Negative electrode sheet, its preparation method and application

Abstract

This application provides a negative electrode sheet and its preparation method, a secondary battery, a battery module, a battery pack, and an electrical device. In the negative electrode sheet, multiple sub-film layers are formed on at least one surface of the negative electrode current collector, and the OI value of each sub-film layer gradually increases along the direction away from the negative electrode current collector. During charging and discharging, gradient pores can be created inside the negative electrode sheet, which helps to reduce the diffusion resistance of lithium ions inside the negative electrode sheet, improve the utilization rate of the active material in the inner sub-film layers, improve the transport rate of lithium ions inside the negative electrode sheet, and thus improve the cycle performance of the secondary battery.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a negative electrode sheet and its preparation method, a secondary battery, a battery module, a battery pack, and an electrical device. Background Technology

[0002] During the charging and discharging process of a secondary battery, the transport of lithium ions within the negative electrode is one of the key factors affecting the battery's performance. If the transport of lithium ions within the negative electrode is restricted, lithium-ion diffusion polarization can easily occur, leading to deterioration in cycle performance. Therefore, improving the transport of lithium ions within the negative electrode is of great significance for improving the cycle performance of secondary batteries. Summary of the Invention

[0003] A first aspect of this application provides a negative electrode sheet. 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 includes multiple sub-film layers stacked sequentially, and the OI value of each sub-film layer gradually increases in the direction away from the negative current collector.

[0004] In the aforementioned negative electrode sheet, by forming multiple sub-film layers on at least one surface of the negative electrode current collector, and with the OI value of each sub-film layer gradually increasing in the direction away from the negative electrode current collector, gradient pores can be created inside the negative electrode sheet during charging and discharging. This helps to reduce the diffusion resistance of lithium ions inside the negative electrode sheet, improve the utilization rate of the active material in the inner sub-film layers, improve the transport rate of lithium ions inside the negative electrode sheet, and thus improve the cycle performance of the secondary battery.

[0005] A second aspect of this application provides a method for preparing a negative electrode sheet. The method includes the following steps: sequentially forming multiple layers of sub-films stacked on at least one surface of a negative electrode current collector, wherein the OI value of each sub-film layer gradually increases in a direction away from the negative electrode current collector.

[0006] A third aspect of this application provides a secondary battery. The secondary battery includes the negative electrode sheet described above or a negative electrode sheet prepared by the aforementioned method.

[0007] A fourth aspect of this application provides a battery module. The battery module includes the secondary battery.

[0008] A fifth aspect of this application provides a battery pack. The battery pack includes the secondary battery or the battery module.

[0009] A sixth aspect of this application provides an electrical device. The electrical device includes at least one of the secondary battery, the battery module, and the battery pack. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application.

[0011] Figure 2 yes Figure 1 An exploded view of a secondary battery according to one embodiment of this application is shown.

[0012] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application.

[0013] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0014] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown.

[0015] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0016] Explanation of reference numerals in the attached figures:

[0017] 1. Battery pack; 2. Upper casing; 3. Lower casing; 4. Battery module; 5. Secondary battery; 51. Housing; 52. Electrode assembly; 53. Top cover assembly. Detailed Implementation

[0018] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the battery assembly, battery cell, secondary battery, and power-consuming 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.

[0019] 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.

[0020] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0021] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0022] 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.

[0023] 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.

[0024] 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 both A and B exist.

[0025] Unless otherwise specified, in this application, the terms "positive electrode sheet" and "positive electrode plate" have the same meaning and can be used interchangeably. The terms "negative electrode sheet" and "negative electrode plate" have the same meaning and can be used interchangeably. The terms "diaphragm" and "separating membrane" have the same meaning and can be used interchangeably.

[0026] One embodiment of this application provides a negative electrode sheet. 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 includes multiple sub-film layers stacked sequentially, and the OI value of each sub-film layer gradually increases in the direction away from the negative current collector.

[0027] In the negative electrode sheet of this embodiment, by forming multiple sub-film layers on at least one surface of the negative electrode current collector, and by gradually increasing the OI value of each sub-film layer in the direction away from the negative electrode current collector, gradient pores can be formed inside the negative electrode sheet during charging and discharging, which is beneficial to reduce the diffusion resistance of lithium ions inside the negative electrode sheet, improve the utilization rate of the active material of the inner sub-film layer, improve the transport rate of lithium ions inside the negative electrode sheet, and thus improve the cycle performance of the secondary battery.

[0028] It is understandable that the OI value of the subfilm represents the ratio of the peak intensity of the 004 crystal plane to the peak intensity of the 110 crystal plane during X-ray diffraction. That is, OI = I004 / I110, where I004 is the peak intensity of the 004 crystal plane of the subfilm during X-ray diffraction, and I110 is the peak intensity of the 110 crystal plane of the subfilm during X-ray diffraction.

[0029] It is also understandable that the negative electrode film layer includes the negative electrode active material. Each sub-film layer includes its own negative electrode active material.

[0030] It can also be understood that the negative electrode film layer includes multiple sub-film layers stacked sequentially. The number of sub-film layers can be at least 2 layers, at least 3 layers, at least 4 layers, or at least 5 layers, etc. That is, the negative electrode film layer includes n sub-film layers stacked sequentially, where n is an integer ≥ 2. Optionally, n can be 2, 3, 4, 5, etc.

[0031] In some embodiments, the OI value of adjacent sub-film layers increases by 5 to 30 in the direction away from the negative electrode current collector. Optionally, the OI value of adjacent sub-film layers increases by 8, 10, 12, 15, 18, 20, 22, 25, 28, 30, etc., in the direction away from the negative electrode current collector. Optionally, the OI value of adjacent sub-film layers increases by 10 to 25 in the direction away from the negative electrode current collector. By using an appropriate increase in OI value between adjacent sub-film layers, the gradient of pore formation inside the negative electrode sheet can be kept within a suitable range during charging and discharging, further promoting the transport of lithium ions inside the negative electrode sheet and further improving the cycle performance of the secondary battery.

[0032] In some embodiments, there are at least three sub-film layers, and the OI value of adjacent sub-film layers gradually increases in the direction away from the negative electrode current collector. This allows for further adjustment of the pore-forming gradient inside the negative electrode, further improving the transport conditions of lithium ions inside the negative electrode and further improving the cycle performance of the secondary battery.

[0033] In some embodiments, there are at least three sub-film layers, and the absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers in the direction away from the negative electrode current collector is ≤25. It is understood that the absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers in the direction away from the negative electrode current collector is greater than 0. Optionally, the absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers in the direction away from the negative electrode current collector is 1, 3, 5, 8, 10, 12, 15, 18, 20, 22, 25, etc. Optionally, the absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers in the direction away from the negative electrode current collector is 5 to 15. It is understandable that when there are at least three sub-film layers, in the direction away from the negative electrode current collector, there is an increase in the OI value between adjacent sub-film layers. This increase is the difference in OI value between the sub-film layer with the larger OI value and the sub-film layer with the smaller OI value. In this case, the absolute value of the difference in the increase in OI value between adjacent sub-film layers represents the absolute value of the difference between these OI values. For example, when there are three sub-film layers, in the direction away from the negative electrode current collector, they are the first, second, and third sub-film layers. The increase in OI value between adjacent sub-film layers is the difference between the OI value of the second sub-film layer and the first sub-film layer, and the difference between the OI value of the third sub-film layer and the second sub-film layer, respectively. In this case, the difference in the increase in OI value between adjacent sub-film layers = (OI value of the third sub-film layer - OI value of the second sub-film layer) - (OI value of the second sub-film layer - OI value of the first sub-film layer).

[0034] In some embodiments, the Dv50 of the active material in each sub-film layer gradually decreases along the direction away from the negative electrode current collector. Optionally, the decrease in Dv50 of the active material in adjacent sub-film layers along the direction away from the negative electrode current collector is 4 μm to 12 μm. Optionally, the decrease in Dv50 of the active material in adjacent sub-film layers along the direction away from the negative electrode current collector is 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, etc.

[0035] In some embodiments, there are at least three sub-film layers, and the rate of decrease in Dv50 of the active material in adjacent sub-film layers gradually decreases along the direction away from the negative electrode current collector. Optionally, there are at least three sub-film layers, and the absolute value of the difference in the rate of decrease in Dv50 of the active material in adjacent sub-film layers is ≤8 μm along the direction away from the negative electrode current collector. It is understood that the absolute value of the difference in the rate of decrease in Dv50 of the active material in adjacent sub-film layers is greater than 0 along the direction away from the negative electrode current collector. Optionally, the absolute value of the difference in the rate of decrease in Dv50 of the active material in adjacent sub-film layers is 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc., along the direction away from the negative electrode current collector. It is understandable that when there are at least three sub-film layers, in the direction away from the negative electrode current collector, there is a decrease in the Dv50 of the active material between adjacent sub-film layers. That is, the difference in Dv50 between the sub-film layer with a larger Dv50 and the sub-film layer with a smaller Dv50. In this case, the absolute value of the difference in the decrease in Dv50 between adjacent sub-film layers represents the absolute value of the difference between these Dv50 values. For example, when there are three sub-film layers, in the direction away from the negative electrode current collector, they are the first sub-film layer, the second sub-film layer, and the third sub-film layer. The decrease in Dv50 between adjacent sub-film layers is the difference in Dv50 between the first and second sub-film layers, and the difference in Dv50 between the second and third sub-film layers, respectively. At this time, the difference in the magnitude of the decrease in Dv50 of the active material of the adjacent sub-film layers = (Dv50 of the active material of the first sub-film layer - Dv50 of the active material of the second sub-film layer) - (Dv50 of the active material of the second sub-film layer - Dv50 of the active material of the third sub-film layer).

[0036] It is understood that in this application, Dv50 refers to the particle size corresponding to the cumulative particle size distribution number reaching 50% in the volume cumulative distribution curve. Physically, it means that 50% of the particles are smaller (or larger) than Dv50. As an example, Dv50 can be conveniently determined using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0037] In some implementations, the thickness of each sub-film layer is equal. Equal thickness of each sub-film layer can improve the manufacturing efficiency of the negative electrode while maintaining both high energy density and good cycle performance.

[0038] In other embodiments, the thickness of each sub-film layer gradually increases in the direction away from the negative electrode current collector. In some embodiments, there are at least three sub-film layers, and the thickness of adjacent sub-film layers increases gradually in the direction away from the negative electrode current collector.

[0039] In some embodiments, the thickness of the negative electrode film is 70 μm to 100 μm. It is understood that the thickness of the negative electrode film refers to the thickness of the negative electrode film on one side of the negative electrode current collector. That is, when a negative electrode film is provided on one surface of the negative electrode current collector, the thickness of the negative electrode film is 70 μm to 100 μm. When a negative electrode film is provided on both surfaces of the negative electrode current collector, the thickness of the negative electrode film on both surfaces is 70 μm to 100 μm. Optionally, the thickness of the negative electrode film is 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, etc.

[0040] In some embodiments, the active material of each sub-film layer includes one or more of single-particle graphite and secondary-granulated graphite. Optionally, the single-particle graphite can be at least one of natural graphite and artificial graphite. Optionally, the secondary-granulated graphite can be at least one of natural graphite and artificial graphite.

[0041] Optionally, when the active material of the subfilm layer includes single-particle graphite and secondary granulated graphite, the mass ratio of single-particle graphite to secondary granulated graphite is 1:9 to 3:7. Optionally, the mass ratio of single-particle graphite to secondary granulated graphite is 1:9, 2:8, 3:7, etc.

[0042] It should be noted that the secondary granulated graphite in this application is prepared by the following method: mixing single-particle graphite precursor with binder asphalt, passing it through a horizontal granulator to obtain secondary particle precursor, then graphitizing it, crushing and classifying it to obtain secondary granulated graphite.

[0043] In some embodiments, the compaction density of each sub-film layer is 1.6 g / cm³.3 ~1.7g / cm 3 Optionally, the compaction density of each sub-membrane layer is 1.65 g / cm³. 3 .

[0044] In some embodiments, the porosity of each sub-film layer is 24% to 29%. After the first charge and discharge, the porosity of each sub-film layer is 32% to 45%.

[0045] In some embodiments, there are at least two sub-film layers, and the OI values ​​of the two sub-film layers are 8-20 and 20-50 respectively in the direction away from the negative electrode current collector; or, there are at least three sub-film layers, and the OI values ​​of the three sub-film layers are 8-20, 20-25 and 25-50 respectively in the direction away from the negative electrode current collector.

[0046] In some embodiments, there are at least two sub-film layers, and the Dv50 of the active materials of the two sub-film layers is 12μm to 24μm and 4μm to 12μm respectively in the direction away from the negative electrode current collector; or, there are at least three sub-film layers, and the Dv50 of the active materials of the three sub-film layers is 20μm to 24μm, 12μm to 20μm and 4μm to 12μm respectively in the direction away from the negative electrode current collector.

[0047] In some embodiments, the sub-film layer has at least two layers. In the direction away from the negative electrode current collector, the active material of the sub-film layer closer to the negative electrode current collector can have a Dv50 of 12 μm, 15 μm, 18 μm, 20 μm, 24 μm, etc. The active material of the other sub-film layer can have a Dv50 of 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, etc.

[0048] In some embodiments, the sub-film layer has at least three layers. In the direction away from the negative electrode current collector, the Dv50 of the active material in the sub-film layer closest to the negative electrode current collector can be 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, etc. The Dv50 of the active material in the next closest sub-film layer can be 12 μm, 15 μm, 18 μm, 20 μm, etc. The Dv50 of the active material in the other sub-film layer can be 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, etc.

[0049] In some embodiments, there are at least two sub-membrane layers, and the thickness ratio of the two sub-membrane layers in the direction away from the negative electrode current collector is 1:9 to 4:6. Optionally, the thickness ratio of the two sub-membrane layers is 1:9, 2:8, 3:7, or 4:6. Alternatively, there are at least three sub-membrane layers, and the thickness ratio of the three sub-membrane layers in the direction away from the negative electrode current collector is (1-3):(1-3):(4-8). Optionally, the thickness ratio of the three sub-membrane layers is 1:1:8, 1:2:7, 1:3:6, 2:2:6, 2:3:5, or 3:3:4.

[0050] In some embodiments, a negative electrode film layer is disposed on at least one surface of the negative electrode current collector, and two sub-film layers are stacked sequentially on corresponding surfaces of the negative electrode current collector. Along the direction away from the negative electrode current collector, the OI values ​​of the two sub-film layers are 8–20 and 20–50, respectively. Optionally, along the direction away from the negative electrode current collector, the Dv50 of the active material of the two sub-film layers is 12 μm–24 μm and 4 μm–12 μm, respectively. Optionally, along the direction away from the negative electrode current collector, the thickness ratio of the two sub-film layers is 1:9 to 4:6.

[0051] In some embodiments, a negative electrode film layer is disposed on at least one surface of the negative electrode current collector, and there are three sub-film layers, which are sequentially stacked on the corresponding surfaces of the negative electrode current collector. Along the direction away from the negative electrode current collector, the OI values ​​of the three sub-film layers are sequentially 8–20, 20–25, and 25–50. Optionally, along the direction away from the negative electrode current collector, the Dv50 of the active material of the three sub-film layers is sequentially 20 μm–24 μm, 12 μm–20 μm, and 4 μm–12 μm. Optionally, along the direction away from the negative electrode current collector, the thickness ratio of the three sub-film layers is (1–3):(1–3):(4–8).

[0052] Another embodiment of this application provides a method for preparing a negative electrode sheet. The preparation method includes the following steps: forming multiple layers of sub-films stacked sequentially on at least one surface of a negative electrode current collector, wherein the OI value of each sub-film layer gradually increases in the direction away from the negative electrode current collector.

[0053] It is understood that forming multiple layers of sequentially stacked sub-films on at least one surface of the negative electrode current collector can be achieved by coating. Optionally, a slurry comprising the active material of each sub-film layer is prepared separately, and then each slurry is coated sequentially.

[0054] Another embodiment of this application provides a secondary battery. The secondary battery includes the negative electrode sheet described above or the negative electrode sheet prepared by the above preparation method.

[0055] In some implementations, the energy density requirements of secondary batteries are constantly increasing. Using lithium-rich materials as the positive electrode material is one method to improve battery energy density. When lithium-rich materials are used as the positive electrode material, a thicker negative electrode sheet is often required. In this case, the negative electrode film layer above at least one surface of the negative electrode current collector needs to be relatively thick. When the negative electrode sheet of this application is matched with a positive electrode sheet including lithium-rich materials, lithium ions can be transported more efficiently in the negative electrode sheet, enabling the battery to maintain high energy density and good cycle efficiency.

[0056] In some embodiments, the active material of the positive electrode of the secondary battery is a lithium-rich material; optionally, the lithium-rich material includes nLi₂MnO₃·(1-n)LiNi. x Mn (1-x-y) M y O2, wherein 0.05≤n≤0.4, 0.3≤x≤1, 0≤y≤0.1, and M is selected from one or more of Na, Mg, Al, Ca, Ba, V, Zn, Ti, Fe, Co, Cr, Nb, W, Mo, Zr, Ta, and Hf.

[0057] Another embodiment of this application provides a battery module. This battery module includes the aforementioned secondary battery.

[0058] Another embodiment of this application provides a battery pack. The battery pack includes the aforementioned secondary battery or the aforementioned battery module.

[0059] Another embodiment of this application provides an electrical device. The electrical device includes at least one of the above-described secondary battery, battery module, and battery pack.

[0060] The following description, with reference to the accompanying drawings, will explain the secondary battery, battery module, battery pack, and electrical device.

[0061] 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.

[0062] [Positive electrode plate]

[0063] 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, the positive electrode film layer including the positive electrode active material of the first aspect of this application.

[0064] 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.

[0065] 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 on the polymer substrate. Optionally, the metal material may include, but is not limited to, one or more of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. Optionally, the polymer substrate may include, but is not limited to, one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0066] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion 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 battery positive electrode active materials 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, at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds. Optionally, lithium cobalt oxide includes LiCoO2. Lithium nickel oxide includes LiNiO2. Lithium manganese oxide includes at least one of LiMnO2 and LiMn2O4. Lithium nickel cobalt manganese oxide includes LiNi... 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM 333 LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM 523 LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM 211 LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622 ) and LiNi 0.8 Co 0.1 Mn0.1 O2(NCM 811 At least one of the following. Lithium nickel cobalt aluminum oxides include LiNi 0.85 Co 0.15 Al 0.05 O2. Examples of lithium phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate, lithium iron phosphate and carbon composites, lithium manganese phosphate, lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites. Optionally, lithium iron phosphate includes LiFePO4 (LFP). Lithium manganese phosphate includes LiMnPO4.

[0067] In some embodiments, when the secondary battery is a sodium-ion battery, the positive electrode active material can be any positive electrode active material known in the art for sodium-ion batteries. As an example, the positive electrode active material can be a single material or a combination of two or more. The positive electrode active material can be selected from sodium-iron composite oxides, sodium-cobalt composite oxides, sodium-chromium composite oxides, sodium-manganese composite oxides, sodium-nickel composite oxides, sodium-nickel-titanium composite oxides, sodium-nickel-manganese composite oxides, sodium-iron-manganese composite oxides, sodium-nickel-cobalt-manganese composite oxides, sodium-iron phosphate compounds, sodium-manganese phosphate compounds, sodium-cobalt phosphate compounds, Prussian blue-based materials, polyanionic materials, etc., but this application is not limited to these materials. Other conventionally known materials that can be used as positive electrode active materials for sodium-ion batteries can also be used. Optionally, the sodium-iron composite oxide includes NaFeO2. The sodium-cobalt composite oxide includes NaCoO2. The sodium-chromium composite oxide includes NaCrO2. The sodium-manganese composite oxide includes NaMnO2. The sodium-nickel composite oxide includes NaNiO2. The sodium-nickel-titanium composite oxide includes NaNi 1 / 2 Ti 1 / 2 O2. Sodium-nickel-manganese composite oxides include NaNi 1 / 2 Mn 1 / 2 O2. Sodium-iron-manganese composite oxides include Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2. Sodium-nickel-cobalt-manganese composite oxides include NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2. Sodium iron phosphate compounds include NaFePO4. Sodium manganese phosphate compounds include NaMnPO4. Sodium cobalt phosphate compounds include NaCoPO4. Polyanionic materials include at least one of phosphates, fluorophosphates, pyrophosphates, and sulfates.

[0068] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0069] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder 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 electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0071] [Negative electrode plate]

[0072] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material. The negative electrode sheet can be the above-described negative electrode sheet.

[0073] As an example, the negative electrode 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 electrode 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material on the polymer material substrate. Optionally, the metal material includes at least one of copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. The polymer material includes at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0075] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0076] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), polyamide-imide (PAI), polyethyleneimine (PEI), polyimide (PI), and tert-butyl polyacrylate-triethoxyvinylsilane (TBATEVS).

[0077] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0078] In some embodiments, the negative electrode film layer may optionally include other additives, such as thickeners. Optionally, the thickener includes sodium carboxymethyl cellulose (CMC-Na). In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent to form a negative electrode slurry; coating the negative electrode slurry onto a negative electrode current collector, and then performing processes such as drying and cold pressing to obtain the negative electrode sheet. Optionally, the solvent includes deionized water.

[0079] Electrolyte

[0080] 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.

[0081] In some embodiments, the electrolyte comprises an electrolyte salt and a solvent.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] [Isolation membrane]

[0086] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0087] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0088] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0089] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0090] 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.

[0091] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0092] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 using a winding or stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0093] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module 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 module.

[0094] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0095] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0096] 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.

[0097] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0098] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. For example, mobile devices include mobile phones, laptops, etc. Electric vehicles include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0099] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0100] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0101] 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.

[0102] Example

[0103] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0104] The preparation method of lithium-ion secondary batteries is as follows:

[0105] (1) Preparation of positive electrode sheet.

[0106] The lithium-rich cathode material 0.4Li2MnO3·0.6LiNi 0.5 Mn 0.5 O2, conductive agent acetylene black, and binder PVDF (polyvinylidene fluoride) are mixed in a weight ratio of 94:4:2. N-methylpyrrolidone solvent is added and the mixture is stirred thoroughly to obtain a positive electrode slurry. The slurry is then coated on both surfaces of the positive electrode current collector aluminum foil, dried, and cold-pressed to obtain the positive electrode sheet.

[0107] (2) Preparation of negative electrode sheet.

[0108] ① Preparation of slurry for each sub-membrane layer: The active material, conductive agent acetylene black, binder SBR (styrene-butadiene rubber), and binder CMC (sodium carboxymethyl cellulose) of each sub-membrane layer are mixed in a weight ratio of 95:1.5:3.1:0.4. Deionized water is added as solvent, and the mixture is stirred thoroughly to obtain the slurry for each sub-membrane layer.

[0109] ② The slurry of each sub-film layer is sequentially coated onto both surfaces of the negative electrode current collector copper foil, dried, and cold-pressed. This forms a multi-layered sub-film layer stacked sequentially on each surface. The compaction density of each sub-film layer is 1.65 g / cm³. 3 .

[0110] (3) Preparation of electrolyte.

[0111] In an argon-atmosphere glove box with a water content of <10ppm, EC (ethylene carbonate), PC (polycarbonate), and DMC (dimethyl carbonate) were mixed in a weight ratio of EC:PC:DMC = 3:3:3. Then, LiPF6, VC (ethylene carbonate), DTD (ethylene sulfate), and PS (propylene sulfite) were added and stirred until homogeneous to obtain the electrolyte. The concentration of LiPF6 in the lithium-ion battery electrolyte was 1 mol / L, and the mass percentages of VC, DTD, and PS were 3%, 1%, and 1%, respectively.

[0112] (4) Preparation of the isolation membrane.

[0113] Polyethylene porous membrane is used as the separation membrane.

[0114] (5) Preparation of lithium-ion secondary batteries.

[0115] The positive electrode sheet obtained in (1), the separator in (4), and the negative electrode sheet obtained in (2) are stacked in sequence, so that the separator is in the middle of the positive and negative electrodes to play a role in isolation, and a bare cell is obtained; the bare cell is placed in the outer packaging, the electrolyte prepared in step (3) is injected, and it is sealed for formation to obtain a lithium-ion secondary battery.

[0116] The examples and comparative examples used the above method to prepare lithium-ion secondary batteries, the difference being the number of sub-film layers and the parameters of the sub-film layers in the negative electrode sheet. The number of sub-film layers and the parameters of the sub-film layers are shown in Table 1.

[0117] In Table 1, the first sub-film layer, the second sub-film layer, the third sub-film layer, and the fourth sub-film layer represent sub-film layers along the direction away from the negative electrode current collector.

[0118] In Table 1, both single-particle graphite and secondary-granulated graphite are artificial graphite. The mixed graphite is composed of single-particle graphite and secondary-granulated graphite mixed at a mass ratio of 3:7.

[0119] Performance testing. The test results are shown in Table 1.

[0120] (1) OI value test of sub-film layer.

[0121] The slurry of each sub-film layer was coated onto both surfaces of the copper foil of the negative electrode current collector, dried, and cold-pressed to 1.65 g / cc to obtain the test electrode. A 2 cm × 2 cm sample was cut from the obtained test electrode and subjected to X-ray diffraction (XRD) to obtain the diffraction pattern. The OI value of the sub-film layer was calculated by reading the peak intensities of the required crystal planes. Wherein, OI = I004 / I110, where I004 is the peak intensity of the 004 crystal plane of the sub-film layer in X-ray diffraction, and I110 is the peak intensity of the 110 crystal plane of the sub-film layer in X-ray diffraction.

[0122] (2) Rate charging performance test.

[0123] Under a constant temperature environment of 25°C, the secondary batteries of the examples and comparative examples were discharged to 2.5V at 1 / 3C. After resting for 5 minutes, they were charged to 4.6V at 1 / 3C, and then charged at 4.6V under constant voltage until the current ≤0.05C. After resting for 5 minutes, the charging capacity at this point was recorded as C0. The batteries were then discharged to 2.5V at 1 / 3C, rested for 5 minutes, and then charged to 4.6V at 3C. After resting for 5 minutes, the charging capacity at this point was recorded as C1. The constant current ratio for 3C charging is (C1 / C0) × 100%.

[0124] (3) Cyclic performance test.

[0125] Under a constant temperature environment of 25℃, the capacitor is charged to 4.6V with a constant current of 1C, then charged to 0.05C with a constant voltage of 4.6V, and then discharged to 2.5V with a constant current of 1C. The discharge specific capacity of the first cycle (Cd1) is obtained. This charging and discharging process is repeated until the 500th cycle, and the discharge specific capacity after 500 cycles is denoted as Cdn.

[0126] Capacity retention rate = (discharge specific capacity after 500 cycles (Cdn) / discharge specific capacity in the first cycle (Cd1)) × 100%.

[0127] (4) Porosity test.

[0128] First, measure the compaction density of the electrode, then calculate its porosity P according to the following formula: P=(1-ρ 压 / ρ 真 )*100%, ρ 真 =1 / (w AM / ρ AM +w CA / ρ CA +w B / ρ B ), where ρ压 ρ is the compaction density of the electrode. 真 w is the weighted true density of the electrode. AM w CA w B These represent the mass fractions of active material, conductive agent, and binder in the electrode, ρ. AM ρ CA ρ B These are the true densities of the active material, conductive agent, and binder in the electrode, respectively.

[0129] (5) Compacted density after the first charge and discharge.

[0130] After the first charge and discharge cycle, the thickness T of the negative electrode sheet was measured, and the compaction density ρ was calculated based on the areal density. 压 The calculation formula is as follows: ρ 压 =m / (TT) 箔 Where m is the areal density of the electrode (g / cm³). 2 T 箔 The thickness of the negative electrode current collector is 1 cm.

[0131] Table 1

[0132]

[0133]

[0134] As can be seen from Table 1, in the negative electrode sheet, when the OI value of each sub-film layer gradually increases along the direction away from the negative electrode current collector, the corresponding battery has a higher capacity retention rate.

[0135] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. The negative electrode film layer includes multiple sub-film layers stacked sequentially, and the OI value of each sub-film layer gradually increases in the direction away from the negative electrode current collector.

2. The negative electrode sheet according to claim 1, characterized in that, Along the direction away from the negative electrode current collector, the OI value of adjacent sub-film layers increases by 5 to 30.

3. The negative electrode sheet according to claim 2, characterized in that, Along the direction away from the negative electrode current collector, the OI value of adjacent sub-film layers increases by 10~25.

4. The negative electrode sheet according to claim 1, characterized in that, The sub-film layer has at least 3 layers, and the OI value of adjacent sub-film layers gradually increases in the direction away from the negative electrode current collector.

5. The negative electrode sheet according to claim 4, characterized in that, The absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers along the direction away from the negative electrode current collector is ≤25.

6. The negative electrode sheet according to claim 5, characterized in that, The absolute value of the difference in the magnitude of the increase in OI value between adjacent sub-film layers along the direction away from the negative electrode current collector is 5 to 15.

7. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, Along the direction away from the negative electrode current collector, the Dv50 of the active material of each sub-film layer gradually decreases.

8. The negative electrode sheet according to claim 7, characterized in that, Along the direction away from the negative electrode current collector, the Dv50 of the active material in the adjacent sub-film layer decreases by 4 μm to 12 μm.

9. The negative electrode sheet according to claim 7, characterized in that, The sub-membrane layer has at least three layers, and the magnitude of the decrease in Dv50 of the active material of adjacent sub-membrane layers gradually decreases in the direction away from the negative electrode current collector.

10. The negative electrode sheet according to claim 9, characterized in that, In the direction away from the negative electrode current collector, the absolute value of the difference in the magnitude of the decrease in Dv50 of the active material of adjacent sub-film layers is ≤8 μm.

11. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The thickness of each of the sub-film layers is equal.

12. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The thickness of each sub-film layer gradually increases in the direction away from the negative electrode current collector.

13. The negative electrode sheet according to claim 12, characterized in that, The sub-film layer has at least three layers, and the thickness of adjacent sub-film layers gradually increases in the direction away from the negative electrode current collector.

14. The negative electrode sheet according to any one of claims 1 to 6, characterized in that, The negative electrode film layer satisfies one or more of the following characteristics: (1) The thickness of the negative electrode film is 70 μm to 100 μm; (2) The active material of each of the sub-film layers includes one or more of single-particle graphite and secondary granulated graphite; (3) The sub-film layer has at least two layers, and the OI values ​​of the two sub-film layers are 8~20 and 20~50 respectively along the direction away from the negative electrode current collector; or, The sub-film layer has at least three layers, and the OI values ​​of the three sub-film layers are 8~20, 20~25 and 25~50 respectively in the direction away from the negative electrode current collector. (4) The sub-film layer has at least two layers, and in the direction away from the negative electrode current collector, the Dv50 of the active material of the two sub-film layers is 12μm~24μm and 4μm~12μm respectively; or, The sub-film layer has at least three layers, and in the direction away from the negative electrode current collector, the Dv50 of the active material of the three sub-film layers is 20μm~24μm, 12μm~20μm and 4μm~12μm respectively. (5) The sub-film layer has at least two layers, and the thickness ratio of the two sub-film layers in the direction away from the negative electrode current collector is 1:9 to 4:6; or, The sub-film layer has at least 3 layers, and the thickness ratio of the 3 sub-film layers in the direction away from the negative electrode current collector is (1~3):(1~3):(4~8).

15. A method for preparing a negative electrode sheet, characterized in that, Includes the following steps: Multiple sub-film layers are sequentially formed on at least one surface of the negative electrode current collector, and the OI value of each sub-film layer gradually increases in the direction away from the negative electrode current collector.

16. A secondary battery, characterized in that, Includes the negative electrode sheet according to any one of claims 1 to 14 or the negative electrode sheet prepared by the preparation method according to claim 15.

17. The secondary battery according to claim 16, characterized in that, The active material of the positive electrode of the secondary battery is a lithium-rich material.

18. The secondary battery according to claim 17, characterized in that, The lithium-rich material includes nLi₂MnO₃·(1-n)LiNi x Mn (1-x-y) M y O2, wherein 0.05≤n≤0.4, 0.3≤x≤1, 0≤y≤0.1, and M is selected from one or more of Na, Mg, Al, Ca, Ba, V, Zn, Ti, Fe, Co, Cr, Nb, W, Mo, Zr, Ta, and Hf.

19. A battery module, characterized in that, The secondary battery includes any one of claims 16 to 18.

20. A battery pack, characterized in that, It includes the secondary battery according to any one of claims 16 to 18 or the battery module according to claim 19.

21. An electrical appliance, characterized in that, It includes at least one of the secondary battery according to any one of claims 16 to 18, the battery module according to claim 19, and the battery pack according to claim 20.

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