A lithium-ion battery

Abstract

The present application relates to the technical field of lithium battery, and provides a lithium ion battery.The battery comprises a positive plate and a negative plate; a lithium iron phosphate material (LFP) in the positive plate is mixed by particles with different particle sizes, and the negative plate is designed by double-layer coating, wherein the upper layer uses first graphite particles with small particle size, and the lower layer uses second graphite particles with large particle size.The positive plate is matched with the double-layer coated negative plate, so that the lithium ion battery has high energy density, good fast charging performance, rate performance and high-temperature storage performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, and in particular to a lithium ion battery. BACKGROUND

[0002] The lithium ion battery is mainly composed of a positive electrode material, a negative electrode material, a separator, an electrolyte and auxiliary materials, wherein the positive electrode material accounts for more than 30% of the cost. So far, the batteries applied to power cars are mainly ternary batteries and lithium iron phosphate batteries. Lithium iron phosphate is widely concerned and applied due to its unique olivine structure, strong P-O bond and very stable structure, and its high safety, long cycle and other characteristics. Lithium iron phosphate positive electrode material is widely used in BEV (pure electric vehicle) and energy storage field due to its high safety, long cycle, pollution-free and other characteristics. With the further application of lithium iron phosphate in the field of BEV, meeting the long endurance and fast charging has become the mainstream demand.

[0003] However, the relatively low energy density and low fast charging performance of lithium iron phosphate lead to the problem of "mileage anxiety" in the application of electric vehicles. To ensure the fast charging performance of the battery, the energy density of the battery also needs to be ensured to increase the endurance. The existing lithium iron phosphate battery design improves the energy density of the battery by preparing thick electrodes and high compaction. However, when the surface density of the electrode coating increases and the compaction improves, the ion transport dynamics of the battery will decrease significantly, affecting the overall performance of the lithium iron phosphate battery. Therefore, it is crucial to research a battery with high energy density, good fast charging cycle performance and excellent rate performance. SUMMARY

[0004] The purpose of the present application is to overcome the above-mentioned problems existing in the prior art, and to provide a lithium ion battery. The lithium iron phosphate material (LFP) in the positive electrode sheet is mixed with particles of different sizes, and the negative electrode sheet is designed with double-layer coating. The upper layer uses first graphite particles with smaller particle size, and the lower layer uses second graphite particles with larger particle size. The positive electrode sheet is matched with the double-layer coated negative electrode sheet, so that the lithium ion battery has high energy density, good fast charging performance, rate performance and high temperature storage performance.

[0005] In order to achieve the above-mentioned purpose, the present application provides a battery comprising a positive electrode sheet and a negative electrode sheet.

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

[0007] The number percentage of the first lithium iron phosphate particles is 70-95%, and the number percentage of the second lithium iron phosphate particles is 5-30%.

[0008] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface 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, and the second negative electrode active material layer is arranged 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.

[0009] The technical scheme disclosed in the application has the following beneficial effects:

[0010] The lithium ion battery provided by the application is characterized in that the lithium iron phosphate material (LFP) in the positive electrode sheet is mixed with particles of different 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 improve the compaction density of the positive electrode sheet, and the lithium ion battery has good high-temperature storage and cycle performance. Meanwhile, the negative electrode sheet is designed in a double-layer coating manner, the first graphite particles with smaller particle sizes are used in the upper layer to improve the rate performance of the battery and the fast charging and cycle performance of the lithium ion battery, and the second graphite particles with larger particle sizes are used in the lower layer to have better high-temperature storage and cycle performance. The positive electrode sheet is matched with the double-layer coated negative electrode sheet, so that the lithium ion battery has high energy density, good fast charging performance, rate performance and high-temperature storage performance.

[0011] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are expected to vary when at least modest variation is derived from variation in size or from the variation in the manufacture of the compositions or formulations. Whenever a range is recited, any intervening value or values between the recited upper and lower values are also specifically contemplated. For values which are less than one, one unit is considered to be a lower limit. For ranges which are less than one, ranges ending in one unit, and less than one, are also considered to be contemplated. The endpoints of all ranges and any values disclosed herein are included within the scope of this disclosure, unless it is specifically stated otherwise. It is specifically intended that the description provided herein be illustrative of the application and not be limiting. BRIEF DESCRIPTION OF DRAWINGS

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

[0013] Figure 2 The structure schematic diagram of the positive electrode coating of the positive electrode sheet in Example 1 is shown.

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

[0015] Figure 4 Figure 2 shows a SEM image of a cross-section of a positive electrode sheet after plasma cutting in Example 1.

[0016] Figure 5 Figure 3 shows a schematic diagram of the structure of a double-coated negative electrode sheet in Example 1.

[0017] Figure 6 Figure 4 shows a topography SEM image of a positive electrode sheet in Example 2c.

[0018] Figure 7 Figure 5 shows a topography SEM image of a positive electrode sheet in Comparative Example 2d. DETAILED DESCRIPTION

[0019] The specific embodiments of the present application are described in detail below. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the application, are given by way of illustration only, and are not by way of limitation.

[0020] Unless otherwise defined, all scientific and technical terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0021] In the present application, the terms "lithium ion battery", "battery", "lithium battery", and "lithium ion secondary battery" all have the same meaning, and all refer to a lithium ion secondary battery, which generally includes an electrode assembly (e.g., a positive electrode sheet, a negative electrode sheet, and a separator), a container (a case) that houses the electrode assembly, and an electrolyte.

[0022] The present application provides a lithium ion battery, which 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 disposed on at least one side 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 first lithium iron phosphate particles have a particle size of 50 nm to 500 nm; and the second lithium iron phosphate particles have a particle size of 500 nm to 5000 nm.

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

[0025] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface 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, and the second negative electrode active material layer is arranged 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; Dv50 of the first graphite particles is less than Dv50 of the second graphite particles.

[0026] In the lithium ion battery provided by the application, lithium iron phosphate material (LFP) in the positive electrode sheet is mixed by particles with 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 bulk density is larger, which improves the compaction density of the positive electrode sheet and makes the lithium ion battery have good high-temperature storage and cycle performance. The first graphite particles with smaller particle sizes are used in the first negative electrode active material layer (also referred to as the upper layer) away from the negative electrode current collector in the negative electrode sheet. The first graphite particles have more defect sites on the surface, the active ion concentration in the upper layer region of the negative electrode sheet is higher, and the rate performance of the battery is improved. Moreover, the specific surface area of the first graphite particles is large, which can provide more channels for deintercalation of active ions and better improve the fast charging and cycle performance of the lithium ion battery. The second graphite particles with larger particle sizes are used in the second negative electrode active material layer (also referred to as the lower layer) close to the negative electrode current collector, and the second graphite particles have less side reactions, better high-temperature storage and cycle performance. The positive electrode sheet is matched with the negative electrode sheet coated with double layers, so that the lithium ion battery has 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 in the range of 50 nm-500 nm, for example, any value in the range of 50 nm-100 nm, 100 nm-200 nm, 200 nm-300 nm, 300 nm-400 nm or 400 nm-500 nm. The particle size of the second lithium iron phosphate particles is any value in the range of 500 nm-5000 nm, for example, any value in the range of 500 nm-1000 nm, 1000 nm-2000 nm, 2000 nm-3000 nm, 3000 nm-4000 nm or 4000 nm-5000 nm. The lithium iron phosphate material is composed of the first lithium iron phosphate particles and the 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 in the above ranges, the lithium iron phosphate material has high bulk density, which can improve the compaction density of the positive electrode sheet and simultaneously ensure the performance of fast deintercalation of lithium of the positive electrode sheet, so that the lithium ion battery has high energy density and fast charging performance.

[0028] In some embodiments, the number ratio of the first lithium iron phosphate particles in the positive electrode active material layer is 70-95%, and the number ratio of the second lithium iron phosphate particles is 5-30%. For example, the number ratio 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 defined by any two of the above values, preferably 80-90%; and the number ratio 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 defined by any two of the above values, preferably 10-20%. When the number ratio of the first lithium iron phosphate particles and the second lithium iron phosphate particles is within the above range, the present application can avoid the problems of poor kinetics and reduced constant current charge ratio caused by adding too many second lithium iron phosphate particles (large particle size). Since the second lithium iron phosphate particles have a large size, the lithium deintercalation path is long, and the lithium in the lithium iron phosphate lattice diffuses mainly along the 010-B axis direction. Too many second lithium iron phosphate particles can cause poor kinetics of the positive electrode active material and reduce the constant current charge ratio. In addition, the present application can avoid the problems of poor cycle performance, poor high-temperature storage, and low energy density caused by adding too few first lithium iron phosphate particles (small particle size). Since the first lithium iron phosphate particles have a small size, the deintercalation lithium path is short, and the kinetics is strong. Too many first lithium iron phosphate particles have a large reaction specific surface area, strong reactivity during charging and discharging, and increased side reactions, which have a greater impact on cycle performance and high-temperature storage. In addition, small particles have low packing density and low content per unit volume, which reduces the energy density of the battery.

[0029] The test method for the particle size and number ratio of the first lithium iron phosphate particles and the second lithium iron phosphate particles can be, for example, as follows: randomly select a specific unit area of 10 μm x 10 μm of the positive electrode sheet under SEM electron microscope field, measure the particle size of all lithium iron phosphate particles in the unit area, and statistically determine the number ratio of the first lithium iron phosphate particles and the second lithium iron phosphate particles according to the particle size range.

[0030] In some embodiments, the number percentage of lithium iron phosphate particles with a particle size of 1000 nm-5000 nm in the positive electrode active material layer is <3%, for example, the number percentage can be 0.5%, 1%, 2%, 2.5%, 2.8%, 2.99%, or any point value in the range consisting of any two of the above-mentioned point values, preferably 1.2%-3% (not including the end point 3%). When different particle sizes and number percentages of lithium iron phosphate particles are used in the positive electrode sheet, further controlling the number percentage of lithium iron phosphate particles with a particle size of 1000 nm-5000 nm (ultra-large lithium iron phosphate particles) in the above range can control the number of ultra-large lithium iron phosphate particles in a lower range, shorten the lithium ion transmission path, reduce the transmission time of lithium ions during charging and discharging, reduce the charging and discharging polarization, improve the dynamics of the battery, and further improve 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. For example, the value of (Dn90-Dn10) / Dn50 can be 0.9, 0.93, 0.95, 1.0, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, 1.2, or any point value in the range consisting of any two of the above-mentioned point values.

[0032] The Dn series parameters are statistical particle size indicators based on the particle number distribution, which are used to quantify the concentration trend and distribution width of the particle population (the size and distribution of lithium iron phosphate particles can be observed and recorded by scanning electron microscopy SEM, and then the statistical graph of the number distribution of lithium iron phosphate particles is obtained). The definition of Dn10: 10% of the particle diameter of the lithium iron phosphate material is less than or equal to this value (i.e., 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, i.e., 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 (i.e., the lower limit of the particle size of the largest 10% of the lithium iron phosphate particle population).

[0033] The value of (Dn90-Dn10) / Dn50 represents the distribution width percentage of the lithium iron phosphate particles. The larger the value, the wider the distribution. The wider the distribution, the greater the difference between the particle sizes of the first lithium iron phosphate particles and the second lithium iron phosphate particles. The first lithium iron phosphate particles and the second lithium iron phosphate particles have different stress levels during charging and discharging, which can cause increased battery polarization. When the value of (Dn90-Dn10) / Dn50 is controlled within the above range, the distribution width percentage 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 improved, and the fast charging performance of the battery can be further improved.

[0034] In some embodiments, the Dn50 of the lithium iron phosphate material is 300-400 nm. The Dn50 of the lithium iron phosphate material may, for example, be 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 defined by any two of the above-mentioned 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 first lithium iron phosphate particles and the second lithium iron phosphate particles are combined for use, and can be obtained based on the statistics of the particle number distribution of the lithium iron phosphate particles.

[0036] When lithium iron phosphate particles with different particle sizes and different quantity proportions are used in the positive electrode sheet, further adjusting the median particle size of the lithium iron phosphate particles within the above-mentioned range makes the particle size of the lithium iron phosphate particles after being used in combination relatively small (basically at the nanometer level), the lithium extraction path shorter, and the kinetic performance stronger. The fully nanometerized lithium iron phosphate particles can ensure that the battery has good fast charging performance and can alleviate the range 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 application further studies have found that by adding some metal oxides (for example, 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, the higher valence metal is doped into LFP through sintering. The doped metal elements often have a valence close to or higher than that of iron, and have 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 and the like, which enhances the bond energy with O, thereby reducing the binding of Li around the oxygen to Li, and improves the lithium diffusion coefficient. Doping the above-mentioned metals can also stabilize the structure of the lithium iron phosphate material and inhibit the abnormal growth of lithium iron phosphate grains during the sintering process, but the doping amount should not be too high, otherwise it will be difficult for the lithium iron phosphate particles to grow during sintering, affecting the final product's specific capacity.

[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.

[0040] In order to further improve the structural stability and capacity of the lithium iron phosphate material mixed with large and small particles, the present application also proposes a technical solution of doping different metal elements in lithium iron phosphate particles with different particle sizes, as follows:

[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, 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 in the range consisting of any two of the above-mentioned point values, and more preferably 500 ppm-2800 ppm. The Ti element is doped in the first lithium iron phosphate particles, Ti is beneficial to further refining the grain, Ti doping occupies Fe site and has greater constraint on O, which can improve the structural stability of the lithium iron phosphate material, inhibit the further growth of the crystal, make the first lithium iron phosphate particles reach a smaller grain level, and also can shorten the ion transmission 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 is ≤5000 ppm, for example, 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 in the range consisting of any two of the above-mentioned point values, and more preferably 3000 ppm-5000 ppm. V doping is N-type doping, has higher chemical valence, and has greater electron cloud density, which can better improve the voltage platform of the lithium iron phosphate material; at the same time, the content of the V element is avoided to be too high, which leads to the increase of the thermal stress in the positive plate and deteriorates the high-temperature performance of the battery.

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

[0044] (a) the powder compaction 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 is <150 ppm, and the magnetic substance content is <1 ppm;

[0047] (d) the compaction density of the positive plate is ≥2.75 g / cc;

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

[0049] In (a), the powder compaction density of the lithium iron phosphate material can be, for example, 2.6 g / cm 32.65g / 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 consisting of any two of the above point values. The lithium iron phosphate material in the positive electrode sheet of the present application is mixed with particles of different sizes, which improves 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 the lithium iron phosphate material includes: using a Sansi vertical and horizontal powder compaction density instrument to test at 3T pressure.

[0051] (b) The powder resistivity of the lithium iron phosphate material may be, for example, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, or any point value in the range consisting of any two of the above point values. The lithium iron phosphate material in the positive electrode sheet of the present application is mixed with particles of different sizes, which makes the packing of the entire lithium iron phosphate material system more compact, helps to reduce the contact resistance between lithium iron phosphate particles, enhances the charge transport efficiency between lithium iron phosphate particles, and thus reduces the overall powder resistivity of lithium iron phosphate, improving the rate performance of the battery.

[0052] The test method for the powder resistivity of the lithium iron phosphate material includes: using a four-probe powder resistivity instrument to test the powder resistance value at 50 mpa.

[0053] In (c), the impurity content in the lithium iron phosphate material is < 150 ppm, and the magnetic substance content is < 1 ppm. The presence of impurities and magnetic substances in the lithium iron phosphate material can accelerate the structural degradation of the battery material and affect the cycle stability of the battery. Controlling the impurity content to be < 150 ppm and the magnetic substance content to be < 1 ppm 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 improve the cycle life of the battery.

[0054] The impurity content in the lithium iron phosphate material 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 ≥ 2.75 g / cc; the lithium iron phosphate material in the positive electrode sheet is mixed with particles of different sizes, which makes the packing of the entire lithium iron phosphate material system more compact and has a higher compaction density, so that the compaction density of the positive electrode sheet is ≥ 2.75 g / cc, and thus the energy density of the battery is improved.

[0056] The compaction density test method of the positive electrode sheet is to punch a round sheet using a round cutter, calculate the area of the round sheet according to the diameter of the round sheet; use a micrometer to measure the thickness of the round sheet, calculate the volume of the two sides of the coating (the thickness is calculated by subtracting the thickness of the foil), then use an electronic balance to weigh the round sheet, and finally calculate the compaction density g / cm according to the weight / volume 3 .

[0057] In (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 point value in the range consisting of any two of the above point values. The high compaction density of the positive electrode sheet makes the contact between the electrode materials more compact, reduces the contact resistance between the lithium iron phosphate particles, thereby reducing the surface resistance of the positive electrode sheet, which helps to improve the power output of the battery, so that it can provide greater current when high-rate charging and discharging, and improve 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 application also provides a preparation method of a lithium iron phosphate material, comprising the following steps:

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

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

[0062] (3) Put the spray-dried materials with a ground particle size of less than 50-500 nm into a roller kiln, sinter them in the roller kiln under a nitrogen gas reducing atmosphere for 15-30 h, and the calcination temperature is 720-740℃;

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

[0064] (5) Use air flow crushing to obtain crushed materials respectively after sintering the materials in steps (3) and (4);

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

[0066] (7) The mixed material is weighed 5-7 kg and placed in a refractory graphite crucible, and sintered in a roller kiln for 15-30 h under a nitrogen atmosphere to reduce the oxygen content to <20 ppm at a calcination temperature of 750-780 °C;

[0067] (8) The sintered material is subjected to jet milling, and the milled material is screened, de-ironed, and packaged to obtain high-density fast-charging lithium iron phosphate, with the milling chamber dew point controlled at -60 °C, and the packaging performed in a constant-temperature low-humidity room at a temperature of 25 °C ± 2 °C and a humidity of 15% RH or less, with vacuum packaging used to maintain the moisture content of the finished product at 1000 ppm or less.

[0068] In some embodiments, the organic carbon source includes glucose. The coating agent includes PEG6000, which can provide carbon coating and surface-active dispersion. The lithium source includes lithium carbonate. The dopant is selected according to the element to be doped, for example including TiO2(doped with Ti element), ammonium metavanadate, and vanadium oxide (doped with V element). The dispersing agent includes citric acid.

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

[0070] In some embodiments, the positive electrode active material layer includes 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 fraction of the lithium iron phosphate material is 96.5%-98%, the mass fraction of the binder is 1.5-2.5%, and the mass fraction of the conductive agent is 1%-2%.

[0071] The positive electrode binder and the positive electrode conductive agent include 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 preferably polyvinylidene fluoride; the positive electrode conductive agent includes but is not limited to one or more mixed conductive agents selected from carbon black or carbon nanotubes, graphene, carbon fiber, and preferably carbon black.

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

[0073] The negative electrode sheet of the present application adopts a double-layer coating design, the first graphite particles with a smaller particle size are used in the upper layer, and the second graphite particles with a larger particle size are used in the lower layer, which improves the fast charging and cycling performance of the lithium ion battery and the high-temperature storage performance.

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

[0075] In some embodiments, the Dv50 of the first graphite particles is 5-15 μm; for example, it can be 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, or any point value in the range consisting of any two of the above point values. And / or, the Dv50 of the second graphite particles is 10-30 μm, for example, it can be 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or any point value in the range consisting of any two of the above point 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 tested by a laser particle size method, and the instrument used is a Mastersizer 3000.

[0077] In some embodiments, the difference between Dv90 and Dv10 of the first graphite particles is denoted as A, the difference between Dv90 and Dv10 of the second graphite particles is denoted as B, and 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 large particle size, and the greater the compaction density and energy density. The difference A between Dv90 and Dv10 of the first graphite particles in the upper layer of the negative electrode sheet is small, that is, graphite particles with smaller particle size are used; the difference B between Dv90 and Dv10 of the second graphite particles in the lower layer of the negative electrode sheet is large, that is, graphite particles with larger particle size are used; the difference in the particle size distribution of the two layers of graphite materials is appropriate, which reduces the polarization of the battery, improves the kinetics of the battery, and improves the cycle performance of the battery.

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

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

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

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

[0082] The Dv series parameters are statistical indicators based on the particle volume distribution, which are used to quantify the volume concentration trend and distribution width of the particle group (the laser particle size method can be used for testing, and then the statistical graph of the volume distribution of the graphite particles is obtained). Dv10 is defined as: 10% of the volume of the graphite particles is less than or equal to the value. Dv50 (volume median particle size) is defined as: 50% of the volume of the graphite particles is less than or equal to the value. Dv90 is defined as: 90% of the volume of the graphite particles is less than or equal to the value.

[0083] The value of (Dv90-Dv10) / Dv50 represents the proportion of the distribution width of the graphite particles, and the larger the value, the wider the distribution, and the more particles with larger particle sizes. By adjusting P1

[0084] In some embodiments, in the first graphite particles, the ratio (Dv90-Dv10) / Dv50 P1 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 point value in the range composed of any two of the above point values.

[0085] In some embodiments, in the second graphite particles, the ratio (Dv90-Dv10) / Dv50 P2 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 point value in the range composed of any two of the above point 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 point value in the range composed of any two of the above point values. The present application further limits the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer to be within the above range, so as to avoid the thickness of the first negative electrode active material layer being too large, which leads to a low energy density of the battery and degrades the high-temperature performance of the battery; and avoid the thickness of the second negative electrode active material layer being too large, which reduces the dynamics of the battery and degrades the 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 point value in the range composed of any two of the above point values.

[0088] In some embodiments, the second negative active material layer has a thickness of 40 μm to 120 μm, for example, 40 μm, 60 μm, 80 μm, 100 μm, 120 μm, or any value within the range defined by any two of the foregoing values.

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

[0090] In some embodiments, the negative active material layer comprises a graphite material (including the first graphite particles and the second graphite particles), a negative binder, and a negative conductive agent. The present application does not specifically limit the types of the negative binder and the negative conductive agent, and the types of the negative binder and the negative conductive agent commonly used in the industry are within the scope of the present application. Illustratively, the types of the negative binder include, but are not limited to, one or more of styrene butadiene rubber, styrene-acrylic emulsion, polyacrylic acid, sodium carboxymethyl cellulose, or lithium carboxymethyl cellulose, and preferably sodium carboxymethyl cellulose and styrene butadiene rubber. Illustratively, the types of the negative conductive agent include, but are not limited to, one or more of carbon black, graphene, graphite, carbon nanotube, and nanocarbon fiber, and preferably carbon black.

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

[0092] In some embodiments, the lithium ion battery further comprises an electrolyte, the electrolyte comprises an additive, the additive comprises at least one of a silicon-containing additive, a sulfate additive, and a lithium salt additive, and the mass fraction of the additive in the total mass of the electrolyte is 0.01% to 5%.

[0093] In some embodiments, the electrolyte comprises a silicon-containing additive, and the mass fraction of the silicon-containing additive in the total mass of the electrolyte is 0.01% to 2%, for example, 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 defined by any two of the foregoing values. The silicon-containing additive includes at least one of hexamethyldisilazane (HDMS), heptamethyldisilazane, tris(trimethylsilyl)phosphate (TMSP), tris(trimethylsilyl)borate (TMSB), ethyl hexamethyldisilazane, and hexamethyldisiloxane. The addition of the silicon-containing additive to the electrolyte can improve the stability of the SEI film and the solvation structure, which helps to improve the cycle stability of the negative electrode and thus the cycle performance of the battery.

[0094] In some embodiments, the electrolyte comprises a sulfonate additive, the mass percentage of the sulfonate additive in the total mass of the electrolyte 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 point value in the range consisting of any two of the above point values. The sulfonate additive comprises at least one of vinyl sulfonate (DTD), diethyl sulfite (DES), lithium methyl sulfonate, 1,3-propanediol sulfonate, and pentaerythritol bisulfonate.

[0095] In some embodiments, the electrolyte comprises a lithium salt type additive, the mass percentage of the lithium salt type additive in the total mass of the electrolyte 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 point value in the range consisting of any two of the above point values. The lithium salt type additive comprises at least one of lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)phosphate (LiODFP), lithium difluoro(oxalate)borate (LiODFB), and lithium tetrafluoro(oxalate)phosphate (LiTFOP). When the above-mentioned lithium salt type additive is added to the electrolyte, the generated inorganic SEI film has good stability, can reduce the generation of side reactions, and has high ionic conductivity, thereby improving the cycle performance and fast charging performance of the battery.

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

[0097] In some embodiments, the electrolyte further comprises a lithium salt, and the lithium salt is one or a combination of lithium hexafluorophosphate and lithium bisfluorosulfonylimide.

[0098] In the present application, the material of the separator is not specifically limited, and various separator materials commonly used in the industry and suitable for secondary batteries are within the protection scope of the present application. Exemplarily, the separator material comprises 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 application is assembled by 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 electric core by winding or stacking commonly used in the industry, then packaged by an aluminum plastic film, and then sequentially subjected to the processes of baking, injecting electrolyte, formation and two-sealing to obtain the lithium ion battery.

[0100] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0101] The materials and reagents used in the following examples can be obtained from commercial channels unless otherwise specified.

[0102] The present application will be described in detail below in combination with specific embodiments, which are used for understanding rather than limiting the present application.

[0103] Preparation of lithium iron phosphate material

[0104] (1) According to the mass ratio, 73-74% of iron phosphate, 8-9% of organic carbon source (glucose), 1-2% of PEG6000, 15-16% of lithium carbonate, 1-2% of dopant, 1-2% of dispersant (citric acid) and ultrapure water are mixed and ground. The ground small particles (50-500 nm) of the dopant are TiO2 powder; the ground large particles (500-5000 nm) of the dopant are ammonium metavanadate and vanadium oxide.

[0105] (2) After batching, grinding is respectively performed using 0.2 mm and 0.3 mm zirconium balls; the ground slurries are respectively spray dried to obtain spray-dried materials with a water content of less than 3%; the spray-dried materials with a ground particle size of less than 50-500 nm are placed in a roller kiln, and sintered in a nitrogen-filled reducing atmosphere for 20 h, with a calcination holding zone temperature of 720-740℃; the spray-dried materials with a ground particle size of 500-5000 nm are placed in a roller kiln, and sintered in a nitrogen-filled reducing atmosphere for 22 h, with a calcination holding zone temperature of 780-810℃; the sintered materials are respectively subjected to air jet crushing to obtain crushed materials; the two crushed materials are mixed and loaded into a pot for secondary sintering, wherein the small particles and the 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. Sinter it in a roller kiln. Sinter in the roller kiln for 20 hours in a nitrogen-filled reducing atmosphere with an oxygen content of <20 ppm. The temperature of the calcination heat preservation zone is 750-780℃. The mixed and sintered material is then subjected to air jet milling. The milled material is sieved to remove iron and packaged to obtain high-pressure fast-charging lithium iron phosphate. During air jet milling, the dew point of the milling chamber is controlled at -60℃. Packaging is carried out in a constant temperature and low humidity room with a temperature of 25℃ and a humidity of less than 15% RH. Vacuum packaging is used to keep the moisture content of the finished product below 1000 ppm.

[0107] (4) Of the prepared lithium iron phosphate particles, those with a particle size in the range of 50-500 nm accounted for 87.78%; those with a particle size in the range of 500-5000 nm (first lithium iron phosphate particles) accounted for 12.22%; specifically, those with a particle size in the range of 1-5 μm (second lithium iron phosphate particles) accounted for 1.88%, resulting in a Dn50 range of 0.35 μm, a (Dn90-Dn10) / Dn50 value of 0.98, and a particle size distribution of lithium iron phosphate with a large particle to small particle mass ratio of 9:1. The large particles (500-5000 nm) had a V element doping content of 4600 ppm; the small particles (50-500 nm) had a Ti element doping content of 1200 ppm. The SEM image of the morphology of this lithium iron phosphate material is shown below. Figure 1 As shown, the obtained lithium iron phosphate powder has a compaction strength of 2.62 g / cm³. 3 .

[0108] Example 1

[0109] (1) Preparation of positive electrode:

[0110] The positive electrode uses the lithium iron phosphate material obtained in Preparation Example 1 as the active material, carbon black and carbon nanotubes (mass ratio 1:1) as the conductive agent, polyvinylidene fluoride (PVDF) as the binder, and N-methylpyrrolidone (NMP) as the solvent. The materials are stirred evenly in a homogenizer in a certain order and then evenly coated onto the carbon-coated aluminum foil current collector using a coating machine. The positive electrode sheet is then pressed using a roller press. Based on the total mass of the lithium iron phosphate positive electrode coating, the positive electrode sheet contains 96.5% lithium iron phosphate particles, 1.5% conductive agent, and 2% binder.

[0111] See the schematic diagram of the obtained positive electrode. Figure 2 The compaction density of the prepared positive electrode sheet was 2.657 g / cm³. 3 The surface density of a single side is 190 g / m³. 3 Its surface SEM image is as follows Figure 3 As shown, the plasma-cut cross-section is obtained using SEM. Figure 4 As shown, the positive electrode morphology is uniformly distributed and the particles are intact.

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

[0113] The negative electrode uses artificial graphite with different particle sizes as the negative electrode material, and conductive carbon black as the conductive agent. Sodium carboxymethyl cellulose (CMC) and polyacrylic acid are used as the binder, and deionized water is used as the solvent. After being mixed uniformly, the mixture is coated on the surface of the copper foil current collector in layers. The active material in the upper layer (far from the current collector) is small particle graphite (first graphite particles), with a particle size Dv50 of about 8 pm, a Dv90 of about 17 pm, and a Dv10 of about 5 pm. The active material in the lower layer (close to the current collector) is large particle graphite (second graphite particles), with a particle size Dv50 of about 12 pm, a Dv90 of about 28 pm, and a Dv10 of about 9 pm. 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 pm). The schematic diagram of the double-layer coating of the negative electrode is shown in Figure 5 .

[0114] (3) Preparation of 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 laid and stacked in a Z-shaped stacking manner. After the core tabs are ultrasonically welded, the positive electrode and the aluminum tab, and the negative electrode and the copper tab are welded. Then, the battery is placed in a shell sealed with an aluminum plastic film and punched into a shell. The core is sealed in the shell by heating. The electrolyte uses a mixed lithium salt system of LiPF6 and LiTFSi (LiPF6 accounts for 6.88%, LiFSI accounts for 8.41%), and an electrolyte of 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 additives (TMSP), 1% sulfonate additives (DTD), and 0.5% LiBOB (the proportion of each component is the mass proportion of the electrolyte). The separator uses a 11 pm separator (7+2+1+1) of PE-based film + aluminum oxide ceramic layer + PVDF glue. The laminated lithium ion battery is prepared. The formation is carried out using a blue electric charging cabinet. After activation at 0.2C, the battery is charged at 0.33C to 3.8V, and discharged at 0.33C to 2.0V.

[0116] Test Example

[0117] (1) The morphology and powder compaction density of the prepared lithium iron phosphate material were tested. The morphology was observed and tested using SEM. The test method of powder compaction density included: using a Sansi vertical and horizontal powder compaction density instrument to test under a pressure of 3T.

[0118] (2) The morphology and compaction density of the positive electrode sheet after coating and rolling were tested. The morphology of the positive electrode sheet was observed and tested using SEM. The compaction density was tested by converting the weight of the punched circle.

[0119] (3) After the battery is activated by the blue electricity charging and discharging cabinet, the rate charging and discharging test is performed, 0.33C constant current discharging is used, and 4C constant current and constant voltage charging is used to charge to 3.8V.

[0120] (4) The capacity of the battery is tested, after the test, the battery is placed in a high temperature environment of 45°C, every 30D, the battery is taken out for capacity test, and the capacity retention rate of 45°C high temperature storage for 365D is recorded.

[0121] (5) The battery is charged at a large rate of 4.5C, and after 20T cycles, the battery is disassembled to observe the lithium precipitation on the negative electrode surface. The evaluation criteria for lithium precipitation on the negative electrode sheet are: 1) no lithium precipitation, no white lithium metal deposition on the surface; 2) lithium precipitation at the top, bottom and crease, which is recorded as slight lithium precipitation; 3) lithium precipitation on the whole surface, which is recorded as serious lithium precipitation.

[0122] (6) A specific step charging system is used to charge to 3.8V, 0.33C / 0.5C constant current discharging to 2.0V, and the capacity retention rate is recorded after 1000 cycles.

[0123] Example 2 group and example 3 group and comparative examples 1-6 are carried out according to example 1, and the main difference is shown in table 1. Among them, the quantity ratio of the first lithium iron phosphate particles and the second lithium iron phosphate particles is adjusted in example 2 group, comparative example 1 and comparative example 2 (in which, the SEM morphology of the positive electrode sheet in example 2c is shown in Figure 6 , and the SEM morphology of the positive electrode sheet in example 2d is shown in Figure 7 ); the Dv50 of the first graphite particles and the Dv50 of the second graphite particles are adjusted in example 3 group; single particle size lithium iron phosphate (particle size of 3-500nm) is used in comparative example 3; single layer coating is used for the negative electrode sheet in comparative example 4, and all the first graphite particles are used; single layer coating is used for the negative electrode sheet in comparative example 5, and all the second graphite particles are used; the second graphite particles are used in the upper layer and the first graphite particles are used in the lower layer in the double layer coating of the negative electrode in comparative example 6.

[0124] Table 1

[0125]

[0126]

[0127] Note: " / " represents that the corresponding parameter is not tested, or the corresponding component is not included.

[0128] The test data of example 2 group and example 3 group 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 application, different particle sizes of lithium iron phosphate particles are mixed in the positive plate, and the compaction density of the positive plate is higher; the negative plate adopts a double-layer coating design, the upper layer uses first graphite particles with a smaller particle size, and the lower layer uses second graphite particles with a larger particle size. The positive plate matched with the double-layer coated negative plate can improve the fast charging performance, rate performance, high-temperature cycle and storage performance of the battery.

[0132] Example 4 group refers to Example 1, the difference is to adjust the Ti element content of the first lithium iron phosphate particle and the V element content of the second lithium iron phosphate particle, see Table 3 for specific differences.

[0133] Table 3

[0134]

[0135] As can be seen from Table 3, in the application, the first lithium iron phosphate particle is mixed with Ti element, the second lithium iron phosphate particle is mixed with V element, and the Ti element and V element are adjusted within the protection scope of the application, which can better balance the fast charging, high-temperature cycle and storage performance of the battery.

[0136] Example 5 group refers to Example 1, the difference is to adjust the Dv90 and Dv10 of the first graphite particle and the Dv90 and Dv10 of the second graphite particle, see Table 4 for specific differences.

[0137] Table 4

[0138]

[0139] The test results of Example 5 group are shown in Table 5.

[0140] Table 5

[0141]

[0142]

[0143] As can be seen from Table 5, in the application, the difference A between Dv90 and Dv10 of the first graphite particle in the upper layer of the negative plate is less than the difference B between Dv90 and Dv10 of the second graphite particle in the lower layer of the negative plate, which can improve the dynamics of the battery and improve the cycle performance of the battery.

[0144] Example 6 group refers to Example 1, the difference is to adjust the Dv90, Dv50 and Dv10 of the first graphite particle and the Dv90, Dv50 and Dv10 of the second graphite particle, and further change the (Dv90-Dv10) / Dv50 ratio P1 and P2, see Table 6 for specific differences.

[0145] Table 6

[0146]

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

[0148] Table 7

[0149]

[0150] As can be seen from Table 7, the value P1 < P2 of the adjustment (Dv90-Dv10) / Dv50 in the present application can improve the fast-charging performance of the negative electrode sheet, reduce lithium precipitation of the negative electrode, and improve the cycle performance of the battery.

[0151] The example 7 group is prepared according to the reference example 1, and the main difference is shown in Table 8. In the example 7 group, the ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer is changed.

[0152] Table 8

[0153]

[0154] As can be seen from the data in Table 8, when the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is controlled within the protection scope of the present application, the present application can reduce lithium precipitation of the negative electrode and improve the high-temperature cycle performance of the battery.

[0155] It should be noted that in this document, the terms "comprise", "comprising", or any other variant thereof are intended to cover non-exclusive inclusions, so that processes, methods, articles, or devices that include a series of elements not only include those elements, but also include other elements not explicitly listed, or inherent to such processes, methods, articles, or devices. Without more limitations, the element defined by the statement "comprising a" does not exclude the presence of additional identical elements in the process, method, article, or device that includes the element. In addition, it should be noted that the scope of the methods and devices in the embodiments of the present application is not limited to the order of performing the functions shown or discussed, and can also include performing the functions in a substantially simultaneous manner or in a reverse order, for example, the described method can be performed in an order different from the described order, and various steps can be added, omitted, or combined. In addition, the features described with reference to certain examples can be combined in other examples.

[0156] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A lithium-ion battery, characterized by, 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 arranged on at least one side surface of the positive electrode current collector; the positive electrode active material layer comprises a lithium iron phosphate material; the lithium iron phosphate material comprises first lithium iron phosphate particles and second lithium iron phosphate particles; the first lithium iron phosphate particles have a particle size of 50 nm-500 nm; the second lithium iron phosphate particles have a particle size of 500 nm-5000 nm; In the positive electrode active material layer, the number percentage of the first lithium iron phosphate particles is 70%-95%, and the number percentage of the second lithium iron phosphate particles is 5%-30%; the number percentage of lithium iron phosphate particles with a particle size of 1000 nm-5000 nm is <3%; The particle size of the lithium iron phosphate material satisfies: 0.9≤(Dn90-Dn10) / Dn50≤1.2; The Dn50 of the lithium iron phosphate material is 300 nm-400 nm; The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface 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, and the second negative electrode active material layer is arranged 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 < the Dv50 of the second graphite particles; The Dv50 of the first graphite particles is 5 μm-15 μm; The Dv50 of the second graphite particles is 10 μm-30 μm.

2. The lithium-ion battery of claim 1, wherein, The number percentage of lithium iron phosphate particles with a particle size of 1000 nm-5000 nm is 1.2%-3%.

3. The lithium-ion battery of claim 1, wherein, The lithium iron phosphate material is doped with one or more of Ti, V, Mg and Al elements.

4. The lithium-ion battery of claim 3, wherein, The first lithium iron phosphate particles are doped with Ti elements, and the content of the Ti elements is 0 ppm-3000 ppm.

5. The lithium-ion battery of claim 4, wherein, The content of the Ti elements is 500 ppm-2800 ppm.

6. The lithium-ion battery of claim 3, wherein, The second lithium iron phosphate particles are doped with V elements, and the content of the V elements is ≤5000 ppm.

7. The lithium-ion battery of claim 6, wherein, The content of the V elements is 3000 ppm-5000 ppm.

8. The lithium-ion battery according to any one of claims 1 to 7, characterized in that, The lithium iron phosphate material or the positive electrode sheet satisfies at least one of the following conditions: (a) the powder compaction density of the lithium iron phosphate material is 2.6 g / cm 3 -2.9 g / cm 3 ; (b) the powder resistivity of the lithium iron phosphate material is 5 Ω.cm-10 Ω.cm; (c) the impurity content in the lithium iron phosphate material is <150 ppm, and the content of magnetic substances is <1 ppm; (d) the compaction density of the positive electrode sheet is ≥2.75 g / cc; (e) the surface resistance of the positive electrode sheet is 5 Ω.cm-15 Ω.cm.

9. The lithium-ion battery of claim 1, wherein, The difference between the Dv90 and the Dv10 of the first graphite particles is denoted as A, and the difference between the Dv90 and the Dv10 of the second graphite particles is denoted as B, and A and B satisfy: A<B.

10. The lithium-ion battery of claim 9, wherein, The Dv90 of the first graphite particles is 15 μm-20 μm, and the Dv10 is 3 μm-5 μm.

11. The lithium-ion battery of claim 9, wherein, The Dv90 of the second graphite particles is 20-30 mu m, and the Dv10 is 8-15 mu m.

12. The lithium-ion battery of claim 9, wherein, In the first graphite particles, the ratio of (Dv90-Dv10) / Dv50 is P1; in the second graphite particles, the ratio of (Dv90-Dv10) / Dv50 is P2, P1 and P2 satisfy: P1 13. The lithium-ion battery of claim 12, wherein, P1 satisfies: 0.6≤P1≤3.5; and / or, P2 satisfies: 0.2≤P2≤2.

2.

14. The lithium-ion battery of claim 9, wherein, The thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer is 1:9-5:

5.

15. The lithium-ion battery of claim 14, wherein, The thickness of the first negative electrode active material layer is 20-60 mu m.

16. The lithium-ion battery of claim 14, wherein, The thickness of the second negative electrode active material layer is 40-120 mu m.

17. The lithium-ion battery of any one of claims 1-7, wherein, The electrolyte comprises an additive, the additive comprises at least one of a silicon-containing additive, a sulfate additive, and a lithium salt type additive; the mass fraction of the additive in the total mass of the electrolyte is 0.01%-5%.

18. The lithium-ion battery of claim 17, wherein, The mass fraction of the silicon-containing additive in the total mass of the electrolyte is 0.01%-2%.

19. The lithium-ion battery of claim 17, wherein, The mass fraction of the sulfate additive in the total mass of the electrolyte is 0.01%-4%.

20. The lithium-ion battery of claim 17, wherein, The mass fraction of the lithium salt type additive in the total mass of the electrolyte is 0.01%-2%.

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