Secondary batteries and electronic devices

Abstract

This application provides a secondary battery and an electronic device. The secondary battery of this application includes a negative electrode, which comprises a current collector and a negative electrode active material layer disposed on the surface of the current collector. The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, with the first negative electrode active material layer disposed between the current collector and the second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode active material, and the maximum rebound rate of the first negative electrode active material is Ra. The second negative electrode active material layer includes a second negative electrode active material, and the maximum rebound rate of the second negative electrode active material is Rb, wherein 5% ≤ Ra ≤ 13%, 10% ≤ Rb ≤ 17%, and Ra < Rb. The secondary battery of this application combines high energy density and excellent fast charging capability.

Description

Technical Field

[0001] This application relates to the field of energy storage. Specifically, this application relates to a secondary battery and an electronic device. Background Technology

[0002] As the secondary battery market continues to expand, the performance requirements are also constantly increasing, with energy density and fast-charging performance being particularly important indicators. Currently, conventional electrode designs in secondary batteries are typically single-layered, and the active materials used often lack both high energy density and fast-charging capability. This often necessitates either reducing coating weight to improve fast-charging performance or increasing coating weight to increase energy density. However, while reducing coating weight to improve fast-charging performance, the limited amount of active material results in significant energy density loss; similarly, increasing coating weight to improve energy density leads to poor fast-charging performance due to the thicker electrode and longer active ion transport paths. Summary of the Invention

[0003] In view of the aforementioned problems in the prior art, this application provides a secondary battery. The secondary battery of this application, by employing a specific multi-layer electrode design, significantly improves the fast-charging performance of the secondary battery while minimizing the impact on the volumetric energy density, thus enabling the secondary battery to possess both high energy density and excellent fast-charging capability.

[0004] The first aspect of this application provides a secondary battery including a negative electrode. The negative electrode includes a current collector and a negative electrode active material layer disposed on the surface of the current collector. The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is disposed between the current collector and the second negative electrode active material layer. The first negative electrode active material layer has a maximum rebound rate of Ra. The second negative electrode active material layer includes a second negative electrode active material, and the maximum rebound rate of the second negative electrode active material is Rb, wherein 5% ≤ Ra ≤ 13%, 10% ≤ Rb ≤ 17%, and Ra < Rb. For negative electrode active materials such as graphite, the maximum rebound rate can reflect its pressure resistance performance after being made into an electrode sheet. A lower maximum rebound rate corresponds to a smaller rebound after cold pressing of the electrode sheet, which is beneficial for maintaining a high compaction density of the electrode sheet. This means that the packing between particles is relatively dense, which reduces the porosity of the electrode sheet, hindering the penetration of electrolyte and the diffusion of active ions, thereby affecting the fast charging capability of the secondary battery. When the maximum rebound rate is high, the electrode has a rich porous structure, which is conducive to electrolyte penetration and active ion diffusion, but the compaction density is low, resulting in a loss of volumetric energy density in the secondary battery. The secondary battery of this application adopts a multi-layer active material design. The upper layer (second negative electrode active material layer) uses a second negative electrode active material with a high maximum rebound rate, while the lower layer (first negative electrode active material layer) uses a first negative electrode active material with a lower maximum rebound rate. Simultaneously, the maximum rebound rates of both the upper and lower active materials are controlled within the aforementioned range, allowing the electrode to achieve both high compaction density and high surface porosity, thus enabling the secondary battery to balance high energy density and excellent fast-charging capability.

[0005] In some embodiments, 5% ≤ Ra ≤ 10%, and 10% ≤ Rb ≤ 14%. In other embodiments, 8% ≤ Ra ≤ 10%, and 12% ≤ Rb ≤ 14%. When the maximum rebound rate of the first negative electrode active material is too low, the packing becomes too dense, reducing the porosity of the electrode sheet and hindering electrolyte penetration and active ion diffusion, thus affecting the fast-charging capability of the secondary battery. When the maximum rebound rate of the second negative electrode active material is too high, although it is beneficial to improve the fast-charging capability, it results in a lower compaction density, leading to a loss of volumetric energy density in the secondary battery.

[0006] In some implementations, 5% ≤ Ra ≤ 10%. In other implementations, 8% ≤ Ra ≤ 10%.

[0007] In some implementations, 10% ≤ Rb ≤ 14%. In other implementations, 12% ≤ Rb ≤ 14%.

[0008] In some embodiments, 0 < Rb - Ra ≤ 10%. When the difference between Rb and Ra is within the above range, the fast-charging performance of the secondary battery can be further improved while ensuring the volumetric energy density. In some embodiments, 2% ≤ Rb - Ra ≤ 6%.

[0009] In some embodiments, the powder compaction density of the first negative electrode active material at which the maximum rebound rate is PDa, where 1.95 g / cm³ 3 ≤PDa≤2.20g / cm 3 In some implementations, 2.00 g / cm³ 3 ≤PDa≤2.20g / cm 3 When the PDa of the first negative electrode active material is within the above range, the first negative electrode active material meets the design requirements for high electrode compaction density and can maintain the integrity of the particle structure.

[0010] In some embodiments, the powder compaction density of the second negative electrode active material at which the maximum rebound rate is PDb, wherein 1.75 g / cm³ 3 ≤PDb≤2.00g / cm 3 In some implementations, 1.80 g / cm³ 3 ≤PDb≤2.00g / cm 3 When the PDb of the second negative electrode active material is within the above range, the second negative electrode active material has a high rebound rate while maintaining a certain compaction density.

[0011] In some embodiments, the first negative electrode active material is selected from artificial graphite, and the second negative electrode active material is selected from artificial graphite.

[0012] In some embodiments, the mass content of the first negative electrode active material layer is 60% to 80% based on the mass of the negative electrode active material layer. In some embodiments, the mass content of the second negative electrode active material layer is 20% to 40% based on the mass of the negative electrode active material layer. When the contents of the first and second negative electrode active material layers are within the above ranges, the secondary battery achieves both high energy density and excellent electrochemical performance. When the mass content of the second negative electrode active material layer is greater than 40% and the mass content of the first negative electrode active material layer is less than 60%, the energy density of the secondary battery is significantly reduced. When the mass content of the second negative electrode active material layer is less than 20% and the mass content of the first negative electrode active material layer is greater than 80%, the fast-charging capability of the secondary battery is significantly reduced.

[0013] In some embodiments, N-methylpyrrolidone is used as the test oil. The oil absorption value of the first negative electrode active material is less than or equal to 65 mL / 100 g, and the oil absorption value of the second negative electrode active material is less than or equal to 65 mL / 100 g. The oil absorption value of the active material reflects the lipophilic nature of the particles. If the oil absorption value is too high, more dispersant is needed to uniformly disperse the active material particles in the slurry, which reduces the proportion of active material in the slurry and thus affects the energy density and kinetic performance of the secondary battery.

[0014] In some embodiments, the adhesion force between the negative electrode active material layer and the current collector is greater than or equal to 8 N / m. If the adhesion force is too low, the electrode is prone to powder shedding and delamination during processing and cycling. Therefore, the adhesion force is limited to greater than or equal to 8 N / m to ensure that the electrode has a more stable structure.

[0015] In some embodiments, the compaction density of the negative electrode is 1.70 g / cm³. 3 Up to 1.85 g / cm 3 When the compaction density of the negative electrode is too high, it can easily lead to over-pressure of the electrode, which will negatively affect the electrode processing and electrical performance. When the compaction density of the negative electrode is too low, it cannot effectively improve the energy density of the secondary battery.

[0016] In some implementations, the electrode rebound rate of the negative electrode at 50% SOC is T, where 15% ≤ T ≤ 20%.

[0017] A second aspect of this application provides an electronic device that includes the secondary battery of the first aspect.

[0018] The secondary battery of this application adopts a multi-layer electrode design. The lower layer near the current collector uses a negative electrode active material with a low maximum rebound rate, which gives the electrode a high compaction density. The upper layer uses a negative electrode active material with a high maximum rebound rate, which gives the electrode surface more pores, which facilitates the wetting and penetration of electrolyte and promotes the diffusion of lithium ions. This allows the electrode to have both high compaction density and high surface porosity, so that the secondary battery can achieve both high energy density and excellent fast charging capability.

[0019] The third aspect of this application provides a method for preparing a negative electrode, the method comprising: preparing a first active material and preparing a second active material; the first active material is prepared by coating natural graphite and / or artificial graphite and then subjecting it to two different heat treatments, and the second active material is prepared by coating artificial graphite and then subjecting it to one heat treatment.

[0020] Specifically, the preparation of the first active material involves mixing artificial graphite and / or natural graphite with a coating agent to obtain a first mixture, wherein the mass content of the coating agent is 1% to 5% based on the mass of the first mixture; subjecting the first mixture to a first heat treatment in an inert atmosphere to obtain a first heat-treated product; and subjecting the first heat-treated product to a second heat treatment in a mixture of CO2 and N2 to obtain the first active material.

[0021] Preparation of the second active material: Artificial graphite is mixed with a coating agent to obtain a second mixture, wherein the mass content of the coating agent is 1% to 5% based on the mass of the second mixture; the second mixture is subjected to a third heat treatment in an inert atmosphere to obtain the second active material.

[0022] The temperature of the first heat treatment is 950℃ to 1200℃, and the time of the first heat treatment is 1h to 5h; the temperature of the second heat treatment is 500℃ to 900℃, and the time of the second heat treatment is 5h to 15h; the temperature of the third heat treatment is 700℃ to 1100℃, and the time of the third heat treatment is 4h to 10h; the coating agent is asphalt.

[0023] Using the first and second active materials as the first and second active material layers of the negative electrode, respectively, and with the second active material layer on the upper surface of the first active material layer (i.e., the surface away from the current collector in the thickness direction of the negative electrode), the secondary battery can achieve both high energy density and fast charging capability. Attached Figure Description

[0024] Figure 1 The rebound rate curves of the first negative electrode active material and the second negative electrode active material of Embodiment 10 of this application are shown, wherein 1 is the first negative electrode active material and 2 is the second negative electrode active material. Detailed Implementation

[0025] In the description of this application, unless otherwise stated, "above" and "below" include the stated number.

[0026] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0027] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0028] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0029] Primary and secondary batteries

[0030] The secondary battery provided in this application includes a negative electrode, which includes a current collector and a negative electrode active material layer disposed on the surface of the current collector. The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, with the first negative electrode active material layer disposed between the current collector and the second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode active material, and the maximum rebound rate of the first negative electrode active material is Ra. The second negative electrode active material layer includes a second negative electrode active material, and the maximum rebound rate of the second negative electrode active material is Rb, wherein 5% ≤ Ra ≤ 13%, 10% ≤ Rb ≤ 17%, and Ra < Rb. For negative electrode active materials such as graphite, the maximum rebound rate can reflect its pressure resistance performance after being made into an electrode sheet. A lower maximum rebound rate corresponds to a smaller rebound after cold pressing of the electrode sheet, which is beneficial for maintaining a high compaction density of the electrode sheet, meaning that the packing between particles is relatively dense. This reduces the porosity of the electrode sheet, which is not conducive to the penetration of electrolyte and the diffusion of active ions, thus affecting the fast charging capability of the secondary battery. When the maximum rebound rate is high, the electrode has a rich porous structure, which is conducive to electrolyte penetration and active ion diffusion, but the compaction density is low, resulting in a loss of volumetric energy density in the secondary battery. The secondary battery of this application adopts a multi-layer active material design. The upper layer (second negative electrode active material layer) uses a second negative electrode active material with a high maximum rebound rate, while the lower layer (first negative electrode active material layer) uses a first negative electrode active material with a lower maximum rebound rate. Simultaneously, the maximum rebound rates of both the upper and lower active materials are controlled within the aforementioned range, allowing the electrode to achieve both high compaction density and high surface porosity, thus enabling the secondary battery to balance high energy density and excellent fast-charging capability.

[0031] In this application, the maximum rebound rate of the active material represents the maximum degree of material rebound after undergoing pressurization and depressurization under different pressure conditions. For specific testing methods, please refer to the test methods in the detailed embodiments below.

[0032] In some embodiments, Ra is a range of 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, or any combination of these values. When the maximum rebound rate of the first negative electrode active material is too low, the packing becomes too dense, reducing the porosity of the electrode sheet, which is detrimental to electrolyte penetration and active ion diffusion, thus affecting the fast-charging capability of the secondary battery. In some embodiments, 5% ≤ Ra ≤ 10%. In some embodiments, 8% ≤ Ra ≤ 10%.

[0033] In some embodiments, Rb is a range of 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, or any combination of these values. While an excessively high maximum rebound rate of the second negative electrode active material is beneficial for improving fast charging capability, it results in a lower compaction density, leading to a loss of volumetric energy density in the secondary battery. In some embodiments, 10% ≤ Rb ≤ 14%. In some embodiments, 12% ≤ Rb ≤ 14%.

[0034] In some embodiments, 5% ≤ Ra ≤ 10%, and 10% ≤ Rb ≤ 14%. In other embodiments, 8% ≤ Ra ≤ 10%, and 12% ≤ Rb ≤ 14%.

[0035] In some embodiments, 0 < Rb-Ra ≤ 10%. In some embodiments, Rb-Ra is a range of 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or any combination of these values. When the difference in Rb-Ra is within the above range, the fast-charging performance of the secondary battery can be further improved while maintaining the volumetric energy density. In some embodiments, 2% ≤ Rb-Ra ≤ 6%.

[0036] In some embodiments, the powder compaction density of the first negative electrode active material at the point where it reaches its maximum rebound rate is PDa, wherein 1.95 g / cm³ 3 ≤PDa≤2.20g / cm 3 In some implementations, PDa is 1.95 g / cm³.3 1.96g / cm 3 1.98g / cm 3 2.00g / cm 3 2.01 g / cm 3 2.02 g / cm 3 2.03 g / cm 3 2.04 g / cm 3 2.05g / cm 3 2.06 g / cm 3 2.07 g / cm 3 2.08 g / cm 3 2.09 g / cm 3 2.10 g / cm 3 2.11 g / cm 3 2.12 g / cm 3 2.13 g / cm 3 2.14 g / cm 3 2.15g / cm 3 2.16 g / cm 3 2.17 g / cm 3 2.18 g / cm 3 2.19 g / cm 3 2.20g / cm 3 Or a range of any two of these values. When the PDa of the first negative electrode active material is within the above range, the first negative electrode active material satisfies the design requirements for high electrode compaction density and can maintain the integrity of the particle structure. In some embodiments, 2.00 g / cm³ 3 ≤PDa≤2.20g / cm 3 .

[0037] In some embodiments, the powder compaction density of the second negative electrode active material at which the maximum rebound rate is PDb, wherein 1.75 g / cm³ 3 ≤PDb≤2.00g / cm 3 In some implementations, PDb is 1.75 g / cm³. 3 1.77g / cm 3 1.79g / cm 3 1.80g / cm 3 1.81 g / cm 3 1.82g / cm 3 1.83g / cm 3 1.84 g / cm 3 1.85g / cm 3 1.86 g / cm 3 1.87 g / cm3 1.88g / cm 3 1.89 g / cm 3 1.90g / cm 3 1.91g / cm 3 1.92g / cm 3 1.93g / cm 3 1.94 g / cm 3 1.95g / cm 3 1.96g / cm 3 1.97g / cm 3 1.98g / cm 3 1.99g / cm 3 2.00g / cm 3 Or a range of any two of these values. When the PDb of the second negative electrode active material is within the above range, the second negative electrode active material exhibits a high rebound rate while maintaining a certain compaction density. In some embodiments, 1.80 g / cm³ 3 ≤PDb≤2.00g / cm 3 .

[0038] In this application, the powder compaction density at which the active material reaches its maximum rebound rate is the material's ultimate compaction density. When further pressure is applied, the particle structure of the active material is disrupted, and the particles change from elastic deformation to inelastic deformation, resulting in a decrease in the rebound rate. For specific testing methods, please refer to the test methods in the detailed embodiments below.

[0039] In some embodiments, the first negative electrode active material is selected from artificial graphite and / or natural graphite. In some embodiments, the second negative electrode active material is selected from artificial graphite.

[0040] In some embodiments, the first negative electrode active material is selected from artificial graphite. In some embodiments, the second negative electrode active material is selected from artificial graphite.

[0041] In some embodiments, the preparation method of the first negative electrode active material includes the following steps:

[0042] S1: Mix artificial graphite and / or natural graphite with a coating agent to obtain a first mixture;

[0043] S2: The first mixture is subjected to a first heat treatment in an inert atmosphere to obtain a first heat-treated product;

[0044] S3: In a mixture of CO2 and N2 gas, the first heat treatment product is subjected to a second heat treatment to obtain the first negative electrode active material.

[0045] In some embodiments, in S1, the mass content of the coating agent is 1% to 5% based on the mass of the first mixture, for example, 2%, 3% or 4%. In some embodiments, the coating agent is bitumen.

[0046] In some embodiments, in S2, the temperature of the first heat treatment is from 950°C to 1200°C, for example, 1000°C, 1050°C, 1100°C, or 1150°C. In some embodiments, in S2, the duration of the first heat treatment is from 1 hour to 5 hours, for example, 2 hours, 3 hours, or 4 hours.

[0047] In some embodiments, in S3, the temperature of the second heat treatment is from 500°C to 900°C, for example, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or 850°C. In some embodiments, in S3, the time of the second heat treatment is from 5 hours to 15 hours, for example, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, or 14 hours.

[0048] In some embodiments, the volume percentage of CO2 in the CO2 and N2 mixture is 3% to 7%, for example, 4%, 5% or 6%.

[0049] In some embodiments, the preparation method of the second negative electrode active material includes the following steps:

[0050] M1: Artificial graphite is mixed with a coating agent to obtain a second mixture;

[0051] M2: The second mixture is subjected to a third heat treatment in an inert atmosphere to obtain the second negative electrode active material.

[0052] In some embodiments, the coating agent in M1 has a mass content of 1% to 5%, for example, 2%, 3%, or 4%, based on the mass of the second mixture. In some embodiments, the coating agent is bitumen.

[0053] In some embodiments, the temperature of the third heat treatment in M2 is between 700°C and 1100°C, for example, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, or 1050°C. In some embodiments, the duration of the second heat treatment in M2 is between 4 hours and 10 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, or 9 hours.

[0054] In some embodiments, artificial graphite is obtained by high-temperature graphitization of carbonaceous raw materials such as needle coke, petroleum coke, and pitch coke. In some embodiments, natural graphite is obtained by acid leaching and spheroidizing of natural flake graphite. In some embodiments, based on the mass of the negative electrode active material layer, the mass content of the first negative electrode active material layer is 60% to 90%, for example, 60%, 65%, 70%, 75%, 80%, 85%, or 90%. In some embodiments, based on the mass of the negative electrode active material layer, the mass content of the second negative electrode active material layer is 10% to 40%, for example, 10%, 15%, 20%, 25%, 30%, or 35% or 40%. When the contents of the first and second negative electrode active material layers are within the above ranges, the secondary battery achieves both high energy density and excellent electrochemical performance. When the mass content of the second negative electrode active material layer is greater than 40% and the mass content of the first negative electrode active material layer is less than 60%, the energy density loss of the secondary battery is significant. When the mass content of the second negative electrode active material layer is less than 10% and the mass content of the first negative electrode active material layer is greater than 90%, the fast charging capability of the secondary battery is significantly reduced.

[0055] In some embodiments, N-methylpyrrolidone is used as the test oil. The oil absorption value of the first negative electrode active material is less than or equal to 65 mL / 100 g, and the oil absorption value of the second negative electrode active material is less than or equal to 65 mL / 100 g. The oil absorption value of the active material reflects the lipophilic nature of the particles. If the oil absorption value is too high, more dispersant is needed to uniformly disperse the active material particles in the slurry, which reduces the proportion of active material in the slurry and thus affects the energy density and kinetic performance of the secondary battery.

[0056] In some embodiments, the adhesion force between the negative electrode active material layer and the current collector is greater than or equal to 8 N / m. If the adhesion force is too low, the electrode is prone to powder shedding and delamination during processing and cycling. Therefore, the adhesion force is limited to greater than or equal to 8 N / m to ensure that the electrode has a more stable structure.

[0057] In some embodiments, the compaction density of the negative electrode is 1.70 g / cm³. 3 Up to 1.85 g / cm 3 In some embodiments, the compaction density of the negative electrode is 1.70 g / cm³. 3 1.73g / cm 3 1.75g / cm 3 1.77g / cm 3 1.80g / cm 3 1.83g / cm 3 1.85g / cm 3Or a range of any two of these values. If the compaction density of the negative electrode is too high, it can easily lead to over-pressure of the electrode, which will negatively affect the electrode processing and electrical performance. If the compaction density of the negative electrode is too low, it will not be able to effectively improve the energy density of the secondary battery.

[0058] In some embodiments, the electrode rebound rate of the negative electrode is T, wherein 15% ≤ T ≤ 20%, for example, 16%, 17%, 18% or 19%.

[0059] In some embodiments, the negative electrode active material layer further includes a binder and a conductive agent. In some embodiments, the binder includes at least one selected from styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamide-imide, polyvinylidene fluoride, polydifluoroethylene, polytetrafluoroethylene, waterborne acrylic resin, polyvinyl alcohol formal, or styrene-acrylic acid copolymer resin. In some embodiments, any conductive material can be used as the conductive material, as long as it does not cause a chemical change. In some embodiments, the conductive material includes at least one selected from conductive carbon black, acetylene black, carbon nanotubes, Ketjen black, conductive graphite, or graphene.

[0060] In some embodiments, the negative current collector may be copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

[0061] In some embodiments, the secondary battery also includes a positive electrode, which comprises a positive electrode current collector and a positive electrode active material layer.

[0062] In some embodiments, the positive electrode active material layer includes a positive electrode active material, a binder, and a conductive agent. In some embodiments, the positive electrode active material may include at least one of lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate. In some embodiments, the binder may include at least one of various adhesive polymers, such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane. In some embodiments, any conductive material may be used as the conductive agent, as long as it does not cause a chemical change. Examples of conductive agents include: carbon-based materials, such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; metal-based materials, such as metal powders or metal fibers including copper, nickel, aluminum, silver, etc.; conductive polymers, such as polyphenylene derivatives, etc.; or mixtures thereof.

[0063] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used. Composite current collectors can be formed by forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate.

[0064] The secondary battery of this application also includes a separator. The material and shape of the separator used in the secondary battery of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0065] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0066] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0067] The secondary battery of this application also includes an electrolyte. The electrolyte that can be used in this application can be any electrolyte known in the prior art.

[0068] According to some embodiments of this application, the electrolyte includes an organic solvent, a lithium salt, and optional additives. The organic solvent in the electrolyte of this application can be any organic solvent known in the prior art that can be used as an electrolyte solvent. There are no limitations on the electrolyte used in the electrolyte of this application; it can be any electrolyte known in the prior art. The additives in the electrolyte of this application can be any additives known in the prior art that can be used as electrolyte additives. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes ether solvents, such as at least one selected from 1,3-dioxane (DOL) and dimethyl glycol ether (DME). In some embodiments, the lithium salt includes at least one selected from organic lithium salts or inorganic lithium salts. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0069] In some implementations, the secondary battery is a wound secondary battery or a stacked secondary battery.

[0070] According to some embodiments of this application, the secondary battery of this application includes, but is not limited to, lithium-ion batteries or sodium-ion batteries. In some embodiments, the secondary battery includes a lithium-ion battery.

[0071] II. Electronic Devices

[0072] This application further provides an electronic device that includes the secondary battery of the first aspect of this application.

[0073] The electronic devices or apparatus described in this application are not particularly limited. In some embodiments, the electronic devices described in this application include, but are not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0074] Unless otherwise specified, all reagents, materials and instruments used in the following examples and comparative examples are commercially available.

[0075] Examples and Comparative Examples

[0076] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0077] Example 1

[0078] 1. Preparation of negative electrode active materials

[0079] Graphite material preparation

[0080] First negative electrode active material: Weigh 5 kg of artificial graphite material and mix it evenly with asphalt as a coating agent to obtain a first mixture, wherein the mass ratio of asphalt to graphite material is 3:97. The first mixture is subjected to a first heat treatment; the first heat treatment conditions are: reaction at 1000℃ in a nitrogen atmosphere for 2 hours. After the reaction is completed and the temperature drops to room temperature, the first heat-treated product is obtained. The first heat-treated product is subjected to a second heat treatment; the second heat treatment conditions are: reaction at 900℃ in a 5% CO2 and 95% N2 mixture atmosphere for 6 hours. After the reaction is completed and cooled to room temperature, the first negative electrode active material is obtained.

[0081] Second negative electrode active material: Weigh 5 kg of artificial graphite material and mix it evenly with asphalt as a coating agent to obtain a second mixture, wherein the mass ratio of asphalt to graphite material is 4:96. The second mixture is then subjected to a third heat treatment under the following conditions: reaction temperature T2 is 700℃, reaction time H2 is 4 h under a nitrogen atmosphere; after the reaction is complete, cool to room temperature to obtain the second negative electrode active material.

[0082] Other examples and comparative examples can be prepared by adjusting the reaction temperature T1 and reaction time H1 in the first negative electrode active material preparation conditions under a mixed atmosphere of 5% CO2 and 95% N2, and the reaction temperature T2 and reaction time H2 in the second negative electrode active material preparation conditions.

[0083] 2. Preparation of the negative electrode

[0084] The first negative electrode active material, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were uniformly dispersed in an appropriate amount of deionized water at a mass ratio of 97.5:1.2:1.3 to obtain slurry 1. The second negative electrode active material, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were uniformly dispersed in an appropriate amount of deionized water at a mass ratio of 97.5:1.2:1.3 to obtain slurry 2. Copper foil was used as a current collector. Slurry 1 was used as the first negative electrode active material layer, and slurry 2 was used as the second negative electrode active material layer, and both were uniformly coated on the current collector. After coating, the material was dried and cold-pressed to obtain the negative electrode sheet, also known as the negative electrode. Based on the total mass of the first and second negative electrode active material layers, the mass content of the first negative electrode active material layer was 70%, and the mass content of the second negative electrode active material layer was 30%.

[0085] 3. Preparation of the positive electrode

[0086] The positive electrode uses lithium cobalt oxide (chemical formula: LiCoO2) as the active material. It is mixed with conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) in a weight ratio of 96.3:2.2:1.5 in an appropriate amount of N-methylpyrrolidone (NMP) solvent to form a uniform positive electrode slurry. The slurry is coated onto the current collector Al foil, dried and cold-pressed to obtain the positive electrode sheet, also known as the positive electrode.

[0087] 4. Preparation of electrolyte

[0088] In a dry argon-atmospheric glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 1:3:3:3. Then, fluoroethylene carbonate and 1,3-propanesulfonyl lactone were added, dissolved, and thoroughly stirred. Lithium salt LiPF6 was then added and mixed evenly to obtain the electrolyte. The mass percentages of LiPF6, fluoroethylene carbonate, and 1,3-propanesulfonyl lactone were all 12.5% ​​and 2% respectively, calculated based on the mass of the electrolyte.

[0089] 5. Preparation of the separating membrane

[0090] Polyethylene porous polymer film is used as the separator.

[0091] 6. Preparation of lithium-ion batteries

[0092] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, the electrode assembly is wound up. After welding the tabs, the electrode assembly is placed in the outer packaging foil aluminum-plastic film. The prepared electrolyte is injected into the dried electrode assembly. After vacuum sealing, settling, formation, shaping, and capacity testing, a soft-pack lithium-ion battery is obtained.

[0093] Examples 2 to 13, Comparative Examples 1 to 9

[0094] Preparation of negative electrode active materials

[0095] The difference lies in that the first negative electrode active material is prepared by adjusting the reaction temperature T1 and the reaction time H1, and the second negative electrode active material is prepared by adjusting the reaction temperature T2 and the reaction time H2. Specific preparation parameters are shown in Table 1. The preparation of the negative electrode, positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 1.

[0096] Examples 14 to 20

[0097] Preparation of negative electrode active materials

[0098] The preparation process of the negative electrode active material is the same as in Example 9.

[0099] The preparation process of the negative electrode is similar to that in Example 9, except that the mass content of the first negative electrode active material layer and the second negative electrode active material layer is adjusted by adjusting the coating weight of the slurry. The specific adjusted mass content of the first negative electrode active material layer and the second negative electrode active material layer is shown in Table 2.

[0100] The preparation of the positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 9.

[0101] Test methods

[0102] Testing of relevant parameters of negative electrode active materials

[0103] Take a fully discharged lithium-ion battery, disassemble it, remove the negative electrode and soak it in DMC (dimethyl carbonate) for 20 minutes, then rinse it with DMC and acetone in turn to remove the electrolyte and the surface SEI film. Then place it in an oven and bake it at 80°C for 12 hours to obtain the treated negative electrode sheet.

[0104] M1 grams of powder within a 10 μm thickness range from the surface of the negative electrode sheet were scraped off with a scraper, and the scraped powder was calcined in air at 500°C for 3 hours to obtain the first negative electrode active material.

[0105] M2 grams of powder within a 10 μm thickness range between the negative electrode sheet and the current collector were scraped off with a scraper, and the scraped powder was calcined in air at 500°C for 3 hours to obtain the second negative electrode active material.

[0106] The obtained first and second anode active materials were subjected to the following tests:

[0107] 1. Rebound rate and powder compaction density at maximum rebound rate

[0108] The rebound rate and the powder compaction density at the maximum rebound rate of the active material were tested using a PRCD3100 powder compaction density tester, i.e., the ultimate compaction density.

[0109] Specifically, m grams of active material are loaded into a special mold for powder compaction testing, with a filling thickness of approximately 1 / 3 of the mold depth. Pressure points are set at 5 MPa, 25 MPa, 45 MPa, 65 MPa, 85 MPa, 105 MPa, 125 MPa, and 130 MPa. Pressure is then gradually applied, and the corresponding active material thickness h1 at each pressure point is recorded. The powder compaction density under pressure (i.e., the holding pressure compaction at that pressure point) can be calculated using the active material mass m, thickness h1, and a fixed filling area s. After releasing the pressure, the thickness h2 of the active material after rebound is recorded, and the corresponding unloaded pressure compaction can be calculated. Powder rebound rate = (holding pressure compaction / unloaded pressure compaction - 1) × 100%.

[0110] By plotting the powder rebound rate and the pressure relief compaction, the change curve of the powder rebound rate of the material can be obtained. The pressure relief compaction at which the maximum rebound rate is reached is the ultimate compaction density of the active material.

[0111] 2. Oil absorption value test

[0112] The oil absorption value of the active material was determined using a DABS-H oil absorption meter. Specifically, N-methylpyrrolidone was added to the sample in the mixing tank of the oil absorption meter using a constant-rate titrator. As the amount of oil absorbed by the sample increased, the viscosity of the mixture continuously increased. When the viscosity reached a predetermined value, the oil absorption meter and titrator were simultaneously shut off. The volume of oil added was read directly from the burette, and the volume of oil absorbed per unit mass of sample was recorded as the oil absorption value of the sample.

[0113] 3. Electrode rebound rate

[0114] The thickness of the negative electrode sheet after cold pressing is recorded as T1. The thickness of the negative electrode sheet of the 50% SOC cell is recorded as T2. The rebound rate of the negative electrode sheet at 50% SOC is T = (T2-T1) / T1*100%.

[0115] Lithium-ion battery performance testing

[0116] 4. Volumetric energy density

[0117] The capacity of lithium-ion batteries is tested according to the following procedure:

[0118] 1) Let it stand at 25℃ for 30 minutes;

[0119] 2) Charge at 0.5C to 4.48V, then maintain constant voltage at 0.05C;

[0120] 3) Let it sit for 5 minutes;

[0121] 4) Discharge at 0.2C to 3.0V;

[0122] 5) Let it sit for 5 minutes, then the test is over.

[0123] Record the discharge capacity as C, the discharge plateau voltage as P, the thickness of the lithium-ion battery as G, the length of the lithium-ion battery as L, and the width of the lithium-ion battery as W. Then, the volumetric energy density E of the lithium-ion battery can be calculated according to the following formula.

[0124] E = (C × P) / (G × L × W).

[0125] 5. Fast charging performance test

[0126] 1) Let the lithium-ion battery rest at 25°C for 10 minutes;

[0127] 2) Discharge at 0.025C to 3.0V;

[0128] 3) Let it sit for 10 minutes;

[0129] 4) Charge to 4.48V using 3C, then maintain constant voltage at 0.025C;

[0130] 5) Let it sit for 10 minutes;

[0131] 6) Discharge at 0.025C to 3.0V;

[0132] 7) Let it sit for 10 minutes;

[0133] 8) Repeat steps 5)-8) 10 times;

[0134] Let D10 be the capacity of the last discharge cycle and C1 be the capacity of the first charge cycle. The irreversible Li loss rate Q is expressed as: Q=(C1-D10) / C1×100%.

[0135] The smaller the Q value, the less irreversible Li loss there is, and the better the fast charging capability of the lithium-ion battery.

[0136] Test Results

[0137] Table 1 shows the effect of the maximum rebound rate Ra of the first negative electrode active material and the maximum rebound rate Rb of the second negative electrode active material on the performance of lithium-ion batteries.

[0138] Table 1

[0139]

[0140] Note: " / " indicates that no heat treatment is performed.

[0141] As can be seen from Examples 1 to 13 in Table 1, when Ra meets the range of 5% ≤ Ra ≤ 13% and Rb meets the range of 10% ≤ Rb ≤ 17%, the lithium-ion battery maintains a low irreversible Li loss rate while having a high energy density, indicating that it can balance high energy density and good fast charging capability.

[0142] Comparative Examples 2 and 6 show that when Ra is greater than 13%, although the lithium-ion battery exhibits excellent fast-charging capability, the electrode rebound rate is relatively high, resulting in significant energy density loss. Comparative Examples 1 and 5 show that when Ra is less than 5%, although the electrode rebound rate is relatively low and the lithium-ion battery has a high energy density, the fast-charging capability is significantly affected.

[0143] Comparative Examples 4 and 6 show that when Rb is greater than 17%, the electrode rebound rate is high, resulting in excellent fast-charging capability of the lithium-ion battery, but also significant energy density loss. Comparative Examples 3 and 5 show that when Rb is less than 9%, the electrode rebound rate is low, resulting in higher energy density of the lithium-ion battery, but the fast-charging capability is significantly affected.

[0144] As can be seen from Comparative Examples 7 to 9, when Ra is greater than or equal to Rb, the energy density and fast charging capability of lithium-ion batteries will be affected to some extent.

[0145] Table 2 further investigates the effects of the mass content Wa of the first negative electrode active material layer and the mass content Wb of the second negative electrode active material layer on lithium-ion performance, based on Example 9.

[0146] Table 2

[0147]

[0148] As can be seen from the data in Table 2, when the mass content Wa of the first negative electrode active material layer is 60% to 80% and the mass content Wb of the second negative electrode active material layer is 20% to 40%, the lithium-ion battery has both high energy density and excellent fast charging performance.

[0149] While some exemplary embodiments of this application have been described and illustrated, this application is not limited to the disclosed embodiments. Rather, those skilled in the art will recognize that modifications and changes may be made to the described embodiments without departing from the spirit and scope of this application as described in the appended claims.

Claims

1. A secondary battery comprising a negative electrode, the negative electrode comprising a current collector and a negative electrode active material layer disposed on the surface of the current collector, the negative electrode active material layer comprising a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer being disposed between the current collector and the second negative electrode active material layer, the first negative electrode active material layer comprising a first negative electrode active material, the first negative electrode active material having a maximum rebound rate Ra, the second negative electrode active material layer comprising a second negative electrode active material, the second negative electrode active material having a maximum rebound rate Rb, wherein... 5%≤Ra≤13%, 10%≤Rb≤17%, Ra<Rb; The first negative electrode active material is selected from artificial graphite and / or natural graphite, and the second negative electrode active material is selected from artificial graphite; The maximum rebound rate of the active material represents the maximum degree of rebound of the material after undergoing pressurization and depressurization under different pressure conditions. The test method is as follows: Take m grams of active material and load it into a special mold for powder compaction testing. The filling thickness should be 1 / 3 of the mold depth. Set pressure points of 5 MPa, 25 MPa, 45 MPa, 65 MPa, 85 MPa, 105 MPa, 125 MPa, and 130 MPa. Then gradually apply pressure and record the corresponding active material thickness h1 at each set pressure point. Calculate the powder compaction density under pressure using the active material mass m, thickness h1, and fixed filling area s, i.e., the pressure holding compaction at that pressure point. After releasing the pressure, record the thickness h2 of the active material after rebound and calculate the corresponding pressure release compaction. Rebound rate = (Pressure holding and compaction / Pressure release and compaction - 1) × 100%.

2. The secondary battery according to claim 1, wherein, 5%≤Ra≤10%, 10%≤Rb≤14%.

3. The secondary battery according to claim 1, wherein, 8%≤Ra≤10%, 12%≤Rb≤14%.

4. The secondary battery according to claim 2, wherein, 0 < Rb - Ra ≤ 9%.

5. The secondary battery according to claim 3, wherein, 2%≤Rb-Ra≤6%.

6. The secondary battery according to claim 1, wherein, The first negative electrode active material is selected from artificial graphite, and the second negative electrode active material is selected from artificial graphite.

7. The secondary battery according to claim 1, wherein, based on the mass of the negative electrode active material layer, the mass content of the first negative electrode active material layer is 60% to 80%, and the mass content of the second negative electrode active material layer is 20% to 40%.

8. The secondary battery according to claim 1, wherein, based on the mass of the negative electrode active material layer, the mass content of the first negative electrode active material layer is 70% to 80%, and the mass content of the second negative electrode active material layer is 20% to 30%.

9. The secondary battery according to claim 1, wherein, The negative electrode satisfies at least one of the following conditions (i) to (iii): (i) The oil absorption value of the first negative electrode active material is less than or equal to 65 mL / 100 g, and the oil absorption value of the second negative electrode active material is less than or equal to 65 mL / 100 g. (ii) The compaction density of the negative electrode is 1.70 g / cm³. 3 Up to 1.85 g / cm 3 ; (iii) The rebound rate of the negative electrode with 50% SOC is T, where 15%≤T≤20%, where the thickness of the negative electrode after cold pressing is recorded as T1, and the thickness of the negative electrode after disassembling the 50% SOC cell is recorded as T2. The rebound rate of the negative electrode with 50% SOC is T=(T2-T1) / T1×100%.

10. An electronic device comprising a secondary battery according to any one of claims 1 to 9.

11. A method for preparing a negative electrode in a secondary battery according to any one of claims 1 to 9, wherein the negative electrode comprises a current collector and a negative electrode active material layer disposed on the surface of the current collector, the negative electrode active material layer comprising a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer being disposed between the current collector and the second negative electrode active material layer, the first negative electrode active material layer comprising a first negative electrode active material, and the second negative electrode active material layer comprising a second negative electrode active material, wherein... The method for preparing the negative electrode includes: Preparation of the first negative electrode active material: Artificial graphite and / or natural graphite are mixed with a coating agent to obtain a first mixture, wherein the coating agent has a mass content of 1% to 5% based on the mass of the first mixture; The first mixture is subjected to a first heat treatment in an inert atmosphere to obtain a first heat-treated product; In a mixture of CO2 and N2, the first heat treatment product is subjected to a second heat treatment to obtain the first negative electrode active material; Preparation of the second negative electrode active material: The artificial graphite is mixed with the coating agent to obtain a second mixture; The second mixture is subjected to a third heat treatment in an inert atmosphere to obtain the second negative electrode active material; The temperature of the first heat treatment is 950°C to 1200°C, and the duration of the first heat treatment is 1 hour to 5 hours. The temperature of the second heat treatment is 500°C to 900°C, and the time of the second heat treatment is 5 hours to 15 hours; The temperature of the third heat treatment is 700°C to 1100°C, and the time of the third heat treatment is 4 hours to 10 hours; The coating agent is asphalt.

12. The preparation method according to claim 11, wherein the temperature of the second heat treatment is 700°C to 900°C, and the time of the second heat treatment is 5h to 7h.

13. The preparation method according to claim 11, wherein the temperature of the third heat treatment is 700°C to 900°C, and the time of the third heat treatment is 4h to 6h.

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