Negative pole piece, battery element, battery, battery pack and electric equipment

Abstract

The invention relates to the technical field of battery negative pole pieces, in particular to a negative pole piece, a battery cell, a battery and electric equipment. The negative pole piece comprises a current collector, a first hard carbon layer and a second hard carbon layer which are sequentially arranged, the first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon; the particle size D50 of the first hard carbon is larger than that of the second hard carbon, the porosity of the first hard carbon is smaller than that of the second hard carbon, at least the surface of the first hard carbon contains oxygen-containing functional groups, and hydrogen bonds are formed between at least part of the oxygen-containing functional groups and the binder in the first hard carbon layer. According to the negative pole piece, the stripping force of the pole piece can be effectively improved, and irreversible capacity loss caused by film formation can be reduced, so that the first effect of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of battery negative electrode technology, specifically to a negative electrode, a battery cell, a battery, a battery pack, and electrical equipment. Background Technology

[0002] Currently, sodium-ion batteries, which are low-cost and highly safe, have shown great potential for development in the field of large-scale energy storage. However, graphite, a negative electrode material widely used in lithium-ion batteries, cannot be used as a negative electrode for sodium-ion batteries because of its small interlayer spacing (0.335nm). Larger sodium ions require more energy to insert into the graphite interlayer and cannot be reversibly inserted or removed within the effective potential window.

[0003] Among many anode materials, hard carbon has attracted widespread attention due to its large interlayer spacing and irregular structure, which is suitable for sodium ion insertion and extraction. However, due to its large specific surface area, numerous defects, and complex microstructure, hard carbon is difficult to meet the peeling force requirements of the prepared electrode. Furthermore, it requires more sodium ions to form the SEI film, resulting in a low initial coulombic efficiency (70-80%), which seriously limits the commercial development of sodium-ion batteries. Summary of the Invention

[0004] This invention provides a negative electrode sheet that has a large peeling force and can reduce irreversible capacity loss caused by film formation, thereby improving the first-time efficiency of the battery.

[0005] The present invention also provides a battery element that can improve the first efficiency of a battery.

[0006] The present invention also provides a battery having a high initial efficiency.

[0007] The present invention also provides an electrical device with strong battery life and long service life.

[0008] The negative electrode sheet provided by the present invention includes a current collector, a first hard carbon layer and a second hard carbon layer arranged sequentially, wherein the first hard carbon layer includes a first hard carbon and the second hard carbon layer includes a second hard carbon.

[0009] The particle size D50 of the first hard carbon is greater than that of the second hard carbon, the porosity of the first hard carbon is less than that of the second hard carbon, and at least the surface of the first hard carbon contains oxygen-containing functional groups, and at least a portion of the oxygen-containing functional groups form hydrogen bonds with the binder in the first hard carbon layer.

[0010] In the negative electrode sheet described above, at least some of the oxygen-containing functional groups exist independently.

[0011] In the negative electrode sheet described above, the oxygen-containing functional group is one or more of the following: ketone group, epoxy group, hydroxyl group, ether group, aldehyde group, carbonyl group, carboxyl group, ester group, amide group, nitro group, and sulfonic acid group.

[0012] In the negative electrode sheet described above, the oxygen element in the oxygen-containing functional group accounts for 1-20% of the mass of the first hard carbon.

[0013] The difference in porosity between the first hard carbon and the second hard carbon in the negative electrode sheet described above is 5-40%.

[0014] The negative electrode sheet described above has a porosity of ≤5% for the first hard carbon and 10-40% for the second hard carbon.

[0015] The negative electrode sheet described above has a particle size D50 of 8-25 μm for the first hard carbon and a particle size D50 of 0.1-5 μm for the second hard carbon.

[0016] The negative electrode sheet described above has a pore size ≥ 2 nm in the second hard carbon.

[0017] The specific surface area of ​​the first hard carbon electrode, as described above, is 1-3 m². 2 / g.

[0018] The ratio of the thickness of the first hard carbon layer to the thickness of the second hard carbon layer in the above-described negative electrode sheet is (1-9):(9-1).

[0019] The present invention also provides a battery element, including the negative electrode sheet described above.

[0020] The present invention also provides a battery, including the above-described negative electrode and battery element.

[0021] The present invention also provides a battery pack, including the above-described negative electrode sheet, the above-described battery element, or the above-described battery.

[0022] The present invention also provides an electrical device including the battery pack described above.

[0023] The negative electrode sheet provided by the present invention adopts a two-layer hard carbon structure, ensuring that the particles of the bottom hard carbon are larger than those of the surface hard carbon and the porosity is smaller than that of the surface hard carbon. Furthermore, oxygen-containing functional groups are loaded on the first hard carbon. This not only effectively improves the peeling force of the electrode sheet, but also significantly reduces the irreversible capacity loss caused by film formation, which is beneficial to improving the first efficiency of the battery. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0025] Existing hard carbon anodes generally suffer from two major problems: low peeling force and the consumption of a large number of active ions during SEI film formation, which affects the initial efficiency of the battery. This invention provides an anode sheet comprising at least a current collector, a first hard carbon layer, and a second hard carbon layer arranged sequentially. The first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon. The particle size D50 of the first hard carbon is larger than that of the second hard carbon, and the porosity of the first hard carbon is smaller than that of the second hard carbon. At least the surface of the first hard carbon contains oxygen-containing functional groups, and at least some of these oxygen-containing functional groups form hydrogen bonds with the binder in the first hard carbon layer.

[0026] The negative electrode sheet provided by the present invention uses large-particle, low-porosity hard carbon as the bottom layer and small-particle, high-porosity hard carbon as the surface layer, and ensures that at least the surface of the first hard carbon contains oxygen-containing functional groups. This not only effectively improves its peeling force, but also significantly reduces the irreversible capacity loss caused by film formation, which is beneficial to improving the first efficiency of the battery.

[0027] In some specific embodiments of the present invention, at least some oxygen-containing functional groups on the surface of the first hard carbon exist independently. These independently existing functional groups can enhance the affinity of the first hard carbon for the electrolyte, help to form a more stable and uniform SEI film, enhance the reversible adsorption of sodium ions, further improve the first efficiency, and reduce irreversible capacity loss.

[0028] There are no specific restrictions on the oxygen-containing functional groups, as long as they can form hydrogen bonds with the binder and have an affinity for the electrolyte. For example, they can be one or more of the following groups: ketone, epoxy, hydroxyl, ether, aldehyde, carbonyl, carboxyl, ester, amide, nitro, and sulfonic acid.

[0029] Oxygen content can be determined by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). Specifically, the electrode is ion-cut, and then the cross-section of the electrode is characterized using SEM-EDS. By detecting the mass percentages of C, H, and O, the mass percentage of oxygen in the oxygen-containing functional groups in the first hard carbon can be calculated. Specifically, the mass percentage of oxygen can be 1%, 3%, 5%, 7%, 10%, 13%, 15%, 17%, 20%, or any value between any two of the above ranges.

[0030] Furthermore, the electrode is ion-cut, and then the cross-section of the electrode is characterized by Fourier transform infrared spectroscopy (FTIR) to determine whether the oxygen-containing functional groups in the electrode exist in the form of hydrogen bonds or in an independent form.

[0031] For example, the surface of the first hard carbon can be modified with citric acid to give it oxygen-containing functional groups. In some specific embodiments, the first hard carbon is subjected to an immersion treatment with a solvent containing citric acid to load its surface with oxygen-containing functional groups.

[0032] In other specific embodiments, the first hard carbon is activated by alkali treatment or solid-state sintering or chemical vapor deposition to load oxygen-containing functional groups on its surface.

[0033] It is easy to understand that, in order to further improve the electrode peeling force, the second hard carbon in the second hard carbon layer can also contain the aforementioned oxygen-containing functional groups. These oxygen-containing functional groups can also partially form hydrogen bonds with the binder in the second hard carbon layer, and partially exist independently, in order to enhance the affinity for the electrolyte. The specific implementation method is as described above, and will not be elaborated further here.

[0034] In this invention, the porosity difference between the first hard carbon and the second hard carbon is not specifically limited. Considering the comprehensive performance of the electrode sheet, such as compaction, it is usually controlled to not exceed 70%. Studies have shown that when the porosity difference between the first hard carbon and the second hard carbon is 5-40%, the comprehensive performance of the negative electrode sheet is better.

[0035] The porosity of the first and second hard carbon electrodes can be detected by the following method: observe the layering of the electrode under a scanning electron microscope, then gently scrape the electrode surface to obtain powder, and further use gas adsorption method to detect its porosity and specific surface area.

[0036] Appropriate particle size is beneficial to further improve the overall performance of the negative electrode sheet. Studies have shown that when the particle size D50 of the first hard carbon is 8-25μm and the particle size D50 of the second hard carbon is 0.1-5μm, the peeling force of the negative electrode sheet is improved and it is more conducive to improving the first efficiency of the battery.

[0037] For example, the particle size D50 of the first hard carbon can be 8 μm, 10 μm, 15 μm, 18 μm, 20 μm, 25 μm, or any value between any two of the above ranges. The particle size D50 of the second hard carbon can be 0.1 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or any value between any two of the above ranges.

[0038] In the above, the particle sizes of the first and second hard carbon can be obtained by directly scanning and measuring the hard carbon layer using a scanning electron microscope.

[0039] Further research has shown that when the porosity of the first hard carbon is ≤5% and the porosity of the second hard carbon is 10-40%, the overall performance of the negative electrode is better.

[0040] For example, the porosity of the first hard carbon can be 1%, 2%, 3%, 4%, 5%, or any value between any two of the above values. The porosity of the second hard carbon can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any value between any two of the above values, preferably 10-40%.

[0041] The preferred pore size of the second hard carbon is ≥2nm. By using a second hard carbon with a pore size ≥2μm, the migration path of sodium ions can be shortened, which is beneficial to further improve the first-effect isoelectric performance.

[0042] In some specific embodiments of the present invention, the specific surface area (BET) of the first hard carbon is 1-3 m². 2 / g, for example, could be 1m 2 / g、2m 2 / g、3m 2 / g and the range between any two of the above values.

[0043] Further research shows that the negative electrode exhibits better performance when the thickness ratio of the first hard carbon layer to the second hard carbon layer is (1-9):(9-1). For example, the thickness ratio of the first hard carbon layer to the second hard carbon layer can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:2, or any range between any two of these values. Specifically, the negative electrode exhibits better overall performance when the thickness ratio of the first hard carbon layer to the second hard carbon layer is (4-7):(3-6).

[0044] It should be noted that the current collector described in this invention is not particularly limited, and can be at least one of aluminum foil, carbon-coated aluminum foil, copper foil, nickel foam, and copper foam. The formation method of the first hard carbon layer and the second hard carbon layer is not particularly limited, and conventional coating formation methods in the art can be used.

[0045] Furthermore, it can be understood that the first and second hard carbon layers, in addition to hard carbon and binders, may also include conductive agents. The types and amounts of binders and conductive agents are not specifically limited; conventional raw materials in the art can be used. For example, binders may include styrene-butadiene rubber (SBR), chitosan, polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), sodium alginate, polyvinyl alcohol resin (PVA), and PVP, etc., while conductive agents may include carbon nanomaterials, carbon black, graphene, graphite nanoribbons, etc.

[0046] The present invention also provides a battery element including the aforementioned negative electrode sheet, which is beneficial for improving the initial efficiency of the battery. The battery element is not specifically limited; it can be a battery cell or a negative electrode sheet containing a solid electrolyte.

[0047] The present invention also provides a battery, including the aforementioned negative electrode or battery element. The type of battery is not specifically limited; it can be a lithium battery, a sodium battery, or a potassium battery, etc., preferably a sodium-ion battery. Furthermore, it can be a liquid battery, a fully solid-state battery, or a quasi-solid-state battery.

[0048] The specific type of battery of this invention is not particularly limited. For example, from the perspective of shape, the battery includes, but is not limited to, prismatic batteries, pouch batteries, and cylindrical batteries. From the perspective of the core structure, the battery core can be a wound core (i.e., a core formed by stacking positive electrode sheets, negative electrode sheets, and separators and then winding them) or a stacked core (i.e., multiple positive electrode sheets, negative electrode sheets, and separators are stacked to form a core). The outer shell can be a hard shell (such as a steel shell, a hard plastic shell, etc.) or a soft shell (such as an aluminum-plastic film shell, a pouch-type soft shell, etc.).

[0049] The present invention also provides a battery pack, including the aforementioned negative electrode plate, battery element, or battery. The battery pack can be in the form of a battery module or a battery pack. In some embodiments, a battery module is assembled from individual batteries (i.e., battery cells), and the number of battery cells contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art based on the application and capacity of the battery module. In some embodiments, the battery pack is a battery pack, and the number of battery cells or 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 based on the application and capacity of the battery pack.

[0050] The present invention also provides an electrical device including the battery described above. This electrical device can be a conventional electrical device in the art, including consumer electronics (mobile communication devices, laptops, tablets, wearable devices, etc.), drones, power tools, energy storage devices, electric bicycles, electric vehicles, etc.

[0051] The negative electrode sheet of the present invention will be described in detail below with reference to specific embodiments.

[0052] Example 1

[0053] A negative electrode includes a current collector, a first hard carbon layer, and a second hard carbon layer sequentially disposed thereon. The first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon. The particle size D50 of the first hard carbon is 12 μm, and the BET is 2 μm. 2 / g, porosity 3%, and SEM-EDS characterization shows that the oxygen element in the oxygen-containing functional groups on the surface of the first hard carbon accounts for 2% of the mass of the first hard carbon. Further FITR analysis shows that some oxygen-containing functional groups form hydrogen bonds with the binder, while some oxygen-containing functional groups exist independently.

[0054] The second hard carbon has a particle size D50 of 2 μm, an average pore size of 2 nm, and a porosity of 10%.

[0055] The thickness ratio of the first hard carbon layer to the second hard carbon layer is 5:5, and the current collector is aluminum foil.

[0056] The preparation method of the above negative electrode sheet is as follows:

[0057] First hard carbon surface treatment: The first hard carbon is immersed in a 0.5 mol / L citric acid solution for a period of time to obtain the first hard carbon with oxygen-containing functional groups (carboxyl groups) on the surface;

[0058] Preparation of the first slurry: The surface-treated first hard carbon, binder PAA and conductive agent carbon black are mixed with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 and mechanically mixed to obtain the first slurry;

[0059] Preparation of the second slurry: The second hard carbon, binder PAA and conductive agent carbon black are mixed mechanically with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 to obtain the second slurry;

[0060] The first slurry and the second slurry are coated sequentially on the aluminum foil. After coating, the foil is dried to obtain the negative electrode sheet.

[0061] Example 2

[0062] A negative electrode includes a current collector, a first hard carbon layer, and a second hard carbon layer sequentially disposed thereon. The first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon. The particle size D50 of the first hard carbon is 8 μm, and the BET is 1 μm. 2 / g, porosity 5%, and SEM-EDS characterization shows that the oxygen element in the oxygen-containing functional groups on the surface of the first hard carbon accounts for 1% of the mass of the first hard carbon. Further FTIR analysis shows that some oxygen-containing functional groups form hydrogen bonds with the binder, while some oxygen-containing functional groups exist independently.

[0063] The second hard carbon has a particle size D50 of 0.5 μm, an average pore size of 2.5 nm, and a porosity of 20%.

[0064] The thickness ratio of the first hard carbon layer to the second hard carbon layer is 5:5, and the current collector is aluminum foil.

[0065] The preparation method of the above negative electrode sheet is as follows:

[0066] First hard carbon surface treatment: The first hard carbon is immersed in a 0.25 mol / L citric acid solution for a period of time to obtain the first hard carbon with oxygen-containing functional groups (carboxyl groups) on the surface;

[0067] Preparation of the first slurry: The surface-treated first hard carbon, binder PAA and conductive agent carbon black are mixed with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 and mechanically mixed to obtain the first slurry;

[0068] Preparation of the second slurry: The second hard carbon, binder PAA and conductive agent carbon black are mixed mechanically with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 to obtain the second slurry;

[0069] The first slurry and the second slurry are coated sequentially on the aluminum foil. After coating, the foil is dried to obtain the negative electrode sheet.

[0070] Example 3

[0071] A negative electrode includes a current collector, a first hard carbon layer, and a second hard carbon layer sequentially disposed thereon. The first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon. The first hard carbon has a particle size D50 of 20 μm and a BET of 3 μm. 2 / g, porosity 1%, and SEM-EDS characterization shows that the oxygen element in the oxygen-containing functional groups on the surface of the first hard carbon accounts for 11% of the mass of the first hard carbon. Further FTIR analysis shows that some oxygen-containing functional groups form hydrogen bonds with the binder, while some oxygen-containing functional groups exist independently.

[0072] The second hard carbon has a particle size D50 of 5 μm, an average pore size of 2.5 nm, and a porosity of 15%.

[0073] The thickness ratio of the first hard carbon layer to the second hard carbon layer is 5:5, and the current collector is aluminum foil.

[0074] The preparation method of the above negative electrode sheet is as follows:

[0075] First hard carbon surface treatment: The first hard carbon is immersed in a 2.5 mol / L citric acid solution for a period of time to obtain the first hard carbon with oxygen-containing functional groups (carboxyl groups) on the surface;

[0076] Preparation of the first slurry: The surface-treated first hard carbon, binder PAA and conductive agent carbon black are mixed with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 and mechanically mixed to obtain the first slurry;

[0077] Preparation of the second slurry: The second hard carbon, binder PAA and conductive agent carbon black are mixed mechanically with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 to obtain the second slurry;

[0078] The first slurry and the second slurry are coated sequentially on the aluminum foil. After coating, the foil is dried to obtain the negative electrode sheet.

[0079] Example 4

[0080] A negative electrode includes a current collector, a first hard carbon layer, and a second hard carbon layer sequentially disposed thereon. The first hard carbon layer comprises first hard carbon, and the second hard carbon layer comprises second hard carbon. The first hard carbon has a particle size D50 of 25 μm and a BET of 3 μm. 2 / g, porosity 2%, and SEM-EDS characterization shows that the oxygen element in the oxygen-containing functional groups on the surface of the first hard carbon accounts for 20% of the mass of the first hard carbon. Further FTIR analysis shows that some oxygen-containing functional groups form hydrogen bonds with the binder, while some oxygen-containing functional groups exist independently.

[0081] The second hard carbon has a particle size D50 of 5 μm, an average pore size of 2.5 nm, and a porosity of 40%.

[0082] The thickness ratio of the first hard carbon layer to the second hard carbon layer is 5:5, and the current collector is aluminum foil.

[0083] The preparation method of the above negative electrode sheet is as follows:

[0084] First hard carbon surface treatment: The first hard carbon is immersed in a 5 mol / L citric acid solution for a period of time to obtain the first hard carbon with oxygen-containing functional groups (carboxyl groups) on the surface;

[0085] Preparation of the first slurry: The surface-treated first hard carbon, the binder polyvinylidene fluoride (PVDF) and the conductive agent graphene are mixed in a mass ratio of 8:1:1, with an appropriate amount of solvent NMP added, and mechanically mixed to obtain the first slurry;

[0086] Preparation of the second slurry: The second hard carbon, binder PAA and conductive agent carbon black are mixed mechanically with an appropriate amount of solvent NMP in a mass ratio of 8:1:1 to obtain the second slurry;

[0087] The first slurry and the second slurry are coated sequentially on the aluminum foil. After coating, the foil is dried to obtain the negative electrode sheet.

[0088] Example 5

[0089] A negative electrode sheet differs from Example 1 in that it has different oxygen-containing functional groups, and FTIR analysis shows that some oxygen-containing functional groups form hydrogen bonds with the binder, while some oxygen-containing functional groups exist independently.

[0090] The surface treatment method of the first hard carbon is as follows: the first hard carbon is immersed in a 0.5 mol / L mixed solution of citric acid and concentrated sulfuric acid for a period of time to obtain the first hard carbon with oxygen-containing functional groups (carboxyl and hydroxyl groups) on the surface.

[0091] Example 6

[0092] A negative electrode sheet, which differs from Example 1, has an oxygen element content of 0.5% in the oxygen-containing functional groups on the surface of the first hard carbon, and further FTIR analysis shows that all oxygen-containing functional groups form hydrogen bonds with the binder in the first hard carbon layer.

[0093] Example 7

[0094] A negative electrode sheet, which differs from Example 1, is provided in that the surfaces of the first hard carbon and the second hard carbon are loaded with the same oxygen-containing functional groups using the same method, and hydrogen bonds are formed between some oxygen-containing functional groups on the surface of the first hard carbon and the binder, while some oxygen-containing functional groups exist independently; hydrogen bonds are formed between some oxygen-containing functional groups on the surface of the second hard carbon and the binder, while some oxygen-containing functional groups exist independently.

[0095] Example 8

[0096] A negative electrode sheet, which differs from Example 1 in that the second hard carbon has a particle size D50 of 12 μm, an average pore size of 2 nm, and a porosity of 70%.

[0097] Example 9

[0098] A negative electrode sheet, which differs from Example 1 in that the second hard carbon has a particle size D50 of 8 μm, an average pore diameter of 2 nm, and a porosity of 10%.

[0099] Example 10

[0100] A negative electrode sheet, which differs from Example 1 in that the second hard carbon has a particle size D50 of 5 μm, an average pore size of 1.6 nm, and a porosity of 5%.

[0101] Example 11

[0102] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 6:4.

[0103] Example 12

[0104] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 7:3.

[0105] Example 13

[0106] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 8:2.

[0107] Example 14

[0108] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 2:8.

[0109] Example 15

[0110] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 3:7.

[0111] Example 16

[0112] A negative electrode sheet, which differs from Example 1 in that the thickness ratio of the first hard carbon layer and the second hard carbon layer is 4:6.

[0113] Comparative Example 1

[0114] The difference between this comparative example and Example 1 is that it only includes the first hard carbon layer, and the thickness of the first hard carbon layer is the same as the thickness of the double hard carbon layer in Example 1.

[0115] Comparative Example 2

[0116] The difference between this comparative example and Example 1 is that the surface of the first hard carbon does not contain oxygen-containing functional groups.

[0117] Performance testing

[0118] (1) Electrode performance testing

[0119] The electrodes obtained in the above examples and comparative examples were subjected to roll pressing to test the peeling force and compaction of the electrodes. The results are shown in Table 1.

[0120] Peel strength test

[0121] Cut the electrode sheet into 40mm*100mm samples, place the samples on the conveyor belt of the electric rolling roller tester, roll them 3 times, and then stick the electrode sheet samples on one side of double-sided tape (the other side of the double-sided tape is pre-attached to the steel plate). Perform a tensile test on a universal testing machine at a speed of 5mm / min with a displacement of 20mm. The measured electrode sheet peel strength is the electrode sheet peel force.

[0122] [Compaction Test]

[0123] During the rolling process, the electrode sheets are rolled with different pressures to test the maximum compaction resistance of the electrode sheets.

[0124] (2) Battery performance test

[0125] The negative electrode sheets obtained from each group of experiments were coated and die-cut, and assembled with the positive electrode sheet, separator, and electrolyte into a pouch battery. The designed capacity of the battery was 2Ah. The pouch battery was subjected to a formation and capacity test to explore the effect of improving the battery's first efficiency. The results are shown in Table 1 below.

[0126] Battery fabrication: The electrode sheets obtained in the examples and comparative examples were rolled and die-cut to obtain negative electrode sheets. A slurry was prepared by mixing lithium iron phosphate (active material), polyvinylidene fluoride (PVDF) (binder), and acetylene black (conductive agent) in a mass ratio of 8:1:1, using N-methylpyrrolidone as the solvent. This slurry was coated onto aluminum foil used as the positive electrode current collector, and then rolled and die-cut to obtain the positive electrode sheet.

[0127] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the positive and negative electrodes separated by the separator. After stacking, the tabs are welded together, the dry cell is placed in an aluminum-plastic film, and then baked in an oven at 105℃ for 24 hours to dry the moisture. Electrolyte is then added and the cell is sealed to obtain a soft-pack battery. The electrolyte used during electrolyte injection includes lithium salt LiPF6 and solvent ethylene carbonate (EC), with a lithium salt concentration of 1.2 mol / L.

[0128] [First Coulomb Efficiency]

[0129] Each battery was immersed in a 45℃ oven for 24 hours, and then subjected to formation testing at room temperature. The formation conditions were: constant current charging at 0.05C for 180 minutes, constant current charging at 0.1C to 3.8V, and constant voltage charging to 0.05C. The initial charge capacity C1 was recorded. The batteries were then placed back into the 45℃ oven for 24 hours of aging, and then discharged at 1 / 3C. The discharge capacity C2 was recorded, and the initial coulombic efficiency was calculated based on C2 / C1.

[0130] Table 1

[0131]

[0132] The results above show that the peeling force, compaction, and first-time efficiency of the electrodes in Examples 1 and 12-17 are all better than those in Comparative Examples 1 and 2. This indicates that the use of a double-layer hard carbon layer, ensuring that the particle size of the hard carbon in the first hard carbon layer is larger than that in the second hard carbon layer, the porosity is smaller than that of the second hard carbon layer, and that the surface of the first hard carbon contains oxygen-containing functional groups, is beneficial to improving the peeling force and compaction of the electrodes and improving the first-time efficiency of the batteries prepared from them.

[0133] In Example 1, the electrode peeling force, compaction, and the initial efficiency of the prepared battery were all better than those in Examples 9, 10, and 11. This indicates that when the particle size and porosity of the first hard carbon and the second hard carbon are further controlled within a suitable range, it is beneficial to further improve the peeling force and compaction of the electrode and improve the initial efficiency of the prepared battery.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector, a first hard carbon layer and a second hard carbon layer arranged sequentially, wherein the first hard carbon layer includes first hard carbon and the second hard carbon layer includes second hard carbon. The particle size D50 of the first hard carbon is greater than that of the second hard carbon, the porosity of the first hard carbon is less than that of the second hard carbon, and at least the surface of the first hard carbon contains oxygen-containing functional groups, and at least a portion of the oxygen-containing functional groups form hydrogen bonds with the binder in the first hard carbon layer.

2. The negative electrode sheet according to claim 1, characterized in that, At least a portion of the oxygen-containing functional groups on the surface of the first hard carbon exist independently.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The oxygen-containing functional group is one or more of the following: ketone, epoxy, hydroxyl, ether, aldehyde, carbonyl, carboxyl, ester, amide, nitro, and sulfonic acid.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, The oxygen element in the oxygen-containing functional group accounts for 1-20% of the mass of the first hard carbon.

5. The negative electrode sheet according to any one of claims 1-4, characterized in that, The porosity difference between the first hard carbon and the second hard carbon is 5-40%.

6. The negative electrode sheet according to any one of claims 1-5, characterized in that, The porosity of the first hard carbon is ≤5%, and the porosity of the second hard carbon is 10-40%; and / or The particle size D50 of the first hard carbon is 8-25 μm, and the particle size D50 of the second hard carbon is 0.1-5 μm.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The pore size of the pores in the second hard carbon is ≥2nm.

8. The negative electrode sheet according to any one of claims 1-7, characterized in that, The specific surface area of ​​the first hard carbon is 1-3 m². 2 / g.

9. The negative electrode sheet according to any one of claims 1-8, characterized in that, The thickness ratio of the first hard carbon layer to the second hard carbon layer is (1-9):(9-1).

10. A battery element, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-9.

11. A battery, characterized in that, It includes the negative electrode sheet as described in any one of claims 1-9 or the battery element as described in claim 10.

12. A battery pack, characterized in that, Includes the negative electrode sheet according to any one of claims 1-9, the battery element according to claim 10, or the battery according to claim 11.

13. An electrical appliance, characterized in that, Includes the battery of claim 11 or the battery pack of claim 12.

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