Negative active material, lithium ion secondary battery and electronic device

Abstract

The invention provides a negative electrode active material, a lithium ion secondary battery and an electronic device. The negative electrode active material is a graphite material with a coating layer on the surface, and the coating layer comprises boron nitride particles; the average equivalent diameter Y of the boron nitride particles is greater than or equal to 20nm and less than or equal to 200nm, and the quantity proportion Z of the particles with the equivalent diameter y greater than or equal to 200nm is less than or equal to 20%. The thermal stability of the lithium ion secondary battery containing the negative electrode active material is improved by regulating and controlling the equivalent diameter of the boron nitride particles in the negative electrode active material and the quantity proportion of the particles with the specific equivalent diameter.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium batteries, and in particular to a negative electrode active material, a lithium ion secondary battery and an electronic device. Background Art

[0002] Lithium-ion secondary batteries are now widely used in all aspects of life, including consumer electronics such as mobile phones, tablet computers, sweeping robots, etc., new energy vehicles and power tools, and household energy storage, industrial and commercial energy storage, etc. Compared with lead-acid batteries and nickel-metal hydride batteries, lithium-ion batteries have high energy density, long cycle life, and excellent performance, but they are slightly lacking in safety. With the large-scale application of lithium-ion batteries, we have also seen things about lithium-ion battery fires in the market, which has also made many people worry about the safety of lithium-ion batteries. The thermal runaway temperature of lithium-ion batteries is an important indicator of safety. Generally, when lithium-ion batteries work in high temperature environments or are charged and discharged at high rates, the battery temperature will rise significantly, and even thermal runaway and fire may occur. How to improve the thermal stability of lithium-ion batteries is also an important issue. At present, the market mainly improves the thermal stability of lithium-ion batteries by increasing the thickness of the separator or the separator coating of the lithium-ion battery and reducing the charge and discharge rate. These methods increase costs and lose the performance of the battery. Summary of the invention

[0003] In view of the above problems in the prior art, the present application improves the thermal stability of lithium-ion secondary batteries from the perspective of negative electrode active materials. This solution has the characteristics of low cost and good performance of lithium-ion secondary batteries, and significantly improves the thermal stability of lithium-ion secondary batteries.

[0004] A first aspect of the present application provides a negative electrode active material, wherein the negative electrode active material is a graphite material having a coating layer on the surface, and the coating layer contains boron nitride particles.

[0005] In some embodiments of the present application, the average equivalent diameter Y of the boron nitride particles satisfies: 20 nm ≤ Y ≤ 280 nm, and the number ratio of particles with an equivalent diameter y ≥ 200 nm is Z, and Z ≤ 20%.

[0006] In some embodiments of the present application, the thickness X of the coating layer satisfies: 20 nm ≤ X ≤ 180 nm.

[0007] In some embodiments of the present application, the thickness X of the coating layer satisfies: 30 nm ≤ X ≤ 160 nm.

[0008] In some embodiments of the present application, the thickness X of the coating layer satisfies: 30 nm ≤ X ≤ 110 nm.

[0009] In some embodiments of the present application, the average equivalent diameter Y of the boron nitride particles satisfies: 30 nm ≤ Y ≤ 250 nm.

[0010] In some embodiments of the present application, the average equivalent diameter Y of the boron nitride particles satisfies: 100 nm≤Y≤160 nm.

[0011] In some embodiments of the present application, the number ratio Z of particles with an equivalent diameter y≥200 nm is ≤15%.

[0012] In some embodiments of the present application, the number ratio Z of particles with an equivalent diameter y≥200 nm is ≤10%.

[0013] In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.2%≤W≤2.0%.

[0014] In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.3%≤W≤1.5%.

[0015] In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.6%≤W≤1.2%.

[0016] In some embodiments of the present application, a ratio of the W to the sphericity S of the negative electrode active material satisfies 0.002≤W / S≤0.025.

[0017] In some embodiments of the present application, a ratio of the W to the sphericity S of the negative electrode active material satisfies 0.006≤W / S≤0.015.

[0018] In some embodiments of the present application, a ratio of the W to the sphericity S of the negative electrode active material satisfies 0.008≤W / S≤0.015.

[0019] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500° C. satisfies: 0.2%≤TG≤2.5%.

[0020] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500° C. satisfies: 0.5%≤TG≤2.5%.

[0021] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500° C. satisfies: 0.5%≤TG≤2.0%.

[0022] In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.50.

[0023] In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.45.

[0024] In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.40.

[0025] In some embodiments of the present application, the specific surface area BET of the negative electrode active material satisfies: 0.5 m 2 / g≤BET≤3.0m 2 / g.

[0026] In some embodiments of the present application, the specific surface area BET of the negative electrode active material satisfies: 0.8 m 2 / g≤BET≤3.0m 2 / g.

[0027] In some embodiments of the present application, the specific surface area BET of the negative electrode active material satisfies: 1.0 m 2 / g≤BET≤2.5m 2 / g.

[0028] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 5.0 μm ≤ Dv50 ≤ 20.0 μm.

[0029] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 8.0 μm ≤ Dv50 ≤ 20.0 μm.

[0030] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 8.0 μm ≤ Dv50 ≤ 16.0 μm.

[0031] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 0.9 μm.

[0032] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 1.5 μm.

[0033] In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤15.

[0034] In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤ 10.

[0035] In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤ 8.

[0036] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: TD ≥ 0.5 g / cm 3

[0037] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: TD ≥ 0.6 g / cm 3 。

[0038] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: 1.3 g / cm 3 ≥ TD ≥ 0.8 g / cm 3 。

[0039] The second aspect of the present application provides a lithium-ion secondary battery, which includes a positive electrode sheet, an electrolyte, a separator, and a negative electrode sheet; the negative electrode sheet includes a binder, a thickener, an additive, a negative electrode current collector, and the negative electrode active material described in the first aspect of the present application.

[0040] In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 15% ≤ K ≤ 40%.

[0041] In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.20 g / cm 3 ≤ PD ≤ 1.80 g / cm 3 。

[0042] In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 18% ≤ K ≤ 40%.

[0043] In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 18% ≤ K ≤ 35%.

[0044] In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.30 g / cm 3 ≤ PD ≤ 1.75 g / cm 3 。

[0045] In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.35 g / cm 3 ≤ PD ≤ 1.70 g / cm 3 。

[0046] The lithium-ion secondary battery composed of the negative electrode sheet containing the above-mentioned negative electrode active material, a positive electrode, an electrolyte, and a separator has excellent thermal stability, and its hot box temperature T satisfies: T≥135°C.

[0047] A third aspect of the present application provides an electronic device, which includes the lithium-ion secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The SEM image and EDS element distribution diagram of the negative electrode active material in Example 1.

[0049] Figure 2 This is a comparison chart of the thermal stability improvement effect of the lithium ion secondary battery in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0050] The technical solution of the present application is further described below through specific embodiments, which do not limit the protection scope of the present application. Some non-essential modifications and adjustments made by others based on the technical solution of the present application are still protected by the present application.

[0051] For simplicity, for a certain parameter, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form an unclearly recorded range, and any lower limit can also be combined with other lower limits to form an unclearly recorded range, and any upper limit can also be combined with any other upper limits to form an unclearly recorded range. In addition, each separately disclosed point or single value itself can be used as a lower limit or upper limit and combined with any other point or single value or with other lower limits or upper limits to form an unclearly recorded range.

[0052] Unless otherwise specified, the terms used in this application have the commonly known meanings generally understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0053] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0054] The terms "first", "second", etc. in the specification and claims of this application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "include" and "comprises" and similar expressions are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units that are not listed, or may optionally include other steps or units inherent to these processes, methods, products or devices.

[0055] In the present application, the "binder" is used to prepare the graphite material having a coating layer containing boron nitride particles on the surface.

[0056] In the present application, the thermal stability of the lithium-ion secondary battery is measured by the hot box temperature. The higher the hot box temperature, the less likely the lithium-ion secondary battery is to experience thermal runaway under high temperature conditions, that is, the better the thermal stability of the lithium-ion secondary battery.

[0057] In the present application, the coating layer containing the boron nitride particles may be completely coated with graphite material or partially coated with graphite material.

[0058] The first aspect of the present application provides a negative electrode active material, the negative electrode active material is a graphite material with a coating layer on the surface, the coating layer comprises boron nitride particles, the average equivalent diameter Y of the boron nitride particles satisfies: 20nm≤Y≤280nm, and the number ratio of particles with an equivalent diameter y≥200nm is Z, Z≤20%. In some embodiments of the present application, the average equivalent diameter Y of the boron nitride particles satisfies: 30nm≤Y≤250nm. In some embodiments of the present application, the average equivalent diameter Y of the boron nitride particles satisfies: 100nm≤Y≤160nm. Specifically, the average equivalent diameter Y of the boron nitride particles can be 20nm, 30nm, 33nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 102nm, 110nm, 145nm, 150nm, 155nm, 157nm, 160nm, 165nm, 200nm, 210nm, 233nm, 245nm, 250nm, 260nm, 276nm, 280nm or a range consisting of any two of the above values. In some embodiments of the present application, the number ratio Z of particles with an equivalent diameter y≥200nm is ≤15%. In some embodiments of the present application, the number ratio Z of particles with an equivalent diameter y≥200nm is ≤10%. Specifically, the proportion Z of the number of particles with an equivalent diameter y≥200nm can be 0.5%, 1%, 2%, 2.5%, 3%, 3.5%, 5.6%, 8%, 10%, 11.5%, 13%, 15%, 18%, 20% or a range consisting of any two of the above values.

[0059] In some embodiments of the present application, the thickness X of the coating layer satisfies: 20nm≤X≤180nm. In some embodiments of the present application, the thickness X of the coating layer satisfies: 30nm≤X≤160nm. In some embodiments of the present application, the thickness X of the coating layer satisfies: 30nm≤X≤110nm. Specifically, the thickness X of the coating layer can be 20nm, 21nm, 25nm, 30nm, 40nm, 50nm, 56nm, 60nm, 70nm, 72nm, 75nm, 80nm, 90nm, 100nm, 109nm, 110nm, 136nm, 150nm, 160nm, 167nm, 180nm or a range consisting of any two of the above values.

[0060] In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.2%≤W≤2.0%. In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.3%≤W≤1.5%. In some embodiments of the present application, the mass ratio W of the boron element in the negative electrode active material satisfies 0.6%≤W≤1.2%. Specifically, the mass ratio W of the boron element in the negative electrode active material can be 0.2%, 0.3%, 0.5%, 0.6%, 0.8%, 1.0%, 1.1%, 1.2%, 1.5%, 1.6%, 2.0% or a range consisting of any two of the above values.

[0061] In some embodiments of the present application, the ratio of the W to the sphericity S of the negative electrode active material satisfies 0.002≤W / S≤0.025. In some embodiments of the present application, the ratio of the W to the sphericity S of the negative electrode active material satisfies 0.006≤W / S≤0.015. In some embodiments of the present application, the ratio of the W to the sphericity S of the negative electrode active material satisfies 0.008≤W / S≤0.015. Specifically, the ratio W / S of the sphericity S of the W to the negative electrode active material can be 0.002, 0.005, 0.006, 0.008, 0.010, 0.011, 0.013, 0.015, 0.020, 0.023, 0.025 or a range consisting of any two of the above values.

[0062] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500°C satisfies: 0.2%≤TG≤2.5%. In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500°C satisfies: 0.5%≤TG≤2.5%. In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 500°C satisfies: 0.5%≤TG≤2.0%. Specifically, the thermogravimetric mass loss TG of the negative electrode active material at 500°C may be 0.2%, 0.4%, 0.5%, 0.9%, 1.0%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2.0%, 2.2%, 2.5%, or a range consisting of any two of the above values.

[0063] In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.50. In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.45. In some embodiments of the present application, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.40. Specifically, the peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material can be 0.20, 0.21, 0.23, 0.25, 0.26, 0.30, 0.35, 0.37, 0.40, 0.43, 0.45 or a range consisting of any two of the above values.

[0064] In some embodiments of the present application, the specific surface area BET of the negative electrode active material satisfies: 0.5 m 2 / g≤BET≤3.0m 2 In some embodiments of the present application, the specific surface area of ​​the negative electrode active material satisfies the following BET: 0.8 m 2 / g≤BET≤3.0m 2 In some embodiments of the present application, the specific surface area BET of the negative electrode active material satisfies: 1.0 m 2 / g≤BET≤2.5m 2 / g. Specifically, the BET specific surface area of ​​the negative electrode active material can be 0.5m 2 / g, 0.6m 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.3m 2 / g, 2.5m 2 / g, 2.7m 2 / g, 2.8m 2 / g, 3.0m 2 / g or a range consisting of any two of the above values.

[0065] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 5.0μm≤Dv50≤20.0μm. In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 8.0μm≤Dv50≤20.0μm. In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 8.0μm≤Dv50≤16.0μm and Dn10≥1.5μm. Specifically, the particle size Dv50 of the negative electrode active material can be 5.0μm, 8.0μm, 10.0μm, 12.0μm, 15.0μm, 16.0μm, 20.0μm or a range consisting of any two of the above values.

[0066] In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 0.9 μm. In some embodiments of the present application, the particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 1.5 μm. Specifically, the particle size Dn10 of the negative electrode active material can be 0.8 μm, 0.9 μm, 1.2 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 2.1 μm, 2.5 μm, 2.7 μm, 3.0 μm, or a range consisting of any two of the above values.

[0067] In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤ 15. In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤ 10. In some embodiments of the present application, the OI value of the negative electrode active material satisfies: OI value ≤ 8. Specifically, the OI value of the negative electrode active material can be 3, 4, 5, 6, 7, 9, 10, 12, 15 or a range consisting of any two of the above values.

[0068] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: TD ≥ 0.5 g / cm 3 In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: TD ≥ 0.6 g / cm 3 In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: 1.3 g / cm 3 ≥TD≥0.8g / cm 3 Specifically, the tap density TD of the negative electrode active material may be 0.5 g / cm 3 , 0.6g / cm 3 , 0.7g / cm 3 , 0.8g / cm 3 , 1.0g / cm 3 , 1.2g / cm 3, 1.3g / cm 3 Or a range consisting of any two of the above values.

[0069] The second aspect of the present application provides a lithium-ion secondary battery, which comprises a positive electrode plate, an electrolyte, a separator and a negative electrode plate; the negative electrode plate comprises a binder, a thickener, an additive, a negative electrode current collector and the negative electrode active material described in the first aspect of the present application.

[0070] In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 15% ≤ K ≤ 40%. In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 18% ≤ K ≤ 40%. In some embodiments of the present application, the porosity K of the negative electrode sheet satisfies: 18% ≤ K ≤ 35%. Specifically, the porosity of the negative electrode sheet can be 15%, 16%, 17%, 18%, 20%, 25%, 26%, 30%, 33%, 35%, 38%, 40%, or a range consisting of any two of the above values.

[0071] In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.20 g / cm 3 ≤PD≤2.80g / cm 3 In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.30 g / cm 3 ≤PD≤1.75g / cm 3 In some embodiments of the present application, the compaction density PD of the negative electrode sheet satisfies: 1.35 g / cm 3 ≤PD≤1.70g / cm 3 Specifically, the compaction density of the negative electrode sheet is 1.20 g / cm 3 , 1.33g / cm 3 , 1.35g / cm 3 , 1.39g / cm 3 , 1.41g / cm 3 , 1.50g / cm 3 、1.53g / cm 3 , 1.55g / cm 3 , 1.60g / cm 3 , 1.65g / cm 3 、1.68g / cm 3 , 1.70g / cm 3 , 1.71g / cm 3 , 1.75g / cm 3 Or a range consisting of any two of the above values.

[0072] The third aspect of the present application provides an electronic device, and the electronic device includes the lithium-ion secondary battery described above.

[0073] The present application provides a method for preparing a negative electrode active material: The negative electrode active material is prepared by a hydrothermal method, wherein the graphite material includes at least one of artificial graphite and natural graphite. The artificial graphite is subjected to crushing, shaping, and grading treatments to reduce its edges and corners. The coating material is boron nitride (BN) nanoparticles with a size less than 200 nm, that is, the average equivalent diameter of the particles is less than 200 nm. The perimeter of a single particle is calculated by image software, and then the equivalent diameter of a single particle is obtained by dividing the perimeter by π. In order to enable the BN nanoparticles to effectively coat the graphite surface, an appropriate amount of binder needs to be added to the water. These binders need to be soluble in water or form a colloid with water and have good affinity with graphite, such as at least one of lithium polyacrylate, polyacrylic acid (PAA), modified polyaniline (such as polyaniline sulfate), polypyrrole, sodium carboxymethyl cellulose, or styrene-butadiene rubber latex. The specific experimental scheme is as follows: First, select the graphite to be coated and weigh an appropriate mass m 1 , then weigh an appropriate mass m 2 of the coating agent BN, and then take an appropriate mass m 3 of the binder. The total mass of these three substances is m, where the proportion m 2 of the total mass m is m 2 / m×100% is 0.2% to 2.5%, and the proportion of m 3 in the total mass is m 3 / m×100% is 1.0% to 10.0%. Place the above three substances together in a rotary evaporator, and add enough deionized water to mix evenly. The mass of deionized water is 10 to 50 times the total mass m. For some adhesives with poor solubility, hydrochloric acid or ammonia water can be appropriately added to adjust its pH value to promote its dissolution. Then start to slowly heat to 60°C to 180°C while stirring, continue stirring until the water evaporates, and take out the mixture after cooling. Place the mixture in a box furnace for heat treatment, pass nitrogen protection, and then heat it to 200°C to 400°C, keep it warm for 4h to 8h, take it out after cooling, and then rinse it with deionized water to obtain the negative electrode active material. The negative electrode active material, additive conductive carbon black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are mixed in a certain ratio, wherein the mass proportion of the additive is 0.5% to 1.5%, the mass proportion of the negative electrode active material is 94.8% to 97.0%, the mass proportion of the binder styrene butadiene rubber (SBR) is 1.2% to 2.2%, and the mass proportion of the thickener sodium carboxymethyl cellulose (CMC) is 0.5% to 1.5%, and the mass proportion of the four components adds up to 100%, and then add them into an appropriate amount of deionized water and stir and mix them thoroughly to form a uniform negative electrode slurry, apply the slurry on the negative electrode collector copper foil, dry and cold press, and then obtain the negative electrode sheet.

[0074] The present application is described in more detail below through examples.

[0075] Example 1

[0076] Preparation of negative electrode active materials

[0077] The coated matrix is ​​primary artificial graphite particles that have been crushed, shaped and graded. The average equivalent diameter of the BN nanoparticles is 150nm, and the adhesive is sodium carboxymethyl cellulose. The mass of primary artificial graphite particles is m 1 is 10.0 g, and then weigh the mass of BN nanoparticles m 2 is 0.10g, and then take the mass m of sodium carboxymethyl cellulose 3 The total weight of the mixture is 0.35g. The three are placed in a rotary evaporator together, and sufficient water is added to mix evenly. The mass of the water is 250g. Then slowly heat to 110°C while stirring, and continue stirring until the water evaporates. Take out the mixture after cooling. Place the mixture in a box furnace for heat treatment, pass nitrogen protection, and then heat to 350°C for 5 hours. Take it out after cooling to obtain the negative electrode active material.

[0078] Preparation of negative electrode

[0079] The negative electrode active material, conductive carbon black additive, styrene butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC) thickener are mixed in the following proportions: the mass proportion of the additive is 1.5%, the mass proportion of the negative electrode active material is 95%, the mass proportion of the styrene butadiene rubber (SBR) binder is 2.0%, and the mass proportion of the sodium carboxymethyl cellulose (CMC) thickener is 1.5%. Then, stir and mix thoroughly with an appropriate amount of deionized water to form a uniform negative electrode slurry. The slurry is coated on the negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0080] Preparation of positive electrode

[0081] Lithium iron phosphate (chemical formula: LiFePO4) is selected as the positive electrode active material, and it is fully stirred and mixed with conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) in a weight ratio of 96.3:2.2:1.5 in an appropriate amount of N-methylpyrrolidone (NMP) solvent to form a uniform positive electrode slurry; this slurry is coated on the current collector Al foil, dried and cold pressed to obtain a positive electrode sheet.

[0082] Preparation of electrolyte

[0083] In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a mass ratio of EC:PC:EMC:DEC=1:3:3:3 to obtain a solvent, and then 1,3-propane sultone was added to the solvent, dissolved and stirred thoroughly, and then lithium salt LiPF was added. 6 , and then mixed evenly to obtain an electrolyte. Among them, based on the mass of the electrolyte, LiPF 6 The mass percentage of is 11.0%, and the mass percentage of 1,3-propane sultone is 2.9%.

[0084] Lithium-ion secondary battery assembly

[0085] A polyethylene film with a thickness of 8 μm is selected as the isolation membrane. The negative electrode sheet and the positive electrode sheet are taken and wound together with the isolation membrane, placed in an aluminum-plastic film, and then injected with liquid, allowed to stand, and formed to make a lithium-ion secondary battery.

[0086] Examples 2 to 10 and Comparative Examples 1 to 3

[0087] The difference between Examples 2 to 10 and Comparative Examples 1 to 3 and Example 1 is that the thickness of the coating layer, the average equivalent diameter of the boron nitride particles, and the ratio of the number of particles with an equivalent diameter y ≥ 200 nm are adjusted by common industry adjustment methods, including the mass of the coating agent boron nitride, the mass of the binder sodium carboxymethyl cellulose, the rotary evaporation temperature, the heat treatment temperature, and the heat treatment time. The corresponding negative electrode active material powder is obtained, as shown in Table 1.

[0088] Example 11 to Example 16

[0089] The difference between Examples 11 to 16 and Example 8 is that the sphericity and boron content of the negative electrode active material can be adjusted in a common way in the industry, including the sphericity and mass ratio of the coating agent boron nitride, so that the negative electrode active material powder can meet the preferred characteristics, as shown in Table 2.

[0090] Example 17 to Example 23

[0091] The difference between Examples 17 to 23 and Example 15 is that the negative electrode active material has a thermogravimetric loss TG and a defect degree Id / Ig at 500°C. Through common industry adjustment methods, including parameter adjustment of the binder type, quality and heat treatment temperature, the negative electrode active material powder can meet the preferred characteristics, as shown in Table 3.

[0092] Example 24 to Example 33

[0093] The difference between Examples 24 to 33 and Example 22 is that the specific surface area, particle size Dv50 and Dn10 of the negative electrode active material are adjusted by the common adjustment method in the industry, and the primary particle artificial graphite is adjusted, including the control of the equipment parameters of the particle crushing process and the adjustment of the parameters of the shaping and grading, so that the negative electrode active material powder indicators can meet the preferred characteristics, as shown in Table 4.

[0094] Example 34 to Example 40

[0095] The difference between Examples 34 to 40 and Example 31 is that the OI value and tap density of the negative electrode active material are adjusted by adjusting the primary particle artificial graphite through the common adjustment method in the industry, including the adjustment of particle shaping and grading parameters, so that the negative electrode active material powder indicators can meet the preferred characteristics, as shown in Table 5.

[0096] Example 41 to Example 50

[0097] The difference between Examples 41 to 50 and Example 39 is that the porosity and compaction density of the negative electrode plate are obtained by adjusting the plate design of the lithium-ion secondary battery, including the coating weight and cold pressing thickness of the plate, so that the negative electrode plate meets the preferred characteristics, as shown in Table 6.

[0098] Test Method

[0099] The negative electrode materials, assembled negative electrode plates and lithium-ion secondary batteries in the above embodiments were subjected to the following performance tests, respectively. The test results are shown in Tables 1 to 6.

[0100] 1. Sphericity test of negative electrode active materials

[0101] The negative electrode active material particles are fully diluted with alcohol, and then coated on copper foil or silicon wafer, and observed with a scanning electron microscope (SEM) after drying, and optical pictures or SEM pictures are taken, and then the sphericity of the particles is analyzed according to the image analysis software. The sphericity is defined as the ratio of the circular circumference of the equivalent projected area of ​​the negative electrode active material particles to the actual circumference of its projection. The magnification of the scanning electron microscope is 1000 times and the number of negative electrode active material particles within the image range is not less than 30. The sphericity S of the negative electrode active material is obtained by calculating the average value of the sphericity of these particles.

[0102] 2. OI value test of negative electrode active material

[0103] According to the Mechanical Industry Standard of the People's Republic of China JB / T 4220-2011 "Method for Determination of Lattice Parameters of Artificial Graphite", the (004) plane diffraction line pattern and the (110) plane diffraction line pattern in the X-ray diffraction spectrum of the negative electrode active material were tested. The test conditions are as follows: X-rays are CuKα radiation, and 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 line pattern, the scanning range of the diffraction angle 2θ is 53°-57°. When recording the 110 diffraction line pattern, the scanning range of the diffraction angle 2θ is 75°-79°. The peak area obtained from the (004) plane diffraction line pattern is recorded as C004. The peak area obtained from the (110) plane diffraction line pattern is recorded as C110. The ratio of C004 / C110 of the negative electrode active material is calculated to be the OI value of the negative electrode active material.

[0104] 3. Specific surface area

[0105] The test method for specific surface area refers to GB / T 19587-2017. The specific process is to weigh 1-8g of negative electrode active material (the sample weight should at least cover 1 / 3 of the volume of the sphere) and place it in a 1 / 2-inch long tube with a bulb (the diameter of the spherical part is 12mm). After pretreatment at 200℃ for 2h, place it in the test equipment TriStar3030 (Mike Company, USA) for testing. The adsorption gas used is N 2(Purity: 99.999%), the test conditions were carried out at 77K, and the specific surface area was tested by the BET calculation method.

[0106] 4. Tap density TD test method

[0107] Weigh 2g of negative electrode active material powder and put it all into a measuring cylinder. Then fix the measuring cylinder with powder on the instrument and vibrate it. After the vibration is finished, calculate the volume according to the height of the powder, and then calculate the tap density TD. The equipment used for the test is Dandong Better BT-301.

[0108] 5. Raman spectroscopy test of negative electrode active materials

[0109] A 50μm×50μm area was selected on the negative electrode active material layer, and the particles in the area were scanned using a laser microscopic confocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instrument Division) to obtain the D peak and G peak of all particles in the area. The data was processed using LabSpec software to obtain the peak intensities of the D peak and G peak of each particle, which are Id and Ig, respectively. The frequency of Id / Ig was counted with a step size of 0.02 to obtain a normal distribution graph, and the average value of Id / Ig was calculated, which is the peak intensity ratio of the D peak to the G peak in the Raman spectrum of the negative electrode active material, Id / Ig. The laser wavelength of the Raman spectrometer can be in the range of 532nm to 785nm. Among them, the D peak: generally at 1350cm -1 Nearby, by the sp 2 Symmetric stretching vibration of carbon atoms caused by radial breathing mode (structural defects); G peak: appears at 1575cm -1 Nearby, by sp 2 The stretching vibration between carbon atoms causes the vibration of E2g optical phonons (in-plane vibration of carbon atoms) at the center of the Brillouin zone.

[0110] 6. Thermogravimetric mass loss TG test of negative electrode active materials

[0111] The negative electrode active material was tested by thermal spectrometry-mass spectrometry (TG-MS). The equipment model was STA449F3-QMS403. The negative electrode active material sample was first dried in a vacuum drying oven at 80 °C for 2 h, and then the negative electrode active material sample was weighed on a balance to obtain the initial mass m 4 Then put the negative electrode active material sample into an aluminum crucible, and press the crucible cover and crucible together on a special tablet press. Pass nitrogen protective atmosphere and heat to 500℃, where the heating rate is 2℃ / min. After reaching 500℃, keep warm for 2min. Record the mass change of the negative electrode active material during the heating process. After cooling, weigh the mass of the remaining negative electrode active material sample m 5, and then through the formula (m 4 -m 5 ) / m 4 ×100% to obtain the mass loss TG of the negative electrode active material, and the position of the mass loss rate peak can be obtained through the mass change curve of the negative electrode active material.

[0112] 7. Granularity test

[0113] The particle size test method refers to GB / T 19077-2016. The specific process is to weigh 1g of the negative electrode active material sample and mix it evenly with 20mL of deionized water and a trace amount of dispersant. After placing it in an ultrasonic device for 5 minutes, the solution is poured into the sampling system Hydro 2000SM for testing. The test equipment used is the Mastersizer 3000 produced by Malvern. During the test, when the laser beam passes through the dispersed particle sample, the particle size measurement is completed by measuring the intensity of the scattered light. The data obtained from the test is used to analyze and calculate the particle size distribution that forms the scattering spectrum, including Dv50 and Dn10. The refractive index of the particles used in the test is 1.8. One sample is tested three times, and the particle size is finally taken as the average of the three tests.

[0114] 8. Negative electrode sheet porosity test

[0115] The negative electrode sheet was cut into small square samples of 10mm×10mm, and the sample mass was weighed. Then, it was placed in a true density tester (AccuPycⅡ1340). The test system was sealed, and nitrogen was introduced according to the degree. The pressure of the gas in the sample chamber and the expansion chamber was detected, and the true volume V was calculated according to Bohr's law (PV=nRT). 2 . Apparent volume V 1 =S×H (S is the surface area of ​​the sample, H is the thickness of the sample), and then calculate the porosity K, K=(V 1 -V 2 ) / V 1 ×100%

[0116] 9. Negative electrode compaction density test

[0117] Compacted density = mass / volume × 100%. Take the negative electrode sheet and punch it into small discs with a punching machine. The radius of the small disc is 1 cm. The thickness of the negative electrode active material layer is L cm. Weigh the mass of the small disc m. 6 g, then soak and clean the small disc with soapy water to remove the negative electrode active material layer, leaving only the clean small disc copper foil, and test the copper foil thickness L 1 , weigh the copper foil weight m 7 g, then according to the formula PD=(m 6 -m 7) / [(3.14×12)×(LL 1 )], repeat the operation 5 times and take the average value to obtain the compaction density PD of the negative electrode sheet.

[0118] 10. Boron element mass ratio W test

[0119] Boron mass ratio W test: Boron mass ratio test method test standard: GB / T 17359-2012, energy spectrum quantitative analysis. Test process: Under the standard experimental environment and test standard requirements, the negative electrode active material is placed in the scanning electron microscope sample chamber according to the standard operating process, the test position is magnified and observed using an acceleration voltage of 15kV, and the sample is analyzed qualitatively and quantitatively by an X-ray energy spectrum analyzer. The surface area is selected for scanning analysis, and the selected area size is 50μm×50μm to obtain the boron mass ratio W.

[0120] 11. Coating thickness X test

[0121] The coating thickness X is tested by SEM and EDS. First, the negative electrode sheet is sliced ​​and sampled by argon ion polishing (CP). The negative electrode sheet is cut perpendicular to the thickness direction of the negative electrode sheet to expose the section. Then the section is observed under SEM, and the elements of the section of the negative electrode sheet are analyzed by X-ray energy spectrum analyzer. The coating layer has a high content of nitrogen and boron elements, and the main element of the coating matrix material is C element. Based on this, the thickness of the coating layer can be measured. Select an image range of a suitable multiple (including 10-20 particle sections), select 10 particles, select 5 positions on each particle section, and measure the thickness of the coating layer at the corresponding positions. Calculate the average value of the thickness of the coating layer at the 5 positions to obtain the coating thickness of each particle, and then arithmetic average the coating thickness of the 10 particles to obtain the coating thickness X of the negative electrode active material.

[0122] 12. Test of equivalent diameter size y and average equivalent diameter size Y of boron nitride particles

[0123] The morphology and structure of the samples were analyzed using JEOL's JSM-6360LV scanning electron microscope and its supporting X-ray spectrometer. The negative electrode active material was placed in the scanning electron microscope sample chamber according to the standard operating procedures, and the test position was magnified and observed using an accelerating voltage of 15kV. The surface area was selected for scanning and analysis, and the area size was selected to be 50μm×50μm. A certain negative electrode active material particle was selected in this area to continue to be magnified and observed. When the magnification is large, the boron nitride particles on the surface of the negative electrode active material can be observed. Combined with the image software ImageJ, the number of boron nitride particles and the equivalent perimeter L of each boron nitride particle were counted within a range of 3μm×3μm on the surface of the negative electrode active material. 2, then y=L 2 / 3.14 is used to calculate the equivalent diameter y of each boron nitride particle, and the number of particles Z with y ≥ 200nm is counted. Then, the equivalent diameters of all the measured boron nitride particles are arithmetic averaged, and the result is the average equivalent diameter Y.

[0124] 13. Hot box temperature T test

[0125] First, the lithium-ion secondary battery is fully charged to 100% SOC (3.60V), and then placed in a hot box and heated at a heating rate of 5°C / min. After heating to T°C, the battery is kept warm for 2 hours. During the heating and insulation process, the battery does not smoke or catch fire. However, at T+1°C, the battery does smoke or catch fire during the heating and insulation process. It can be considered that the hot box temperature of the lithium-ion secondary battery is T°C.

[0126] Table 1 Negative electrode active material coating process parameters and hot box temperature test results of Examples 1-10 and Comparative Examples 1-3

[0127]

[0128]

[0129] Table 2 Test results of mass ratio and sphericity of boron element in negative electrode active materials of Examples 8 and 11 to 16 and hot box temperature

[0130]

[0131] Table 3 Thermogravimetric loss, defectivity and hot box temperature test results of negative electrode active materials of Examples 15 and 17 to 23 at 500°C

[0132]

[0133] Table 4 Specific surface area, particle size Dv50, Dn10 and hot box temperature test results of negative electrode active materials of Example 22 and Examples 24 to 33

[0134]

[0135]

[0136] Table 5 OI value, tap density and hot box temperature test results of negative electrode active materials of Example 31 and Examples 34 to 40

[0137]

[0138] Table 6 Test results of porosity, compaction density and hot box temperature of negative electrode sheets of Example 39 and Examples 41 to 50

[0139]

[0140] From Example 1 to Example 10 and the comparative example in Table 1, it can be found that when the negative electrode active material satisfies the following relationship, the hot box temperature T of the battery is significantly improved: the negative electrode active material is graphite coated with boron nitride (BN), the average equivalent diameter Y of the boron nitride particles satisfies: 20nm≤Y≤280nm, and the number ratio of particles with an equivalent diameter y≥200nm is Z, Z≤20%, in particular, the average equivalent diameter Y of the boron nitride particles satisfies: 30nm≤Y≤250nm, and the number ratio of particles with an equivalent diameter y≥200nm is Z≤15%, more particularly, the average equivalent diameter Y of the boron nitride particles satisfies: 100nm≤Y≤160nm, and the number ratio of particles with an equivalent diameter y≥200nm is Z≤10%.

[0141] The SEM image and EDS element distribution diagram of the negative electrode active material in Example 1 are as follows: Figure 1 shown.

[0142] When lithium-ion secondary batteries are at high temperatures, lithium ions embedded in the negative electrode graphite material will migrate, and lithium ions will migrate from the inside of the graphite to the surface. When the amount of migration is large enough, they will precipitate from the surface of the negative electrode plate, pierce the diaphragm or react with the electrolyte to intensify heat release, resulting in smoking and fire. The present application is to suppress the precipitation of lithium by covering the surface of the graphite particles with a layer of boron nitride nano-coating. Boron nitride has good insulation and thermal stability, and is also stable in the electrolyte and does not react with the electrolyte. By selecting a specific boron nitride particle size and a specific equivalent diameter ratio, boron nitride can be evenly covered on the graphite surface to obtain a coating with good consistency and uniformity. After meeting the above conditions, the hot box temperature T of the lithium-ion secondary battery is significantly improved.

[0143] When the thickness X of the coating layer satisfies: 20nm≤X≤180nm, especially the thickness X of the coating layer satisfies: 30nm≤X≤160nm, more especially the thickness X of the coating layer satisfies: 30nm≤X≤110nm, the thermal stability of the lithium-ion secondary battery can be further improved. In some embodiments of the present application, by selecting a specific boron nitride coating thickness, the coating strength is improved, the coating is prevented from breaking and falling off during the slurry processing, and the electronic conduction between particles and the conductivity of the negative electrode are improved.

[0144] From Example 11 to Example 16 in Table 2, it can be found that when the negative electrode active material satisfies the following relationship, the hot box temperature T of the battery is further improved: the mass proportion W of the boron element (B) in the negative electrode active material satisfies 0.2%≤W≤2.0%, and / or the ratio of W to the sphericity S of the negative electrode active material satisfies 0.002≤W / S≤0.025, especially W satisfies 0.3%≤W≤1.5%, and / or W / S satisfies 0.006≤W / S≤0.015, more especially W satisfies 0.6%≤W≤1.2%, and / or W / S satisfies 0.008≤W / S≤0.015.

[0145] In some embodiments of the present application, the hot box temperature can be further improved by adjusting the amount of coating agent, and the larger the sphericity S1, the smaller the amount of coating agent. The coating layer of boron nitride contains boron element (B), and the content of B element is also related to the thickness of the coating layer. The main factor affecting its thickness is the coating amount, and the amount of coating can be characterized by the mass ratio W of boron element in the negative electrode active material. The larger W means the more coating amount, and the smaller W means the less coating amount. In some embodiments of the present application, by selecting a specific mass ratio of boron element in the negative electrode active material, and / or the ratio of the mass ratio of boron element in the negative electrode active material to the sphericity S of the negative electrode active material satisfies 0.002≤W / S≤0.025, boron nitride can be effectively coated on the graphite surface, and the hot box temperature T of the lithium-ion secondary battery is further improved.

[0146] It can be found from Examples 17 to 23 in Table 3 that when the negative electrode active material satisfies the following relationship, the hot box temperature T of the battery is further improved:

[0147] (1) The negative electrode active material has a thermogravimetric mass loss TG at 500° C. that satisfies: 0.2% ≤ TG ≤ 2.5%, in particular, TG satisfies: 0.5% ≤ TG ≤ 2.5%, and more particularly, TG satisfies: 0.5% ≤ TG ≤ 2.0%; and / or

[0148] (2) The peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the condition: 0.20≤Id / Ig≤0.50, particularly Id / Ig satisfies 0.20≤Id / Ig≤0.45, and more particularly Id / Ig satisfies 0.20≤Id / Ig≤0.40.

[0149] That is, when one of the TG and Id / Ig index conditions is met or both index conditions are met at the same time, the hot box temperature T of the battery is further improved.

[0150] In some embodiments of the present application, the thermogravimetric loss TG and defectivity Id / Ig of the negative electrode active material at 500°C can be controlled by changing the proportion of the coating amount. As the amount of binder added to the coating increases, TG and Id / Ig increase.

[0151] Boron nitride is an inorganic substance, and it has no good bonding with graphite, and it is difficult to firmly adhere to the graphite surface. In order to solve the problem of stable bonding, in some embodiments of the present application, boron nitride is coated on the graphite surface by bonding with an adhesive. The adhesive has good affinity with graphite and can be stably fixed on the graphite surface after heat treatment. Through the characteristics of the adhesive, boron nitride can be effectively bonded to the surface of graphite particles. In some embodiments of the present application, the amount of adhesive can be characterized by the mass loss of thermogravimetric testing. By selecting a specific amount of adhesive, the boron nitride coating can effectively bond to the graphite surface, improving the first coulomb efficiency and thermal stability of the lithium-ion secondary battery. The defectivity of the negative active material is generally represented by the peak intensity ratio Id / Ig of the D peak and the G peak in the Raman spectrum. In some embodiments, by selecting a specific Id / Ig, the balance between the crystallinity of the negative active material and the diffusion of lithium ions on the particle surface is adjusted, so that the thermal stability of the lithium-ion secondary battery is further improved.

[0152] It can be found from Examples 24 to 33 in Table 4 that when the negative electrode active material satisfies the following relationship, the hot box temperature T of the battery is further improved:

[0153] (1) The specific surface area of ​​the negative electrode active material meets the BET requirement of 0.5 m 2 / g≤BET≤3.0m 2 / g, especially the specific surface area BET meets: 0.8m 2 / g≤BET≤3.0m 2 / g, more particularly, the specific surface area BET satisfies: 1.0m 2 / g≤BET≤2.5m 2 / g; and / or

[0154] (2) the particle size of the negative electrode active material satisfies: 5.0 μm ≤ Dv50 ≤ 20.0 μm, in particular the particle size of the negative electrode active material satisfies: 8.0 μm ≤ Dv50 ≤ 20.0 μm, more particularly the particle size of the negative electrode active material satisfies: 8.0 μm ≤ Dv50 ≤ 16.0 μm; and / or

[0155] (3) The particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 0.9 μm, and in particular, the particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 1.5 μm.

[0156] That is, when one, two or three of the specific surface area BET, Dv50 and Dn10 are met, the hot box temperature T of the lithium ion secondary battery is further improved.

[0157] The specific surface area BET and particle size of the negative electrode active material also affect the thermal stability of the lithium-ion secondary battery. Among them, Dv50 is one of the indicators of particle size. The smaller the particle size of the negative electrode active material, the larger the contact area with the electrolyte under the same mass. Dn10 is an indicator of the fine powder content in the particles. The smaller the Dn10, the more small particles in the negative electrode active material. In some embodiments of the present application, by selecting the specific surface area BET, Dv50 and Dn10 of a specific negative electrode active material, the coating of boron nitride can form a SEI film with good thermal stability. It can be found from Table 4 that when the above conditions are met, the thermal stability of the battery is improved.

[0158] It can be found from Examples 34 to 40 in Table 5 that when the negative electrode active material satisfies the following relationship, the hot box temperature T of the lithium ion secondary battery is further improved:

[0159] (1) The OI value of the negative electrode active material satisfies: OI value ≤ 15, in particular, the OI value satisfies: OI value ≤ 10, and more particularly, the OI value satisfies: OI value ≤ 8; and / or

[0160] (2) The tap density TD of the negative electrode active material satisfies: TD ≥ 0.5 g / cm 3 , especially TD meets: TD ≥ 0.6g / cm 3 , TD meets: TD ≥ 0.8g / cm 3 , more specifically TD meets: 1.3g / cm 3 ≥TD≥0.8g / cm 3 .

[0161] That is, when one of the OI value and the tap density TD is met or both of the two index conditions are met at the same time, the hot box temperature T of the battery is further improved.

[0162] The crystal orientation of the negative electrode active material is generally expressed by the OI value, and the crystal orientation will affect the lithium insertion direction of the lithium ions in the negative electrode active material. In some embodiments of the present application, by selecting the OI value of a specific negative electrode active material, the lithium insertion speed and rate performance are improved. The tap density TD of the negative electrode active material will affect the processing properties of the material, especially the surface-treated material. In some embodiments of the present application, by selecting the tap density TD of a specific negative electrode active material, the coating quality of the negative electrode slurry is improved and the performance of the battery is optimized. It can be found from Table 5 that when the above conditions are met, the thermal stability of the battery is improved.

[0163] It can be found from Examples 41 to 50 in Table 6 that when the negative electrode plate satisfies the following relationship, the hot box temperature T of the lithium-ion secondary battery is further improved:

[0164] (1) The porosity K of the negative electrode sheet satisfies: 15% ≤ K ≤ 40%, in particular, the porosity K satisfies: 18% ≤ K ≤ 40%, and more particularly, the porosity K satisfies: 18% ≤ K ≤ 35%; and / or

[0165] (2) The compaction density PD of the negative electrode sheet meets the following requirements: 1.20 g / cm 3 ≤PD≤1.80g / cm 3 , especially the compaction density PD meets: 1.30g / cm 3 ≤PD≤1.75g / cm 3 , especially the compaction density PD meets: 1.35g / cm 3 ≤PD≤1.70g / cm 3 .

[0166] That is, when one of the index conditions of the porosity K and the compaction density PD of the negative electrode sheet is met or both index conditions are met at the same time, the hot box temperature of the battery is further improved.

[0167] The porosity of the negative electrode sheet will directly affect the ability of the sheet to store electrolyte. In some embodiments of the present application, by selecting a specific negative electrode sheet porosity, the high temperature storage and high temperature cycle performance of the battery are improved. The compaction density of the negative electrode sheet will affect the processing stability of the sheet and the stability of the negative electrode active material. In some embodiments of the present application, by selecting a specific compaction density of the negative electrode sheet, the first coulomb efficiency and cycle performance are optimized, the demolding of the negative electrode sheet is reduced, and the battery energy density is improved. It can be found from Table 6 that when the above conditions are met, the hot box temperature of the lithium-ion secondary battery is also improved.

[0168] The above-mentioned implementation modes are only preferred implementation modes of the present application and cannot be used to limit the scope of protection of the present application. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present application shall fall within the scope of protection required by the present application.

Claims

1. A negative electrode active material, characterized in that: The negative electrode active material is a graphite material having a coating layer on the surface, and the coating layer contains boron nitride particles; The average equivalent diameter Y of the boron nitride particles satisfies: 20nm≤Y≤280nm, and the number ratio of particles with an equivalent diameter y≥200nm is Z, and Z≤20%.

2. The negative electrode active material according to claim 1, characterized in that The thickness X of the coating layer satisfies: 20nm≤X≤180nm; preferably, 30nm≤X≤160nm; more preferably, 30nm≤X≤110nm.

3. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following conditions: (1) 30nm≤Y≤250nm; preferably, 100nm≤Y≤160nm; (2) Z≤15%; preferably, Z≤10%.

4. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following conditions: (3) The mass ratio W of the boron element in the negative electrode active material satisfies 0.2%≤W≤2.0%; preferably, 0.3%≤W≤1.5%; more preferably, 0.6%≤W≤1.2%; (4) The ratio of W to the sphericity S of the negative electrode active material satisfies 0.002≤W / S≤0.025; preferably, 0.006≤W / S≤0.015; more preferably, 0.008≤W / S≤0.

015.

5. The negative electrode active material according to claim 1, characterized in that The thermogravimetric mass loss TG of the negative electrode active material at 500° C. satisfies: 0.2%≤TG≤2.5%; preferably, 0.5%≤TG≤2.5%; more preferably, 0.5%≤TG≤2.0%.

6. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following conditions: (5) The specific surface area BET of the negative electrode active material satisfies: 0.5 m 2 / g≤BET≤3.0m 2 / g; preferably, 0.8m 2 / g≤BET≤3.0m 2 / g; more preferably, 1.0m 2 / g≤BET≤2.5m 2 / g; (6) The particle size of the negative electrode active material satisfies: 5.0 μm ≤ Dv50 ≤ 20.0 μm; preferably, 8.0 μm ≤ Dv50 ≤ 20.0 μm; more preferably, 8.0 μm ≤ Dv50 ≤ 16.0 μm; (7) The particle size of the negative electrode active material satisfies: 3.0 μm ≥ Dn10 ≥ 0.9 μm; more preferably, 3.0 μm ≥ Dn10 ≥ 1.5 μm.

7. The negative electrode active material according to claim 1, characterized in that The negative electrode active material satisfies at least one of the following conditions: (8) The OI value of the negative electrode active material satisfies: OI value ≤ 15; preferably, OI value ≤ 10; more preferably, OI value ≤ 8; (9) The tap density TD of the negative electrode active material satisfies: TD ≥ 0.5 g / cm 3 ; Preferably, TD ≥ 0.6 g / cm 3 More preferably, 1.3 g / cm 3 ≥TD≥0.8g / cm 3 ; (10) The peak intensity ratio Id / Ig of the D peak to the G peak in the Raman spectrum of the negative electrode active material satisfies the following conditions: 0.20≤Id / Ig≤0.50; preferably, 0.20≤Id / Ig≤0.45; more preferably, 0.20≤Id / Ig≤0.

40.

8. A lithium ion secondary battery, characterized in that: The lithium-ion secondary battery comprises a positive electrode sheet, an electrolyte, a separator and a negative electrode sheet; the negative electrode sheet comprises a negative electrode active material as described in any one of claims 1 to 7.

9. The lithium ion secondary battery according to claim 8, characterized in that: The porosity K of the negative electrode sheet satisfies: 15%≤K≤40%; and / or The compaction density PD of the negative electrode sheet satisfies: 1.20 g / cm 3 ≤PD≤1.80g / cm 3 ; Preferably, 18%≤K≤40%; more preferably, 18%≤K≤35%; Preferably, 1.30 g / cm 3 ≤PD≤1.75g / cm 3 More preferably, 1.35 g / cm 3 ≤PD≤1.70g / cm 3 .

10. An electronic device, characterized in that: The electronic device comprises the lithium ion secondary battery according to any one of claims 8 to 9.

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