and water to prepare the lower slurry; the upper active material described in S2 is mixed with the upper conductive agent, the upper binder and water to prepare the upper slurry; S4, the lower slurry and the upper slurry described in S3 are sequentially coated on at least one surface of the current collector to obtain a double-coated negative electrode sheet.

[0013] In one optional embodiment, the mass ratio of the lower active material, the lower conductive agent, and the lower binder in S3 is (94-98):1:(1-5), preferably 96:1:3; And / or, the mass ratio of the upper active material, the upper conductive agent and the upper binder is (94-98):1:(1-5), preferably 96:1:3; And / or, the lower conductive agent and the upper conductive agent are each independently selected from at least one of carbon black, acetylene black, graphite, carbon fiber, graphene, and carbon nanotubes.

[0014] In one optional embodiment, the solid content of the lower slurry in S3 is 40%-60%, preferably 45%-55%, and more preferably 50%. And / or, the solid content of the upper slurry is 40%-60%, preferably 45%-55%, and more preferably 50%.

[0015] In an optional embodiment, the segmented heat treatment step in S1 includes holding at 80℃-150℃ for 1h-3h, and then holding at 200℃-400℃ for 1h-2h; wherein, the purpose of holding at 80℃-150℃ for 1h-3h is to allow the lower coating material to spread fully on the surface of the lower negative electrode substrate; and the purpose of continuing to hold at 200℃-400℃ for 1h-2h is to allow the lower coating material (such as asphalt) to crosslink and carbonize on the surface of the lower negative electrode substrate to form a dense carbon layer, thereby obtaining the lower active material coated with the lower coating material.

[0016] And / or, the heat treatment in S2 is to hold at 200℃-300℃ for 1h-5h; before the heat treatment, a mixing and filtration process is required. The mixing is to fully disperse the lithium ions from the upper coating material into the particle voids of the upper negative electrode substrate, and the filtration is to remove excess free lithium ions. Then, the heat treatment forms a lithium-containing coating layer such as lithium oxide on the surface of the upper negative electrode material, thereby obtaining the upper active material coated with the upper coating material.

[0017] And / or, the organic solvent in S1 is cyclohexane, and the dispersant is polyethylene glycol, preferably the polyethylene glycol has a molecular weight of 1800-2000, more preferably the polyethylene glycol has a molecular weight of 1800; And / or, the organic solvent in S2 is dimethyl carbonate, and the dispersant is polyvinylpyrrolidone, preferably with a molecular weight of 8,000-700,000, more preferably with a molecular weight of 400,000.

[0018] Specifically, the preparation method of the double-layer coated negative electrode sheet and the lithium-ion battery comprising the double-layer coated negative electrode sheet of the present invention is as follows: Preparation of the lower active material: (1) The lower negative electrode substrate is dried using a vacuum dryer to remove the adsorbed water on the surface of the lower negative electrode substrate. The vacuum drying conditions are: temperature 120℃-140℃, duration 2h-6h, vacuum degree -0.05-0.08MPa.

[0019] (2) Add organic solvent to the reaction vessel and stir at a speed of 100 r / min-500 r / min. Add the lower coating material and stir for 60-90 min until completely dissolved. Then add dispersant and continue stirring for 30-60 min until a uniform dispersion is formed. The mass ratio of organic solvent, lower coating material and dispersant in the dispersion is (16-24):1:(0.04-0.006). (3) Take the lower negative electrode substrate after drying treatment (1) and add it to the dispersion prepared in (2). Stir it thoroughly for 30-60 minutes at a speed of 100r / min-500r / min until it is uniform, so that the lower negative electrode substrate is uniformly dispersed in the dispersion prepared in step (2). (4) Inert gas is introduced into the reactor to replace the air (oxygen) in the reactor. The temperature is raised to 80-150℃ at a rate of 5℃ / min and maintained for 1-3h. At the same time, the stirring speed is reduced to 50-100r / min so that the lower coating material is fully spread on the surface of the lower negative electrode substrate. (5) Stop stirring, raise the temperature to 200-400℃ at a rate of 3℃ / min, keep it at the temperature for 1-2 hours, so that the lower coating material cross-links and carbonizes on the surface of the lower negative electrode substrate to form a dense carbon layer, and cool naturally to room temperature. (6) Transfer the material in the reactor to a vacuum filtration device or a centrifugal filter for filtration. The filter residue is a wet cake of the lower negative electrode substrate with a surface coating material (containing a small amount of residual solvent). Place the filter residue in a vacuum drying oven. The vacuum drying conditions are: temperature 80℃, duration 2h, vacuum degree -0.08MPa, to completely remove the solvent and obtain the filter cake. (7) The filter cake is crushed using an air jet mill and then sieved through an ultrasonic vibrating grading screen to remove a small amount of agglomerates. The sieved powder is then vacuum-packed and stored in a dry environment for later use.

[0020] Preparation of the upper active material: (1) The upper negative electrode substrate is dried using a vacuum dryer to remove the adsorbed water on the surface of the upper negative electrode substrate. The vacuum drying conditions are: temperature 120℃-140℃, duration 2h-6h, vacuum degree -0.05-0.08MPa.

[0021] (2) Add the organic solvent and dispersant into the reactor and stir at a stirring speed of 100r / min-500r / min for 20-40min. Slowly add the raw material of the upper coating material and continue stirring for 30-90min. Slowly add deionized water (DIW) and stir to form a uniform dispersion. In the preparation of the dispersion, (5.0-5.5)g of dispersant is needed to uniformly disperse 1 mole of lithium, and the mass ratio of organic solvent to dispersant in the dispersion is (4000:45000):(5.0-5.5).

[0022] (3) Take the upper negative electrode substrate after (1) drying and add it to the above dispersion. Stir at a stirring speed of 300-800 r / min for 20-40 min to form a uniform mixture. During the stirring process, cooling water needs to be circulated to maintain the temperature of the reactor at 10-15℃ and maintain the reaction state for 1-5 h. (4) The above reaction solution was filtered using a plate and frame press with a filter cloth pore size of 5-10 μm, a pressure of 0.1-0.5 MPa, and a filtration time of 20-40 min. The filter cake was retained and washed with acetone to remove free lithium ions. (5) Place the filter cake in a mesh belt furnace or muffle furnace, dry it at 80°C for 1-2 hours, then introduce inert gas and heat it to 200-300°C at a rate of 5°C / min. Hold it for 2 hours and then stop heating. Allow it to cool naturally to room temperature. (6) Use an air jet mill to pulverize the above filter cake, pass it through an ultrasonic vibration grading sieve to remove a small amount of agglomerates, vacuum package the sieved powder, and store it in a dry environment for later use.

[0023] Preparation of double-layer coated negative electrode sheet: The lower active material, lower conductive agent, and lower binder are added to a mixing tank at a mass ratio of (94-98):1:(1-5), and water is added and stirred to prepare a lower slurry with a solid content of 40%-60%. The upper active material, upper conductive agent, and upper binder are added to a mixing tank at a mass ratio of (94-98):1:(1-5), and water is added and stirred to prepare an upper slurry with a solid content of 40%-60%. A double-layer coating device is used to coat the lower and upper layers of slurry at a 5:5 areal density ratio on both sides of the current collector (4-8µm copper foil), with a double-sided areal density of 100-300 g / m². 2 After drying, rolling, slitting, and sheet forming, a double-coated negative electrode sheet is obtained.

[0024] Preparation of the positive electrode sheet: The positive electrode active material (lithium iron phosphate), carbon black (SP), and polyvinylidene fluoride (PVDF) are added to a stirred tank at a mass ratio of (94-98):(1-3):(1-3) (e.g., 96.4:1.8:1.8). N-methylpyrrolidone (NMP) is added and stirred to prepare a positive electrode slurry with a solid content of 60-80%. The slurry is then coated on both sides of the positive electrode current collector (12 μm aluminum foil) using a positive electrode coating device, with a double-sided areal density of 200-500 g / m³. 2 After drying, rolling, slitting, and sheet forming, positive electrode sheets are obtained. Lithium-ion battery manufacturing: The above-mentioned double-coated negative electrode sheet, positive electrode sheet and separator are stacked together to form a battery cell. After hot pressing, it is packaged with aluminum-plastic film. After processes such as liquid injection, aging, formation, secondary packaging and capacity testing, it becomes a battery. The preparation environment requires a temperature of 20-30℃ and a humidity of ≤40%RH. The equipment used includes a mixer, coating machine, roller press, slitting machine, stacking machine, hot press, liquid injection machine, ultrasonic spot welding machine, top and side sealing machine, formation cabinet, capacity testing cabinet, vacuum oven, etc.

[0025] Thirdly, the present invention provides a lithium-ion battery, comprising the above-described double-coated negative electrode sheet or the double-coated negative electrode sheet prepared by the above-described preparation method.

[0026] The technical solution of this invention has the following advantages: 1. This invention provides a double-layer coated negative electrode sheet, comprising a current collector and a lower coating layer and an upper coating layer sequentially disposed on at least one surface of the current collector. The lower coating layer comprises a lower active material and a lower binder, and the upper coating layer comprises an upper active material and an upper binder. The lower active material consists of a lower negative electrode substrate and asphalt coating the surface of the lower negative electrode substrate. The lower coating material increases the polarity of the surface of the lower negative electrode substrate particles, improves the compatibility of the lower active material surface with the electrolyte, improves the wettability of the electrolyte to the lower active material, solves the problem of insufficient wetting of the lower active material after rolling, and allows the lower active material to fully undergo lithium insertion / extraction reactions under limited lithium ion concentration, fully utilizing the specific capacity of the lower active material and improving the overall energy density of the cell. The upper active material consists of an upper negative electrode substrate and upper coating materials such as lithium oxide, lithium nitride, and lithium fluoride coated on the surface of the upper negative electrode substrate. The F / O / N elements in the upper coating material can preferentially participate in the formation of the SEI film during the first charge and discharge of the electrode sheet, effectively reducing the consumption of lithium salt in the electrolyte and avoiding the increase in internal resistance caused by excessive decomposition of lithium salt. At the same time, it promotes the desolvation process of solvated lithium ions on the surface of the upper active material, further reducing the lithium ion transport impedance, promoting the formation of a structurally stable and low-impedance SEI film on the surface of the upper active material, and ensuring the fast charging performance and cycle stability of the upper active material.

[0027] This invention controls the coating amount of the lower coating material within the range of 0.2-2 wt%. This ensures that the lower coating material uniformly covers the surface of the lower negative electrode substrate particles, effectively improving the polarity of the negative electrode substrate and enhancing its compatibility with the electrolyte. Simultaneously, it considers the structural stability and energy density of the negative electrode sheet, while also optimizing the bonding effect between the lower adhesive and the negative electrode substrate. Meanwhile, the upper coating material is controlled within the range of 0.1-1 wt%, with a minimum coating amount of 0.1 wt%. This ensures, on the one hand, that the upper coating material preferentially participates in SEI film formation during the first charge-discharge cycle, avoiding insufficient SEI film formation due to insufficient F / O / N content, leading to increased lithium salt consumption and high upper layer impedance. On the other hand, it prevents excessively thick coating layers from hindering lithium-ion transport, and also prevents excessive coating material from increasing the electrode interface impedance and hindering the capacity utilization of the upper active material.

[0028] 2. This invention provides a double-layer coated negative electrode sheet. The lower binder is selected from modified carboxymethyl cellulose and / or polyacrylic acid, and the upper binder is selected from at least one of modified polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC). The lower binder has strong adhesion and good compatibility with the lower coating material, adapting to the polar surface of the coated lower active material and helping to enhance the bonding force between the binder and the current collector and the particles of the lower active material. The upper binder has moderate adhesion and low interfacial impedance, ensuring the roll forming properties of the upper active material without hindering lithium ions from passing through the upper coating, thus meeting the design requirements of fast charging and low impedance for the upper active material.

[0029] 3. This invention provides a method for preparing a double-layer coated negative electrode sheet, comprising: coating a lower layer coating material onto the surface of a lower negative electrode substrate to prepare a lower active material; coating an upper layer coating material onto the surface of an upper negative electrode substrate to prepare an upper active material; mixing the lower active material with a lower binder, and the upper active material with an upper binder with a conductive agent and water respectively to prepare a lower slurry and an upper slurry; and then sequentially coating the lower slurry and the upper slurry onto at least one surface of a current collector to obtain a double-layer coated negative electrode sheet. This invention addresses the technical challenges of high internal resistance, low energy efficiency, and uneven electrolyte wetting in existing double-layer coated negative electrode sheets by performing targeted coating treatments on the lower and upper negative electrode substrates and selecting adhesives with different properties in the preparation of the lower and upper slurries. Furthermore, it fully leverages the inherent advantages of double-layer coating technology in high energy density and long cycle performance, promoting the large-scale application of double-layer coated negative electrode sheets in high-energy-density fast-charging cells. Attached Figure Description

[0030] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the double-layer coated negative electrode structure provided by the present invention; Figure 2 This is a schematic diagram of the surface coating structure of the upper active material and / or lower active material in the double-layer coated negative electrode sheet provided by the present invention.

[0032] The attached figures are labeled as follows: 1. Current collector; 2. Lower coating layer; 3. Upper coating layer; 4. Negative electrode substrate; 5. Coating layer. Detailed Implementation

[0033] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0034] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0035]

[0036] Example 1 This embodiment provides a double-layer coated negative electrode sheet and its preparation method, the steps of which are as follows: Preparation of the lower active material of the negative electrode sheet (using modified medium-temperature pitch as the coating material). (1) The artificial graphite was dried using a vacuum dryer to remove the adsorbed water on the surface of the graphite particles. The vacuum drying conditions were: temperature 130℃, duration 4h, vacuum degree -0.06MPa. (2) Add 20 kg of cyclohexane to the reactor and stir at 300 r / min. Add 1 kg of modified medium-temperature asphalt and stir for 75 min until completely dissolved. Add 0.05 kg of polyethylene glycol (molecular weight 1800) and continue stirring for 45 min until a uniform dispersion is formed. The mass ratio of cyclohexane, modified medium-temperature asphalt and polyethylene glycol (molecular weight 1800) in the dispersion is 20:1:0.05.

[0037] (3) Take 100 kg of graphite particles after drying treatment (1) and add them to the dispersion prepared in (2). Stir at 500 r / min for 45 min until a uniform mixture is achieved. (4) Introduce nitrogen into the reactor to replace the air (oxygen) in the reactor, raise the temperature to 120°C at a rate of 5°C / min, maintain for 2 hours, and at the same time reduce the stirring speed to 75 r / min so that the modified medium-temperature asphalt can be fully spread on the surface of the graphite particles. (5) Stop stirring, raise the temperature to 300°C at a rate of 3°C / min, keep it at the temperature for 1.5h, so that the modified medium-temperature asphalt cross-links and carbonizes on the surface of graphite particles to form a dense carbon layer, and cool it naturally to room temperature. (6) The material in the reactor was transferred to a vacuum filtration device for filtration. The filter residue was a graphite wet cake coated with modified medium-temperature asphalt (containing a small amount of residual solvent). The filter residue was placed in a vacuum drying oven. The vacuum drying conditions were: temperature 80℃, duration 2h, vacuum degree -0.08MPa. The solvent was completely removed, and 100.5kg of filter cake was obtained (indicating that 0.5kg of modified medium-temperature asphalt was coated on the surface of the graphite particles). (7) The filter cake was crushed using an air jet mill and then sieved through an ultrasonic vibrating grading sieve (sieve mesh size 45μm / 325 mesh, stainless steel material, sieve surface diameter 1000mm). The sieve was sieved in a single layer and 36kHz ultrasonic waves were turned on to prevent clogging and remove a small amount of agglomerates. The sieved powder was then vacuum-packed and stored in a dry environment for later use.

[0038] Preparation of the upper active material of the negative electrode sheet (lithium oxide was selected as the coating material). (1) The artificial graphite was dried using a vacuum dryer to remove the adsorbed water on the surface of the graphite particles. The vacuum drying conditions were: temperature 130℃, duration 4h, vacuum degree -0.06MPa. (2) 40 kg of dimethyl carbonate and 50 g of polyvinylpyrrolidone (PVP, molecular weight 400,000) were added to the reactor and stirred at 300 r / min for 30 min. 390 g of hydrated lithium hydroxide (LiOH·H2O) was slowly added and stirred for another 60 min. 1 kg of deionized water (DIW) was slowly added and stirred to form a uniform dispersion. In order to achieve uniform dispersion, 5.38 g of polyvinylpyrrolidone was required per mole of lithium during the preparation of the dispersion. The mass ratio of dimethyl carbonate, polyvinylpyrrolidone and deionized water in the dispersion was 4304:5.38:107.6.

[0039] (3) Take 50 kg of graphite particles dried in (1) and add them to the precursor solution prepared in (2). Stir at a stirring speed of 500 r / min for 30 min to form a uniform mixture. During the stirring process, cooling water needs to be circulated to maintain the temperature of the reactor at 12℃ and maintain the reaction state for 3 h. (4) The above reaction solution was filtered using a plate and frame press with filter cloth pore size, pressure of 0.3 MPa, filtration time of 30 min, filter cake was retained, and the filter cake was washed with acetone to remove free lithium ions; (5) The filter cake was placed in a mesh belt furnace and dried at 80°C for 1.5 hours. Nitrogen gas was introduced and the temperature was raised to 250°C at a rate of 5°C / min. The temperature was maintained for 2 hours and then the heating was stopped. The filter cake was allowed to cool naturally to room temperature to obtain 50.1 kg of dried filter cake (indicating that 0.1 kg of lithium oxide was coated on the surface of the graphite particles). (6) The filter cake is crushed using an air jet mill and then sieved through an ultrasonic vibrating grading sieve (sieve mesh size 45μm / 325 mesh, stainless steel material, sieve surface diameter 1000mm). The sieve is a single-layer sieve, and the ultrasonic wave is turned on at 36kHz to prevent clogging and remove a small amount of agglomerates. The sieved powder is then vacuum-packed and stored in a dry environment for later use.

[0040] Preparation of the lower slurry of the negative electrode sheet The lower active material obtained by coating artificial graphite (lower negative electrode substrate) with modified medium-temperature asphalt, carbon black (SP), modified hydroxymethyl cellulose (CMC-2200, containing a small amount of short-chain hydrophobic groups, such as alkyl, ester, epoxy, etc.), and polyacrylic acid (PAA) are added to a mixing tank in a mass ratio of 96:1:1.5:1.5, and water is added and stirred to prepare a lower slurry with a solid content of 50%.

[0041] Preparation of the upper slurry of the negative electrode sheet The upper active material obtained by coating artificial graphite with lithium oxide (upper negative electrode substrate), carbon black (SP), styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are added to a stirred tank in a mass ratio of 96:1:1.8:1.2, and water is added and stirred to prepare an upper slurry with a solid content of 50%.

[0042] Preparation of negative electrode sheet Using a double-layer coating equipment, a lower layer of slurry and an upper layer of slurry are sequentially coated on both sides of the current collector 1 (6μm copper foil) at a 5:5 areal density ratio, with a double-sided areal density of 200g / m². 2After drying, rolling, slitting, and sheet forming, a double-coated negative electrode sheet is obtained. The double-coated negative electrode sheet includes a current collector 1 and a coating disposed on the surface of the current collector. The coating thickness is 130μm (testing instrument: Mitutoyo, Japan, model: 293-240-30). The coating includes a lower coating 2 on the side closer to the current collector 1 and an upper coating 3 on the side farther from the current collector 1. The lower coating 2 and the upper coating 3 respectively include a negative electrode substrate 4 and a coating layer 5 covering the surface of the negative electrode substrate 4.

[0043] Preparation of positive electrode sheet The positive electrode active material (lithium iron phosphate), carbon black (SP), and polyvinylidene fluoride (PVDF) were added to a stirred tank at a mass ratio of 96.4:1.8:1.8. N-methylpyrrolidone (NMP) was added and stirred to prepare a positive electrode slurry with a solid content of 68%. The slurry was then coated on both sides of the positive electrode current collector (12 μm aluminum foil) using a positive electrode coating device, with a double-sided areal density of 430 g / m³. 2 After drying, rolling, slitting, and sheet forming, positive electrode sheets are obtained. Battery manufacturing The prepared double-layer coated negative electrode sheet, positive electrode sheet, and separator (Celgard 2325 PP / PE / PP three layers, with a thickness of 25 μm) are stacked together to form a battery cell. After hot pressing, it is packaged with aluminum-plastic film. After processes such as liquid injection, aging, formation, secondary packaging, and capacity testing, it becomes a battery (battery size is 145mm x 90mm x 2mm). The preparation environment requires a temperature of 20-30℃ and a humidity of ≤40%RH. The equipment used includes a mixer, coating machine, roller press, slitting machine, stacking machine, hot press, liquid injection machine, ultrasonic spot welding machine, top and side sealing machine, formation cabinet, capacity testing cabinet, vacuum oven, etc.

[0044] The difference between the preparation methods of Examples 2-8 and Example 1 lies in the different mass ratios of the lower coating material based on the mass of the lower negative electrode substrate, or the different mass ratios of the upper coating material based on the mass of the upper negative electrode substrate. The specific parameter settings are shown in Table 1.

[0045] Example 9 The difference between the preparation methods of Example 9 and Example 1 is that lithium nitride (Li3N) is used instead of lithium oxide as the upper coating material.

[0046] Example 10 The difference between the preparation methods of Example 10 and Example 1 is that lithium nitride (Li3N) is used instead of lithium oxide as the upper coating material, and alumina is used instead of modified medium-temperature asphalt as the lower coating material.

[0047] Example 11 The difference between the preparation method of Example 11 and Example 1 is that the binder used in the lower layer slurry is styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC), and the mass ratio of SBR to CMC is 1.8:1.2, which is the same as the binder used in the upper layer slurry.

[0048] Example 12 The difference between the preparation method of Example 12 and Example 1 is that the binder used in the upper slurry is modified hydroxymethyl cellulose and polyacrylic acid (PAA), and the mass ratio of modified hydroxymethyl cellulose and polyacrylic acid (PAA) is 1.5:1.5, which is the same as the binder used in the lower slurry.

[0049] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that neither the upper active material nor the lower active material was coated; otherwise, the preparation method was the same as in Example 1.

[0050] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the upper active material was not coated, but the preparation method was the same as that of Example 1.

[0051] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the lower active material was not coated, but the preparation method was the same as that of Example 1.

[0052] Examples 1-12 are similar to the preparation methods of Comparative Examples 1-3, except that the types of upper and lower coating materials are different, as are the mass percentages of the upper coating material in the upper negative electrode substrate (i.e., the mass percentages of the upper coating material in Table 1), the mass percentages of the lower coating material in the lower negative electrode substrate (i.e., the mass percentages of the lower coating material in Table 1), and the types and mass ratios of the upper and lower binders. These are detailed in the tables below. The mass percentage of the upper coating material in the upper negative electrode substrate refers to the percentage of the mass of the upper coating material in the mass of the upper negative electrode substrate; the mass percentage of the lower coating material in the lower negative electrode substrate refers to the percentage of the mass of the lower coating material in the mass of the lower negative electrode substrate.

[0053] Table 1

[0054] Test case

[0055] 1. Calibration capacity test method: At an ambient temperature of 20-25℃, the batteries prepared in each embodiment and comparative example were subjected to constant current and constant voltage charge and discharge three times using a charge and discharge cabinet. The voltage range was 2.0-3.65V, the current was 0.33C, the charging cutoff condition was that the voltage reached 3.65V and the current was less than 0.05C, and the discharge cutoff condition was 2.0V. The average discharge capacity of the last two cycles was taken as the initial capacity of the battery.

[0056] 2. Cyclic performance testing method: At an ambient temperature of 20-25℃, the batteries prepared in each embodiment and comparative example were subjected to cyclic charging and discharging using a charge-discharge cabinet. The voltage range was 2.0-3.65V, the current was 0.5C, the charging cut-off condition was when the voltage reached 3.65V and the current was less than 0.05C, and the discharging cut-off condition was 2.0V. The discharge capacity of the first cycle was taken as the first cycle capacity of the battery, and the capacity retention rate of each cycle was obtained by comparing the discharge capacity of each cycle with the first cycle capacity.

[0057] 3. SEI membrane impedance testing method: At 25℃, the battery was charged at a 0.5C rate to 50% SOC (determined through charge / discharge capacity calculations), and then allowed to stand for 4 hours to reach thermodynamic equilibrium. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 1Hz-100kHz with AC amplitude. Nyquist plots were fitted; the high-frequency semicircle (typically in the 10kHz-100kHz range) corresponded to ohmic impedance, and the mid-frequency semicircle (typically in the 1Hz-1000Hz range) corresponded to SEI film impedance. The SEI film impedance RSEI value was extracted. Normalized to the effective electrode area, expressed in Ω·cm 2 The interfacial impedance is expressed in units of 1 / 2. The lower the interfacial impedance, the better the ion conductivity of the solid electrolyte interfacial film, and the less it limits battery performance.

[0058] Table 2

[0059] Table 2 summarizes the initial capacity, capacity after 500 cycles, capacity retention after 500 cycles, and SEI film impedance data of the lithium-ion batteries in each embodiment and comparative example.

[0060] Examples 1-8 all used modified medium-temperature asphalt as the lower coating material and lithium oxide as the upper coating material.

[0061] Data from Examples 1, 4, 5, 7, and 8 show that when the mass percentage of the upper coating material is controlled at 0.20%, the battery performance is best when the mass percentage of the lower coating material is 0.5wt%, 0.8wt%, and 2.0wt%. Therefore, a mass percentage of 0.5wt%-2.0wt% for the lower coating material is a preferred embodiment of the present invention. Data from Examples 1, 2, 3, and 6 show that when the mass percentage of the lower coating material is controlled at 0.5wt%, the battery performance is best when the mass percentage of the upper coating material is 0.1wt%, 0.2wt%, and 0.3wt%. However, when the mass percentage of the upper coating material increases to 1.0wt%, the battery performance significantly decreases. Therefore, a mass percentage of 0.1wt%-0.3wt% for the upper coating material is a preferred embodiment of the present invention.

[0062] In Example 9, lithium nitride was used as the top coating material. The initial capacity (60.6 mAh), capacity retention after 500 cycles (93.20%), and SEI film impedance (9.2 Ω·cm) of Example 9 were also observed. 2 All three data points were worse than in Example 1, indicating that using lithium oxide as the top coating material is more beneficial to improving battery performance. Therefore, the present invention prefers lithium oxide as the top coating material.

[0063] In Example 1, the upper binder was styrene-butadiene rubber and sodium carboxymethyl cellulose, and the lower binder was modified carboxymethyl cellulose and polyacrylic acid. Its initial capacity (61.4 mAh), capacity retention after 500 cycles (95.0%), and SEI film impedance (8.2 Ω·cm) were [not specified in the original text]. 2 All three data points are superior to those of Examples 11 and 12, which use the same upper and lower adhesives. A comparison of Examples 11 and 12 with Example 1 shows that the use of an incompatible adhesive for either the upper or lower slurry alone leads to a decrease in battery performance, demonstrating that the differentiated design of the upper and lower adhesives can further improve battery capacity, cycle performance, and reduce membrane impedance.

[0064] In Comparative Example 2, the lower active material was coated with modified medium-temperature asphalt, while the upper active material was not coated. Its initial capacity (59.5 mAh), capacity retention after 500 cycles (91.10%), and SEI film impedance (12.3 Ω·cm) were significantly improved. 2 All three indicators were significantly lower than those in Example 1; in Comparative Example 3, the upper active material was coated with lithium oxide (Li2O), while the lower active material was not coated. Its initial capacity (59.8 mAh), capacity retention after 500 cycles (91.10%), and SEI film impedance (10.8 Ω·cm) were significantly lower. 2All three indicators were significantly lower than those in Example 1. The comparison between Comparative Examples 2 and 3 and Example 1 showed that if either the upper or lower active material was not coated, the battery performance would decrease. This proves that targeted coating of the upper and lower active materials can improve the battery capacity, cycle performance, and reduce membrane impedance.

[0065] In Comparative Example 1, neither the upper nor lower active material was coated, resulting in an initial capacity of 58.2 mAh. In Comparative Example 2, the upper active material was uncoated, while the lower active material was coated with modified medium-temperature asphalt, resulting in an initial capacity of 59.5 mAh. In Comparative Example 3, the upper active material was coated with lithium oxide (Li2O), while the lower active material was uncoated, resulting in an initial capacity of 59.8 mAh. Comparing Comparative Example 2 with Comparative Example 1 shows that coating the lower active material with modified medium-temperature asphalt increased the battery capacity by 1.3 mAh; comparing Comparative Example 3 with Comparative Example 1 shows that coating the upper active material with lithium oxide (Li2O) increased the battery capacity by 1.6 mAh.

[0066] In Example 1, the upper active material was coated with lithium oxide (Li2O), and the lower active material was coated with modified medium-temperature asphalt. The initial capacity was 61.4 mAh. Compared with Comparative Example 1, the upper active material coated with lithium oxide (Li2O) and the lower active material coated with modified medium-temperature asphalt resulted in a 3.2 mA increase in battery capacity. Since the battery capacity increase was greater than the sum of the 2.9 mAh increase from the lower active material coated with modified medium-temperature asphalt and the upper active material coated with lithium oxide (Li2O) alone, it indicates that the upper active material coated with lithium oxide (Li2O) and the lower active material coated with modified medium-temperature asphalt can have a synergistic effect on improving battery capacity.

[0067] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A double-layer coated negative electrode sheet, comprising a current collector and a lower coating layer and an upper coating layer sequentially disposed on at least one surface of the current collector, characterized in that, The lower coating layer includes a lower active material and a lower binder, and the upper coating layer includes an upper active material and an upper binder; The lower active material consists of a lower negative electrode substrate and a lower coating material coated on the surface of the lower negative electrode substrate. The lower coating material is selected from at least one of asphalt and metal oxides. Based on the mass of the lower negative electrode substrate, the mass fraction of the lower coating material is 0.2wt%-2.0wt%; The upper active material is composed of an upper negative electrode substrate and an upper coating material coated on the surface of the upper negative electrode substrate. The upper coating material is selected from at least one of lithium oxide, lithium nitride, and lithium fluoride. Based on the mass of the upper negative electrode substrate, the mass fraction of the upper coating material is 0.1wt%-1.0wt%; The upper and lower adhesives are each independently selected from at least one of modified carboxymethyl cellulose, polyacrylic acid, modified polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

2. The double-layer coated negative electrode sheet according to claim 1, characterized in that, The lower negative electrode substrate and the upper negative electrode substrate are each independently selected from at least one of graphite, silicon carbide, and silicon oxide; And / or, the underlying coating material is selected from at least one of bitumen and alumina; And / or, based on the mass of the lower negative electrode substrate, the mass fraction of the lower coating material is 0.5wt%-2.0wt%; And / or, the upper coating material is selected from at least one of lithium oxide and lithium nitride; And / or, based on the mass of the upper negative electrode substrate, the mass fraction of the upper coating material is 0.1wt%-0.3wt%; And / or, the mass ratio of the upper adhesive to the upper active material is (1-5):(94-98); And / or, the mass ratio of the lower adhesive layer to the lower active material is (1-5):(94-98).

3. The double-layer coated negative electrode sheet according to claim 1 or 2, characterized in that, The lower adhesive is selected from at least one of modified carboxymethyl cellulose and polyacrylic acid; And / or, the upper adhesive is selected from at least one of modified polyvinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

4. The double-layer coated negative electrode sheet according to any one of claims 1-3, characterized in that, The underlying coating material is selected from asphalt, preferably medium-temperature asphalt; And / or, the upper coating material is selected from lithium oxide; And / or, the lower adhesive layer is modified carboxymethyl cellulose and polyacrylic acid; And / or, the upper adhesive is styrene-butadiene rubber and sodium carboxymethyl cellulose.

5. A double-layer coated negative electrode sheet as described in any one of claims 1-4, characterized in that, The thickness of the lower coating layer is 60μm-70μm; And / or, the thickness of the upper coating layer is 60μm-70μm; And / or, the areal density of the lower coating layer is 25-75 g / m³. 2 Preferably 40-60g / m 2 ; And / or, the areal density of the upper coating layer is 25-75 g / m³. 2 Preferably 40-60g / m 2 .

6. A method for preparing a double-layer coated negative electrode sheet as described in any one of claims 1-5, characterized in that, Includes the following steps: S1, the lower negative electrode substrate and the lower coating material are subjected to segmented heat treatment in the presence of organic solvent and dispersant in an inert atmosphere to obtain the lower active material; S2, the upper negative electrode substrate, the upper coating material, the organic solvent and the dispersant are mixed in a solution, filtered and heat-treated to obtain the upper active material; S3, the lower active material described in S1 is mixed with the lower conductive agent, the lower binder and water to prepare the lower slurry; the upper active material described in S2 is mixed with the upper conductive agent, the upper binder and water to prepare the upper slurry; S4, the lower slurry and the upper slurry described in S3 are sequentially coated on at least one surface of the current collector to obtain a double-coated negative electrode sheet.

7. The preparation method according to claim 6, characterized in that, The mass ratio of the lower active material, the lower conductive agent, and the lower binder in S3 is (94-98):1:(1-5); And / or, the mass ratio of the upper active material, the upper conductive agent and the upper binder is (94-98):1:(1-5); And / or, the lower conductive agent and the upper conductive agent are each independently selected from at least one of carbon black, acetylene black, graphite, carbon fiber, graphene, and carbon nanotubes.

8. The preparation method according to claim 6 or 7, characterized in that, The solid content of the lower layer slurry in S3 is 40%-60%, preferably 45%-55%; And / or, the solid content of the upper slurry is 40%-60%, preferably 45%-55%.

9. The preparation method according to claim 6, characterized in that, The segmented heat treatment step described in S1 includes holding at 80℃-150℃ for 1h-3h, and then holding at 200℃-400℃ for 1h-2h. And / or, the heat treatment described in S2 is to hold at 200℃-300℃ for 1h-5h; And / or, the organic solvent in S1 is cyclohexane, and the dispersant is polyethylene glycol; And / or, the organic solvent in S2 is dimethyl carbonate, and the dispersant is polyvinylpyrrolidone.

10. A lithium-ion battery, characterized in that, This includes the double-coated negative electrode sheet according to any one of claims 1-5 or the double-coated negative electrode sheet prepared by the preparation method according to any one of claims 6-9.

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