Composite negative plate, preparation method thereof and lithium ion battery

Abstract

The invention relates to a composite negative plate, a preparation method thereof and a lithium ion battery. The composite negative plate comprises a current collector and a negative active material layer arranged on at least one side surface of the current collector; a negative electrode active material in the negative electrode active material layer comprises a first negative electrode active material and a second negative electrode active material; the first negative active material comprises graphite and / or silicon; the second negative electrode active material comprises hard carbon; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material. Small-particle-size hard carbon and large-particle-size graphite are matched for use, the power performance of the composite negative electrode plate under the low SOC is improved by utilizing a terminal lithium storage mechanism of the hard carbon, the DCR of the composite negative electrode plate is reduced, meanwhile, the hard carbon can be inserted into gaps of the graphite, the compaction density of the composite negative electrode plate is improved, and the service life of the composite negative electrode plate is prolonged. Therefore, the lithium ion battery is ensured to have relatively high energy density.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to composite negative electrode sheets, and more particularly to a composite negative electrode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] In lithium-ion batteries, the negative electrode active material is mainly graphite or a mixture of graphite and silicon. These batteries exhibit excellent kinetic performance at high states of charge (SOC), characterized by low DC internal resistance and high discharge power. However, when the battery is at a low SOC, such as <15%, its DC resistance (DCR) increases significantly, and the discharge power drops sharply. This is primarily due to the poor intrinsic kinetics of graphite or silicon materials in the high delithiation state. This problem severely limits the application of these batteries in scenarios requiring high-rate discharge to low-charge conditions.

[0003] CN118983518A discloses an electrolyte and a lithium-ion battery for a high-power lithium-ion battery. The electrolyte comprises a lithium salt, an organic solvent, and additives. The lithium salt includes lithium dioxalatoborate, and the additives include fluorofluorophosphate-based triazine organic compounds. The electrolyte and lithium-ion battery provided by this invention, when lithium dioxalatoborate is used in combination with fluorofluorophosphate-based triazine organic compounds, can form an SEI layer rich in inorganic substances such as F, P, and B. Compared to organic SEI, this inorganic SEI is thinner and denser, exhibiting rapid Li-ion conductivity. + This reduces charge migration resistance and makes the battery less prone to cracking during cycling, thus improving the battery's power performance.

[0004] CN114725349A discloses the preparation of a modified cathode material for lithium-ion batteries and the application of the obtained material, including LNCM811 (LiNi). 0.8 Co 0.1 Mn 0.1 The LNCM811 cathode material contains a ternary cathode material (O2) and a composite coating layer on its surface, wherein the composite coating layer is LaCo / NFCo-CDs. Modifying the LNCM811 cathode material with a LaCo / NFCo-CDs coating layer enhances the structural stability of the material, optimizes the particle morphology of the LNCM811 cathode material, and produces particles with narrow, rounded, and angular particle size distribution. This effectively improves the stress concentration along the C-axis of the layered structure during charge and discharge, enhances the reversibility of the H2-H3 phase transition in the material, and solves the problem of high DCR growth under low temperature, low SOC, and long cycling conditions.

[0005] CN115000386A discloses a negative electrode active material, a negative electrode sheet, a lithium-ion battery, and an electrical device. This invention utilizes a specific surface area of ​​1.0~8.0 m². 2 The lithium-ion battery prepared with a dibutyl phthalate / 100g negative electrode active material has excellent high-rate charge-discharge performance, power performance and high-temperature storage performance, which can effectively meet the lithium-ion battery requirements of hybrid vehicles and has broad application prospects.

[0006] Existing technologies typically improve the kinetics of lithium-ion batteries to some extent by optimizing the electrolyte, cathode material, or increasing the electrode surface area. However, these methods are either costly, have limited effectiveness, or may sacrifice the battery's energy density or high-temperature performance.

[0007] Therefore, there is an urgent need for an economical and efficient solution that can effectively improve the power performance of lithium-ion batteries, especially their kinetic performance at low SOC, without significantly sacrificing battery energy density. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a composite negative electrode sheet, its preparation method, and a lithium-ion battery. This invention utilizes a combination of small-particle-size hard carbon and large-particle-size graphite. By leveraging the "end-of-cell" lithium storage mechanism of hard carbon, the power performance of the composite negative electrode sheet at low SOC is improved, reducing the DCR of the composite negative electrode sheet. Simultaneously, the hard carbon can be interspersed within the gaps in the graphite, increasing the compaction density of the composite negative electrode sheet, thereby ensuring a high energy density for the lithium-ion battery.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a composite negative electrode sheet, the composite negative electrode sheet comprising a current collector and a negative electrode active material layer disposed on at least one side surface of the current collector; the negative electrode active material layer comprises a first negative electrode active material and a second negative electrode active material; the first negative electrode active material comprises graphite and / or silicon; the second negative electrode active material comprises hard carbon; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material.

[0011] Compared to graphite and silicon, hard carbon has a large number of nanopores and defect sites. Its lithium storage mechanism is different from the interlayer intercalation of graphite or the alloying reaction between silicon anode and lithium metal. Even when the overall SOC of the battery is very low, lithium ions can still be rapidly intercalated and rapidly deintercalated at the nanopores and defect sites of hard carbon.

[0012] This invention introduces hard carbon into a negative electrode sheet comprising graphite and / or silicon, effectively providing lithium ions with an additional, kinetically optimized "fast channel" and "reservoir." Utilizing the "end-of-life" lithium storage mechanism of hard carbon, it effectively improves the power performance of the composite negative electrode sheet at low SOC and reduces the DCR of the composite negative electrode sheet. Hard carbon and graphite and / or silicon form a complementary effect. Graphite and / or silicon ensure high capacity and high energy density of the battery in the high to medium SOC range, while hard carbon effectively optimizes the power performance in the low SOC range, thereby achieving a more stable and superior power output across the entire SOC range. Furthermore, this invention uses small-particle-size hard carbon in combination with large-particle-size graphite and / or silicon. The hard carbon can be interspersed in the gaps between graphite and / or silicon, increasing the compaction density of the composite negative electrode sheet, thereby improving the volumetric energy density of the battery. This compensates for the decrease in battery mass energy density caused by the introduction of hard carbon, maintaining a high energy density in the lithium-ion battery.

[0013] Preferably, in the negative electrode active material, the mass percentage a of the first negative electrode active material is 80wt%~90wt%, and the mass percentage b of the second negative electrode active material is 10wt%~20wt%.

[0014] Preferably, the D50 particle size of the first negative electrode active material is 5μm~20μm, and more preferably 6μm~10μm.

[0015] Preferably, the D50 particle size of the second negative electrode active material is 2μm~15um, and more preferably 3μm~9um.

[0016] Preferably, if the compaction density of the negative electrode sheet with the same loading amount of the first negative electrode active material as the composite negative electrode sheet is x, and the compaction density of the negative electrode sheet with the same loading amount of the second negative electrode active material as the composite negative electrode sheet is y, then the compaction density of the composite negative electrode sheet is w = a×x + b×y.

[0017] Preferably, the compaction density x of the negative electrode sheet having the same loading of the first negative electrode active material as the composite negative electrode sheet is 1.3 g / cm³. 3 ~1.6g / cm 3 .

[0018] Preferably, the compaction density γ of the negative electrode sheet having the same loading of the second negative electrode active material as the composite negative electrode sheet is 0.8 g / cm³. 3 ~1.0g / cm 3 .

[0019] Preferably, the compaction density of the composite negative electrode sheet is 1.2 g / cm³. 3 ~1.55g / cm 3 .

[0020] Preferably, the negative electrode active material layer of the composite negative electrode sheet further includes a binder and a conductive agent, and the total mass of the first negative electrode active material and the second negative electrode active material is in the mass ratio of the binder and the conductive agent as (92~97):(1~3):(2~5).

[0021] Preferably, the adhesive comprises any one or a combination of at least two of polyacrylic acid, sodium carboxymethyl cellulose, or styrene-butadiene rubber.

[0022] Preferably, the conductive agent includes any one or a combination of at least two of conductive carbon black, Ketjen black, carbon nanotubes, or graphene.

[0023] Preferably, the current collector includes a copper current collector.

[0024] In a second aspect, the present invention provides a method for preparing a composite negative electrode sheet as described in the first aspect, the method comprising:

[0025] A negative electrode slurry is coated onto at least one side of the current collector and dried to obtain the composite negative electrode sheet; the negative electrode slurry includes a first negative electrode active material and a second negative electrode active material; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material.

[0026] Preferably, the method for preparing the negative electrode slurry includes dispersing a first negative electrode active material, a second negative electrode active material, a binder, and a conductive agent in a solvent.

[0027] Preferably, the solvent includes water.

[0028] Preferably, the solid content of the negative electrode slurry is 45% to 65%.

[0029] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the composite negative electrode sheet as described in the first aspect.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] This invention introduces hard carbon into a negative electrode sheet comprising graphite and / or silicon, effectively providing lithium ions with an additional, kinetically optimized "fast channel" and "reservoir." Utilizing the "end-of-life" lithium storage mechanism of hard carbon, it effectively improves the power performance of the composite negative electrode sheet at low SOC and reduces the DCR of the composite negative electrode sheet. Simultaneously, this invention uses small-particle-size hard carbon in combination with large-particle-size graphite and / or silicon, allowing the hard carbon to intersect within the gaps in the graphite and / or silicon, increasing the compaction density of the composite negative electrode sheet and thus improving the volumetric energy density of the battery. This compensates for the decrease in battery mass energy density caused by the introduction of hard carbon, maintaining a high energy density in the lithium-ion battery. Detailed Implementation

[0032] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0033] The "range" disclosed in this invention can be defined in the form of 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 the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning 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 specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" 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" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0034] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0036] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0037] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can 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.

[0038] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0039] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0040] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0041] In this invention, "optional" means that something is optional, that is, it refers to either "with" or "without". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0042] In this invention, unless otherwise specified, it is assumed that the experiments are conducted at room temperature or a temperature conventionally set in the art. "Room temperature" generally refers to 4°C to 35°C, and may refer to 20°C ± 5°C. In some embodiments of this invention, room temperature refers to 20°C to 30°C.

[0043] In one specific embodiment, the present invention provides a composite negative electrode sheet, the composite negative electrode sheet comprising a current collector and a negative electrode active material layer disposed on at least one side surface of the current collector; the negative electrode active material layer comprises a first negative electrode active material and a second negative electrode active material; the first negative electrode active material comprises graphite and / or silicon; the second negative electrode active material comprises hard carbon; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material.

[0044] Compared to graphite and silicon, hard carbon has a large number of nanopores and defect sites. Its lithium storage mechanism is different from the interlayer intercalation of graphite or the alloying reaction between silicon anode and lithium metal. Even when the overall SOC of the battery is very low, lithium ions can still be rapidly intercalated and rapidly deintercalated at the nanopores and defect sites of hard carbon.

[0045] This invention introduces hard carbon into a negative electrode sheet comprising graphite and / or silicon, effectively providing lithium ions with an additional, kinetically optimized "fast channel" and "reservoir." Utilizing the "end-of-life" lithium storage mechanism of hard carbon, it effectively improves the power performance of the composite negative electrode sheet at low SOC and reduces the DCR of the composite negative electrode sheet. Hard carbon and graphite and / or silicon form a complementary effect. Graphite and / or silicon ensure high capacity and high energy density of the battery in the high to medium SOC range, while hard carbon effectively optimizes the power performance in the low SOC range, thereby achieving a more stable and superior power output across the entire SOC range. Furthermore, this invention uses small-particle-size hard carbon in combination with large-particle-size graphite and / or silicon. The hard carbon can be interspersed in the gaps between graphite and / or silicon, increasing the compaction density of the composite negative electrode sheet, thereby improving the volumetric energy density of the battery. This compensates for the decrease in battery mass energy density caused by the introduction of hard carbon, maintaining a high energy density in the lithium-ion battery.

[0046] This invention, by controlling the mass ratio of the first and second negative electrode active materials and the D50 particle size, fully leverages the effect of hard carbon in improving low SOC power performance while ensuring the battery has high energy density and initial efficiency. Excessive addition of hard carbon will lead to a decrease in the battery's specific capacity, making it impossible to balance power performance and energy density and ensure excellent overall battery performance. Conversely, if the D50 particle size of hard carbon is mismatched with that of graphite and / or silicon, the compaction density of the composite negative electrode cannot be effectively reduced, thus failing to improve the battery's volumetric energy density.

[0047] In some embodiments, the mass percentage a of the first negative electrode active material is 80wt% to 90wt%, for example, it can be 80wt%, 82wt%, 84wt%, 86wt%, 88wt%, or 90wt%, and the mass percentage b of the second negative electrode active material is 10wt% to 20wt%, for example, it can be 10wt%, 12wt%, 14wt%, 16wt%, 18wt%, or 20wt%.

[0048] In some embodiments, the D50 particle size of the first negative electrode active material is 5μm to 20μm, for example, it can be 5μm, 8μm, 10μm, 12μm, 15μm, 17μm or 20μm, preferably 6μm to 10μm.

[0049] In some embodiments, the D50 particle size of the second negative electrode active material is 2μm to 15μm, for example, it can be 2μm, 4μm, 6μm, 7μm, 10μm, 12μm, 14μm or 15μm, preferably 3μm to 9μm.

[0050] In some embodiments, if the compaction density of the negative electrode sheet with the same loading amount of the first negative electrode active material as the composite negative electrode sheet is x, and the compaction density of the negative electrode sheet with the same loading amount of the second negative electrode active material as the composite negative electrode sheet is y, then the compaction density of the composite negative electrode sheet is w = a × x + b × y.

[0051] In the composite negative electrode sheet provided by the present invention, by adjusting the particle size relationship between the first negative electrode active material and the second negative electrode active material, the compaction density of the first negative electrode active material and the second negative electrode active material in the composite electrode sheet can be made comparable to that of an electrode sheet containing only the first negative electrode active material and the second negative electrode active material, without the introduction of the second negative electrode active material causing a decrease in the compaction density of the first negative electrode active material itself.

[0052] In some embodiments, the compaction density x of the negative electrode sheet having the same loading of the first negative electrode active material as the composite negative electrode sheet is 1.3 g / cm³. 3 ~1.6g / cm 3 For example, it could be 1.3 g / cm³ 3 1.35g / cm 3 1.4g / cm 3 1.45g / cm 3 1.5g / cm 3 1.55g / cm 3 Or 1.6g / cm 3 .

[0053] In some embodiments, the compaction density γ of the negative electrode sheet having the same loading of the second negative electrode active material as the composite negative electrode sheet is 0.8 g / cm³. 3 ~1.0g / cm 3 For example, it could be 0.8 g / cm³. 3 0.85g / cm 3 0.9g / cm 3 0.95g / cm 3 Or 1.0g / cm 3 .

[0054] In some embodiments, the compaction density of the composite negative electrode is 1.2 g / cm³. 3 ~1.55g / cm 3 For example, it could be 1.2 g / cm³. 3 1.25g / cm 3 1.3g / cm 3 1.35g / cm 3 1.4g / cm 3 1.45g / cm 3 1.5g / cm3 Or 1.55g / cm 3 .

[0055] In some embodiments, the negative electrode active material layer of the composite negative electrode sheet further includes a binder and a conductive agent, wherein the total mass of the first negative electrode active material and the second negative electrode active material is in the mass ratio of the binder and the conductive agent as (92~97):(1~3):(2~5), for example, it can be 92:3:5, 93:2.5:4.5, 94:2:4, 95:1.5:3.5, 96:1:3 or 97:1:2.

[0056] In some embodiments, the adhesive comprises any one or a combination of at least two of polyacrylic acid, sodium carboxymethyl cellulose, or styrene-butadiene rubber. Typical but not limited combinations include a combination of polyacrylic acid and sodium carboxymethyl cellulose, or a combination of sodium carboxymethyl cellulose and styrene-butadiene rubber.

[0057] In some embodiments, the conductive agent includes any one or a combination of at least two of conductive carbon black, Ketjen black, carbon nanotubes, or graphene. Typical but non-limiting combinations include a combination of conductive carbon black and Ketjen black, a combination of conductive carbon black and carbon nanotubes, a combination of conductive carbon black and graphene, or a combination of conductive carbon black, carbon nanotubes, and graphene.

[0058] In some embodiments, the current collector includes a copper current collector.

[0059] In another specific embodiment, the present invention provides a method for preparing a composite negative electrode sheet as described in one of the foregoing specific embodiments, the method comprising:

[0060] A negative electrode slurry is coated onto at least one side of the current collector and dried to obtain the composite negative electrode sheet; the negative electrode slurry includes a first negative electrode active material and a second negative electrode active material; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material.

[0061] In some embodiments, the method for preparing the negative electrode slurry includes dispersing a first negative electrode active material, a second negative electrode active material, a binder, and a conductive agent in a solvent.

[0062] In some embodiments, the solvent includes water.

[0063] In some embodiments, the solid content of the negative electrode slurry is 45% to 65%, for example, it may be.

[0064] In yet another embodiment, the present invention provides a lithium-ion battery comprising a composite negative electrode as described in one of the preceding embodiments.

[0065] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0066] Example 1

[0067] This embodiment provides a composite negative electrode sheet, which includes a copper current collector and a negative electrode active material layer coated on both sides of the copper current collector. The negative electrode active material layer includes a negative electrode active material, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 95:1.5:1.5:2. The negative electrode active material is composed of 85wt% graphite and 15wt% hard carbon. The D50 particle size of the graphite is 8μm and the D50 particle size of the hard carbon is 5μm.

[0068] The compacted density of the graphite anode sheet with the same loading as the composite anode sheet is 1.5 g / cm³. 3 The compaction density of the hard carbon anode sheet is 0.95 g / cm³. 3 The compaction density of the composite negative electrode sheet is 1.42 g / cm³. 3 .

[0069] The method for preparing the composite negative electrode includes:

[0070] According to the mass ratio, graphite, hard carbon, conductive carbon black sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed evenly in water to obtain a negative electrode slurry with a solid content of 52%. The negative electrode slurry is coated on both sides of the copper current collector and dried to obtain the composite negative electrode sheet.

[0071] Example 2

[0072] This embodiment provides a composite negative electrode sheet, which includes a copper current collector and a negative electrode active material layer coated on both sides of the copper current collector. The negative electrode active material layer includes negative electrode active material, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 96:2:1:1. The negative electrode active material is composed of 90 wt% graphite and 10 wt% hard carbon. The D50 particle size of the graphite is 6 μm and the D50 particle size of the hard carbon is 3 μm.

[0073] The compacted density of the graphite anode sheet with the same loading as the composite anode sheet is 1.3 g / cm³. 3 The compaction density of the hard carbon anode sheet is 0.8 g / cm³. 3 The compaction density of the composite negative electrode sheet is 1.25 g / cm³. 3 .

[0074] The method for preparing the composite negative electrode includes:

[0075] According to the mass ratio, graphite, hard carbon, conductive carbon black sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed evenly in water to obtain a negative electrode slurry with a solid content of 51%. The negative electrode slurry is coated on both sides of the copper current collector and dried to obtain the composite negative electrode sheet.

[0076] Example 3

[0077] This embodiment provides a composite negative electrode sheet, which includes a copper current collector and a negative electrode active material layer coated on both sides of the copper current collector. The negative electrode active material layer includes negative electrode active material, conductive carbon black, sodium carboxymethyl cellulose and polyacrylic acid in a mass ratio of 95:1.5:1.5:2. The negative electrode active material is composed of 75wt% graphite, 10wt% silicon and 15wt% hard carbon. The D50 particle size of graphite is 10μm, the D50 particle size of silicon is 20μm and the D50 particle size of hard carbon is 9μm.

[0078] The compaction density of the graphite and silicon anode sheet with the same loading as the composite anode sheet is 1.6 g / cm³. 3 The compaction density of the hard carbon anode sheet is 1.0 g / cm³. 3 The compaction density of the composite negative electrode sheet is 1.51 g / cm³. 3 .

[0079] The method for preparing the composite negative electrode includes:

[0080] According to the mass ratio, graphite, silicon, hard carbon, conductive carbon black sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed evenly in water to obtain a negative electrode slurry with a solid content of 52%. The negative electrode slurry is coated on both sides of the copper current collector and dried to obtain the composite negative electrode sheet.

[0081] Example 4

[0082] This embodiment provides a composite negative electrode sheet. Except for the negative electrode active material, which includes 80 wt% graphite and 20 wt% hard carbon, the composite negative electrode sheet is the same as that in Example 1.

[0083] Example 5

[0084] This embodiment provides a composite negative electrode sheet. Except for the graphite having a D50 particle size of 5 μm and the hard carbon having a D50 particle size of 2 μm, the composite negative electrode sheet is the same as that in Example 1.

[0085] Example 6

[0086] This embodiment provides a composite negative electrode sheet. Except for the graphite having a D50 particle size of 20 μm and the hard carbon having a D50 particle size of 15 μm, the composite negative electrode sheet is the same as that in Example 1.

[0087] Example 7

[0088] This embodiment provides a composite negative electrode sheet. Except for the negative electrode active material, which includes 95 wt% graphite and 5 wt% hard carbon, the composite negative electrode sheet is the same as that in Example 1.

[0089] Example 8

[0090] This embodiment provides a composite negative electrode sheet, in which, except for the negative electrode active material comprising 70wt% graphite and 30wt% hard carbon, the rest are the same as in Example 1.

[0091] Comparative Example 1

[0092] This comparative example provides a composite negative electrode sheet, in which, except that the negative electrode active material does not include hard carbon, everything else is the same as in Example 1.

[0093] Comparative Example 2

[0094] This comparative example provides a composite negative electrode sheet, in which, except that the negative electrode active material does not include hard carbon, everything else is the same as in Example 3.

[0095] Comparative Example 3

[0096] This comparative example provides a composite negative electrode sheet, in which, except for the graphite having a D50 particle size of 5 μm and the hard carbon having a D50 particle size of 15 μm, all other aspects are the same as in Example 1.

[0097] Performance testing:

[0098] Ni 0.8 Co 0.1 Mn 0.1 O2 (NCM811) and lithium iron phosphate (LFP) are used as positive electrode active materials. They are dispersed in NMP with polyvinylidene fluoride and conductive carbon black in a mass ratio of 96:2:2 to prepare NCM811 positive electrode slurry and LFP positive electrode slurry, respectively. The NCM811 positive electrode slurry is coated on the surface of an aluminum current collector to prepare an NCM811 positive electrode sheet. The LFP positive electrode slurry is coated on the surface of a carbon-coated aluminum current collector to prepare an LFP positive electrode sheet.

[0099] Using lithium hexafluorophosphate electrolyte and polyethylene separator, with N / P=1.05, the negative electrode sheets provided in all the above examples and comparative examples were assembled with NCM811 positive electrode sheets to form NCM lithium-ion batteries; and the negative electrode sheets provided in Example 1 and Comparative Example 1 were assembled with LFP positive electrode sheets to form LFP lithium-ion batteries.

[0100] At 25℃ and within a voltage range of 2.5V to 4.25V, the initial coulombic efficiency and initial discharge specific capacity of the above-mentioned NCM lithium-ion battery were tested by charging and discharging at a rate of 0.1C. Then, the SOC of the NCM lithium-ion battery was adjusted to 10%, and pulse discharge was performed at a rate of 2C for 10s to test the DC internal resistance and maximum power of the NCM lithium-ion battery at 10% SOC. The test results are shown in Table 1.

[0101] The initial coulombic efficiency of the LFP lithium-ion battery was tested at 25℃ and within a voltage range of 2.5V~3.65V, using a charge-discharge rate of 0.1C. Then, the SOC of the LFP lithium-ion battery was adjusted to 10%, and a pulse discharge at a 2C rate was performed for 10s. The DC internal resistance and maximum power of the LFP lithium-ion battery at 10% SOC were tested. The test results are shown in Table 2.

[0102] Table 1

[0103]

[0104] Table 2

[0105]

[0106] In summary, by introducing hard carbon into the negative electrode sheet including graphite and / or silicon, this invention provides lithium ions with an additional, kinetically superior "fast channel" and "reservoir." Utilizing the "end-of-life" lithium storage mechanism of hard carbon, it effectively improves the power performance of the composite negative electrode sheet at low SOC and reduces the DCR of the composite negative electrode sheet. Simultaneously, this invention uses small-particle-size hard carbon in combination with large-particle-size graphite and / or silicon, allowing the hard carbon to intersect within the gaps in the graphite and / or silicon, increasing the compaction density of the composite negative electrode sheet and thus improving the volumetric energy density of the battery. This compensates for the decrease in battery mass energy density caused by the introduction of hard carbon, maintaining a high energy density in the lithium-ion battery.

[0107] According to the test results of Examples 1, 7 and 8, if the mass ratio of hard carbon is too small, the power performance cannot be fully improved. If the mass ratio of hard carbon is too large, the specific capacity and compaction density of the battery will decrease significantly, that is, the energy density will decrease significantly. Furthermore, excessive hard carbon doping will result in a lower open circuit voltage of the whole cell at low SOC, ultimately leading to a decrease in the power performance of the cell.

[0108] Combining the test results of Example 1 and Comparative Example 1 in Tables 1 and 2, the composite anode sheet provided by this invention significantly improves the power performance of the battery at low SOC in both NCM and LFP battery systems. Specifically, after introducing small-particle hard carbon into the graphite anode sheet, the small hard carbon particles can fill the gaps between large graphite particles, so the battery's compaction density does not decrease significantly due to the mixing of hard carbon, and there is no decrease in electron and ion diffusion rate due to the increase in electrode thickness at the same areal density. At the same time, the "end" lithium storage mechanism of hard carbon effectively reduces the DCR of the battery at 10% SOC. Although the specific capacity and first-efficiency of the battery decrease slightly, the power performance of the battery is significantly improved.

[0109] Based on the test results of Example 3 and Comparative Example 2, for anode sheets with graphite and silicon as the negative electrode active materials, the composite anode sheet obtained by introducing small-particle hard carbon showed a significant decrease in DCR at 10% SOC and a significant improvement in power performance.

[0110] Based on the test results of Example 1 and Comparative Example 3, if large-particle hard carbon and small-particle graphite are used in combination, the power performance of large-particle hard carbon is far inferior to that of small-particle hard carbon. Therefore, the improvement effect on the DCR and power performance of the composite electrode at 10% SOC is not good, and the improvement effect on the compaction density of the composite negative electrode is not good.

[0111] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A composite negative electrode, characterized in that, The composite negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one side surface of the current collector; The negative electrode active material in the negative electrode active material layer includes a first negative electrode active material and a second negative electrode active material; The first negative electrode active material includes graphite and / or silicon; The second negative electrode active material includes hard carbon; The D50 particle size of the second negative electrode active material is smaller than that of the first negative electrode active material.

2. The composite negative electrode sheet as described in claim 1, characterized in that, In the negative electrode active material, the mass percentage a of the first negative electrode active material is 80wt%~90wt%, and the mass percentage b of the second negative electrode active material is 10wt%~20wt%.

3. The composite negative electrode sheet as described in claim 1, characterized in that, The D50 particle size of the first negative electrode active material is 5μm~20μm; And / or, the D50 particle size of the second negative electrode active material is 2μm~15um.

4. The composite negative electrode sheet as described in claim 3, characterized in that, The D50 particle size of the first negative electrode active material is 6μm~10μm; And / or, the D50 particle size of the second negative electrode active material is 3μm~9um.

5. The composite negative electrode sheet as described in claim 2, characterized in that, If the compaction density of the negative electrode sheet with the same loading amount of the first negative electrode active material as the composite negative electrode sheet is x, and the compaction density of the negative electrode sheet with the same loading amount of the second negative electrode active material as the composite negative electrode sheet is y, then the compaction density of the composite negative electrode sheet is w = a × x + b × y.

6. The composite negative electrode sheet as described in claim 5, characterized in that, The compaction density x of the negative electrode sheet having the same loading of the first negative electrode active material as the composite negative electrode sheet is 1.3 g / cm³. 3 ~1.6g / cm 3 ; And / or, the compaction density γ of the negative electrode sheet having the same loading of the second negative electrode active material as the composite negative electrode sheet is 0.8 g / cm³. 3 ~1.0g / cm 3 ; And / or, the compaction density of the composite negative electrode is 1.2 g / cm³. 3 ~1.55g / cm 3 .

7. The composite negative electrode sheet as described in claim 1, characterized in that, The composite negative electrode sheet also includes a binder and a conductive agent in the negative electrode active material layer. The total mass of the first negative electrode active material and the second negative electrode active material is in the mass ratio of the binder and the conductive agent as (92~97):(1~3):(2~5).

8. The composite negative electrode sheet as described in claim 7, characterized in that, The adhesive comprises any one or a combination of at least two of polyacrylic acid, sodium carboxymethyl cellulose, or styrene-butadiene rubber. And / or, the conductive agent includes any one or a combination of at least two of conductive carbon black, Ketjen black, carbon nanotubes, or graphene.

9. A method for preparing a composite negative electrode sheet as described in any one of claims 1 to 8, characterized in that, The preparation method includes: The negative electrode slurry is coated onto at least one surface of the current collector and dried to obtain the composite negative electrode sheet. The negative electrode slurry includes a first negative electrode active material and a second negative electrode active material; the D50 particle size of the second negative electrode active material is smaller than the D50 particle size of the first negative electrode active material.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the composite negative electrode sheet as described in any one of claims 1 to 8.

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