Carbon-coated aluminum foil for improving cycle rate capability of lithium battery and preparation method of carbon-coated aluminum foil

Abstract

The invention relates to the technical field of battery materials, in particular to a carbon-coated aluminum foil for improving the cycle rate capability of a lithium battery and a preparation method of the carbon-coated aluminum foil. A bottom-layer material, a middle-layer material and a top-layer material are sequentially arranged on the surface of an aluminum foil substrate; the bottom layer material comprises conductive graphite, the middle layer material comprises nano conductive carbon black, modified graphene, a polyurethane elastomer and boron nitride micro powder, the top layer material comprises carboxylated carbon nanotubes, and the bottom layer material, the middle layer material and the top layer material further comprise a binder and a dispersant. The bottom layer large-particle graphite in the carbon coating layer constructs a stable conductive framework, a stable path is provided for electron transmission, the middle layer nano-scale carbon black is matched with the modified graphene, electrons are captured through the high specific surface area of the carbon black, the modified graphene rapidly conducts the electrons by means of the excellent electron migration characteristic, and the conductivity of the carbon coating layer is improved. And the carboxylated carbon nanotubes on the top layer regulate the electrode interface environment and promote uniform diffusion of lithium ions, so that the cycle rate performance of the lithium battery is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a carbon-coated aluminum foil for improving the cycle rate performance of a lithium battery and a preparation method thereof. Background Art

[0002] Lithium batteries are a type of battery that uses lithium metal or lithium alloy as positive / negative electrode materials and uses non-aqueous electrolyte solutions. According to the form of lithium in the battery, lithium batteries can be divided into two categories: lithium metal batteries and lithium ion batteries. Lithium metal batteries are usually not rechargeable and contain metallic lithium, while lithium ion batteries are rechargeable and do not contain metallic lithium. Lithium batteries have the advantages of being light, durable, and relatively less polluting to the environment. The carbon-coated aluminum foil in lithium batteries is a composite material in which a conductive carbon material is coated on an aluminum foil substrate. It is mainly used in battery manufacturing, especially in lithium batteries and power batteries. Carbon-coated aluminum foil has many advantages, including reducing battery internal resistance, inhibiting battery polarization, increasing battery energy density, and extending battery cycle life.

[0003] Now with the widespread application of lithium batteries in electric vehicles, portable electronic devices and many other fields, there are many problems with traditional aluminum foil as a current collector during high-rate charging and discharging. For example, the contact resistance between the electrode and the current collector is large, resulting in poor electron transmission and severe battery heating, which in turn affects the battery's cycle life and rate performance. In addition, when lithium batteries are charged and discharged in high and low temperature environments, the charging and discharging performance of lithium batteries deteriorates significantly, which greatly limits the cycle rate performance of lithium batteries during the charging and discharging process. Summary of the invention

[0004] In order to solve the defect of insufficient cycle rate performance of lithium batteries in the prior art, the present invention provides a carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries and a preparation method thereof.

[0005] The technical solution adopted by the present invention is as follows: a carbon-coated aluminum foil for improving the cycle rate performance of a lithium battery, wherein a bottom layer material, a middle layer material and a top layer material are sequentially arranged on the surface of the aluminum foil substrate; the bottom layer material comprises conductive graphite, the middle layer material comprises nano conductive carbon black, modified graphene, polyurethane elastomer and boron nitride powder, the top layer material comprises carboxylated carbon nanotubes, and the bottom layer material, the middle layer material and the top layer material also comprise a binder and a dispersant.

[0006] Furthermore, in some embodiments, the mass ratio of conductive graphite, binder and dispersant in the bottom layer material is 70:2:10; the mass ratio of nano conductive carbon black, modified graphene, polyurethane elastomer, boron nitride powder, binder and dispersant in the middle layer material is 35:25:15:10:10:5; the mass ratio of carboxylated carbon nanotubes, binder and dispersant in the top layer material is 80:15:5.

[0007] Furthermore, in some embodiments, the thickness of the bottom layer material is 1-2 μm, the thickness of the middle layer material is 0.8-1.5 μm, and the thickness of the top layer material is 0.2-0.4 μm.

[0008] Furthermore, in some embodiments, the binder is styrene-butadiene rubber latex, and the dispersant is polyethylene glycol.

[0009] Furthermore, in some embodiments, the preparation method of the modified graphene is:

[0010] A1, 100g natural graphite flakes were crushed, and 1000mL of a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1 was added thereto, and the mixture was stirred at room temperature at a speed of 300r / min for 48h until the reaction was completed;

[0011] A2, adding a strong oxidant consisting of 30g potassium permanganate and 100mL concentrated sulfuric acid, stirring continuously at 35°C at a speed of 250r / min for oxidation reaction for 5h, then adding 50mL of a mixed solution consisting of ethylene glycol and acetic acid in a molar ratio of 1:1 to prepare ethyl hydroxyacetate, wherein the molar concentration of ethylene glycol and acetic acid is 2mol / L, then heating to 90°C, adding 25 parts of clean water dropwise at a speed of 5mL / min during the heating process, hydrolyzing reaction for 3h, finally adding 50mL of hydrogen peroxide with a mass fraction of 30% to terminate the reaction, and obtaining graphite oxide after centrifugation, washing and impurity removal;

[0012] A3. Add 20 g of graphite oxide to 1000 mL of deionized water and perform ultrasonic dispersion for 2 h at an ultrasonic power of 500 W to form a uniform dispersion. Add 5 g of a reducing agent to the dispersion to perform a reduction reaction on part of the graphite oxide for 4 h, retain part of the ethyl hydroxyacetate, and obtain modified graphene.

[0013] Furthermore, in some embodiments, the concentration of concentrated sulfuric acid is 98% by mass, the concentration of concentrated nitric acid is 68% by mass, the reducing agent is sodium borohydride, and the sodium borohydride is added to form a sodium borohydride aqueous solution with a mass fraction of 10%.

[0014] The present invention also provides a method for preparing the aluminum foil, comprising the following steps:

[0015] B1. Clean the aluminum foil;

[0016] B2, vacuum drying the conductive graphite and nano-conductive carbon black for standby use;

[0017] B3, adding the carboxylated carbon nanotubes into an ultrasonic disperser, and adding 25 parts by mass of 99% N-methylpyrrolidone to disperse the carboxylated carbon nanotubes;

[0018] B4. Preparation of coating slurry:

[0019] B401, bottom slurry: put conductive graphite, binder and dispersant into an ultrasonic reactor, add deionized water, perform ultrasonic stirring and heating, and obtain bottom slurry;

[0020] B402, middle layer slurry: nano conductive carbon black, modified graphene, polyurethane elastomer and boron nitride powder are mixed in an ultrasonic reactor, and then a binder, a dispersant and deionized water are added, and ultrasonic stirring and heating are performed to obtain a middle layer slurry;

[0021] B403, top slurry: ultrasonically stirring and heating the carboxylated carbon nanotubes, binder, dispersant and deionized water in an ultrasonic reactor to obtain a top slurry;

[0022] B5. Coating: first apply the bottom layer slurry on the surface of the aluminum foil, let it stand and dry, and form a bottom layer material on the surface of the aluminum foil; then apply the middle layer slurry on the bottom layer slurry, let it stand and dry, and form a middle layer material on the surface of the bottom layer material; then apply the top layer slurry on the middle layer slurry, and form a top layer material on the surface of the middle layer material; finally, put the coated aluminum foil into an oven to dry, and obtain carbon-coated aluminum foil.

[0023] Furthermore, in some embodiments, the solid content of the bottom layer slurry, the middle layer slurry and the top layer slurry is 30% to 40%.

[0024] Furthermore, in some embodiments, the ultrasonic stirring frequency in the ultrasonic reactor in step B4 is 50-80 kHz, the stirring time is 1.5-3 h, and the heating temperature is 35-50° C.

[0025] Furthermore, in some embodiments, the coating machine is used to coat the surface of the aluminum foil. When coating the bottom layer slurry, the coating machine scraper gap is 0.7-1 mm, the coating speed is 2-3 m / min, and the coating thickness of the bottom layer slurry is 1-2 μm; when coating the middle layer slurry, the coating machine scraper gap is 0.5-0.8 mm, the coating speed is 2-2.5 m / min, and the coating thickness of the middle layer slurry is 0.8-1.5 μm; when coating the top layer slurry, the coating machine scraper gap is 0.3-0.6 mm, the coating speed is 1-2 m / min, and the coating thickness of the top layer slurry is 0.2-0.4 μm.

[0026] Beneficial effects: 1. The present invention designs the carbon-coated layer of the carbon-coated aluminum foil into a multi-layer composite structure. The bottom large-particle graphite in the carbon-coated layer constructs a stable conductive skeleton, which fits tightly with the aluminum foil and provides a stable path for electron transmission. The nano-scale carbon black in the middle layer cooperates with the modified graphene to capture electrons by utilizing the high specific surface area of ​​carbon black. The modified graphene conducts electrons quickly by virtue of its excellent electron migration characteristics. The two work together to greatly improve the electron transmission efficiency. The carboxylated carbon nanotubes in the top layer adjust the electrode interface environment and promote the uniform diffusion of lithium ions, so that the transmission of ions and electrons is smoother during the high-rate charge and discharge process of the battery, thereby significantly improving the cycle rate performance of the lithium battery.

[0027] 2. The present invention adds boron nitride powder to the middle layer, and utilizes its stable chemical thermal stability and high ignition point characteristics. In the high temperature environment of high temperature charging and discharging of lithium batteries, the boron nitride powder acts as a heat insulation protective layer, which can effectively prevent heat from being transferred to the aluminum foil substrate too quickly, reduce the negative impact of high temperature on the conductivity and mechanical properties of the aluminum foil, and ensure that the battery can still maintain stable performance output under high temperature conditions, thereby improving the charging and discharging stability of lithium batteries in high temperature environments;

[0028] 3. Taking advantage of the adaptive buffering properties of polyurethane elastomers, during the charge and discharge process of lithium batteries, as the volume of the electrode material expands and contracts, these elastic particles can dynamically adjust their own shape, fill the microscopic gaps caused by volume changes, and always maintain close contact between the electrode and the current collector, effectively preventing the problem of a sharp increase in resistance caused by poor contact, ensuring stable battery performance under high-rate cycles, and further improving the overall performance and service life of the battery. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with specific implementation methods. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative changes belong to the protection scope of the present invention.

[0030] Example 1

[0031] Preparation of modified graphene:

[0032] A1. Crush 100g of natural graphite flakes, add 1000mL of a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1, wherein the concentration of concentrated sulfuric acid is 98% by mass, and the concentration of concentrated nitric acid is 68% by mass, and stir at room temperature at a speed of 300r / min for 48h until the reaction is completed;

[0033] A2, adding a strong oxidant consisting of 30g potassium permanganate and 100mL concentrated sulfuric acid, stirring continuously at 35°C at a speed of 250r / min for oxidation reaction for 5h, then adding 50mL of a mixed solution consisting of ethylene glycol and acetic acid in a molar ratio of 1:1 to prepare ethyl hydroxyacetate, wherein the molar concentration of ethylene glycol and acetic acid is 2mol / L, then heating to 90°C, adding 25 parts of clean water dropwise at a speed of 5mL / min during the heating process, hydrolyzing reaction for 3h, finally adding 50mL of hydrogen peroxide with a mass fraction of 30% to terminate the reaction, and obtaining graphite oxide after centrifugation, washing and impurity removal;

[0034] A3. Add 20 g of graphite oxide to 1000 mL of deionized water and perform ultrasonic dispersion for 2 hours at an ultrasonic power of 500 W to form a uniform dispersion. Add 5 g of reducing agent sodium borohydride to the dispersion. When sodium borohydride is added, it is configured to have a sodium borohydride aqueous solution with a mass fraction of 10%. Perform reduction reaction on part of the graphite oxide for 4 hours, retain part of the ethyl hydroxyacetate, and obtain modified graphene, which contains 15% by mass of unmodified graphene.

[0035] The preparation of carbon-coated aluminum foil includes the following steps:

[0036] B1. Select an aluminum foil with a purity of 99.5% and a thickness of 15 μm, then put the aluminum foil into anhydrous ethanol and soak it for 30 minutes. After soaking, use clean water to clean the impurities and dirt on the surface of the aluminum foil, then take it out and place it in a dust-free environment to dry naturally to obtain a clean aluminum foil base layer;

[0037] B2, 35 parts (weight parts, the same below) of conductive graphite (particle size of 8 μm) were placed in a vacuum oven for drying, and the vacuum oven was started, and the drying temperature was set to 80 ° C. The drying time was 2 hours, and moisture was removed. After the conductive graphite was dried, 17.5 parts of nano conductive carbon black (particle size of 50nm) were placed in a vacuum oven for drying under the same drying conditions, and the drying was completed and taken out for standby use;

[0038] B3, select 40 parts of finished carboxylated carbon nanotubes, prepare an ultrasonic disperser, put the carboxylated carbon nanotubes into the ultrasonic disperser, add 25 parts of N-methylpyrrolidone, and then start the ultrasonic disperser to disperse the carboxylated carbon nanotubes to prevent them from agglomerating;

[0039] B4, then carry out the slurry preparation of carbon-coated aluminum foil, which comprises the following steps:

[0040] B401, bottom slurry: put the dried conductive graphite into an ultrasonic reactor, then add 1 part of a binder and 5 parts of a dispersant into the reactor, then add deionized water, and then start the ultrasonic reactor for ultrasonic stirring and heating, wherein the ultrasonic frequency is 50 kHz, the heating temperature is 35°C, and the stirring is performed for 1.5 hours to obtain a bottom slurry with a solid content of 30% (mass fraction);

[0041] B402, middle layer slurry: 17.5 parts of dried nano conductive carbon black, 12.5 parts of modified graphene, 7.5 parts of polyurethane elastomer (produced by Covestro Bayer AG, Germany, the specific model is TPU polyurethane elastomer 5377 transparent raw material, the same below), 5 parts of boron nitride powder (particle size is 0.5 μm, produced by Yingkou Liaobin Fine Chemical Co., Ltd., the specific model is boron nitride 10043-11-5 raw material, the same below), mix, then add 5 parts of binder and 2.5 parts of dispersant, put into an ultrasonic reactor and add deionized water, then start the ultrasonic reactor for ultrasonic stirring and heating, wherein the ultrasonic frequency is 50 kHz, the heating temperature is 35°C, stir for 1.5 hours, and obtain a middle layer slurry with a solid content of 35% (mass fraction);

[0042] B403, top slurry: 40 parts of carboxylated carbon nanotubes were put into an ultrasonic reactor, and 7.5 parts of a binder, 2.5 parts of a dispersant and deionized water were added, and then the ultrasonic reactor was started for ultrasonic stirring and heating, wherein the ultrasonic frequency was 50 kHz and the heating temperature was 35° C. The mixture was stirred for 1.5 hours to obtain a top slurry having a solid content of 40% (mass fraction);

[0043] B5. Prepare the coating machine, install the aluminum foil base layer on the coating machine's work station, then lay the aluminum foil base layer flat on the coating machine's conveyor belt and transport it forward, then evenly coat the bottom slurry on the aluminum foil surface, then wait for the bottom slurry to be coated successfully, let the aluminum foil base layer coated with the bottom slurry stand and dry to form the bottom material, then put the aluminum foil base layer coated with the bottom slurry on the coating machine again for coating of the middle layer slurry, then repeat the above drying and loading process to form the middle layer material, finally coat the top layer slurry on the middle layer material to form the top layer material, complete the attachment of the carbon coating layer, and the coating scraper gap, speed and coating thickness are in order of bottom and bottom. Layer slurry: 0.8mm, 2.5m / min, 1μm; middle layer slurry: 0.5mm, 2m / min, 0.8μm; top layer slurry: 0.3mm, 1.5m / min, 0.4μm. After the three layers of slurry are coated, the aluminum foil base layer and the material coated with the carbon coating layer are placed in an oven for drying. The drying temperature is 100°C and the drying time is 2h. Finally, the carbon-coated aluminum foil is obtained. Subsequently, low-temperature plasma treatment is used. The carbon-coated aluminum foil is placed in a low-temperature device, and then a mixed gas of argon and hydrogen is introduced to clean the surface of the carbon-coated aluminum foil. After cleaning, the carbon-coated aluminum foil is cut to produce the corresponding lithium battery.

[0044] Under standard charge and discharge conditions, after 500 cycles, the battery capacity retention rate reached 90%, the 3C high-rate discharge capacity retention rate was 85%, and under high temperature of 60°C environment, after 200 cycles of 1C rate charge and discharge, the capacity retention rate was 80%, the battery internal resistance increased slowly, the electrode and the current collector were in good contact, and the heat generation phenomenon was not obvious. The contact resistance was measured using a four-probe resistance tester, and the resistance was 0.75mΩ / cm2. The adhesion was measured by a tensile testing machine, reaching 2.45N / cm. The coating was tightly bonded to the electrode material, and no undesirable phenomena such as falling off and stratification occurred during the cycle.

[0045] Example 2

[0046] The preparation method of modified graphene is the same as that in Example 1.

[0047] The preparation of carbon-coated aluminum foil includes the following steps:

[0048] B1. Select an aluminum foil with a purity of 99.2% and a thickness of 18 μm, then put the aluminum foil into anhydrous ethanol and soak it for 60 minutes. After soaking, use clean water to clean the impurities and dirt on the surface of the aluminum foil, then take it out and place it in a dust-free environment to dry naturally to obtain a clean aluminum foil base layer;

[0049] B2, 35 parts of conductive graphite (particle size of 6 μm) were placed in a vacuum oven for drying, the vacuum oven was started, the drying temperature was set to 100 ° C, the drying time was 3 hours, and the moisture was removed. After the conductive graphite was dried, 17.5 parts of nano conductive carbon black (particle size of 60nm) were placed in a vacuum oven for drying under the same drying conditions, and after drying, they were taken out for standby use;

[0050] B3, select 40 parts of finished carboxylated carbon nanotubes, prepare an ultrasonic disperser, put the carboxylated carbon nanotubes into the ultrasonic disperser, add 25 parts of N-methylpyrrolidone, and then start the ultrasonic disperser to disperse the carboxylated carbon nanotubes to prevent them from agglomerating;

[0051] B4, then carry out the slurry preparation of carbon-coated aluminum foil, which comprises the following steps:

[0052] B401, bottom slurry: put the dried conductive graphite into an ultrasonic reactor, then add 1 part of a binder and 5 parts of a dispersant in the corresponding amount into the reactor, then add deionized water, and then start the ultrasonic reactor for ultrasonic stirring and heating, wherein the ultrasonic frequency is 70kHz, the heating temperature is 35°C, and the stirring is performed for 2 hours to obtain a bottom slurry with a solid content of 35% (mass fraction);

[0053] B402, middle layer slurry: 17.5 parts of dried nano conductive carbon black, 12.5 parts of modified graphene, 7.5 parts of polyurethane elastomer and 5 parts of boron nitride powder (particle size of 0.8 μm) were mixed, and then 5 parts of binder and 2.5 parts of dispersant were added, put into an ultrasonic reactor and deionized water was added, and then the ultrasonic reactor was started for ultrasonic stirring and heating, wherein the ultrasonic frequency was 70 kHz and the heating temperature was 45° C. The mixture was stirred for 3 hours to obtain a middle layer slurry with a solid content of 40% (mass fraction);

[0054] B403, top layer slurry: 40 parts of carboxylated carbon nanotubes were put into an ultrasonic reactor, and 7.5 parts of a binder, 2.5 parts of a dispersant and deionized water were added, and then the ultrasonic reactor was started for ultrasonic stirring and heating; wherein the ultrasonic frequency was 70 kHz, the heating temperature was 45°C, and the stirring was performed for 2 hours to obtain a middle layer slurry with a solid content of 30% (mass fraction);

[0055] B5. Prepare the coating machine, install the aluminum foil base layer on the coating machine's work station, then lay the aluminum foil base layer flat on the coating machine's conveyor belt and transport it forward, then evenly coat the bottom slurry on the aluminum foil surface, and then wait for the bottom slurry to be coated successfully, let the aluminum foil base layer coated with the bottom slurry stand and dry, then put the aluminum foil base layer coated with the bottom slurry on the coating machine again for coating of the middle layer slurry, and then repeat the above drying and loading process, and finally coat the top layer slurry on the middle layer slurry to complete the attachment of the carbon coating layer. The coating scraper gap, speed and coating thickness are as follows: bottom slurry: 1mm, 2m / min, 2μm; middle layer slurry: 0.6mm, 2.5m / min, 1μm; top layer slurry: 0.4mm, 2m / min, 0.3μm. After the three layers of slurry are coated, the aluminum foil base layer and the material coated with the carbon coating layer are placed in an oven for drying at a temperature of 90°C and a drying time of 2.5h. Finally, the carbon-coated aluminum foil is obtained. Subsequently, low-temperature plasma treatment is adopted. The carbon-coated aluminum foil is placed in a low-temperature device, and then a mixed gas of argon and hydrogen is introduced to clean the surface of the carbon-coated aluminum foil. After cleaning, the carbon-coated aluminum foil is cut to produce the corresponding lithium battery.

[0056] After 500 cycles under standard conditions, the battery capacity retention rate is 88%, the 3C high-rate discharge capacity retention rate is 82%, and under a high temperature of 60°C, after 200 cycles of 1C rate charge and discharge, the capacity retention rate is 78%. The battery internal resistance is stable, the contact between the electrode and the current collector is stable, the heat generation is controllable, the test resistance is 0.84mΩ / cm2, the adhesion measurement value is 1.95N / cm, the coating has good stability, and meets the requirements for the use of lithium batteries.

[0057] Example 3

[0058] The preparation method of modified graphene is the same as that in Example 1.

[0059] The preparation of carbon-coated aluminum foil includes the following steps:

[0060] B1. Select an aluminum foil with a purity of 99.8% and a thickness of 12 μm, then put the aluminum foil into anhydrous ethanol and soak it for 50 minutes. After soaking, use clean water to clean the impurities and dirt on the surface of the aluminum foil, then take it out and place it in a dust-free environment to dry naturally to obtain a clean aluminum foil base layer;

[0061] B2, 35 parts of conductive graphite (particle size of 9 μm) were placed in a vacuum oven for drying, the vacuum oven was started, the drying temperature was set to 90 ° C, the drying time was 2.5 hours, and the moisture was removed. After the conductive graphite was dried, 17.5 parts of nano conductive carbon black (particle size of 40 nm) were placed in a vacuum oven for drying under the same drying conditions, and after drying, they were taken out for standby use;

[0062] B3, select 40 parts of finished carboxylated carbon nanotubes, prepare an ultrasonic disperser, put the carboxylated carbon nanotubes into the ultrasonic disperser, add 25 parts of N-methylpyrrolidone, and then start the ultrasonic disperser to disperse the carboxylated carbon nanotubes to prevent them from agglomerating;

[0063] B4, then carry out the slurry preparation of carbon-coated aluminum foil, which comprises the following steps:

[0064] B401, bottom slurry: put the dried conductive graphite into an ultrasonic reactor, then add 1 part of a binder and 5 parts of a dispersant in the reactor, then add a corresponding amount of deionized water, and then start the ultrasonic reactor for ultrasonic stirring and heating; the ultrasonic frequency is 80kHz, the heating temperature is 50°C, and stirring is performed for 3 hours to obtain a bottom slurry with a solid content of 40% (mass fraction).

[0065] B402, middle layer slurry: 17.5 parts of dried nano conductive carbon black, 12.5 parts of modified graphene, 7.5 parts of polyurethane elastomer and 5 parts of boron nitride powder (particle size of 0.3 μm) were mixed, and then 5 parts of binder and 2.5 parts of dispersant were added, and the mixture was put into an ultrasonic reactor and deionized water was added, and then the ultrasonic reactor was started for ultrasonic stirring and heating; wherein the ultrasonic frequency was 80 kHz, the heating temperature was 50° C., and the mixture was stirred for 3 hours to obtain a middle layer slurry with a solid content of 30% (mass fraction);

[0066] B403, top slurry: 40 parts of carboxylated carbon nanotubes were put into an ultrasonic reactor, and 7.5 parts of a binder, 2.5 parts of a dispersant and deionized water were added, and then the ultrasonic reactor was started for ultrasonic stirring and heating; wherein the ultrasonic frequency was 80 kHz, the heating temperature was 35° C., and the stirring was performed for 1.5 hours to obtain a top slurry with a solid content of 35% (mass fraction);

[0067] B5. Prepare the coating machine, install the aluminum foil base layer on the coating machine station, and then lay the aluminum foil base layer flat on the conveyor belt of the coating machine for forward conveyance, and then evenly coat the bottom slurry on the surface of the aluminum foil. After waiting for the bottom slurry to be coated successfully, let the aluminum foil base layer coated with the bottom slurry stand and dry, and then put the aluminum foil base layer coated with the bottom slurry on the coating machine again for coating of the middle layer slurry, and then repeat the above drying and loading process, and finally coat the top layer slurry on the middle layer slurry to complete the attachment of the carbon coating layer. The coating scraper gap, speed and coating thickness are as follows: bottom slurry: 0.7mm, 3 m / min, 1.5μm; middle layer slurry: 0.8mm, 2.5m / min, 1.5μm; top layer slurry: 0.6mm, 1m / min, 0.2μm. After the three layers of slurry are coated, the aluminum foil base layer and the material coated with the carbon coating layer are placed in an oven for drying at a temperature of 110°C and a drying time of 1.5h. Finally, the carbon-coated aluminum foil is obtained. Subsequently, low-temperature plasma treatment is adopted. The carbon-coated aluminum foil is placed in a low-temperature device, and then a mixed gas of argon and hydrogen is introduced to clean the surface of the carbon-coated aluminum foil. After cleaning, the carbon-coated aluminum foil is cut to produce the corresponding lithium battery.

[0068] After 500 cycles under standard conditions, the battery capacity retention rate is 92%, the 3C high-rate discharge capacity retention rate is 88%, and under a high temperature of 60°C, after 200 cycles of 1C rate charge and discharge, the capacity retention rate is 82%. The battery internal resistance changes little, the electrode and the current collector always maintain close contact, the heat dissipation is good, the test resistance is 0.56mΩ / cm2, and the bonding force reaches 2.94N / cm. Under high-rate charge and discharge and high-temperature environments, the carbon-coated aluminum foil and the electrode material work together excellently.

[0069] Comparative Example 1

[0070] Traditional pure aluminum foil is used as the current collector of lithium batteries without any carbon coating treatment. Commercially available aluminum foil with a purity of 99.5% and a thickness of 15 μm is directly selected and cut into a suitable size for lithium battery assembly.

[0071] Comparative Example 2

[0072] A conventional pure aluminum foil is used and a single layer of carbon black is coated on its surface. The carbon black particle size for coating is 50 nm. An adhesive is used for coating and bonding. No other functional materials are added. Other conditions are the same as in Example 1.

[0073] Comparative Example 3

[0074] A conventional pure aluminum foil is used and double-layer coating is applied on its surface. The bottom layer is large-particle conductive graphite (particle size is 8 μm), and the top layer is ordinary conductive carbon black (particle size is 50 nm). No other materials in Examples 1-3 are added, and other conditions are the same as in Example 1.

[0075] Table 1 shows the performance of resistance, adhesion and heating of carbon-coated aluminum foil:

[0076] Category / Project Contact resistance (mΩ / cm2) Adhesion force (N / cm) Fever Comparative Example 1 2.4 - Severe fever Comparative Example 2 1.56 1.45 Severe fever Comparative Example 3 1.08 1.75 Fever controllable Example 1 0.75 2.45 Fever is not obvious Example 2 0.84 1.95 Fever controllable Example 3 0.56 2.94 Low heat generation

[0077] As can be seen from Table 1, compared with Comparative Examples 1-3, the performance of contact resistance, adhesion and heat generation of Examples 1-3 is significantly improved, and Example 3 has a significant performance improvement compared with Example 1-2;

[0078] Table 2 shows the battery capacity retention rate data of lithium batteries after charging and discharging at different rates:

[0079] Category / Project Standard charge and discharge retention rate (%) High rate charge and discharge retention rate (%) High temperature charge and discharge retention rate (%) Comparative Example 1 79 73 70 Comparative Example 2 82 79 76 Comparative Example 3 85 80 78 Example 1 90 85 80 Example 2 88 82 78 Example 3 92 88 82

[0080] Among them, the standard charge and discharge retention rate is the battery capacity retention rate after 500 charge and discharge cycles using the original standard power charger under standard environmental conditions, the high-rate charge and discharge retention rate is the battery capacity retention rate after 200 charge and discharge cycles using a high-rate charger that has passed 3C under standard environmental conditions, and the high-temperature charge and discharge retention rate is the battery capacity retention rate after 200 charge and discharge cycles using a 1C rate charger under a high temperature environment of 60°C;

[0081] As can be seen from Table 2, the battery capacity retention rates of Examples 1-3 after charge and discharge under different standard environments are compared with those of Comparative Examples 1-3, and the battery capacity retention rates are significantly improved, among which Example 3 has a significant performance improvement compared with Examples 1-2.

[0082] Although the disclosure is disclosed as above, the protection scope of the disclosure is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the disclosure, and these changes and modifications will fall within the protection scope of this application.

Claims

1. A carbon-coated aluminum foil for improving the cycle rate performance of a lithium battery, characterized in that: The surface of the aluminum foil substrate is sequentially provided with a bottom layer material, a middle layer material and a top layer material; the bottom layer material comprises conductive graphite, the middle layer material comprises nano conductive carbon black, modified graphene, polyurethane elastomer and boron nitride powder, the top layer material comprises carboxylated carbon nanotubes, and the bottom layer material, the middle layer material and the top layer material also comprise a binder and a dispersant.

2. The carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries according to claim 1, characterized in that: The mass ratio of conductive graphite, binder and dispersant in the bottom layer material is 70:2:10; the mass ratio of nano conductive carbon black, modified graphene, polyurethane elastomer, boron nitride powder, binder and dispersant in the middle layer material is 35:25:15:10:10:5; the mass ratio of carboxylated carbon nanotubes, binder and dispersant in the top layer material is 80:15:

5.

3. The carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries according to claim 1 or 2, characterized in that: The thickness of the bottom layer material is 1-2 μm, the thickness of the middle layer material is 0.8-1.5 μm, and the thickness of the top layer material is 0.2-0.4 μm.

4. The carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries according to claim 1, characterized in that: The binder is styrene-butadiene rubber emulsion, and the dispersant is polyethylene glycol.

5. The carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries according to claim 1, characterized in that: The preparation method of the modified graphene is: A1, 100g natural graphite flakes were crushed, and 1000mL of a mixed acid solution of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1 was added thereto, and the mixture was stirred at room temperature at a speed of 300r / min for 48h until the reaction was completed; A2, adding a strong oxidant consisting of 30g potassium permanganate and 100mL concentrated sulfuric acid, stirring continuously at 35°C at a speed of 250r / min for oxidation reaction for 5h, then adding 50mL of a mixed solution consisting of ethylene glycol and acetic acid in a molar ratio of 1:1 to prepare ethyl hydroxyacetate, wherein the molar concentration of ethylene glycol and acetic acid is 2mol / L, then heating to 90°C, adding 25 parts of clean water dropwise at a speed of 5mL / min during the heating process, hydrolyzing reaction for 3h, finally adding 50mL of hydrogen peroxide with a mass fraction of 30% to terminate the reaction, and obtaining graphite oxide after centrifugation, washing and impurity removal; A3. Add 20 g of graphite oxide to 1000 mL of deionized water and perform ultrasonic dispersion for 2 h at an ultrasonic power of 500 W to form a uniform dispersion. Add 5 g of a reducing agent to the dispersion to perform a reduction reaction on part of the graphite oxide for 4 h, retain part of the ethyl hydroxyacetate, and obtain modified graphene.

6. The carbon-coated aluminum foil for improving the cycle rate performance of lithium batteries according to claim 5, characterized in that: The concentration of the concentrated sulfuric acid is 98% by mass, the concentration of the concentrated nitric acid is 68% by mass, the reducing agent is sodium borohydride, and the sodium borohydride is added to form a sodium borohydride aqueous solution with a mass fraction of 10%.

7. The method for preparing a carbon-coated aluminum foil for improving the cycle rate performance of a lithium battery according to any one of claims 1 to 6, characterized in that: The following steps are involved: B1. Clean the aluminum foil; B2, vacuum drying the conductive graphite and nano-conductive carbon black for standby use; B3, adding the carboxylated carbon nanotubes into an ultrasonic disperser, and adding 25 parts by mass of 99% N-methylpyrrolidone to disperse the carboxylated carbon nanotubes; B4. Preparation of coating slurry: B401, bottom slurry: put conductive graphite, binder and dispersant into an ultrasonic reactor, add deionized water, perform ultrasonic stirring and heating, and obtain bottom slurry; B402, middle layer slurry: nano conductive carbon black, modified graphene, polyurethane elastomer and boron nitride powder are mixed in an ultrasonic reactor, and then a binder, a dispersant and deionized water are added, and ultrasonic stirring and heating are performed to obtain a middle layer slurry; B403, top slurry: ultrasonically stirring and heating the carboxylated carbon nanotubes, binder, dispersant and deionized water in an ultrasonic reactor to obtain a top slurry; B5. Coating: first apply the bottom layer slurry on the surface of the aluminum foil, let it stand and dry, and form a bottom layer material on the surface of the aluminum foil; then apply the middle layer slurry on the bottom layer slurry, let it stand and dry, and form a middle layer material on the surface of the bottom layer material; then apply the top layer slurry on the middle layer slurry, and form a top layer material on the surface of the middle layer material; finally, put the coated aluminum foil into an oven to dry, and obtain carbon-coated aluminum foil.

8. The preparation method according to claim 7, characterized in that: The solid contents of the bottom slurry, the middle slurry and the top slurry are 30% to 40%.

9. The preparation method according to claim 7, characterized in that: In step B4, the ultrasonic stirring frequency in the ultrasonic reactor is 50-80 kHz, the stirring time is 1.5-3 hours, and the heating temperature is 35-50°C.

10. The preparation method according to claim 7, characterized in that: The coating machine is used to coat the surface of the aluminum foil. When coating the bottom layer slurry, the coating machine scraper gap is 0.7-1mm, the coating speed is 2-3m / min, and the coating thickness of the bottom layer slurry is 1-2μm; when coating the middle layer slurry, the coating machine scraper gap is 0.5-0.8mm, the coating speed is 2-2.5m / min, and the coating thickness of the middle layer slurry is 0.8-1.5μm; when coating the top layer slurry, the coating machine scraper gap is 0.3-0.6mm, the coating speed is 1-2m / min, and the coating thickness of the top layer slurry is 0.2-0.4μm.