Electrolyte, lithium ion secondary battery and electric equipment

Abstract

The invention belongs to the field of batteries, and particularly discloses an electrolyte, a lithium ion secondary battery and electric equipment. The electrolyte comprises a lithium salt and a solvent, the solvent comprises a carbonic ester solvent and a carboxylic ester solvent in a mass ratio of 1: (1-3); and the conductivity of the electrolyte is greater than or equal to 12.7 mS / cm. The lithium ion secondary battery comprises a positive pole piece, an electrolyte and a negative pole piece, the negative pole piece contains a negative active material; the lithium ion secondary battery meets the requirements. According to the lithium ion secondary battery disclosed by the invention, parameters such as the conductivity of the electrolyte, the surface density of the negative pole piece and the porosity of the negative pole piece are regulated and controlled, so that the lithium ion battery has relatively high ion transmission efficiency and relatively high constant-current charging capacity ratio when being quickly charged at the multiplying power of 4C or above, and has relatively high capacity retention ratio when being discharged at a low temperature.

Description

Technical Field

[0001] This invention belongs to the field of batteries, specifically relating to an electrolyte, a lithium-ion secondary battery, and an electrical device. Background Technology

[0002] With the rapid development of new energy vehicles, portable electronic devices, and other fields, the market has placed higher demands on the fast charging performance of lithium-ion batteries. Fast charging technology with a rate of 4C and above has become a research hotspot in the industry. However, in the process of achieving high-rate charging, the synergistic matching problem between the battery's internal ion transport efficiency, electrode structure, and electrolyte performance has become increasingly prominent.

[0003] During the operation of a lithium-ion battery, lithium ions need to be transported through the electrolyte and electrode pores. Electrode porosity directly affects the degree of electrolyte wetting and the smoothness of the ion transport path; electrolyte conductivity determines the conduction speed of ions in the electrolyte; and negative electrode areal density is related to electrode thickness, thus affecting the ion migration distance. In lithium-ion battery design, electrolyte conductivity, electrode porosity, and negative electrode areal density are all factors that need to be considered. For example, low electrode porosity hinders effective electrolyte wetting, increasing ion transport resistance; excessively high negative electrode areal density makes the electrode too thick, lengthening the ion migration path and reducing ion diffusion efficiency; while insufficient electrolyte conductivity cannot provide a good environment for rapid ion transport. Therefore, it is necessary to optimize the electrolyte conductivity, electrode porosity, and negative electrode areal density of lithium-ion batteries to improve their 4C and higher rate fast charging performance. Summary of the Invention

[0004] In order to overcome at least one of the technical problems existing in the prior art, one of the objectives of the present invention is to provide an electrolyte.

[0005] The second objective of this invention is to provide a lithium-ion secondary battery.

[0006] The third objective of this invention is to provide an electrical device.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A first aspect of the present invention provides an electrolyte comprising a lithium salt and a solvent; The solvents include carbonate solvents and carboxylic acid ester solvents in a mass ratio of 1:(1~3); The carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate; The carboxylic acid ester solvent is selected from at least one of methyl acetate, methyl propionate, ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. The conductivity of the electrolyte is ≥12.7 mS / cm.

[0008] In some embodiments of the present invention, the concentration of the lithium salt is 0.8~1.5 mol / L.

[0009] In some embodiments of the present invention, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorobis(oxalate)phosphate, lithium bis(oxalate)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

[0010] In some embodiments of the present invention, the electrolyte further contains additives, wherein the mass of the additives is 0.5% to 10% of the total mass of the electrolyte.

[0011] In some embodiments of the present invention, the additive is selected from at least one of vinyl sulfate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, lithium difluorooxalate borate, vinylene carbonate, ethylene ethylene carbonate, fluoroethylene carbonate, and 1,3-propanesulfonate lactone.

[0012] A second aspect of the present invention provides a lithium-ion secondary battery, comprising a positive electrode, an electrolyte, and a negative electrode; the negative electrode contains a negative electrode active material; the physical properties of the electrolyte and the negative electrode of the lithium-ion secondary battery satisfy the following relationship:

[0013] Where κ is the conductivity of the electrolyte, in mS / cm; b Porosity of the negative electrode sheet; ρ is the surface density of the negative electrode plate, in g / m³. 2 ; c is the true density of the negative electrode active material, in g / cm³. 3 ; j is the current density, in mA / cm². 2 .

[0014] In some embodiments of the present invention, the areal density of the negative electrode sheet is 120~220 g / m³. 2 .

[0015] In some embodiments of the present invention, the true density of the negative electrode active material in the negative electrode sheet is 2.0~2.4 g / cm³. 3 .

[0016] In some embodiments of the present invention, the negative electrode active material in the negative electrode sheet is selected from at least one of graphite and silicon-doped graphite.

[0017] In some embodiments of the present invention, the porosity of the negative electrode sheet is 28-38%.

[0018] In some embodiments of the present invention, the electrolyte is the electrolyte described in the first aspect of the present invention.

[0019] In some embodiments of the present invention, the positive electrode active material of the positive electrode sheet includes at least one of lithium iron phosphate and lithium manganese iron phosphate.

[0020] In some embodiments of the present invention, the porosity of the positive electrode sheet is 25% to 35%.

[0021] In some embodiments of the present invention, the current density = (charging rate × areal density of the negative electrode × specific capacity of the negative electrode active material) ÷ ​​1000; wherein, the specific capacity of the negative electrode active material is in mAh / g, and the charging rate is in C.

[0022] In some embodiments of the present invention, the charging rate is ≥4C.

[0023] A third aspect of the present invention provides an electrical device. In some embodiments of the present invention, the electrical device includes a battery; the battery includes the electrolyte described in the first aspect of the present invention.

[0024] In some embodiments of the present invention, the electrical device includes a battery, which is a lithium-ion secondary battery as described in the second aspect of the present invention.

[0025] The beneficial effects of the present invention are as follows: The electrolyte of the present invention uses a specific ratio of carbonate solvent with high dielectric constant and carboxylic acid ester solvent, which is compounded with other components in the electrolyte such as lithium salt, thereby obtaining an electrolyte with high conductivity. When the electrolyte is applied to lithium-ion batteries, it can meet the fast charging requirements of 4C and above and improve the low temperature performance and cycle life of the battery.

[0026] The lithium-ion secondary battery of this invention achieves high ion transport efficiency, high constant current charging capacity ratio, and high capacity retention during low-temperature discharge by adjusting parameters such as electrolyte conductivity, negative electrode areal density, and negative electrode porosity. Furthermore, the lithium-ion secondary battery of this invention can reduce polarization voltage during operation, avoiding adverse phenomena such as battery polarization, lithium deposition on the negative electrode surface, and excessive heat generation, thus improving the battery's fast-charging performance. This achieves a balance between fast-charging performance at high rates above 4C and battery cycle life and energy density. Attached Figure Description

[0027] Figure 1 This is an interface diagram of the negative electrode sheet in Example 2 after 50 cycles at 25°C and 4C.

[0028] Figure 2 This is an interface diagram of the negative electrode sheet in Example 10 after 50 cycles at 25°C and 4C.

[0029] Figure 3 This is an interface diagram of the negative electrode sheet in Example 11 after 50 cycles at 25°C and 4C.

[0030] Figure 4 This is an interface diagram of the negative electrode sheet in Example 12 after 50 cycles at 25°C and 4C.

[0031] Figure 5 This is an interface diagram of the negative electrode sheet in Example 13 after 50 cycles at 25°C and 4C.

[0032] Figure 6 This is an interface diagram of the negative electrode sheet in Example 14 after 50 cycles at 25°C and 4C.

[0033] Figure 7 The diagram shows the interface of the negative electrode in Comparative Example 8 after 50 cycles at 25°C and 4C.

[0034] Figure 8 The diagram shows the interface of the negative electrode in Comparative Example 9 after 50 cycles at 25°C and 4C.

[0035] Figure 9 The diagram shows the interface of the negative electrode in Comparative Example 10 after 50 cycles at 25°C and 4C.

[0036] Figure 10 The diagram shows the interface of the negative electrode in Comparative Example 11 after 50 cycles at 25°C and 4C. Detailed Implementation

[0037] The following examples provide a more detailed description of the specific implementation of the present invention, but the implementation and protection of the present invention are not limited thereto. It should be noted that any processes not specifically described below are methods that can be implemented or understood by those skilled in the art by referring to existing technology. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0038] In some embodiments of the present invention, an electrolyte is provided, comprising a lithium salt and a solvent; the solvent comprises a carbonate solvent and a carboxylic acid ester solvent in a mass ratio of 1:(1~3); the carbonate solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the carboxylic acid ester solvent is selected from at least one of methyl acetate (MA), methyl propionate (MP), ethyl acetate (EA), propyl acetate (PA), ethyl propionate (EP), and propyl propionate (PP); the conductivity of the electrolyte is ≥12.7 mS / cm.

[0039] In some embodiments of the present invention, the mass ratio of carbonate solvent to carboxylic acid ester solvent is any value or a range formed by any two of the following: 1:1, 1:1.2, 1:1.4, 1:1.5, 1:1.6, 1:1.8, 1:2, 1:2.2, 1:2.4, 1:2.5, 1:2.6, 1:2.8, 1:3.

[0040] In some embodiments of the present invention, the conductivity of the electrolyte is 12.7~25 mS / cm; in some embodiments of the present invention, the conductivity of the electrolyte is any value or a range formed by any two of the following: 12.7 mS / cm, 13 mS / cm, 14 mS / cm, 15 mS / cm, 16 mS / cm, 17 mS / cm, 18 mS / cm, 19 mS / cm, 20 mS / cm, 21 mS / cm, 22 mS / cm, 23 mS / cm, 24 mS / cm, and 25 mS / cm.

[0041] In some embodiments of the present invention, the concentration of lithium salt is 0.8~1.5 mol / L; in some embodiments of the present invention, the concentration of lithium salt is any value or a range formed by any two of 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, and 1.5 mol / L.

[0042] In some embodiments of the present invention, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorobis(oxalate) phosphate (LiDFOP), lithium bis(oxalate) borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0043] In some embodiments of the present invention, the electrolyte further contains an additive, the mass of which is 0.5% to 10% of the total mass of the electrolyte; in some embodiments of the present invention, the mass of the additive is any value or a range formed by any two of the following: 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10% of the total mass of the electrolyte.

[0044] In some embodiments of the present invention, the additive is selected from at least one of vinyl sulfate (DTD), tris(trimethylsilane)phosphate (TMSP), tris(trimethylsilane)borate (TMSB), lithium difluorooxalate borate (LiDFOB), vinylene carbonate (VC), ethylene ethylene carbonate (VEC), fluoroethylene carbonate (FEC), and 1,3-propanesulfonate lactone (PS). Adding compounds containing sulfate / phosphate / borate groups, fluorinated / silyl-substituted carbonate skeletons, or sulfonate lactone ring structures can further improve electrolyte stability and ion transport efficiency, thereby achieving a synergistic improvement in energy density, cycle life, and safety.

[0045] In some embodiments of the present invention, the additives include 1.5 to 3% ethylene carbonate, 0.5 to 2% fluoroethylene carbonate, and 0.5 to 2% 1,3-propanesulfonic acid lactone, based on the total mass of the electrolyte as 100%.

[0046] In some embodiments of the present invention, the mass percentage of ethylene carbonate, based on the total mass of the electrolyte as 100%, is any value or a range formed by any two of the following: 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, and 3%.

[0047] In some embodiments of the present invention, the mass percentage of fluoroethylene carbonate, based on the total mass of the electrolyte as 100%, is any value or a range formed by any combination of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2%.

[0048] In some embodiments of the present invention, the mass percentage of 1,3-propanesulfonic acid lactone, based on the total mass of the electrolyte as 100%, is any value or a range formed by any combination of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2%.

[0049] In some embodiments of the present invention, a lithium-ion secondary battery is provided, comprising a positive electrode, an electrolyte, and a negative electrode; the negative electrode contains a negative electrode active material; the physical properties of the electrolyte and the negative electrode of the lithium-ion secondary battery satisfy the following relationship:

[0050] Where κ is the conductivity of the electrolyte, in mS / cm; b Porosity of the negative electrode sheet; ρ is the surface density of the negative electrode plate, in g / m³. 2 ; c is the true density of the negative electrode active material, in g / cm³. 3 ; j is the current density, in mA / cm². 2 .

[0051] To meet the high energy density requirements of lithium-ion batteries, the negative electrode sheet is required to have high compaction and high areal density during battery design. However, high compaction leads to a decrease in the porosity of the negative electrode sheet and hinders the effective wetting of the electrolyte, increasing the resistance to ion transport. Excessive areal density of the negative electrode sheet will make the negative electrode sheet too thick, prolonging the ion migration path and reducing the ion diffusion efficiency. When the porosity of the negative electrode sheet, the areal density of the negative electrode sheet, the true density of the negative electrode active material, the current density, and the conductivity of the electrolyte meet the above formula, the battery can have advantages such as high ion transport efficiency, low polarization voltage, no lithium plating, and no heat generation during fast charging, achieving a balance between 4C and above fast charging performance and battery cycle life and energy density.

[0052] In some embodiments of the present invention, the areal density of the negative electrode sheet is 120~220 g / m³. 2 In some embodiments of the present invention, the areal density of the negative electrode sheet is 120 g / m². 2 130g / m 2 140g / m 2 150g / m 2 160g / m 2 170g / m 2 180g / m 2 190g / m 2 200g / m 2 210g / m 2 220g / m 2 The range of values ​​in the range, or any combination thereof; in some embodiments of the present invention, the areal density of the negative electrode sheet is 140~200 g / m². 2 .

[0053] In some embodiments of the present invention, the true density of the negative electrode active material is 2.0~2.4 g / cm³. 3 In some embodiments of the present invention, the true density of the negative electrode active material is 2.0 g / cm³. 3 2.1g / cm 3 2.2g / cm 3 2.3g / cm3 2.4g / cm 3 The value in the range formed by any one or both; in some embodiments of the present invention, the true density of the negative electrode active material is 2.15~2.35 g / cm³. 3 .

[0054] In some embodiments of the present invention, the negative electrode active material is selected from at least one of graphite and silicon-doped graphite; in some embodiments of the present invention, the negative electrode active material is graphite with a true density of 2.2 g / cm³. 3 .

[0055] In some embodiments of the present invention, the average particle size of the negative electrode active material is 5-30 μm; in some embodiments of the present invention, the average particle size of the negative electrode active material is 15-20 μm.

[0056] In some embodiments of the present invention, the porosity of the negative electrode sheet is 28-38%; in some embodiments of the present invention, the porosity of the negative electrode sheet is any value of 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38% or a range formed by any two of these values.

[0057] In some embodiments of the present invention, the electrolyte used in the lithium-ion secondary battery is the electrolyte of the present invention.

[0058] In some embodiments of the present invention, the positive electrode active material of the positive electrode sheet includes at least one of lithium iron phosphate (LiFePO4) and lithium manganese iron phosphate (LMFP).

[0059] In some embodiments of the present invention, the porosity of the positive electrode sheet is 25% to 35%; in some embodiments of the present invention, the porosity of the positive electrode sheet is any value of 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35% or a range formed by any two of these values.

[0060] In some embodiments of the present invention, the current density = (charging rate × areal density of the negative electrode × specific capacity of the negative electrode active material) ÷ ​​1000; wherein, the specific capacity of the negative electrode active material is in mAh / g, and the charging rate is in C.

[0061] The derivation of the formula for calculating current density is as follows: Current density j = Charging rate (C) × Aspect capacity (mAh / cm²) 2 The areal capacity = mass of negative electrode active material (mg) × specific capacity of negative electrode active material (mAh / g) ÷ area of ​​negative electrode sheet (cm²) 2The mass of the negative electrode active material = the areal density of the negative electrode sheet × the area of ​​the negative electrode sheet; therefore, the areal capacity (mAh / cm²) 2 = Aspect density of the negative electrode (mg / cm³) 2 The formula for calculating current density is derived by multiplying the specific capacity of the negative electrode active material (mAh / g) by 1000.

[0062] When the negative electrode active material is graphite, its specific capacity is 350 mAh / g, and its true density is c = 2.2 g / cm³. 3 .

[0063] In some embodiments of the present invention, the charging rate is ≥4C.

[0064] In some embodiments of the present invention, the present invention provides an electrical device that includes a battery; the battery includes the electrolyte of the present invention.

[0065] In some embodiments of the present invention, the present invention provides an electrical device that includes a battery; the battery is a lithium-ion secondary battery as described in the present invention.

[0066] To meet the 4C fast charging requirement and maximize the energy density of lithium-ion rechargeable batteries, it is necessary to increase the areal density and decrease the porosity of the negative electrode sheet. In the following embodiments, the porosity of the negative electrode sheet ranges from 28% to 38%, and the areal density ranges from 140 to 200 g / m³. 2 The true density range of the negative electrode active material is 2.15~2.35 g / cm³. 3 .

[0067] When fast charging at 4C rate, the porosity of the negative electrode sheet is limited. b =28%, the surface density of the negative electrode is ρ=200g / m². 2 (i.e., 20mg / cm) 2 The true density of the negative electrode active material (i.e., graphite) is c = 2.2 g / cm³. 3 Current density j = charging rate (C) × areal density of the negative electrode (mg / cm³) 2 ) × Specific capacity of negative electrode active material (mAh / g) ÷ 1000, i.e., j = (4C × (20mg / cm³) 2 )×(350mAh / g))÷1000=28mA / cm 2 At that time, by It is known that κ ≥ 22.2 mS / cm. Conventional carbonate solvents are difficult to meet the conductivity requirements of this electrolyte. Therefore, this invention uses carboxylic acid ester MA as the main solvent and LiFSI as the main salt, so that the conductivity of the electrolyte meets the requirements.

[0068] When fast charging at 4C rate, the porosity of the negative electrode sheet is limited. b =28%, the surface density of the negative electrode is ρ=170g / m². 2 (i.e., 17 mg / cm) 2 The true density of the negative electrode active material is c = 2.2 g / cm³. 3 Current density j = (4C × (17mg / cm³)) 2 )×(350mAh / g))÷1000=23.8mA / cm 2 At that time, by It is known that κ ≥ 16 mS / cm. Conventional carbonate solvents are still insufficient to meet the conductivity requirements of this electrolyte. This invention uses carboxylic acid esters MA or EA as the main solvent and LiFSI or LiPF6 as the composite main salt, thereby enabling the electrolyte to meet the conductivity requirements.

[0069] When fast charging at 4C rate, the porosity of the negative electrode sheet is limited. b =32%, the surface density of the negative electrode is ρ=170g / m². 2 (i.e., 17 mg / cm) 2 The true density of the negative electrode active material is c = 2.2 g / cm³. 3 Current density j = (4C × (17mg / cm³)) 2 )×(350mAh / g))÷1000=23.8mA / cm 2 At that time, by It is known that κ ≥ 12.7 mS / cm. Conventional carbonate solvents are still insufficient to meet the conductivity requirements of this electrolyte. This invention uses carboxylic acid esters MA or EA as the main solvent and LiFSI or LiPF6 as the composite main salt, thereby enabling the electrolyte to meet the conductivity requirements.

[0070] Among them, 4C fast charging refers to the average 4C fast charging rate of the battery at 10~80% SOC, that is, 10~80% SOC, charging for 10.5 minutes.

[0071] The method for testing the conductivity of the electrolyte in this invention is based on the principle of alternating current impedance (to avoid electrolysis effects caused by direct current). The core is to calculate the conductivity by measuring the resistance of the electrolyte (conductivity σ = electrode constant K / electrolyte resistance R). Test conditions: The temperature of the electrolyte is controlled at 25℃±0.1℃ using a constant temperature water bath, and then a conductivity meter is used for testing.

[0072] Test method for porosity of negative electrode sheet: Based on the "quantitative relationship between pore volume and total volume", the liquid displacement method (immersion method) is used for testing. Test steps: (1) Use alcohol cotton to gently wipe the surface of the negative electrode sheet to avoid damage to the negative electrode sheet and to ensure that the surface of the negative electrode sheet is free of dust, oil and other impurities, so as to avoid interference of oil, dust and other impurities with liquid wetting; (2) Dry the negative electrode sheet in a vacuum drying oven at 60℃ for two hours to ensure that the negative electrode sheet is fully dried and to remove moisture (moisture will compete with the liquid for pores, resulting in incomplete filling); (3) Use the liquid displacement method (immersion method) at 25±2℃ to test, so as to avoid changes in liquid density caused by temperature changes. First, measure the length, width and thickness of the negative electrode sheet and calculate the apparent volume V. a Weigh the negative electrode sheet to obtain a dry weight m1; then completely immerse the negative electrode sheet in a liquid that does not dissolve the electrode components (e.g., anhydrous ethanol), and degas it under vacuum (removing air from the pores) to fill the pores with liquid; remove the electrode sheet, wipe off excess liquid from the surface, and weigh it to obtain a wet weight m2; calculate the pore volume V based on the liquid density (ρ). p =(m2-m1) / ρ), porosity ε=V p / V a ×100%. Testing equipment: analytical balance (accuracy 0.1mg), beaker (for liquid), vacuum drying oven (for sample pretreatment); also includes auxiliary tools such as: punch (for sampling), filter paper (for removing excess liquid), tweezers (for handling samples), etc.

[0073] The porosity of the negative electrode sheet is affected by multiple factors, such as: negative electrode active material, conductive agent, binder, negative electrode slurry, coating thickness and other parameters, specifically: (1) The particle size, specific surface area, morphology and other factors of the negative electrode active material directly affect the initial pore structure of the negative electrode sheet, which is the basis for porosity control. When large-diameter graphite (such as 20~30μm) is stacked, the "inter-particle pores" formed are larger, and the initial porosity is higher; small-diameter graphite (such as 5~10μm) is stacked tightly, and the initial porosity is lower. Using "multi-particle size gradation" (such as large particles + small particles mixed in a certain proportion) can optimize the pore distribution: small particles fill the gaps between large particles, reduce ineffective large pores, and at the same time retain an appropriate amount of interconnected pores (which is conducive to electrolyte penetration). Spherical graphite (good fluidity, tight stacking) has a lower initial porosity and more uniform distribution than flake graphite (easy interlayer stacking, irregular pores). To increase porosity, a small amount of flake graphite can be added (to increase the interparticle spacing). High specific surface area graphite (e.g., SSA > 10 μm) 2 / g) The surface is rough and there are more "micropores in the particles", and the initial porosity is relatively high (but it should be avoided to avoid being too large, otherwise it will easily lead to excessive consumption of electrolyte). By controlling the spheroidization of graphite or coating treatment (such as carbon coating), the SSA can be reduced and the proportion of micropores can be reduced. (2) The morphology and amount of conductive agent will change the "skeleton structure" of the negative electrode sheet, which will indirectly affect the porosity. (3) The role of the binder is to bond the negative electrode active material and the current collector. Its amount and film-forming characteristics will affect the degree of pore filling. (4) The solid content and dispersion state of the negative electrode slurry will affect the structure of the wet coating after coating, and thus determine the initial porosity of the dry coating after drying.

[0074] (5) Coating determines the thickness and uniformity of the wet coating, while drying determines the solvent evaporation path. Both factors together affect the initial pore structure of the electrode after drying. (6) Rolling is a "precise adjustment" step for porosity: the electrode is compressed by mechanical pressure to reduce the porosity to the target value (the porosity is usually 50%~60% before rolling and drops to 20%~40% after rolling). Therefore, the porosity of the negative electrode can be controlled by adjusting one or more of the above parameters.

[0075] True density testing of negative electrode active materials: The gas displacement method is used, the core of which is to accurately measure the "volume of a solid without any pores," and then calculate the true density (true density = sample mass / true solid volume) based on the sample mass. Testing conditions: Graphite surfaces easily adsorb moisture, air, and oil from the preparation process. These occupy the gas / liquid space, leading to volume measurement errors. Therefore, graphite pretreatment is necessary before testing: vacuum drying at 60~100℃ for 2-4 hours (temperature should not be too high to avoid graphite oxidation; ≤80℃ for natural graphite, ≤100℃ for artificial graphite); if there is oil on the surface, it can be ultrasonically cleaned with anhydrous ethanol for 10 minutes, then dried (to avoid residual ethanol affecting the measurement). Particles that are too large (e.g., >1mm) may cause uneven gas distribution in the sample chamber; particles that are too fine (e.g., <1μm) are prone to agglomeration, forming gaps that are difficult for gas to penetrate. It is recommended that the particle size be controlled between 10~100μm (this can be achieved through sieving). In the gas displacement method, temperature fluctuations affect gas density (following the ideal gas law PV=nRT), so testing must be conducted in a constant temperature environment (25±0.1℃). Testing equipment: true density meter; auxiliary tools: vacuum drying oven (for sample pretreatment); agate mortar (for lightly grinding agglomerated particles to avoid excessive breakage affecting the structure); analytical balance (accuracy 0.01mg, used for weighing sample mass).

[0076] Negative electrode sheet areal density test: The areal density is calculated by measuring the mass difference of the electrode sheet before and after coating and the coating area using the direct weighing method. Calculation formula: Areal density = (Mass of electrode sheet after coating - Mass of current collector before coating) ÷ Coating area. Test conditions: The electrode sheet is blown clean with compressed air or gently wiped with an alcohol swab to avoid dust, oil, or electrolyte residue on the surface, which could affect the mass measurement. The electrode sheet is then thoroughly dried (vacuum drying at 60℃ for 2 hours) to remove moisture (moisture increases mass, leading to a higher areal density measurement). Samples must have neat edges, no wrinkles or damage (especially important for the weighing method, where accurate area measurement is crucial). Trim the sample with a cutter if necessary. The test must be conducted in a constant temperature and humidity environment (25±2℃, relative humidity 30%~50%) to avoid the influence of air buoyancy due to temperature changes or moisture absorption by the electrode sheet due to humidity. Testing equipment: Analytical balance (accuracy ≥ 0.1 mg) for measuring electrode mass; also includes auxiliary tools such as: cutter or punch (for preparing standard-sized samples, such as a circle with a diameter of 16 mm or a square of 50 mm × 50 mm), vacuum drying oven (for drying samples), and anti-static tweezers (for handling samples to avoid hand contamination).

[0077] The areal density of the negative electrode sheet is a core parameter that determines the battery capacity, rate performance and safety. Its control needs to run through the entire process of negative electrode slurry preparation and coating process. The target value is achieved by "precisely controlling the ratio of coating mass to area". The areal density of the negative electrode sheet can be controlled from the following aspects, specifically: (1) Control of negative electrode slurry characteristics: The solid content, viscosity and uniformity of the negative electrode slurry directly affect the "wet coating thickness" during coating. The wet coating thickness is positively correlated with the areal density of the dry coating after drying (under the same formula, the thicker the wet coating, the higher the areal density of the dry coating). (2) Control of coating process: Coating is the key link in the formation of areal density. By controlling the "wet coating area" and "wet coating thickness", the areal density of the dry coating after drying is directly determined (area density = solid mass per unit area of ​​wet coating = wet coating thickness × solid content of negative electrode slurry × solvent density). (3) Control of drying process: The drying process converts the wet coating into a dry coating by removing the solvent. If the drying is not done properly, the areal density uniformity will deteriorate (such as local residual solvent causing quality deviation).

[0078] The specific implementation of the present invention will be further described in detail below with reference to specific embodiments: Example 1 This example provides an electrolyte and its preparation method, as well as a lithium-ion secondary battery containing the electrolyte.

[0079] The electrolyte in this example consists of lithium salt, additives, and solvent; The lithium salt is lithium hexafluorophosphate (LiPF6), with a molar concentration of 1 mol / L. The mass of the additive is 4.5% of the total mass of the electrolyte; The solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl propionate (EP) in a mass ratio of 3:2:5. Based on the total mass of the electrolyte, the additive consists of 2.5% by mass of vinylene carbonate (VC), 1% by mass of fluoroethylene carbonate (FEC), and 1% by mass of 1,3-propanesulfonic acid lactone (PS).

[0080] The conductivity of the electrolyte in this example was tested. It is 12.9 mS / cm.

[0081] The electrolyte preparation method in this example includes the following steps: (1) Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl propionate (EP) in a mass ratio of EC:EMC:EP=3:2:5, and purify and remove impurities and water using molecular sieves; (2) At room temperature, 12.5% ​​by mass of conductive lithium salt lithium hexafluorophosphate (LiPF6) is dissolved in the solvent obtained in step (1) and stirred evenly to obtain the basic electrolyte. (3) Add vinylene carbonate (VC), fluoroethylene carbonate (FEC) and 1,3-propanesulfonic acid lactone (PS) to the basic electrolyte prepared in step (2) to obtain an electrolyte for lithium-ion secondary batteries.

[0082] The lithium-ion secondary battery in this example is a three-electrode soft-pack battery. The positive electrode is LiFePO4, the negative electrode is graphite, the reference electrode is lithium-plated copper wire, and the electrolyte is the same as in this example. The porosity of the negative electrode in this lithium-ion secondary battery is... b The negative electrode has a surface density ρ of 32% and a surface area density ρ of 170 g / m². 2 (i.e., 17 mg / cm) 2 The true density c of the negative electrode active material is 2.2 g / cm³. 3 The specific capacity of the negative electrode active material is 350mAh / g; Current density j = charging rate (C) × areal capacity (mAh / cm²) 2 The areal capacity = mass of active material on the negative electrode (mg) × specific capacity of the negative electrode active material (mAh / g) ÷ electrode area (cm²) 2 The mass of the negative electrode active material is related to its areal density; the mass of the negative electrode active material = areal density of the negative electrode sheet × electrode area; therefore, the areal capacity (mAh / cm²) is related to the areal density of the negative electrode sheet. 2 = Aspect density of the negative electrode (mg / cm³) 2) × specific capacity of negative electrode active material (mAh / g) ÷ 1000, therefore, the current density j when charging at 4C is j = 4C × (17mg / cm³) 2 )×(350mAh / g)÷1000=23.8mA / cm 2 ; In this example, the physical properties of the electrolyte and the negative electrode of the lithium-ion secondary battery satisfy the following relationship: .

[0083] When preparing the graphite anode in a lithium-ion secondary battery, it is necessary to calculate the wet coating thickness and dry coating thickness of the anode coating based on set parameters. Specifically: Compacted density calculation: Compacted density = True density of negative electrode active material × (1 - Porosity of negative electrode sheet) = 2.2 g / cm³ 3 ×(1-0.32)=1.496g / cm 3 ; Theoretical thickness calculation for dry coating: Thickness = Negative electrode surface density ÷ Compacted density = (170g / m³) 2 ) ÷ (1.496g / cm 3 ≈113.6μm; The conversion relationship between wet and dry coatings: (1) Relationship between solid volume fraction and mass solid content in wet coating (the mass solid content in this paper refers to the solid content of the negative electrode slurry): Solid volume fraction in wet coating = (mass solid content ÷ true density of negative electrode active material) ÷ ​​((mass solid content ÷ true density of negative electrode active material) + (1 - mass solid content) ÷ density of solvent water) = (0.45 ÷ 2.2) ÷ ((0.45 ÷ 2.2) + (1 - 0.45) ÷ 1) = 0.2045 ÷ (0.2045 + 0.55) = 0.2709 = 27.09%; (2) Wet coating thickness = theoretical dry coating thickness × (1 - porosity) ÷ solid volume fraction in wet coating = 113.6 μm × (1 - 0.32) ÷ 0.2709 = 285.1 μm; The negative electrode sheet is prepared according to the above-mentioned dry coating thickness and wet coating thickness. The specific preparation method is as follows: Raw material preparation and pretreatment: (1) Selection of particle size Dv 50 =15~20μm artificial graphite, large particles are removed by sieving (to avoid uneven coating), and moisture is removed by vacuum drying (120℃, 2h); (2) SuperP is selected and ground before premixing with graphite (particle size <5μm after grinding) to enhance conductivity; (3) Sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) are selected as binders; deionized water is used as solvent. (4) Copper foil (thickness of 6~8μm) is cleaned with plasma or wiped with ethanol to improve surface energy.

[0084] Preparation of negative electrode slurry: (1) Add graphite (96.5wt%) and conductive agent SuperP (1wt%) to a planetary mixer and dry stir (300rpm, 30min) to make the conductive agent uniformly adhere to the graphite surface and form a preliminary conductive network. (2) Dissolve CMC (1wt%) in water and stir at high speed (2000rpm, 30min) until a transparent liquid is formed. Then add the mixture prepared in (1) to it and planet stir (500rpm, 60min) until a uniform paste is formed. (3) Add SBR (1.5wt%) emulsion and stir at low speed (300rpm, 30min) to avoid demulsification. (4) Add deionized water to adjust the solid content to 45% and control the viscosity to 3000-5000mPa·s (25℃).

[0085] Coating process: Coating is performed using a coating machine, with a wet coating thickness of approximately 285.1 μm (calculated based on the conversion relationship between the wet and dry coatings mentioned above); coating speed is 5~8 m / min; then, segmented drying is carried out in an oven at a temperature gradient of 60℃→80℃→100℃, with each segment controlled for 1~3 min; the areal density is controlled at 170±2 g / m³. 2 (Online beta-ray monitoring).

[0086] Roll pressing process: The thickness is adjusted to the target value (approximately 113.6 μm) by rolling with a roller press. The linear pressure of the roller press is 80~100 kN / m, and the target compaction density is 1.496 g / cm³. 3 .

[0087] Slitting and post-processing: The cells are slit according to their size and vacuum baked at 120℃ for 12 hours (moisture content ≤300ppm), with a dew point ≤-40℃, thus producing a graphite negative electrode sheet with a porosity of 32% as shown in this example.

[0088] This invention allows for rapid adjustment of process parameters and ensures production stability during the preparation of the negative electrode sheet. A beta-ray thickness gauge is installed at the coating machine outlet to measure the wet and dry coating thicknesses in real time using beta rays. If the deviation exceeds the limit, the measurement is automatically fed back to the coating system, allowing for adjustments to the coating flow rate or speed. Furthermore, the thickness data measured by the beta-ray thickness gauge is verified using a direct weighing method.

[0089] Example 2 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC) and ethyl propionate (EP) in a mass ratio of 3:7.

[0090] The conductivity of the electrolyte in this example was tested. It is 13.3 mS / cm.

[0091] This example provides a lithium-ion secondary battery, which is a three-electrode soft-pack battery. The positive electrode is LiFePO4, the negative electrode is graphite, the reference electrode is lithium-plated copper wire, and the electrolyte is the electrolyte described in this example. In this lithium-ion secondary battery, the porosity of the negative electrode sheet is... b The negative electrode has a surface density ρ of 32% and a surface area density ρ of 170 g / m². 2 (i.e., 17 mg / cm) 2 The true density c of the negative electrode active material is 2.2 g / cm³. 3 The specific capacity of the negative electrode active material is 350mAh / g; Current density j = rate of increase (C) × areal capacity (mAh / cm³) 2 The areal capacity = mass of active material on the negative electrode (mg) × specific capacity of the negative electrode active material (mAh / g) ÷ electrode area (cm²) 2 The mass of the negative electrode active material is related to its areal density; the mass of the negative electrode active material = areal density of the negative electrode sheet × electrode area; therefore, the areal capacity (mAh / cm²) is related to the areal density of the negative electrode sheet. 2 = Aspect density of the negative electrode (mg / cm³) 2 ) × specific capacity of negative electrode active material (mAh / g) ÷ 1000, therefore, the current density j when charging at 4C is j = 4C × (17mg / cm³) 2 )×(350mAh / g)÷1000=23.8mA / cm 2 ; In this example, the physical properties of the electrolyte and the negative electrode of the lithium-ion secondary battery satisfy the following relationship: .

[0092] Example 3 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) in a mass ratio of 3:2:5.

[0093] The conductivity of the electrolyte in this example was tested. It is 14.1 mS / cm.

[0094] Example 4 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC) and ethyl acetate (EA) in a mass ratio of 3:7.

[0095] The conductivity of the electrolyte in this example was tested. It is 15.7 mS / cm.

[0096] Example 5 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl acetate (EA), and methyl acetate (MA) in a mass ratio of 3:5:2.

[0097] The conductivity of the electrolyte in this example was tested. It is 17.1 mS / cm.

[0098] Example 6 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl acetate (EA), and methyl acetate (MA) in a mass ratio of 3:2:5.

[0099] The conductivity of the electrolyte in this example was tested. It is 19.1 mS / cm.

[0100] Example 7 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC) and methyl acetate (MA) in a mass ratio of 3:7.

[0101] The conductivity of the electrolyte in this example was tested. It is 20.2 mS / cm.

[0102] Example 8 The electrolyte in this example differs from that in Example 1 only in that, based on the total mass of the electrolyte, the lithium salt in this example consists of 6.25% lithium hexafluorophosphate (LiPF6) and 7.5% lithium bisfluorosulfonylimide (LiFSI) by mass. The concentration of lithium hexafluorophosphate and lithium bisfluorosulfonylimide in the electrolyte is 0.5 mol / L.

[0103] The conductivity of the electrolyte in this example was tested. It is 20.8 mS / cm.

[0104] Example 9 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that, based on the total mass of the electrolyte, the lithium salt in this example is lithium bis(fluorosulfonyl)imide (LiFSI) with a mass percentage of 15% and a concentration of 1 mol / L.

[0105] The conductivity of the electrolyte in this example was tested. It is 21.3 mS / cm.

[0106] The electrolytes in Examples 2-9 were all assembled into three-electrode soft-pack batteries according to the structure and composition of the lithium-ion secondary battery in Example 1. The only difference between them and the lithium-ion secondary battery in Example 1 was the electrolyte.

[0107] The physical properties of the electrolyte and negative electrode of the lithium-ion secondary batteries in Examples 2-9 satisfy the following relationship: .

[0108] Comparative Example 1 The electrolyte in this example differs from the electrolyte in Example 1 only in that the solvent in this example is a cyclic carbonate solvent ethylene carbonate (EC), a linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 3:5:2.

[0109] The conductivity of the electrolyte in this example was tested. It is 7.8 mS / cm.

[0110] Comparative Example 2 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 3:7.

[0111] The conductivity of the electrolyte in this example was tested. It is 8.4 mS / cm.

[0112] Comparative Example 3 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a mass ratio of 3:5:2.

[0113] The conductivity of the electrolyte in this example was tested. It is 9.2 mS / cm.

[0114] Comparative Example 4 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a mass ratio of 3:2:5.

[0115] The conductivity of the electrolyte in this example was tested. It is 10.3 mS / cm.

[0116] Comparative Example 5 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC) and dimethyl carbonate (DMC) in a mass ratio of 3:7.

[0117] The conductivity of the electrolyte in this example was tested. It is 12.4 mS / cm.

[0118] Comparative Example 6 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl propionate (EP) in a mass ratio of 3:5:2.

[0119] The conductivity of the electrolyte in this example was tested. It is 11.9 mS / cm.

[0120] Comparative Example 7 The only difference between the electrolyte in this example and the electrolyte in Example 1 is that the solvent in this example is ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) in a mass ratio of 3:5:2.

[0121] The conductivity of the electrolyte in this example was tested. It is 12.2 mS / cm.

[0122] The electrolytes in Comparative Examples 1 to 7 were all assembled into three-electrode soft-pack batteries according to the structure and composition of the lithium-ion secondary battery in Example 1. The only difference between them and the lithium-ion secondary battery in Example 1 was the electrolyte.

[0123] The physical properties of the electrolyte and negative electrode of the lithium-ion secondary batteries in Comparative Examples 1-7 satisfy the following relationship: .

[0124] The lithium-ion secondary batteries in Examples 1-9 and Comparative Examples 1-7 were tested for fast charging time at 10%-80% SOC under normal temperature conditions (2.0-3.75V), capacity percentage during 4C rate charging constant current period, and capacity retention rate during low-temperature discharge. The specific test methods are as follows: 10%~80% SOC fast charging time: This refers to the time required to charge the battery from 10% to 80% SOC. The specific testing steps are as follows: Step (1): Connect the positive terminal of the voltage acquisition line of one of the channels of the test equipment to the negative terminal of the battery under test, and connect the negative terminal of the voltage acquisition line to the reference electrode of the battery under test. Step (2): Let stand at 25℃ for 1 hour; Step (3): Discharge DC to 2V at 0.33C; Step (4): Let it sit for 5 minutes; Step (5): Charge at a constant current (CC) rate of 0.5C to 3.75V or negative parameter potential ≤0mV; Step (6): Let it sit for 10 minutes; Step (7): Discharge to 2V using 0.33C DC; Step (8): Let it sit for 5 minutes; Step (9): Charge at a constant current rate of 1C to 3.75V or negative parameter potential ≤0mV; Step (10): Let it sit for 10 minutes; Step (11): Repeat steps (3) to (10) a total of four times, changing the charging rate of steps (5) and (9) each time, as shown in Table 1 below: Table 1 Cyclic Test Conditions

[0125] Step (12): Discharge DC to 2V at a charging rate of 0.33C; Step (13): Let it sit for 5 minutes, then it's done; Step (14): Based on the test data, the corresponding fast charging time can be obtained after processing.

[0126] 4C rate charging constant current segment capacity ratio: In the 4C rate charging process, the constant current charging (CC) ratio is the proportion of the constant current charging (CC+CV) in the entire charging stage, i.e., CC / (CC+CV)×100%. The larger the CC ratio, the smaller the polarization. CV refers to constant voltage charging. The specific test method is as follows: Step (1): After connecting the battery to be tested to the test equipment, let it stand at 25℃ for 1 hour; Step (2): Discharge to 2V at 0.33C; Step (3): Let it stand for 5 minutes; Step (4): Charge to 3.75V at a constant current charging rate of 4C; Step (5): Charge to 0.05C at a constant voltage; Step (6): Let it stand for 5 minutes, and then stop; Step (7): According to the test data, the corresponding constant current charging capacity ratio can be obtained after processing.

[0127] Low temperature (-10℃) discharge capacity retention rate: The discharge capacity of the battery at 0.2C rate at 25℃ is tested, and then the discharge capacity at 0.2C rate at -10℃ is tested. The low temperature discharge capacity retention rate is calculated as (-10℃ discharge capacity / 25℃ discharge capacity) × 100%. The specific test steps are as follows: Step (1): After connecting the battery to be tested to the test equipment, let it stand at 25℃ for 1 hour; Step (2): Charge it at a constant current rate of 0.2C to 3.75V and charge it at a constant voltage rate of 0.05C; Step (3): Let it stand at 25℃ for 1 hour; Step (4): Discharge it at a discharge rate of 0.2C to 2V; Step (5): Let it rest for 30 minutes; Step (6): Charge it at a constant current rate of 0.2C to 3.75V and charge it at a constant voltage rate of 0.05C; Step (7): Thermal equilibrium at -10℃; Step (8): Let it stand for 1 hour; Step (9): Discharge it at a discharge rate of 0.2C to 2V; Step (10): Let it stand for 5 minutes and then stop; Step (11): According to the test data, the corresponding low temperature discharge capacity retention rate can be obtained after processing.

[0128] The performance test results of the lithium-ion secondary batteries in Examples 1-9 and Comparative Examples 1-7, obtained according to the above test methods, are shown in Table 2 below.

[0129] Table 2 Performance test results of lithium-ion secondary batteries

[0130] Table 2 shows that the relationship between the conductivity of the electrolyte, the porosity of the negative electrode, and the areal density of the negative electrode satisfies the following formula: At this time, lithium-ion secondary batteries can achieve fast charging at 4C, with a fast charging time of 7.5~10.4 minutes for 10%~80% SOC. The capacity ratio at 4C rate charging is 95.2%~98.1%, and the capacity retention rate at low temperature discharge is 62.6%~77.4%. Both the capacity ratio at 4C rate charging and the capacity retention rate at low temperature discharge are higher than those of comparative examples 1~7, and the charging time is shorter. Furthermore, when the relationship between the conductivity of the electrolyte, the porosity of the negative electrode, and the areal density of the negative electrode satisfies the formula... When required, lithium-ion secondary batteries are less prone to problems such as increased polarization, severe heat generation, lithium plating, and shortened cycle life during fast charging, thus improving the fast charging performance of lithium-ion secondary batteries.

[0131] Example 10 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the porosity of the negative electrode sheet in this example is 34%.

[0132] Example 11 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the porosity of the negative electrode sheet in this example is 38%.

[0133] The preparation method of the negative electrode sheet in the lithium-ion secondary battery of Examples 10-11 can be carried out with reference to Example 1.

[0134] Example 12 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the porosity of the negative electrode sheet in this example is 28%, and the areal density of the negative electrode sheet is 140 g / m³. 2 The current density is 19.6 mA / cm². 2 .

[0135] The method for preparing the negative electrode sheet in the lithium-ion secondary battery in this example is as follows: First, calculate the compaction density and the theoretical thickness of the dry coating according to the following formulas: Compacted density = True density of negative electrode active material × (1 - porosity) = 2.2 g / cm³ 3 ×(1-0.28)=1.584g / cm 3 ; Theoretical thickness of dry coating = negative electrode surface density ÷ compacted density = (140g / m³) 2 ) ÷ (1.584g / cm 3 ≈88.4μm; Then, the thickness of the wet coating is obtained according to the following conversion relationship between wet and dry coatings.

[0136] (1) Relationship between solid volume fraction and mass solid content in wet coating: Solid volume fraction in wet coating = (mass solid content ÷ true density of negative electrode active material) ÷ ​​((mass solid content ÷ true density of negative electrode active material) + (1 - mass solid content) ÷ density of solvent water) = (0.45 ÷ 2.2) ÷ ((0.45 ÷ 2.2) + (1 - 0.45) ÷ 1) = 0.2045 ÷ (0.2045 + 0.55) = 27.09%; (2) Wet coating thickness = theoretical dry coating thickness × (1 - porosity) ÷ solid volume fraction in wet coating = 88.4 μm × (1 - 0.28) ÷ 0.2709 = 235 μm; Then, the negative electrode sheet is prepared according to the thicknesses of the wet coating and dry coating described above. The specific steps are as follows: Raw material preparation and pretreatment: (1) Selection of particle size Dv 50 =15~20μm artificial graphite, large particles are removed by sieving (to avoid uneven coating), and moisture is removed by vacuum drying (120℃, 2h); (2) SuperP is selected and ground before premixing with graphite (particle size <5μm after grinding) to enhance conductivity; (3) CMC and SBR are selected as binders; deionized water is used as solvent; (4) Copper foil (thickness 6~8μm) is cleaned with plasma or wiped with ethanol to improve surface energy.

[0137] Preparation of negative electrode slurry: (1) Add graphite (96.5wt%) and conductive agent SuperP (1wt%) to a planetary mixer and dry stir (300rpm, 30min) to make the conductive agent uniformly adhere to the graphite surface and form a preliminary conductive network. (2) Dissolve CMC (1wt%) in water and stir at high speed (2000rpm, 30min) until a transparent liquid is formed. Then add the mixture prepared in (1) to it and planet stir (500rpm, 60min) until a uniform paste is formed. (3) Add SBR (1.5wt%) emulsion and stir at low speed (300rpm, 30min) to avoid demulsification. (4) Add deionized water to adjust the solid content to 45% and control the viscosity to 3000-5000mPa·s (25℃).

[0138] Coating process: Coating is performed using a coating machine, with a wet coating thickness of approximately 235 μm; coating speed is 5~8 m / min; after coating, drying is carried out in stages at a temperature gradient of 60℃→80℃→100℃ in an oven, with each stage controlled for 1~3 min; areal density control: 140±2 g / m³ 2 (Online beta-ray monitoring).

[0139] Roll pressing process: The thickness is adjusted to the target value (approximately 88.4 μm) by rolling the rollers, with a linear pressure of 80~100 kN / m and a target compaction density of 1.584 g / cm³. 3 This achieves a porosity of 28%.

[0140] Slitting and post-processing: Slitting is carried out according to the cell size, and vacuum baking is performed at 120℃ for 12 hours (moisture content ≤300ppm), dew point ≤-40℃.

[0141] Example 13 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the areal density of the negative electrode in this example is 160 g / m³. 2 The current density is 22.4 mA / cm². 2 .

[0142] The negative electrode sheet in the lithium-ion secondary battery in this example can be prepared by referring to Example 1.

[0143] Example 14 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the negative electrode sheet in this example has a porosity of 30% and an areal density of 160 g / m². 2 The current density is 22.4 mA / cm². 2 .

[0144] The method for preparing the negative electrode sheet in the lithium-ion secondary battery in this example is as follows: First, calculate the compaction density and the theoretical thickness of the dry coating according to the following formulas: Compacted density = True density of negative electrode active material × (1 - porosity) = 2.2 g / cm³ 3 ×(1-0.3)=1.54g / cm 3 ; Theoretical thickness of dry coating = negative electrode surface density ÷ compacted density = (160g / m³) 2 ) ÷ (1.54g / cm 3 ≈103.9μm; Then, the thickness of the wet coating is obtained according to the following conversion relationship between wet and dry coatings.

[0145] The relationship between the volume fraction of solids and the mass solid content in the wet coating: Volume fraction of solids in the wet coating = (mass solid content ÷ true density of the negative electrode active material) ÷ ​​((mass solid content ÷ true density of the negative electrode active material) + (1 - mass solid content) ÷ density of the solvent water) = (0.45 ÷ 2.2) ÷ ((0.45 ÷ 2.2) + (1 - 0.45) ÷ 1) = 0.2045 ÷ (0.2045 + 0.55) = 0.2709 = 27.09%; Wet coating thickness = theoretical dry coating thickness × (1 - porosity) ÷ solid volume fraction in wet coating = 103.9 μm × (1 - 0.3) ÷ 0.2709 = 268.5 μm; The negative electrode sheet is prepared according to the thicknesses of the dry coating and wet coating as described above. The specific steps are as follows: Raw material preparation and pretreatment: (1) Selection of particle size Dv 50 =15~20μm artificial graphite, large particles are removed by sieving (to avoid uneven coating), and moisture is removed by vacuum drying (120℃, 2h); (2) SuperP is selected and ground before premixing with graphite (particle size <5μm after grinding) to enhance conductivity; (3) CMC and SBR are selected as binders; deionized water is used as solvent; (4) Copper foil (thickness 6~8μm) is cleaned with plasma or wiped with ethanol to improve surface energy.

[0146] Preparation of negative electrode slurry: (1) Add graphite (96.5wt%) and conductive agent SuperP (1wt%) to a planetary mixer and dry stir (300rpm, 30min) to make the conductive agent uniformly adhere to the graphite surface and form a preliminary conductive network. (2) Dissolve CMC (1wt%) in water and stir at high speed (2000rpm, 30min) until a transparent liquid is formed. Then add the mixture prepared in (1) to it and planet stir (500rpm, 60min) until a uniform paste is formed. (3) Add SBR (1.5wt%) emulsion and stir at low speed (300rpm, 30min) to avoid demulsification. (4) Add deionized water to adjust the solid content to 45% and control the viscosity to 3000-5000mPa·s (25℃).

[0147] Coating process: Coating is performed using a coating machine, with a wet coating thickness of approximately 268.5 μm; coating speed is 5-8 m / min; after coating, drying is carried out in stages at a temperature gradient of 60℃→80℃→100℃ in an oven, with each stage controlled for 1~3 min; areal density control: 160±2 g / m³ 2 (Online beta-ray monitoring).

[0148] Roll pressing process: The thickness is adjusted to the target value (approximately 103.9 μm) by rolling the roller press, with a linear pressure of 80~100 kN / m and a target compaction density of 1.54 g / cm³. 3 This achieves a porosity of 30%.

[0149] Slitting and post-processing: Slitting is carried out according to the cell size, and vacuum baking is performed at 120℃ for 12 hours (moisture content ≤300ppm), dew point ≤-40℃.

[0150] The physical properties of the electrolyte and negative electrode of the lithium-ion secondary batteries in Examples 10-14 satisfy the following relationship: .

[0151] Comparative Example 8 The only difference between the lithium-ion secondary battery in this example and that in Example 2 is that the porosity of the negative electrode sheet in this example is 28%.

[0152] The method for preparing the negative electrode sheet in the lithium-ion secondary battery in this example is as follows: First, calculate the compaction density and the theoretical thickness of the dry coating according to the following formulas: Compacted density = True density of negative electrode active material × (1 - porosity) = 2.2 g / cm³ 3 × (1 - 0.28) = 1.584 g / cm³ 3 ; Theoretical thickness of dry coating = negative electrode surface density ÷ compacted density = (170g / m³) 2 ) ÷ (1.584g / cm 3≈107.3μm; Then, the thickness of the wet coating is obtained according to the following conversion relationship between wet and dry coatings.

[0153] The relationship between the volume fraction of solids and the mass solid content in the wet coating: Volume fraction of solids in the wet coating = (mass solid content ÷ true density of the negative electrode active material) ÷ ​​((mass solid content ÷ true density of the negative electrode active material) + (1 - mass solid content) ÷ density of the solvent water) = (0.45 ÷ 2.2) ÷ ((0.45 ÷ 2.2) + (1 - 0.45) ÷ 1) = 0.2045 ÷ (0.2045 + 0.55) = 0.2709 = 27.09%; Wet coating thickness = theoretical dry coating thickness × (1 - porosity) ÷ solid volume fraction in wet coating = 107.3 μm × (1 - 0.28) ÷ 0.2709 = 285.1 μm; The negative electrode sheet is prepared according to the above-mentioned dry coating thickness and wet coating thickness, and the specific steps are as follows: Raw material preparation and pretreatment: (1) Selection of particle size Dv 50 =15~20μm artificial graphite, large particles are removed by sieving (to avoid uneven coating), and moisture is removed by vacuum drying (120℃, 2h); (2) SuperP is selected and ground before premixing with graphite (particle size <5μm after grinding) to enhance conductivity; (3) CMC and SBR are selected as binders; deionized water is used as solvent; (4) Copper foil (thickness 6~8μm) is cleaned with plasma or wiped with ethanol to improve surface energy.

[0154] Preparation of negative electrode slurry: (1) Add graphite (96.5wt%) and conductive agent SuperP (1wt%) to a planetary mixer and dry stir (300rpm, 30min) to make the conductive agent uniformly adhere to the graphite surface and form a preliminary conductive network. (2) Dissolve CMC (1wt%) in water and stir at high speed (2000rpm, 30min) until a transparent liquid is formed. Then add the mixture prepared in (1) to it and planet stir (500rpm, 60min) until a uniform paste is formed. (3) Add SBR (1.5wt%) emulsion and stir at low speed (300rpm, 30min) to avoid demulsification. (4) Add deionized water to adjust the solid content to 45% and control the viscosity to 3000-5000mPa·s (25℃).

[0155] Coating process: Coating is performed using a coating machine, with a wet coating thickness of approximately 285.1 μm; coating speed is 5~8 m / min; after coating, the coating is dried in stages under an oven temperature gradient of 60℃→80℃→100℃, with each stage controlled for 1~3 minutes; areal density control: 170±2 g / m³ 2 (Online beta-ray monitoring).

[0156] Roll pressing process: The thickness is adjusted to the target value (approximately 107.3μm) by rolling the roller press, with a linear pressure of 80-100kN / m and a compaction density target value of 1.584g / cm³, thereby achieving a porosity of 28%.

[0157] Slitting and post-processing: Slitting is carried out according to the cell size, and vacuum baking is performed at 120℃ for 12 hours (moisture content ≤300ppm), dew point ≤-40℃.

[0158] Comparative Example 9 The only difference between the lithium-ion secondary battery in this example and that in Example 12 is that the areal density of the negative electrode sheet in this example is 160 g / m³. 2 The current density is 22.4 mA / cm². 2 .

[0159] Comparative Example 10 The only difference between the lithium-ion secondary battery in this example and that in Example 12 is that the areal density of the negative electrode in this example is 180 g / m². 2 The current density is 25.2 mA / cm². 2 .

[0160] Comparative Example 11 The only difference between the lithium-ion secondary battery in this example and that in Example 12 is that the anode surface density in this example is 200 g / m². 2 The current density is 28 mA / cm². 2 .

[0161] The preparation method of the negative electrode sheet in the lithium-ion secondary batteries of Comparative Examples 9-11 can be referred to Comparative Example 8.

[0162] The physical properties of the electrolyte and negative electrode of the lithium-ion secondary batteries in Comparative Examples 8-11 satisfy the following relationship: .

[0163] Following the methods described above, the capacity percentage of the lithium-ion secondary batteries in Examples 10-14 and Comparative Examples 8-11 was tested at room temperature (2.0-3.75V) and a 4C rate. The presence of lithium plating at the negative electrode interface after 50 cycles at 25°C and 4C was also tested. Four parallel tests were performed, and the presence of lithium plating was determined based on the results of the four tests. The specific test results are shown in Table 3 below. The physical images of the negative electrode interfaces in Examples 2, 10-14, and Comparative Examples 8-11 after 50 cycles at 25°C and 4C are shown below. Figures 1-10 As shown.

[0164] Table 3 Test results of lithium plating at the interface of lithium-ion secondary batteries

[0165] From Table 3 and Figures 1-10 It can be seen that when the lithium-ion secondary battery in this invention satisfies the formula: At 4C rate, no lithium deposition occurs at the interface during fast charging, and the 4C rate charging capacity ratio is 95.1%~96.8%. If the lithium-ion secondary battery does not meet the formula... At 4C rate, there will be a more serious lithium plating phenomenon during fast charging, and the capacity ratio of 4C rate charging will be reduced, thus affecting the cycle performance of the battery.

[0166] The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. An electrolyte, characterized in that: Including lithium salts and solvents; The solvents include carbonate solvents and carboxylic acid ester solvents in a mass ratio of 1:(1~3); The carbonate solvent is selected from at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate; The carboxylic acid ester solvent is selected from at least one of methyl acetate, methyl propionate, ethyl acetate, propyl acetate, ethyl propionate, and propyl propionate. The conductivity of the electrolyte is ≥12.7 mS / cm.

2. The electrolyte according to claim 1, characterized in that: The concentration of the lithium salt is 0.8~1.5 mol / L; And / or, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorobis(oxalate)phosphate, lithium bis(oxalate)borate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide.

3. The electrolyte according to claim 1, characterized in that: The electrolyte also contains additives, the mass of which is 0.5% to 10% of the total mass of the electrolyte; And / or, the electrolyte further contains an additive selected from at least one of vinyl sulfate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, lithium difluorooxalate borate, vinylene carbonate, ethylene ethylene carbonate, fluoroethylene carbonate, and 1,3-propanesulfonate lactone.

4. A lithium-ion secondary battery, comprising a positive electrode, an electrolyte, and a negative electrode; wherein the negative electrode contains a negative electrode active material; characterized in that: The physical properties of the electrolyte and the negative electrode of the lithium-ion secondary battery satisfy the following relationship: Where κ is the conductivity of the electrolyte, in mS / cm; b Porosity of the negative electrode sheet; ρ is the surface density of the negative electrode plate, in g / m³. 2 ; c is the true density of the negative electrode active material, in g / cm³. 3 ; j is the current density, in mA / cm². 2 .

5. The lithium-ion secondary battery according to claim 4, characterized in that: The negative electrode sheet has at least one of the following characteristics: (a1) The areal density of the negative electrode sheet is 120~220 g / m³. 2 ; (a2) In the negative electrode sheet, the true density of the negative electrode active material is 2.0~2.4 g / cm³. 3 ; (a3) In the negative electrode sheet, the negative electrode active material is selected from at least one of graphite and silicon-doped graphite; (a4) The porosity of the negative electrode sheet is 28-38%.

6. The lithium-ion secondary battery according to claim 4, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 3.

7. The lithium-ion secondary battery according to claim 4, characterized in that: The positive electrode sheet has at least one of the following characteristics: (a1) The positive electrode active material of the positive electrode sheet includes at least one of lithium iron phosphate and lithium manganese iron phosphate; (a2) The porosity of the positive electrode sheet is 25%~35%.

8. The lithium-ion secondary battery according to claim 4, characterized in that: The current density is calculated as follows: (charge rate × areal density of the negative electrode × specific capacity of the negative electrode active material) ÷ ​​1000; where the specific capacity of the negative electrode active material is expressed in mAh / g and the charge rate is expressed in C.

9. The lithium-ion secondary battery according to claim 8, characterized in that: The charging rate is ≥4C.

10. An electrical appliance, characterized in that: The electrical equipment contains a battery; The battery contains the electrolyte according to any one of claims 1 to 3, or the battery is a lithium-ion secondary battery according to any one of claims 4 to 9.

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