Negative active material, preparation method thereof, negative slurry, negative electrode sheet, secondary battery, and electric device

Abstract

The application discloses a negative electrode active material and a preparation method thereof, a negative electrode slurry, a negative electrode sheet, a secondary battery and an electric device, and relates to the technical field of batteries. The negative electrode active material comprises a carbon coating layer and an inner core arranged in the carbon coating layer, and the material of the inner core comprises graphite and a first alkali metal salt. The first alkali metal salt and the graphite help to form a thin and dense SEI film. The alkali metal can be embedded in the edge of the graphite particle, expand the graphite layer spacing, and help the embedding and extraction of lithium ions. Due to the electrostatic shielding effect, the alkali metal ion can effectively inhibit the growth of lithium dendrites, form a uniform and smooth SEI film, help to improve the attenuation of the negative electrode, and improve the cycle performance. The carbon coating layer helps the embedding and extraction of lithium ions, on the one hand, improves the rate performance and cycle performance of the graphite; and on the other hand, forms a synergistic effect with the first alkali metal salt, helps to form a more stable SEI film, and reduces the irreversible lithium loss.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a negative electrode active material and its preparation method, a negative electrode slurry, a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] The cycle performance of a battery is closely related to the state of the SEI film on the negative electrode surface, with the quality of the initial film formation having a significant impact on battery life. Furthermore, as the battery is used and ages, the SEI film on the negative electrode continuously breaks down and grows, leading to lithium dendrite growth and a rapid decline in battery performance. Summary of the Invention

[0003] This application is made in view of the above-mentioned problems, and its purpose is to provide a negative electrode active material that promotes the formation of a thin and dense SEI during the initial film formation process of the negative electrode, and inhibits the growth of lithium dendrites during cycling, reduces the repeated breakage and growth of SEI, and extends the battery cycle performance.

[0004] To achieve the above objectives, embodiments of this application provide a negative electrode active material and its preparation method, a negative electrode slurry, a negative electrode sheet, a secondary battery, and an electrical device.

[0005] In a first aspect, embodiments of this application propose a negative electrode active material, including a carbon coating layer and a core disposed within the carbon coating layer, wherein the core is made of graphite and a first alkali metal salt.

[0006] Therefore, in the technical solution of this application embodiment, the first alkali metal salt and graphite, during the initial film formation process, react with fluorine in the electrolyte to form an alkali metal fluoride salt within the SEI, which helps to form a thin and dense SEI film, improving the Young's modulus and ion migration rate of the SEI film. Simultaneously, the alkali metal can embed into the edges of graphite particles, expanding the graphite interlayer spacing, which facilitates lithium ion insertion and extraction. During the aging process of the battery cell, due to the electrostatic shielding effect, alkali metal ions can effectively protect the surface structure of the negative electrode, inhibit lithium dendrite growth, and form a uniform and smooth SEI film, which helps to improve the degradation of the negative electrode and enhance cycle performance. Furthermore, the carbon coating layer facilitates lithium ion insertion and extraction, improving the rate performance and cycle performance of graphite on one hand; on the other hand, it synergistically interacts with the first alkali metal salt to help form a more stable SEI film and reduce irreversible lithium loss.

[0007] In any embodiment, the first alkali metal salt includes at least one of NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3. Experiments show that using at least one of the above alkali metal salts can effectively protect the surface structure of the negative electrode, inhibit the growth of lithium dendrites, form a uniform and smooth SEI film, and help improve the degradation of the negative electrode and enhance cycle performance.

[0008] The mass ratio of the first alkali metal salt to the mass of the graphite is X, where 0.5% ≤ X ≤ 5%, and further, 0.8% ≤ X ≤ 3%. This effectively protects the surface structure of the negative electrode, inhibits lithium dendrite growth, forms a uniform and smooth SEI film, and helps improve the attenuation of the negative electrode and enhance cycle performance.

[0009] The mass ratio of the carbon coating to the graphite is Z, where 2% ≤ Z ≤ 15%, and further, 5% ≤ X ≤ 12%. The carbon coating facilitates the insertion and extraction of lithium ions, improving the rate performance and cycle performance of graphite on the one hand; on the other hand, it synergizes with the first alkali metal salt to help form a more stable SEI film and reduce irreversible lithium loss.

[0010] In any embodiment, the carbon coating layer is made of at least one of pitch and phenolic resin. Using at least one of pitch and phenolic resin facilitates lithium-ion insertion and extraction, improving both the rate performance and cycle performance of graphite. Furthermore, it synergizes with the first alkali metal salt to form a more stable SEI film, reducing irreversible lithium loss.

[0011] In any embodiment, the mass ratio of the first alkali metal salt to the mass of graphite is X, and the specific gravity of the graphite is v, 1.6 × 10⁻⁶. -6 ≤X / v≤1.6×10 -4 Optionally, 8×10- 6 ≤X / v≤3×10 -5 Therefore, when the relationship between X and v satisfies 1.6 × 10 -6 ≤X / v≤1.6×10 -4 During the initial film formation process, alkali metal fluoride salts are formed within the SEI, contributing to the formation of a thin and dense SEI film. Simultaneously, a small amount of alkali metal can embed into the edges of graphite particles, widening the interlayer spacing and facilitating lithium ion insertion and extraction. Furthermore, during cell cycling, due to electrostatic shielding, alkali metal ions effectively protect the negative electrode surface structure, inhibiting lithium dendrite growth and forming a uniform and smooth SEI film, which helps improve negative electrode degradation and enhance cycle performance. When X / v < 1.6 × 10⁻⁶-6 When the content of dopant ions is too low, a dense SEI layer cannot be formed, and no improvement effect can be achieved during cycling. When X / v > 1.6 × 10 -4 Excessive doping can hinder lithium-ion transport in the SEI and graphite, and reduce the specific capacity.

[0012] Secondly, this application provides a method for preparing the negative electrode active material described in the above embodiments, comprising the following steps:

[0013] The first alkali metal salt, carbon source and graphite are mixed and then heated and carbonized under a protective atmosphere to obtain the negative electrode active material.

[0014] By mixing the three substances, heating and carbonizing them, the organic matter in the graphite is decomposed to form residual carbon, which improves the kinetic properties of the graphite. The three substances are then sintered together to form a coating structure.

[0015] In any embodiment, the step of mixing the first alkali metal salt, carbon source, and graphite, and then heating and carbonizing them under a protective atmosphere to obtain the negative electrode active material includes:

[0016] The first alkali metal salt and pitch were mixed to obtain a mixture;

[0017] After mixing graphite and the aforementioned mixture, the mixture is heated and carbonized under a protective atmosphere to obtain a negative electrode active material. This process enhances the kinetic properties of graphite and sintersects the three substances together to form a coated structure.

[0018] In any embodiment, after mixing the graphite and the mixture in the step, the mixture is heated and carbonized under a protective atmosphere to obtain the negative electrode active material.

[0019] The heating rate is less than or equal to 10℃ / 111; the heating temperature is 800-81300℃; and the heating time is 286 hours. Under these conditions, a negative electrode active material with complete coating and good morphology can be obtained.

[0020] Thirdly, embodiments of this application provide a negative electrode slurry, including the negative electrode active material described in the first aspect of embodiments of this application.

[0021] In any embodiment, the negative electrode slurry further includes an additive, which comprises a second alkali metal salt. The first alkali metal salt forms an alkali metal fluoride salt within the SEI during the initial film formation process, contributing to the formation of a thin and dense SEI film. The second alkali metal salt improves the battery's cycle performance; the two work synergistically to promote the formation of a thin and dense SEI on the negative electrode during the initial film formation process, thereby extending the battery's cycle performance.

[0022] In any embodiment, the second alkali metal salt includes at least one of NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3. Experiments show that using at least one of the above alkali metal salts can effectively improve the cycle performance of the battery.

[0023] In any embodiment, the mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.01% ≤ Y ≤ 5%. Within this range, the cycle performance of the battery can be effectively improved.

[0024] In any embodiment, the mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.55% ≤ X + Y ≤ 5%. Studies have shown that by controlling the sum of the amounts of both within the above range, X mainly targets the initial film formation, while Y mainly targets cycle improvement, resulting in a better synergistic effect between the two.

[0025] Fourthly, embodiments of this application provide a negative electrode sheet, including the negative electrode active material of the first aspect of this application.

[0026] Fifthly, embodiments of this application provide a secondary battery, including the negative electrode sheet of the fourth aspect of this application.

[0027] Sixthly, embodiments of this application provide an electrical device, including a secondary battery according to embodiments of the fifth aspect of this application. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a scanning electron microscope image of the negative electrode of the battery in Embodiment 1 of this application after 100 cycles;

[0030] Figure 2 This is a scanning electron microscope image of the negative electrode of the battery in Embodiment 2 of this application after 100 cycles;

[0031] Figure 3 This is a scanning electron microscope image of the negative electrode of the battery in Comparative Example 1 of this application after 100 cycles;

[0032] Figure 4This is a scanning electron microscope image of the negative electrode of the battery in Comparative Example 2 of this application after 100 cycles;

[0033] Figure 5 This is a scanning electron microscope image of the negative electrode of the battery in Comparative Example 3 of this application after 100 cycles.

[0034] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0035] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the adhesive, separator, electrode, electrode assembly, battery cell, battery, and power supply device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0036] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0037] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0038] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0039] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0040] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0041] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0042] The anode material is one of the key factors in evaluating the overall performance of lithium-ion batteries. Currently, the main anode materials used in commercially available lithium-ion batteries are carbon-based materials, including easily graphitized carbon, difficult-to-graphitize carbon, and graphite. Graphite materials have good electrical conductivity, high crystallinity, high theoretical lithium intercalation capacity, and a good layered structure and charge / discharge voltage plateau, making them one of the key materials for lithium-ion battery research in recent years. The cycle performance of the battery is closely related to the state of the SEI film on the anode surface, with the quality of the initial film formation having a significant impact on battery life. Furthermore, as the battery is used and ages, the SEI film on the anode will continuously break down and grow, leading to lithium dendrite growth and rapid performance degradation.

[0043] Therefore, research on graphite is constantly emerging. For example, there is the preparation and application of a lithium battery anode material with asphalt liquid-phase coating and modification of artificial graphite. The method includes mechanically crushing petroleum coke, classifying it, mixing it with an appropriate amount (3%-88%) of asphalt powder, reacting it in a medium-temperature reactor at 500-8650℃ for 68-10 hours, then performing a second shaping and classification treatment, followed by a high-temperature graphitization treatment at 2400-83000℃ for 368-72 hours. Finally, after classification, unmodified artificial graphite material is obtained. Then, the unmodified artificial graphite material and asphalt are further dissolved in a dispersant and stirred to obtain a homogeneous mixture. The homogeneous mixture is then processed into a composite powder using a closed-loop spray dryer, followed by heating at 700-1100℃ for 1-5 hours to finally obtain the asphalt liquid-phase coated and modified artificial graphite material.

[0044] However, the modified graphite prepared above still suffers from the problem of continuous breakage and growth of the SEI film and lithium dendrite growth at the negative electrode. In other words, the inventors found in their practical work that the graphite used in the prior art has poor performance during the first film formation, resulting in continuous breakage and growth of the SEI film and lithium dendrite growth at the negative electrode, leading to rapid degradation of battery performance.

[0045] Surprisingly, after extensive experimentation, the inventors discovered that adding alkali metal salts to graphite facilitates the formation of alkali metal fluoride salts within the SEI during the initial film formation process, contributing to the formation of a thin and dense SEI film. Simultaneously, the alkali metals can embed into the edges of graphite particles, widening the interlayer spacing and aiding in the insertion and extraction of lithium ions. During the aging process of the battery cell, due to electrostatic shielding, alkali metal ions effectively protect the surface structure of the negative electrode, inhibiting lithium dendrite growth and forming a uniform and smooth SEI film, thus helping to improve negative electrode degradation and enhance cycle performance.

[0046] Based on this, this application provides a negative electrode active material and its preparation method, a negative electrode slurry, a negative electrode sheet, a secondary battery, and an electrical device.

[0047] In a first aspect, this application proposes a negative electrode active material, comprising a carbon coating layer and a core disposed within the carbon coating layer, wherein the core is made of graphite and a first alkali metal salt.

[0048] Therefore, in the technical solution of this application embodiment, the first alkali metal salt and graphite, during the initial film formation process, react with fluorine in the electrolyte to form an alkali metal fluoride salt within the SEI, which helps to form a thin and dense SEI film, improving the Young's modulus and ion migration rate of the SEI film. Simultaneously, the alkali metal can embed into the edges of graphite particles, expanding the graphite interlayer spacing, which facilitates lithium ion insertion and extraction. During the aging process of the battery cell, due to the electrostatic shielding effect, alkali metal ions can effectively protect the surface structure of the negative electrode, inhibit lithium dendrite growth, and form a uniform and smooth SEI film, which helps to improve the degradation of the negative electrode and enhance cycle performance. Furthermore, the carbon coating layer facilitates lithium ion insertion and extraction, improving the rate performance and cycle performance of graphite on one hand; on the other hand, it synergistically interacts with the first alkali metal salt to help form a more stable SEI film and reduce irreversible lithium loss.

[0049] This application does not limit the type of graphite. Graphite materials are divided into two main categories: natural graphite and artificial graphite. Compared with natural graphite, artificial graphite has a larger interlayer spacing and a lower degree of graphitization (≤93%). Easily graphitizable carbon is obtained through high-temperature graphitization. At the same time, artificial graphite has a rough and porous surface, a large specific surface area, and is more sensitive to solvents in the electrolyte, resulting in lower initial efficiency and specific capacity (≤3501 Ah / g) for artificial graphite.

[0050] Understandably, this application does not limit the arrangement of graphite and the first alkali metal salt in the core. They can be a uniform mixture or a coating. In this embodiment, the first alkali metal salt is distributed on the outer surface of the graphite. During the initial film formation process, alkali metal fluoride salts are formed within the SEI, which further facilitates the formation of a thin and dense SEI film. Simultaneously, alkali metals can more easily embed into the edges of graphite particles, increasing the interlayer spacing and aiding in the insertion and extraction of lithium ions. During the aging process of the battery cell, due to electrostatic shielding, alkali metal ions can effectively protect the surface structure of the negative electrode, inhibit lithium dendrite growth, and form a uniform and smooth SEI film, which helps improve the degradation of the negative electrode and enhance cycle performance.

[0051] In any embodiment, the first alkali metal salt includes at least one of NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3. Experiments show that using at least one of the above alkali metal salts can effectively protect the surface structure of the negative electrode, inhibit the growth of lithium dendrites, form a uniform and smooth SEI film, and help improve the degradation of the negative electrode and enhance cycle performance.

[0052] The mass ratio of the first alkali metal salt to the mass of the graphite is X, where 0.5% ≤ X ≤ 5%. For example, X can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc. Further, 0.8% ≤ X ≤ 3%. Within the above range, the surface structure of the negative electrode can be effectively protected, lithium dendrite growth can be inhibited, a uniform and smooth SEI film can be formed, which helps to improve the degradation of the negative electrode and enhance cycle performance.

[0053] The mass ratio of the carbon coating to the mass of the graphite is Z, where 2% ≤ Z ≤ 15%. For example, Z can be 2%, 3%, 5%, 7%, 8%, 10%, 12%, 13%, 14%, 15%, etc., and further, 5% ≤ X ≤ 12%. The carbon coating facilitates the insertion and extraction of lithium ions, improving the rate performance and cycle performance of graphite on the one hand; on the other hand, it synergizes with the first alkali metal salt to help form a more stable SEI film and reduce irreversible lithium loss.

[0054] In any embodiment, the carbon coating layer is made of at least one of pitch and phenolic resin. Using at least one of pitch and phenolic resin facilitates lithium-ion insertion and extraction, improving both the rate performance and cycle performance of graphite. Furthermore, it synergizes with the first alkali metal salt to form a more stable SEI film, reducing irreversible lithium loss.

[0055] In any embodiment, the mass ratio of the first alkali metal salt to the mass of graphite is X, and the specific gravity of the graphite is v, 1.6 × 10⁻⁶. -6 ≤X / v≤1.6×10 -4 For example, it could be 1.6 × 10 -6 5×10 -6 1×10 -5 1.6×10 -5 5×10 -5 1.6×10 -4 Optionally, 8×10- 6 ≤X / v≤3×10 -5 As the demand for lithium-ion energy density increases, the specific capacity of graphite anode materials has gradually improved, reaching 3601 Ah / g, approaching the theoretical specific capacity of 3721 Ah / g. However, high specific capacity graphite is often accompanied by continuous breakage and regrowth of the SEI (Sediment-Insulated Plate), leading to rapid capacity decay and severely affecting the cycle life of the battery cell. Therefore, it is necessary to optimize its performance by doping and modifying graphite.

[0056] Therefore, when the relationship between X and v satisfies 1.6 × 10 -6 ≤X / v≤1.6×10 -4During the initial film formation process, alkali metal fluoride salts are formed within the SEI, contributing to the formation of a thin and dense SEI film. Simultaneously, a small amount of alkali metal can embed into the edges of graphite particles, widening the interlayer spacing and facilitating lithium ion insertion and extraction. Furthermore, during cell cycling, due to electrostatic shielding, alkali metal ions effectively protect the negative electrode surface structure, inhibiting lithium dendrite growth and forming a uniform and smooth SEI film, which helps improve negative electrode degradation and enhance cycle performance. When X / v < 1.6 × 10⁻⁶ -6 When the content of dopant ions is too low, a dense SEI layer cannot be formed, and no improvement effect can be achieved during cycling. When X / v > 1.6 × 10 -4 Excessive doping can hinder lithium-ion transport in the SEI and graphite, and reduce the specific capacity.

[0057] In other words, this application allows for the addition of different amounts of the first alkali metal salt based on different graphite weights. During the initial film formation process, alkali metal fluoride salts are formed within the SEI, which helps to form a thin and dense SEI film. This improves the applicability to different types of graphite.

[0058] Secondly, this application provides a method for preparing the negative electrode active material described in the above embodiments, comprising the following steps:

[0059] The first alkali metal salt, carbon source and graphite are mixed and then heated and carbonized under a protective atmosphere to obtain the negative electrode active material.

[0060] By mixing the three substances, heating and carbonizing them, the organic matter in the graphite is decomposed to form residual carbon, which improves the kinetic properties of the graphite. The three substances are then sintered together to form a coating structure.

[0061] In any embodiment, the step of mixing the first alkali metal salt, carbon source, and graphite, and then heating and carbonizing them under a protective atmosphere to obtain the negative electrode active material includes:

[0062] The first alkali metal salt and pitch were mixed to obtain a mixture;

[0063] After mixing graphite and the aforementioned mixture, the mixture is heated and carbonized under a protective atmosphere to obtain a negative electrode active material. This process enhances the kinetic properties of graphite and sintersects the three substances together to form a coated structure.

[0064] The mixing equipment can be either a VC mixer or a fusion machine. The protective gas is one or a mixture of more than one of nitrogen, argon, or helium. The heating rate is less than 10℃ / 111, the carbonization temperature is 800-1300℃, and the carbonization time is 286h. Under the above conditions, a negative electrode active material with complete coating and good morphology can be obtained.

[0065] Furthermore, in this embodiment, after heating and carbonization, the material is naturally cooled to room temperature, and the undersize material obtained by sieving through a sieve is the negative electrode active material. The room temperature is less than or equal to 60°C. The sieve is a sieve with a mesh size greater than or equal to 325.

[0066] Thirdly, embodiments of this application provide a negative electrode slurry, including the negative electrode active material described in the first aspect of embodiments of this application.

[0067] In any embodiment, the negative electrode slurry further includes an additive, which comprises a second alkali metal salt. The first alkali metal salt forms an alkali metal fluoride salt within the SEI during the initial film formation process, contributing to the formation of a thin and dense SEI film. The second alkali metal salt improves the battery's cycle performance; the two work synergistically to promote the formation of a thin and dense SEI on the negative electrode during the initial film formation process, thereby extending the battery's cycle performance.

[0068] In any embodiment, the second alkali metal salt includes at least one of NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3. Experiments show that using at least one of the above alkali metal salts can effectively improve the cycle performance of the battery.

[0069] In any embodiment, the mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.01% ≤ Y ≤ 5%. For example, Y can be 0.01%, 0.05%, 0.1%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%. Within the above range, the cycle performance of the battery can be effectively improved.

[0070] In any embodiment, the mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.55% ≤ X + Y ≤ 5%. For example, X + Y can be 0.55%, 0.6%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%. Studies have shown that controlling the sum of the two additions within the above range allows X to primarily target the initial film formation, while Y primarily targets cycle improvement, resulting in a better synergistic effect between the two.

[0071] In addition, the negative electrode slurry also includes a binder, a conductive agent, a solvent, and an optional thickener. The binder can be a binder commonly used by those skilled in the art, and the conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The thickener can be, for example, sodium carboxymethyl cellulose (CMC-Na).

[0072] Fourthly, embodiments of this application provide a negative electrode sheet, including the negative electrode active material of the first aspect of this application.

[0073] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer being formed by coating the negative electrode slurry as described above onto the negative current collector. As an example, the negative current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0074] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0075] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as binder, conductive agent, negative electrode active material described in the first aspect of this application, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing, and other processes.

[0076] Fifthly, embodiments of this application provide a secondary battery, including the negative electrode sheet of the fourth aspect of this application.

[0077] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0078] The secondary battery can be prepared by methods commonly used in the art. For example, the positive electrode, negative electrode and separator can be made into an electrode assembly by a winding process or a stacking process, and then an electrolyte can be injected into the electrode assembly and sealed to obtain a secondary battery.

[0079] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector.

[0080] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0081] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0082] In some embodiments, the positive electrode active material may be a known positive electrode active material for batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as L1CoO2), lithium nickel oxides (such as L1N1O2), lithium manganese oxides (such as L1M1O2, L1M12O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as L1N1). 1 / 3 Co 1 / 3 M1 1 / 3 O2 (also known as NCM) 333 L1N1 0.5 Co 0.3 M1 0.2 O2 (also known as NCM) 523 L1N1 0.5 Co 0.25 M1 0.25 O2 (also known as NCM) 211 L1N1 0.6 Co 0.2 M1 0.2 O2 (also known as NCM) 622 L1N1 0.8 Co 0.1 M1 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as L1N1) 0.85 Co 0.15 Al0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as L1FePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as L1M1PO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0083] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive active material, conductive agent, binder described in the first aspect of this application, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive current collector, and then obtaining the positive electrode sheet after drying, cold pressing, and other processes.

[0084] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0085] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0086] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0087] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0088] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0089] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0090] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0091] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0092] At least one of the secondary batteries in the above embodiments can be assembled into a battery module and a battery pack. The number of battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0093] In a battery module, multiple secondary batteries can be arranged sequentially along the length of the module. Alternatively, they can be arranged in any other manner. Furthermore, these multiple secondary battery cells can be secured using fasteners.

[0094] In any embodiment, the battery module may further include a housing with a receiving space in which multiple secondary battery cells are housed.

[0095] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0096] Sixthly, embodiments of this application provide an electrical device, including a secondary battery according to embodiments of the fifth aspect of this application.

[0097] The secondary battery, battery pack, and battery module can all be used as power sources for the electrical device, or as energy storage units for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0098] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0099] The technical solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0100] The relevant parameters of the negative electrode active materials of Examples 1814 and Comparative Example 185 are shown in Table 1 below.

[0101] Table 1. Parameters of Example 1814 and Comparative Example 185

[0102]

[0103]

[0104] Example 1

[0105] Sodium carbonate, asphalt, and graphite were mixed in a mass ratio of 1:5:100. First, sodium carbonate and asphalt were added to a low-speed mixer and mixed at 100 rpm for 10 minutes to obtain a homogeneous mixture A. Then, mixture A and graphite were added to a high-speed mixer and mixed at 600 rpm for 60 minutes to obtain mixture B. Mixture B was placed under nitrogen protection and heated to 1000℃ at a heating rate of 5℃ / 1 minute and held at that temperature for 3 hours. Then, it was allowed to cool naturally to room temperature (30℃). The sample was then removed and sieved through a 325-mesh sieve. The material that passed through the sieve was the negative electrode active material.

[0106] The negative electrode active material, conductive agent Super-P, thickener CMC, binder SBR, and sodium carbonate are added to the solvent deionized water in a mass ratio of 96:1.0:1.0:1.5:0.5 and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry is coated on the current collector copper foil and dried at 85°C. Then, it is trimmed, cut into sheets, and slit. It is then dried under vacuum at 110°C for 4 hours. The tabs are welded to produce a secondary battery negative electrode sheet that meets the requirements.

[0107] Example 2

[0108] Cesium chloride, asphalt, and graphite were mixed in a mass ratio of 1:5:100. First, cesium chloride and asphalt were added to a low-speed mixer and mixed at 100 rpm for 10 minutes to obtain a homogeneous mixture A. Then, mixture A and graphite were added to a high-speed mixer and mixed at 600 rpm for 60 minutes to obtain mixture B. Mixture B was placed under nitrogen protection and heated to 1000℃ at a heating rate of 5℃ / 1 minute and held at that temperature for 3 hours. Then, it was allowed to cool naturally to room temperature (30℃). The sample was then removed and sieved through a 325-mesh sieve. The material that passed through the sieve was the negative electrode active material.

[0109] The negative electrode active material, conductive agent Super-P, thickener CMC, binder SBR, and cesium chloride are added to deionized water in a mass ratio of 96:1.0:1.0:1.5:0.5 and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry is coated on the current collector copper foil and dried at 85°C. Then, it is trimmed, cut into sheets, and slit. It is then dried under vacuum at 110°C for 4 hours. The tabs are welded to produce a secondary battery negative electrode sheet that meets the requirements.

[0110] Example 3

[0111] Sodium carbonate, asphalt, and graphite were mixed in a molar ratio of 1:5:100. First, sodium carbonate and asphalt were added to a low-speed mixer and mixed at 100 rpm for 10 minutes to obtain a homogeneous mixture A. Then, mixture A and graphite were added to a high-speed mixer and mixed at 600 rpm for 60 minutes to obtain mixture B. Mixture B was placed under nitrogen protection and heated to 1000℃ at a heating rate of 5℃ / 1 minute and held at that temperature for 3 hours. Then, it was allowed to cool naturally to room temperature (30℃). The sample was then removed and sieved through a 325-mesh sieve. The material that passed through the sieve was the negative electrode active material.

[0112] The negative electrode active material, conductive agent Super-P, thickener CMC, and binder SBR are added to deionized water in a mass ratio of 96.5:1.0:1.0:1.5 and mixed evenly to prepare a negative electrode slurry. The negative electrode slurry is coated on the current collector copper foil and dried at 85°C. Then, it is trimmed, cut into sheets, and slit. It is then dried under vacuum at 110°C for 4 hours and the tabs are welded to produce a secondary battery negative electrode sheet that meets the requirements.

[0113] The preparation of the negative electrode active material, negative electrode slurry and negative electrode sheet in Examples 4 to 14 is the same as in Example 1, except that they are prepared according to the parameters in Table 1.

[0114] The preparation of polymers, binders, and separators in Comparative Examples 1 to 5 were the same as in Example 1, except that they followed the parameters in Table 1. In Comparative Example 1, the negative electrode active material did not contain the first alkali metal, and the negative electrode slurry did not contain the second alkali metal. In Comparative Example 2, the negative electrode active material did not contain the first alkali metal, but the negative electrode slurry contained the second alkali metal. In Comparative Examples 3 and 4, the amount of the first alkali metal added was outside the range. Comparative Example 5 did not have a carbon coating layer.

[0115] The negative electrode sheets of Examples 1 to 14 and Comparative Examples 1 to 5 were subjected to the following operations and tests:

[0116] Preparation of the positive electrode of a secondary battery: 1) Polyvinylidene fluoride (PVDF) and lithium-ion positive electrode material L1N1 are used. 0.6 Co 0.2 M1 0.2O2 and conductive agent (Super P carbon black) are mixed in a mass ratio of 90:5:5, using N-methyl-pyrrolidone (NMP) as a solvent. The amount of solvent added is adjusted to control the slurry viscosity at 100-20000 Pa·s. The slurry is then coated onto the cathode current collector using a coating machine or sprayer. After drying at 85°C, it is cold-pressed, then trimmed, cut into sheets, and slit. It is then dried again under vacuum at 85°C for 4 hours, and the tabs are welded to produce a secondary battery positive electrode sheet that meets the requirements.

[0117] Preparation of electrolyte for secondary batteries: A mixture of ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) was used as a non-aqueous organic solvent, with the mass ratio of each component being EC:PC:DEC = 30:30:40. Lithium hexafluorophosphate (L1PF6) was used as the lithium salt to prepare an electrolyte with a concentration of 1M.

[0118] Preparation of the secondary battery: Using a 12μm polypropylene film as the separator, the positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The resulting structure is then wound into a square bare cell with a thickness of 811μm, a width of 6011μm, and a length of 13011μm. The bare cell is placed in an aluminum foil bag and vacuum-baked at 75℃ for 10 hours. Electrolyte is then injected, and the cell is vacuum-sealed and allowed to stand for 24 hours. Afterward, it is charged to 4.5V with a constant current of 0.1C (1601A), then charged at a constant voltage of 4.5V until the current drops to 0.05C (801A). It is then discharged to 2.8V with a constant current of 0.1C (1601A), and this charge-discharge cycle is repeated twice. Finally, it is charged to 3.8V with a constant current of 0.1C (1601A), thus completing the preparation of the secondary battery.

[0119] Cyclic performance test:

[0120] The cycle life test conditions were as follows: the secondary battery was subjected to 1C / 1C cycle test at 25℃ and 45℃, with a charge / discharge voltage range of 2.88-4.35V. The test was stopped when the capacity decayed to 80% of the initial discharge specific capacity. The cycle life at 25℃ and 45℃ was measured and obtained in Table 2.

[0121] Table 2 Performance measurements of lithium-ion batteries in Examples 1 to 14 and Comparative Examples 1 to 5

[0122]

[0123]

[0124] As can be seen from Table 2, by comparing the comparative examples and the embodiments, through Examples 1 to 14, it can be seen that the addition of the first alkali metal salt to the negative electrode active material significantly improves the cycle performance of the battery. Compared with Examples 3 and 1, the electrolyte does not contain the second alkali metal salt, and its cycle performance is somewhat reduced. However, compared with Comparative Examples 1 to 4, the cycle performance is still better. It can be seen that the addition of the second alkali metal salt to the electrolyte also has a synergistic effect on improving the cycle performance.

[0125] In Comparative Example 1, the negative electrode active material lacks the first alkali metal, and the negative electrode slurry lacks the second alkali metal. In Comparative Example 2, the negative electrode active material lacks the first alkali metal, but the negative electrode slurry contains the second alkali metal. In Comparative Examples 3 and 4, the amount of the first alkali metal added is outside the acceptable range. This indicates that the first alkali metal salt and graphite form alkali metal fluoride salts within the SEI during the initial film formation process, which helps to form a thin and dense SEI film. Simultaneously, the alkali metal can embed into the edges of graphite particles, expanding the graphite interlayer spacing and facilitating lithium ion insertion and extraction. During the cell aging process, due to electrostatic shielding, alkali metal ions can effectively protect the negative electrode surface structure, inhibit lithium dendrite growth, and form a uniform and smooth SEI film, helping to improve negative electrode degradation and enhance cycle performance. Comparative Example 5 lacks a carbon coating layer and exhibits poor cycle performance. This demonstrates that the carbon coating layer facilitates lithium ion insertion and extraction, improving both the rate performance and cycle performance of graphite, and synergistically interacting with the first alkali metal salt to form a more stable SEI film and reduce irreversible lithium loss.

[0126] After cycling at 25°C for 100 cycles, scanning electron microscopy (SEM) images were obtained for the negative electrode sheets of Examples 1 and 2 and Comparative Examples 1, 2, and 3, respectively. Figure 1 , 2 As can be seen from Comparative Examples 1 to 3, the negative electrode sheets of Comparative Examples 1 to 3 have many spots and are relatively rough, indicating that the SEI film formed on them is relatively uneven, with more side reactions, which is not conducive to the subsequent cycling of lithium ions and has poor cycle performance. Compared with Comparative Examples 1 to 3, the negative electrode sheets of Examples 1 and 2 are relatively smooth, which is conducive to cycling and has better cycle performance. It can be seen that the addition of the first alkali metal salt is crucial.

[0127] In summary, the negative electrode active material proposed in this application, in which the first alkali metal salt and graphite form an alkali metal fluoride salt within the SEI during the initial film formation process, facilitates the formation of a thin and dense SEI film. Simultaneously, the alkali metal can embed into the edges of graphite particles, expanding the interlayer spacing and aiding in lithium ion insertion and extraction. During cell aging, due to electrostatic shielding, alkali metal ions effectively protect the negative electrode surface structure, inhibiting lithium dendrite growth and forming a uniform and smooth SEI film, which helps improve negative electrode degradation and enhance cycle performance. The carbon coating layer facilitates lithium ion insertion and extraction, improving both the rate performance and cycle performance of graphite; furthermore, it synergizes with the first alkali metal salt to form a more stable SEI film and reduce irreversible lithium loss.

[0128] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A negative electrode active material, characterized in that, It includes a carbon coating layer and a core disposed within the carbon coating layer, wherein the core is made of graphite and a first alkali metal salt.

2. The negative electrode active material as described in claim 1, characterized in that, The first alkali metal salt includes at least one selected from NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3; and / or, The mass ratio of the first alkali metal salt to the mass of graphite is X, where 0.5% ≤ X ≤ 5%; and / or, The mass ratio of the carbon coating to the mass of the graphite is Z, where 2% ≤ Z ≤ 15%; and / or, The carbon coating material includes at least one of asphalt and phenolic resin.

3. The negative electrode active material as described in claim 1, characterized in that, The mass ratio of the first alkali metal salt to the mass of the graphite is X, where 0.8% ≤ X ≤ 3%; and / or, The ratio of the mass of the carbon coating to the mass of the graphite is Z, where 5% ≤ Z ≤ 12%.

4. The negative electrode active material as described in claim 1, characterized in that, The mass ratio of the first alkali metal salt to the mass of the graphite is X, and the specific gravity of the graphite is v, 1.6 × 10⁻⁶. -6 ≤X / v≤1.6×10 -4 .

5. The negative electrode active material as described in claim 1, characterized in that, The mass ratio of the first alkali metal salt to the mass of the graphite is X, and the specific gravity of the graphite is v, 8 × 10⁻⁶. 6 ≤X / v≤3×10 -5 .

6. A method for preparing a negative electrode active material as described in any one of claims 1 to 5, characterized in that, Includes the following steps: The first alkali metal salt, carbon source and graphite are mixed and then heated and carbonized under a protective atmosphere to obtain the negative electrode active material.

7. The method for preparing the negative electrode active material as described in claim 6, characterized in that, The first step involves mixing the alkali metal salt, carbon source, and graphite, and then heating and carbonizing the mixture under a protective atmosphere to obtain the negative electrode active material, which includes: The first alkali metal salt and pitch were mixed to obtain a mixture; After mixing graphite and the mixture, the mixture is heated and carbonized under a protective atmosphere to obtain a negative electrode active material.

8. The method for preparing the negative electrode active material as described in claim 7, characterized in that, After mixing graphite and the mixture in the step, the mixture is heated and carbonized under a protective atmosphere to obtain the negative electrode active material. The heating rate is less than or equal to 10°C / min; and / or, The heating temperature is 800~1300℃; and / or, The heating time is 2 to 6 hours.

9. A negative electrode slurry, characterized in that, Includes the negative electrode active material as described in any one of claims 1 to 5.

10. The negative electrode slurry as described in claim 9, characterized in that, The negative electrode slurry also includes additives, including a second alkali metal salt.

11. The negative electrode slurry as described in claim 10, characterized in that, The second alkali metal salt includes at least one selected from NaCl, NaNO3, Na2SO4, Na2CO3, KCl, KNO3, K2SO4, K2CO3, RbCl, RbNO3, Rb2SO4, Rb2CO3, CsCl, CsNO3, Cs2SO4, and Cs2CO3; and / or, The mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.01%≤Y≤5%.

12. The negative electrode slurry as described in claim 10, characterized in that, The mass ratio of the second alkali metal salt to the mass of the negative electrode active material is Y, where 0.55%≤X+Y≤5%.

13. A negative electrode sheet, characterized in that, Includes the negative electrode active material as described in any one of claims 1 to 5.

14. A secondary battery, characterized in that, Including the negative electrode sheet as described in claim 13.

15. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 14.

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