Negative electrode active material, secondary battery, and electronic device

Abstract

The present application provides a negative electrode active material, a secondary battery, and an electronic device, belonging to the field of battery technology. The negative electrode active material provided by the present application includes a silicon-based material and an outer layer material; the outer layer material includes a conductive agent and a nitrile polymer; the infrared spectrum of the negative electrode active material includes an infrared characteristic peak of a cyano group; the negative electrode active material includes nitrogen element and silicon element; based on the mass of the negative electrode active material, the mass content of the nitrogen element is D1%, and the mass content of the silicon element is G%, and 0.02 ≤ D1 / G ≤ 0.5. By forming an outer layer material composed of a conductive agent and a nitrile polymer on the surface of the silicon-based material, the present application can improve the conductivity and stability of the negative electrode active material, enable the secondary battery to have excellent cycle capacity retention rate and swelling inhibition performance, and further improve the high-temperature storage performance of the secondary battery.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and particularly relates to a negative electrode active material, a secondary battery, and an electronic device. Background Art

[0002] With the increasing demands for large-scale storage, electric vehicles, and portable electronic devices, the development of energy storage devices with higher energy density has become a top priority. Due to their long cycle life and environmental friendliness, secondary batteries have been widely used in various fields. However, as a traditional negative electrode active material for secondary batteries, graphite has limitations in its further application due to its low capacity (372 mAh / g) and safety hazards such as the precipitation of active substances. Therefore, the development of negative electrode active materials with high energy density and high safety has become the key to the current development of secondary battery technology.

[0003] Compared with carbon-based materials such as graphite, elemental silicon has an ultra-high theoretical specific capacity (Li 15 Si4, 3579 mAh / g) and a suitable working voltage (<0.5V vs. Li / Li + ), and is considered to be the most promising negative electrode active material to replace graphite. However, the huge volume expansion and low conductivity of elemental silicon during the alloying / dealloying process severely limit its large-scale application in secondary batteries.

[0004] To solve the above problems, the existing technologies mainly adopt methods such as silicon nanosizing, constructing porous silicon or silicon-carbon negative electrode active materials, and introducing transition metal oxides. Although silicon nanosizing and porous silicon can alleviate the volume expansion problem of elemental silicon to a certain extent, their high specific surface area and low tap density limit their large-scale application. Although silicon-carbon composites and the introduction of transition metal oxides can reduce the volume expansion of silicon, due to the unremarkable mechanical properties and chemical stability of carbon and metal oxides, the ideal buffering effect cannot be achieved. Summary of the Invention

[0005] To solve the above problems, this application provides a negative electrode active material, a secondary battery, and an electronic device, which can improve the conductivity of the negative electrode active material, reduce the volume expansion during charge and discharge, improve the first Coulomb efficiency, inhibit the swelling performance, cycle performance, and high-temperature storage performance of the secondary battery.

[0006] In the first aspect, this application provides a negative electrode active material, which includes a silicon-based material and an outer layer material; the outer layer material is located on at least part of the surface of the silicon-based material; the outer layer material includes a conductive agent and a nitrile polymer; the infrared spectrum of the negative electrode active material includes an infrared characteristic peak of a cyano group, and the infrared characteristic peak of the cyano group is located at 2250 cm -1 to 2400 cm -1Between; the negative electrode active material includes nitrogen and silicon elements; based on the mass of the negative electrode active material, the mass content of nitrogen is D1%, and the mass content of silicon is G%, where 0.02 ≤ D1 / G ≤ 0.5. In this application, a conductive agent and a nitrile polymer containing a cyano group are used in combination to form a first outer shell. The conductive network formed by the conductive agent on the surface of the silicon-based material can improve the overall conductivity of the negative electrode active material, reduce the internal resistance of the negative electrode, and improve the first Coulomb efficiency of the secondary battery. Among them, when controlling the mass contents of nitrogen and silicon elements in the negative electrode active material to meet the above range, the conductive agent and the nitrile polymer can form a highly flexible and tightly intertwined outer layer material on the surface of the silicon-based material, and can also promote the formation of a stable solid electrolyte interface film (SEI film) on the surface of the negative electrode active material, which helps to buffer the volume expansion and stress of the silicon-based material under high-temperature conditions and during the cycling process, improve the interface stability of the negative electrode active material, reduce the structural damage of the negative electrode active material and the decomposition of the electrolyte, improve the swelling problem of the silicon-containing secondary battery, and improve the high-temperature storage performance and cycling stability of the secondary battery.

[0007] In some embodiments, the negative electrode active material satisfies at least one of the following conditions:

[0008] (1) 1.1 ≤ D1 ≤ 18.4; controlling the content of nitrogen in the negative electrode active material within this range can improve the quality of the SEI film, reduce the decomposition of the electrolyte and the generation of by-products, and enable the secondary battery to exhibit higher cycling performance and first Coulomb efficiency.

[0009] (2) 36.8 ≤ G ≤ 68.3; controlling the content of silicon in the negative electrode active material within this range can enable the negative electrode active material to have a high specific capacity, and at the same time, when combined with the conductive agent and the nitrile polymer in the outer layer material, can significantly improve the swelling suppression performance and cycling performance of the secondary battery.

[0010] (3) 0.04 ≤ D1 / G ≤ 0.37. Adjusting the mass ratio of nitrogen and silicon elements within this range can further improve the film-forming quality of the SEI film, enable the negative electrode active material to maintain high cycling stability, and can optimize the flexible conductive network structure of the outer layer material, absorb the volume change of the silicon-based material, improve the swelling suppression performance of the secondary battery, and optimize the cycling performance and high-temperature storage performance of the secondary battery.

[0011] In some more preferred embodiments, the negative electrode active material satisfies at least one of the following conditions:

[0012] (1) 2.3 ≤ D1 ≤ 9.2; the nitrile polymer with nitrogen in this content range can further improve the first Coulomb efficiency and cycling performance of the secondary battery.

[0013] (2) 43.2 ≤ G ≤ 51.3; The silicon element within this content range enables the secondary battery to exhibit more excellent swelling inhibition performance and cycling performance.

[0014] (3) 0.06 ≤ D1 / G ≤ 0.19. Controlling the mass ratio of nitrogen element and silicon element in the negative electrode active material to satisfy this range can further improve the swelling inhibition performance, cycling performance and high-temperature storage performance of the secondary battery.

[0015] In some embodiments, the sphericity of the negative electrode active material is S, 0.8 ≤ S ≤ 1.0; when the sphericity of the negative electrode active material is regulated to meet this condition and is combined with the outer layer material including a conductive agent and a nitrile polymer, it can release the stress of the negative electrode active material more uniformly under high-temperature conditions or during cycling, buffer volume expansion and reduce the structural damage of the negative electrode active material, thereby improving the swelling inhibition performance, cycling performance and high-temperature storage performance of the secondary battery.

[0016] In some embodiments, in the negative electrode active material, the thickness of the outer layer material is T nm, 0.5 ≤ T ≤ 200.0. By regulating the thickness of the outer layer material in this application to meet this range, the negative electrode active material can have a high specific capacity, improve the conductivity of the negative electrode active material, and is also beneficial to improving the interfacial stability of the negative electrode active material, optimizing the swelling inhibition performance, cycling performance and high-temperature storage performance of the secondary battery.

[0017] In some embodiments, the negative electrode active material satisfies at least one of the following conditions:

[0018] (1) 0.9 ≤ S ≤ 1.0; when further regulating the sphericity of the secondary battery to meet this range, the secondary battery can have higher swelling inhibition performance, cycling performance and high-temperature storage performance.

[0019] (2) 0.5 ≤ T ≤ 100.0; regulating the thickness T of the outer layer material to meet this range can more significantly improve the swelling inhibition performance, cycling performance and high-temperature storage performance of the secondary battery.

[0020] (3) 0.5 ≤ S × T ≤ 90.0; preferably, 10.4 ≤ S × T ≤ 67.8; controlling the value of S × T to meet this range can promote the close cooperation of each component in the negative electrode active material, uniformly release the stress in the negative electrode active material, and improve the first Coulomb efficiency, swelling inhibition performance, high-temperature storage performance and cycling performance of the secondary battery.

[0021] (4) The average particle size of the negative electrode active material is R μm; 5.6 ≤ R ≤ 10.3. Controlling the average particle size of the negative electrode active material within this range can enable the negative electrode active material to achieve a suitable specific surface area, reduce the consumption of the electrolyte, and is also beneficial to improving the tap density of the negative electrode active material, making the secondary battery have a high energy density and cycle performance.

[0022] In some embodiments, the negative electrode active material satisfies at least one of the following conditions:

[0023] (1) The conductive agent includes single-walled carbon nanotubes and / or multi-walled carbon nanotubes; using these carbon nanotubes as the conductive agent in this application can achieve long-range conductivity performance, further improve the conductivity of the negative electrode active material, and enable the secondary battery to exhibit a higher initial Coulomb efficiency and cycle performance when combined with the nitrile polymer of this application.

[0024] (2) The nitrile polymer includes at least one of polyacrylonitrile, polycyanoacrylate, and polycyanobenzene; these nitrile polymers can promote the improvement of the interface stability during the cycling process, thereby improving the cycle performance and anti-swelling performance of the secondary battery. Among them, the nitrile polymer is preferably polyacrylonitrile. Using polyacrylonitrile can rivet the outer layer material on the silicon-carbon material, form a well-performing coating on the silicon-based material, improve the interface stability of the negative electrode active material and reduce the generation of interface by-products, thereby improving the cycle stability, high-temperature storage performance, and anti-swelling performance of the secondary battery.

[0025] (3) The silicon-based material includes a porous carbon skeleton and a silicon material; the silicon material is located on the surface and / or inside of the porous carbon skeleton. This type of silicon-based material can better cooperate with the outer layer material, improving the cycle performance, high-temperature storage performance, and anti-swelling performance of the secondary battery.

[0026] In a second aspect, this application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on at least a part of the surface of the negative electrode current collector, and the negative electrode mixture layer includes any one of the negative electrode active materials provided in the first aspect of this application.

[0027] In some embodiments, the negative electrode mixture layer includes sodium element, and based on the mass of the negative electrode mixture layer, the mass content of nitrogen element is D2%, and the mass content of sodium element is N%; the negative electrode mixture layer satisfies at least one of the following conditions:

[0028] (1) 0.58 ≤ N ≤ 1.74; preferably, 0.81 ≤ N ≤ 1.04; regulating the content of sodium element in the negative electrode mixture layer within the above range can enable the dispersant to cooperate with the negative electrode active material of this application and fully exert the improvement effect on the initial Coulomb efficiency of the secondary battery.

[0029] (2) 1.6 ≤ D2 ≤ 8.1; preferably, 5.0 ≤ D2 ≤ 7.8; by controlling the mass content of nitrogen element in the negative electrode mixture layer to meet the above range, the specific capacity and conductivity of the secondary battery can be improved, and its cycle performance and swelling inhibition performance can be improved.

[0030] (3) 2.35 ≤ D2 / N ≤ 11.74; preferably, 4.81 ≤ D2 / N ≤ 9.63; controlling the mass ratio of nitrogen element and sodium element in the negative electrode mixture layer to meet the above range can improve the energy density of the secondary battery, and improve the first Coulombic efficiency, cycle performance and swelling inhibition performance of the secondary battery.

[0031] (4) The sodium element comes from at least one of sodium alginate, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose or sodium hydroxyethyl carboxymethyl cellulose.

[0032] In some embodiments, the electrolyte includes at least one of fluorobenzene or fluoroethylene carbonate; based on the mass of the electrolyte, the electrolyte satisfies at least one of the following conditions:

[0033] (1) The mass content of fluorobenzene is F1%, 0.9 ≤ F1 ≤ 5.2; preferably, 2.1 ≤ F1 ≤ 3.1; when controlling the electrolyte of the secondary battery to contain fluorobenzene and regulating the mass content of fluorobenzene in the electrolyte to meet the above range, the high-temperature storage performance of the secondary battery can be further optimized, and its first Coulombic efficiency, cycle performance and swelling inhibition performance can be improved.

[0034] (2) The mass content of fluoroethylene carbonate is F2%, 3.8 ≤ F2 ≤ 7.4. Regulating the mass content of fluoroethylene carbonate (FEC) in the electrolyte to meet the above range can enable the secondary battery to achieve more excellent high-temperature storage performance, cycle performance and swelling inhibition performance.

[0035] In the third aspect, the present application provides an electronic device including the secondary battery provided in the second aspect of the present application.

[0036] Based on the negative electrode active material, secondary battery and electronic device of the present application above, by forming an outer layer material including a conductive agent and a nitrile polymer on the surface of the silicon-based material, the conductivity of the negative electrode active material, as well as the stability of the interface and structure can be improved simultaneously, enabling the secondary battery to have excellent cycle capacity retention rate and swelling inhibition performance at high energy density. In addition, it can also promote the formation of a more stable SEI film, further improving the cycle performance and high-temperature storage performance of the secondary battery. Description of the Drawings

[0037] Figure 1 It is the infrared spectrogram of the negative electrode active material provided in Example 1-1 of the present application;

[0038] Figure 2 This is a partially enlarged view of the infrared spectrum of the negative electrode active material provided in Embodiment 1-1 of the present application. Detailed implementation manners

[0039] In order to make the objectives, technical solutions and advantages of the present application more clear and understandable, the present application will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0040] In a first aspect, the present application provides a negative electrode active material, including a silicon-based material and an outer layer material; the outer layer material is located on at least part of the surface of the silicon-based material; the outer layer material includes a conductive agent and a nitrile polymer; the infrared spectrum of the negative electrode active material includes an infrared characteristic peak of a cyano group, and the infrared characteristic peak of the cyano group is located at 2250 cm -1 to 2400 cm -1 ; the negative electrode active material includes nitrogen element and silicon element; based on the mass of the negative electrode active material, the mass content of the nitrogen element is D1%, and the mass content of the silicon element is G%, and 0.02 ≤ D1 / G ≤ 0.5. In the present application, an outer layer material including a conductive agent and a nitrile polymer is formed on the surface of the silicon-based material, which can form a conductive network structure of the conductive agent on the surface of the negative electrode active material, improve the conductivity of the negative electrode active material, improve the electron conduction performance, cooperate with the silicon-based material to improve the energy density of the secondary battery, and at the same time can promote the ion transport rate during the charge and discharge process of the secondary battery and improve the first Coulomb efficiency. On this basis, controlling the mass ratio of the silicon element and the nitrogen element in the negative electrode active material to satisfy the above relationship can improve the flexibility and structural stability of the outer layer material, and promote the formation of a stable SEI film on the surface of the negative electrode active material, which helps to buffer the volume expansion and stress of the silicon-based material under high temperature conditions and during the cycling process, reduce the structural damage of the negative electrode active material and the decomposition of the electrolyte, and improve the swelling inhibition performance, high temperature storage performance and cycling stability of the secondary battery.

[0041] In the present application, the mass contents of the silicon element and the nitrogen element in the negative electrode active material satisfy 0.02 ≤ D1 / G ≤ 0.5, preferably 0.04 ≤ D1 / G ≤ 0.37, and more preferably 0.06 ≤ D1 / G ≤ 0.19. For example, the value of D1 / G is 0.02, 0.04, 0.06, 0.09, 0.12, 0.14, 0.16, 0.19, 0.20, 0.23, 0.26, 0.30, 0.31, 0.33, 0.36, 0.37, 0.42, 0.43, 0.46, 0.48, 0.5 or a value within the range formed by any two of these values. Controlling the content ratio of the silicon element and the nitrogen element within the above range can improve the cooperation effect between the nitrile polymer and the silicon-based material, and further improve the cycling performance and high temperature storage performance of the secondary battery during the cycling process.

[0042] In some embodiments, 1.1 ≤ D1 ≤ 18.4; preferably 2.3 ≤ D1 ≤ 9.2. Exemplarily, D1 is 1.1, 1.3, 2.3, 3.8, 4.6, 6.8, 7.1, 9.2, 11.2, 11.8, 13.6, 14.9, 15.7, 17.3, 18.4 or a value within the range formed by any two of these values. By controlling the content of nitrogen element in the negative electrode active material within the above range, the stability of the SEI film can be improved, the side reaction between the silicon-based material and the electrolyte can be reduced, and at the same time, the volume stability of the silicon-based material can be improved, further improving the high-temperature storage performance, cycle performance and swelling inhibition performance of the secondary battery.

[0043] In some embodiments, 36.8 ≤ G ≤ 68.3, preferably 43.2 ≤ G ≤ 51.3. For example, G can be 36.8, 38.8, 40.0, 43.2, 44.3, 47.0, 49.2, 51.3, 53.4, 55.4, 58.1, 59.8, 62.5, 65.2, 67.1, 68.3 or a value within the range formed by any two of these values. After controlling the content of silicon element in the negative electrode active material within the above range and cooperating with the nitrile polymer, the specific capacity of the negative electrode active material can be improved, the volume expansion effect can be reduced, and the volume stability of the negative electrode active material during the cycling process can be improved, thereby enhancing the swelling inhibition performance and cycle performance of the secondary battery.

[0044] In this application, there is no particular limitation on the method for controlling the mass contents of silicon element and nitrogen element in the negative electrode active material, as long as the purpose of this application can be achieved. For example, the mass content of silicon element can be adjusted by controlling the silicon element content in the silicon-based material or changing the addition ratio of the silicon-based material, and the mass content of nitrogen element can be adjusted by controlling the addition ratio of the nitrile compound.

[0045] The element contents (such as silicon element, nitrogen element, etc.) of the negative electrode active material in this application can be tested by methods known in the art. For example, a Shimadzu / Kratos X-ray photoelectron spectrometer AXIS SUPRA+ can be used to quantitatively test the element contents of the negative electrode active material sample.

[0046] In some embodiments, the sphericity of the negative electrode active material is S, where 0.8 ≤ S ≤ 1.0; preferably 0.9 ≤ S ≤ 1.0; for example, S is 0.8, 0.81, 0.82, 0.84, 0.85, 0.87, 0.89, 0.91, 0.92, 0.93, 0.94, 0.96, 0.97, 0.99, 1.0, or a value within the range formed by any two of these values. Controlling the sphericity of the negative electrode active material to meet the above range, in combination with the outer layer material including a conductive agent and a nitrile polymer, can release the stress of the negative electrode active material more uniformly under high-temperature conditions or during cycling, buffer volume expansion, reduce structural damage to the negative electrode active material, and improve the swelling inhibition performance, cycling performance, and high-temperature storage performance of the secondary battery.

[0047] In this application, the sphericity S of the negative electrode active material can be tested by methods known in the art. For example, the sphericity of the negative electrode active material can be tested using the equivalent diameter method. The testing method includes: observing the particle sample of the negative electrode active material using a ZEISS-SEM (sigma-02-33) scanning electron microscope, randomly selecting 20 negative electrode active material particles, and calculating their perimeter equivalent diameter and area equivalent diameter; or, cutting out the test surface from the negative electrode sheet, performing argon ion polishing on the test surface to obtain a test sample, then observing the negative electrode active material in the test sample using a scanning electron microscope, randomly selecting 20 negative electrode active material particles, and calculating the perimeter equivalent diameter and area equivalent diameter of the negative electrode active material particles. The sphericity of each negative electrode active material particle = perimeter equivalent diameter / area equivalent diameter. Calculate the arithmetic mean of the sphericities of 20 negative electrode active material particles, which is the sphericity of the negative electrode active material.

[0048] In some embodiments, in the negative electrode active material, the thickness of the outer layer material is T nm, where 0.5 ≤ T ≤ 200.0; preferably 0.5 ≤ T ≤ 100.0. Exemplarily, T is 0.5, 4.7, 17.6, 27.2, 37.5, 50.5, 54.4, 67.9, 76.2, 92.5, 100.0, 104.7, 111.3, 125.7, 129.8, 143.3, 150.1, 160.9, 171.7, 185.6, 189.9, 200, or a value within the range formed by any two of these values. Controlling the thickness of the outer layer material in this application to meet the above range can enable the negative electrode active material to have a high specific capacity, improve the conductivity of the negative electrode active material, and is also beneficial to improving the interfacial stability of the negative electrode active material, optimizing the swelling inhibition performance, cycling performance, and high-temperature storage performance of the secondary battery.

[0049] The test method for the thickness of the outer layer material of the negative electrode active material in this application is not particularly limited, and known methods in the art can be used for testing. For example, 20 negative electrode active materials can be randomly selected, 20 cross-sectional samples of the negative electrode active materials are prepared, and the maximum thickness and the minimum thickness of the outer layer material on the cross-section of each negative electrode active material are observed and counted by using a scanning electron microscope. The arithmetic mean of the maximum thickness and the minimum thickness is calculated as the thickness of the outer layer material in a single negative electrode active material, and then the arithmetic mean of the 20 negative electrode active materials is calculated and denoted as the thickness T nm of the outer layer material.

[0050] In some embodiments, 0.5 ≤ S×T ≤ 90.0; preferably, 10.4 ≤ S×T ≤ 67.8. Exemplarily, the value of S×T is 0.5, 2.0, 6.7, 12.6, 14.9, 22.0, 24.9, 33.1, 34.9, 40.3, 45.2, 48.0, 56.6, 60.4, 65.7, 69.6, 75.2, 77.4, 82.8, 88.6, 90.0 or a value within the range formed by any two of these values. Controlling the value of S×T to satisfy the above range can promote the close cooperation of each component in the negative electrode active material, uniformly release the stress in the negative electrode active material, improve the first Coulombic efficiency of the secondary battery, inhibit the swelling performance, high-temperature storage performance and cycling performance.

[0051] In some embodiments, the average particle size of the negative electrode active material is R μm, and 5.6 ≤ R ≤ 10.3. For example, R can be 5.6, 5.9, 6.2, 6.6, 7.0, 7.1, 7.7, 8.1, 8.3, 8.7, 8.9, 9.3, 9.8, 10.0, 10.3 or a value within the range formed by any two of these values. On the basis of controlling the element content of the negative electrode active material as described above, further regulating the average particle size of the negative electrode active material within the above range can enable the negative electrode active material to have an appropriate specific surface area, which is beneficial to improving the synergistic cooperation effect between elements, reducing the occurrence of side reactions and the consumption of electrolyte during the cycling process, and also helps to increase the tap density of the negative electrode active material, which is beneficial to improving the conductivity of the negative electrode active material and enhancing the cycling performance and the first Coulombic efficiency of the secondary battery.

[0052] In this application, the average particle size of the negative electrode active material can be measured by methods known in the art. For example, it can be measured by a method including the following steps: Select a sample area on the negative electrode sheet, take an SEM photo of the sample area through a scanning electron microscope, and then, using image analysis software, randomly select 10 particles of the negative electrode active material from the SEM photo as samples, calculate the area of each of these negative electrode active material samples, and then, assuming that the negative electrode active material samples are spherical, calculate the particle size R0 (diameter) of each of these negative electrode active material samples through the following formula: R0 = 2×(S / π); where S is the area of the negative electrode active material sample. Perform the process of calculating the particle size R0 of the above-mentioned negative electrode active material samples on 3 SEM images, and perform an arithmetic average on the particle sizes of the obtained 30 (10×3) negative electrode active material samples, so as to obtain the average particle size R of the negative electrode active material to be measured. 1 / 2 ; where S is the area of the negative electrode active material sample. Perform the process of calculating the particle size R0 of the above-mentioned negative electrode active material samples on 3 SEM images, and perform an arithmetic average on the particle sizes of the obtained 30 (10×3) negative electrode active material samples, so as to obtain the average particle size R of the negative electrode active material to be measured.

[0053] In some embodiments, the conductive agent includes single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The use of single-walled carbon nanotubes or multi-walled carbon nanotubes can further improve the conductivity of the negative electrode active material. When combined with the nitrile polymer in this application, it can optimize the structural stability of the outer layer material, further improve the cycle stability of the negative electrode active material, and enable the secondary battery to exhibit higher initial Coulomb efficiency, cycle performance, and anti-swelling performance.

[0054] In some embodiments, the nitrile polymer includes polyacrylonitrile. After polyacrylonitrile is combined with the conductive agent and silicon-based material in this application, it can further promote the improvement of the interface stability during the cycle, and improve the cycle stability, high-temperature storage performance, and anti-swelling performance of the secondary battery.

[0055] The silicon-based material in this application can be a material known in the art, such as silicon-carbon material, silicon-oxygen material, and silicon. In some preferred embodiments, the silicon-based material includes a porous carbon skeleton and a silicon material; the silicon material is located on the surface and / or inside of the porous carbon skeleton. This type of silicon-carbon material can better cooperate with the outer layer material. By cooperating with the outer layer material through the porous carbon skeleton to perform multiple fixation and support on the silicon, it can further limit the swelling effect during the silicon alloying process, improve the anti-swelling performance and cycle stability of the secondary battery. In addition, it is also beneficial to improve the overall conductivity of the negative electrode active material, promote the insertion and extraction of active substances, and improve the energy density and initial Coulomb efficiency of the secondary battery.

[0056] The silicon-carbon material used in this application can be prepared by methods known in the art. This application does not make special restrictions as long as the purpose of this application can be achieved. For example, it can include chemical vapor deposition of a silicon source and a carbon source.

[0057] In some embodiments, the negative electrode active material is prepared by a method including the following steps:

[0058] S1: Provide silicon-carbon material; the silicon-based material includes a porous carbon framework and silicon material; the silicon material is located on the surface and inside of the porous carbon framework.

[0059] S2: Conduct a first dispersion treatment on the conductive agent and nitrile polymer in a solvent to obtain a dispersion system; wherein the solvent includes at least one of water, ethanol, propanol, butanol, ethylene glycol, propylene glycol, dimethyl methanol, dimethyl sulfoxide, ethyl acetate.

[0060] S3: Add the silicon-carbon material in S1 into the dispersion system in S2 for a second dispersion treatment, and then conduct a drying treatment to obtain the negative electrode active material of the present application.

[0061] The present application does not particularly limit the specific parameters of the first dispersion treatment, the second dispersion treatment, and the drying treatment in the preparation method of the negative electrode active material, as long as the purpose of the present application can be achieved. During the first dispersion treatment, there is no particular limitation on the addition order of the conductive agent and the nitrile polymer, as long as the purpose of the present application can be achieved. For example, the addition order of the conductive agent and the nitrile polymer can be adjusted so that they have different first dispersion treatment times, and the area ratio of the conductive agent and the nitrile polymer in the outer layer material can be regulated.

[0062] In the second aspect, the present application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the negative electrode includes a negative electrode current collector and a negative electrode mixture layer provided on at least a part of the surface of the negative electrode current collector, and the negative electrode mixture layer includes any one of the negative electrode active materials provided by the present application in the first aspect.

[0063] The secondary battery of the present application is not particularly limited. For example, it may include, but is not limited to, lithium-ion secondary batteries (lithium-ion batteries) and sodium-ion batteries, etc.

[0064] In some embodiments, the negative electrode mixture layer includes sodium element. Based on the mass of the negative electrode mixture layer, the mass content of nitrogen element is D2%, and the mass content of sodium element is N%; 2.35 ≤ D2 / N ≤ 11.74; preferably, 4.81 ≤ D2 / N ≤ 9.63; for example, the value of D2 / N is 2.35, 2.95, 3.61, 4.41, 4.81, 4.93, 5.31, 6.26, 7.09, 7.82, 8.76, 9.14, 9.63, 10.07, 11.00, 11.74 or a value within the range composed of any two of these values. Controlling the mass ratio of nitrogen element and sodium element in the negative electrode mixture layer to meet the above range can improve the energy density of the secondary battery, and improve the first Coulomb efficiency, cycle performance, and swelling inhibition performance of the secondary battery.

[0065] In some embodiments, 0.58 ≤ N ≤ 1.74; preferably, 0.81 ≤ N ≤ 1.04; for example, N is 0.58, 0.61, 0.70, 0.76, 0.81, 0.86, 0.99, 1.04, 1.15, 1.25, 1.37, 1.40, 1.53, 1.57, 1.70, 1.74 or a value within the range formed by any two of these values. By regulating the content of sodium element in the negative electrode mixture layer within the above range, the dispersant can cooperate with the negative electrode active material of the present application, fully exert the improvement effect on the first Coulombic efficiency of the secondary battery, and is beneficial to the cycle performance and high-temperature storage performance of the secondary battery.

[0066] In some embodiments, 1.6 ≤ D2 ≤ 8.1; preferably, 5.0 ≤ D2 ≤ 7.8; for example, D2 is 1.6, 2.0, 2.5, 2.8, 3.4, 3.7, 4.4, 5.0, 5.2, 5.6, 6.4, 6.7, 7.2, 7.8, 8.1 or a value within the range formed by any two of these values. By controlling the mass content of nitrogen element in the negative electrode mixture layer to meet the above range, the specific capacity and conductivity of the secondary battery can be improved, and its cycle performance, high-temperature storage performance and swelling inhibition performance can be improved.

[0067] In some embodiments, the sodium element comes from at least one of sodium alginate, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose or sodium hydroxyethyl carboxymethyl cellulose. The above dispersant can better cooperate with the negative electrode active material of the present application to improve the first Coulombic efficiency, cycle performance, high-temperature storage performance and swelling inhibition performance of the secondary battery.

[0068] In the present application, the negative electrode active material is included in the negative electrode mixture layer of the negative electrode sheet. The negative electrode sheet of the present application further includes a negative electrode current collector. In the present application, the negative electrode mixture layer can be disposed on the surface of one side in the thickness direction of the negative electrode current collector, or can be disposed on the surfaces of both sides in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of the negative electrode current collector or a partial area of the negative electrode current collector, and the present application has no special limitation as long as the purpose of the present application can be achieved. The present application has no special limitation on the thickness of the negative electrode mixture layer as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided negative electrode mixture layer can be 30 μm to 160 μm.

[0069] The present application has no particular limitation on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include copper foil, aluminum foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, composite current collector (such as carbon-copper composite current collector, nickel-copper composite current collector, titanium-copper composite current collector), polymer substrate coated with a conductive metal, or any combination thereof. The present application has no particular limitation on the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 10 μm.

[0070] In the present application, the negative electrode binder layer may further include a negative electrode binder, and the negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0071] In the present application, the negative electrode binder layer may further include a conductive agent. The present application has no particular limitation on the type of the conductive agent in the negative electrode binder layer, as long as the purpose of the present application can be achieved. For example, the conductive agent may include, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative. The present application has no particular limitation on the mass ratio of the negative electrode active material, conductive agent, and negative electrode binder in the negative electrode binder layer, as long as the purpose of the present application can be achieved. For example, the loading amount of the negative electrode active material in the negative electrode sheet is 1.0 mg / cm 2 to 1.5 mg / cm 2 .

[0072] In some embodiments, the electrolyte includes at least one of fluorobenzene or fluoroethylene carbonate; based on the mass of the electrolyte, the mass content of fluorobenzene is F1%, and 0.9 ≤ F1 ≤ 5.2; preferably, 2.1 ≤ F1 ≤ 3.1; for example, F1 is 0.9, 1.0, 1.2, 1.5, 1.7, 2.0, 2.1, 2.5, 2.7, 2.9, 2.9, 3.3, 3.5, 3.8, 4.0, 4.2, 4.4, 4.6, 4.9, 5.0, 5.2, or a value within the range composed of any two of these values. When controlling the electrolyte of the secondary battery to contain fluorobenzene and regulating the mass content of fluorobenzene in the electrolyte to meet the above range, the high-temperature storage performance of the secondary battery can be further optimized, and its first Coulomb efficiency, cycle performance, and swelling inhibition performance can be improved.

[0073] In some embodiments, based on the mass of the electrolyte, the mass content of fluoroethylene carbonate is F2%, where 3.8 ≤ F2 ≤ 7.4. For example, F2 is 3.8, 4.0, 4.2, 4.3, 4.4, 4.6, 4.9, 5.0, 5.3, 5.4, 5.5, 5.7, 6.0, 6.1, 6.4, 6.6, 6.7, 6.8, 7.2, 7.3, 7.4, or a value within the range formed by any two of these values. Controlling the mass content of fluoroethylene carbonate (FEC) in the electrolyte to meet the above range can enable the secondary battery to achieve more excellent high-temperature storage performance, cycling performance, and swelling inhibition performance.

[0074] According to some embodiments of the present application, the electrolyte further includes a lithium salt and a non-aqueous solvent. The lithium salt may include, but is not limited to, at least one of 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(oxalato)borate LiB(C2O4)2 (LiBOB), lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB), LiNO3, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiC(SO2CF3)3, Li2SiF6, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), or lithium difluoroborate. The present application does not limit the content of the lithium salt in the electrolyte, as long as the object of the present application can be achieved. The present application has no particular limitation on the non-aqueous solvent, as long as the object of the present application can be achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylate compounds, ether compounds, or other organic solvents. The above carbonate compounds may include, but are not limited to, at least one of linear carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The above linear carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, or ethyl methyl carbonate (EMC). The above cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, or ethylene glycol ethylidene carbonate. The fluorinated carbonate compounds may include, but are not limited to, at least one of 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethyl ethylene carbonate. The above carboxylate compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone. The above ether compounds may include, but are not limited to, at least one of 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (1,2-dimethoxyethane, DME), dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.The above other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0075] This application has no particular limitation on the positive electrode, as long as the purpose of this application can be achieved. For example, the positive electrode includes a positive electrode current collector and a positive electrode mixture layer located on at least one surface of the positive electrode current collector. The above "positive electrode mixture layer located on at least one surface of the positive electrode current collector" means that the positive electrode mixture layer can be located on one surface of the positive electrode current collector along its own thickness direction, or can be located on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the positive electrode current collector surface, or a partial area of the positive electrode current collector surface. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0076] This application has no particular limitation on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the positive electrode current collector can include aluminum foil, aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate.

[0077] The positive electrode mixture layer of this application contains a positive electrode material. This application has no particular limitation on the type of the positive electrode material, as long as the purpose of this application can be achieved. For example, the positive electrode material may include at least one of lithium nickel cobalt manganese oxide (LiNi 0.90 Co 0.05 Mn 0.05 O2 (NCM955), NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type manganese oxide, spinel-type nickel manganese oxide, and lithium titanate. In this application, the positive electrode material may further contain non-metallic elements. For example, the non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur. In this application, there is no particular limitation on the thickness of the positive electrode current collector and the positive electrode mixture layer, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the single-sided positive electrode mixture layer is 30 μm to 120 μm.

[0078] In the present application, the positive electrode mixture layer may further include a positive electrode binder and a conductive agent. There is no particular limitation on the type of the positive electrode binder in the positive electrode mixture layer of the present application, as long as the object of the present application can be achieved. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified styrene-butadiene rubber (SBR), or polyurethane. In some embodiments, the polyolefin binder includes at least one of polyethylene, polypropylene, polyacrylate, polyvinyl alcohol, or polyacrylic acid.

[0079] There is no particular limitation on the type of the conductive agent in the positive electrode mixture layer of the present application, as long as the object of the present application can be achieved. For example, the conductive agent may be the same as the type of the conductive agent in the negative electrode mixture layer described above. In some embodiments, the conductive agent includes carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, or carbon fiber; metal-based materials such as metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof. There is no particular limitation on the mass ratio of the positive electrode material, the conductive agent, and the positive electrode binder in the positive electrode mixture layer of the present application, and those skilled in the art can select according to actual needs as long as the object of the present application can be achieved. For example, the loading amount of the positive electrode material in the positive electrode sheet is 4.0 mg / cm 2 to 10.0 mg / cm 2 .

[0080] The secondary battery of the present application further includes a separator. There is no particular limitation on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

[0081] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0082] A surface treatment layer is provided on at least one surface of the base material layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silica, magnesia, 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 vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride - hexafluoropropylene).

[0083] The secondary battery of the present application further includes a packaging bag for accommodating the positive electrode plate, the negative electrode plate, the separator, and the electrolyte, as well as other components known in the art in the secondary battery. The present application does not limit the above - mentioned other components. There is no particular limitation on the packaging bag of the present application, and it can be a packaging bag well - known in the art as long as it can achieve the purpose of the present application.

[0084] The preparation process of the secondary battery of the present application is well - known to those skilled in the art, and there is no particular limitation in the present application. For example, it can include but is not limited to the following steps: stacking the positive electrode plate, the separator, and the negative electrode plate in sequence, and performing operations such as winding and folding according to needs to obtain a wound - structure electrode assembly, putting the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain the secondary battery; or stacking the positive electrode plate, the separator, and the negative electrode plate in sequence, and then fixing the four corners of the entire laminated structure with tape to obtain a laminated - structure electrode assembly, putting the electrode assembly into the packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain the secondary battery. In addition, an over - current protection element, a guide plate, etc. can be placed in the packaging bag according to needs to prevent the pressure inside the secondary battery from rising and over - charge and discharge.

[0085] In a third aspect, the present application provides an electronic device including the secondary battery provided in the second aspect of the present application.

[0086] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, a laptop computer, a pen input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset stereo, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium-ion capacitor.

[0087] The solution of the present application will be described below with reference to the following specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments are all from ordinary commercially available products, and the devices or equipment used are all purchased from conventional market sales channels. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0088] Example 1-1

[0089] This example provides a negative electrode active material, including a silicon-based material and an outer layer material; the outer layer material is located on at least part of the surface of the silicon-based material; the outer layer material includes multi-walled carbon nanotubes (conductive agent) and polyacrylonitrile (nitrile polymer).

[0090] Refer to Figure 1 In the infrared spectrum of the negative electrode active material shown in the infrared spectrum diagram of Example 1, there is an infrared characteristic peak of cyano group. Through the Figure 2 local enlarged view, it can be seen that the infrared characteristic peak of cyano group is located at 2340 cm -1 to 2300 cm -1 ; the negative electrode active material includes nitrogen element and silicon element; based on the mass of the negative electrode active material, the mass content D1% of nitrogen element is 1.11%, and the mass content G% of silicon element is 54.2%, where D1 / G = 0.02.

[0091] In the negative electrode active material, the sphericity S = 0.85; the thickness T nm of the outer layer material is 0.5 nm. The average particle size R μm of the negative electrode active material is 5.3 μm.

[0092] The negative electrode active material of this example is prepared by a method including the following steps:

[0093] S1: Provide a silicon-carbon material; the silicon-based material includes a porous carbon skeleton and a silicon material; the silicon material is located on the surface and inside of the porous carbon skeleton.

[0094] S2: Perform the first dispersion treatment on multi-walled carbon nanotubes and polyacrylonitrile in a solvent for 4 h to obtain a dispersion system; the solvent includes ethanol and butanol (mass ratio 1:1).

[0095] S3: Add the silicon-carbon material in S1 into the dispersion system in S2 for the second dispersion treatment for 4 h, and then perform a drying treatment at 200 °C for 4 h to obtain the negative electrode active material of this example.

[0096] Preparation of the negative electrode: Use the negative electrode active material prepared above as the active material, acetylene black as the conductive agent, and sodium alginate as the binder. The mass ratio of the active material, acetylene black, and sodium alginate is 70:20:10. After fully mixing the active material and acetylene black in proportion and grinding evenly, add an aqueous sodium alginate solution in proportion and stir for 4 h to obtain a negative electrode mixture slurry. Finally, coat the above slurry evenly on a copper foil and vacuum dry it at 70 °C for 12 h. After cold pressing, a negative electrode is obtained, where the loading amount of the active material is 1.2 mg / cm 2 . The mass content of nitrogen element in the negative electrode binder layer is 0.77%, and the mass content of sodium element is 1.6%.

[0097] Preparation of the positive electrode: Use super-P as the conductive agent and PVDF as the binder. The mass ratio of the active material (LiFePO4), super-P, and PVDF is 70:20:10. After fully mixing the active material and super-P in proportion and grinding evenly, add the prepared 10 wt.% PVDF solution in proportion and stir for 4 h to obtain a positive electrode mixture slurry. Finally, coat the positive electrode mixture slurry evenly on an aluminum foil and vacuum dry it at 70 °C for 12 h. After cold pressing, a positive electrode is obtained. The loading amount of the active material is 8 mg / cm 2 .

[0098] Preparation of the electrolyte: Use ethylene carbonate and dimethyl carbonate (EC and DMC, volume ratio 1:1) as the base solvent, and add 1 mol / L of LiPF6 to the base solvent to obtain the electrolyte.

[0099] Preparation of the separator: Use a 25-μm microporous single-layer polypropylene (PP) membrane as the separator.

[0100] Preparation of the secondary battery: Cut the negative electrode into circular electrode sheets with a diameter of 10 mm, and use a lithium metal sheet and a circular electrode sheet as counter electrodes to assemble a button-type half cell in a glove box.

[0101] Alternatively, stack the above positive electrode, separator, and negative electrode in sequence, with the separator in the middle of the positive electrode and the negative electrode to play a role in isolation, and wind it to obtain a bare battery core. Place the bare battery core in an outer package, inject the electrolyte, encapsulate it, and obtain a lithium-ion battery through processes such as formation, degassing, and trimming.

[0102] Test method:

[0103] Specific capacity test of negative electrode active material

[0104] Place the coin-type half-cell on a Blue-Electric Tester for testing. The test process is as follows: In the first step, discharge at 0.05C until the voltage reaches 0V. In the second step, let it stand for 5 minutes. In the third step, discharge at 50 μA until the voltage reaches 0V. In the fourth step, let it stand for 5 minutes. In the fifth step, discharge at 20 μA until the voltage reaches 0V. In the sixth step, let it stand for 5 minutes. In the seventh step, charge at 0.1C until the voltage reaches 2V. In the eighth step, let it stand for 5 minutes. Repeat steps one to eight twice. Then discharge at 1C until the voltage reaches 0V, let it stand for 5 minutes, and charge at 0.1C until the voltage reaches 2V to end the test. The cumulative capacity of the first, third, and fifth discharge processes in the first cycle is the discharge capacity. Charging to 2.0V at 0.1C gives the charge capacity. Dividing the charge capacity by the weight of the negative electrode active material gives the specific capacity of the negative electrode active material. Dividing the charge capacity by the discharge capacity gives the first Coulombic efficiency.

[0105] Cycling capacity retention rate and cycling expansion rate test:

[0106] Place the lithium-ion battery in an incubator at 25 °C ± 1 °C and let it stand for 30 minutes. Charge it at a constant current of 0.5C until the voltage reaches 4.35V, then charge it at a constant voltage of 4.35V until the current reaches 0.025C, and let it stand for 5 minutes. Then discharge it at 0.5C until the voltage reaches 3.0V. This is one charge-discharge cycle process. Record the first-cycle discharge capacity C0 and the initial thickness H of the lithium-ion battery. 10 , After that, cycle 600 times according to the above cycle process. Record the cycle discharge capacity C1 and the thickness H after cycling in the 600th cycle. 11 .

[0107] Cycling capacity retention rate after 600 cycles = C1 / C0 × 100%.

[0108] Cycling expansion rate = (H 11 - H 10 ) / H 10 × 100%.

[0109] High-temperature storage expansion rate test:

[0110] Place the lithium-ion battery in an incubator at 25 °C ± 1 °C and let it stand for 30 minutes. Charge it at a constant current of 0.5C until the voltage reaches 4.35V, then charge it at a constant voltage of 4.35V until the current reaches 0.025C, and let it stand for 5 minutes. Measure the initial thickness H of the lithium-ion battery. 20 . Store it in an environment at 60 °C for 21 days and measure the thickness H of the lithium-ion battery after storage. 21 . Storage expansion rate = (H 21 - H 20) / H 20 × 100%.

[0111] The negative electrode active materials in the following examples and comparative examples are different from those in Example 1-1 only in that: the mass contents of silicon element and / or nitrogen element, sphericity, outer layer material thickness, and average particle size in the negative electrode active material are adjusted according to Table 1. When adjusting the element contents, only the addition amounts of the silicon-based material, nitrile polymer, and conductive agent are changed accordingly. The addition amount of nitrogen element in the negative electrode active material of Comparative Example 1-1 is 0, but the same mass of nitrile polymer as in Example 1-1 is added to the negative electrode mixture slurry to ensure that the mass ratio of the nitrile polymer in the negative electrode plates of Comparative Example 1-1 and Example 1-1 is the same.

[0112] Table 1

[0113]

[0114] As can be seen from Table 1, the negative electrode active material of the present application forms an outer layer material including a conductive agent and a nitrile polymer on the surface of the silicon-based material. Based on the negative electrode active material, the mass content of nitrogen element is D1%, and the mass content of silicon element is G%. When controlling 0.02 ≤ D1 / G ≤ 0.5, the cycle capacity retention rate, swelling inhibition performance, and high-temperature storage performance of the secondary battery can be improved. Preferably, when 0.04 ≤ D1 / G ≤ 0.37 is satisfied, the cycle capacity retention rate, swelling inhibition performance, and high-temperature storage performance of the secondary battery can be further improved. More preferably, when 0.06 ≤ D1 / G ≤ 0.19 is satisfied, the secondary battery has more excellent cycle capacity retention rate, swelling inhibition performance, and high-temperature storage performance.

[0115] Particularly, when controlling to satisfy 1.1 ≤ D1 ≤ 18.4, especially when satisfying the condition of 2.3 ≤ D1 ≤ 9.2, the secondary battery exhibits higher cycle performance and high-temperature performance. When controlling to satisfy 36.8 ≤ G ≤ 68.3, especially when satisfying 43.2 ≤ G ≤ 51.3, the silicon element can cooperate better with the nitrogen element to improve the swelling inhibition performance and cycle performance of the secondary battery.

[0116] Particularly, when regulating the sphericity S of the negative electrode active material to meet 0.8 ≤ S ≤ 1.0, the swelling inhibition performance, cycle performance, and high-temperature storage performance of the secondary battery can be improved. When further increasing the sphericity to 0.9 ≤ S ≤ 1.0, the secondary battery can have higher swelling inhibition performance, cycle performance, and high-temperature storage performance. Particularly, when regulating the thickness T nm of the outer layer material to meet 0.5 ≤ T ≤ 200.0, the swelling inhibition performance, cycle performance, and high-temperature storage performance of the secondary battery can be optimized. Especially when regulating the thickness T of the outer layer material to satisfy 0.5 ≤ T ≤ 100.0, the swelling inhibition performance, cycle performance, and high-temperature storage performance of the secondary battery can be improved more significantly.

[0117] In particular, when the negative electrode active material satisfies 0.5 ≤ S×T ≤ 90.0, the initial Coulombic efficiency of the secondary battery can be improved, and the swelling performance, high-temperature storage performance, and cycling performance can be inhibited. Preferably, when 10.4 ≤ S×T ≤ 67.8 is satisfied, the swelling inhibition performance, high-temperature storage performance, and cycling performance of the secondary battery can be further improved. In particular, when the average particle size R μm of the negative electrode active material is controlled within the range of 5.6 ≤ R ≤ 10.3, the secondary battery can have higher cycling performance, swelling inhibition performance, and high-temperature storage performance.

[0118] The secondary batteries of the following examples are different from those of Example 1-1 only in that: the mass contents of nitrogen element and / or sodium element in the negative electrode binder layer are adjusted according to Table 2 (wherein, the adjustment method of the mass contents of nitrogen element and sodium element in the negative electrode binder layer is to adjust the type of the negative electrode active material according to Table 2, and to adjust the ratio of the negative electrode active material to sodium alginate, and the balance is multi-walled carbon nanotubes), and / or, the mass contents of fluorobenzene and / or fluoroethylene carbonate in the electrolyte are adjusted (the adjustment method is to add the corresponding mass of fluorobenzene and / or fluoroethylene carbonate to the base solvent).

[0119] Table 2

[0120]

[0121] As can be seen from Table 2, by controlling the mass content D2% of nitrogen element and the mass content N% of sodium element in the negative electrode binder layer to satisfy 2.35 ≤ D2 / N ≤ 11.74, the cycling performance and swelling inhibition performance of the secondary battery can be improved. In particular, under the condition of 4.81 ≤ D2 / N ≤ 9.63, the cycling performance and swelling inhibition performance of the secondary battery can be further improved.

[0122] In particular, adjusting the content of sodium element in the negative electrode binder layer within the range of 0.58 ≤ N ≤ 1.74 is beneficial to the cycling performance and high-temperature storage performance of the secondary battery. In particular, when 0.81 ≤ N ≤ 1.04, the secondary battery exhibits more excellent cycling performance and high-temperature storage performance. In particular, when the secondary battery satisfies 1.6 ≤ D2 ≤ 8.1, its cycling performance and swelling inhibition performance can be improved. In particular, when 5.0 ≤ D2 ≤ 7.8, the cycling performance and swelling inhibition performance of the secondary battery are significantly improved.

[0123] In particular, adding fluorobenzene and / or fluoroethylene carbonate to the electrolyte in combination with the negative electrode system of the present application can optimize the high-temperature storage performance of the secondary battery, and can improve its first Coulombic efficiency, cycle performance, and swelling inhibition performance. Preferably, when the mass content F1% of fluorobenzene in the electrolyte is adjusted to meet 0.9 ≤ F1 ≤ 5.2, the high-temperature storage performance, cycle performance, and swelling inhibition performance of the secondary battery can be further optimized; preferably, when 2.1 ≤ F1 ≤ 3.1, the high-temperature storage performance, cycle performance, and swelling inhibition performance of the secondary battery can be significantly improved.

[0124] In particular, adjusting the mass content F2% of fluoroethylene carbonate in the electrolyte to satisfy 3.8 ≤ F2 ≤ 7.4 can enable the secondary battery to achieve more excellent high-temperature storage performance, cycle performance, and swelling inhibition performance. In particular, when the electrolyte contains both fluorobenzene and fluoroethylene carbonate and meets the above preferred ranges, the secondary battery has extremely excellent high-temperature storage performance, cycle performance, and swelling inhibition performance.

[0125] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the principle of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode active material, characterized in that, It includes a silicon-based material and an outer layer material; the outer layer material is located on at least part of the surface of the silicon-based material; the outer layer material includes a conductive agent and a nitrile polymer; The infrared spectrum of the negative electrode active material includes an infrared characteristic peak of a cyano group, and the infrared characteristic peak of the cyano group is located at 2250 cm -1 to 2400 cm -1 between; The negative electrode active material includes nitrogen element and silicon element; based on the mass of the negative electrode active material, the mass content of the nitrogen element is D1%, and the mass content of the silicon element is G%, where 0.02 ≤ D1 / G ≤ 0.

5.

2. The negative electrode active material according to claim 1, wherein The negative electrode active material satisfies at least one of the following conditions: （1）1.1≤D1≤18.4； （2）36.8≤G≤68.3； (3)0.04 ≤ D1 / G ≤ 0.

37.

3. The negative electrode active material according to claim 2, wherein The negative electrode active material satisfies at least one of the following conditions: （1）2.3≤D1≤9.2； （2）43.2≤G≤51.3； (3)0.06 ≤ D1 / G ≤ 0.

19.

4. The negative electrode active material according to any one of claims 1 to 3, characterized in that, The sphericity of the negative electrode active material is S, where 0.8 ≤ S ≤ 1.0; and / or, In the negative electrode active material, the thickness of the outer layer material is T nm, where 0.5 ≤ T ≤ 200.

0.

5. The negative electrode active material according to claim 4, characterized in that, The negative electrode active material satisfies at least one of the following conditions: （1）0.9≤S≤1.0； （2）0.5≤T≤100.0； (3)0.5 ≤ S×T ≤ 90.0; (4)The average particle size of the negative electrode active material is R μm; 5.6≤R≤10.3。 6. The negative electrode active material according to claim 5, wherein 10.4 ≤ S×T ≤ 67.

8.

7. The negative electrode active material according to any one of claims 1 to 3, characterized in that, The negative electrode active material satisfies at least one of the following conditions: (1)The conductive agent includes single-walled carbon nanotubes and / or multi-walled carbon nanotubes; (2)The nitrile polymer includes at least one of polyacrylonitrile, polycyanoacrylate, and polycyanobenzene; (3)The silicon-based material includes a porous carbon framework and a silicon material; the silicon material is located on the surface and / or inside of the porous carbon framework.

8. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte; The negative electrode includes a negative electrode current collector and a negative electrode mixture layer provided on at least part of the surface of the negative electrode current collector, and the negative electrode mixture layer includes the negative electrode active material according to any one of claims 1 to 7.

9. The secondary battery according to claim 8, characterized in that, The negative electrode mixture layer includes sodium element. Based on the mass of the negative electrode mixture layer, the mass content of the nitrogen element is D2%, and the mass content of the sodium element is N%; The negative electrode mixture layer satisfies at least one of the following conditions: （1）0.58≤N≤1.74； （2）1.6≤D2≤8.1； (3)2.35 ≤ D2 / N ≤ 11.74; (4)The sodium element comes from at least one of sodium alginate, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose, or sodium hydroxyethyl carboxymethyl cellulose.

10. The secondary battery according to claim 9, characterized in that, The negative electrode mixture layer satisfies at least one of the following conditions: （1）0.81≤N≤1.04； （2）5.0≤D2≤7.8； (3)4.81 ≤ D2 / N ≤ 9.

63.

11. According to the secondary battery of claim 8, the electrolyte includes at least one of fluorobenzene or fluoroethylene carbonate; based on the mass of the electrolyte, the electrolyte satisfies at least one of the following conditions: (1)The mass content of the fluorobenzene is F1%, where 0.9 ≤ F1 ≤ 5.2; (2)The mass content of the fluoroethylene carbonate is F2%, where 3.8 ≤ F2 ≤ 7.

4.

12. The secondary battery according to claim 11, characterized in that, 2.1≤F1≤3.1。 13. An electronic device, characterized in that, It includes the secondary battery according to any one of claims 8 to 12.

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