Lithium ion battery

Abstract

The invention relates to the technical field of lithium batteries, and provides a lithium ion battery. The battery comprises a positive plate and a negative plate; a lithium iron phosphate (LFP) material in the positive plate is formed by mixing particles with different particle sizes, the negative plate adopts a double-layer coating design, the upper layer uses first graphite particles with smaller particle sizes, and the lower layer uses second graphite particles with larger particle sizes. The positive plate is matched with a double-layer coated negative plate, so that the lithium ion battery has relatively high energy density, good quick charge performance, rate capability and high-temperature storage performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly relates to a lithium-ion battery. Background Art

[0002] Lithium-ion batteries mainly consist of a positive electrode material, a negative electrode material, a separator, an electrolyte, and auxiliary materials, among which the positive electrode material accounts for more than 30% of the cost. So far, the batteries applied to electric vehicles are mainly ternary batteries and lithium iron phosphate batteries. Due to its unique olivine structure and strong P-O bonds, the structure of lithium iron phosphate is very stable. Therefore, the characteristics of high safety and long cycle life of lithium iron phosphate have attracted wide attention and application. Due to the characteristics of high safety, long cycle life, and pollution-free, the lithium iron phosphate positive electrode material is widely applicable to BEV (battery electric vehicle) and energy storage fields. With the further application of lithium iron phosphate in the BEV field, meeting long endurance and fast charging has increasingly become the mainstream demand.

[0003] However, the relatively low energy density and fast charging performance of lithium iron phosphate lead to the problem of "range anxiety" in electric vehicle applications. To ensure the fast charging performance of the battery and at the same time ensure the energy density of the battery and increase the endurance, the existing lithium iron phosphate battery design improves the battery energy density by preparing thick electrodes and high compaction. However, when the surface density of the electrode coating increases and the compaction improves, the battery ion transport kinetics will drop significantly, affecting the overall performance of the lithium iron phosphate battery. Based on this, it is crucial to research a battery with high energy density, good fast charging cycle performance, and excellent rate performance. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above problems existing in the prior art, and provide a lithium-ion battery. The lithium iron phosphate material (LFP) in the positive electrode sheet uses a mixture of large and small particles with different particle sizes, and the negative electrode sheet adopts a double-layer coating design. The upper layer uses the first graphite particles with smaller particle sizes, and the lower layer uses the second graphite particles with larger particle sizes. The combination of the positive electrode sheet and the double-layer coated negative electrode sheet enables the lithium-ion battery to have high energy density, good fast charging performance, rate performance, and high-temperature storage performance.

[0005] To achieve the above purpose, the present invention provides a battery, including a positive electrode sheet and a negative electrode sheet;

[0006] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a lithium iron phosphate material; the lithium iron phosphate material includes a first lithium iron phosphate particle and a second lithium iron phosphate particle; the particle size of the first lithium iron phosphate particle is 50nm - 500nm; the particle size of the second lithium iron phosphate particle is 500nm - 5000nm;

[0007] For the positive electrode active material layer, the proportion of the number of the first lithium iron phosphate particles is 70%-95%, and the proportion of the number of the second lithium iron phosphate particles is 5%-30%.

[0008] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, and the second negative electrode active material layer is provided between the negative electrode current collector and the first negative electrode active material layer; the first negative electrode active material layer contains first graphite particles, and the second negative electrode active material layer contains second graphite particles; Dv50 of the first graphite particles < Dv50 of the second graphite particles.

[0009] The present invention adopts the above technical solution and has the following beneficial effects:

[0010] For the lithium ion battery provided by the present invention, the lithium iron phosphate material (LFP) in the positive electrode sheet is admixed with large and small particles with different particle sizes. The first lithium iron phosphate particles with smaller particle sizes can improve the fast charging performance of the lithium ion battery; the second lithium iron phosphate particles with larger particle sizes can increase the compaction density of the positive electrode sheet, so that the lithium ion battery has good high-temperature storage and cycling performance. At the same time, the negative electrode sheet adopts a double-layer coating design. The upper layer uses the first graphite particles with smaller particle sizes to improve the rate performance of the battery and improve the fast charging and cycling performance of the lithium ion battery; the lower layer uses the second graphite particles with larger particle sizes, which has better high-temperature storage and cycling performance. The combination of this positive electrode sheet and the double-layer coated negative electrode sheet enables the lithium ion battery to have a high energy density, good fast charging performance, rate performance and high-temperature storage performance.

[0011] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In this article, unless otherwise specified, the data ranges include endpoints. Description of the Drawings

[0012] Figure 1 Shown is the SEM image of the morphology of the lithium iron phosphate material obtained in Preparation Example 1.

[0013] Figure 2 Shown is the schematic structural diagram of the positive electrode coating of the positive electrode sheet in Example 1.

[0014] Figure 3 Shown is the SEM image of the morphology of the positive electrode sheet in Example 1.

[0015] Figure 4 Shown is the SEM electron micrograph of the cross-section of the positive electrode sheet in Example 1 after plasma cutting.

[0016] Figure 5 Shown is the schematic structural diagram of the double-layer coating of the negative electrode sheet in Example 1.

[0017] Figure 6 Shown is the SEM micrograph of the morphology of the positive electrode sheet in Example 2c.

[0018] Figure 7 Shown is the SEM micrograph of the morphology of the positive electrode sheet in Comparative Example 2d. Detailed Description of the Invention

[0019] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not intended to limit the present invention.

[0020] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0021] In the present invention, the terms "lithium-ion battery", "battery", "lithium battery", and "lithium-ion secondary battery" all have the same meaning, referring to a lithium-ion secondary battery, which generally includes an electrode assembly (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (housing) for accommodating the electrode assembly, and an electrolyte.

[0022] The present invention provides a lithium-ion battery, and the battery includes a positive electrode sheet and a negative electrode sheet;

[0023] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a lithium iron phosphate material; the lithium iron phosphate material includes first lithium iron phosphate particles and second lithium iron phosphate particles; the particle size of the first lithium iron phosphate particles is 50 nm - 500 nm; the particle size of the second lithium iron phosphate particles is 500 nm - 5000 nm;

[0024] In the positive electrode active material layer, the proportion of the number of the first lithium iron phosphate particles is 70% - 95%, and the proportion of the number of the second lithium iron phosphate particles is 5% - 30%;

[0025] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; the negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer, and the second negative electrode active material layer is provided between the negative electrode current collector and the first negative electrode active material layer; the first negative electrode active material layer contains first graphite particles, and the second negative electrode active material layer contains second graphite particles; the Dv50 of the first graphite particles < the Dv50 of the second graphite particles.

[0026] In the lithium-ion battery provided by the present invention, the lithium iron phosphate material (LFP) in the positive electrode sheet is blended with large and small particles of different particle sizes. The first lithium iron phosphate particles with smaller particle sizes can provide more reaction sites, improve the kinetics of the positive electrode sheet, and further improve the fast charging performance of the lithium-ion battery; the second lithium iron phosphate particles with larger particle sizes can provide a compaction basis. The lithium iron phosphate particles with smaller particle sizes are filled in the gaps between the lithium iron phosphate particles with larger particle sizes, and the theoretical packing density is larger, which improves the compaction density of the positive electrode sheet and enables the lithium-ion battery to have good high-temperature storage and cycling performance. In the first negative electrode active material layer (also called the upper layer) of the negative electrode sheet far from the negative electrode current collector, first graphite particles with smaller particle sizes are used. The surface of the first graphite particles has many defect sites, and the concentration of active ions in the upper region of the negative electrode sheet is relatively high, which improves the rate performance of the battery; moreover, the specific surface area of the first graphite particles is large, which can provide more deintercalation and intercalation active ion channels and better improve the fast charging and cycling performance of the lithium-ion battery; the second negative electrode active material layer (also called the lower layer) close to the negative electrode current collector uses second graphite particles with larger particle sizes, with fewer side reactions and better high-temperature storage and cycling performance. The combination of this positive electrode sheet and the double-coated negative electrode sheet enables the lithium-ion battery to have a high energy density, good fast charging performance, rate performance, and high-temperature storage performance.

[0027] In some embodiments, the particle size of the first lithium iron phosphate particles is any value within the range of 50 nm - 500 nm, for example, it can be any point value within the ranges of 50 nm - 100 nm, 100 nm - 200 nm, 200 nm - 300 nm, 300 nm - 400 nm, 400 nm - 500 nm; the particle size of the second lithium iron phosphate particles is any value within the range of 500 nm - 5000 nm, for example, it can be any point value within the ranges of 500 nm - 1000 nm, 1000 nm - 2000 nm, 2000 nm - 3000 nm, 3000 nm - 4000 nm, 4000 nm - 5000 nm. The lithium iron phosphate material is composed of a combination of first lithium iron phosphate particles and second lithium iron phosphate particles with different particle sizes. When the particle sizes of the first lithium iron phosphate particles and the second lithium iron phosphate particles are within the above ranges, they have a relatively high packing density, can improve the compaction density of the positive electrode sheet, and at the same time take into account the performance of fast lithium deintercalation and intercalation of the positive electrode sheet, enabling the lithium-ion battery to have a high energy density and fast charging performance.

[0028] In some embodiments, in the positive electrode active material layer, the proportion of the number of the first lithium iron phosphate particles is 70%-95%, and the proportion of the number of the second lithium iron phosphate particles is 5%-30%. For example, the proportion of the number of the first lithium iron phosphate particles can be 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95% or any value within the range composed of any two of the above values, preferably 80%-90%; for example, the proportion of the number of the second lithium iron phosphate particles can be 5%, 6%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30% or any value within the range composed of any two of the above values, preferably 10%-20%. When the present invention controls the proportion of the number of the first lithium iron phosphate particles and the second lithium iron phosphate particles within the above range, it can avoid the problems of poor kinetics and reduced constant current charging ratio caused by adding a large number of the second lithium iron phosphate particles (large particle size). Since the size of the second lithium iron phosphate particles is relatively large, the path of lithium deintercalation / insertion becomes longer. The diffusion of lithium in the lithium iron phosphate basically diffuses along the 010-B axis direction inside the crystal lattice. Too many second lithium iron phosphate particles easily lead to poor kinetics of the positive electrode active material and a reduced constant current charging ratio; and it can avoid the problems of poor cycle performance, poor high-temperature storage performance and low energy density caused by adding a small number of the first lithium iron phosphate particles (small particle size). Since the size of the first lithium iron phosphate particles is relatively small, the path of lithium deintercalation / insertion is short and the kinetic performance is strong. Too many first lithium iron phosphate particles have a large reaction specific surface area, strong reaction activity during charge and discharge, an increase in side reactions, which has a greater impact on the cycle performance and high-temperature storage performance. Moreover, the packing density of small particles is relatively low, and the capacity per unit volume is relatively low, reducing the energy density of the battery.

[0029] The test method for the particle size and the proportion of the number of the first lithium iron phosphate particles and the second lithium iron phosphate particles can be, for example: Arbitrarily select a specific unit area of 10μm×10μm of the positive electrode sheet under the SEM electron microscope field of view, and obtain the particle sizes of all lithium iron phosphate particles within this unit area through statistical measurement, and count the proportion of the number of the first lithium iron phosphate particles and the second lithium iron phosphate particles according to the particle size range.

[0030] In some embodiments, in the positive electrode active material layer, the proportion of lithium iron phosphate particles with a particle size of 1000 nm - 5000 nm is less than 3%, for example, the proportion can be 0.5%, 1%, 2%, 2.5%, 2.8%, 2.99% or any value within the range formed by any two of the above values, preferably 1.2% - 3% (excluding the endpoint 3%). When lithium iron phosphate particles with different particle sizes and different proportion are mixed and used in the positive electrode sheet, when the proportion of lithium iron phosphate particles with a particle size of 1000 nm - 5000 nm (ultra-large lithium iron phosphate particles) is further controlled within the above range, the number of ultra-large lithium iron phosphate particles can be controlled within a lower range, shortening the lithium ion transmission path, reducing the transmission time of lithium ions during charge and discharge, reducing charge-discharge polarization, enhancing the dynamics of the battery, and further enhancing the fast charging performance of the battery.

[0031] In some embodiments, the particle size of the lithium iron phosphate material satisfies: 0.9 ≤ (Dn90 - Dn10) / Dn50 ≤ 1.2. The value of (Dn90 - Dn10) / Dn50 can be, for example, 0.9, 0.93, 0.95, 1.0, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, 1.2 or any value within the range formed by any two of the above values.

[0032] The Dn series parameters are statistical particle size indicators based on the particle number distribution, used to quantify the central tendency and distribution width of the particle population (the size and distribution of lithium iron phosphate particles can be observed and recorded by scanning electron microscope SEM, and then the statistical graph of the lithium iron phosphate particle number distribution can be obtained). Among them, the definition of Dn10: 10% of the particle diameter of the lithium iron phosphate material is less than or equal to this value (that is, the upper limit of the particle size of the smallest 10% of the lithium iron phosphate particle population). The definition of Dn50 (median particle size): 50% of the particle diameter is less than or equal to this value, that is, the number median of the lithium iron phosphate particle population. The definition of Dn90: 90% of the particle diameter is less than or equal to this value (that is, the lower limit of the particle size of the largest 10% of the lithium iron phosphate particle population).

[0033] (Dn90 - Dn10) / Dn50 represents the proportion of the distribution width of lithium iron phosphate particles. The larger the value, the wider the distribution. The wider the distribution, the greater the difference in particle size between the first lithium iron phosphate particle and the second lithium iron phosphate particle, and the different stress levels of the first lithium iron phosphate particle and the second lithium iron phosphate particle during charge and discharge will cause an increase in battery polarization. When the value of (Dn90 - Dn10) / Dn50 is controlled within the above range, the proportion of the distribution width of the lithium iron phosphate material in the positive electrode sheet can be reduced, the battery polarization can be reduced, the dynamics of the battery can be enhanced, and further the fast charging performance of the battery can be enhanced.

[0034] In some embodiments, the Dn50 of the lithium iron phosphate material is 300 nm - 400 nm. The Dn50 of the lithium iron phosphate material can be, for example, 300 nm, 310 nm, 320 nm, 330 nm, 340 nm, 350 nm, 360 nm, 370 nm, 380 nm, 390 nm, 400 nm, or any value within the range formed by any two of the above values.

[0035] It should be noted that the Dn50 of the lithium iron phosphate material refers to the median particle size of the lithium iron phosphate material obtained after the combined use of the first lithium iron phosphate particles and the second lithium iron phosphate particles, and can be obtained based on the statistical graph of the particle number distribution of the lithium iron phosphate particles.

[0036] When lithium iron phosphate particles with different particle sizes and different quantity ratios are blended and used in the positive electrode sheet, and the median particle size of the lithium iron phosphate particles is further adjusted within the above range, the particle size of the blended lithium iron phosphate particles is relatively small (basically in the nanometer range), the lithium deintercalation / insertion path is shorter, the kinetic performance is stronger, and the fully nanocrystallized lithium iron phosphate particles can ensure that the battery has good fast charging performance and alleviate the mileage anxiety problem.

[0037] In some embodiments, the lithium iron phosphate material is doped with one or more of Ti, V, Mg, and Al elements.

[0038] The present invention further studies and finds that by adding some metal oxides (such as metal oxides containing one or more of Ti, V, Mg, and Al elements) during the preparation and grinding stage of the lithium iron phosphate material, and doping higher-valent metals into LFP through sintering, the doped metal elements often have a valence state similar to or higher than that of iron, with a larger electron cloud density, which can improve the voltage platform of the lithium iron phosphate material. In addition, the doping is mainly for Fe vacancies, etc., to enhance the bond energy with O, thereby reducing the binding of oxygen around Li to Li and increasing the lithium diffusion coefficient. Doping the above metals can also stabilize the structure of the lithium iron phosphate material and inhibit the abnormal growth of lithium iron phosphate grains during sintering, but it is necessary to control the doping amount not to be too high, otherwise it will be difficult for the lithium iron phosphate particles to grow during sintering, affecting the specific capacity of the final product.

[0039] The test method for the content of the doped metal elements in the lithium iron phosphate material includes: testing the content of the doped metal elements by ICP inductively coupled testing.

[0040] In order to further improve the structural stability and capacity performance of the blended lithium iron phosphate material with different particle sizes, the present invention also proposes a technical solution of doping different metal elements into lithium iron phosphate particles with different particle sizes, as described below:

[0041] In some embodiments, the first lithium iron phosphate particles are doped with Ti element, and the content of the Ti element is 0 ppm - 3000 ppm. For example, it can be 0 ppm, 10 ppm, 100 ppm, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm or any point value within the range composed of any two of the above values. More preferably, it is 500 ppm - 2800 ppm. When the first lithium iron phosphate particles are doped with Ti element, Ti is beneficial to further refinement of the crystal grains. The Ti doping occupies the Fe site and has a greater binding force on O, which can improve the structural stability of the lithium iron phosphate material, inhibit further growth of the crystal, make the first lithium iron phosphate particles reach a smaller crystal grain level, and at the same time can shorten the ion transport path, thereby further improving the rate performance of the battery.

[0042] In some embodiments, the second lithium iron phosphate particles are doped with V element, and the content of the V element ≤ 5000 ppm. For example, it can be 10 ppm, 100 ppm, 500 ppm, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm or any point value within the range composed of any two of the above values. More preferably, it is 3000 ppm - 5000 ppm. V doping is N-type doping, with a higher chemical valence and a greater electron cloud density, which can better improve the voltage platform of the lithium iron phosphate material; at the same time, it is necessary to avoid too high content of V element, which may lead to an increase in the internal thermal stress of the positive electrode sheet and deteriorate the high-temperature performance of the battery.

[0043] In some embodiments, the lithium iron phosphate material or the positive electrode sheet satisfies at least one of the following conditions:

[0044] (a) The powder tap density of the lithium iron phosphate material is 2.6 g / cm 3 -2.9 g / cm 3 ;

[0045] (b) The powder resistivity of the lithium iron phosphate material is 5 Ω·cm - 10 Ω·cm;

[0046] (c) The impurity content in the lithium iron phosphate material < 150 ppm, and the magnetic substance content < 1 ppm;

[0047] (d) The tap density of the positive electrode sheet ≥ 2.75 g / cc;

[0048] (e) The surface resistance of the positive electrode sheet is 5 Ω·cm - 15 Ω·cm.

[0049] (a), the powder tap density of the lithium iron phosphate material can be, for example, 2.6 g / cm 3, 2.65g / cm 3 , 2.7g / cm 3 , 2.75g / cm 3 , 2.8g / cm 3 , 2.85g / cm 3 , 2.9g / cm 3 Or any point value in the range formed by the above two point values. The lithium iron phosphate material in the positive electrode sheet of the present invention is mixed with particles of different sizes, which increases the powder compaction density of the lithium iron phosphate material and further improves the energy density of the battery.

[0050] The test method for the powder compaction density of lithium iron phosphate materials includes: using a Sansi Zongheng powder compaction density instrument to test at a pressure of 3T.

[0051] (b) The powder resistivity of the lithium iron phosphate material can be, for example, 5Ω.cm, 6Ω.cm, 7Ω.cm, 8Ω.cm, 9Ω.cm, 10Ω.cm or any point value in the range of the above two point values. The lithium iron phosphate material in the positive electrode sheet of the present invention is mixed with large and small particles of different particle sizes, and the small particles are filled into the gaps between the large particles, so that the stacking of the entire lithium iron phosphate material system is more compact, which helps to reduce the contact resistance between the lithium iron phosphate particles, enhance the charge transfer efficiency between the lithium iron phosphate particles, and thus reduce the overall powder resistivity of the lithium iron phosphate, and improve the rate performance of the battery.

[0052] The test method of the powder resistivity of the lithium iron phosphate material includes: using a four-probe powder resistivity meter to test the powder resistance value under 50 MPa.

[0053] In (c), the impurity content in the lithium iron phosphate material is less than 150ppm, and the magnetic substance content is less than 1ppm. The presence of impurities and magnetic substances in the lithium iron phosphate material will accelerate the structural degradation of the battery material and affect the cycle stability of the battery. Controlling the impurity content to less than 150ppm and the magnetic substance content to less than 1ppm can significantly reduce the damage of these harmful substances to the structure of the lithium iron phosphate material, improve the structural stability of the positive electrode sheet, and increase the cycle life of the battery.

[0054] The impurity content in lithium iron phosphate materials can be tested by ICP; the magnetic substance content can be tested by magnetic rod adsorption + ICP.

[0055] In (d), the compaction density of the positive electrode sheet is ≥2.75g / cc; the lithium iron phosphate material in the positive electrode sheet is mixed with particles of different sizes, and the entire lithium iron phosphate material system is more densely stacked and has a higher compaction density, so that the compaction density of the positive electrode sheet is ≥2.75g / cc, thereby improving the energy density of the battery.

[0056] The method for testing the tap density of the positive electrode sheet is to first use a circular cutter to punch out circular wafers, calculate the area of the circular wafers according to the diameter of the wafers; measure the thickness of the wafers using a micrometer, calculate the volume of the two-sided coatings (the thickness of the foil material should be subtracted when calculating the thickness), then weigh the circular wafers using an electronic balance, and finally calculate the tap density g / cm according to weight / volume 3 。

[0057] (e), the surface resistance of the positive electrode sheet is, for example, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm or any value within the range composed of any two of the above values. The high tap density of the positive electrode sheet makes the contact between the electrode materials closer, reduces the contact resistance between the lithium iron phosphate particles, thereby reducing the surface resistance of the positive electrode sheet, helping to improve the power output of the battery, enabling it to provide a larger current during high-rate charge and discharge, and enhancing the rate performance of the battery.

[0058] The surface resistance of the positive electrode sheet can be tested using a resistance tester.

[0059] The present invention also provides a preparation method for preparing lithium iron phosphate materials, including the following steps:

[0060] (1) Add iron phosphate to an organic carbon source, a coating agent, a lithium source, a doping agent, a dispersant, and ultrapure water, grind small particles (50 nm - 500 nm), the addition amount of the doping element is 0 - 3000 ppm; grind large particles (500 - 5000 nm), the addition amount of the doping element is 3000 - 5000 ppm; after batching, grind them separately;

[0061] (2) Spray-dry the ground slurries separately to obtain spray-dried materials with a moisture content of less than 3%;

[0062] (3) Put the spray-dried materials with a grinding particle size below 50 - 500 nm into a roller hearth kiln, and sinter them in a nitrogen-reducing atmosphere in the roller hearth kiln for 15 - 30 h, and the calcination temperature is 720 - 740 °C;

[0063] (4) Put the spray-dried materials with a grinding particle size of 500 - 5000 nm into a roller hearth kiln, and sinter them in a nitrogen-reducing atmosphere in the roller hearth kiln for 18 - 30 h, and the calcination temperature is 780 - 810 °C;

[0064] (5) Subject the materials sintered in steps (3) and (4) to air-flow pulverization respectively to obtain pulverized materials;

[0065] (6) Mix the two pulverized materials and load them into a crucible for secondary sintering, where the small particles and the large particles are mixed in a weight ratio of 1:9 to 2:8;

[0066] (7) Weigh 5 - 7 kg of the well - mixed materials and put them into a refractory graphite crucible, and sinter them in a roller hearth kiln. Sinter for 15 - 30 h in a nitrogen - reducing atmosphere in the roller hearth kiln, with an oxygen content < 20 ppm and a calcination temperature of 750 - 780 °C;

[0067] (8) Air - flow crush the materials after mixed sintering; screen, remove iron from the crushed materials, and then package them to obtain high - density and fast - charging lithium iron phosphate. When air - flow crushing, control the dew point of the crushing cavity to - 60 °C. The packaging is carried out in a constant - temperature and low - humidity room at a temperature of 25 °C ± 2 °C and a humidity within 15% RH. The packaging uses vacuum packaging to keep the moisture content of the finished product within 1000 ppm.

[0068] In some embodiments, the organic carbon source includes glucose. The coating agent includes PEG6000, which can provide the functions of carbon coating and surface - active dispersion. The lithium source includes lithium carbonate. The dopant is selected according to the doped element. For example, it includes TiO 2 (doping Ti element), ammonium metavanadate and vanadium oxide (doping V element). The dispersant includes citric acid.

[0069] Among the lithium iron phosphate particles obtained by the above - mentioned preparation method, in terms of the number of particles, the first lithium iron phosphate particles (small particles) with a particle size in the range of 100 - 500 nm account for 80 - 90%; the second lithium iron phosphate particles (large particles) with a particle size in the range of 500 - 5000 nm account for 10 - 20%. In particular, the proportion of lithium iron phosphate particles with a particle size in the range of 1 - 5 μm is < 3%. The large particles (500 - 5000 nm) and small particles (50 - 500 nm) are mixed and graded according to a quantity ratio of 2:8 or 1:9 to obtain lithium iron phosphate with a Dn50 range of 0.3 - 0.4 μm and a (Dn90 - Dn10 / Dn50) range between 0.9 and 1.2, and a mass ratio of large particles to small particles of 9:1. The doping amount of large particles (500 - 5000 nm) is 3000 - 5000 ppm; the doping amount range of small particles (100 - 500 nm) is 0 - 3000 ppm.

[0070] In some embodiments, the positive electrode active material layer includes a lithium iron phosphate material (including the first lithium iron phosphate particles and the second lithium iron phosphate particles), a positive electrode binder, and a positive electrode conductive agent. Based on the total mass of the positive electrode active material layer, the mass proportion of the lithium iron phosphate material is 96.5% - 98%, the mass proportion of the binder is 1.5 - 2.5%, and the mass proportion of the conductive agent is 1% - 2%.

[0071] The positive electrode binder and the positive electrode conductive agent include the binders and conductive agents commonly used in the art. The positive electrode binder includes, but is not limited to, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene oxide, sodium carboxymethyl cellulose, styrene-butadiene rubber (SBR), etc., and polyvinylidene fluoride is preferred; the positive electrode conductive agent includes, but is not limited to, one or a mixture of conductive agents such as carbon black or carbon nanotubes, graphene, and carbon fiber, and carbon black is preferred.

[0072] In some embodiments, the positive electrode current collector includes metal aluminum foil or coated carbon metal aluminum foil of metal aluminum foil and carbon black, and coated carbon metal aluminum foil is preferred.

[0073] The negative electrode sheet of the present invention adopts a double-layer coating design. Smaller-sized first graphite particles are used in the upper layer, and larger-sized second graphite particles are used in the lower layer, improving the fast charging and cycling performance of the lithium-ion battery, as well as the high-temperature storage performance.

[0074] In order to further optimize the electrochemical performance of the negative electrode sheet, the present invention further proposes the following technical solutions:

[0075] In some embodiments, the Dv50 of the first graphite particles is 5 μm - 15 μm; for example, it can be 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, or any value within the range composed of any two of the above values. And / or, the Dv50 of the second graphite particles is 10 μm - 30 μm, for example, it can be 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or any value within the range composed of any two of the above values. When the negative electrode sheet adopts a double-layer coating design, further adjusting the Dv50 of the first graphite particles and the Dv50 of the second graphite particles to meet the above range can shorten the diffusion path of lithium ions in the negative electrode sheet, accelerate the transmission speed of lithium ions, and further improve the rate performance of the battery.

[0076] The particle size Dv50 of the first graphite particles and the second graphite particles can be measured by laser particle size analysis, and the instrument model used is Mastersizer 3000.

[0077] In some embodiments, the difference between Dv90 and Dv10 of the first graphite particles is denoted as A, and the difference between Dv90 and Dv10 of the second graphite particles is denoted as B, where A and B satisfy: A < B. The difference between Dv90 and Dv10 of the graphite material represents the distribution width of the particle size of the graphite particles. The larger the difference, the wider the particle distribution, the more graphite particles with larger particle sizes, and the greater the tap density and energy density. By adjusting, the difference A between Dv90 and Dv10 of the first graphite particles in the upper layer of the negative electrode sheet is smaller, that is, graphite particles with smaller particle sizes are used; the difference B between Dv90 and Dv10 of the second graphite particles in the lower layer of the negative electrode sheet is larger, that is, graphite particles with larger particle sizes are used; the particle size distribution differences of the two layers of graphite materials are appropriately distributed, reducing battery polarization, improving the kinetics of the battery, and improving the cycle performance of the battery.

[0078] In some embodiments, Dv90 of the first graphite particles is 15 μm - 20 μm, for example, it can be 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or any value within the range composed of any two of the above values. Dv10 of the first graphite particles is 3 μm - 5 μm, for example, it can be 3 μm, 4 μm, 5 μm, or any value within the range composed of any two of the above values.

[0079] In some embodiments, Dv90 of the second graphite particles is 20 μm - 30 μm, for example, it can be 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, or any value within the range composed of any two of the above values. Dv10 of the second graphite particles is 8 μm - 15 μm, for example, it can be 8 μm, 9 μm, 11 μm, 13 μm, 15 μm, or any value within the range composed of any two of the above values.

[0080] Exemplarily, Dv10, Dv50, and Dv90 of the first graphite particles and Dv10, Dv50, and Dv90 of the second graphite particles can all be tested using a laser particle size analyzer.

[0081] In some embodiments, for the first graphite particles, the ratio of (Dv90 - Dv10) / Dv50 is P1; for the second graphite particles, the ratio of (Dv90 - Dv10) / Dv50 is P2, where P1 and P2 satisfy: P1 < P2.

[0082] The Dv series parameters are statistical indicators based on the particle volume distribution, used to quantify the volume central tendency and distribution width of the particle population (which can be measured by laser particle size analysis to obtain the statistical graph of the graphite particle volume distribution). Definition of Dv10: 10% of the graphite particle volume is less than or equal to this value. Definition of Dv50 (volume median diameter): 50% of the graphite particle volume is less than or equal to this value. Definition of Dv90: 90% of the graphite particle volume is less than or equal to this value.

[0083] (The value of (Dv90 - Dv10) / Dv50 represents the proportion of the distribution width of the graphite particles. The larger the value, the wider the distribution and the more particles with larger particle sizes. Adjust P1 < P2. Smaller-sized graphite particles are used in the upper layer of the negative electrode sheet to improve the fast charging performance of the negative electrode sheet and reduce lithium plating on the negative electrode; larger-sized graphite particles are used in the lower layer of the negative electrode sheet, so that the negative electrode sheet has a greater tap density, improves the energy density of the battery, and has good porosity and ionic conductivity, which is beneficial to lithium diffusion.

[0084] In some embodiments, in the first graphite particles, the ratio P1 of (Dv90 - Dv10) / Dv50 satisfies: 0.6 ≤ P1 ≤ 3.5. For example, it can be 0.6, 1.0, 1.5, 2, 2.5, 3, 3.5 or any value within the range composed of any two of the above values.

[0085] In some embodiments, in the second graphite particles, the ratio P2 of (Dv90 - Dv10) / Dv50 satisfies: 0.2 ≤ P2 ≤ 2.2. For example, it can be 0.2, 0.5, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0, 2.2 or any value within the range composed of any two of the above values.

[0086] In some embodiments, the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:9 - 5:5. For example, the thickness ratio can be 1:9, 2:8, 3:7, 4:6, 5:5 or any value within the range composed of any two of the above values. The present invention further limits the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer within the above range, which can avoid the thickness of the first negative electrode active material layer being too large, resulting in low energy density of the battery and deteriorating the high-temperature performance of the battery; and avoid the thickness of the second negative electrode active material layer being too large, resulting in reduced battery kinetics and poor cycle performance.

[0087] In some embodiments, the thickness of the first negative electrode active material layer is 20μm - 60μm. For example, it can be 20μm, 30μm, 40μm, 50μm, 60μm or any value within the range composed of any two of the above values.

[0088] In some embodiments, the thickness of the second negative electrode active material layer is 40 μm - 120 μm, and for example, it can be 40 μm, 60 μm, 80 μm, 100 μm, 120 μm, or any value within the range formed by any two of the above point values.

[0089] In some embodiments, the first negative electrode active material and the second negative electrode active material may each independently include at least one of silicon, silicon monoxide, hard carbon, and soft carbon.

[0090] In some embodiments, the negative electrode active material layer includes a graphite material (including first graphite particles and second graphite particles), a negative electrode binder, and a negative electrode conductive agent. The present invention does not specifically limit the types of the negative electrode binder and the negative electrode conductive agent, and all types of negative electrode binders and negative electrode conductive agents commonly used in this industry are within the protection scope of the present invention. Exemplarily, the types of the negative electrode binder include, but are not limited to, one or more of styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylic acid-based, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose, and preferably sodium carboxymethyl cellulose and styrene-butadiene rubber. Exemplarily, the types of the negative electrode conductive agent include, but are not limited to, one or more of carbon black, graphene, graphite, carbon nanotubes, and nanofibrous carbon, and preferably carbon black.

[0091] In some embodiments, the negative electrode current collector includes at least one of copper foil or composite copper foil.

[0092] In some embodiments, the lithium-ion battery further includes an electrolyte, the electrolyte includes additives, and the additives include at least one of silicon-containing additives, sulfate esters, and lithium salt-type additives; based on the total mass of the electrolyte, the mass percentage of the additives is 0.01% - 5%.

[0093] In some embodiments, the electrolyte includes silicon-containing additives, and based on the total mass of the electrolyte, the mass percentage of the silicon-containing additives is 0.01% - 2%, and for example, it can be 0.01%, 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2%, or any value within the range formed by any two of the above point values. The silicon-containing additives include at least one of hexamethyldisilazane (HDMS), heptamethyldisilazane, tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), ethylhexamethyldisilazane, and hexamethyldisiloxane. Adding silicon-containing additives to the electrolyte can improve the stability of the SEI film and the solvation structure, help improve the cycling stability of the negative electrode, and thus improve the cycling performance of the battery.

[0094] In some embodiments, the electrolyte includes a sulfate additive. Based on the total mass of the electrolyte, the mass percentage of the sulfate additive is 0.01% - 4%, for example, it can be 0.01%, 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2%, 2.5%, 3%, 3.5%, 4% or any value within the range composed of any two of the above values. The sulfate additive includes at least one of vinylene sulfate (DTD), diethyl sulfite (DES), lithium methyl sulfate, 1,3 - propanediol sulfate, and pentaerythritol bisulfate.

[0095] In some embodiments, the electrolyte includes a lithium salt additive. Based on the total mass of the electrolyte, the mass percentage of the lithium salt additive is 0.01% - 2%, for example, it can be 0.01%, 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2% or any value within the range composed of any two of the above values. The lithium salt additive includes at least one of lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF 4 )), lithium difluoro(oxalato)phosphate (LiODFP), lithium difluoro(oxalato)borate (LiODFB), and lithium tetrafluoro(oxalato)phosphate (LiTFOP). When adding the above - mentioned types of lithium salt additives to the electrolyte, the generated inorganic SEI film has good stability, can reduce the generation of side reactions, and at the same time has high ionic conductivity, improving the cycle performance and fast - charging performance of the battery.

[0096] In some embodiments, the electrolyte further includes an organic solvent. The organic solvent includes short - chain carbonates (carbonates with less than 10 carbon atoms), and the short - chain carbonates include one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Selecting short - chain carbonates as the organic solvent can reduce the viscosity and solvation energy of the electrolyte, improve the conductivity and ion migration rate, thereby enhancing the fast - charging performance of the battery.

[0097] In some embodiments, the electrolyte further includes a lithium salt, and the lithium salt is one or a combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0098] In the present invention, the material of the separator is not specifically limited, and various separator materials commonly used in the industry and applicable to secondary batteries are within the protection scope of the present invention. Exemplarily, the separator material includes but is not limited to at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber.

[0099] The lithium-ion battery of the present invention is obtained by assembling a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte. For example, the positive electrode sheet, the negative electrode sheet, and the separator are assembled into an electrode core by the commonly used winding or laminating method in the industry, and then encapsulated with an aluminum-plastic film. Then, through baking, injecting the electrolyte, forming, and secondary sealing processes, the lithium-ion battery is obtained.

[0100] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0101] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions.

[0102] The present invention will be described in detail below with specific embodiments, and these embodiments are for understanding rather than limiting the present invention.

[0103] Preparation Example 1 Preparation of Lithium Iron Phosphate Material

[0104] (1) According to the mass ratio, 73-74% of iron phosphate, 8-9% of glucose as the organic carbon source, 1-2% of PEG6000, 15-16% of lithium carbonate, 1-2% of the dopant, 1-2% of citric acid as the dispersant, and ultrapure water are mixed and ground. The dopant for grinding small particles (50-500 nm) is TiO 2 powder; the dopants for grinding large particles (500-5000 nm) are ammonium metavanadate and vanadium oxide.

[0105] (2) After batching, grinding is carried out respectively. Grinding is carried out using 0.2 mm and 0.3 mm zirconia balls respectively; the grinding slurries are respectively spray-dried to obtain spray-dried materials with a moisture content of less than 3%; the spray-dried materials with a grinding particle size below 50-500 nm are put into a roller hearth kiln, and sintered in a nitrogen-reducing atmosphere in the roller hearth kiln for 20 h, and the temperature in the calcination holding zone is 720-740 °C; the spray-dried materials with a grinding particle size of 500-5000 nm are put into a roller hearth kiln, and sintered in a nitrogen-reducing atmosphere in the roller hearth kiln for 22 h, and the temperature in the calcination holding zone is 780-810 °C; the sintered materials are respectively pulverized by air flow to obtain pulverized materials; the two pulverized materials are mixed and put into a crucible for secondary sintering, and the small particles and large particles are mixed according to a weight ratio of 1:9.

[0106] (3) Weigh 5-7 kg of the mixed material and put it into a refractory graphite sagger, and sinter it in a roller kiln. The roller kiln is sintered in a nitrogen reducing atmosphere for 20 hours, the oxygen content is less than 20 ppm, and the temperature of the calcination and insulation zone is 750-780° C. The mixed and sintered materials are subjected to air flow pulverization; the pulverized materials are screened, iron is removed, and packaged to obtain high-density fast-charge lithium iron phosphate. The dew point of the pulverization chamber is controlled at -60° C. during air flow pulverization. The packaging is carried out in a constant temperature and low humidity room with a temperature of 25° C. and a humidity of less than 15% RH. The packaging is vacuum packed to keep the moisture content of the finished product within 1000 ppm;

[0107] (4) Among the prepared lithium iron phosphate particles, in terms of the number of particles, lithium iron phosphate particles with a particle size in the range of 50-500nm accounted for 87.78%; lithium iron phosphate particles with a particle size in the range of 500-5000nm (first lithium iron phosphate particles) accounted for 12.22%; in particular, the number of lithium iron phosphate particles with a particle size in the range of 1-5μm (second lithium iron phosphate particles) accounted for 1.88%, and a particle grading lithium iron phosphate with a Dn50 range of 0.35um, a (Dn90-Dn10) / Dn50 value of 0.98, and a mass ratio of large particles to small particles of 9:1 was obtained. The V element doping amount of large particles (500-5000nm) is 4600ppm; the Ti element doping amount of small particles (50-500nm) is 1200ppm. The SEM image of the morphology of this lithium iron phosphate material is as follows Figure 1 As shown, the obtained lithium iron phosphate powder has a compaction density of 2.62 g / cm 3 .

[0108] Example 1

[0109] (1) Preparation of positive electrode sheet:

[0110] The positive electrode uses the lithium iron phosphate material obtained in the above-mentioned Preparation Example 1 as the active material, the conductive agent uses carbon black and carbon nanotubes (mass ratio 1:1), the binder uses polyvinylidene fluoride PVDF, and the solvent uses N-methylpyrrolidone NMP. They are stirred evenly in a homogenizer in a certain order, and are evenly coated on the carbon-coated aluminum foil current collector by a coater, and are pressed by a roller press to obtain a positive electrode sheet; wherein, based on the total mass of the lithium iron phosphate positive electrode coating, the positive electrode sheet contains 96.5% of lithium iron phosphate particles, 1.5% of the conductive agent and 2% of the binder.

[0111] The schematic diagram of the obtained positive electrode is shown in Figure 2 The compaction density of the prepared positive electrode sheet is 2.657 g / cm 3 , single surface density is 190g / m 3 , its surface SEM picture is as follows Figure 3 As shown, plasma cutting cross-section SEM electron microscope Figure 4 As shown, it can be seen that the morphology of the positive electrode sheet is evenly distributed and the particles are complete.

[0112] (2) Preparation of the negative electrode sheet:

[0113] The negative electrode uses artificial graphite with different particle sizes as the negative electrode material, conductive carbon black as the conductive agent, sodium carboxymethyl cellulose CMC and polyacrylic acid as the binder, and deionized water as the solvent. After mixing evenly, it is coated on the surface of the copper foil current collector in layers. Among them, the active material in the upper layer (far from the current collector) is small particle graphite (the first graphite particle), with a Dv50 of about 8 μm, a Dv90 of about 17 μm, and a Dv10 of about 5 μm; the active material in the lower layer (close to the current collector) is large particle graphite (the second graphite particle), with a Dv50 of about 12 μm, a Dv90 of about 28 μm, and a Dv10 of about 9 μm. The thickness ratio of the upper layer to the lower layer is 2:8 (the total thickness of the negative electrode active material layer is about 100 μm). For the schematic diagram of the double-layer coating of the negative electrode, see Figure 5 .

[0114] (3) Preparation of the laminated lithium-ion battery:

[0115] The positive electrode sheet and the negative electrode sheet are cut into the designed shape by die-cutting. The positive electrode sheet, the separator, and the negative electrode sheet are alternately stacked in a Z-shaped stacking method. After the tabs of the electrode core are ultrasonically welded, the positive electrode is welded to the aluminum tab, and the negative electrode is welded to the copper tab. Then, it is placed in a housing made of heat-sealed aluminum-plastic film by stamping, and the stacked core is sealed in the housing by heating. The electrolyte uses a mixed lithium salt system of LiPF 6 and LiTFSi (LiPF 6 accounts for 6.88%, and LiFSI accounts for 8.41%). The electrolyte uses EC, DMC, EMC, and additives. EC:DMC:EMC is mixed in a volume ratio of 3:3:4; the additives include 0.5% Si-containing additive (TMSP), 1% sulfate additive (DTD), and 0.5% LiBOB (the proportions of the above components are all mass proportions of the electrolyte). The separator uses an 11-μm separator (7+2+1+1) of a PE-based film + aluminum oxide ceramic layer + PVDF glue to prepare the laminated lithium-ion battery. The formation is carried out using a Blue Power charging cabinet. After activation at 0.2C, the battery is charged at a constant current and constant voltage of 0.33C to 3.8V and discharged at 0.33C to 2.0V.

[0116] Test example

[0117] (1) The prepared lithium iron phosphate material is tested for its morphology and powder tap density. The morphology is observed and tested using an SEM electron microscope. The test method for the powder tap density includes: using a Sansi Zongheng powder tap density meter to conduct the test under a pressure of 3T.

[0118] (2) The coated and roll-pressed positive electrode sheets were subjected to morphology and tap density tests. The morphology of the positive electrode sheets was observed and tested using an SEM electron microscope. The tap density was tested by converting the weight measured by the round punching test.

[0119] (3) After activating the battery using a Blue Power charge-discharge cabinet, rate charge-discharge tests were carried out. A constant current discharge at 0.33C was used, and a constant current and constant voltage charge at 4C was used until 3.8V.

[0120] (4) The battery was subjected to a capacity test. After the test, the battery was placed in a high-temperature environment at 45°C. The battery was taken out every 30 days for a capacity test, and the capacity retention rate after 365 days of high-temperature storage at 45°C was recorded.

[0121] (5) The battery was charged at a high rate of 4.5C. After 20 cycles, the battery was disassembled to observe the lithium deposition on the surface of the negative electrode. The criteria for judging the lithium deposition phenomenon on the negative electrode sheet were as follows: 1) No lithium deposition, no white lithium metal deposition on the surface; 2) Lithium deposition at the top, bottom, and creases was recorded as slight lithium deposition; 3) Lithium deposition on the entire surface was recorded as severe lithium deposition.

[0122] (6) A specific stepped charging regime was adopted, charging to 3.8V, followed by a constant current discharge at 0.33C / 0.5C to 2.0V, and the capacity retention rate was recorded after 1000 cycles.

[0123] The second and third groups of examples and Comparative Examples 1-6 were carried out with reference to Example 1. The main differences are shown in Table 1. Among them, in the second group of examples, Comparative Example 1, and Comparative Example 2, the proportion of the number of the first lithium iron phosphate particles and the second lithium iron phosphate particles was adjusted (wherein, the SEM morphology of the positive electrode sheet in Example 2c is as Figure 6 shown, and the SEM morphology of the positive electrode sheet in Example 2d is as Figure 7 shown); in the third group of examples, the Dv50 of the first graphite particles and the Dv50 of the second graphite particles were adjusted; in Comparative Example 3, lithium iron phosphate with a single particle size (particle size of 3-500nm) was used; in Comparative Example 4, the negative electrode sheet was coated in a single layer and all the first graphite particles were used; in Comparative Example 5, the negative electrode sheet was coated in a single layer and all the second graphite particles were used; in Comparative Example 6, in the double-layer coating of the negative electrode, the second graphite particles were used in the upper layer and the first graphite particles were used in the lower layer.

[0124] Table 1

[0125]

[0126]

[0127] Note: " / " indicates that the corresponding parameter was not tested or the corresponding component was not included.

[0128] The test data of the second and third groups of examples and Comparative Examples 1-6 are shown in Table 2.

[0129] Table 2

[0130]

[0131] As can be seen from Table 2, in the battery of the present invention, lithium iron phosphate large and small particles with different particle sizes are mixed in the positive electrode sheet, and the compaction density of the positive electrode sheet is relatively high; the negative electrode sheet adopts a double-layer coating design, with the upper layer using first graphite particles with smaller particle sizes and the lower layer using second graphite particles with larger particle sizes. The positive electrode sheet is paired with the double-layer coated negative electrode sheet, which can improve the fast charging performance, rate performance, high-temperature cycle and storage performance of the battery.

[0132] Four groups of Examples were carried out with reference to Example 1, the differences being that the content of Ti element doped in the first lithium iron phosphate particles was adjusted, and the content of V element doped in the second lithium iron phosphate particles was adjusted. The specific differences are shown in Table 3.

[0133] Table 3

[0134]

[0135] As can be seen from Table 3, in the present invention, when Ti element is mixed in the first lithium iron phosphate particles, V element is doped in the second lithium iron phosphate particles, and the Ti element and V element are adjusted within the protection scope of the present invention, the fast charging, high-temperature cycle and storage performance of the battery can be better balanced.

[0136] Five groups of Examples were carried out with reference to Example 1, the differences being that the Dv90 and Dv10 of the first graphite particles were adjusted, and the Dv90 and Dv10 of the second graphite particles were adjusted. The specific differences are shown in Table 4.

[0137] Table 4

[0138]

[0139] The test results of five groups of Examples are shown in Table 5.

[0140] Table 5

[0141]

[0142]

[0143] As can be seen from Table 5, in the present invention, when the difference A between Dv90 and Dv10 of the first graphite particles in the upper layer of the negative electrode sheet is less than the difference B between Dv90 and Dv10 of the second graphite particles in the lower layer of the negative electrode sheet, the kinetics of the battery can be improved and the cycle performance of the battery can be enhanced.

[0144] The 6th group of examples was carried out with reference to Example 1, the difference being that the Dv90, Dv50, and Dv10 of the first graphite particles were adjusted, and the Dv90, Dv50, and Dv10 of the second graphite particles were adjusted, thereby changing the ratios P1 and P2 of (Dv90 - Dv10) / Dv50. The specific differences are shown in Table 6.

[0145] Table 6

[0146]

[0147] The test results of the 6th group of examples are shown in Table 7.

[0148] Table 7

[0149]

[0150] It can be seen from Table 7 that in the present invention, by adjusting the values of P1 < P2 of (Dv90 - Dv10) / Dv50, the fast charging performance of the negative electrode sheet can be improved, the lithium deposition on the negative electrode can be reduced, and the cycle performance of the battery can be improved.

[0151] The 7th group of examples was carried out with reference to Example 1, and the main differences are shown in Table 8. Among them, in the 7th group of examples, the ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer was adjusted.

[0152] Table 8

[0153]

[0154] It can be seen from the data in Table 8 that when the present invention controls the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer within the protection scope of the present invention, the lithium deposition on the negative electrode can be reduced, and the high-temperature cycle performance of the battery can be improved.

[0155] It should be noted that in this article, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent in such a process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including that element. In addition, it should be pointed out that the methods and devices in the embodiments of the present application are not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0156] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lithium ion battery, characterized in that: The battery comprises a positive electrode sheet and a negative electrode sheet; The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector; the positive electrode active material layer comprises a lithium iron phosphate material; the lithium iron phosphate material comprises a first lithium iron phosphate particle and a second lithium iron phosphate particle; the particle size of the first lithium iron phosphate particle is 50nm-500nm; the particle size of the second lithium iron phosphate particle is 500nm-5000nm; In the positive electrode active material layer, the first lithium iron phosphate particles account for 70%-95%, and the second lithium iron phosphate particles account for 5%-30%; The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector; the negative electrode active material layer comprises a first negative electrode active material layer and a second negative electrode active material layer, the second negative electrode active material layer being disposed between the negative electrode current collector and the first negative electrode active material layer; the first negative electrode active material layer comprises first graphite particles, and the second negative electrode active material layer comprises second graphite particles; The Dv50 of the first graphite particles is less than the Dv50 of the second graphite particles.

2. The lithium-ion battery according to claim 1, characterized in that The number of lithium iron phosphate particles with a particle size of 1000nm-5000nm accounts for <3%, preferably 1.2%-3%.

3. The lithium-ion battery according to claim 1, characterized in that The particle size of the lithium iron phosphate material satisfies: 0.9≤(Dn90-Dn10) / Dn50≤1.2; And / or, the Dn50 of the lithium iron phosphate material is 300nm-400nm.

4. The lithium-ion battery according to claim 1, characterized in that: The lithium iron phosphate material is doped with one or more of Ti, V, Mg, and Al elements; Preferably, the first lithium iron phosphate particles are doped with Ti element, and the content of the Ti element is 0ppm-3000ppm, more preferably 500ppm-2800ppm; Preferably, the second lithium iron phosphate particles are doped with V element, and the content of V element is ≤5000ppm, more preferably 3000ppm-5000ppm.

5. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The lithium iron phosphate material or the positive electrode sheet meets at least one of the following conditions: (a) The powder compaction density of lithium iron phosphate material is 2.6 g / cm 3 -2.9g / cm 3 ; (b) The powder resistivity of lithium iron phosphate material is 5Ω.cm-10Ω.cm; (c) The impurity content in the lithium iron phosphate material is less than 150ppm, and the magnetic material content is less than 1ppm; (d) The compaction density of the positive electrode is ≥ 2.75 g / cc; (e) The surface resistance of the positive electrode sheet is 5Ω.cm-15Ω.cm.

6. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The Dv50 of the first graphite particles is 5 μm-15 μm; And / or, the Dv50 of the second graphite particles is 10 μm-30 μm.

7. The lithium-ion battery according to claim 6, characterized in that: The difference between Dv90 and Dv10 of the first graphite particle is recorded as A, and the difference between Dv90 and Dv10 of the second graphite particle is recorded as B. A and B satisfy: A <B; Preferably, the Dv90 of the first graphite particles is 15 μm-20 μm, and the Dv10 is 3 μm-5 μm; Preferably, the second graphite particles have a Dv90 of 20 μm-30 μm and a Dv10 of 8 μm-15 μm.

8. The lithium-ion battery according to claim 7, characterized in that: In the first graphite particle, the ratio of (Dv90-Dv10) / Dv50 is P1; in the second graphite particle, the ratio of (Dv90-Dv10) / Dv50 is P2, and P1 and P2 satisfy: P1 <P2; Preferably, P1 satisfies: 0.6≤P1≤3.5; and / or, P2 satisfies: 0.2≤P2≤2.

2.

9. The lithium-ion battery according to claim 7, characterized in that: The thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:9-5:5; Preferably, the thickness of the first negative electrode active material layer is 20 μm-60 μm; Preferably, the second negative electrode active material layer has a thickness of 40 μm-120 μm.

10. The lithium ion battery according to any one of claims 1 to 4, characterized in that: The electrolyte comprises an additive, wherein the additive comprises at least one of a silicon-containing additive, a sulfate-containing additive, and a lithium salt-type additive; the mass proportion of the additive is 0.01%-5% based on the total mass of the electrolyte; Preferably, based on the total mass of the electrolyte, the mass proportion of the silicon-containing additive is 0.01%-2%; Preferably, based on the total mass of the electrolyte, the mass proportion of the sulfate ester additive is 0.01%-4%; preferably, based on the total mass of the electrolyte, the mass proportion of the lithium salt additive is 0.01%-2%.

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