Negative electrode for lithium ion secondary battery, preparation method thereof, and lithium ion secondary battery containing the same

Abstract

The present invention relates to a negative electrode for a lithium ion secondary battery and a preparation method thereof, comprising: a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material; the negative electrode active material comprises amorphous carbon-coated graphite; wherein the porosity P of the negative electrode is 34.0%-55.0%; the specific surface area S of the negative electrode active material is 0.80m 2 / g‑3.20m 2 / g; the amorphous carbon coating amount C of the negative electrode active material is 0.80%-3.35%; the graphitization degree G of the negative electrode active material is 83.0%-95.0%; and the particle size D of the negative electrode active material is 3.0μm-14.0μm. The present invention also relates to a lithium-ion secondary battery and an electric device comprising the negative electrode of the present invention. The present invention significantly improves the power performance of the lithium-ion secondary battery by rationally matching the material layer and the electrode layer of the negative terminal.

Description

Technical Field

[0001] The present application belongs to the field of lithium-ion secondary batteries, and specifically relates to a negative electrode for a lithium-ion secondary battery, a preparation method thereof, and a lithium-ion secondary battery comprising the negative electrode. Background Art

[0002] Lithium-ion secondary batteries have the advantages of high energy density, long cycle life, good low-temperature performance, and are green and safe. Therefore, they are widely used in power batteries, mobile phones, computers, power tools, wind and solar energy storage and other fields.

[0003] Plug-in hybrid electric vehicles (PHEVs) and hybrid electric vehicles (HEVs) are attracting widespread attention for their promise of lower fuel consumption and a superior driving experience. These electric vehicles primarily require high power performance and high charge and discharge rates from lithium-ion batteries, making improving the power performance of lithium-ion batteries a pressing challenge in the lithium-ion battery field. Therefore, the development of lithium-ion secondary batteries with even better power performance is crucial.

[0004] Improving the negative terminal of lithium-ion batteries is a viable approach to improving the power performance of lithium-ion secondary batteries. CN115188920A discloses a negative electrode sheet in which the active layer is composed of coatings with different amounts of fast-charging material and conductive agent. This allows the fast-charging material to be combined with the conductive agent, with the inner portion having less fast-charging material and more conductive agent than the outer portion, ensuring lithium ion transmission through the inner coating while maximizing the electrode sheet's energy density. Summary of the Invention

[0005] Currently, improvements to negative terminals generally involve reducing the particle size of the active material and performing surface coating to increase the material's power performance. To further improve the power performance of lithium-ion secondary batteries, this application utilizes a rational combination of material and electrode layers to achieve a negative electrode for a lithium-ion secondary battery, significantly enhancing the power performance of lithium-ion secondary batteries.

[0006] In one aspect, the present invention relates to a negative electrode for a lithium ion secondary battery, comprising: a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material; the negative electrode active material comprises: graphite coated with amorphous carbon; wherein the porosity P of the negative electrode is 34.0%-55.0%; the specific surface area S of the negative electrode active material is 0.80m 2 / g-3.20m 2 / g; the amorphous carbon coating amount C of the negative electrode active material is 0.80%-3.35%; the graphitization degree G of the negative electrode active material is 83.0%-95.0%; and the particle size D of the negative electrode active material is 3.0 μm-14.0 μm.

[0007] In one embodiment, the negative electrode further satisfies one or more of the following conditions: the porosity P of the negative electrode is 35.0%-53.0%; the specific surface area S of the negative electrode active material is 1.50m 2 / g-3.00m 2 / g; the amorphous carbon coating amount C of the negative electrode active material is 1.00%-2.50%; the graphitization degree G of the negative electrode active material is 85.0%-93.0%; and the particle size D of the negative electrode active material is 5.0 μm-12.0 μm.

[0008] In one embodiment, the porosity P of the negative electrode and the specific surface area S, amorphous carbon coating amount C, graphitization degree G and particle size D of the negative electrode active material satisfy the following mathematical formula: 0.020≤P×(S×C+2×(1-G) / D)≤0.030.

[0009] On the other hand, the present invention also relates to a method for preparing a negative electrode for a lithium-ion secondary battery, which comprises the following steps: providing a negative electrode slurry comprising: a negative electrode active material and an optional additive; coating the negative electrode slurry on a negative electrode current collector and then drying it to obtain a negative electrode coating sheet; and rolling the negative electrode coating sheet to obtain a negative electrode for a lithium-ion secondary battery.

[0010] In yet another aspect, the present invention also relates to a lithium ion secondary battery having a power generation element comprising: the negative electrode for the lithium ion secondary battery of the present invention, a positive electrode, an electrolyte solution, and a separator.

[0011] In yet another aspect, the present invention also relates to an electric device comprising the lithium-ion secondary battery of the present invention. DETAILED DESCRIPTION

[0012] General Definitions and Terminology

[0013] Unless otherwise indicated, all publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0014] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In the event of a conflict, the definitions provided herein will prevail.

[0015] Unless otherwise indicated, all percentages, parts, ratios, etc. are by weight.

[0016] When quantity, concentration or other value or parameter is given as range, preferred range or preferred upper limit and lower limit or specific value, it should be understood that all ranges formed from the paired values ​​of any upper limit range or preferred value and any lower limit range or preferred value are specifically disclosed, regardless of whether the range is disclosed separately. Unless otherwise stated, when numerical range is quoted herein, the range is meant to include its endpoints and integers and fractions within all such ranges. The scope of the present invention is not limited to the specific numerical value quoted when defining the range. For example, "1-8" encompasses 1, 2, 3, 4, 5, 6, 7, 8 and any sub-range consisting of any two values ​​thereof, such as 2-6, 3-5.

[0017] The terms "comprises," "includes," "has," "contains," or "involves," and other variations thereof herein, are inclusive or open-ended and do not exclude other unrecited elements or method steps. It will be understood by those skilled in the art that the above terms such as "comprising" encompass the meaning of "consisting of." The expression "consisting of" excludes any element, step, or ingredient not specified. The expression "consisting essentially of" means that the scope is limited to the specified elements, steps, or ingredients, plus the optional presence of elements, steps, or ingredients that do not materially affect the basic and novel characteristics of the claimed subject matter. It will be understood that the expression "comprising" encompasses the expressions "consisting essentially of" and "consisting of."

[0018] The term "selected from..." means one or more elements from the group listed thereafter, selected independently, and may include combinations of two or more elements.

[0019] As used herein, the term "optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes both occurring and not occurring.

[0020] When values ​​or end-points of a range are described herein, it should be understood that the disclosure includes the specific value or end-point recited.

[0021] As used herein, the terms "one or more" or "at least one" refer to one, two, three, four, five, six, seven, eight, nine or more.

[0022] Unless otherwise indicated, the terms "combinations thereof" and "mixtures thereof" refer to multi-component mixtures of the elements described, such as two-, three-, four- and up to the maximum possible multi-component mixtures.

[0023] In addition, if the number of parts or components of the present invention is not indicated before, it means that there is no limit to the number of occurrences (or existence) of the parts or components. Therefore, it should be interpreted as including one or at least one, and the singular form of the parts or components also includes the plural form unless the value obviously represents the singular.

[0024] The term "lithium-ion secondary battery" refers to a type of rechargeable battery in which lithium ions move from the anode to the cathode during discharge and from the cathode to the anode during charge.

[0025] The term "anode" refers to the electrode in a secondary battery at which oxidation occurs during discharge and at which reduction occurs during charge.

[0026] The term "cathode" refers to the electrode in a secondary battery at which reduction occurs during discharge and oxidation occurs during charge.

[0027] The term "formation" refers to the process of charging and discharging a battery with a low current after it is obtained during the battery manufacturing process. This formation treatment helps stabilize the battery's electrical performance.

[0028] The term "electric vehicle" refers to a vehicle that uses electricity as a power source. Examples of electric vehicles include, but are not limited to, electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0029] negative electrode

[0030] In one aspect, the present invention relates to a negative electrode for a lithium-ion secondary battery, comprising: a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode refers to an electrode with a lower potential from which electrons flow from an external circuit during discharge.

[0031] negative electrode current collector

[0032] In the present invention, there are no particular restrictions on the material constituting the negative electrode current collector; for example, metals may be used. Specifically, the metals selected herein include, but are not limited to, aluminum, nickel, iron, stainless steel, titanium, copper, and combinations thereof. When a combination of two or more metals is used as the negative electrode current collector, this may refer to an alloy, a cladding, a plating, or the like. From the perspectives of electronic conductivity and battery operating potential, the metal of the negative electrode current collector is preferably copper, aluminum, or stainless steel. In one specific embodiment, the negative electrode current collector is copper foil.

[0033] The size of the negative electrode current collector can be adjusted according to the actual application of the battery. For example, if the battery is used in a large battery requiring high energy density, a current collector with a larger area can be used. The thickness of the negative electrode current collector is not particularly limited and can be, for example, 1-100 μm, such as 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, 80 μm, 100 μm, etc. In a specific embodiment, the thickness of the negative electrode current collector is 10 μm.

[0034] Negative electrode active material layer

[0035] The negative electrode active material layer is formed on the surface of the negative electrode current collector, and its shape and area are not particularly limited. The negative electrode active material layer can cover one or both sides of the negative electrode current collector. The thickness of the negative electrode active material layer is not particularly limited, and can be, for example, 1-100 μm, such as 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, 80 μm, 100 μm, etc. In a specific embodiment, the thickness of the negative electrode current collector is 10 μm.

[0036] Negative electrode active material

[0037] The negative electrode active material layer may contain a negative electrode active material. The negative electrode active material is generally a material that can embed or deintercalate lithium ions. The negative electrode active material that can be used herein may be a carbon material, including but not limited to amorphous carbon-coated graphite (i.e., graphite having an amorphous amorphous carbon layer coated on its surface). In one embodiment, the negative electrode active material is amorphous carbon-coated graphite. The graphite described herein may include: natural graphite, artificial graphite, modified graphite, silicon-oxygen composite artificial graphite, or a combination thereof. The negative electrode active material may be in the form of particles, powder, etc., for example, amorphous carbon-coated graphite particles.

[0038] The power performance of lithium-ion batteries is related to the transport of lithium ions in the liquid phase, at the solid-liquid interface, and in the solid phase. By manipulating the negative electrode at the material level (such as the specific surface area S of the negative electrode active material, the amount of amorphous carbon coating C, the degree of graphitization G, and the particle size D) and the electrode level (such as the negative electrode electrode porosity P), the battery's power performance can be improved.

[0039] Lithium ion transport in the liquid phase is affected by the porosity of the electrode. Herein, the "porosity of the negative electrode" may also be referred to as the "porosity of the negative electrode sheet". In one embodiment, the porosity P of the negative electrode is 34.0%-55.0%, preferably 35.0%-53.0%, for example, 34.0%, 35.0%, 36.0%, 37.0%, 38.0%, 38.1%, 39.0%, 39.2%, 39.4%, 39.6%, 39.7%, 40.0%, 40.6%, 42.0%, 42.1%, 42.3%, 42.8%, 43.5%, 44.0%, 45.0%, 45.4%, 48.0%, 50.0%, 51.0%, 52.0%, 53.0%, 54.0%, 55.0%, etc. Increasing the porosity of the negative electrode increases the liquid phase transmission pathways and reduces the liquid phase transmission resistance, thereby improving power performance. Negative electrode porosity that is too low or too high is not conducive to improving power performance. Too low a porosity may lead to poor electrolyte wetting, preventing the electrolyte from fully entering the electrode, which is not conducive to reducing liquid phase transmission resistance, thus reducing the power performance of the lithium-ion battery. Too high a porosity may affect the full contact between the particles of the negative electrode active material. Poor particle contact may cause the electronic conductivity of the negative electrode sheet to decrease, thereby reducing the power performance of the lithium-ion battery.

[0040] As used herein, "porosity" has the meaning commonly understood in the art, and refers, for example, to the ratio of the pores within a material to the total apparent volume of a sample (e.g., a negative electrode sheet). The porosity of a negative electrode sheet can be measured by conventional methods, such as mercury intrusion porosimetry according to the national standard GB / T 21650.1-2008 / ISO 15901-1:2005.

[0041] At the solid-liquid interface, the transport of lithium ions is affected by the interfacial lithium ion transport channels. The specific surface area S and amorphous carbon coating C of the negative electrode active material influence this transport. A suitable specific surface area S and amorphous carbon coating C of the negative electrode active material can help improve the solid-liquid interfacial transport of lithium ions, thereby enhancing the battery's power performance.

[0042] A suitable specific surface area S of the negative electrode active material helps to increase the interface transmission channel, thereby improving the power performance. In one embodiment, the specific surface area S of the negative electrode active material is 0.80 m 2 / g-3.20m 2 / g, preferably 1.50m 2 / g-3.00m 2 / g, for example 0.80m 2 / g, 0.90m 2 / g, 1.00m 2 / g, 1.10m2 / g, 1.20m 2 / g, 1.30m 2 / g, 1.40m 2 / g, 1.50m 2 / g, 1.60m 2 / g, 1.63m 2 / g, 1.64m 2 / g, 1.76m 2 / g, 1.78m 2 / g, 1.80m 2 / g, 1.82m 2 / g, 1.90m 2 / g, 1.91m 2 / g, 1.96m 2 / g, 2.00m 2 / g, 2.01m 2 / g, 2.03m 2 / g, 2.10m 2 / g, 2.11m 2 / g, 2.12m 2 / g, 2.19m 2 / g, 2.20m 2 / g, 2.21m 2 / g, 2.42m 2 / g, 2.50m 2 / g, 2.53m 2 / g, 2.80m 2 / g, 3.00m 2 / g, 3.10m 2 / g, 3.20m 2 / g, etc. A negative electrode active material with an excessively large or small specific surface area S can negatively impact the battery's power performance. An excessively large specific surface area (S) impairs the dispersion and processing of the negative electrode active material particles, hindering adequate contact between the particles and reducing the electronic conductivity of the negative electrode sheet, leading to a decrease in the power performance of the lithium-ion battery. An excessively small specific surface area reduces interfacial transmission channels, hindering the transport of lithium ions at the solid-liquid interface and thus reducing the power performance of the lithium-ion battery.

[0043] As used herein, "specific surface area" has the meaning generally understood by those skilled in the art, i.e., the total surface area per unit mass of a material. Specific surface area can be measured using conventional methods and equipment in the art, for example, using the multi-point BET method.

[0044] Amorphous carbon materials have a larger interlayer spacing than graphite, which improves the diffusion of lithium ions within them. Therefore, coating graphite with amorphous carbon forms a lithium ion buffer layer on the outer surface of the graphite, thereby enhancing the high-current charge and discharge performance of the graphite material. Furthermore, coating graphite with amorphous carbon prevents the exfoliation of the graphite layers caused by the co-intercalation of solvent molecules, expanding the range of electrolyte systems available and improving the cycling stability of the electrode material.

[0045] Properly increasing the amorphous carbon coating amount C of the negative electrode active material can effectively reduce the charge transfer impedance at the solid-liquid interface, which is beneficial to the transmission of lithium ions at the solid-liquid interface, thereby improving the power performance. In one embodiment, the amorphous carbon coating amount C of the negative electrode active material is 0.80%-3.35%, preferably 1.00%-2.50%, for example, 0.80%, 0.90%, 1.00%, 1.20%, 1.45%, 1.48%, 1.50%, 1.52%, 1.53%, 1.55%, 1.58%, 1.59%, 1.60%, 1.62%, 1.63%, %, 1.69%, 1.70%, 1.71%, 1.77%, 1.80%, 1.82%, 1.90%, 1.92%, 2.00%, 2.02%, 2.10%, 2.20%, 2.30%, 2.40%, 2.50%, 2.60%, 2.70%, 2.80%, 2.90%, 3.00%, 3.10%, 3.20%, 3.30%, 3.35%, etc. Too high or too low amorphous carbon coating amount C of the negative electrode active material will adversely affect the power performance of the battery. If the amorphous carbon coating amount is too high, it may affect the dispersion processing effect of the negative electrode active material particles, which is not conducive to sufficient contact between the particles. The deterioration of particle contact may cause the electronic conductivity of the negative electrode sheet to decrease, and thus lead to a decrease in the power performance of the lithium-ion battery; if the amorphous carbon coating amount is too low, it cannot fully improve the charge transfer impedance at the solid-liquid interface, and the transmission of lithium ions at the solid-liquid interface is hindered, which may lead to a decrease in the power performance of the lithium-ion battery.

[0046] The "amorphous carbon coating amount" herein refers to the ratio of the mass of the coated amorphous carbon layer to the mass of the negative electrode active material in the negative electrode active material. Taking amorphous carbon-coated graphite particles as an example, the "amorphous carbon coating amount" refers to the ratio of the mass of the amorphous carbon layer to the mass of the entire graphite particles (including the amorphous carbon layer). The amount of amorphous carbon coating can be measured using conventional methods and equipment in the art, for example, according to the national standard GB / T 8727-2008, the coking value of the coating agent is determined, and the product of the coking value and the amount of coating agent added is calculated to obtain the amorphous carbon coating amount.

[0047] The solid-phase transport of lithium ions is affected by the bulk transport distance and transport resistance of the solid phase (such as graphite).

[0048] The particle size of the negative electrode active material affects the bulk transport distance. A suitable particle size of the negative electrode active material helps improve the power performance of the battery. Properly reducing the particle size of the negative electrode active material helps shorten the bulk transport distance, reduce the transport resistance, and thus improve the power performance. In one embodiment, the particle size D of the negative electrode active material is 3.0 μm-14.0 μm, preferably 5.0 μm-12.0 μm, for example, 3.0 μm, 4.0 μm, 5.0 μm, 6.0 μm, 6.2 μm, 6.6 μm, 6.8 μm, 7.0 μm, 7.3 μm, 7.7 μm, 8.0 μm, 8.1 μm, 8.6 μm, 9.0 μm, 9.3 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.1 μm, 11.2 μm, 11.5 μm, 12.0 μm, 13.0 μm, 14.0 μm, etc. If the particle size of the negative electrode active material is too large or too small, it is not conducive to the power performance of the battery. If the particle size of the negative electrode active material is too large, the bulk phase transmission distance will increase and the transmission resistance will increase, which is not conducive to the solid phase transmission of lithium ions, resulting in the inability to replenish lithium ions in time for lithium insertion, thereby causing the power performance of the lithium-ion battery to deteriorate; if the particle size is too small, the particle dispersion processing effect is poor, affecting the full contact between the particles, which may cause the electronic conductivity of the negative electrode to decrease, thereby causing the power performance of the lithium-ion battery to deteriorate.

[0049] "Particle size" herein refers to the particle size at which 50% of the sample's volume distribution of smallest to largest particles occurs, i.e., the particle size corresponding to the cumulative particle size distribution percentage reaching 50%. "Particle size" herein may also be referred to as "D50 particle size." "Particle size" herein can be measured using conventional methods and equipment in the art, such as using a laser particle size analyzer to measure particle size volume distribution.

[0050] Appropriately reducing the degree of graphitization of the negative electrode active material increases the interlayer spacing, reduces the solid-phase transport resistance of lithium ions, and thus improves power performance. In one embodiment, the degree of graphitization G of the negative electrode active material is 83.0%-95.0%, preferably 85.0%-93.0%, such as 83.0%, 84.0%, 85.0%, 86.0%, 87.0%, 87.2%, 88.0%, 89.0%, 89.2%, 89.6%, 90.0%, 90.3%, 90.5%, 90.9%, 91.0%, 91.2%, 91.7%, 91.8%, 92.0%, 90.2%, 92.4%, 92.7%, 93.0%, 94.0%, 94.4%, 95.0%, etc. Neither too low nor too high a graphitization level of the negative electrode active material is conducive to improving the power performance of the battery. If the graphitization level of the negative electrode active material is too low, the particle dispersion processing effect is poor, affecting the sufficient contact between the particles, which is not conducive to power performance and may cause the electronic conductivity of the negative electrode to decrease, thereby deteriorating the power performance of the lithium-ion battery. If the graphitization level is too high, the interlayer spacing becomes larger, the solid transmission resistance is greater, which is not conducive to the solid-phase transmission of lithium ions, resulting in the inability to replenish lithium ions for lithium insertion in a timely manner, thus causing the power performance of the lithium-ion battery to deteriorate.

[0051] As used herein, "degree of graphitization" has the meaning generally understood by those skilled in the art, namely, the degree of perfection of the graphite crystal structure. It can also refer to the regularity of the arrangement of carbon atoms in the graphite structure. It can generally be calculated using the 002 interplanar spacing of graphite. The smaller the interplanar spacing, the higher the degree of graphitization. The degree of graphitization can be measured using conventional methods and equipment in the art, for example, using the formula for degree of graphitization as described in GB / T 24533-2019.

[0052] As used herein, when the negative electrode active material is a coated material, the specific surface area, degree of graphitization, and particle size of the negative electrode active material refer to the specific surface area, degree of graphitization, and particle size of the coated material, respectively. For example, if graphite is coated with amorphous carbon, the amorphous carbon-coated graphite obtained is used as the negative electrode active material. The specific surface area, degree of graphitization, and particle size of the negative electrode active material refer to the specific surface area, degree of graphitization, and particle size of the amorphous carbon-coated graphite, respectively.

[0053] The applicant has found that when the porosity, specific surface area, amorphous carbon coating amount, graphitization degree, and particle size meet a certain range (as described above), the negative electrode can have an optimal material structure and electrode structure, which helps to improve the battery power performance.

[0054] In one embodiment, the negative electrode of the present invention satisfies the following conditions: the porosity P of the negative electrode is 34.0%-55.0%, the specific surface area S of the negative electrode active material is 0.80m 2 / g-3.20m2 / g, the amorphous carbon coating amount C of the negative electrode active material is 0.80%-3.35%, the graphitization degree G of the negative electrode active material is 83.0%-95.0% and the particle size D of the negative electrode active material is 3.0μm-14.0μm, and the battery using the negative electrode can have better power performance.

[0055] Furthermore, in a preferred embodiment, the negative electrode also satisfies one or more of the following conditions, such as one, two, three, four or all of the following, which can further improve the power performance of the battery: the porosity P of the negative electrode is 35.0%-53.0%; the specific surface area S of the negative electrode active material is 1.50m 2 / g-3.00m 2 / g; the amorphous carbon coating amount C of the negative electrode active material is 1.00%-2.50%; the graphitization degree G of the negative electrode active material is 85.0%-93.0%; and the particle size D of the negative electrode active material is 5.0 μm-12.0 μm.

[0056] In a more preferred embodiment, the negative electrode of the present invention has the following characteristics: the porosity P of the negative electrode is 35.0%-53.0%; the specific surface area S of the negative electrode active material is 1.50m 2 / g-3.00m 2 / g; the amorphous carbon coating amount C of the negative electrode active material is 1.00%-2.50%; the graphitization degree G of the negative electrode active material is 85.0%-93.0%; and the particle size D of the negative electrode active material is 5.0μm-12.0μm. This can significantly improve the power performance of the battery using the negative electrode of the present invention.

[0057] An appropriate content of the negative electrode active material in the negative electrode active material layer contributes to achieving good power performance of the negative electrode. The weight of the negative electrode active material can be greater than 80% of the weight of the negative electrode active material layer, preferably greater than 90%, and more preferably greater than 92%. In one embodiment, the weight ratio of the negative electrode active material to the negative electrode active material layer can be 0.92-0.97, for example, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, etc.

[0058] Furthermore, the applicant unexpectedly discovered that for the following aspects: pole piece factors (including porosity P, which affects the liquid phase transmission process), interface factors (including: amorphous carbon coating amount C and graphitization degree G, which affect the solid-liquid interface transmission process), internal factors (including: particle size D and specific surface area S, which affect the solid phase transmission process), when the relevant parameters of these aspects meet a specific relationship, the power performance of the battery can be further improved. The negative electrode for lithium-ion secondary batteries of the present invention, the porosity P of the negative electrode, the amorphous carbon coating amount C of the negative electrode active material, the graphitization degree G, the particle size D, and the specific surface area S meet the following mathematical formula: 0.020≤P×(S×C+2×(1-G) / D)≤0.030, wherein the unit of porosity P is "1"; the unit of specific surface area S is m 2 / g; the unit of amorphous carbon coating amount C and graphitization degree G is "1"; the unit of particle size D is micrometer (μm). When the negative electrode satisfies the mathematical formula: 0.020≤P×(S×C+2×(1-G) / D)≤0.030, the power performance of the lithium-ion secondary battery is better.

[0059] As needed, the negative electrode active material layer may further contain other additives (such as a conductive agent, a thickener, a binder, etc.) to impart appropriate properties to the negative electrode.

[0060] Conductive agent

[0061] Conductive agent refers to an additive added to improve the conductivity of the negative electrode active material layer. When the negative electrode active material layer contains a conductive agent, the internal electronic network of the active material layer can be effectively formed, which contributes to the power characteristics of the battery. Conductive agents usable herein include, but are not limited to, conductive carbon black, carbon nanotubes, carbon fibers, acetylene black, Ketjen black, preferably conductive carbon black. These conductive agents can be used alone or in combination. In one embodiment, the conductive agent is conductive carbon black. Using conductive carbon black as a conductive agent, in combination with the negative electrode active material herein, can achieve a good power improvement effect.

[0062] In the negative electrode active material layer, a suitable conductive agent content helps to fully improve the conductivity of the negative electrode active material layer and avoid adverse effects. In one embodiment, the weight ratio of the conductive agent to the negative electrode active material layer is 0.015-0.025, for example, 0.015, 0.018, 0.02, 0.023, 0.025, etc.

[0063] binder

[0064] The binder is used to provide a bonding effect, which helps to obtain good bonding force inside the negative electrode active material layer and between the negative electrode active material layer and the negative electrode current collector. The binders that can be used herein include, but are not limited to, polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC) and its salts, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, ethylene-propylene rubber, ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and its hydride, styrene-isoprene-styrene block copolymer and its hydride, etc. thermoplastic polymers, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE Fluororesins such as polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber), and epoxy resins may be used. These binders may be used alone or in combination.

[0065] In the negative electrode active material layer, the binder is preferably one that can form an aqueous binder system. An aqueous binder system refers to a system that uses water as a solvent or dispersion medium to provide an adhesive effect. The aqueous binder system has high bonding strength, and uses water as a solvent or dispersion medium, which is inexpensive and environmentally friendly. In addition, since water vapor is generated during drying, the equipment investment in the production line can be significantly reduced, which can reduce the environmental load. Herein, the binder using water as a dispersion medium includes all types of latex or emulsion, and refers to polymers that are emulsified with water or suspended in water, such as polymer latexes that can be obtained by emulsion polymerization in a self-emulsifying system.

[0066] Binders used to form the aqueous binder system include, but are not limited to: styrene-based polymers (styrene-butadiene rubber, styrene-vinyl acetate copolymer, styrene-acrylic copolymer, etc.), acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, (meth)acrylic polymers (polyethyl acrylate, polyethyl methacrylate, polypropyl acrylate, polymethyl methacrylate (methyl methacrylate rubber), polypropyl methacrylate, polyisopropyl acrylate, polyisopropyl methacrylate, polybutyl acrylate, polybutyl methacrylate, polyhexyl acrylate, polyhexyl methacrylate, polyethylhexyl acrylate, polyethylhexyl methacrylate, polylauryl acrylate, polylauryl methacrylate, etc.), polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene copolymer, polybutadiene, butyl rubber, fluororubber, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, polyester resin, phenolic resin, epoxy resin.

[0067] From the perspective of adhesion, the adhesive herein may include, but is not limited to, one or more combinations of styrene-butadiene rubber, acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, and methyl methacrylate rubber. To achieve better adhesion, the adhesive may be styrene-butadiene rubber.

[0068] In order to provide a good binding effect, the content of the binder should be maintained within an appropriate range. In one embodiment, the weight ratio of the binder to the negative electrode active material layer is 0.015-0.025.

[0069] thickener

[0070] When a binder for forming a water-based binder system is used, the binder may be used in combination with a thickener to improve coating properties, which helps to achieve a good coating effect during the preparation process.

[0071] The thickener can improve the viscosity of the negative electrode slurry, which helps to improve the coating properties of the slurry. The optional thickeners herein include, but are not limited to: polyvinyl alcohol (average degree of polymerization is preferably 200-4000, more preferably 1000-3000, saponification degree is preferably 80 mol% or more, more preferably 90 mol% or more) and its modified bodies (1-80 mol% saponified products in the vinyl acetate unit of the copolymer with ethylene / vinyl acetate = 2 / 98-30 / 70 molar ratio, 1-50 mol% partial acetal of polyvinyl alcohol, etc.), starch and its modified bodies (oxidized starch, phosphated starch, cationized starch, etc.), cellulose derivatives (carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose and its salts, etc.), polyethylene glycol, polyvinyl alcohol, polyvinyl acetate ... Water-soluble polymers such as pyrrolidone, polyacrylic acid (salt), polyethylene glycol, copolymers of (meth)acrylamide and / or (meth)acrylate [(meth)acrylamide polymer, (meth)acrylamide-(meth)acrylate copolymer, (meth)acrylate alkyl (1-4 carbon atoms) ester-(meth)acrylate copolymer, etc.], styrene-maleate copolymer, Mannich modified polyacrylamide, formaldehyde condensation resin (urea-formaldehyde resin, melamine-formaldehyde resin, etc.), polyamide polyamine or dialkylamine-epichlorohydrin copolymer, polyethyleneimine, casein, soy protein, synthetic protein, and galactomannan derivatives. These thickeners may be used alone or in combination.

[0072] Thickening agent preferably can comprise: polyvinyl alcohol and its modified body, starch and its modified body, cellulose derivative (carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose and their salt etc.), polyvinyl pyrrolidone, polyacrylic acid (salt) or polyethylene glycol or its combination, preferably sodium carboxymethyl cellulose.When using styrene-butadiene rubber as binding agent, these thickening agents can obtain good bonding effect in combination with it. In a preferred embodiment, use styrene-butadiene rubber as binding agent, carboxymethyl cellulose (salt) is as thickening agent. In a more preferred embodiment, use styrene-butadiene rubber as binding agent, use sodium carboxymethyl cellulose as thickening agent.

[0073] In the negative electrode active material layer, the weight ratio of the binder to the thickener is not particularly limited. In one embodiment, the weight ratio of the binder to the thickener is 1:0.1-10, more preferably 1:0.2-1, for example, 1:0.5. Preferably, the ratio of styrene-butadiene rubber to sodium carboxymethyl cellulose is 1:0.1-10, more preferably 1:0.2-1, for example, 1:0.5.

[0074] Method for preparing negative electrode for lithium ion secondary battery

[0075] In another aspect, the present invention also relates to a method for preparing a negative electrode for a lithium ion secondary battery, comprising the following steps:

[0076] Providing a negative electrode slurry, which comprises: a negative electrode active material and an optional additive;

[0077] The negative electrode slurry is coated on the negative electrode current collector and then dried to obtain a negative electrode coating sheet;

[0078] The negative electrode coating sheet was roll-pressed to obtain a negative electrode for a lithium ion secondary battery.

[0079] Wherein, the negative electrode active material and additives are as described above.

[0080] The negative electrode slurry may also contain a solvent. The solvent is used to disperse the negative electrode active material and additives to form a dispersed system. In one embodiment, water is used as the solvent. Using water as a solvent has a number of advantages, such as being inexpensive, environmentally friendly, and generating water vapor during the subsequent drying process, which can significantly reduce equipment investment in the production line and reduce environmental impact.

[0081] The components of the negative electrode slurry can be mixed to obtain the negative electrode slurry. For example, the negative electrode active material, additives, and solvent are mixed to obtain the negative electrode slurry. During the mixing process, the stirring speed can be controlled according to actual conditions, for example, 100-2500 rpm, such as 100 rpm, 200 rpm, 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, etc. Thorough mixing and stirring helps to obtain a uniformly dispersed negative electrode slurry, which in turn helps to obtain a negative electrode coated sheet with a uniformly distributed coating.

[0082] In order to obtain a better dispersion effect, the negative electrode active material and additives (such as a conductive agent, a binder, and a thickener) can be fully mixed first, and then the solvent is added, which helps to shorten the stirring time for obtaining a uniformly dispersed negative electrode slurry.

[0083] The negative electrode slurry can be applied using conventional methods to form a coating layer on all or part of the negative electrode current collector, such as die coating such as slide die coating, comma direct coating, comma reverse coating, gravure coating, gravure reverse coating, and the like.

[0084] The obtained negative electrode coating sheet is dried to remove the volatile phase (such as solvent) in the coating layer, and the remaining components (such as negative electrode active material, additives, etc.) are retained to form a negative electrode coating sheet. Drying can be carried out in a conventional manner in the field. In order to improve the drying efficiency, heating can usually be used for drying, such as vacuum drying, infrared heating drying, etc. During the drying process, an appropriate drying temperature can speed up the drying efficiency and achieve sufficient drying in a shorter time. However, the drying temperature should not be too high to avoid decomposition of the raw materials, and to avoid uneven surface caused by violent volatilization of the solvent. In one embodiment, the drying temperature is 50-70°C, for example: 50°C, 55°C, 60°C, 65°C or 70°C, etc.

[0085] Rolling the negative electrode coating sheet can further compact the negative electrode sheet to achieve the target porosity, thereby obtaining the target lithium ion secondary battery negative electrode. The rolling process can be carried out using, for example, a metal roller, an elastic roller, a heating roller (hot roller), etc. The temperature during rolling can be lower than the temperature at which the coating layer of the active material layer is dried. It can be carried out at room temperature or under heating conditions, preferably under heating conditions. By rolling (hot pressing) under heating conditions, the deviation of the porosity can be reduced, which may be because the softening of the binder can be promoted and the pore distribution tends to become uniform.

[0086] During the coating process and / or rolling process, an electric field can be applied to further improve the performance of the negative electrode. An electric field perpendicular to the current collector plane can be applied during the coating process and / or rolling process. Due to the anisotropy of the conductivity of the graphite particles, the graphite layer tends to be oriented in the direction perpendicular to the current collector, thereby reducing the orientation degree OI value of the graphite negative electrode sheet, thereby achieving the effect of improving fast charging, reducing expansion, and improving circulation. The electric field can be applied by continuous pressure. During the coating process, the voltage of the applied electric field is 5-10V, for example: 5V, 6V, 7V, 8V, 9V or 10V, etc. During the rolling process, the voltage of the applied electric field can be 10-20V, for example 10V, 12V, 15V, 18V or 20V, etc.

[0087] lithium-ion secondary batteries

[0088] In yet another aspect, the present invention also relates to a lithium ion secondary battery having a power generation element, the power generation element comprising: a negative electrode for a lithium ion secondary battery as described herein, a positive electrode, an electrolyte, and a separator.

[0089] positive electrode

[0090] The positive electrode refers to an electrode with a higher potential, into which electrons flow from an external circuit during discharge. The positive electrode is not specifically defined herein. For example, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector.

[0091] The positive electrode active material layer may include a positive electrode active material. The positive electrode active material usable herein may be a compound containing one or more selected from iron, cobalt, manganese, and nickel and lithium. The positive electrode active material may be a single lithium-containing compound or a mixture of multiple lithium-containing compounds. In one embodiment, the positive electrode active material includes lithium iron phosphate (LiFePO4).

[0092] The positive electrode active material layer may further include additives such as a conductive agent, a binder, etc. Other additives are not particularly limited, and may be, for example, those described above for use in the negative electrode.

[0093] In one embodiment, the conductive agent used in the positive electrode active material layer comprises conductive carbon black or carbon nanotubes. In another embodiment, the binder used in the positive electrode active material layer comprises polyvinylidene fluoride (PVDF).

[0094] diaphragm

[0095] The separator has the function of retaining the electrolyte to ensure lithium ion conductivity between the positive and negative electrodes, and also functions as a partition between the positive and negative electrodes. The separator can be in the form of a porous sheet made of polymers and / or fibers, a non-woven fabric separator, etc.

[0096] The diaphragm of the porous sheet formed by polymers and / or fibers can use microporous materials (microporous membranes). Specifically, it can be a microporous (microporous membrane) diaphragm formed by the following substances: polyolefins such as polyethylene (PE) and polypropylene (PP); a laminate obtained by laminating multiple layers of the above polyolefins (for example, a laminate made of a 3-layer structure of PP / PE / PP, etc.), hydrocarbon resins such as polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), glass fiber, etc. There is no special restriction on the thickness of the diaphragm of the porous sheet. Non-woven fabric diaphragms can use cotton, rayon, acetate, nylon, polyester, polyolefins (such as PP, PE, etc.), polyimide, aramid, etc., alone or in combination. The volume density of the non-woven fabric is not particularly limited as long as sufficient battery characteristics can be obtained by the impregnated electrolyte. Therefore, the thickness of the non-woven fabric diaphragm can be the same as that of the electrolyte layer. In a preferred embodiment, the diaphragm is a 9μm polyethylene diaphragm coated with a ceramic coating with a thickness of 2μm on both sides.

[0097] electrolyte

[0098] The electrolyte is used to transport ions between the positive and negative electrodes and conduct current. There are no specific restrictions on the electrolyte used herein. Liquid electrolytes or gel polymer electrolytes can be used.

[0099] The liquid electrolyte has the function of being a carrier of lithium ions. The liquid electrolyte may comprise: a solvent (e.g., an organic solvent, which may serve as a plasticizer), a lithium salt dissolved in the solvent as a supporting salt, and other optional additives. Organic solvents available in the liquid electrolyte include, but are not limited to, carbonates such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), preferably a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). Lithium salts available in the electrolyte include, but are not limited to, compounds that can be added to the active material layer of the electrode, such as Li(CF3SO2)2N, Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, LiTaF6, and LiCF3SO3, preferably LiPF6. In a preferred embodiment, the electrolyte is a liquid electrolyte comprising: ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and LiPF6.

[0100] By using a gel polymer electrolyte, the distance between electrodes can be stabilized, the generation of polarization can be suppressed, and the durability (cycle characteristics) can be improved. The gel polymer electrolyte can be obtained as follows: the above-mentioned liquid electrolyte is injected into a matrix polymer (main polymer) containing an ion-conducting polymer. When a gel polymer electrolyte is used as an electrolyte, the electrolyte no longer has fluidity and is prone to blocking the ion conductivity between layers. The ion-conducting polymer of the matrix polymer (main polymer) includes but is not limited to: polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene (PVDF-HEP), poly (methyl methacrylate (PMMA) and their copolymers. The matrix polymer of the gel electrolyte can exhibit excellent mechanical strength by forming a cross-linked structure. In order to form a cross-linked structure, a suitable polymerization initiator can be used to perform polymerization treatments such as thermal polymerization, ultraviolet polymerization, radiation polymerization, and electron beam polymerization on the polymeric polymer used to form the polymer electrolyte (such as PEO, PPO).

[0101] Preparation of lithium-ion secondary batteries

[0102] The lithium-ion secondary battery of the present invention can be prepared in a conventional manner. As an example, the positive electrode, separator, negative electrode and electrolyte can be assembled in sequence, with the positive electrode, separator, negative electrode and separator being alternately combined, and then formed into a soft pack battery.

[0103] Power performance of lithium-ion secondary batteries

[0104] The power performance of lithium-ion secondary batteries can be evaluated using the battery's room-temperature DC resistance (DCR) and low-temperature DC resistance (DCR). Lower room-temperature and low-temperature DCR values ​​indicate better power performance. The room-temperature and low-temperature DCR can be measured as follows: At a constant temperature of 25°C, adjust the lithium-ion battery's state of charge (SOC) to 50%, let it rest for one hour, and record the voltage V0 after the rest period. Then, discharge it at a 10C rate (current I0) for 10 seconds, record the voltage V1 after the discharge period, and denote (V1-V0) / I0 as the room-temperature DCR. At a constant temperature of 25°C, adjust the lithium-ion battery's state of charge (SOC) to 50%, adjust the temperature to -20°C, let it rest for three hours, and record the voltage V2 after the rest period. Then, discharge it at a 4C rate (current I1) for 10 seconds, record the voltage V3 after the discharge period, and denote (V3-V2) / I1 as the low-temperature DCR. In one embodiment, the room temperature DC resistance (DCR) of the lithium ion secondary battery of the present invention is 65 mΩ or less, preferably 62 mΩ or less, more preferably 60 mΩ or less, more preferably 57 mΩ or less, and most preferably 50 mΩ or less. In one embodiment, the low temperature DC resistance (DCR) of the lithium ion secondary battery of the present invention is 630 mΩ or less, preferably 600 mΩ or less, more preferably 550 mΩ or less, and most preferably 520 mΩ or less.

[0105] Electric device

[0106] In yet another aspect, the present invention also relates to an electric device comprising the lithium-ion secondary battery of the present invention. Electric devices include, but are not limited to, electric vehicles, electric two-wheeled vehicles, power storage systems, or combinations thereof. Electric vehicles include, but are not limited to, plug-in hybrid electric vehicles, hybrid electric vehicles, or combinations thereof.

[0107] Beneficial effects

[0108] The present invention, starting with the negative electrode of a lithium-ion secondary battery, rationally coordinates materials and electrode layers. Specifically, by finely controlling the negative electrode porosity, the specific surface area (S) of the negative electrode active material, the amorphous carbon coating (C), the degree of graphitization (G), and the particle size (D), the power performance of the lithium-ion secondary battery is significantly improved. Furthermore, the inventors discovered that when the negative electrode parameters satisfy the following mathematical formula (I): 0.020 ≤ P × (S × C + 2 × (1-G) / D) ≤ 0.030, the power performance of the lithium-ion secondary battery can be further improved.

[0109] Example

[0110] The solution of the present invention is further described in detail below with reference to specific embodiments.

[0111] It should be noted that the following examples are merely examples for clearly illustrating the technical solutions of the present invention, and are not intended to limit the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is not necessary and impossible to exhaustively enumerate all embodiments here, and the obvious variations or modifications derived therefrom are still within the scope of protection of the present invention. Unless otherwise indicated, the instruments, equipment, and reagents used herein are all commercially available or can be obtained by conventional means in the art.

[0112] preparation

[0113] 1. Preparation of positive electrode

[0114] The positive electrode active material (LiFePO4 lithium iron phosphate), the conductive agent (conductive carbon black (Super P), carbon nanotubes (CNT), and the binder (polyvinylidene fluoride (PVDF)) were mixed in a mass ratio of 94:3:1:2. N-methylpyrrolidone (NMP) was added as a solvent and stirred to obtain a positive electrode slurry. The slurry was evenly coated onto a 16μm aluminum foil current collector. After drying, rolling, and cutting, the positive electrode sheet was prepared.

[0115] 2. Preparation of negative electrode sheet

[0116] The negative electrode active material (graphite coated or uncoated with amorphous carbon) (the specific surface area S, amorphous carbon coating amount C, graphitization degree G, and particle size D of the graphite coated or uncoated with amorphous carbon used in the Examples and Comparative Examples are shown in Table 1), a conductive agent (conductive carbon black (Super P), a thickener (sodium carboxymethyl cellulose (CMC), and a binder (styrene-butadiene rubber (SBR)) were mixed in a mass ratio of 95:2:1:2. A solvent (deionized water) was added and stirred to mix uniformly to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on a 10 μm copper foil current collector and dried. The electrode sheet was rolled to a certain porosity P (the porosity of the negative electrode in the Examples and Comparative Examples is shown in Table 1). After cutting and other processes, a negative electrode sheet was prepared.

[0117] 3. Preparation of electrolyte

[0118] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are uniformly mixed in a volume ratio of 1:1:1, and then dry high-purity lithium salt LiPF6 is dissolved in the above mixed solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0119] 4. Preparation of diaphragm

[0120] The diaphragm is a 9μm polyethylene diaphragm coated on both sides with a ceramic coating with a thickness of 2μm.

[0121] 5. Preparation of lithium-ion secondary batteries

[0122] The prepared positive electrode sheet, separator, negative electrode sheet and electrolyte were assembled in sequence, and the positive electrode sheet, separator, negative electrode sheet and separator were alternately combined, and the soft-pack lithium-ion secondary batteries of Examples 1-23 and Comparative Examples 1-5 with a capacity of 1 Ah were obtained through formation.

[0123] Table 1

[0124]

[0125]

[0126] test

[0127] The lithium-ion secondary battery of the present invention can be tested by the following method, and the test results are shown in Table 2:

[0128] 1. Specific surface area test method

[0129] 2 g of the negative electrode active material (i.e., graphite coated or not coated with amorphous carbon) was placed in a 9 mm ball sample tube and degassed at 200°C for 2 h. The specific surface area of ​​the sample was determined by the multi-point BET method (P / P0 was 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30). The adsorbent was nitrogen.

[0130] 2. Particle size test method

[0131] To 50 mg of the negative electrode active material (i.e., graphite coated or uncoated with amorphous carbon), add 5 drops of a 1% aqueous solution of ethylphenyl polyethylene glycol and 20 mL of deionized water. The mixture is thoroughly dispersed and the particle size distribution is measured using a laser particle size analyzer. The particle refractive index is 2.68, the solvent refractive index is 1.33, and the light shielding is 5-10%. The particle size at which 50% of the volume distribution, from smallest to largest, is determined as the particle size of the material.

[0132] 3. Amorphous carbon coating amount test method

[0133] The negative electrode active material (i.e., amorphous carbon-coated graphite) was used as a sample, and the coking value of the coating agent was determined according to the national standard GB / T8727-2008. The product of the coking value and the amount of coating agent added was calculated as the amorphous carbon coating amount.

[0134] 4. Graphitization degree test method

[0135] Referring to the national standard GB / T 24533-2019, 0.15g of silicon powder and 0.35g of the negative electrode active material (i.e., graphite coated or uncoated with amorphous carbon) were weighed and thoroughly ground in an agate mortar for 10 minutes. The sample was then placed in a sample holder and flattened. The sample was scanned using an X-ray diffractometer at 25°-30° with a step size of 0.02°. The graphite 002 peak was corrected using the silicon 111 peak to obtain the 002 peak 2θ value, and the 002 interlayer spacing was calculated using the Bragg equation. The degree of graphitization was calculated using the formula for the degree of graphitization.

[0136] 5. Pole piece porosity test method

[0137] With reference to the national standard GB / T 21650.1-2008 / ISO 15901-1:2005, the porosity of the negative electrode sheet was measured using the mercury intrusion method.

[0138] 6. Test methods for DC resistance (DCR) at room temperature and DC resistance (DCR) at low temperature

[0139] Under a constant temperature of 25°C, adjust the lithium-ion battery's state of charge (SOC) to 50%, leave it for 1 hour, and record the voltage V0 after the rest period. Discharge it at a rate of 10C (current I0) for 10 seconds, record the voltage V1 after the discharge period, and record (V1-V0) / I0 as the room-temperature DCR. Under a constant temperature of 25°C, adjust the lithium-ion battery's state of charge (SOC) to 50%, adjust the temperature to -20°C, leave it for 3 hours, and record the voltage V2 after the rest period. Discharge it at a rate of 4C (current I1) for 10 seconds, record the voltage V3 after the discharge period, and record (V3-V2) / I1 as the low-temperature DCR.

[0140] Table 2

[0141]

[0142]

[0143] Room-temperature DC resistance (DCR) and low-temperature DC resistance (DCR) are parameters used to evaluate battery power performance. The lower the room-temperature DCR and low-temperature DCR values, the better the battery's power performance.

[0144] In Comparative Example 1, amorphous carbon was not used to coat the graphite, and the graphite particle size was too high and the electrode porosity P was too low. The room temperature DCR and low temperature DCR of the lithium ion secondary battery were relatively high, and the power performance was poor.

[0145] In Comparative Example 2, amorphous carbon was not used to coat the graphite, the electrode porosity P was too low, and the power performance of the battery was relatively poor.

[0146] Comparative Example 3 uses graphite not coated with amorphous carbon. The room-temperature DCR and low-temperature DCR of the lithium-ion secondary battery are relatively high, and the power performance is poor.

[0147] Comparative Example 4 uses amorphous carbon-coated graphite, but the electrode porosity P is too low, and the room-temperature DCR and low-temperature DCR of the lithium-ion secondary battery are still poor.

[0148] Comparative Example 5 also uses amorphous carbon-coated graphite, but the graphitization degree of the amorphous carbon-coated graphite is slightly high, the particle size D is too high, and the electrode porosity P is too low, so the room temperature DCR and low temperature DCR of the lithium ion secondary battery are not ideal.

[0149] In Examples 1-10, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D and electrode porosity P of the amorphous carbon-coated graphite are relatively appropriate, and the room temperature DCR and low temperature DCR values ​​of the lithium-ion secondary battery are reduced, and the power performance of the battery is improved.

[0150] In Examples 11-20, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D and electrode porosity P of the amorphous carbon-coated graphite are more appropriate. From the test results, it can be seen that the room temperature DCR and low temperature DCR of the lithium-ion secondary battery are further reduced, and the power performance is better.

[0151] In Example 21, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and electrode porosity P of the amorphous carbon-coated graphite were all suitable, but these parameters did not satisfy the mathematical formula: 0.020 ≤ P × (S × C + 2 × (1-G) / D) ≤ 0.030. This lithium-ion secondary battery exhibited low room-temperature and low-temperature DCRs and good power performance.

[0152] In Examples 22 and 23, not only do the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and pole piece porosity P of the amorphous carbon-coated graphite all have very suitable values, but these parameters also satisfy the mathematical formula: 0.020 ≤ P × (S × C + 2 × (1-G) / D) ≤ 0.030. Compared to Example 21, Examples 22 and 23 further reduce the room temperature DCR and low temperature DCR, demonstrating excellent power performance.

[0153] The above description is only a specific embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent transformation made by using the present invention, or directly or indirectly applied in other related technical fields, is also included in the patent protection scope of the present invention.

Claims

1. A negative electrode for a lithium ion secondary battery, comprising: a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector, wherein: The negative electrode active material layer contains a negative electrode active material; The negative electrode active material includes: graphite coated with amorphous carbon; in, The porosity P of the negative electrode is 34.0%-55.0%; The specific surface area S of the negative electrode active material is 0.80 m 2 / g-3.20m 2 / g; The amorphous carbon coating amount C of the negative electrode active material is 0.80%-3.35%; The graphitization degree G of the negative electrode active material is 83.0%-95.0%; The particle size D of the negative electrode active material is 3.0 μm-14.0 μm; The porosity P of the negative electrode, the specific surface area S, the amorphous carbon coating amount C, the graphitization degree G and the particle size D of the negative electrode active material satisfy the following mathematical formula: 0.020≤P×(S×C+2×(1-G) / D)≤0.

030.

2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein in, The negative electrode further satisfies one or more of the following conditions: The porosity P of the negative electrode is 35.0%-53.0%; The specific surface area S of the negative electrode active material is 1.50 m 2 / g-3.00m 2 / g; The amorphous carbon coating amount C of the negative electrode active material is 1.00%-2.50%; The graphitization degree G of the negative electrode active material is 85.0%-93.0%; The particle size D of the negative electrode active material is 5.0 μm-12.0 μm.

3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The graphite includes natural graphite, artificial graphite, modified graphite, silicon-oxygen composite artificial graphite or a combination thereof.

4. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The weight ratio of the negative electrode active material to the negative electrode active material layer is 0.92-0.

97.

5. The negative electrode for a lithium ion secondary battery according to claim 1, wherein The negative electrode active material layer further comprises an additive, The additives include: a conductive agent, a binder, a thickener or a combination thereof; in, The conductive agent includes: conductive carbon black, carbon nanotubes, carbon fibers or a combination thereof; The binder includes: styrene-butadiene rubber, acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, methyl methacrylate rubber or a combination thereof; The thickener includes one or more of polyvinyl alcohol, modified polyvinyl alcohol, starch, modified starch, cellulose derivatives, polyvinyl pyrrolidone, polyacrylic acid, polyacrylate, and polyethylene glycol.

6. The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein The negative electrode current collector includes: a copper current collector, an aluminum current collector, a nickel current collector, an iron current collector or a combination thereof.

7. A method for preparing a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6, comprising the following steps: Providing a negative electrode slurry, which includes: a negative electrode active material; coating the negative electrode slurry on a negative electrode current collector and then drying the negative electrode to obtain a negative electrode coating sheet; The negative electrode coating sheet is roll-pressed to obtain a negative electrode for a lithium ion secondary battery.

8. A lithium-ion secondary battery comprising a power generation element, the power generation element comprising: The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 6; positive electrode; electrolyte; and diaphragm. 9 . An electric device comprising the lithium-ion secondary battery according to claim 8 .

Images