Secondary battery and power-consuming device

Abstract

The present application discloses a secondary battery and an electrical device, belonging to the field of electrochemical technology. A secondary battery includes a positive electrode plate, an electrolyte, a separator, and a negative electrode plate. The negative electrode plate includes: a negative electrode current collector; a composite negative electrode material layer region, wherein the composite negative electrode material layer region includes a composite negative electrode material layer; the composite negative electrode material layer region includes a first negative electrode material layer region and a second negative electrode material layer region, wherein the first negative electrode material layer region is disposed between the negative electrode current collector and the second negative electrode material layer region, the first negative electrode material layer region includes a first protective film, and the second negative electrode material layer region includes a second protective film. The average thickness of the first protective film is A nm, and the average thickness of the second protective film is B nm, where B>A. The negative electrode plate of the secondary battery of the present application uses a composite negative electrode material layer, which makes the secondary lithium-ion battery have the characteristics of long cycle life, high energy density, and fast charging capability.

Description

Technical Field

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

[0002] In recent years, with the rise of the electric vehicle industry, the demand for secondary batteries, such as lithium-ion batteries, has continued to increase in terms of energy density, charging speed, lifespan, and safety. Improving the charging speed of lithium-ion batteries is crucial to further promoting the marketization of electric vehicles. Energy density, fast charging capability, and cycle life are mutually influential. However, the development of fast charging technology has also brought new problems. It is easy to increase side reactions and cause lithium deposition, which reduces cycle life and even poses safety risks.

[0003] Therefore, the present application urgently needs to provide a secondary battery with the characteristics of long cycle life, high energy density and fast charging capability to overcome the shortcomings of the existing technology. Summary of the Invention

[0004] The purpose of this application is to provide a secondary battery and an electrical device. The secondary battery has excellent dynamic performance and can solve the above technical problems.

[0005] An embodiment of the present application provides a secondary battery, including a positive electrode plate, an electrolyte, a separator and a negative electrode plate, wherein the negative electrode plate includes: a negative electrode current collector; a composite negative electrode material layer region, wherein the composite negative electrode material layer region has a composite negative electrode material layer; the composite negative electrode material layer region includes a first negative electrode material layer region and a second negative electrode material layer region, wherein the first negative electrode material layer region is arranged between the negative electrode current collector and the second negative electrode material layer region, wherein the first negative electrode material layer region has a first protective film, and the second negative electrode material layer region has a second protective film, wherein the average thickness of the first protective film is A nm, and the average thickness of the second protective film is B nm, and B>A.

[0006] Optionally, in some embodiments of the present application, the composite negative electrode material layer region satisfies the following characteristics: 2.4≤PD×1.5+B / A×2×P≤15.8;

[0007] Where PD g / cm 3 is the compaction density of the composite negative electrode material layer,

[0008] P is the porosity of the composite negative electrode material layer,

[0009] A nm is the average thickness of the first protective film,

[0010] B nm is the average thickness of the second protective film.

[0011] Optionally, in some embodiments of the present application, the composite negative electrode material layer region satisfies the following characteristics: 3≤PD×1.5+B / A×2×P≤9.

[0012] Optionally, in some embodiments of the present application, the compaction density of the composite negative electrode material layer is 1.4 to 1.7 g / cm 3 .

[0013] Optionally, in some embodiments of the present application, the porosity of the composite negative electrode material layer is 20% to 50%.

[0014] Optionally, in some embodiments of the present application, the average thickness of the first protective film is 15 to 100 nm.

[0015] Optionally, in some embodiments of the present application, the average thickness of the second protective film is 70 to 200 nm.

[0016] Optionally, in some embodiments of the present application, the thickness of the first negative electrode material layer region is 40 to 80 μm.

[0017] Optionally, in some embodiments of the present application, the thickness of the second negative electrode material layer region is 40 to 80 μm.

[0018] Optionally, in some embodiments of the present application, the gram capacity of the composite negative electrode material layer is 340 to 370 mAh / g.

[0019] Optionally, in some embodiments of the present application, the orientation degree of the composite negative electrode material layer is 3-40.

[0020] Optionally, in some embodiments of the present application, the first negative electrode material layer region contains first negative electrode active material particles, the average particle size of the first negative electrode active material particles is D1μm, and the second negative electrode material layer region contains second negative electrode active material particles, the average particle size of the second negative electrode active material particles is D2μm, satisfying D1≥D2.

[0021] Optionally, in some embodiments of the present application, 15≤D1≤30, 6≤D2≤15.

[0022] Optionally, in some embodiments of the present application, the electrolyte contains at least one of an unsaturated cyclic carbonate or a silane compound.

[0023] Correspondingly, an embodiment of the present application further provides an electrical device comprising the secondary battery described above.

[0024] The beneficial effects of this application are:

[0025] The present application provides a secondary battery in which the negative electrode plate utilizes a composite negative electrode material layer, resulting in a secondary lithium-ion battery having a long cycle life, high energy density, rapid charging capability, and excellent kinetic performance. Specifically, the present application achieves high energy density, long cycle life, and rapid charging capability by matching the relationship between the average thickness, compaction density, and porosity of the protective film of the composite negative electrode material layer in the negative electrode plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 This is a schematic diagram of the structure of the negative electrode provided in the embodiment of the present application. Figure 1 .

[0028] Figure 2 This is a schematic diagram of the structure of the negative electrode provided in the embodiment of the present application. Figure 2 .

[0029] Figure 3 This is a schematic diagram of the structure of the negative electrode provided in the embodiment of the present application. Figure 3 .

[0030] Figure 4 This is a transmission electron microscope image of the first protective film in the first negative electrode material layer area in the negative electrode plate of Example 1 of the present application.

[0031] Figure 5 This is a transmission electron microscope image of the second protective film in the second negative electrode material layer area in the negative electrode plate of Example 1 of the present application.

[0032] Figure 6 This is a rate performance curve of the first negative electrode active material and the second negative electrode active material in the negative electrode sheet of Example 1 of the present application.

[0033] The numbers in the figure are: 10, negative electrode sheet; 100, negative electrode current collector; 300, composite negative electrode material layer region; 310, first negative electrode material layer region; 320, second negative electrode material layer region. DETAILED DESCRIPTION

[0034] The following, in conjunction with the accompanying drawings, provides a clear and complete description of the technical solutions in the embodiments of the present application. It should be noted that the described embodiments are only a portion of the embodiments of the present application, and are not intended to be exhaustive. All other embodiments derived by persons skilled in the art based on the embodiments of the present application without inventive effort are intended to fall within the scope of protection of the present application. Furthermore, in the description of the present application, the term "including" means "including but not limited to." The terms "first," "second," and "third," etc., are used merely as designations and do not impose numerical requirements or establish a sequence. Various embodiments of the present application may be presented in the form of a range. It should be understood that describing in a range format is merely for convenience and brevity and should not be construed as a rigid limitation on the scope of the present application. Therefore, the range description should be considered to specifically disclose all possible subranges and individual numerical values within the range. For example, a range description of 1 to 6 should be considered to specifically disclose subranges, such as 1 to 3, 1 to 4, 1 to 5, 2 to 4, 2 to 6, 3 to 6, etc., as well as individual numerical values within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range.

[0035] Currently, graphite is the most widely used negative electrode material in the market. This is primarily due to the minimal volume change of graphite negative electrodes during battery charge and discharge. Furthermore, a solid electrolyte membrane (SEI) forms at the interface between the graphite negative electrode and the electrolyte, preventing macromolecular groups from embedding into the graphite negative electrode along with lithium ions, thus preventing graphite exfoliation and maintaining battery cycle stability. The structure and performance of the SEI membrane directly impact the battery's irreversible capacity loss, rate capability, cycle performance, graphite exfoliation, and battery safety. Therefore, improving battery performance requires finding a balance between fast charging and high energy density.

[0036] The high-performance negative electrode active material and the reasonable negative electrode plate design can better achieve the effect of fast charging of the battery. How to obtain fast charging capability without sacrificing energy density is the key to battery design. To improve energy density, on the one hand, a single material can increase the active material content by increasing the thickness of the electrode, but it will make the electrolyte penetration difficult, thus affecting the rate performance under high current; on the other hand, the use of synergistic effects between materials for material compounding can improve the overall performance to a certain extent, but it requires strict evaluation of materials and formulas to maximize high performance, which is time-consuming and labor-intensive. Therefore, the inventors of this application have developed a high-capacity and high-safety secondary battery with both energy density and fast charging performance, which is of great significance to this field.

[0037] During the research and practice of the technology, the inventors of this application discovered that:

[0038] During the battery charging process, the negative electrode needs to go through the following electrochemical processes: (1) The lithium ions released from the positive electrode active material enter the electrolyte and enter the pores of the negative electrode membrane along with the electrolyte, completing the liquid phase conduction of the active ions in the pores. The liquid phase conduction includes liquid phase diffusion and electromigration; (2) The active ions and electrons complete the charge exchange on the surface of the negative electrode active material; (3) The active ions are solid-phase conducted from the surface of the negative electrode active material to the interior of the negative electrode active material crystals.

[0039] Low compaction density helps change the porosity distribution of the electrode itself, improve the electrolyte's infiltration ability, and enhance battery filling efficiency and battery consistency. Higher compaction density helps reduce the material's contact internal resistance, which helps the material perform better. However, as the electrolyte has difficulty infiltrating into the electrode, the path for ion movement cannot be ensured, making it difficult to achieve smooth ion movement, which can lead to reduced battery performance and life characteristics.

[0040] The liquid-phase conduction of ions within the porous electrode pores of the negative electrode material layer has an important influence on the improvement of the battery's fast charging capability, and the liquid-phase conduction of ions within the porous electrode pores of the negative electrode material layer is closely related to the pore structure morphology of the negative electrode plate. A large porosity is conducive to the liquid-phase conduction of ions, but the electronic contact between the particles of the negative electrode active material will be affected, resulting in a significant impact on the charge exchange rate between ions and electrons on the surface of the negative electrode active material, affecting the electronic conductivity of the negative electrode plate. A small porosity is conducive to the electronic conductivity of the negative electrode plate, but the pore structure inside the negative electrode plate will be more tortuous, which is not conducive to the liquid-phase conduction of ions. During the battery charging process, the ions may be reduced and precipitated directly on the negative electrode surface due to liquid-phase polarization.

[0041] The structure and properties of the SEI film have a crucial impact on battery performance. On the one hand, the formation of the SEI film increases the irreversible capacity of the initial charge and discharge, reducing the charge and discharge rate of the electrode material. On the other hand, it prevents the organic electrolyte from embedding into the electrode, helping to maintain the electrode's mechanical stability and improving the battery's cycle life and safety. The SEI film does not conduct electrons but allows the smooth passage of lithium ions, effectively inhibiting the co-intercalation of solvated lithium ions and preventing the shedding of the graphite layer during cycling. This imparts good kinetic stability to the active material surface, effectively protecting the electrode from side reactions and suppressing the formation of lithium dendrites. The characteristics of the negative electrode further influence the formation of the SEI film, as well as its structure and properties.

[0042] The embodiments of the present application provide a secondary battery and an electrical device. These are described in detail below. It should be noted that the order in which the following embodiments are described does not limit the preferred order of the embodiments.

[0043] The present invention provides a secondary battery comprising a positive electrode sheet, an electrolyte, a separator, and a negative electrode sheet. The negative electrode sheet comprises a negative electrode current collector and a composite negative electrode material layer, wherein the composite negative electrode material layer is disposed on the negative electrode current collector. In other words, the negative electrode sheet comprises a composite negative electrode material layer region disposed on the negative electrode current collector, wherein the composite negative electrode material layer region comprises a composite negative electrode material layer.

[0044] Furthermore, the composite negative electrode material layer includes at least two negative electrode material layers. In the composite negative electrode material layer, each negative electrode material layer is formed with a solid electrolyte membrane (SEI membrane, also known as a protective membrane). The thickness of the solid electrolyte membrane formed by the negative electrode material layer close to the negative electrode current collector is less than the thickness of the solid electrolyte membrane formed by the negative electrode material layer far from the negative electrode current collector.

[0045] In one embodiment, see Figure 1 The negative electrode sheet 10 includes a negative electrode current collector 100 and a composite negative electrode material layer region 300. The composite negative electrode material layer region includes a composite negative electrode material layer. The composite negative electrode material layer region 300 includes a first negative electrode material layer region 310 and a second negative electrode material layer region 320. The first negative electrode material layer region 310 is disposed between the negative electrode current collector 100 and the second negative electrode material layer region 320. It is conceivable that in the embodiment of the present application, the first negative electrode material layer region 310 includes a first negative electrode material layer, and the second negative electrode material layer region 320 includes a second negative electrode material layer.

[0046] Furthermore, the first negative electrode material layer region 310 is disposed between the negative electrode current collector 100 and the second negative electrode material layer region 320. A first protective film (first SEI film) is formed on the first negative electrode material layer region 310, and a second protective film (second SEI film) is formed on the second negative electrode material layer region 320. The average thickness of the first protective film is A nm, and the average thickness of the second protective film is B nm, where B>A. That is, the average thickness of the second protective film is greater than the average thickness of the first protective film.

[0047] It is conceivable that the first negative electrode material layer region 310 may include a first negative electrode material layer and a first protective film, and the second negative electrode material layer region may include a second negative electrode material layer and a second protective film.

[0048] In the embodiment of the present application, the thin SEI film can effectively reduce the loss of lithium ions during the charging cycle and improve the energy density of the battery; reduce the transmission impedance of lithium ions, have a stronger binding force with the negative electrode active material, avoid the phenomenon of graphite volume expansion and layer peeling, which is beneficial to improve the reversibility of the negative electrode reaction and extend the service life. Since the outer layer of active material has better kinetic performance, the side reactions of the electrolyte on the negative electrode surface increase, resulting in an increase in the thickness of the SEI film. The thick SEI film has more defects that are conducive to electron transmission, ensuring its conduction between the electrolyte and the negative electrode layer, improving Li+ The insertion / deintercalation dynamics can avoid uneven ion deposition and the generation of lithium dendrites during charge and discharge, and at the same time help to hinder the reduction and decomposition of the electrolyte on the negative electrode surface, thereby improving safety performance.

[0049] In some embodiments, the composite negative electrode material layer region satisfies the following characteristics:

[0050] 2.4≤PD×1.5+B / A×2×P≤15.8; where PD g / cm 3 is the compaction density of the composite negative electrode material layer, P is the porosity of the composite negative electrode material layer, A nm is the average thickness of the first protective film, B nm is the average thickness of the second protective film, and B>A.

[0051] In some embodiments, the composite negative electrode material layer area satisfies: 3≤PD×1.5+B / A×2×P≤9. Where, PDg / cm 3 is the compaction density of the composite negative electrode material layer, P is the porosity of the composite negative electrode material layer, Anm is the average thickness of the first protective film, B nm is the average thickness of the second protective film, and B>A. When the composite negative electrode material layer satisfies the above relationship, the battery has better overall performance.

[0052] In some embodiments, 3.4≤PD×1.5+B / A×2×P≤9. When the composite negative electrode material layer satisfies the above relationship, the overall performance of the battery can be further improved.

[0053] In some embodiments, the compaction density (PD g / cm 3 ) is 1.4~1.7g / cm 3 When the above range is maintained, the porous negative electrode can maintain a reasonable pore structure while avoiding the negative electrode active material and the electrolyte from producing more side reactions that affect the improvement of battery performance.

[0054] In some embodiments, the compaction density (PD g / cm 3 ) is 1.45~1.65g / cm 3 .

[0055] In some embodiments, the porosity (P) of the composite negative electrode material layer is 20% to 50%. When the porosity of the composite negative electrode material layer is within this range, the negative electrode sheet is better able to retain the electrolyte, and good electrolyte wettability between the negative electrode active material particles is ensured. The interfacial charge transfer impedance between the negative electrode active material and the electrolyte is also lower, further improving the battery's fast charging capability and cycle life.

[0056] In some embodiments, the porosity (P) of the composite negative electrode material layer is 25% to 48%.

[0057] In some embodiments, the porosity (P) of the composite negative electrode material layer is 25% to 45%.

[0058] In some embodiments, the average thickness (nm) of the first protective film is 15 to 100 nm. When the average thickness of the first protective film is within the above range, the resulting battery can have a long cycle life, high energy density, and fast charging capability. The first protective film (i.e., the thin inner SEI film) can effectively reduce lithium ion loss during the charging cycle, thereby improving the energy density of the battery. It also reduces the transmission impedance of lithium ions, has a stronger bond with the electrode material, and avoids the phenomenon of graphite volume expansion and flaking, which is conducive to improving the reversibility of the negative electrode reaction and extending the service life.

[0059] In some embodiments, the average thickness (Anm) of the first protection film is 20-90 nm.

[0060] In some embodiments, the average thickness (B nm) of the second protective film is 70 to 200 nm. Within this preferred range, the resulting battery can achieve long cycle life, high energy density, and fast charging capability. The second protective film (i.e., the thick outer SEI film) contains more defects that are conducive to electron transport, ensuring its conduction between the electrolyte and the negative electrode layer, improving the insertion / extraction dynamics of lithium ions, and avoiding uneven ion deposition during charge and discharge to form lithium dendrites. It also helps prevent the reduction and decomposition of the electrolyte on the negative electrode surface, thereby improving safety performance.

[0061] In some embodiments, the second protective film has an average thickness (B nm) of 80-180 nm.

[0062] In some embodiments, the thickness of the first negative electrode material layer region is 40 to 80 μm. The thickness of the second negative electrode material layer region is 40 to 80 μm. Furthermore, a first protective film is formed within the first negative electrode material layer region; and a second protective film is formed within the second negative electrode material layer region. Furthermore, the first protective film is formed within a 40 to 60 μm range in the first negative electrode material layer region close to the negative electrode current collector; and the second protective film is formed within a 40 to 60 μm range in the second negative electrode material layer region away from the negative electrode current collector.

[0063] In some embodiments, the first negative electrode material layer region contains first negative electrode active material particles, and the average particle size of the first negative electrode active material particles is D1 μm. The second negative electrode material layer region contains second negative electrode active material particles, and the average particle size of the second negative electrode active material particles is D2 μm. The first negative electrode active material particles and the second negative electrode active material particles satisfy: D1 ≥ D2. It is conceivable that the first negative electrode material layer contains the first negative electrode active material particles, and the second negative electrode material layer contains the second negative electrode active material particles.

[0064] Furthermore, the average particle size of the first negative electrode active material particles is 15 to 30 μm; and the average particle size of the second negative electrode active material particles is 6 to 15 μm.

[0065] In the embodiments of the present application, in the composite negative electrode material layer, the kinetic performance of the first negative electrode material layer is lower than the kinetic performance of the second negative electrode material layer. Alternatively, the kinetic performance of the negative electrode material layer closer to the negative electrode current collector is lower than the kinetic performance of the negative electrode material layer farther from the negative electrode current collector. This is the case when the composite negative electrode material layer has two negative electrode material layer regions. Furthermore, when the composite negative electrode material layer of the negative electrode sheet has two or more negative electrode material layer regions, the kinetic performance of the active material in the negative electrode material layer regions gradually increases as it moves outward from the negative electrode current collector.

[0066] In the embodiment of the present application, the composite negative electrode material layer of the negative electrode plate may have three negative electrode material layer regions. Figure 2 Specifically, the negative electrode sheet 10 includes a negative electrode current collector 100 and a composite negative electrode material layer region 300 located on one side of the negative electrode current collector 100, the composite negative electrode material layer region comprising a composite negative electrode material layer. The composite negative electrode material layer region 300 on the negative electrode current collector 100 includes a first negative electrode material layer region 310, a second negative electrode material layer region 320, and a third negative electrode material layer region 330. Similarly, the composite negative electrode material layer region can be formed to have four, five, six, or other layers.

[0067] In the embodiment of the present application, the composite negative electrode material layer in the negative electrode plate can be on either side of the negative electrode current collector, or the composite negative electrode material layer can be arranged on both sides of the negative electrode current collector. Figure 3 The negative electrode sheet 10 includes: a negative electrode current collector 100, and composite negative electrode material layer regions 300 located on both sides of the negative electrode current collector 100. The composite negative electrode material layer region 300 on each negative electrode current collector 100 includes a first negative electrode material layer region 310 and a second negative electrode material layer region 320. Furthermore, the kinetic performance of the negative electrode material layer regions gradually improves from the negative electrode current collector outward. That is, in the composite negative electrode material layer region 300 on either side of the negative electrode current collector 100, the kinetic performance of the second negative electrode material layer region 320 is higher than the kinetic performance of the first negative electrode material layer region 310.

[0068] In some embodiments, the composite negative electrode material layer region comprises multiple negative electrode material layers, for example, each negative electrode material layer region comprises one negative electrode material layer; each negative electrode material layer comprises a negative electrode active material. Furthermore, the mass fraction of the negative electrode active material in each negative electrode material layer may be 93% to 98%; for example, the mass fraction may be 93%, 94%, 95%, 96%, 97%, or 98%. Furthermore, the negative electrode active material is independently selected from, but not limited to, one or more of, artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, soft carbon, and carbon fiber. Furthermore, the materials of each negative electrode material layer further comprise a conductive agent, a binder, and an additive. For example, the conductive agent is carbon black; the binder is styrene-butadiene rubber; and the additive is sodium carboxymethyl cellulose.

[0069] In some embodiments, the composite negative electrode material layer has a gram capacity of 340 to 370 mAh / g.

[0070] In some embodiments, the orientation degree (OI value) of the composite negative electrode material layer is 3 to 40. The OI value in the negative electrode sheet can characterize the active reaction sites of the composite negative electrode material layer. Generally, the smaller the OI value of the composite negative electrode material layer, the more end faces in the composite negative electrode material layer are available for active ion deintercalation, the more active reaction sites in the negative electrode material layer, the faster the charge exchange rate between active ions and electrons on the surface of the negative electrode active material during charging, the better the battery's kinetic performance, and the greater the ability to withstand higher charging rates.

[0071] As can be seen from this, the multilayered negative electrode sheet in this application can improve battery energy density. By coating the current collector layer by layer with materials of varying kinetic properties, from low to high, the SEI film formed by the surface active material layer is thicker, with more defects that facilitate electron transport, and exhibits excellent ion conductivity. This also helps prevent the electrolyte from undergoing reduction and decomposition on the negative electrode surface, improving rate performance. The thin inner SEI film reduces lithium ion transmission impedance, prevents graphite volume expansion and flake exfoliation, and helps improve the reversibility of the negative electrode reaction, thereby extending the battery's cycle life.

[0072] In some embodiments, the electrolyte comprises at least one of an unsaturated cyclic carbonate or a silane compound.

[0073] In some embodiments, the unsaturated cyclic carbonate comprises vinylene carbonate (VC) and / or vinyl vinylene carbonate. When the negative electrode plate meets the above characteristics, the electrolyte further includes an unsaturated cyclic carbonate, which can optimize the composition of the protective film formed by the electrolyte on the composite negative electrode material layer, thereby further improving the overall performance of the battery.

[0074] In some embodiments, the content of the unsaturated cyclic carbonate is a%, based on the mass of the electrolyte, with 0.05≤a≤3. When the content of the unsaturated cyclic carbonate is within the above range, the composition and thickness of the protective film can be further controlled, while maintaining the protective film in a more optimal state, thereby improving the battery performance.

[0075] In some embodiments, the silane compound includes at least one of tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite (TMSPi), trimethylfluorosilane, or tris(trimethylsilyl)borate. When the negative electrode plate meets the aforementioned characteristics, further inclusion of a silane compound in the electrolyte can further enhance the overall performance of the battery.

[0076] In some embodiments, the content of the silane compound is b% based on the mass of the electrolyte, and 0.1≤b≤4.

[0077] In some embodiments, the electrolyte comprises an unsaturated cyclic carbonate and a silane compound.

[0078] In some embodiments, the electrolyte comprises a lithium salt additive, and the lithium salt additive comprises at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, or lithium difluorophosphate.

[0079] In some embodiments, the electrolyte comprises an unsaturated cyclic carbonate and a silane compound.

[0080] In the embodiment of the present application, the negative electrode current collector may be made of copper foil.

[0081] In the embodiment of the present application, the materials of the positive electrode plate include but are not limited to positive electrode active material (such as NCM811), conductive agent (such as carbon black), and binder (such as PVDF).

[0082] In the embodiments of the present application, other materials such as the separator in the secondary battery can be made of conventional types or materials in the art.

[0083] The embodiment of the present application further provides an electric device, comprising the above secondary battery, wherein the secondary battery comprises the above negative electrode plate.

[0084] The present application also provides a method for preparing a secondary battery, wherein the method for preparing a negative electrode sheet includes the following steps:

[0085] The first layer of negative electrode slurry and the second layer of negative electrode slurry are sequentially coated on the negative electrode current collector, rolled, and cut into sheets to obtain negative electrode sheets. It can be imagined that the first layer of negative electrode slurry and the second layer of negative electrode slurry are sequentially coated on the same side surface of the negative electrode current collector.

[0086] Furthermore, the first layer of negative electrode slurry and the second layer of negative electrode slurry can be coated on the negative electrode current collector at the same time, or can be coated layer by layer.

[0087] Furthermore, the preparation of the first layer of negative electrode slurry includes: mixing the first negative electrode active material with a conductive agent, a binder, and additives, adding deionized water, stirring, and sieving to obtain the first layer of negative electrode slurry. The preparation of the second layer of negative electrode slurry includes: mixing the second negative electrode active material with a conductive agent, a binder, and additives, adding deionized water, stirring, and sieving to obtain the second layer of negative electrode slurry. It is contemplated that the specific selection of the conductive agent, binder, and additives in the first layer of negative electrode slurry can be the same as or different from those in the second negative electrode active material.

[0088] Furthermore, the preparation method of the secondary battery includes the following steps: stacking the positive electrode sheet, the separator and the prepared negative electrode sheet in sequence, using a separator (such as Celgard 2300), then winding them into a battery cell and placing them in a soft package shell, and after top and side sealing, liquid injection, formation, sorting and other processes, a lithium-ion battery is obtained, that is, a secondary battery is obtained.

[0089] The present invention has been tested many times, and some of the test results are now cited as reference to further describe the invention in detail, and the following is a detailed description of the invention in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0090] Example 1

[0091] This embodiment provides a secondary battery, including a positive electrode plate, an electrolyte, a separator and a negative electrode plate, the negative electrode plate including: a negative electrode current collector; a composite negative electrode material layer region, the composite negative electrode material layer region having a composite negative electrode material layer; the composite negative electrode material layer region including a first negative electrode material layer region and a second negative electrode material layer region, the first negative electrode material layer region being arranged between the negative electrode current collector and the second negative electrode material layer region, the first negative electrode material layer region having a first protective film, the second negative electrode material layer region having a second protective film, the average thickness of the first protective film being A nm, the average thickness of the second protective film being B nm, and B>A.

[0092] The preparation of a secondary battery includes the following steps:

[0093] The positive electrode sheet, isolation membrane and negative electrode sheet are stacked in sequence, using Celgard 2300 separator, then wound into a battery cell and placed in a soft-pack shell. After top and side sealing, liquid injection, formation, sorting and other processes, a secondary battery is obtained.

[0094] Preparation of positive electrode:

[0095] The positive electrode active material NCM811, the conductive agent carbon black, and the binder PVDF are mixed in a mass ratio of 96:2:2, the solvent NMP is added, and the mixture is stirred under the action of a vacuum mixer to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying, and then rolled and cut into pieces to obtain a positive electrode sheet.

[0096] Preparation of electrolyte:

[0097] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of 1:1:1, and then LiPF6 is added and mixed evenly to prepare an electrolyte.

[0098] Preparation of negative electrode sheet:

[0099] The artificial graphite negative electrode material (the average particle size of the first negative electrode active material particles is 16 μm), the conductive agent carbon black, the binder styrene-butadiene rubber and the additive sodium carboxymethyl cellulose are mixed in a mass ratio of 96.5:1.5:1.5:0.5, and deionized water is added as a solvent. The mixture is fully stirred under the action of a vacuum mixer and passed through a 200-mesh sieve to obtain a first layer of negative electrode slurry;

[0100] The artificial graphite negative electrode material (the average particle size of the second negative electrode active material particles is 7 μm), the conductive agent carbon black, the binder styrene-butadiene rubber and the additive sodium carboxymethyl cellulose are mixed in a mass ratio of 96.5:1.5:1.5:0.5, and the solvent deionized water is added. The mixture is fully stirred under the action of a vacuum mixer and passed through a 200-mesh sieve to obtain a second layer of negative electrode slurry;

[0101] The prepared first layer of negative electrode slurry and second layer of negative electrode slurry are coated within 24 hours after discharge, and are coated on the surface of the negative electrode current collector copper foil in the order of the first layer of negative electrode slurry and the second layer of negative electrode slurry by a coating machine; then, the negative electrode sheet with a composite negative electrode material layer is obtained through rolling and cutting.

[0102] In this embodiment, the compaction density of the composite negative electrode material layer of the negative electrode plate is 1.6 g / cm 3 , porosity is 30%, OI value is 10, the average thickness of the first protective film formed in the first negative electrode material layer region is 30 nm, and the average thickness of the second protective film formed in the second negative electrode material layer region is 90 nm, that is, PD×1.5+B / A×2×P is 4.2. PD is the compaction density of the composite negative electrode material layer, P is the porosity of the composite negative electrode material layer, A is the average thickness of the first protective film, and B is the average thickness of the second protective film.

[0103] The performance parameters of the secondary battery in this embodiment are tested.

[0104] See also Figure 4 and Figure 5 , Figure 4 The first protective film is shown; Figure 5 The second protective film is shown in FIG. It can be seen that the thickness of the first protective film is smaller than that of the second protective film.

[0105] See also Figure 6 , Figure 6 The rate performance curves for the first and second negative electrode active materials in the negative electrode sheet of Example 1 of the present application are shown below. Specifically, the rate performance test method involves assembling the first and second negative electrode active materials into a button-type half-cell. The cells are then placed at 25°C for 30 minutes, charged with a 1C constant current and constant voltage charge, with a cutoff current of 0.05C, and allowed to rest for 5 minutes. The cells are then discharged at the following rates: 1 / 3, 0.5, 1, 2, 3, and 4C, completing each discharge rate in sequence.

[0106] Examples 2 to 18

[0107] Examples 2 to 18 respectively provide a secondary battery, which differs from the secondary battery of Example 1 in that: in the negative electrode sheets of Examples 2 to 12, the compaction density, porosity, OI value, average thickness of the first protective film, average thickness of the second protective film of the composite negative electrode material layer, and the average particle size of the first negative electrode active material particles and the average particle size of the second negative electrode active material particles used are different from those in Example 1, as shown in Table 1 for details.

[0108] The structures and preparation methods of the secondary batteries of Examples 2 to 18 are the same as those of Example 1.

[0109] Comparative Examples 1 to 3

[0110] Comparative Examples 1 to 3 each provide a secondary battery. These secondary batteries differ from the secondary battery of Example 1 in that the compaction density, porosity, OI value, average thickness of the first protective film, average thickness of the second protective film, and average particle size of the first negative electrode active material particles and average particle size of the second negative electrode active material particles used in the composite negative electrode material layer are different from those in Example 1, as shown in Table 1. Other conditions are the same as those of Example 1.

[0111] Example 19 to Example 29

[0112] The preparation method is similar to that of Example 3, except that other substances are added to the electrolyte. The specific composition and content of the substances are shown in Table 3.

[0113] The thickness test of the protective film of the secondary battery of the embodiment of the present application: The average thickness of the first protective film and the second protective film in the secondary battery of the embodiment of the present application was tested by TEM.

[0114] The gram capacity testing method of the secondary battery of the embodiment of the present application can be: the battery is allowed to stand at 25°C for 30 minutes, discharged at a constant current of 1C, allowed to stand for 10 minutes, charged at a constant current of 1C, discharged at a constant voltage of 0.05C, allowed to stand for 5 minutes, and discharged at a constant current of 1C. The discharge capacity at this time is the capacity of the battery under 1C.

[0115] Table 1

[0116]

[0117]

[0118]

[0119] Test Example 1

[0120] The performance parameters of the secondary batteries obtained in Examples 1 to 18 and Comparative Examples 1 to 3 were measured respectively, as shown in Table 2.

[0121] Performance testing method of the secondary battery of this application:

[0122] Rate performance test:

[0123] The battery is left at rest for 30 minutes at 25°C, discharged at a constant current of 1C, then charged at a constant current and constant voltage of 1C, left at rest for 5 minutes, and discharged at a constant current of 1C. The discharge capacity at this time is the actual capacity C0 of the battery at 1C. The battery is charged at a constant current and constant voltage of 1C, left at rest for 5 minutes, and discharged at a constant current of 2C. The discharge capacity C1 is the 2C discharge capacity. 2C rate capacity retention rate = (2C discharge capacity / 1C discharge capacity) × 100% = C1 / C0 × 100%.

[0124] Dynamic performance test:

[0125] The battery was left at rest at 25°C for 30 minutes. The battery was then fully charged at xC and fully discharged at 1C, repeated 10 times. The battery was then fully charged at xC. The negative electrode was then disassembled and the surface of the negative electrode observed for lithium deposition. If no lithium deposition was observed, the charge rate xC was increased by 0.1C and the test was repeated until lithium deposition occurred. The maximum charge rate of the battery was then determined by subtracting 0.1C from the charge rate xC.

[0126] Cyclic performance test:

[0127] The battery was left at rest at 25°C for 30 min, discharged at 1C constant current, left at rest for 10 min, charged at 1C constant current and constant voltage, left at rest for 10 min, and then fully charged and discharged. The capacity retention rate after 1000 cycles was recorded.

[0128] Table 2

[0129]

[0130]

[0131] It can be seen from the test results in Table 2 that the negative electrode plates in the secondary batteries of Examples 1 to 18 all satisfy 2.4≤PD×1.5+B / A×4×P≤15.8, and the corresponding batteries can have long cycle life, high energy density and fast charging capability. This is because the average thickness of the protective film formed by the first negative electrode material layer and the average thickness of the protective film formed by the second negative electrode material layer match well, the pore structure of the plate is relatively reasonable, the liquid phase conduction resistance of lithium ions in the negative electrode is small, and the kinetic performance of the negative electrode plate is good, so that the obtained battery can have long cycle life, high energy density and fast charging capability.

[0132] Compared with Examples 1 to 18, in Comparative Examples 1 to 3, when one or more of the parameters including the compaction density PD of the negative electrode sheet, the porosity P of the negative electrode sheet, and the average thickness A and B of the protective film of the inner and outer layers of the active slurry layer fail to meet the above-mentioned preferred range, the battery cycle or lithium plating rate deteriorates, and it is impossible to simultaneously take into account long cycle life, high energy density, and fast charging capability.

[0133] Test Example 2

[0134] The performance parameters of the secondary batteries obtained in Examples 19 to 29 and Example 3 were measured respectively, as shown in Table 3.

[0135] High temperature cycle performance test: The battery was placed at 45°C for 30 minutes, discharged at 1C constant current, allowed to stand for 10 minutes, charged at 1C constant current and constant voltage, allowed to stand for 10 minutes, and then fully charged and discharged. The capacity retention rate after 1000 cycles was recorded.

[0136] Table 3

[0137]

[0138] It can be seen from the test results in Table 3 that Examples 19 to 29 all have excellent cycle performance.

[0139] According to Examples 19 to 26, by adding vinylene carbonate (VC) among unsaturated cyclic carbonates or tris(trimethylsilyl)phosphite (TMSPi) among silane compounds to the electrolyte, the overall performance of the battery is basically improved; according to Examples 27 to 28, by adding unsaturated cyclic carbonates and silane compounds to the electrolyte, the comprehensive performance of the battery is significantly improved; according to Example 29, by adding unsaturated cyclic carbonates, silane compounds and lithium salt additives to the electrolyte, the comprehensive performance of the battery is further improved.

[0140] In summary, the negative electrode sheet of the present application has a multi-layer composite layer structure, so it can have a high energy density, wherein different layers are coated layer by layer from the inside to the outside with the corresponding electrode structure from low to high kinetic performance, so that the kinetic performance of the active material increases slowly from the inside to the outside, and the corresponding solid electrolyte interface film gradually becomes thicker from the inside to the outside. The kinetic performance of the surface active material layer is better, and the side reactions of the electrolyte on the negative electrode surface increase, resulting in an increase in the average thickness of the protective film, and there are more defects that are conducive to electron transmission, thereby improving Li + The insertion / deinsertion kinetics of the battery are optimized. The inner protective film is thin and dense, which helps reduce the internal resistance of the material and improve the reversibility of the negative electrode reaction, thereby extending the battery's service life. The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer disposed on one or both surfaces of the negative electrode current collector. The formula 2.4≤PD×1.5+B / A×2×P≤15.8 is satisfied, where A and B are the average thickness of the protective film formed by the active material particles of the first and second negative electrode material layers, PD is the compacted density of the negative electrode material layer, and P is the porosity of the negative electrode material layer. By adjusting these relationships to further satisfy the formula, the battery can achieve higher fast charging capabilities while improving energy density.

[0141] In the present application, the composite negative electrode material layer comprises a structure of two or more layers. The characteristics of different electrode materials having different kinetic properties are utilized to prepare composite pole pieces coated with kinetics ranging from low to high. The design of different kinetic active material layers can improve the charging characteristics by reducing the interface resistance of the negative electrode. The pole piece has high capacity and high rate performance, and the preparation process is simple. These negative electrode active material layers with different kinetics can better play their respective roles and achieve the purpose of taking both high energy density and fast charging capability into account. It can be seen that the negative electrode pole piece of the present application adopts a composite negative electrode material layer, which makes the secondary lithium-ion battery have the characteristics of long cycle life, high energy density and fast charging capability.

[0142] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0143] The above is a detailed introduction to a secondary battery and an electrical device provided in the embodiments of the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of the present application. At the same time, for those skilled in the art, based on the ideas of the present application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A secondary battery comprising a positive electrode sheet, an electrolyte, a separator and a negative electrode sheet, characterized in that: The negative electrode plate comprises: negative electrode current collector; A composite negative electrode material layer region, wherein the composite negative electrode material layer region comprises a composite negative electrode material layer; the composite negative electrode material layer region comprises a first negative electrode material layer region and a second negative electrode material layer region, wherein the first negative electrode material layer region is disposed between the negative electrode current collector and the second negative electrode material layer region, wherein the first negative electrode material layer region comprises a first protective film, and the second negative electrode material layer region comprises a second protective film, wherein the average thickness of the first protective film is A nm, and the average thickness of the second protective film is B nm, and B>A; The composite negative electrode material layer region satisfies the following characteristics: 2.4≤PD×1.5+B / A×2×P≤15.8; Where PD g / cm 3 is the compaction density of the composite negative electrode material layer, P is the porosity of the composite negative electrode material layer, A nm is the average thickness of the first protective film, B nm is the average thickness of the second protective film; The compaction density of the composite negative electrode material layer is 1.4 to 1.7 g / cm 3 ; The porosity of the composite negative electrode material layer is 20% to 50%; The average thickness of the first protective film is 15 to 100 nm; The average thickness of the second protective film is 70-200 nm.

2. The secondary battery according to claim 1, wherein The thickness of the first negative electrode material layer region is 40 to 80 μm; and / or the thickness of the second negative electrode material layer region is 40 to 80 μm.

3. The secondary battery according to any one of claims 1 to 2, characterized in that The gram capacity of the composite negative electrode material layer is 340 to 370 mAh / g; and / or the orientation degree of the composite negative electrode material layer is 3 to 40.

4. The secondary battery according to claim 1, wherein The first negative electrode material layer region contains first negative electrode active material particles, the average particle size of the first negative electrode active material particles is D1 μm, the second negative electrode material layer region contains second negative electrode active material particles, the average particle size of the second negative electrode active material particles is D2 μm, and D1≥D2 is satisfied.

5. The secondary battery according to claim 4, wherein 15≤D1≤30, 6≤D2≤15.

6. The secondary battery according to claim 1, wherein The electrolyte solution contains at least one of an unsaturated cyclic carbonate or a silane compound.

7. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 5.

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