Secondary battery, preparation method of secondary battery and electronic device

Abstract

The invention provides a secondary battery, a preparation method of the secondary battery and an electronic device, the secondary battery comprises a positive electrode plate, a negative electrode plate, a diaphragm and an electrolyte, the negative electrode plate comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, and the negative electrode material layer comprises hydrophobic modified polyacrylic acid; powder of the negative electrode material layer is dissolved in deionized water to obtain first slurry with the solid content of 45 wt%-50 wt%, the shear rate is 0.1 S <-1 >, the shear viscosity eta of the first slurry is 1 mPa.s, and eta is larger than or equal to 1000 and smaller than or equal to 30000. When the negative electrode material layer comprises the hydrophobic modified polyacrylic acid, in the preparation process of the negative electrode plate, the negative electrode slurry containing the hydrophobic modified polyacrylic acid has the characteristic of high viscosity under low shear, so that the risk of edge thinning after coating is reduced, and the electrode assembly interface in the later period of circulation is improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a secondary battery, a method for preparing a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, offer high specific energy, high operating voltage, low self-discharge, compact size, and light weight, making them widely used in portable consumer electronics. With the rapid development of electric vehicles and mobile electronic devices in recent years, the demand for the long-cycle performance of lithium-ion batteries has become increasingly stringent.

[0003] Currently, the secondary battery field generally uses a slurry system of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). Due to the strong low-shear flow of the slurry, this system will form a thinning area at the edge of the coated electrode, which often affects the electrode processing and subsequent interfaces. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery, a method for preparing a secondary battery, and an electronic device, which utilizes the high viscosity characteristics of the electrode slurry under low shear during the secondary battery preparation process and applies it to electrode coating, thereby reducing the risk of thinning of the edge of the material layer and improving the interface problems of the secondary battery.

[0005] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a secondary battery, the secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, the negative electrode material layer comprising hydrophobically modified polyacrylic acid; powder of the negative electrode material layer is dissolved in deionized water to obtain a first slurry with a solid content of 45 wt% to 50 wt%, and the shear rate is 0.1S -1 The shear viscosity of the first slurry is η1 mPa·s, 1000≤η1≤30000. When the negative electrode material layer includes hydrophobically modified polyacrylic acid, the negative electrode slurry including the hydrophobically modified polyacrylic acid exhibits high viscosity under low shear conditions during the preparation of the negative electrode sheet, which helps reduce the risk of edge thinning after coating, thereby improving the electrode assembly interface in the later stages of cycling.

[0007] In some embodiments of the present application, along the width direction of the unfolded negative electrode sheet, the negative electrode material layer includes a main region and an edge region; along the thickness direction of the negative electrode sheet, the region where the thickness fluctuation difference of the negative electrode material layer is ≤1 μm from the center position of the negative electrode material layer is the main region, and the average thickness of the main region is H μm; along the width direction of the unfolded negative electrode sheet, the width of the main region is W μm; along the width direction of the unfolded negative electrode sheet, the width of the edge region is L μm; 1.04×10 -4 ≤L / W≤2.08×10 -4 and / or, 0.33 ≤ L / H ≤ 1.25. When the values of L / W and / or L / H are within the above ranges, the risk of edge thinning after coating is reduced, the area of the thinned area formed at the edge of the negative electrode material layer is reduced, and thus the electrode assembly interface is improved in the later stages of cycling.

[0008] In some embodiments of the present application, 50≤L≤100. When the value of L is within the above range, the thinning length of the negative electrode sheet is shorter, which reduces the area of the thinned area formed at the edge of the negative electrode material layer, thereby improving the electrode assembly interface in the later stage of the cycle.

[0009] In some embodiments of the present application, the negative electrode material layer includes a negative electrode active material and a conductive agent, the negative electrode active material includes at least one of artificial graphite, natural graphite, pure silicon, silicon oxides, silicon carbon compounds or intermediate phase microspheres, and the conductive agent includes at least one of conductive carbon black, conductive carbon nanotubes or graphene.

[0010] In some embodiments of the present application, the negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200°C to 350°C, and the decomposition ratio of the negative electrode material layer is X, 0.8% ≤ X ≤ 3.5%. In some embodiments of the present application, the negative electrode material layer further comprises at least one of styrene-butadiene rubber or styrene-propylene rubber. In some embodiments of the present application, the negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200°C to 350°C, and the decomposition ratio of the negative electrode material layer is X, 0.8% ≤ X ≤ 1.5%. When the value of X is within the above range, it is beneficial to reduce the risk of edge thinning at the end of coating, reduce the area of the thinned area formed at the edge of the negative electrode material layer, and thus improve the electrode assembly interface in the later stage of the cycle.

[0011] In some embodiments of the present application, the hydrophobically modified polyacrylic acid is an alkalized hydrophobically modified polyacrylic acid, and the monomers forming the alkalized hydrophobically modified polyacrylic acid include a hydrophobic monomer, a hydrophilic monomer, and a functional monomer. The hydrophobic monomer includes a C4 to C21 long carbon chain monomer, the hydrophilic monomer includes an acrylic monomer, and the functional monomer includes at least one of acrylonitrile or acrylamide. In some embodiments of the present application, the C4 to C21 long carbon chain monomer includes at least one of an ether, ester, or hydrocarbon containing an unsaturated double bond; and / or the acrylic monomer includes at least one of methacrylic acid, ethacrylic acid, or acrylic acid. By adding a hydrophobic segment, i.e., a long carbon chain monomer, to the alkalized hydrophobically modified polyacrylic acid, the long carbon chain molecule opens the molecular segment of the hydrophilic monomer, i.e., acrylic acid (PAA) monomer, through alkali neutralization, forming a hydrophobic association effect, thereby increasing the viscosity of the solution. Moreover, due to the introduction of the long carbon chain monomer, the hydrophobic monomer and the hydrophilic monomer form another form of hydrogen bond, thereby increasing the viscosity. At the same time, the intramolecular and / or intermolecular hydrogen bonding of the hydrophilic monomer is weakened, the free negative charge on the surface increases, resulting in an increase in the electrokinetic potential and a higher Zeta potential, thereby enhancing the dispersibility of the alkalized hydrophobically modified polyacrylic acid. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, further reducing the risk of edge thinning at the end of coating, and reducing the area of the thinned area formed at the edge of the negative electrode material layer, thereby improving the electrode assembly interface in the later stage of the cycle.

[0012] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2 wt% to 10 wt%, and the pH of the second slurry is 6.0 to 8.0; (2) the weight average molecular weight of the alkalized hydrophobically modified polyacrylic acid is Mw g / mol, 3×10 5 ≤Mw≤1.5×10 6 ; (3) Alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2wt% to 10wt%, and the viscosity of the second slurry at 25°C and 12rpm is ηmPa·s, 30000≤η≤100000; (4) The elastic modulus of the film of the alkaline hydrophobically modified polyacrylic acid is E GPa, 5≤E≤20; (5) The swelling degree of the alkaline hydrophobically modified polyacrylic acid in the electrolyte is c, 1%≤c≤10%. Through the above settings, the alkaline hydrophobically modified polyacrylic acid has high viscosity and good dispersibility. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, reduce the risk of thinning at the end of coating, and reduce the area of the thinning zone formed at the edge of the negative electrode material layer, thereby helping to improve the electrode assembly interface in the later stage of the cycle.

[0013] The second aspect of the present application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, and assembling the resultant secondary battery; wherein the method for preparing the negative electrode sheet comprises the following steps: preparing a negative electrode slurry, adding hydrophobically modified polyacrylic acid to the negative electrode slurry, then adding an alkali, mixing to obtain a negative electrode slurry comprising alkalized hydrophobically modified polyacrylic acid, coating the negative electrode slurry comprising alkalized hydrophobically modified polyacrylic acid on the surface of a negative electrode current collector, and drying and cold pressing to obtain a negative electrode sheet. The alkali comprises at least one of lithium hydroxide, sodium hydroxide, or potassium hydroxide, and the mass ratio of the alkali to the hydrophobically modified polyacrylic acid is a, 1 / 8≤a≤1 / 4. The secondary battery is prepared using the preparation method provided in the second aspect of the present application, and the value of a is controlled within the above range. The negative electrode slurry comprising the alkalized hydrophobically modified polyacrylic acid has high viscosity under low shear, which helps reduce the risk of edge thinning at the end of coating, thereby helping to improve the electrode assembly interface in the later stages of cycling.

[0014] In some embodiments of the present application, the shear rate is 0.1S -1 The shear viscosity of the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid is η2 mPa·s, 100,000 ≤ η2 ≤ 400,000. When the value of η2 is within the above range, the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has a high viscosity under low shear conditions, which helps reduce the risk of edge thinning after coating, and thus helps improve the electrode assembly interface in the later stages of cycling.

[0015] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in the first aspect of the present application, or the secondary battery prepared according to the preparation method provided in the second aspect of the present application. Therefore, the electronic device of the present application has good performance.

[0016] Beneficial effects of this application:

[0017] The present application provides a secondary battery, a method for preparing a secondary battery, and an electronic device. The secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector. The negative electrode material layer includes hydrophobically modified polyacrylic acid. The powder of the negative electrode material layer is dissolved in deionized water to obtain a first slurry with a solid content of 45 wt% to 50 wt%. The shear rate is 0.1S -1 The shear viscosity of the first slurry is η1 mPa·s, 1000≤η1≤30000. When the negative electrode material layer includes hydrophobically modified polyacrylic acid, the negative electrode slurry including the hydrophobically modified polyacrylic acid exhibits high viscosity under low shear conditions during the preparation of the negative electrode sheet, which helps reduce the risk of edge thinning after coating, thereby improving the electrode assembly interface in the later stages of cycling.

[0018] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0020] Figure 1 This is a partial schematic diagram of a negative electrode sheet in one embodiment of the present application after being unfolded along its width direction;

[0021] Figure 2 for Figure 1 AA cross-sectional diagram of .

[0022] Reference numerals: negative electrode sheet 10 ; negative electrode current collector 11 ; negative electrode material layer 12 ; main region 121 ; edge region 122 . DETAILED DESCRIPTION

[0023] The following will be combined with the embodiments of the present application and the accompanying drawings to clearly and completely describe the technical solutions in this application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0024] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0025] The manufacturing level of the previous process has a great influence on the performance of the electrode assembly. In the preparation process of the negative electrode, the secondary battery field generally adopts sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) slurry system. When using the above slurry system for coating, based on the slurry's own performance and the limitations of the coating process, the coating edge area of the slurry on the current collector often forms an area with gradually decreasing thickness, that is, a thinning area. This phenomenon is mainly due to the low viscosity and poor uniformity of the slurry at low shear rates, which causes the slurry to flow to the edge during the coating process, forming a material layer with uneven thickness, thereby affecting the performance of the secondary battery, especially the interface of the electrode assembly after subsequent cycles. Based on this, the present application provides a secondary battery, a method for preparing a secondary battery, and an electronic device, which utilizes the high viscosity characteristics of the electrode slurry under low shear in the preparation process of the secondary battery and is applied to the electrode coating, thereby reducing the risk of thinning the edge of the material layer, thereby improving the interface problem of the secondary battery. The specific technical solution is as follows:

[0026] The first aspect of the present application provides a secondary battery, the secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, the negative electrode material layer comprising hydrophobically modified polyacrylic acid; powder of the negative electrode material layer is dissolved in deionized water to obtain a first slurry with a solid content of 45 wt% to 50 wt%, and the shear rate is 0.1S -1 , the shear viscosity of the first slurry is η1 mPa·s, 1000≤η1≤30000. For example, the solid content of the first slurry can be 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%, or a range consisting of any two values therein; the value of η1 can be 1000, 3000, 5000, 8000, 10000, 12000, 15000, 18000, 20000, 22000, 25000, 28000, 30000, or a range consisting of any two values therein.

[0027] The inventors have found that by adding hydrophobically modified polyacrylic acid, especially alkalized hydrophobically modified polyacrylic acid, to the negative electrode material layer, the alkalized hydrophobically modified polyacrylic acid has high viscosity and good dispersibility. The negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear. When preparing the negative electrode sheet, it is beneficial to the uniform coating of the negative electrode slurry on the negative electrode current collector, reducing the risk of edge thinning at the end of coating, and reducing the area of the thinning zone formed at the edge of the negative electrode material layer, thereby helping to improve the problems of cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, and thus helping to improve the electrode assembly interface in the later stage of the cycle.

[0028] In some embodiments of the present application, along the width direction of the unfolded negative electrode sheet, the negative electrode material layer includes a main region and an edge region; along the thickness direction of the negative electrode sheet, the region where the thickness fluctuation difference of the negative electrode material layer is ≤1 μm from the center position of the negative electrode material layer is the main region, the average thickness of the main region is H μm, along the width direction of the unfolded negative electrode sheet, the width of the main region is W μm; along the width direction of the unfolded negative electrode sheet, the width of the edge region is L μm. For example, Figure 1 and Figure 2 As shown, Figure 2 for Figure 1 In the AA section, the negative electrode sheet 10 includes a negative electrode current collector 11 and a negative electrode material layer 12. Along the width direction Y of the negative electrode sheet 10 after it is unfolded, the negative electrode material layer 12 includes a main region 121 and an edge region 122, wherein the main region 121 is Figure 1In the area within the dotted line, along the thickness direction Z of the negative electrode sheet 10, the average thickness of the main region 121 is H μm, along the width direction Y of the unfolded negative electrode sheet 10, the width of the main region 121 is W μm; along the width direction Y of the unfolded negative electrode sheet 10, the width of the edge region 122 is L μm. -4 ≤L / W≤2.08×10 -4 For example, the value of L / W can be 1.04×10 -4 , 1.05×10 -4 , 1.10×10 -4 , 1.20×10 -4 , 1.30×10 -4 , 1.40×10 -4 , 1.50×10 -4 , 1.60×10 -4 , 1.70×10 -4 , 1.80×10 -4 , 1.90×10 -4 , 2×10 -4 , 2.05×10 -4 , 2.08×10 -4 Or it can be a range consisting of any two values. When the value of L / W is within the above range, the thinning length of the negative electrode sheet is short, indicating that the characteristics of the negative electrode slurry match the size of the negative electrode sheet, which is beneficial to reduce the risk of edge thinning after coating and reduce the area of the thinned area formed at the edge of the negative electrode material layer, thereby improving the problems of cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, and further improving the electrode assembly interface in the later stage of the cycle.

[0029] In some embodiments of the present application, along the width direction of the unfolded negative electrode sheet, the negative electrode material layer includes a main region and an edge region; along the thickness direction of the negative electrode sheet, the region where the thickness fluctuation difference of the negative electrode material layer is ≤1 μm from the center of the negative electrode material layer is the main region, the average thickness of the main region is H μm, the width of the main region along the width direction of the unfolded negative electrode sheet is W μm; along the width direction of the unfolded negative electrode sheet, the width of the edge region is L μm; 0.33 ≤ L / H ≤ 1.25. For example, the value of L / H can be 0.33, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, or a range consisting of any two of these values. When the value of L / H is within the above range, the proportion of the edge area within the electrode size range can be regulated. The smaller the value of H, the smaller the width of the edge area. At this time, the impact on the negative electrode interface is also smaller, and the thinning length of the negative electrode is shorter, indicating that the characteristics of the negative electrode slurry match the thickness of the negative electrode, which is beneficial to reduce the risk of edge thinning at the end of coating and reduce the area of the thinning zone formed at the edge of the negative electrode material layer, thereby improving the cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, thereby improving the electrode assembly interface in the later stage of the cycle.

[0030] In some embodiments of the present application, along the width direction of the unfolded negative electrode sheet, the negative electrode material layer includes a main region and an edge region; along the thickness direction of the negative electrode sheet, the region where the thickness fluctuation difference of the negative electrode material layer is ≤1 μm from the center position of the negative electrode material layer is the main region, and the average thickness of the main region is H μm; along the width direction of the unfolded negative electrode sheet, the width of the main region is W μm; along the width direction of the unfolded negative electrode sheet, the width of the edge region is L μm; 1.04×10 -4 ≤L / W≤2.08×10 -4 , for example, the value of L / W can be 1.04×10 -4 , 1.05×10 -4 , 1.10×10 -4 , 1.20×10 -4 , 1.30×10 -4 , 1.40×10 -4 , 1.50×10 -4 , 1.60×10 -4 , 1.70×10 -4 , 1.80×10 -4 , 1.90×10 -4 , 2×10 -4 , 2.05×10 -4 , 2.08×10 -4Or a range consisting of any two of the values therein; and, 0.33≤L / H≤1.25. For example, the value of L / H can be 0.33, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25 or a range consisting of any two of the values therein. When the values of L / W and L / H are within the above ranges, the thinning length of the negative electrode sheet is shorter, indicating that the characteristics of the negative electrode slurry match the size and thickness of the negative electrode sheet, which is beneficial to reducing the risk of edge thinning at the end of coating and reducing the area of the thinning zone formed at the edge of the negative electrode material layer, thereby improving the problems of cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, thereby improving the electrode assembly interface in the later stage of the cycle.

[0031] In some embodiments of the present application, 50≤L≤100. For example, the value of L can be 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, or a range consisting of any two of these values. When the value of L is within the above range, the length of the edge area of the negative electrode pole piece is small, and the thinning length of the negative electrode pole piece is short, indicating that the addition of alkaline hydrophobically modified polyacrylic acid to the negative electrode material layer is beneficial to reducing the risk of edge thinning at the end of coating and reducing the area of the thinning zone formed at the edge of the negative electrode material layer, thereby improving the cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, thereby improving the electrode assembly interface in the later stage of the cycle.

[0032] In the present application, along the width direction of the unfolded negative electrode sheet, if the negative electrode sheet includes a main area and an edge area, L refers to the width of one edge area; if the negative electrode sheet includes a main area and two edge areas, it means that both edge areas have a thinning and symmetrical effect, which can be approximately understood as equivalent, then L refers to the width of any edge area.

[0033] In some embodiments of the present application, the negative electrode material layer includes a negative electrode active material and a conductive agent, the negative electrode active material includes at least one of artificial graphite, natural graphite, pure silicon, silicon oxide, silicon carbon compound or mesophase microspheres, and the conductive agent includes at least one of conductive carbon black, conductive carbon nanotubes or graphene. By selecting the above-mentioned types of negative electrode active materials and conductive agents, the negative electrode plate has good structural stability and conductivity. Among them, the silicon carbon compound is a silicon carbon composite material, based on the mass of the silicon carbon composite material, the mass percentage of silicon element is 30% to 70%, and the mass percentage of carbon element is 30% to 70%; the silicon oxide compound includes SiOx, wherein 0<x<2, and illustratively, the silicon oxide compound can include silicon monoxide (SiO, the molar ratio of silicon and oxygen is 1:1).

[0034] In some embodiments of the present application, based on the mass of the negative electrode material layer, the mass percentage of the negative electrode active material is 96% to 98.7%, and the mass percentage of the conductive agent is 0.05% to 0.5%. By regulating the mass percentage of the negative electrode active material and the mass percentage of the conductive agent within the above ranges, the negative electrode sheet has a high energy density, good structural stability, and good conductivity.

[0035] In some embodiments of the present application, the negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200°C to 350°C, and the decomposition ratio of the negative electrode material layer is X, 0.8%≤X≤3.5%. For example, the value of X can be 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5% or a range consisting of any two values therein. The thermal decomposition temperature of the alkalized hydrophobically modified polyacrylic acid is generally 250°C to 300°C. When the value of X is within the above range, it is beneficial to reduce the risk of edge thinning at the end of coating and reduce the area of the thinning zone formed at the edge of the negative electrode material layer, thereby improving the problems of cold pressing decarburization (insufficient cold pressing window) and edge bulging caused by thinning, thereby improving the electrode assembly interface in the later stage of the cycle.

[0036] In some embodiments of the present application, the negative electrode material layer further comprises at least one of styrene-butadiene rubber (SBR) or styrene-propylene rubber (SPR). Furthermore, the aforementioned arrangement, in which the alkalized hydrophobically modified polyacrylic acid is combined with the SBR and / or SPR in the negative electrode material layer, further increases the viscosity of the negative electrode slurry. This facilitates uniform coating of the negative electrode slurry on the negative electrode current collector during the preparation of the negative electrode sheet, further reducing the risk of edge thinning at the end of coating and the area of the thinned area formed at the edge of the negative electrode material layer, thereby improving the electrode assembly interface in the later stages of cycling.

[0037] It can be understood that in the process of preparing the negative electrode slurry, styrene-butadiene rubber emulsion and / or styrene-acrylic rubber emulsion is added to the negative electrode slurry, and the negative electrode plates in the secondary battery need to undergo a drying step during the preparation process to remove the solvent used in the plate preparation process, thereby improving the safety and reliability of the secondary battery, and the negative electrode material layer prepared includes styrene-butadiene rubber and / or styrene-acrylic rubber.

[0038] In some embodiments of the present application, the negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200°C to 350°C, and the decomposition ratio of the negative electrode sheet is X, 0.8%≤X≤1.5%. For example, the value of X can be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or a range consisting of any two values therein. When the value of X is within the above range, the content of the alkalized hydrophobically modified polyacrylic acid in the negative electrode material layer is low, and the viscosity of the negative electrode slurry is further improved by combining the styrene-butadiene rubber and / or styrene-propylene rubber in the negative electrode material layer. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, further reducing the risk of edge thinning at the end of coating, and reducing the area of the thinned area formed at the edge of the negative electrode material layer, thereby helping to improve the electrode assembly interface in the later stage of the cycle.

[0039] In some embodiments of the present application, the hydrophobically modified polyacrylic acid is an alkalized hydrophobically modified polyacrylic acid, and the monomers forming the alkalized hydrophobically modified polyacrylic acid include hydrophobic monomers, hydrophilic monomers, and functional monomers. The hydrophobic monomers include long carbon chain monomers from C4 to C21, preferably, the hydrophobic monomers include long carbon chain monomers from C7 to C15; the hydrophilic monomers include acrylic monomers, and the functional monomers include at least one of acrylonitrile or acrylamide. By adding hydrophobic segments, i.e., long carbon chain monomers, to the alkalized hydrophobically modified polyacrylic acid, the long carbon chain molecules open the molecular segments of the hydrophilic monomers, i.e., acrylic acid (PAA) monomers, through alkali neutralization, to form hydrophobic association. This interaction forms a physical cross-linked network through the interaction of intermolecular hydrophobic groups, increases the volume of the fluid mechanics, and thus increases the viscosity of the solution. The use of functional monomers in combination with hydrophilic monomers is conducive to the formation of more hydrogen bonds, further increasing the viscosity of the alkalized hydrophobically modified polyacrylic acid; in addition, the acrylic monomers contain functionalized carboxylic acid groups and have good water solubility. It is beneficial to improve the hydrophilicity of the alkaline hydrophobically modified polyacrylic acid, and it is beneficial for the alkaline hydrophobically modified polyacrylic acid to form a uniform dispersion in the aqueous system, thereby improving the uniformity and dispersibility of the negative electrode slurry; and due to the introduction of long carbon chain monomers, the hydrophobic monomer and the hydrophilic monomer form another form of hydrogen bond, thereby increasing the viscosity. At the same time, it weakens the intramolecular and / or intermolecular hydrogen bonding of the hydrophilic monomer, and the free negative charge on the surface increases, resulting in an increase in the electrokinetic potential (ZP, i.e., Zeta potential). The Zeta potential is higher, and the dispersibility of the alkaline hydrophobically modified polyacrylic acid is enhanced. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, due to the introduction of the hydrophobic monomer, the hydrophobic association effect is enhanced, the viscosity is greatly improved, and the viscosity is higher at low shear rates. This characteristic makes it weaker in fluid flow after the coating is completed, reduces the degree of thinning at the end of the coating, and reduces the area of the thinning zone formed at the edge of the negative electrode material layer, which is beneficial to improve the electrode assembly interface in the later stage of the cycle. In the present application, it is understood that the alkalized hydrophobically modified polyacrylic acid is prepared by polymerizing a hydrophobic monomer, a hydrophilic monomer, and a functional monomer.

[0040] In some embodiments of the present application, the structural formula of the structural unit of the hydrophobically modified polyacrylic acid is as shown in Formula I:

[0041]

[0042] wherein Y' is selected from a long carbon chain monomer of C4 to C21, preferably, Y' is selected from a long carbon chain monomer of C7 to C15; n1 is selected from a natural number within the range of 3000 to 20000, n3 is selected from a natural number within the range of 800 to 5000, and n4 is selected from a natural number within the range of 0 to 5000. By selecting the hydrophobically modified polyacrylic acid having the above structure, the long carbon chain monomer opens the molecular chain segments of the hydrophilic monomer, i.e., the acrylic monomer, through alkali neutralization, forming a hydrophobic association, and can form a physical cross-linked network through the interaction of the intermolecular hydrophobic groups, thereby increasing the hydrodynamic volume of the solution and thus increasing the solution viscosity. Furthermore, due to the introduction of the long carbon chain monomer, the hydrophobic monomer and the hydrophilic monomer form another form of hydrogen bond, thereby increasing the viscosity. At the same time, the hydrophilic monomer is weakened, thereby weakening the intramolecular and / or intermolecular hydrogen bonds of the hydrophilic monomer, increasing the free negative charge on the surface, and thus increasing the zeta potential. The higher the zeta potential, the more dispersibility of the alkalized hydrophobically modified polyacrylic acid is enhanced. Thus, the viscosity and dispersibility of the alkalized hydrophobically modified polyacrylic acid are improved, and when applied to negative electrode slurries, the negative electrode slurries have good dispersibility and stability. It is understood that the number of monomers in the hydrophobically modified polyacrylic acid meets the requirements of n1, n3, and n4 in the molecular formula, but the actual connection of the monomer units can be disordered. The structural formula in this application is drawn for schematic purposes only.

[0043] In some embodiments of the present application, the structural formula of the structural unit of the hydrophobically modified polyacrylic acid is shown in II:

[0044]

[0045] wherein Y' is selected from a long carbon chain monomer of C4 to C21, preferably, Y' is selected from a long carbon chain monomer of C7 to C15; n1 is selected from a natural number within the range of 3000 to 20000, n2 is selected from a natural number within the range of 0 to 5000, and n3 is selected from a natural number within the range of 800 to 5000. By selecting the hydrophobically modified polyacrylic acid having the above structure, the long carbon chain monomer opens the molecular chain segments of the hydrophilic monomer, i.e., the acrylic monomer, through alkali neutralization, forming a hydrophobic association, and can form a physical cross-linked network through the interaction of the intermolecular hydrophobic groups, thereby increasing the hydrodynamic volume of the solution and thus increasing the solution viscosity. Furthermore, due to the introduction of the long carbon chain monomer, the hydrophobic monomer and the hydrophilic monomer form another form of hydrogen bond, thereby increasing the viscosity. At the same time, the hydrophilic monomer is weakened, thereby weakening the intramolecular and / or intermolecular hydrogen bonds of the hydrophilic monomer, increasing the free negative charge on the surface, and thus increasing the zeta potential. The higher the zeta potential, the more dispersibility of the alkalized hydrophobically modified polyacrylic acid is enhanced. Thus, the viscosity and dispersibility of the alkalized hydrophobically modified polyacrylic acid are improved, and when applied to negative electrode slurries, the negative electrode slurries have good dispersibility and stability. It is understood that the number of each monomer in the hydrophobically modified polyacrylic acid meets the requirements of n1, n2, and n3 in the molecular formula, but the actual connection of the monomer units can be disordered. The structural formula in this application is drawn for schematic purposes only.

[0046] In some embodiments of the present application, the long carbon chain monomers of C4 to C21 include at least one of ethers, esters or hydrocarbons containing unsaturated double bonds. By selecting the above-mentioned types of hydrophobic monomers, the long carbon chain monomers open the molecular segments of the hydrophilic monomers, i.e., acrylic acid (PAA) monomers, through alkali neutralization to form a hydrophobic association. This effect forms a physical cross-linked network through the interaction of intermolecular hydrophobic groups, thereby increasing the fluid dynamics volume of the solution, thereby further increasing the solution viscosity. In addition, due to the introduction of long carbon chain monomers, the hydrophobic monomers and the hydrophilic monomers form another form of hydrogen bonds, thereby increasing the viscosity. At the same time, the hydrophilic monomers are weakened, so that the intramolecular and / or intermolecular hydrogen bonds of the hydrophilic monomers are weakened, and the surface free negative charge increases, resulting in an increase in the Zeta potential. The Zeta potential is higher, and the dispersibility of the alkalized hydrophobically modified polyacrylic acid is further enhanced. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, further reducing the risk of edge thinning at the end of coating, and reducing the area of the thinned zone formed at the edge of the negative electrode material layer, thereby helping to improve the electrode assembly interface in the later stage of the cycle.

[0047] In some embodiments of the present application, the acrylic monomer includes at least one of methacrylic acid, ethacrylic acid or acrylic acid. By selecting the above-mentioned types of hydrophilic monomers, the acrylic monomer contains a functionalized carboxylic acid group and has good water solubility, which is beneficial to improving the hydrophilicity of the alkalized hydrophobically modified polyacrylic acid, and is beneficial to the alkalized hydrophobically modified polyacrylic acid to form a uniform dispersion in the aqueous system, thereby improving the uniformity and dispersibility of the negative electrode slurry. At the same time, it synergizes with the hydrophobic monomer in the alkalized hydrophobically modified polyacrylic acid to form a physical cross-linked network through the interaction of intermolecular hydrophobic groups, thereby increasing the fluid dynamics volume of the solution and thus increasing the solution viscosity. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is beneficial to the uniform coating of the negative electrode slurry on the negative electrode current collector, further reducing the risk of thinning at the end of coating, and reducing the area of the thinning zone formed at the edge of the negative electrode material layer, which is beneficial to improving the electrode assembly interface in the later stage of the cycle.

[0048] In some embodiments of the present application, the C4 to C21 long carbon chain monomer includes at least one of ethers, esters or hydrocarbons containing unsaturated double bonds; and / or the acrylic monomer includes at least one of methacrylic acid or acrylic acid. By selecting the aforementioned types of hydrophobic and hydrophilic monomers, the hydrophobic and hydrophilic monomers act synergistically. The long-chain monomers, through alkali neutralization, open the molecular segments of the hydrophilic monomers, namely, acrylic acid (PAA) monomers, forming hydrophobic associations. This interaction forms a physical cross-linked network through the interaction of intermolecular hydrophobic groups, increasing the hydrodynamic volume of the solution, thereby further increasing the solution viscosity. In addition, the acrylic acid monomers contain functionalized carboxylic acid groups and have good water solubility, which helps to improve the hydrophilicity of the alkalized hydrophobically modified polyacrylic acid and facilitate the formation of a uniform dispersion of the alkalized hydrophobically modified polyacrylic acid in the aqueous system, thereby improving the uniformity and dispersibility of the negative electrode slurry. In addition, due to the introduction of the long-chain monomers, the hydrophobic and hydrophilic monomers form another form of hydrogen bonding, thereby increasing the viscosity. At the same time, the hydrophilic monomers are weakened, resulting in weakened intramolecular and / or intermolecular hydrogen bonds in the hydrophilic monomers, an increase in surface free negative charge, and an increase in the zeta potential. The higher the zeta potential, the further enhanced dispersibility of the alkalized hydrophobically modified polyacrylic acid. When applied to negative electrode slurries, the negative electrode slurry has good dispersibility and stability. This facilitates uniform coating of the negative electrode slurry on the negative electrode current collector during the preparation of negative electrode sheets, further reducing the risk of edge thinning at the end of coating and the area of thinned areas formed at the edges of the negative electrode material layer, thereby improving the electrode assembly interface in the later stages of cycling.

[0049] In some embodiments of the present application, the C4 to C21 long carbon chain monomer includes at least one of butadiene, isoprene, styrene, vinyl acetate, butyl acrylate, n-octyl acrylate, isooctyl acrylate, lauryl acrylate, hexadecyl acrylate, octadecyl acrylate, hydroxybutyl vinyl ether, butyl vinyl ether, triethylene glycol diacetate, butyl acrylate or glyceryl acrylate. By selecting the above-mentioned types of long carbon chain monomers, the long carbon chain monomers open the molecular chain segments of the hydrophilic monomers, namely acrylic acid (PAA) monomers, through alkali neutralization to form hydrophobic association. This effect forms a physical cross-linked network through the interaction of intermolecular hydrophobic groups, which increases the hydrodynamic volume of the solution, thereby further increasing the solution viscosity. In addition, due to the introduction of long carbon chain monomers, the hydrophobic monomers and the hydrophilic monomers form another form of hydrogen bond, thereby increasing the viscosity. At the same time, the hydrophilic monomers are weakened, so that the hydrogen bonds within and / or between the hydrophilic monomer molecules are weakened, and the free negative charge on the surface increases, resulting in an increase in the Zeta potential. The Zeta potential is higher, and the dispersibility of the alkaline hydrophobically modified polyacrylic acid is further enhanced. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is conducive to the uniform coating of the negative electrode slurry on the negative electrode current collector, further reducing the risk of thinning at the end of coating, and reducing the area of the thinning area formed at the edge of the negative electrode material layer, which is conducive to improving the electrode assembly interface in the later stage of the cycle.

[0050] Y'In some embodiments of the present application, the alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2 wt% to 10 wt%, and the pH of the second slurry is 6.0 to 8.0. For example, the pH of the second slurry can be 6.0, 6.2, 6.5, 6.8, 7.0, 7.2, 7.5, 7.8, 8.0 or a range consisting of any two values thereof. By regulating the pH of the alkalized hydrophobically modified polyacrylic acid within the above range, it is beneficial for the hydrophilic monomer, i.e., acrylic acid monomer, in the alkalized hydrophobically modified polyacrylic acid to further exert its dispersing effect, and it is beneficial for the carboxylic acid group in the acrylic acid monomer to dissociate and form a negative charge, thereby increasing the electrostatic repulsion between particles and reducing the possibility of particle agglomeration. While taking into account the viscosity, the dispersibility of the alkalized hydrophobically modified polyacrylic acid is further improved.

[0051] In some embodiments of the present application, the weight average molecular weight of the alkalized hydrophobically modified polyacrylic acid is Mw g / mol, 3×10 5 ≤Mw≤1.5×10 6 For example, the value of Mw can be 3×10 5 , 4×10 5 , 5×10 5 , 6×10 5 , 7×10 5, 8×10 5 , 9×10 5 , 1×10 6 , 1.1×10 6 , 1.2×10 6 , 1.3×10 6 , 1.4×10 6 , 1.5×10 6 Or a range consisting of any two of the values. By regulating the value of Mw within the above range, the alkaline hydrophobically modified polyacrylic acid has a longer molecular chain, which is conducive to better adsorption on the surface of the negative electrode active material, reducing the risk of agglomeration of the negative electrode active material particles, and improving the dispersibility of the negative electrode slurry. The high molecular weight polymer is conducive to forming a relatively stable three-dimensional network structure, which enhances the stability of the negative electrode slurry. Therefore, when applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is conducive to the uniform coating of the negative electrode slurry on the negative electrode current collector, further reducing the risk of thinning at the end of coating, and reducing the area of the thinning zone formed at the edge of the negative electrode material layer, which is conducive to improving the electrode assembly interface in the later stage of the cycle.

[0052] In some embodiments of the present application, the alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2 wt% to 10 wt%, and the viscosity of the second slurry at 25 ° C and 12 rpm is η mPa s, 30000 ≤ η ≤ 100000. For example, the solid content of the second slurry can be 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or a range consisting of any two values therein; the value of η can be 30000, 40000, 50000, 60000, 70000, 80000, 90000, 100000, or a range consisting of any two values therein. By regulating the value of η within the above range, the second slurry has a higher viscosity and good dispersibility. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, further reducing the risk of edge thinning at the end of coating, and reducing the area of the thinned zone formed at the edge of the negative electrode material layer, thereby helping to improve the electrode assembly interface in the later stage of the cycle.

[0053] In some embodiments of the present application, the elastic modulus of the film of the alkalized hydrophobically modified polyacrylic acid (film size is 80mm×20mm×1mm) is E GPa, 5≤E≤20. For example, the value of E can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or a range consisting of any two values therein. By regulating the value of E within the above range, the viscosity of the alkalized hydrophobically modified polyacrylic acid is higher. When applied to the negative electrode slurry to prepare the negative electrode sheet, the binding ability of the negative electrode active material particles is stronger. During the charge and discharge process of the secondary battery, it is beneficial to reduce the expansion of the negative electrode sheet, thereby improving the electrode assembly interface in the later stage of the cycle.

[0054] In some embodiments of the present application, the degree of swelling of the alkalized hydrophobically modified polyacrylic acid in the electrolyte is c, 1% ≤ c ≤ 10%. For example, the value of c can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of these values. By regulating the value of c within the above range, the interaction between the alkalized hydrophobically modified polyacrylic acid and the electrolyte is smaller, which can reduce the side reactions of the electrolyte, thereby improving the cycle life of the secondary battery and the negative electrode interface; in addition, the smaller swelling has little effect on the bonding of the alkalized hydrophobically modified polyacrylic acid, which correspondingly reduces the risk of debonding and accelerated cycling due to bonding failure at the negative electrode interface after cycling.

[0055] The swelling degree refers to the ratio of the mass of the polymer molecules after swelling to the mass before swelling, when the polymer molecules adsorb solvent molecules and reach swelling equilibrium. This application does not specifically limit the electrolyte used to test the swelling degree, as long as it can achieve the objectives of this application. For example, a conventional electrolyte composition can be used. Specifically, the electrolyte composition can be the same as the electrolyte composition described in the Examples section of this application.

[0056] In some embodiments of the present application, the secondary battery satisfies the following characteristics: the alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2 wt% to 10 wt%, and the pH of the second slurry is 6.0 to 8.0, for example, the pH of the second slurry can be 6.0, 6.2, 6.5, 6.8, 7.0, 7.2, 7.5, 7.8, 8.0 or a range consisting of any two values thereof; and / or, the weight average molecular weight of the alkalized hydrophobically modified polyacrylic acid is Mw g / mol, 3×10 5 ≤Mw≤1.5×10 6 , for example, the value of Mw can be 3×10 5 , 4×10 5 , 5×10 5 , 6×10 5 , 7×10 5 , 8×105 , 9×10 5 , 1×10 6 , 1.1×10 6 , 1.2×10 6 , 1.3×10 6 , 1.4×10 6 , 1.5×10 6 or a range consisting of any two values thereof; and / or, the alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 2 wt% to 10 wt%, and the viscosity of the second slurry at 25 ° C and a rotation speed of 12 rpm is η mPa·s, 30000≤η≤100000, for example, the solid content of the second slurry can be 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt% or a range consisting of any two values thereof, and the value of η can be 30000, 40000, 50000, 60000, 70000, 80000, 90000, 100000 or a range consisting of any two values thereof; and / or, the elastic modulus of the film of the alkalized hydrophobically modified polyacrylic acid (film size is 80 mm×20 mm×1 mm) is E GPa, 5≤E≤20, for example, the value of E can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two values therein; and / or, the swelling degree of the alkalized hydrophobically modified polyacrylic acid in the electrolyte is c, 1%≤c≤10%, for example, the value of c can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two values therein. Through the above settings, the alkalized hydrophobically modified polyacrylic acid has high viscosity and good dispersibility. When applied to the negative electrode slurry, the negative electrode slurry has good dispersibility and stability. When preparing the negative electrode sheet, it is beneficial to uniformly coat the negative electrode slurry on the negative electrode current collector, reduce the risk of edge thinning at the end of coating, and reduce the area of the thinned area formed at the edge of the negative electrode material layer, thereby improving the electrode assembly interface in the later stage of cycling.

[0057] The second aspect of the present application provides a method for preparing a secondary battery, comprising the following steps: preparing a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, and assembling the resultant secondary battery; wherein the method for preparing the negative electrode sheet comprises the following steps: preparing a negative electrode slurry, adding an aqueous dispersion of hydrophobically modified polyacrylic acid to the negative electrode slurry, then adding an alkali, and mixing to obtain a negative electrode slurry comprising an alkalized hydrophobically modified polyacrylic acid, applying the alkalized hydrophobically modified polyacrylic acid to the surface of a negative electrode current collector, and drying and cold pressing to obtain a negative electrode sheet. The alkali comprises at least one of lithium hydroxide, sodium hydroxide, or potassium hydroxide, and the mass ratio of the alkali to the hydrophobically modified polyacrylic acid is a, 1 / 8≤a≤1 / 4. For example, the value of a can be 1 / 8, 3 / 20, 7 / 40, 1 / 5, 9 / 40, 1 / 4, or a range consisting of any two of these values.

[0058] The inventors have found that after the dispersant is alkalized in the slurry, the polymer chains are opened, the hydrophobic monomers show a strong hydrophobic association effect in the aqueous slurry, and the viscosity of the slurry is greatly improved. When the slurry is used to coat the pole pieces in the electrode assembly, after coating, the shear rate of the slurry is relatively low. At this time, the higher viscosity of the slurry makes its fluidity low, and the slurry can quickly stop spreading at the coating edge, so the edge of the pole piece has a smaller thinning area. The preparation method provided in the second aspect of the present application is used to prepare a secondary battery, and the value of a is regulated within the above range. The negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, which is beneficial to reduce the risk of edge thinning at the end of coating. The thinning length of the negative electrode piece is relatively short, which is beneficial to improve the electrode assembly interface in the later stage of the cycle.

[0059] In some embodiments of the present application, the shear rate is 0.1S -1 The shear viscosity of the negative electrode slurry comprising the alkalized hydrophobically modified polyacrylic acid is η2 mPa·s, 100,000 ≤ η2 ≤ 400,000. For example, the value of η2 can be 100,000, 120,000, 150,000, 180,000, 200,000, 220,000, 250,000, 280,000, 300,000, 320,000, 350,000, 380,000, 400,000, or a range consisting of any two values thereof. When the value of η2 is within the above range, the negative electrode slurry comprising the alkalized hydrophobically modified polyacrylic acid has a high viscosity characteristic under low shear, thereby reducing the risk of edge thinning at the end of coating, shortening the thinning length of the negative electrode sheet, and thus improving the electrode assembly interface in the later stage of the cycle.

[0060] The present application has no particular restrictions on the preparation method of hydrophobically modified polyacrylic acid, as long as the purpose of the present application can be achieved. For example, the preparation method of hydrophobically modified polyacrylic acid includes but is not limited to the following steps: (1) taking 900 to 4900 parts of deionized water as a solvent; adding a chain transfer agent and an initiator to the solvent to obtain a mixture, wherein the chain transfer agent includes at least one of n-dodecyl mercaptan, secondary dodecyl mercaptan, tert-dodecyl mercaptan, mercaptoethanol or thioglycolic acid, and the addition amount of the chain transfer agent W2 is 0.2 to 0.8 parts; the initiator includes at least one of potassium persulfate, sodium persulfate, ammonium persulfate, azobisisobutyronitrile or azobisisoheptonitrile, and the initiator The amount of the added agent W3 is 0.5 to 1 part; (2) adding the hydrophobic monomer, the hydrophilic monomer and the functional monomer to the mixture, wherein the mass fraction W4 of the hydrophilic monomer is 10 to 50 parts, the mass fraction W5 of the hydrophobic monomer is 20 to 60 parts, the mass fraction W6 of the functional monomer is 20 to 60 parts; and the total mass fraction of the hydrophobic monomer, the hydrophilic monomer and the functional monomer is 100 parts; continuing to heat the polymerization reaction to a temperature T of 60°C to 80°C and a time t of 2h to 10h to obtain an aqueous dispersion of hydrophobically modified polyacrylic acid. The aqueous dispersion of the hydrophobically modified polyacrylic acid can subsequently be obtained by heat treatment or other methods to obtain hydrophobically modified polyacrylic acid.

[0061] When preparing the negative electrode slurry, the hydrophobically modified polyacrylic acid is generally added in the form of an aqueous dispersion of the hydrophobically modified polyacrylic acid.

[0062] The present application has no particular restrictions on the various parameter regulation of alkalized hydrophobically modified polyacrylic acid, as long as the purpose of the application can be achieved. For example, the pH of the alkalized hydrophobically modified polyacrylic acid can be regulated by regulating the addition of a hydrophilic monomer, i.e., an acrylic monomer, and illustratively, when other conditions are fixed, the addition of the hydrophilic monomer increases, and the addition of the hydrophobic monomer and the functional monomer decreases accordingly, and now the pH of the negative electrode dispersant decreases therewith, and vice versa. The weight average molecular weight Mw of the alkalized hydrophobically modified polyacrylic acid can be regulated by regulating and controlling the polymerization reaction temperature and time, and illustratively, when other conditions are fixed, the higher the polymerization reaction temperature, the longer the time, the larger the weight average molecular weight, and vice versa smaller; it is understandable that when the addition and / or type of each monomer in the alkalized hydrophobically modified polyacrylic acid changes, the weight average molecular weight Mw of the polymer also changes therewith. The viscosity η of the alkalized hydrophobically modified polyacrylic acid at 25 ° C and 12 rpm can be adjusted by regulating the amount of hydrophobic monomer added, the polymerization reaction temperature and time. For example, when other conditions remain unchanged, the amount of hydrophobic monomer added increases, and the amount of hydrophilic monomer and functional monomer added decreases accordingly. The higher the polymerization temperature and the longer the time, the greater the viscosity of the alkalized hydrophobically modified polyacrylic acid at 25 ° C and 12 rpm, and vice versa. The elastic modulus E of the film of the alkalized hydrophobically modified polyacrylic acid can be adjusted by regulating the addition ratio of the hydrophilic monomer and the hydrophobic monomer, the polymerization reaction temperature and time. For example, when other conditions remain unchanged, the greater the addition ratio of the hydrophilic monomer and the hydrophobic monomer, the higher the polymerization reaction temperature and the longer the time, the higher the elastic modulus, and vice versa. The swelling degree c of the alkalized hydrophobically modified polyacrylic acid in the electrolyte can be adjusted by regulating the addition amount of the hydrophilic monomer, the polymerization reaction temperature and the time. For example, when other conditions remain unchanged, the addition amount of the hydrophilic monomer increases, and the addition amount of the hydrophobic monomer and the functional monomer decreases accordingly. The higher the polymerization reaction temperature and the longer the time, the smaller the swelling degree of the alkalized hydrophobically modified polyacrylic acid in the electrolyte, and vice versa.

[0063] In the present application, it is understood that the difference between the hydrophobically modified polyacrylic acid and the alkalized hydrophobically modified polyacrylic acid lies in whether or not a base is added to the slurry for neutralization. After the neutralization reaction, the content of the alkalized hydrophobically modified polyacrylic acid is substantially the same as that of the hydrophobically modified polyacrylic acid.

[0064] It is understood that the negative electrode sheets in secondary batteries require a drying step during their preparation to remove the solvents used in the preparation process, thereby improving the safety and reliability of the secondary battery. Therefore, when the negative electrode sheet is prepared from the negative electrode slurry, the negative electrode material layer in the resulting negative electrode sheet only includes non-volatile solids.

[0065] In the present application, “the negative electrode material layer located on at least one surface of the negative electrode current collector” means that the negative electrode material layer can be provided on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector itself, or can be provided on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector itself. It should be noted that the “surface” here can be the entire area of the surface of the negative electrode current collector, or it can be a partial area of the surface of the negative electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector (such as 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.), etc.

[0066] In the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer arranged on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be arranged on one surface of the positive electrode current collector along the thickness direction of itself, or can be arranged on two surfaces of the positive electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of the surface of the positive electrode current collector, or it can be a partial area of the surface of the positive electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved. This application has no special restrictions on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector). The positive electrode material layer of this application contains positive electrode active materials. This application has no special restrictions on the type of positive electrode active materials, as long as the purpose of this application can be achieved. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05O2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganate, lithium iron manganese phosphate or lithium titanate, etc. In the present application, the positive electrode active material may also contain non-metallic elements, for example, non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode material layer, as long as the purpose of the present application can be achieved. In the present application, the positive electrode material layer may also include a binder and a conductive agent. The present application has no particular restriction on the type of binder in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the binder may include but is not limited to at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The present application has no particular restrictions on the type of conductive agent in the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the 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, Ketjen black, graphene, metal materials or conductive polymers. The above-mentioned carbon nanotubes may include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymers may include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The present application has no particular restrictions on the mass ratio of the positive active material, conductive agent and binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0067] In the present application, the electrolyte includes a lithium salt and a non-aqueous solvent. The present application has no particular restrictions on the lithium salt, as long as the purpose of the present application can be achieved. For example, the lithium salt may 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(oxalatoborate) (LiBOB) or lithium difluoroborate. The present application has no particular restrictions on the content of the lithium salt in the electrolyte, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, the non-aqueous solvent may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may 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 linear carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC) or methylethyl carbonate (MEC). The above-mentioned cyclic carbonate may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinylethylene carbonate (VEC). The fluorocarbonate compound may include but is not limited to at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate. The above-mentioned carboxylate compounds may include but are not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid, valerolactone or caprolactone. The above-mentioned ether compounds may include but are not limited to at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may 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 present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the purpose of the present application can be achieved.

[0068] The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include, but is not limited to, polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or at least one of aramid. The type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane. In some embodiments of the present application, the diaphragm may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven membrane or a composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. In some embodiments of the present application, the inorganic layer includes inorganic particles and a binder. The present application has no particular restrictions on inorganic particles. For example, inorganic particles can include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application has no particular restrictions on the binder. For example, the binder can be at least one of the above-mentioned binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyvinyl ether or polyvinylidene fluoride or poly (vinylidene fluoride-hexafluoropropylene). In the present application, the thickness of the diaphragm is not particularly limited, as long as the purpose of the present application can be achieved, for example, the thickness of the diaphragm can be 3 μm to 30 μm.

[0069] The secondary battery also includes a shell for accommodating the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the above-mentioned other components. This application does not particularly limit the shell, and it can be a shell known in the art, as long as it can achieve the purpose of this application. For example, the shell can be a hard shell or a flexible shell. The material of the hard shell can be metal. This application does not limit the type of metal. A metal hard shell known in the art can be used, as long as it can achieve the purpose of this application. The flexible shell can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0070] The secondary battery of the present application is not particularly limited and may include any device that generates an electrochemical reaction. In one or more embodiments, the secondary battery may include but is not limited to: a lithium ion secondary battery (lithium ion battery), a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0071] The third aspect of the present application provides an electronic device, which includes the secondary battery provided in the first aspect of the present application, or the secondary battery prepared according to the preparation method provided in the second aspect of the present application. Therefore, the electronic device of the present application has good performance.

[0072] The present application does not particularly limit the type of electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0073] Example

[0074] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0075] Test methods and equipment:

[0076] Shear viscosity test:

[0077] At 25°C, the lithium-ion battery was discharged at a constant current of 0.5C to a discharge cut-off voltage. The lithium-ion battery was disassembled under an argon atmosphere, and the negative electrode sheet was soaked in dimethyl carbonate solvent for 4 hours and dried at 60°C for 1 hour to obtain the negative electrode sheet. The negative electrode material layer on the negative electrode sheet was scraped off and the powder of the negative electrode material layer was collected.

[0078] The discharge cut-off voltage of the lithium-ion batteries in the examples and comparative examples of this application is 3.0 V. It is understood that when the voltage range marked on the factory battery packaging is 3.0 V to 4.5 V, the charge cut-off voltage is 4.5 V and the discharge cut-off voltage is 3.0 V. Unless otherwise specified, the charge cut-off voltage of the lithium-ion batteries used as examples in this application is 4.5 V and the discharge cut-off voltage is 3.0 V.

[0079] At an ambient temperature of 25°C, the powder of the negative electrode material layer was dissolved in deionized water to prepare a first slurry with a solid content of 48 wt%. 2 mL of the first slurry was taken with a pipette and placed in the sample tray of the rotational rheometer. The rotational rheometer "shear test" was turned on and the speed was set to 12 rpm. The rheometer was tested on the first slurry at a shear rate of 0.1 S. -1 to 100S -1 After the value stabilizes for 2 minutes, record the corresponding viscosity and draw a shear curve. The shear rate is recorded as 0.1S -1 The shear viscosity η1 of the first slurry at , the unit is mPa · s.

[0080] At an ambient temperature of 25°C, the shear viscosity of the negative electrode slurries prepared in each group of examples and comparative examples was tested. 2 mL of the negative electrode slurries prepared in each group were taken with a pipette and placed in the sample tray of the rotational rheometer. The rotational rheometer was turned on for "shear test" and the speed was set to 12 rpm. The rheometer was used to test the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid at a shear rate of 0.1 S / min. -1 to 100S -1 After the value stabilizes for 2 minutes, record the corresponding viscosity and draw a shear curve. The shear rate is recorded as 0.1S -1 The shear viscosity η2 of the negative electrode slurry prepared by the group is mPa·s.

[0081] It can be understood that the negative electrode slurry finally prepared in the embodiment is the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid.

[0082] Negative electrode edge area test:

[0083] The negative electrode edge area test method uses a laminated electrode assembly as an example, but is not limited to a specific electrode assembly structure. The specific test steps are as follows:

[0084] At 25°C, the lithium-ion battery was discharged to the discharge cut-off voltage at a constant current of 0.5C. The lithium-ion battery was disassembled under an argon atmosphere, and the negative electrode plate was soaked in dimethyl carbonate solvent for 4 hours and dried at 60°C for 1 hour to obtain the negative electrode plate. Among them, the discharge cut-off voltage of the lithium-ion battery in the examples and comparative examples of the present application is 3.0V. It is understandable that when the voltage range marked on the factory battery packaging is 3.0V to 4.5V, the charge cut-off voltage is 4.5V and the discharge cut-off voltage is 3.0V. Unless otherwise specified, the charge cut-off voltage of the lithium-ion battery used as an example in this application is 4.5V, and the discharge cut-off voltage is 3.0V.

[0085] Place it on an offline laser thickness gauge, scan from one end of the negative electrode sheet to the other end in the corresponding direction parallel to the ear after the negative electrode sheet is unfolded, measure the corresponding thickness, and obtain a thickness curve. Record the maximum length in the width direction of the negative electrode sheet from the main area of the negative electrode sheet (the area where the thickness fluctuation difference of the negative electrode material layer is ≤1μm from the center position of the negative electrode sheet is the main area), which is the width L of the negative electrode material layer in the edge area.

[0086] Thermal decomposition test:

[0087] At 25°C, the lithium-ion battery was discharged at a constant current of 0.5C to a discharge cut-off voltage. The lithium-ion battery was disassembled under an argon atmosphere, and the negative electrode sheet was soaked in dimethyl carbonate solvent for 4 hours and dried at 60°C for 1 hour to obtain the negative electrode sheet. The negative electrode material layer on the negative electrode sheet was scraped off and the powder of the negative electrode material layer was collected.

[0088] The discharge cut-off voltage of the lithium-ion batteries in the examples and comparative examples of this application is 3.0 V. It is understood that when the voltage range marked on the factory battery packaging is 3.0 V to 4.5 V, the charge cut-off voltage is 4.5 V and the discharge cut-off voltage is 3.0 V. Unless otherwise specified, the charge cut-off voltage of the lithium-ion batteries used as examples in this application is 4.5 V and the discharge cut-off voltage is 3.0 V.

[0089] Weigh 2 mg of the collected negative electrode material layer powder into a sample tank (a precision electronic balance with automatic weighing function). Place the sample in a thermogravimeter and introduce nitrogen (N2) at a purge current of 60 mL / min. Turn on the thermogravimeter and set the temperature ramp from 35°C to 800°C at a rate of 10°C / min. After the instrument finishes, it automatically records the mass versus temperature curve during the process, and records and calculates the decomposition ratio X of the negative electrode material layer from 200°C to 350°C.

[0090] Test of alkalized hydrophobically modified polyacrylic acid:

[0091] At an ambient temperature of 25°C, alkaline hydrophobically modified polyacrylic acid was dissolved in deionized water to obtain a slurry with a solid content of 6 wt%. The second slurry was placed in a rectangular glass template (specifications: 80 mm × 20 mm × 1 mm). After air-drying until there was no obvious moisture on the slurry surface, it was placed in an 80°C oven and baked until the quality no longer decreased, thereby preparing a 20 mm × 80 mm × 1 mm film. The film was placed in the injection crucible of a pyrolysis gas chromatography-mass spectrometer (PY-GCMS), the injector was installed in the pyrolysis instrument, the instrument was started, the injection button was pressed, and the instrument was run at the same time. After the equipment was completed, the spectrum was exported to obtain the composition of the hydrophobic monomer.

[0092] The alkalized hydrophobically modified polyacrylic acid was dissolved in deionized water to obtain a slurry with a solid content of 6 wt%. The infrared spectrometer and its corresponding software were turned on and the acquisition wave number range was set to 4000 cm -1 Up to 600cm -1 , then click on the optical table, the projected light intensity is required to be greater than 5.0, and then the test can be carried out. With air as the background, click on the background collection. Add the slurry to the attenuated total reflection crystal (ATR crystal), click on the sample collection, and test to get the spectrum. Infrared at ~2500cm -1 to 3500cm -1 (OH stretch), ~1700cm -1 (C=O stretching) peaks were observed, confirming the presence of carboxylic acid groups.

[0093] Open the nuclear magnetic resonance (NMR) and its corresponding software, dissolve the above film in deuterated dimethyl sulfoxide (DMSO-d6) or deuterated N,N-dimethylformamide (DMF-d7) as solvent, and heat at 60 to 80 ° C to dissolve it into a solution with a mass ratio of 3% to 5%. Place the above solution into the nuclear magnetic resonance (NMR) sample cell and adjust 1 In H mode, click Collect, and the test is completed to obtain the NMR spectrum. 1 H NMR observed the main chain methylene signal (δ 1.5ppm to 2.5ppm). 13 In C mode, the test was carried out in the same manner, and the signal at δ175ppm to 185ppm confirmed the appearance of the carboxylic acid carbon peak.

[0094] Weigh 2 mg of alkaline hydrophobically modified polyacrylic acid into a sample holder (equipped with a precision electronic balance for automatic weighing). Place the sample in a thermogravimeter and introduce nitrogen gas at a purge current of 60 mL / min. Turn on the thermogravimeter and program the temperature to rise from 35°C to 800°C at a rate of 10°C / min. Once the temperature has finished rising, the instrument automatically records the mass change over temperature. Polyacrylic acid (PAA) typically begins to decompose (decarboxylation of the carboxylic acid groups) between 200°C and 350°C.

[0095] The hydrophilic monomer, namely the polyacrylic acid structure, can be determined by combining nuclear magnetic resonance, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TG) tests.

[0096] The alkalized hydrophobically modified polyacrylic acid was dissolved in deionized water to obtain a slurry with a solid content of 6 wt%. The infrared spectrometer and its corresponding software were turned on and the acquisition wave number range was set to 4000 cm -1 Up to 600cm -1, then click on the optical table, the projected light intensity is required to be greater than 5.0, and then the test can be started. With air as the background, click on the background collection. Add the slurry to the ATR crystal, click on the sample collection, and test to get the spectrum. Infrared at about 2240cm -1 Peak, ~1700cm -1 (C=O stretching) peaks were observed, confirming the presence of nitrile groups.

[0097] Open the nuclear magnetic resonance (NMR) and its corresponding software, dissolve the above film in deuterated dimethyl sulfoxide (DMSO-d6) or deuterated N,N-dimethylformamide (DMF-d7) as solvent, and heat at 60℃ to 80℃ to dissolve it into a solution with a mass percentage of 3% to 5%. Place the above solution into the nuclear magnetic resonance (NMR) sample cell and adjust 1 In H mode, click Collect, and the test is completed to obtain the NMR spectrum. 1 H NMR shows two sets of multiple peaks (CH2 and CH) at δ2.0ppm to 3.5ppm. 13 The C mode, tested in the same manner, showed a sharp single peak of nitrile carbon at δ 120 ppm to 125 ppm.

[0098] The functional monomer, namely the structure of polyacrylonitrile, can be determined by combining nuclear magnetic resonance, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TG).

[0099] Weight average molecular weight test:

[0100] The weight-average molecular weight of the alkalized hydrophobically modified polyacrylic acid was determined using an Agilent 1200 Series liquid chromatograph. The alkalized hydrophobically modified polyacrylic acid was dissolved in deionized water to prepare a 6 wt% solids solution. The solution was filtered through an aqueous filter membrane and then injected into the Agilent 1200 Series liquid chromatograph for analysis. The weight-average molecular weight of the alkalized hydrophobically modified polyacrylic acid was determined.

[0101] Viscosity test:

[0102] At an ambient temperature of 25°C, the alkalized hydrophobically modified polyacrylic acid was dissolved in deionized water to obtain a second slurry with a solid content of 6 wt%. The viscosity of the second slurry was measured at 25°C and 12 rpm using a Brookfield DV1 viscometer. The sample was placed horizontally, and the viscosity of the second slurry was estimated. An appropriate spindle was selected. For example, if the viscosity of the second slurry was estimated to be between 0 and 2000 mPa·s, a #61 spindle was selected; if the viscosity of the second slurry was estimated to be between 50 and 10,000 mPa·s, a #62 spindle was selected; if the viscosity of the second slurry was estimated to be between 10,000 and 40,000 mPa·s, a #63 spindle was selected; and if the viscosity of the second slurry was estimated to be between 10,000 and 200,000 mPa·s, a #64 spindle was selected. Insert the rotor into the measurement sample, set the rotor speed to 12 rpm, start the test, and test for 5 minutes. When the rotor viscosity value stabilizes for 2 minutes, the reading is the viscosity η of the second slurry being measured, in mPa·s.

[0103] Elastic modulus test of alkalized hydrophobically modified polyacrylic acid film:

[0104] At an ambient temperature of 25°C, alkalized hydrophobically modified polyacrylic acid is dissolved in deionized water to obtain a second slurry with a solid content of 6wt%. The second slurry is placed in a rectangular template made of glass (specifications: 80mm×20mm×1mm), air-dried until there is no obvious moisture on the surface of the slurry, and then placed in an 80°C oven and baked until the quality no longer decreases, thereby preparing a 20mm×80mm×1mm film. First, use a measuring tool with appropriate accuracy to measure the initial size of the sample, clamp the sample on the testing machine fixture, and ensure that the axis of the sample coincides with the direction of the tensile force. Set the test parameter tensile speed to 50mm / min. Start the testing machine to perform a tensile test, record the force-displacement data, calculate the stress-strain curve through data processing software, and calculate the elastic modulus of the alkalized hydrophobically modified polyacrylic acid film from the linear part of the curve.

[0105] Swelling test of alkalized hydrophobically modified polyacrylic acid in electrolyte:

[0106] At 25°C, alkalized hydrophobically modified polyacrylic acid was dissolved in deionized water to produce a second slurry with a solid content of 6 wt%. This second slurry was placed in a rectangular glass template (60 mm × 150 mm × 1 mm) and air-dried until no significant moisture was present on the surface. The slurry was then baked in an 80°C oven until the mass stopped decreasing, yielding a film. The film was weighed, recorded as m1, and then sealed and immersed in an electrolyte solution. The film was then placed in an 85°C oven for 48 hours. The film was then removed and the electrolyte on the film surface was blotted dry with paper until no electrolyte remained. The mass was then weighed and recorded as m2. The degree of swelling of the alkalized hydrophobically modified polyacrylic acid in the electrolyte, c, was calculated to be (m2 - m1) / m1 × 100%.

[0107] 800cls cycle capacity retention and negative electrode interface test:

[0108] The test temperature was maintained at 25°C. The lithium ion batteries in the examples and comparative examples were charged at a constant current of 0.7C to 4.5V, then charged at a constant voltage of 4.5V to a current of 0.025C, left to stand for 30 minutes, and discharged at a constant current of 0.5C to 3.0V, left to stand for 30 minutes. The above steps were recorded as one cycle, i.e., one circle. The discharge capacity C1 of the first circle was recorded. Each group of lithium ion batteries was cycled 800 circles (cls) according to the above cycle process, and the discharge capacity C1 of each group of lithium ion batteries was recorded. 800 800cls capacity retention rate (%) = C 800 / C1×100%.

[0109] After the cycle is completed, each group of lithium-ion batteries is disassembled, the negative electrode plates are taken out, and the surface conditions of each group of negative electrode plates are observed and photographed to record whether the negative electrode plates have problems such as purple spots / black spots on the interface.

[0110] The interface condition of the negative electrode sheet is determined by the area ratio of purple / black spots, which is calculated based on the total area of the negative electrode material layer. A purple / black spot area of ≤0.5% of the total negative electrode material layer is defined as no interface problem; a purple / black spot area of ≤2% of the total negative electrode material layer is defined as a mild interface problem; a purple / black spot area of ≤5% of the total negative electrode material layer is defined as a moderate interface problem; and a purple / black spot area of >5% of the total negative electrode material layer is defined as a severe interface problem.

[0111] Example 1

[0112] <Preparation of Hydrophobically Modified Polyacrylic Acid>

[0113] (1) 1566.7 parts of deionized water (W1) was used as a solvent, and a chain transfer agent, n-dodecyl mercaptan, and an initiator, sodium persulfate, were added to the solvent to obtain a mixture, wherein the amount of the chain transfer agent added, W2, was 0.5 parts, and the amount of the initiator added, W3, was 0.5 parts;

[0114] (2) Adding a hydrophobic monomer, isooctyl acrylate, a hydrophilic monomer, acrylic acid, and a functional monomer, acrylonitrile, to the mixture, wherein the mass fraction W4 of the hydrophilic monomer is 40 parts; the mass fraction W5 of the hydrophobic monomer is 30 parts, the mass fraction W6 of the functional monomer is 30 parts, and the total mass fraction of the hydrophobic monomer, the hydrophilic monomer, and the functional monomer is 100 parts; continuing to heat to a polymerization reaction temperature of 80°C for a time t of 4 hours, and performing reduced pressure distillation for 4 hours to remove residual monomers to obtain an aqueous dispersion of hydrophobically modified polyacrylic acid.

[0115] <Preparation of negative electrode sheet>

[0116] The negative electrode active material artificial graphite, conductive agent conductive carbon black, aqueous dispersion of hydrophobically modified polyacrylic acid, and styrene butadiene emulsion were mixed in a solid mass ratio of 97:0.5:1.0:1.5, and deionized water was added as a solvent to prepare a slurry with a solid content of 45wt%. After that, alkaline lithium hydroxide was added and stirred evenly in a vacuum mixer to obtain a negative electrode slurry including alkaline hydrophobically modified polyacrylic acid. The negative electrode slurry was evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6μm and dried at 120℃ to obtain a negative electrode sheet with a single-sided negative electrode material layer. The coating mass of the negative electrode material layer was 160mg / 1540.25mm 2 Then repeat the above steps on the other surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode material layer on both sides. Dry at 120℃ and then cold press. Cut the sheet after cold pressing. Place the negative electrode sheet in a high-temperature vacuum furnace for heat treatment at 320℃ to prepare a negative electrode sheet with a specification of 42.5mm×13mm for use. Among them, the cold pressing density of the negative electrode material layer is 1.80mg / mm 2 The mass ratio a of the alkali to the hydrophobically modified polyacrylic acid is 0.1875.

[0117] <Preparation of positive electrode sheet>

[0118] The positive electrode active material lithium titanate, the conductive agent Super P, and the binder polyvinylidene fluoride were mixed in a mass ratio of 97.9:0.9:1.2, and N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75wt%. After vacuum stirring, the positive electrode slurry was obtained. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 6μm and dried at 120℃ to obtain a positive electrode sheet with a single-sided positive electrode material layer. The coating mass of the positive electrode material layer was 267.8mg / 1540mm 2 Repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. Dry at 120°C and cold press, then cut into pieces to obtain a positive electrode sheet with a specification of 41.9mm×25mm for use. The cold pressed density of the positive electrode material layer is 4.15mg / mm 2 .

[0119] <Preparation of Separator>

[0120] A 5 μm thick polyethylene film was used as the base layer of the diaphragm. A 2 μm thick alumina ceramic layer was coated on one surface of the base layer, and then a 2.5 mg / 1540.25 mm thick alumina ceramic layer was coated on the surface of the alumina ceramic layer and the other surface of the base layer. 2The adhesive layer is dried to obtain a diaphragm. The alumina ceramic layer comprises aluminum oxide and polyvinylidene fluoride in a ratio of 70:30 by mass; the adhesive layer is made of polyvinylidene fluoride-hexafluoroethylene, and the porosity of the diaphragm is 39%.

[0121] <Preparation of Electrolyte>

[0122] In an environment with a water content of less than 10 ppm, ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl propionate are mixed in a mass ratio of 3:1:3:3 to obtain an organic solvent. The electrolyte salt LiPF6 is then added to the organic solvent and mixed uniformly to obtain an electrolyte solution. The electrolyte salt concentration is 1 mol / L, and the remainder is the organic solvent.

[0123] <Preparation of lithium-ion batteries>

[0124] Place the single-sided positive electrode sheet in the "Preparation of Positive Electrode Sheets" on the outermost side of the electrode assembly as the outermost electrode sheet of the electrode assembly. The remaining positive electrode sheets in the electrode assembly are all double-sided positive electrode sheets. Stack the electrodes and diaphragms in sequence, wherein the total number of layers of the electrode assembly is 16. Then use tape to fix the four corners of the entire laminate structure to obtain an electrode assembly with a laminate structure. Place the electrode assembly in a steel shell and place it in a vacuum oven at 80°C to dry for 12 hours to remove moisture. Inject the above-prepared electrolyte and obtain a lithium-ion battery after vacuum packaging, standing, formation, degassing, and trimming.

[0125] Example 2 to Example 19

[0126] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1.

[0127] Example 20

[0128] The preparation method is the same as that in Example 1, except that the alkali lithium hydroxide in the preparation of the negative electrode sheet is replaced by sodium hydroxide and the relevant preparation parameters are adjusted according to Table 1 and Table 2.

[0129] Example 21

[0130] The preparation process was the same as in Example 1, except that the negative electrode active material artificial graphite, the conductive agent conductive carbon black, the aqueous dispersion of hydrophobically modified polyacrylic acid, and the styrene-butadiene emulsion were mixed in a solid mass ratio of 97.7:0.5:0.8:1.5, the mass ratio a of the alkali to the hydrophobically modified polyacrylic acid was 0.1875, and the relevant preparation parameters were adjusted according to Tables 1 and 2.

[0131] Example 22

[0132] The negative electrode slurry in <Preparation of Negative Electrode Sheet> does not include styrene-butadiene emulsion, and the negative electrode active material artificial graphite, the conductive agent conductive carbon black, and the aqueous dispersion of hydrophobically modified polyacrylic acid are mixed in a solid mass ratio of 96:0.5:3.5, the mass ratio a of alkali to hydrophobically modified polyacrylic acid is 0.1875, and the relevant preparation parameters are adjusted according to Tables 1 and 2. The rest is the same as Example 1.

[0133] Example 23 to Example 29

[0134] Except for adjusting the relevant preparation parameters according to Table 1 and Table 2, the rest is the same as Example 1.

[0135] Example 30

[0136] The negative electrode slurry in <Preparation of Negative Electrode Sheet> does not include styrene-butadiene emulsion, and the negative electrode active material artificial graphite, conductive agent conductive carbon black, and a dispersion of hydrophobically modified polyacrylic acid are mixed in a solid mass ratio of 98:0.5:1.5, the mass ratio a of alkali to hydrophobically modified polyacrylic acid is 0.1875, and the relevant preparation parameters are adjusted according to Tables 1 and 2. The rest is the same as Example 1.

[0137] Example 31

[0138] The preparation process was the same as in Example 1, except that the negative electrode active material artificial graphite, the conductive agent conductive carbon black, the aqueous dispersion of hydrophobically modified polyacrylic acid, and the styrene-butadiene emulsion were mixed in a solid mass ratio of 97.5:0.5:1.5:0.5, the mass ratio a of the alkali to the hydrophobically modified polyacrylic acid was 0.1875, and the relevant preparation parameters were adjusted according to Tables 1 and 2.

[0139] Example 32

[0140] Except that the negative electrode active material in <Preparation of Negative Electrode Sheet> was replaced from artificial graphite to natural graphite and the relevant preparation parameters were adjusted according to Table 1 and Table 2, the rest was the same as Example 1.

[0141] Example 33

[0142] Except that the conductive agent in <Preparation of Negative Electrode Sheet> is replaced by carbon nanotubes instead of conductive carbon black, and the relevant preparation parameters are adjusted according to Table 1 and Table 2, the rest is the same as Example 1.

[0143] Example 34

[0144] Except that in <Preparation of Negative Electrode Sheet>, the negative electrode active material is replaced from artificial graphite to artificial graphite and SiC, where the mass ratio of artificial graphite to SiC is 9:1, the conductive agent is replaced from conductive carbon black to carbon nanotubes, and the relevant preparation parameters are adjusted according to Tables 1 and 2, the rest are the same as Example 1.

[0145] Comparative Example 1

[0146] The process was the same as in Example 1 except that the polymer was prepared according to the following steps, the hydrophobically modified polyacrylic acid in Example 1 was replaced, and no base was added.

[0147] <Preparation of Polymer>

[0148] Plant cellulose, a 75% ethanol solution, and a sodium hydroxide solution are mixed in 3 parts, 8 parts, and 4.5 parts by weight, and allowed to react at room temperature and pressure for 60 minutes. Four parts of a 45% monochloroacetic acid-ethanol solution are then added, and an etherification reaction is carried out at room pressure for 30 minutes. The reaction product is then washed with a 50% ethanol solution and purified twice by centrifugation to obtain sodium carboxymethyl cellulose (CMC). The mass percentage of sodium hydroxide based on the mass of the sodium hydroxide solution is 30%.

[0149] Comparative Example 2

[0150] The process was the same as in Example 1 except that the polymer was prepared according to the following steps, the hydrophobically modified polyacrylic acid in Example 1 was replaced, and no base was added.

[0151] <Preparation of Polymer>

[0152] 100 g of acrylic acid was added to 900 g of water and heated to 68° C. to prepare an acrylic acid aqueous solution with a mass fraction of 10% acrylic acid. Then, 1 g of ammonium persulfate was added to the acrylic acid aqueous solution at 68° C. and a rotation speed of 100 rpm to cause the acrylic acid to undergo self-polymerization to prepare a solution containing polyacrylic acid. Subsequently, using a peristaltic pump, 170 g of a 30% mass fraction sodium hydroxide aqueous solution was dropwise added to the polyacrylic acid solution over 20 minutes at the same temperature and rotation speed, and stirring was continued for 0.5 hour to prepare a solution containing sodium polyacrylate.

[0153] Comparative Example 3

[0154] The process was the same as in Example 1 except that the polymer was prepared according to the following steps, the hydrophobically modified polyacrylic acid in Example 1 was replaced, and no base was added.

[0155] <Preparation of Polymer>

[0156] 90 parts of vinyl acetate, 5 parts of methanol, and 5 parts of a 0.013 g / L isopropyl hydroperoxide-methanol solution (methanol as the solvent) (all previously deoxygenated using N2 bubbling) were added to a pressure reactor equipped with a stirrer, liquid and gas feed ports, cooling, and addition equipment. The air in the reactor was replaced with N2, and the internal temperature of the reactor was raised to 150°C to initiate polymerization. Maintaining the reactor temperature constant, after 8 hours of reaction, 90 parts of methanol was added to the reactor via a feed pump. The mixture was then cooled to 25°C to terminate polymerization, and the residual monomer was blown out to obtain a methanol solution of polyvinyl acetate. The mass fraction of the polyvinyl acetate was adjusted to 20%, and alcoholysis was carried out using a 40 g / L methanolic sodium hydroxide solution. The solution was then dried and pulverized to obtain polyvinyl alcohol powder.

[0157] Comparative Example 4

[0158] The preparation process was the same as in Example 1, except that in the <Preparation of Negative Electrode Sheet>, the negative electrode active material artificial graphite, the conductive agent conductive carbon black, the aqueous dispersion of hydrophobically modified polyacrylic acid, and the styrene acrylic emulsion were mixed in a solid mass ratio of 97:0.5:1.0:1.5, deionized water was added as a solvent, and a slurry with a solid content of 45 wt% was prepared. The negative electrode slurry was uniformly stirred with a vacuum mixer, that is, no alkali was added during the preparation of the negative electrode slurry, the aqueous dispersion of hydrophobically modified polyacrylic acid was dispersed at a high speed of 2000 rpm for 10 hours before use, and the relevant preparation parameters were adjusted according to Tables 1 and 2.

[0159] It can be understood that in the above embodiments and comparative examples, the mass ratio of the substances added to prepare the negative electrode slurry is the mass ratio of the solids contained in the substances.

[0160] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0161]

[0162]

[0163]

[0164] Table 2

[0165]

[0166]

[0167] Note: “ / ” in Table 2 indicates no relevant preparation or result parameters.

[0168] From Examples 1 to 34 and Comparative Examples 1 to 4, it can be seen that by adding alkaline hydrophobically modified polyacrylic acid to the negative electrode material layer, the width L of the negative electrode material layer in the edge region is smaller, the 800cls cycle capacity retention rate is higher, and the interface problems of the negative electrode plate after the cycle are less severe. This indicates that during the preparation of the negative electrode plate, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the edge thinning length after coating and improve the electrode assembly interface in the later stage of the cycle. In Comparative Examples 1 to 3, conventional negative electrode dispersants are used. At this time, the width L of the negative electrode material layer in the edge region is larger, and the interface problems of the negative electrode plate after the cycle are more severe, indicating that the electrode plates in Comparative Examples 1 to 3 have a longer edge thinning length after coating, and the interface problems of the electrode assembly in the later stage of the cycle are serious. The shear viscosity of the first slurry in Comparative Example 4 is outside the scope of this application. The width L of the negative electrode material layer in the edge region is large, and the 800cls cycle capacity retention rate is low, indicating that the electrode sheet in Comparative Example 43 has a long thinning length at the edge after coating, resulting in poor cycling performance. In contrast, the negative electrode material layer in Examples 1 to 34 includes alkaline hydrophobically modified polyacrylic acid. The negative electrode slurry has a high viscosity, making it difficult for the slurry to flow at the coating edge, reducing the thinning length at the coating edge and improving the electrode assembly interface in the later stages of the cycle.

[0169] The value of L / W usually affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. From Examples 1 to 34, it can be seen that when the value of L / W is within the range of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface problem after the cycle is relatively mild. This indicates that during the preparation process of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0170] The value of L / H generally affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1 to 34, when the value of L / H is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface problem after the cycle is relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, indicating that it can reduce the thinning length of the edge at the end of the coating and improve the electrode assembly interface in the later stages of the cycle.

[0171] The value of L typically affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1 to 34, when the value of L is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface issues after the cycle are relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, which can reduce the thinning length of the edge at the end of the coating process and improve the electrode assembly interface in the later stages of the cycle.

[0172] The value of X usually affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. It can be seen from Examples 1, 21 to 22, and 30 to 31 that when the value of X is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface problem after the cycle is relatively mild, indicating that during the preparation process of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0173] The type of hydrophobic monomer usually affects the thinning length of the negative electrode sheet, and thus affects the interface of the electrode assembly in the later stages of the cycle. It can be seen from Example 1, Example 8 to Example 19, Example 23, and Example 26 that when the type of hydrophobic monomer is within the scope of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode sheet after the cycle is relatively mild, indicating that in the preparation process of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the interface of the electrode assembly in the later stages of the cycle. Among them, the carbon chain length of the hydrophobic monomer in Example 26 is relatively long, and the bonding force between the prepared negative electrode material layer and the negative electrode current collector is relatively low, resulting in a large area of purple spots / black spots on the interface of the negative electrode sheet after the lithium-ion battery is cycled. The type of hydrophilic monomer usually affects the thinning length of the negative electrode sheet, and thus affects the interface of the electrode assembly in the later stages of the cycle. It can be seen from Example 1 and Example 27 that when the type of hydrophilic monomer is within the scope of this application, the width L of the negative electrode material layer in the edge area is smaller, the 800cls cycle capacity retention rate is higher, and the interface problem of the negative electrode plate after the cycle is lighter, indicating that in the preparation process of the negative electrode plate, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that the edge thinning length at the end of coating can be reduced, and the electrode assembly interface in the later stage of the cycle can be improved.

[0174] The type of functional monomer usually affects the thinning length of the negative electrode plate, and thus affects the electrode assembly interface in the later stage of the cycle. It can be seen from Example 1, Example 28 and Example 29 that when the type of functional monomer is within the scope of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode plate after the cycle is relatively mild, indicating that in the preparation process of the negative electrode plate, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stage of the cycle. Among them, the bonding force between the negative electrode material layer prepared in Example 29 and the negative electrode current collector is low, resulting in a large area of purple spots / black spots on the interface of the negative electrode plate after the lithium-ion battery is cycled.

[0175] The pH of the second slurry typically affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1 to 31, when the pH of the second slurry is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface issues after the cycle are relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry containing the alkaline hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, indicating that it can reduce the thinning length of the edge at the end of the coating and improve the electrode assembly interface in the later stages of the cycle.

[0176] The weight-average molecular weight of the alkalized hydrophobically modified polyacrylic acid usually affects the thinning length of the negative electrode sheet, and thus affects the electrode assembly interface in the later stages of the cycle. From Examples 1 to 31, it can be seen that when the weight-average molecular weight of the alkalized hydrophobically modified polyacrylic acid is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode sheet after the cycle is relatively mild, indicating that during the preparation process of the negative electrode sheet, the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0177] The viscosity of the alkaline hydrophobically modified polyacrylic acid at 25°C and a rotation speed of 12 rpm usually affects the thinning length of the negative electrode sheet, and thus affects the electrode assembly interface in the later stage of the cycle. From Examples 1 to 31, it can be seen that when the viscosity of the alkaline hydrophobically modified polyacrylic acid at 25°C and a rotation speed of 12 rpm is within the range of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode sheet after the cycle is relatively mild, indicating that in the preparation process of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has a high viscosity characteristic under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stage of the cycle.

[0178] The elastic modulus of the alkalized hydrophobically modified polyacrylic acid film usually affects the thinning length of the negative electrode sheet, and thus affects the electrode assembly interface in the later stages of the cycle. From Examples 1 to 31, it can be seen that when the elastic modulus of the alkalized hydrophobically modified polyacrylic acid film is within the range of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode sheet after the cycle is relatively mild, indicating that in the preparation process of the negative electrode sheet, the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has a high viscosity characteristic under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0179] The degree of swelling of the alkaline hydrophobically modified polyacrylic acid in the electrolyte usually affects the thinning length of the negative electrode sheet, and thus affects the electrode assembly interface in the later stages of the cycle. From Examples 1 to 31, it can be seen that when the degree of swelling of the alkaline hydrophobically modified polyacrylic acid in the electrolyte is within the scope of this application, the width L of the negative electrode material layer in the edge area is small, the 800cls cycle capacity retention rate is high, and the interface problem of the negative electrode sheet after the cycle is relatively mild, indicating that in the preparation process of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity characteristics under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0180] The type of alkali typically affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1 and 20, when the type of alkali is within the scope of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface issues after the cycle are relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, indicating that it can reduce the thinning length of the edge after coating and improve the electrode assembly interface in the later stages of the cycle.

[0181] The value of a typically affects the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1, 24, and 25, when the value of a is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface issues after the cycle are relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry comprising alkalinized hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, which can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0182] The shear viscosity of the negative electrode slurry comprising an alkalized hydrophobically modified polyacrylic acid generally affects the thinning length of the negative electrode sheet, thereby affecting the electrode assembly interface in the later stages of the cycle. From Examples 1 to 31, it can be seen that when the shear viscosity of the negative electrode slurry comprising an alkalized hydrophobically modified polyacrylic acid is within the range of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface problem after the cycle is relatively mild, indicating that during the preparation process of the negative electrode sheet, the negative electrode slurry comprising an alkalized hydrophobically modified polyacrylic acid has a high viscosity characteristic under low shear, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0183] The type of negative electrode active material and the type of conductive agent generally affect the thinning length of the negative electrode sheet, which in turn affects the electrode assembly interface in the later stages of the cycle. As can be seen from Examples 1 and 32 to 34, when the type of negative electrode active material and the type of conductive agent are within the scope of this application, the width L of the negative electrode material layer in the edge region is small, the 800cls cycle capacity retention rate is high, and the negative electrode sheet interface problems after the cycle are relatively mild. This indicates that during the preparation of the negative electrode sheet, the negative electrode slurry including the alkaline hydrophobically modified polyacrylic acid has high viscosity under low shear conditions, indicating that it can reduce the thinning length of the edge at the end of coating and improve the electrode assembly interface in the later stages of the cycle.

[0184] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0185] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0186] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises hydrophobically modified polyacrylic acid; The powder of the negative electrode material layer is dissolved in deionized water to obtain a first slurry with a solid content of 45 wt % to 50 wt %, and the shear rate is 0.1S -1 , the shear viscosity of the first slurry is η1mPa·s, 1000≤η1≤30000.

2. The secondary battery according to claim 1, wherein Along the width direction of the unfolded negative electrode sheet, the negative electrode material layer includes a main region and an edge region; along the thickness direction of the negative electrode sheet, an area where the thickness fluctuation difference of the negative electrode material layer is ≤1 μm from the center position of the negative electrode material layer is the main region, the average thickness of the main region is H μm, and along the width direction of the unfolded negative electrode sheet, the width of the main region is W μm; Along the width direction of the unfolded negative electrode sheet, the width of the edge area is L μm; 1.04×10 -4 ≤L / W≤2.08×10 -4 ; and / or, 0.33≤L / H≤1.

25.

3. The secondary battery according to claim 2, wherein 50≤L≤100。 4. The secondary battery according to claim 1, wherein The negative electrode material layer includes a negative electrode active material and a conductive agent, wherein the negative electrode active material includes at least one of artificial graphite, natural graphite, pure silicon, silicon oxides, silicon carbon compounds or mesophase microspheres, and the conductive agent includes at least one of conductive carbon black, conductive carbon nanotubes or graphene.

5. The secondary battery according to claim 4, wherein The negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200° C. to 350° C., and the decomposition ratio of the negative electrode material layer is X, where 0.8%≤X≤3.5%.

6. The secondary battery according to claim 4, wherein The negative electrode material layer further comprises at least one of styrene-butadiene rubber or styrene-propylene rubber.

7. The secondary battery according to claim 6, wherein The negative electrode material layer is thermally decomposed in a nitrogen atmosphere at 200° C. to 350° C., and the decomposition ratio of the negative electrode material layer is X, 0.8%≤X≤1.5%.

8. The secondary battery according to claim 1, wherein The hydrophobically modified polyacrylic acid is an alkalized hydrophobically modified polyacrylic acid. The monomers forming the alkalized hydrophobically modified polyacrylic acid include a hydrophobic monomer, a hydrophilic monomer, and a functional monomer. The hydrophobic monomer includes a long carbon chain monomer of C4 to C21, the hydrophilic monomer includes an acrylic monomer, and the functional monomer includes at least one of acrylonitrile or acrylamide.

9. The secondary battery according to claim 8, wherein The C4 to C21 long carbon chain monomer includes at least one of ethers, esters or hydrocarbons containing unsaturated double bonds; and / or the acrylic monomer includes at least one of methacrylic acid, ethacrylic acid or acrylic acid.

10. The secondary battery according to claim 8, which satisfies at least one of the following characteristics: (1) dissolving the alkalized hydrophobically modified polyacrylic acid in deionized water to obtain a second slurry having a solid content of 2 wt % to 10 wt %, wherein the pH of the second slurry is 6.0 to 8.0; (2) The weight average molecular weight of the alkalized hydrophobically modified polyacrylic acid is Mw g / mol, 3×10 5 ≤Mw≤1.5×10 6 ; (3) dissolving the alkalized hydrophobically modified polyacrylic acid in deionized water to obtain a second slurry having a solid content of 2 wt % to 10 wt %, wherein the viscosity of the second slurry at 25° C. and 12 rpm is η mPa·s, 30000≤η≤100000; (4) The elastic modulus of the alkalized hydrophobically modified polyacrylic acid film is E GPa, 5≤E≤20; (5) The swelling degree of the alkaline hydrophobically modified polyacrylic acid in the electrolyte is c, 1%≤c≤10%.

11. A method for preparing a secondary battery according to any one of claims 1 to 10, comprising the following steps: preparing the positive electrode sheet, the negative electrode sheet, the electrolyte and the separator, and assembling them to obtain the secondary battery; The method for preparing the negative electrode sheet comprises the following steps: preparing a negative electrode slurry, adding the hydrophobically modified polyacrylic acid to the negative electrode slurry, then adding alkali, and mixing to obtain a negative electrode slurry comprising alkalized hydrophobically modified polyacrylic acid, coating the negative electrode slurry comprising the alkalized hydrophobically modified polyacrylic acid on the surface of the negative electrode current collector, and drying and cold pressing to obtain the negative electrode sheet; The base includes at least one of lithium hydroxide, sodium hydroxide or potassium hydroxide, and the mass ratio of the base to the hydrophobically modified polyacrylic acid is a, 1 / 8≤a≤1 / 4.

12. The preparation method according to claim 11, wherein The shear rate is 0.1S -1 The shear viscosity of the negative electrode slurry including the alkalized hydrophobically modified polyacrylic acid is η2mPa·s, 100000≤η2≤400000.

13. An electronic device comprising the secondary battery according to any one of claims 1 to 10, or the secondary battery prepared by the preparation method according to claim 11 or 12.

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