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 comprises a graphite material with calcium fluoride (CaF2) particles on the surface, and the mass ratio X of the CaF2 particles in the raw materials of the negative electrode active material satisfies 0.15% < = X < = 2.0%; the quantity proportion Y of the particles with the equivalent diameter D smaller than or equal to 500 nm in the CaF2 particles is larger than or equal to 50%. The first coulombic efficiency of the lithium ion secondary battery containing the negative electrode active material is improved by regulating and controlling the mass ratio of the CaF2 particles in the negative electrode active material and the quantity ratio of the particles with the equivalent diameter D smaller than or equal to 500 nm in the CaF2 particles.

Description

Technical Field

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

[0002] Lithium-ion secondary battery is a battery with high energy density. Compared with lead-acid battery and nickel-metal hydride battery, lithium-ion secondary battery has higher energy density and better cycle life. Its comprehensive performance advantage is obvious, but its cost is much higher than that of lead-acid battery and nickel-metal hydride battery, which also limits the application of lithium-ion secondary battery in some scenarios. Among the material costs of lithium-ion secondary batteries, negative electrode active materials account for a large proportion, which also directly affects the price of lithium-ion secondary batteries. The first coulomb efficiency of lithium-ion secondary battery is directly related to the energy density and cost of the battery. The high first coulomb efficiency means that the energy density of lithium-ion secondary battery is higher. In addition, the high first coulomb efficiency means that the negative electrode active material has a high capacity. Under the same battery capacity design, it requires less negative electrode active materials, which can reduce costs. In order to achieve the above purpose, the current main methods are to increase the gram capacity of the positive electrode and increase the battery voltage, but these methods put forward higher requirements for battery safety design, especially high-temperature storage safety under full charge, which may bring additional cost increases. Summary of the invention

[0003] In view of the above problems in the prior art, the present application realizes a high first coulombic efficiency of lithium-ion secondary batteries from the perspective of negative electrode active materials. This solution can not only improve the energy density of the battery but also reduce the cost per unit energy of the battery without changing the battery design.

[0004] A first aspect of the present application provides a negative electrode active material, which includes a graphite material having calcium fluoride particles on the surface.

[0005] In some embodiments of the present application, the mass proportion of the calcium fluoride particles in the negative electrode active material is X, 0.15%≤X≤2.0%; and the number ratio of particles with an equivalent diameter D≤500nm in the calcium fluoride particles is Y, Y≥50%.

[0006] In some embodiments of the present application, the mass proportion X of the calcium fluoride particles in the negative electrode active material satisfies 0.5%≤X≤2.0%.

[0007] In some embodiments of the present application, the mass proportion X of the calcium fluoride particles in the negative electrode active material satisfies 0.5%≤X≤1.0%.

[0008] In some embodiments of the present application, the number ratio Y of particles with an equivalent diameter D≤500 nm in the calcium fluoride particles satisfies Y≥55%.

[0009] In some embodiments of the present application, the number ratio Y of particles with an equivalent diameter D≤500 nm in the calcium fluoride particles satisfies Y≥60%.

[0010] In some embodiments of the present application, the negative electrode active material has an oxygen element, and the mass ratio of the oxygen element in the negative electrode active material is Z, 0.15%≤Z≤2.0%.

[0011] In some embodiments of the present application, the mass ratio Z of the oxygen element in the negative electrode active material satisfies 0.5%≤Z≤2.0%.

[0012] In some embodiments of the present application, the mass ratio Z of the oxygen element in the negative electrode active material satisfies 1.0%≤Z≤2.0%.

[0013] In some embodiments of the present application, the thermogravimetric mass loss of the negative electrode active material at 600° C. is TG, 0.2%≤TG≤1.5%.

[0014] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 600° C. satisfies 0.8%≤TG≤1.5%.

[0015] In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 600° C. satisfies 1.2%≤TG≤1.5%.

[0016] In some embodiments of the present application, the peak ratio of the D peak to the G peak in the Raman spectrum of the negative electrode active material is Id / Ig, 0.10≤Id / Ig≤0.45; preferably, 0.15≤Id / Ig≤0.45; more preferably, 0.15≤Id / Ig≤0.40.

[0017] In some embodiments of the present application, the oil absorption value of the negative electrode active material is S, 20 mL / 100 g≤S≤70 mL / 100 g.

[0018] In some embodiments of the present application, the oil absorption value S of the negative electrode active material satisfies 25 mL / 100 g≤S≤65 mL / 100 g.

[0019] In some embodiments of the present application, the oil absorption value S of the negative electrode active material satisfies 30 mL / 100 g≤S≤65 mL / 100 g.

[0020] In some embodiments of the present application, the specific surface area of ​​the negative electrode active material is BET, 0.2 m2 / g≤BET≤3.0m 2 / g; preferably, 0.5m 2 / g≤BET≤2.5m 2 / g; more preferably, 1.0m 2 / g≤BET≤2.5m 2 / g.

[0021] In some embodiments of the present application, the tap density of the negative electrode active material is TD, TD≥0.6 g / cm 3 .

[0022] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies 1.3 g / cm 3 ≥TD≥0.7g / cm 3 .

[0023] 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 the negative electrode active material described in the first aspect of the present application.

[0024] In some embodiments of the present application, the OI value of the negative electrode plate satisfies: OI value ≤ 20; preferably, 5 ≤ OI value ≤ 16; more preferably, 6 ≤ OI value ≤ 13.

[0025] In some embodiments of the present application, the porosity of the negative electrode sheet is H, 20%≤H≤40%; preferably, 25%≤H≤40%; more preferably, 30%≤H≤40%.

[0026] In some embodiments, a negative electrode sheet containing the negative electrode active material, a positive electrode, an electrolyte, and a separator-composed lithium-ion secondary battery has excellent first coulombic efficiency, and its first coulombic efficiency F satisfies: F≥92.5%.

[0027] A third aspect of the present application provides an electronic device, which includes the lithium-ion secondary battery described in the second aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The SEM image and EDS element distribution diagram of the negative electrode active material in Example 1.

[0029] Figure 2 This is a comparison chart of the first coulombic efficiency data of the lithium-ion secondary batteries in Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0030] 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.

[0031] 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.

[0032] 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).

[0033] 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.

[0034] In the present application, a "binder" is used to prepare the graphite material having calcium fluoride on the surface.

[0035] The first aspect of the present application provides a negative electrode active material, the negative electrode active material comprising a graphite material having calcium fluoride particles on the surface. In some embodiments of the present application, the mass proportion of the calcium fluoride particles in the negative electrode active material is X, 0.15%≤X≤2.0%. In some embodiments of the present application, the mass proportion X of the calcium fluoride particles in the negative electrode active material satisfies 0.5%≤X≤2.0%. In some embodiments of the present application, the mass proportion X of the calcium fluoride particles in the negative electrode active material satisfies 0.5%≤X≤1.0%. Specifically, the mass proportion X of the calcium fluoride particles in the negative electrode active material can be 0.15%, 0.18%, 0.2%, 0.45%, 0.5%, 0.7%, 0.8%, 0.88%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.68%, 1.8%, 2.0% or a range consisting of any two of the above values; and the number ratio of particles with an equivalent diameter D≤500nm in the calcium fluoride particles is Y, Y≥50%. In some embodiments of the present application, the number ratio Y of particles with an equivalent diameter D≤500nm in the calcium fluoride particles satisfies Y≥55%. In some embodiments of the present application, the number ratio Y of particles with an equivalent diameter D≤500nm in the calcium fluoride particles satisfies Y≥60%. In the present application, by selecting a specific CaF 2 Mass fraction of particles and CaF with a specific equivalent diameter 2 The particle quantity ratio can effectively reduce the consumption of active lithium ions during subsequent formation and significantly improve the initial coulombic efficiency of lithium-ion secondary batteries.

[0036] In some embodiments of the present application, the negative electrode active material has an oxygen element, and the mass ratio of the oxygen element in the negative electrode active material is Z, 0.15%≤Z≤2.0%. In some embodiments of the present application, the mass ratio Z of the oxygen element in the negative electrode active material satisfies 0.5%≤Z≤2.0%. In some embodiments of the present application, the mass ratio Z of the oxygen element in the negative electrode active material satisfies 1.0%≤Z≤2.0%. Specifically, the mass ratio Z of the oxygen element in the negative electrode active material can be 0.15%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.3%, 1.5%, 1.9%, 2.0% or a range consisting of any two of the above values. By limiting the mass ratio Z of the oxygen element in the negative electrode active material, the amorphous carbon content in the negative electrode active material is controlled, and an ideal bonding effect is achieved, so that the first coulomb efficiency of the lithium-ion secondary battery is further improved.

[0037] In some embodiments of the present application, the thermogravimetric mass loss of the negative electrode active material at 600°C is TG, 0.2%≤TG≤1.5%. In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 600°C satisfies 0.8%≤TG≤1.5%. In some embodiments of the present application, the thermogravimetric mass loss TG of the negative electrode active material at 600°C satisfies 1.2%≤TG≤1.5%. Specifically, the thermogravimetric mass loss TG of the negative electrode active material at 600°C may be 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, or a range consisting of any two of the above values.

[0038] In some embodiments of the present application, the peak intensity ratio of the D peak to the G peak in the Raman spectrum of the negative electrode active material is Id / Ig, 0.10≤Id / Ig≤0.45; preferably, 0.15≤Id / Ig≤0.45; more preferably, 0.15≤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 0.1, 0.15, 0.2, 0.25, 0.26, 0.3, 0.35, 0.4, 0.45 or a range consisting of any two of the above values.

[0039] In some embodiments of the present application, the oil absorption value S of the negative electrode active material satisfies 25 mL / 100 g ≤ S ≤ 65 mL / 100 g. In some embodiments of the present application, the oil absorption value S of the negative electrode active material satisfies 30 mL / 100 g ≤ S ≤ 65 mL / 100 g. Specifically, the oil absorption value S of the negative electrode active material can be 20mL / 100g, 23mL / 100g, 25mL / 100g, 30mL / 100g, 35mL / 100g, 36mL / 100g, 40mL / 100g, 45mL / 100g, 50mL / 100g, 55mL / 100g, 60mL / 100g, 65mL / 100g, 70mL / 100g or a range consisting of any two of the above values.

[0040] In some embodiments of the present application, the specific surface area of ​​the negative electrode active material is BET, 0.2 m 2 / g≤BET≤3.0m 2 / g; preferably, 0.5m 2 / g≤BET≤2.5m 2 / g; more preferably, 1.0m 2 / g≤BET≤2.5m 2 / g. Specifically, the BET specific surface area of ​​the negative electrode active material can be 0.2m 2 / g, 0.3m 2 / g, 0.4m 2 / g, 0.5m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.6m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.5m 2 / g, 3.0m 2 / g or a range consisting of any two of the above values.

[0041] In some embodiments of the present application, the tap density TD of the negative electrode active material satisfies: TD ≥ 0.6 g / cm 3 ; Preferably, 1.3g / cm 3 ≥TD≥0.7g / cm 3 Specifically, the tap density TD of the negative electrode active material may be 0.6 g / cm 3 , 0.7g / cm 3 , 0.8g / cm 3 , 0.9g / cm 3 , 1.0g / cm 3 , 1.1g / cm 3 , 1.2g / cm 3 , 1.3g / cm 3 Or a range consisting of any two of the above values.

[0042] 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 and the negative electrode active material described in the first aspect of the present application.

[0043] In some embodiments of the present application, the OI value of the negative electrode plate satisfies: OI value ≤ 20; preferably, 5 ≤ OI value ≤ 16; more preferably, 6 ≤ OI value ≤ 13. Specifically, the OI value of the negative electrode plate can be 5, 6, 8, 10, 13, 15, 16, 18, 20 or a range consisting of any two of the above values.

[0044] In some embodiments of the present application, in some embodiments of the present application, the porosity H of the negative electrode sheet satisfies: 20% ≤ H ≤ 40%. In some embodiments of the present application, the porosity H of the negative electrode sheet satisfies: 25% ≤ H ≤ 40%. In some embodiments of the present application, the porosity H of the negative electrode sheet satisfies: 30% ≤ H ≤ 40%. Specifically, the porosity H of the negative electrode sheet can be 20%, 22%, 24%, 25%, 26%, 27%, 28%, 30%, 35%, 36%, 38%, 40% or a range consisting of any two of the above values.

[0045] A third aspect of the present application provides an electronic device, which includes the lithium-ion secondary battery.

[0046] The present application provides a method for preparing a negative electrode active material and a negative electrode sheet: the coating matrix material is a graphite material, the graphite material includes at least one of artificial graphite and natural graphite, and the artificial graphite is preferably primary particle artificial graphite. CaF 2 The material has nanometer size, CaF 2 The particle equivalent diameter D of the material is less than 700nm. 2 The negative electrode active material is prepared by a hydrothermal method, wherein the binder can be selected from organic matter having affinity with graphite, such as lithium polyacrylate, polyacrylic acid (PAA), modified polyaniline (such as polyaniline sulfate), polypyrrole, sodium carboxymethyl cellulose or styrene-butadiene rubber emulsion. The specific preparation method is as follows: firstly, a graphite material is selected and its mass is weighed as m 1 , then weigh CaF 2 The mass of the material is m 2 , then take an appropriate amount of adhesive and weigh the mass of the adhesive as m 3 . Graphite materials, CaF 2 Materials and adhesives are raw materials for the preparation of negative electrode active materials, graphite materials, CaF 2 The sum of the mass of the material and the adhesive is m, where m 2 The proportion m of the total mass m 2 / m×100% is 0.1% to 3.0%, m 3 The proportion of the total mass m 3 / m×100% is 0.5% to 10.0%. The above three materials are placed together in a rotary evaporator, and sufficient deionized water is added to mix evenly. The mass of deionized water is 20 to 60 times the mass of the raw materials for preparing the negative electrode active material. For some adhesives with poor solubility, hydrochloric acid can be appropriately added to promote their dissolution. Then start to slowly heat to 80 to 200°C while stirring, continue stirring until the water is evaporated, 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 300 to 500°C, keep it warm for 2 to 5 hours, and take it out after cooling to obtain the negative electrode active material. The negative electrode active material, conductive carbon black additive, styrene butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC) thickener are mixed in a certain ratio, wherein the mass ratio of the additive is 0.5-1.5%, the mass ratio of the negative electrode active material is 94.8%-97%, the mass ratio of the binder styrene butadiene rubber (SBR) is 1.2%-2.2%, and the mass ratio of the thickener sodium carboxymethyl cellulose (CMC) is 0.5%-1.5%, and the mass ratio of the four components adds up to 100%, and then add to an appropriate amount of deionized water and stir and mix thoroughly to form a uniform negative electrode slurry, and apply the slurry on the negative electrode current collector copper foil, dry, and cold press to obtain a negative electrode sheet.

[0047] The present application is described in more detail below through examples.

[0048] Example 1

[0049] Preparation of negative electrode active materials

[0050] The coating matrix is ​​made of primary particle artificial graphite material. CaF 2 The average equivalent diameter of the material particles is 200nm, and the adhesive is lithium polyacrylate. The mass of primary artificial graphite is m 1 is 10.0g, and then weigh CaF 2 Material mass m 2 is 0.15g, and then take the mass of lithium polyacrylate m 3 The total weight of the mixture is 1.0 g. The three are placed together in a rotary evaporator, and deionized water is added to mix evenly. The mass of the deionized water is 300 g. Then slowly heat to 90 ° C while stirring, continue stirring until the water evaporates, and take out the mixture after cooling. The mixture is placed in a box furnace for heat treatment, nitrogen is passed through, and the heat treatment is heated to 450 ° C, kept warm for 3 hours, and taken out after cooling to obtain the negative electrode active material.

[0051] Preparation of negative electrode

[0052] 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 negative electrode slurry is coated on the negative electrode current collector copper foil, dried, and cold pressed to obtain a negative electrode sheet.

[0053] Preparation of positive electrode

[0054] The positive electrode is lithium iron phosphate (LiFePO 4 ) is used as the positive electrode active material, which is fully stirred and mixed with the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) in a proper amount of N-methylpyrrolidone (NMP) solvent in a weight ratio of 96.3:2.2:1.5 to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector Al foil, dried and cold pressed to obtain a positive electrode sheet.

[0055] Preparation of electrolyte

[0056] In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC=1:3:3:3 to obtain a solvent, and then 1,3-propane sultone was added, 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 proportion of isocyanate is 11.5%, and the mass proportion of 1,3-propane sultone is 3%.

[0057] Assembly of lithium-ion secondary batteries

[0058] 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.

[0059] Example 2 to Example 10, Comparative Example 1 to Comparative Example 5

[0060] The difference between Examples 2 to 10 and Comparative Examples 1 to 5 and Example 1 is that the mass proportion of calcium fluoride particles and the number proportion of particles with an equivalent diameter D≤500nm in the calcium fluoride particles are adjusted by the common industry method: including CaF 2The parameters of the mass of the particles, the mass of the binder lithium polyacrylate, the rotary evaporation temperature, the heat treatment temperature, and the heat treatment time were adjusted to obtain the corresponding negative electrode active material powder, as shown in Table 1.

[0061] Example 11 to Example 15

[0062] The difference between Examples 11 to 15 and Example 5 is that the mass proportion of oxygen in the negative electrode active material can be adjusted in a common way in the industry, including the type and mass proportion of the binder and the parameter adjustment of the heat treatment temperature, so that the negative electrode active material powder can meet the preferred characteristics, as shown in Table 2.

[0063] Example 16 to Example 21

[0064] The difference between Examples 16 to 21 and Example 12 is that the 600°C thermogravimetric loss and defectivity of the negative electrode active material can be adjusted by common industry adjustments, including the mass ratio of the binder, the type of the binder and the heat treatment temperature, so that the negative electrode active material powder can meet the preferred characteristics, as shown in Table 3.

[0065] Example 22 to Example 35

[0066] The difference between Examples 22 to 35 and Example 20 is that the oil absorption value, specific surface area and tap density can be adjusted in the industry in common ways, including the degree of particle shaping, particle size and surface coating amount, so that the negative electrode active material powder can meet the preferred characteristics, as shown in Table 4.

[0067] Example 36 to Example 46

[0068] The difference between Examples 36 to 46 and Example 34 is that the OI value and porosity of the negative electrode plate can be adjusted by adjusting the battery plate design, including the compaction density of the plate and the parameter adjustment of the material morphology, so that the negative electrode active material powder can meet the preferred characteristics, as shown in Table 5.

[0069] Test Method

[0070] The negative electrode materials, assembled negative electrode plates and lithium-ion secondary batteries in the above embodiments were subjected to the following performance tests, and the test results are shown in Tables 1 to 5.

[0071] 1. Porosity H test of negative electrode sheet

[0072] Cut the negative electrode into small square samples of 10mm×10mm and weigh the sample mass. Then put it in the true density test instrument (AccuPycⅡ1340), seal the test system, introduce nitrogen according to the degree, detect the pressure of the gas in the sample chamber and the expansion chamber, and then calculate the true volume V according to Bohr's law (PV=nRT). 2 . Apparent volume V 1 =S 1 ×L(S 1 is the surface area of ​​the sample, L is the thickness of the sample), and then calculate the porosity H, H = (V 1 -V 2 ) / V 1 ×100%.

[0073] 2. OI value test of negative electrode

[0074] According to the mechanical industry standard JB / T 4220-2011 of the People's Republic of China "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 layer 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 layer is calculated to be the OI value of the negative electrode sheet.

[0075] 3. Specific surface area

[0076] 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.

[0077] 4. Tap density test method

[0078] 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. The equipment used for the test is Dandong Better BT-301.

[0079] 5. Raman spectroscopy test of negative electrode active materials

[0080] A 100μm×100μm 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 length of 0.02 to obtain a normal distribution graph, and the average value of Id / Ig was calculated, which is the ratio of the D peak to the G peak intensity 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 It is caused by the stretching vibration between carbon atoms, which corresponds to the vibration of E2g optical phonon in the center of the Brillouin zone (in-plane vibration of carbon atoms).

[0081] 6. Thermogravimetric mass loss TG test of negative electrode active materials

[0082] 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 600℃, where the heating rate is 2℃ / min. After reaching 600℃, 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% can be used 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.

[0083] 7. Oil absorption value S test of negative electrode active material

[0084] Reference standard: GB / T 3780.2-2007 Determination of oil absorption value. The oil absorption value S of the negative electrode active material is measured by the oil absorption volume per unit mass of the negative electrode active material, and the value is expressed in mL / 100g. Calculation formula: S = V / m 6 ×100, V represents the volume of consumed oil, in milliliters (mL), m 6 It is the value of the mass of the negative electrode active material in grams (g). The equipment parameters are DABS-H oil absorption meter, and the temperature range is 23±5℃.

[0085] 8. Test of the mass ratio of calcium fluoride particles in the negative electrode active material

[0086] Test of the mass proportion of calcium fluoride particles in the negative electrode active material: Test method for Ca element percentage and mass ratio Test standard: GB / T 17359-2012, quantitative analysis by energy spectrum method. 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, and the test position is magnified and observed using an acceleration voltage of 15kV, and the negative electrode active material sample is subjected to elemental qualitative and quantitative analysis using an X-ray energy spectrum analyzer. Select a surface area for scanning analysis, and the size of the selected surface area is 100μm×100μm. Obtain the Ca element mass ratio m 7 Afterwards, CaF 2 Calculate the mass percentage of Ca in the molecular mass of CaF 2 The mass ratio of particles in the negative electrode active material is X, X = m 7 / (40 / 78)×100%.

[0087] 9. SEM test and calculation of Y

[0088] The morphology and structure of the samples were analyzed using a JEOL JSM-6360LV scanning electron microscope and its matching X-ray spectrometer. During use, the sample of the negative electrode active material was evenly coated on a conductive tape, then glued to the sample stage and placed in the electron microscope for testing. The SEM showed that CaF 2 Particles, combined with the image software ImageJ, the image software can count the CaF 2 The number of particles and the equivalent diameter size, when selecting the SEM image area for image processing, the magnification is selected as 10,000 times. At this magnification, 5 areas on the graphite surface are photographed, the area size is 3μm×3μm, and the CaF in each area is calculated by the image software. 2The number ratio of particles with equivalent diameter D≤500nm in the particles is calculated by averaging the number ratios of the five regions to obtain Y, where D is the equivalent diameter of the particles. The circumference of the particles is calculated using imaging software, and then D is obtained by dividing the circumference by π.

[0089] 10. Test of the mass ratio Z of oxygen element in negative electrode active material

[0090] Test of the mass ratio Z of oxygen in negative electrode active materials: Test method for the mass ratio of oxygen in negative electrode active materials Test standard: GB / T 17359-2012, quantitative analysis by energy spectrum method. Test process: Under the standard experimental environment and test standard requirements, the negative electrode active material is placed in the sample chamber of the scanning electron microscope according to the standard operating process, and the test position is magnified and observed using an accelerating voltage of 15kV, and the sample is subjected to elemental qualitative and quantitative analysis using an X-ray energy spectrum analyzer. Select a surface area for scanning analysis, and select a surface area size of 100μm×100μm to obtain the mass ratio Z of oxygen in the negative electrode active material.

[0091] 11. First Coulomb efficiency F test

[0092] The lithium-ion secondary battery after injection is formed:

[0093] Temperature: 45℃

[0094] 1.0.02CC 10min, charging capacity C 1 mAh;

[0095] 2.---Rest 10min;

[0096] 3.0.2C CC 60min, charging capacity C 2 mAh;

[0097] 4.---Rest 10min;

[0098] 5.0.5C CC to 3.6V,CV to 0.05C,Charging capacity C 3 mAh;

[0099] 6.0.5C DC to 2.5V, discharge capacity D 1 mAh;

[0100] The first coulombic efficiency of lithium-ion secondary batteries is F = D 1 / (C 1 +C 2 +C 3 )×100%

[0101] Table 1 Negative electrode active material coating process parameters and first coulombic efficiency test results of Examples 1 to 10 and Comparative Examples 1 to 5

[0102]

[0103] Table 2 Results of the mass ratio test of oxygen element in the negative electrode active material and the first coulombic efficiency test of Example 5 and Examples 11 to 15

[0104]

[0105] Table 3 Thermogravimetric loss TG at 600°C, peak intensity ratio Id / Ig of D peak to G peak of the negative electrode active materials of Example 12 and Examples 16 to 21, and the first coulombic efficiency test results

[0106]

[0107] Table 4 Oil absorption value, specific surface area, tap density and first coulombic efficiency test results of negative electrode active materials of Example 20 and Examples 22 to 34

[0108]

[0109] Table 5 OI value, porosity and first coulombic efficiency test results of negative electrode active materials of Examples 34 and Examples 36 to 46

[0110]

[0111]

[0112] From Examples 1 to 10 in Table 1, it can be found that the negative electrode active material satisfies: the negative electrode active material is calcium fluoride (CaF 2 ) coated graphite material, CaF 2 The mass proportion X in the negative electrode active material satisfies 0.15%≤X≤2.0%, particularly 0.5%≤X≤2.0%, and more particularly 0.5%≤X≤1.0%; the CaF 2 When the number ratio of particles with equivalent diameter D≤500nm in the particles is Y≥50%, especially Y≥55%, and more especially Y≥60%, the first coulombic efficiency of the lithium ion secondary battery is significantly improved. When comparative examples 1 to 5 do not meet the above conditions, the first coulombic efficiency of the corresponding lithium ion secondary battery is low.

[0113] The SEM image and EDS element distribution diagram of the negative electrode active material in Example 1 are as follows: Figure 1 As shown, the EDS image shows the element distribution, and F and Ca elements are evenly distributed on the surface of graphite particles.

[0114] In the negative electrode of the lithium-ion secondary battery, the deintercalation of lithium ions occurs in the graphite particles. In addition, during the initial formation of the lithium-ion battery, a layer of solid electrolyte (SEI) is formed on the surface of the negative electrode material graphite. The composition of SEI includes LiF / Li 2 CO 3 The formation process of various inorganic salts will consume active lithium ions, resulting in the loss of the first coulomb efficiency. The inorganic salts in the SEI film have the effect of electronic insulation and ion conduction, and the ion conduction is achieved through the gaps and grain boundaries of the inorganic salts; based on this, in this application, by coating CaF on the surface of graphite 2 Nanoparticles are used to form inorganic salts in the SEI film on the graphite surface, CaF 2 Nanoparticles have good electronic insulation, and they have more grain boundaries and more pores where the particles are piled up, which are very similar to the characteristics of inorganic components in the SEI film and can play the role of SEI film. 2 Mass fraction of particles and CaF with a specific equivalent diameter 2 Particle number ratio, CaF 2 The coating can effectively play the role of SEI film, reduce the consumption of active lithium ions during subsequent formation, and improve the first coulombic efficiency of lithium-ion secondary batteries. When the above two conditions are met at the same time, the first coulombic efficiency of the battery is significantly improved.

[0115] It can be found from Examples 11 to 15 in Table 2 that when the negative electrode active material satisfies: the mass ratio of the oxygen element in the negative electrode active material satisfies 0.15%≤Z≤2.0%, particularly satisfies 0.5%≤Z≤2.0%, and more particularly satisfies 1.0%≤Z≤2.0%, the first coulombic efficiency of the lithium ion secondary battery is further improved.

[0116] CaF in negative electrode active materials 2 Nanoparticles are inorganic substances, and there is only a small van der Waals force between them and graphite, but this force is not enough to make CaF 2 The nanoparticles are stably coated on the graphite surface. In order to solve this problem, in some embodiments of the present application, the nanoparticles are uniformly adhered to the surface of the graphite particles by bonding, so that the CaF 2 Nanoparticles can be stably coated on the graphite surface. Adhesives are organic substances that have affinity with graphite. These organic substances contain oxygen. CaF 2 Nanoparticles have a good bonding effect on the graphite surface, CaF 2The nanoparticles are stably and evenly present on the surface of the graphite particles to obtain the negative electrode active material. In some embodiments of the present application, the bonding effect between the adhesive and the graphite can be changed by adjusting the type and mass ratio of the adhesive. First, the adhesive itself has different oxygen contents, and different selections will produce different effects. Secondly, the same adhesive will also produce different effects if added in different amounts.

[0117] The mass ratio of oxygen in the negative electrode active material can indirectly indicate the residual amount of the binder after heat treatment. Relatively speaking, in some embodiments of the present application, by limiting the mass ratio Z of oxygen in the negative electrode active material, the amorphous carbon content in the negative electrode active material is controlled to achieve an ideal bonding effect. After meeting the above conditions, the first coulombic efficiency of the battery is further improved.

[0118] It can be found from Examples 16 to 21 in Table 3 that when the negative electrode active material satisfies the following relationship, the first coulombic efficiency of the lithium ion secondary battery is further improved:

[0119] (1) The thermogravimetric loss TG of the negative electrode active material at 600° C. satisfies 0.2%≤TG≤1.5%, particularly 0.8%≤TG≤1.5%, and more particularly 1.2%≤TG≤1.5%; and / or

[0120] (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 0.10≤Id / Ig≤0.45, particularly 0.15≤Id / Ig≤0.45, and more particularly 0.15≤Id / Ig≤0.40.

[0121] 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 first coulombic efficiency of the battery can be further improved.

[0122] The negative electrode active material is a binder and CaF 2 The composite coating of nanoparticles will have a certain thermogravimetric loss at high temperature. This is because the groups of the binder will decompose at high temperature. When the temperature is high enough, most of the groups and carbon-carbon bonds will break. Except for carbon, other elements will volatilize due to the decomposition of the groups. At the same time, the water content of the negative electrode active material will also decrease in weight due to volatilization. The thermogravimetric mass loss TG of the negative electrode active material can be used to indirectly judge the content of non-carbon elements in the negative electrode active material. Non-carbon elements are important for CaF 2It helps to bond nanoparticles to graphite. The defect degree of the negative electrode active material is represented by the peak intensity ratio Id / Ig between the D peak and the G peak in the Raman spectrum, which also affects the first coulombic efficiency of the battery. In some embodiments of the present application, the thermogravimetric loss of the material can be changed by adjusting the amount of adhesive and selecting different types of adhesives. At the same time, Id / Ig will also change. As the amount of adhesive increases, the thermogravimetric loss will also increase, and Id / Ig will also increase with the increase in the amount of adhesive, but it is also affected by the heat treatment temperature. The higher the temperature, the smaller the Id / Ig. In some embodiments of the present application, by selecting a specific negative electrode active material at 600°C Thermogravimetric loss TG and Id / Ig, the content of non-carbon elements in the negative electrode active material is adjusted to assist in the bonding of CaF 2 Nanoparticles and graphite materials. By selecting the peak intensity ratio Id / Ig of the D peak and the G peak in the Raman spectrum of a specific negative electrode active material, the balance between the amorphous structure content and lithium ion deintercalation in the negative electrode active material is adjusted to improve the low temperature performance and cycle performance of the battery. When the above conditions are met, the first coulomb efficiency of the battery is further improved.

[0123] It can be found from Examples 22 to 35 in Table 4 that when the negative electrode active material satisfies the following relationship, the first coulombic efficiency of the lithium ion secondary battery is further improved:

[0124] (1) the oil absorption value S of the negative electrode active material satisfies 20 mL / 100 g ≤ S ≤ 65 mL / 100 g, particularly 25 mL / 100 g ≤ S ≤ 65 mL / 100 g, and more particularly 30 mL / 100 g ≤ S ≤ 65 mL / 100 g; and / or

[0125] (2) The specific surface area of ​​the negative electrode active material meets the BET requirement of 0.2 m 2 / g≤BET≤2.5m 2 / g, especially meeting 0.5m 2 / g≤BET≤2.5m 2 / g, more particularly, meeting 1.0m 2 / g≤BET≤2.5m 2 / g; and / or

[0126] (3) The tap density TD of the negative electrode active material satisfies TD ≥ 0.6 g / cm 3 , especially meeting 1.3g / cm 3 ≥TD≥0.7g / cm 3 .

[0127] That is, when one of the oil absorption value S, specific surface area BET and tap density TD is met or two of the index conditions are met at the same time, the first coulombic efficiency of the lithium-ion secondary battery is improved; when all three index conditions are met at the same time, the first coulombic efficiency of the battery is further improved.

[0128] The oil absorption value S is an important indicator of the negative electrode active material, which indirectly indicates the amount of dispersant consumed by the negative electrode active material in the slurry, thereby affecting the processing performance of the negative electrode active material. The specific surface area BET of the negative electrode active material also affects the performance of the battery, and the tap density TD of the negative electrode active material is also related to the processing performance of the slurry. In some embodiments of the present application, by selecting a specific oil absorption value S, the dispersion of the negative electrode active material is improved, the occurrence of particle aggregation is reduced, the proportion of the negative electrode active material in the negative electrode slurry is increased, and the loss of energy density is reduced. By selecting a specific specific surface area BET, the effective lithium insertion area of ​​the negative electrode active material is adjusted, the side reactions of the battery are reduced, and the first coulomb efficiency, subsequent cycle performance, storage performance, low temperature performance and rate performance of the battery are improved. By selecting a specific tap density TD, the processability of the negative electrode slurry is improved, abnormalities such as slurry stratification are reduced, and the coating quality is improved. After meeting the above conditions, the first coulomb efficiency of the battery is further improved.

[0129] It can be found from Examples 36 to 46 in Table 5 that when the negative electrode active material satisfies the following relationship, the first coulombic efficiency of the lithium ion secondary battery is further improved:

[0130] (1) The OI value of the negative electrode plate satisfies OI value ≤ 20, particularly satisfies 5 ≤ ​​OI value ≤ 16, and more particularly satisfies 6 ≤ OI value ≤ 13; and / or

[0131] (2) The porosity of the negative electrode sheet satisfies 20%≤H≤40%, particularly 25%≤H≤40%, and more particularly 30%≤H≤40%.

[0132] That is, when one of the OI value and the porosity H is met or both of the two index conditions are met at the same time, the first coulombic efficiency of the battery is significantly improved.

[0133] The OI value of the negative electrode sheet is also an important indicator that affects battery performance, indicating the orientation of the negative active material in the negative electrode sheet. The porosity H of the negative electrode sheet has a significant effect on the first coulombic efficiency and cycle performance of the lithium-ion secondary battery. The porosity of the negative electrode sheet is directly related to the battery's ability to store electrolyte. In some embodiments of the present application, by selecting a specific OI value, the deintercalation direction of lithium ions in the negative electrode active material is increased, which is beneficial to the deintercalation of lithium ions in the negative electrode active material and improves the dynamics of the negative electrode sheet. By selecting a specific porosity H, the battery's electrolyte storage capacity is adjusted, the formation is sufficient, the first coulombic efficiency, cycle performance, and high-temperature storage performance of the lithium-ion secondary battery are improved, and the risks of side reactions, cycle gas production, and deterioration of the cycle during formation are reduced. After meeting the above conditions, the first coulombic efficiency of the lithium-ion secondary battery is further improved.

[0134] 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 comprises a graphite material having calcium fluoride particles on the surface; The mass proportion of the calcium fluoride particles in the negative electrode active material is X, 0.15%≤X≤2.0%; and The proportion of the number of particles with an equivalent diameter D≤500 nm in the calcium fluoride particles is Y, and Y≥50%.

2. The negative electrode active material according to claim 1, characterized in that 0.5%≤X≤2.0%; preferably, 0.5%≤X≤1.0%.

3. The negative electrode active material according to claim 1, characterized in that Y≥55%; preferably, Y≥60%.

4. The negative electrode active material according to claim 1, characterized in that The negative electrode active material contains oxygen element, and the mass ratio of the oxygen element in the negative electrode active material is Z, 0.15%≤Z≤2.0%; preferably, 0.5%≤Z≤2.0%; more preferably, 1.0%≤Z≤2.0%.

5. The negative electrode active material according to claim 1, characterized in that The thermogravimetric mass loss of the negative electrode active material at 600° C. is TG, 0.2%≤TG≤1.5%; preferably, 0.8%≤TG≤1.5%; more preferably, 1.2%≤TG≤1.5%.

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: (1) The oil absorption value S of the negative electrode active material satisfies: 20 mL / 100 g ≤ S ≤ 70 mL / 100 g; preferably, 25 mL / 100 g ≤ S ≤ 65 mL / 100 g; more preferably, 30 mL / 100 g ≤ S ≤ 65 mL / 100 g; (2) The specific surface area BET of the negative electrode active material satisfies: 0.2 m 2 / g≤BET≤3.0m 2 / g; preferably, 0.5m 2 / g≤BET≤2.5m 2 / g; more preferably, 1.0m 2 / g≤BET≤2.5m 2 / g; (3) The peak intensity ratio of the D peak to the G peak in the Raman spectrum of the negative electrode active material is Id / Ig, 0.10≤Id / Ig≤0.45; preferably, 0.15≤Id / Ig≤0.45; more preferably, 0.15≤Id / Ig≤0.

40.

7. The negative electrode active material according to claim 1, characterized in that The tap density of the negative electrode active material is TD, TD≥0.6g / cm 3 ; Preferably, 1.3g / cm 3 ≥TD≥0.7g / cm 3 .

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 negative electrode sheet satisfies at least one of the following conditions: (4) The OI value of the negative electrode plate satisfies: OI value ≤ 20; preferably, 5 ≤ OI value ≤ 16; more preferably, 6 ≤ OI value ≤ 13; (5) The porosity H of the negative electrode sheet satisfies: 20%≤H≤40%; preferably, 25%≤H≤40%; more preferably, 30%≤H≤40%.

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

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