Negative electrode sheet, secondary battery and device

Abstract

A negative electrode sheet. The negative electrode sheet comprises a current collector and a negative electrode active material layer, which is arranged on the surface of the current collector, wherein the negative electrode active material layer comprises artificial graphite. The negative electrode sheet satisfies: 0.2≤S×E / (D×P)≤1.2, wherein S is the structural strength of the artificial graphite, and the structural strength refers to the ratio of the volume-average particle size Dv50 of the artificial graphite after powder pressing under a pressure of 20 KN to the volume-average particle size Dv50 thereof before powder pressing; E is the resistivity of the negative electrode sheet; D is the surface defect degree of the negative electrode sheet, and the surface defect degree refers to the peak area ratio of the D peak to the G peak of the artificial graphite; and P is the porosity of the negative electrode sheet.

Description

Negative electrode sheet, secondary battery and device

[0001] This application claims priority to Chinese patent application CN202410178053.6, entitled “Negative Electrode Plate, Secondary Battery and Device,” filed on February 8, 2024. The entire contents of the above-mentioned Chinese patent application are incorporated into this application by reference. Technical Field

[0002] The present application relates to a negative electrode plate, and in particular to a negative electrode plate, a secondary battery, and a device including the secondary battery, belonging to the field of batteries. Background Art

[0003] For new energy vehicles to replace traditional fuel vehicles, charging speed is one of the key technologies that must be mastered. It is also a very important indicator affecting the user experience. In addition, high-temperature performance (including high-temperature cycling and high-temperature storage) is extremely important to the user experience throughout the entire life cycle. To this end, major power battery companies have successively invested significant human and material resources. In a specific power battery system, the key to its charging speed is the negative electrode.

[0004] Currently, most research focuses on negative electrode active material design, with very little attention paid to negative electrode plate design. Numerous studies have shown that negative electrode active materials with excellent power performance may not achieve sustained fast charging if used with an inappropriate plate design. However, negative electrode active materials with average power performance can also achieve fast charging with an appropriate plate design. Therefore, the selection of negative electrode active materials and negative electrode plate design are key to achieving fast charging.

[0005] The negative electrode system using pure graphite has a lower theoretical capacity (372mAh·g -1 ) cannot meet the high-energy development needs, and the development of new, higher-capacity anode materials has become a current research hotspot. In recent years, the development of silicon-based materials such as silicon-oxygen, silicon-carbon, and nano-silicon has made rapid progress, but their practical application has been plagued by problems, especially in some high-end applications (such as pure electric vehicles and smartphones).

[0006] Therefore, there is an urgent need to develop a novel secondary battery. Summary of the Invention

[0007] The technical challenge to be solved by this application is to overcome the problem that the fast-charging graphite negative electrode in the prior art has defects such as low structural strength and large surface defects, which make it difficult to achieve high-energy fast charging, long cycle life, and good high-temperature storage characteristics. This application provides a negative electrode plate, a secondary battery, and a device including the secondary battery. The secondary battery of this application effectively improves its storage performance, long cycle life, and high-energy fast charging performance at high temperatures by controlling the structural strength of the artificial graphite in the negative electrode plate, the peak area ratio of the D peak to the G peak, and the resistivity and porosity of the negative electrode plate.

[0008] The first aspect of the present application provides a negative electrode plate, comprising a current collector and a negative electrode active material layer arranged on the surface of the current collector, the negative electrode active material layer comprising artificial graphite, and the negative electrode plate satisfies: 0.2≤S×E / (D×P)≤1.2, wherein S is the structural strength of the artificial graphite, wherein the structural strength refers to the ratio of the volume average particle size Dv50 of the artificial graphite secondary particles after powder pressing under a pressure of 20KN to the volume average particle size Dv50 before powder pressing; E is the resistivity of the negative electrode plate; D is the surface defectivity of the negative electrode plate, wherein the surface defectivity refers to the peak area ratio of the D peak to the G peak of the artificial graphite; and P is the porosity of the negative electrode plate.

[0009] A second aspect of the present application provides a secondary battery, comprising the negative electrode sheet described in the first aspect.

[0010] A third aspect of the present application provides a device, comprising the secondary battery described in the second aspect.

[0011] The negative electrode provided in this application has low surface defectivity, moderate membrane resistance and porosity. In addition, the lithium-ion battery provided in this application also exhibits high energy density, good fast charging capability, excellent cycle stability and good high-temperature storage performance. DETAILED DESCRIPTION

[0012] For the sake of clarity, this application only specifically discloses certain numerical ranges. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0013] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0014] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0015] The term "structural strength" refers to the ratio of the volume average particle size Dv50 of the artificial graphite after powder compaction under a pressure of 2t (20KN) to the volume average particle size Dv50 before powder compaction;

[0016] The term "surface defectivity" refers to the peak area ratio of the D peak to the G peak of the artificial graphite;

[0017] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0018] The negative electrode sheet provided in the present application includes a current collector and a negative electrode active material layer disposed on the surface of the current collector, wherein the negative electrode active material layer includes artificial graphite. The negative electrode sheet satisfies the following conditions: 0.2≤S×E / (D×P)≤1.2, wherein S is the structural strength of the artificial graphite, which refers to the ratio of the volume average particle size Dv50 of the artificial graphite after powder compaction under a pressure of 20 kN to the volume average particle size Dv50 before powder compaction; E is the resistivity (resistance) of the negative electrode sheet; D is the surface defectivity of the negative electrode sheet, which refers to the peak area ratio of the D peak to the G peak of the artificial graphite; and P is the porosity of the negative electrode sheet. High structural strength not only results in a lower lithium insertion expansion rate, but also effectively mitigates the degree of damage to graphite particles during the rolling process, while reducing the peeling of graphite flakes during cycling and high-temperature storage, reducing graphite surface defects, and thus improving cycling and high-temperature storage performance. At the same time, the reduced destruction of the graphite particle structure will also lead to a reduction in particle contact resistance and contact resistance, resulting in the membrane resistance and porosity of the negative electrode sheet being well maintained. The negative electrode sheet provided in this application has a low surface defect degree, moderate membrane resistance and porosity. In addition, the lithium-ion battery provided in this application also exhibits high energy density, good fast charging capability, excellent cycle stability and good high-temperature storage performance.

[0019] In some embodiments, S×E / (D×P) is 0.2, 0.25, 0.4, 0.55, 0.7, 0.85, 0.9, 1.0, 1.2 or any range therebetween; if S×E / (D×P) is too large, it indicates that the structural strength of the graphite material and the resistivity of the electrode are too large, and the electrode porosity is low, which is not conducive to dynamic performance; if S×E / (D×P) is too small, it is not conducive to long-term cycle stability; in some embodiments, 0.25≤S×E / (D×P)≤0.97.

[0020] In some embodiments, 0.5≤D≤0.92; in some embodiments, D is 0.52, 0.54, 0.56, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.92 or any range therebetween; when the surface defect degree D of the negative electrode sheet after rolling is too small, the dynamic performance is relatively poor, and the manufacturing requirements are high, which increases the manufacturing cost; when the surface defect degree D of the negative electrode sheet after rolling is too large, the reaction between such defects and the electrolyte increases, thereby leading to side effects, such as degradation of life characteristics; in some embodiments, 0.55≤D≤0.9.

[0021] In some embodiments, 0.07 ≤ E ≤ 0.16; in some embodiments, E is 0.07, 0.09, 0.11, 0.13, 0.15, 0.16, or any range therebetween. If the resistivity of the negative electrode plate is too high, electron transport between negative electrode particles is impaired, affecting kinetic performance. If the resistivity of the negative electrode plate is too low, side reactions between the negative electrode plate and the electrolyte are excessive, impairing long-term performance. In some embodiments, 0.08 ≤ E ≤ 0.15.

[0022] In some embodiments, 26.3% ≤ P ≤ 30%; in some embodiments, P is 26.3%, 26.8%, 27.3%, 27.8%, 28.3%, 28.8%, 29.3%, 30% or any range therebetween; if the porosity of the electrode is too large, the contact between the particles and between the particles and the binder is not firm, the electrode expands greatly, and it is not conducive to cycle stability; if the porosity of the electrode is too small, it is not conducive to electrolyte infiltration, affects the reaction kinetics, and causes a decrease in rate performance; in some embodiments, 26.3% ≤ P ≤ 29%.

[0023] In some embodiments, 0.7 ≤ S ≤ 0.96; in some embodiments, S is 0.7, 0.75, 0.8, 0.85, 0.9, 0.93, 0.96, or any range therebetween. High structural strength not only results in a lower lithium-insertion expansion rate but also effectively mitigates damage to graphite particles during the rolling process. It also reduces graphite flake exfoliation during cycling and high-temperature storage, reduces graphite surface defects, and improves cycling and high-temperature storage performance. However, excessive structural strength impedes lithium ion diffusion, resulting in reduced rate performance.

[0024] In some embodiments, the negative electrode active material layer further includes a silicon-based material.

[0025] In some embodiments, the silicon-based material includes at least one of silicon, a silicon alloy, a silicon-carbon compound, and a silicon-oxygen compound.

[0026] In some embodiments, the silicon-based material accounts for 0-40% by mass of the negative electrode active material.

[0027] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material accounts for greater than or equal to 95%.

[0028] In some embodiments, the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a nickel-cobalt-based ternary material and a phosphate-based material.

[0029] In some embodiments, the nickel-cobalt ternary material includes LiNi m Co n At least one of A(1-mn)O2 materials, A is selected from at least one of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver and niobium, 0.5≤m≤1, 0≤n≤0.5, m+n≤1.

[0030] In some embodiments, m is 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two of these values. In some embodiments, n is 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, or a range consisting of any two of these values.

[0031] In some embodiments, the nickel-cobalt ternary material includes at least one of NCA, NCM111, NCM523, NCM622, NCM811, Ni90, Ni92, and Ni95.

[0032] In some embodiments, the phosphate-based material includes LiMn k B (1-k) PO4, wherein 0≤k≤1, and the B element is selected from at least one of iron, cobalt, magnesium, calcium, zinc, chromium, and lead. In some embodiments, k is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or a range consisting of any two of these values. In some embodiments, the phosphate-based material includes lithium iron phosphate, LiMn 0.6 Fe 0.4 PO4 and LiMn 0.8 Fe 0.2 At least one of PO4.

[0033] In some embodiments, the positive electrode plate includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode active material, and the positive electrode active material includes at least one of lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese cobalt magnesium oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium iron phosphate and lithium manganese iron phosphate.

[0034] In some embodiments, the positive electrode active material layer further includes a binder, and optionally a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector.

[0035] In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0036] In some embodiments, the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0037] In some embodiments, the positive electrode further includes a positive electrode current collector, which can be a metal foil or a composite current collector. For example, aluminum foil can be used. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0038] In some embodiments, the negative electrode includes a negative electrode active particle layer, the negative electrode active particle layer includes negative electrode active particles, and the negative electrode active particles include silicon-based materials, or a mixture of silicon-based materials and at least one material selected from carbon-based materials, tin-based materials, phosphorus-based materials, and metallic lithium.

[0039] In some embodiments, the negative electrode active particle layer further comprises a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0040] In some embodiments, the conductive agent includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0041] In some embodiments, the negative electrode further includes a negative electrode current collector, and the negative electrode current collector includes: copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0042] In some embodiments, a separator is provided between the positive electrode and the negative electrode to prevent short circuits. The material and shape of the separator that can be used in the embodiments of the present application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material formed from a material that is stable to the electrolyte of the present application. In some embodiments, the separator can be selected from polyethylene film, polypropylene film, polyvinylidene fluoride film, and multilayer composite films thereof.

[0043] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, film, or composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric, or polypropylene-polyethylene-polypropylene porous composite film can be used.

[0044] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic layer.

[0045] The inorganic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene.

[0046] The polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride and poly(vinylidene fluoride-hexafluoropropylene).

[0047] When the secondary battery is a lithium-ion secondary battery, a lithium salt solution dissolved in an organic solvent is usually used as a non-aqueous electrolyte. The lithium salt can be an inorganic lithium salt (such as LiClO4, LiPF6, LiBF4, LiAsF6, LiSbF6), or an organic lithium salt (such as LiCF3SO3, LiCF3CO2, Li2C2F4(SO3)2, LiN(CF3SO2)2, LiC(CF3SO2)3, LiCnF2n+1SO3 (n≥2)). The organic solvent used in the non-aqueous electrolyte includes cyclic carbonates (such as ethylene carbonate, propylene carbonate, butylene carbonate or vinylene carbonate), chain carbonates (such as dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate), (chain esters, such as methyl propionate), cyclic esters (such as γ-butyrolactone), dimethoxyethane, chain ethers (such as diethyl ether, diethylene glycol dimethyl ether or triethylene glycol dimethyl ether), cyclic ethers (such as tetrahydrofuran or 2-methyltetrahydrofuran), acetonitrile, propionitrile or a combination thereof.

[0048] Lithium-ion secondary batteries consist of a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte. The positive and negative electrodes are immersed in the electrolyte, and lithium ions move between them through the electrolyte, enabling the battery to charge and discharge. To prevent the positive and negative electrodes from short-circuiting through the electrolyte, a separator is required to separate them. Lithium-ion secondary batteries can be cylindrical (square or cylindrical) with an aluminum or steel shell, or soft-pack batteries with an aluminum-plastic film shell.

[0049] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to: a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0050] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0051] In some embodiments, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other shape.

[0052] In some embodiments, the present application also provides a battery module. The battery module includes the aforementioned secondary battery. The battery module of the present application utilizes the aforementioned secondary battery and therefore has at least the same advantages as the aforementioned secondary battery. The battery module of the present application may include multiple secondary batteries, the specific number of which can be adjusted based on the application and capacity of the battery module.

[0053] In some embodiments, the present application further provides a battery pack comprising the above-mentioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0054] 2. Device

[0055] The present application also provides a device comprising at least one of the above-mentioned secondary battery, battery module or battery pack.

[0056] In some embodiments, the device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. To meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0057] In other embodiments, the device may be a mobile phone, a tablet computer, a laptop computer, etc. The device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0058] In order to make the purpose, technical solutions and beneficial technical effects of the present application clearer, the present application is further described in detail below with reference to the examples. However, it should be understood that the examples of the present application are only for the purpose of explaining the present application and are not intended to limit the present application, and the examples of the present application are not limited to the examples given in the specification. Specific experimental conditions or operating conditions not specified in the examples were prepared under conventional conditions or under conditions recommended by the material supplier.

[0059] The artificial graphite secondary particles in the following examples and comparative examples are all commercially available.

[0060] Examples and Comparative Examples

[0061] Example 1

[0062] (1) Preparation of positive electrode sheet:

[0063] The positive electrode active material LiNi 0.9 Co 0.03 Mn 0.07 O2, Super P, multi-walled carbon tubes, and PVDF are dispersed in an appropriate amount of NMP at a mass ratio of 97.5:0.9:0.5:1.1 to form a uniform positive electrode slurry. The positive electrode slurry is coated on aluminum foil and dried, rolled, and other processes to obtain a positive electrode sheet. The compacted density of the positive electrode sheet is 3.5 g / cm 3 , with a surface density of 25 mg / cm 2 .

[0064] (2) Preparation of negative electrode sheet

[0065] Artificial graphite secondary particles and pre-lithium silicon oxide compounds (commercially available) are pre-mixed in a mass ratio of 100:25 to prepare a negative electrode active material, and the negative electrode active material is homogenized with carbon black, carbon nanotubes, CMC, SBR and PAA in a mass ratio of 96:0.9:0.1:0.6:1.2:1.2 to prepare a uniformly dispersed slurry. Finally, the above slurry is coated on a 6μm copper foil, dried at 90°C, and then rolled to obtain a silicon-containing negative electrode plate for use. The double-sided surface density of the above-mentioned silicon-containing negative electrode plate is 22mg / cm2, the compaction density is 1.65g / cm3, the surface defectivity (median) is 0.76, the membrane resistance is 0.12Ωcm, and the plate porosity is 27.2%. The preparation of the artificial graphite secondary particles is as follows:

[0066] a) Raw material crushing: The raw coke is crushed using a mechanical mill and then categorized to produce specific aggregates;

[0067] b) Aggregate shaping: The aggregate is shaped using a shaping machine with a processing time of 30 minutes and a rotation speed of 1500 rpm;

[0068] c) Granulation: The shaped aggregate and asphalt are mixed in a mass ratio of 100:9, added to a drum-type, horizontal or vertical reactor, and heated to 600°C at a rate of 5°C / min for 2 hours to obtain an artificial graphite secondary particle precursor A, wherein the artificial graphite secondary particle precursor A has a Dv50 of 14.5 μm;

[0069] d) Graphitization treatment: adding the artificial graphite secondary particle precursor A to a box-type or Acheson graphitization furnace, performing high-temperature graphitization treatment at 2800° C. for 20 h, and then cooling to obtain an artificial graphite secondary particle precursor B;

[0070] e) Coating: The artificial graphite secondary particle precursor B and liquid asphalt are mixed in a mass ratio of 100:10, and then carbonized in a roller kiln heat treatment device, wherein the heating rate is 10°C / min, the heat treatment temperature is 1250°C, the holding time is 2h, and the temperature is lowered to obtain artificial graphite secondary particles, wherein the artificial graphite secondary particles have a Dv50 of 13.5μm and a structural strength of 0.92.

[0071] (3) Preparation of electrolyte:

[0072] LiPF6 was dissolved in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate (volume ratio 1:1:1), and 10 wt% of fluoroethylene carbonate was added as a film-forming additive to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0073] (4) Isolation film: polyethylene isolation film.

[0074] (5) Outer packaging: aluminum-plastic film.

[0075] (6) Preparation of lithium-ion batteries (full batteries):

[0076] The positive electrode sheet, separator, and negative electrode sheet are wound or stacked in the order of the three to form a battery cell; the battery cell is placed in an outer package, injected with electrolyte and sealed to obtain the secondary battery of the present application. The shape of the secondary battery described in the present application can be cylindrical, square or other arbitrary shapes, such as a square soft pack.

[0077] Example 2

[0078] The steps of Example 2 are the same as those of Example 1, except that in Example 2, during step c) granulation, the mass ratio of aggregate to asphalt is 100:3.

[0079] The Dv50 of the artificial graphite secondary particles prepared in Example 2 was 11.8 μm, and the structural strength was 0.74.

[0080] The median surface defectivity of the negative electrode plate obtained was 0.89, the membrane resistance was 0.09 Ωcm, and the plate porosity was 28.5%.

[0081] Example 3

[0082] The steps of Example 3 are the same as those of Example 1, except that in Example 3, during step c) granulation, the mass ratio of aggregate to asphalt is 100:6.

[0083] The Dv50 of the artificial graphite secondary particles prepared in Example 3 is 12.5 μm, the structural strength is 0.87, the median surface defectivity of the obtained negative electrode plate is 0.89, the membrane resistance is 0.11 Ωcm, and the plate porosity is 27.7%.

[0084] Example 4

[0085] The steps of Example 4 are the same as those of Example 1, except that in Example 4, during step c) granulation, the mass ratio of aggregate to asphalt is 100:12.

[0086] The Dv50 of the artificial graphite secondary particles prepared in Example 4 is 17.8 μm, the structural strength is 0.95, the median surface defectivity of the obtained negative electrode plate is 0.89, the membrane resistance is 0.15 Ωcm, and the plate porosity is 26.4%.

[0087] Example 5

[0088] The steps of Example 5 are the same as those of Example 1, except that in Example 5, no pre-lithium silicon oxide compound is added during the preparation of the negative electrode plate in step (2).

[0089] Example 6

[0090] The steps of Example 6 are the same as those of Example 1, except that in Example 6, during step (2) of preparing the negative electrode sheet, the mass ratio of the artificial graphite secondary particles to the pre-lithium silicon oxide compound is 100:15.

[0091] Example 7

[0092] The steps of Example 7 are the same as those of Example 1, except that in Example 7, during step (2) of preparing the negative electrode sheet, the mass ratio of the artificial graphite secondary particles to the pre-lithium silicon oxide compound is 100:35.

[0093] Example 8

[0094] The steps of Example 8 are the same as those of Example 1, except that in Example 8, during the preparation of the negative electrode plate in step (2), the silicon-based material is a silicon oxide compound (not pre-lithium).

[0095] Comparative Example 1

[0096] The steps of Comparative Example 1 are the same as those of Example 1, except that, in Comparative Example 1, the preparation method of artificial graphite secondary particles is as follows:

[0097] a) Raw coke pulverization: The raw coke is pulverized using a jet mill, mechanical mill or roller mill, and then subjected to a classification process to produce aggregates with a Dv50 of 8.0 μm and a Dv10 of 3-4 μm;

[0098] b) Aggregate shaping: The aggregate is shaped using a fusion machine with a processing time of 30 minutes and a rotation speed of 1500 rpm;

[0099] c) Graphitization treatment: The shaped aggregate is added into a box-type or Acheson graphitization furnace and subjected to high-temperature graphitization treatment at 2800°C for 20 hours. After cooling, the fast-filling graphite precursor A is obtained.

[0100] d) Coating: The above-mentioned fast-charging graphite precursor A and asphalt are mixed in a mass ratio of 100:8, and then carbonized in a heat treatment device such as a box furnace / roller kiln / rotary kiln. The heating rate is 5°C / min, the heat treatment temperature is 1200°C, and the holding time is 2h. After cooling, fast-charging artificial graphite secondary particles are obtained, whose Dv50 is 13.5μm and the structural strength is 0.60.

[0101] The median surface defectivity of the negative electrode plate obtained was 0.94, the membrane resistance was 0.065 Ωcm, and the plate porosity was 30.2%.

[0102] Comparative Example 2

[0103] The steps of Comparative Example 2 are the same as those of Example 1, except that in Comparative Example 2, during granulation, the mass ratio of aggregate to asphalt is 100:15.

[0104] The Dv50 of the artificial graphite secondary particles prepared in Comparative Example 2 was 19.7 μm, the structural strength was 1.0, the median surface defectivity of the negative electrode obtained was 0.43, the membrane resistance was 0.166 Ωcm, and the electrode porosity was 26.2%.

[0105] Test method:

[0106] 1. Test of structural strength S of artificial graphite:

[0107] 3g of graphite powder was pressed with a 2t weight on a powder compactor for 30s. The pressed graphite powder was taken out and the particle size distribution was tested. Structural strength S = Dv50 of graphite after 2t pressure treatment (denoted as Dv50-2t) / Dv50 of graphite before treatment (denoted as Dv50-initial)

[0108] 2. Test of surface defect D of negative electrode:

[0109] The rolled negative electrode was placed directly on a Raman spectrometer with an excitation wavelength of 532nm, and the median surface defectivity (Id / Ig, the ratio of the graphite D peak to the graphite G peak) was recorded. Specifically, 1200 points were sampled during the test and a histogram was plotted. The median value (median) in the plot is the median defectivity.

[0110] 3. Test of negative electrode (diaphragm) resistivity E:

[0111] The diaphragm resistance is tested using the four-probe method. The specific steps are as follows: a. Place the electrode on the test device and make its surface flat; b. Adjust the pressure gauge to 0.001 MPa to fix the test sample and ensure good contact between the probe and the sample; c. Turn on the test device, connect the circuit, introduce current into the sample to be tested, and record the voltage change; d. Calculate the resistance value of the sample according to Ohm's law.

[0112] 4. Test of the porosity P of the negative electrode:

[0113] The mercury intrusion method is used to test membrane porosity. The basic principle is as follows: Mercury does not wet general solids. External pressure is required to force mercury into pores. The greater the external pressure, the smaller the pore radius that mercury can enter. Measuring the amount of mercury entering the pore under different external pressures provides the pore volume of the corresponding pore size. The mercury intrusion instrument used has a maximum operating pressure of approximately 200 MPa and can measure pores ranging from 0.0064 μm to 950 μm (pore diameter).

[0114] 5. Cycle performance test:

[0115] At 25°C, first charge at a constant current of 1C to 4.3V, then charge at a constant voltage to a current of 0.05C. After standing for 10 minutes, discharge at a constant current of 1C to 2.5V. Perform a cycle test in this full charge and discharge form until the discharge capacity of the lithium-ion battery decays to 80% of the initial capacity, and record the number of cycles at this time.

[0116] 6. High temperature storage performance test:

[0117] The full battery after fractionation was charged to 100% state of charge in the form of 0.33C CC+CV, and the discharge capacity after fractionation was recorded as C1. After being placed at 60℃ for 14 days, the battery cell was taken out and placed at 25℃ for 4h, then discharged to 2.5V at a rate of 0.33C, placed for 10min, and then charged to 4.3V at a rate of 0.33C, placed for 10min, and then charged at a constant voltage to a current of 0.05C. Finally, it was discharged to 2.5V at a rate of 0.33C. The discharge capacity at this time was recorded as C2, and the capacity recovery rate after storage = C2 / C1*100%.

[0118] 7. 3C / 0.33C capacity ratio test:

[0119] At 25°C, the full battery was charged at constant voltage to a current of 0.05C, then discharged at a rate of 0.33C to 2.5V. This full charge and discharge cycle was performed for three weeks to activate the battery. The battery was charged at a rate of 3C to 4.3V, left to rest for 10 minutes, and then charged at constant voltage to a current of 0.05C. The battery was then discharged at a rate of 0.33C to 2.5V. The 3C / 0.33C capacity ratio = the capacity of the 3C constant current charge / the total capacity of the 0.33C charge in the third week. The larger the 3C / 0.33C ratio, the better the fast charging capability.

[0120] Table 1

[0121] As can be seen from Table 1, the artificial graphite anode material provided herein has high structural strength and low lithium insertion expansion rate. Furthermore, the anode electrode provided herein has low surface defectivity, moderate membrane resistance, and porosity. Consequently, the lithium-ion battery provided herein exhibits excellent cycling stability and good high-temperature storage performance.

[0122] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A negative electrode plate, comprising a current collector and a negative electrode active material layer disposed on a surface of the current collector, wherein the negative electrode active material layer comprises artificial graphite, and the negative electrode plate satisfies the following conditions: 0.2≤S×E / (D×P)≤1.2, wherein: S is the structural strength of the artificial graphite, wherein the structural strength refers to the ratio of the volume average particle size Dv50 of the artificial graphite after powder pressing under a pressure of 20 kN to the volume average particle size Dv50 before powder pressing; E is the resistivity of the negative electrode sheet; D is the surface defectivity of the negative electrode sheet, wherein the surface defectivity refers to the peak area ratio of the D peak to the G peak of the artificial graphite; P is the porosity of the negative electrode sheet.

2. The negative electrode sheet according to claim 1, characterized in that: 0.25≤S×E / (D×P)≤0.

97.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: 0.5≤D≤0.92。 4. The negative electrode sheet according to claim 1 or 2, characterized in that: 0.07≤E≤0.16。 5. The negative electrode sheet according to claim 3, characterized in that: 0.55≤D≤0.9。 6. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: 0.08≤E≤0.15。 7. The negative electrode sheet according to claim 1 or 2, characterized in that: 26.3％≤P≤30％。 8. The negative electrode sheet according to claim 1 or 2, characterized in that: 0.7≤S≤0.96。 9. The negative electrode sheet according to claim 7, characterized in that: 26.3％≤P≤29％。 10. The negative electrode sheet according to any one of claims 1 to 9, characterized in that: The negative electrode active material layer further includes a silicon-based material.

11. The negative electrode sheet according to claim 10, characterized in that: The silicon-based material includes at least one of silicon, silicon alloys, silicon-carbon compounds and silicon-oxygen compounds. 12 . A secondary battery comprising the negative electrode sheet and the positive electrode sheet according to claim 1 . 13 . A device comprising the secondary battery according to claim 12 .

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