A negative electrode material, a negative electrode sheet and lithium ion battery

Abstract

The present invention provides a negative electrode material, a negative electrode sheet, and a lithium-ion battery. By introducing silicon particles and carbon particles of four different particle size ranges into the negative electrode active material and adjusting the silicon content of particles of different size ranges, the characteristics of particles of different sizes and the gaps between particles are effectively utilized to increase the energy density of the battery cell and disperse the local stress caused by silicon particle expansion, thereby improving the cycle life and safety performance of the lithium battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and in particular to a negative electrode material, a negative electrode sheet, and a lithium-ion battery. Background Art

[0002] Lithium batteries, a common power source for portable power tools, utilize the back-and-forth movement of lithium ions between positive and negative electrodes to charge and discharge. They offer advantages such as high energy density, low self-discharge, and a long cycle life. Portable power tools are increasingly used in our lives, and their longevity has become a focus of attention. The question of how to effectively increase the energy density of lithium-ion batteries while ensuring safety, thereby extending their battery life, has become a hot topic in the field of lithium battery technology.

[0003] Increasing the energy density of lithium-ion battery cells by introducing silicon into the negative electrode active material is a common method. However, as the battery heats up during charging and discharging, the silicon particles in the negative electrode active material expand, causing mechanical stress to accumulate in the negative electrode sheet of the lithium-ion battery. At the same time, the expansion of the particles will form local mutual extrusion, resulting in uneven stress, causing local binder breakage, electrode cracking, and even collector deformation, posing a huge hidden danger to battery safety. Summary of the Invention

[0004] To address the existing problem of silicon particles expanding and damaging the battery structure when added to the negative electrode active material of lithium batteries, the present invention provides a negative electrode material, a negative electrode sheet, and a lithium-ion battery. By introducing silicon particles and carbon particles of four different particle size ranges into the negative electrode active material and adjusting the silicon content of particles of different size ranges, the present invention effectively utilizes the properties of particles of different sizes and the gaps between particles, thereby increasing the energy density of the battery cell and dissipating the localized stress caused by silicon particle expansion.

[0005] The specific technical solutions of the present invention are:

[0006] First, the present invention provides a negative electrode active material, which includes carbon particles and silicon particles. The negative electrode active material can be divided into four different particle size ranges of particles D1, D2, D3, and D4 according to the particle size of the negative electrode active material, and meets the following requirements:

[0007] D1>18um;

[0008] 10um<D2≤18um;

[0009] 5um<D3≤10um;

[0010] 0um<D4≤5um;

[0011] The silicon content of the D1 particles is ≤5% by weight, the silicon content of the D2 particles is 2-15% by weight, the silicon content of the D3 particles is 3-20% by weight, and the silicon content of the D4 particles is ≤6% by weight. The silicon particles comprise one or more of elemental silicon or silicon oxide particles; the carbon particles comprise one or more of artificial graphite, natural graphite, hard carbon, soft carbon, or mesophase carbon microbeads. The silicon and carbon particles fuse during the electrode preparation process, producing silicon-carbon composite particles.

[0012] D1 particles with a particle size greater than 18um are called super-large particles. The expansion of silicon particles of this size will produce huge local stress, which will affect the structure of the negative electrode sheet. Therefore, the proportion of silicon in the D1 particles needs to be relatively reduced. D2 particles with a particle size between 10-18um are called large particles. If the silicon content in the D2 particles is too little, the compaction density of the electrode sheet will be reduced, which is not conducive to achieving high specific energy of the battery cell. However, if the proportion of silicon in the D2 particles is too high, it will also cause the problem of excessive expansion stress. Therefore, the most appropriate proportion of silicon in the D2 particles is 3-20%. D3 particles with a particle size between 5-10um are called medium particles, and D4 particles with a particle size less than 5um are called small particles. Small particles are conducive to filling gaps and improving energy density. At the same time, their expansion can be effectively dispersed, thereby reducing local strain and avoiding local electrode deformation. Therefore, the present invention improves the strain capacity of the negative electrode sheet by adjusting the proportion of silicon in the D1, D2, D3, and D4 particles, thereby improving the cycle life and safety performance of the battery cell.

[0013] Preferably, the negative electrode active material of the present invention further includes a metal element, and the metal element is at least one of Al, Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La, and Y, which can further improve the conductivity of the negative electrode active material.

[0014] Second, the present invention provides a negative electrode sheet, comprising a negative electrode current collector, wherein at least one surface of the negative electrode current collector is coated with a negative electrode coating containing the negative electrode active material.

[0015] Preferably, the negative electrode coating is divided into a lower half coating close to the negative electrode current collector and an upper half coating away from the negative electrode current collector, and the average particle sizes of the negative electrode active material in the upper half coating and the lower half coating are d1 and d2 respectively, and satisfy: 1.4d1≥d2≥1.05d1.

[0016] By satisfying d1 and d2 within the range of the above formula, the gap ratio of the upper half coating of the negative electrode coating can be slightly larger than that of the lower half coating, thereby making the lower half coating more compact and more structurally stable, thereby improving the fast charging performance and safety of the lithium battery as a whole.

[0017] Preferably, the average values ​​of ID / IG of the upper and lower coating layers are Z1 and Z2, respectively, and satisfy 0.7Z2≤Z1≤1.7Z2. Within this range, the negative electrode sheet exhibits superior chemical properties and more uniform stress distribution, resulting in improved cycle life and safety.

[0018] Preferably, the negative electrode coating is divided into a first coating, a second coating, and a third coating of equal thickness, extending away from the surface of the negative electrode current collector. The average particle sizes of the negative electrode active material in the first, second, and third coatings are D-1, D-2, and D-3, respectively, and D-3 ≤ D-2 ≤ D-1. If the upper layer contains more large particles and the lower layer contains more small particles, the strain in the upper layer will be excessive, causing deformation and delamination of the active materials in the upper and lower layers, reducing the cycle life and safety. Therefore, satisfying the above formula is necessary to improve the cycle life and safety of the battery cell.

[0019] Third, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps:

[0020] (1) Adding the negative electrode active material, the first conductive agent, the second conductive agent, the dispersant, and the binder to deionized water respectively, mixing and stirring in a homogenizer until the solid content is 40-60%, to obtain a negative electrode slurry;

[0021] (2) The slurry is applied to at least one surface of a negative electrode current collector, baked, and roll-pressed to obtain the layered negative electrode sheet.

[0022] Preferably, the first conductive agent is acetylene black, the second conductive agent is single-walled carbon nanotubes, the dispersant is carboxymethyl cellulose, and the binder is polyacrylic acid or styrene-butadiene rubber.

[0023] Through the above preparation method, the surfaces of the silicon particles and carbon particles in the negative electrode active material can be fully wrapped by polyacrylic acid or styrene-butadiene rubber, and after the slurry is baked, the interaction between the polyacrylic acid or styrene-butadiene rubber and the solvent is stronger, which can carry small-sized particles upward, thereby achieving the particle size of the upper half coating of the negative electrode coating is not larger than that of the lower half coating.

[0024] Fourth, the present invention also provides a lithium-ion battery, which includes a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, wherein the electrolyte contains fluorine element, the mass ratio of fluorine element to carbon element in the upper half coating is A1, the mass ratio of fluorine element to carbon element in the lower half coating is B1, and satisfies 1.1B1≥A1≥0.9B1.

[0025] Preferably, the silicon particles in the negative electrode active material are coated with a carbon-containing layer, the thickness of which is T. Starting from the surface of the silicon particles, the carbon content of the first 1 / 2T of the carbon-containing layer is M1, and the carbon content of the second 1 / 2T of the carbon-containing layer is M2, with M1 ≥ 1.04M2. Within this range, the chemical properties of the negative electrode active material are superior, the stress distribution is more uniform, and thus, the cycle life and safety are improved. If M1 is less than 1.04M2, the conductivity of the battery cell will deteriorate, causing cycle degradation.

[0026] Preferably, the silicon particles in the negative electrode active material are coated with a fluorine-containing layer. The thickness of the fluorine-containing layer is T. Starting from the surface of the silicon particle, the fluorine content of the first 1 / 2T of the fluorine-containing layer is N1, and the fluorine content of the second 1 / 2T of the fluorine-containing layer is N2, and the fluorine content satisfies 10% ≤ N1 + N2 ≤ 50%. Within the range of the above formula, the chemical properties of the negative electrode active material are better, the stress distribution is more uniform, and thus, the cycle life and safety are improved. If N1 + N2 exceeds 50%, the cycle life will deteriorate.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] (1) The present invention introduces silicon particles and carbon particles of four different particle size ranges into the negative electrode active material, and simultaneously adjusts the proportion of silicon in particles of different size ranges, effectively utilizing the characteristics of particles of different particle sizes and the gaps between particles, thereby improving the energy density of the battery cell and dispersing the local stress generated by the expansion of silicon particles;

[0029] (2) The present invention optimizes the negative electrode sheet preparation process, so that the negative electrode coating of the prepared negative electrode sheet is stratified, and the upper half coating has a larger porosity and a higher liquid absorption rate, and the lower half coating has a higher compactness and a more stable structure, thereby improving the overall fast charging performance of the battery;

[0030] (3) The negative electrode active material of the present invention includes metal elements, which further improves the conductive properties of the active material. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solution of this application, the following is a brief introduction to the drawings required for the description of this application.

[0032] Figure 1 is a schematic diagram of particle distribution in the cross section of the negative electrode sheet in this application;

[0033] Figure 2 It is a schematic diagram of the structure of the negative electrode sheet in this application;

[0034] Figure 3 1 is a graph of cycle retention rates of Example 1 and Comparative Example 1 in this application;

[0035] Figure numerals: 1-D1 particles, 2-D2 particles, 3-D3 particles, 4-D4 particles, 5-upper half coating, 6-lower half coating, 7-negative electrode coating, 8-current collector, 9-negative electrode sheet, 11-silicon particles, 12-carbon particles. DETAILED DESCRIPTION

[0036] Examples of this embodiment are described in detail below. The examples are only used to explain this embodiment and are not to be construed as limiting this embodiment.

[0037] In the description of the embodiments of the present application, it should be noted that all ranges disclosed in this application will be understood to encompass any and all subranges therein. For example, the stated range "3% to 20%" should be deemed to include any and all subranges that begin with a minimum value of 3% or greater and end with a maximum value of 20% or less, such as 4% to 19%, or 4.5% to 15%, or 12% to 13%. At the same time, all ranges disclosed in this application are also deemed to include the endpoints of the ranges, unless otherwise expressly stated. For example, the ranges "between 4 and 6" or "4 to 6" or "4-6" should generally be deemed to include the endpoints 4 and 6.

[0038] In the description of this embodiment, "several" means one or more, "many" means more than two, "greater than," "less than," and "exceed" are understood to exclude the number itself, while "above," "below," and "within" are understood to include the number itself. The use of "first" and "second" in the description is solely for the purpose of distinguishing technical features and is not to be construed as indicating or implying relative importance, implicitly specifying the number of the indicated technical features, or implicitly specifying the order of the indicated technical features.

[0039] Figure 1 A preferred embodiment of the negative electrode sheet of the present invention is shown, wherein the negative electrode active material in the negative electrode coating 7 coated on the surface of the current collector 8 includes particles D1 particles 1, D2 particles 2, D3 particles 3, and D4 particles 4 in four different particle size ranges. It will be understood that the particles in the negative electrode active material are divided into particles of at least four different particle size ranges, and can also be divided into particles of five or more particle size ranges.

[0040] The silicon particles include one or more of elemental silicon and / or silicon-oxygen particles, and the carbon particles are selected from a mixture of one or more of artificial graphite, natural graphite, hard carbon, soft carbon or mesophase carbon microspheres.

[0041] In some embodiments, in order to further improve the conductivity of the negative electrode material, the negative electrode active material also includes a metal element, and the metal element is at least one of Al (aluminum), Zr (zirconium), Ti (titanium), Sr (strontium), Mg (magnesium), Na (sodium), Mo (molybdenum), Ca (calcium), Ba (barium), La (lanthanum), and Y (yttrium).

[0042] Figure 2 A preferred embodiment of the negative electrode sheet of the present invention is shown, wherein the negative electrode sheet 9 includes a current collector 8, and the upper and lower surfaces of the current collector 8 are coated with a negative electrode coating 7. The negative electrode coating 7 can be divided into a lower half coating 6 close to the surface of the current collector 8 and an upper half coating 5 away from the surface of the current collector 8.

[0043] In some embodiments, the negative electrode coating is divided into a first coating, a second coating, and a third coating of the same thickness in a direction away from the surface of the negative electrode current collector, and the average particle size of the negative electrode active material in the first coating, the second coating, and the third coating is D-1, D-2, and D-3, respectively, and D-3≤D-2≤D-1 is satisfied.

[0044] In some embodiments, the method for preparing the negative electrode sheet includes the following steps:

[0045] (1) Add the above-mentioned negative electrode active material, first conductive agent, second conductive agent, dispersant, and binder to deionized water respectively, mix and stir evenly in a homogenizer, and obtain a negative electrode slurry with a solid content of 40-60%;

[0046] (2) The slurry is applied to at least one surface of a negative electrode current collector, baked, and roll-pressed to obtain the layered negative electrode sheet.

[0047] The first conductive agent is acetylene black, the second conductive agent is single-walled carbon nanotubes, and the dispersant is carboxymethyl cellulose.

[0048] In some embodiments, a binder composed of carboxymethyl cellulose and / or styrene-butadiene rubber and / or polyacrylic acid is further introduced in step (1). Preferably, the binder is polyacrylic acid and the solvent is deionized water. In some embodiments, the slurry is baked at a temperature of 80-100° C. for a time of 6-10 hours in step (2). Preferably, the slurry is baked at a temperature of 85° C. for a time of 6-8 hours.

[0049] In some embodiments, the lithium-ion battery is composed of a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the electrolyte contains fluorine element. Preferably, at least one of fluoroethylene carbonate, lithium salt and fluorophenyl acetate is introduced into the electrolyte. Furthermore, the lithium salt includes one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium difluorooxalatoborate and lithium difluorodioxalatophosphate. Furthermore, an additive is also introduced into the electrolyte, and preferably, the additive is tris(trimethylsilyl)phosphate.

[0050] In some embodiments, the average values ​​of ID / IG of the upper and lower coatings are Z1 and Z2, respectively, and satisfy 0.7Z2≤Z1≤1.7Z2. ID / IG in this application refers to the intensity ratio of the D-peak and the G-peak in the Raman spectrum, where I represents intensity and the D-peak represents the defects of the lattice. The larger the ID / IG value, the more defects the carbon atom crystal has. However, when ID / IG is too large, indicating that there are too many surface defects, it will trigger too many side reactions, increase the ionic conductivity of the interface, and deteriorate the cycle performance. The average values ​​of ID / IG of the upper and lower coatings are within the range that satisfies the above formula, and a better cycle life can be obtained.

[0051] In some embodiments, the silicon particles in the negative electrode active material are coated with a carbon-containing layer, the thickness of the carbon-containing layer is T, and taking the surface of the silicon particles as the starting point, the carbon content of the carbon-containing layer in the first 1 / 2T thickness is M1, and the carbon content of the carbon-containing layer in the last 1 / 2T thickness is M2, and M1≥1.04M2 is satisfied.

[0052] In some embodiments, the silicon particles in the negative electrode active material are coated with a fluorine-containing layer, the thickness of the fluorine-containing layer is T, and taking the surface of the silicon particles as the starting point, the fluorine content of the fluorine-containing layer with a thickness of the first 1 / 2T is N1, and the fluorine content of the fluorine-containing layer with a thickness of the last 1 / 2T is N2, and 10%≤N1+N2≤50% is satisfied.

[0053] The present invention will be further explained with reference to the following examples.

[0054] Example 1

[0055] Embodiment 1 of the present invention provides a negative electrode sheet comprising a negative electrode active material. The negative electrode active material is a mixture of silicon particles and graphite particles doped with metal elements, wherein, by weight percentage, the silicon particles account for 10 wt % and the rest is graphite doped with metal elements.

[0056] Silicon particles are prepared by the following route:

[0057] (1) Purchase silicon particle A precursor, which contains silicon particles with particle sizes of 7 μm, 25 μm, and 1.1 μm; purchase silicon particle B precursor, which contains silicon particles with particle sizes of 12 μm, 23 μm, and 4.6 μm;

[0058] (2) 20 kg of silicon particles B were placed in a ball mill, 1 kg of ethanol was added, and the mixture was ground for 2 hours;

[0059] (3) Add 10 kg of silicon particles A and 0.5 kg of ethanol, and continue grinding for 1 hour;

[0060] (4) Place the dried powder in a rotary furnace and introduce N2 at a rate of 10 L / h. Gradually heat the furnace to 300°C at a rate of 5°C / min and keep it at that temperature for 1 hour.

[0061] (5) Add methane at a rate of 0.5 L / h, gradually heat to 850°C at a rate of 5°C / min, and keep at this temperature for 12 hours;

[0062] (6) Turn off the methane gas, turn off the heating, and gradually cool to below 50°C;

[0063] (7) Turn off N2, turn off the power, take out the powder, grind it through a 200-mesh sieve, and obtain silicon particles.

[0064] Graphite doped with metal elements and coated with asphalt:

[0065] 1) Purchase graphitized graphite particles;

[0066] 2) Add 10g of Al2O3 to 80g of asphalt and mix well;

[0067] 3) Add 90g of the above mixture to 2kg of graphite particles and disperse them evenly;

[0068] 4) Treating it at 1200°C for 48 hours under N2 atmosphere;

[0069] 5) crushing, grinding, and sieving to obtain graphite material doped with metal elements.

[0070] The negative electrode active material is prepared by mixing silicon particles and graphite particles doped with metal elements, and specifically includes the following steps:

[0071] (1) Silicon particles and graphite particles were added to a centrifugal mixer at a ratio of 10:90 and mixed, and 10% ethanol and 0.5% isopropanol were added;

[0072] (2) Mix the mixture at 200 r / min for 10 minutes and let it stand for 10 minutes;

[0073] (3) the mixture was further mixed at 1000 r / min for 30 minutes;

[0074] (4) After mixing, the mixture was baked in a vacuum oven at 80°C for 24 h, and then the resultant was sieved through 200 mesh to obtain the target product.

[0075] The following analysis and statistics were performed on the prepared negative electrode sheets:

[0076] 1. Interface Particle Size Statistics: SEM analysis was performed on the cross-section of the negative electrode sheet to measure its thickness. The sheet was then divided into two or three equal sections as required. Particles within each divided section were then counted to obtain the average particle sizes d1 and d2, or D-1, D-2, and D-3. The statistical method was as follows: a random location was selected as the starting point, and the statistics were performed from top to bottom and left to right. Particles on the boundary line were counted only if at least 50% of them were within this region; otherwise, they were counted as another region. Particles with a size below 1 μm were not counted. Counts were performed according to the following rules up to 100 particles, and the average value was calculated. Through testing and statistics, it was found that for the negative electrode sheet of Example 1, when divided into two equal sections, the average particle size d1 of the upper half coating was 10.4 μm, and the average particle size d2 of the lower half coating was 12.4 μm. For the negative electrode sheet divided into three equal sections, the average particle size D-1 of the first coating was 12.8 μm, the average particle size D-2 of the second coating was 11.0 μm, and the average particle size D-3 of the third coating was 9.9 μm.

[0077] 2. Raman spectroscopy test: The average value Z1 of the ID / IG ratio of the upper half coating and the average value Z2 of the ID / IG ratio of the lower half coating were measured by Raman spectroscopy. The test showed that Z1 of Example 1 was 1.45 and Z2 was 0.93.

[0078] 3. Fluorine to carbon mass ratio in the upper and lower coating layers: The mass ratio of fluorine to carbon in the upper coating layer is A1, and the mass ratio of fluorine to carbon in the lower coating layer is B1. Testing revealed that A1 for Example 1 was 0.44, and B1 was 0.43.

[0079] 4. Carbon content in the carbon-containing layer of silicon particles in the negative electrode active material and fluorine content in the fluorine-containing layer: Prepare a cross-section of the negative electrode sheet, randomly select 10 silicon particles with a particle size between 3-30um, and perform elemental analysis within a 5um thickness range around them; use EDS to perform point scans within the inner layer area of ​​0-2.5um and the outer layer area of ​​2.5-5um around them. The point scans must not contain other active materials. The point scans are taken at 3 different locations and the average value is calculated. Finally, by sorting all the results of the above tests and removing the 3 minimum and 3 maximum values, the average value of the remaining 27 data is calculated to obtain the inner carbon content M1 and outer carbon content M2 of the carbon-containing layer, and the inner fluorine content N1 and outer fluorine content N2 of the fluorine-containing layer. The test results show that M1 is 0.43, M2 is 0.31, N1 is 0.14, and N2 is 0.22.

[0080] Example 2

[0081] Example 2 provides a negative electrode sheet, which differs from Example 1 in that the weight ratio of silicon particles is 16 wt %, and a total of 10 kg of silicon particles A and 10 kg of silicon particles B are added.

[0082] Example 3

[0083] Example 3 provides a negative electrode sheet, which differs from Example 1 in that 30 kg of silicon particles A and 10 kg of silicon particles B are added.

[0084] Example 4

[0085] Example 4 provides a negative electrode sheet, which differs from Example 1 in that 10 kg of silicon particles A and 30 kg of silicon particles B are added.

[0086] Example 5

[0087] Example 5 provides a negative electrode sheet, which differs from Example 1 in that 10 kg of silicon particles A and 40 kg of silicon particles B are added.

[0088] Example 6

[0089] Example 6 provides a negative electrode sheet, which differs from Example 1 in that 10 kg of silicon particles A and 20 kg of silicon particles B are added.

[0090] Example 7

[0091] Example 7 provides a negative electrode sheet, which differs from Example 1 in that the weight ratio of silicon particles is 16 wt %, and a total of 10 kg of silicon particles A and 20 kg of silicon particles B are added.

[0092] Example 8

[0093] Example 8 provides a negative electrode sheet, which differs from Example 1 in that 10 kg of silicon particles A and 40 kg of silicon particles B are added.

[0094] Example 9

[0095] Example 9 provides a negative electrode sheet, which differs from Example 1 in that, when the negative electrode sheet of Example 9 is divided into two equal parts, the average particle size d1 of the upper half coating is 10.5um, and the average particle size d2 of the lower half coating is 11.1um; when the negative electrode sheet is divided into three equal parts, the average particle size D-1 of the first coating is 12.2um, the average particle size D-2 of the second coating is 10.4um, and the average particle size D-3 of the third coating is 9.7um.

[0096] Example 10

[0097] Example 10 provides a negative electrode sheet, which differs from Example 1 in that, when the negative electrode sheet of Example 10 is divided into two equal parts, the average particle size d1 of the upper half coating is 10.1 um, and the average particle size d2 of the lower half coating is 13.9 um; when the negative electrode sheet is divided into three equal parts, the average particle size D-1 of the first coating is 14.1 um, the average particle size D-2 of the second coating is 11.5 um, and the average particle size D-3 of the third coating is 9.4 um.

[0098] Example 11

[0099] Example 11 provides a negative electrode sheet, which differs from Example 1 in that, in Example 11, Raman spectroscopy testing shows that Z1 is 1.53 and Z2 is 1.33.

[0100] Example 12

[0101] Example 12 provides a negative electrode sheet, which differs from Example 1 in that, in Example 12, Raman spectroscopy testing shows that Z1 is 1.53 and Z2 is 1.33.

[0102] Example 13

[0103] Example 13 provides a negative electrode sheet, which differs from Example 1 in that, according to the test results of Example 13, A1 is 0.44 and B1 is 0.45.

[0104] Example 14

[0105] Example 14 provides a negative electrode sheet, which differs from Example 1 in that the metal element in Example 14 is Ti, and through testing, it is found that M1 is 0.47, M2 is 0.35, N1 is 0.18, and N2 is 0.21.

[0106] Example 15

[0107] Example 15 provides a negative electrode sheet, which differs from Example 1 in that the metal element in Example 15 is Mn, and through testing, it is found that M1 is 0.44, M2 is 0.32, N1 is 0.15, and N2 is 0.22.

[0108] Comparative Example 1

[0109] Comparative Example 1 provides a negative electrode active material, which differs from Example 1 in that the silicon particles in Comparative Example 1 are 15 wt %, and a total of 10 kg of silicon particles A and 150 kg of silicon particles B are added.

[0110] Comparative Example 2

[0111] Comparative Example 2 provides a negative electrode active material, which differs from Example 1 in that the silicon particles in Comparative Example 2 are 15 wt %, and a total of 50 kg of silicon particles A and 10 kg of silicon particles B are added.

[0112] Comparative Example 3

[0113] Comparative Example 3 provides a negative electrode active material, which differs from Example 1 in that, when the negative electrode sheet of Comparative Example 3 is divided into two equal parts, the average particle size d1 of the upper half coating is 11.5um, and the average particle size d2 of the lower half coating is 13.9um; when the negative electrode sheet is divided into three equal parts, the average particle size D-1 of the first coating is 11.7um, the average particle size D-2 of the second coating is 11.1um, and the average particle size D-3 of the third coating is 11.6um.

[0114] Comparative Example 4

[0115] Comparative Example 4 provides a negative electrode active material, which differs from Example 1 in that, when the negative electrode sheet of Comparative Example 4 is divided into two equal parts, the average particle size d1 of the upper half coating is 14.3um, and the average particle size d2 of the lower half coating is 12.2um; when the negative electrode sheet is divided into three equal parts, the average particle size D-1 of the first coating is 11.2um, the average particle size D-2 of the second coating is 13.4um, and the average particle size D-3 of the third coating is 14.9um.

[0116] Comparative Example 5

[0117] Comparative Example 5 provides a negative electrode active material, which differs from Example 1 in that, when the negative electrode sheet of Comparative Example 5 is divided into two equal parts, the average particle size d1 of the upper half coating is 9.5um, and the average particle size d2 of the lower half coating is 16um; when the negative electrode sheet is divided into three equal parts, the average particle size D-1 of the first coating is 18.4um, the average particle size D-2 of the second coating is 15.5um, and the average particle size D-3 of the third coating is 8.3m.

[0118] Comparative Example 6

[0119] Comparative Example 6 provides a negative electrode active material, which differs from Example 1 in that, in the test of Comparative Example 6, A1 is 0.38 and B1 is 0.22.

[0120] Comparative Example 7

[0121] Comparative Example 7 provides a negative electrode active material, which differs from Example 1 in that, according to the test results of Comparative Example 7, A1 is 0.45 and B1 is 0.61.

[0122] Comparative Example 8

[0123] Comparative Example 8 provides a negative electrode active material, which differs from Example 1 in that, according to Raman spectroscopy testing, Z1 of Comparative Example 8 is 0.9 and Z2 is 2.2.

[0124] Comparative Example 9

[0125] Comparative Example 9 provides a negative electrode active material, which differs from Comparative Example 8 in that, according to testing, Comparative Example 9 has M1 of 0.36 and M2 of 0.45.

[0126] Comparative Example 10

[0127] Comparative Example 10 provides a negative electrode active material, which differs from Comparative Example 8 in that, in Comparative Example 10, N1 is 0.32 and N2 is 0.34 as determined by testing.

[0128] The parameters of the above examples and comparative examples are compared in Table 1, Table 2 and Table 3.

[0129] Table 1

[0130] D1 D2 D3 D4 Silicon A:B Silicon ratio Example 1 3.50％ 9.20％ 13.10％ 8.20％ 1:2 10.00％ Example 2 4.80％ 14.20％ 19.70％ 9.70％ 1:1 16.00％ Example 3 3.30％ 2.60％ 16.90％ 11.50％ 3:1 10.00％ Example 4 3.10％ 13.00％ 11.10％ 6.00％ 1:3 10.00％ Example 5 7.50％ 10.10％ 9.80％ 8.40％ 1:4 10.00％ Example 6 3.30％ 18％ 7.80％ 6.50％ 1:2 10.00％ Example 7 3.50％ 10.10％ 22.50％ 7.20％ 1:2 15.00％ Example 8 4.10％ 9.20％ 12.20％ 1.50％ 1:4 10.00％ Comparative Example 1 13.20％ 19.50％ 1.20％ 1.10％ 1:15 15.00％ Comparative Example 2 1.30％ 1.50％ 24.50％ 13.30％ 5：1 15.00％

[0131] Table 2

[0132]

[0133] Table 3

[0134]

[0135] Performance evaluation

[0136] The negative electrode active materials provided in the above examples and comparative examples were prepared into negative electrode sheets, and the positive electrode sheets, separators, and negative electrode sheets were assembled into a battery core, which was then injected with electrolyte. Finally, the battery was assembled into a housing to form a lithium battery for testing. The preparation method is as follows:

[0137] (1) Preparation of positive electrode sheet: 97% lithium nickel cobalt manganese oxide, 0.4% carbon nanotube conductive agent, 1.6% conductive carbon black, and 1% polyvinylidene fluoride (PVDF) binder were taken as the solid matter of the positive electrode slurry by weight percentage, and the solid matter was dispersed in N-methyl-2-pyrrolidone to a solid content of 60%. The mixture was mixed in a homogenizer, and the slurry was coated on both sides of aluminum foil and dried to obtain a positive electrode sheet;

[0138] (2) Preparation of negative electrode sheet: 96% of the negative electrode active material, 1% of the acetylene black conductive agent, 1% of the dispersant (carboxymethyl cellulose), and 2% of the polyacrylic acid binder were taken as the solid matter of the negative electrode slurry by weight percentage. The solid matter was dispersed in deionized water to a solid content of 40%, and mixed evenly in a homogenizer. The slurry was coated on both sides of the copper foil and dried to obtain the negative electrode sheet;

[0139] (3) Preparation of electrolyte: Ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, 1 wt.% of lithium bis(fluorosulfonyl)imide as an electrolyte additive was added, and then fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L;

[0140] (4) Assembly: The positive and negative electrode sheets are rolled separately, cut and die-cut according to size requirements, and then wound together with the separator to obtain a cylindrical battery roll core. The battery roll core tabs are welded to the electrical connection sheet, and the battery roll core is placed in the shell. The electrolyte is injected, sealed, and formed to obtain an experimental battery.

[0141] Test 1: Cyclic performance test

[0142] Take a fresh battery, place it in a 25℃ constant temperature box for more than 4 hours, and test it according to the following steps:

[0143] (1) Discharge the battery at a constant current of 0.1C to a cutoff of 2.5V and let it stand for 5 minutes;

[0144] (2) Charge the battery at a constant current of 0.2C to a cutoff of 4.2V, and at a constant voltage of 0.05C to a cutoff, and let it rest for 5 minutes;

[0145] (3) Discharge the battery at a constant current of 0.2C to a cutoff of 2.5V, let it stand for 5 minutes, and read the capacity value C0 at this time;

[0146] (4) Charge the battery at a constant current of 1.0C to 4.2V, and charge it at a constant voltage of 0.05C, and let it rest for 5 minutes.

[0147] (5) Discharge the battery at a constant current of 2.0C to a cutoff of 2.5V and let it stand for 5 minutes;

[0148] (6) Repeat steps (4) and (5) 600 times;

[0149] (7) The cycle performance of a single battery, i.e., the capacity retention rate, is obtained by calculating the ratio of the 600th discharge capacity to the first discharge capacity in steps (4) and (5).

[0150] Test 2: Hot box test

[0151] Take a fresh battery, place it in a 25℃ constant temperature box for more than 4 hours, and test it according to the following steps:

[0152] (1) Discharge the battery at a constant current of 0.1C to a cutoff of 2.5V and let it stand for 5 minutes;

[0153] (2) Charge the battery at a constant current of 0.2C to a cutoff of 4.2V, and at a constant voltage of 0.05C to a cutoff, and let it rest for 5 minutes;

[0154] (3) Place the battery in a constant temperature box, set the heating rate to 5K / min, heat it to 130°C, and keep it for 1 hour, then stop heating and allow it to cool to below 30°C by self-heating;

[0155] (4) If the battery does not catch fire or emit smoke, it is considered passed; otherwise, it is failed. At least 5 batteries are tested and the pass rate is recorded.

[0156] The test results are shown in Table 4.

[0157] Table 4

[0158]

[0159]

[0160] By comparing Examples 1-4, it can be seen that the values ​​within the scope of protection of this application have excellent cycle life and safety performance. Figure 3 As shown, the discharge capacity cycle retention rate of Example 1 is better than that of Comparative Example 1. By comparing Example 1 and Example 5, it can be seen that when there are too many large particles, it is easy to cause excessive local stress, thereby reducing safety performance. By comparing Example 1 and Example 6, it can be seen that when there are too many silicon-carbon composite particles in any particle size range, the stress uniformity of the particles will be reduced, thereby reducing cycle performance. By comparing Example 1 and Example 7, it can be seen that when there are excessive D3 composite particles, the deterioration of stress uniformity will cause a reduction in safety performance. By comparing Example 1 and Example 8, it can be seen that when there are too few small particles, their dynamics decrease, thereby reducing the cycle retention rate. By comparing Examples 1 and 2, it can be seen that when there are particularly many large particles or particularly many small particles, the stress is too concentrated and the uniformity is extremely poor, thereby greatly reducing the cycle and safety performance.

[0161] It can be seen from Example 1, Example 9 and Example 10 that the battery cell has excellent cycle and safety performance at an average particle size within the scope of protection of this application. It can be seen from Comparative Example 1 and Comparative Example 3 that when there is no significant difference in particle size, the cumulative stress of the lower layer will be greater than that of the upper layer, thereby causing strain imbalance and reducing cycle performance. It can be seen from Comparative Example 4 that when there are many large particles in the upper layer and many small particles in the lower layer, the strain of the upper layer will be too large, thereby causing deformation and stratification of the active materials in the upper and lower layers, reducing cycle and safety. When the particle size of the lower layer is significantly larger than that of the upper layer, the stress accumulation of the lower layer will cause the current collector to deform, greatly deteriorating the cycle and safety performance. It can be concluded from Table 4 that the 600 discharge capacity cycle retention rate of the lithium-ion battery protected by this application is between 83-96%, and the hot box test pass rate is greater than or equal to 80%.

[0162] By comparing Table 3, it can be seen that when the values ​​are within the protection range of this application, the performance of the negative electrode sheet is relatively excellent, the stress distribution is uniform, and the cycle and safety are relatively excellent. Comparing Example 1 with Comparative Examples 6 and 7, it can be seen that when the F / C ratio is not within the set range, it indicates that there are more silicon side reactions in the upper or lower layer, thereby deteriorating the cycle performance and safety performance. Comparative Example 8 shows that when there are more defects in the electrode sheet in the lower half layer, its conductivity is poor, thereby deteriorating the cycle performance; Comparative Examples 9 and 10 show that when the carbon content of the inner layer is low, the conductivity is poor, causing the cycle to deteriorate; when there are more fluorine-containing layers, the cycle and safety will be deteriorated.

[0163] Although examples of the present embodiment have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and intent of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A lithium-ion battery, characterized in that: The lithium-ion battery includes a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. The negative electrode sheet includes a negative electrode current collector. At least one surface of the negative electrode current collector is coated with a negative electrode coating containing a negative electrode active material. The negative electrode active material includes carbon particles and silicon particles. The silicon-carbon composite particles are classified into at least four different particle size ranges D1, D2, D3, and D4 according to the particle size of the negative electrode active material, and meet the following requirements: D1>18um; 10um<D2≤18um; 5um<D3≤10um; 0um<D4≤5um; The silicon content of the D1 particles is ≤5% by weight, the silicon content of the D2 particles is 2-15% by weight, the silicon content of the D3 particles is 3-20% by weight, and the silicon content of the D4 particles is ≤6% by weight. The negative electrode coating is divided into a lower half coating close to the negative electrode current collector and an upper half coating away from the negative electrode current collector, and the average values ​​of ID / IG of the upper half coating and the lower half coating are Z1 and Z2 respectively, and satisfy: 0.7Z2≤Z1≤1.7Z2; The electrolyte contains fluorine element, the mass ratio of fluorine element to carbon element in the upper half coating is A1, the mass ratio of fluorine element to carbon element in the lower half coating is B1, and the following conditions are satisfied: 1.1B1≥A1≥0.9B1.

2. The lithium-ion battery according to claim 1, wherein The negative electrode active material further includes a metal element, and the metal element is at least one of Al, Zr, Ti, Sr, Mg, Na, Mo, Ca, Ba, La, and Y.

3. The lithium-ion battery according to claim 1, wherein The silicon particles include one or more of elemental silicon or silicon-oxygen particles; the carbon particles include one or more of artificial graphite, natural graphite, hard carbon, soft carbon or mesophase carbon microspheres.

4. The lithium-ion battery according to claim 1, wherein The average particle sizes of the negative electrode active material in the upper and lower coating layers are d1 and d2, respectively, and satisfy the following: 1.4d1≥d2≥1.05d1.

5. The lithium-ion battery according to claim 1, wherein The negative electrode coating is divided into a first coating, a second coating and a third coating of the same thickness in a direction away from the surface of the negative electrode current collector, and the average particle sizes of the negative electrode active material in the first coating, the second coating and the third coating are D-1, D-2 and D-3 respectively, and satisfy: D-3≤D-2≤D-1.

6. The lithium-ion battery according to any one of claims 1 to 5, characterized in that: The method for preparing the negative electrode sheet comprises the following steps: (1) Add the negative electrode active material, the first conductive agent, the second conductive agent, the dispersant, and the binder according to claim 1 to deionized water, mix and stir them evenly in a homogenizer, and obtain a negative electrode slurry with a solid content of 40-60%. (2) applying the negative electrode slurry to at least one surface of the negative electrode current collector, baking, and rolling to obtain the layered negative electrode sheet.

7. The lithium-ion battery according to claim 6, characterized in that The first conductive agent is acetylene black, the second conductive agent is single-walled carbon nanotubes, the dispersant is carboxymethyl cellulose, and the binder is polyacrylic acid or styrene-butadiene rubber.

8. The lithium-ion battery according to claim 1, wherein The silicon particles in the negative electrode active material are coated with a carbon-containing layer, the thickness of the carbon-containing layer is T, and taking the surface of the silicon particles as the starting point, the carbon content of the carbon-containing layer in the first 1 / 2T thickness is M1, and the carbon content of the carbon-containing layer in the last 1 / 2T thickness is M2, and the following conditions are satisfied: M1≥1.04M2.

9. The lithium-ion battery according to claim 1, wherein The silicon particles in the negative electrode active material are coated with a fluorine-containing layer, the thickness of the fluorine-containing layer is T, and taking the surface of the silicon particles as the starting point, the fluorine content of the fluorine-containing layer in the first 1 / 2T thickness is N1, and the fluorine content of the fluorine-containing layer in the last 1 / 2T thickness is N2, and the following conditions are satisfied: 10%≤N1+N2≤50%.

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