Secondary batteries and electronic devices

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery of the present application includes a negative electrode, which includes a current collector and a negative electrode active material layer provided on the current collector. The negative electrode active material layer includes a first active material layer and a second active material layer. The first active material layer is disposed between the current collector and the second active material layer. The first active material layer includes a first active material. A charge-discharge test is performed on a first button cell using lithium as the negative electrode and the first active material as the positive electrode. The slope of the end potential of the de-lithiation curve of the first button cell is Sa. The second active material layer includes a second active material. A charge-discharge test is performed on a second button cell using lithium as the negative electrode and the second active material as the positive electrode. The slope of the end potential of the de-lithiation curve of the second button cell is Sb, and Sa < Sb, where 2 mAh·g ‑1 / V ≤ Sa ≤ 7 mAh·g ‑1 / V, 5 mAh·g ‑1 / V ≤ Sb ≤ 14 mAh·g ‑1 / V. The secondary battery of the present application has both a high energy density and excellent fast charging ability.

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 Art

[0002] With the wide application of secondary batteries in fields such as consumer electronics, electric vehicles, power tools, and energy storage, the market's requirements for their performance are also getting higher and higher. Among them, to meet the longest possible battery life and short charging time for various products, it is required that secondary batteries have as high an energy density as possible and excellent fast charging performance. In existing secondary batteries, the conventional electrode design is usually single-layer, and it is difficult for the active materials used to have both high energy density and fast charging ability. Usually, it is necessary to reduce the coating weight to improve the fast charging ability of the secondary battery, or increase the coating weight to improve the energy density of the lithium secondary battery. However, when reducing the coating weight to improve the fast charging ability of the secondary battery, due to less active material, the energy density loss is serious; similarly, when increasing the coating weight to improve the energy density of the secondary battery, due to the large thickness of the electrode and the too long active ion transport path, its fast charging ability is poor. Summary of the Invention

[0003] In view of the above problems existing in the prior art, this application provides a secondary battery. The secondary battery of this application adopts a specific multi-layer electrode design, which significantly improves the fast charging performance of the secondary battery while minimizing the impact on the volumetric energy density of the secondary battery, making the secondary battery have both high energy density and excellent fast charging ability.

[0004] In the first aspect of this application, a secondary battery is provided, which includes a negative electrode. The negative electrode includes a current collector and a negative electrode active material layer provided on the current collector. The negative electrode active material layer includes a first active material layer and a second active material layer. The first active material layer is disposed between the current collector and the second active material layer. The first active material layer includes a first active material. A first coin cell with lithium as the negative electrode and the first active material as the positive electrode is used for charge and discharge tests. The slope of the end potential of the de-lithiation curve of the first coin cell is Sa. The second active material layer includes a second active material. A second coin cell with lithium as the negative electrode and the second active material as the positive electrode is used for charge and discharge tests. The slope of the end potential of the de-lithiation curve of the second coin cell is Sb is Sb, and Sa < Sb, where 2 mAh·g -1 / V ≤ Sa ≤ 7 mAh·g -1 / V, 5 mAh·g -1 / V ≤ Sb ≤ 14 mAh·g -1 / V. For negative electrode active materials, the slope of the lithium stripping curve at the end of the coin cell reflects the ease with which lithium ions are extracted from the structure of the negative electrode active material. When the lithium stripping potential slope is small, lithium ions can be basically extracted at a potential below 1V. This characteristic is usually exhibited by negative electrode active materials such as graphite intercalation lithium during extraction. The more complete the interlayer structure of the negative electrode active material, the smaller its lithium stripping potential slope, and the higher the available specific capacity and initial coulombic efficiency in the secondary battery. However, when the structure is too complete, the distance for lithium ion insertion increases, thereby inhibiting the fast charging capability of the secondary battery. A larger lithium stripping potential slope is conducive to the rapid transfer of lithium ions and improves the kinetic performance of the secondary battery. However, a larger lithium stripping potential slope means that some lithium ions need to be completely extracted at a higher potential above 1V. The specific capacity at this higher potential is difficult to utilize under the 3.0V cutoff voltage condition of the secondary battery, resulting in a loss of energy density in the secondary battery. The secondary battery of this application adopts a multi-layer active material design. The upper layer (second active material layer) uses a second active material with a higher delithiation potential slope, while the lower layer (first active material layer) uses a first active material with a lower delithiation potential slope. At the same time, the potential slopes at the end of the delithiation curves of the upper and lower active materials are controlled within the above range. This allows the electrode to have both high compaction density and the ability for lithium ions to diffuse and transfer rapidly on the electrode surface, thus enabling the secondary battery to achieve both high energy density and excellent kinetic performance.

[0005] In some implementations, 2 mAh·g -1 / V≤Sa≤5mAh·g -1 / V and 5mAh·g -1 / V≤Sb≤12mAh·g -1 / V. When the delithiation potential slope of the first active material is in a relatively low range, the energy density of the secondary battery can be further improved, and its kinetic performance is also within an acceptable range.

[0006] In some implementations, 8mAh·g -1 / V≤Sb≤14mAh·g -1 / V. When the delithiation potential slope of the second active material is too high, the available specific capacity below 1V decreases, resulting in a loss of energy density in the secondary battery. Furthermore, the excessive number of active sites in the second active material leads to excessive side reactions with the electrolyte, which in turn affects the initial coulombic efficiency and cycle performance of the secondary battery.

[0007] In some implementations, 4 mAh·g -1 / V≤Sa≤7mAh·g -1 / V and 8mAh·g -1 / V≤Sb≤12mAh·g -1 / V. When the delithiation potential slopes of the first and second active materials simultaneously meet the above-mentioned range, the energy density and kinetic performance of the secondary battery can be further improved.

[0008] In some implementations, 2 mAh·g -1 / V≤Sb-Sa≤12mAh·g -1 / V. The difference between Sb and Sa represents the difference in delithiation capability between the upper second active material and the lower first active material in the electrode. When the difference between Sb and Sa meets the above range, the secondary battery has high energy density while also maintaining excellent kinetic performance. When the difference between Sb and Sa is small, the delithiation capabilities of the upper and lower active materials are more similar, meaning the electrode as a whole tends towards interlayer lithium intercalation in a graphite structure, resulting in higher specific capacity but poor lithium-ion diffusion performance, or the electrode as a whole tends towards adsorption lithium intercalation in amorphous carbon, resulting in good lithium-ion diffusion performance but lower specific capacity. When the difference between Sb and Sa is large, the difference in delithiation capability between the upper and lower active materials is greater. The high specific capacity of the first active material cannot compensate for the low specific capacity of the second active material, leading to a lower energy density, or the high lithium-ion diffusion capability of the second active material cannot compensate for the low lithium-ion diffusion capability of the first active material, leading to a higher electrochemical impedance. In some embodiments, 5 mAh·g -1 / V≤Sb-Sa≤8mAh·g -1 / V.

[0009] In some embodiments, the thickness of the negative electrode active material layer is T, the thickness of the first active material layer is Ta, and the thickness of the second active material layer is Tb, wherein 60% ≤ Ta / T ≤ 85%, and 15% ≤ Tb / T ≤ 40%. When the thickness ratio of the first and second active material layers is within the above range, the secondary battery achieves both high energy density and excellent electrochemical performance. When the thickness ratio of the second active material layer (Tb) is greater than 40% and the thickness ratio of the first active material layer (Ta) is less than 60%, the energy density of the secondary battery is significantly reduced. When the thickness ratio of the second active material layer (Tb) is less than 15% and the thickness ratio of the first active material layer (Ta) is greater than 85%, the kinetic performance of the secondary battery is significantly reduced.

[0010] In some embodiments, the first active material comprises artificial graphite. In some embodiments, the second active material comprises artificial graphite.

[0011] In some implementations, the specific capacity of the negative electrode is 335 mAh·g. -1 Up to 365mAh·g -1 When the specific capacity of the negative electrode is too small, it has good lithium-ion diffusion ability, but the energy density is too low; when the specific capacity of the negative electrode is too large, the energy density is high, but the lithium-ion diffusion ability is poor.

[0012] In some implementations, the porosity of the negative electrode is 20% to 40%. If the porosity of the negative electrode is too small, it is not conducive to the wetting and penetration of the electrolyte, making lithium-ion diffusion difficult and thus reducing the kinetic performance of the secondary battery; if the porosity is too large, the contact area between the electrolyte and the active material is too large, and too many side reactions occur, which will cause the capacity to decay too quickly during cycling.

[0013] In some embodiments, the compaction density of the negative electrode is 1.60 g / cm³. 3 Up to 1.80 g / cm 3 When the compaction density of the negative electrode is too high, it can easily lead to over-pressure on the electrode, affecting 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.

[0014] A second aspect of this application provides an electronic device that includes the secondary battery of the first aspect.

[0015] 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 delithiation potential slope, which can provide the secondary battery with higher energy density and first coulombic efficiency, thereby improving its energy density. The upper layer uses a negative electrode active material with a high delithiation potential slope, which allows lithium ions to diffuse and transfer quickly on the electrode surface, thereby improving the fast charging capability of the secondary battery. Thus, the secondary battery can achieve both high energy density and excellent fast charging capability.

[0016] 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.

[0017] 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.

[0018] 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 3% to 8% 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.

[0019] 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 750℃ to 1100℃, and the time of the second heat treatment is 2h to 8h; the temperature of the third heat treatment is 700℃ to 1150℃, and the time of the third heat treatment is 4h to 10h; the coating agent is asphalt.

[0020] 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

[0021] Figure 1 The delithiation curves of the first and second coin cells of Embodiment 2 of this application are shown, where 1-first coin cell and 2-second coin cell.

[0022] Figure 2 This is a comparison chart of the fast charging performance of the secondary batteries in Example 14 and Comparative Example 1 of this application. Detailed Implementation

[0023] In the description of this application, unless otherwise stated, "above" and "below" include the stated number.

[0024] 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).

[0025] 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.

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0027] I. Secondary battery

[0028] The secondary battery of the present application includes a negative electrode, and the negative electrode includes a current collector, a first active material layer, and a second active material layer. The first active material layer is disposed between the current collector and the second active material layer. The first active material layer includes a first active material. A first coin cell using lithium as the negative electrode and the first active material as the positive electrode is used for charge and discharge tests. The slope of the end potential of the de-lithiation curve of the first coin cell is Sa. The second active material layer includes a second active material. A second coin cell using lithium as the negative electrode and the second active material as the positive electrode is used for charge and discharge tests. The slope of the end potential of the de-lithiation curve of the second coin cell is Sb, and Sa < Sb, where 2 mAh·g -1 / V ≤ Sa ≤ 7 mAh·g -1 / V, 5 mAh·g -1 / V ≤ Sb ≤ 14 mAh·g -1 / V. For the negative electrode active material, the slope of the end potential of the de-lithiation curve of the coin cell reflects the difficulty of lithium ions escaping from the structure of the negative electrode active material. When the slope of the de-lithiation potential is small, lithium ions can be basically removed at a potential below 1V. Usually, negative electrode active materials such as lithium intercalation in graphite layers exhibit this characteristic when de-lithiated. The more complete the interlayer structure of the negative electrode active material, the smaller the slope of its de-lithiation potential. The available gram capacity and the first Coulomb efficiency in the secondary battery are higher. However, when the structure is too complete, the distance for lithium ion insertion will increase, thus inhibiting the fast charging ability of the secondary battery. When the slope of the de-lithiation potential is large, it is beneficial to the rapid transfer of lithium ions and improves the kinetic performance of the secondary battery. However, a large slope of the de-lithiation potential indicates that some lithium ions need to be completely removed at a higher potential above 1V, and the gram capacity at this part of the high potential is difficult to be utilized under the cut-off voltage condition of 3.0V of the secondary battery, resulting in an energy density loss of the secondary battery. The secondary battery of the present application adopts a multi-layer active material layer design. A second active material with a higher slope of the end potential of the de-lithiation curve is selected for the upper layer (the second active material layer), and a first active material with a lower slope of the end potential of the de-lithiation curve is selected for the lower layer (the first active material layer). At the same time, the slopes of the end potentials of the de-lithiation curves of the upper and lower layer active materials are controlled within the above range, so that the electrode sheet has a higher compaction density and enables lithium ions to quickly diffuse and transfer on the surface layer of the electrode sheet, so that the secondary battery takes into account both high energy density and excellent kinetic capabilities.

[0029] In this application, the delithiation potential slope of the active material is obtained through the delithiation curve of a coin cell containing the active material. For specific testing methods, please refer to the test methods in the detailed embodiments below.

[0030] In some implementations, Sa is 2.0 mAh·g -1 / V, 2.3mAh·g -1 / V, 2.5mAh·g -1 / V, 2.7mAh·g -1 / V、3.0mAh·g -1 / V, 3.1mAh·g -1 / V、3.3mAh·g -1 / V、3.5mAh·g -1 / V, 3.7mAh·g -1 / V, 4.0mAh·g -1 / V, 4.1mAh·g -1 / V, 4.3mAh·g -1 / V, 4.5mAh·g -1 / V, 4.7mAh·g -1 / V、5.0mAh·g -1 / V、5.3mAh·g -1 / V、5.5mAh·g -1 / V, 5.7mAh·g -1 / V、6.0mAh·g -1 / V、6.3mAh·g -1 / V、6.5mAh·g -1 / V、7.0mAh·g -1 / V or a range of any two of these values.

[0031] In some implementations, 2 mAh·g -1 / V≤Sa≤5mAh·g -1 / V and 5mAh·g -1 / V≤Sb≤12mAh·g -1 / V. When the delithiation potential slope of the first active material is in a relatively low range, the energy density of the secondary battery can be further improved, and its kinetic performance is also within an acceptable range.

[0032] In some implementations, Sb is 5.0 mAh·g -1 / V、5.5mAh·g -1 / V、6mAh·g -1 / V、6.5mAh·g -1 / V, 7mAh·g -1 / V, 7.5mAh·g -1 / V、8mAh·g -1 / V, 8.5mAh·g -1 / V、9mAh·g -1 / V, 9.5mAh·g -1 / V, 10mAh·g -1 / V, 10.5mAh·g -1 / V, 11mAh·g -1 / V, 11.5mAh·g -1 / V, 12mAh·g -1 / V, 12.5mAh·g -1 / V, 13mAh·g -1 / V, 13.5mAh·g -1 / V, 14mAh·g -1 / V or a range of any two of these values. In some implementations, 8 mAh·g -1 / V≤Sb≤14mAh·g -1 / V. When the delithiation potential slope of the second active material is too high, the available specific capacity below 1V decreases, resulting in a loss of energy density in the secondary battery. Furthermore, the excessive number of active sites in the second active material leads to excessive side reactions with the electrolyte, which in turn affects the initial coulombic efficiency and cycle performance of the secondary battery.

[0033] In some implementations, 4 mAh·g -1 / V≤Sa≤7mAh·g -1 / V and 8mAh·g -1 / V≤Sb≤12mAh·g -1 / V. When the delithiation potential slopes of the first and second active materials simultaneously meet the above-mentioned range, the energy density and kinetic performance of the secondary battery can be further improved.

[0034] In some implementations, 2 mAh·g -1 / V≤Sb-Sa≤12mAh·g -1 / V. In some implementations, Sb-Sa is 2.0 mAh·g. -1 / V, 2.5mAh·g -1 / V, 3mAh·g -1 / V、3.5mAh·g -1 / V, 4mAh·g -1 / V, 4.5mAh·g -1 / V、5mAh·g -1 / V、5.5mAh·g -1 / V、6mAh·g -1 / V、6.5mAh·g -1 / V, 7mAh·g-1 / V, 7.5mAh·g -1 / V、8mAh·g -1 / V, 8.5mAh·g -1 / V、9mAh·g -1 / V, 9.5mAh·g -1 / V, 10mAh·g -1 / V, 10.5mAh·g -1 / V, 11mAh·g -1 / V, 11.5mAh·g -1 / V, 12mAh·g -1 / V or a range of any two of these values. The difference between Sb and Sa represents the difference in delithiation capability between the upper second active material and the lower first active material in the electrode. When the difference between Sb and Sa meets the above range, the secondary battery has high energy density while also maintaining excellent kinetic performance. When the difference between Sb and Sa is small, the delithiation capabilities of the upper and lower active materials are more similar, that is, the electrode as a whole tends to be interlayer lithium intercalation of a graphite structure, with higher specific capacity but poor lithium-ion diffusion performance, or the electrode as a whole tends to be adsorbed lithium intercalation of amorphous carbon, with good lithium-ion diffusion performance but lower specific capacity. When the difference between Sb and Sa is large, the difference in delithiation capability between the upper and lower active materials is greater, and the high specific capacity of the first active material cannot compensate for the low specific capacity of the second active material, or the high lithium-ion diffusion capability of the second active material cannot compensate for the low lithium-ion diffusion capability of the first active material. In some embodiments, 5 mAh·g -1 / V≤Sb-Sa≤8mAh·g -1 / V.

[0035] In some embodiments, the first active material includes natural graphite and / or artificial graphite.

[0036] In some embodiments, the method for preparing the first active material includes the following steps:

[0037] S1: Mix artificial graphite and / or natural graphite with a coating agent to obtain a first mixture;

[0038] S2: The first mixture is subjected to a first heat treatment in an inert atmosphere to obtain a first heat-treated product;

[0039] 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 active material.

[0040] 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.

[0041] 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.

[0042] In some embodiments, in S3, the temperature of the second heat treatment is 750°C to 1100°C, for example, 800°C, 850°C, 900°C, 950°C, 1000°C, or 1050°C. In some embodiments, in S3, the time of the second heat treatment is 2 hours to 8 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours, or 7 hours.

[0043] In some embodiments, the volume percentage of CO2 in the CO2 and N2 mixture is 3% to 7%, for example, 4%, 5% or 6%.

[0044] In some embodiments, the second active material includes artificial graphite. In some embodiments, the second active material is selected from amorphous carbon-coated artificial graphite.

[0045] In some embodiments, the preparation method of the second active material includes the following steps:

[0046] M1: Artificial graphite is mixed with a coating agent to obtain a second mixture;

[0047] M2: The second mixture is subjected to a third heat treatment in an inert atmosphere to obtain the second active material.

[0048] In some embodiments, the coating agent in M1 has a mass content of 3% to 8%, for example, 4%, 5%, 6%, or 7%, based on the mass of the second mixture. In some embodiments, the coating agent is bitumen.

[0049] In some embodiments, the temperature of the third heat treatment in M2 is between 700°C and 1150°C, for example, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1110°C, or 1150°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.

[0050] 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.

[0051] In some embodiments, the thickness of the negative electrode active material layer is T, the thickness of the first active material layer is Ta, and the thickness of the second active material layer is Tb, wherein 60% ≤ Ta / T ≤ 85%, and 15% ≤ Tb / T ≤ 40%. In some embodiments, Ta / T is 60%, 65%, 70%, 75%, 80%, or 85%. In some embodiments, Tb / T is 15%, 20%, 25%, 30%, 35%, or 40%. When the thickness ratio of the first and second active material layers is within the above ranges, the secondary battery achieves both high energy density and excellent electrochemical performance. When the thickness ratio of the second active material layer (Tb) is greater than 40% and the thickness ratio of the first active material layer (Ta) is less than 60%, the energy density of the secondary battery is significantly reduced. When the thickness ratio of the second active material layer (Tb) is less than 15% and the thickness ratio of the first active material layer (Ta) is greater than 85%, the kinetic performance of the secondary battery is significantly reduced.

[0052] In some embodiments, the specific capacity of the negative electrode active material layer is 335 mAh·g. -1 Up to 365mAh·g -1 In some embodiments, the specific capacity of the negative electrode active material layer is 340 mAh·g. -1 345mAh·g -1 350mAh·g -1 355mAh·g -1 Or 360mAh·g -1 When the specific capacity of the negative electrode active material layer is too small, it has good lithium-ion diffusion ability, but the energy density is too low; when the specific capacity of the negative electrode active material layer is too large, the energy density is high, but the lithium-ion diffusion ability is poor.

[0053] In some embodiments, the porosity of the negative electrode is between 20% and 40%. In some embodiments, the porosity of the negative electrode is 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, or any combination of these values. If the porosity of the negative electrode is too low, it hinders the wetting and penetration of the electrolyte, making lithium-ion diffusion difficult and thus reducing the kinetic performance of the secondary battery. If the porosity is too high, the contact area between the electrolyte and the active material is too large, leading to excessive side reactions and excessively rapid capacity decay during cycling.

[0054] In some embodiments, the compaction density of the negative electrode active material layer is 1.60 g / cm³. 3 Up to 1.80 g / cm 3 In some implementations, 1.63 g / cm³ 3 1.65g / cm 3 1.67 g / cm 3 1.70g / cm3 1.73g / cm 3 1.75g / cm 3 1.77g / cm 3 Or a range of any two of these values. When the compaction density of the negative electrode active material layer is too high, it can easily lead to over-pressure of the electrode, affecting electrode processing and electrical performance. When the compaction density of the negative electrode active material layer is too low, it cannot effectively improve the energy density of the secondary battery.

[0055] In some embodiments, the first active material layer and the second active material layer further include 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] In some implementations, the secondary battery is a wound secondary battery or a stacked secondary battery.

[0066] 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.

[0067] II. Electronic Devices

[0068] This application further provides an electronic device that includes the secondary battery of the first aspect of this application.

[0069] 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.

[0070] Unless otherwise specified, all reagents, materials and instruments used in the following examples and comparative examples are commercially available.

[0071] Examples and Comparative Examples

[0072] 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.

[0073] Example 1

[0074] 1. Preparation of negative electrode active materials

[0075] First active material: Weigh 5 kg of artificial graphite material and mix it evenly with asphalt as a coating agent to form a first mixture, wherein the mass ratio of asphalt to graphite material is 2:98. Subject the first mixture to a first heat treatment; the first heat treatment conditions are: reaction at 1100℃ under a nitrogen atmosphere for 2 hours. After the reaction is complete and the temperature drops to room temperature, the first heat-treated product is obtained. Subject the first heat-treated product to a second heat treatment; the second heat treatment conditions are: reaction atmosphere of 5% CO2 and 95% N2 mixture, reaction temperature T1 of 1000℃, and reaction time H1 of 6 hours; after the reaction is complete and cooled to room temperature, the first active material is obtained.

[0076] Second 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 5:95. Then, subject the second mixture to a third heat treatment under the following conditions: reaction temperature T2 is 800℃, reaction time H2 is 4h under a nitrogen atmosphere; after the reaction is completed, cool to room temperature to obtain the second active material.

[0077] Other examples and comparative examples can be prepared by adjusting the coating agent pitch ratio W1, reaction temperature T1, and reaction time H1 in the first active material preparation conditions under a mixed atmosphere of 5% CO2 and 95% N2 in Example 1, and the coating agent pitch ratio W2, reaction temperature T2, and reaction time H2 in the second active material preparation conditions.

[0078] 2. Preparation of the negative electrode

[0079] The first 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 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 the current collector, and slurry 1 was used as the first active material layer, while slurry 2 was used as the second active material layer. Both layers were uniformly coated onto the current collector, dried, and cold-pressed to obtain the negative electrode sheet, also known as the negative electrode. The thickness of the first active material layer accounted for 60% of the total active material layer thickness, and the thickness of the second active material layer accounted for 40% of the total active material layer thickness.

[0080] 3. Preparation of the positive electrode

[0081] 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.

[0082] 4. Preparation of electrolyte

[0083] 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.

[0084] 5. Preparation of the separating membrane

[0085] Polyethylene porous polymer film is used as the separator.

[0086] 6. Preparation of lithium-ion batteries

[0087] 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.

[0088] Examples 2 to 18, Comparative Examples 1 to 4

[0089] Preparation of negative electrode active materials

[0090] The preparation process of the first and second active materials is similar to that in Example 1, except that the corresponding negative electrode active materials are prepared by adjusting the reaction temperature T1, reaction time H1, reaction temperature T2, and reaction time H2. Specific preparation parameters are shown in Tables 1 and 2.

[0091] The preparation of the negative electrode, positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 1.

[0092] Examples 19 to 25

[0093] Preparation of negative electrode active materials

[0094] The preparation process of the negative electrode active material is the same as in Example 14.

[0095] The negative electrode preparation process is similar to that in Example 14, except that the thickness of the first active material layer and the second active material layer is adjusted by adjusting the coating weight of the slurry. The specific mass content of the first active material layer and the second active material layer after adjustment is shown in Table 3.

[0096] The preparation of the positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 14.

[0097] Examples 26 to 32

[0098] Preparation of negative electrode active materials

[0099] The preparation process of the negative electrode active material is the same as in Example 21.

[0100] The preparation process of the negative electrode is similar to that in Example 21, except that the ratio of the coating agent is adjusted.

[0101] The specific capacity of the resulting negative electrode sheet is adjusted using W1 and W2. The specific ratios and specific capacities of the negative electrode sheet after adjustment are shown in Table 4. The preparation of the positive electrode, separator, electrolyte, and lithium-ion battery are the same as in Example 21.

[0102] Test methods

[0103] Testing of relevant parameters of negative electrode active materials

[0104] 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.

[0105] 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 active material.

[0106] 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 active material.

[0107] All M grams of powder above the current collector 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 negative electrode active material.

[0108] The obtained first active material, second active material, and negative electrode active material were subjected to the following tests:

[0109] 1. Test of the slope of the potential at the end of the lithium removal curve

[0110] The first active material, the second active material, and the negative electrode active material are respectively mixed with SBR, CMC, and conductive carbon in a weight ratio of 93:2.5:2.5:2, and then thoroughly stirred in an appropriate amount of deionized water solvent to form a uniform negative electrode slurry. This slurry is coated onto a current collector Cu foil, dried, and cold-pressed to obtain the first negative electrode sheet of the first active material, the second negative electrode sheet of the second active material, and the negative electrode sheet of all active materials.

[0111] First button cell battery:

[0112] The first negative electrode sheet was used as the positive electrode of the coin cell. It was assembled with lithium sheet, separator (same as in Example 1), electrolyte (same as in Example 1), steel sheet, nickel foam and coin cell casing to obtain the first coin cell. It was left to stand for 6 hours before testing.

[0113] Second button cell battery:

[0114] The prepared second negative electrode sheet was used as the positive electrode of the coin cell. It was assembled with lithium sheet, separator (same as in Example 1), electrolyte (same as in Example 1), steel sheet, nickel foam and coin cell casing to obtain the second coin cell. It was left to stand for 6 hours before testing.

[0115] Integral electrode coin cell:

[0116] All the prepared active material negative electrode sheets were used as the positive electrode of the coin cell, and assembled together with lithium sheet, separator (same as in Example 1), electrolyte (same as in Example 1), steel sheet, nickel foam and coin cell case to obtain an integral electrode coin cell, which was left to stand for 6 hours before testing.

[0117] Potential slope test:

[0118] The assembled coin cells were tested using a blue battery tester. The test procedure was as follows: discharge to 5mV at 0.05C, let stand for 5 minutes, discharge to 5mV at 0.05mA, discharge to 5mV at 0.01mA, and charge to 2.0V at 0.1C to obtain the charging capacity. Finally, divide by the weight of the active material to obtain the specific capacity of the sample. Plotting the charging voltage against the specific capacity yielded the delithiation curve of the material. The specific capacity at 2V was recorded as Cap2, and the specific capacity at 1V was recorded as Cap1. The slope of the delithiation end potential S = (Cap2-Cap1) / (2V-1V)mAh / g / V. The potential slopes Sa of the first active material and Sb of the second active material were tested and recorded respectively.

[0119] Overall electrode capacity test:

[0120] The assembled coin cell with electrode sheet was placed on a Blue Electricity Tester for testing. The testing procedure was as follows: discharge to 5mV at 0.05C, let stand for 5 minutes, discharge to 5mV at 0.05mA, discharge to 5mV at 0.01mA, and charge to 2.0V at 0.1C to obtain the charging capacity. Finally, divide by the weight of the active material to obtain the total specific capacity of the electrode sheet.

[0121] 2. Porosity test of negative electrode sheet

[0122] The porosity of the electrode was determined using a McMurray Tick true density analyzer (ACCUPYC II1345) and the gas displacement method. The percentage of pore volume in the electrode to the total volume of the electrode is the electrode porosity, calculated using the formula: P = (V - V0) / V × 100%, where V0 is the true volume and V is the apparent volume.

[0123] Lithium-ion battery performance testing

[0124] 3. Volumetric energy density

[0125] The capacity of lithium-ion batteries is tested according to the following procedure:

[0126] 1) Let it stand at 25℃ for 30 minutes;

[0127] 2) Charge at 0.5C to 4.48V, then maintain constant voltage at 0.05C;

[0128] 3) Let it sit for 5 minutes;

[0129] 4) Discharge at 0.2C to 3.0V;

[0130] 5) Let it sit for 5 minutes, then the test is over.

[0131] 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:

[0132] E = (C × P) / (G × L × W).

[0133] 4. Electrochemical impedance spectroscopy

[0134] 1) The test temperature is 25℃;

[0135] 2) Let stand for 60 minutes;

[0136] 3) Constant current (CC) up to 4.48V, constant voltage (CV) up to 0.025C;

[0137] 4) Let stand for 10 minutes;

[0138] 5) 0.1C DC to 3V;

[0139] 6) Let stand for 10 minutes;

[0140] 7) 0.5C CC to 4.48V, CV to 0.025C;

[0141] 8) Let stand for 1 hour;

[0142] 9) 0.1C DC to 10s;

[0143] 10) 1C DC to 1s;

[0144] 11) Let stand for 1 hour;

[0145] 12) 0.5C DC for 6 minutes;

[0146] 13) If the voltage is ≤2.5V, then proceed to step 15;

[0147] 14) Repeat steps 8 to 13 26 times;

[0148] 15) Let stand for 10 minutes;

[0149] 16) 0.5C CC to 3.95V, CV to 0.025C;

[0150] 17) Let stand for 10 minutes;

[0151] The 1s DC impedance of the battery at 70% SOC is called the electrochemical impedance Rct. The smaller the Rct, the better the kinetic performance of the lithium-ion battery.

[0152] Test Results

[0153] Table 1 shows the effect of the terminal potential slope Sa of the coin cell delithiation curve of the first active material and the terminal potential slope Sb of the coin cell delithiation curve of the second active material on the performance of lithium-ion batteries.

[0154] Table 1

[0155]

[0156] Note: " / " indicates that the corresponding heat treatment step is not performed.

[0157] As can be seen from Examples 1 to 11 in Table 1, the magnitude of Sa mainly affects the energy density of the lithium-ion battery, while the magnitude of Sb mainly affects the electrochemical impedance of the lithium-ion battery. When Sa meets the range of 2 mAh·g... -1 / V≤Sa≤7mAh·g -1 / V,Sb meets the range of 5mAh·g -1 / V≤Sb≤14mAh·g -1 At a voltage of 1 / V, lithium-ion batteries exhibit high energy density and an electrochemical impedance Rct ≤ 35mΩ, indicating excellent kinetic performance.

[0158] When 2mAh·g -1 / V≤Sa≤5mAh·g -1 / V and 5mAh·g -1 / V≤Sb≤12mAh·g -1 At / V, lithium-ion batteries have higher energy density, with a volumetric energy density exceeding 750Wh / L. When 2mAh·g -1 / V≤Sa≤7mAh·g -1 / V and 8mAh·g -1 / V≤Sb≤14mAh·g -1 At / V, the electrochemical impedance of lithium-ion batteries is relatively small, Rct≤30mΩ.

[0159] Comparative Examples 1 and 2 show that when Sb is less than 5 mAh·g -1 When the voltage is / V, the Rct of the lithium-ion battery is too high, which greatly affects its dynamic performance.

[0160] Table 2, based on Example 2, further investigates the effect of the difference between the terminal potential slope Sa of the coin-cell delithiation curve of the first active material and the terminal potential slope Sb of the coin-cell delithiation curve of the second active material on lithium-ion performance by controlling the reaction time of the active material to regulate the potential slope at the end of the delithiation curve.

[0161] Table 2

[0162]

[0163] Table 2 shows that the difference between Sb and Sa needs to be controlled to further balance the volumetric energy density and electrochemical impedance of lithium-ion batteries. Combining Tables 1 and 2 yields the same conclusion. When 4 mAh·g -1 / V≤Sa≤7mAh·g -1 / V and 8mAh·g -1 / V≤Sb≤12mAh·g -1 / V, 5mAh·g -1 / V≤Sb-Sa≤8mAh·g -1 At / V, the volumetric energy density and kinetic performance of lithium-ion batteries can be further improved.

[0164] Table 3 further investigates the effects of the thickness of the first active material layer and the thickness of the second active material layer on lithium-ion performance, based on Example 14.

[0165] Table 3

[0166]

[0167]

[0168] A comparison of the data from Examples 14, 19 to 24 and 25 shows that when the thickness of the first active material layer is in the range of 60% to 85% and the thickness of the second active material layer is in the range of 15% to 40%, the lithium-ion battery has high volumetric energy density and excellent kinetic performance.

[0169] Table 4 further investigates the effect of the specific capacity of the negative electrode on lithium-ion performance based on Example 21.

[0170] Table 4

[0171]

[0172] As can be seen from Example 32, when the specific capacity of the negative electrode is less than 355 mAh / g, the Rct of the lithium-ion battery is low, but the energy density loss is significant.

[0173] As can be seen from Examples 21, 26 to 31, when the specific capacity of the negative electrode is in the range of 355mAh / g-365mAh / g, the lithium-ion battery has high volumetric energy density and excellent kinetic performance.

[0174] 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 provided on the current collector, the negative electrode active material layer comprising a first active material layer and a second active material layer, the first active material layer being provided between the current collector and the second active material layer, the first active material layer comprising a first active material, a first coin cell using lithium as a negative electrode and the first active material as a positive electrode is subjected to a charge-discharge test, the charge-discharge test being a discharge to 5 mV at 0.05 C, a rest for 5 min, a discharge to 5 mV at 0.05 mA, a discharge to 5 mV at 0.01 mA, a charge to 2.0 V at 0.1 C, and a stop, a delithiation curve of the first coin cell is plotted with a charge voltage and a gram capacity, a gram capacity corresponding to a 2 V potential is recorded as Cap2, a gram capacity corresponding to a 1 V potential is recorded as Cap1, a delithiation end potential slope S = (Cap2-Cap1) / (2V-1V) mAh / g / V, and a delithiation curve potential slope of the first coin cell is Sa, the first active material is prepared by: mixing artificial graphite and / or natural graphite with a coating agent to obtain a first mixture, a mass content of the coating agent being 1% to 5% based on a mass of the first mixture; subjecting the first mixture to a first heat treatment in an inert atmosphere to obtain a first heat treatment product; and subjecting the first heat treatment product to a second heat treatment in a mixed gas of CO2 and N2 to obtain the first active material. 2.The secondary battery according to claim 1, wherein the second active material is prepared by: mixing the artificial graphite with the coating agent to obtain a second mixture, a mass content of the coating agent being 3% to 8% based on a mass of the second mixture; and subjecting the second mixture to a third heat treatment in an inert atmosphere to obtain the second active material. The second active material layer comprises a second active material, a second button cell using lithium as a negative electrode and the second active material as a positive electrode is subjected to charge-discharge test, the charge-discharge test is discharging to 5mV at 0.05C, standing for 5min, discharging to 5mV at 0.05mA, discharging to 5mV at 0.01mA, charging to 2.0V at 0.1C to stop, and the delithiation curve of the second button cell is plotted with the charging voltage and gram capacity, the corresponding gram capacity at 2V potential is recorded as Cap2, the corresponding gram capacity at 1V potential is recorded as Cap1, the delithiation end potential slope S=(Cap2-Cap1) / (2V-1V) mAh / g / V, the potential slope of the delithiation curve of the second button cell is Sb, and Sa<Sb, wherein, 2 mAh-g -1 / V≤Sa≤7 mAh-g -1 / V, 5 mAh-g -1 / V≤Sb≤14 mAh-g -1 / V; 3.The secondary battery according to claim 1 or 2, wherein a temperature of the first heat treatment is 950℃ to 1200℃, and a time of the first heat treatment is 1h to 5h. 4.The secondary battery according to claim 1 or 2, wherein a temperature of the second heat treatment is 750℃ to 1100℃, and a time of the second heat treatment is 2h to 8h. 5.The secondary battery according to claim 1 or 2, wherein a temperature of the third heat treatment is 700℃ to 1150℃, and a time of the third heat treatment is 4h to 10h. 6.The secondary battery according to any one of claims 1 to 5, wherein the coating agent is pitch. 7.The secondary battery according to any one of claims 1 to 6, wherein a thickness of the negative electrode active material layer is T, a thickness of the first active material layer is Ta, and a thickness of the second active material layer is Tb, wherein 60% ≤ Ta / T ≤ 85%, and 15% ≤ Tb / T ≤ 40%. 8.The secondary battery according to any one of claims 1 to 7, wherein the negative electrode satisfies at least one of conditions (i) to (iii) below: (i) a capacity of the first active material layer is 20% to 40% of a total capacity of the negative electrode; (ii) a porosity of the negative electrode is 20% to 40%; and (iii) a capacity of the second active material layer is 15% to 40% of the total capacity of the negative electrode. 9.An electronic device comprising the secondary battery according to any one of claims 1 to 8. 10.A method for manufacturing a negative electrode, the method comprising: a first active material preparation: mixing artificial graphite and / or natural graphite with a coating agent to obtain a first mixture, a mass content of the coating agent being 1% to 5% based on a mass of the first mixture; subjecting the first mixture to a first heat treatment in an inert atmosphere to obtain a first heat treatment product; and subjecting the first heat treatment product to a second heat treatment in a mixed gas of CO2 and N2 to obtain the first active material. 11.The method for manufacturing a negative electrode according to claim 10, wherein the second active material is prepared by: mixing the artificial graphite with the coating agent to obtain a second mixture, a mass content of the coating agent being 3% to 8% based on a mass of the second mixture; and subjecting the second mixture to a third heat treatment in an inert atmosphere to obtain the second active material. 12.The method for manufacturing a negative electrode according to claim 10 or 11, wherein a temperature of the first heat treatment is 950℃ to 1200℃, and a time of the first heat treatment is 1h to 5h. 13.The method for manufacturing a negative electrode according to claim 10 or 11, wherein a temperature of the second heat treatment is 750℃ to 1100℃, and a time of the second heat treatment is 2h to 8h.

2. The secondary battery according to claim 1, wherein 2 mAh-g -1 / V≤Sa≤5 mAh-g -1 / V and 5 mAh-g -1 / V≤Sb≤12 mAh-g -1 / V.

3. The secondary battery according to claim 1, wherein 8 mAh-g -1 / V ≤ Sb ≤ 14 mAh-g -1 / V.

4. The secondary battery according to claim 1, wherein 4 mAh-g -1 / V < Sa < 7 mAh-g -1 / V and 8 mAh-g -1 / V < Sb < 12 mAh-g -1 / V.

5. The secondary battery according to claim 1, wherein 2 mAh.g -1 / V ≤ Sb - Sa ≤ 12 mAh.g -1 / V.

6. The secondary battery according to claim 4, wherein 5 mAh.g -1 / V ≤ Sb - Sa ≤ 8 mAh.g -1 / V.

7. The secondary battery according to claim 1, wherein 14.The method for manufacturing a negative electrode according to claim 10 or 11, wherein a temperature of the third heat treatment is 700℃ to 1150℃, and a time of the third heat treatment is 4h to 10h.

8. The secondary battery according to claim 1, wherein 15.The method for manufacturing a negative electrode according to any one of claims 10 to 14, wherein the coating agent is pitch. (i) the capacity per gram of the negative electrode is 335 mAh g -1 to 365 mAh g -1 ; ​ (iii) the compaction density of the negative electrode is 1.60 g / cm 3 to 1.80 g / cm 3 . ​ 10. A method for producing a negative electrode of a secondary battery as described in any one of claims 1 to 9, the negative electrode comprising a current collector and a negative electrode active material layer provided on a surface of the current collector, the negative electrode active material layer comprising a first active material layer and a second active material layer, the first active material layer being provided between the current collector and the second active material layer, the first active material layer comprising a first active material, the second active material layer comprising a second active material, wherein, ​ ​ ​ ​ The first heat-treated product is subjected to a second heat treatment in a mixed gas of CO2 and N2 to obtain the first active material; Preparation of the second active material: The artificial graphite is mixed with the coating agent to obtain a second mixture, and the mass content of the coating agent in the second mixture is 3% to 8%; The second mixture is subjected to a third heat treatment in an inert atmosphere to obtain the second active material; The temperature of the first heat treatment is 950°C to 1200°C, and the time of the first heat treatment is 1h to 5h; The temperature of the second heat treatment is 750°C to 1100°C, and the time of the second heat treatment is 2h to 8h; The temperature of the third heat treatment is 700°C to 1150°C, and the time of the third heat treatment is 4h to 10h; The coating agent is pitch.

11. The preparation method of claim 10, wherein the temperature of the second heat treatment is 800°C to 1000°C.

12. The preparation method of claim 10, wherein the time of the second heat treatment is 5h to 6h.

13. The production method according to claim 10, wherein In the preparation of the first active material, the mass content of the coating agent in the first mixture is 1% to 3%; In the preparation of the second active material, the mass content of the coating agent in the second mixture is 3% to 5%.

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