Negative electrode for lithium ion secondary battery, method for preparing same, and lithium ion secondary battery containing same

Abstract

The present invention relates to a negative electrode for a lithium ion secondary battery and a method for preparing the same. The negative electrode for a lithium ion secondary battery includes 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 active material layer including a negative electrode active material. The negative electrode active material includes graphite coated with amorphous carbon. Here, the porosity P of the negative electrode is 34.0 to 55.0%, and the specific surface area S of the negative electrode active material is 0.80 to 3.20 m 2 / g, the coating amount C of the amorphous carbon of the negative electrode active material is 0.80 to 3.35%, the graphitization degree G of the negative electrode active material is 83.0 to 95.0%, and the particle size D of the negative electrode active material is 3.0 to 14.0 μm. The present invention also relates to a lithium ion secondary battery and an electric device containing the negative electrode of the present invention. The present invention significantly improves the output performance of the lithium ion secondary battery by rationally combining the material level and electrode sheet level of the negative electrode end.

Description

[Technical Field]

[0001] Cross-Citation of Related Applications This application claims priority from a Chinese application filed on March 29, 2023, bearing application number 202310322863.X and entitled "Negative electrode for lithium ion secondary battery, preparation method thereof, and lithium ion secondary battery containing the same," the entire text of which is incorporated herein by reference.

[0002] The present invention relates to the field of lithium ion secondary batteries, and more particularly to a negative electrode for a lithium ion secondary battery, a method for preparing the same, and a lithium ion secondary battery containing the negative electrode. [Background technology]

[0003] Lithium-ion secondary batteries have advantages such as high energy density, long cycle life, good low-temperature performance, environmental friendliness and safety, and are therefore widely used in fields such as power batteries, mobile phones, computers, power tools, wind and solar energy storage, etc.

[0004] Among these, plug-in hybrid electric vehicles (PHEVs) and hybrid electric vehicles (HEVs) have attracted widespread attention due to their ability to achieve lower fuel consumption and a better driving experience. The main requirements for lithium-ion batteries in these types of electric vehicles are high power output and a large charge / discharge ratio. Therefore, improving the power output of lithium-ion batteries has become an urgent issue in the field of lithium-ion batteries. Therefore, there is a need to develop lithium-ion secondary batteries with better power output.

[0005] Improvements to the negative electrode of lithium-ion batteries are one viable way to improve the output performance of lithium-ion secondary batteries. Patent document application number CN115188920A discloses a negative electrode sheet in which the active material layer is composed of coating layers with different contents of fast-charging material and conductive agent, allowing the fast-charging material to be combined with the conductive agent, with the inner portion containing less fast-charging material and more conductive agent than the outer portion, ensuring lithium ion transmission in the inner coating layer while allowing the electrode sheet to have a maximum high energy density. Summary of the Invention

[0006] Currently, improvements to the negative electrode generally involve reducing the particle size of the active material, applying a surface coating, etc. to improve the output performance of the material. In order to further improve the output performance of lithium ion secondary batteries, the present invention uses a negative electrode for lithium ion secondary batteries obtained by rationally combining materials and electrode sheet levels, thereby significantly improving the output performance of lithium ion secondary batteries.

[0007] In one aspect, the present invention relates to a negative electrode for a lithium ion secondary battery. The negative electrode for a lithium ion secondary battery includes 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 active material layer includes a negative electrode active material, and the negative electrode active material includes graphite coated with amorphous carbon. Here, the porosity P of the negative electrode is 34.0 to 55.0%, and the specific surface area S of the negative electrode active material is 0.80 to 3.20 m 2 / g, the coating amount C of amorphous carbon of the negative electrode active material is 0.80 to 3.35%, the graphitization degree G of the negative electrode active material is 83.0 to 95.0%, and the particle size D of the negative electrode active material is 3.0 to 14.0 μm.

[0008] 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 to 53.0%. The specific surface area S of the negative electrode active material is 1.50 to 3.00 m 2 / g. The coating amount C of the amorphous carbon of the negative electrode active material is 1.00 to 2.50%. The graphitization degree G of the negative electrode active material is 85.0 to 93.0%. The particle size D of the negative electrode active material is 5.0 to 12.0 μm.

[0009] In one embodiment, the porosity P of the negative electrode, the specific surface area S of the negative electrode active material, the coating amount C of the amorphous carbon, the degree of graphitization G, and the particle size D satisfy the formula "0.020≦P×(S×C+2×(1−G) / D)≦0.030".

[0010] In another aspect, the present invention also relates to a method for preparing a negative electrode for a lithium ion secondary battery, comprising the steps of: providing a negative electrode slurry containing a negative electrode active material and optional additives; coating the negative electrode slurry on a negative electrode current collector and then drying the coated negative electrode sheet; The negative electrode coated sheet is rolled to obtain a negative electrode for a lithium ion secondary battery.

[0011] In yet another aspect, the present invention also relates to a lithium ion secondary battery including a power generating element, the power generating element including the negative electrode for the lithium ion secondary battery of the present invention, a positive electrode, an electrolyte, and a separator.

[0012] In yet another aspect, the present invention also relates to an electrically powered device, which includes the lithium ion secondary battery of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] General Definitions and Terminology Unless otherwise indicated, all publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. 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. In case of conflict, the definitions provided herein shall prevail.

[0014] Unless otherwise stated, all percentages, parts, ratios, etc. are by weight. When a quantity, concentration, or other value or parameter is given as a range, a preferred range, or upper and lower preferred values, or as a specific value, it should be understood that all ranges formed from the logarithmic values ​​of any upper range or preferred value and any lower range or preferred value are specifically disclosed, regardless of whether the range is disclosed alone. Unless otherwise specified, when a numerical range is recited herein, the range is intended to include its endpoints, and all integers and fractions within the range. The scope of the present invention is not limited to the specific numerical values ​​recited when defining the range. For example, "1 to 8" includes 1, 2, 3, 4, 5, 6, 7, 8, and any subrange consisting of any two values ​​therein, such as 2 to 6 and 3 to 5.

[0015] The terms "comprising," "containing," "having," "including," or "relating to," and other variations thereof herein, are inclusive or open-ended and do not exclude other unrecited elements or method steps. Those skilled in the art should understand that the above terms, such as "comprising," encompass the meaning of "consisting of." The expression "consisting of" excludes any unspecified elements, steps, or ingredients. The expression "consisting essentially of" indicates that the scope is limited to the specified elements, steps, or ingredients, plus optionally present elements, steps, or ingredients that do not materially affect the basic and novel characteristics of the claimed subject matter. The expression "comprising" should be understood to encompass the expressions "consisting essentially of" and "consisting of."

[0016] The term "selected from" refers to one or more elements in a group listed thereafter, selected independently, and can include combinations of two or more elements. As used herein, the term "optionally" or "optionally" refers to the possibility that the subsequently described event or circumstance may or may not occur, and the description includes the occurrence of the event or circumstance and the absence of the occurrence of the event or circumstance.

[0017] When numerical values ​​or range limits are stated herein, the published content should be understood to include the specific value or limit recited. 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 species. Unless otherwise specified, the terms "combinations thereof" and "mixtures thereof" refer to multi-component mixtures of the aforementioned elements, such as two-, three-, four- and up to the maximum possible multi-component mixtures. Furthermore, the absence of a specified number before a component or ingredient of the present invention indicates that there is no limit to the number of occurrences (or presences) of the component or ingredient, and therefore should be interpreted as including one or at least one, and the singular form of a component or ingredient also includes the plural, unless the numerical value clearly indicates the singular.

[0018] 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 charging. The term "anode" refers to the electrode in a secondary battery in which oxidation occurs during discharge and reduction occurs during charge. The term "cathode" refers to the electrode in a secondary battery in which reduction occurs during discharge and oxidation occurs during charge.

[0019] The term "formation" refers to the process of performing a small current charge / discharge on a battery after it is obtained in the battery preparation process. The formation treatment is advantageous for stabilizing the electrical performance of the battery. The term "electric vehicle" refers to a vehicle powered by electricity, and 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.

[0020] negative electrode 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 negative electrode current collector. The negative electrode refers to an electrode from which electrons flow out of an external circuit during discharge and which has a lower potential.

[0021] negative electrode current collector In the present invention, the material constituting the negative electrode current collector is not particularly limited, and for example, a metal can be used. Specifically, selectable metals in this specification include, but are not limited to, aluminum, nickel, iron, stainless steel, titanium, copper, or a combination thereof. When a combination of two or more metals is used for the negative electrode current collector, this may refer to the use of an alloy, or the use of a coating layer, plating layer, or the like. From the viewpoints of electronic conductivity and battery operating potential, the metal for the negative electrode current collector is preferably copper, aluminum, or stainless steel. In one specific embodiment, the negative electrode current collector is copper foil.

[0022] The dimensions of the negative electrode current collector can be adjusted depending on the actual use of the battery. For example, when used in a large battery requiring high energy density, a current collector with a large area can be used. There are no particular limitations on the thickness of the negative electrode current collector, and it can be, for example, 1 to 100 μm, such as 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, 80 μm, 100 μm, etc. In one specific embodiment, the thickness of the negative electrode current collector is 10 μm.

[0023] Negative electrode active material layer The negative electrode active material layer is formed on the surface of the negative electrode current collector, and there are no particular limitations on its shape or area. The negative electrode active material layer can cover one or both surfaces of the negative electrode current collector. There are no particular limitations on the thickness of the negative electrode active material layer, and it can be, for example, 1 to 100 μm, such as 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, 80 μm, 100 μm, etc. In one specific embodiment, the thickness of the negative electrode current collector is 10 μm.

[0024] negative electrode active material The negative electrode active material layer may include a negative electrode active material. A negative electrode active material is typically a material capable of inserting or extracting 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 coated with an amorphous carbon layer 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, or the like, such as amorphous carbon-coated graphite particles.

[0025] The output performance of a lithium-ion battery is related to the transport of lithium ions in the liquid phase, solid-liquid interface, and solid phase. Adjustments to the negative electrode material level (e.g., specific surface area S of the negative electrode active material, coating amount C of amorphous carbon, degree of graphitization G, particle size D, etc.) and electrode sheet level (e.g., porosity P of the negative electrode sheet) contribute to improving the battery output performance.

[0026] The liquid phase transport of lithium ions is affected by the porosity of the electrode sheet. In this specification, 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 to 55.0%, preferably 35.0 to 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 number of liquid phase transmission paths, reduces liquid phase transmission resistance, and further contributes to improving output performance. Both excessively low and excessively high porosity of the negative electrode are detrimental to improving power performance. If the porosity is too low, the electrolyte infiltration effect will be impaired, preventing the electrolyte from fully penetrating the electrode sheet, which will be detrimental to the reduction of liquid phase transmission resistance and may lead to a reduction in the power performance of the lithium-ion battery. If the porosity is too high, it will affect the sufficient contact between particles of the negative electrode active material, resulting in poor particle contact and a reduction in the electronic conductivity of the negative electrode sheet, which may further lead to a reduction in the power performance of the lithium-ion battery.

[0027] The term "porosity" as used herein has the meaning commonly understood in the art, and refers to, for example, the ratio of the pores within a material to the total apparent volume of a sample (such as a negative electrode sheet). The porosity of a negative electrode can be measured by conventional methods, for example, the porosity of a negative electrode sheet is measured using mercury porosimetry in accordance with Chinese national standard GB / T 21650.1-2008 / ISO 15901-1:2005.

[0028] At the solid-liquid interface, the lithium ion transfer process at the solid-liquid interface is affected by the interfacial lithium ion transfer channel. The specific surface area (S) of the negative electrode active material and the coating amount (C) of amorphous carbon affect the solid-liquid interfacial transfer. Appropriate specific surface area (S) of the negative electrode active material and coating amount (C) of amorphous carbon contribute to improving the lithium ion transfer process at the solid-liquid interface, further improving the output performance of the battery.

[0029] The specific surface area S of the appropriate negative electrode active material contributes to increasing the interfacial transmission channel, thereby improving the output performance. In one embodiment, the specific surface area S of the negative electrode active material is 0.80 to 3.20 m 2 / g, preferably 1.50 to 3.00m 2 / g, for example, 0.80m 2 / g, 0.90m 2 / g, 1.00m 2 / g, 1.10m 2 / 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. If the specific surface area S of the negative electrode active material is too large or too small, both will have a detrimental effect on the battery's output performance. If the specific surface area is too large, the dispersion processing effect of the negative electrode active material particles will be poor, affecting sufficient contact between the particles, reducing the electronic conductivity of the negative electrode sheet and further reducing the output performance of the lithium-ion battery. If the specific surface area is too small, there will be few interfacial transmission channels, which will be unfavorable for the transmission of lithium ions at the solid-liquid interface, thereby reducing the output performance of the lithium-ion battery.

[0030] The term "specific surface area" as used herein has the meaning commonly understood by those skilled in the art, i.e., the total area of ​​a material per unit mass. Measurements can be performed using standard methods and equipment in the art, for example, by using the multi-point BET method.

[0031] Amorphous carbon materials have a larger interlayer distance than graphite, which can improve the diffusion of lithium ions therein. Therefore, using amorphous carbon to coat graphite is equivalent to forming a lithium ion buffer layer on the outer surface of the graphite, thereby improving the high-current charge / discharge performance of the graphite material. Furthermore, coating graphite with amorphous carbon can prevent graphite delamination caused by co-intercalation of solvent molecules, increasing the range of electrolyte system options and improving the cycling stability of electrode materials.

[0032] Appropriately increasing the coating amount C of the amorphous carbon on the negative electrode active material can effectively reduce the charge transfer resistance at the solid-liquid interface, favor the transmission of lithium ions at the solid-liquid interface, and further improve output performance. In one embodiment, the coating amount C of the amorphous carbon on the negative electrode active material is 0.80 to 3.35%, preferably 1.00 to 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. If the coating amount C of the amorphous carbon of the negative electrode active material is too high or too low, it will have an adverse effect on the output performance of the battery. If the coating amount of amorphous carbon is too high, it may affect the dispersion processing effect of the negative electrode active material particles, which may be unfavorable to sufficient contact between the particles, resulting in poor particle contact and a decrease in the electronic conductivity of the negative electrode sheet, further leading to a decrease in the output performance of the lithium ion battery.If the coating amount of amorphous carbon is too low, it may not be possible to sufficiently improve the charge transfer resistance at the solid-liquid interface, which may hinder the transmission of lithium ions at the solid-liquid interface, leading to a decrease in the output performance of the lithium ion battery.

[0033] In this specification, the term "amorphous carbon coating amount" refers to the ratio of the mass of the amorphous carbon layer coated on the negative electrode active material to the mass of the negative electrode active material. Taking graphite particles coated with amorphous carbon as an example, the term "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). This can be measured according to standard methods and equipment in the art, such as measuring the coking value of the coating agent according to Chinese National Standard GB / T 8727-2008, and calculating the product of the coking value and the amount of coating agent added to obtain the amorphous carbon coating amount.

[0034] The solid-state transport of lithium ions is affected by the bulk phase transport distance and transport resistance of the solid phase (e.g., graphite).

[0035] The particle size of the negative electrode active material affects the bulk phase transmission distance. A suitable particle size of the negative electrode active material contributes to improving the battery's output performance. A moderate reduction in the particle size of the negative electrode active material contributes to shortening the bulk phase transmission distance and reducing transmission resistance, further improving output performance. In one embodiment, the particle size D of the negative electrode active material is 3.0 to 14.0 μm, preferably 5.0 to 12.0 μm, such as 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 both detrimental to the output performance of the battery. If the particle size of the negative electrode active material is too large, it will lead to an increase in the bulk phase transmission distance and an increase in transmission resistance, which is unfavorable to the solid phase transmission of lithium ions, leading to the inability to timely replenish lithium ions for lithium insertion, and therefore to a deterioration in the output performance of the lithium ion battery.If the particle size is too small, the particle dispersion processing effect will be relatively poor, affecting sufficient contact between particles, which may lead to a decrease in the electronic conductivity of the negative electrode, and therefore to a deterioration in the output performance of the lithium ion battery.

[0036] The term "particle size" as used herein refers to the particle size at which the proportion of small to large particles in a sample's volume particle size distribution is 50%, i.e., the particle size corresponding to when the cumulative particle size distribution percentage reaches 50%. The term "particle size" as used herein may also be referred to as "D50 particle size." The term "particle size" as used herein can be measured according to standard methods and equipment in the art, for example, by using a laser particle sizer to measure the particle size volume distribution.

[0037] By appropriately reducing the graphitization degree of the negative electrode active material, the interlayer distance increases, the solid-state transmission resistance of lithium ions decreases, and output performance can be further improved. In one embodiment, the graphitization degree G of the negative electrode active material is 83.0 to 95.0%, preferably 85.0 to 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. An excessively low or high degree of graphitization of the negative electrode active material is both detrimental to improving the output performance of the battery. If the degree of graphitization of the negative electrode active material is too low, the particle dispersion processing effect is relatively poor, affecting sufficient contact between particles, which is detrimental to output performance and may lead to a decrease in the electronic conductivity of the negative electrode, further deteriorating the output performance of the lithium-ion battery. If the degree of graphitization is too high, the interlayer distance becomes large, which leads to a relatively large solid-state transmission resistance, which is detrimental to solid-state lithium ion transmission, leading to the inability to timely replenish lithium ions for lithium insertion, thereby deteriorating the output performance of the lithium-ion battery.

[0038] The term "degree of graphitization" as used herein has the meaning commonly understood by those skilled in the art, i.e., the degree of perfection of the graphite crystal structure, and can also represent the degree of regularity of the carbon atom arrangement in the graphite structure. It can generally be calculated based on the distance between the 002 crystal planes of graphite. The smaller the distance between the crystal planes, the higher the degree of graphitization. It can be measured according to standard methods and equipment in the art, for example, the graphitization formula can be used to calculate the degree of graphitization with reference to the Chinese national standard GB / T24533-2019.

[0039] In this specification, 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 correspond to the specific surface area, degree of graphitization, and particle size of the material having the coating layer, respectively. For example, when graphite is coated with amorphous carbon and the resulting amorphous carbon-coated graphite 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.

[0040] The applicant has found that when the porosity, specific surface area, coating amount of amorphous carbon, degree of graphitization, and particle size satisfy certain ranges (see above), the negative electrode can have an optimal material structure and electrode sheet structure, which contributes to improving the output performance of the battery.

[0041] In one embodiment, the negative electrode of the present invention satisfies the following conditions: the porosity P of the negative electrode is 34.0 to 55.0%, and the specific surface area S of the negative electrode active material is 0.80 to 3.20 m 2 / g, the coating amount C of the amorphous carbon of the negative electrode active material is 0.80 to 3.35%, the graphitization degree G of the negative electrode active material is 83.0 to 95.0%, and the particle size D of the negative electrode active material is 3.0 to 14.0 μm. A battery using this negative electrode can have relatively good output performance.

[0042] Furthermore, in one preferred embodiment, the negative electrode satisfies one or more of the following conditions, for example, one, two, three, four, or all of the following, to further improve the output performance of the battery: The porosity P of the negative electrode is 35.0 to 53.0%. The specific surface area S of the negative electrode active material is 1.50 to 3.00 m 2 / g. The coating amount C of amorphous carbon of the negative electrode active material is 1.00 to 2.50%. The graphitization degree G of the negative electrode active material is 85.0 to 93.0%. The particle size D of the negative electrode active material is 5.0 to 12.0 μm.

[0043] In one more preferred embodiment, the negative electrode of the present invention has the following characteristics: The porosity P of the negative electrode is 35.0 to 53.0%. The specific surface area S of the negative electrode active material is 1.50 to 3.00 m 2 / g. The coating amount C of amorphous carbon of the negative electrode active material is 1.00 to 2.50%. The graphitization degree G of the negative electrode active material is 85.0 to 93.0%. The particle size D of the negative electrode active material is 5.0 to 12.0 μm. This allows the output performance of a battery using the negative electrode of the present invention to be significantly improved.

[0044] In the negative electrode active material layer, an appropriate content of the negative electrode active material contributes to realizing good output performance of the negative electrode. The weight of the negative electrode active material may be 80% or more of the weight of the negative electrode active material layer, preferably 90% or more, and more preferably 92% or more. In one embodiment, the weight ratio of the negative electrode active material to the negative electrode active material layer may be 0.92 to 0.97, for example, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, etc.

[0045] Furthermore, the applicant unexpectedly discovered the following aspect: When the relevant parameters of the electrode sheet factors (including porosity P, which affects the liquid-phase transfer process), interface factors (including amorphous carbon coating amount C and graphitization degree G, which affects the solid-liquid interfacial transfer process), and internal factors (including particle size D and specific surface area S, which affects the solid-phase transfer process) satisfy a specific relationship, the battery output performance can be further improved. In the negative electrode for a lithium ion secondary battery 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 satisfy the formula "0.020≦P×(S×C+2×(1−G) / D)≦0.030", where the unit of porosity P is "1" and the unit of specific surface area S is m 2 / g, the amorphous carbon coating amount C, the degree of graphitization G are in units of "1", and the particle size D is in units of micrometers (μm). When the negative electrode satisfies the above formula "0.020≦P×(S×C+2×(1-G) / D)≦0.030", the output performance of the lithium-ion secondary battery is better.

[0046] If necessary, the negative electrode active material layer may further contain other additives (for example, a conductive agent, a thickener, a binder, etc.) to impart appropriate performance to the negative electrode.

[0047] Conductive agent A conductive agent refers to an additive added to improve the conductivity of the negative electrode active material layer. When a conductive agent is contained in the negative electrode active material layer, an internal electronic network is effectively formed within the active material layer, contributing to the output characteristics of the battery. Conductive agents usable herein include, but are not limited to, the following: Examples include conductive carbon black, carbon nanotubes, carbon fiber, acetylene black, and ketjen black, with conductive carbon black being preferred. These conductive agents can be used alone or in combination. In one embodiment, the conductive agent is conductive carbon black. By using conductive carbon black as a conductive agent, a good output improvement effect can be achieved in combination with the negative electrode active material of this specification.

[0048] In the negative electrode active material layer, an appropriate content of the conductive agent contributes to sufficiently improving the conductivity of the negative electrode active material layer and avoiding adverse effects. In one embodiment, the weight ratio of the conductive agent to the negative electrode active material layer is 0.015 to 0.025, for example, 0.015, 0.018, 0.02, 0.023, 0.025, etc.

[0049] binder The binder is used to provide an adhesive effect and contributes to obtaining good adhesion within the negative electrode active material layer and between the negative electrode active material layer and the negative electrode current collector. Binders that can be used in this specification include, but are not limited to, thermoplastic polymers such as 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 hydrogenated product, and styrene-isoprene-styrene block copolymer and its hydrogenated product. Examples include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF). For example, vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber), as well as epoxy resins, etc.These binders can be used alone or in combination of two or more kinds.

[0050] In the negative electrode active material layer, the binder is preferably capable of forming an aqueous binder system. An aqueous binder system is a system that provides adhesion using water as a solvent or dispersion medium. Aqueous binder systems have high adhesive strength and are inexpensive and environmentally friendly because they use water as a solvent or dispersion medium. Furthermore, since only water vapor is generated during drying, capital investment in production lines can be significantly reduced, thereby reducing the environmental impact. In this specification, the term "water-based binder" refers to all types of binders that appear as latexes or emulsions, and refers to polymers that are emulsified with or suspended in water, such as polymer latexes obtained by emulsion polymerization in self-emulsifying systems.

[0051] Binders used to form the aqueous binder system can include, but are not limited to, the following: styrene-based polymers (styrene-butadiene rubber, styrene-vinyl acetate copolymer, styrene-acrylic acid copolymer, etc.), acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, (meth)acrylic acid-based polymers (polyethyl acrylate, polyethyl methacrylate, polypropyl acrylate, polymethyl methacrylate (methyl methacrylate rubber), polypropyl methacrylate, polyisopropyl acrylate, polyisopropyl methacrylate, polybutyl acrylate, polybutyl methacrylate, polyhexyl acrylate, polymethacrylate, etc.), Polyethylene, polypropylene, ethylene-propylene copolymer, polybutadiene, butyl rubber, fluororubber, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, polyester resin, phenolic resin, epoxy resin.

[0052] In terms of adhesiveness, the binder herein may contain, but is not limited to, one or more of the following: styrene butadiene rubber, acrylonitrile butadiene rubber, methyl methacrylate butadiene rubber, and methyl methacrylate rubber. To obtain better adhesiveness, the binder may be styrene butadiene rubber.

[0053] In order to provide a good adhesive effect, the content of the binder should be kept within an appropriate range. In one embodiment, the weight ratio of the binder to the negative electrode active material layer is 0.015 to 0.025.

[0054] thickener When using a binder to form an aqueous binder system, the binder can be used in combination with a thickener to improve coating properties, which is beneficial to achieve good coating effect during the preparation process.

[0055] A thickener can improve the viscosity of the negative electrode slurry, and is therefore useful for improving the coatability of the slurry. Selectable thickeners herein include, but are not limited to, polyvinyl alcohol (the average polymerization degree is preferably 200 to 4000, more preferably 1000 to 3000, and the saponification degree is preferably 80 mol % or more, more preferably 90 mol % or more) and modified products thereof (e.g., 1 to 80 mol % saponified products of vinyl acetate units in copolymers with a molar ratio of ethylene / vinyl acetate = 2 / 98 to 30 / 70, 1 to 50 mol % partial acetalization products of polyvinyl alcohol), starch and modified products thereof (e.g., oxidized starch, phosphated starch, cationized starch), cellulose derivatives (e.g., carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, hydroxyethyl cellulose, and salts thereof), polyvinylpyrrolidone, polyvinyl alcohol, etc. These thickeners contain acrylic acid (salt), polyethylene glycol, (meth)acrylamide and / or (meth)acrylate copolymers [(meth)acrylamide polymer, (meth)acrylamide-(meth)acrylate copolymer, (meth)acrylate alkyl (C1-4) ester-(meth)acrylate copolymer, etc.], styrene-maleate copolymer, Mannich-modified polyacrylamide, formaldehyde condensation resin (urea-formaldehyde resin, melamine-formaldehyde resin, etc.), polyamidepolyamine or dialkylamine-epichlorohydrin copolymer, polyethyleneimine, casein, soy protein, synthetic protein, and water-soluble polymers such as galactomannan derivatives. These thickeners can be used alone or in combination.

[0056] The thickener may preferably contain polyvinyl alcohol and its modified products, starch and its modified products, cellulose derivatives (such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and their salts), polyvinylpyrrolidone, polyacrylic acid (salts), polyethylene glycol, or a combination thereof, and is preferably sodium carboxymethyl cellulose. When styrene-butadiene rubber is used as a binder, a good adhesive effect can be obtained by combining it with these thickeners. In a preferred embodiment, styrene-butadiene rubber is used as the binder and carboxymethyl cellulose (salts) is used as the thickener. In a more preferred embodiment, styrene-butadiene rubber is used as the binder and carboxymethyl cellulose sodium is used as the thickener.

[0057] 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 to 10, more preferably 1:0.2 to 1, for example 1:0.5. The ratio of the styrene butadiene rubber to the sodium carboxymethyl cellulose is preferably 1:0.1 to 10, more preferably 1:0.2 to 1, for example 1:0.5.

[0058] Method for preparing negative electrode for lithium-ion secondary battery In another aspect, the present invention also relates to a method for preparing a negative electrode for a lithium ion secondary battery, which comprises the steps of: Providing an anode slurry comprising an anode active material and optional additives. The negative electrode slurry is applied onto a negative electrode current collector and then dried to obtain a negative electrode coated sheet. The negative electrode coated sheet is rolled to obtain a negative electrode for a lithium ion secondary battery. Here, the negative electrode active material and additives are as described above.

[0059] The negative electrode slurry may further contain a solvent. The solvent is used to disperse the negative electrode active material and additives and form a dispersion system. In one embodiment, water is used as the solvent. Using water as the solvent has several advantages, such as low cost, environmental friendliness, and the subsequent drying process produces only water vapor, significantly reducing capital investment for production lines and reducing environmental impact.

[0060] 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 on-site requirements, for example, 100 to 2500 rpm, 100 rpm, 200 rpm, 500 rpm, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, etc. Sufficient mixing and stirring contributes to obtaining a uniformly dispersed negative electrode slurry, which in turn contributes to obtaining a negative electrode coating sheet with a uniformly distributed coating film.

[0061] To achieve a better dispersion effect, the negative electrode active material and additives (conductive agent, binder, thickener, etc.) can be first thoroughly mixed and then the solvent can be added, which contributes to shortening the stirring time required to obtain a uniformly dispersed negative electrode slurry.

[0062] A conventional method can be used to apply the negative electrode slurry to form a coating layer on all or part of the negative electrode current collector, such as die coating (e.g., slide die coating), comma direct coating, comma reverse coating, gravure coating, or gravure reverse coating.

[0063] The resulting negative electrode coated sheet is dried to remove the volatile phase (e.g., solvent) from the coating layer and retain the other components (e.g., negative electrode active material, additives), forming a negative electrode coated sheet. Drying can be performed using conventional methods in the art. To improve drying efficiency, heating methods, such as vacuum drying and infrared heating, can be used. During the drying process, an appropriate drying temperature can accelerate drying efficiency and achieve sufficient drying within a shorter time. However, the drying temperature should not be excessively high. This can avoid decomposition of the raw materials and surface irregularities due to rapid solvent evaporation. In one embodiment, the drying temperature is 50 to 70°C, such as 50°C, 55°C, 60°C, 65°C, or 70°C.

[0064] By performing roll pressing on the negative electrode coated sheet, the negative electrode sheet can be further compressed to achieve the desired porosity, thereby obtaining the desired negative electrode for a lithium-ion secondary battery. The roll pressing process can be performed using, for example, a metal roll, an elastic roll, a heated roll (hot roll), or the like. The temperature used in the roll pressing process should be lower than the temperature at which the active material layer is dried. The roll pressing process can be performed at room temperature or under heated conditions, with heated conditions being preferred. Performing roll pressing (hot pressing) under heated conditions can reduce porosity deviation, which is thought to be due to the fact that it promotes binder softening and makes the pore distribution more uniform.

[0065] During the coating 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 and / or rolling process. Due to the anisotropy of the electrical conductivity of graphite particles, the graphite layer planes tend to be oriented perpendicular to the current collector, thereby reducing the degree of orientation (OI) of the graphite negative electrode sheet and achieving the effects of improved fast charging, reduced expansion, and improved cycle life. The electric field can be applied in a continuous manner. During the coating process, the voltage of the applied electric field is 5 to 10 V, for example, 5 V, 6 V, 7 V, 8 V, 9 V, or 10 V. During the rolling process, the voltage of the applied electric field can be 10 to 20 V, for example, 10 V, 12 V, 15 V, 18 V, or 20 V.

[0066] Lithium-ion secondary battery In yet another aspect, the present invention relates to a lithium ion secondary battery having a power generating element, the power generating element including the negative electrode for the lithium ion secondary battery described herein, a positive electrode, an electrolyte, and a separator.

[0067] positive electrode The positive electrode refers to an electrode into which electrons flow from an external circuit during discharge and which has a higher potential. In this specification, there is no particular limitation on the positive electrode. 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.

[0068] The positive electrode active material layer may contain a positive electrode active material. The positive electrode active material usable herein may be a compound composed of lithium and one or more selected from iron, cobalt, manganese, and nickel. 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 contains lithium iron phosphate (LiFePO4).

[0069] The positive electrode active material layer may further contain additives such as a conductive agent, a binder, etc. There are no particular limitations on the other additives, and they may be, for example, those usable for the negative electrode described above.

[0070] In one embodiment, the conductive agent used in the positive electrode active material layer includes conductive carbon black or carbon nanotubes. In another embodiment, the binder used in the positive electrode active material layer includes polyvinylidene fluoride (PVDF).

[0071] Separator The separator has the function of ensuring lithium ion conductivity between the positive electrode and the negative electrode by retaining the electrolyte, and the function of acting as a partition between the positive electrode and the negative electrode. The separator can be in the form of a porous sheet made of polymer and / or fiber, a nonwoven fabric separator, or the like.

[0072] The porous sheet separator made of polymer and / or fiber can be a microporous membrane. Specifically, the microporous membrane separator can be made of the following materials: For example, polyolefins such as polyethylene (PE) and polypropylene (PP) can be used. Examples include laminates obtained by laminating multiple layers of the polyolefins (e.g., a three-layer laminate of PP / PE / PP), hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and glass fibers. There are no particular limitations on the thickness of the porous sheet separator. For example, known materials such as cotton, rayon, acetate, nylon, polyester, polyolefins (e.g., PP, PE), polyimide, and aramid can be used alone or in combination for the nonwoven fabric separator. The volume density of the nonwoven fabric is not particularly limited, as long as sufficient battery characteristics can be obtained with the impregnated electrolyte. Therefore, the thickness of the nonwoven separator only needs to be the same as that of the electrolyte layer. In one preferred embodiment, the separator is a 9 μm polyethylene separator with a 2 μm thick ceramic coating on both sides.

[0073] electrolyte The electrolyte is used to transport ions between the positive and negative electrodes and conduct current. There are no particular limitations on the electrolyte in this specification. A liquid electrolyte or a gel polymer electrolyte can be used.

[0074] Liquid electrolytes function as carriers of lithium ions. Liquid electrolytes can contain the following: a solvent (e.g., an organic solvent, which can act as a plasticizer), a lithium salt dissolved in the solvent as a supporting salt, and other optional additives. Organic solvents that can be used in liquid electrolytes 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). A mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) is preferred. Lithium salts that can be used in the electrolyte include, but are not limited to, compounds that can be added to the active material layer of an electrode, such as Li(CF3SO2)2N, Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, LiTaF6, and LiCF3SO3. Preferably, it is LiPF6. In one preferred embodiment, the electrolyte is a liquid electrolyte and contains ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and LiPF6.

[0075] The use of a gel polymer electrolyte stabilizes the distance between electrodes, suppresses polarization, and improves durability (cyclability). Gel polymer electrolytes can be obtained by injecting the above-mentioned liquid electrolyte into a matrix polymer (main polymer) containing an ion-conducting polymer. When a gel polymer electrolyte is used as the electrolyte, the electrolyte no longer has fluidity and is prone to inhibiting ionic conductivity between the layers. Ion-conducting polymers in the matrix polymer (main polymer) include, but are 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 a gel electrolyte exhibits excellent mechanical strength by forming a crosslinked structure. To form the crosslinked structure, the polymerizable polymer (e.g., PEO, PPO) used to form the polymer electrolyte can be subjected to polymerization treatments such as thermal polymerization, ultraviolet polymerization, radiation polymerization, and electron beam polymerization using an appropriate polymerization initiator.

[0076] Preparation of lithium-ion secondary batteries The lithium ion secondary battery of the present invention can be prepared by a conventional method, for example, by assembling a positive electrode, a separator, a negative electrode, and an electrolyte in this order, and then alternately combining the positive electrode, the separator, the negative electrode, and the separator, and then performing chemical formation to obtain a soft-pack battery.

[0077] Lithium-ion secondary battery output performance The output performance of a lithium-ion secondary battery can be evaluated using its room-temperature direct current resistance (DCR) and low-temperature direct current resistance (DCR). The smaller the room-temperature and low-temperature DCR values, the better the battery's output performance. Room-temperature and low-temperature DCRs can be measured using the following method: In a constant temperature environment of 25°C, adjust the state of charge (SOC) of a lithium-ion battery to 50%, leave it standing for one hour, and record the voltage (V0) after the setting. Discharge it at 10C (current is I0) for 10 seconds, record the voltage (V1) after the setting, and define the room-temperature DCR as (V1 - V0) / I0. In a constant temperature environment of 25°C, adjust the state of charge (SOC) of a lithium-ion battery to 50%, adjust the temperature to -20°C, leave it standing for three hours, and record the voltage (V2) after the setting. The battery is discharged at a 4C rate (where the current is I1) for 10 seconds, and the voltage V3 after discharge is recorded. The low-temperature DCR is (V3 - V2) / I1. 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, even 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.

[0078] electric equipment In yet another aspect, the present invention relates to an electrically powered device containing the lithium-ion secondary battery of the present invention. The electrically powered device includes, but is not limited to, an electric vehicle, an electric motorcycle, a power storage system, or a combination thereof. The electric vehicle includes, but is not limited to, a plug-in hybrid electric vehicle, a hybrid electric vehicle, or a combination thereof.

[0079] Beneficial effects In the present invention, starting from the negative electrode of a lithium ion secondary battery, a rational combination is made at the material level and electrode sheet level, specifically, by precisely controlling the porosity of the negative electrode, the specific surface area S of the negative electrode active material, the coating amount C of the amorphous carbon, the graphitization degree G, and the particle size D, the output performance of the lithium ion secondary battery is significantly improved. Furthermore, the inventors have also found that the output performance of a lithium ion secondary battery can be further improved when each parameter of the negative electrode satisfies the mathematical formula I, "0.020≦P×(S×C+2×(1−G) / D)≦0.030."

[0080] Example The present invention will be described in more detail below in conjunction with specific examples.

[0081] It should be noted that the following examples are merely illustrative examples prepared to clearly explain the technical solutions of the present invention, and are not intended to limit the present invention. Those skilled in the art can make various changes or variations based on the above description, and it is not necessary or possible to cover all implementation modes here, but obvious changes or variations resulting therefrom are still within the scope of protection of the present invention. Unless otherwise specified, all equipment and reagent materials used herein are commercially available or can be prepared by standard methods in the art.

[0082] preparation 1. Preparation of Positive Electrode Sheet The positive electrode active material is LiFePO4 lithium iron phosphate, the conductive agent is conductive carbon black (Super P), carbon nanotubes (CNT), and the binder is polyvinylidene fluoride (PVDF) in a mass ratio of 94:3:1:2. N-methylpyrrolidone (NMP) is added as a solvent and stirred to homogenize the mixture to obtain positive electrode slurry. The positive electrode slurry is then uniformly applied to a 16 μm aluminum foil current collector. After drying, rolling, cutting, and other processes, a positive electrode sheet is prepared.

[0083] 2. Preparation of Negative Electrode Sheet The negative electrode active material was 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 amorphous carbon-coated or uncoated graphite used in the Examples and Comparative Examples are shown in Table 1), conductive carbon black (Super P) as a conductive agent, sodium carboxymethyl cellulose (CMC) as a thickener, and styrene butadiene rubber (SBR) as a binder. The mixture was then mixed in a mass ratio of 95:2:1:2, followed by the addition of deionized water as a solvent and stirring to homogeneously mix the mixture to obtain a negative electrode slurry. The negative electrode slurry was then uniformly applied to a 10 μm copper foil current collector and dried. The electrode sheet was then rolled to a certain porosity P (the porosity of the negative electrodes in the Examples and Comparative Examples is shown in Table 1), followed by cutting and other processes to prepare a negative electrode sheet.

[0084] 3. Electrolyte Preparation Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed uniformly in a volume ratio of 1:1:1, and dried high-purity lithium salt LiPF6 is dissolved in the mixed solvent to prepare an electrolyte solution with a concentration of 1 mol / L.

[0085] 4. Separator Preparation The separator is a 9 μm polyethylene separator with a 2 μm thick ceramic coating applied to both sides.

[0086] 5. Preparation of Lithium-ion Secondary Batteries The prepared positive electrode sheet, separator, negative electrode sheet, and electrolyte solution were assembled in this order, and the positive electrode sheet, separator, negative electrode sheet, and separator were alternately combined. After chemical formation, soft-pack lithium-ion secondary batteries with a capacity of 1 Ah were obtained for Examples 1 to 23 and Comparative Examples 1 to 5.

[0087] [Table 1]

[0088] measurement 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.

[0089] 1.Specific surface area test method Two grams of the negative electrode active material (i.e., amorphous carbon-coated or uncoated graphite) was placed in a sample tube with a 9 mm ball and degassed at 200°C for two hours. The specific surface area of ​​the sample was measured using the multipoint BET method (P / P0 = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30). The adsorbate was nitrogen gas.

[0090] 2. Grain size test method 50 mg of negative electrode active material (i.e., graphite coated or uncoated with amorphous carbon) is mixed with 5 drops of a 1% mass fraction aqueous solution of ethylphenyl polyethylene glycol and 20 mL of deionized water, thoroughly dispersed and homogenized, and the particle size volume distribution is measured using a laser granulometer. The particle refractive index is 2.68, the solvent refractive index is 1.33, and the light blocking rate is 5-10%. The particle size at which the occupancy ratio from the smallest to the largest in the volume particle size distribution is 50% is the particle size of the material.

[0091] 3. Test method for amorphous carbon coating amount Using the negative electrode active material (i.e., graphite coated with amorphous carbon) as a sample, the coking value of the coating agent is measured with reference to the Chinese national standard GB / T8727-2008, and the product of the coking value and the amount of coating agent added is calculated to determine the amount of amorphous carbon coating.

[0092] 4. Graphitization degree test method Using the Chinese National Standard GB / T 24533-2019 as a reference, 0.15 g of silicon powder and 0.35 g of negative electrode active material (i.e., amorphous carbon-coated or uncoated graphite) were thoroughly ground in an agate mortar for 10 minutes and then placed on a sample stage to flatten. The sample was scanned using an X-ray diffractometer at a scanning angle of 25-30° with a step width of 0.02°. The 111 peak of silicon was used to calibrate the 002 peak of graphite, and the 2θ value of the 002 peak was obtained. The 002 interlayer distance was then calculated using the Bragg equation. The graphitization degree was calculated using the graphitization degree formula.

[0093] 5.Porosity test method for electrode sheets The porosity of the negative electrode sheet is measured using the mercury intrusion method, with reference to the Chinese national standard GB / T 21650.1-2008 / ISO 15901-1:2005.

[0094] 6. Test method for room temperature DC resistance (DCR) and low temperature DC resistance (DCR) In a constant temperature environment of 25°C, adjust the state of charge (SOC) of a lithium-ion battery to 50%, leave it to stand for 1 hour, and record the voltage V0 after standing. Discharge it for 10 seconds at a rate of 10C (current I0), record the voltage V1 after discharge, and use (V1 - V0) / I0 as the room temperature DCR. In a constant temperature environment of 25°C, adjust the state of charge (SOC) of a lithium-ion battery to 50%, adjust the temperature to -20°C, leave it to stand for 3 hours, and record the voltage V2 after standing. Discharge it for 10 seconds at a rate of 4C (current I1), record the voltage V3 after discharge, and use (V3 - V2) / I1 as the low temperature DCR.

[0095] [Table 2]

[0096] Room temperature direct current resistance (DCR) and low temperature direct current resistance (DCR) are parameters used to evaluate the output performance of a battery. The lower the room temperature DCR and low temperature DCR values, the better the output performance of the battery.

[0097] In Comparative Example 1, the graphite was not coated with amorphous carbon, and the particle size of the graphite was too high, resulting in an excessively low porosity P of the electrode sheet. As a result, the room temperature DCR and low temperature DCR of the lithium ion secondary battery were relatively high, and the output performance was poor.

[0098] In Comparative Example 2, graphite was not coated with amorphous carbon, and the porosity P of the electrode sheet was too low, resulting in relatively poor output performance of the battery.

[0099] Comparative Example 3 uses graphite that is not coated with amorphous carbon, and the room temperature DCR and low temperature DCR of the lithium ion secondary battery are relatively high, and the output performance is poor.

[0100] Comparative Example 4 employed graphite coated with amorphous carbon, but the porosity P of the electrode sheet was too low, so the room temperature DCR and low temperature DCR of the lithium ion secondary battery were still poor.

[0101] Comparative Example 5 also employed graphite coated with amorphous carbon, but the degree of graphitization of the amorphous carbon-coated graphite was somewhat high, the particle size D was excessively high, and the porosity P of the electrode sheet was excessively low, resulting in less than ideal room temperature DCR and low temperature DCR of the lithium ion secondary battery.

[0102] In Examples 1 to 10, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and porosity P of the electrode sheet of the amorphous carbon-coated graphite were all relatively appropriate, and the room temperature DCR and low temperature DCR of the lithium ion secondary batteries were reduced, improving the battery output performance.

[0103] In Examples 11 to 20, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and porosity P of the electrode sheet of the amorphous carbon-coated graphite were all more appropriate, and the test results showed that the room temperature DCR and low temperature DCR of the lithium ion secondary battery were further reduced, and the output performance was good.

[0104] In Example 21, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and electrode sheet porosity P of the amorphous carbon-coated graphite are all very suitable, but these parameters do not satisfy the formula "0.020≦P×(S×C+2×(1−G) / D)≦0.030." The resulting lithium-ion secondary battery has small room temperature and low temperature DCRs and good power performance.

[0105] In Examples 22 and 23, the amorphous carbon coating amount C, graphitization degree G, specific surface area S, particle size D, and electrode sheet porosity P of the amorphous carbon-coated graphite all have very suitable values, and these parameters further satisfy the formula "0.020≦P×(S×C+2×(1−G) / D)≦0.030." Compared to Example 21, the room temperature DCR and low temperature DCR of Examples 22 and 23 are further reduced, and their output performance is excellent.

[0106] The above-described embodiments are merely specific examples of the present invention and are not intended to limit the scope of the patent of the present invention. Any equivalent transformation using the present invention, or any direct or indirect application in other related technical fields, is also included in the scope of patent protection 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 a surface of the negative electrode current collector, the negative electrode active material layer contains a negative electrode active material, the negative electrode active material includes graphite coated with amorphous carbon, The porosity P of the negative electrode is 34.0 to 55.0%, The specific surface area S of the negative electrode active material is 0.80 to 3.20 m ２ / g, The coating amount C of the amorphous carbon of the negative electrode active material is 0.80 to 3.35%; The degree of graphitization G of the negative electrode active material is 83.0 to 95.0%, The particle size D of the negative electrode active material is 3.0 to 14.0 μm. Negative electrode for lithium-ion secondary batteries.

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

3. The porosity P of the negative electrode, the specific surface area S of the negative electrode active material, the coating amount C of the amorphous carbon, the degree of graphitization G, and the particle size D are calculated by the following formulas: 0.020≦P×(S×C+2×(1-G) / D)≦0.030 The negative electrode for a lithium ion secondary battery according to claim 1 , which satisfies the above.

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

5. the weight ratio of the negative electrode active material to the negative electrode active material layer is 0.92 to 0.97; The negative electrode for a lithium ion secondary battery according to claim 1 .

6. the negative electrode active material layer further contains an additive, the additives include a conductive agent, a binder, a thickener, or a combination thereof; the conductive agent comprises conductive carbon black, carbon nanotubes, carbon fibers, acetylene black, ketjen black, or a combination thereof; the binder comprises styrene butadiene rubber, acrylonitrile butadiene rubber, methyl methacrylate butadiene rubber, methyl methacrylate rubber, or a combination thereof; The thickener includes polyvinyl alcohol and its modifications, starch and its modifications, cellulose derivatives, polyvinylpyrrolidone, polyacrylic acid (salts), polyethylene glycol, or a combination thereof. The negative electrode for a lithium ion secondary battery according to claim 1 .

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

8. A method for preparing a negative electrode for a lithium ion secondary battery according to any one of claims 1 to 7, comprising: providing an anode slurry containing an anode active material and optional additives; applying the negative electrode slurry onto a negative electrode current collector and then performing a drying treatment to obtain a negative electrode coated sheet; rolling the negative electrode coated sheet with a roll to obtain a negative electrode for a lithium ion secondary battery; A preparation method comprising:

9. A lithium ion secondary battery including a power generating element, The power generating element is The negative electrode for a lithium ion secondary battery according to any one of claims 1 to 7, A positive electrode and Electrolyte, and Separator, and A lithium-ion secondary battery comprising:

10. An electrically powered device comprising the lithium ion secondary battery according to claim 9.

Images