Negative plate and preparation method thereof, electrochemical device and electronic equipment

Abstract

The invention discloses a negative plate and a preparation method thereof, an electrochemical device and electronic equipment. The negative plate comprises a first negative electrode material layer and a second negative electrode material layer; wherein the first negative electrode material layer is in direct contact with at least one side of a negative electrode current collector, and the second negative electrode material layer is in direct contact with the first negative electrode material layer; the surface density of the negative plate is 0.15 mg / 1540.25 cm < 2 > to 0.18 mg / 1540.25 cm < 2 >; the mass ratio of the first negative electrode active material to the second negative electrode active material is (4: 6)-(6: 4). The negative plate has good cycle performance, storage performance and energy density when applied to the lithium ion battery.

Description

Technical Field

[0001] This invention specifically relates to negative electrode sheets and their preparation methods, electrochemical devices, and electronic devices. Background Technology

[0002] With the increasing global demand for clean energy and sustainable development, secondary batteries such as lithium-ion batteries have been widely used in electric vehicles, portable electronic devices, and energy storage systems due to their advantages such as high energy density, long cycle life, and no memory effect. As a key component of lithium-ion batteries, the performance of the negative electrode directly affects the overall performance of the battery, including energy density, power density, cycle life, and safety. Therefore, developing high-performance negative electrode materials and manufacturing processes is of significant practical importance.

[0003] Graphite, as an important anode material for lithium-ion batteries, is generally divided into artificial graphite and natural graphite. Artificial graphite is derived from petrochemical byproducts and requires high-temperature graphitization for production. This process is energy-intensive and highly polluting. Furthermore, the graphitization process vaporizes impurities from the raw material coke, which adhere to the graphite surface. As the temperature cools, this deposit solidifies into a thick, hard shell. This shell is highly disordered and contains various surface defects and impurities, increasing the material's specific surface area and accelerating surface degradation. Its in-situ growth on the graphite surface persists even after the battery is manufactured, significantly impacting its lifespan. Natural graphite, on the other hand, comes from natural mineral deposits, has a limited production volume, and has other important applications, such as metal smelting and nuclear energy, making it difficult to meet the ever-growing demand for graphite materials.

[0004] Therefore, how to make good use of artificial graphite and recycled graphite to obtain high-performance negative electrode sheets has received widespread attention in this field. Summary of the Invention

[0005] This invention primarily aims to overcome the low energy density of batteries caused by defects in artificial graphite in existing negative electrode materials. It provides a negative electrode material, its preparation method, an electrochemical device, and an electronic device. When applied to lithium-ion batteries, the negative electrode material of this invention exhibits excellent energy density, as well as good cycle performance and storage performance.

[0006] In a first aspect, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a first negative electrode material layer and a second negative electrode material layer; wherein the first negative electrode material layer directly contacts at least one side of the negative electrode current collector, and the second negative electrode material layer directly contacts the first negative electrode material layer;

[0007] The areal density of the negative electrode is 0.15 mg / 1540.25 cm³. 2 -0.18 mg / 1540.25cm 2 ;

[0008] The first negative electrode material layer includes a first negative electrode active material, and the first artificial graphite material includes first artificial graphite particles, wherein the first artificial graphite particles satisfy the following conditions:

[0009] The breakage coefficient P1 is 0.75-0.95, P1=A1 / B1, where A1 and B1 are the specific surface areas of the first artificial graphite particles before and after powder pressing, and the powder pressing pressure is 5t.

[0010] The roundness R is 0.58-0.70;

[0011] The wear resistance S is 0.55-0.72;

[0012] The second negative electrode material layer includes a second negative electrode active material, which includes second artificial graphite particles, and the second artificial graphite particles satisfy the following conditions:

[0013] The crushing coefficient P2 is 0.5-0.7, P2=A2 / B2, where A2 and B2 are the specific surface areas of the second artificial graphite particles before and after pressing, and the pressing pressure is 5t.

[0014] The surface hydroxyl abundance N is 0.7 at%-1.8 at%;

[0015] The lattice spacing d is 0.338-0.342 nm;

[0016] The mass ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4.

[0017] Secondly, the present invention provides a method for preparing a negative electrode sheet as described above, the method comprising the following steps:

[0018] S1. A first negative electrode slurry is coated on at least one side of the negative electrode current collector, dried and cold-pressed to form a first negative electrode material layer; the first negative electrode slurry includes a first negative electrode active material;

[0019] S2. Coat the first negative electrode material layer with a second negative electrode slurry, dry and cold press to form a second negative electrode material layer, and obtain a negative electrode precursor; the second negative electrode slurry includes a second negative electrode active material;

[0020] S3. Heat-treat the negative electrode precursor obtained in step S2 to obtain the negative electrode; the heat treatment time is 30-80ms; the heat treatment temperature is 100-150℃; and the heat treatment heating rate is 1600-2600℃ / s.

[0021] Thirdly, the present invention provides an electrochemical device comprising the negative electrode as described above.

[0022] Fourthly, the present invention provides an electronic device comprising the electrochemical device as described above.

[0023] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0024] The reagents and raw materials used in this invention are all commercially available.

[0025] The positive and progressive effects of this invention are as follows:

[0026] The negative electrode sheet provided by this invention has two layers of negative electrode material, using different artificial graphite as active materials in a specific mass ratio, and meeting a certain areal density. When applied to lithium-ion batteries, it has good cycle performance, storage performance and energy density. Detailed Implementation

[0027] In the negative electrode sheet provided in the first aspect of the present invention, the negative electrode sheet includes a first negative electrode material layer and a second negative electrode material layer; wherein, the first negative electrode material layer directly contacts at least one side of the negative electrode current collector, and the second negative electrode material layer directly contacts the first negative electrode material layer;

[0028] The areal density of the negative electrode is 0.15 mg / 1540.25 cm³. 2 -0.18 mg / 1540.25cm 2 ;

[0029] The first negative electrode material layer includes a first negative electrode active material, which includes first artificial graphite particles, and the first artificial graphite particles satisfy the following conditions:

[0030] The breakage coefficient P1 is 0.75-0.95, P1=A1 / B1, where A1 and B1 are the specific surface areas of the first artificial graphite particles before and after powder pressing, and the powder pressing pressure is 5t.

[0031] The roundness R is 0.58-0.70;

[0032] The wear resistance S is 0.55-0.72;

[0033] The second negative electrode material layer includes a second negative electrode active material, which includes second artificial graphite particles, and the second artificial graphite particles satisfy the following conditions:

[0034] The crushing coefficient P2 is 0.5-0.7, P2=A2 / B2, where A2 and B2 are the specific surface areas of the second artificial graphite particles before and after pressing, and the pressing pressure is 5t.

[0035] The surface hydroxyl abundance N is 0.7 at%-1.8 at%;

[0036] The lattice spacing d is 0.338-0.342 nm;

[0037] The mass ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4.

[0038] In this invention, the mass ratio of the first negative electrode active material to the second negative electrode active material can be 4:6, 5:5, or 6:4.

[0039] In this invention, the areal density of the negative electrode sheet can be 0.15 mg / 1540.25 cm³. 2 -0.17 mg / 1540.25cm 2 For example, 0.15 mg / 1540.25 cm 2 0.165 mg / 1540.25cm 2 Or 0.18 mg / 1540.25cm 2 .

[0040] In this invention, the adhesion strength of the negative electrode sheet after formation and capacity testing can be 2N-8N, for example, 2N, 3.2N, 3.4N, 5.7N, 6.8N, 7N, or 7.9N. After formation and capacity testing, the negative electrode sheet is prone to large-scale detachment and peeling, mainly due to the swelling of the binder and the slippage between graphite particles. Theoretically, the greater the adhesion strength of the electrode sheet after disassembly, the more complete the structure, which is more beneficial to the long-cycle charge-discharge behavior of the electrode sheet. Adding binders is usually used to improve adhesion strength, but this deteriorates other performance characteristics.

[0041] In this invention, the mass percentage of the first artificial graphite material in the first negative electrode material layer is 97.5%-99.0%, preferably 97.8%-98.6%, for example 97.8%.

[0042] In some specific embodiments, the areal density of the negative electrode is 0.165 mg / 1540.25 cm³. 2 After formation and disassembly, the adhesive strength is 2-8N, for example 2.0 N, 3.2 N, 3.4 N, 5.7 N, 6.8 N, 7.0 N or 7.9 N.

[0043] In some specific embodiments, the areal density of the negative electrode is 0.15 mg / 1540.25 cm³. 2 The adhesive strength after formation and capacity separation is 3.4N.

[0044] In some specific embodiments, the areal density of the negative electrode is 0.18 mg / 1540.25 cm³. 2 The adhesive strength after formation and disassembly is 7.0 N.

[0045] First artificial graphite particles

[0046] In this invention, the breakage coefficient P1 reflects the degree of fragmentation of the powder under extreme pressure. The closer this value is to 1.0, the less likely the material is to break. The testing method is as follows: using the ASTM D6556 multi-point BET method (77K nitrogen adsorption), the original specific surface area of ​​the artificial graphite particles is first tested and recorded as A1; then, the powder is pressed under a pressure of 5t, and the specific surface area is tested again and recorded as B1. P1 = A1 / B1.

[0047] In this invention, the roundness R describes the sharpness of the edges of artificial graphite particles; a higher value indicates a smoother edge. The roundness range is typically 0 to 1, where 0 represents very sharp edges and 1 represents perfectly round edges. The testing method is as follows: SEM images of artificial graphite particles are acquired using a scanning electron microscope (SEM), and then the images are analyzed using the Malvern Morphologi 4 software module to obtain the roundness R.

[0048] In this invention, the roughness S describes the degree to which the particles are close to a sphere; a higher value indicates a closer approximation to a sphere, and it is used to describe the unevenness of the particle surface. The testing method is as follows: SEM images of artificial graphite particles are obtained using a scanning electron microscope (SEM), and then the images are analyzed using the Malvern Morphologi 4 software module to obtain the roughness S.

[0049] In this invention, the breakage coefficient P1 of the first artificial graphite particle can be 0.80-0.88.

[0050] In some specific implementations, the breakage coefficient P1 of the first artificial graphite particle can be 0.91, 0.88, 0.85, 0.8 or 0.75.

[0051] In this invention, the roundness R of the first artificial graphite particles can be 0.59-0.66;

[0052] In some specific implementations, the roundness R of the first artificial graphite particles can be 0.58, 0.59, 0.6, 0.63 or 0.69.

[0053] In this invention, the roughness S of the first artificial graphite particle can be 0.65-0.70.

[0054] In some specific implementations, the roughness S of the first artificial graphite particle can be 0.62, 0.63, 0.64, 0.65 or 0.68.

[0055] In some specific implementations, the first artificial graphite particles meet the following conditions: the breakage coefficient P1 is 0.85, the roundness R is 0.63, and the convexity S is 0.68.

[0056] In some specific implementations, the first artificial graphite particles meet the following conditions: the breakage coefficient P1 is 0.88, the roundness R is 0.58, and the convexity S is 0.62.

[0057] In some specific implementations, the first artificial graphite particles meet the following conditions: the breakage coefficient P1 is 0.80, the roundness R is 0.60, and the roughness S is 0.64.

[0058] In some specific implementations, the first artificial graphite particles meet the following conditions: the breakage coefficient P1 is 0.75, the roundness R is 0.69, and the convexity S is 0.63.

[0059] In some specific implementations, the first artificial graphite particles meet the following conditions: the breakage coefficient P1 is 0.91, the roundness R is 0.59, and the convexity S is 0.65.

[0060] In this invention, the first artificial graphite particles can be commercially available or made in-house.

[0061] In this invention, the preparation method of the first artificial graphite particles includes the following steps:

[0062] (1) The raw material coke is graphitized to obtain product I;

[0063] (2) The product I is subjected to a first shaping to obtain product II; the first rotational speed R1 of the first shaping is 2500 r / min-3500 r / min;

[0064] (3) The product II is subjected to a “heating-first heat preservation-cooling-second heat preservation” cycle treatment, and then a second shaping is performed to obtain the first artificial graphite particles; the second rotation speed R2 of the second shaping is 5000r / min-7000r / min; the cooling rate is 8-18℃ / min; the first heat preservation time is 0.5-2h.

[0065] In this invention, the raw material coke can be any raw material conventionally used in the preparation of artificial graphite materials in the art, preferably needle coke, such as coal-based needle coke.

[0066] In this invention, the volatile matter content of the raw material coke is preferably less than 5%, for example, 3.5%; the Dv50 of the raw material coke is preferably 8μm-15μm, for example, 13μm.

[0067] In step (1), the graphitization temperature can be a temperature conventionally used in the art, preferably 2500℃-3000℃, for example 2800℃.

[0068] In step (2), the first shaping can be performed in an air jet mill.

[0069] In step (2), the temperature of the first shaping can be 400℃-500℃, for example, 500℃. The main purpose of heating during the shaping is to remove the water of crystallization adsorbed on the product I.

[0070] In step (2), the first rotational speed R1 can be 2700 r / min - 3300 r / min. The main purpose of controlling the appropriate rotational speed in the first shaping is to remove the large, irregular edges on the surface of product I.

[0071] In some specific implementations, the first rotational speed R1 is 2500 r / min, 3000 r / min, or 3500 r / min.

[0072] In step (2), the roundness R of product II is ≥0.45.

[0073] In step (2), the roughness S of product II is ≥0.55.

[0074] In step (3), the number of cycles of the “heating-first heat preservation-cooling-second heat preservation” cycle can be 2-3 times.

[0075] In step (3), the temperature of the first insulation can be 900℃-1100℃, for example 1000℃.

[0076] In step (3), the first heat preservation time can be 1-2 hours, for example, 1 hour.

[0077] In step (3), the heating rate can be 3-8℃ / min, for example 5℃ / min.

[0078] In step (3), the temperature of the second insulation can be 550℃-700℃, for example 600℃.

[0079] In step (3), the second heat preservation time can be 1-3 hours, for example 2 hours.

[0080] In step (3), the cooling method is to introduce gas for cooling. The flow rate of the introduced gas can be 15-30 L / min, preferably 20-30 L / min, for example 20 L / min; the gas is preferably nitrogen.

[0081] In step (3), the cooling rate can be 9-18℃ / min, preferably 10-18℃ / min, for example 10℃ / min.

[0082] In step (3), the temperature of the second shaping can be 550℃-700℃, for example 600℃.

[0083] In step (3), the second rotational speed R2 can be 6000 r / min - 7000 r / min, for example 7000 r / min.

[0084] In some specific implementations, the second rotational speed R2 is 5000 r / min, 6000 r / min, or 7000 r / min.

[0085] In step (3), the second shaping time can be 1h-3h, for example 2h.

[0086] In step (3), after the second shaping, the obtained material can be post-processed, including one or more of grading, washing and ultrasonic vibration.

[0087] The particle size distribution width d of the first artificial graphite particles obtained by grading can satisfy: d < 1.10; preferably 1.0 < d < 1.10.

[0088] The particle size of the first artificial graphite particles obtained by grading satisfies: (Dv90-Dv50 / Dv50-Dv10)>1.4.

[0089] In this invention, the particle size distribution widths d, Dv10, Dv50, and Dv90 can be obtained by using a conventional laser particle size analyzer in the art.

[0090] Second artificial graphite particles

[0091] In this invention, the breakage coefficient P2 reflects the degree of fragmentation of the powder under extreme pressure. The closer this value is to 1.0, the less likely the material is to break. The testing method is the ASTM D6556 multi-point BET method (77K nitrogen adsorption). First, the original specific surface area of ​​the second artificial graphite particle is measured and recorded as A2; then, the powder is pressed under a pressure of 5t, and the specific surface area is measured again and recorded as B2. P2 = A2 / B2.

[0092] In this invention, the surface hydroxyl abundance N reflects the number or coverage density of hydroxyl (-OH) functional groups per unit area on the surface of artificial graphite particles (the surface refers to a region with a thickness of 100 nm from the surface layer towards the center). The surface hydroxyl abundance is analyzed by XPS and expressed as the percentage (at%) of oxygen atoms in the hydroxyl groups relative to the total number of detected atoms within the analysis depth. In lithium-ion battery anodes, the presence of hydroxyl groups significantly affects interfacial reactivity, initial coulombic efficiency (ICE), and cycle stability.

[0093] In this invention, the lattice spacing d reflects the ease with which lithium ions embed into artificial graphite particles. A smaller lattice spacing indicates poorer kinetics and higher repair costs, while a larger lattice spacing indicates more defects, which is detrimental to storage performance. The test method for lattice spacing refers to the national standard GB / T 24533-2019.

[0094] In this invention, the breakage coefficient P2 of the second artificial graphite particles can be 0.55-0.60.

[0095] In some specific implementations, the breakage coefficient P2 of the second artificial graphite particles can be 0.52, 0.55, 0.62, 0.66 or 0.7.

[0096] In this invention, the surface hydroxyl abundance N of the second artificial graphite particles can be 1.0at%-1.5at%.

[0097] In some specific embodiments, the surface hydroxyl abundance N of the second artificial graphite particles may be 0.88 at%, 0.96 at%, 1.1 at%, 1.31 at%, or 1.54 at%.

[0098] In some specific embodiments, the lattice spacing d of the second artificial graphite particles may be 0.3383 nm, 0.3385 nm, 0.3393 nm, 0.3405 nm or 0.3413 nm.

[0099] In some specific embodiments, the second artificial graphite particles meet the following conditions: the breakage coefficient P2 is 0.62, the surface hydroxyl abundance N is 1.1 at%, and the lattice spacing d is 0.3393 nm.

[0100] In some specific embodiments, the second artificial graphite particles meet the following conditions: a breakage coefficient P2 of 0.55, a surface hydroxyl abundance N of 1.31 at%, and a lattice spacing d of 0.3405 nm.

[0101] In some specific embodiments, the second artificial graphite particles meet the following conditions: the breakage coefficient P2 is 0.66, the surface hydroxyl abundance N is 0.96 at%, and the lattice spacing d is 0.3385 nm.

[0102] In some specific embodiments, the second artificial graphite particles meet the following conditions: the breakage coefficient P2 is 0.52, the surface hydroxyl abundance N is 1.54 at%, and the lattice spacing d is 0.3413 nm.

[0103] In some specific embodiments, the second artificial graphite particles meet the following conditions: the breakage coefficient P2 is 0.70, the surface hydroxyl abundance N is 0.88 at%, and the lattice spacing d is 0.3383 nm.

[0104] In this invention, the secondary particles account for 50-100% of the second artificial graphite particles, for example, 75%.

[0105] In this invention, the second artificial graphite particles can be commercially available or made in-house.

[0106] In this invention, the method for preparing the second artificial graphite particles includes the following steps:

[0107] (1) The graphite recycled powder is first shaped to obtain product I; the first rotation speed R1 of the first shaped is 600 r / min-1200 r / min;

[0108] (2) The product I is subjected to heat treatment, rinsing and second shaping in sequence to obtain the artificial graphite material; the second rotation speed R2 of the second shaping is 2000 r / min - 3600 r / min.

[0109] In this invention, the recycled graphite powder can be derived from the negative electrode sheet of a waste battery.

[0110] In some implementations, the method for obtaining the graphite recycled powder from the negative electrode sheet of the waste battery includes: first treating the negative electrode sheet of the waste battery with ultraviolet light, sieving, collecting the sieve material and washing it with water.

[0111] The intensity of the ultraviolet light can be 90 μW / cm²-150 μW / cm², for example 123 μW / cm².

[0112] The temperature of the irradiation treatment can be 30-45℃, for example 38℃.

[0113] The sieving can be performed using methods conventional in the art. Preferably, the sieving is a vibrating sieve. Preferably, the mesh size of the sieve used is 30 mesh.

[0114] The number of times the water is washed is preferably 3 times.

[0115] In this invention, the carbon content of the recycled graphite powder can be greater than 98%.

[0116] In this invention, the recycled graphite powder includes secondary particles, wherein the number of secondary particles preferably accounts for 50-100%, for example 75%.

[0117] In step (1), the first shaping can be performed in an air jet mill.

[0118] In step (1), the temperature of the first shaping can be 400℃-500℃, for example, 500℃. The main purpose of heating in the first shaping is to remove the water of crystallization adsorbed on the graphite recycled powder.

[0119] In step (1), the time for the first shaping can be 1h-5h, for example 4h.

[0120] In step (1), the first rotational speed R1 can be 800 r / min - 1200 r / min. The main purpose of controlling the appropriate rotational speed in the first shaping is to remove the irregular large edges on the surface of the graphite recycled powder.

[0121] In some specific implementations, the first rotational speed R1 is 600 r / min, 1000 r / min, or 1200 r / min.

[0122] In step (1), the breakage coefficient P2 of product I can be 0.3 or higher.

[0123] In step (2), the temperature of the heat treatment can be 900℃-1100℃, for example 1000℃.

[0124] In step (2), the heat treatment time can be 0.5-3h, for example 2h.

[0125] In step (2), the heating rate of the heat treatment can be 2-5℃ / min, for example 2℃ / min.

[0126] In step (2), the detergent used for rinsing may include subcritical water, the temperature of which is 150-200°C, for example, 200°C.

[0127] In step (2), the flow rate of the detergent used for rinsing can be 8-30 L / min, preferably 10-30 L / min, for example 20 L / min.

[0128] In step (2), the rinsing time can be 0.5-3h, for example 3h.

[0129] In step (2), the temperature of the second shaping can be 300℃-500℃, for example 400℃.

[0130] In step (2), the second rotational speed R2 can be 2000 r / min - 3000 r / min, for example 2000 r / min.

[0131] In some specific implementations, the second rotational speed R2 is 2000 r / min, 2600 r / min, or 3600 r / min.

[0132] In step (2), the second shaping time can be 0.5h-3h, for example 3h.

[0133] In this invention, the second artificial graphite particles are further subjected to post-processing, which includes one or more of grading, water washing, and demagnetization.

[0134] Preferably, the particle size distribution width d of the second artificial graphite particles obtained by the grading satisfies: d < 1.10.

[0135] Preferably, the particle size of the second artificial graphite particles obtained by the grading satisfies: (Dv90-Dv50 / Dv50-Dv10)>1.4.

[0136] In this invention, the particle size distribution width, Dv10, Dv50, and Dv90 can be obtained by using a conventional laser particle size analyzer in the art.

[0137] The method for preparing the negative electrode sheet as described above, provided in the second aspect of the present invention, includes the following steps:

[0138] S1. A first negative electrode slurry is coated on at least one side of the negative electrode current collector, dried and cold-pressed to form a first negative electrode material layer; the first negative electrode slurry includes a first negative electrode active material;

[0139] S2. Coat the first negative electrode material layer with a second negative electrode slurry, dry and cold press to form a second negative electrode material layer, and obtain a negative electrode precursor; the second negative electrode slurry includes a second negative electrode active material;

[0140] S3. Heat-treat the negative electrode precursor obtained in step S2 to obtain the negative electrode; the heat treatment time is 30-80ms; the heat treatment temperature is 100-150℃; and the heat treatment heating rate is 1600-2600℃ / s.

[0141] In step S3, the heat treatment time can be 40-70ms, for example, 50ms.

[0142] In step S3, the temperature of the heat treatment can be 110℃-150℃.

[0143] In some specific embodiments, the temperature of the heat treatment may be 100°C, 120°C or 150°C.

[0144] In step S3, the heating rate of the heat treatment can be 1800-2600℃ / s, for example 2000℃ / s.

[0145] In step S3, the heat treatment is preferably carried out in a halogen lamp array.

[0146] The electrochemical device provided in the third aspect of the present invention includes the artificial graphite material as described above or the negative electrode as described above.

[0147] In this invention, the electrochemical device is preferably a battery.

[0148] In some embodiments, the electrochemical device is a lithium-ion battery.

[0149] In some embodiments, the lithium-ion battery may be a liquid lithium-ion battery, an all-solid-state lithium-ion battery, or a semi-solid-state lithium-ion battery. The battery type does not limit the scope of protection of this invention.

[0150] In some specific embodiments, the liquid lithium-ion battery includes a positive electrode, a negative electrode as described above, a separator, and an electrolyte.

[0151] In some specific embodiments, the all-solid-state lithium-ion battery includes a positive electrode, a negative electrode as described above, and a solid electrolyte membrane.

[0152] In this invention, the liquid lithium-ion battery includes a positive electrode, a negative electrode as described above, a separator, and an electrolyte.

[0153] Positive electrode film

[0154] In this invention, the positive electrode sheet may include a positive current collector and a positive electrode material layer, wherein the positive electrode material layer is disposed on at least one surface of the positive current collector; the positive electrode material layer includes a positive electrode active material.

[0155] In some embodiments, the positive electrode active material may be a conventionally used positive electrode active material in the art, such as one or more of lithium nickel cobalt manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, and lithium cobalt oxide. In some embodiments, the positive electrode material layer further includes a conductive agent. The conductive agent is a reagent used to ensure that the electrode has good charge and discharge performance. It may be selected from graphite materials such as natural graphite and artificial graphite, carbon black materials such as Super P, conductive carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black, conductive fibers such as carbon nanotubes (CNTs), carbon fibers, and metal fibers, metal powders such as fluorinated carbon powder, aluminum powder, nickel powder, conductive whiskers such as zinc oxide and potassium titanate, and conductive metal oxides or polyphenylene derivatives such as titanium dioxide, for example, Super P and CNTs.

[0156] In some embodiments, the positive electrode material layer further includes a binder. The binder may be a component that facilitates bonding between the positive electrode material and the conductive agent, and also facilitates bonding between the positive electrode material and the positive electrode current collector. It can typically be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers, such as PVDF.

[0157] In some embodiments, the positive electrode material layer includes a positive electrode active material, Super P, CNT, and PVDF.

[0158] In some specific embodiments, the mass ratio of the positive electrode active material, Super P, CNT and PVDF is 97:1.0:0.5:1.5.

[0159] In this invention, the positive electrode current collector can be a conventional positive electrode current collector in the art. For the positive electrode current collector, materials that do not cause chemical changes and have high conductivity can be used without limitation. For example, commonly used materials include stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel materials surface-treated with carbon, nickel, titanium, silver, etc. To enhance adhesion, micro-embossing can be formed on the surface of the positive electrode current collector. The positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, or porous bodies.

[0160] In some alternative embodiments, the positive current collector is aluminum foil.

[0161] In this invention, the positive electrode sheet can be prepared using methods conventional in the art.

[0162] In some optional embodiments, the method for preparing the positive electrode sheet includes the following steps: mixing the positive electrode material, binder and conductive agent in a certain mass ratio, adding solvent and mixing evenly to obtain a positive electrode slurry; then uniformly coating the positive electrode slurry onto at least one surface of the positive electrode current collector; and then preparing the positive electrode sheet by drying, rolling, slitting and other processes.

[0163] electrolyte

[0164] In some embodiments, the electrolyte may be a conventional electrolyte used in batteries, typically including non-aqueous solvents and lithium salts.

[0165] In this invention, the non-aqueous solvent can be a conventional non-aqueous solvent in the art.

[0166] In some embodiments, the non-aqueous solvent preferably comprises ester solvents and / or dimethyl sulfoxide (DMSO), more preferably carbonate solvents. The carbonate solvent may optionally be one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate, propylene carbonate, and butyl carbonate (BC). The non-aqueous solvent may also include ethyl acetate.

[0167] In this invention, the lithium salt can be a conventional lithium salt in the art, preferably one or more of LiPF6, LiBF4, LiClO4, LiCF3SO3 and LiN(CF3SO2)2, for example, LiPF6; the concentration of the lithium salt is preferably 1 mol / L.

[0168] In this invention, the electrolyte may include additives, which may be conventional additives in the art, such as fluoroethylene carbonate (FEC).

[0169] In some embodiments, the electrolyte includes LiPF6, ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate.

[0170] In some preferred embodiments, the electrolyte comprises EC, EMC, and DEC. The volume ratio of EC, EMC, and DEC is, for example, 1:1:1.

[0171] In some embodiments, the electrolyte can be prepared by conventional methods in the art. Optionally, it can be prepared by the following method: mixing the various non-aqueous solvents in proportion in an argon atmosphere glove box with a water content of <10 ppm, and then adding a fully dried lithium salt and mixing evenly to obtain the electrolyte.

[0172] diaphragm

[0173] In some alternative embodiments, the diaphragm may be a polypropylene membrane or a polyethylene membrane.

[0174] In some alternative embodiments, the thickness of the diaphragm may be 9 μm-18 μm, for example, 11 μm.

[0175] In some alternative embodiments, the air permeability of the diaphragm can be 180s / 100mL-380s / 100mL.

[0176] In some alternative embodiments, the porosity of the membrane may be 30%-50%.

[0177] In this invention, the method for preparing the lithium-ion battery can be a conventional method in the art. It can be that the positive electrode, separator, and negative electrode are wound in sequence to obtain the battery cell, which is then packaged in a packaging shell and injected with the electrolyte. Alternatively, the positive electrode, separator, and negative electrode are stacked in sequence to obtain the battery cell, which is then packaged in a packaging shell and injected with the electrolyte. After that, the lithium-ion battery is obtained through processes such as settling, hot and cold pressing, formation, clamping, and capacity testing.

[0178] The electronic device provided in the fourth aspect of the present invention includes the electrochemical device as described above.

[0179] For example, the electronic devices described in this invention may be, but are not limited to, mobile devices (such as mobile phones, tablets, laptops, video recorders, portable printers / copiers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems and backup power supplies, etc.

[0180] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0181] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0182] Example 1

[0183] 1. Negative electrode sheet

[0184] S1. Preparation of the first negative electrode material layer

[0185] (1) Coal-based needle coke (A-series needle coke, purchased from Baoshun Technology Co., Ltd., with a volatile content of 3.6%) was crushed into Dv50 of 13μm, and then graphitized to obtain product I; the graphitization temperature was 2800℃;

[0186] (2) The product I obtained in step S1 is placed in an air jet mill and heated to 500°C. At the same time, the rotation speed of the air jet mill is kept at the first rotation speed R1, which is 3000 r / min. The first shaping is performed to obtain product II. The roundness of the obtained product II is R≥0.45 and the convexity is S≥0.55.

[0187] (3) The product II obtained in step (2) is heated to 1000℃ at a rate of 5℃ / min and kept at that temperature for 1h (first heat preservation). Then, nitrogen is introduced at a flow rate of 20L / min to cool it down to 600℃ (cooling rate of 10℃ / min) and kept at that temperature for 2h (second heat preservation). The above-mentioned “heating-first heat preservation-cooling-second heat preservation” cycle is repeated 3 times. Then, the temperature is kept at 600℃, and the rotation speed of the splitting mill is adjusted to the second rotation speed R2, R2 is 7000r / min. The second shaping is carried out for 2h. The material obtained from the second shaping is cooled naturally and then graded. The particle size distribution width d is controlled to be less than 1.10 and (Dv90-Dv50 / Dv50-Dv10)>1.4. Then, it is washed with water and ultrasonically vibrated to obtain the first artificial graphite particles.

[0188] (4) The first artificial graphite particles obtained in the above steps are used as the negative electrode active material and are mixed with conductive agent acetylene black, thickener CMC, binder SBR and PAA in a mass ratio of 97.8:0.4:0.4:0.4:1.0. Then, deionized water is added as solvent and the mixture is stirred and mixed thoroughly to obtain the first negative electrode slurry. The first negative electrode slurry is uniformly coated on one surface of the negative electrode current collector copper foil and then dried, cold pressed and cut to form the first negative electrode material layer.

[0189] S2. Preparation of negative electrode precursor

[0190] (0) Disassemble the waste battery to obtain waste negative electrode sheets. First, treat the waste negative electrode sheets under ultraviolet light with an intensity of 123 W / cm² and a treatment temperature of 38°C. Then, use a 30-mesh sieve to vibrate and sieve the material to obtain primary graphite recovery powder. Then, wash the material with water three times to obtain graphite recovery powder with a carbon content greater than 98% and a secondary granular graphite mass ratio of 75%.

[0191] (1) The obtained graphite recycled powder is fed into an air jet mill and heated to 500℃. At the same time, the rotation speed of the air jet mill is kept at the first rotation speed R1, R1 is 1000r / min. The first shaping is carried out for 4h to obtain product I. The crushing coefficient P2 of product I is greater than 0.3.

[0192] (2) The product I obtained in step (1) is heated to 1000℃ at a rate of 2℃ / min and kept at the temperature for 2h for heat treatment. Then, the air jet mill is vacuumed to keep the internal pressure less than 0.1 atmospheres. Subcritical water preheated and pressurized to 200℃ is used for rinsing. The flow rate of subcritical water is 20L / min and the rinsing time is 3h. Then, the temperature is kept at 400℃ and the speed of the splitting mill is adjusted to the second speed R2, which is 2000r / min. The second shaping is carried out for 3h. The crushing coefficient P2 of the obtained material is greater than 0.5 and the surface hydroxyl abundance N is less than 3.0 at%; the material obtained by the second shaping is naturally cooled and then graded. The particle size distribution width d is controlled to be less than 1.10 and (Dv90-Dv50 / Dv50-Dv10)>1.4. Then, it is washed with water, dried and demagnetized to obtain the second artificial graphite particles.

[0193] (3) The second artificial graphite particles obtained in the above steps are used as the negative electrode active material and are mixed with conductive agent acetylene black, thickener CMC, binder SBR and PAA in a mass ratio of 97.8:0.4:0.4:0.4:1.0. Then, deionized water is added as solvent and the mixture is stirred thoroughly to obtain the second negative electrode slurry.

[0194] The second negative electrode slurry is uniformly coated onto the first negative electrode material layer obtained in step S1, and is in direct contact with the first negative electrode material layer. After drying, cold pressing, slitting and other processes, the second negative electrode material layer is formed, and the negative electrode precursor is obtained.

[0195] S3. Heat Treatment

[0196] The negative electrode precursor obtained in step S2 is heat-treated in a halogen lamp array to obtain the negative electrode. The heat treatment conditions are as follows: heat treatment time is 50 ms, heat treatment temperature is 120 ℃, and heating rate is 2000 ℃ / s.

[0197] 2. Positive electrode plate

[0198] Lithium iron phosphate (LFP) positive electrode active material, conductive agent Super P, CNT, and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:1.0:0.5:1.5. N-methylpyrrolidone (NMP) solvent was added and the mixture was stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry was coated on one surface of the positive electrode current collector aluminum foil, and the positive electrode sheet was prepared by drying, cold pressing, and slitting.

[0199] 3. Diaphragm

[0200] A porous PE membrane with a thickness of 11 μm was used as the separator.

[0201] 4. Electrolyte

[0202] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0203] 5. Lithium-ion batteries

[0204] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to act as a separator. Then, an aluminum-plastic film is wrapped around the separator, dried, and injected with the electrolyte prepared above. After encapsulation, settling, and formation processes, a 1 Ah soft-pack battery, i.e., a lithium-ion battery, is finally produced.

[0205] Examples 2-15 and Comparative Examples 1-5

[0206] The process parameters for each embodiment and comparative example are shown in Table 1. Other unlisted process parameters and preparation methods are the same as in Example 1.

[0207] Examples 2, 3, 1, and 2 differ from Example 1 in the mass ratio of the first artificial graphite particles to the second artificial graphite particles, while the other steps and process conditions are the same as in Example 1.

[0208] Examples 4, 5, 3, and 4 differ from Example 1 in areal density, while the other steps and process conditions are the same as in Example 1.

[0209] Examples 6 and 7 differ from Example 1 in the heat treatment temperature and heating rate, while the other steps and process conditions are the same as in Example 1.

[0210] The difference between Example 8 and Example 1 is that in the preparation method of the first artificial graphite particles, the first rotation speed R1 is adjusted to 2500 r / min.

[0211] The difference between Example 9 and Example 1 is that in the preparation method of the first artificial graphite particles, the first rotation speed R1 is adjusted to 3500 r / min.

[0212] The difference between Example 10 and Example 1 is that in the preparation method of the first artificial graphite particles, the second rotation speed R2 is adjusted to 6000 r / min.

[0213] The difference between Example 11 and Example 1 is that in the preparation method of the first artificial graphite particles, the second rotation speed R2 is adjusted to 5000 r / min.

[0214] The difference between Example 12 and Example 1 is that in the preparation method of the second artificial graphite particles, the first rotation speed R1 is adjusted to 600 r / min.

[0215] The difference between Example 13 and Example 1 is that in the preparation method of the second artificial graphite particles, the first rotation speed R1 is adjusted to 1200 r / min.

[0216] The difference between Example 14 and Example 1 is that in the preparation method of the second artificial graphite particles, the second rotation speed R2 is adjusted to 2600 r / min.

[0217] The difference between Example 15 and Example 1 is that in the preparation method of the second artificial graphite particles, the second rotation speed R2 is adjusted to 3600 r / min.

[0218] The difference between Comparative Example 5 and Example 1 is that the heat treatment in step S3 is not performed, while the other steps and process conditions are the same as in Example 1.

[0219] Table 1

[0220]

[0221] Table 2

[0222]

[0223] Example 1: Material Characterization

[0224] The negative electrode sheets prepared in Examples 1-15 and Comparative Examples 1-5 were characterized as follows:

[0225] 1. Surface density

[0226] (1) Tools and equipment:

[0227] High-precision analytical balance: with an accuracy of at least 0.1 mg (parts per ten thousand balance).

[0228] Standard sample cutter: round or square, with an area (S) precisely known, e.g., 1540.25 cm².

[0229] Cleaning tools: tweezers, gloves, brush, ear syringe.

[0230] Solvent for cleaning the coating: Use N-methylpyrrolidone (NMP).

[0231] Drying equipment: vacuum oven or forced-air drying oven.

[0232] (2) Test method

[0233] Step 1: Cut the sample

[0234] Use a sample cutter to cut a circular sample of standard area from the negative electrode sheet prepared in Examples 1-15 or Comparative Examples 1-5. Select an area with good appearance, no exposed foil, no wrinkles, and uniform coating. When cutting the sample, be sure to ensure that the edges are neat.

[0235] Step 2: Weigh the initial mass (m1)

[0236] Gently blow away the dust from the sample surface using a bulb syringe. Place the sample on the analytical balance using tweezers and record the mass m1 (unit: mg) after stabilization. This is the total mass of the current collector + negative electrode material layer.

[0237] Step 3: Clean the negative electrode material layer

[0238] Chemical dissolution method: Immerse the weighed sample (known m1) in an appropriate amount of NMP, let it stand or shake it slightly until the negative electrode material layer is completely dissolved and detached, and the current collector (copper foil) regains its full metallic luster without any residue.

[0239] Step 4: Cleaning and Drying

[0240] Rinse the cleaned current collector foil with fresh solvent to remove surface residues, then place it in a vacuum oven and dry at 105°C for 15-30 minutes to ensure complete solvent evaporation. After drying, the sample must be cooled to room temperature in a desiccator before weighing, as hot samples will affect weighing accuracy due to airflow.

[0241] Step 5: Weigh the final mass (m2)

[0242] Use tweezers to place the cooled, clean current collector foil back onto the analytical balance. After stabilization, record the mass m2 (unit: g), which is the mass of the current collector itself.

[0243] Step 6: Calculation

[0244] The areal density was calculated using the following formula, and the results are recorded in Table 1.

[0245] Surface density = (m1 - m2) / S

[0246] m1-m2: Net mass of the positive electrode material layer (mg)

[0247] S: Area of ​​the cut sample piece (cm²);

[0248] 2. Adhesion

[0249] The adhesion strength of the negative electrode sheets prepared in Examples 1-15 or Comparative Examples 1-5 was tested according to the method of GB / T 2792-2014, and the results are recorded in Table 1.

[0250] 3. Crushing coefficient

[0251] The ASTM D6556 multi-point BET method (77K nitrogen adsorption) was used. First, the original specific surface area of ​​the sample was tested and recorded as A. Then, the powder was pressed at a pressure of 5t (held for 30s), and the specific surface area was tested again and recorded as B. P=A / B. The results are recorded in Table 2.

[0252] 4. Surface hydroxyl abundance N

[0253] (1) Sample preparation: The powder compression method was used for sample preparation.

[0254] Artificial graphite material is ultrasonically dispersed in high-purity anhydrous ethanol to form a uniform suspension. A small amount of the suspension is taken with a dropper and dropped onto a clean substrate of a silicon wafer. The substrate is then thoroughly dried in a vacuum drying oven to remove all ethanol solvent. A small amount of dried artificial graphite material is taken and evenly spread on gold foil. Another flat foil or glass plate is used to gently press the powder from above to form a flat, dense sheet with good contact with the foil. The prepared sample is then dried in a vacuum drying oven at 60-80°C for at least 12 hours to remove physically adsorbed water and solvent to the maximum extent.

[0255] (2) Sample loading into the preparation chamber for pretreatment

[0256] After drying, place the sample into the rapid injection chamber of the XPS instrument as soon as possible to prevent re-adsorption of water and contaminants from the air. Allow the sample to stand in the rapid injection chamber (vacuum of approximately 10⁻⁵ Pa) for a period of time, or subject it to gentle heating (e.g., 80°C) to further remove adsorbed gases from the sample surface. Once the required vacuum level is achieved, transfer the sample to an ultra-high vacuum analysis chamber (vacuum level typically better than 5 × 10⁻⁵ Pa). -8 Pa).

[0257] (3) Data acquisition and analysis

[0258] First, a low-resolution, fast, broad-spectrum scan is performed in the range of 0-1100 eV or 0-1400 eV to identify all elements present on the sample surface (such as C, O, N, Si, etc.) and to check for any obvious contaminants; then, a high-resolution, slow scan is performed on the C 1s and O 1s orbitals.

[0259] Key parameters: Power: 20-40 eV (to ensure sufficient resolution and signal-to-noise ratio), Step size: 0.05-0.1 eV, Number of scans: Multiple scans are stacked to improve the signal-to-noise ratio (usually 5-20 times).

[0260] Based on the collected data, XPS processing software (such as CasaXPS, Avantage) automatically processes and calculates the surface hydroxyl abundance N, and the results are recorded in Table 2.

[0261] 5. Lattice spacing d

[0262] Referring to the national standard GB / T 24533-2019, the results are recorded in Table 2.

[0263] 6. Percentage of secondary particles

[0264] (1) Wet dispersion:

[0265] Disperse a small amount of recycled graphite powder in anhydrous ethanol using ultrasonication (power should not be too high, and the time should be short, such as 30-60 seconds, to prevent breakage of secondary particles). Place 1-2 drops of this suspension onto a silicon wafer or polished aluminum foil and allow it to dry naturally. If the subsequent SEM image quality is poor, a very thin layer of gold or platinum can be sputtered onto it to enhance the signal.

[0266] (2) SEM image acquisition

[0267] Images were acquired using a scanning electron microscope (SEM, manufactured by Nippon Electron, model JSM-7610FPlus).

[0268] Low-magnification observation: First, scan the entire sample stage at a low magnification (500-2000) to find statistically representative areas (i.e., uniformly dispersed particles without large-area aggregation or blank areas). Multi-area sampling: Randomly select 30 such representative fields of view at different locations on the sample for photography;

[0269] High magnification confirmation: Switch the magnification to 5000-20000x or even higher to clearly distinguish the primary particle structure on the surface of secondary particles and confirm the particle types that are difficult to identify at low magnification.

[0270] The key points for identifying single particles and secondary particles are: single particles have clear and independent boundaries; secondary particles have relatively blurred boundaries and can be seen to be "bonded" together by smaller units.

[0271] (3) Statistical analysis

[0272] Print the SEM image or open it in image software. Use a counter to manually classify and count all particles in a single image. To ensure statistical significance, at least 300 particles need to be counted in total (counting all particles in 30 sampling areas).

[0273] Calculation ratio:

[0274] Percentage of secondary particles = (Number of secondary particles / Total number of particles) × 100%

[0275] 7. Roundness R

[0276] SEM images of the artificial graphite material were acquired using a scanning electron microscope (SEM, JSM-7610FPlus, NEC). The images were then analyzed using the Malvern Morphologi 4 software module. At least 30 particles were selected from different regions of the SEM images, and each particle was tested individually. The average value was taken as the final roundness R. The results are recorded in Table 2.

[0277] 8. Grinding Surface S

[0278] SEM images of the artificial graphite material were acquired using a scanning electron microscope (SEM, JEOL JSM-7610FPlus), and then analyzed using the Malvern Morphologi 4 software module. At least 30 particles were selected from different regions of the SEM images, and each particle was tested individually. The average value was taken as the final abrasion roughness S, and the results are recorded in Table 2.

[0279] After preparing 1Ah soft-pack batteries using the examples and comparative examples, activation and capacity testing were performed. The activation and capacity testing process included cycling the prepared lithium-ion batteries three times at a current of 0.5Ah within a voltage range of 2.5-3.65V, and recording the average discharge capacity of the three cycles as C. Then, the following tests were performed:

[0280] Example 2: Cyclic Performance Test

[0281] (1) Fast charging cycle

[0282] The lithium-ion batteries prepared in Examples 1-15 and Comparative Examples 1-5 were cycled at 2C and 2.5-3.65V under 25°C conditions. After 3000 cycles, the discharge capacity of the battery at this point was divided by the discharge capacity of the first cycle to obtain the battery's capacity retention rate after 3000 cycles. The results are recorded in Table 3.

[0283] (2) High-temperature cycling capacity retention

[0284] The lithium-ion batteries prepared in Examples 1-15 and Comparative Examples 1-5 were cycled at 45°C using a charge-discharge cycle of 1C and 2.5-3.65V. After 2000 cycles, the discharge capacity of the battery at this point was divided by the discharge capacity of the first cycle to obtain the battery's capacity retention rate after 2000 cycles. The results are recorded in Table 3.

[0285] Example 3: Storage Performance Test

[0286] At 25°C, the lithium-ion batteries prepared in Examples 1-15 and Comparative Examples 1-5 were subjected to a 0.33C current for capacity determination, denoted as C0. The cells were then stored at a high temperature of 60°C. Every 7 days thereafter, the cells were taken out and their capacity was tested at room temperature, denoted as C1, C2, ..., Cn. The number of days until Cn first fell below 80% of C0 was used as the standard for measuring storage capacity. The results are recorded in Table 3.

[0287] Example 4: Energy Density Test

[0288] The energy density in this invention is the volumetric energy density, calculated according to the following method, and the results are recorded in Table 3.

[0289] (1) Testing equipment: charge-discharge tester, constant temperature chamber.

[0290] (2) Test conditions:

[0291] Temperature: Usually carried out under standard conditions of 25°C.

[0292] (3) Charging and discharging system:

[0293] Charging: The standard method specified by the manufacturer is used, which is usually constant current and constant voltage charging to the upper limit voltage of 3.65V and then cut off to a very small current.

[0294] Let it stand: After fully charging, let it stand for a period of time;

[0295] Discharge: Discharge at a constant current of the rated discharge current (e.g., 0.2C) until the cutoff voltage of 2.5V.

[0296] (4) Data Recording:

[0297] Discharge capacity: Records the total capacity released during the entire discharge process, in ampere-hours.

[0298] Average voltage: The charge / discharge tester software will automatically calculate and provide the average voltage throughout the entire discharge process, in volts. If the software does not provide this value, it can be obtained by integrating the discharge curve and dividing by the discharge time.

[0299] (5) Calculate energy

[0300] Energy = Discharge Capacity × Average Voltage

[0301] For example, a battery cell with a capacity of 5 Ah and an average voltage of 3.2 V has an energy of 1 Ah × 3.2 V = 3.2 Wh.

[0302] (6) Physical volume measurement (obtaining volume)

[0303] Tools: Digital caliper.

[0304] Measurement: Measure its length, width, and thickness (excluding any additional tabs, safety valve protrusions, etc.).

[0305] Calculate the volume:

[0306] Volume = Length × Width × Thickness

[0307] (7) Calculate the volumetric energy density and record the results in Table 2.

[0308] Volumetric energy density = energy / volume

[0309] Example: Energy = 3.2 Wh

[0310] Volume (measured geometrically) = 0.007 L (i.e. 7 cm³)

[0311] Volumetric energy density = 3.2 Wh / 0.007 L = 457 Wh / L

[0312] Table 3

[0313] serial number Fast charging cycles (%) High-temperature cycling capacity retention (%) Storage days (days) Volumetric energy density (Wh / L) Example 1 98.3 96.7 504 462 Example 2 98.7 96.0 469 448 Example 3 97.7 96.1 525 476 Example 4 98.2 96.7 490 425 Example 5 97.2 95.5 539 482 Example 6 96.9 95.2 455 462 Example 7 97.5 94 497 462 Example 8 98.2 96.4 490 460 Example 9 98.1 96.6 497 460 Example 10 98.2 96.1 490 465 Example 11 98.1 96.2 476 464 Example 12 97.8 96.3 504 455 Example 13 97.7 96.1 511 462 Example 14 97.5 96.2 490 456 Example 15 97.1 96 476 438 Comparative Example 1 94.2 89.5 350 416 Comparative Example 2 94.1 91.6 420 490 Comparative Example 3 95.2 92.5 350 416 Comparative Example 4 94.6 90.2 434 485 Comparative Example 5 92.3 88.7 399 455

[0314] As can be seen from the data in the table above, the negative electrode sheet provided by this invention has two layers of negative electrode material, using different artificial graphite as active materials in a specific mass ratio, and meeting a certain areal density. When applied to lithium-ion batteries, it has good cycle performance (fast charging cycle capacity retention rate of over 96%, high temperature cycle capacity retention rate of over 93%), storage performance (storage days of over 450 days at a high temperature of 60℃), and energy density (over 420 Wh / L).

[0315] Compared with Example 1, the difference in Comparative Example 1 is that the proportion of artificial graphite particles in the first example of Comparative Example 1 is too large, and the performance of the lithium-ion battery obtained is deteriorated to varying degrees, especially the storage performance, which is reduced by about 31%.

[0316] Compared with Example 1, Comparative Example 2 differs in that the proportion of the second artificial graphite particles in Comparative Example 2 is too large, which has a significant impact on the cycle performance and storage performance of the prepared lithium-ion battery, reducing the storage performance by about 17%.

[0317] Compared with Example 1, Comparative Example 3 has a lower areal density, and the performance of the lithium-ion battery prepared in Comparative Example 3 deteriorates to varying degrees in all aspects, especially the storage performance, which is reduced by about 31%.

[0318] Compared with Example 1, Comparative Example 4 differs in that its areal density is too high, which has a significant impact on the cycle performance and storage performance of the prepared lithium-ion battery, especially the cycle performance.

[0319] Compared to Example 1, Comparative Example 5 did not undergo heat treatment, resulting in a lithium-ion battery whose performance in all aspects deteriorated to varying degrees, especially its cycle performance. This may be because the surface material had excessively high compaction density, leading to poor cycle performance.

[0320] The difference between Examples 1-3 is that the mass ratio of the first artificial graphite material and the second artificial graphite material is different. It can be seen that, within a certain range, increasing the proportion of the second artificial graphite material is beneficial to improving the bonding force and the storage performance and energy density of the lithium-ion battery; increasing the proportion of the first artificial graphite particles is beneficial to improving the fast charging cycle performance.

[0321] The difference between Examples 1, 4 and 5 lies in the areal density. It can be seen that increasing the areal density is beneficial to improving storage performance and energy density.

[0322] The difference between Examples 1, 6 and 7 lies in the heat treatment time and the heating rate. It can be seen that there is no significant impact on energy density. However, using a lower heat treatment temperature and a lower heating rate is beneficial to cycle performance and storage performance, but it is not always better to use the lowest possible temperature.

[0323] The difference between Example 1 and Examples 8-11 is that the first person had different crushing coefficient, roundness and roughness of graphite particles. When the crushing coefficient is in the range of 0.75-0.88, the lithium-ion battery can obtain better storage performance.

[0324] The difference between Examples 1 and Examples 12-15 lies in the different fragmentation coefficient, surface hydroxyl abundance, and lattice spacing of the second artificial graphite particles. When the fragmentation coefficient is in the range of 0.5-0.66 and the surface hydroxyl abundance is in the range of 0.9-1.8 at%, the lithium-ion battery can obtain better cycle performance, storage performance, and higher energy density.

[0325] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a first negative electrode material layer and a second negative electrode material layer; wherein, the first negative electrode material layer directly contacts at least one side of the negative electrode current collector, and the second negative electrode material layer directly contacts the first negative electrode material layer; The areal density of the negative electrode is 0.15 mg / 1540.25 cm³. 2 -0.18 mg / 1540.25cm 2 ; The first negative electrode material layer includes a first negative electrode active material, which includes first artificial graphite particles, and the first artificial graphite particles satisfy the following conditions: The breakage coefficient P1 is 0.75-0.95, P1=A1 / B1, where A1 and B1 are the specific surface areas of the first artificial graphite particles before and after powder pressing, and the powder pressing pressure is 5t. The roundness R is 0.58-0.70; The wear resistance S is 0.55-0.72; The second negative electrode material layer includes a second negative electrode active material, which includes second artificial graphite particles, and the second artificial graphite particles satisfy the following conditions: The crushing coefficient P2 is 0.5-0.7, P2=A2 / B2, where A2 and B2 are the specific surface areas of the second artificial graphite particles before and after pressing, and the pressing pressure is 5t. The surface hydroxyl abundance N is 0.7 at%-1.8 at%; The lattice spacing d is 0.338-0.342 nm; The mass ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:

4.

2. The negative electrode sheet according to claim 1, characterized in that, The first artificial graphite particle satisfies one or more of the following conditions: a. The breakage coefficient P1 is 0.80-0.88; b. The roundness R is 0.59-0.66; c. The roughness S is 0.65-0.

70.

3. The negative electrode sheet according to claim 1, characterized in that, The second artificial graphite particles satisfy one or more of the following conditions: a. The breakage coefficient P2 is 0.55-0.60; b. Surface hydroxyl abundance N is 1.0 at%-1.5 at%; c. The second artificial graphite particles include secondary particles.

4. The negative electrode sheet according to claim 1, characterized in that, The negative electrode sheet satisfies one or more of the following conditions ae: a. The areal density of the negative electrode is 0.15 mg / 1540.25 cm³. 2 -0.17 mg / 1540.25cm 2 ; b. The adhesion strength of the negative electrode sheet after formation and capacity testing is 2N-8N; c. The mass ratio of the first negative electrode active material to the second active material is 5:5; d. In the first negative electrode material layer, the mass percentage of the first negative electrode active material is 97.5%-99.0%; e. In the second negative electrode material layer, the mass percentage of the second negative electrode active material is 97.5%-99.0%.

5. The negative electrode sheet according to claim 1, characterized in that, The preparation method of the first artificial graphite particles includes the following steps: (1) The raw material coke is graphitized to obtain product I; (2) The product I is subjected to a first shaping to obtain product II; the first rotational speed R1 of the first shaping is 2500 r / min-3500 r / min; (3) The product II is subjected to a “heating-first heat preservation-cooling-second heat preservation” cycle treatment, and then a second shaping is performed to obtain the first artificial graphite particles; the second rotation speed R2 of the second shaping is 5000r / min-7000r / min; the cooling rate is 8-18℃ / min; the first heat preservation time is 0.5-2h.

6. The negative electrode sheet according to claim 1, characterized in that, The method for preparing the second artificial graphite particles includes the following steps: (1) The graphite recycled powder is subjected to a first shaping to obtain product I; the first rotation speed R1 of the first shaping is 600 r / min - 1200 r / min; (2) The product I is subjected to heat treatment, rinsing and second shaping in sequence to obtain the second artificial graphite particles; the second rotation speed R2 of the second shaping is 2000 r / min - 3600 r / min.

7. A method for preparing a negative electrode sheet according to any one of claims 1-6, characterized in that, The method for preparing the negative electrode sheet includes the following steps: S1. A first negative electrode slurry is coated on at least one side of the negative electrode current collector, dried and cold-pressed to form a first negative electrode material layer; the first negative electrode slurry includes a first negative electrode active material; S2. Coat the first negative electrode material layer with a second negative electrode slurry, dry and cold press to form a second negative electrode material layer, and obtain a negative electrode precursor; the second negative electrode slurry includes a second negative electrode active material; S3. The negative electrode precursor obtained in step S2 is subjected to heat treatment to obtain the negative electrode; the heat treatment time is 30-80ms; the heat treatment temperature is 100-150℃; and the heat treatment heating rate is 1600-2600℃ / s.

8. The method for preparing the negative electrode sheet according to claim 7, characterized in that, The method for preparing the negative electrode sheet satisfies one or more of the following conditions: a. The heat treatment time is 40-70 ms; b. The temperature of the heat treatment is 110℃-150℃; c. The heating rate of the heat treatment is 1800℃ / s-2600℃ / s; d. The heat treatment is carried out in a halogen lamp array.

9. An electrochemical device, characterized in that, It includes the negative electrode sheet as described in any one of claims 1-6.

10. An electronic device, characterized in that, It includes the electrochemical device as described in claim 9.

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