Lithium-ion battery

Abstract

Disclosed in the present application is a lithium-ion battery, which comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode active material, which involves the gradation of four size fractions, including a first size fraction, a second size fraction, a third size fraction and a fourth size fraction; and the negative electrode sheet comprises a graphite negative electrode material, the surface of the graphite negative electrode material having a residual carbon content of 0%-5%, and the degree of graphitization of the graphite negative electrode material being 90%-97%.

Description

A lithium ion battery

[0001] The present application claims priority to the Chinese patent application No. 2024113753559 filed on September 29, 2024 with the China Patent Office, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a lithium ion battery. BACKGROUND

[0003] Under the pressure of energy crisis and environmental pollution problems, safety, environmental protection and energy saving have become the theme of today's automobile development. New energy vehicles are highly valued and strongly supported by the transportation and energy sectors due to their energy-saving, environmentally friendly and pollution-free advantages. Lithium batteries, as the key of new energy vehicles, play a very important role in them, and with the continuous development of new energy field, the performance requirements of lithium batteries are also getting higher and higher.

[0004] Lithium iron phosphate, as a positive material of lithium ion battery, has the characteristics of rich raw material source, high theoretical capacity, stable structure, good safety performance and environmental friendliness, etc., and is the most potential positive material that can be widely used in the field of energy storage and power battery in the future. TECHNICAL PROBLEM

[0005] The theoretical capacity of lithium iron phosphate material is relatively low (about 170 mAh / g), and its electrical conductivity is poor, which is not conducive to the improvement of energy density and rate performance of lithium battery, and cannot meet the demand of long-range electric vehicles for high energy density. In addition, the discharge performance of lithium iron phosphate battery is poor at high temperature. TECHNICAL SOLUTION

[0006] In a first aspect, the present application provides a lithium ion battery, comprising a positive electrode sheet and a negative electrode sheet;

[0007] The positive electrode sheet comprises a positive electrode active material, and the positive electrode active material is a four-stage particle grading, comprising a first-stage particle, a second-stage particle, a third-stage particle and a fourth-stage particle.

[0008] The primary particle size R1 of the first-level particles satisfies R1 < 100 nm, and the number ratio of the first-level particles in the positive electrode active material is 5%-30%; the primary particle size R2 of the second-level particles satisfies 100 nm≤R2 < 400 nm, and the number ratio of the second-level particles in the positive electrode active material is 10%-50%; the primary particle size R3 of the third-level particles satisfies 400 nm≤R3 < 1000 nm, and the number ratio of the third-level particles in the positive electrode active material is 5%-30%; the primary particle size R4 of the fourth-level particles satisfies 1000 nm≤R4 ≤4000 nm, and the number ratio of the fourth-level particles in the positive electrode active material is 5%-20%; the areal density of the positive electrode sheet is 180-350 g / m2; the compaction density of the positive electrode sheet is 2.45-2.85 g / cm3; the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a graphite negative electrode material; the residual carbon content of the surface of the graphite negative electrode material is 0%-5%, and the graphitization degree of the graphite negative electrode material is 90%-97%.

[0009] The residual carbon content refers to the amount of carbon residues left after evaporation or cracking of a substance under specific conditions.

[0010] In a second aspect, the application provides a power utilization device comprising the lithium ion battery. Advantages

[0011] First, by adjusting the proportion of positive electrode active materials with different particle sizes, the positive electrode active material can be tightly packed, and in this state, the distribution of various particle sizes of the lithium supplement and the positive electrode active material tends to be consistent, which not only helps to improve the rate of lithium ion insertion and extraction in the positive electrode sheet and improve the kinetic performance of the lithium battery, but also achieves the ideal compaction density of the electrode sheet, which is beneficial to reducing the thickness of the positive electrode sheet and improving the energy density of the lithium battery. Specifically, in the same type of lithium battery, the use of the positive electrode sheet of the application significantly increases the mass of the positive electrode active material in the lithium battery (for a wound type lithium battery, the overall thickness of the positive electrode sheet is thinned, the number of wound coils of the positive electrode sheet is increased, and accordingly, the length of the positive electrode sheet that can be stored in the winding core is increased, and the energy density of the lithium battery is increased; for a lithium battery of a stacked type, the overall thickness of the positive electrode sheet is thinned, and the number of positive electrode sheets that can be stacked in the lithium battery is increased, and accordingly, the energy density of the lithium battery is increased), thereby helping to improve the energy density of the lithium battery and obtaining a lithium battery with a volume energy density of 420-490 Wh / L.

[0012] Secondly, by adjusting the residual carbon amount on the surface of the graphite negative electrode material, the residual carbon helps to improve the graphitization degree of the graphite negative electrode material, which can not only improve the desolvation rate of lithium ions on the interface between the electrode and the electrolyte and enhance the desolvation capacity, but also improve the high temperature resistance of the lithium battery; and the particle size of the positive electrode active material in the application matches the positive electrode active material with a suitable specific surface area, and a distance conducive to the diffusion of the lithium battery is formed in the positive plate, so as to adjust the charge and discharge rate of the positive electrode to match the above-mentioned graphite negative electrode material, and avoid the occurrence of negative electrode lithium precipitation, which helps to improve the rate performance and cycle performance of the lithium battery. Embodiments of the application

[0013] As used herein, "and / or" means one or all of the listed items.

[0014] As used herein, "includes" and "comprises" encompass the case where only the referenced element is present and the case where additional un-referenced elements are also present.

[0015] All percentages in the present application are weight percentages, unless otherwise specified.

[0016] Unless otherwise indicated, "a", "an", "the", and "this" are intended to encompass "at least one" or "one or more" in the specification herein. For example, "a component" means one or more components, so that more than one component can be considered and can be used or employed in the implementation of the described embodiments.

[0017] In some embodiments, the specific capacity of the negative plate is 345-365 mAh / g.

[0018] In some embodiments, the residual carbon amount on the surface of the graphite negative electrode material is 0.1%-3.5%.

[0019] By further optimizing the residual carbon amount on the surface of the negative electrode, the internal resistance of the negative plate can be reduced, which helps to further improve the cycle performance and high temperature resistance of the lithium battery.

[0020] In some embodiments, the negative electrode active material is prepared by a method comprising the following steps: mixing graphite with a coating agent and then performing first heat treatment, and performing second heat treatment after the first heat treatment is completed, to obtain the graphite negative electrode material.

[0021] In some embodiments, the coating agent comprises pitch, and the graphite comprises artificial graphite; the temperature of the first heat treatment is 2800-3000 DEG C, and the time of the first heat treatment is 60-100 h; the temperature of the second heat treatment is 800-1600 DEG C, and the time of the second heat treatment is 3-10 h; the graphite negative electrode material obtained has a residual carbon amount of 0.3%-2.0% and a graphitization degree of 93%-96%.

[0022] By selecting asphalt as the coating agent, the defects such as pores, grooves and cracks in the graphite can be modified after heat treatment, the electrochemical reversible capacity and cycle performance of the graphite are improved, in addition, asphalt as the coating agent can positively regulate the residual carbon content on the surface of the graphite, and the cost of asphalt is low, the raw material is easy to obtain, and the production cost can be reduced.

[0023] The graphite negative electrode material is prepared by high-temperature heat treatment and then low-temperature heat treatment, which can promote the complete development of graphite crystallites, improve the graphitization degree of the material, increase the graphite layer spacing, and be beneficial to the rapid embedding and stripping of lithium ions, thereby helping to improve the kinetic performance and electrochemical performance of the lithium battery.

[0024] In some embodiments, the compaction density of the positive electrode active material is 2.45-2.75 g / cm3.

[0025] The compaction density of the positive electrode active material in the application is controlled within the above range, which helps to improve the energy per unit volume of the battery and the energy density of the lithium battery.

[0026] In some embodiments, the positive electrode active material comprises lithium iron phosphate.

[0027] In some embodiments, the areal density of the positive electrode sheet is 200-350 g / m2, and / or the compaction density of the positive electrode sheet is 2.55-2.85 g / m3.

[0028] The application controls the areal density and compaction density of the positive electrode active material, adjusts the areal density and compaction density of the positive electrode active material layer, controls the porosity in the positive electrode active material layer within a certain range, forms a channel structure in the positive electrode active material layer that is beneficial to the transmission of lithium ions, balances the lithium ion stripping rate at the positive electrode with the lithium ion embedding rate at the negative electrode, thereby avoiding the occurrence of lithium precipitation at the negative electrode, and improving the cycle performance and rate performance of the lithium battery.

[0029] In some embodiments, the volumetric energy density of the lithium battery is 460-480 Wh / L.

[0030] In some embodiments, the CB value of the lithium battery is 1.1-1.4.

[0031] By controlling the CB value within the above range, the adaptability of the positive electrode active material and the negative electrode active material can be adjusted, the phenomenon of lithium precipitation during the cycle of the lithium battery can be avoided, and the cycle performance of the lithium battery can be improved.

[0032] Example 1

[0033] 1. Preparation of negative electrode active material

[0034] The graphite is mixed with pitch and then heat treated at 2900℃ for 80h, followed by heat treatment at 1200℃ for 7h, to obtain a graphite negative electrode material with a residual carbon content of 3%.

[0035] 2. Positive electrode active material preparation

[0036] The positive electrode active material (lithium iron phosphate) is divided into first grade particles, second grade particles, third grade particles and fourth grade particles according to particle size;

[0037] The primary particle size R1 of the first grade particles satisfies R1 < 100nm, and the number ratio of the first grade particles in the positive electrode active material is 20%; the primary particle size R2 of the second grade particles satisfies 100nm≤R2 < 400nm, and the number ratio of the second grade particles in the positive electrode active material is 40%; the primary particle size R3 of the third grade particles satisfies 400nm≤R3 < 1000nm, and the number ratio of the third grade particles in the positive electrode active material is 30%; the primary particle size R4 of the fourth grade particles satisfies 1000nm≤R4 ≤4000nm, and the number ratio of the fourth grade particles in the positive electrode active material is 10%;

[0038] The positive electrode active materials with different particle size distributions are mixed, and the positive electrode active material of the present embodiment is obtained.

[0039] 3. Negative electrode sheet preparation

[0040] The above graphite negative electrode material, conductive agent acetylene black, thickening agent CMC, and binder SBR are added into a vacuum stirrer in a mass ratio of 96.4:1:1.2:1.4 for mixing, then solvent deionized water is added into the mixture obtained, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thereby obtaining the negative electrode slurry of the present embodiment;

[0041] The above negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil, and after air drying at room temperature, it is transferred to an oven for continuous drying. After drying in the oven, the negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold-pressed and cut to obtain the negative electrode sheet to be assembled.

[0042] 4. Positive electrode sheet preparation

[0043] The above lithium iron phosphate positive electrode active material, conductive agent (acetylene black), and polyvinylidene fluoride PVDF (molecular weight 500000Da) are added into a vacuum stirrer in a mass ratio of 97.9:0.9:1.2 for mixing, then solvent NMP is added into the mixed slurry, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thereby obtaining the positive electrode slurry of the present embodiment;

[0044] The above positive electrode slurry is uniformly coated on both surfaces of the positive electrode current collector aluminum foil, transferred to an oven for continuous drying, and after drying in the oven, a positive electrode sheet semi-finished product is obtained, and then the positive electrode sheet semi-finished product is cold-pressed and cut to obtain a positive electrode sheet to be assembled (the area density of the positive electrode sheet is 310 g / cm 3 , and the compacted density is 2.83 g / cm 3 ).

[0045] 5. Lithium battery assembly

[0046] A commercially available polyethylene film is used as the separator of the lithium ion battery, and an electrolyte suitable for a 4.2V (upper limit of charging voltage) voltage system battery is used as the electrolyte. The above positive electrode sheet, negative electrode sheet (CB value is 1.14) and separator are wound together to obtain a bare cell, and the bare cell is packaged, injected, placed, formed, and divided to obtain a finished battery.

[0047] Example 2

[0048] 1. Preparation of negative active material

[0049] The graphite is mixed with pitch and then heat treated at 2800°C for 100h, and then heat treated at 1600°C for 3h to obtain a graphite negative electrode material with a residual carbon content of 0.5%.

[0050] 2. Preparation of positive active material

[0051] The positive active material (lithium iron phosphate) is divided into first grade particles, second grade particles, third grade particles and fourth grade particles according to particle size;

[0052] The primary particle size R1 of the first grade particles satisfies R1 < 100 nm, and the number ratio of the first grade particles in the positive active material is 10%; the primary particle size R2 of the second grade particles satisfies 100 nm ≤ R2 < 400 nm, and the number ratio of the second grade particles in the positive active material is 40%; the primary particle size R3 of the third grade particles satisfies 400 nm ≤ R3 < 1000 nm, and the number ratio of the third grade particles in the positive active material is 30%; the primary particle size R4 of the fourth grade particles satisfies 1000 nm ≤ R4 ≤ 4000 nm, and the number ratio of the fourth grade particles in the positive active material is 20%;

[0053] Mixing the above positive active materials with different particle size distributions, i.e.

[0054] 3. Preparation of negative electrode sheet

[0055] The above graphite negative electrode material, conductive agent acetylene black, thickening agent CMC, binder SBR are added into a vacuum stirrer in a mass ratio of 96.4:1:1.2:1.4 for mixing, then deionized water as solvent is added into the mixture thus obtained, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thus obtaining the negative electrode slurry of the present example.

[0056] The above negative electrode slurry is uniformly coated on both surfaces of the negative electrode current collector copper foil, transferred to an oven for continuous drying, and the negative electrode sheet semi-finished product is obtained after drying in the oven, then the negative electrode sheet semi-finished product is cold-pressed and cut to obtain the negative electrode sheet to be assembled.

[0057] 4. Preparation of positive electrode sheet

[0058] The above lithium iron phosphate positive electrode active material, conductive agent (acetylene black), and polyvinylidene fluoride (PVDF) with a molecular weight of 500,000 Da are added into a vacuum stirrer in a mass ratio of 97.9:0.9:1.2 for mixing, then solvent NMP is added into the mixed slurry, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thus obtaining the positive electrode slurry of the present example.

[0059] The above positive electrode slurry is uniformly coated on both surfaces of the positive electrode current collector aluminum foil, transferred to an oven for continuous drying, and the positive electrode sheet semi-finished product is obtained after drying in the oven, then the positive electrode sheet semi-finished product is cold-pressed and cut to obtain the positive electrode sheet to be assembled (the areal density of the positive electrode sheet is 300 g / cm 3 , and the compacted density is 2.85 g / cm 3 ).

[0060] 5. Assembly of lithium battery

[0061] A commercially available polyethylene film is used as the separator of the lithium ion battery, and a commercially available electrolyte suitable for a 4.2 V (upper limit of charging) voltage system battery is used as the electrolyte. The above positive electrode sheet and negative electrode sheet (CB value is 1.11) are wound together with the separator to obtain a bare cell, and the bare cell is subjected to packaging, liquid injection, standing, formation, and capacity distribution to obtain a finished battery.

[0062] Example 3

[0063] 1. Preparation of negative electrode active material

[0064] The graphite is mixed with pitch and then heat-treated at 3000°C for 60 h, and then heat-treated at 800°C for 10 h to obtain a graphite negative electrode material with a residual carbon content of 5%.

[0065] 2. Preparation of positive electrode active material

[0066] The positive active material (lithium iron phosphate) is divided into first grade particles, second grade particles, third grade particles and fourth grade particles according to particle size;

[0067] The primary particle size R1 of the first grade particles satisfies R1<100 nm, and the number ratio of the first grade particles in the positive active material is 30%; the primary particle size R2 of the second grade particles satisfies 100 nm≤R2<400 nm, and the number ratio of the second grade particles in the positive active material is 50%; the primary particle size R3 of the third grade particles satisfies 400 nm≤R3<1000 nm, and the number ratio of the third grade particles in the positive active material is 15%; the primary particle size R4 of the fourth grade particles satisfies 1000 nm≤R4≤4000 nm, and the number ratio of the fourth grade particles in the positive active material is 5%;

[0068] The positive active materials with different particle size distributions are mixed, and the positive active material of the embodiment is obtained.

[0069] 3. Preparation of negative electrode sheet

[0070] The above graphite negative electrode material, conductive agent acetylene black, thickening agent CMC and binder SBR are added into a vacuum stirrer in a mass ratio of 96.4:1:1.2:1.4 for mixing, then deionized water is added into the mixture obtained, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, so as to obtain the negative electrode slurry of the embodiment;

[0071] The above negative electrode slurry is uniformly coated on the two surfaces of the negative electrode current collector copper foil, and is transferred to an oven for continuous drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold-pressed and cut to obtain a negative electrode sheet to be assembled.

[0072] 4. Preparation of positive electrode sheet

[0073] The above lithium iron phosphate positive active material, conductive agent (acetylene black) and polyvinylidene fluoride PVDF (molecular weight 500000 Da) are added into a vacuum stirrer in a mass ratio of 97.9:0.9:1.2 for mixing, then solvent NMP is added into the mixed slurry, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, so as to obtain the positive electrode slurry of the embodiment;

[0074] The above positive electrode slurry is uniformly coated on the two surfaces of the positive electrode current collector aluminum foil, and is transferred to an oven for continuous drying. After drying in the oven, a positive electrode sheet semi-finished product is obtained, and then the positive electrode sheet semi-finished product is cold-pressed and cut to obtain a positive electrode sheet to be assembled (the areal density of the positive electrode sheet is 315 g / cm 3 , and the compacted density is 2.82 g / cm 3 ).

[0075] 5. Assembly of lithium battery

[0076] The commercially available polyethylene film is used as the separator of the lithium ion battery, and the electrolyte is the commercially available electrolyte suitable for the 4.2V (upper limit of charging voltage) voltage system battery. The above positive electrode sheet, negative electrode sheet (CB value is 1.19) and separator are subjected to a winding process to obtain a bare battery cell. The bare battery cell is subjected to packaging, liquid injection, standing, formation and capacity distribution operations to obtain a finished battery.

[0077] Example 4

[0078] 1. Preparation of negative active material

[0079] The graphite is mixed with pitch and then heat treated at 3000°C for 60h, and then heat treated at 1000°C for 5h to obtain a graphite negative material with a residual carbon content of 2%.

[0080] 2. Preparation of positive active material

[0081] The positive active material (lithium iron phosphate) is divided into first grade particles, second grade particles, third grade particles and fourth grade particles according to particle size;

[0082] The primary particle size R1 of the first grade particles satisfies R1 < 100nm, and the number ratio of the first grade particles in the positive active material is 25%; the primary particle size R2 of the second grade particles satisfies 100nm ≤ R2 < 400nm, and the number ratio of the second grade particles in the positive active material is 25%; the primary particle size R3 of the third grade particles satisfies 400nm ≤ R3 < 1000nm, and the number ratio of the third grade particles in the positive active material is 30%; the primary particle size R4 of the fourth grade particles satisfies 1000nm ≤ R4 ≤ 4000nm, and the number ratio of the fourth grade particles in the positive active material is 20%;

[0083] The above positive active materials with different particle size distributions are mixed to obtain the positive active material.

[0084] 3. Preparation of negative electrode sheet

[0085] The above graphite negative material, conductive agent acetylene black, thickening agent CMC and binder SBR are added into a vacuum stirrer in a mass ratio of 96.4:1:1.2:1.4 for mixing, and then deionized water is added into the obtained mixture, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thereby obtaining the negative electrode slurry of the present embodiment;

[0086] The above negative electrode slurry is uniformly coated on the two surfaces of the negative current collector copper foil, and then transferred to an oven for continuous drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain a negative electrode sheet to be assembled.

[0087] 4. Preparation of positive electrode sheet

[0088] The above lithium iron phosphate positive electrode active material, conductive agent (acetylene black), polyvinylidene fluoride PVDF (molecular weight 500000 Da) are added into a vacuum stirrer in a mass ratio of 97.9:0.9:1.2 for mixing, and then a solvent NMP is added into the mixed slurry, and the mixed slurry is stirred to be uniform under the action of the vacuum stirrer, thereby obtaining the positive electrode slurry of the present example;

[0089] The above positive electrode slurry is uniformly coated on both surfaces of the positive electrode current collector aluminum foil, and after air drying at room temperature, it is transferred to an oven for continuous drying, and after drying in the oven, a positive electrode sheet semi-finished product is obtained, and then the positive electrode sheet semi-finished product is cold-pressed and cut to obtain a positive electrode sheet to be assembled (the area density of the positive electrode sheet is 305 g / cm 3 , and the compacted density is 2.83 g / cm 3 ).

[0090] 5. Lithium battery assembly

[0091] A commercially available polyethylene film is used as the separator of the lithium ion battery, and an electrolyte suitable for a 4.2 V (upper limit of charging) voltage system battery is used as the electrolyte. The above positive electrode sheet, negative electrode sheet (CB value is 1.15) and separator are subjected to a winding process to obtain a bare cell, and the bare cell is subjected to packaging, liquid injection, standing, formation and capacity distribution operations to obtain a finished battery.

[0092] Example 5

[0093] The difference between the present example and Example 1 is that in the preparation process of the graphite negative electrode material, an equal weight of resin is used instead of the pitch used in Example 1, and the other steps and parameter settings remain the same as in Example 1.

[0094] Example 6

[0095] The difference between the present example and Example 1 is that in the preparation process of the graphite negative electrode material, the temperature in the two heat treatment processes is always kept at 2300℃ for 60h, and then heat treated at 300℃ for 5h; the other steps and parameter settings remain the same as in Example 1.

[0096] Comparative Example 1

[0097] The difference between the present comparative example and Example 1 is that the particle size D of the positive electrode active material satisfies 50nm

[0098] Comparative Example 2

[0099] The difference between the present comparative example and Example 1 is that the particle size distribution of the positive electrode active material is different, specifically, the positive electrode active material includes first-level particles, second-level particles and third-level particles;

[0100] The primary particle size R1 of the first-level particles satisfies R1 < 300 nm, and the number ratio of the first-level particles in the positive electrode active material is 35-45%; the primary particle size R2 of the second-level particles satisfies 300 nm≤R2 < 1700 nm, and the number ratio of the second-level particles in the positive electrode active material is 45-65%; the primary particle size R3 of the third-level particles satisfies 1700 nm≤R3≤4000 nm, and the number ratio of the third-level particles in the positive electrode active material is < 10%; other steps and parameter settings are consistent with those of Example 1.

[0101] Comparative Example 3

[0102] The difference between the present comparative example and Example 1 is that the particle size distribution of the positive electrode active material is different, specifically, the positive electrode active material includes first-level particles, second-level particles, third-level particles, fourth-level particles and fifth-level particles;

[0103] The primary particle size R1 of the first-level particles satisfies R1 < 100 nm, and the number ratio of the first-level particles in the positive electrode active material is 30-35%; the primary particle size R2 of the second-level particles satisfies 100 nm≤R2 < 300 nm, and the number ratio of the second-level particles in the positive electrode active material is 25-30%; the primary particle size R3 of the third-level particles satisfies 300 nm≤R3 < 600 nm, and the number ratio of the third-level particles in the positive electrode active material is 25-30%; the primary particle size R4 of the fourth-level particles satisfies 600 nm≤R4 < 1200 nm, and the number ratio of the fourth-level particles in the positive electrode active material is 5-15%; the primary particle size R5 of the fifth-level particles satisfies 1200 nm≤R5≤4000 nm, and the number ratio of the fifth-level particles in the positive electrode active material is < 5%; other steps and parameter settings are consistent with those of Example 1.

[0104] Comparative Example 4

[0105] The difference between the present example and Example 1 is that the particle size distribution of the positive electrode active material is different, specifically:

[0106] The primary particle size R1 of the first-level particles satisfies R1 < 200 nm, and the number ratio of the first-level particles in the positive electrode active material is 15%-20%; the primary particle size R2 of the second-level particles satisfies 200 nm≤R2 < 450 nm, and the number ratio of the second-level particles in the positive electrode active material is 35%-45%; the primary particle size R3 of the third-level particles satisfies 500 nm≤R3≤700 nm, and the number ratio of the third-level particles in the positive electrode active material is 30%-45%; the primary particle size R4 of the fourth-level particles satisfies R4 > 1200 nm, and the number ratio of the fourth-level particles in the positive electrode active material is < 8%; and other steps and parameter settings are consistent with those in Example 1.

[0107] Test method

[0108] I. Volume energy density test

[0109] At 25°C, the battery is charged at a current of 0.5C to 3.75V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a current of 0.5C to 2.0V, to obtain the discharge energy; the volume energy density = discharge energy / battery volume.

[0110] II. High temperature performance test

[0111] At 25°C, the battery is charged at a current of 1C to 3.75V, then charged at a constant voltage until the current drops to 0.05C, and then the battery is transferred to a 60°C oven for 30 days, and after the battery cools to room temperature, it is discharged at a current of 1C to 2.0V, and the discharge capacity after high temperature storage is compared with the discharge capacity before high temperature storage to calculate the capacity retention rate; the capacity retention rate = discharge capacity after high temperature storage / discharge capacity before high temperature storage x 100%;

[0112] Then the battery is charged at a current of 1C to 3.75V, then charged at a constant voltage until the current drops to 0.05C, and then discharged at a current of 1C to 2.0V, and the discharge capacity is compared with the discharge capacity before high temperature storage to calculate the capacity recovery rate; that is, the capacity recovery rate = discharge capacity after high temperature storage and recharging and discharging / discharge capacity before high temperature storage x 100%.

[0113] III. Residual carbon content test

[0114] In the following examples and comparative examples, the residual carbon content = coking value x addition amount.

[0115] The test method of coking value: 1.0000 g of dry sample with particle size of 1-3 mm is weighed and placed in a 20 ml porcelain crucible with constant weight, covered with a lid, and then placed in a 100 mL porcelain crucible or graphite crucible pre-coated with 10 mm thick coke particles. The gap between the two crucibles is filled with coke particles, and the crucible is completely buried in the coke particles, and the outer crucible cover is covered. The crucible is placed on a nickel-chromium wire support and placed in a 550℃±l0℃ muffle furnace, with the bottom of the crucible about 25 mm from the bottom of the furnace. After the sample is placed, the furnace temperature must be restored to constant temperature within 10 min, and continue to heat for 2 h. The crucible is taken out and cooled in air for about 15 min, the inner crucible is taken out, the coke powder attached to the outside of the crucible is swept away, and then placed in a desiccator and cooled to room temperature. The coking value is calculated according to the following formula:

[0116] K(%)=(m1-m2) / m×100%;

[0117] In the formula, K is the coking value of the pitch / %; m is the mass of the sample / g; m1 is the mass of the inner crucible / g; and m2 is the mass of the inner crucible and residue / g.

[0118] IV. Particle size test

[0119] The particle size of the positive electrode active material is tested by the following method: SEM test is performed on the positive electrode active material, and 3-10 pictures are randomly taken at a resolution of 20K. The size and number of the positive electrode active material in each picture are marked, and the particle size of each level of the positive electrode active material and the number ratio of each level of the positive electrode active material are calculated.

[0120] Table 1

[0121]

[0122] In combination with Examples 1-4, Comparative Examples 1-4 and Table 1, it can be seen that the distribution and ratio of particle size will affect the high-temperature performance of the battery. The first, second, third and fourth particles with the above particle size range are used in the present application, and the number ratio of each in the positive electrode active material is controlled, so that the positive electrode active material can achieve good bulk density without external high pressure, which helps to improve the lithium ion battery insertion and extraction rate, and improve the kinetic performance of the lithium battery. In addition, the residual carbon content of the negative electrode active material used in the negative electrode sheet is controlled within a suitable range, at which the positive electrode sheet can cooperate with the negative electrode sheet, not only can the lithium ion balance transfer between the positive and negative electrodes, but also helps to exert the positive and negative electrode capacity, and helps to improve the capacity of the lithium battery. At the same time, the positive electrode active material with the above particle size distribution and ratio can reduce the thickness of the active material layer on the surface of the positive electrode sheet to a certain extent. For the fixed size of the battery structure, the number of positive electrode sheet stacking or the number of winding increases, which is beneficial to improve the volume energy density of the lithium battery.

[0123] In combination with Example 1, Examples 5-6 and Table 1, it can be seen that the selection of the coating agent and the setting of the heat treatment temperature during the preparation of the negative electrode both have an impact on the negative electrode, especially a negative effect on the kinetic performance of the negative electrode, affecting the high-temperature capacity retention rate and the high-temperature capacity recovery rate of the battery.

Claims

1. A lithium ion battery, comprising a positive electrode sheet and a negative electrode sheet; the positive electrode sheet comprises a positive electrode active material, the positive electrode active material is a quaternary particle size distribution, comprising a first grade particle, a second grade particle, a third grade particle and a fourth grade particle; a primary particle size R1 of the first grade particle satisfies R1<100 nm, and the number ratio of the first grade particle in the positive electrode active material is 5%-30%; a primary particle size R2 of the second grade particle satisfies 100 nm≤R2<400 nm, and the number ratio of the second grade particle in the positive electrode active material is 10%-50%; a primary particle size R3 of the third grade particle satisfies 400 nm≤R3<1000 nm, and the number ratio of the third grade particle in the positive electrode active material is 5%-30%; a primary particle size R4 of the fourth grade particle satisfies 1000 nm≤R4≤4000 nm, and the number ratio of the fourth grade particle in the positive electrode active material is 5%-20%. The areal density of the positive electrode sheet is 180-350 g / m 2 ; the compaction density of the positive electrode sheet is 2.45-2.85 g / cm 3 ; the negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises a graphite negative electrode material; the residual carbon content of the surface of the graphite negative electrode material is 0%-5%, and the graphitization degree of the graphite negative electrode material is 90%-97%.

2. The lithium-ion battery of claim 1, wherein: the residual carbon content of the surface of the graphite negative electrode material is 0.1%-3.5%.

3. The lithium-ion battery of claim 1, wherein: the negative electrode active material is prepared by a method comprising the following steps: mixing graphite and a coating agent, and then performing first heat treatment, and then performing second heat treatment after the first heat treatment to obtain a graphite negative electrode material.

4. The lithium-ion battery of claim 3, wherein: the coating agent comprises pitch; the temperature of the first heat treatment is 2800-3000 ℃, and the time of the first heat treatment is 60-100 h; the temperature of the second heat treatment is 800-1600 ℃, and the time of the second heat treatment is 3-10 h.

5. The lithium-ion battery of claim 1, wherein: The compacted density of the positive electrode active material is 2.45-2.75 g / cm 3 .

6. The lithium-ion battery of claim 1, wherein: the positive electrode active material comprises a lithium iron phosphate material.

7. The lithium-ion battery of claim 1, wherein: The areal density of the positive electrode sheet is 200-350 g / m 2 , and / or the compaction density of the positive electrode sheet is 2.55-2.85 g / m 3 .

8. The lithium-ion battery of claim 1, wherein: the volume energy density of the lithium battery is 460-480 Wh / L.

9. The lithium-ion battery of claim 1, wherein: the CB value of the lithium battery is 1.1-1.

4. 10.A power device comprising the lithium ion battery according to any one of claims 1-9.

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