Negative electrode sheet, secondary battery, battery pack, battery module, and electrical device

Abstract

The present application provides a negative electrode plate, which includes an active layer 1 and an active layer 2. The active layer 1 includes an active material 1, and the active layer 2 includes an active material 2. The OI value of the powder of the active material 1 is in the range of 8-32, and the OI value of the powder of the active material 2 is in the range of 2-7. The specific capacity of the active material 1 is in the range of 290-350 mAh / g. The specific capacity of the active material 2 is in the range of 350-368 mAh / g. The ratio α of the OI value of the powder of the active material 1 to the OI value of the powder of the active material 2 ranges from 2.00 to 6.25. The negative electrode plate provided by the present application has a low expansion rate during the cycling process, significantly improving the cycling performance and energy density of the secondary battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium batteries, and in particular to a negative electrode sheet that has reduced swelling during cycling. In addition, the present application also relates to a method for preparing the negative electrode sheet, a secondary battery including the negative electrode sheet, a battery pack, a battery module, and an electrical device including the secondary battery. Background Art

[0002] In recent years, with the increasingly wide application range of lithium-ion batteries, lithium-ion batteries are widely used in energy storage power systems such as hydraulic, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Due to the great development of lithium-ion batteries, higher requirements are put forward for their endurance.

[0003] In a lithium-ion battery, the negative electrode sheet swells as the number of cycles of the lithium-ion battery increases, resulting in a continuous increase in the swelling force of the battery, thereby deteriorating the cycle performance of the battery and reducing the endurance of the battery. Therefore, reducing the swelling of the negative electrode sheet is considered the best choice to improve the endurance of the battery. Improving the swelling performance of the negative electrode by means such as coating or binder matching is a relatively effective means at present. However, the existing methods will cause varying degrees of damage to the performance of lithium-ion batteries. For example, the energy density of lithium-ion batteries becomes worse, etc., and it is difficult to achieve the effect of continuously reducing swelling. Therefore, the existing negative electrode sheets still need to be improved. Summary of the Invention

[0004] The present application is made in view of the above problems, and its purpose is to provide a negative electrode sheet that can continuously maintain a low swelling effect during cycling, thereby improving the endurance of the battery.

[0005] To achieve the above object, the present application provides a negative electrode sheet.

[0006] In a first aspect of the present application, there is provided a negative electrode sheet, which includes

[0007] a current collector;

[0008] an active layer 1, located on at least one surface of the current collector and including an active material 1,

[0009] an active layer 2, located on at least one surface of each of the two active layers 1 away from the current collector and including an active material 2;

[0010] The powder OI value of the active material 1 is in the range of 8 - 32, optionally in the range of 10 - 25, and the powder OI value of the active material 2 is in the range of 2 - 7, optionally in the range of 3 - 6;

[0011] The specific capacity of the active material 1 is in the range of 290-350 mAh / g,

[0012] The specific capacity of the active material 2 is in the range of 350-368 mAh / g.

[0013] By limiting the OI value and the specific capacity of the active material within the above ranges, the negative electrode sheet provided by this application can ensure good adhesion to the current collector and a large capacity required for the secondary battery, while effectively reducing the volume expansion of the negative electrode sheet during cycling, thereby improving the cycling performance and energy density of the secondary battery.

[0014] In any embodiment, the ratio α of the powder OI values of the active material 1 to the active material 2 ranges from 2.00 to 6.25.

[0015] Therefore, by limiting the ratio of the powder OI values of the active material 1 to the active material 2 within a certain range, this application effectively reduces the expansion of the negative electrode sheet during cycling.

[0016] In any embodiment, in the negative electrode sheet, the ratio β of the specific capacity of the active material 1 to the specific capacity of the active material 2 ranges from 0.80 to 1.00.

[0017] This application further limits the range of the ratio β of the specific capacity of the active material in the active layer 1 to the specific capacity of the active material in the active layer 2, and further achieves the effect of reducing the expansion of the negative electrode sheet while ensuring the battery performance.

[0018] In any embodiment, the value of the ratio α / β of α to β ranges from 2.0 to 6.7.

[0019] By limiting the differences in the specific capacity and the OI value between the active layer 1 and the active layer 2 of the negative electrode sheet within a certain range, unexpectedly, high adhesion can be ensured, and at the same time, the effects of low expansion and high energy density can be achieved. When this electrode sheet is used in a secondary battery, it can improve the endurance of the secondary battery.

[0020] In this application, artificial graphite is the active material used in the active layer for preparing the negative electrode sheet. In any embodiment, the volume average particle size Dv50 of the artificial graphite for the active material 1 can be 8-24 μm, optionally 10-20 μm; the graphitization degree is 85.0%-90.0%, optionally 86.0%-89.9%. The volume average particle size Dv50 of the artificial graphite for the active material 2 can be 6-24 μm, optionally 8-20 μm; the graphitization degree is 90.0%-97.5%, optionally 90.2%-96.8%.

[0021] In any embodiment, the powder resistivity of the artificial graphite for active material 1 under a pressure of 8 MPa can be 0.035 Ω·cm or less, and can optionally be 0.025 Ω·cm or less. The powder resistivity of the artificial graphite for active material 2 under a pressure of 8 MPa can be 0.035 Ω·cm or less, and can optionally be 0.025 Ω·cm or less.

[0022] In any embodiment, the areal density of active layer 1 is 3 - 10 mg / cm 2 , and can optionally be 4 - 8 mg / cm 2 . The areal density of active layer 2 is 3 - 10 mg / cm 2 , and can optionally be 4 - 8 mg / cm 2 .

[0023] In any embodiment, in the negative electrode tab, based on the total weight of active layer 1, the proportion of active material 1 ranges from 92.0 to 98.99 wt%. In some embodiments, in the negative electrode tab, based on the total weight of active layer 2, the proportion of active material 2 ranges from 92.0 to 98.99 wt%.

[0024] In any embodiment, the porosity of the negative electrode tab is 18.0% - 40.2%, and can optionally be 22.5% - 35.0%.

[0025] In any embodiment, the tap density of the negative electrode tab is 1.45 - 1.90 g / cm 3 , and can optionally be 1.55 - 1.80 g / cm 3 .

[0026] In any embodiment, the thickness of active layer 1 after cold pressing is 20 - 135 μm, and can optionally be 30 - 120 μm. The thickness of active layer 2 after cold pressing is 20 - 135 μm, and can optionally be 30 - 120 μm.

[0027] In any embodiment, active layer 1 and active layer 2 also respectively include a conductive agent, a binder, and a thickener that are conventionally used in the negative electrode tab in the art.

[0028] In any embodiment, in addition to artificial graphite, active material 1 in active layer 1 also includes a small amount (mass content ≤ 10 wt%, based on all active materials) of other carbon materials, such as hard carbon and soft carbon.

[0029] In any embodiment, in addition to artificial graphite, active material 2 in active layer 2 also includes a small amount (mass content ≤ 10 wt%, based on all active materials) of other carbon materials, such as hard carbon and soft carbon.

[0030] The second aspect of the present application provides a secondary battery, which includes the negative electrode sheet described in the first aspect of the present application.

[0031] The third aspect of the present application provides a battery module, which includes the secondary battery described in the second aspect of the present application.

[0032] The fourth aspect of the present application provides a battery pack, which includes the battery module described in the third aspect of the present application.

[0033] The fifth aspect of the present application provides an electrical device, which includes at least one of the secondary battery described in the fourth aspect of the present application, the battery module described in the fourth aspect of the present application, or the battery pack described in the fifth aspect of the present application.

[0034] [Advantageous Effects]

[0035] For the negative electrode sheet designed by the inventor of the present application, by reasonably selecting the active materials of the active layer 1 and the active layer 2, and comprehensively regulating the specific capacity and OI value to be within a reasonable range, the swelling balance of the entire negative electrode sheet is regulated, and the swelling of the overall negative electrode sheet is significantly reduced. By comprehensively regulating the powder OI value and specific capacity of the active material 1 and utilizing the synergistic effect of the two, a negative electrode sheet with high specific capacity and low expansion rate is developed. The secondary battery prepared therefrom has both high energy density and cycling performance.

[0036] The battery module, battery pack and electrical device of the present application include the lithium-ion battery provided by the present application, and thus have at least the same advantages as the lithium-ion battery. Description of the Drawings

[0037] Figure 1 is a cross-sectional view of the negative electrode sheet along the thickness direction according to an embodiment of the present application.

[0038] Figure 2 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0039] Figure 3 is Figure 2 the exploded view of the secondary battery according to an embodiment of the present application shown.

[0040] Figure 4 is a schematic diagram of a battery module according to an embodiment of the present application.

[0041] Figure 5 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0042] Figure 6 is Figure 5 the exploded view of the battery pack according to an embodiment of the present application shown.

[0043] Figure 7Schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application.

[0044] 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed implementation mode

[0045] Hereinafter, embodiments of the negative electrode sheet, its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack, and electrical device of the present application will be specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to avoid the following descriptions from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the accompanying drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0046] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a specific range. The ranges defined in this way can include or exclude the end values and can be combined arbitrarily, 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, ranges of 60 - 110 and 80 - 120 are also contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 6. In the present application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" have been fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when it is stated that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] If there is no special instruction, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0048] If there is no special instruction, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0049] Unless otherwise specified, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) carried out sequentially, or may also include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or may include steps (a), (c), and (b), or may also include steps (c), (a), and (b), etc.

[0050] Unless otherwise specified, the terms "comprising" and "including" mentioned in this application are open-ended and can also be closed-ended. For example, the terms "comprising" and "including" may mean that other components not listed may also be included or contained, or may only include or contain the listed components.

[0051] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) while B is true (or exists); or both A and B are true (or exist).

[0052] The inventors of this application found that during the cycling process of a secondary battery, due to the repeated lithium insertion and extraction processes of the negative electrode plate, the irreversible lattice change of the negative electrode active material occurs, resulting in the swelling of the volume of the negative electrode active layer. On the one hand, the relatively large swelling of the electrode plate increases the risk of the negative electrode active material layer peeling off from the surface of the current collector. On the other hand, it also causes a large number of cracks in all directions inside the electrode plate, significantly deteriorating the cycling performance of the battery. Secondly, due to the large number of cracks inside the swollen electrode plate, the electrolyte will be concentrated at the gaps rather than evenly dispersed into the pore structures of the negative electrode active material layer, resulting in poor electrolyte infiltration, thereby deteriorating the cycling performance of the battery.

[0053] In particular, in order to continuously increase the specific capacity of the negative electrode active material to improve the overall energy density of the battery, the above-mentioned adverse conditions become more serious, and the degree of deterioration of the battery performance is also more significant.

[0054] Based on the above-mentioned discovered technical problems, the inventors of this application started from the modification of the negative electrode plate and developed a negative electrode plate with a high specific capacity and a low expansion rate at the same time. The secondary battery prepared therefrom has both a high energy density and a good cycling performance.

[0055] [Negative electrode plate]

[0056] Referring to Figure 1 , this application provides a negative electrode plate, which includes

[0057] Current collector;

[0058] Active layer 1, located on at least one surface of the current collector and comprising active material 1,

[0059] Active layer 2, located on at least one surface of each of the two active layers 1 away from the current collector and comprising active material 2;

[0060] The OI value of the powder of the active material 1 is in the range of 8 - 32, optionally in the range of 10 - 25,

[0061] The OI value of the powder of the active material 2 is in the range of 2 - 7, optionally in the range of 3 - 6;

[0062] The specific capacity of the active material 1 is in the range of 290 - 350 mAh / g,

[0063] The specific capacity of the active material 2 is in the range of 350 - 368 mAh / g.

[0064] The negative electrode plate of the present application comprises active layer 1 and active layer 2, and the OI value of the powder of the active material 1 in active layer 1 is greater than the OI value of the powder of the active material 2 in active layer 2, and the specific capacity of the active material 2 is not lower than the specific capacity of the active material 1. The design reasons are as follows:

[0065] The OI value of the powder of the negative electrode active material in active layer 2 is low and the degree of isotropy is high. Even if the microstructure undergoes irreversible lattice expansion, the expansion stress can be dispersed in all directions, thereby preventing the generation of cracks in all directions in the negative electrode active layer; However, unexpectedly, the negative electrode active material with a smaller OI value has a poor adhesion force with the current collector, so active layer 2 is prone to detachment from the current collector. Therefore, in the present application, active layer 1 is provided between active layer 2 and the surface of the current collector. The OI value of the powder of the active material 1 in active layer 1 is high, which can significantly improve the adhesion force between the entire active layer and the surface of the current collector.

[0066] However, when the OI value of the powder of the active material 1 is designed to be large in order to improve the adhesion force between the overall active layer and the surface of the current collector, the expansion rate of active layer 1 will be aggravated. Therefore, in order to reduce the expansion caused by the large OI value of the negative electrode active material, the specific capacity of the active material 1 in active layer 1 is designed to be small in the present application, so as to slow down the irreversible expansion of the active layer caused by the repeated insertion and extraction of lithium ions.

[0067] In addition, from the perspective of overall improving the energy density of the negative electrode plate, the specific capacity of the active material 2 in active layer 2 is designed to be large in the present application, so as to overall improve the energy density of the negative electrode plate.

[0068] In summary, the inventors of the present application designed a negative electrode plate. By reasonably selecting the active materials of active layer 1 and active layer 2, and comprehensively regulating the specific capacity and OI value to be within a reasonable range, the swelling balance of the entire negative electrode plate is regulated, and the swelling of the overall negative electrode plate is significantly reduced. Through comprehensive regulation of the powder OI value and specific capacity of active material 1, and utilizing the synergistic effect of the two, a negative electrode plate with high specific capacity and low expansion rate is developed. The secondary battery prepared therefrom has both high energy density and cycling performance.

[0069] In the present application, the defined OI value of the active material powder is: in the X-ray diffraction pattern of the active material powder, the ratio of the peak area C004 of the diffraction peak of the (004) crystal plane to the peak area C110 of the diffraction peak of the (110) crystal plane, that is, OI value = C004 / C110.

[0070] In the present application, the test process of the OI value of the active material is as follows: It is tested using an X-ray diffractometer, and the test process is carried out according to JISK0131-1996. The X-ray diffraction pattern of the active material is obtained by testing, and after analysis and calculation by XRD pattern analysis software such as Highscore Plus or Jade, the peak area C004 of the diffraction peak of the (004) crystal plane and the peak area C110 of the diffraction peak of the (110) crystal plane are obtained. OI value = C004 / C110.

[0071] In the present application, the defined specific capacity of the active material is: the ratio of the capacitance (mAh) that the active material can release to the mass (g) of the active material.

[0072] In the present application, the specific capacity of the active material can be determined by the following method: The active material, conductive agent, and binder are mixed evenly with a solvent according to a predetermined mass ratio to form a slurry. The prepared slurry is coated on a copper foil current collector and dried in an oven for standby. A lithium metal sheet is used as the counter electrode. A polyethylene (PE) film is used as the separator. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1, and then LiPF6 is uniformly dissolved in the above solution to obtain an electrolyte, where the concentration of LiPF6 is 1 mol / L. The above components are assembled into a CR2430 type button cell in a glove box under argon protection.

[0073] After the obtained button cell is left standing for 12 hours, it is discharged at a constant current of 0.05C until 0.005V, and left standing for 10 minutes. It is then discharged at a constant current of 50 μA until 0.005V, and left standing for 10 minutes. It is then discharged at a constant current of 10 μA until 0.005V. Then it is charged at a constant current of 0.1C until 2V. The charging capacity is recorded. The ratio of the charging capacity to the mass of artificial graphite is the specific capacity of the prepared artificial graphite.

[0074] Optionally, the OI value of the powder of the active substance 1 can be 8, 9, 10, 11, 12, 16, 25, 32, or the value can be within the range formed by any two of the above values. Optionally, the OI value of the powder of the active substance 2 can be 2, 3, 4, 6, 7, or the value can be within the range formed by any two of the above values. Optionally, the gram capacity of the active substance 1 can be 290, 310, 320, 340, 350, or the value can be within the range formed by any two of the above values. Optionally, the gram capacity of the active substance 2 can be 350, 355, 360, 365, 368, or the value can be within the range formed by any two of the above values.

[0075] In some embodiments, the range of the ratio α of the OI value of the powder of the active substance 1 to the OI value of the powder of the active substance 2 is 2.00 - 6.25.

[0076] In terms of the OI value, the present application uses the ratio α of the OI value of the active substance in the active layer 1 to the OI value of the active substance in the active layer 2 to characterize the difference in the ability of the two active layers to reduce swelling. The inventors found that only when the difference in the OI values of the active substances in the two active layers is within a certain range can the swelling of the negative electrode tab be reduced while ensuring the battery performance.

[0077] Optionally, the value of α can be 2.00, 2.29, 2.50, 2.75, 3.00, 4.00, 4.50, 5.33, 6.25, or the value can be within the range formed by any two of the above values.

[0078] In some embodiments, in the negative electrode tab, the range of the ratio β of the gram capacity of the active substance 1 to the gram capacity of the active substance 2 is 0.80 - 1.00.

[0079] In terms of the gram capacity, the present application uses the ratio β of the gram capacity of the active substance in the active layer 1 to the gram capacity of the active substance in the active layer 2 to characterize the difference in the ability of the two active layers to reduce swelling. The inventors found that only when the difference in the gram capacities of the active substances in the two active layers is within a certain range can the swelling of the negative electrode tab be reduced while ensuring the battery performance.

[0080] In some embodiments, the value range of the ratio α / β of α to β is 2.0 - 6.7.

[0081] The OI value represents the degree of order of the material in a certain direction, and this degree of order determines the distribution and transfer ability of the electrical properties of the material in the whole material; during the dynamic operation of the electrode, a reasonable match is required between the gram capacity and the OI value between the two layers to ensure a smooth transition of the electrical properties between the two layers without "drop-off", thereby ensuring the stable operation of the electrode sheet. By observing the relationship between the gram capacity and the OI value, unexpectedly, the inventors of the present application also found that the difference degree characterized by the OI value and the difference degree characterized by the gram capacity should not be too small or too large. If the difference is too small or too large, it will cause large expansion of the negative electrode sheet. Therefore, the value range of the ratio α / β of α to β is 2.0 - 6.7.

[0082] Optionally, the active layer 1 is located on two surfaces of the current collector, and the active layer 2 is located on two surfaces of the two active layers 1 away from the current collector respectively.

[0083] In the negative electrode sheet of the present application, the areal density of the active layer 1 is 3 - 10 mg / cm 2 , and can be optionally 4 - 8 mg / cm 2 . The areal density of the active layer 2 is 3 - 10 mg / cm 2 , and can be optionally 4 - 8 mg / cm 2 .

[0084] The areal density of the active layer indicates the coating quality of the electrode sheet. If it is too low, it will affect the energy density of the battery; if it is too high, it will lead to too poor kinetic performance of the battery. Therefore, the areal density of the active layer needs to be controlled within a certain range. In the present application, the areal density is measured according to the methods commonly used in the art, and can also be tested by the following method: Punch 15 negative electrode sheets and current collectors (from the same production batch as the current collector used for the negative electrode sheet) with a certain area S (unit: cm 2 ), weigh the mass, and calculate the average value. The average mass of the negative electrode sheet is M1 (unit: mg), and the average mass of the current collector is M2 (unit: mg); when the active material layer is only provided on one side of the current collector, the areal density is: (M1 - M2) / S, and when the active material layer is provided on both sides of the current collector, the areal density is: (M1 - M2) / 2S.

[0085] In some embodiments, in the negative electrode tab, based on the total weight of the active layer 1, the proportion of the active material 1 ranges from 92.0% to 98.99% by weight. In some embodiments, in the negative electrode tab, based on the total weight of the active layer 2, the proportion of the active material 2 ranges from 92.0% to 98.99% by weight. When the proportion of the active material in the active layer of the negative electrode tab is relatively low, for example, lower than the above range, the coating quality of the negative electrode is relatively high. However, the energy density of the secondary battery is low; when the proportion of the active material in the active layer of the negative electrode tab is relatively high, for example, higher than the above range, the processability of the slurry formed by the active material and other raw materials for preparing the active layer is poor.

[0086] In some embodiments, in the negative electrode tab, the porosity of the negative electrode tab is 18.0% to 40.2%, and optionally 22.5% to 35.0%. When the porosity of the negative electrode tab is relatively low, it is difficult for the electrolyte to infiltrate, the liquid-phase conduction of lithium ions is restricted, and the kinetic performance of the tab is poor; when the porosity of the negative electrode tab is relatively high, more mass of the electrolyte is required to fill the pores, resulting in a lower energy density of the secondary battery.

[0087] The porosity of the negative electrode tab described in this application refers to the porosity of the entire active layer on the surface of the negative electrode current collector. The specific test process can be carried out according to the following steps: Calculate V1: Punch 30 small round pieces with a diameter of 13 mm from the negative electrode tab, and place them in a true density meter to test the true volume of the samples. Calculate the average true volume of the above 30 small round pieces, and record this average value as V1; Calculate V2: V2 is the average apparent volume of the above 30 small round pieces, and V2 = S * H * A, where S is the average area of the above 30 small round pieces; H is the thickness of the tab; A is the number of samples; then the porosity P = (V2 - V1) / V2 * 100%. The test process of the porosity of the negative electrode tab refers to GB / T 24586-2009.

[0088] In addition, according to this application, the higher the tap density of the negative electrode tab, the higher the energy density of the battery. However, if the tap density is too high, some power performance will be lost. Therefore, the tap density of the negative electrode tab should be controlled within a suitable range. Therefore, in the negative electrode tab of this application, the tap density is 1.45 - 1.90 g / cm 3 , and optionally 1.55 - 1.80 g / cm 3 .

[0089] In this article, the tap density of the negative electrode tab can be tested according to the following method: Use a micrometer to measure the thickness of the cold-pressed negative electrode tab and the current collector (the same production batch as the current collector used for the negative electrode tab), take 15 points, and the average thickness of the negative electrode tab is H A (unit: μm), and the average thickness of the current collector is H B(unit: μm); when the active material is only disposed on one side of the current collector, the tap density is: 10 × areal density / (H A -H B ); when the active material is disposed on both sides of the current collector, the tap density is: 20 × areal density / (H A -H B ). The tap density of the negative electrode sheet can be controlled by conventional technical means in the art, for example, by adjusting the raw materials, graphitization degree, and particle structure to regulate the tap density.

[0090] In this application, the thickness of the active layer 1 after cold pressing is 20 - 135 μm, optionally 30 - 120 μm, and the thickness of the active layer 2 after cold pressing is 20 - 135 μm, optionally 30 - 120 μm.

[0091] In addition, the active layer 1 and the active layer 2 also respectively include a conductive agent, a binder, and a thickener that are conventionally used for negative electrode sheets in the art. In the active layer 1 and the active layer 2, the proportion of the active material is in the range of 92.0 - 98.99% by weight, respectively based on all components used to prepare the active layer 1 and 2. The ratio of the active material to the conductive agent, binder, and thickener in the active layer 1 and the active layer 2 is (92.0 - 98.99% by weight): (0.01 - 2% by weight): (0.5 - 3.5% by weight): (0.5 - 2.5% by weight), and the optional ratio is 96.5% by weight: 0.6% by weight: 1.8% by weight: 1.1% by weight.

[0092] The conductive agent can be a conductive agent conventionally used for negative electrode sheets in the art. Optionally, the conductive agent can be one or more of graphene, acetylene black, single-walled carbon nanotubes, multi-walled carbon nanotubes, superconducting carbon, carbon black, Ketjen black, carbon dots, and carbon nanofibers.

[0093] The binder can be a conductive agent conventionally used for negative electrode sheets in the art. Optionally, the binder can be one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0094] The thickener can be a thickener conventionally used for negative electrode sheets in the art. Optionally, the thickener can be one or more of sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose, and sodium alginate.

[0095] In some embodiments, in addition to artificial graphite, the active material 1 in the active layer 1 also includes a small amount (mass content ≤ 10% by weight, based on all active materials) of other carbon materials, such as hard carbon and soft carbon.

[0096] In some embodiments, in addition to artificial graphite, the active material 2 in the active layer 2 further includes a small amount (mass content ≤ 10% by weight, based on all active materials) of other carbon materials, such as hard carbon and soft carbon.

[0097] In the present application, the active material involved is a carbon material, especially graphite, particularly artificial graphite, hard carbon, soft carbon, etc. Graphite is composed of carbon elements and has a layered structure. Carbon atoms within the layer form σ bonds through SP2 heterocycles, with strong forces, and the van der Waals forces between layers are relatively weak. In particular, artificial graphite is used in the present application, which can be a single artificial graphite or a mixture prepared by blending two or more artificial graphites with different structures and properties. The artificial graphite or the mixture of artificial graphites in the present application has a specific powder OI value and specific capacity, and can achieve the effect of reducing swelling during the cycling process. Soft carbon, namely easily graphitizable carbon, refers to amorphous carbon that can be graphitized at a high temperature above 2500 °C. Soft carbon has a low crystallinity (i.e., graphitization degree), small grain size, and a large interplanar spacing (d 002 ), has good compatibility with the electrolyte, but has a relatively high irreversible capacity during the first discharge, a lower output voltage, and no obvious charge-discharge platform potential. Common soft carbons include petroleum coke, needle coke, carbon fiber, carbon microspheres, etc. Hard carbon refers to difficult-to-graphitize carbon, which is the pyrolytic carbon of polymer polymers. Such carbon is difficult to graphitize even at a high temperature above 2500 °C. Common hard carbons include resin carbon, such as phenolic resin, epoxy resin, polyfurfuryl alcohol PEA-C, etc. The inventors of the present application have found that mixing specific artificial graphite with a small amount of hard carbon and soft carbon can also achieve the effect of reducing the swelling of the negative electrode plate during the cycling process.

[0098] In some embodiments, artificial graphite with an OI value and specific capacity within the above ranges of the present application can be selected from commercially available artificial graphites, or artificial graphite with the said OI value and specific capacity can be prepared artificially. For example, by adjusting the raw materials of artificial graphite (such as petroleum green coke, needle green coke, calcined petroleum coke, calcined needle coke, metallurgical coke, pitch coke, etc.) and the amount of binder added, artificial graphite with the said OI value can be obtained. For example, in the case of using needle green coke, the larger the amount of binder pitch added during granulation, the lower the OI value of the obtained graphite; when no binder pitch is added to calcined petroleum coke or the amount of binder pitch added is relatively low, the OI value of the obtained graphite is larger.

[0099] In some embodiments, the volume average particle size Dv50 of the artificial graphite for active material 1 may be 8 - 24 μm, optionally 10 - 20 μm; the graphitization degree is 85.0% - 97.5%, optionally 86.0% - 89.9%. The volume average particle size Dv50 of the artificial graphite for active material 2 may be 6 - 24 μm, optionally 8 - 20 μm; the graphitization degree is 85.0% - 97.5%, optionally 90.2% - 96.8%. The artificial graphite is the artificial graphite before preparing the active layer of the negative electrode plate. When the average particle size of the artificial graphite is small, the compaction density of the electrode plate is low, and the energy density of the secondary battery is correspondingly low. On the contrary, when the average particle size is large, the kinetics is poor. In addition, the higher the graphitization degree of the artificial graphite, the higher the specific capacity per gram of the artificial graphite material, the greater the compaction density of the electrode plate, and the higher the energy density of the corresponding secondary battery. On the contrary, when the graphitization degree is low, the specific capacity of the artificial graphite material is low, the compaction density of the electrode plate is low, and the energy density of the corresponding secondary battery is low and cannot be used. Therefore, it is necessary to limit the average particle size, graphite layer spacing, and graphitization degree of the artificial graphite within the above ranges.

[0100] The graphitization degree is a measure of the degree to which a carbonaceous material approaches a perfect graphite crystal through structural rearrangement from amorphous carbon. The level of graphitization can be a necessary condition for being able to serve as a negative electrode material for lithium-ion batteries. In addition, the level of graphitization also affects the charge and discharge capacity of the carbon negative electrode. The graphitization degree can be tested using the X-ray diffraction method: First, measure the interplanar spacing d 002 of the graphite (002) crystal plane, and then calculate using the Franklin formula (Mering-Maire formula): G = (0.3440 - d 002 ) / (0.3440 - 0.3354) × 100%, where G is the graphitization degree (%), 0.3440 is the interplanar spacing of non-graphitized carbon (nm), 0.3354 is the interplanar spacing of an ideal graphite crystal (half of the lattice constant of the c-axis of hexagonal graphite, nm), and d 002 is the interplanar spacing of the (002) crystal plane of the carbon material (nm). It should be noted that in order to obtain a more accurate d 002 value, Si powder is usually incorporated to correct the diffraction angle to reduce errors.

[0101] In some embodiments, the powder resistivity of artificial graphite for active material 1 under a pressure of 8 MPa can be 0.035 Ω·cm or less, and optionally 0.025 Ω·cm or less. The powder resistivity of artificial graphite for active material 2 under a pressure of 8 MPa can be 0.035 Ω·cm or less, and optionally 0.025 Ω·cm or less. The lower the powder resistivity of the artificial graphite, the higher the conductivity, and the higher the conductivity of the corresponding prepared negative electrode sheet. Moreover, the polarization of the battery using the negative electrode sheet is smaller, and the kinetic performance is better, so that it can have a higher cycle life. The powder resistivity of the artificial graphite can be tested by methods known in the art. For example, a resistivity tester (such as ST2722) can be used to test based on the four-probe method. A certain mass of the sample is placed in the feeding cup of the resistivity tester, the pressure is applied to 8 MPa, and the data is manually collected to record the test result of the powder resistivity. The test refers to GB / T 30835-2014.

[0102] If active material 1 and active material 2 also include a small amount of other carbon materials, the preparation method of the active material having the OI value and the specific capacity is similar to the above content.

[0103] In the present application, the negative electrode current collector can be made of a material with good conductivity and mechanical strength, which play the roles of conduction and current collection. In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0104] The negative electrode sheet provided by the present application can be prepared by a method including the following steps:

[0105] (1) Select active materials with the required OI value and specific capacity from commercially available products, or prepare active materials with the required OI value and specific capacity.

[0106] (2) Mix active material 1, conductive agent, binder, and thickener in a certain mass ratio in an appropriate amount of deionized water and stir well to form a uniform negative electrode slurry.

[0107] (3) Coat this negative electrode slurry on the surface of the negative electrode current collector as active layer 1.

[0108] (4) The active material 2, conductive agent, binder, and thickener are fully stirred and mixed in an appropriate amount of deionized water according to a certain mass ratio to form a uniform negative electrode slurry.

[0109] (5) This negative electrode slurry is coated on the surface of the active layer 1 as the active layer 2.

[0110] (6) After the above-mentioned electrode sheet is dried and cold-pressed, a negative electrode sheet is obtained.

[0111] It should be understood that the negative electrode sheet of the present application can be used not only in lithium-ion batteries but also in any other battery, battery module, battery pack, or electrical device that requires reducing swelling and improving cycle performance.

[0112] [Positive Electrode Sheet]

[0113] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

[0114] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode film layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0115] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0116] In some embodiments, the positive electrode active material can be a positive electrode active material for batteries well-known in the art. As an example, the positive electrode active material can include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides can include but are not limited to lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3Mn 1 / 3 O2 (which can also be abbreviated as NCM 333 )、LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be abbreviated as NCM 523 )、LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be abbreviated as NCM 211 )、LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be abbreviated as NCM 622 )、LiNi 0.8 Co 0.1 Mn[[ID=3३]] 0.1 O2 (which can also be abbreviated as NCM 811 )、lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and at least one of its modified compounds, etc. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO4 (which can also be abbreviated as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon.

[0117] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0118] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0119] In some embodiments, the positive electrode plate can be prepared by the following method: dispersing the above components for preparing the positive electrode plate, such as the positive electrode active material, conductive agent, binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode plate can be obtained.

[0120] [Electrolyte]

[0121] The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. There is no specific limitation on the type of electrolyte in this application, and it can be selected according to requirements. For example, the electrolyte can be selected from at least one of solid electrolytes and liquid electrolytes (i.e., electrolytic solutions).

[0122] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0123] In some embodiments, the electrolyte salt can be selected from one or several of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro(dioxalato)phosphate (LiDFOP), and lithium tetrafluoro(oxalato)phosphate (LiTFOP).

[0124] In some embodiments, the solvent can be selected from one or several of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE).

[0125] In some embodiments, the electrolytic solution may also optionally include additives. For example, the additives can include negative electrode film-forming additives, can also include positive electrode film-forming additives, and can also include additives that can improve certain battery performances, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, additives for improving the low-temperature performance of the battery, etc.

[0126] [Separator membrane]

[0127] In some embodiments, the secondary battery further includes a separator membrane. The separator membrane is disposed between the positive electrode plate and the negative electrode plate to play a role in isolation. There is no particular limitation on the type of separator membrane in this application, and any well-known porous structure separator membrane with good chemical stability and mechanical stability can be selected.

[0128] In some embodiments, the material of the separator membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0129] [Outer packaging]

[0130] In some embodiments, the secondary battery can include an outer packaging for encapsulating the positive electrode plate, the negative electrode plate, and the electrolyte. As an example, the positive electrode plate, the negative electrode plate, and the separator membrane can be laminated or wound to form a laminated structure secondary battery or a wound structure secondary battery, and the secondary battery is encapsulated in the outer packaging; the electrolyte can be an electrolytic solution, and the electrolytic solution is infiltrated in the secondary battery. The number of secondary batteries in the secondary battery can be one or several, which can be adjusted according to requirements.

[0131] In one embodiment, the present application provides an electrode assembly. In some embodiments, the positive electrode plate, the negative electrode plate, and the separator membrane can be made into an electrode assembly by a winding process or a lamination process. The outer packaging can be used to encapsulate the above electrode assembly and the electrolyte.

[0132] In some embodiments, the outer packaging of the secondary battery can be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as including one or several of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc. In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.

[0133] [Secondary battery]

[0134] The present application does not particularly limit the shape of the secondary battery, which can be cylindrical, square, or any other shape. For example, Figure 2 is a secondary battery 5 with a square structure as an example.

[0135] In some embodiments, referring to Figure 3 , the outer packaging can include a housing 51 and a cover plate 53. Among them, the housing 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate, and the separator membrane can form an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolytic solution is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual requirements.

[0136] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries included in the battery module can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery module.

[0137] Figure 4 is a battery module 4 as an example. Refer to Figure 4 , in the battery module 4, a plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0138] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0139] In some embodiments, the above battery module can be further assembled into a battery pack. The number of battery modules included in the battery pack can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery pack.

[0140] Figure 5 and Figure 6 is a battery pack 1 as an example. Refer to Figure 6 , in the battery pack 1, it can include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any way.

[0141] In addition, the present application also provides an electric device, and the electric device includes at least one of the secondary battery, battery module, or battery pack provided by the present application. The secondary battery, battery module, or battery pack can be used as the power source of the electric device or as the energy storage unit of the electric device. The electric device can include mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto.

[0142] As the electric device, the secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0143] Figure 7 is an example of an electric device. The electric device is a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle, etc. To meet the high-power and high-energy density requirements of the secondary battery for this electric device, a battery pack or battery module can be adopted.

[0144] Another example of the device can be a mobile phone, a tablet computer, a laptop computer, etc. The device usually requires being thin and light, and a secondary battery can be used as the power source.

[0145] Embodiment

[0146] The preparation methods of the negative electrode sheet, the positive electrode sheet, the separator, the electrolyte, and the lithium-ion battery of the present application will be described in detail below through Embodiment 1. For the relevant preparation parameters of other embodiments and comparative examples, refer to Table 1. It should be noted that in the following description, the negative electrode sheet 1 corresponds to the negative electrode sheet in Embodiment 1, and the battery 1 corresponds to the lithium-ion battery in Embodiment 1.

[0147] Embodiment 1

[0148] I. Preparation of Lithium-Ion Batteries

[0149] 1. Preparation of the negative electrode sheet

[0150] (1) Active material 1

[0151] The active material 1 is artificial graphite with a specific capacity of 340 mAh / g and an OI value of 12. Its preparation process is as follows: The graphite precursor needle coke is mechanically ground, the fine powder is removed by a classification device, and then it is subjected to shaping treatment by a shaper. Finally, the shaped needle coke is graphitized at 2650 °C to obtain the active material 1 of Embodiment 1.

[0152] (2) Active material 2

[0153] The active material 1 is artificial graphite with a specific capacity of 360 mAh / g and an OI value of 4. Its preparation process is as follows: The calcined needle coke of the graphite precursor is roll-pressed and ground, the fine powder is removed by a classification device, and then it is subjected to shaping treatment by a shaper. After that, pitch with a softening point of 250 °C (the addition amount is 15%, based on the total amount of the graphite precursor and the pitch) is added to a horizontal reactor for granulation, and then it is graphitized at 3100 °C to obtain the active material 2 of Embodiment 1.

[0154] (3) Negative electrode sheet

[0155] Artificial graphite with a specific capacity of 340 mAh / g and a powder OI value of 12, a conductive agent (Super P), a binder (SBR), and a thickener (CMC) are fully stirred and mixed in an appropriate amount of deionized water according to a mass ratio of 96.2:0.8:1.8:1.2 to form a uniform negative electrode slurry. This negative electrode slurry is coated on both surfaces of the negative electrode current collector copper foil, as Figure 1 shown in, as the active layer 1;

[0156] Artificial graphite with a specific capacity of 360 mAh / g and a powder OI value of 4, a conductive agent (Super P), a binder (SBR), and a thickener (CMC) were thoroughly stirred and mixed in deionized water in a mass ratio of 96.2:0.8:1.8:1.2 to form a uniform negative electrode slurry. This negative electrode slurry was coated on the surface of the active layer 1 as the active layer 2;

[0157] After drying and cold pressing the above-mentioned electrode sheet, a negative electrode sheet was obtained.

[0158] 2. Preparation of the positive electrode sheet

[0159] The positive electrode active material LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), a conductive agent (Super P), and a binder PVDF were thoroughly stirred and mixed in a weight ratio of 96.2:2.7:1.1 in an appropriate amount of NMP to form a uniform positive electrode slurry. This positive electrode slurry was coated on the surface of the positive electrode current collector aluminum foil and, after drying and cold pressing, a positive electrode sheet was obtained.

[0160] 3. Preparation of the electrolyte

[0161] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte, where the concentration of LiPF6 was 1 mol / L.

[0162] 4. Separator

[0163] A polyethylene (PE) film was used.

[0164] 5. Preparation of the secondary battery

[0165] The above-mentioned positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, and after winding, an electrode assembly was obtained. The electrode assembly was placed in an outer package, the above-mentioned electrolyte was added, and after processes such as encapsulation, standing, formation, and aging, the secondary battery of Example 1 was obtained. The outer package was a hard shell with dimensions of length × width × height = 148 mm × 28.5 mm × 97.5 mm.

[0166] Other examples and comparative examples

[0167] Active materials 1 and 2 in Examples 2 - 17 and Comparative Examples 1 - 13 were prepared in a manner similar to that of Example 1. The specific raw materials, binders, binder addition amounts, graphitization temperatures, and graphitization methods required are shown in Table 1 below.

[0168]

[0169]

[0170] II. Related Parameters and Battery Performance Tests

[0171] 1. Determination of OI value

[0172] The OI values of different active substances in this application are tested using an X-ray diffractometer (Bruker D8 Discover). The testing process is carried out according to JB / T 4220-2011. The X-ray diffraction pattern of artificial graphite is obtained by testing, and the peak area C004 of the diffraction peak of the (004) crystal plane and the peak area C110 of the diffraction peak of the (110) crystal plane are integrated. The OI value of artificial graphite is C004 / C110. The measurement data are shown in Table 2 for details.

[0173] The OI value of the active substance 2 in the active layer 2 of the negative electrode sheet of this application is tested as follows: Scrape the substance with a thickness of 20 μm on the surface layer of the negative electrode sheet, and ultrasonically clean it with solvents DMC (dimethyl carbonate), NMP (N-methylpyrrolidone), and deionized water respectively, and then dry it in a blast drying oven to obtain the active substance 2. The OI value of the active substance 2 is tested using the above testing method.

[0174] The OI value of the active substance 1 in the active layer 1 of the negative electrode sheet of this application is tested as follows: Scrape off the upper layer of the electrode sheet, retain the active layer 1 with a thickness of 20 μm, and scrape the remaining substance with a thickness of 20 μm, and ultrasonically clean it with solvents DMC (dimethyl carbonate), NMP, and deionized water respectively, and then dry it in a blast drying oven to obtain the active substance 1. The OI value of the active substance 1 is tested using the above testing method.

[0175] 2. Determination of specific capacity

[0176] Mix the active substance 1 (or active substance 2), conductive agent Super P, and binder PVDF evenly with the solvent NMP according to a mass ratio of 91.6:1.8:6.6 to make a slurry. Coat the prepared slurry on a copper foil current collector and dry it in an oven for later use. Use a lithium metal sheet as the counter electrode. Use a polyethylene (PE) film as the separator. Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) according to a volume ratio of 1:1:1, and then dissolve LiPF6 evenly in the above solution to obtain an electrolyte, where the concentration of LiPF6 is 1 mol / L. Assemble the above parts into a CR2430 type button cell in a glove box under argon protection.

[0177] After leaving the obtained button battery to stand for 12 hours, it is discharged at a constant current of 0.05C until 0.005V, and then left to stand for 10 minutes. It is then discharged at a constant current of 50μA until 0.005V, and left to stand for 10 minutes. It is further discharged at a constant current of 10μA until 0.005V. Then it is charged at a constant current of 0.1C until 2V. Record the charging capacity. The ratio of the charging capacity to the mass of artificial graphite is the gram capacity of the prepared artificial graphite. The measurement data are shown in Table 2 for details.

[0178] 3. Test of the cyclic swelling rate of the negative electrode sheet

[0179] Record the thickness of the cold-pressed negative electrode sheet as H0. Then, make a secondary battery with the cold-pressed negative electrode sheet, the positive electrode sheet, the separator, and the electrolyte. The specific process is as described in III. At 25°C, perform 1C / 1C cycling with 100% DOD (100% depth of discharge, that is, fully charged and then fully discharged) on the prepared secondary battery on a Neware charge and discharge machine. Record the discharge capacity of the first cycle (i.e., the initial capacity) as 100%. When the cyclic capacity retention rate reaches 80% of the initial capacity, stop cycling. Then charge the secondary battery to 100% SOC (State of Charge), disassemble the secondary battery and measure the thickness of the corresponding negative electrode sheet, denoted as H1. Then the cyclic swelling rate of the negative electrode sheet is: (H1 / H0 - 1)×100%. The measurement data are shown in Tables 2 - 6 for details.

[0180] 4. Test of the adhesion between the active layer and the current collector

[0181] Stick double-sided tape (Nitto 5000NS; width: 2 cm) on a stainless steel plate. Cut the test electrode sheet to the same shape and size as the double-sided tape (roughly rectangular) and stick it on the double-sided tape. Make a slit on the electrode sheet along the direction parallel to the short side of the electrode sheet, and gently peel off the contact area between the active layer and the copper foil (about 1 cm in length) using a blade. Insert a 2 cm wide strip of paper into the slit and bond it firmly with crepe tape. Place the steel plate vertically in the middle of the lower card slot of a high-speed tensile machine, and place the strip of paper in the middle of the upper card slot of the high-speed tensile machine. Set the speed to 50 mm / min and the displacement to 60 mm. After zeroing on the computer, click start to test, and read the adhesion data after the test. The measurement data are shown in Tables 2 - 6 for details.

[0182] 5. Test of the energy density of the secondary battery

[0183] At 25 °C, the secondary battery is charged at a constant current of 1 / 3C to 4.3V, then charged at a constant voltage of 4.3V until the current reaches 0.05C, left standing for 5 min, and then discharged at a constant current of 1 / 3C to 2.8V. Record the battery discharge energy at this time. The battery discharge energy divided by the weight of the battery is the weight energy density of the battery, with the unit of Wh / kg. See Tables 2 - 6 for the measurement data.

[0184] 6. Retention rate of the cycle capacity of the secondary battery

[0185] At 25 °C, charge-discharge tests are performed on the lithium-ion batteries of all the examples and comparative examples. One charge-discharge cycle process is as follows: Charge at a constant current of 1C to 4.3V, then charge at a constant voltage of 4.3V until the current reaches 0.05C, left standing for 5 min, and then discharge at a constant current of 1C to 2.8V. Record the battery capacity at this time as C1. The above is one charge-discharge cycle of the battery. Cycle 1500 times according to the above process, and record the battery capacity C1500 at this time. Then the retention rate of the cycle capacity = C1500 / C1×100%. See Tables 2 - 6 for the measurement data.

[0186]

[0187]

[0188]

Claims

1. A negative electrode plate, characterized in that, including a current collector; an active layer 1, located on at least one surface of the current collector and including active material 1, an active layer 2, located on at least one surface of each of the two active layers 1 that is away from the current collector and including active material 2; the powder OI value of the active material 1 is in the range of 8 - 32, and the powder OI value of the active material 2 is in the range of 2 - 7; the specific capacity of the active material 1 is in the range of 290 - 350 mAh / g, the specific capacity of the active material 2 is in the range of 350 - 368 mAh / g.

2. The negative electrode sheet according to claim 1, characterized in that The powder OI value of the active material 1 is in the range of 10 - 25, and the powder OI value of the active material 2 is in the range of 3 - 6.

3. The negative electrode tab according to claim 1, wherein the ratio α of the powder OI value of the active material 1 to the powder OI value of the active material 2 ranges from 2.00 to 6.

25.

4. The negative electrode tab according to claim 1, wherein the ratio β of the specific capacity of the active material 1 to the specific capacity of the active material 2 ranges from 0.80 to 1.

00.

5. The negative electrode tab according to claim 3, wherein the ratio β of the specific capacity of the active material 1 to the specific capacity of the active material 2 ranges from 0.80 to 1.00, and the ratio α / β of α to β ranges from 2.0 to 6.

7.

6. The negative electrode tab according to claim 1, wherein The areal density of the active layer 1 is 3-10 mg / cm 2 ; and The areal density of the active layer 2 is 3-10 mg / cm 2 .

7. The negative electrode plate according to claim 6, characterized in that, The areal density of the active layer 1 is 4-8 mg / cm 2 ; and The areal density of the active layer 2 is 4-8 mg / cm 2 .

8. The negative electrode tab according to claim 1, wherein based on the total weight of the active layer 1, the proportion of the active material 1 ranges from 92.0% to 98.99% by weight; and based on the total weight of the active layer 2, the proportion of the active material 2 ranges from 92.0% to 98.99% by weight.

9. The negative electrode sheet according to claim 1, wherein, The porosity of the negative electrode tab is 18.0% - 40.2%.

10. The negative electrode sheet according to claim 9, characterized in that, The porosity of the negative electrode tab is 22.5% - 35.0%.

11. The negative electrode tab according to claim 1, wherein The compaction density of the negative electrode sheet is 1.45 - 1.90 g / cm 3 .

12. The negative electrode tab according to claim 11, wherein The compaction density of the negative electrode sheet is 1.55 - 1.80 g / cm 3 .

13. The negative electrode tab according to claim 1, wherein the thickness of the active layer 1 after cold pressing is 20 - 135 μm; the thickness of the active layer 2 after cold pressing is 20 - 135 μm.

14. The negative electrode tab according to claim 13, wherein the thickness of the active layer 1 after cold pressing is 30 - 120 μm; the thickness of the active layer 2 after cold pressing is 30 - 120 μm.

15. The negative electrode sheet according to claim 1, characterized in that, The active layer 1 or the active layer 2 further includes a conductive agent, a binder, and a thickener.

16. The negative electrode sheet according to claim 1, characterized in that, The active material 1 includes at least one selected from artificial graphite, hard carbon, and soft carbon.

17. The negative electrode plate according to claim 1, wherein, The active material 2 includes at least one selected from artificial graphite, hard carbon, and soft carbon.

18. A secondary battery, characterized in that, including the negative electrode tab according to any one of claims 1 - 17.

19. A battery module, characterized in that, including the secondary battery according to claim 18.

20. A battery pack, characterized in that, including the battery module according to claim 19.

21. An electrical device, characterized in that, including at least one selected from the secondary battery according to claim 18, the battery module according to claim 19, or the battery pack according to claim 20.

Images