Secondary battery and electric device

Abstract

The invention provides a secondary battery and an electric device, and relates to the field of batteries. The secondary battery comprises a positive pole piece, a negative pole piece and an isolating membrane, the positive pole piece comprises a positive current collector and a positive membrane layer arranged on at least one side surface of the positive current collector, and the compaction density of the positive membrane layer is 2.0-3.0 g / cm < 3 >; the negative pole piece comprises a negative current collector and a negative film layer located on at least one surface of the negative current collector, and the compaction density of the negative film layer is 1.4 g / cm < 3 >-1.85 g / cm < 3 >; the isolating membrane comprises a base membrane, the thickness of the base membrane ranges from 4 micrometers to 12 micrometers, the base membrane is provided with nanopores, and the average pore size of the nanopores ranges from 10 nm to 300 nm. The secondary battery has high energy density and improved safety performance.

Description

Technical Field

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

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

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

[0004] In related technologies, increasing the compaction density of the membrane layer is often used to improve the energy density of the battery. However, this method has obvious drawbacks, because increasing the compaction density will lead to increased heat release 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 a technical problem that needs to be solved urgently. Summary of the Invention The present application is made in view of the above-mentioned problems, and its object is to provide a secondary battery and an electric device. The secondary battery has both high energy density and improved safety performance.

[0006] In order to achieve the above-mentioned object, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer provided on at least one side surface of the positive electrode current collector, and the compaction density of the positive electrode film layer is 2.0 g / cm 3 Up to 3.0g / cm 3 The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, and the compaction density of the negative electrode film layer is 1.4g / cm 3 Up to 1.85g / cm 3 ; The isolation membrane includes a base membrane, the thickness of the base membrane is 4μm to 12μm, and the base membrane has nanopores, and the average pore size of the nanopores is 10nm to 300nm.

[0007] In this application, by controlling the compaction density of the positive electrode film layer to 2.0g / cm 3 Up to 3.0g / cm 3 The compaction density of the negative electrode film is 1.4g / cm 3 Up to 1.85g / cm 3, which is conducive to the battery achieving high energy density. The present application adopts a base film with a thickness of 4μm to 12μm, which can reduce the impact of the base film thickness being too thin (less than 4μm) on the battery safety performance, and can also reduce the adverse effects of the base film thickness being too thick (greater than 12μm) on the battery energy density. The battery has enhanced battery safety performance and can ensure a higher energy density. In addition, the base film has nanopores, and the average pore size of the nanopores is 10nm to 300nm. This helps to improve the migration rate of lithium ions while also helping to inhibit the growth of lithium dendrites, thereby further improving the safety performance of the battery.

[0008] Specifically, a base film with a thickness within the aforementioned range can maintain its structural stability at high temperatures, effectively reducing the risk of short circuits between the positive and negative electrodes. In particular, during thermal runaway, the stable base film can inhibit side reactions through physical isolation, thereby lowering the maximum temperature of thermal runaway, reducing the risk of heat spread, and improving battery safety. Furthermore, a base film thickness within the aforementioned range helps minimize its adverse effects on battery energy density, thereby helping the battery maintain a high energy density.

[0009] In some embodiments, the base film has a thickness of 5 μm to 9 μm. The base film thickness within the above range is beneficial for further considering the energy density and safety performance of the battery.

[0010] In some embodiments, the porosity of the base film is 20% to 70%, which helps to increase the migration rate of lithium ions while also helping to inhibit the growth of lithium dendrites, thereby further improving the safety performance of the battery.

[0011] In some embodiments, the separator further includes a coating disposed on at least one side of the base membrane, the coating comprising heat-resistant particles interwoven to form a porous structure. Compared to coatings without a porous structure, in this implementation, the interwoven heat-resistant particles form a porous structure, which helps reduce the coating's resistance to ion transport, thereby improving ion transport efficiency. Furthermore, the coating includes heat-resistant particles, whose volume changes little with temperature. Disposing the coating containing heat-resistant particles on the surface of the base membrane helps suppress the thermal shrinkage of the base membrane, thereby further reducing the risk of short circuits between the positive and negative electrodes and improving battery safety.

[0012] In some embodiments, the thickness of the coating disposed on one side of the base film is 0.011 μm to 3 μm, which not only helps the battery achieve a high energy density, but also helps the coating better suppress the thermal shrinkage of the base film, thereby further improving the safety performance of the battery.

[0013] In some embodiments, the heat-resistant particles have an average particle size of 5 nm to 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 not only supports the base film but also suppresses thermal shrinkage of the base film, thereby improving battery safety.

[0014] In some embodiments, the particle area density of the heat-resistant particles is 0.5 mg / 1540.25 cm 2 Up to 2.5mg / 1540.25cm 2 This is beneficial for achieving energy density in the battery while also helping the heat-resistant particles to suppress the thermal shrinkage of the base film, thereby improving the safety performance of the battery.

[0015] In some embodiments, the heat-resistant particles include first heat-resistant particles along the thickness of the base film. The first heat-resistant particles are distributed on the surface of the base film and can directly inhibit thermal shrinkage of the base film, thereby further improving the safety performance of the battery while also taking into account the energy density of the battery.

[0016] In some embodiments, along the thickness of the base film, the heat-resistant particles include first and second heat-resistant particles; the first heat-resistant particles are distributed on the surface of the base film, while 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 stacked arrangement of the first and second heat-resistant particles along the thickness of the base film facilitates the formation of a thicker coating. This thicker coating acts as a more effective physical barrier, reducing chemical erosion of the base film by the electrolyte. It also buffers friction between the electrode sheets (positive and negative electrodes) and the base film, thereby further improving battery safety.

[0017] In some embodiments, the heat-resistant particles include inorganic particles and bonding particles. In this implementation, the bonding particles act as a bridge, tightly connecting the inorganic particles together. This allows the inorganic particles to form a stable overall structure, thereby improving the mechanical strength and stability of the coating and enabling it to better suppress thermal shrinkage of the base film.

[0018] In some embodiments, the heat-resistant particles include inorganic particles and a bonding layer disposed on at least a portion of the surface of the inorganic particles. In this embodiment, the bonding layer is disposed on the surface of the inorganic particles, thereby improving the bonding strength between the inorganic particles and the base film, reducing the risk of the inorganic particles falling, and improving the safety and service life of the battery.

[0019] 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. These materials exhibit minimal volume changes with temperature, and their use as inorganic particles can further suppress thermal shrinkage of the base film, thereby further improving battery safety.

[0020] In some embodiments, the inorganic particles account for 5% to 30% by weight of the coating. This helps the inorganic particles form a heat-resistant skeleton structure in the coating, effectively suppressing thermal shrinkage of the coating, improving battery safety, and reducing the risk of inorganic particles falling off.

[0021] In some embodiments, the negative electrode film layer includes a first film layer and a second film layer arranged 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 second film layer includes a second negative electrode active material, the second negative electrode active material includes a second graphite material, and the I of the second graphite material D / I G 0.05 to 0.2; among which I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at .

[0022] The first graphite material I used in this application D / I G The ID / IG ratio is 0.4 to 0.9, indicating that there are a large number of defects on the surface of the first graphite material, which can serve as additional active sites for lithium ion embedding. Placing this first graphite material with more surface defects in the outer layer (first film layer) of the negative electrode plate is beneficial to increasing the contact probability between the additional active sites and the electrolyte, thereby improving the fast charging performance of the battery. On this basis, the inner layer (second film layer) of the negative electrode plate adopts a second graphite material with an ID / IG ratio of 0.05 to 0.2. The second graphite material has fewer surface defects and a relatively complete structure, which is beneficial to reducing the irreversible loss of lithium ions, thereby extending the service life of the battery.

[0023] 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, wherein the negative electrode coating layer includes amorphous carbon. The structure of the amorphous carbon is relatively loose and has a rich pore structure. These pores can provide more diffusion channels for lithium ions, shortening the diffusion path of lithium ions in the electrode material. By disposing the amorphous carbon on the surface of the first graphite material, lithium ions can pass through the negative electrode coating layer more quickly to the interior of the first graphite material, thereby improving the fast charging performance of the battery.

[0024] In some embodiments, the thickness of the negative electrode coating layer is 10 nm to 100 nm. The thickness of the negative electrode coating layer within the above range is beneficial for the secondary battery to have both fast charging performance and energy density.

[0025] 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, arranging the second graphite material and the first graphite material in the second film layer can improve both the fast-charging performance of the battery and the battery itself.

[0026] 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 service life of the battery.

[0027] In some embodiments, the coating weight of the negative electrode film layer is 80 mg / 1540.25 mm 2 Up to 170mg / 1540.25mm 2 This helps increase the amount of lithium ions released per unit area of the negative electrode film, thereby increasing the energy density of the battery.

[0028] In some embodiments, the thickness of the negative electrode film layer disposed on a single side of the negative electrode current collector is 40 μm to 75 μm, which is beneficial for the negative electrode film layer to have both high capacity and high lithium ion and electron transport properties, thereby facilitating the secondary battery to have both high energy density and fast charging performance.

[0029] In some embodiments, the thickness of the negative electrode current collector is 4 μm to 8 μm. The thickness of the negative electrode current collector within the above range is conducive to achieving a high energy density of the battery and reducing the risk of cracks in the negative electrode current collector, thereby extending the service life of the battery.

[0030] In some embodiments, the positive electrode film layer includes a positive electrode active material, the positive electrode active material includes a first positive electrode active material, and the first positive electrode active material includes an olivine-structured lithium-containing phosphate. The olivine-structured lithium-containing phosphate has a stable three-dimensional lattice structure. During the insertion and extraction of lithium ions, the structure can remain relatively stable and is not susceptible to structural collapse or deformation. This allows the olivine-structured lithium-containing phosphate to withstand multiple charge and discharge cycles without damage, thereby extending the service life of the battery.

[0031] In some embodiments, the olivine-structured lithium-containing phosphate comprises a compound represented by formula (I): LiFe 1-x-y Mn x M 1 y PO4, formula (I); In formula (I), M 1 At least one selected from V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn, and Pb, 0≤x≤1, 0≤y<1. The lithium-containing phosphate has a stable lattice structure and is not susceptible to phase change, thereby further improving the safety performance of the secondary battery.

[0032] In some embodiments, 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-containing phosphate, and includes at least one of a fast ion conductor material and a carbon material.

[0033] The carbon material is loose and porous, which enables the electrolyte and the lithium iron phosphate matrix to fully and effectively contact each other, thereby improving the electrolyte's infiltration performance into the positive electrode film layer, and further improving the fast charging performance of the secondary battery.

[0034] Fast ion conductor materials have high ionic conductivity. Selecting a positive electrode coating layer containing fast ion conductor materials is beneficial to improving the fast charging performance of the battery.

[0035] In some embodiments, the mass ratio of the fast ion conductor material to the carbon material in the positive electrode coating layer is (0-100):(100-0). By controlling the mass ratio of the fast ion conductor material to the carbon material within the above range, the fast charging performance of the battery is improved.

[0036] In some embodiments, the positive electrode coating 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 arranged between the lithium phosphate and the second coating layer; carbon materials generally have good bonding properties. In this embodiment, the second coating layer containing carbon material is arranged on the outer surface of the first positive electrode active material, which is beneficial to enhancing the bonding force between the first positive electrode active material and the current collector, reducing the risk of the first positive electrode active material falling off during the use of the battery, and improving the service life of the battery.

[0037] In some embodiments, the positive electrode coating includes a first coating and a second coating; the first coating includes a fast ion conductor material, the second coating includes a carbon material, and the second coating is disposed between the lithium-containing phosphate and the first coating. In this embodiment, during the conduction process, lithium ions first pass through the first coating composed of the fast ion conductor material, then enter the second coating and finally diffuse into the interior of the lithium-containing phosphate, thereby accelerating the migration speed of lithium ions within the first positive electrode active material and improving the fast charging performance of the battery. In addition, the carbon material of the second coating has good electrical conductivity, and it can form a continuous conductive network between the lithium-containing phosphate and the first coating to accelerate the transmission of electron conduction, thereby further improving the fast charging performance of the battery.

[0038] In some embodiments, the fast ion conductor material comprises a compound represented by formula (II): Li 3-b Fe 2-b M 2 b (PO4)3 formula (II); in formula (II), M 2 At least one selected from Ti, Zr, Hf, Ge, and Sn, 0≤b≤1. The above fast ion conductor material has excellent ionic conductivity. By selecting the above fast ion conductor material, it is beneficial to further improve the fast charging performance of the battery.

[0039] In some embodiments, the mass percentage of carbon in the first positive electrode active material is 1% to 1.5%. This, on the one hand, helps the first positive electrode active material achieve a high capacity, thereby helping the secondary battery achieve a high energy density, and on the other hand, helps improve the electronic conductivity of the first positive electrode active material, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0040] In some embodiments, the BET specific surface area of the first positive electrode active material is 12 m 2 / g to 16m 2 / g; thus, the surface of the first positive electrode active material provides more channels for lithium ion insertion and extraction. During fast charging, lithium ions can more quickly embed into the first positive electrode active material through these channels, reducing the lithium ion transmission distance and resistance, thereby improving the battery's fast charging performance.

[0041] In some embodiments, the tap density of the first positive electrode active material is 0.8 g / cm 3 to 1.3g / cm 3 ; This is conducive to the formation of a rich pore structure in the positive electrode film layer, which can ensure that the battery has better fast charging performance.

[0042] 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 a rich pore structure between the particles of the first positive electrode active material, thereby improving the lithium ion and electron transmission performance in the positive electrode film layer, and thus improving the kinetic performance of the secondary battery.

[0043] In some embodiments, the first positive electrode active material has a compaction density of 2.4 g / cm2 at 50,000 N. 3 Up to 2.6g / cm 3 This makes the contact between the first positive electrode materials closer, which is beneficial to improving the energy density of the battery.

[0044] In some embodiments, the positive electrode active material further includes a second positive electrode active material, and the second positive electrode active material includes a lithium transition metal oxide. Lithium transition metal oxide has a higher specific capacity. By selecting lithium transition metal oxide as the second positive electrode active material, the energy density of the secondary battery can be further improved.

[0045] 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), which helps to balance the service life and energy density of the secondary battery.

[0046] In some embodiments, the thickness of the positive electrode film layer arranged on a single side of the positive electrode current collector is 100 μm to 200 μm, which is beneficial for the positive electrode film layer to have high capacity, high lithium ion and electron transmission performance, and thus beneficial for the secondary battery to have high energy density and fast charging performance.

[0047] 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.

[0048] In some embodiments, the compaction density of the positive electrode film layer is 2 g / cm 3 Up to 3g / cm 3 This helps the positive electrode film layer maintain a better pore structure, reduces the tortuosity of the positive electrode film layer, shortens the lithium ion transmission path, thereby improving the battery's fast charging performance and life performance, while taking into account high energy density.

[0049] In some embodiments, the thickness of the positive electrode current collector is 10 μm to 18 μm, which is beneficial for achieving a high energy density of the battery and reducing the risk of cracks in the positive electrode current collector, thereby extending the battery life.

[0050] In some embodiments, the positive electrode film layer further includes a positive electrode dispersant, which includes at least one of polyethylene glycol octylphenyl ether, polyvinyl pyrrolidone, and sodium carboxymethyl cellulose. The positive electrode dispersant has good flexibility and elasticity, and can disperse the stress experienced by the positive electrode active material during compaction. The use of such a positive electrode dispersant facilitates achieving a high compaction density of the positive electrode film layer, thereby increasing the energy density of the battery.

[0051] In some embodiments, the mass ratio of the positive electrode dispersant to the positive electrode film layer is 0.3% to 5%, thereby facilitating the battery to achieve high energy density.

[0052] In some embodiments, the secondary battery further comprises an electrolyte, the electrolyte comprising an electrolyte salt and a solvent; the solvent comprises at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone; the electrolyte salt comprises 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 difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate and lithium tetrafluorooxalatophosphate.

[0053] In some embodiments, the electrolyte further includes an additive, including at least one of barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tris(trimethylsilyl)phosphate, and vinylene carbonate. These additives undergo electrochemical reduction reactions on the surface of the negative electrode prior to the solvent molecules in the electrolyte, forming a dense and stable SEI film, thereby inhibiting the growth of lithium dendrites and improving battery safety.

[0054] In some embodiments, the additive accounts for 1% to 10% by weight of the electrolyte. This facilitates the formation of a SEI film of moderate thickness on the electrode surface, which, on the one hand, helps inhibit the growth of lithium dendrites and improve battery safety. On the other hand, it helps balance the transmission resistance of lithium ions in the negative electrode sheet, thereby taking into account the fast charging performance of the battery.

[0055] In some embodiments, the conductivity of the electrolyte is 10 mS / cm to 18.5 mS / cm, which helps reduce the internal impedance of the battery and reduces energy loss caused by resistance during charging and discharging.

[0056] The second aspect of the present application further provides an electrical device comprising the secondary battery of the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a schematic diagram of a secondary battery according to one embodiment of the present application; Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of the present application is shown; Figure 3 is a schematic diagram of a battery module according to one embodiment of the present application; Figure 4 is a schematic diagram of a battery pack according to one embodiment of the present application; Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown; Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to one embodiment of the present application; Figure 7 This is a scanning electron microscope (SEM) image of the isolation film of Example 1 of the present application.

[0058] Description of reference numerals: 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery cell; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0059] Below, the embodiments of the secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the present application.

[0060] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0061] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0062] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0063] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, a method includes steps (a) and (b), which indicates that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, a method may further include step (c), which indicates that step (c) may be added to the method in any order, for example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may include steps (c), (a), and (b), etc.

[0064] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may indicate that other components not listed may also be included or that only the listed components are included.

[0065] In recent years, as the application scope of secondary batteries has become increasingly wider, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace and other fields.

[0066] With the remarkable development of secondary batteries, people have put forward higher requirements on the energy density and safety performance of secondary batteries in practical applications.

[0067] Increasing the compaction density of the membrane layer is often used to improve battery energy density. However, this approach has significant drawbacks. This increased compaction density also increases heat release within the battery, which in turn degrades battery safety. For example, separators are typically made of polymers such as polyethylene (PE) and polypropylene (PP). These polymers consist of long chains. At room temperature, the chains are relatively stable, with certain interactions between them that maintain the separator's shape. However, as the temperature rises, the molecules gain more energy, intensifying thermal motion, increasing the spacing between chains and making their arrangement more disordered. This molecular motion and change in arrangement causes macroscopic dimensional changes in the separator, known as thermal shrinkage. This thermal shrinkage can cause overlap between the positive and negative electrode sheets, degrading battery safety. Therefore, balancing battery energy density and safety has become a pressing technical challenge.

[0068] In order to solve the above technical problems, the present application provides a secondary battery and an electrical device. The secondary battery takes both energy density and safety performance into consideration.

[0069] The first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer provided on at least one side surface of the positive electrode current collector, and the compaction density of the positive electrode film layer is 2.0 g / cm 3 Up to 3.0g / cm 3 The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, and the compaction density of the negative electrode film layer is 1.4g / cm 3 Up to 1.85g / cm 3 ; The isolation film includes a base film, and the thickness of the base film is 4μm to 12μm.

[0070] In this application, by controlling the compaction density of the positive electrode film layer to 2.0g / cm 3 Up to 3.0g / cm 3 The compaction density of the negative electrode film is 1.4g / cm 3 Up to 1.85g / cm 3, which is conducive to the battery achieving high energy density.

[0071] While the aforementioned compaction density helps to closely arrange the electrode material particles and improve the battery's energy density, it can also hinder the penetration of the electrolyte, increase the battery's internal resistance, and lead to excessive heat generation during discharge, affecting battery safety.

[0072] To this end, this application utilizes a base film with a thickness of 4 to 12 μm. This can reduce the impact of thin base film thickness (less than 4 μm) on battery safety, while also reducing the adverse effects of excessive base film thickness (greater than 12 μm) on battery energy density. This battery enhances safety while maintaining a high energy density. Specifically, a base film with a thickness within this range can maintain its structural stability at high temperatures, effectively reducing the risk of positive and negative electrode short circuits. In particular, during thermal runaway, the stable base film can inhibit side reactions through physical isolation, thereby lowering the maximum temperature of thermal runaway, reducing the risk of heat spread, and improving battery safety. Furthermore, a base film thickness within this range helps minimize its adverse effects on battery energy density, thereby helping the battery maintain a high energy density. The base film also has nanopores with an average pore size of 10 to 300 nm. Using a base film with an average nanopore size within this range helps increase the migration rate of lithium ions while also suppressing the growth of lithium dendrites, further improving battery safety.

[0073] In this application, "compacted density" refers to the mass per unit volume of active materials (positive and negative electrode active materials) after compaction under certain pressure conditions. In battery systems, it primarily describes the compactness of positive and negative electrode film layers, reflecting the amount of active material loaded per unit volume of the electrode film.

[0074] In this application, the compaction density can be tested using methods known in the art. The electrode sheets (positive electrode sheets, negative electrode sheets) to be tested can be prepared electrode sheets or electrode sheets obtained by disassembling the battery. For example, the positive electrode sheets are obtained by disassembling the battery, and the positive electrode sheet layers are cut into pieces with an area of 1540.25 mm. 2 Weigh the disc and assign its mass to m1. Use a micrometer to measure its thickness at different locations, taking the average value and recording it as d1. Then, remove the positive electrode film layer on one side of the disc and weigh the disc to m2. Use a micrometer to measure its thickness at different locations, taking the average value and recording it as d2. Use (m1 - m2) / (d1 - d2) as the compacted density of the positive electrode film layer.

[0075] In this application, the compaction density of the positive electrode film layer is 2g / cm 3 Up to 3g / cm3 For example, the compaction density of the positive electrode film layer can be 2g / cm 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 a value between any two of the values. Optionally, the compaction density of the positive electrode film is 2.3 g / cm 3 Up to 2.5g / cm 3 .

[0076] In this application, the compaction density of the negative electrode film layer is 1.4g / cm 3 Up to 1.85g / cm 3 For example, the compaction density of the negative electrode film layer can be 1.4 g / cm 3 , 1.5g / cm 3 , 1.55g / cm 3 , 1.60g / cm 3 , 1.62g / cm 3 , 1.64g / cm 3 , 1.65g / cm 3 , 1.85g / cm 3 Or a value between any two of them.

[0077] In this application, the thickness of the base film can be measured using a micrometer or a ten-thousandth caliper. For example, the thickness of the base film can be measured multiple times using a micrometer at different locations on the base film, and the average value is taken as the thickness of the base film.

[0078] In the present application, the thickness of the base film is 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 range consisting of any two of these values. Optionally, the thickness of the base film is 5 μm to 9 μm.

[0079] In this application, the average pore size can be measured 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 average pore size of the separator (base membrane) obtained by disassembling a battery can be measured using a pore size tester (model: PMI Porometer) in accordance with GB / T 21650.2-2008.

[0080] Illustratively, the average pore diameter of the nanopores is 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 a range consisting of any two of these values. The term "secondary battery" referred to herein refers to a battery cell, battery module, or battery pack. Each of these is described below. Unless otherwise specified, the batteries referred to in this application refer to secondary batteries.

[0081] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, lithium ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0082] Isolation film The present application has no particular limitation on the type of isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0083] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven 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.

[0084] In some embodiments, the base film includes at least one of a polyethylene base film, a polypropylene base film, a polyethylene-polypropylene composite base film, a polyethylene non-woven base film, a polypropylene non-woven base film, a polypropylene-polyethylene-polypropylene composite base film, a polyimide base film, a polyimide non-woven base film, a polytetrafluoroethylene-based film, a polytetrafluoroethylene non-woven base film, a polyvinyl chloride-based film, or a polyvinyl chloride non-woven base film.

[0085] In some embodiments, the porosity of the base membrane is 20% to 70%. By using a base membrane with a porosity within the above range, it helps to improve the migration rate of lithium ions while also helping to inhibit the growth of lithium dendrites, thereby further improving the safety performance of the battery. Exemplarily, the porosity of the base membrane is 20%, 30%, 40%, 50%, 60%, 70%, or a value between the ranges consisting of any two of these values. Optionally, the porosity of the base membrane is 35% to 42%.

[0086] In some embodiments, the separator further includes a coating disposed on at least one side of the base membrane, the coating comprising heat-resistant particles interwoven to form a porous structure. Compared to coatings without a porous structure, in this implementation, the interwoven heat-resistant particles form a porous structure, which helps reduce the coating's resistance to ion transport, thereby improving ion transport efficiency. Furthermore, the coating includes heat-resistant particles, whose volume changes little with temperature. Disposing the coating containing heat-resistant particles on the surface of the base membrane helps suppress the thermal shrinkage of the base membrane, thereby further reducing the risk of short circuits between the positive and negative electrodes and improving battery safety.

[0087] In some embodiments, the thickness of the coating provided on one side of the base film is 0.011 μm to 3 μm. By selecting a coating with a thickness in the above range, on the one hand, it is beneficial for the battery to achieve a high energy density, and on the other hand, it is beneficial for the coating to better suppress the thermal shrinkage of the base film, thereby further improving the safety performance of the battery. Exemplarily, the thickness of the coating 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 between the ranges consisting of any two of them.

[0088] In some embodiments, the average particle size of the heat-resistant particles is 5nm to 185nm. The heat-resistant particles with an average particle size of 5nm to 185nm can form a coating with a stable supporting network structure, which can support the base film while suppressing the thermal shrinkage of the base film, thereby improving the safety performance of the battery. Exemplarily, the average particle size of the heat-resistant particles is 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 85nm, 90nm, 95nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 185nm or a value between the ranges consisting of any two of them.

[0089] In some embodiments, the particle area density of the heat-resistant particles is 0.5 mg / 1540.25 cm 2 Up to 2.5mg / 1540.25cm 2 By controlling the particle surface density within the above range, it is beneficial for the battery to achieve energy density while also helping the heat-resistant particles to exert their function of inhibiting the thermal shrinkage of the base film, thereby improving the safety performance of the battery. For example, the particle surface density of the heat-resistant particles is 0.5 mg / 1540.25 cm 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 a value between any two of the values. Optionally, the particle surface density of the heat-resistant particles is 1.25 mg / 1540.25 cm 2 to 1.85mg / 1540.25cm 2 .

[0090] In some embodiments, the heat-resistant particles include first heat-resistant particles along the thickness of the base film, and the first heat-resistant particles are distributed on the surface of the base film. In this implementation, the first heat-resistant particles are distributed on the surface of the base film and can directly inhibit thermal shrinkage of the base film, thereby further improving the safety performance of the battery while also taking into account the energy density of the battery.

[0091] In some embodiments, the heat-resistant particles include first and second heat-resistant particles; the first heat-resistant particles are distributed on the surface of the base film, while 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 stacking of the first and second heat-resistant particles across the thickness of the base film facilitates the formation of a thicker coating. This thicker coating acts as a more effective physical barrier, reducing chemical erosion of the base film by the electrolyte. It also buffers friction between the electrodes (positive and negative electrodes) and the base film, thereby further improving battery safety.

[0092] In some embodiments, the heat-resistant particles include inorganic particles and bonding particles. In this implementation, the bonding particles act as a bridge, tightly connecting the inorganic particles together. This allows the inorganic particles to form a stable overall structure, thereby improving the mechanical strength and stability of the coating and enabling it to better suppress thermal shrinkage of the base film.

[0093] In some embodiments, the first glass transition temperature of the bonded particles is from 30°C to 75°C; and / or the second glass transition temperature of the bonded particles is from -10°C to 25°C.

[0094] In some embodiments, the bonding particles include one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane, or polystyrene.

[0095] In some embodiments, the heat-resistant particles include inorganic particles and a bonding layer disposed on at least a portion of the surface of the inorganic particles. In this embodiment, the bonding layer is disposed on the surface of the inorganic particles, thereby improving the bonding strength between the inorganic particles and the base film, reducing the risk of the inorganic particles falling, and improving the safety and service life of the battery.

[0096] In some embodiments, the first glass transition temperature of the bonding layer is 30°C to 75°C; and / or the second glass transition temperature of the bonding layer is -10°C to 25°C.

[0097] In some embodiments, the tie layer includes one or more of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane, or polystyrene.

[0098] 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. These materials exhibit minimal volume changes with temperature, and their use as inorganic particles can further suppress thermal shrinkage of the base film, thereby further improving battery safety.

[0099] In some embodiments, the mass proportion of inorganic particles in the coating is 5% to 30%. By controlling the mass proportion of inorganic particles within the above range, it is beneficial for the inorganic particles to form a heat-resistant skeleton structure in the coating, effectively suppressing the thermal shrinkage of the coating, improving the safety performance of the battery, and reducing the risk of inorganic particles falling off. Exemplarily, the mass proportion of inorganic particles is 5%, 10%, 15%, 20%, 25%, 30%, or a value between the ranges consisting of any two of these values.

[0100] In this application, the mass fraction of inorganic particles can be tested using methods known in the art. The isolation membrane to be tested can be a prepared isolation membrane or an isolation membrane obtained by disassembling a battery. For example, the isolation membrane is obtained by disassembling the battery, the coating is peeled off from the base membrane, the coating is collected, and the mass of the coating is weighed as m3. The adhesive particles or adhesive layer are dissolved in an appropriate 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 fraction of the inorganic particles.

[0101] Negative electrode In a first aspect, the present application provides a secondary battery, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer comprising a first film layer and a second film layer arranged between the first film layer and the negative electrode current collector, the first film layer comprising a first negative electrode active material, the first negative electrode active material comprising a first graphite material, the first graphite material I D / I G The second film layer includes a second negative electrode active material, the second negative electrode active material includes a second graphite material, and the I of the second graphite material D / I G 0.05 to 0.2; among which I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at .

[0102] The first graphite material I used in this application D / I G The ratio is between 0.4 and 0.9, indicating that the first graphite material has a large number of surface defects, which can serve as additional active sites for lithium ion embedding. Placing this first graphite material with a high number of surface defects on the outer layer (first film layer) of the negative electrode increases the contact probability between the additional active sites and the electrolyte, thereby improving the battery's fast-charging performance.

[0103] On this basis, the inner layer (second film layer) of the negative electrode sheet adopts I D / I GThe second graphite material has a carbon content of 0.05 to 0.2, and the second graphite material has fewer surface defects and a relatively complete structure, which is beneficial to reducing the irreversible loss of lithium ions and thus extending the service life of the battery.

[0104] In the present application, the morphology of the negative electrode plate can be tested by methods known in the art. The negative electrode plate to be tested can be a prepared negative electrode plate, or a negative electrode plate obtained by disassembling a battery. The test process is described below using the latter as an example. Specifically, the battery is disassembled to obtain the negative electrode plate, the negative electrode plate is placed in a sample holder and locked and fixed, an argon ion cross-section polisher (such as the IB-09010 CP argon ion cross-section polisher from JEOL, Japan) is turned on to cut a cross section of the negative electrode plate, and a scanning electron microscope (HR-TEM Talos F200) is used to collect a cross-sectional SEM photograph of the negative electrode plate. From the cross-sectional SEM photograph, it can be seen that the negative electrode plate includes a negative electrode current collector, a second film layer disposed on the surface of the negative electrode current collector, and a first film layer disposed on the surface of the second film layer.

[0105] In this application, the graphite material (first graphite material, second graphite material) D / I G The value can be tested by 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 describes the test process using the latter as an example. Specifically, the battery is disassembled to obtain the negative electrode sheet, the first film layer is scraped off with a scraper, the first film layer is dissolved with an appropriate solvent, and then the first graphite material is filtered out, and the I value of the first graphite material is tested. D / I G Use a scraper to scrape off the second film layer, use a suitable solvent to dissolve the second film layer, and then filter out the second graphite material, and test the I of the second graphite material. 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 film layers. To reduce sampling errors, the sampling area for the second film layer is 2.5 μm from the surface of the negative electrode film layer close to the current collector toward the negative electrode film layer; the sampling area for the first film layer is 2.5 μm from the surface of the negative electrode film layer away from the current collector toward the negative electrode film layer.

[0106] The test conditions are: excitation wavelength of 532nm, grating of 600 lines, objective lens of 50 times, integration time of 10s, accumulation times of 3 times, surface scanning, obtain the D peak and G peak intensity of 100 points, calculate the I of 100 points D / I G , remove the largest and smallest 30 I D / I G The average value of the remaining 40 points is the ID / I G The testing instrument may be a Horiba Lab RAMHR800 Raman spectrometer.

[0107] In this application, the first graphite material I D / I G is 0.4 to 0.9. For example, the I of the first graphite material D / I G The value may be 0.4, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9 or a range between any two values, but is not limited thereto.

[0108] In this application, the second graphite material I D / I G is 0.05 to 0.2. For example, the I of the second graphite material D / I G The amount may be 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 a range between any two values, but is not limited thereto.

[0109] As an example, the negative electrode current collector has two surfaces opposite to each other in its 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.

[0110] In some embodiments, the volume average particle size of the first graphite material is 9.2 μm to 15.5 μm. This is conducive to the formation of a rich pore structure between the particles of the first graphite material, thereby improving the lithium ion and electron transport performance in the first film layer, and thus improving the dynamic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the first graphite material is 9.2 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 15.5 μm or a value between the range consisting of any two values, but is not limited thereto.

[0111] In some embodiments, the volume average particle size Dv50 of the second graphite material is 16.3 μm to 25.5 μm. This is beneficial to improving the compaction density of the second film layer, thereby improving the energy density of the secondary battery. In addition, it is also beneficial to form a rich pore structure between the particles of the second graphite material, improving the lithium ion and electron transport performance in the negative electrode film layer, and thus improving the kinetic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the second graphite material is 16.3 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25.5 μm or a value between the range consisting of any two values, but is not limited thereto.

[0112] In this application, the volume distribution particle size Dv50 of a material represents the particle size corresponding to the cumulative volume distribution percentage of the material reaching 50%. It can be measured using instruments and methods known in the art. For example, it can be measured using a laser particle size analyzer in accordance with GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd. in the United Kingdom.

[0113] In some embodiments, the BET specific surface area of the first graphite material is 0.3 m 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 lithium ion insertion and extraction channels. During fast charging, lithium ions can be more quickly embedded in the first graphite material through these channels, reducing the transmission distance and resistance of lithium ions, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of the first graphite material is 0.3m 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 a value between any two values, but not limited thereto.

[0114] In some embodiments, the BET specific surface area of the second graphite material is 0.5 m 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 lithium ion embedding and extraction channels. During the fast charging process, lithium ions can be embedded into the second graphite material more quickly through these channels, reducing the transmission distance and resistance of lithium ions, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of the second graphite material is 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, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g or a value between any two values, but not limited thereto.

[0115] In this application, the BET specific surface area of materials (second graphite material, first graphite material, first positive electrode active material) is well known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method in accordance with GB / T 19587-2017 and calculated using the BET (Brunauer-Emmett-Teller) method. The testing instrument can be a Tri-Star 3020 specific surface area pore size analyzer from Micromeritics, Inc., USA.

[0116] In some embodiments, the tap density of the first graphite material is 0.7 g / cm 3 Up to 1.6g / cm 3 The tap density of the first graphite material is within the above range, which is conducive to the formation of a rich pore structure in the first film layer, and can ensure that the battery has better fast charging performance. For example, the tap density of the first graphite material is 0.7g / cm 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 a value between any two numerical values, but not limited thereto.

[0117] In some embodiments, the tap density of the second graphite material is 0.8 g / cm 3 Up to 1.5g / cm 3 The tap density of the second graphite material is within the above range, and the second graphite material can be densely packed in the second film layer, which is conducive to accommodating more second graphite material (active material) in 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 / 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 or a value between any two numerical values, but not limited thereto.

[0118] In this application, the tap density of materials (second graphite material, first graphite material, and first positive electrode active material) has a well-known meaning in the art and can be measured using instruments and methods known in the art. For example, it can be measured using a powder tap density tester in accordance with GB / T 5162-2006. A Dandong Better BT-301 tester can be used, with the following test parameters: vibration frequency of 250 ± 15 times / minute, amplitude of 3 ± 0.2 mm, number of vibrations of 5000, and a 25 mL graduated cylinder.

[0119] 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, wherein the negative electrode coating layer includes amorphous carbon. The structure of the amorphous carbon is relatively loose and has a rich pore structure. These pores can provide more diffusion channels for lithium ions, shortening the diffusion path of lithium ions in the electrode material. By disposing the amorphous carbon on the surface of the first graphite material, lithium ions can pass through the negative electrode coating layer more quickly to the interior of the first graphite material, thereby improving the fast charging performance of the battery.

[0120] In the present application, TEM can be used to characterize the negative electrode coating layer on the surface of the first graphite material. Microscopic morphology information of the first negative electrode active material is collected using SEM. A periodically repeating lattice structure observed by TEM is the first graphite material. If a regular lattice structure is found on the surface of the first graphite material, it indicates that amorphous carbon is provided on the surface of the first graphite material.

[0121] In some embodiments, the thickness of the negative electrode coating layer is 10 nm to 100 nm. The thickness of the negative electrode coating layer in the above range is beneficial for the secondary battery to have both fast charging performance and energy density. Exemplarily, the thickness of the negative electrode coating layer is 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or a range between any two values, but is not limited thereto.

[0122] In this application, the thickness of the negative electrode coating layer can be characterized using a scanning electron microscope (SEM) combined with a focused ion beam (FIB). The sample (first negative electrode active material) is fixed on a sample stage, and a flat cross section is cut from the sample surface using the FIB. The cross section is then imaged using the SEM, and the thickness of the negative electrode coating layer is determined by measuring the width of the image.

[0123] 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 and discharge cycles, slowing the rate of battery capacity decay and extending the battery's service life. The first graphite material can improve the battery's fast-charging performance. In this embodiment, the arrangement of the second graphite material and the first graphite material in the second film layer facilitates improved fast-charging performance of the battery.

[0124] In the present 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 one obtained by disassembling a battery. Specifically, the negative electrode sheet is placed in a sample holder and locked securely. An argon ion cross-section polisher (e.g., the IB-09010 CP Argon Ion Cross-Section Polisher from JEOL, Japan) is used to obtain a cross-section of the negative electrode sheet. A scanning electron microscope (HR-TEM Talos F200) is used to capture a cross-sectional SEM image of the negative electrode sheet. The cross-sectional SEM image shows 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 a large-particle material and a small-particle material. The large-particle material is the second graphite material, and the small-particle material is the first graphite material.

[0125] 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 service life of the battery. Exemplarily, 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 a range between any two values, but is not limited thereto.

[0126] In the present application, the mass ratio of the second graphite material to the first graphite material in the second film layer can be tested by methods known in the art. The negative electrode plate to be tested can be a prepared negative electrode plate, or a negative electrode plate obtained by disassembling the battery. Specifically, the battery is disassembled to obtain the negative electrode plate, and the first film layer is removed with a scraper to expose the second film layer. The second film layer is removed with a scraper and collected. The second film layer is dissolved with an appropriate solvent, and filtered to obtain a mixture of the second graphite material and the first graphite material. The second graphite material and the first graphite material are separated by using the difference in particle size between the two, and the mass of the second graphite material and the mass of the first graphite material are weighed respectively, thereby obtaining the mass ratio of the second graphite material to the first graphite material in the second film layer.

[0127] 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 service life 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 5:5, 4:6, 3:7, or a range consisting of any two values, but is not limited to this.

[0128] 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 measured using methods known in the art. The negative electrode sheet to be tested can be a pre-prepared negative electrode sheet or one obtained by disassembling a battery. Specifically, a cross-sectional SEM photograph of the negative electrode sheet is captured using a scanning electron microscope (HR-TEM Talos F200). The cross-sectional SEM photograph clearly shows that the negative electrode sheet consists of a negative electrode current collector, a second film layer located on the current collector surface, and a first film layer disposed on the surface of the second film layer. Subsequently, four sampling points are evenly selected on each of the first and second film layers, and the film thickness at each sampling point is accurately measured using image analysis software. The thickness values at the four sampling points of the first film layer are averaged to obtain the average thickness of the first film layer. The average thickness of the second film layer is obtained similarly. 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.

[0129] In some embodiments, the first film layer also includes a first binder, which includes at least one of styrene-butadiene rubber, styrene-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 binder, the risk of the first film layer separating from the second film layer can be reduced, which is beneficial to further improve the service life of the secondary battery.

[0130] In some embodiments, the first binder includes at least one of styrene-butadiene rubber, lithium polyacrylate, and polyacrylate.

[0131] Styrene-butadiene rubber has good flexibility and elasticity. It can disperse the stress borne by the first negative electrode active material during the compaction process during the charge and discharge process of the electrode, so that the first negative electrode active material can withstand greater pressure. By selecting styrene-butadiene rubber as the first dispersant, it is beneficial for the first film layer to achieve a high compaction density, thereby improving the energy density of the battery.

[0132] Lithium polyacrylate has high ionic conductivity, and its selection as the first binder is beneficial to improving the fast charging performance of the battery.

[0133] Polyacrylate can increase the compression modulus of the first negative electrode active material, reduce the adverse effects on the dynamic performance of the negative electrode plate during compression, and thus improve the fast charging performance of the battery.

[0134] In some embodiments, the second film layer further includes a second binder, and the second binder includes at least one of styrene-butadiene rubber, styrene-butadiene rubber, lithium polyacrylate, polyacrylate, polyacrylic acid, sodium polyacrylate, polyacrylamide, polyvinyl alcohol, sodium alginate, polymethacrylic acid, or carboxymethyl chitosan. These second binders have high viscosity. By using these materials as the second binder, the risk of the second film layer detaching from the negative electrode current collector can be reduced, thereby further improving the service life of the secondary battery.

[0135] In some embodiments, the second binder includes at least one of styrene-butadiene rubber, lithium polyacrylate, and polyacrylate. In this embodiment, the use of the above substances as the second binder helps to balance the fast charging performance, service life, and energy density of the battery.

[0136] In some embodiments, the mass proportion of the first binder in the first film layer is 0.1% to 2%. By controlling the mass proportion of the first binder in the first film layer within the above range, on the one hand, it is beneficial to prevent the pulverization and shedding of the first negative electrode active material, thereby extending the service life of the battery. On the other hand, it is beneficial for the battery to achieve high energy density. Exemplarily, the mass proportion 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 a value between the ranges consisting of any two values.

[0137] In some embodiments, the mass proportion of the second binder in the second film layer is 0.1% to 2%. By controlling the mass proportion of the second binder in the second film layer within the above range, on the one hand, it is beneficial to prevent the pulverization and shedding of the second negative electrode active material, thereby extending the service life of the battery. On the other hand, it is beneficial to achieve a high energy density of the battery. Exemplarily, the mass proportion 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 a value between the ranges consisting of any two values.

[0138] In some embodiments, the first film layer further includes a first dispersant, and the first dispersant includes at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, and a polymer represented by formula (III).

[0139] The structural formula of formula (III) is: , wherein n is 1000-10000, preferably 3000-6000.

[0140] The above-mentioned substance has high ionic conductivity, and selecting it as the first dispersant is beneficial to improving the fast charging performance of the battery.

[0141] In some embodiments, the second film layer further includes a second dispersant comprising at least one of lithium carboxymethyl cellulose, sodium carboxymethyl cellulose, and a polymer represented by formula (III). These materials have high ionic conductivity, and their use as the second dispersant can improve the fast-charging performance of the battery.

[0142] 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 caused by the formation of the SEI film during the first charge, thereby improving the first charge efficiency of the battery.

[0143] 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 caused by the formation of the SEI film during the first charge, thereby improving the first charge efficiency of the battery.

[0144] 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, the battery can 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 consisting of any two values.

[0145] In some embodiments, the second dispersant comprises 0.3% to 1.5% by weight of the second film layer. Controlling the second dispersant's mass fraction within this range facilitates achieving a high energy density in the battery. For example, the second dispersant's mass fraction is 0.3%, 0.5%, 1%, 1.5%, or a range consisting of any two of these values.

[0146] In some embodiments, the first film layer further includes a first conductive agent, which includes at least one of superconducting carbon, acetylene black, Ketjen black, carbon dots, graphene, carbon nanofibers, carbon nanotubes, and conductive carbon black. The above substances have good electrical conductivity. By using the above substances as the first conductive agent, the fast charging performance of the battery is further improved. Optionally, the first conductive agent includes carbon nanotubes and conductive carbon black. By using the above substances as the first conductive agent, the fast charging performance of the battery is further improved.

[0147] In some embodiments, the second film layer further includes a second conductive agent, which includes at least one of superconducting carbon, acetylene black, Ketjen black, carbon dots, graphene, carbon nanofibers, carbon nanotubes, and conductive carbon black. These materials have excellent electrical conductivity, and their use as the second conductive agent can further enhance the fast-charging performance of the battery. Alternatively, the second conductive agent includes carbon nanotubes and conductive carbon black.

[0148] In some embodiments, the mass proportion of the first conductive agent in the first film layer is 0.1% to 2%. By controlling the mass proportion 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. Exemplarily, the mass proportion 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 between the ranges consisting of any two values.

[0149] In some embodiments, the mass proportion of the second conductive agent in the second film layer is 0.1% to 2%. By controlling the mass proportion 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. Exemplarily, the mass proportion 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 between the ranges consisting of any two values.

[0150] In some embodiments, the thickness of the negative electrode film layer disposed on a single side of the negative electrode current collector is 40 μm to 75 μm. By ensuring that the thickness of the negative electrode film layer is within this range, the negative electrode film layer can achieve both high capacity and high lithium ion and electron transport performance, thereby facilitating 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 range consisting of any two of these values.

[0151] 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 layer within the above range, the amount of lithium ions released per unit area of the negative electrode film layer can be increased, thereby increasing the energy density of the battery. For example, the coating weight of the negative electrode film layer is 80mg / 1540.25mm 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 a value between any two values.

[0152] In this application, the coating weight of the negative electrode 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 negative electrode sheet layer is cut into pieces with an area of 1540.25 mm 2 The mass of the disc is weighed as m5, and then the negative electrode film layer set on one side of the disc is removed, and the mass of the disc is weighed as m6, and m5-m6 is used as the coating weight of the negative electrode film layer.

[0153] In some embodiments, the thickness of the negative electrode current collector is 4 μm to 8 μm. The thickness of the negative electrode current collector in the above range is conducive to achieving a high energy density of the battery on the one hand, and on the other hand, it is conducive to reducing the risk of cracks in the negative electrode current collector, thereby extending the service life of the battery. Exemplarily, 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 between any two of these values.

[0154] 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 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 (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0155] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0156] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0157] Positive electrode In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, the positive electrode active material includes a first positive electrode active material, and the first positive electrode active material includes an olivine-structured lithium-containing phosphate. The olivine-structured lithium-containing phosphate has a stable three-dimensional lattice structure. During the insertion and extraction of lithium ions, the structure can remain relatively stable and is not prone to structural collapse or deformation. This allows the olivine-structured lithium-containing phosphate to withstand multiple charge and discharge cycles without damage, thereby extending the service life of the battery.

[0158] In the present application, examples of lithium-containing 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)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0159] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0160] In some embodiments, the olivine-structured lithium-containing phosphate comprises a compound as shown in formula (I): LiFe 1-x-y Mn x M 1 y PO4, formula (I); in formula (I), M 1 At least one selected from V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb, 0≤x≤1, 0≤y<1. The lattice structure of the above-mentioned lithium-containing phosphate is stable and not prone to phase change, thereby further improving the safety performance of the secondary battery. Exemplarily, x is 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range between any two of them. Exemplarily, y is 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 or a range between any two of them.

[0161] In some embodiments, 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-containing phosphate and includes at least one of a fast ion conductor material and a carbon material. The carbon material is loose and porous, enabling sufficient and effective contact between the electrolyte and the lithium iron phosphate matrix, thereby improving the electrolyte's wettability of the positive electrode film layer, thereby enhancing the fast-charging performance of the secondary battery. Fast ion conductor materials have high ionic conductivity, and selecting a positive electrode coating layer comprising a fast ion conductor material can help improve the fast-charging performance of the battery.

[0162] In some embodiments, a positive electrode coating layer is provided on the surface of the lithium-containing phosphate, and the positive electrode film layer includes a fast ion conductor material and a carbon material.

[0163] 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-containing phosphate and the second coating layer. Carbon materials generally have good bonding properties. In this embodiment, disposing the second coating layer comprising a carbon material on the outer surface of the first positive electrode active material helps enhance the bonding 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 thereby increasing the battery's service life.

[0164] 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-containing phosphate and the first coating layer.

[0165] In this embodiment, during the conduction process, 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 interior of the lithium-containing phosphate. This helps to accelerate the migration speed of lithium ions within the first positive electrode active material and improve the fast-charging performance of the battery. In addition, the carbon material of the second coating layer has good conductivity. It can form a continuous conductive network between the lithium-containing phosphate and the first coating layer, accelerating the transmission of electrons, thereby further improving the fast-charging performance of the battery.

[0166] In some embodiments, in the positive electrode coating layer, the mass ratio of the fast ion conductor material to the carbon material is (0-100): (100-0). By controlling the mass ratio of the fast ion conductor material to the carbon material within the above range, it is beneficial to improve the fast charging performance of the battery. Exemplarily, the mass ratio of the fast ion conductor material to the 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 between the ranges consisting of any two of these values.

[0167] In some embodiments, the fast ion conductor material comprises a compound represented by formula (II): Li 3-b Fe 2-b M 2 b (PO4)3 formula (II); In formula (II), M 2 At least one selected from Ti, Zr, Hf, Ge, and Sn, with 0≤b≤1. These fast ion conductor materials have excellent ionic conductivity, and the selection of these fast ion conductor materials can further enhance the fast charging performance of the battery. For example, b is 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or a range consisting of any two of these values.

[0168] 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, on the one hand, it is beneficial to achieve a high capacity of the first positive electrode active material, thereby facilitating a high energy density of the secondary battery, and on the other hand, it is beneficial to improve the electronic conductivity of the first positive electrode active material, thereby improving the fast charging performance of the lithium-ion secondary battery.

[0169] In the present application, the mass proportion of carbon elements can be tested using methods known in the art. The positive electrode plate to be tested can be a prepared positive electrode plate, or a positive electrode plate obtained by disassembling a battery. Specifically, the battery is disassembled to obtain the positive electrode plate, the positive electrode film layer and the positive electrode current collector are peeled off, the positive electrode film layer is collected, and then the positive electrode film layer is dissolved with an appropriate solvent, and then the first positive electrode active material is separated. The mass proportion of carbon elements in the first positive electrode active material can be determined by using a carbon-sulfur analyzer with reference to the standard GB / T20123-2006.

[0170] In some embodiments, the BET specific surface area of the first positive electrode active material is 12 m 2 / g to 16m 2 / g.

[0171] When the BET specific surface area of the first positive electrode active material is within the above range, the surface of the first positive electrode active material can provide more lithium ion insertion and extraction channels. During fast charging, lithium ions can be more quickly embedded in the first positive electrode active material through these channels, reducing the transmission distance and resistance of lithium ions, thereby improving the fast charging performance of the battery. For example, the BET specific surface area of the first positive electrode active material is 12m 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 a value between any two values, but not limited thereto.

[0172] In some embodiments, the tap density of the first positive electrode active material is 0.8 g / cm 3 to 1.3g / cm 3 The tap density of the first positive electrode active material is within the above range, which is conducive to the formation of a rich pore structure in the positive electrode film layer, and can ensure that the battery has better fast charging performance. For example, the tap density of the first positive electrode active material is 0.8g / cm 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 / cm3 or a value between any two numerical values, but not limited thereto.

[0173] In some embodiments, the volume average particle size of the first positive electrode active material is 1 μm to 3 μm. This is conducive to the formation of a rich pore structure between the particles of the first positive electrode active material, improving the lithium ion and electron transport performance in the positive electrode film layer, and thus improving the kinetic performance of the secondary battery. Exemplarily, the volume average particle size Dv50 of the first positive electrode active material is 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 a value between the ranges consisting of any two values, but is not limited thereto.

[0174] In some embodiments, the first positive electrode active material has a compaction density of 2.5 g / cm2 at 50,000 N. 3 Up to 2.6g / cm 3 The compaction density of the first positive electrode active material is within the above range, which makes the first positive electrode materials more closely contacted, and is conducive to improving the energy density of the battery. For example, the compaction density of the first positive electrode active material is 2.5g / cm 3 , 2.51g / cm 3 , 2.52g / cm 3 , 2.53g / cm 3 , 2.54g / cm 3 , 2.55g / cm 3 , 2.56g / cm 3 , 2.57g / cm 3 , 2.58g / cm 3 , 2.59g / cm 3 , 2.60g / cm 3 Or a value between any two of them.

[0175] In some embodiments, the positive electrode active material further includes a second positive electrode active material, and the second positive electrode active material includes a lithium transition metal oxide. Lithium transition metal oxide has a higher specific capacity. By selecting lithium transition metal oxide as the second positive electrode active material, the energy density of the secondary battery can be further improved.

[0176] In the present application, examples of lithium transition metal oxides may include but are not limited to lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 )、LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.1 Al 0.05 O2) and its modified compounds, etc.

[0177] The battery's charge and discharge processes involve the intercalation and deintercalation of lithium, leading to different molar Li contents at different discharge states. The molar Li contents listed in this application for the positive electrode active materials refer to the initial state of the material, i.e., the state before addition of the materials. When the positive electrode active materials are used in a battery system, the molar Li content will change after charge and discharge cycles.

[0178] In the list of positive electrode active materials in this application, the molar content of oxygen is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of oxygen will fluctuate.

[0179] 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. Exemplarily, 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 consisting of any two of these values.

[0180] In some embodiments, the thickness of the positive electrode film layer disposed on a single side of the positive electrode current collector is 100 μm to 200 μm. By setting the thickness of the positive electrode film layer within the above range, it is beneficial for the positive electrode film layer to have both high capacity and high lithium ion and electron transport performance, thereby facilitating the secondary battery to have both high energy density and fast charging performance. Exemplarily, 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.

[0181] 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 layer within the above range, the amount of lithium ions released per unit area of the positive electrode film layer can be increased, thereby increasing the energy density of the battery. For example, the coating weight of the positive electrode film layer is 200 mg / 1540.25 mm 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 2 、350mg / 1540.25mm 2 、360mg / 1540.25mm 2 、370mg / 1540.25mm 2 、380mg / 1540.25mm2 、390mg / 1540.25mm 2 , 400mg / 1540.25mm 2 Or a value between any two values. Optionally, the coating weight of the positive electrode film layer is 250 / 1540.25mm 2 Up to 320 / 1540.25mm 2 .

[0182] In this application, the coating weight of the positive electrode film layer can be tested using methods known in the art. The positive electrode sheet to be tested can be a prepared positive electrode sheet or a positive electrode sheet obtained by disassembling a battery. Specifically, the positive electrode sheet is obtained by disassembling the battery and cut into pieces with an area of 1540.25 mm 2 The mass of the disc is weighed as m7, and then the positive electrode film layer set on one side of the disc is removed, and the mass of the disc is weighed as m8, and m7-m8 is taken as the coating weight of the positive electrode film layer.

[0183] In some embodiments, the compaction density of the positive electrode film layer is 2 g / cm 3 Up to 3g / cm 3 By making the compaction density of the positive electrode film layer within the above range, it is beneficial for the positive electrode film layer to maintain a good pore structure, reduce the tortuosity of the positive electrode film layer, shorten the lithium ion transmission path, thereby improving the fast charging performance and life performance of the battery while taking into account high energy density. For example, the compaction density of the positive electrode film layer can be 2g / cm 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 a value between any two of the values. Optionally, the compaction density of the positive electrode film is 2.3 g / cm 3 Up to 2.5g / cm 3 .

[0184] In some embodiments, the thickness of the positive electrode current collector is 10μm to 18μm. The thickness of the positive electrode current collector in the above range is beneficial to the battery to achieve high energy density on the one hand, and on the other hand it is beneficial to reduce the risk of cracks in the positive electrode current collector, thereby extending the service life of the battery. Exemplarily, the thickness of the positive electrode current collector is 10μm, 11μm, 12μm, 13μm, 14μm, 14.5μm, 15μm, 15.5μm, 16μm, 16.5μm, 17μm, 17.5μm, 18μm or a value between the ranges consisting of any two of them. Optionally, the thickness of the positive electrode current collector is 13μm to 15μm.

[0185] In some embodiments, the positive electrode 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) on a polymer substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0186] In some embodiments, the positive electrode film layer may further optionally include a binder. For example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0187] In some embodiments, the positive electrode film layer may further include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0188] In some embodiments, the positive electrode film layer further includes a positive electrode dispersant, which includes at least one of polyethylene glycol octylphenyl ether, polyvinyl pyrrolidone, and sodium carboxymethyl cellulose. The positive electrode dispersant has good flexibility and elasticity, and can disperse the stress experienced by the positive electrode active material during compaction. The use of such a positive electrode dispersant facilitates achieving a high compaction density of the positive electrode film layer, thereby increasing the energy density of the battery.

[0189] In some embodiments, the mass proportion of the positive electrode dispersant relative to the positive electrode film layer is 0.3% to 5%. Alternatively, it is 0.1% to 2%, further optionally 0.1% to 1%, and further optionally 0.5% to 0.8%. By controlling the mass proportion 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 proportion of the positive electrode dispersant is 0.3%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5%, or a value between the ranges formed by any two of these values.

[0190] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0191] electrolytes The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0192] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0193] 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 difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0194] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0195] In some embodiments, the electrolyte further includes an additive, including at least one of barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tris(trimethylsilyl)phosphate, and vinylene carbonate. During the initial charge and discharge process of the battery, the additive undergoes an electrochemical reduction reaction on the surface of the negative electrode before the solvent molecules in the electrolyte, forming a dense and stable SEI film, thereby inhibiting the growth of lithium dendrites and improving the safety performance of the battery.

[0196] In some embodiments, in the electrolyte, the mass proportion of the additive is 1% to 10%. By controlling the mass proportion of the additive within the above range, it is beneficial to form a SEI film of moderate thickness on the electrode surface, which is beneficial to inhibit the growth of lithium dendrites and improve the safety performance of the battery. On the other hand, it is beneficial to take into account the transmission resistance of lithium ions in the negative electrode sheet, thereby taking into account the fast charging performance of the battery. Exemplarily, the mass proportion of the additive is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a value between the ranges consisting of any two of them.

[0197] In some embodiments, the electrolyte has a conductivity of 10 mS / cm to 18.5 mS / cm. By selecting an electrolyte with a conductivity within the above range, the internal impedance of the battery can be reduced, reducing energy loss due to resistance during charge and discharge, and extending the battery life. Exemplarily, the electrolyte has a conductivity of 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 a range consisting of any two of these values. Optionally, the electrolyte has a conductivity of 14 mS / cm to 16.8 mS / cm.

[0198] In some embodiments, the electrolyte may further optionally include additives, such as additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0199] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0200] In some embodiments, the battery cell may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0201] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the battery cell may be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0202] The present application has no particular restrictions on the shape of the battery cell, which can be cylindrical, square or any other shape. For example, Figure 1 The battery cell 5 is a square structure as an example.

[0203] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0204] In some embodiments, battery cells may be assembled into a battery module. The battery module may contain one or more battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0205] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of battery cells 5 may further be fixed by fasteners.

[0206] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0207] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0208] Figure 4 and Figure 5 The battery pack 1 is used as an example. Figure 4 and Figure 5The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0209] Electrical devices The present application also provides an electrical device, comprising the secondary battery provided in the first aspect of the present application. The secondary battery may include at least one of a battery cell, a battery module, or a battery pack. The secondary battery may serve as a power source for the electrical device or as an energy storage unit for 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, satellites, energy storage systems, and the like.

[0210] As an electrical device, a battery cell, battery module or battery pack can be selected according to its usage requirements.

[0211] Figure 6 This is an example of an electric device. This device is 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, a battery pack or battery module can be used.

[0212] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0213] Example Below, the embodiment of the present application is described. The embodiment described below is exemplary, is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0214] 1. Preparation of the first graphite material and the second graphite material: Preparation Example 1-1 Preparation of the first graphite material (material 1-1): Step 1: crush and shape the oil-based calcined needle coke raw material using a jet mill to obtain a shaped material with a Dv50 of 15 μm; Step 2: Use granulation asphalt (softening point of 125°C; coking value of 51%) as a granulating agent to granulate the above-mentioned shaping material together through a granulation kettle to obtain granulated material, wherein the mass ratio of the shaping material to the granulating agent is 88:7.

[0215] Step 3: The granulated material was pre-carbonized at 1450° C. for 5.2 h under a nitrogen atmosphere to obtain an intermediate.

[0216] Step 4: graphitizing the intermediate at a high temperature of 3000° C., screening and demagnetizing the graphitized particles to obtain a first graphite material.

[0217] Preparation Example 2-1 Preparation of the second graphite material (material 2-1): Step 1: crush and shape the oil-based calcined needle coke raw material using a jet mill to obtain a shaped material with a Dv50 of 22 μm; Step 2: Use granulation asphalt (softening point of 113° C.; coking value of 66%) as a granulating agent to granulate the above-mentioned shaping materials together through a granulation kettle to obtain granulated materials, wherein the mass ratio of the shaping materials to the granulating agent is 90:6.

[0218] Step 3: The granulated material was pre-carbonized at 1480° C. for 4.7 h under a nitrogen atmosphere to obtain an intermediate.

[0219] Step 4: graphitizing the intermediate at a high temperature of 3000° C., screening and demagnetizing the graphitized particles to obtain a second graphite material.

[0220] 2. Testing of the first graphite material and the second graphite material: (1) I D / I G The test: The samples were tested using a Raman spectrometer.

[0221] The test conditions are: excitation wavelength of 532nm, grating of 600 lines, objective lens of 50 times, integration time of 10s, accumulation times of 3 times, surface scanning, obtain the D peak and G peak intensity of 100 points, calculate the I of 100 points D / I G , remove the largest and smallest 30 I D / I G The average value of the remaining 40 points is the I D / I G The testing instrument may be a Horiba LabRAMHR800 Raman spectrometer.

[0222] Material 1-1 I D / I GThe I of material 2-1 is 0.51. D / I G is 0.13.

[0223] (2) Test of volume average particle size Dv50: The measurement is performed using a laser particle size analyzer in accordance with GB / T 19077-2016. The test instrument may be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0224] The Dv50 of material 1-1 was 11.8 μm, and the Dv50 of material 2-1 was 18.3 μm.

[0225] (3) BET specific surface area test: The surface area analysis method based on nitrogen adsorption is used in accordance with GB / T 19587-2017, and the surface area is calculated using the Brunauer Emmett Teller (BET) method. The test instrument can be a Micromeritics Tri-Star 3020 surface area pore size analyzer.

[0226] The BET specific surface area of material 1-1 is 0.94 m 2 / g. The BET specific surface area of material 2-1 is 1.78m 2 / g.

[0227] (4) Tap density test: Refer to GB / T 5162-2006 and use a powder tap density tester. The test instrument can be the Dandong Better BT-301, with the following test parameters: vibration frequency 250 ± 15 times / minute, amplitude 3 ± 0.2 mm, vibration count 5000 times, and a 25 mL graduated cylinder.

[0228] The tap density of material 1-1 is 1.1 g / cm 3 The tap density of material 2-1 is 1.05 g / cm 3 .

[0229] Table 1

[0230] 3. Preparation of negative electrode slurry Preparation Example 3-1 A first negative electrode slurry was prepared by mixing a first graphite material (see material 1-1 in Table 1), a first binder (styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 2:1), a first dispersant (lithium carboxymethyl cellulose), and a 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.

[0231] Preparation Example 3-2 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) were mixed in a mass ratio of 97.8:1:0.7:0.5 to prepare a second negative electrode slurry.

[0232] Preparation Example 3-3 The negative electrode active material (material 1-1 and material 2-1 in a mass ratio of 5:5), the second binder (styrene-butadiene rubber and lithium polyacrylate in a mass ratio of 2:1), the second dispersant (lithium carboxymethyl cellulose), and the second conductive agent (carbon nanotubes and conductive carbon black in a mass ratio of 5:5) were mixed in a mass ratio of 96.9:1.8:0.8:0.5 to prepare a third negative electrode slurry.

[0233] 4. Preparation of secondary batteries: Example 1 1. Preparation of positive electrode sheet: 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 was added as a solvent 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, dried, and cold pressed to obtain a positive electrode sheet.

[0234] 2. Preparation of negative electrode sheet: Using a dual-chamber coating machine, a first and second negative electrode slurry are extruded simultaneously, with a mass ratio of 5:5. After drying and cold pressing, negative electrode sheets are obtained. The second negative electrode slurry is coated on the negative electrode current collector (copper foil) to form a second film layer, while the first negative electrode slurry is coated on 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 sheets are obtained.

[0235] 3. Preparation of electrolyte: A mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was mixed in 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 above organic solution to obtain an electrolyte with a LiPF6 concentration of 1 mol / L.

[0236] 4. Isolation film: A polyethylene base film with a thickness of 5 μm is used. Both sides of the base film are coated with a coating. The coating includes heat-resistant particles. The heat-resistant particles include boehmite particles (inorganic particles) and a bonding layer arranged on the surface of the boehmite particles.

[0237] 5. Preparation of secondary batteries: The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide insulation. The electrode assembly is then bent and wound to form an electrode assembly. The electrode assembly is placed in an outer package, dried, and then injected with the prepared electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped.

[0238] 5. Test of isolation film morphology: The isolation diaphragm was placed in the sample holder and locked in place. The SEM photograph of the isolation diaphragm cross section was collected using a scanning electron microscope (HR-TEM Talos F200). Figure 7 As shown, it can be seen from the cross-sectional SEM photograph that the coating includes heat-resistant particles, and the heat-resistant particles are interwoven to form a porous structure.

[0239] 6. Compaction density test: (1) Test of the compaction density of the positive electrode film layer: First, wipe off the positive electrode sheet. Punch the positive electrode film from one side into a small disc with an area of S21. Weigh this disc, record it as M21, and measure its thickness H21. Then wipe off the positive electrode film from the weighed positive electrode sheet. Weigh the positive electrode current collector, record it as M20, and measure its thickness H20. The single-sided coating weight of the positive electrode sheet = (M21 - weight of the positive electrode current collector M20) / S21. The thickness of the positive electrode film = H21 - H20. The compacted density of the positive electrode film = the single-sided coating weight of the positive electrode film / the thickness of the positive electrode film.

[0240] (2) Test of the compaction density of the negative electrode film layer: First, wipe off the negative electrode sheet. Punch the negative electrode film from one side into a small disc with an area of S11. Weigh this disc, record it as M11, and measure its thickness H11. Then wipe off the negative electrode film from the weighed negative electrode sheet. Weigh the negative electrode current collector, record it as M10, and measure its thickness H10. The single-sided coating weight of the negative electrode sheet = (M11 - M10) / S11. The thickness of the negative electrode film = H11 - H10. The compacted density of the negative electrode film = the single-sided coating weight of the negative electrode film / the thickness of the negative electrode film.

[0241] 7. Performance test of secondary batteries: (1) Energy density test: At 25°C, charge the battery at a constant current of 0.33C to a cutoff voltage of 3.65V. Then, charge at a constant voltage of 3.65V to a current of 0.05C. At this point, the secondary battery is fully charged. After the fully charged secondary battery is left to rest for 5 minutes, discharge 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, recorded as C0.

[0242] The secondary battery was then charged at a constant current of 0.33C0 to a cutoff voltage of 3.65V. Constant voltage charging was continued until the current reached 0.05C, at which point the secondary battery was fully charged. The fully charged secondary battery was allowed to rest for 5 minutes before being discharged at a constant current of 0.33C0 to a cutoff voltage of 2.5V. The discharge energy Q of the secondary battery was obtained. The energy density (Wh / Kg) of the secondary battery is calculated as: discharge energy Q of the secondary battery / mass M of the secondary battery. The test results are recorded in Table 2 below.

[0243] (2) Safety performance test: Place the fully charged secondary battery in a heating box; Gradually increase the temperature of the heating box at a certain heating rate (3℃ / min); Observe the changes in the secondary battery. When the secondary battery shows thermal runaway, the heating box stops heating. Record the highest temperature when the secondary battery runs away from thermal runaway.

[0244] Examples 2 to 3 Secondary batteries were prepared in the same manner as in Example 1, with the following differences: The cold pressing process of the positive electrode sheet was adjusted so that the compaction density of the positive electrode film layer was as shown in Table 2.

[0245] The cold pressing process of the negative electrode sheet was adjusted so that the compaction density of the negative electrode film layer was as shown in Table 2.

[0246] Examples 4 to 7 Secondary batteries were prepared in Examples 4 to 7 using the same method as in Example 1, with the difference being that base films of corresponding thicknesses were used as described in Table 2.

[0247] Comparative Examples 1 and 2 Secondary batteries were prepared in Comparative Examples 1 and 2 using the same method as Example 1, except that base films of corresponding thicknesses were used as described in Table 2.

[0248] 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. The test results are recorded in Table 2.

[0249] Table 2

[0250] In the embodiments of this application, the maximum temperature during runaway is used to characterize the safety performance of the secondary battery. The higher the maximum temperature during runaway, the worse the battery safety performance. A maximum temperature during thermal runaway exceeding 300°C will cause heat spread and seriously deteriorate the safety performance of the battery.

[0251] Compared with Comparative Example 1 (the thickness of the base film is less than 4 μm), the secondary batteries prepared in Examples 1 to 7 have improved safety performance.

[0252] By comparing the data of Ratio 2, it can be found that when the base film thickness exceeds 12 μm, increasing the base film thickness has limited effect on improving the safety performance of the battery, and will also reduce the energy density of the battery.

[0253] In Examples 1 to 7, the thickness of the prepared secondary battery base film is between 4 μm and 12 μm. Such batteries ensure high safety performance while maintaining high energy density.

[0254] Examples 8 to 11 Secondary batteries were prepared in Examples 8 to 11 using the same method as in Example 1, with the difference being that the coating parameters were adjusted according to the description in Table 3-1.

[0255] Example 12 A secondary battery was prepared in Example 12 using the same method as in Example 1, except that the heat-resistant particles included boehmite particles and PVDF particles.

[0256] The performance of the secondary batteries prepared in Examples 8 to 12 was tested in the same manner as in Example 1. The test results are recorded in Table 3-2.

[0257] Table 3-1

[0258] Table 3-2

[0259] The data in Table 3-1 and Table 3-2 show that the thickness of the coating is set at 0.011 μm to 2 μm, the average particle size of the heat-resistant particles is 5 nm to 185 nm, and the particle surface density of the heat-resistant particles is 0.5 mg / 1540.25 cm 2 Up to 2.5mg / 1540.25cm 2 When the secondary battery is prepared, high safety performance and high energy density can be achieved.

[0260] Example 13 The secondary battery was prepared in the same manner as in Example 1, except that: During the preparation of the negative electrode sheet, the second negative electrode slurry is replaced by the third negative electrode slurry.

[0261] Examples 14 to 17 Secondary batteries were prepared in the same manner as in Example 13, except that: 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) were adjusted according to the records in Table 4.

[0262] Fast charging performance test: At 25°C, the secondary battery was charged at a constant current of 0.33C to a charge cut-off voltage of 3.65V, 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 a discharge cut-off voltage of 2.5V, and its actual capacity was recorded as C0.

[0263] The secondary battery was then charged with a constant current of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2C0, 2.3C0, 2.5C0, and 3.0C0 in sequence to a charge cut-off voltage of 3.65V or a negative electrode cut-off potential of 0mV (whichever was reached first). After each charge was completed, it was discharged with 1C0 to a discharge cut-off voltage of 2.5V. The negative electrode potential corresponding to charging to 10% SOC, 20% SOC, 30% SOC, 40% SOC, 50% SOC, 60% SOC, 70% SOC, and 80% SOC (State of Charge) at different charge rates was recorded.

[0264] The charge rate-negative electrode potential curves of different SOC states are drawn, and the charge rate corresponding to the negative electrode potential of 0V under different SOC states is obtained after linear fitting. The charge rate is the charging window under the SOC state, which is recorded 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 .

[0265] According to the following formula:

[0266] The charging time T for the secondary battery to charge from 10% SOC to 80% SOC (assuming no lithium deposition in the secondary battery) was calculated in minutes. The test results are recorded in Table 4.

[0267] The safety performance and energy density of the secondary batteries prepared in Examples 13 to 17 were tested in the same manner as in Example 1. The test results are recorded in Table 4.

[0268] Table 4

[0269] The “mass ratio” in Table 4 is the mass ratio of the second graphite material to the first graphite material.

[0270] It can be seen from the data in Table 4 that when the second film layer includes the first graphite material and the second graphite material, the prepared secondary battery not only takes into account both safety performance and energy density, but also has further improved fast charging performance.

[0271] Examples 18 to 22 Secondary batteries were prepared in the same manner as in Example 13, except that: Batteries were prepared using the first positive electrode active materials listed in Table 5-1. Parameters of each first positive electrode active material are listed in Table 5-2.

[0272] The performance of the secondary batteries prepared in Examples 18 to 22 was tested in the same manner as in Example 1. The test results are recorded in Table 5-2.

[0273] Table 5-1

[0274] Table 5-2

[0275] It can be seen from the data in Table 5-1 to Table 5-2 that when the positive electrode coating layer includes a fast ion conductor material and / or a carbon material, the fast charging performance of the prepared secondary battery is further improved.

[0276] Examples 23 to 25 Secondary batteries were prepared in the same manner as in Example 13, except that at least one of the ratio of the components in the positive electrode active material, the coating weight of the positive electrode slurry, and the thickness of the positive electrode current collector was adjusted according to the description in Table 6-1.

[0277] The performance of the secondary batteries prepared in Examples 23 to 25 was tested in the same manner as in Example 1. The test results are recorded in Table 6-2.

[0278] Table 6-1

[0279] The “mass ratio” in Table 6-1 is the mass ratio of the first positive electrode active material to the second positive electrode active material.

[0280] Table 6-2

[0281] The data in Table 6-1 and Table 6-2 show that the positive electrode film is fully added with lithium transition metal oxide (NCM 811 ), the energy density of the resulting secondary battery is further improved.

[0282] Examples 26 and 27 Examples 26 and 27 were prepared using the same method as Example 13, with the following differences: the type or addition amount of the positive electrode dispersant (mass percentage in the positive electrode film layer) was adjusted according to the records in Table 7, and the type or addition amount of the additive (mass percentage in the electrolyte) was adjusted according to the records in Table 7.

[0283] The performance of the secondary batteries prepared in Examples 26 and 27 were tested in the same manner as in Example 1. The test results are recorded in Table 7.

[0284] Table 7

[0285] From the data in Table 7, it can be seen that when the mass proportion of the positive electrode dispersant in the positive electrode sheet is 0.3% to 5%, and the mass proportion of the additive in the electrolyte is 1% to 10%, the resulting secondary battery can take into account both energy density and safety performance.

[0286] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present application, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present application. In addition, within the scope of the subject matter of the present application, various modifications that can be conceived by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, characterized in that: It includes a positive electrode sheet, a negative electrode sheet and a separator, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided 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 Up to 3.0g / cm 3 ; The negative electrode plate includes a negative electrode current collector and a negative electrode film layer located 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 Up to 1.85g / cm 3 ; The isolation film includes a base film, the base film has a thickness of 4 μm to 12 μm, the base film has nanopores, and the average pore diameter of the nanopores is 10 nm to 300 nm.

2. The secondary battery according to claim 1, wherein The base film has a thickness of 5 μm to 9 μm.

3. The secondary battery according to claim 1, wherein The porosity of the base film is 20% to 70%.

4. The secondary battery according to claim 1, wherein The isolation film further includes a coating layer provided on at least one side of the base film, wherein the coating layer includes heat-resistant particles, and the heat-resistant particles are interwoven to form a pore structure.

5. The secondary battery according to claim 4, wherein The coating layer provided on one side of the base film has a thickness of 0.011 μm to 3 μm.

6. The secondary battery according to claim 4, characterized in that The average particle size of the heat-resistant particles is 5 nm to 185 nm.

7. The secondary battery according to claim 4, characterized in that The particle surface density of the heat-resistant particles is 0.5 mg / 1540.25 cm 2 Up to 2.5mg / 1540.25cm 2 .

8. The secondary battery according to claim 4, wherein Along the thickness direction of the base film, the heat-resistant particles include first heat-resistant particles; or the heat-resistant particles include first heat-resistant particles and second heat-resistant particles; The first heat-resistant particles are distributed on the surface of the base film, and the second heat-resistant particles are stacked on a side of the first heat-resistant particles away from the base film.

9. The secondary battery according to claim 4, wherein The heat-resistant particles include inorganic particles and bonding particles, and / or, The heat-resistant particles include inorganic particles and a bonding layer disposed on at least a portion of the surface of the inorganic particles.

10. The secondary battery according to claim 9, wherein 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.

11. The secondary battery according to claim 9, wherein The inorganic particles account for 5% to 30% by mass in the coating.

12. The secondary battery according to claim 1, wherein The negative electrode film layer includes a first film layer and a second film layer arranged 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 has a D / I G 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 has a D / I G 0.05 to 0.2; Among them I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at .

13. The secondary battery according to claim 12, 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, and the negative electrode coating layer includes amorphous carbon.

14. The secondary battery according to claim 13, wherein: The thickness of the negative electrode coating layer is 10 nm to 100 nm.

15. The secondary battery according to claim 14, characterized in that The second film layer also includes a first graphite material.

16. The secondary battery according to claim 15, 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).

17. The secondary battery according to claim 12, wherein: 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).

18. The secondary battery according to claim 12, wherein: The negative electrode film layer satisfies at least one of the following: (1) The coating weight of the negative electrode film layer provided 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 provided on one side of the negative electrode current collector is 40 μm to 75 μm.

19. The secondary battery according to claim 1, wherein The thickness of the negative electrode current collector is 4 μm to 8 μm.

20. The secondary battery according to claim 1, wherein The positive electrode film layer includes a positive electrode active material, the positive electrode active material includes a first positive electrode active material, and the first positive electrode active material includes an olivine-structured lithium-containing phosphate.

21. The secondary battery according to claim 20, characterized in that The olivine-structured lithium-containing phosphate comprises a compound as shown in formula (I): LiFe 1-x-y Mn x M 1 y PO4, formula (I); In the formula (I), M 1 At least one selected from V, Nb, Ti, Co, Ni, Sc, Ge, Mg, Al, Zr, Mn, Hf, Ta, Mo, W, Ru, Ag, Sn and Pb, 0≤x≤1, 0≤y<1.

22. The secondary battery according to claim 20, 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-containing phosphate, and includes at least one of a fast ion conductor material and a carbon material.

23. The secondary battery according to claim 22, 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).

24. The secondary battery according to claim 22, wherein: 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-containing phosphate and the second coating layer; Alternatively, the second coating layer is disposed between the lithium-containing phosphate and the first coating layer.

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

26. The secondary battery according to claim 22, characterized in that The mass proportion of carbon element in the first positive electrode active material is 1% to 1.5%.

27. The secondary battery according to claim 20, 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 12 m 2 / g to 16m 2 / g; (2) The tap density of the first positive electrode active material is 0.8 g / cm 3 to 1.3g / 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 50,000 N is 2.5 g / cm 3 Up to 2.6g / cm 3 .

28. The secondary battery according to claim 20, characterized in that The positive electrode active material further includes a second positive electrode active material, and the second positive electrode active material includes a lithium transition metal oxide.

29. The secondary battery according to claim 28, 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).

30. The secondary battery according to claim 20, characterized in that The positive electrode film layer satisfies at least one of the following: (1) The thickness of the positive electrode film layer provided 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 .

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

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

33. The secondary battery according to claim 32, characterized in that The mass proportion of the positive electrode dispersant relative to the positive electrode film layer is 0.3% to 5%.

34. The secondary battery according to claim 1, wherein The secondary battery further includes an electrolyte solution including an electrolyte salt and a solvent; The solvent comprises at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, 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 bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

35. The secondary battery according to claim 34, characterized in that The electrolyte further includes an additive, and the additive includes at least one of barium sulfate, polytrifluoroethyl methacrylate, bicyclic sulfate, tricyclic sulfate, tris(trimethylsilyl)phosphate, and vinylene carbonate.

36. The secondary battery according to claim 35, characterized in that In the electrolyte, the mass proportion of the additive is 1% to 10%.

37. The secondary battery according to claim 36, characterized in that The conductivity of the electrolyte is 10 mS / cm to 18.5 mS / cm.

38. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 37.

Images