Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, the positive electrode material layer comprises a positive electrode binder, the positive electrode binder comprises a polymer, the polymer comprises vinylidene fluoride and a fluorine-containing structure unit, and the fluorine-containing structure unit comprises a fluorine-containing structure unit. The fluorine-containing structural unit comprises at least one of-CH = CF <->,-CFH-CF2 <-> or-CFCl-CF2 <->. Based on the total amount of substance of the polymer, the molar fraction of the fluorine-containing structural unit is M%, and M is more than or equal to 20 and less than or equal to 80. The characteristics are met by regulating and controlling the structure of the polymer, the type and mole fraction of the fluorine-containing structural unit are within the range of the invention, the flexibility of the positive pole piece can be improved, the initial compaction density of the positive pole piece is improved, and the volume energy density of the secondary battery is improved.

Description

Technical Field

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

[0002] With the further expansion of the application field of lithium-ion batteries, the demand for lightweight, small size and long battery life in portable electronic devices has further increased. In particular, to save space, volume energy density is the focus of current production. Increasing the coating weight of the electrode per unit area and the initial compaction density of the electrode are effective means to improve the volume energy density of lithium-ion batteries. However, high compaction density will cause damage to the electrode. In the subsequent lithium-ion battery preparation process, there is a risk of brittle fracture of the electrode and delamination of the electrode material, which affects the capacity, life and safety of the lithium-ion battery. Therefore, how to make the electrode have better flexibility while increasing the compaction density of the electrode is a research focus of lithium-ion batteries.

[0003] Flexibility can be improved by redesigning the battery structure, such as using segmented batteries, or reducing the thickness of the pole pieces to avoid large strains during bending. However, while segmented batteries or reducing the thickness of the pole pieces can improve the flexibility of lithium-ion batteries, this also affects the volumetric energy density of lithium-ion batteries. Replacing metal current collectors with carbon-based flexible materials can improve pole piece flexibility, but directly using carbon-based flexible materials as current collectors also faces problems such as low load rate and low conductivity. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and electronic device that can improve the flexibility of the positive electrode sheet, increase the initial compaction density of the positive electrode sheet, and increase the volume energy density of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides a secondary battery, which includes a positive electrode plate, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, the positive electrode material layer includes a positive electrode binder, the positive electrode binder includes a polymer, the polymer includes vinylidene fluoride and a fluorine-containing structural unit, the fluorine-containing structural unit includes at least one of -CH=CF-, -CFH-CF2- or -CFCl-CF2-. Based on the total amount of the polymer, the mole fraction of the fluorine-containing structural unit is M%, 20≤M≤80, preferably, 30≤M≤50. By regulating the structure of the polymer to meet the above characteristics, the type and mole fraction of the fluorine-containing structural unit are within the scope of the present application, the energy barrier for the movement of the polymer molecular chain is low, and the appropriate content of the fluorine-containing structural unit can better promote the transformation of the vinylidene fluoride (VDF) phase structure to the β crystal form, so that the mobility of the polymer molecular chain is stronger, which can improve the flexibility of the positive electrode plate, increase the initial compaction density of the positive electrode plate, and increase the volume energy density of the secondary battery.

[0006] In some embodiments of the present application, the ratio of the number of fluorine atoms in the fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit is A, and 0.5≤A≤0.75. By regulating the ratio of the number of fluorine atoms in the fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit within the above range, the VDF phase structure in the polymer is better promoted to transform to the β crystal form, the free movement ability of the polymer molecular chain is enhanced, the flexibility of the positive electrode plate can be improved, the initial compaction density of the positive electrode plate is increased, and the volume energy density of the secondary battery is increased.

[0007] In some embodiments of the present application, the polymer includes at least one of polyvinylidene fluoride-double bond copolymer (P(VDF-DB)), poly(vinylidene fluoride-trifluoroethylene), or poly(vinylidene fluoride-chlorotrifluoroethylene). The above polymers are selected as fluorinated polymers containing a β-crystal structure, and the polymer molecular chain has a low energy barrier for motion and a strong mobility of the polymer molecular chain, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0008] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, the first characteristic peaks are present in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 6.1ppm to 6.8ppm. 1 The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer includes VDF and a fluorine-containing structural unit DB (double bond). The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing β crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The movement ability of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0009] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, there are second characteristic peaks in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm, and 5.0ppm to 6.0ppm. 1 The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer contains VDF and a fluorine-containing structural unit -CFH-CF2-. The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The movement ability of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0010] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, there are third characteristic peaks in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm, and 3.3ppm to 3.5ppm. 1 The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer contains VDF and a fluorine-containing structural unit -CFCl-CF2-. The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The movement ability of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0011] In some embodiments of the present application, the average molecular weight of the polymer is Mz, the weight average molecular weight of the polymer is Mw, 1.8≤Mz / Mw≤3.5, 700000≤Mw≤1200000. By regulating the values ​​of Mz / Mw and Mw within the above ranges, the average molecular weight of the polymer, the weight average molecular weight of the polymer, and the ratio of the average molecular weight to the weight average molecular weight of the polymer are relatively appropriate, the polymer has an appropriate molecular chain length, and the steric hindrance of the polymer is small, which can further reduce the energy barrier for the movement of the polymer molecular chain, further enhance the mobility of the polymer molecular chain, further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0012] In some embodiments of the present application, the crystallinity of the polymer is X%, and 20≤X≤70. By regulating the crystallinity of the polymer within the above range, the polymer itself has good flexibility, the energy barrier for the movement of the polymer molecular chain is low, and the mobility of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0013] In some embodiments of the present application, in the X-ray diffraction spectrum of the polymer, a fourth characteristic peak exists in the range of 19° to 21° and 34° to 35°. The X-ray diffraction spectrum of the polymer satisfies the above characteristics, the fourth characteristic peak is the characteristic peak of the polar phase (β phase), the dielectric constant of the polymer mainly comes from the β phase, and the high dielectric constant can promote the dissociation of lithium salts; at the same time, the β phase represents the neat arrangement of electronegative groups, which can provide ion transmission channels, shorten the ion transmission path, and improve the kinetic performance of the secondary battery; and the energy barrier for the movement of the polymer molecular chain is low, and the mobility of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0014] In some embodiments of the present application, in the infrared spectrum of the polymer, at 470 cm -1 Up to 475cm -1 、507cm -1 Up to 515cm -1 、830cm -1 Up to 850cm -1 、1275cm -1 to 1290cm -1 There is a fifth characteristic peak within the range. The infrared spectrum of the polymer meets the above characteristics. The groups in the polymer are neatly arranged, and the polymer contains a β phase. The dielectric constant of the polymer mainly comes from the β phase. The high dielectric constant can promote the dissociation of lithium salts. At the same time, the groups in the polymer are neatly arranged, which can provide ion transmission channels, shorten the ion transmission path, and improve the kinetic performance of the secondary battery. In addition, the energy barrier of the polymer molecular chain movement is low, and the polymer molecular chain has strong mobility, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0015] In some embodiments of the present application, the melting point of the polymer is Tm°C, 120≤Tm≤160. By regulating the melting point of the polymer within the above range, the processing performance of the positive electrode slurry can be optimized, the stability of the electrode structure can be enhanced, and the cycle life of the secondary battery can be extended.

[0016] In some embodiments of the present application, the polymer includes silicon, iron, and zinc, and the sum of the mass of the silicon, iron, and zinc in the polymer is W, where W is less than 200 ppm. By regulating the sum of the mass of the silicon, iron, and zinc in the polymer to fall within the above range, the content of impurity elements in the polymer is low, which can reduce the possibility of increased self-discharge of the secondary battery or precipitation of impurity elements on the surface of the negative electrode, thereby improving the safety performance of the secondary battery.

[0017] In some embodiments of the present application, the compaction density of the positive electrode material layer is PDg / cm 3 , 3.0≤PD≤4.5, preferably, 3.8≤PD≤4.4. By regulating the compaction density of the positive electrode material layer within the above range, the positive electrode material layer has a higher compaction density, which can enable the secondary battery to have a higher volumetric energy density. At the same time, the positive electrode sheet also has good flexibility, which can better reduce the risk of brittle fracture of the positive electrode sheet and delamination of the positive electrode material layer, and thus provide the secondary battery with better cycle performance and safety performance.

[0018] In some embodiments of the present application, the positive electrode material layer also includes a positive electrode active material, and the Dv50 of the positive electrode active material satisfies the following conditions: 0.5μm≤Dv50≤35μm, preferably, 5μm≤Dv50≤30μm, and more preferably, 10μm≤Dv50≤25μm. By regulating the Dv50 of the positive electrode active material within the above range, the particle size of the positive electrode active material is moderate, the stress of the positive electrode material layer is evenly distributed, the possibility of cracks occurring when the positive electrode plate is bent is small, and the flexibility of the positive electrode plate is improved; at the same time, the density of the positive electrode plate is moderate, and the deformation ability of the positive electrode plate is strong, which further improves the flexibility of the positive electrode plate, further increases the initial compaction density of the positive electrode plate, and further increases the volume energy density of the secondary battery.

[0019] In some embodiments of the present application, the thickness of the positive electrode current collector is H1 μm, 7≤H1≤20, preferably, 8≤H1≤12. By adjusting the thickness of the positive electrode current collector within the above range, the thickness of the positive electrode current collector is smaller, which can enable the secondary battery to have a higher volume energy density.

[0020] In some embodiments of the present application, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, or lithium iron phosphate. The selection of these positive electrode active materials and their application in the positive electrode material layer with a polymer can enable the secondary battery to have a higher volumetric energy density.

[0021] In some embodiments of the present application, the positive electrode material layer further includes a positive electrode conductive agent. Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode active material is w1%, the mass percentage of the positive electrode binder is w2%, and the mass percentage of the positive electrode conductive agent is w3%, 97.6≤w1≤98.8, 0.7≤w2≤1.3, and 0.5≤w3≤1.1. By regulating the mass percentages of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent within the above ranges, the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent all have appropriate mass percentages, the energy barrier for polymer molecular chain motion is low, and the polymer molecular chain has strong mobility, which can make the positive electrode sheet have good flexibility; and the mass percentage of the positive electrode active material is relatively high, and the secondary battery has a high volume energy density.

[0022] In some embodiments of the present application, the thickness of the positive electrode material layer is H2 μm, and 40.5 ≤ H2 ≤ 70. By regulating the thickness of the positive electrode material layer within the above range, the positive electrode material layer has an appropriate thickness and a high compaction density, which can enable the secondary battery to have a high volume energy density. At the same time, the positive electrode sheet also has good flexibility, which can better reduce the risk of brittle fracture of the positive electrode sheet and delamination of the positive electrode material layer, and thus provide the secondary battery with good cycle performance and safety performance.

[0023] In some embodiments of the present application, the bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, 13≤F≤35. By regulating the bonding force between the positive electrode material layer and the positive electrode current collector within the above range, the bonding force between the positive electrode material layer and the positive electrode current collector is greater, the positive electrode material layer and the positive electrode current collector are in closer contact, the risk of the positive electrode material layer peeling is reduced, and the secondary battery has better cycle performance and safety performance.

[0024] In some embodiments of the present application, along the thickness direction of the positive electrode sheet, the positive electrode material layer includes a first surface and a second surface facing each other; from the first surface to the second surface, the positive electrode material layer includes a first region, a second region, and a third region in sequence, with the third region being located on the surface of the positive electrode current collector; the thickness of the first region accounts for 1 / 3 of the thickness of the positive electrode material layer, the thickness of the second region accounts for 1 / 3 of the thickness of the positive electrode material layer, and the thickness of the third region accounts for 1 / 3 of the thickness of the positive electrode material layer. The weight loss rate of the positive electrode material layer particles in the first region from 0°C to 600°C is TG1, the weight loss rate of the positive electrode material layer particles in the second region from 0°C to 600°C is TG2, and the weight loss rate of the positive electrode material layer particles in the third region from 0°C to 600°C is TG3, 0.85≤TG1 / TG2≤1.15, and 0.85≤TG1 / TG3≤1.15. The values ​​of TG1 / TG2 and TG1 / TG3 are within the above range, indicating that the upper, middle and lower regions of the positive electrode material layer are all distributed with positive electrode binders of similar contents. The positive electrode binders include the above polymers, and the energy barrier for the movement of the polymer molecular chains is low. The polymer molecular chains have strong mobility, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0025] In some embodiments of the present application, the area on the surface of the positive electrode current collector where the positive electrode material layer is provided is the fourth area, which satisfies at least one of the following characteristics:

[0026] (1) the ratio of the specific surface areas of the positive electrode active materials on any two positive electrode material layers with an area of ​​250 mm×250 mm on the fourth region is 0.97 to 1.03;

[0027] (2) The ratio of the roughness of the positive electrode current collector of any two fourth regions with an area of ​​250 mm×250 mm is 0.93 to 1.07.

[0028] In some embodiments of the present application, the contact angle between the positive electrode sheet and the electrolyte is θ, 10°≤θ≤30°. By regulating the contact angle between the positive electrode sheet and the electrolyte within the above range, the contact angle between the positive electrode sheet and the electrolyte is small, the positive electrode sheet has good wettability, the electrolyte can better infiltrate the polymer, the polymer has high ionic conductivity, can promote lithium ion transmission, reduce the impedance of the positive electrode sheet, improve the charge and discharge capacity of the secondary battery under high current, and improve the dynamic performance of the secondary battery.

[0029] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the aforementioned embodiments. Therefore, the electronic device provided by the present application has good performance.

[0030] Beneficial effects of this application:

[0031] The present application provides a secondary battery and an electronic device, wherein the secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, the positive electrode material layer includes a positive electrode binder, the positive electrode binder includes a polymer, the polymer includes vinylidene fluoride and a fluorine-containing structural unit, the fluorine-containing structural unit includes at least one of -CH=CF-, -CFH-CF2- or -CFCl-CF2-. Based on the total amount of the polymer, the mole fraction of the fluorine-containing structural unit is M%, 20≤M≤80. By regulating the structure of the polymer to meet the above characteristics, the type and mole fraction of the fluorine-containing structural unit are within the scope of the present application, the flexibility of the positive electrode sheet can be improved, the initial compaction density of the positive electrode sheet can be increased, and the volume energy density of the secondary battery can be increased.

[0032] 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

[0033] 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.

[0034] Figure 1 For some embodiments of the present application, the polymer 1 H NMR spectrum;

[0035] Figure 2 X-ray diffraction spectra of polymers according to some embodiments of the present application;

[0036] Figure 3 This is an infrared spectrum of the polymer according to some embodiments of the present application. DETAILED DESCRIPTION

[0037] 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.

[0038] 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.

[0039] The first aspect of the present application provides a secondary battery, which includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, the positive electrode material layer includes a positive electrode binder, the positive electrode binder includes a polymer, the polymer includes vinylidene fluoride (VDF) and a fluorine-containing structural unit, the fluorine-containing structural unit includes at least one of -CH=CF-, -CFH-CF2- or -CFCl-CF2-. Based on the total amount of the polymer, the mole fraction of the fluorine-containing structural unit is M%, 20≤M≤80, preferably, 30≤M≤50. For example, the value of M can be 20, 23, 26, 29, 30, 33, 36, 39, 40, 43, 46, 49, 50, 53, 56, 59, 60, 63, 66, 69, 70, 73, 76, 79, 80 or a range consisting of any two of the above values. The phrase "a positive electrode material layer disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along its thickness direction, or on both surfaces of the positive electrode current collector along its thickness direction. It should be noted that the "surface" herein can refer to the entire area of ​​the positive electrode current collector or a portion of the positive electrode current collector. This is not particularly limited in this application, as long as the objectives of this application can be achieved.

[0040] The inventors have discovered that introducing the aforementioned fluorine-containing structural units into the structure of the polymer polyvinylidene fluoride can reduce the energy barrier for polymer molecular chain motion. Furthermore, the fluorine-containing structural units promote the transformation of the VDF phase structure to the β-crystal form, further enhancing the mobility of the polymer molecular chains, improving the flexibility of the positive electrode sheet and increasing the initial compaction density of the positive electrode sheet, thereby increasing the volumetric energy density of the secondary battery. When the mole fraction of the fluorine-containing structural units is too low, for example, less than 20%, the fluorine-containing structural unit content is too low, and the energy barrier for polymer molecular chain motion, the mobility of the polymer molecular chains, and the flexibility of the positive electrode sheet cannot be effectively reduced, the initial compaction density of the positive electrode sheet cannot be effectively improved, and the improvement in the volumetric energy density of the secondary battery is also small. When the mole fraction of the fluorine-containing structural units is too high, for example, greater than 80%, the fluorine-containing structural unit content is too high, and the dipole interaction between fluorine atoms is enhanced, restricting the motion of the chain segments, failing to effectively improve the flexibility of the positive electrode sheet and failing to effectively increase the volumetric energy density of the secondary battery. Therefore, by regulating the structure of the polymer to meet the above characteristics, the type and molar fraction of the fluorine-containing structural unit are within the scope of this application, the energy barrier for the movement of the polymer molecular chain is low, and the appropriate content of fluorine-containing structural units can better promote the transformation of the VDF phase structure to the β crystal form, so that the polymer molecular chain has stronger mobility, can improve the flexibility of the positive electrode sheet, increase the initial compaction density of the positive electrode sheet, and increase the volume energy density of the secondary battery.

[0041] In some embodiments of the present application, the ratio of the number of fluorine atoms in the fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit is A, 0.5≤A≤0.75. For example, the value of A can be 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, 0.72, 0.74, 0.75 or a range consisting of any two of the above values. By regulating the ratio of the number of fluorine atoms in the fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit within the above range, the VDF phase structure in the polymer is better promoted to transform to the β crystal form, the free movement ability of the polymer molecular chain is improved, the flexibility of the positive electrode sheet can be improved, the initial compaction density of the positive electrode sheet can be increased, and the volume energy density of the secondary battery can be increased.

[0042] In some embodiments of the present application, the polymer includes at least one of polyvinylidene fluoride-double bond copolymer (P(VDF-DB)), poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), or poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)). The structural formula of the structural unit DB in P(VDF-DB) is -CH=CF-. The above-mentioned polymer is selected, and the above-mentioned polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low, and the mobility of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0043] In this application, P(VDF-DB) can be obtained by removing HCl from the polymer P(VDF-CTFE) through an elimination reaction. P(VDF-TrFE) can be obtained by copolymerizing vinylidene fluoride monomer with trifluoroethylene, a small molecule monomer. P(VDF-CTFE) can be obtained by copolymerizing vinylidene fluoride monomer with chlorotrifluoroethylene, a small molecule monomer.

[0044] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, the first characteristic peaks are present in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 6.1ppm to 6.8ppm. 1 In the H NMR spectrum, the peaks in the range of 2.3ppm to 2.7ppm and 2.7ppm to 3.3ppm are characteristic peaks of VDF, and the peak in the range of 6.1ppm to 6.8ppm is the characteristic peak of DB. 1The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer contains VDF and a fluorine-containing structural unit DB. The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The polymer molecular chain has a strong movement ability, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0045] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, there are second characteristic peaks in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 5.0ppm to 6.0ppm. 1 In the H NMR spectrum, the peaks in the range of 2.3ppm to 2.7ppm and 2.7ppm to 3.3ppm are characteristic peaks of VDF, and the peak in the range of 5.0ppm to 6.0ppm is the characteristic peak of -CFH-CF2-. 1 The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer contains VDF and a fluorine-containing structural unit -CFH-CF2-. The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The movement ability of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0046] In some embodiments of the present application, in the polymer 1 In the H NMR spectrum, there are third characteristic peaks in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 3.3ppm to 3.5ppm. 1 In the H NMR spectrum, the peaks in the range of 2.3ppm to 2.7ppm and 2.7ppm to 3.3ppm are characteristic peaks of VDF, and the peak position of 3.3ppm to 3.5ppm corresponds to the chemical shift of the hydrogen atoms in the vinylidene fluoride connected to trifluorochloroethylene, which can be used to characterize the presence of trifluorochloroethylene. 1 The H nuclear magnetic resonance spectrum meets the above characteristics, indicating that the polymer contains VDF and a fluorine-containing structural unit -CFCl-CF2-. The above polymer is used as a positive electrode binder in the positive electrode material layer. The above polymer is a fluorine-containing polymer containing a β-crystal form, and the energy barrier for the movement of the polymer molecular chain is low. The movement ability of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0047] In some embodiments of the present application, the average molecular weight of the polymer is Mz, the weight average molecular weight of the polymer is Mw, 1.8≤Mz / Mw≤3.5, 700000≤Mw≤1200000. For example, the value of Mz / Mw can be 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, or a range consisting of any two of the above values; the value of Mw can be 700000, 750000, 800000, 850000, 900000, 950000, 1000000, 1050000, 1100000, 1150000, 1200000, or a range consisting of any two of the above values. By regulating the values ​​of Mz / Mw and Mw within the above ranges, the average molecular weight of the polymer, the weight-average molecular weight of the polymer, and the ratio of the average molecular weight to the weight-average molecular weight of the polymer are relatively appropriate, the polymer has an appropriate molecular chain length, and the steric hindrance of the polymer is small, which can further reduce the energy barrier for the movement of the polymer molecular chain, further enhance the mobility of the polymer molecular chain, further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery. In this application, 1260000≤Mz≤4200000.

[0048] In some embodiments of the present application, the crystallinity of the polymer is X%, 20≤X≤70. For example, the value of X can be 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or a range consisting of any two of the above values. By regulating the crystallinity of the polymer within the above range, the polymer itself has good flexibility, the energy barrier for the movement of the polymer molecular chain is low, and the mobility of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0049] In some embodiments of the present application, in the X-ray diffraction spectrum of the polymer, a fourth characteristic peak exists in the range of 19° to 21° and 34° to 35°. The X-ray diffraction spectrum of the polymer satisfies the above characteristics, the fourth characteristic peak is the characteristic peak of the polar phase (β phase), the dielectric constant of the polymer mainly comes from the β phase, and the high dielectric constant can promote the dissociation of lithium salts; at the same time, the β phase represents the neat arrangement of electronegative groups, which can provide ion transmission channels, shorten the ion transmission path, and improve the kinetic performance of the secondary battery; and the energy barrier for the movement of the polymer molecular chain is low, and the mobility of the polymer molecular chain is strong, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0050] In some embodiments of the present application, in the infrared spectrum of the polymer, at 470 cm -1 Up to 475cm -1 、507cm -1 Up to 515cm -1 、830cm -1 Up to 850cm -1 、1275cm -1 to 1290cm -1 There is a fifth characteristic peak within the range. The infrared spectrum of the polymer meets the above characteristics. The groups in the polymer are neatly arranged, and the polymer contains a β phase. The dielectric constant of the polymer mainly comes from the β phase. The high dielectric constant can promote the dissociation of lithium salts. At the same time, the groups in the polymer are neatly arranged, which can provide ion transmission channels, shorten the ion transmission path, and improve the kinetic performance of the secondary battery. In addition, the energy barrier of the polymer molecular chain movement is low, and the polymer molecular chain has strong mobility, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0051] In some embodiments of the present application, the melting point of the polymer is Tm°C, 120≤Tm≤160. For example, the value of Tm can be 120, 123, 125, 127, 130, 133, 135, 137, 140, 143, 145, 147, 150, 153, 155, 157, 160, or a range consisting of any two of the above values. By regulating the melting point of the polymer within the above range, the processing performance of the positive electrode slurry can be optimized, the stability of the electrode structure can be enhanced, and the cycle life of the secondary battery can be extended.

[0052] In some embodiments of the present application, the polymer includes silicon, iron and zinc, and the sum of the mass of silicon, iron and zinc in the polymer accounts for W, W < 200ppm. Exemplarily, the value of W can be less than 200ppm, 180ppm, 160ppm, 140ppm, 120ppm, 100ppm, 80ppm, 60ppm, 40ppm, 20ppm. Silicon, iron and zinc are impurity elements in the polymer. By regulating the sum of the mass of silicon, iron and zinc in the polymer to be within the above range, the content of impurity elements in the polymer is small, which can reduce the possibility of increased self-discharge of the secondary battery or precipitation of impurity elements on the surface of the negative electrode sheet causing a short circuit in the secondary battery, thereby improving the safety performance of the secondary battery.

[0053] In some embodiments of the present application, the compaction density of the positive electrode material layer is PDg / cm 3, 3.0≤PD≤4.5, preferably, 3.8≤PD≤4.4. Exemplarily, the value of PD can be 3.0, 3.2, 3.4, 3.6, 3.8, 4.0, 4.2, 4.4, 4.5 or a range consisting of any two of the above values. By regulating the compaction density of the positive electrode material layer within the above range, the positive electrode material layer has a higher compaction density, which can enable the secondary battery to have a higher volume energy density; at the same time, the positive electrode sheet also has good flexibility, which can better reduce the risk of brittle fracture of the positive electrode sheet and delamination of the positive electrode material layer, so that the secondary battery has better cycle performance and safety performance.

[0054] In some embodiments of the present application, the positive electrode material layer further includes a positive electrode active material, and the Dv50 of the positive electrode active material satisfies the following conditions: 0.5 μm ≤ Dv50 ≤ 35 μm, preferably, 5 μm ≤ Dv50 ≤ 30 μm, and more preferably, 10 μm ≤ Dv50 ≤ 25 μm. For example, the value of Dv50 can be 0.5, 1, 5, 9, 10, 11, 15, 19, 20, 21, 25, 29, 30, 31, 35, or a range consisting of any two of the above values. By regulating the Dv50 of the positive electrode active material within the above range, the particle size of the positive electrode active material is moderate, the stress of the positive electrode material layer is evenly distributed, the possibility of cracks when the positive electrode plate is bent is reduced, and the flexibility of the positive electrode plate is improved; at the same time, the density of the positive electrode plate is moderate, the deformation ability of the positive electrode plate is strong, the flexibility of the positive electrode plate is further improved, the initial compaction density of the positive electrode plate is further increased, and the volume energy density of the secondary battery is further increased.

[0055] In the present application, Dv50 means the particle size at which the volume accumulation reaches 50% from the smallest particle size in the volume-based particle size distribution of the material.

[0056] In some embodiments of the present application, the thickness of the positive electrode current collector is H1 μm, 7 ≤ H1 ≤ 20, preferably, 8 ≤ H1 ≤ 12. For example, the value of H1 can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two of the above values. By regulating the thickness of the positive electrode current collector within the above range, the thickness of the positive electrode current collector is small, which can enable the secondary battery to have a higher volume energy density.

[0057] In some embodiments of the present application, the positive electrode active material includes at least one of lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide or lithium iron phosphate. The above-mentioned lithium nickel cobalt manganese oxide may include LiNi 0.95 Co 0.03 Mn 0.02 O2(Ni95),LiNi 0.91 Co 0.03 Mn0.06 O2(Ni91), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 The above-mentioned positive electrode active material is selected and applied with the polymer in the positive electrode material layer, which can make the secondary battery have a higher volume energy density.

[0058] In some embodiments of the present application, the positive electrode material layer also includes a positive electrode conductive agent. Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode active material is w1%, the mass percentage of the positive electrode binder is w2%, and the mass percentage of the positive electrode conductive agent is w3%, 97.6≤w1≤98.8, 0.7≤w2≤1.3, 0.5≤w3≤1.1. Illustratively, the value of w1 can be 97.6, 97.7, 97.8, 97.9, 98.0, 98.1, 98.2, 98.3, 98.4, 98.5, 98.6, 98.7, 98.8, or a range consisting of any two of the above values; the value of w2 can be 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, or a range consisting of any two of the above values; the value of w3 can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, or a range consisting of any two of the above values. By regulating the mass percentage of the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent within the above range, the positive electrode active material, the positive electrode binder, and the positive electrode conductive agent all have appropriate mass percentages, the energy barrier for the movement of the polymer molecular chain is low, and the movement ability of the polymer molecular chain is strong, which can make the positive electrode sheet have better flexibility; and the mass percentage of the positive electrode active material is relatively high, and the secondary battery has a higher volume energy density.

[0059] The present application does not particularly limit the positive electrode conductive agent, as long as it can achieve the purpose of the present application. For example, the positive electrode conductive agent may include, but is not limited to, at least one of Super P, carbon nanofibers, flake graphite, acetylene black, Ketjen black, carbon dots, graphene, or carbon nanotubes. The above-mentioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0060] In some embodiments of the present application, the thickness of the positive electrode material layer is H2μm, 40.5≤H2≤70. For example, the value of H2 can be 40.5, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 57, 59, 60, 65, 67, 69, 70 or a range consisting of any two of the above values. By regulating the thickness of the positive electrode material layer within the above range, the positive electrode material layer has a suitable thickness, the positive electrode material layer has a higher compaction density, and the secondary battery can have a higher volume energy density; at the same time, the positive electrode sheet also has good flexibility, which can better reduce the risk of brittle fracture of the positive electrode sheet and delamination of the positive electrode material layer, so that the secondary battery has better cycle performance and safety performance.

[0061] In some embodiments of the present application, the bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, 13≤F≤35. For example, the value of F can be 13, 15, 17, 19, 20, 23, 25, 27, 29, 30, 33, 35, or a range consisting of any two of the above values. By regulating the bonding force between the positive electrode material layer and the positive electrode current collector within the above range, the bonding force between the positive electrode material layer and the positive electrode current collector is greater, the positive electrode material layer and the positive electrode current collector are in closer contact, the risk of the positive electrode material layer delaminating is smaller, and the secondary battery has better cycle performance and safety performance.

[0062] In some embodiments of the present application, along the thickness direction of the positive electrode sheet, the positive electrode material layer includes a first surface and a second surface facing each other; from the first surface to the second surface, the positive electrode material layer includes a first region, a second region, and a third region, with the third region located on the surface of the positive electrode current collector; wherein the first region is the upper region, the second region is the middle region, and the third region is the lower region. The thickness of the first region accounts for 1 / 3 of the thickness of the positive electrode material layer, the thickness of the second region accounts for 1 / 3 of the thickness of the positive electrode material layer, and the thickness of the third region accounts for 1 / 3 of the thickness of the positive electrode material layer. The weight loss rate of the positive electrode material layer particles in the first region from 0°C to 600°C is TG1, the weight loss rate of the positive electrode material layer particles in the second region from 0°C to 600°C is TG2, and the weight loss rate of the positive electrode material layer particles in the third region from 0°C to 600°C is TG3, with 0.85≤TG1 / TG2≤1.15 and 0.85≤TG1 / TG3≤1.15. Illustratively, the value of TG1 / TG2 can be 0.85, 0.87, 0.89, 0.90, 0.93, 0.95, 0.97, 0.99, 1.00, 1.03, 1.05, 1.07, 1.09, 1.10, 1.13, 1.15, or a range consisting of any two of the above values; the value of TG1 / TG3 can be 0.85, 0.87, 0.89, 0.90, 0.93, 0.95, 0.97, 0.99, 1.00, 1.03, 1.05, 1.07, 1.09, 1.10, 1.13, 1.15, or a range consisting of any two of the above values. The values ​​of TG1 / TG2 and TG1 / TG3 are within the above range, indicating that the upper, middle and lower regions of the positive electrode material layer are all distributed with positive electrode binders of similar contents. The positive electrode binders include the above polymers, and the energy barrier for the movement of the polymer molecular chains is low. The polymer molecular chains have strong mobility, which can further improve the flexibility of the positive electrode sheet, further increase the initial compaction density of the positive electrode sheet, and further increase the volume energy density of the secondary battery.

[0063] In some embodiments of the present application, the area on the surface of the positive electrode current collector where the positive electrode material layer is disposed is the fourth region. The ratio (B1 / B2) of the specific surface area of ​​the positive electrode active material on any two 250 mm x 250 mm positive electrode material layers in the fourth region is between 0.97 and 1.03. For example, the value of B1 / B2 can be 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, or a range consisting of any two of the above values. The value of B1 / B2 can reflect the breakage of the positive electrode active material particles in different regions of the positive electrode sheet. Within the above range, the specific surface area of ​​the positive electrode active material particles in any two regions is similar, the positive electrode sheet has good consistency, and can improve electrolyte consumption and other issues caused by particle breakage, thereby increasing the cycle life of the secondary battery. It also reduces the local accumulation of large and small particles, reduces the risk of brittle fracture of the positive electrode sheet due to stress concentration, and improves the flexibility of the positive electrode sheet.

[0064] In some embodiments of the present application, the area on the surface of the positive electrode current collector where the positive electrode material layer is provided is the fourth area, and the roughness ratio Ra1 / Ra2 of the positive electrode current collector of any two fourth areas with an area of ​​250 mm × 250 mm is 0.93 to 1.07. For example, the value of Ra1 / Ra2 can be 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07 or a range consisting of any two of the above values. When the value of Ra1 / Ra2 is within the above range, the surface state of the positive electrode current collector is more consistent, which can reduce the risk of fracture of the positive electrode sheet and improve the flexibility of the positive electrode sheet.

[0065] In some embodiments of the present application, the area on the surface of the positive electrode collector on which the positive electrode material layer is provided is the fourth area, and the ratio B1 / B2 of the specific surface area of ​​the positive electrode active material on any two positive electrode material layers with an area of ​​250 mm×250 mm in the fourth area is 0.97 to 1.03; and the ratio Ra1 / Ra2 of the roughness of any two positive electrode collectors in the fourth area with an area of ​​250 mm×250 mm is 0.93 to 1.07. Illustratively, the value of B1 / B2 can be 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, or a range consisting of any two of the above values; the value of Ra1 / Ra2 can be 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, or a range consisting of any two of the above values. When the values ​​of B1 / B2 and Ra1 / Ra2 are within the above ranges, the specific surface areas of the positive electrode active material particles in any two regions are similar, and the positive electrode sheets have good consistency, which can improve problems such as electrolyte consumption caused by particle breakage and increase the cycle life of the secondary battery; at the same time, it reduces the risk of local accumulation of large / small particles and stress concentration leading to brittle fracture of the positive electrode sheets; and the surface state of the positive electrode collector has good consistency, which can reduce the risk of positive electrode sheet fracture and further improve the flexibility of the positive electrode sheet.

[0066] In some embodiments of the present application, the contact angle between the positive electrode plate and the electrolyte is θ, 10°≤θ≤30°. For example, θ can be 10°, 11°, 13°, 15°, 17°, 19°, 20°, 21°, 23°, 25°, 27°, 29°, 30° or a range consisting of any two of the above values. By regulating the contact angle between the positive electrode plate and the electrolyte within the above range, the contact angle between the positive electrode plate and the electrolyte is small, the positive electrode plate has good wettability, the electrolyte can better infiltrate the polymer, the polymer has high ionic conductivity, can promote lithium ion transmission, reduce the impedance of the positive electrode plate, improve the charge and discharge capacity of the secondary battery under large current, and improve the kinetic performance of the secondary battery.

[0067] The present application does not particularly limit the method for regulating the molar fraction of the fluorinated structural unit, as long as the purpose of the present application can be achieved. For example, when the fluorinated structural unit includes -CH=CF-, the molar fraction of the fluorinated structural unit can be regulated by regulating the ratio of VDF and CTFE in P(VDF-CTFE); when the fluorinated structural unit includes -CFH-CF2-, the molar fraction of the fluorinated structural unit can be regulated by regulating the ratio of VDF and the small molecule monomer trifluoroethylene during copolymerization; when the fluorinated structural unit includes -CFCl-CF2-, the molar fraction of the fluorinated structural unit can be regulated by regulating the ratio of VDF and the small molecule monomer chlorotrifluoroethylene during copolymerization.

[0068] The present application does not particularly limit the method for regulating the weight average molecular weight and average molecular weight of the polymer, as long as the purpose of the present application can be achieved. For example, commercially available polymers with different weight average molecular weights can be selected, and the weight average molecular weight of the polymer can be tested in combination with the test method of "Molecular Weight and Molecular Weight Distribution Test" in this application, and a polymer with a desired weight average molecular weight can be selected. For example, commercially available polymers with different average molecular weights can be selected, and the average molecular weight of the polymer can be tested in combination with the test method of "Molecular Weight and Molecular Weight Distribution Test" in this application, and a polymer with a desired average molecular weight can be selected.

[0069] The present application does not particularly limit the method for regulating Mz / Mw, as long as the purpose of the present application can be achieved. For example, the Mz / Mw value can be regulated by regulating the respective values ​​of Mz and Mw, and the regulation method is as described above.

[0070] The present application does not particularly limit the method for regulating the crystallinity of the polymer, as long as the objectives of the present application can be achieved. For example, the crystallinity of the polymer can be regulated by controlling the cooling rate, reaction temperature, and reaction time during the polymer preparation process. For example, the preparation method and molecular structure of the polymer can also affect the crystallinity of the polymer.

[0071] The present application does not particularly limit the method for regulating the melting point of the polymer, as long as the purpose of the present application can be achieved. For example, the average molecular weight, weight average molecular weight, molecular chain structure, and crystal distribution of the polymer will affect the melting point of the polymer.

[0072] The present application does not particularly limit the method for regulating the compaction density of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the compaction density of the positive electrode material layer can be regulated by regulating the cold pressing pressure during the cold pressing process.

[0073] This application does not particularly limit the method for regulating the Dv50 of the positive electrode active material, as long as the objectives of this application can be achieved. For example, the Dv50 of the positive electrode active material can be regulated by grinding the positive electrode active material. For example, the Dv50 of the positive electrode active material can be regulated by controlling the grinding time. For example, when other conditions remain unchanged, extending the grinding time decreases the Dv50 of the positive electrode active material; shortening the grinding time increases the Dv50 of the positive electrode active material.

[0074] The present application does not particularly limit the method for adjusting the thickness of the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, commercially available positive electrode current collectors of different thicknesses can be selected.

[0075] The present application does not particularly limit the method for regulating the mass percentage of the positive electrode active material, positive electrode binder, and positive electrode conductive agent, as long as the purpose of the present application can be achieved. For example, the mass percentage of the positive electrode active material can be regulated by regulating the mass of the added positive electrode active material; the mass percentage of the positive electrode binder can be regulated by regulating the mass of the added positive electrode binder; and the mass percentage of the positive electrode conductive agent can be regulated by regulating the mass of the added positive electrode conductive agent.

[0076] The present application does not particularly limit the method for regulating the thickness of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode material layer can be regulated by regulating the compaction density of the positive electrode material layer.

[0077] The present application does not particularly limit the method for regulating the bonding force between the positive electrode material layer and the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the bonding force between the positive electrode material layer and the positive electrode current collector can be regulated by regulating the type and / or mass percentage of the positive electrode binder.

[0078] The present application does not particularly limit the method for regulating the contact angle between the positive electrode sheet and the electrolyte, as long as the purpose of the present application can be achieved. For example, the contact angle between the positive electrode sheet and the electrolyte can be regulated by regulating the type and / or mass percentage of the positive electrode binder in the positive electrode material layer.

[0079] The present application has no particular limitation on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, it may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector).

[0080] In the present application, the secondary battery also includes a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer provided 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 itself, or on two surfaces of the negative electrode current collector along the thickness direction of itself. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a partial area 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, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector, etc.

[0081] The negative electrode material layer of the present application includes a negative electrode active material. The present application has no particular limitation on the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5<x<1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium. The negative electrode material layer of the present application also includes a negative electrode binder and a negative electrode conductive agent. The present application has no particular restrictions on the negative electrode binder and the negative electrode conductive agent in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to polyvinylidene fluoride (PVDF), a copolymer of vinylidene fluoride and hexafluoropropylene, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyamide, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber (SBR), polypropylene, polyethylene, polyetherimide, a copolymer of propylene derivatives or at least one of carboxymethyl cellulose salts. The above-mentioned carboxymethyl cellulose salts may include but are not limited to at least one of sodium carboxymethyl cellulose, potassium carboxymethyl cellulose or lithium carboxymethyl cellulose. The negative electrode conductive agent may be at least one of the above-mentioned positive electrode conductive agents. The present application has no particular restrictions on the mass ratio of the negative electrode active material, the negative electrode binder and the negative electrode conductive agent in the negative 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.

[0082] The present application does not particularly limit the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm. The present application does not particularly limit the thickness of the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 30 μm to 250 μm.

[0083] In the present application, the secondary battery also includes an electrolyte. The electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may include various lithium salts commonly used in the art, such as 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 non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvent. 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 fluorocarbonate compound. The above-mentioned chain carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or ethyl methyl carbonate (EMC). The cyclic carbonate compound may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include but is not limited to at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate. The carboxylate compound may include but is 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, decanolactone, valerolactone or caprolactone. The above-mentioned ether compound may include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents 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 weight percentage of the lithium salt and the non-aqueous solvent, as long as the purpose of the present application can be achieved.

[0084] In the present application, the secondary battery also includes a diaphragm. The diaphragm is used to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no special 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 (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or 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.

[0085] In the present application, the diaphragm may include a base film and a surface treatment layer. The base film may be a non-woven fabric or a composite film having a porous structure, and the material of the base film may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the base film, 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. For example, the inorganic layer includes inorganic particles and a binder for the diaphragm. The present application does not particularly limit the above-mentioned inorganic particles, and for example, it may 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 does not particularly limit the separator binder, and for example, it can be at least one of the aforementioned negative electrode binders. The polymer layer comprises a first polymer, and the material of the first polymer comprises at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0086] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components of the secondary battery known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0087] The present application does not particularly limit the type of secondary battery, which may include any device that generates an electrochemical reaction. In the present application, secondary batteries may include but are not limited to: lithium metal secondary batteries, lithium ion secondary batteries (lithium ion batteries), lithium polymer secondary batteries or lithium ion polymer secondary batteries, etc.

[0088] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, guide plates, etc. may also be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging. Among them, the packaging bag is a packaging bag known in the art, and the present application does not limit this.

[0089] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the aforementioned embodiments. Therefore, the electronic device provided by the present application has good performance.

[0090] The present application does not particularly limit the type of electronic device, and the electronic device may be any electronic device known in the art. In some embodiments, the electronic device may 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 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, an automobile, 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.

[0091] Example

[0092] 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.

[0093] Test methods and equipment:

[0094] Positive electrode binder sampling method:

[0095] Disassemble the lithium-ion battery and take out the positive electrode sheet. Cut the positive electrode sheet into 1cm×1cm samples, and use a blade to scrape off the positive electrode material layer on the positive electrode current collector aluminum foil to separate the positive electrode material layer from the positive electrode current collector. Place the positive electrode material layer powder in a vacuum dryer at 60°C for 12 hours to remove the electrolyte and moisture. Dissolve the dried positive electrode material layer powder in N-methylpyrrolidone (NMP), where the mass ratio of the positive electrode material layer powder to NMP is 1:20; centrifuge at 5000rpm for 10 minutes to separate the supernatant (containing positive electrode binder) and the precipitate (containing positive electrode active material, positive electrode conductive agent). After evaporating the solvent from the supernatant at 50°C, vacuum dry at 60°C for 6 hours to obtain the positive electrode binder powder. The following 1 The positive electrode binder was sampled using the above method in H nuclear magnetic resonance spectroscopy test, molecular weight and molecular weight distribution test, crystallinity test, X-ray diffraction test, infrared spectroscopy test, melting point test, and element mass ratio test.

[0096] 1 H NMR spectroscopy test:

[0097] The test was performed using a nuclear magnetic resonance spectrometer (model BRUKER AVANCE 400). Take 20 mg of positive electrode binder powder, dissolve it in 0.5 mL of deuterated solvent CDCl3 to obtain a sample solution, use a 5 mm outer diameter NMR sample tube, load 0.7 mL of sample solution, the liquid column height is about 4 cm, and place the NMR sample tube vertically into the probe in the magnet cavity to ensure consistent depth. Set parameters: spectral width (SW): 15 ppm; number of sampling points (TD): 64k; relaxation delay (D1): 3 seconds; number of scans (NS): 64 times. Data processing: Collect free induction decay (FID) signals, perform Fourier transform on the FID signals, generate frequency domain spectra, and perform phase correction and baseline correction. Compare the nuclear magnetic resonance according to the position of the characteristic peaks. 1 The H chemical shift determines the corresponding structural unit; the ratio of the number of fluorine atoms to the number of atoms other than carbon atoms is calculated from the structural unit structure; the characteristic peaks are integrated to calculate the molar fractions of different structural units.

[0098] Molecular weight and molecular weight distribution test:

[0099] In accordance with the national standard "Gel Permeation Chromatography (GPC) Using Tetrahydrofuran as Eluent" (GB / T 21863-2008), an ultra-high performance polymer chromatograph (ACQUITY APC) and an ACQUITY differential refractive index detector (DID) were used. The test steps are as follows: (1) Preheating: Install the chromatographic column and tubing, turn on the console, test power, and then open the Empower test software. (2) Parameter setting: injection volume: 10 μL to 50 μL (depending on sample concentration); pump flow rate: 0.2 mL / min; mobile phase: 30 mol / L LiBr in N-methylpyrrolidone (NMP); sealing cleaning liquid: isopropanol; pre-column: PL gel 10 μm MiniMIX-B Guard (size: 50 mm × 4.6 mm × 2); analytical phase: PL gel 10 μm MiniMIX-B (size: 250 mm × 4.6 mm); standard: polystyrene; run time: 30 min; detector: ACQUITY differential refractive index (RI) detector; column oven temperature: 90 °C; detector temperature: 55 °C. (3) Sample testing: a. Preparation of standard sample solution and test sample solution: Weigh 0.005g of standard sample narrow distribution polystyrene (PS) and test sample positive electrode binder powder respectively and add 2mL of mobile phase liquid to prepare 2.5mg / mL standard sample solution and test sample solution, and place them in the refrigerator for more than 8h; b. Standard / test sample solution test: Edit the sample group to be tested, select the established sample group method, wait for the baseline to stabilize, click the run queue, and start testing the sample. (4) Data processing: Based on the relationship between retention time and molecular weight, use the chemical workstation to establish a calibration curve, integrate and quantify the sample spectrum, and the chemical workstation automatically generates molecular weight (including weight average molecular weight and average molecular weight) and molecular weight distribution results.

[0100] Crystallinity test:

[0101] Differential scanning calorimetry (DSC) was used to determine the crystallinity of the positive electrode binder. First, 10 mg of the positive electrode binder powder was sealed in an aluminum crucible and melted and quenched at 200°C to eliminate the thermal history. Then, the temperature was raised to 200°C at 10°C / min under a nitrogen atmosphere. The melting peak was recorded and the melting enthalpy (ΔH) was calculated by integration. f ), crystallinity of positive electrode binder = ΔH f / ΔH 0 f , ΔH 0 f The melting enthalpy for 100% crystallinity is 100%. After testing, the initial and secondary scan data must be compared to ensure repeatability is less than 5%. Calibration with a Sn standard ensures instrument accuracy. The instrument used for this test is a differential scanning calorimeter (NETZSCH, DSC214 Polyma).

[0102] X-ray diffraction (XRD) test:

[0103] The cathode binder was tested using an X-ray powder diffractometer (Bruker D8 ADVANCE) with a Cu Kα target. The voltage was 40 kV, the current was 40 mA, the scanning angle range was 5° to 80°, the scanning step size was 0.00836°, and the time per step was 0.3 s. Silicon powder (15% by weight of the cathode binder) was added. Using the internal standard method, the peak position of the silicon standard was used to calibrate the instrument and test errors. The characteristic peak positions of the cathode binder were then calculated to obtain the X-ray diffraction spectrum of the cathode binder.

[0104] Infrared spectrum test:

[0105] The positive electrode binder powder was tested by Fourier transform infrared spectroscopy using a Thermo Fisher Nicolet iS20 FTIR spectrometer. The reference standard was the national standard "Fourier transform infrared spectrometer" (GB / T 21186-2007). The infrared spectrum of the positive electrode binder was obtained by the potassium bromide tablet method.

[0106] Melting point test:

[0107] The melting point of the positive electrode binder was tested using differential scanning calorimetry (DSC). An aluminum crucible was used, the positive electrode binder powder weighed 10 mg, and the test temperature range was from room temperature to 400°C at a heating rate of 10°C / min. The instrument used for the test was a differential scanning calorimeter (Netzsch, model DSC214 Polyma).

[0108] Element mass ratio test:

[0109] The element types and mass proportions of the different elements in the positive electrode binder were determined using an inductively coupled plasma emission spectrometer (ICP, PE7000DV spectrometer manufactured by PerkinElmer, USA). 0.2 g of the positive electrode binder powder was weighed and dissolved in a 42% nitric acid solution. The nitric acid solution in which the positive electrode binder was dissolved was then tested to determine the element types and mass proportions of the different elements in the positive electrode binder, and the sum of the mass proportions of the corresponding elements was calculated.

[0110] Compaction density test:

[0111] Disassemble the lithium-ion battery, remove the positive electrode sheet, clean the positive electrode sheet with dimethyl carbonate (DMC), and dry the positive electrode sheet. Select the area on the positive electrode sheet including the double-sided positive electrode material layer and cut it into 10 pieces with an area of ​​1540.25mm using a cutting machine. 2Weigh the small discs and take the average value m1. Then use a caliper to measure the thickness of the discs and take the average value H3. Then wipe off the positive electrode material layer from the ten discs, weigh them, and take the average value m2. Then use a caliper to measure the thickness of the positive electrode current collector and take the average value H1. The compacted density of the positive electrode material layer, PD, = (m1 - m2) / [1540.25 × (H3 - H1)].

[0112] Dv50 test:

[0113] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V and then disassembled. The positive electrode sheet was removed, cleaned with DMC, and air-dried. The dried positive electrode sheet was randomly cut from the area on the surface of the positive electrode sheet where the positive electrode material layer was provided. The sheet was placed in a muffle furnace and calcined at 600°C for 2 hours. The powder on the surface of the positive electrode sheet was scraped off with a knife to obtain positive electrode active material particles. The particle size of the positive electrode active material particles was measured using a Malvern particle size analyzer (model: MasterSizer2000). 0.02g of positive electrode active material particles was added to a 50mL clean beaker, followed by 20mL of dispersant ethanol. The sample dispersion was completely dispersed in the ethanol using a 120W ultrasonic cleaner. The sample dispersion was then tested using a Malvern particle size analyzer to obtain the particle size Dv50 of the positive electrode active material particles.

[0114] Adhesion test:

[0115] (1) Disassemble the lithium-ion battery, remove the positive electrode, clean the positive electrode with DMC, and then dry the positive electrode;

[0116] (2) Take the dried positive electrode sheet and cut a sample with a width of 30 mm and a length of 100 mm using a blade;

[0117] (3) Apply special double-sided tape to the steel plate with a width of 20 mm and a length of 90 mm;

[0118] (4) Stick the sample cut in step (2) on the double-sided tape with the test surface facing downward;

[0119] (5) Insert a paper tape with a width equal to the width of the specimen and a length of 180 mm under the specimen and secure it with wrinkle glue;

[0120] (6) Turn on the power of the Sansi tensile testing machine, the indicator light will light up, and adjust the position of the limit block;

[0121] (7) Fix the sample prepared in step (5) on the test bench, set the speed to 10 mm / min, the test range to 0 mm to 40 mm, and pull the paper tape in a 90° direction until the test is completed to obtain the bonding force between the positive electrode material layer and the positive electrode current collector.

[0122] Thermogravimetric testing:

[0123] Discharge the lithium-ion battery at a constant current of 0.2C to 3.0V, then disassemble the lithium-ion battery, remove the positive electrode sheet, clean the positive electrode sheet with DMC, and dry the positive electrode sheet. Take the dried positive electrode sheet and use a knife to evenly scrape the powder from the upper, middle, and lower regions of the positive electrode sheet. The thickness of the upper region accounts for 1 / 3 of the thickness of the positive electrode material layer, the thickness of the middle region accounts for 1 / 3 of the thickness of the positive electrode material layer, and the thickness of the lower region accounts for 1 / 3 of the thickness of the positive electrode material layer. Using a thermogravimetric analyzer (model: Netzsch TG209F1 Libra), the powders in the upper region, middle region, and lower region were respectively heated to 600°C at a rate of 10°C / min in a nitrogen atmosphere, and the weight loss curves of the powders in the upper region, middle region, and lower region were tested to obtain the weight loss rates of the powders in the upper region, middle region, and lower region, that is, the weight loss rate TG1 of the positive electrode material layer particles in the first region, the weight loss rate TG2 of the positive electrode material layer particles in the second region, and the weight loss rate TG3 of the positive electrode material layer particles in the third region.

[0124] Specific surface area test:

[0125] The lithium-ion battery was discharged at a constant current of 0.2C to 3.0V, and then the lithium-ion battery was disassembled, the positive electrode sheet was removed, the positive electrode sheet was cleaned with DMC, and the positive electrode sheet was dried. The dried positive electrode sheet was taken, and two areas of 250mm×250mm were randomly cut out from the area where the positive electrode material layer was provided on the surface of the positive electrode sheet. One of the positive electrode sheets was designated as the first positive electrode sheet, and the other was designated as the second positive electrode sheet. The first positive electrode sheet and the second positive electrode sheet were then placed in a muffle furnace and calcined at 600°C for 2h. The powder on the surface of the first positive electrode sheet and the second positive electrode sheet was scraped off with a knife to obtain the first positive electrode active material and the second positive electrode active material, respectively. In accordance with the national standard "Determination of the specific surface area of ​​solid substances by gas adsorption BET method" (GB / T 19587-2017), a specific surface area analyzer (model: TristarⅡ3020M) was used to measure the specific surface area B1 of the first positive electrode active material and the specific surface area B2 of the second positive electrode active material by gas adsorption method, and the value of B1 / B2 was calculated based on the test results.

[0126] Roughness test:

[0127] (1) The lithium-ion battery is discharged at a constant current of 0.2C to 3.0V, and then the discharged lithium-ion battery to be tested is disassembled, and the positive electrode sheet is removed. The positive electrode sheet is cleaned with DMC and then dried. Take the dried positive electrode sheet and randomly cut two areas of 250mm×250mm in the area where the positive electrode material layer is provided on the surface of the positive electrode sheet to obtain a first positive electrode sheet and a second positive electrode sheet. The first positive electrode sheet and the second positive electrode sheet were immersed in dimethyl carbonate (DMO) for 30 minutes to remove the electrolyte and byproducts on the surface of the first positive electrode sheet and the second positive electrode sheet, and then dried at 25°C in a fume hood for 4 hours. The dried first positive electrode sheet and the second positive electrode sheet were taken out and immersed in N-methylpyrrolidone (NMP) for 30 minutes; then the first positive electrode sheet and the second positive electrode sheet were taken out and laid flat on a glass plate, the positive electrode material layer was wiped off with dust-free paper (Clearoom wipe-0609), and NMP was used to rinse until no black block active material was visible to the naked eye. Then, they were naturally dried to obtain a first positive electrode current collector and a second positive electrode current collector.

[0128] (2) Cut a 50 mm × 50 mm first positive electrode current collector as the sample to be tested, use a high-power microscope (model VK-S100), lay the sample to be tested flat under the microscope lens, select a 10X magnification lens, and adjust the focal length until the interface is clear; use 3D scanning, open the microscope operation interface, adjust the upper and lower limits of the 3D scan, and then click the "3D Scan" button to perform 3D scanning; after the 3D scan is completed, the roughness is automatically calculated, which is the roughness Ra1 of the first positive electrode current collector.

[0129] A second positive electrode current collector of 50 mm×50 mm was cut as a sample to be tested. The roughness Ra2 of the second positive electrode current collector was tested using the above method, and the value of Ra1 / Ra2 was calculated based on the test results.

[0130] Contact angle test between positive electrode and electrolyte:

[0131] The contact angle between the positive electrode and the electrolyte is measured using the sessile drop method. First, the positive electrode is laid flat on a substrate (keeping it flat), then 0.02 mL of electrolyte is dropped onto the positive electrode sample. The image of the drop is captured by a high-resolution camera, and the angle is automatically measured by software using an optical contact angle meter. The test result is the contact angle between the positive electrode and the electrolyte.

[0132] Positive electrode brittle fracture test:

[0133] Under the conditions of 25°C and 40% RH (relative humidity), the lithium-ion battery was disassembled to obtain the positive electrode sheet. The positive electrode sheet was dried in a fume hood for 4 hours, and the dried positive electrode sheet was taken out. The positive electrode sheet was then cut into 4cm×25cm samples, pre-folded in half along the longitudinal direction of the sample, and the pre-folded sample was placed on the flat surface of the laboratory table. A 2kg cylinder was used to roll the pre-folded sample twice, and the sample was folded back along the longitudinal crease. The positive electrode sheet was spread out and observed against the light. Among them, if the positive electrode sheet is broken after folding, or the translucent part is connected into a line, it is defined as severe; if the positive electrode sheet is point-like translucent after folding, it is defined as mild; if the positive electrode sheet is not translucent or broken after folding, it is defined as none.

[0134] Volume energy density test:

[0135] At (25±3)℃, the lithium-ion battery is first charged at a constant current rate of 0.5C to a voltage of 4.52V, then charged at a constant voltage rate to a cutoff current of 0.025C, left for 30 minutes, and discharged at a current rate of 0.2C to a voltage of 3.0V, left for 5 minutes. The energy of the above discharge process is recorded as the discharge energy E. Calculate the volume V (mm) of the lithium-ion battery. 3 ) = length × width × height.

[0136] Volumetric energy density (Wh / L) = E / V × 10 6 .

[0137] Example 1-1

[0138] <Preparation of positive electrode sheet>

[0139] The positive electrode active material lithium cobalt oxide (LiCoO2, LCO), the positive electrode conductive agent Super P, the positive electrode conductive agent multi-walled carbon nanotubes, and the positive electrode binder polymer polyvinylidene fluoride-double bond copolymer (P(VDF-DB)) were mixed in a weight ratio of 97.6:0.6:0.5:1.3, and N-methylpyrrolidone (NMP) was added as a solvent. The mixture was stirred and mixed to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry was 73wt%; the Dv50 of the positive electrode active material was 15.2μm. The positive electrode slurry was evenly coated on one surface of the positive electrode current collector aluminum foil and dried at 120℃ for 1h to obtain a positive electrode sheet with a single-sided positive electrode material layer; wherein the coating weight of the positive electrode material layer was 0.02g / cm 2 The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. After drying under vacuum conditions at 120°C for 1 hour, cold pressing, cutting, and slitting, a positive electrode sheet with a specification of 74mm×867mm is obtained. Among them, the thickness H1 of the positive electrode current collector is 10μm; the compaction density PD of the positive electrode material layer is 4.4g / cm3 ; The thickness H2 of the single-sided positive electrode material layer is 45.45μm.

[0140] <Preparation of negative electrode sheet>

[0141] The negative electrode active material, artificial graphite, the negative electrode binder, sodium carboxymethyl cellulose (CMC-Na), and the negative electrode binder, styrene-butadiene rubber (SBR), were mixed in a weight ratio of 95:2:3. Deionized water was added as a solvent and the mixture was stirred to obtain a negative electrode slurry with a solid content of 75 wt%. The negative electrode slurry was evenly coated on one surface of a 12 μm thick negative electrode current collector copper foil and dried at 120°C to obtain a negative electrode sheet coated with a 120 μm thick negative electrode material layer on one side. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode material layer on both sides. The sheet was dried under vacuum at 120°C for 1 hour, and then cold pressed, cut, and slit to obtain a negative electrode sheet with a size of 78 mm × 875 mm. The compaction density during the cold pressing process was 1.75 g / cm 3 .

[0142] <Preparation of Electrolyte>

[0143] In an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 1:1:1 to create a base solvent. Lithium hexafluorophosphate (LiPF6) was then added and mixed thoroughly to create an electrolyte. The weight percentage of the lithium salt, LiPF6, was 12.5% ​​based on the mass of the electrolyte, with the remainder being the base solvent.

[0144] <Preparation of Separator>

[0145] A polyethylene (PE) film with a thickness of 15 μm is used.

[0146] <Preparation of lithium-ion batteries>

[0147] The prepared positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide isolation. The electrodes are then wound to form an electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then filled with electrolyte. The lithium-ion battery is then vacuum packaged, allowed to stand, formed, degassed, and trimmed.

[0148] Example 1-2 to Example 1-3

[0149] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0150] Example 1-4 to Example 1-9

[0151] The process was the same as Example 1-1 except that the ratio of VDF to CTFE in P(VDF-CTFE) was adjusted so that the molar fraction of the fluorine-containing structural unit was as shown in Table 1.

[0152] Example 1-10 to Example 1-17

[0153] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0154] Example 1-18 to Example 1-21

[0155] The process was the same as Example 1-1 except that the cooling rate and reaction temperature during the polymer preparation were adjusted so that the crystallinity of the polymer was as shown in Table 1.

[0156] Examples 1-22

[0157] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0158] Example 2-1 to Example 2-4

[0159] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0160] Example 2-5 to Example 2-12

[0161] Except that the milling time was adjusted so that the Dv50 of the positive electrode active material was as shown in Table 2, the rest was the same as Example 1-1.

[0162] Example 2-13 to Example 2-17

[0163] Except that the cold pressing pressure in the cold pressing process was adjusted so that the compaction density of the positive electrode material layer was as shown in Table 2, the rest was the same as Example 1-1.

[0164] Example 2-18 to Example 2-23

[0165] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0166] Comparative Example 1

[0167] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0168] Comparative Examples 2 to 4

[0169] Except for adjusting the ratio of VDF and CTFE in P(VDF-CTFE) so that the molar fraction of the fluorine-containing structural unit is as shown in Table 1 and adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0170] The preparation parameters and electrical performance parameters of each embodiment and comparative example are shown in Tables 1 and 2.

[0171]

[0172]

[0173] From Examples 1-1 to 1-22 and Comparative Examples 1 to 4, it can be seen that when the polymer structure meets the above-mentioned characteristics and the type and mole fraction of the fluorine-containing structural unit are within the range of this application, the brittle fracture of the positive electrode sheet is relatively mild, and the prepared lithium-ion battery has a high volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a high volumetric energy density. In Comparative Examples 1 to 4, the type of fluorine-containing structural unit in Comparative Examples 1 and 4 is outside the range of this application, and the mole fraction of the fluorine-containing structural unit in Comparative Examples 2 and 3 is outside the range of this application. The brittle fracture of the positive electrode sheet is relatively severe, and the prepared lithium-ion battery has a low volumetric energy density, indicating that the flexibility of the positive electrode sheet is poor and the volumetric energy density of the lithium-ion battery is low.

[0174] The ratio of the number of fluorine atoms in a fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit generally affects the flexibility of the positive electrode and the volumetric energy density of the lithium-ion battery. As can be seen from Examples 1-1 to 1-3, by regulating the ratio of the number of fluorine atoms in a fluorine-containing structural unit to the number of atoms other than carbon atoms in the fluorine-containing structural unit within the scope of this application, the brittle fracture of the positive electrode is relatively mild, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0175] The values ​​of Mw and Mz / Mw generally affect the flexibility of the positive electrode sheet and the volumetric energy density of the lithium-ion battery. As can be seen from Examples 1-1, 1-10, and 1-17, by adjusting the values ​​of Mw and Mz / Mw within the range of this application, the brittle fracture of the positive electrode sheet is relatively mild, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a high volumetric energy density.

[0176] The crystallinity of a polymer generally affects the flexibility of the positive electrode sheet and the volumetric energy density of a lithium-ion battery. As can be seen from Examples 1-1, 1-18, and 1-21, by regulating the crystallinity of the polymer within the scope of this application, the brittle fracture of the positive electrode sheet is reduced, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0177] The melting point of a polymer typically affects the flexibility of the positive electrode sheet, the volumetric energy density, and the cycle life of a lithium-ion battery. As can be seen from Examples 1-1, 1-2, 1-3, and 1-22, by adjusting the melting point of the polymer within the scope of this application, the brittle fracture of the positive electrode sheet is reduced, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0178] Depend on Figure 1 It can be seen that in the polymer P(VDF-DB) 1 In the H NMR spectrum, the first characteristic peaks are present in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 6.1ppm to 6.8ppm; in the range of 1 In the H NMR spectrum, there are second characteristic peaks in the ranges of 2.3ppm to 2.7ppm, 2.7ppm to 3.3ppm and 5.0ppm to 6.0ppm; in the polymer P(VDF-CTFE), 1 In the H nuclear magnetic resonance spectrum, there are third characteristic peaks in the ranges of 2.3 ppm to 2.7 ppm, 2.7 ppm to 3.3 ppm, and 3.3 ppm to 3.5 ppm.

[0179] Depend on Figure 2 It can be seen that in the X-ray diffraction spectrum of the polymer P(VDF-CTFE), there are fourth characteristic peaks in the ranges of 19° to 21° and 34° to 35°.

[0180] Depend on Figure 3 It can be seen that in the infrared spectrum of polymer P(VDF-TrFE), at 470 cm -1 Up to 475cm -1 、507cm -1 Up to 515cm -1 、830cm -1 Up to 850cm -1 、1275cm -1 to 1290cm -1 There is a fifth characteristic peak in the range.

[0181]

[0182]

[0183] The mass percentages of the positive electrode active material, positive electrode binder, and positive electrode conductive agent typically affect the flexibility of the positive electrode sheet and the volumetric energy density of the lithium-ion battery. As can be seen from Examples 1-1, 2-1, and 2-4, by adjusting the mass percentages of the positive electrode active material, positive electrode binder, and positive electrode conductive agent within the range of this application, the brittle fracture of the positive electrode sheet is reduced, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0184] The Dv50 of the positive electrode active material usually affects the flexibility of the positive electrode sheet and the volumetric energy density of the lithium-ion battery. It can be seen from Examples 1-1, 2-5 to 2-12 that by regulating the Dv50 of the positive electrode active material within the scope of this application, the brittle fracture of the positive electrode sheet is relatively mild, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density. In Examples 1-1, 2-5 to 2-12, as the Dv50 of the positive electrode active material increases, the bonding force between the positive electrode material layer and the positive electrode current collector first increases and then decreases. This is because when the Dv50 of the positive electrode active material is small, the positive electrode active material absorbs more positive electrode binder, thereby reducing the bonding force between the positive electrode material layer and the positive electrode current collector; when the Dv50 of the positive electrode active material is large, the contact area between the positive electrode active material and the positive electrode current collector is small, thereby reducing the bonding force between the positive electrode material layer and the positive electrode current collector.

[0185] The compaction density of the positive electrode material layer generally affects the flexibility of the positive electrode sheet and the volumetric energy density of the lithium-ion battery. As can be seen from Examples 1-1, 2-13, and 2-17, by adjusting the compaction density of the positive electrode material layer within the scope of this application, the brittle fracture of the positive electrode sheet is reduced, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0186] The thickness of the positive electrode current collector generally affects the volumetric energy density of a lithium-ion battery. As can be seen from Examples 1-1, 2-18, and 2-23, by adjusting the thickness of the positive electrode current collector within the scope of this application, the brittle fracture of the positive electrode sheet is reduced, and the prepared lithium-ion battery has a higher volumetric energy density, indicating that the positive electrode sheet has good flexibility and the lithium-ion battery has a higher volumetric energy density.

[0187] 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.

[0188] 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.

[0189] 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 plate, the positive electrode plate comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a positive electrode binder, the positive electrode binder comprising a polymer, the polymer comprising vinylidene fluoride and a fluorine-containing structural unit, the fluorine-containing structural unit comprising at least one of -CH=CF-, -CFH-CF2-, or -CFCl-CF2-; based on the total amount of the polymer, the mole fraction of the fluorine-containing structural unit is M%, and 20≤M≤80.

2. The secondary battery according to claim 1, wherein 30≤M≤50。 3. The secondary battery according to claim 1, wherein The ratio of the number of fluorine atoms in the fluorine-containing structural unit to the number of other atoms in the fluorine-containing structural unit excluding carbon atoms is A, and 0.5≤A≤0.

75.

4. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1) The polymer comprises at least one of polyvinylidene fluoride-double bond copolymer, poly(vinylidene fluoride-trifluoroethylene) or poly(vinylidene fluoride-chlorotrifluoroethylene); (2) In the polymer 1 In the H nuclear magnetic resonance spectrum, first characteristic peaks are present in the ranges of 2.3 ppm to 2.7 ppm, 2.7 ppm to 3.3 ppm, and 6.1 ppm to 6.8 ppm; (3) In the polymer 1 In the H nuclear magnetic resonance spectrum, second characteristic peaks are present in the ranges of 2.3 ppm to 2.7 ppm, 2.7 ppm to 3.3 ppm, and 5.0 ppm to 6.0 ppm; (4) in the polymer 1 In the H nuclear magnetic resonance spectrum, there are third characteristic peaks in the ranges of 2.3 ppm to 2.7 ppm, 2.7 ppm to 3.3 ppm, and 3.3 ppm to 3.5 ppm; (5) The average molecular weight of the polymer is Mz, the weight average molecular weight of the polymer is Mw, 1.8≤Mz / Mw≤3.5, 700000≤Mw≤1200000; (6) The crystallinity of the polymer is X%, 20≤X≤70.

5. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1) In the X-ray diffraction spectrum of the polymer, there is a fourth characteristic peak in the range of 19° to 21° and 34° to 35°; (2) In the infrared spectrum of the polymer, at 470 cm -1 Up to 475cm -1 、507cm -1 Up to 515cm -1 、830cm -1 Up to 850cm -1 、1275cm -1 to 1290cm -1 There is a fifth characteristic peak in the range; (3) The melting point of the polymer is Tm°C, 120≤Tm≤160; (4) The polymer includes silicon, iron and zinc, and the sum of the mass of the silicon, iron and zinc in the polymer accounts for W, and W is less than 200 ppm.

6. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1) The compaction density of the positive electrode material layer is PD g / cm 3 , 3.0≤PD≤4.5; (2) The positive electrode material layer further includes a positive electrode active material, and the Dv50 of the positive electrode active material satisfies the following: 0.5 μm ≤ Dv50 ≤ 35 μm; (3) The thickness of the positive electrode current collector is H1 μm, 7≤H1≤20.

7. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1) The compaction density of the positive electrode material layer is PD g / cm 3 , 3.8≤PD≤4.4; (2) The positive electrode material layer further includes a positive electrode active material, and the Dv50 of the positive electrode active material satisfies: 5 μm ≤ Dv50 ≤ 30 μm; (3) The thickness of the positive electrode current collector is H1 μm, 8≤H1≤12.

8. The secondary battery according to claim 1, wherein The positive electrode material layer further includes a positive electrode active material, and the Dv50 of the positive electrode active material satisfies: 10 μm≤Dv50≤25 μm.

9. The secondary battery according to claim 8, wherein The positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide or lithium iron phosphate.

10. The secondary battery according to claim 8, wherein The positive electrode material layer also includes a positive electrode conductor. Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode active material is w1%, the mass percentage of the positive electrode binder is w2%, and the mass percentage of the positive electrode conductor is w3%. 97.6≤w1≤98.8, 0.7≤w2≤1.3, 0.5≤w3≤1.

1.

11. The secondary battery according to claim 1, wherein The thickness of the positive electrode material layer is H2 μm, 40.5≤H2≤70.

12. The secondary battery according to claim 1, wherein The bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, 13≤F≤35.

13. The secondary battery according to claim 1, wherein Along the thickness direction of the positive electrode sheet, the positive electrode material layer includes a first surface and a second surface opposite to each other; From the first surface to the second surface, the positive electrode material layer includes a first region, a second region and a third region in sequence, and the third region is located on the surface of the positive electrode current collector; The thickness of the first region accounts for 1 / 3 of the thickness of the positive electrode material layer, the thickness of the second region accounts for 1 / 3 of the thickness of the positive electrode material layer, and the thickness of the third region accounts for 1 / 3 of the thickness of the positive electrode material layer; The weight loss rate of the positive electrode material layer particles in the first region from 0°C to 600°C is TG1, the weight loss rate of the positive electrode material layer particles in the second region from 0°C to 600°C is TG2, and the weight loss rate of the positive electrode material layer particles in the third region from 0°C to 600°C is TG3, 0.85≤TG1 / TG2≤1.15, 0.85≤TG1 / TG3≤1.

15.

14. The secondary battery according to claim 1, wherein The area on the surface of the positive electrode current collector where the positive electrode material layer is provided is a fourth area, which satisfies at least one of the following characteristics: (1) the ratio of the specific surface areas of the positive electrode active materials on any two positive electrode material layers with an area of ​​250 mm×250 mm on the fourth region is 0.97 to 1.03; (2) The ratio of the roughness of the positive electrode current collector of any two of the fourth regions with an area of ​​250 mm×250 mm is 0.93 to 1.

07.

15. The secondary battery according to claim 1, wherein The contact angle between the positive electrode plate and the electrolyte is θ, 10°≤θ≤30°. 16 . An electronic device comprising the secondary battery according to claim 1 .

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