Secondary battery and electric device

Abstract

The application provides a secondary battery and a power utilization device, and relates to the field of batteries. The secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator film, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side surface of the positive electrode current collector, the compaction density of the positive electrode film layer is 2.0 g / cm 3 to 3.0 g / cm 3 ; the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, the compaction density of the negative electrode film layer is 1.4 g / cm 3 to 1.85 g / cm 3 ; and the separator film comprises a base film, the thickness of the base film is 4 to 12 mu m, and the base film is provided with nano-pores with an average pore diameter of 10 to 300 nm. The secondary battery has high energy density and improved safety performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and more particularly to a secondary battery and an electrical device. Background Technology

[0002] In recent years, as the application scope of secondary batteries has become increasingly wide, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, and aerospace.

[0003] With the significant development of secondary batteries, people have put forward higher requirements for their energy density and safety performance in practical applications.

[0004] In related technologies, increasing the compaction density of the film layer is a common method to improve the energy density of batteries. However, this method has significant drawbacks, as increasing the compaction density leads to an increase in the heat released inside the battery, thereby deteriorating the battery's safety performance.

[0005] Therefore, how to balance the energy density and safety performance of batteries has become an urgent technical problem to be solved. Summary of the Invention

[0006] This application was made in view of the above-mentioned problems, and its object is to provide a secondary battery and an electrical device. This secondary battery combines high energy density with improved safety performance.

[0007] To achieve the above objectives, the first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, and the compaction density of the positive electrode film layer is 2.0 g / cm³. 3 Up to 3.0 g / cm 3 The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the compaction density of the negative electrode film layer being 1.4 g / cm³. 3 Up to 1.85 g / cm 3 The separator includes a base film with a thickness of 4 μm to 12 μm and nanopores with an average pore size of 10 nm to 300 nm.

[0008] In this application, the compaction density of the positive electrode film is controlled at 2.0 g / cm³. 3 Up to 3.0 g / cm 3 The compaction density of the negative electrode film is 1.4 g / cm³. 3 Up to 1.85 g / cm 3This technology facilitates the achievement of high energy density in batteries. The application employs a base film with a thickness of 4μm to 12μm, which reduces the impact of an excessively thin base film (less than 4μm) on battery safety performance, while also mitigating the adverse effects of an excessively thick base film (greater than 12μm) on battery energy density. This results in a battery that enhances safety while maintaining high energy density. Furthermore, the base film contains nanopores with an average pore size of 10nm to 300nm. This helps to improve the migration rate of lithium ions while simultaneously suppressing the growth of lithium dendrites, thereby further enhancing battery safety performance.

[0009] Specifically, base films with thicknesses within the aforementioned range can maintain structural stability at high temperatures, effectively reducing the risk of short circuits between the positive and negative electrodes. Especially during battery thermal runaway, a base film with a stable structure can suppress side reactions through physical isolation, thereby lowering the maximum temperature of battery thermal runaway, reducing the risk of heat propagation, and improving battery safety. Furthermore, base film thicknesses within the aforementioned range help reduce the adverse effects of the base film on battery energy density, thus helping the battery maintain a high energy density.

[0010] In some embodiments, the thickness of the base film is 5 μm to 9 μm. A base film thickness within this range is advantageous for further balancing the energy density and safety performance of the battery.

[0011] In some embodiments, the porosity of the base film is 20% to 70%. This helps to improve the migration rate of lithium ions while also suppressing the growth of lithium dendrites, thereby further improving the safety performance of the battery.

[0012] In some embodiments, the separator further includes a coating disposed on at least one side of the base film. The coating comprises heat-resistant particles interwoven to form a porous structure. Compared to a coating without a porous structure, this implementation, where the heat-resistant particles interweave to form a porous structure, helps reduce the coating's obstruction to ion transport, thereby improving ion transport efficiency. Furthermore, the heat-resistant particles have a small volume change with temperature; disposing of the coating containing these particles on the base film surface helps suppress the thermal shrinkage rate of the base film, thereby further reducing the risk of short circuits between the positive and negative electrodes and improving battery safety.

[0013] In some embodiments, the coating thickness on one side of the base film is from 0.011 μm to 3 μm. This is beneficial for achieving high energy density in the battery and for the coating to better suppress the thermal shrinkage rate of the base film, thereby further improving the battery's safety performance.

[0014] In some embodiments, the average particle size of the heat-resistant particles is between 5 nm and 185 nm. Heat-resistant particles with an average particle size of 5 nm to 185 nm can form a coating with a stable supporting network structure, which can support the base film and suppress the thermal shrinkage of the base film, thereby improving the safety performance of the battery.

[0015] In some embodiments, the areal density of the heat-resistant particles is 0.5 mg / 1540.25 cm³. 2 Up to 2.5mg / 1540.25cm 2 This not only helps the battery achieve higher energy density, but also allows the heat-resistant particles to effectively suppress the thermal shrinkage of the base film, thus improving the battery's safety performance.

[0016] In some embodiments, the heat-resistant particles include first heat-resistant particles along the thickness direction of the base film. The first heat-resistant particles are distributed on the surface of the base film and can directly suppress the thermal shrinkage of the base film, thereby helping to further improve the safety performance of the battery while also taking into account the energy density of the battery.

[0017] In some embodiments, along the thickness direction of the base film, the heat-resistant particles include first heat-resistant particles and second heat-resistant particles; wherein the first heat-resistant particles are distributed on the surface of the base film, and the second heat-resistant particles are stacked on the side of the first heat-resistant particles away from the base film. In this implementation, the first and second heat-resistant particles are stacked in the thickness direction of the base film, which is beneficial to forming a thicker coating. The thicker coating can act as a more effective physical barrier, reducing the chemical erosion of the base film by the electrolyte, and also buffering the friction of the electrode sheets (positive electrode sheet and negative electrode sheet) on the base film, thereby further improving the safety performance of the battery.

[0018] In some embodiments, the heat-resistant particles include inorganic particles and binder particles. In this implementation, the binder particles act as a bridge, tightly connecting the inorganic particles together. This allows the inorganic particles to form a stable, integral structure, thereby improving the mechanical strength and stability of the coating and enabling it to better suppress the thermal shrinkage of the base film.

[0019] In some embodiments, the heat-resistant particles include inorganic particles and an adhesive layer disposed on at least a portion of the surface of the inorganic particles. In this implementation, the adhesive layer is disposed on the surface of the inorganic particles, which helps to improve the adhesion between the inorganic particles and the base film, reduces the risk of the inorganic particles falling off, and helps to improve the safety performance and service life of the battery.

[0020] In some embodiments, the inorganic particles include at least one of aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, boehmite, magnesium oxide, zinc oxide, barium sulfate, magnesium nitride, or barium titanate. The volume of these substances changes little with temperature, and using them as inorganic particles helps to further suppress the thermal shrinkage of the base film, thereby further improving the safety performance of the battery.

[0021] In some embodiments, the inorganic particles constitute 5% to 30% of the coating by mass. This facilitates the formation of a heat-resistant skeletal structure by the inorganic particles in the coating, effectively suppressing thermal shrinkage of the coating, improving battery safety performance, and reducing the risk of inorganic particles detaching.

[0022] In some embodiments, the negative electrode film layer includes a first film layer and a second film layer disposed between the first film layer and the negative electrode current collector. The first film layer includes a first negative electrode active material, which includes a first graphite material. D / I G The value is 0.4 to 0.9; the second film layer includes a second negative electrode active material, which includes a second graphite material, and the second graphite material I D / I G It ranges from 0.05 to 0.2; where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location.

[0023] The first graphite material used in this application is I D / I G A value of 0.4 to 0.9 indicates that the surface of the first graphite material has numerous defects, which can serve as additional active sites for lithium-ion insertion. Placing this first graphite material with numerous surface defects in the outer layer (first film) of the negative electrode increases the probability of contact between the additional active sites and the electrolyte, thereby improving the battery's fast-charging performance. Furthermore, the inner layer (second film) of the negative electrode uses a second graphite material with an ID / IG ratio of 0.05 to 0.2. This second graphite material has fewer surface defects and a relatively complete structure, which helps reduce irreversible lithium-ion loss, thereby extending the battery's lifespan.

[0024] In some embodiments, the first negative electrode active material further includes a negative electrode coating layer distributed on the surface of the first graphite material, the negative electrode coating layer comprising amorphous carbon. Amorphous carbon has a relatively loose structure with abundant pores. These pores can provide more diffusion channels for lithium ions, shortening the diffusion path of lithium ions in the electrode material. By placing amorphous carbon on the surface of the first graphite material, lithium ions can more quickly pass through the negative electrode coating layer to reach the interior of the first graphite material, thereby improving the fast-charging performance of the battery.

[0025] In some embodiments, the thickness of the negative electrode coating layer is between 10 nm and 100 nm. A thickness within this range is beneficial for the secondary battery to balance fast-charging performance and energy density.

[0026] In some embodiments, the second film layer further includes a first graphite material. The first graphite material can improve the fast-charging performance of the battery. In this embodiment, by arranging the second graphite material and the first graphite material in the second film layer, it is beneficial to improve the fast-charging performance of the battery.

[0027] In some embodiments, the mass ratio of the second graphite material to the first graphite material in the second film layer is (3-5):(5-7). By controlling the mass ratio of the second graphite material to the first graphite material in the second film layer within the above range, it is beneficial to further improve the fast charging performance and lifespan of the battery.

[0028] In some embodiments, the coating weight of the negative electrode film is 80 mg / 1540.25 mm. 2 Up to 170mg / 1540.25mm 2 This helps to increase the number of lithium ions released per unit area of ​​the negative electrode film, thereby increasing the energy density of the battery.

[0029] In some embodiments, the thickness of the negative electrode film layer disposed on one side of the negative electrode current collector is 40 μm to 75 μm. This is beneficial for the negative electrode film layer to achieve both high capacity, high lithium-ion and electron transport performance, and thus for the secondary battery to achieve both high energy density and fast charging performance.

[0030] In some embodiments, the thickness of the negative electrode current collector is 4 μm to 8 μm. A thickness within this range is beneficial for achieving high energy density in the battery, and also helps reduce the risk of cracking in the negative electrode current collector, thereby extending the battery's lifespan.

[0031] In some embodiments, the positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material, comprising a lithium phosphate with an olivine structure. The lithium phosphate with an olivine structure has a stable three-dimensional crystal lattice structure. During lithium ion insertion and extraction, this structure remains relatively stable and is not prone to structural collapse or deformation. This allows the lithium phosphate with an olivine structure to withstand multiple charge-discharge cycles without damage, thereby extending the battery's lifespan.

[0032] In some embodiments, the lithium phosphate with an olivine structure comprises a compound as shown in formula (I):

[0033] LiFe 1-x-y Mn x M 1 y PO4, formula (I);

[0034] In equation (I), M 1 The lithium phosphate is selected from at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb, where 0 ≤ x ≤ 1 and 0 ≤ y < 1. The crystal structure of the above lithium phosphates is stable and not prone to phase transitions, thereby further improving the safety performance of secondary batteries.

[0035] In some embodiments, the first positive electrode active material further includes a positive electrode coating layer disposed on at least a portion of the surface of the lithium phosphate-containing material, and the positive electrode coating layer includes at least one of a fast ion conductor material and a carbon material.

[0036] The carbon material is loose and porous, which allows the electrolyte and lithium iron phosphate substrate to fully and effectively contact each other, thereby improving the wetting performance of the electrolyte on the positive electrode film and thus improving the fast charging performance of the secondary battery.

[0037] Fast ion conductor materials have high ionic conductivity, and selecting a positive electrode coating containing fast ion conductor materials can help improve the fast charging performance of the battery.

[0038] In some embodiments, the mass ratio of fast ion conductor material to carbon material in the positive electrode coating layer is (0-100):(100-0). By controlling the mass ratio of fast ion conductor material to carbon material within the above range, it is beneficial to improve the fast charging performance of the battery.

[0039] In some embodiments, the positive electrode coating layer includes a first coating layer and a second coating layer; the first coating layer includes a fast ion conductor material, and the second coating layer includes a carbon material; the first coating layer is disposed between the lithium phosphate and the second coating layer; carbon materials generally have good adhesion properties, and in this embodiment, disposing the second coating layer containing carbon material on the outer surface of the first positive electrode active material is beneficial to enhancing the adhesion between the first positive electrode active material and the current collector, reducing the risk of the first positive electrode active material falling off during battery use, and improving the battery's service life.

[0040] In some embodiments, the positive electrode coating layer includes a first coating layer and a second coating layer; the first coating layer includes a fast-ion conductor material, and the second coating layer includes a carbon material, with the second coating layer disposed between the lithium phosphate and the first coating layer. In this embodiment, during lithium ion conduction, lithium ions first pass through the first coating layer composed of the fast-ion conductor material, then enter the second coating layer, and finally diffuse into the lithium phosphate, thereby accelerating the migration speed of lithium ions within the first positive electrode active material and improving the battery's fast-charging performance. Furthermore, the carbon material in the second coating layer has good conductivity, which can form a continuous conductive network between the lithium phosphate and the first coating layer, accelerating electron conduction and further improving the battery's fast-charging performance.

[0041] In some embodiments, the fast ion conductor material comprises a compound as shown in formula (II):

[0042] Li 3-b Fe 2-b M 2 b (PO4)3 Equation (II); In Equation (II), M 2 The material is selected from at least one of Ti, Zr, Hf, Ge, and Sn, where 0 ≤ b ≤ 1. These fast ion conductor materials exhibit excellent ionic conductivity, and their selection can further improve the fast-charging performance of batteries.

[0043] In some embodiments, the mass percentage of carbon in the first positive electrode active material is 1% to 1.5%. This is beneficial in two ways: firstly, it helps the first positive electrode active material achieve high capacity, which in turn helps the secondary battery achieve high energy density; secondly, it helps improve the electronic conductivity of the first positive electrode active material, which in turn improves the fast-charging performance of the lithium-ion secondary battery.

[0044] In some embodiments, the BET specific surface area of ​​the first positive electrode active material is 12 m². 2 / g to 16m 2 / g; This provides more lithium-ion insertion and extraction channels on the surface of the first positive electrode active material. During fast charging, lithium ions can be inserted into the first positive electrode active material more quickly through these channels, reducing the lithium-ion transport distance and resistance, thereby improving the battery's fast charging performance.

[0045] In some embodiments, the tap density of the first positive electrode active material is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 This facilitates the formation of a rich porous structure in the positive electrode film, ensuring that the battery has superior fast-charging performance.

[0046] In some embodiments, the volume average particle size Dv50 of the first positive electrode active material is 1 μm to 3 μm, which is conducive to the formation of abundant pore structures between the particles of the first positive electrode active material, improving the lithium ion and electron transport performance in the positive electrode film, and thus enhancing the kinetic performance of the secondary battery.

[0047] In some embodiments, the compaction density of the first positive electrode active material at 50,000 N is 2.4 g / cm³. 3 Up to 2.6 g / cm 3 This results in a closer contact between the first cathode materials, which helps to improve the energy density of the battery.

[0048] In some embodiments, the positive electrode active material further includes a second positive electrode active material, which includes a lithium transition metal oxide. Lithium transition metal oxides have a higher specific capacity, and by selecting a lithium transition metal oxide as the second positive electrode active material, it is beneficial to further improve the energy density of the secondary battery.

[0049] In some embodiments, the mass ratio of the first positive electrode active material to the second positive electrode active material is (99-90):(1-10). This is beneficial for balancing the lifespan and energy density of the secondary battery.

[0050] In some embodiments, the thickness of the positive electrode film layer disposed on one side of the positive electrode current collector is 100 μm to 200 μm, which is beneficial for the positive electrode film layer to take into account high capacity, high lithium-ion and electron transport performance, and thus beneficial for the secondary battery to take into account high energy density and fast charging performance.

[0051] In some embodiments, the coating weight of the positive electrode film layer disposed on one side of the positive electrode current collector is 200 mg / 1540.25 mm. 2 Up to 400mg / 1540.25mm 2 This helps to increase the number of lithium ions released per unit area of ​​the positive electrode film, thereby increasing the energy density of the battery.

[0052] In some embodiments, the compaction density of the positive electrode film is 2 g / cm³. 3 Up to 3g / cm 3 This helps the positive electrode film maintain a better pore structure, reduces the tortuosity of the positive electrode film, shortens the lithium-ion transport path, thereby improving the battery's fast charging performance and lifespan, while also ensuring high energy density.

[0053] In some implementations, the thickness of the positive electrode current collector is 10 μm to 18 μm. This is beneficial for achieving high energy density in the battery and also helps reduce the risk of cracking in the positive electrode current collector, thereby extending the battery's lifespan.

[0054] In some embodiments, the positive electrode film layer further includes a positive electrode dispersant, which includes at least one of polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, and sodium carboxymethyl cellulose. The above-mentioned positive electrode dispersant has good flexibility and elasticity, and can disperse the stress borne by the positive electrode active material during compaction. By selecting the above-mentioned positive electrode dispersant, it is beneficial to achieve a high compaction density of the positive electrode film layer, thereby improving the energy density of the battery.

[0055] In some implementations, the mass percentage of the positive electrode dispersant relative to the positive electrode film layer is 0.3% to 5%. This is beneficial for achieving high energy density in the battery.

[0056] In some embodiments, the secondary battery further includes an electrolyte comprising an electrolyte salt and a solvent; the solvent includes at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone; the electrolyte salt includes at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0057] In some embodiments, the electrolyte further includes additives, including at least one selected from barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tris(trimethylsilane) phosphate, and vinylene carbonate. These additives preferentially undergo electrochemical reduction reactions on the surface of the negative electrode, prior to solvent molecules in the electrolyte, to form a dense and stable SEI film. This helps suppress lithium dendrite growth and improves battery safety performance.

[0058] In some implementations, the additive accounts for 1% to 10% of the electrolyte by mass. This facilitates the formation of an SEI film of moderate thickness on the electrode surface, which on the one hand helps suppress the growth of lithium dendrites and improves the safety performance of the battery. On the other hand, it helps to balance the transport resistance of lithium ions on the negative electrode, thereby ensuring the fast charging performance of the battery.

[0059] In some embodiments, the electrolyte conductivity ranges from 10 mS / cm to 18.5 mS / cm. This helps to reduce the battery's internal impedance and decrease energy loss due to resistance during charging and discharging.

[0060] A second aspect of this application also provides an electrical device, including the secondary battery of the first aspect of this application. Attached Figure Description

[0061] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0062] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown;

[0063] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application;

[0064] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0065] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of this application is shown;

[0066] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application;

[0067] Figure 7 This is a scanning electron microscope (SEM) image of the isolation membrane of Embodiment 1 of this application.

[0068] Explanation of reference numerals in the attached figures:

[0069] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0070] Hereinafter, embodiments of the secondary battery and power-consuming device of this application are disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter described in this application.

[0071] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0072] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0073] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0074] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0075] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0076] In recent years, as the application scope of secondary batteries has become increasingly wide, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, and aerospace.

[0077] With the significant development of secondary batteries, people have put forward higher requirements for their energy density and safety performance in practical applications.

[0078] In related technologies, increasing the compaction density of the membrane layer is a common method to improve the energy density of batteries. However, this method has significant drawbacks, as increased compaction density leads to increased heat release within the battery, thereby deteriorating its safety performance. For example, separators are typically made of polymers such as polyethylene (PE) and polypropylene (PP). These polymers are composed of long-chain molecular structures. At room temperature, these chains are relatively stable, with certain interactions that maintain the separator's stable shape. As the temperature rises, the molecules gain more energy, intensifying the thermal motion of the chains, increasing the spacing between them, and making their arrangement more disordered. This change in chain motion and arrangement causes the separator to exhibit dimensional changes on a macroscopic scale, i.e., thermal shrinkage. This thermal shrinkage can cause the positive and negative electrodes to overlap, worsening battery safety. Therefore, balancing battery energy density and safety performance has become a pressing technical problem.

[0079] To address the aforementioned technical problems, this application provides a secondary battery and an electrical device. This secondary battery balances energy density and safety performance.

[0080] The first aspect of this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, and the compaction density of the positive electrode film layer is 2.0 g / cm³. 3 Up to 3.0 g / cm 3 The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the compaction density of the negative electrode film layer being 1.4 g / cm³. 3 Up to 1.85 g / cm 3 The separator includes a base membrane with a thickness of 4 μm to 12 μm.

[0081] In this application, the compaction density of the positive electrode film is controlled at 2.0 g / cm³. 3 Up to 3.0 g / cm 3 The compaction density of the negative electrode film is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 This is beneficial for batteries to achieve high energy density.

[0082] While the aforementioned compaction density helps to ensure close packing of electrode material particles and improves battery energy density, it may also hinder electrolyte penetration, increase battery internal resistance, and consequently lead to excessive heat generation during discharge, affecting battery safety performance.

[0083] Therefore, this application employs a base film with a thickness of 4μm to 12μm. This reduces the impact of an excessively thin base film (less than 4μm) on battery safety performance, while also mitigating the adverse effects of an excessively thick base film (greater than 12μm) on battery energy density. This results in a battery with enhanced safety performance while maintaining a high energy density. Specifically, a base film with a thickness within the above range maintains structural stability at high temperatures, effectively reducing the risk of short circuits between the positive and negative electrodes. Particularly during battery thermal runaway, a base film with a stable structure can suppress side reactions through physical isolation, thereby lowering the maximum temperature of battery thermal runaway, reducing the risk of heat propagation, and improving battery safety performance. Furthermore, a base film thickness within the above range helps reduce the adverse effects of the base film on battery energy density, thus helping the battery maintain a high energy density. The base film also possesses nanopores with an average pore size of 10nm to 300nm. By employing a base film with an average pore size within this range, it helps to improve the migration rate of lithium ions while also suppressing the growth of lithium dendrites, thereby further enhancing battery safety performance.

[0084] In this application, "compacted density" refers to the mass per unit volume of active materials (positive electrode active material and negative electrode active material) after being compacted under certain pressure conditions. In battery systems, it is mainly used to describe the density of the positive electrode film and the negative electrode film, reflecting the amount of active material loaded per unit volume of electrode film.

[0085] In this application, the compaction density can be tested using methods known in the art. The electrode to be tested (positive electrode, negative electrode) can be a pre-prepared electrode or an electrode obtained by disassembling a battery. For example, a positive electrode can be obtained by disassembling a battery, and the positive electrode layer can be cut into pieces with an area of ​​1540.25 mm². 2A circular wafer is weighed, and its mass is recorded as m1. The thickness of the wafer is measured multiple times at different locations using a micrometer, and the average value is recorded as d1. Then, the positive electrode film layer disposed on one side of the wafer is removed, and the mass of the wafer is recorded as m2. The thickness of the wafer is measured multiple times at different locations using a micrometer, and the average value is recorded as d2. (m1-m2) / (d1-d2) is taken as the compaction density of the positive electrode film layer.

[0086] In this application, the compaction density of the positive electrode film is 2 g / cm³. 3 Up to 3g / cm 3 For example, the compaction density of the positive electrode film can be 2 g / cm³. 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm 3 2.4g / cm 3 2.5g / cm 3 2.60g / cm 3 2.7 / cm 3 2.8g / cm 3 2.9g / cm 3 3g / cm 3 Or values ​​within a range formed by any two of these values. Optionally, the compaction density of the positive electrode film is 2.3 g / cm³. 3 Up to 2.5g / cm 3 .

[0087] In this application, the compaction density of the negative electrode film is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 For example, the compaction density of the negative electrode film can be 1.4 g / cm³. 3 1.5g / cm 3 1.55g / cm 3 1.60g / cm 3 1.62g / cm 3 1.64 g / cm 3 1.65g / cm 3 1.85g / cm 3 Or the value between any two of them within a range.

[0088] In this application, the thickness of the base film can be tested using a micrometer or a ten-thousandth-degree meter. For example, the thickness can be measured multiple times at different locations on the base film using a micrometer, and the average value can be recorded as the thickness of the base film.

[0089] In this application, the thickness of the base film is from 4 μm to 12 μm. Exemplarily, the thickness of the base film is 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, or a value within a range of any two of these values. Optionally, the thickness of the base film is from 5 μm to 9 μm.

[0090] In this application, the average pore size can be tested using methods known in the art. The separator to be tested can be a pre-prepared separator or a separator obtained by disassembling a battery. For example, if the separator (base membrane) is obtained by disassembling a battery, the average pore size of the base membrane can be tested using a pore size meter (model: PMI Porometer) in accordance with GB / T 21650.2-2008.

[0091] For example, the average pore size of the nanopores is a value within the range of 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 200 nm, 300 nm, or any two of these values. The term "secondary battery" as used herein refers to a single battery cell, a battery module, or a battery pack. These will be described separately below. Unless otherwise specified, the battery referred to in this application is a secondary battery.

[0092] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, lithium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0093] Separating membrane

[0094] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0095] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0096] In some embodiments, the base film includes at least one of polyethylene base film, polypropylene base film, polyethylene-polypropylene composite base film, polyethylene nonwoven base film, polypropylene nonwoven base film, polypropylene-polyethylene-polypropylene composite base film, polyimide base film, polyimide nonwoven base film, polytetrafluoroethylene film, polytetrafluoroethylene nonwoven base film, polyvinyl chloride film, or polyvinyl chloride nonwoven base film.

[0097] In some embodiments, the porosity of the base film is 20% to 70%. Using a base film with a porosity within this range helps to improve the migration rate of lithium ions while also suppressing the growth of lithium dendrites, thereby further improving the safety performance of the battery. Exemplarily, the porosity of the base film is a value between 20%, 30%, 40%, 50%, 60%, 70%, or any two of these values. Optionally, the porosity of the base film is 35% to 42%.

[0098] In some embodiments, the separator further includes a coating disposed on at least one side of the base film. The coating comprises heat-resistant particles interwoven to form a porous structure. Compared to a coating without a porous structure, this implementation, where the heat-resistant particles interweave to form a porous structure, helps reduce the coating's obstruction to ion transport, thereby improving ion transport efficiency. Furthermore, the heat-resistant particles have a small volume change with temperature; disposing of the coating containing these particles on the base film surface helps suppress the thermal shrinkage rate of the base film, thereby further reducing the risk of short circuits between the positive and negative electrodes and improving battery safety.

[0099] In some embodiments, the coating thickness disposed on one side of the base film is from 0.011 μm to 3 μm. By selecting a coating with a thickness within this range, it is beneficial for the battery to achieve a high energy density, and also for the coating to better suppress the thermal shrinkage rate of the base film, thereby further improving the battery's safety performance. For example, the coating thickness is 0.011 μm, 0.02 μm, 0.03 μm, 0.04 μm, 0.05 μm, 0.06 μm, 0.07 μm, 0.08 μm, 0.09 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 2 μm, 3 μm, or a value within a range of any two of these values.

[0100] In some embodiments, the average particle size of the heat-resistant particles is between 5 nm and 185 nm. Heat-resistant particles with an average particle size of 5 nm to 185 nm can form a coating with a stable supporting network structure, supporting the base film while also suppressing thermal shrinkage of the base film, thereby improving the battery's safety performance. For example, the average particle size of the heat-resistant particles is 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 185 nm, or a value within a range consisting of any two of these values.

[0101] In some embodiments, the areal density of the heat-resistant particles is 0.5 mg / 1540.25 cm³. 2 Up to 2.5mg / 1540.25cm 2 By controlling the particle areal density within the aforementioned range, it is beneficial for the battery to achieve the desired energy density while also allowing the heat-resistant particles to effectively suppress the thermal shrinkage of the base film, thereby improving battery safety. For example, the areal density of the heat-resistant particles is 0.5 mg / 1540.25 cm³. 2 0.6mg / 1540.25cm 2 0.7mg / 1540.25cm 2 0.8mg / 1540.25cm 2 0.9mg / 1540.25cm 2 1mg / 1540.25cm 2 1.1mg / 1540.25cm 2 1.2mg / 1540.25cm 2 1.25mg / 1540.25cm 2 1.3mg / 1540.25cm 2 1.4mg / 1540.25cm 2 1.5mg / 1540.25cm 2 1.6mg / 1540.25cm 2 1.7mg / 1540.25cm 2 1.8mg / 1540.25cm 2 1.85mg / 1540.25cm 2 Or values ​​within a range formed by any two of these values. Optionally, the areal density of the heat-resistant particles is 1.25 mg / 1540.25 cm³. 2Up to 1.85 mg / 1540.25 cm 2 .

[0102] In some embodiments, along the thickness direction of the base film, the heat-resistant particles include first heat-resistant particles distributed on the surface of the base film. In this implementation, the first heat-resistant particles distributed on the surface of the base film can directly suppress the thermal shrinkage of the base film, thereby further improving the safety performance of the battery while also maintaining the battery's energy density.

[0103] In some embodiments, the heat-resistant particles include first heat-resistant particles and second heat-resistant particles; wherein the first heat-resistant particles are distributed on the surface of the base film, and the second heat-resistant particles are stacked on the side of the first heat-resistant particles away from the base film. In this implementation, the first and second heat-resistant particles are stacked in the thickness direction of the base film, which is beneficial to forming a thicker coating. The thicker coating can act as a more effective physical barrier, reducing the chemical erosion of the base film by the electrolyte, and also buffering the friction of the electrode sheets (positive electrode sheet and negative electrode sheet) on the base film, thereby further improving the safety performance of the battery.

[0104] In some embodiments, the heat-resistant particles include inorganic particles and binder particles. In this implementation, the binder particles act as a bridge, tightly connecting the inorganic particles together. This allows the inorganic particles to form a stable, integral structure, thereby improving the mechanical strength and stability of the coating and enabling it to better suppress the thermal shrinkage of the base film.

[0105] In some embodiments, the first glass transition temperature of the binder particles is 30°C to 75°C; and / or, the second glass transition temperature of the binder particles is -10°C to 25°C.

[0106] In some embodiments, the binder particles include one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane, or polystyrene ester.

[0107] In some embodiments, the heat-resistant particles include inorganic particles and an adhesive layer disposed on at least a portion of the surface of the inorganic particles. In this implementation, the adhesive layer is disposed on the surface of the inorganic particles, which helps to improve the adhesion between the inorganic particles and the base film, reduces the risk of the inorganic particles falling off, and helps to improve the safety performance and service life of the battery.

[0108] In some embodiments, the first glass transition temperature of the adhesive layer is 30°C to 75°C; and / or, the second glass transition temperature of the adhesive layer is -10°C to 25°C.

[0109] In some embodiments, the adhesive layer includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane, or polystyrene ester.

[0110] In some embodiments, the inorganic particles include at least one of aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, boehmite, magnesium oxide, zinc oxide, barium sulfate, magnesium nitride, or barium titanate. The volume of these substances changes little with temperature, and using them as inorganic particles helps to further suppress the thermal shrinkage of the base film, thereby further improving the safety performance of the battery.

[0111] In some embodiments, the inorganic particles constitute 5% to 30% of the coating by mass. Controlling the mass percentage of inorganic particles within this range facilitates the formation of a heat-resistant skeletal structure within the coating, effectively suppressing thermal shrinkage of the coating, improving battery safety performance, and reducing the risk of inorganic particle detachment. For example, the mass percentage of inorganic particles may be 5%, 10%, 15%, 20%, 25%, 30%, or a value within a range of any two of these values.

[0112] In this application, the mass percentage of inorganic particles can be tested using methods known in the art. The separator to be tested can be a pre-prepared separator or a separator obtained by disassembling a battery. For example, the separator is obtained by disassembling a battery, the coating is peeled off from the base film, the coating is collected, and its mass m3 is weighed. The binder particles or binder layer are dissolved using a suitable solvent, and then the inorganic particles are filtered out. The mass of the inorganic particles is weighed as m4, and m4 / m3 is the mass percentage of inorganic particles.

[0113] Negative electrode sheet

[0114] This application provides a secondary battery, including a negative electrode sheet, the negative electrode sheet including a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer including a first film layer and a second film layer disposed between the first film layer and the negative current collector, the first film layer including a first negative electrode active material, the first negative electrode active material including a first graphite material, the first graphite material I D / I G The value is 0.4 to 0.9; the second film layer includes a second negative electrode active material, which includes a second graphite material, and the second graphite material I D / I G It ranges from 0.05 to 0.2; where I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location.

[0115] The first graphite material used in this application is I D / I GA value of 0.4 to 0.9 indicates that the surface of the first graphite material has a large number of defects, which can serve as additional active sites for lithium-ion intercalation. Placing this first graphite material with more surface defects in the outer layer (first film layer) of the negative electrode sheet is beneficial to increasing the probability of contact between the additional active sites and the electrolyte, thereby improving the fast-charging performance of the battery.

[0116] Based on this, the inner layer (second film layer) of the negative electrode sheet adopts I D / I G The second graphite material has a content of 0.05 to 0.2. This second graphite material has fewer surface defects and a relatively complete structure, which helps to reduce the irreversible loss of lithium ions and thus extend the battery's lifespan.

[0117] In this application, the morphology of the negative electrode sheet can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. The latter will be used as an example to illustrate the testing process below. Specifically, the negative electrode sheet is obtained by disassembling the battery, placed in a sample holder and locked in place, and a cross-section of the negative electrode sheet is cut using an argon ion cross-section polisher (e.g., the IB-09010 CP type argon ion cross-section polisher from JEOL Corporation of Japan). A scanning electron microscope (HR-TEM Talos F200) is used to acquire a SEM image of the cross-section of the negative electrode sheet. From the cross-section SEM image, it can be seen that the negative electrode sheet includes a negative current collector, a second film layer disposed on the surface of the negative current collector, and a first film layer disposed on the surface of the second film layer.

[0118] In this application, the graphite material (first graphite material, second graphite material) I D / I G The value can be tested using methods known in the art. The graphite material to be tested can be a prepared graphite material or a graphite material obtained by disassembling a battery. The following description uses the latter as an example to illustrate the testing process. Specifically, the negative electrode sheet is obtained by disassembling the battery, the first film layer is scraped off using a scraper, the first film layer is dissolved using a suitable solvent, and then the first graphite material is filtered out. The Ig value of the first graphite material is then tested. D / I G The second film layer was scraped off using a doctor blade, dissolved in a suitable solvent, and then the second graphite material was filtered out. The I content of the second graphite material was then tested. D / I G It should be noted that there may or may not be a clear interface at the junction of the first and second membrane layers. To reduce sampling error, the sampling area for the second membrane layer is defined as extending 2.5 μm from the surface of the negative electrode membrane layer closest to the current collector towards the negative electrode membrane layer; the sampling area for the first membrane layer is defined as extending 2.5 μm from the surface of the negative electrode membrane layer furthest from the current collector towards the negative electrode membrane layer.

[0119] The test conditions were: excitation wavelength of 532 nm, grating of 600 lines, objective lens of 50x, integration time of 10 s, cumulative count of 3, area scan, obtaining the D peak and G peak intensities of 100 points, and calculating the I values ​​of 100 points. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G The testing instrument can be a Horiba Lab RAMHR800 Raman spectrometer.

[0120] In this application, the first graphite material I D / I G The value is between 0.4 and 0.9. For example, the first graphite material I... D / I G The value is a range of 0.4, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, or any two values, but is not limited to these.

[0121] In this application, the second graphite material I D / I G The value is between 0.05 and 0.2. For example, the second graphite material I... D / I G The value is a range of 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, or any two values, but is not limited to these.

[0122] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0123] In some embodiments, the volume average particle size of the first graphite material is between 9.2 μm and 15.5 μm. This facilitates the formation of abundant porous structures between the particles of the first graphite material, thereby improving the lithium-ion and electron transport performance in the first film layer, and thus enhancing the kinetic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the first graphite material is a value between 9.2 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 15.5 μm, or any two of these values, but is not limited thereto.

[0124] In some embodiments, the volume average particle size Dv50 of the second graphite material is between 16.3 μm and 25.5 μm. This is beneficial for increasing the compaction density of the second film layer, thereby increasing the energy density of the secondary battery. Furthermore, it facilitates the formation of abundant porous structures between the particles of the second graphite material, improving the lithium-ion and electron transport performance in the negative electrode film layer, and thus enhancing the kinetic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the second graphite material is a value between 16.3 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25.5 μm, or any two of these values, but is not limited thereto.

[0125] In this application, the volumetric particle size Dv50 represents the particle size corresponding to a cumulative volumetric distribution percentage of 50%, which can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0126] In some embodiments, the BET specific surface area of ​​the first graphite material is 0.3 m². 2 / g to 3m 2 / g. When the BET specific surface area of ​​the first graphite material is within the above range, the surface of the first graphite material can provide more channels for lithium-ion insertion and extraction. During fast charging, lithium ions can be inserted into the first graphite material more quickly through these channels, reducing the lithium-ion transport distance and resistance, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of ​​the first graphite material is 0.3m². 2 / g, 0.4m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g、1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g or any range of values ​​between two numbers, but not limited to these.

[0127] In some embodiments, the BET specific surface area of ​​the second graphite material is 0.5 m². 2 / g to 5m 2 / g. When the BET specific surface area of ​​the second graphite material is within the above range, the surface of the second graphite material can provide more channels for lithium-ion insertion and extraction. During fast charging, lithium ions can be inserted into the second graphite material more quickly through these channels, reducing the lithium-ion transport distance and resistance, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of ​​the second graphite material is 0.5m². 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g、1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g or any range of values ​​between two numbers, but not limited to these.

[0128] In this application, the specific surface area (BET) of the materials (second graphite material, first graphite material, and first positive electrode active material) has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0129] In some embodiments, the tap density of the first graphite material is 0.7 g / cm³. 3 Up to 1.6 g / cm 3 The tap density of the first graphite material is within the aforementioned range, which is beneficial for the formation of a rich porous structure in the first film layer, ensuring that the battery has superior fast-charging performance. For example, the tap density of the first graphite material is 0.7 g / cm³. 3 0.8g / cm 3 0.9g / cm 3 1g / cm 3 1.1g / cm 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm3 Or the value between any two values ​​within a range, but not limited to this.

[0130] In some embodiments, the tap density of the second graphite material is 0.8 g / cm³. 3 Up to 1.5g / cm 3 The tap density of the second graphite material is within the aforementioned range. This allows the second graphite material to be densely packed within the second film layer, which is beneficial for accommodating more second graphite material (active material) within the second film layer, thereby improving the energy density of the battery. For example, the tap density of the second graphite material is 0.8 g / cm³. 3 0.9g / cm 3 1g / cm 3 1.1g / cm 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 Or the value between any two values ​​within a range, but not limited to this.

[0131] In this application, the tap density of the materials (second graphite material, first graphite material, and first positive electrode active material) has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a powder tap density tester according to GB / T 5162-2006. The testing instrument can be Dandong Baite BT-301, with the following testing parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, number of vibrations 5000 times, and a measuring cylinder of 25mL.

[0132] In some embodiments, the first negative electrode active material further includes a negative electrode coating layer distributed on the surface of the first graphite material, the negative electrode coating layer comprising amorphous carbon. Amorphous carbon has a relatively loose structure with abundant pores. These pores can provide more diffusion channels for lithium ions, shortening the diffusion path of lithium ions in the electrode material. By placing amorphous carbon on the surface of the first graphite material, lithium ions can more quickly pass through the negative electrode coating layer to reach the interior of the first graphite material, thereby improving the fast-charging performance of the battery.

[0133] In this application, TEM can be used to characterize the negative electrode coating layer on the surface of the first graphite material. SEM is used to collect the microstructure information of the first negative electrode active material. TEM observations show that the first graphite material has a periodically repeating lattice structure. If a regular lattice structure is found on the surface of the first graphite material, it indicates that amorphous carbon is present on the surface of the first graphite material.

[0134] In some embodiments, the thickness of the negative electrode coating layer is between 10 nm and 100 nm. A thickness within this range is beneficial for the secondary battery to balance fast-charging performance and energy density. Exemplarily, the thickness of the negative electrode coating layer is a value between 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two of these values, but is not limited to this.

[0135] In this application, scanning electron microscopy (SEM) combined with focused ion beam (FIB) can be used to characterize the thickness of the negative electrode coating. The sample (first negative electrode active material) is fixed on the sample stage, and a flat cross-section is cut out of the sample surface using FIB. Then, SEM is used to image the cross-section, and the thickness of the negative electrode coating is determined by measuring the width of the negative electrode coating in the image.

[0136] In some embodiments, the second film layer further includes a first graphite material. The second graphite material can reduce the irreversible loss of active lithium ions during multiple charge-discharge cycles, slowing down the rate of battery capacity decay and extending battery life. The first graphite material can improve the battery's fast-charging performance. In this embodiment, by arranging the second graphite material and the first graphite material in the second film layer, it is beneficial to improve the battery's fast-charging performance.

[0137] In this application, the second film layer comprises a second graphite material and a first graphite material, and can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. Specifically, the negative electrode sheet is mounted and locked in a sample holder, and an argon ion cross-section polisher (e.g., the JEOL IB-09010 CP type argon ion cross-section polisher) is used to cut a cross-section of the negative electrode sheet. A scanning electron microscope (HR-TEM Talos F200) is used to acquire a SEM image of the negative electrode sheet cross-section. From the cross-section SEM image, it can be seen that the negative electrode sheet includes a negative current collector, a second film layer disposed on the surface of the negative current collector, and a first film layer disposed on the surface of the second film layer. The second film layer comprises large particles and small particles. Among them, the large particles are the second graphite material, and the small particles are the first graphite material.

[0138] In some embodiments, the mass ratio of the second graphite material to the first graphite material in the second film layer is (3-5):(5-7). By controlling the mass ratio of the second graphite material to the first graphite material in the second film layer within the above range, it is beneficial to further improve the fast-charging performance and lifespan of the battery. For example, the mass ratio of the second graphite material to the first graphite material in the second film layer is 5:5, 4:6, 3:7, or any value within a range of two such values, but is not limited thereto.

[0139] In this application, the mass ratio of the second graphite material to the first graphite material in the second film layer can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. Specifically, the negative electrode sheet is obtained by disassembling the battery, and the first film layer is removed using a scraper to expose the second film layer. The second film layer is then removed using a scraper and collected. The second film layer is dissolved using a suitable solvent, filtered to obtain a mixture of the second graphite material and the first graphite material, and separated using the particle size difference between the two materials. The mass of the second graphite material and the mass of the first graphite material are weighed separately to determine the mass ratio of the second graphite material to the first graphite material in the second film layer.

[0140] In some embodiments, the ratio of the average thickness of the first film layer to the average thickness of the second film layer is (3-5):(5-7). By controlling the ratio of the average thickness of the first film layer to the average thickness of the second film layer within the above range, it is beneficial to further improve the fast-charging performance and lifespan of the battery. Exemplarily, the ratio of the average thickness of the first film layer to the average thickness of the second film layer is a value between 5:5, 4:6, 3:7, or any two of these values, but is not limited thereto.

[0141] In this application, the ratio of the average thickness of the first film layer to the average thickness of the second film layer can be tested using methods known in the art. The negative electrode sheet to be tested can be a fabricated negative electrode sheet or a negative electrode sheet obtained by disassembling a battery. Specifically, a scanning electron microscope (HR-TEM Talos F200) is used to acquire a cross-sectional SEM image of the negative electrode sheet. The cross-sectional SEM image clearly shows that the negative electrode sheet consists of a negative current collector, a second film layer located on the surface of the current collector, and a first film layer disposed on the surface of the second film layer. Subsequently, four sampling points are uniformly selected on the first and second film layers respectively, and the thickness of the film layer at each sampling point is accurately measured using image analysis software. The average thickness of the first film layer is obtained by averaging the thickness values ​​of the four sampling points; similarly, the average thickness of the second film layer is obtained. Finally, by calculating the ratio of the two average thicknesses, the ratio of the average thickness of the first film layer to the average thickness of the second film layer can be accurately determined.

[0142] In some embodiments, the first membrane layer further includes a first adhesive, which includes at least one of styrene-butadiene rubber, polystyrene-butadiene rubber, lithium polyacrylate, polyacrylate, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan. The above substances have high viscosity. By selecting the above substances as the first adhesive, the risk of the first membrane layer separating from the second membrane layer can be reduced, thereby helping to further improve the service life of the secondary battery.

[0143] In some embodiments, the first adhesive includes at least one of styrene-butadiene rubber, lithium polyacrylate, and polyacrylate.

[0144] Styrene-butadiene rubber (SBR) has good flexibility and elasticity. It can disperse the stress borne by the first negative electrode active material during the compaction process during electrode charging and discharging, so that the first negative electrode active material can withstand greater pressure. By selecting SBR as the first dispersant, it is beneficial to achieve a high compaction density of the first film layer, thereby improving the energy density of the battery.

[0145] Lithium polyacrylate has high ionic conductivity, and using it as the first binder is beneficial to improving the fast charging performance of the battery.

[0146] Polyacrylate can improve the compressive modulus of the first negative electrode active material and reduce the adverse effects of compression on the dynamic performance of the negative electrode sheet, thereby improving the fast charging performance of the battery.

[0147] In some embodiments, the second membrane layer further includes a second binder, which includes at least one selected from styrene-butadiene rubber, polystyrene-butadiene rubber, lithium polyacrylate, polyacrylate, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethyl methacrylate, or carboxymethyl chitosan. The aforementioned second binders have high viscosity; by selecting these substances as the second binder, the risk of the second membrane layer detaching from the negative electrode current collector can be reduced, thereby further improving the service life of the secondary battery.

[0148] In some embodiments, the second binder includes at least one of styrene-butadiene rubber, lithium polyacrylate, and polyacrylate. In this embodiment, selecting the above-mentioned substances as the second binder is beneficial for balancing the battery's fast-charging performance, lifespan, and energy density.

[0149] In some embodiments, the mass percentage of the first binder in the first film layer is 0.1% to 2%. By controlling the mass percentage of the first binder in the first film layer within the above range, it is beneficial to prevent the pulverization and detachment of the first negative electrode active material, thereby extending the battery's lifespan. Furthermore, it is beneficial for the battery to achieve a high energy density. For example, the mass percentage of the first binder in the first film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, or any value within a range of two such values.

[0150] In some embodiments, the mass percentage of the second binder in the second film layer is 0.1% to 2%. By controlling the mass percentage of the second binder in the second film layer within the above range, it is beneficial to prevent the pulverization and detachment of the second negative electrode active material, thereby extending the battery's lifespan. Furthermore, it is beneficial for the battery to achieve a high energy density. For example, the mass percentage of the second binder in the second film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, or any value within a range of two such values.

[0151] In some embodiments, the first film layer further includes a first dispersant, which includes at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, and a polymer of formula (III).

[0152] The structural formula of equation (III) is: Where n is 1000-10000, preferably 3000-6000.

[0153] The aforementioned substances have high ionic conductivity, and using them as the first dispersant is beneficial to improving the fast-charging performance of the battery.

[0154] In some embodiments, the second film layer further includes a second dispersant, which includes at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, and the polymer shown in formula (III). These substances have high ionic conductivity, and selecting them as the second dispersant is beneficial for improving the fast-charging performance of the battery.

[0155] In some embodiments, the first dispersant includes lithium carboxymethyl cellulose. Lithium carboxymethyl cellulose contains lithium ions, which can compensate for the loss of active lithium ions during the first charge due to the formation of the SEI film, thereby improving the first charge efficiency of the battery.

[0156] In some embodiments, the second dispersant includes lithium carboxymethyl cellulose. Lithium carboxymethyl cellulose contains lithium ions, which can compensate for the loss of active lithium ions during the first charge due to the formation of the SEI film, thereby improving the first charge efficiency of the battery.

[0157] In some embodiments, the mass percentage of the first dispersant in the first film layer is 0.3% to 1.5%. By controlling the mass percentage of the first dispersant in the first film layer within the above range, it is beneficial for the battery to achieve a high energy density. For example, the mass percentage of the first dispersant is 0.3%, 0.5%, 1%, 1.5%, or a value within a range of any two values.

[0158] In some embodiments, the mass percentage of the second dispersant in the second film layer is 0.3% to 1.5%. By controlling the mass percentage of the second dispersant in the second film layer within the above range, it is beneficial for the battery to achieve a high energy density. For example, the mass percentage of the second dispersant is 0.3%, 0.5%, 1%, 1.5%, or a value within a range of any two values.

[0159] In some embodiments, the first film layer further includes a first conductive agent, which includes at least one selected from superconducting carbon, acetylene black, Ketjen black, carbon dots, graphene, carbon nanofibers, carbon nanotubes, and conductive carbon black. These substances possess good electrical conductivity, and using them as the first conductive agent helps to further improve the battery's fast-charging performance. Optionally, the first conductive agent may include carbon nanotubes and conductive carbon black; selecting these substances as the first conductive agent further enhances the battery's fast-charging performance.

[0160] In some embodiments, the second film layer further includes a second conductive agent, which includes at least one selected from superconducting carbon, acetylene black, Ketjen black, carbon dots, graphene, carbon nanofibers, carbon nanotubes, and conductive carbon black. These substances possess excellent electrical conductivity, and using them as the second conductive agent helps to further improve the fast-charging performance of the battery. Optionally, the second conductive agent includes carbon nanotubes and conductive carbon black.

[0161] In some embodiments, the mass percentage of the first conductive agent in the first film layer is 0.1% to 2%. By controlling the mass percentage of the first conductive agent in the first film layer within the above range, it is beneficial to improve the overall conductivity of the first film layer, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of the first conductive agent in the first film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, or a value within any range of two such values.

[0162] In some embodiments, the mass percentage of the second conductive agent in the second film layer is 0.1% to 2%. By controlling the mass percentage of the second conductive agent in the second film layer within the above range, it is beneficial to improve the overall conductivity of the second film layer, thereby further improving the fast-charging performance of the secondary battery. For example, the mass percentage of the second conductive agent in the second film layer is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, or a value within any range of two such values.

[0163] In some embodiments, the thickness of the negative electrode film layer disposed on one side of the negative electrode current collector is 40 μm to 75 μm. By keeping the thickness of the negative electrode film layer within the above range, it is beneficial for the negative electrode film layer to achieve both high capacity, high lithium-ion and electron transport performance, thereby benefiting the secondary battery to achieve both high energy density and fast charging performance. For example, the thickness of the negative electrode film layer can be 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, or a value within a range consisting of any two of these values.

[0164] In some embodiments, the coating weight of the negative electrode film layer disposed on one side of the negative electrode current collector is 80 mg / 1540.25 mm. 2 Up to 170mg / 1540.25mm 2 By controlling the coating weight of the negative electrode film within the aforementioned range, the number of lithium ions released per unit area of ​​the negative electrode film can be increased, thereby improving the energy density of the battery. For example, the coating weight of the negative electrode film is 80 mg / 1540.25 mm². 2 90mg / 1540.25mm 2 100mg / 1540.25mm 2 110mg / 1540.25mm 2 120mg / 1540.25mm 2 130mg / 1540.25mm 2 140mg / 1540.25mm 2 150mg / 1540.25mm 2 160mg / 1540.25mm 2 170mg / 1540.25mm 2 Or the value between any two values ​​within a range.

[0165] In this application, the coating weight of the negative electrode film can be tested using methods known in the art. The negative electrode sheet to be tested can be a prepared negative electrode sheet or a negative electrode sheet obtained by disassembling the battery. Specifically, the negative electrode sheet is obtained by disassembling the battery, and the negative electrode sheet layer is cut into pieces with an area of ​​1540.25 mm². 2 Take a disc, weigh it as m5, then remove the negative electrode film layer on one side of the disc, weigh the disc as m6, and take m5-m6 as the coating weight of the negative electrode film layer.

[0166] In some embodiments, the thickness of the negative electrode current collector is between 4 μm and 8 μm. A thickness within this range is beneficial for achieving high energy density in the battery and also helps reduce the risk of cracking in the negative electrode current collector, thereby extending the battery's lifespan. For example, the thickness of the negative electrode current collector is 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, or a value within a range of any two of these values.

[0167] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0168] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0169] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0170] Positive electrode sheet

[0171] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material comprising a lithium phosphate with an olivine structure. The lithium phosphate with an olivine structure has a stable three-dimensional crystal lattice structure. During lithium ion insertion and extraction, this structure remains relatively stable and is not prone to structural collapse or deformation. This allows the lithium phosphate with an olivine structure to withstand multiple charge-discharge cycles without damage, thereby extending the battery's lifespan.

[0172] In this application, examples of lithium phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0173] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0174] In some embodiments, the olivine-structured lithium phosphate comprises a compound as shown in formula (I): LiFe 1-x-y Mn x M 1 y PO4, Equation (I); in Equation (I), M 1 The lithium phosphate is selected from at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb, where 0 ≤ x ≤ 1 and 0 ≤ y < 1. The crystal structure of the above-mentioned lithium phosphate is stable and not prone to phase transitions, thereby further improving the safety performance of the secondary battery. For example, x is a value between 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any two of these values. For example, y is a value between 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any two of these values.

[0175] In some embodiments, the first positive electrode active material further includes a positive electrode coating layer disposed on at least a portion of the surface of the lithium phosphate-containing material, and the positive electrode coating layer includes at least one of a fast ion conductor material and a carbon material. The carbon material is porous, enabling sufficient and effective contact between the electrolyte and the lithium iron phosphate substrate, thereby improving the wetting performance of the electrolyte on the positive electrode film layer and thus enhancing the fast-charging performance of the secondary battery. Fast ion conductor materials have high ionic conductivity; therefore, selecting a positive electrode coating layer containing a fast ion conductor material is beneficial for improving the fast-charging performance of the battery.

[0176] In some embodiments, a positive electrode coating layer is disposed on the surface of the lithium phosphate, and the positive electrode film layer includes a fast ion conductor material and a carbon material.

[0177] In some embodiments, the positive electrode coating layer includes a first coating layer and a second coating layer; the first coating layer includes a fast ion conductor material, and the second coating layer includes a carbon material; the first coating layer is disposed between the lithium phosphate and the second coating layer. Carbon materials generally have good adhesion properties. In this embodiment, placing the second coating layer containing carbon material on the outer surface of the first positive electrode active material helps to enhance the adhesion between the first positive electrode active material and the current collector, reducing the risk of the first positive electrode active material detaching during battery use and improving the battery's lifespan.

[0178] In some embodiments, the positive electrode coating layer includes a first coating layer and a second coating layer; the first coating layer includes a fast ion conductor material, the second coating layer includes a carbon material, and the second coating layer is disposed between the lithium phosphate and the first coating layer.

[0179] In this embodiment, during the conduction process, lithium ions first pass through the first coating layer composed of a fast-ion conductor material, then enter the second coating layer, and finally diffuse into the lithium phosphate-containing interior. This facilitates faster migration of lithium ions within the first positive electrode active material, improving the battery's fast-charging performance. Furthermore, the carbon material in the second coating layer possesses excellent conductivity, allowing it to form a continuous conductive network between the lithium phosphate-containing interior and the first coating layer, accelerating electron transport and further enhancing the battery's fast-charging performance.

[0180] In some embodiments, the mass ratio of fast ion conductor material to carbon material in the positive electrode coating layer is (0-100):(100-0). Controlling the mass ratio of fast ion conductor material to carbon material within this range is beneficial for improving the fast-charging performance of the battery. Exemplarily, the mass ratio of fast ion conductor material to carbon material is 0:100, 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, 90:10, or a value within a range of any two of these values.

[0181] In some embodiments, the fast ion conductor material comprises a compound as shown in formula (II):

[0182] Li 3-b Fe 2-b M 2 b (PO4)3 (II);

[0183] In equation (II), M 2The material is selected from at least one of Ti, Zr, Hf, Ge, and Sn, where 0 ≤ b ≤ 1. The aforementioned fast ion conductor materials possess excellent ionic conductivity, and selecting these materials can further improve the fast charging performance of the battery. For example, b is a value between 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any two of these values.

[0184] In some embodiments, the mass percentage of carbon in the first positive electrode active material is 1% to 1.5%. By controlling the mass percentage of carbon in the first positive electrode active material within the above range, it is beneficial to achieve high capacity in the first positive electrode active material, which in turn is beneficial to achieve high energy density in the secondary battery. On the other hand, it is beneficial to improve the electronic conductivity of the first positive electrode active material, which in turn improves the fast charging performance of the lithium-ion secondary battery.

[0185] In this application, the mass percentage of carbon can be tested using methods known in the art. The positive electrode to be tested can be a prepared positive electrode or a positive electrode obtained by disassembling a battery. Specifically, the positive electrode is obtained by disassembling the battery, the positive electrode film is peeled off from the positive electrode current collector, the positive electrode film is collected, and then the positive electrode film is dissolved with a suitable solvent. The first positive electrode active material is then separated, and the mass percentage of carbon in the first positive electrode active material can be determined using a carbon-sulfur analyzer, referring to standard GB / T20123-2006.

[0186] In some embodiments, the BET specific surface area of ​​the first positive electrode active material is 12 m². 2 / g to 16m 2 / g.

[0187] When the BET specific surface area of ​​the first positive electrode active material is within the aforementioned range, the surface of the first positive electrode active material can provide more channels for lithium-ion insertion and extraction. During fast charging, lithium ions can be inserted into the first positive electrode active material more quickly through these channels, reducing the lithium-ion transport distance and resistance, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of ​​the first positive electrode active material is 12m². 2 / g, 12.2m 2 / g, 12.4m 2 / g, 12.6m 2 / g, 12.8m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g, 16m 2 / g or any range of values ​​between two numbers, but not limited to these.

[0188] In some embodiments, the tap density of the first positive electrode active material is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 The tap density of the first positive electrode active material is within the above-mentioned range, which is beneficial for the formation of a rich porous structure in the positive electrode film, ensuring that the battery has better fast-charging performance. For example, the tap density of the first positive electrode active material is 0.8 g / cm³. 3 1.1g / cm 3 0.9g / cm 3 1.0g / cm 3 1.05g / cm 3 1.15g / cm 3 1.2g / cm 3 1.25g / cm 3 1.28g / cm 3 1.3g / cm 3 Or the value between any two values ​​within a range, but not limited to this.

[0189] In some embodiments, the volume average particle size of the first positive electrode active material is 1 μm to 3 μm. This facilitates the formation of abundant porous structures between the particles of the first positive electrode active material, improving the lithium-ion and electron transport performance in the positive electrode film, thereby enhancing the kinetic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the first positive electrode active material is a value between 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, or any two of these values, but is not limited thereto.

[0190] In some embodiments, the compaction density of the first positive electrode active material at 50,000 N is 2.5 g / cm³. 3 Up to 2.6 g / cm 3 The compaction density of the first positive electrode active material is within the aforementioned range, resulting in a tighter contact between the first positive electrode materials, which is beneficial for improving the energy density of the battery. For example, the compaction density of the first positive electrode active material is 2.5 g / cm³. 3 2.51g / cm 3 2.52g / cm 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 32.58g / cm 3 2.59g / cm 3 2.60g / cm 3 Or the value between any two of them within a range.

[0191] In some embodiments, the positive electrode active material further includes a second positive electrode active material, which includes a lithium transition metal oxide. Lithium transition metal oxides have a higher specific capacity, and by selecting a lithium transition metal oxide as the second positive electrode active material, it is beneficial to further improve the energy density of the secondary battery.

[0192] In this application, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.1 Al 0.05 At least one of O2 and its modified compounds.

[0193] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0194] In the examples of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of oxygen will fluctuate.

[0195] In some embodiments, the mass ratio of the first positive electrode active material to the second positive electrode active material is (99-90):(1-10). By controlling the mass ratio of the first positive electrode active material to the second positive electrode active material within the above range, it is beneficial to balance the service life and energy density of the secondary battery. For example, the mass ratio of the first positive electrode active material to the second positive electrode active material is 99:1, 98:2, 97:3, 96:4, 95:5, 94:6, 93:7, 92:8, 91:9, 90:10, or a value within a range of any two of these values.

[0196] In some embodiments, the thickness of the positive electrode film layer disposed on one side of the positive electrode current collector is 100 μm to 200 μm. By keeping the thickness of the positive electrode film layer within this range, it is beneficial for the positive electrode film layer to achieve both high capacity, high lithium-ion and electron transport performance, thereby benefiting the secondary battery to achieve both high energy density and fast charging performance. For example, the thickness of the positive electrode film layer can be 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, or a value within a range consisting of any two of these values.

[0197] In some embodiments, the coating weight of the positive electrode film layer disposed on one side of the positive electrode current collector is 200 mg / 1540.25 mm. 2 Up to 400mg / 1540.25mm 2 By controlling the coating weight of the positive electrode film within the aforementioned range, the number of lithium ions released per unit area of ​​the positive electrode film can be increased, thereby improving the energy density of the battery. For example, the coating weight of the positive electrode film is 200 mg / 1540.25 mm². 2 220mg / 1540.25mm 2 240mg / 1540.25mm 2 260mg / 1540.25mm 2 280mg / 1540.25mm 2 300mg / 1540.25mm 2 310mg / 1540.25mm 2 320mg / 1540.25mm 2 330mg / 1540.25mm 2 340mg / 1540.25mm 2350mg / 1540.25mm 2 360mg / 1540.25mm 2 370mg / 1540.25mm 2 380mg / 1540.25mm 2 390mg / 1540.25mm 2 400mg / 1540.25mm 2 Or a value within a range formed by any two values. Optionally, the coating weight of the positive electrode film is 250 / 1540.25 mm. 2 Up to 320 / 1540.25mm 2 .

[0198] In this application, the coating weight of the positive electrode film can be tested using methods known in the art. The positive electrode sheet to be tested can be a pre-prepared positive electrode sheet or a positive electrode sheet obtained by disassembling the battery. Specifically, the positive electrode sheet is obtained by disassembling the battery and cut into pieces with an area of ​​1540.25 mm². 2 The disc is weighed and its mass is m7. Then the positive electrode film layer set on one side of the disc is removed and the mass of the disc is weighed and its mass is m8. The difference between m7 and m8 is taken as the coating weight of the positive electrode film layer.

[0199] In some embodiments, the compaction density of the positive electrode film is 2 g / cm³. 3 Up to 3g / cm 3 By maintaining the compaction density of the positive electrode film within the aforementioned range, it is beneficial to preserve a good pore structure, reduce the tortuosity of the positive electrode film, and shorten the lithium-ion transport path, thereby improving the battery's fast-charging performance and lifespan while also achieving high energy density. For example, the compaction density of the positive electrode film can be 2 g / cm³. 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm 3 2.4g / cm 3 2.5g / cm 3 2.60g / cm 3 2.7 / cm 3 2.8g / cm 3 2.9g / cm 3 3g / cm 3 Or values ​​within a range formed by any two of these values. Optionally, the compaction density of the positive electrode film is 2.3 g / cm³. 3 Up to 2.5g / cm 3 .

[0200] In some embodiments, the thickness of the positive electrode current collector is between 10 μm and 18 μm. A thickness within this range is beneficial for achieving high energy density in the battery and also helps reduce the risk of cracking in the positive electrode current collector, thereby extending the battery's lifespan. Exemplarily, the thickness of the positive electrode current collector is 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, 18 μm, or a value within the range of any two of these values. Optionally, the thickness of the positive electrode current collector is between 13 μm and 15 μm.

[0201] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0202] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0203] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0204] In some embodiments, the positive electrode film layer further includes a positive electrode dispersant, which includes at least one of polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, and sodium carboxymethyl cellulose. The above-mentioned positive electrode dispersant has good flexibility and elasticity, and can disperse the stress borne by the positive electrode active material during compaction. By selecting the above-mentioned positive electrode dispersant, it is beneficial to achieve a high compaction density of the positive electrode film layer, thereby improving the energy density of the battery.

[0205] In some embodiments, the mass percentage of the positive electrode dispersant relative to the positive electrode film layer is 0.3% to 5%. Optionally, it is 0.1% to 2%, further preferably 0.1% to 1%, and even more preferably 0.5% to 0.8%. By controlling the mass percentage of the positive electrode dispersant in the positive electrode film layer within the above range, it is beneficial for the battery to achieve a high energy density. Exemplarily, the mass percentage of the positive electrode dispersant is a value within the range of any two values ​​formed by 0.3%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, and 5%.

[0206] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0207] electrolytes

[0208] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0209] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0210] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0211] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0212] In some embodiments, the electrolyte further includes additives, including at least one selected from barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tris(trimethylsilane) phosphate, and vinylene carbonate. During the first charge and discharge of the battery, the above-mentioned additives preferentially undergo electrochemical reduction reactions on the surface of the negative electrode plate, forming a dense and stable SEI film, which helps to suppress the growth of lithium dendrites and improve the safety performance of the battery.

[0213] In some embodiments, the additive comprises 1% to 10% of the electrolyte by mass. Controlling the additive's mass percentage within this range facilitates the formation of an SEI film of appropriate thickness on the electrode surface. This helps suppress lithium dendrite growth and improve battery safety. Furthermore, it helps to mitigate the lithium-ion transport resistance on the negative electrode, thereby ensuring the battery's fast-charging performance. Exemplarily, the additive's mass percentage is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within a range of any two of these values.

[0214] In some embodiments, the conductivity of the electrolyte is from 10 mS / cm to 18.5 mS / cm. By selecting an electrolyte with a conductivity within this range, the internal impedance of the battery can be reduced, thereby reducing energy loss due to resistance during charging and discharging and extending the battery's lifespan. Exemplarily, the conductivity of the electrolyte is a value between 10 mS / cm, 11 mS / cm, 12 mS / cm, 13 mS / cm, 14 mS / cm, 15 mS / cm, 16 mS / cm, 17 mS / cm, 18 mS / cm, 18.5 mS / cm, or any two of these values. Optionally, the conductivity of the electrolyte is from 14 mS / cm to 16.8 mS / cm.

[0215] In some embodiments, the electrolyte may optionally include additives. For example, it may include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0216] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0217] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0218] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0219] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 The example shown is a square-structured battery cell 5.

[0220] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0221] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0222] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0223] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0224] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0225] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0226] Electrical appliances

[0227] This application also provides an electrical device, which includes the secondary battery provided in the first aspect of this application. The secondary battery may include at least one of a battery cell, a battery module, or a battery pack. The secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0228] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0229] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0230] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0231] Example

[0232] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0233] I. Preparation of the first and second graphite materials:

[0234] Preparation Example 1-1

[0235] Preparation of the first graphite material (Material 1-1):

[0236] Step 1: The oil-based calcined needle coke raw material is crushed and shaped using an air jet mill to obtain a shaped material with a Dv50 of 15μm;

[0237] Step 2: Using granulated asphalt (softening point 125℃; coking value 51%) as a granulating agent, the above-mentioned shaping material is granulated together in a granulation kettle to obtain granulated material, wherein the mass ratio of shaping material to granulating agent is 88:7.

[0238] Step 3: The above granulated material is pre-carbonized at 1450℃ for 5.2 hours under a nitrogen atmosphere to obtain an intermediate.

[0239] Step 4: The above intermediate is graphitized at a high temperature of 3000℃. The graphitized particles are then sieved and demagnetized to obtain the first graphite material.

[0240] Preparation Example 2-1

[0241] Preparation of the second graphite material (Material 2-1):

[0242] Step 1: The oil-based calcined needle coke raw material is crushed and shaped using an air jet mill to obtain a shaped material with a Dv50 of 22μm;

[0243] Step 2: Using granulated asphalt (softening point 113℃; coking value 66%) as a granulating agent, the above-mentioned shaping material is granulated together in a granulation kettle to obtain granulated material, wherein the mass ratio of shaping material to granulating agent is 90:6.

[0244] Step 3: The above granulated material is pre-carbonized at 1480℃ for 4.7 hours under a nitrogen atmosphere to obtain an intermediate.

[0245] Step 4: The above intermediate is graphitized at a high temperature of 3000℃. The graphitized particles are then sieved and demagnetized to obtain the second graphite material.

[0246] II. Testing of the first and second graphite materials:

[0247] (1) I D / I G The test:

[0248] The sample was tested using a Raman spectrometer.

[0249] The test conditions were: excitation wavelength of 532 nm, grating of 600 lines, objective lens of 50x, integration time of 10 s, cumulative count of 3, area scan, obtaining the D peak and G peak intensities of 100 points, and calculating the I values ​​of 100 points. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I GThe testing instrument can be a Horiba LabRAMHR800 Raman spectrometer.

[0250] Material 1-1 of I D / I G It is 0.51. The I of material 2-1 D / I G It is 0.13.

[0251] (2) Test of volume average particle size Dv50:

[0252] The particle size distribution was performed using a laser particle size analyzer, in accordance with GB / T 19077-2016. The suitable instrument is the Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0253] The Dv50 of material 1-1 is 11.8 μm. The Dv50 of material 2-1 is 18.3 μm.

[0254] (3) Test of BET specific surface area:

[0255] The nitrogen adsorption specific surface area was measured according to GB / T 19587-2017, and the result was calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0256] The BET specific surface area of ​​material 1-1 is 0.94 m². 2 / g. The BET specific surface area of ​​material 2-1 is 1.78m². 2 / g.

[0257] (4) Test of tap density:

[0258] The density of the powder was determined using a powder tap density tester in accordance with GB / T 5162-2006. The testing instrument can be Dandong Baite BT-301, with the following parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, number of vibrations 5000, and a measuring cylinder of 25mL.

[0259] The tap density of material 1-1 is 1.1 g / cm³. 3 The tap density of material 2-1 is 1.05 g / cm³. 3 .

[0260] Table 1

[0261]

[0262] III. Preparation of Negative Electrode Slurry

[0263] Preparation Example 3-1

[0264] The first negative electrode slurry is prepared by mixing the first graphite material (see material 1-1 in Table 1), the first binder (styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 2:1), the first dispersant (lithium carboxymethyl cellulose), and the first conductive agent (carbon nanotubes and conductive carbon black in a mass ratio of 5:5) in a mass ratio of 96.6:1.8:0.9:0.7.

[0265] Preparation Example 3-2

[0266] The second negative electrode slurry was prepared by mixing the second graphite material (see material 2-1 in Table 1), the second binder (styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 2:1.3), the second dispersant (lithium carboxymethyl cellulose), and the second conductive agent (carbon nanotubes and conductive carbon black in a mass ratio of 5:5) in a mass ratio of 97.8:1:0.7:0.5.

[0267] Preparation Example 3-3

[0268] The third negative electrode slurry is prepared by mixing the negative electrode active materials (material 1-1 and material 2-1 with a mass ratio of 5:5), the second binder (styrene-butadiene rubber and lithium polyacrylate with a mass ratio of 2:1), the second dispersant (lithium carboxymethyl cellulose), and the second conductive agent (carbon nanotubes and conductive carbon black with a mass ratio of 5:5) in a mass ratio of 96.9:1.8:0.8:0.5.

[0269] IV. Preparation of Secondary Batteries:

[0270] Example 1

[0271] 1. Preparation of the positive electrode sheet:

[0272] The positive electrode active material (first positive electrode active material, lithium iron phosphate), conductive carbon black, polyvinylidene fluoride (PVDF), and positive electrode dispersant (polyethylene glycol octylphenyl ether) were mixed in a mass ratio of 97.6:0.1:1.8:0.5, and then N-methylpyrrolidone solvent was added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0273] 2. Preparation of the negative electrode sheet:

[0274] Using a dual-cavity coating device, a first negative electrode slurry and a second negative electrode slurry are simultaneously extruded, with a mass ratio of 5:5. After drying and cold pressing, a negative electrode sheet is obtained. The second negative electrode slurry is coated onto the negative electrode current collector (copper foil) to form a second film layer, while the first negative electrode slurry is coated onto the side of the second negative electrode slurry away from the negative electrode current collector to form a first film layer. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0275] 3. Preparation of electrolyte:

[0276] A mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was mixed at a volume ratio of 3:7 to form an organic solvent. A film-forming agent (barium sulfate, 2.5% by mass) was added, and LiPF6 was dissolved in the organic solvent to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0277] 4. Separating membrane:

[0278] A 5μm thick polyethylene base film is used, with a coating on both sides of the base film. The coating includes heat-resistant particles, which include boehmite particles (inorganic particles) and an adhesive layer set on the surface of the boehmite particles.

[0279] 5. Preparation of secondary batteries:

[0280] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then formed by bending and winding. The electrode assembly is placed in outer packaging, dried, and then injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0281] V. Testing of the morphology of the separating membrane:

[0282] The separator membrane was mounted and secured in the sample holder. A cross-sectional SEM image of the separator membrane was then acquired using a scanning electron microscope (HR-TEM Talos F200). Figure 7 As shown in the cross-sectional SEM image, the coating consists of heat-resistant particles that interweave to form a porous structure.

[0283] VI. Test of compacted density:

[0284] (1) Test of the compaction density of the positive electrode film:

[0285] First, wipe off the positive electrode sheet. Cut the positive electrode film layer on one side into a small circular piece with an area of ​​S21, weigh it, and record its weight as M21. Measure its thickness H21. Then, wipe off the positive electrode film layer of the weighed positive electrode sheet, weigh the positive current collector, and record its weight as M20. Measure its thickness H20. The coating weight on one side of the positive electrode sheet = (M21 - weight of the positive current collector M20) / S21, the thickness of the positive electrode film layer = H21 - H20, and the compaction density of the positive electrode film layer = coating weight on one side of the positive electrode film layer / thickness of the positive electrode film layer.

[0286] (2) Test of compaction density of negative electrode film:

[0287] First, wipe off the negative electrode sheet. Cut the negative electrode film layer on one side into a small circular piece with an area of ​​S11, weigh it, and record its weight as M11. Measure its thickness H11. Then, wipe off the negative electrode film layer of the weighed negative electrode sheet, weigh the negative current collector, and record its weight as M10. Measure its thickness H10. The single-sided coating weight of the negative electrode sheet = (M11 - M10) / S11, the thickness of the negative electrode film layer = H11 - H10, and the compaction density of the negative electrode film layer = single-sided coating weight of the negative electrode film layer / thickness of the negative electrode film layer.

[0288] VII. Performance Testing of Secondary Batteries:

[0289] (1) Energy density test:

[0290] At 25℃, the battery cell is charged at a constant current of 0.33C to a cutoff voltage of 3.65V, and then charged at a constant voltage of 3.65V until the current is 0.05C. At this point, the secondary battery is fully charged. After the fully charged secondary battery is left to stand for 5 minutes, it is discharged at a constant current of 0.33C to a cutoff voltage of 2.5V. The discharge capacity at this point is the actual capacity of the secondary battery at 0.33C, denoted as C0.

[0291] The secondary battery was then charged at a constant current of 0.33C0 to a cutoff voltage of 3.65V, and then charged at a constant voltage until the current reached 0.05C, at which point the secondary battery was fully charged. After the fully charged secondary battery was left to stand for 5 minutes, it was discharged at a constant current of 0.33C0 to a cutoff voltage of 2.5V, and the discharge energy Q of the secondary battery was obtained. The energy density of the secondary battery (Wh / Kg) = discharge energy Q of the secondary battery / mass M of the secondary battery. The test results are recorded in Table 2 below.

[0292] (2) Safety performance testing:

[0293] Place the fully charged secondary battery in the heating box;

[0294] The temperature of the heating chamber is gradually increased at a certain heating rate (3℃ / min);

[0295] Observe the changes in the secondary battery. When the secondary battery experiences thermal runaway, the heating box will stop heating.

[0296] Record the highest temperature at which the secondary battery experiences thermal runaway.

[0297] Examples 2 to 3

[0298] Examples 2 and 3 were prepared using the same method as Example 1, with the following differences:

[0299] The cold pressing process of the positive electrode sheet was adjusted so that the compaction density of the positive electrode film layer is shown in Table 2.

[0300] The cold pressing process of the negative electrode sheet was adjusted so that the compaction density of the negative electrode film layer is shown in Table 2.

[0301] Examples 4 to 7

[0302] Examples 4 to 7 were prepared using the same method as Example 1, except that a base film of the corresponding thickness was used as described in Table 2.

[0303] Comparative Examples 1 and 2

[0304] Comparative Examples 1 and 2 were prepared using the same method as Example 1, except that a base film of the corresponding thickness was used as described in Table 2.

[0305] The performance of the secondary batteries prepared in Examples 2 to 7 and Comparative Examples 1 and 2 were tested in the same manner as in Example 1, and the test results are recorded in Table 2.

[0306] Table 2

[0307]

[0308] In this embodiment, the highest temperature during thermal runaway is used to characterize the safety performance of the secondary battery. The higher the highest temperature during runaway, the worse the battery's safety performance. A thermal runaway temperature greater than 300°C will cause thermal propagation, severely deteriorating the battery's safety performance.

[0309] Compared with Comparative Example 1 (where the thickness of the base film is less than 4 μm), the secondary batteries prepared in Examples 1 to 7 have improved safety performance.

[0310] By comparing the data from ratio 2, it can be found that when the base film thickness exceeds 12μm, increasing the base film thickness has limited effect on improving the safety performance of the battery, and at the same time, it will reduce the energy density of the battery.

[0311] In Examples 1 to 7, the thickness of the secondary battery substrate film prepared is between 4 μm and 12 μm. Such batteries ensure both high safety performance and maintain high energy density.

[0312] Examples 8 to 11

[0313] Examples 8 to 11 were prepared using the same method as Example 1, except that the coating parameters were adjusted according to the description in Table 3-1.

[0314] Example 12

[0315] Example 12 uses the same method as Example 1 to prepare a secondary battery, the difference being that the heat-resistant particles include boehmite particles and PVDF particles.

[0316] The performance of the secondary batteries prepared in Examples 8 to 12 was tested using the same method as in Example 1, and the test results are recorded in Table 3-2.

[0317] Table 3-1

[0318]

[0319] Table 3-2

[0320]

[0321] The data in Tables 3-1 and 3-2 show that the coating thickness is set between 0.011 μm and 2 μm, the average particle size of the heat-resistant particles is between 5 nm and 185 nm, and the areal density of the heat-resistant particles is 0.5 mg / 1540.25 cm³. 2 Up to 2.5mg / 1540.25cm 2 At that time, the prepared secondary battery can achieve both high safety performance and high energy density.

[0322] Example 13

[0323] Example 13 and Example 1 were prepared using the same method to prepare secondary batteries, the difference being:

[0324] In the preparation of the negative electrode sheet, the second negative electrode slurry is replaced with the third negative electrode slurry.

[0325] Examples 14 to 17

[0326] Examples 14 to 17 were prepared using the same method as Example 13, with the following differences:

[0327] During the preparation of the negative electrode sheet, the mass ratio of the first graphite material to the second graphite material in the third negative electrode slurry and / or the mass ratio of the first negative electrode slurry to the second negative electrode slurry (corresponding to the thickness ratio of the first film layer to the second film layer in the negative electrode sheet) are adjusted according to Table 4.

[0328] Fast charging performance test:

[0329] At 25°C, the secondary battery was charged at a constant current of 0.33C to the charging cutoff voltage of 3.65V, and then charged at a constant voltage to a current of 0.05C. After standing for 5 minutes, the secondary battery was discharged at a constant current of 0.33C to the discharge cutoff voltage of 2.5V, and its actual capacity was recorded as C0.

[0330] Then, the secondary battery is charged sequentially at constant current rates of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2C0, 2.3C0, 2.5C0, and 3.0C0 until the charging cutoff voltage of 3.65V or the negative electrode cutoff potential of 0mV (whichever comes first). After each charging is completed, it is discharged at 1C0 until the discharge cutoff voltage of 2.5V. The negative electrode potential corresponding to the charging at different charging rates to 10%SOC, 20%SOC, 30%SOC, 40%SOC, 50%SOC, 60%SOC, 70%SOC, and 80%SOC (State of Charge) is recorded.

[0331] Plot the charge rate-negative electrode potential curves for different SOC states. After linear fitting, obtain the charge rate corresponding to a negative electrode potential of 0V under different SOC states. This charge rate is the charging window under that SOC state, denoted as C. 10%SOC C 20%SOC C 30%SOC C 40%SOC C 50%SOC C 60%SOC C 70%SOC C 80%SOC .

[0332] According to the following formula:

[0333]

[0334] The charging time T from 10% SOC to 80% SOC of the secondary battery was calculated (assuming no lithium plating), in minutes. The test results are recorded in Table 4.

[0335] The safety performance and energy density of the secondary batteries prepared in Examples 13 to 17 were tested using the same method as in Example 1, and the test results are recorded in Table 4.

[0336] Table 4

[0337]

[0338] The "mass ratio" in Table 4 refers to the mass ratio of the second graphite material to the first graphite material.

[0339] The data in Table 4 shows that when the second film layer includes both the first and second graphite materials, the prepared secondary battery achieves a balance between safety performance and energy density, while also further improving fast charging performance.

[0340] Examples 18 to 22

[0341] Examples 18 to 22 and Example 13 were prepared using the same method for secondary batteries, the difference being:

[0342] The battery was prepared using the first positive electrode active material listed in Table 5-1. The parameters of each first positive electrode active material are listed in Table 5-2.

[0343] The performance of the secondary batteries prepared in Examples 18 to 22 was tested using the same method as in Example 1, and the test results are recorded in Table 5-2.

[0344] Table 5-1

[0345]

[0346] Table 5-2

[0347]

[0348] The data in Tables 5-1 and 5-2 show that when the positive electrode coating includes fast ion conductor materials and / or carbon materials, the fast charging performance of the prepared secondary battery is further improved.

[0349] Examples 23 to 25

[0350] Examples 23 to 25 were prepared using the same method as Example 13, except that at least one of the following was adjusted according to Table 6-1: the ratio of each component in the positive electrode active material, the coating weight of the positive electrode slurry, and the thickness of the positive electrode current collector.

[0351] The performance of the secondary batteries prepared in Examples 23 to 25 was tested in the same manner as in Example 1, and the test results are recorded in Table 6-2.

[0352] Table 6-1

[0353]

[0354] The “mass ratio” in Table 6-1 is the mass ratio of the first positive electrode active material and the second positive electrode active material.

[0355] Table 6-2

[0356]

[0357] The data in Tables 6-1 to 6-2 show that: The positive electrode film is fully filled with lithium transition metal oxide (NCM) 811 When the energy density of the secondary battery is further improved, the energy density of the secondary battery is further improved.

[0358] Examples 26 and 27

[0359] Examples 26 and 27 were prepared using the same method as Example 13, except that the type or amount of the positive electrode dispersant (mass percentage in the positive electrode film) was adjusted according to Table 7, and the type or amount of the additive (mass percentage in the electrolyte) was adjusted according to Table 7.

[0360] The performance of the secondary batteries prepared in Examples 26 and 27 was tested in the same manner as in Example 1, and the test results are recorded in Table 7.

[0361] Table 7

[0362]

[0363] The data in Table 7 shows that when the mass percentage of the positive electrode dispersant in the positive electrode sheet is 0.3% to 5% and the mass percentage of the additive in the electrolyte is 1% to 10%, the resulting secondary battery can balance energy density and safety performance.

[0364] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator, among which, The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, wherein the compaction density of the positive electrode film layer is 2.0 g / cm³. 3 Up to 3.0 g / cm 3 ; The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, wherein the compaction density of the negative electrode film layer is 1.4 g / cm³. 3 Up to 1.85 g / cm 3 ; The isolation membrane includes a base membrane with a thickness of 4 μm to 12 μm and nanopores with an average pore size of 10 nm to 300 nm; the isolation membrane also includes a coating disposed on at least one side of the base membrane, the coating comprising heat-resistant particles. The heat-resistant particles interweave to form a porous structure. The heat-resistant particles include inorganic particles. The inorganic particles account for 5% to 30% of the mass of the coating, and the average particle size of the heat-resistant particles is 5 nm to 185 nm.

2. The secondary battery according to claim 1, characterized in that, The thickness of the base film is 5 μm to 9 μm.

3. The secondary battery according to claim 1, characterized in that, The porosity of the base membrane is 20% to 70%.

4. The secondary battery according to claim 1, characterized in that, The coating, which is applied to one side of the base film, has a thickness of 0.011 μm to 3 μm.

5. The secondary battery according to claim 1, characterized in that, The areal density of the heat-resistant particles is 0.5 mg / 1540.25 cm³. 2 Up to 2.5mg / 1540.25cm 2 .

6. The secondary battery according to claim 1, characterized in that, Along the thickness direction of the base film, the heat-resistant particles include first heat-resistant particles; Or the heat-resistant particles may include a first heat-resistant particle and a second heat-resistant particle; The first heat-resistant particles are distributed on the surface of the base film, and the second heat-resistant particles are stacked on the side of the first heat-resistant particles away from the base film.

7. The secondary battery according to claim 1, characterized in that, The heat-resistant particles also include binder particles, and / or, The heat-resistant particles also include an adhesive layer disposed on at least a portion of the surface of the inorganic particles.

8. The secondary battery according to claim 1, characterized in that, The inorganic particles include at least one of aluminum oxide, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, boehmite, magnesium oxide, zinc oxide, barium sulfate, magnesium nitride, or barium titanate.

9. The secondary battery according to claim 1, characterized in that, The negative electrode film layer includes a first film layer and a second film layer disposed between the first film layer and the negative electrode current collector. The first film layer includes a first negative electrode active material, the first negative electrode active material includes a first graphite material, and the first graphite material I D / I G The value ranges from 0.4 to 0.

9. The second film layer includes a second negative electrode active material, the second negative electrode active material includes a second graphite material, and the second graphite material I D / I G The value ranges from 0.05 to 0.

2. Among them I D This indicates that the Raman spectrum is at 1350±50 cm⁻¹. -1 D peak intensity at I G This indicates that the Raman spectrum is at 1580±50 cm⁻¹. -1 The intensity of the G peak at that location.

10. The secondary battery according to claim 9, characterized in that, The first negative electrode active material further includes a negative electrode coating layer distributed on the surface of the first graphite material, the negative electrode coating layer comprising amorphous carbon.

11. The secondary battery according to claim 10, characterized in that, The thickness of the negative electrode coating layer is 10 nm to 100 nm.

12. The secondary battery according to claim 11, characterized in that, The second film layer also includes a first graphite material.

13. The secondary battery according to claim 12, characterized in that, The mass ratio of the second graphite material to the first graphite material in the second film layer is (3-5):(5-7).

14. The secondary battery according to claim 9, characterized in that, The ratio of the average thickness of the first film layer to the average thickness of the second film layer is (3-5):(5-7).

15. The secondary battery according to claim 1, characterized in that, The negative electrode film layer satisfies at least one of the following: (1) The coating weight of the negative electrode film layer disposed on one side of the negative electrode current collector is 80 mg / 1540.25 mm. 2 Up to 170mg / 1540.25mm 2 ; (2) The thickness of the negative electrode film layer disposed on one side of the negative electrode current collector is 40 μm to 75 μm.

16. The secondary battery according to claim 1, characterized in that, The thickness of the negative electrode current collector is 4 μm to 8 μm.

17. The secondary battery according to claim 1, characterized in that, The positive electrode film layer includes a positive electrode active material, which includes a first positive electrode active material, and the first positive electrode active material includes a lithium phosphate with an olivine structure.

18. The secondary battery according to claim 17, characterized in that, The lithium phosphate with the olivine structure comprises compounds as shown in formula (I): LiFe 1-x-y Mn x M 1 y PO4, formula (I); In the above formula (I), M 1 It is selected from at least one of V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb, where 0 ≤ x ≤ 1 and 0 ≤ y < 1.

19. The secondary battery according to claim 17, characterized in that, The first positive electrode active material further includes a positive electrode coating layer, which is disposed on at least a portion of the surface of the lithium phosphate-containing material, and the positive electrode coating layer includes at least one of a fast ion conductor material and a carbon material.

20. The secondary battery according to claim 19, characterized in that, In the positive electrode coating layer, the mass ratio of the fast ion conductor material to the carbon material is (0-100):(100-0).

21. The secondary battery according to claim 19, characterized in that, The positive electrode coating layer includes a first coating layer and a second coating layer; the first coating layer includes a fast ion conductor material, and the second coating layer includes a carbon material. The first coating layer is disposed between the lithium phosphate and the second coating layer; Alternatively, the second coating layer may be disposed between the lithium phosphate and the first coating layer.

22. The secondary battery according to claim 19, characterized in that, The fast ion conductor material comprises a compound as shown in formula (II): Li 3-b Fe 2-b M 2 b (PO4)3 of formula (II); In the above formula (II), M 2 It is selected from at least one of Ti, Zr, Hf, Ge and Sn, where 0 ≤ b ≤ 1.

23. The secondary battery according to claim 19, characterized in that, The mass percentage of carbon in the first positive electrode active material is 1% to 1.5%.

24. The secondary battery according to claim 17, characterized in that, The first positive electrode active material satisfies at least one of the following: (1) The BET specific surface area of ​​the first positive electrode active material is 12m². 2 / g to 16m 2 / g; (2) The tap density of the first positive electrode active material is 0.8 g / cm³. 3 Up to 1.3 g / cm 3 ; (3) The volume average particle size Dv50 of the first positive electrode active material is 1 μm to 3 μm; (4) The compaction density of the first positive electrode active material at 50000N is 2.5g / cm³. 3 Up to 2.6 g / cm 3 .

25. The secondary battery according to claim 17, characterized in that, The positive electrode active material further includes a second positive electrode active material, which includes a lithium transition metal oxide.

26. The secondary battery according to claim 25, characterized in that, The mass ratio of the first positive electrode active material to the second positive electrode active material is (99-90):(1-10).

27. The secondary battery according to claim 1, characterized in that, The positive electrode film layer satisfies at least one of the following: (1) The thickness of the positive electrode film layer disposed on one side of the positive electrode current collector is 100 μm to 200 μm; (2) The coating weight of the positive electrode film is 200 mg / 1540.25 mm. 2 Up to 400mg / 1540.25mm 2 .

28. The secondary battery according to claim 1, characterized in that, The thickness of the positive current collector is 10 μm to 18 μm.

29. The secondary battery according to claim 1, characterized in that, The positive electrode film layer further includes a positive electrode dispersant, which includes at least one of polyethylene glycol octylphenyl ether, polyvinylpyrrolidone, and sodium carboxymethyl cellulose.

30. The secondary battery according to claim 29, characterized in that, The mass percentage of the positive electrode dispersant relative to the positive electrode film layer is 0.3% to 5%.

31. The secondary battery according to claim 1, characterized in that, The secondary battery also includes an electrolyte, which comprises an electrolyte salt and a solvent; The solvent includes at least one selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone. The electrolyte salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

32. The secondary battery according to claim 31, characterized in that, The electrolyte also includes additives, which include at least one of barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tri(trimethylsilane) phosphate, and vinylene carbonate.

33. The secondary battery according to claim 32, characterized in that, In the electrolyte, the additive accounts for 1% to 10% of the total mass.

34. The secondary battery according to claim 33, characterized in that, The conductivity of the electrolyte is from 10 mS / cm to 18.5 mS / cm.

35. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 34.

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