Negative active material, negative electrode sheet, secondary battery, and electronic device

Abstract

The application provides a negative electrode active material, a negative electrode sheet, a secondary battery and an electronic device. The negative electrode active material comprises: graphite particles, and a silicon carbide layer arranged on the surface of the graphite particles, wherein the mass percentage content Y% of silicon in the negative electrode active material and the average height Lc nm of the negative electrode active material along the c-axis direction satisfy: 0.3x(32-Lc)≤Y≤0.9x(32-Lc). The negative electrode active particle can effectively reduce the reaction of the graphite particle and the electrolyte, reduce the consumption of active ions, and thus improve the initial coulomb efficiency of the battery by arranging the silicon carbide layer on the surface of the graphite particle and controlling the thickness of the silicon carbide layer in the negative electrode active material based on the average height of the graphite particle along the c-axis direction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode active material, a negative electrode sheet, a secondary battery and an electronic device. BACKGROUND

[0002] The secondary battery represented by lithium ion battery is widely used in digital electronic products, energy storage, unmanned aerial vehicles, electric tools, electric vehicles and other products due to its high energy density, long cycle life, high safety, fast charging capability and other characteristics. At present, the negative electrode active material of lithium ion battery is mainly graphite, but it will form a solid electrolyte interface film (SEI film) in the formation process, resulting in the loss of active lithium ions and the reduction of the first coulomb efficiency of the battery. Therefore, it is necessary to improve the first coulomb efficiency of the battery. SUMMARY

[0003] The present application provides a negative electrode active material, a negative electrode sheet, a secondary battery and an electronic device, which aims to improve the first coulomb efficiency of the battery by reducing the side reaction of the negative electrode active material and the electrolyte.

[0004] In a first aspect, the present application provides a negative electrode active material, comprising: graphite particles, and a silicon carbide layer arranged on the surface of the graphite particles, wherein the mass percentage content Y% of silicon in the negative electrode active material and the average height Lc nm of the negative electrode active material along the c-axis direction satisfy: 0.3x(32-Lc)≤Y≤0.9x(32-Lc).

[0005] According to the present application, by arranging a silicon carbide layer on the surface of the graphite particles and controlling the thickness of the silicon carbide layer in the negative electrode active material based on the average height of the graphite particles along the c-axis direction, the reaction of the graphite particles and the electrolyte can be effectively reduced, the consumption of active ions can be reduced, and the first coulomb efficiency of the battery can be improved.

[0006] In some embodiments, the thickness of the silicon carbide layer is 5nm-100nm; preferably 15nm-40nm.

[0007] In some embodiments, the crystal phase of silicon carbide in the silicon carbide layer is β-phase silicon carbide.

[0008] In some embodiments, the specific surface area BET m 2 / g and the mass percentage content Y% of silicon in the negative electrode active material satisfy: 1.2≤BET-0.3xY≤2.0.

[0009] In some embodiments, the powder compaction density P g / cm 3The mass percentage of silicon Y% in the negative electrode active material satisfies: 1.80≤P+0.08×Y≤2.10.

[0010] In some embodiments, the volumetric particle size of the negative electrode active material satisfies: 0.2≤(Dv90-Dv50) / Dv99≤0.5.

[0011] In some embodiments, the Raman spectrum of the negative electrode active material has a D peak intensity I d The Raman spectrum of the negative electrode active material shows the intensity of the G peak I. g Satisfy: I d / I g <0.5.

[0012] In some embodiments, the tap density TD of the negative electrode active material is [value missing] g / cm³. 3 It satisfies: 0.7≤TD≤1.2.

[0013] In some embodiments, the orientation degree (OI) value of the negative electrode active material is ≤6.

[0014] In a second aspect, this application provides a negative electrode sheet, comprising: a negative electrode film layer, the negative electrode film layer comprising the negative electrode active material according to any embodiment of the first aspect.

[0015] In some embodiments, the compaction density PD of the negative electrode sheet is... 3 It satisfies: 1.45≤PD≤1.75.

[0016] In some embodiments, the porosity K% of the negative electrode sheet satisfies: 25≤K≤40.

[0017] Thirdly, this application provides a secondary battery, including: a negative electrode sheet according to any embodiment of the second aspect.

[0018] Fourthly, this application provides an electronic device, including: a secondary battery according to any embodiment of the third aspect. Attached Figure Description

[0019] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0020] FIG. 1 This is a SEM image of the negative electrode active material in one embodiment of this application.

[0021] FIG. 2 The image shows the Si element spectrum obtained by EDS elemental analysis of the negative electrode active material in one embodiment of this application. Detailed Implementation

[0022] The embodiments or examples in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0023] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in connection with the implementation or example are included in at least one implementation or example of the present application. In the specification, the exemplary description of the above terms does not necessarily mean the same implementation or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more implementations or examples in a suitable manner.

[0024] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0025] In the present application, the battery can include a lithium ion secondary battery, a lithium ion primary battery, a lithium-sulfur battery, a sodium lithium ion, a sodium ion battery or a magnesium ion battery, etc. The present application embodiments are not limited thereto. The battery can be in the shape of a cylinder, a flat body, a cuboid or other shapes, etc. The present application embodiments are also not limited thereto.

[0026] As described in the background section above, the negative electrode active material represented by graphite consumes active ions during the formation process, resulting in a decrease in the first coulomb efficiency of the battery.

[0027] Based on the above problems, in the related art, the amount of consumed active ions is reduced to improve the coulomb efficiency of the battery by increasing the particle size of the negative electrode active material and particle shaping, etc. However, the above methods will affect the dynamics performance of the battery, and the improvement effect is limited and the cost is high.

[0028] Based on this, the present application provides a negative electrode active material, a negative electrode sheet, a secondary battery and an electronic device, which is not easy to react with the electrolyte, can reduce the consumption of active ions during the formation process, thereby improving the first coulomb efficiency of the battery. The embodiments of the present application are described in detail below.

[0029] Negative active material

[0030] In a first aspect, the present application provides a negative electrode active material, comprising: graphite particles, and a silicon carbide layer provided on the surface of the graphite particles, wherein the mass percentage content Y% of silicon in the negative electrode active material and the average height Lc nm of the negative electrode active material along the c-axis direction satisfy: 0.3 x (32 - Lc) ≤ Y ≤ 0.9 x (32 - Lc).

[0031] According to the present application, the structure of the negative active material is to set a silicon carbide layer on the surface of the graphite particles, and the silicon carbide layer on the surface of the negative active material can effectively reduce the contact between the active surface of the graphite particles and the electrolyte, thereby reducing the formation of the SEI film, reducing the consumption of active ions, and improving the first coulombic efficiency (first efficiency) of the battery. At the same time, since silicon carbide has good active ion deintercalation performance, the surface silicon carbide layer will not significantly affect the kinetic performance of the battery; the mass percentage of silicon Y% in the negative active material and the average height Lc nm of the negative active material along the c-axis direction satisfy: 0.3x(32-Lc)≤Y≤0.9x(32-Lc). It can be understood that the mass percentage of silicon in the negative active material is positively correlated with the content of silicon carbide in the negative active material. Since the main material of the negative active material is graphite particles, the main influencing factor of the average height Lc nm of the negative active material along the c-axis direction is the average height of the graphite particles along the c-axis direction. The higher Lc is, that is, the higher the average height of the graphite particles along the c-axis direction, the higher the crystallinity of the graphite particles, the better the stability, and the thinner the silicon carbide layer can effectively reduce the reaction between the graphite particles and the electrolyte. In addition, when the crystallinity of the graphite particles is high, the volume expansion rate of the embedded active ions is large. Since the rigidity of the silicon carbide layer is large, the thickness of the silicon carbide layer should not be too large to adapt to the volume expansion of the graphite particles with high crystallinity and improve the stability of the negative active material, thereby reducing the reduction of the first efficiency caused by the rupture of the silicon carbide layer. Based on this, the appropriate content of silicon carbide needs to be selected according to the average height Lc of the negative active material along the c-axis direction to improve the first efficiency of the battery. In addition, since the height of the high-crystalline graphite particles along the c-axis direction is about 32 nm, when the mass percentage of silicon Y% in the negative active material and the average height Lc nm of the negative active material along the c-axis direction satisfy: 0.3x(32-Lc)≤Y≤0.9x(32-Lc), the battery has a high first efficiency. In addition, it should be noted that the Lc nm of the negative active material satisfies 0

[0032] The average height of the negative active material along the c-axis direction has the meaning known in the art and can be detected according to known methods and instruments. As an example, the average height of the negative active material along the c-axis direction can be detected by X-ray powder diffraction (XRD) test. Specifically, the negative active material is tested by an X-ray powder diffractometer (XRD, instrument model: Bruker D8 ADVANCE), with Cu Kα as the target material, a voltage current of 40 KV / 40 mA, a scanning angle range of 5° to 80°, a scanning step of 0.00836°, and a time of 0.3 s for each step, to obtain an X-ray diffraction spectrum of the negative active material, a half-peak width of the 002 peak (full width at 50% between the lowest and highest points of the peak intensity of the 002 peak), and the average height Lc nm of the negative active material along the c-axis direction calculated according to Lc = Kλ / α(2θ) / cosα, wherein K = scherrer constant (K = 0.9), α is the half-peak width of the 002 peak, λ is the wavelength (0.154056), and θ is the maximum peak position angle of the 002 peak.

[0033] In some embodiments, the thickness of the silicon carbide layer is 5 nm to 100 nm.

[0034] In some embodiments, the thickness of the silicon carbide layer is further limited. Since the silicon carbide layer has strong rigidity, the thickness of the silicon carbide layer will affect the powder compaction density of the negative active material, leading to easier crushing. Therefore, the thickness of the silicon carbide layer should not be too thick. In addition, a suitable thickness of the silicon carbide layer can further improve the thermal stability of the negative active material, thereby further improving the thermal runaway temperature of the battery. For example, the thickness of the silicon carbide layer can be 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, or a range formed by any of the above values. In addition, it should be noted that the graphite particles with suitable crystallinity can be selected based on the suitable thickness of the silicon carbide layer to further improve the initial efficiency and thermal reliability of the battery.

[0035] The thickness of the silicon carbide layer has the meaning known in the art and can be detected according to known methods and instruments. As an example, the thickness of the silicon carbide in the sample can be observed and calculated by transmission electron microscopy.

[0036] In some embodiments, the crystal phase of the silicon carbide in the silicon carbide layer is β-phase silicon carbide. The metal phase of the silicon carbide can be β-phase, which not only has good mechanical properties but also has good active ion deintercalation capacity. The β-phase silicon carbide can improve the initial efficiency of the battery without significantly affecting the kinetic performance of the battery.

[0037] In some embodiments, the specific surface area BET m 2 / g and the mass percentage content of silicon Y% in the negative electrode active material satisfy: 1.2≤BET-0.3×Y≤2.0.

[0038] In some of the above embodiments, the relationship between the specific surface area of the negative electrode active material and the mass percentage content of silicon in the negative electrode active material is further limited. Due to the silicon carbide layer arranged on the surface of the graphite particles, the adsorption capacity of nitrogen gas will be stronger, thereby increasing the specific surface area of the negative electrode active material. Generally, the higher the mass percentage content of silicon in the negative electrode active material, i.e. the greater the content of silicon carbide, the greater the specific surface area of the negative electrode active material. Since the specific surface area of the graphite particles will affect the uniformity of the deposition of silicon carbide, and through a large number of experiments, the influence factor of the mass percentage content of silicon in the negative electrode active material on the specific surface area of the negative electrode active material is determined to be 0.3, i.e. BET-0.3×Y can be approximately understood as the specific surface area of the graphite particles. By controlling the specific surface area of the graphite particles, the silicon carbide layer on the surface can be more uniform. When the negative electrode active material satisfies 1.2≤BET-0.3×Y≤2.0, the consumption of active ions of the negative electrode active material during the formation process of the battery is less, and the initial efficiency of the battery is further improved. For example, BET-0.3×Y can be 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, or a range formed by any of the above values.

[0039] The specific surface area of the negative electrode active material has a meaning known in the art and can be detected according to methods and instruments known in the art. As an example, it can be detected according to GB / T 19587-2017. Specifically, 1-8 g of the sample (the sample is weighed to be at least 1 / 3 of the volume of the sphere) is placed in a 1 / 2 inch long tube with a ball bubble (the diameter of the spherical part of the tube is 12 mm), and after pretreatment at 200°C for 2 h, it is placed in a test device TriStar3030 (USA, Micromeritics) for testing. The adsorption gas used is N2 (purity: 99.999%), and the test condition is carried out at 77K. The specific surface area is tested by the BET calculation method.

[0040] In some embodiments, the powder compaction density Pg / cm 3 and the mass percentage content of silicon Y% in the negative electrode active material satisfy: 1.80≤P+0.08×Y≤2.10.

[0041] In some of the above embodiments, the relationship between the powder compaction density of the negative active material under a test condition of a pressure of 5 tons and the mass percentage of silicon in the negative active material is further limited, as described above, because the silicon carbide layer has strong rigidity, the thickness of the silicon carbide layer will affect the powder compaction density of the negative active material, the greater the content of silicon carbide, the smaller the powder compaction density, through a large number of experiments, the influence factor of the mass percentage of silicon in the negative active material on the powder compaction density of the negative active material under a test condition of a pressure of 5 tons is determined to be 0.08, that is, P+0.08×Y can be approximately understood as the powder compaction density of the graphite particles, by controlling the powder compaction density of the graphite particles, the stability of the negative active material can be further improved, when the negative active material satisfies 1.80≤P+0.08×Y≤2.10, the negative active material is less likely to deform and break during cold pressing, and the initial efficiency of the battery is further improved. For example, P+0.08×Y can be 1.80, 1.85, 1.90, 1.95, 2.00, 2.05, 2.10, or a range formed by any of the above values, and further preferably, 1.95≤P+0.08×Y≤2.05.

[0042] The powder compaction density of the negative active material under a test condition of a pressure of 5 tons has a meaning known in the art and can be detected according to methods and instruments known in the art, as an example, GB / T 24533-2009 can be referred to for detection, specifically: 1.0000±0.0500g of the sample is weighed and placed in a test mold (CARVER #3619(13mm)), and then the sample is placed in a test device, the test device is a three-dimensional UTM7305 test tonnage of 5 tons, a pressure increasing rate of 10mm / min, a pressure holding time of 30s, a pressure releasing rate of 30mm / min, and a pressure holding time of 10s. The calculation formula of the compaction density is: compaction density=mass of negative active material / force area of negative active material / thickness of sample.

[0043] In some embodiments, the volume particle size of the negative active material satisfies: 0.2≤(Dv90-Dv50) / Dv99≤0.5.

[0044] In some of the above embodiments, the volume particle size distribution of the negative active material is further limited. Generally, a wide particle size distribution of the negative active material is not conducive to the processability and coulombic efficiency of the negative active material, and a too narrow particle size distribution is not conducive to further improving the compaction density of the negative electrode sheet. Therefore, when the volume particle size of the negative active material satisfies 0.2≤(Dv90-Dv50) / Dv99≤0.5, the initial efficiency of the battery can be further improved. For example, (Dv90-Dv50) / Dv99 can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or a range formed by any of the above values.

[0045] The volume particle sizes Dv50, Dv90 and Dv99 of the negative active material have meanings known in the art and can be detected according to methods and instruments known in the art. As an example, GB / T 19077-2016 can be referred to for detection. Specifically, 1 g of the sample is weighed and mixed uniformly with 20 mL of deionized water and a small amount of dispersant, placed in an ultrasonic device for ultrasonic treatment for 5 min, and then the solution is poured into the sample injection system Hydro2000SM for testing. The testing device used is Mastersizer 3000 produced by Malvern Company. During the test, the particle size measurement is completed by measuring the intensity of scattered light when the laser beam passes through the dispersed particle sample. Then the data is used to analyze and calculate the particle size distribution of the particles forming the scattering spectrum. The refractive index of the particles used for testing is 1.8. One sample is tested three times, and the final particle size is the average of the three tests. Dv50, Dv90 and Dv99 are obtained respectively.

[0046] In some embodiments, the Raman spectrum D peak intensity I d of the negative active material and the Raman spectrum G peak intensity I g satisfy: I d / I g <0.5.

[0047] In some of the above embodiments, the defect degree of the negative active material is further limited. I d / I g The value can represent the degree of defect of the negative active material. I d / I g The greater the value, the higher the degree of surface defects of the negative active material, which will increase the side reaction of the negative active material with the electrolyte, thereby affecting the initial efficiency of the battery. Therefore, when I d / I g <0.5, the initial efficiency of the battery can be further improved.

[0048] The Raman spectrum D peak intensity I d of the negative active material and the Raman spectrum G peak intensity Ig with meanings commonly known in the art, can be detected according to methods and instruments known in the art, as an example, scanning the sample particles with a laser confocal Raman microscope (Raman, HR Evolution, HORIBA Scientific), obtaining the D peak and G peak of all particles in the area range, and processing the data with LabSpec software to obtain the peak intensity of the D peak and G peak of each particle, respectively, I d and I g , I d / I g The frequency of Id / Ig is counted with a step of 0.02 to obtain a normal distribution graph, and the average value of Id / Ig is calculated, that is, the D peak and G peak intensity ratio I d / I g of the active material. The laser wavelength of the Raman spectrometer can be in the range of 532 nm to 785 nm. Among them, the D peak: generally around 1350 cm -1 , caused by the symmetric stretching vibration of sp2 carbon atoms in the aromatic ring (structural defects); the G peak: appears at 1575 cm -1 , caused by the stretching vibration between sp2 carbon atoms, which corresponds to the vibration of E2g optical phonon at the center of the Brillouin zone (carbon atom in-plane vibration).

[0049] In some embodiments, the tap density TD of the negative electrode active material g / cm 3 satisfies: 0.7≤TD≤1.2.

[0050] In some of the above embodiments, the tap density of the negative electrode active material is further limited. The tap density will affect the processability of the negative electrode active material. When the tap density TD of the negative electrode active material g / cm 3 satisfies 0.7≤TD≤1.2, the processability of the negative electrode active material is better, and the performance of the negative electrode sheet obtained is more stable. For example, TD can be 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, or a range consisting of any of the above values.

[0051] The tap density of the negative electrode active material has meanings commonly known in the art, which can be detected according to methods and instruments known in the art. As an example, 50 g of powder is weighed, all of which is loaded into a measuring cylinder, then the measuring cylinder containing the powder is fixed to the instrument for vibration, after vibration, the volume is visually measured according to the height of the powder surface, and then the tap density is calculated. The testing device can be Dandong Bitai BT-301.

[0052] In some embodiments, the orientation degree OI of the negative electrode active material is ≤6.

[0053] In some of the above embodiments, the orientation degree OI value of the negative electrode active material is further limited, the larger the OI value, the higher the orientation degree of the negative electrode active material, a high orientation degree means that the deintercalation direction of active ions in the negative electrode active material is relatively single, which is not conducive to the deintercalation of active ions, thereby affecting the kinetic performance of the battery. Therefore, when the OI value of the negative electrode active material is ≤6, the kinetic performance of the battery is better.

[0054] The orientation degree OI value of the negative electrode active material has a meaning known in the art and can be detected according to methods and instruments known in the art. As an example, the (004) plane diffraction pattern and the (110) plane diffraction pattern in the X-ray diffraction pattern of the negative electrode active material layer can be tested according to the industry standard JB / T 4220-2011. The specific test conditions are as follows: X-ray uses CuKα radiation, and the CuKα radiation is removed by a filter or a monochromator. The working voltage of the X-ray tube is (30-35) kV, and the working current is (15-20) mA. The scanning speed of the counter is 1 / 4° / min. When recording the 004 diffraction pattern, the scanning range of the diffraction angle 2θ is 53°-57°. When recording the 110 diffraction pattern, the scanning range of the diffraction angle 2θ is 75°-79°. The peak area obtained from the (004) plane diffraction pattern is denoted as C004. The peak area obtained from the (110) plane diffraction pattern is denoted as C110. The ratio of C004 / C110 of the negative electrode active material is calculated, which is the OI value of the negative electrode active material.

[0055] Method of preparing a negative active material

[0056] The negative electrode active material of the first aspect can be prepared using methods and instruments known in the art. As an example, it can be prepared by the following method:

[0057] Preparation of graphite particles: petroleum coke with sulfur content less than 1% can be selected as the raw material of graphite particles. The petroleum coke is crushed to Dv50 of 9-12 μm, and then subjected to pre-carbonization treatment at a temperature of 900-1100 °C for a time of ≥6 h. The treated sample is then subjected to graphitization, and the graphitization temperature is controlled at 2700-3000 °C and the holding time is controlled at 36-72 h. After graphitization, the graphite particles are shaped and classified to narrow the particle size range, and the shaping and classification time is 3-8 h. According to the actual powder parameter requirements, the yield is adjusted, and the classification yield is controlled at 50%-90%. Thus, the graphite particles are obtained. It can be understood that different graphite particles can be obtained by controlling the above process parameters.

[0058] Preparation of the negative active material: the suitable graphite particles are coated by vapor deposition, the vapor deposition temperature is selected as 900-1200℃, the carrier gas is selected as argon (Ar), the reaction gas is selected as silane and acetylene, the flow rate ratio of silane and acetylene is controlled as 5:4. The flow rate of silane is controlled as 25-75sccm, the flow rate of acetylene is controlled as 20-60sccm, and the deposition time is controlled as 3-15h. By controlling the gas flow rate and the reaction time, the coating thickness of the silicon carbide deposition layer on the surface of the graphite particles can be controlled.

[0059] FIG. 1 The SEM image of the negative active material in an embodiment of the application has no obvious difference from the general graphite particles, which indicates that the silicon carbide layer does not obviously affect the morphology of the graphite particles. FIG. 2 The Si element spectrum of the EDS element test of the negative active material in an embodiment of the application is shown in FIG. 6, and the red color indicates the presence of Si elements. It can be found that the graphite particles are fully coated by the silicon carbide, which indicates that the deposition effect of the silicon carbide is good. Negative electrode sheet

[0060] In a second aspect, the application provides a negative electrode sheet, comprising: a negative electrode film layer, wherein the negative electrode film layer comprises the negative active material according to any one of the embodiments of the first aspect.

[0061] According to the application, since the negative electrode sheet comprises the silicon-carbon negative electrode material according to any one of the embodiments of the first aspect, the beneficial effects of the first aspect are achieved.

[0062] In some embodiments, the compaction density PD g / cm3 of the negative electrode sheet satisfies: 1.45≤PD≤1.75. 3 satisfies: 1.45≤PD≤1.75.

[0063] In some embodiments, the compaction density PD g / cm3 of the negative electrode sheet satisfies: 1.45≤PD≤1.75.

[0064] In some embodiments, the porosity K% of the negative electrode sheet satisfies: 25≤K≤40.

[0065] In some embodiments, the porosity K% of the negative electrode sheet satisfies: 25≤K≤40.

[0066] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0067] In some embodiments, the negative electrode film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0068] In some embodiments, the negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0069] In some embodiments, the negative electrode film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0070] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and after processes such as drying, cold pressing, and the like, the negative electrode sheet can be obtained.

[0071] In a third aspect, the present application provides a secondary battery, including the negative electrode sheet according to any one of the embodiments of the second aspect.

[0072] According to the present application, the battery includes the negative electrode sheet according to any one of the embodiments of the second aspect, and thus the battery has the beneficial effects of the second aspect.

[0073] Generally, the battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator.

[0074]

Positive electrode sheet

[0075] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, and the positive electrode film layer includes a positive electrode active material.

[0076] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is provided on either one or both of the two surfaces of the positive electrode current collector.

[0077] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0078] In some embodiments, the positive electrode active material can employ a positive electrode active material for a battery known in the art. As an example, the positive electrode active material can include at least one of a lithium-containing phosphate of olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode active material for a battery can also be used. These positive electrode active materials can be used alone only or in combination of two or more. Among them, examples of the lithium transition metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2(also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2(also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2(also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2(also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co 0.15 Al0.05 O2) and modified compounds thereof, etc. Examples of the lithium-containing phosphate of olivine structure can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0079] In some embodiments, the positive electrode film layer further optionally includes a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0080] In some embodiments, the positive electrode film layer further optionally includes a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0081] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., obtaining the positive electrode tab.

[0082]

Negative electrode tab

[0083] According to any of the embodiments of the second aspect.

[0084]

Separator

[0085] The separator is disposed between the positive electrode tab and the negative electrode tab, and mainly functions to prevent short circuiting of the positive and negative electrodes, while allowing active ions to pass through. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0086] In some embodiments, the material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, polyvinylidene fluoride, but is not limited to these. Optionally, the material of the separator can include polyethylene and / or polypropylene. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of the layers can be the same or different. In some embodiments, a ceramic coating or a metal oxide coating can be further provided on the separator.

[0087]

Electrolyte

[0088] The electrolyte functions to conduct active ions between the positive electrode sheet and the negative electrode sheet. The electrolyte used in the secondary battery according to the present application can be an electrolyte known in the art.

[0089] In some embodiments, the electrolyte can include an organic solvent, an electrolyte salt, and an optional additive, and the kinds of the organic solvent, the lithium salt, and the additive are not particularly limited and can be selected as desired.

[0090] In some embodiments, the secondary battery is a lithium ion battery, and the electrolyte salt can include a lithium salt. As an example, the lithium salt includes, but is not limited to, at least one of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDODFP (lithium difluoro(dioxalato)phosphate), and LiOTFP (lithium tetrafluoro(oxalato)phosphate). The above lithium salt can be used alone or two or more kinds thereof can be used simultaneously.

[0091] In some embodiments, the secondary battery is a sodium ion battery, and the electrolyte salt can include a sodium salt. As an example, the sodium salt can be selected from at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, and Na(CH3)C6H4SO3.

[0092] In some embodiments, as an example, the organic solvent includes, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). The above organic solvent can be used alone or two or more kinds thereof can be used simultaneously. Alternatively, two or more kinds of the above organic solvent can be used simultaneously.

[0093] In some embodiments, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0094] As an example, the additive includes, but is not limited to, at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), divinyl sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3-propane sultone (PS), 1,3-propene sultone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl)phosphate (TMSP), tris(trimethylsilyl)borate (TMSB).

[0095] The electrolyte solution can be prepared according to a method conventional in the art. For example, the organic solvent, the electrolyte salt, and the optional additive can be mixed uniformly to obtain the electrolyte solution. The order of addition of the materials is not particularly limited, for example, the electrolyte salt and the optional additive can be added to the organic solvent and mixed uniformly to obtain the electrolyte solution; or the electrolyte salt can be added to the organic solvent first, and then the optional additive can be added to the organic solvent and mixed uniformly to obtain the electrolyte solution.

[0096] Electronic device

[0097] In a fourth aspect, the present application provides an electronic device, characterized in that comprising the secondary battery according to any one of the third aspect.

[0098] According to the present application, since the electronic device comprises the secondary battery according to any one of the third aspect, the electronic device has the beneficial effects of the third aspect.

[0099] The electronic device of the present application is not particularly limited, and can be any electronic device known in the art. In some embodiments, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0100] Hereinafter, the examples of the present application will be described. The examples described below are illustrative and are given for the purpose of illustration and explanation only and are not to be understood as limiting the present application. Unless otherwise indicated, the techniques and conditions described in the examples are in accordance with those described in the literature or in accordance with the product instructions. Unless otherwise indicated, the reagents or instruments used are conventional products that can be obtained commercially.

[0101] Test of the first coulombic efficiency of the battery: the assembled lithium ion battery was subjected to formation and capacity test, charged at a rate of 0.2C until the voltage reached 3.6V and the current was lower than 0.05C, to obtain the charge capacity Q1, then rested for 5 min, and discharged at a rate of 0.2C until the voltage reached 2.5V, to obtain the discharge capacity Q2, the first coulombic efficiency = Q2 / Q1 x 100%.

[0102] Test of the thermal runaway temperature of the battery: the battery was fully charged, and then placed in a high-temperature oven, the initial temperature of the high-temperature oven was 25℃, the temperature was raised at a rate of 5℃ / min, and raised to the set temperature, the set temperature was 120-180℃, then kept for 60 min, if the battery smoked or caught fire or even exploded, it was determined to be thermal runaway, otherwise it was determined not to have thermal runaway.

[0103] Example 1-1

[0104] Preparation of graphite particles: petroleum coke was selected as the raw material of the graphite particles, and the sulfur content of the petroleum coke was 0.8%. The petroleum coke was crushed to a Dv50 of 10 μm, and then subjected to a pre-carbonization treatment, the treatment temperature was 950℃, and the treatment time was 8h. The treated sample was then subjected to graphitization, the temperature and time of the graphitization were controlled, the temperature of the graphitization was controlled at 2900℃, and the holding time was controlled at 48h. After the graphitization was completed, the graphite particles were shaped and classified to narrow the particle size range, the shaping and classifying time was 4h, and the classification yield was controlled at 65%, and the graphite particles were obtained.

[0105] Preparation of the negative active material: the prepared graphite particles were subjected to gas phase deposition coating, the gas phase deposition temperature was selected to be 1000℃, the carrier gas was selected to be argon (Ar), the reaction gas was selected to be silane and acetylene, and the flow rate ratio of the silane and acetylene was controlled at 5:4. The flow rate of the silane was controlled at 45sccm, the flow rate of the acetylene was controlled at 36sccm, and the deposition time was controlled at 4h, and the negative active material was obtained, and part of the parameters are shown in Table 1.

[0106] Preparation of the negative electrode sheet: the negative electrode active material, conductive carbon, binder styrene butadiene rubber (abbreviated as SBR), thickening agent sodium carboxymethyl cellulose (abbreviated as CMC) are mixed in a certain ratio, wherein the mass ratio of conductive carbon is 1.5%, the mass ratio of negative electrode active material is 96%, the mass ratio of binder styrene butadiene rubber (abbreviated as SBR) is 1.5%, and the mass ratio of thickening agent sodium carboxymethyl cellulose (abbreviated as CMC) is 1.0%, then a proper amount of deionized water solvent is used to fully stir and mix, so that a uniform negative electrode slurry is formed, the slurry is coated on the current collector Cu foil, dried and cold-pressed to obtain the negative electrode sheet.

[0107] Preparation of the positive electrode sheet: lithium iron phosphate is selected as the positive electrode active material, which is fully stirred and mixed with conductive agent acetylene black and binder polyvinylidene fluoride (abbreviated as PVDF) in a weight ratio of 96.3:2.2:1.5 in a proper amount of N-methyl pyrrolidone (abbreviated as NMP) solvent to form a uniform positive electrode slurry; the slurry is coated on the current collector Al foil, dried and cold-pressed to obtain the positive electrode sheet.

[0108] Preparation of the electrolyte: in a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC: EMC: DEC = 1:3:3:3, then 1,3-propane sultone is added, dissolved and fully stirred, and then lithium salt LiPF6 is added, and the mixture is uniformly mixed to obtain the electrolyte. The mass percentage of LiPF6 is 11.0%, the mass percentage of 1,3-propane sultone is 2.9%, and the mass percentage of each substance is calculated based on the mass of the electrolyte.

[0109] Preparation of the separator film: the separator film is selected as an 8 μm PE porous polymer film.

[0110] Preparation of the lithium ion battery: the above-mentioned negative electrode sheet and positive electrode sheet are wound together with the separator film, placed in an aluminum plastic film, then liquid injection, standing and formation are carried out to prepare a lithium ion secondary battery.

[0111] The first coulomb efficiency and thermal runaway temperature of the above-mentioned lithium ion battery are detected, and the results are shown in Table 1.

[0112] Examples 1-2 to 1-9 and Comparative Examples 1 to 6

[0113] The same as Example 1-1, except that the thickness, Lc and Y of the silicon carbide layer of the negative electrode active material are changed by controlling the graphitization process and gas deposition conditions. The graphitization temperature and time affect the size of Lc, and increasing the temperature and holding time increases Lc, and vice versa. The specific parameters are shown in Table 1.

[0114] The first coulombic efficiency and thermal runaway temperature of the lithium ion battery are detected, and the results are shown in Table 1.

[0115] Table 1

[0116]

[0117] According to Table 1, the first efficiency and thermal runaway temperature of the lithium ion battery obtained by each embodiment are higher than those of each comparative example, which indicates that the negative electrode active material provided by the application can effectively improve the first efficiency and thermal stability of the battery. Among them, in each comparative example, because Lc and Y do not meet the requirements, it may lead to that the silicon carbide layer cannot effectively protect the negative electrode active material, or the silicon carbide layer cannot well adapt to the expansion of the graphite particles to cause cracking, and then lead to the decrease of the first efficiency and thermal runaway temperature.

[0118] Examples 2-1 to 2-7

[0119] The specific parameters are shown in Table 2.

[0120] The first coulombic efficiency and thermal runaway temperature of the lithium ion battery are detected, and the results are shown in Table 2.

[0121] Table 2

[0122]

[0123] According to Table 2, the relationship between the specific surface area of the negative electrode active material and the mass percentage of silicon also affects the first efficiency and thermal runaway temperature of the battery. When BET-0.3Y meets 1.2-2, the first efficiency and thermal runaway temperature of the battery are better.

[0124] Examples 3-1 to 3-6

[0125] The specific parameters are shown in Table 3.

[0126] The first coulombic efficiency and thermal runaway temperature of the lithium ion battery are detected, and the results are shown in Table 3.

[0127] Table 3

[0128]

[0129] According to Table 3, the powder compaction density of the negative active material and the mass percentage of silicon meet the relationship that also affects the initial efficiency and thermal runaway temperature of the battery. When P+0.08Y meets 1.8-2.1, the initial efficiency and thermal runaway temperature of the battery are better. Further, when P+0.08Y meets 1.95-2.05, the initial efficiency and thermal runaway temperature of the battery are better.

[0130] Examples 4-1-4-6

[0131] The same as Example 3-4, except that by controlling the shaping time and the classification yield of the graphitization material, the (Dv90-Dv50) / Dv99 and I d / I g , and the specific parameters are as shown in Table 4.

[0132] The initial coulombic efficiency and thermal runaway temperature of the above lithium ion battery were detected, and the results are shown in Table 4.

[0133] Table 4

[0134]

[0135]

[0136] According to Table 4, the particle size distribution and surface defect degree of the negative active material also affect the initial efficiency and thermal runaway temperature of the battery. When 0.2≤(Dv90-Dv50) / Dv99≤0.5, the initial efficiency and thermal runaway temperature of the battery are better. When I d / I g <0.5, the initial efficiency and thermal runaway temperature of the battery are better.

[0137] Examples 5-1-5-6

[0138] The same as Example 4-5, except that by controlling the yield and classification frequency of the shaping classification (the classification frequency affects the particle size distribution, and the lower the frequency, the wider the particle size distribution of the particles), the TD and OI values of the negative active material can be changed. Removing more fine powder can reduce the yield, but can increase the TD of the negative active material. The wider particle size distribution of the particles can reduce the OI value of the active material. The specific parameters are as shown in Table 5.

[0139] The initial coulombic efficiency and thermal runaway temperature of the above lithium ion battery were detected, and the results are shown in Table 5.

[0140] Table 5

[0141]

[0142] According to Table 5, the tap density and OI value of the negative active material also affect the initial efficiency and thermal runaway temperature of the battery, and the initial efficiency and thermal runaway temperature of the battery are better when 0.7≤TD≤1.2, and the initial efficiency and thermal runaway temperature of the battery are better when OI≤6.

[0143] Examples 6-1 to 6-6

[0144] The same as Example 5-5, except that the PD and K of the negative electrode tab are changed, and the specific parameters are as shown in Table 6.

[0145] The initial coulombic efficiency and thermal runaway temperature of the above lithium ion battery were detected, and the results are shown in Table 6.

[0146] Table 6

[0147]

[0148] According to Table 6, the tap density and porosity of the negative electrode tab also affect the initial efficiency and thermal runaway temperature of the battery, and the initial efficiency and thermal runaway temperature of the battery are better when 1.45≤PD≤1.75, and the initial efficiency and thermal runaway temperature of the battery are better when 25≤K≤40.

[0149] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A negative electrode active material, characterized by, Comprising: graphite particles, and a silicon carbide layer provided on a surface of the graphite particles, wherein a mass percentage content Y% of silicon in the negative electrode active material and an average height Lc nm of the negative electrode active material in a direction along a c-axis satisfy: 0.3 x (32 - Lc) ≤ Y ≤ 0.9 x (32 - Lc).

2. The negative electrode active material according to claim 1, characterized by The thickness of the silicon carbide layer is 5 nm to 100 nm; and a crystal phase of silicon carbide in the silicon carbide layer is β-phase silicon carbide.

3. The negative electrode active material according to claim 2, characterized by The thickness of the silicon carbide layer is 15 nm to 40 nm.

4. The negative electrode active material according to any one of claims 1 to 3, characterized by, the specific surface area BET m of the negative active material 2 and the mass percentage content Y% of silicon in the negative active material satisfies: 1.2≤BET-0.3×Y≤2.

0.

5. The negative electrode active material according to any one of claims 1 to 3, characterized by, The negative active material has a powder compaction density P g / cm under test conditions of a pressure of 5 tons 3 and the mass percentage content Y% of silicon in the negative active material satisfies: 1.80 ≤ P + 0.08 × Y ≤ 2.

10.

6. The negative electrode active material according to any one of claims 1 to 3, characterized by, The volume particle size of the negative electrode active material satisfies: 0.2 ≤ (Dv90 - Dv50) / Dv99 ≤ 0.

5.

7. The negative electrode active material according to claim 1, characterized by The negative electrode active material satisfies at least one of the following conditions: (1) the Raman spectrum D peak intensity I d and the Raman spectrum G peak intensity I g satisfies: I d / I g < 0.5; (2) the tap density TD of the negative electrode active material is 0.7 to 1.2 g / cm3 3 satisfies: 0.7 ≤ TD ≤ 1.2; (3) The orientation degree OI value of the negative electrode active material is ≤ 6.

8. A negative electrode sheet characterized by comprising: Comprising: a negative electrode film layer including the negative electrode active material according to any one of claims 1 to 7.

9. The negative electrode sheet according to claim 8, characterized by, The negative electrode sheet satisfies at least one of the following conditions: (4) the compaction density PD g / cm3 of the negative electrode sheet 3 satisfies: 1.45 ≤ PD ≤ 1.75; (5) The porosity K% of the negative electrode sheet satisfies: 25 ≤ K ≤ 40.

10. A secondary battery characterized by comprising: Comprising: the negative electrode sheet according to claim 8 or 9.

11. An electronic device, comprising: Comprising: the secondary battery according to claim 10.

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