Secondary battery and electric equipment

Abstract

The invention provides a secondary battery and electric equipment. The secondary battery comprises a positive pole piece, the positive pole piece comprises a positive current collector and a positive active material layer arranged on at least one surface of the positive current collector, and the positive active material layer comprises lithium-containing phosphate with an olivine structure, a first binder and a second binder; the first binder comprises a polyvinylidene fluoride copolymer, and the second binder comprises a copolymer containing a tetrafluoroethylene structural unit. The compound binder is used in the positive electrode active material layer, so that the problem of thick coating cracking of the positive electrode plate when the small-particle-size lithium-containing phosphate is used as the positive electrode active material is solved, and the energy density and high-temperature cycle performance of the secondary battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, and in particular to a secondary battery and a power consumption device. BACKGROUND

[0002] With the rapid development of secondary batteries, the market demand for energy density is also increasing. Lithium-containing phosphates with olivine structure are considered as ideal positive electrode materials to replace ternary materials (NCM) due to their low cost. However, the particle size of lithium-containing phosphates with olivine structure is usually small, which is prone to cause coating cracking due to uneven inter-particle stress at high load thick coating (>20 mg / cm2), and the traditional binder is mainly PVDF, which has too strong rigidity, and the internal stress is too strong to shrink seriously during baking, so the cracking problem cannot be effectively inhibited, resulting in reduced electrode mechanical strength and deteriorated cycle life, and high energy density cannot be achieved. Therefore, it is of great significance to improve the thick coating cracking problem by fine design and regulation of the binder. SUMMARY

[0003] The purpose of the present application is to provide a secondary battery and a power consumption device to improve the energy density and high-temperature cycle performance of a secondary battery using lithium-containing phosphates as positive electrode active materials.

[0004] To achieve the above-mentioned purpose, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising lithium-containing phosphates with olivine structure, a first binder and a second binder. The first binder comprises a polyvinylidene fluoride copolymer, and the second binder comprises a copolymer containing a tetrafluoroethylene structural unit.

[0005] As an embodiment of the present application, the mass ratio of the polyvinylidene fluoride copolymer to the copolymer containing a tetrafluoroethylene structural unit in the positive electrode active material layer is (20-50):(50-80).

[0006] As an embodiment of the present application, the swelling rate of the polyvinylidene fluoride copolymer is 20%-100%.

[0007] As an embodiment of the present application, the weight average molecular weight of the polyvinylidene fluoride copolymer is 80W-200W.

[0008] As an embodiment of the present application, the elongation at break of the polyvinylidene fluoride copolymer is 5%-50%.

[0009] As an embodiment of the present application, the tensile strength of the polyvinylidene fluoride copolymer is 20 MPa-60 MPa.

[0010] As an embodiment of the present application, the polyvinylidene fluoride copolymer contains a polar group, which at least includes a carboxyl group and an anhydride.

[0011] As an embodiment of the present application, the mole ratio of the tetrafluoroethylene structural unit in the copolymer containing the tetrafluoroethylene structural unit is 20 mol% to 50 mol%.

[0012] As an embodiment of the present application, the copolymer containing the tetrafluoroethylene structural unit further contains a vinylidene fluoride structural unit, and the mole ratio of the vinylidene fluoride structural unit is 50 mol% to 80 mol%.

[0013] As an embodiment of the present application, the swelling rate of the copolymer containing the tetrafluoroethylene structural unit is 50% to 200%.

[0014] As an embodiment of the present application, the weight average molecular weight of the copolymer containing the tetrafluoroethylene structural unit is 80 W to 300 W.

[0015] As an embodiment of the present application, the elongation at break of the copolymer containing the tetrafluoroethylene structural unit is 200% to 1000%.

[0016] As an embodiment of the present application, the tensile strength of the copolymer containing the tetrafluoroethylene structural unit is 0.5 MPa to 10 MPa.

[0017] As an embodiment of the present application, the Dv50 of the lithium-containing phosphate with olivine structure is 0.3 μm to 2 μm.

[0018] As an embodiment of the present application, the powder resistivity of the lithium-containing phosphate with olivine structure is 5 Ω·cm to 100 Ω·cm.

[0019] As an embodiment of the present application, the maximum bending stress of the positive electrode tab is 223 to 289 N / m.

[0020] In a second aspect of the present application, a power utilization device is provided, which includes the secondary battery of the first aspect of the present application.

[0021] Compared with the prior art, the present application has the following beneficial effects: By using the polyvinylidene fluoride copolymer and the copolymer containing the tetrafluoroethylene structural unit as the compounded binder in the positive electrode active material layer, the present application improves the problems of cracking of thick coating and poor electrode structure stability at high temperature when using small particle size lithium-containing phosphate (Dv50 is 0.3 μm to 2 μm, and the powder resistivity is 5 Ω·cm to 100 Ω·cm) as the positive electrode active material, thereby improving the energy density and high-temperature cycle performance of the secondary battery. DETAILED DESCRIPTION

[0022] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0023] In the present application, the technical features described in an open way include both the closed technical solutions consisting of the listed features and the open technical solutions containing the listed features.

[0024] In the present application, as no specific description is given, the numerical range is regarded as continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all the ranges disclosed herein should be understood to include any and all sub-ranges falling within the range.

[0025] The reagents or instruments used in the present application are all conventional products that can be obtained from the market, unless otherwise specified.

[0026] The embodiments of the present application provide a secondary battery, which comprises a positive electrode sheet including a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, the positive electrode active material layer containing a lithium-containing phosphate having an olivine structure, a first binder and a second binder. The first binder contains a polyvinylidene fluoride (PVDF) copolymer, and the second binder contains a copolymer containing a tetrafluoroethylene (TFE) structural unit.

[0027] The inventors of the present application have found that the copolymer containing tetrafluoroethylene structural unit has a weak intermolecular interaction force (small van der Waals force) due to the TFE segment composed of C-C bond and symmetrical F atoms, the segment is easy to slide, has low friction coefficient, self-lubricating property and ultra-low shrinkage, which can improve the wrapping property of small particle size main material and eliminate the internal stress generated by shrinkage during coating baking, but there is a risk of weak adhesion when used alone. The polyvinylidene fluoride copolymer can enhance the combination with the main material through van der Waals force and has high adhesion. By using the copolymer containing tetrafluoroethylene structural unit and the polyvinylidene fluoride copolymer in combination, the combination of high flexibility of the copolymer containing tetrafluoroethylene structural unit due to low friction coefficient and self-lubricating property (reducing coating internal stress) and high adhesion of the polyvinylidene fluoride copolymer, the two form an interpenetrating network structure, thereby enhancing the coating crack resistance, improving the problem of cracking of thick coating, and further improving the energy density of the secondary battery. In addition, the tetrafluoroethylene structural unit is composed of perfluorocarbon chain, which can improve the thermal decomposition temperature of the polymer, and the tetrafluoroethylene has strong chemical inertness and almost does not react with any substance, which can reduce the side reaction with electrolyte in high temperature environment. At the same time, the polyvinylidene fluoride copolymer with high adhesion strength is used in combination, and the two synergistically can ensure that the electrode has excellent adhesion and flexibility, release the stress generated by the shrinkage and expansion of the positive material during the charging and discharging process of the battery, and improve the stability of the electrode structure at high temperature, thereby improving the cycle life.

[0028] In some embodiments, the mass ratio of the polyvinylidene fluoride copolymer to the copolymer containing tetrafluoroethylene structural unit in the positive active material layer is (20-50):(50-80). Exemplarily, the mass ratio of the polyvinylidene fluoride copolymer to the copolymer containing tetrafluoroethylene structural unit in the positive active material layer can be any one value or a range value between any two values in 20:80, 25:75, 30:70, 35:65, 40:60, 50:50. The mass ratio of the polyvinylidene fluoride copolymer to the copolymer containing tetrafluoroethylene structural unit in the positive active material layer in the above range can further improve the flexibility of the positive electrode sheet and ensure sufficient adhesion.

[0029] In some embodiments, the polyvinylidene fluoride copolymer has a swelling ratio of 20% to 100%. Illustratively, the polyvinylidene fluoride copolymer can have a swelling ratio of any one of 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% or a range value between any two of them. The swelling ratio of the polyvinylidene fluoride copolymer within the above range can ensure sufficient ion transmission channels and electrolyte wettability, while reducing the mechanical strength and dimensional instability problems caused by excessive swelling. In some embodiments, the swelling ratio is 30% to 50%. The swelling ratio of the polyvinylidene fluoride copolymer within this range can further achieve a balance between swelling performance and mechanical strength, improving the kinetics, safety and service life of the secondary battery.

[0030] In some embodiments, the polyvinylidene fluoride copolymer has a weight average molecular weight of 80W to 200W. Illustratively, the polyvinylidene fluoride copolymer can have a weight average molecular weight of any one of 80W, 100W, 120W, 140W, 160W, 180W, 200W or a range value between any two of them. The weight average molecular weight of the polyvinylidene fluoride copolymer within the above range can ensure sufficient melt strength and film formability, reduce excessive melt viscosity and processing difficulty caused by too high molecular weight, while balancing electrolyte wettability and structural stability to ensure excellent ion conductivity. In some embodiments, the weight average molecular weight of the polyvinylidene fluoride copolymer is 1 million to 1.6 million. Within this range, the material exhibits the best synergistic effect of melt flowability, mechanical strength and ion conductivity, improving the kinetics, safety and service life of the secondary battery.

[0031] In some embodiments, the polyvinylidene fluoride copolymer has an elongation at break of 5% to 50%. Illustratively, the polyvinylidene fluoride copolymer can have an elongation at break of any one of 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or a range value between any two of them. The elongation at break of the polyvinylidene fluoride copolymer within the above range can ensure the basic deformation ability of the material, avoid brittle fracture, and prevent excessive elongation of the material from causing dimensional stability and mechanical strength to decrease. In some embodiments, the elongation at break is 20% to 40%. Within this range, the material can maintain structural integrity, effectively absorb mechanical stress, ensure the integrity of the conductive network during long cycle, inhibit electrode instability and powdering and shedding, thereby improving the safety and service life of the secondary battery.

[0032] In some embodiments, the polyvinylidene fluoride copolymer has a tensile strength of 20 MPa to 60 MPa. Illustratively, the polyvinylidene fluoride copolymer can have a tensile strength of any one of 20 MPa, 30 MPa, 40 MPa, 50 MPa, 60 MPa or a range between any two of them. The tensile strength of the polyvinylidene fluoride copolymer within the above range can ensure sufficient structural strength and dimensional stability of the material during application, while reducing the increase in material brittleness and difficulty in forming and processing caused by excessively high strength. In some embodiments, the tensile strength is 30 MPa to 50 MPa, which can ensure excellent mechanical properties of the material while maintaining good processability and durability, ensuring the integrity of the conductive network during long cycles, while inhibiting electrode powdering and shedding, improving the safety and service life of the secondary battery.

[0033] In some embodiments, the polyvinylidene fluoride copolymer contains polar groups, which at least include carboxyl groups and anhydride groups. When the polyvinylidene fluoride copolymer contains carboxyl groups and anhydride groups, the binding with the main material can be further enhanced by van der Waals forces, thereby having high adhesion.

[0034] In some embodiments, the copolymer containing tetrafluoroethylene structural units has a molar ratio of tetrafluoroethylene structural units of 20 mol% to 50 mol%. Illustratively, the copolymer containing tetrafluoroethylene structural units can have a molar ratio of tetrafluoroethylene structural units of any one of 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol% or a range between any two of them. The molar ratio of tetrafluoroethylene structural units in the copolymer containing tetrafluoroethylene structural units within the above range can ensure sufficient chemical and thermal stability, while maintaining appropriate molecular chain flexibility and processing performance. In some embodiments, the tetrafluoroethylene unit has a ratio of 30 mol% to 40 mol%, which can exhibit the best balance of electrochemical stability and mechanical properties of the copolymer, which is beneficial to improve the safety and service life of the secondary battery.

[0035] In some embodiments, the copolymer containing tetrafluoroethylene structural units further contains vinylidene fluoride (VDF) structural units, wherein the molar percentage of the VDF structural units is 50 mol% to 80 mol%. Exemplarily, the molar percentage of the VDF structural units in the copolymer containing tetrafluoroethylene structural units can be any one or a range between any two of 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, and 80 mol%. A molar percentage of VDF structural units in the copolymer containing tetrafluoroethylene structural units within the above range ensures sufficient polar group content to achieve good bonding performance, while reducing the increase in brittleness caused by excessive crystallinity. In some embodiments, the VDF unit percentage is 60 mol% to 70 mol%. Within this range, the copolymer exhibits both excellent bonding performance and processability, inhibits electrode pulverization and detachment, ensures electrode structural stability, and thereby improves the safety and service life of the secondary battery.

[0036] In some embodiments, the swelling ratio of the copolymer containing tetrafluoroethylene structural units is 50% to 200%. Exemplarily, the swelling ratio of the copolymer containing tetrafluoroethylene structural units can be any one or a range between any two of the following values: 50%, 60%, 70%, 80%, 90%, 100%, 120%, 140%, 160%, 180%, and 200%. A swelling ratio within the above range ensures sufficient electrolyte penetration and ion transport channels, while reducing mechanical property degradation and dimensional deformation due to excessive swelling. In some embodiments, the swelling ratio is 70% to 120%, within which the material exhibits better swelling balance performance, maintaining the structural stability and ion conductivity of the electrode during charge and discharge, and improving the kinetic performance, safety, and service life of the secondary battery.

[0037] In some embodiments, the weight-average molecular weight of the copolymer containing tetrafluoroethylene structural units is 80W to 300W. Exemplarily, the weight-average molecular weight of the copolymer containing tetrafluoroethylene structural units can be any one or a range between any two of the following values: 80W, 100W, 150W, 180W, 200W, 220W, 250W, 280W, and 300W. A weight-average molecular weight within this range ensures sufficient melt strength and film-forming properties, while reducing processing difficulties caused by excessively high molecular weight. Simultaneously, it balances electrolyte wettability and structural stability, ensuring excellent ionic conductivity. In some embodiments, the weight-average molecular weight is 1 million to 2 million. Materials within this range exhibit excellent processing and mechanical properties, improving the kinetic performance, safety, and lifespan of the secondary battery.

[0038] In some embodiments, the elongation at break of the copolymer containing tetrafluoroethylene structural units is 200% to 1000%. Exemplarily, the elongation at break of the copolymer containing tetrafluoroethylene structural units can be any one or a range between any two of 200%, 300%, 400%, 500%, 600%, 700%, 800%, 900%, and 1000%. An elongation at break within the above range ensures that the material has sufficient deformation capacity under stress and maintains appropriate rigidity and dimensional recovery, ensuring the stability and recoverability of the electrode structure during charging and discharging, and improving the safety and lifespan of the secondary battery. In some embodiments, the elongation at break is 400% to 600%, within which the material maintains high elasticity while exhibiting better structural stability.

[0039] In some embodiments, the tensile strength of the copolymer containing tetrafluoroethylene structural units is 0.5 MPa to 10 MPa. Exemplarily, the tensile strength of the copolymer containing tetrafluoroethylene structural units can be any one or a range between any two of the following values: 0.5 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, and 10 MPa. A tensile strength within this range balances the flexibility and rigidity of the material, ensuring the bonding network in the electrode does not collapse, maintaining electrode structural stability, and improving safety and cycle performance. In some embodiments, the tensile strength is 2 MPa to 6 MPa, within which the material exhibits a better balance between mechanical and electrical properties.

[0040] In some embodiments, the Dv50 of the lithium phosphate with an olivine structure is 0.3 μm to 2 μm. Exemplarily, the Dv50 of the lithium phosphate with an olivine structure can be any one or a range between any two of the following values: 0.3 μm, 0.5 μm, 0.8 μm, 1.2 μm, 1.5 μm, 1.8 μm, and 2 μm. Having a Dv50 within the above range avoids the agglomeration effect of excessively small nanoparticles and the problems of low compaction and poor kinetics caused by excessively large particles, thus enabling the material to possess both excellent processing performance and electrochemical activity. Dv50 has a well-known meaning in the art and can be tested using methods known in the art. For example, it can be determined using a laser particle size analyzer. Dv50 represents the particle size corresponding to the cumulative 50% volume distribution percentage of particles starting from the smallest particle size.

[0041] In some embodiments, the resistivity of the lithium phosphate powder with an olivine structure is 5 Ω·cm to 100 Ω·cm. Exemplarily, the resistivity of the lithium phosphate powder with an olivine structure can be any one or any two values ​​from 5 Ω·cm, 10 Ω·cm, 20 Ω·cm, 30 Ω·cm, 40 Ω·cm, 50 Ω·cm, 60 Ω·cm, 70 Ω·cm, 80 Ω·cm, 90 Ω·cm, and 100 Ω·cm. When the resistivity of the lithium phosphate powder with an olivine structure is within the above range, the material has a suitable carbon coating layer, avoiding a decrease in the proportion of active material due to excessive carbon coating, while ensuring that the intrinsic conductivity of the material meets the requirements for electrode conductive network construction, thus ensuring the material exhibits optimal electrochemical performance.

[0042] In some embodiments, the maximum bending stress of the positive electrode sheet is 223–289 N / m. Exemplarily, the maximum bending stress of the positive electrode sheet can be any one of 223 N / m, 239 N / m, 247 N / m, 256 N / m, 265 N / m, 278 N / m, or 289 N / m, or a range between any two of these values.

[0043] In some embodiments, the lithium phosphate with an olivine structure is a lithium phosphate with a carbon layer at least partially formed on its surface, and the mass percentage of the carbon layer in the lithium phosphate with an olivine structure is 1.0% to 1.8%. Exemplarily, the mass percentage of the carbon layer can be any value from 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, and 1.8%, or a range consisting of any two of the above values. A carbon layer content within the above range can significantly improve the overall electronic conductivity and powder compaction density of the cathode material, alleviate the problem of cracking in thick coatings of the cathode sheet, and thereby increase the energy density of the secondary battery.

[0044] In some embodiments, this application provides a secondary battery, the secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a lithium phosphate having an olivine structure, a first binder and a second binder; The first adhesive comprises a polyvinylidene fluoride (PVDF) copolymer, and the second adhesive comprises a copolymer containing tetrafluoroethylene (TFE) structural units; The lithium phosphate with olivine structure has a Dv50 of 0.3 μm to 2 μm and a powder resistivity of 5 Ω·cm to 100 Ω·cm.

[0045] The inventors of this application have discovered that for lithium phosphates with an olivine structure and a Dv50 of 0.3μm to 2μm and a powder resistivity of 5Ω·cm to 100Ω·cm, the use of the compound binder described in this application can significantly increase the coating limit, thereby increasing the energy density of the secondary battery.

[0046] In some embodiments, the areal density of the positive electrode active material layer is 19 mg / cm³. 2 ~27mg / cm 2 For example, the areal density of the positive electrode active material layer may be 19 mg / cm³. 2 20mg / cm 2 21mg / cm 2 22mg / cm 2 23mg / cm 2 24mg / cm 2 25mg / cm 2 26mg / cm 2 27mg / cm 2 The value is any one of the values ​​or a range between any two values. In some embodiments, the areal density of the positive electrode active material layer is 22 mg / cm³. 2 ~27mg / cm 2 .

[0047] In some embodiments, the positive electrode active material layer further includes a conductive agent, which comprises carbon black and carbon nanotubes, wherein the mass ratio of carbon black to carbon nanotubes in the conductive agent is (0.2–1.5):(0.2–1). The conductive agent in the positive electrode active material layer is composed of carbon black and carbon nanotubes. By employing this specific ratio of composite conductive agent system, carbon black provides point contact conductive pathways, and carbon nanotubes form linear conductive bridges. Together, they construct a three-dimensional conductive network, reducing interfacial contact resistance. Simultaneously, the fibrous structure of the carbon nanotubes can form a mesh framework, improving the tensile strength of the electrode and mitigating cracking.

[0048] In some embodiments, the positive electrode active material layer comprises, by weight percentage, the following components: 95%–98% lithium phosphate having an olivine structure; 0.5%–1% first binder; 0.5%–1.5% second binder; and 0.7%–2.5% conductive agent, calculated based on the total mass of the positive electrode active material layer.

[0049] In some embodiments, lithium phosphates having an olivine structure include lithium iron phosphate and / or lithium manganese iron phosphate.

[0050] Conventional positive electrode current collectors in this field can be used in this application, including but not limited to metallic materials, carbon materials, and composite materials formed by polymers and metal layers. The metallic materials include aluminum, stainless steel, nickel plating, titanium, tantalum, etc., and the carbon materials include carbon cloth, carbon paper, etc. In some embodiments, the positive electrode current collector comprises aluminum foil or carbon-coated aluminum foil.

[0051] In some embodiments, the secondary battery further includes a negative electrode, a separator, and an electrolyte.

[0052] In some embodiments, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material, a negative conductive agent, and a negative binder.

[0053] In some embodiments, the negative current collector comprises copper foil or carbon-coated copper foil.

[0054] In some embodiments, the negative electrode active material includes, but is not limited to, at least one of silicon material, carbon material, and silicon-carbon composite material, wherein the silicon material includes at least one of elemental silicon, silicon oxide, and silicon carbide; and the carbon material includes graphite.

[0055] This application does not limit the types of negative electrode conductive agents and negative electrode binders; conventional negative electrode conductive agents and negative electrode binders in the art can all be used in this application. The negative electrode conductive agents include, but are not limited to, at least one of acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene-reduced graphene oxide, and carbon nanofibers. The negative electrode binders include, but are not limited to, at least one of polyacrylic acid, styrene-butadiene rubber, and carboxymethyl cellulose.

[0056] In some embodiments, the electrolyte comprises a lithium salt and an organic solvent, and may also contain additives. The types and compositions of the lithium salt and organic solvent are not particularly limited and can be selected according to actual needs. The lithium salt may include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, etc.; the solvent may include ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and propyl propionate, etc.; and the additives may include lithium difluorophosphate, lithium bis(oxalato)borate, succinate, 1,3-propanesulfonyl lactone, and ethylene sulfate, etc.

[0057] In this application, the type of separator is not particularly limited and can be selected according to actual needs. The separator can be a polypropylene film, a polyethylene film, a polyvinylidene fluoride film, a spandex film, an aramid film, or a multilayer composite film modified with a coating.

[0058] In some embodiments of this application, the preparation of the secondary battery includes: stacking the positive electrode, separator, and negative electrode in sequence, so that the separator is between the positive and negative electrode to play a role in isolation, then winding it into a square bare cell, installing it into a battery casing, then vacuum drying, injecting electrolyte, and after processes such as standing, hot and cold pressing, formation, secondary sealing, and capacity testing, a secondary battery is obtained.

[0059] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging may also be a soft pack, such as a pouch-type soft pack. The soft pack may be made of plastic, such as one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate, or an aluminum-plastic film, such as an aluminum-plastic film formed by a composite of a PA layer, an aluminum layer, and a PP layer. The shape of the secondary battery is not particularly limited; it may be cylindrical, square, or any other arbitrary shape.

[0060] In this application, the positive electrode sheet is obtained by coating a positive electrode slurry onto a current collector, followed by drying, rolling, and other processes. In some embodiments, the positive electrode slurry includes a lithium phosphate having an olivine structure, a first binder, and a second binder. The first binder comprises a polyvinylidene fluoride copolymer, and the second binder comprises a copolymer containing tetrafluoroethylene structural units.

[0061] In some embodiments, the surface tension of the positive electrode slurry is 27–82 mN / m. Exemplarily, the surface tension of the positive electrode slurry can be any one or a range between any two of the following values: 27 mN / m, 34 mN / m, 45 mN / m, 57 mN / m, 69 mN / m, 74 mN / m, and 82 mN / m. A surface tension within this range ensures sufficient wetting, spreading, and leveling properties of the slurry on the current collector surface, preventing pinhole defects during coating and guaranteeing uniform distribution of the active material on the current collector surface.

[0062] In some embodiments, the adhesion strength of the positive electrode slurry is 380–690 μN. Exemplarily, the adhesion strength of the positive electrode slurry can be any one or a range between any two of the following values: 380 μN, 430 μN, 450 μN, 480 μN, 500 μN, 570 μN, 610 μN, and 690 μN. An adhesion strength within this range ensures sufficient bonding strength between the slurry and the current collector, preventing peeling or powdering during coating, drying, or rolling. It also prevents excessively strong adhesion from causing rapid viscosity rebound and resulting in poor manufacturability.

[0063] A second aspect of this application provides an electrical device comprising the secondary battery described in the first aspect of this application. The electrical device can be an application device such as a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, and power tool. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; spacecraft include airplanes, rockets, space shuttles, and spacecraft; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0064] The following are specific embodiments of this application, and the technical solutions of this application are further described in conjunction with the embodiments. However, this application is not limited to these embodiments. Unless otherwise specified, the reagents, methods, and equipment used in this application are all conventional reagents, methods, and equipment in this technical field.

[0065] Example 1 This embodiment provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode sheet Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 PO4 (Dv50 of 0.6 μm, powder resistivity of 40 Ω·cm), carbon black, carbon nanotubes, and binder were mixed in a ratio of 96%:1%:1%:2%, NMP solvent was added, and then the mixture was transferred to a vacuum mixer for high-speed dispersion. After dispersion, vacuum reversal defoaming was performed to obtain the positive electrode slurry. The mass ratio of PVDF copolymer (first binder) to TFE-VDF copolymer (second binder) in the binder was 20%:80%. The physical properties of PVDF copolymer and TFE-VDF copolymer are shown in Table 1. The obtained positive electrode slurry was uniformly coated on both sides of aluminum foil using an extrusion coating machine, and then dried and rolled to obtain the positive electrode sheet.

[0066] (2) Preparation of negative electrode sheet Artificial graphite, carbon black, carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 94.3:1.5:1.2:3. Then, deionized water was added as a solvent, and the mixture was stirred until it became homogeneous to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated onto both sides of a copper foil using an extrusion coating machine. After drying and rolling, the negative electrode sheet was obtained.

[0067] (3) Preparation of electrolyte Ethylene carbonate, methyl ethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the mixture to obtain the electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L.

[0068] (4) Preparation of secondary batteries The above-mentioned positive electrode sheet, separator (polypropylene) and negative electrode sheet are wound in sequence to obtain a bare cell; the bare cell is placed in an outer packaging shell, vacuum dried and then injected with electrolyte. After standing, hot and cold pressing, formation, secondary sealing and capacity testing, etc., a secondary battery is obtained.

[0069] Examples 2-11 and Comparative Examples 1-4 provide a series of secondary batteries. The difference from Example 1 is that the physical properties of the positive electrode active material or the first binder and the second binder are different, as shown in Table 1. Among them, Comparative Examples 3 and 4 use homopolymer polyvinylidene fluoride.

[0070] Table 1 The test methods for the physical property parameters in Table 1 are as follows: 1. Dv50 of the main material: The Dv50 was determined by laser diffraction using a Malvern Mastersizer 3000 laser particle size analyzer. PVP aqueous solution was used as the dispersion medium. The material was ultrasonically dispersed for 3 minutes and then tested. The test was performed 3 times, and the median particle size was taken.

[0071] 2. Powder resistivity of main material: The volume resistivity of compacted powder was measured using a powder resistivity tester under a pressure of 30 MPa. The average value was taken from three measurements.

[0072] 3. Weight-average molecular weight of binder: determined by gel permeation chromatography, using DMF as the mobile phase (flow rate 1.0 mL / min) and narrow-distribution polystyrene as the standard, at a column temperature of 35℃.

[0073] 4. Elongation at break of adhesive: The adhesive was dissolved into a 7% concentration solution using a universal testing machine, poured into a mold and dried to form a dumbbell-shaped specimen (Type IBA). The specimen was tested at a tensile speed of 50 mm / min, and the average value of three tests was taken.

[0074] 5. Tensile strength of adhesive: Determined simultaneously with elongation at break. Using a universal testing machine, the adhesive is dissolved into a 7% concentration solution, poured into a mold, dried and shaped to obtain a dumbbell-shaped specimen (Type IBA). The specimen is tested at a tensile speed of 50 mm / min, and the ratio of the maximum load to the original cross-sectional area of ​​the specimen is calculated.

[0075] 6. Adhesive swelling rate: Cut the adhesive material into 20mm×20mm samples, place them in an electrolyte (ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate in a volume ratio of 1:1:1, lithium hexafluorophosphate concentration of 1mol / L), and let them stand and soak for 7 days at room temperature. Calculate the swelling rate, where swelling rate = (weight after swelling - original weight) / original weight × 100%.

[0076] The surface tension, adhesion force, maximum bending stress of the positive electrode sheet, and electrochemical performance of the secondary batteries prepared in each embodiment and comparative example were tested. The test methods are shown below, and the test results are shown in Table 2.

[0077] Slurry surface tension: Adjust the surface tension tester, hang the platinum plate under the hook of the equipment, place the prepared liquid to be tested directly below the platinum plate, and then immerse the platinum plate into the liquid to be tested; due to the surface tension of the liquid, as the platinum plate is pulled out of the liquid surface, the precision balance inside the equipment will display the change of its mass in real time, and the surface tension of the liquid can be calculated by software.

[0078] Slurry adhesion: The liquid to be tested is drawn into the adhesion test probe, and then the slurry is controlled to flow out of the probe to form a droplet-like shape. At the same time, the foil to be tested is fixed on the sample stage. Then the sample stage is raised to make the droplet contact the foil, and then the sample stage is lowered to pull it apart. The force value when it separates is recorded as the adhesion force of the material.

[0079] Upper limit of coating weight on one side of the electrode: Using slit extrusion coating, coating is carried out at 15m / min, and the coating weight is gradually increased until cracks appear in the coated film area. The coating weight value before cracking is taken as the upper limit of coating weight.

[0080] Maximum bending stress of the electrode: Cut the positive electrode into 40mm×80mm pieces, place them on the support of the bending stress testing machine, adjust the loading head of the testing machine so that it is located at the midpoint of the specimen, apply a load to the specimen, and record the bending stress.

[0081] The test method for 45℃ cycle capacity retention is as follows: In a 45℃ test environment, the test battery is charged at 0.2C for 3 cycles to obtain the average capacity C0. Then, it is charged at a constant rate of 1C0 to 4.4V, allowed to stand for 10 minutes, and discharged at a constant rate of 1C0 to 2.5V. After standing for 10 minutes, this charge-discharge cycle is repeated until the capacity retention rate reaches 80%. The number of cycles at this point is recorded. The cycle capacity retention rate of the battery is calculated using the following formula: Cycle capacity retention rate = Last cycle discharge capacity / First cycle discharge capacity × 100%.

[0082] The test method for the storage volume expansion rate at 70℃ is as follows: In a 25℃ test environment, the test battery is charged at 0.2C for 3 cycles to obtain the average capacity C0 (charged at a constant rate of 0.2C to 4.4V, and discharged at a constant rate of 0.2C to 2.5V). After being fully charged at 0.2C0, it is left to stand for 5 minutes. The initial volume V0 of the battery is measured by the water displacement method. The fully charged battery is placed in a 70℃ high and low temperature chamber and the battery volume V is measured after 60 days. The storage volume expansion rate (%) of the battery at 70℃ is calculated using the following formula: Storage volume expansion rate (%) = (V - V0) / V0 × 100%.

[0083] Table 2 As can be seen from the above examples and comparative data, using polyvinylidene fluoride copolymer and copolymer containing tetrafluoroethylene structural units as a compound binder in the positive electrode active material layer can achieve better coating performance. When the PVDF content is too high, the surface tension, adhesion, and bending stress of the slurry are too strong, the slurry viscosity is high and difficult to process, and the coating process is prone to cracking, making it difficult to achieve a thick coating effect. Compounding TFE-VDF can reduce surface tension, improve the problem of cracking caused by excessive shrinkage stress during slurry drying, and at the same time reduce bending stress, improve the flexibility of the electrode sheet, thereby achieving excellent coating effect. As can be seen from Example 5 and Comparative Example 4, for main materials with small particle size, homopolymeric PVDF has weak adhesion, and the coating weight can only meet 16.50 mg / cm³. 2 The use of a blend of copolymerized PVDF and TFE-VDF ensures sufficient adhesion and flexibility even with high surface tension of small-particle-size substrates, significantly improving thick coating performance. The maximum single-sided coating strength is at least 22.07 mg / cm². 2 .

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a positive electrode, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a lithium phosphate having an olivine structure, a first binder, and a second binder. The first adhesive comprises a polyvinylidene fluoride copolymer, and the second adhesive comprises a copolymer containing tetrafluoroethylene structural units.

2. The secondary battery according to claim 1, characterized in that, The mass ratio of the polyvinylidene fluoride copolymer to the copolymer containing tetrafluoroethylene structural units in the positive electrode active material layer is (20-50):(50-80).

3. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies at least one of the following characteristics: (1) The swelling rate of the polyvinylidene fluoride copolymer is 20% to 100%; (2) The weight-average molecular weight of the polyvinylidene fluoride copolymer is 80W to 200W; (3) The elongation at break of the polyvinylidene fluoride copolymer is 5% to 50%; (4) The tensile strength of the polyvinylidene fluoride copolymer is 20 MPa to 60 MPa.

4. The secondary battery according to claim 1, characterized in that, The polyvinylidene fluoride copolymer contains polar groups, which include at least carboxyl groups and acid anhydrides.

5. The secondary battery according to claim 1, characterized in that, The copolymer containing tetrafluoroethylene structural units has a molar percentage of 20 mol% to 50 mol% of tetrafluoroethylene structural units.

6. The secondary battery according to claim 1 or 5, characterized in that, The copolymer containing tetrafluoroethylene structural units also contains vinylidene fluoride structural units, wherein the molar percentage of the vinylidene fluoride structural units is 50 mol% to 80 mol%.

7. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies at least one of the following characteristics: (I) The swelling rate of the copolymer containing tetrafluoroethylene structural units is 50% to 200%; (II) The weight-average molecular weight of the copolymer containing tetrafluoroethylene structural units is 80W to 300W; (III) The elongation at break of the copolymer containing tetrafluoroethylene structural units is 200% to 1000%; (IV) The tensile strength of the copolymer containing tetrafluoroethylene structural units is 0.5 MPa to 10 MPa.

8. The secondary battery according to claim 1, characterized in that, The Dv50 of the lithium phosphate with olivine structure is 0.3 μm to 2 μm.

9. The secondary battery according to claim 1, characterized in that, The resistivity of the lithium phosphate powder with olivine structure is 5 Ω·cm to 100 Ω·cm.

10. The secondary battery according to claim 1, characterized in that, The maximum bending stress of the positive electrode sheet is 223–289 N / m.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 10.

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