A negative electrode sheet, a secondary battery, and an electronic device

Abstract

The application provides a negative electrode sheet, a secondary battery and an electronic device. The negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprises a dispersing agent, the dispersing agent comprises a first dispersing agent and a second dispersing agent, the first dispersing agent comprises carboxymethyl cellulose, the second dispersing agent comprises at least one of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, the mass ratio of the first dispersing agent to the second dispersing agent is (1:1) to (1:5); the negative electrode sheet is subjected to heat treatment, the temperature of the heat treatment is 200 DEG C to 400 DEG C; the mass percentage of lithium in the negative electrode material layer is 0% to 0.051%, and / or the mass percentage of sodium in the negative electrode material layer is 0% to 0.158%. The secondary battery has good kinetic performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and in particular to a negative electrode sheet, a secondary battery and an electronic device. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, are widely used in the fields of smart phones, wearable devices, consumer drones and electric vehicles due to their high energy density, long cycle life and no memory effect. With the wide application of lithium ion batteries in the above fields, the market has higher and higher requirements for the charging speed of lithium ion batteries, and therefore the requirements for the kinetic performance of lithium ion batteries are also higher and higher.

[0003] However, there is often residual dispersant in the negative electrode material layer of the existing negative electrode sheet, and the lithium compound or sodium compound of the residual dispersant will affect the transmission of lithium ions in the negative electrode sheet, thereby affecting the kinetic performance of the lithium ion battery. SUMMARY

[0004] The purpose of the present application is to provide a negative electrode sheet, a secondary battery and an electronic device to improve the kinetic performance of the secondary battery. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a dispersant, the dispersant comprising a first dispersant and a second dispersant, the first dispersant comprising carboxymethyl cellulose (CMC-H), the second dispersant comprising at least one of lithium carboxymethyl cellulose (CMC-Li) or sodium carboxymethyl cellulose (CMC-Na), the mass ratio of the first dispersant to the second dispersant being (1:1) to (1:5); the negative electrode sheet is subjected to heat treatment, the temperature of the heat treatment being 200℃C to 400℃C; the mass percentage content of lithium in the negative electrode material layer is 0% to 0.051%, and / or the mass percentage content of sodium in the negative electrode material layer is 0% to 0.158%. By selecting the above first dispersant and second dispersant and adjusting the mass ratio thereof within the above range, and by subjecting the negative electrode sheet to heat treatment and adjusting the temperature of the heat treatment within the above range, the uniformity and stability of the negative electrode slurry can be improved, the distribution of each component of the prepared negative electrode material layer is uniform, the negative electrode material layer and the negative electrode current collector have high adhesion, the negative electrode sheet has high cohesion, and the lithium compound and / or sodium compound of the residual dispersant can be reduced, the mass percentage content of lithium and / or the mass percentage content of sodium in the negative electrode material layer is within the above range and is kept at a low level, which is beneficial to the transmission of lithium ions in the negative electrode sheet, thereby improving the kinetic performance of the secondary battery.

[0006] In some embodiments of the present application, the mass ratio of the first dispersant to the second dispersant is (1:1) to (1:2). By regulating the mass ratio of the first dispersant to the second dispersant within the above range, a higher adhesion between the negative material layer and the negative current collector can be maintained, the negative electrode sheet has a higher cohesion, and at the same time, the residual lithium compound and / or sodium compound of the dispersant can be further reduced, which is more conducive to the transmission of lithium ions in the negative electrode sheet, thereby further improving the kinetic performance of the secondary battery.

[0007] In some embodiments of the present application, the mass percentage of lithium element in the negative material layer is 0.012% to 0.051%, or the mass percentage of sodium element in the negative material layer is 0.038% to 0.158%. By regulating the mass percentage of lithium element or sodium element in the negative material layer within the above range, the transmission of lithium ions in the negative electrode sheet is more conducive, thereby further improving the kinetic performance of the secondary battery.

[0008] In some embodiments of the present application, the heat treatment time is 2h to 10h. By regulating the heat treatment time within the above range, the preparation cost of the negative electrode sheet can be reduced, and the preparation time of the negative electrode sheet can be shortened, and at the same time, the first dispersant and the second dispersant can also be decomposed by the heat treatment reaction, reducing the residual lithium compound and / or sodium compound of the dispersant, which is conducive to the transmission of lithium ions in the negative electrode sheet, thereby improving the kinetic performance of the secondary battery.

[0009] In some embodiments of the present application, the mass percentage of the dispersant based on the mass of the negative material layer is 0.8% to 2%. By regulating the mass percentage of the dispersant within the above range, the uniformity and stability of the negative electrode slurry can be improved, the distribution of each component of the prepared negative material layer is uniform, a higher adhesion between the negative material layer and the negative current collector can be maintained, the negative electrode sheet has a higher cohesion, and at the same time, the residual lithium compound and / or sodium compound of the dispersant can be reduced, which is conducive to the transmission of lithium ions in the negative electrode sheet, thereby improving the kinetic performance of the secondary battery; in addition, the mass percentage of the negative active material in the negative material layer can be higher, thereby improving the energy density of the negative electrode sheet.

[0010] In some embodiments of the present application, the number average molecular weight of the first dispersant is 300,000 to 1,000,000. By regulating the number average molecular weight of the first dispersant within the above range, the negative electrode slurry can have a suitable viscosity, which is conducive to improving the uniformity and stability of the negative electrode slurry, the distribution of each component of the prepared negative material layer is uniform, a higher adhesion between the negative material layer and the negative current collector can be maintained, the negative electrode sheet has a higher cohesion, and the kinetic performance of the secondary battery is further improved.

[0011] In some embodiments of this application, at 25°C, when the mass percentage of the first dispersant is 1%, the pH of the aqueous solution of the first dispersant is 2 to 6, and the viscosity is 1000 mPa·s to 30000 mPa·s. By adjusting the pH and viscosity of the aqueous solution of the first dispersant within the above ranges, it is beneficial to improve the uniformity and stability of the negative electrode slurry, ensure that the components of the prepared negative electrode material layer are evenly distributed, maintain a high bonding force between the negative electrode material layer and the negative electrode current collector, and ensure that the negative electrode sheet has a high cohesive force, thereby improving the kinetic performance of the secondary battery.

[0012] In some embodiments of this application, the single-sided coating areal density of the negative electrode material layer is 80 mg / 1540.25 mm. 2 Up to 160mg / 1540.25mm 2 By adjusting the surface density of the single-sided coating of the negative electrode material layer within the above range, it is possible to maintain a high adhesion between the negative electrode material layer and the negative electrode current collector, and to ensure that the negative electrode sheet has a high cohesive force, thereby improving the dynamic performance of the secondary battery.

[0013] In some embodiments of this application, the compaction density of the negative electrode material layer is 1.6 g / cm³. 3 Up to 1.9 g / cm 3 By adjusting the compaction density of the negative electrode material layer within the aforementioned range, the porosity of the negative electrode material layer can be kept within a suitable range, improving the wettability of the electrolyte to the negative electrode sheet. This also increases the capacity of the negative electrode active material per unit volume, thereby improving the energy density of the secondary battery. Furthermore, it maintains high adhesion between the negative electrode material layer and the negative electrode current collector, resulting in high cohesion of the negative electrode sheet, which in turn improves the kinetic performance of the secondary battery.

[0014] In some embodiments of this application, the negative electrode material layer includes a negative electrode active material, which includes at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesophase carbon microspheres, elemental silicon, silicon-oxygen, or silicon-carbon. Using the above-mentioned negative electrode active material is beneficial for improving the kinetic performance of the secondary battery.

[0015] A second aspect of this application provides a secondary battery including the negative electrode provided in the first aspect of this application. The negative electrode provided in this application enables the secondary battery to have good kinetic performance.

[0016] A third aspect of this application provides an electronic device that includes the secondary battery provided in the second aspect of this application. The secondary battery provided in this application has good dynamic performance, thereby providing the electronic device with a long service life and good performance.

[0017] The beneficial effects of this application are:

[0018] The application provides a negative electrode sheet, a secondary battery and an electronic device. The negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprises a dispersing agent, the dispersing agent comprises a first dispersing agent and a second dispersing agent, the first dispersing agent comprises carboxymethyl cellulose, the second dispersing agent comprises at least one of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, the mass ratio of the first dispersing agent to the second dispersing agent is (1:1) to (1:5); the negative electrode sheet is subjected to heat treatment, the temperature of the heat treatment is 200°C to 400°C; the mass percentage of lithium in the negative electrode material layer is 0% to 0.051%, and / or the mass percentage of sodium in the negative electrode material layer is 0% to 0.158%. The application can improve the uniformity and stability of the negative electrode slurry by selecting the first dispersing agent and the second dispersing agent and adjusting the mass ratio of the first dispersing agent to the second dispersing agent in the above range, and by subjecting the negative electrode sheet to heat treatment and adjusting the temperature of the heat treatment in the above range, so that the components of the prepared negative electrode material layer are uniformly distributed, the negative electrode material layer and the negative electrode current collector have high adhesion, the negative electrode sheet has high cohesion, and the residual lithium compound and / or sodium compound of the dispersing agent can be reduced, so that the mass percentage of lithium in the negative electrode material layer and / or the mass percentage of sodium in the negative electrode material layer is in the above range and is kept at a low level, which is beneficial to the transmission of lithium ions in the negative electrode sheet, thereby improving the kinetic performance of the secondary battery.

[0019] Of course, implementing any product or method of the application does not necessarily require achieving all the advantages described above at the same time. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the application will be clearly and completely described below. Obviously, the described embodiments are only some of the embodiments of the application, but not all the embodiments of the application. All other embodiments obtained by those skilled in the art based on the application belong to the scope of protection of the application.

[0021] It should be noted that in the following content, the lithium ion battery is taken as an example of the secondary battery to explain the application, but the secondary battery of the application is not limited to the lithium ion battery. The specific technical solutions are as follows:

[0022] The first aspect of the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a dispersant, the dispersant comprising a first dispersant and a second dispersant, the first dispersant comprising carboxymethyl cellulose (CMC-H), the second dispersant comprising at least one of lithium carboxymethyl cellulose (CMC-Li) or sodium carboxymethyl cellulose (CMC-Na), the mass ratio of the first dispersant to the second dispersant being (1:1) to (1:5), preferably, the mass ratio of the first dispersant to the second dispersant being (1:1) to (1:2). For example, the mass ratio of the first dispersant to the second dispersant can be 1:1, 1:1.3, 1:1.5, 1:1.6, 1:1.8, 1:2, 1:2.5, 1:3, 1:3.4, 1:4, 1:4.5, 1:5, or a range consisting of any two of the aforementioned values. The negative electrode sheet is subjected to heat treatment, the temperature of the heat treatment being 200 °C to 400 °C. For example, the temperature of the heat treatment can be 200 °C, 230 °C, 260 °C, 300 °C, 340 °C, 370 °C, 400 °C, or a range consisting of any two of the aforementioned values. The mass percentage content of lithium in the negative electrode material layer is 0% to 0.051%, and / or the mass percentage content of sodium in the negative electrode material layer is 0% to 0.158%. For example, the mass percentage content of lithium in the negative electrode material layer can be 0%, 0.001%, 0.005%, 0.010%, 0.015%, 0.020%, 0.024%, 0.030%, 0.036%, 0.040%, 0.047%, 0.051%, or a range consisting of any two of the aforementioned values, and the mass percentage content of sodium in the negative electrode material layer can be 0%, 0.001%, 0.005%, 0.01%, 0.020%, 0.031%, 0.040%, 0.062%, 0.080%, 0.100%, 0.122%, 0.140%, 0.158%, or a range consisting of any two of the aforementioned values.

[0023] The inventors have found that the dispersant used in the negative material layer of the negative electrode plate usually contains lithium and / or sodium elements, and the lithium compound and / or sodium compound remaining in the dispersant can coat the negative active material, affecting the transmission of lithium ions in the negative electrode plate. By selecting the first dispersant and the second dispersant and adjusting the mass ratio thereof within the above range, and by heat treating the negative electrode plate and adjusting the heat treatment temperature within the above range, on the one hand, the risk of agglomeration of the negative slurry during the preparation of the negative material layer can be reduced, the uniformity and stability of the negative slurry can be improved, the distribution of each component of the prepared negative material layer is uniform, the negative material layer and the negative current collector have high adhesion, the negative electrode plate has high cohesion, and thus the kinetic performance of the secondary battery is improved; on the other hand, the first dispersant and the second dispersant can be decomposed by heat treatment reaction after the negative electrode plate is heat treated, and compared with the second dispersant, the first dispersant is more completely decomposed, thereby reducing the lithium compound and / or sodium compound remaining in the dispersant, keeping the mass percentage of lithium and / or the mass percentage of sodium in the negative material layer within the above range at a low level, which is beneficial to the transmission of lithium ions in the negative electrode plate, and thus the kinetic performance of the secondary battery is improved. Therefore, the negative electrode plate of the present application selects the above dispersant, and heat treats the negative electrode plate, which is beneficial to improve the kinetic performance of the secondary battery.

[0024] When the mass ratio of the first dispersant to the second dispersant is too small, for example, less than 1:5, the lithium compound and / or the sodium compound remaining in the negative electrode sheet after the heat treatment of the dispersant is too much, which is not conducive to the transmission of lithium ions in the negative electrode sheet, thereby affecting the kinetic performance of the secondary battery. When the mass ratio of the first dispersant to the second dispersant is too large, for example, greater than 1:1, the content of the first dispersant carboxymethyl cellulose is too high, and the negative electrode slurry is prone to agglomeration during the preparation of the negative electrode material layer, the uniformity and stability of the negative electrode slurry are poor, the distribution of each component of the prepared negative electrode material layer is uneven, the adhesion between the negative electrode material layer and the negative electrode current collector is too low, and the cohesion of the negative electrode sheet is too low, thereby the process difficulty of the preparation of the negative electrode sheet is large. When the temperature of the heat treatment is too low, for example, less than 200℃, the first dispersant and the second dispersant are difficult to be removed by the heat treatment reaction, and the lithium compound and / or the sodium compound remaining in the dispersant is too much, which is not conducive to the transmission of lithium ions in the negative electrode sheet, thereby affecting the kinetic performance of the secondary battery. When the temperature of the heat treatment is too high, for example, higher than 400℃, the negative electrode binder in the negative electrode material layer will be invalid under the action of high temperature, the bonding network of the negative electrode sheet is destroyed, the negative electrode material layer is easy to separate from the negative electrode current collector, the adhesion between the negative electrode material layer and the negative electrode current collector is too low, and the cohesion of the negative electrode sheet is too low, thereby the process difficulty of the preparation of the negative electrode sheet is large. When the mass percentage of lithium in the negative electrode material layer is too high, for example, higher than 0.051%, and / or, the mass percentage of sodium in the negative electrode material layer is too high, for example, higher than 0.158%, it indicates that the lithium compound and / or the sodium compound remaining in the dispersant after the heat treatment of the negative electrode sheet is too much, which is not conducive to the transmission of lithium ions in the negative electrode sheet, thereby affecting the kinetic performance of the secondary battery.

[0025] In some embodiments of the present application, the second dispersant is lithium carboxymethyl cellulose, the mass percentage of lithium element in the negative electrode material layer is 0.012% to 0.051%, and the mass percentage of sodium element in the negative electrode material layer is 0%; in other embodiments, the second dispersant is sodium carboxymethyl cellulose, the mass percentage of sodium element in the negative electrode material layer is 0.038% to 0.158%, and the mass percentage of lithium element in the negative electrode material layer is 0%. In still other embodiments, the second dispersant comprises both lithium carboxymethyl cellulose and sodium carboxymethyl cellulose, and the present application does not particularly limit the mass ratio of lithium carboxymethyl cellulose to sodium carboxymethyl cellulose, as long as the purpose of the present application can be achieved, for example, the mass ratio of lithium carboxymethyl cellulose to sodium carboxymethyl cellulose can be (1:99) to (99:1). For example, the mass ratio of lithium carboxymethyl cellulose to sodium carboxymethyl cellulose can be 1:99, 15:85, 30:70, 40:60, 50:50, 65:35, 80:20, 99:1, or a range composed of any two of the above values. It can be understood that, since the second dispersant comprises at least one of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, the mass percentage of lithium element and the mass percentage of sodium element in the negative electrode material layer are not both 0% at the same time.

[0026] In the present application, the negative electrode material layer can be arranged on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or on both surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, and the present application does not particularly limit it, as long as the purpose of the present application can be achieved.

[0027] In the present application, the heat treatment of the negative electrode sheet can be performed between any process after the negative electrode material layer is coated and before the secondary battery is injected with electrolyte. For example, the heat treatment of the negative electrode sheet can be performed after the negative electrode sheet is cold-pressed and before the sheet is cut, or can be performed after the sheet is cut and before the tab is welded.

[0028] In some embodiments of the present application, the heat treatment is performed for 2h to 10h. For example, the heat treatment can be performed for 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, or a range composed of any two of the above values. By adjusting the heat treatment time within the above range, the preparation cost of the negative electrode sheet can be reduced, and the preparation time of the negative electrode sheet can be shortened, while the first dispersant and the second dispersant can also be decomposed by the heat treatment reaction, reducing the residual lithium compounds and / or sodium compounds of the dispersants, which is beneficial to the transmission of lithium ions in the negative electrode sheet, thereby being beneficial to improving the kinetic performance of the secondary battery.

[0029] In some embodiments of the present application, the mass percentage content of the dispersant is 0.8% to 2% based on the mass of the negative electrode material layer. For example, the mass percentage content of the dispersant can be 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2.0%, or a range formed by any two of the above values. By adjusting the mass percentage content of the dispersant within the above range, the risk of agglomeration of the negative electrode slurry during the preparation of the negative electrode material layer can be reduced, the uniformity and stability of the negative electrode slurry can be improved, the distribution of the components of the prepared negative electrode material layer is uniform, the adhesion between the negative electrode material layer and the negative electrode current collector is high, the negative electrode sheet has high cohesion, the residual lithium compound and / or sodium compound of the dispersant can be reduced, which is conducive to the transmission of lithium ions in the negative electrode sheet, thereby improving the kinetic performance of the secondary battery; in addition, the mass percentage content of the negative electrode active material in the negative electrode material layer can be high, thereby improving the energy density of the negative electrode sheet.

[0030] In some embodiments of the present application, the number average molecular weight of the first dispersant is 300,000 to 1,000,000. For example, the number average molecular weight of the first dispersant can be 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, or a range formed by any two of the above values. By adjusting the number average molecular weight of the first dispersant within the above range, the negative electrode slurry can have a suitable viscosity, which is conducive to improving the uniformity and stability of the negative electrode slurry, the distribution of the components of the prepared negative electrode material layer is uniform, the adhesion between the negative electrode material layer and the negative electrode current collector is high, the negative electrode sheet has high cohesion, thereby improving the kinetic performance of the secondary battery.

[0031] In some embodiments of the present application, the pH of the aqueous solution of the first dispersant is 2 to 6 and the viscosity of the aqueous solution of the first dispersant is 1000 mPa s to 30000 mPa s when the mass percentage of the first dispersant is 1% at 25°C. For example, the pH of the aqueous solution of the first dispersant can be 2, 3, 4, 5, 6 or a range defined by any two of the above numbers, and the viscosity of the aqueous solution of the first dispersant can be 1000 mPa s, 4000 mPa s, 7000 mPa s, 10000 mPa s, 13000 mPa s, 17000 mPa s, 20000 mPa s, 24000 mPa s, 28000 mPa s, 30000 mPa s or a range defined by any two of the above numbers. By adjusting the pH and the viscosity of the aqueous solution of the first dispersant within the above ranges, the uniformity and stability of the negative electrode slurry can be improved, the components of the prepared negative electrode material layer are uniformly distributed, the adhesion between the negative electrode material layer and the negative electrode current collector is high, and the negative electrode sheet has high cohesion, thereby improving the kinetic performance of the secondary battery. In some embodiments, the viscosity of the aqueous solution of the first dispersant is 4500 mPa s to 18000 mPa s when the mass percentage of the first dispersant is 1% at 25°C.

[0032] The preparation method of the first dispersant is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the first dispersant can include but is not limited to the following preparation steps: taking natural cellulose (such as cotton pulp) as the first dispersant raw material, and under the action of NaOH and sodium chloroacetate, the CMC-Na is obtained through two-step reactions of alkalization and etherification, and then the first dispersant CMC-H is prepared by neutralization reaction with sulfuric acid.

[0033] Generally, the number average molecular weight of the first dispersant can be adjusted by adjusting the number average molecular weight of the first dispersant raw material during the preparation of the first dispersant. For example, when other conditions are unchanged, the number average molecular weight of the first dispersant increases as the number average molecular weight of the first dispersant raw material during the preparation of the first dispersant increases, and the number average molecular weight of the first dispersant decreases as the number average molecular weight of the first dispersant raw material during the preparation of the first dispersant decreases.

[0034] Generally, the pH of the aqueous solution of the first dispersant at 25°C when the mass percentage of the first dispersant is 1% can be adjusted by adjusting the amount of sulfuric acid used during the preparation of the first dispersant. For example, when other conditions are unchanged, the pH of the aqueous solution of the first dispersant at 25°C when the mass percentage of the first dispersant is 1% decreases as the amount of sulfuric acid used during the preparation of the first dispersant increases, and the pH of the aqueous solution of the first dispersant at 25°C when the mass percentage of the first dispersant is 1% increases as the amount of sulfuric acid used during the preparation of the first dispersant decreases.

[0035] Generally, the viscosity of the aqueous solution of the first dispersant at 25°C and 1% of the mass percentage of the first dispersant can be controlled by adjusting the number average molecular weight of the first dispersant raw material in the process of preparing the first dispersant. For example, when other conditions remain unchanged, the number average molecular weight of the first dispersant raw material in the process of preparing the first dispersant increases, the number average molecular weight of the first dispersant increases, and the viscosity of the aqueous solution of the first dispersant at 25°C and 1% of the mass percentage of the first dispersant increases; the number average molecular weight of the first dispersant raw material in the process of preparing the first dispersant decreases, the number average molecular weight of the first dispersant decreases, and the viscosity of the aqueous solution of the first dispersant at 25°C and 1% of the mass percentage of the first dispersant decreases.

[0036] In the present application, the first dispersants with different number average molecular weights, pH and viscosity of the aqueous solution can be purchased and tested by the “test of the number average molecular weight of the first dispersant”, “test of the pH and viscosity of the aqueous solution of the first dispersant” provided in the present application to test the number average molecular weight, pH and viscosity of the aqueous solution of the first dispersant, and select the first dispersant with the desired number average molecular weight, pH and viscosity of the aqueous solution.

[0037] In some embodiments, the number average molecular weight of the second dispersant CMC-Li is 150,000 to 1,000,000. The pH of the aqueous solution of CMC-Li is 7.0 to 9.0, and the viscosity is 1,500 mPa·s to 30,000 mPa·s at 25°C and 1% of the mass percentage of CMC-Li.

[0038] In some embodiments of the present application, the single-side coating area density of the negative electrode material layer is 80 mg / 1540.25 mm 2 to 160 mg / 1540.25 mm 2 . For example, the single-side coating area density of the negative electrode material layer can be 80 mg / 1540.25 mm 2 , 90 mg / 1540.25 mm 2 , 100 mg / 1540.25 mm 2 , 110 mg / 1540.25 mm 2 , 115 mg / 1540.25 mm 2 , 120 mg / 1540.25 mm 2 , 124 mg / 1540.25 mm 2 , 130 mg / 1540.25 mm 2 , 140 mg / 1540.25 mm 2 , 150 mg / 1540.25 mm 2 , 160 mg / 1540.25 mm 2or a range consisting of any two of the above numerical values. By regulating the single-sided coating area density of the negative material layer within the above range, a higher bonding force between the negative material layer and the negative current collector can be maintained, and the negative electrode sheet has a higher cohesion, thereby facilitating the improvement of the kinetic performance of the secondary battery.

[0039] In some embodiments of the present application, the compaction density of the negative material layer is 1.6 g / cm 3 to 1.9 g / cm 3 . For example, the compaction density of the negative material layer can be 1.6 g / cm 3 , 1.65 g / cm 3 , 1.7 g / cm 3 , 1.74 g / cm 3 , 1.8 g / cm 3 , 1.86 g / cm 3 , 1.9 g / cm 3 , or a range consisting of any two of the above numerical values. By regulating the compaction density of the negative material layer within the above range, the porosity of the negative material layer can be within a suitable range, the wettability of the electrolyte to the negative electrode sheet is improved, and the capacity of the negative active material per unit volume of the negative electrode sheet is also improved, thereby facilitating the improvement of the energy density of the secondary battery, and a higher bonding force between the negative material layer and the negative current collector can be maintained, and the negative electrode sheet has a higher cohesion, thereby facilitating the improvement of the kinetic performance of the secondary battery.

[0040] Generally, the compaction density of the negative material layer can be regulated by regulating the cold-pressing pressure of the negative electrode sheet. For example, when other conditions remain unchanged, as the cold-pressing pressure increases, the compaction density of the negative material layer increases; as the cold-pressing pressure decreases, the compaction density of the negative material layer decreases.

[0041] In some embodiments of the present application, the negative material layer includes a negative active material, and the negative active material includes at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads, elemental silicon, silicon oxide SiO x (0 < x < 2), or silicon carbide SiC. The use of the above negative active material facilitates the improvement of the kinetic performance of the secondary battery.

[0042] In the present application, the mass percentage content of the negative active material can be 88% to 99% based on the mass of the negative material layer. The negative material layer includes a dispersant and a negative active material, and can also include a negative binder and a negative conductive agent. The mass percentage content of the negative binder can be 0.1% to 5%, and the mass percentage content of the negative conductive agent can be 0.1% to 5% based on the mass of the negative material layer.

[0043] This application does not impose any particular limitation on the types of negative electrode binders and negative electrode conductive agents, as long as they can achieve the purpose of this application. For example, the negative electrode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. For example, the negative electrode conductive agent may include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. Conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole.

[0044] This application does not impose any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. For example, the composite current collector may be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector may be from 4 μm to 15 μm.

[0045] Optionally, the negative electrode sheet may further include a conductive layer located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer, which may be at least one of the aforementioned negative electrode conductive agent and negative electrode binder.

[0046] In some embodiments, the adhesion force between the negative electrode material layer and the negative electrode current collector is between 7 N / m and 21 N / m. For example, the adhesion force between the negative electrode material layer and the negative electrode current collector can be 7 N / m, 8 N / m, 10 N / m, 12 N / m, 14 N / m, 16 N / m, 18 N / m, 20 N / m, 21 N / m, or any combination of two of the above values. Controlling the adhesion force between the negative electrode material layer and the negative electrode current collector within the above range is beneficial for improving the kinetic performance of the secondary battery.

[0047] In some embodiments, the cohesion of the negative electrode tab is 12 N / m to 27 N / m. For example, the cohesion of the negative electrode tab can be 12 N / m, 13 N / m, 15 N / m, 17 N / m, 19 N / m, 21 N / m, 23 N / m, 25 N / m, 27 N / m, or a range defined by any two of the above values. Controlling the cohesion of the negative electrode tab within the above range is conducive to improving the kinetic performance of the secondary battery.

[0048] The second aspect of the present application provides a secondary battery comprising the negative electrode tab provided by the first aspect of the present application. The negative electrode tab provided by the present application can enable the secondary battery to have good kinetic performance.

[0049] In the present application, the secondary battery further comprises a positive electrode tab, which comprises a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The "positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along the thickness direction of the positive electrode current collector, or can be disposed on both surfaces of the positive electrode current collector along the thickness direction of the positive electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the positive electrode current collector, or can be part of the area of the surface of the positive electrode current collector, which is not particularly limited in the present application as long as the purpose of the present application can be achieved.

[0050] The positive electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the positive electrode current collector can comprise an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. The thickness of the positive electrode current collector is not particularly limited in the present application as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector can be 5 μm to 20 μm.

[0051] The positive electrode material layer comprises a positive electrode active material, which is not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, the positive electrode active material can comprise, but is not limited to, at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based material, lithium cobaltate (LiCoO2), lithium manganate, lithium manganese iron phosphate, or lithium titanate.

[0052] The positive electrode material layer can further comprise a positive electrode conductive agent and a positive electrode binder, which are not particularly limited in the present application as long as the purpose of the present application can be achieved, for example, can be at least one of the above-mentioned negative electrode conductive agent and the above-mentioned negative electrode binder. The mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer is not particularly limited in the present application, which can be selected by a person skilled in the art according to actual needs as long as the purpose of the present application can be achieved.

[0053] The present application does not particularly limit the single-sided coating area density of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the single-sided coating area density of the positive electrode material layer can be 150 mg / 1540.25 mm 2 to 350 mg / 1540.25 mm 2 The present application does not particularly limit the compaction density of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the compaction density of the positive electrode material layer can be 3 g / cm 3 to 5 g / cm 3

[0054] Optionally, the positive electrode sheet can further include a conductive layer, which is located between the positive electrode current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be a commonly used conductive layer in the art. The conductive layer includes a conductive agent and a binder. The present application does not particularly limit the conductive agent and the binder in the conductive layer, which can be, for example, at least one of the above-mentioned negative electrode conductive agents and the above-mentioned negative electrode binders.

[0055] In the present application, the secondary battery further includes a separator. The present application does not particularly limit the separator, as long as the purpose of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spunlaid film.

[0056] In some embodiments, the separator can include a substrate layer and a surface treatment layer. The substrate layer can be a non-woven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0057] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

[0058] ​In some embodiments, the inorganic layer includes inorganic particles and a binder. The inorganic particles are not particularly limited in the present application, and for example, the inorganic particles can include at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder in the inorganic layer is not particularly limited in the present application, and for example, the binder in the inorganic layer can be at least one of the negative electrode binders described above. In some embodiments, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0059] In the present application, the thickness of the separator film is not particularly limited as long as the object of the present application can be achieved, and for example, the thickness of the separator film can be 3 μm to 30 μm.

[0060] In the present application, the secondary battery further includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the lithium salt can include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate.

[0061] The non-aqueous solvent is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The above-mentioned carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The above-mentioned chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The above-mentioned cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The above-mentioned fluorinated carbonate compound can include, but is not limited to, at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The above-mentioned carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The above-mentioned ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the non-aqueous solvent in the electrolyte is not particularly limited in the present application, as long as the object of the present application can be achieved.

[0062] In the present application, the secondary battery further includes a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte, and other components known in the art of secondary batteries, and the present application does not limit the above-mentioned other components. The case is not particularly limited in the present application, and can be a case known in the art, as long as the object of the present application can be achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be a metal, and the kind of the metal is not particularly limited in the present application, and a metal hard case known in the art can be used, as long as the object of the present application can be achieved. The flexible case can be a metal plastic film, for example, an aluminum plastic film, a steel plastic film, or the like.

[0063] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited. For example, the preparation process of the secondary battery can include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, and performing an operation such as winding, folding, etc. as needed to obtain an electrode assembly of a wound structure, placing the electrode assembly in a case, injecting an electrolyte into the case and sealing, to obtain a secondary battery. Alternatively, the positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, and then the four corners of the entire stack structure are fixed with a tape to obtain an electrode assembly of a stack structure, the electrode assembly is placed in a case, an electrolyte is injected into the case and sealed, to obtain a secondary battery. In addition, a current overflow prevention element, a guide plate, etc. can be placed in the case as needed, thereby preventing the pressure inside the secondary battery from rising, overcharging and discharging.

[0064] The third aspect of the present application provides an electronic device including the secondary battery of the second aspect of the present application. The secondary battery provided by the present application has good kinetic performance, and thus the electronic device provided by the present application has a long service life and good performance.

[0065] The kind of the electronic device of the present application is not particularly limited, and it can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0066] Embodiment

[0067] Hereinafter, the embodiments of the present application will be described more specifically by citing examples and comparative examples. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0068] Test methods and apparatus:

[0069] Test of the number average molecular weight of the first dispersant

[0070] The first dispersant, carboxymethyl cellulose (CMC-H), was dissolved in deionized water to obtain a 1% (w / w) dispersant solution. This solution was then injected into a GPC (gel permeation chromatography) instrument. The polymeric first dispersant was separated according to molecular size in the chromatographic column, with larger molecular weights eluting first and smaller molecular weights eluting later. The elutants were detected, and the molecular weight was automatically calculated to obtain a chromatogram. Using the chromatogram and a known molecular weight standard curve, the number-average molecular weight of the first dispersant was calculated.

[0071] pH and viscosity tests of the aqueous solution of the first dispersant

[0072] At 25°C, the first dispersant, carboxymethyl cellulose (CMC-H), was dissolved in deionized water to obtain a test solution with a mass percentage of 1% for the first dispersant. The pH of the above test solution was tested using a pH meter (pHS-3C, provided by Shanghai Leici Company), and the viscosity of the above test solution was tested using a viscometer (DV1, provided by Bollefeld Company, USA) (rotor speed 12 rpm / min).

[0073] Testing of the surface density of single-sided coating of negative electrode material layer

[0074] After discharging the lithium-ion batteries of each embodiment and comparative example to 3V at 0.5C, the negative electrode sheets were disassembled. These negative electrode sheets were cleaned with dimethyl carbonate (DMC) and then dried at 60°C. A punch with an area of ​​1540.25 mm² was then made at any location on both sides of the negative electrode sheet where the negative electrode material layer was coated. 2 The mass of the test sample is recorded as m1 mg. Then, the negative electrode material layers on both sides of the test sample are peeled off, and the mass of the negative electrode current collector is recorded as m2 mg. The surface density of the single-sided coating of the negative electrode material layer is calculated using the following formula: Surface density of the single-sided coating of the negative electrode material layer = (m1 - m2) mg / (2 × 1540.25) mm 2 .

[0075] Testing the compaction density of the negative electrode material layer

[0076] After discharging the lithium-ion batteries of each embodiment and comparative example to 3V at 0.5C, the negative electrode sheets were disassembled. These negative electrode sheets were cleaned with dimethyl carbonate (DMC) and then dried at 60°C. A punch with an area of ​​1540.25 mm² was then made at any location on the negative electrode sheet coated with the negative electrode material layer. 2 The test sample was weighed, its mass recorded as m3 mg, and its thickness recorded as h1 μm. Then, the negative electrode material layer of the test sample was peeled off, the mass of the negative electrode current collector was recorded as m4 mg, and its thickness recorded as h2 μm. The compaction density of the negative electrode material layer was calculated using the following formula: Compaction density of negative electrode material layer = (m3 - m4) × 103 / (1540.25×(h1-h2)), in g / cm³ 3 .

[0077] Test of the adhesion between the negative electrode material layer and the negative electrode current collector

[0078] The adhesion between the negative electrode material layer and the negative electrode current collector was measured using a 180° peel test. After discharging the lithium-ion batteries in each embodiment and comparative example to 3V at 0.5C, the negative electrode sheets were disassembled, soaked in dimethyl carbonate for 20 minutes, dried, and then cut into 20mm × 100mm test strips for testing the adhesion between the negative electrode material layer and the negative electrode current collector. A 20mm × 100mm double-sided adhesive tape (NITTO.NO5000NS) was attached to a steel plate, and the test strips were then attached to the tape with the negative electrode material layer facing down. A 20mm × 100mm paper tape was connected to one end of the test strip via the double-sided adhesive tape, and the test strips were rolled back and forth four times with a roller to obtain the test samples. Tensile testing was then performed. The test sample was fixed on the test stage, the paper tape was folded upwards 180° and secured with clamps, and then the tensile testing machine was started to pull the paper tape at a speed of 50 mm / min until the negative electrode material layer and the negative electrode current collector on the double-sided adhesive surface separated, at which point the test ended and the test data was saved. The adhesion force between the negative electrode material layer and the negative electrode current collector was calculated based on the tensile force and the displacement during separation, in N / m.

[0079] Testing the cohesive force of the negative electrode sheet

[0080] The lithium-ion batteries in the various embodiments and comparative examples were discharged to 3V at 0.5C, and the negative electrode sheets were obtained by disassembly. These negative electrode sheets were soaked in dimethyl carbonate for 20 minutes, dried, and then cut into 20mm × 100mm test strips for cohesive strength testing. A 20mm × 80mm single-sided adhesive tape for testing the cohesive strength of the negative electrode sheet was centered and covered the test strip. A 20mm × 60mm strip of paper was cut and inserted into the gap between the negative electrode sheet and the single-sided adhesive tape, with an overlap length of 15mm. The test strip was rolled back and forth four times with a roller to obtain the test sample. A tensile testing machine was used. The paper strip was fixed with a clamp, and then the tensile testing machine was used to pull the paper strip at a speed of 50mm / min until the single-sided adhesive tape bonding the negative electrode material layer separated from the negative electrode sheet. The test data was then saved. The cohesive force of the negative electrode sheet is calculated based on the tensile force and displacement during the separation of the single-sided adhesive paper of the bonding negative electrode material layer and the negative electrode sheet, with the unit being N / m.

[0081] Test of the mass percentage of lithium in the negative electrode material layer

[0082] The lithium ion battery of each example and comparative example was discharged at 0.5C to 3V, and then disassembled to obtain a negative electrode sheet. The negative electrode sheet was cleaned with dimethyl carbonate (DMC) to remove the electrolyte, and then dried at 60°C. The negative electrode material layer of the negative electrode sheet was scraped off to obtain a negative electrode material layer powder. The negative electrode material layer powder was placed in aqua regia at 185°C for digestion. The aqua regia was obtained by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) at a volume ratio of 3:1. For example, 0.4 g of the negative electrode material layer powder was digested with 10 mL of aqua regia for 30 min. Then, the volume was adjusted to 100 mL. The obtained liquid sample was injected into an inductively coupled plasma spectrometer (ICP) for comparison with a standard graph to test the mass percentage content of lithium in the negative electrode material layer.

[0083] Test of mass percentage content of sodium in the negative electrode material layer

[0084] The lithium ion battery of each example and comparative example was discharged at 0.5C to 3V, and then disassembled to obtain a negative electrode sheet. The negative electrode sheet was cleaned with dimethyl carbonate (DMC) to remove the electrolyte, and then dried at 60°C. The negative electrode material layer of the negative electrode sheet was scraped off to obtain a negative electrode material layer powder. The negative electrode material layer powder was placed in aqua regia at 185°C for digestion. The aqua regia was obtained by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) at a volume ratio of 3:1. For example, 0.4 g of the negative electrode material layer powder was digested with 10 mL of aqua regia for 30 min. Then, the volume was adjusted to 100 mL. The obtained liquid sample was injected into an inductively coupled plasma spectrometer (ICP) for comparison with a standard graph to test the mass percentage content of lithium in the negative electrode material layer.

[0085] Test of impedance

[0086] The kinetic performance of the lithium ion battery was evaluated by its impedance. The smaller the impedance of the lithium ion battery, the better the kinetic performance. The larger the impedance, the worse the kinetic performance. At 25°C, the lithium ion battery was charged at 0.7C to a voltage of 4.5V, and then charged at 4.5V to a current of 0.05C until the lithium ion battery was fully charged. The lithium ion battery was discharged at 0.1C for 5s, and the voltage value at this time was recorded as V1 and the current value was recorded as I1. Then, the lithium ion battery was discharged at 1C for 1s, and the voltage value at this time was recorded as V2 and the current value was recorded as I2. The above operation was repeated until the lithium ion battery was discharged to 3.0V. The impedance (DCR) of the lithium ion battery was calculated by the following formula: DCR = (V1-V2) / (I1-I2).

[0087] Example 1-1

[0088] Preparation of negative electrode sheet

[0089] The negative active material artificial graphite, the negative conductive agent conductive carbon black (Super P), the first dispersant carboxymethyl cellulose (CMC-H), the second dispersant carboxymethyl cellulose sodium (CMC-Na, CMC2200, provided by DKS Co., Ltd.), and the negative binder styrene-butadiene rubber were mixed in a mass ratio of 97.3:0.5:0.6:0.6:1, deionized water was added as a solvent, and a slurry with a solid content of 45 wt% was prepared. The negative slurry was uniformly coated on one surface of a negative current collector copper foil with a thickness of 4 μm, dried at 120°C, and a negative electrode sheet with a single-side coated negative electrode material layer was obtained. Then the above steps were repeated on the other surface of the copper foil, and a negative electrode sheet with a double-side coated negative electrode material layer was obtained. After drying at 120°C and cold pressing, the cold-pressed negative electrode sheet was placed in a high-temperature vacuum furnace for heat treatment. After cutting and welding the tabs, a negative electrode sheet with a size of 78 mm x 875 mm was obtained for use. The heat treatment temperature and time are shown in Table 1, the number average molecular weight of the first dispersant, the pH and viscosity of the aqueous solution of the first dispersant (1% by mass of the first dispersant at 25°C), the single-side coating surface density of the negative electrode material layer, and the compacted density of the negative electrode material layer are shown in Table 2.

[0090] <Preparation of the positive electrode sheet>

[0091] The positive active material LiCoO2, the positive conductive agent conductive carbon black (Super P), and the positive binder polyvinylidene fluoride were mixed in a mass ratio of 95:3:2, N-methylpyrrolidone (NMP) was added as a solvent, and a slurry with a solid content of 75 wt% was prepared. The positive slurry was uniformly coated on one surface of a positive current collector aluminum foil with a thickness of 6 μm, dried at 120°C, and a positive electrode sheet with a single-side coated positive electrode material layer was obtained. Then the above steps were repeated on the other surface of the aluminum foil, and a positive electrode sheet with a double-side coated positive electrode material layer was obtained. After drying at 120°C and cold pressing, the cold-pressed positive electrode sheet was cut and the tabs were welded, and a positive electrode sheet with a size of 74 mm x 867 mm was obtained for use. The single-side coating surface density of the positive electrode material layer was 224 mg / 1540.25 mm 2 , and the compacted density of the positive electrode material layer was 4.15 g / cm 3 .

[0092] <Preparation of the separator>

[0093] A porous polyethylene film (provided by Celgard Co.) with a thickness of 5 μm was used as the separator.

[0094] <Preparation of the electrolyte>

[0095] In a dry argon atmosphere, first, non-aqueous solvents ethylene carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC) were mixed uniformly in a mass ratio of 3:5:2 to obtain a base solvent, then lithium salt lithium hexafluorophosphate (LiPF6) was added to the base solvent to dissolve and mix uniformly to obtain an electrolyte with a lithium salt concentration of 1 mol / L.

[0096] <Preparation of lithium ion battery>

[0097] The positive electrode sheet, the separator and the negative electrode sheet prepared above were stacked and wound in sequence to obtain an electrode assembly with a wound structure. The electrode assembly was placed in an aluminum plastic film packaging bag, dried, and then injected with electrolyte. After vacuum packaging, standing, formation (0.3C constant current charging to 3.5V, then 1C constant current charging to 3.9V, 1.5C constant current charging to 90% state of charge (SOC)), capacity, degassing, edge cutting and other processes, a lithium ion battery was obtained.

[0098] Examples 1-2 to 1-16

[0099] Except that the relevant preparation parameters were adjusted according to Table 1 in <Preparation of negative electrode sheet>, the rest was the same as Example 1-1. Among them, when the mass percentage content of the dispersant changed, the sum of the mass percentage contents of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder changed accordingly, and the mass ratio of the three remained unchanged.

[0100] Examples 2-1 to 2-9

[0101] Except that the relevant preparation parameters were adjusted according to Table 2 in <Preparation of negative electrode sheet>, the rest was the same as Example 1-1. Among them, the compaction density of the negative electrode material layer was adjusted by adjusting the cold pressing pressure of the negative electrode sheet.

[0102] Comparative Example 1

[0103] Except that no first dispersant was added in <Preparation of negative electrode sheet>, the rest was the same as Example 1-1.

[0104] Comparative Examples 2 to 3

[0105] Except that the relevant preparation parameters were adjusted according to Table 1 in <Preparation of negative electrode sheet>, the rest was the same as Example 1-1.

[0106] Comparative Example 4

[0107] Except that no first dispersant was added in <Preparation of negative electrode sheet>, the rest was the same as Example 1-4.

[0108] Comparative Examples 5 to 6

[0109] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 1 in the <preparation of negative electrode sheet>.

[0110] Comparative Example 7

[0111] The rest was the same as Example 1-1 except that no heat treatment was performed in the <preparation of negative electrode sheet>.

[0112] Comparative Examples 8 to 9

[0113] The rest was the same as Example 1-1 except that the relevant preparation parameters were adjusted according to Table 1 in the <preparation of negative electrode sheet>.

[0114] Comparative Example 10

[0115] The rest was the same as Example 1-1 except that no first dispersant was added and no heat treatment was performed in the <preparation of negative electrode sheet>.

[0116] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 2.

[0117] Table 1

[0118]

[0119] Note: " / " in Table 1 means no corresponding parameter. The number average molecular weight of the second dispersant CMC-Li is 300,000. At 25°C, when the mass percentage of CMC-Li is 1%, the pH of the aqueous solution of CMC-Li is 7.0 and the viscosity is 5500 mPa·s. "-" in Table 1 means that the performance data of the lithium ion battery cannot be measured.

[0120] As can be seen from Examples 1-1 to 1-3, 1-7 to 1-16, Comparative Examples 1 to 3 and 7 to 10, when the second dispersant comprises sodium carboxymethyl cellulose, the first dispersant and the second dispersant are selected and the mass ratio thereof is adjusted within the scope of the present application, and the negative electrode sheet is heat treated and the heat treatment temperature is adjusted within the scope of the present application, the mass percentage content of sodium element in the negative electrode material layer is smaller, the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance, while the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet do not change much. Similarly, as can be seen from Examples 1-4 to 1-6, 1-14 to 1-16, Comparative Examples 4 to 6, the same is true when the second dispersant comprises lithium carboxymethyl cellulose, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet do not change much, the mass percentage content of lithium element in the negative electrode material layer is smaller, the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance. In addition, it can also be seen that when the mass ratio of the first dispersant and the second dispersant and the mass percentage content of the dispersant are the same, the impedance of the lithium ion battery is lower and the kinetic performance is better when the second dispersant comprises lithium carboxymethyl cellulose.

[0121] The first dispersant in the negative electrode material layer and the mass ratio of the first dispersant to the second dispersant will generally affect the kinetic performance of the lithium ion battery. As can be seen from Examples 1-1 to 1-6 and Comparative Examples 1 to 6, when the negative electrode material layer does not contain the first dispersant, for example, Comparative Example 1 and Comparative Example 4, or when the mass ratio of the first dispersant to the second dispersant is too small, for example, Comparative Example 3 and Comparative Example 6, the second dispersant comprises sodium carboxymethyl cellulose, or the second dispersant comprises lithium carboxymethyl cellulose, while the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet do not change much, the mass percentage content of lithium element or sodium element in the negative electrode material layer is larger, and the impedance of the lithium ion battery is larger. When the mass ratio of the first dispersant to the second dispersant is too large, for example, Comparative Example 2 and Comparative Example 5, the negative electrode sheet is difficult to prepare, and the performance data of the lithium ion battery cannot be measured. When the mass ratio of the first dispersant to the second dispersant is within the scope of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet do not change much, the mass percentage content of lithium element or sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0122] The heat treatment of the negative electrode sheet and the temperature of the heat treatment generally affect the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-7 to Example 1-8, and Comparative Example 7 to Comparative Example 9, when the negative electrode sheet is not subjected to heat treatment, for example, Comparative Example 7, although the mass percentage content of sodium elements in the negative electrode material layer is small, the first dispersant and the second dispersant are not decomposed by the heat treatment reaction, and the amount of sodium compounds in the negative electrode material layer is excessive, resulting in a larger impedance of the lithium ion battery; when the temperature of the heat treatment is too low, for example, Comparative Example 8, although the mass percentage content of sodium elements in the negative electrode material layer is small, the first dispersant and the second dispersant cannot be decomposed at a too low heat treatment temperature, and the residual sodium compounds in the negative electrode material layer are also excessive, resulting in a larger impedance of the lithium ion battery, the inventors speculate that the excessive residual sodium compounds in the negative electrode material layer adhere to the surface of the negative electrode active material, hinder the transmission of lithium ions in the negative electrode sheet, and increase the impedance of the lithium ion battery, thereby affecting the kinetic performance of the secondary battery; when the temperature of the heat treatment is too high, for example, Comparative Example 9, the negative electrode binder fails at high temperature, and the negative electrode sheet is difficult to prepare, and the performance data of the lithium ion battery cannot be measured. When the temperature of the heat treatment is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet change little, the mass percentage content of sodium elements in the negative electrode material layer is small, and the impedance of the lithium ion battery is small, indicating that the lithium ion battery has good kinetic performance.

[0123] The time of the heat treatment generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-9 to Example 1-10, when the time of the heat treatment is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet change little, the mass percentage content of sodium elements in the negative electrode material layer is small, and the impedance of the lithium ion battery is small, indicating that the lithium ion battery has good kinetic performance.

[0124] The mass percentage content of the dispersant generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 1-11 to Example 1-12, when the mass percentage content of the dispersant is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet change little, the mass percentage content of sodium elements in the negative electrode material layer is small, and the impedance of the lithium ion battery is small, indicating that the lithium ion battery has good kinetic performance.

[0125] Table 2

[0126]

[0127] The number average molecular weight of the first dispersant generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-2, when the number average molecular weight of the first dispersant is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector is greater, the cohesion of the negative electrode sheet is greater, the mass percentage content of sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0128] The pH and viscosity of the aqueous solution of the first dispersant generally affect the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-3 to Example 2-4, when the pH and viscosity of the aqueous solution of the first dispersant are within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet are not greatly affected, while the mass percentage content of sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0129] The single-side coating area density of the negative electrode material layer generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-5 to Example 2-6, when the single-side coating area density of the negative electrode material layer is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector is greater, the cohesion of the negative electrode sheet is greater, the mass percentage content of sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0130] The compaction density of the negative electrode material layer generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-7 to Example 2-8, when the compaction density of the negative electrode material layer is within the range of the present application, the adhesion between the negative electrode material layer and the negative electrode current collector is greater, the cohesion of the negative electrode sheet is greater, the mass percentage content of sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0131] The type of negative electrode active material generally affects the kinetic performance of the lithium ion battery. As can be seen from Example 1-1, Example 2-9, when the negative electrode active material within the range of the present application is selected, the adhesion between the negative electrode material layer and the negative electrode current collector and the cohesion of the negative electrode sheet are not greatly affected, while the mass percentage content of sodium element in the negative electrode material layer is smaller, and the impedance of the lithium ion battery is smaller, indicating that the lithium ion battery has good kinetic performance.

[0132] It is to be noted that, in the present document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0133] Various embodiments are described herein with reference to particular applications with a specific configuration and contents for convenience. It is to be understood that the application is not limited to those embodiments but cover any technical equivalents in principle as far as they are within the scope of a patent protection. The same or similar parts or features between the embodiments are designated by the same reference numerals, and a repeated explanation of these parts or features is omitted.

[0134] The above description is merely illustrative of the application and is not to be taken in a limiting sense. It is contemplated that departures from the specific design choices disclosed can still come within the scope of the application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a dispersant, the dispersant comprising a first dispersant and a second dispersant, the first dispersant comprising carboxymethyl cellulose, the second dispersant comprising at least one of lithium carboxymethyl cellulose or sodium carboxymethyl cellulose, a mass ratio of the first dispersant to the second dispersant being (1:1) to (1:5). The negative electrode sheet is subjected to heat treatment at a temperature of 200 ℃ to 400 ℃, a mass percentage of lithium in the negative electrode material layer is 0% to 0.051%, and / or a mass percentage of sodium in the negative electrode material layer is 0% to 0.158%.

2. The negative electrode sheet according to claim 1, wherein The mass ratio of the first dispersant to the second dispersant is (1:1) to (1:2).

3. The negative electrode sheet according to claim 1, wherein The mass percentage of lithium in the negative electrode material layer is 0.012% to 0.051%, or the mass percentage of sodium in the negative electrode material layer is 0.038% to 0.158%.

4. The negative electrode sheet according to claim 1, wherein The heat treatment is performed for 2 h to 10 h.

5. The negative electrode sheet according to claim 1, wherein A mass percentage of the dispersant based on a mass of the negative electrode material layer is 0.8% to 2%.

6. The negative electrode sheet according to claim 1, wherein A number average molecular weight of the first dispersant is 300,000 to 1,000,000.

7. The negative electrode sheet according to claim 1, wherein At 25 ℃, when a mass percentage of the first dispersant is 1%, a pH of an aqueous solution of the first dispersant is 2 to 6, and a viscosity of the aqueous solution of the first dispersant is 1,000 mPa·s to 30,000 mPa·s.

8. The negative electrode sheet according to claim 1, wherein The negative electrode sheet satisfies at least one of the following characteristics: (1) the single-sided coating area density of the negative electrode material layer is 80 mg / 1540.25 mm 2 to 160 mg / 1540.25 mm 2 ; (2) the compacted density of the negative electrode material layer is 1.6 g / cm 3 to 1.9 g / cm 3 .

9. The negative electrode sheet according to claim 1, wherein The negative electrode material layer comprises a negative electrode active material, the negative electrode active material comprising at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads, elemental silicon, silicon oxide, or silicon carbon. 10.A secondary battery comprising the negative electrode sheet according to any one of claims 1 to 9. 11.An electronic device comprising the secondary battery according to claim 10.

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