Secondary battery and device containing the same

Abstract

The present application discloses a secondary battery and a device containing the secondary battery. The secondary battery includes a positive electrode sheet and a negative electrode sheet. The positive electrode sheet includes a positive electrode current collector and a positive electrode membrane disposed on at least one surface of the positive electrode current collector and including a positive electrode active material. The negative electrode sheet includes a negative electrode current collector and a negative electrode membrane disposed on at least one surface of the negative electrode current collector and including a negative electrode active material. The negative electrode active material includes a first material and a second material, the first material includes artificial graphite, the second material includes natural graphite, and the degree of graphitization of the negative electrode active material is 92% to 96%. The positive electrode active material includes one or more of layered lithium transition metal oxides and modified compounds thereof. The resistance R of the negative electrode sheet satisfies the following conditions: 6.0 mΩ≤R≤12.0 mΩ.

Description

[0001] This application is a divisional application based on the invention with application number 202080005562.9, application date March 27, 2020, applicant is Contemporary Amperex Technology Co., Ltd., and invention name is “Secondary battery and device containing the same”. Technical Field

[0002] The present application belongs to the technical field of energy storage devices, and specifically relates to a secondary battery and a device containing the secondary battery. Background Art

[0003] Secondary batteries are widely used due to their reliable performance, pollution-free operation, and lack of memory effect. For example, as environmental protection issues are increasingly valued and new energy vehicles become increasingly popular, the demand for power-type secondary batteries will experience explosive growth. However, as the application scope of secondary batteries becomes wider and wider, higher requirements are placed on the low-temperature power performance and energy density of secondary batteries. The low low-temperature power performance of secondary batteries will affect their use in low-temperature environments. Energy density will affect the battery's endurance, that is, the time it can be used after a single charge. Therefore, how to improve the low-temperature power performance of secondary batteries while maintaining a high energy density has become a technical problem that needs to be solved urgently. Summary of the Invention

[0004] In a first aspect, the present application provides a secondary battery, which includes a positive electrode plate and a negative electrode plate, the positive electrode plate includes a positive electrode collector and a positive electrode membrane provided on at least one surface of the positive electrode collector and including a positive electrode active material, the negative electrode plate includes a negative electrode collector and a negative electrode membrane provided on at least one surface of the negative electrode collector and including a negative electrode active material, wherein the negative electrode active material includes a first material and a second material, the first material includes artificial graphite, and the second material includes natural graphite; the positive electrode active material includes one or more of layered lithium transition metal oxides and modified compounds thereof; the resistance R of the negative electrode plate satisfies: 6.0mΩ≤R≤12.0mΩ.

[0005] Surprisingly, the secondary battery of this application utilizes a specific positive electrode active material for the positive electrode membrane, and a negative electrode active material comprising both artificial and natural graphite for the negative electrode membrane. Furthermore, the resistance of the negative electrode is controlled within a specific range. This allows the negative electrode to have a higher energy density while effectively improving its active ion transport and electron conductivity. This allows the secondary battery to achieve high low-temperature power performance while maintaining a high energy density. Furthermore, the secondary battery can also simultaneously exhibit low high-temperature cyclic expansion and high high-temperature storage performance.

[0006] In any embodiment of the first aspect of the present application, the resistance R of the negative electrode plate satisfies 7.0 mΩ ≤ R ≤ 11.0 mΩ. Optionally, 8.0 mΩ ≤ R ≤ 10.0 mΩ. When the resistance R of the negative electrode plate is within an appropriate range, the battery's high-temperature cycling expansion can be further reduced and its low-temperature power performance can be improved.

[0007] In any embodiment of the first aspect of the present application, the mass proportion of natural graphite in the negative electrode active material is 10% to 50%, and optionally 15% to 30%. The inclusion of an appropriate amount of natural graphite in the negative electrode active material can enhance the cohesion and adhesion of the negative electrode sheet while reducing side reactions on the negative electrode surface, thereby further reducing battery cycle expansion. This can also further improve the battery's high-temperature cycling performance and safety.

[0008] In any embodiment of the first aspect of the present application, the powder resistivity of the natural graphite at a pressure of 8 MPa is 10.0 mΩ·cm to 14.0 mΩ·cm, and may be 11.0 mΩ·cm to 13.0 mΩ·cm. A powder resistivity of the natural graphite within an appropriate range can further improve the power performance, energy density, and cycle performance of the battery.

[0009] In any embodiment of the first aspect of the present application, the artificial graphite has a powder resistivity of 11.0 mΩ·cm to 16.0 mΩ·cm, optionally 13.0 mΩ·cm to 15.0 mΩ·cm, at a pressure of 8 MPa. When the powder resistivity of the artificial graphite is within an appropriate range, the power performance, energy density, and cycle performance of the battery can be further improved.

[0010] In any embodiment of the first aspect of the present application, the volume average particle size D of the negative electrode active material is v 50 is 11μm~15μm, and can be 12μm~14μm. v 50 is appropriate, so that it has a lower powder resistivity and a higher ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the side reaction of the electrolyte on the surface of the material, so that the battery can take into account higher power performance, cycle performance and energy density at the same time.

[0011] In any embodiment of the first aspect of the present application, the negative electrode active material has a degree of graphitization of 92% to 96%, optionally 93% to 95%. A graphitization degree of the negative electrode active material within an appropriate range can further improve the power performance of the battery. Furthermore, the negative electrode sheet using this negative electrode active material can also have high cohesion and adhesion, improving the battery's low cyclic expansion performance.

[0012] In any embodiment of the first aspect of the present application, the surface density of the negative electrode film is 10 mg / cm 2 ~13mg / cm2 , optional 10.5mg / cm 2 ~11.5mg / cm 2 The negative electrode membrane's areal density within the given range can provide the battery with a higher energy density and further improve the battery's kinetic performance. Furthermore, the negative electrode's design can reduce polarization and side reactions, further improving the battery's cycling performance.

[0013] In any embodiment of the first aspect of the present application, the compaction density of the negative electrode membrane is 1.6 g / cm 3 ~1.8g / cm 3 , optional 1.65g / cm 3 ~1.75g / cm 3 A negative electrode membrane compaction density within the given range can ensure a high energy density for the battery and further enhance the battery's power performance. Furthermore, a suitable compaction density can further improve the battery's cycle life and safety performance.

[0014] In any embodiment of the first aspect of the present application, the layered lithium transition metal oxide includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof.

[0015] In any embodiment of the first aspect of the present application, any two circular regions of equal area on the negative electrode sheet are designated as a first region and a second region, respectively, with a center-to-center distance of 20 cm between the first region and the second region. The resistance R1 of the first region and the resistance R2 of the second region satisfy the following relationship: |R1-R2|≤3. Alternatively, |R1-R2|≤1. The small resistance difference between any two circular regions of equal area and a center-to-center distance of 20 cm on the negative electrode sheet can substantially equalize the active ion transport performance and electron conduction performance at different locations on the negative electrode sheet, thereby improving the capacity utilization, cycle and storage life, and kinetic performance at each location on the negative electrode sheet.

[0016] A second aspect of the present application provides a secondary battery, which includes a positive electrode plate and a negative electrode plate, the positive electrode plate includes a positive electrode collector and a positive electrode membrane provided on at least one surface of the positive electrode collector and including a positive electrode active material, the negative electrode plate includes a negative electrode collector and a negative electrode membrane provided on at least one surface of the negative electrode collector and including a negative electrode active material, wherein the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and its modified compounds; the negative electrode active material includes a first material and a second material, the first material includes artificial graphite, and the second material includes natural graphite; the resistance of the negative electrode plate is 3.0mΩ≤R≤7.0mΩ.

[0017] Surprisingly, the secondary battery of this application utilizes a specific positive electrode active material for the positive electrode membrane, and a negative electrode active material comprising both artificial and natural graphite for the negative electrode membrane. Furthermore, the resistance of the negative electrode is controlled within a specific range. This allows the negative electrode to have a higher energy density while effectively improving its active ion transport and electron conductivity. This allows the secondary battery to achieve high low-temperature power performance while maintaining a high energy density. Furthermore, the secondary battery can also simultaneously exhibit low high-temperature cyclic expansion and high high-temperature storage performance.

[0018] In any embodiment of the second aspect of the present application, the resistance R of the negative electrode sheet satisfies 4.0 mΩ≤R≤6.0 mΩ. When the resistance R of the negative electrode sheet is within an appropriate range, the low-temperature power performance and high-temperature cycle performance of the battery can be further improved.

[0019] In any embodiment of the second aspect of the present application, the mass proportion of natural graphite in the negative electrode active material is 10% to 50%, and optionally 35% to 50%. The inclusion of an appropriate amount of natural graphite in the negative electrode active material is beneficial for the battery to simultaneously achieve high low-temperature power performance, energy density, and high-temperature cycle performance.

[0020] In any embodiment of the second aspect of the present application, the powder resistivity of the natural graphite at a pressure of 8 MPa is 8.0 mΩ·cm to 12.0 mΩ·cm, and may be 9.0 mΩ·cm to 11.0 mΩ·cm. A powder resistivity of the natural graphite within an appropriate range can further improve the power performance, energy density, and cycle performance of the battery.

[0021] In any embodiment of the second aspect of the present application, the powder resistivity of the artificial graphite at a pressure of 8 MPa is 15 mΩ·cm to 20 mΩ·cm, and optionally 16 mΩ·cm to 18 mΩ·cm. When the powder resistivity of the artificial graphite is within an appropriate range, the power performance, energy density, and cycle performance of the battery can be further improved.

[0022] In any embodiment of the second aspect of the present application, the volume average particle size D of the negative electrode active material is v 50 is 15μm to 19μm, and can be 16μm to 18μm. v 50 is appropriate, so that it has lower powder resistivity and higher lithium ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the side reaction of the electrolyte on the surface of the material, thereby improving the power performance, cycle performance and energy density of the battery.

[0023] In any embodiment of the second aspect of the present application, the degree of graphitization of the negative electrode active material is 92% to 95%, optionally 93% to 94%. A graphitization degree of the negative electrode active material within an appropriate range can further improve the power performance of the battery. A negative electrode sheet using this negative electrode active material also exhibits high cohesion and adhesion, reducing battery expansion during cycling, thereby further improving the battery's cycling performance.

[0024] In any embodiment of the second aspect of the present application, the surface density of the negative electrode film is 7 mg / cm 2 ~10mg / cm 2 , optional 7mg / cm 2 ~8mg / cm 2 The surface density of the negative electrode film is within the given range, which can make the battery have a higher energy density and further improve the power performance of the battery. In addition, the negative electrode sheet meets the above design and can further improve the cycle performance of the battery.

[0025] In any embodiment of the second aspect of the present application, the compaction density of the negative electrode film is 1.5 g / cm 3 ~1.7g / cm 3 , optional 1.55g / cm 3 ~1.65g / cm 3 The compaction density of the negative electrode membrane within the given range can ensure that the battery has a high energy density and further improve the battery's power performance. In addition, the appropriate compaction density can further improve the battery's cycle life and safety performance.

[0026] In any embodiment of the second aspect of the present application, the lithium-containing phosphate with an olivine structure includes one or more of lithium iron phosphate and a composite material of lithium iron phosphate and carbon.

[0027] In any embodiment of the second aspect of the present application, any two circular regions of equal area on the negative electrode sheet are designated as a first region and a second region, respectively, with a center-to-center distance of 20 cm between the first region and the second region. The resistance R1 of the first region and the resistance R2 of the second region satisfy the following relationship: |R1-R2|≤3. Alternatively, |R1-R2|≤1. The small resistance difference between any two circular regions of equal area and a center-to-center distance of 20 cm on the negative electrode sheet can substantially equalize the active ion transport performance and electron conduction performance at different locations on the negative electrode sheet, thereby improving the capacity utilization, cycle and storage life, and kinetic performance at each location on the negative electrode sheet.

[0028] A third aspect of the present application provides a device comprising the secondary battery according to the first aspect and / or the second aspect of the present application.

[0029] The device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0031] Figure 1 This is a schematic diagram of one embodiment of a secondary battery.

[0032] Figure 2 yes Figure 1 Exploded diagram of .

[0033] Figure 3 is a schematic diagram of one embodiment of a battery module.

[0034] Figure 4 is a schematic diagram of one embodiment of a battery pack.

[0035] Figure 5 yes Figure 4 Exploded diagram of .

[0036] Figure 6 This is a schematic diagram of one embodiment of a device using a secondary battery as a power source. DETAILED DESCRIPTION

[0037] In order to make the invention purpose, technical solution and beneficial technical effect of this application clearer, this application is further described in detail with reference to the following embodiments. It should be understood that the embodiments described in this specification are only for explaining this application and are not intended to limit this application.

[0038] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and similarly, any upper limit can be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, each point or individual value between the endpoints of a range is included in the range. Thus, each point or individual value can serve as its own lower limit or upper limit and be combined with any other point or individual value, or with other lower limits or upper limits, to form an unspecified range.

[0039] In the description of this article, it should be noted that, unless otherwise specified, “above” and “below” are inclusive of the number themselves, and “several” in “one or several” means two or more.

[0040] The above summary of the invention of this application is not intended to describe every disclosed embodiment or every implementation in this application. The following description more specifically illustrates exemplary embodiments. In many places throughout the application, guidance is provided by a series of examples, which can be used in various combinations. In each example, the enumeration is intended only as a representative group and should not be construed as exhaustive.

[0041] secondary batteries

[0042] A first aspect of the present application provides a secondary battery. The secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the battery's charge and discharge processes, active ions are intercalated and released back and forth between the positive and negative electrode sheets. The electrolyte acts as an ion conductor between the positive and negative electrode sheets.

[0043] [Positive electrode]

[0044] The positive electrode sheet may include a positive electrode current collector and a positive electrode membrane disposed on at least one surface of the positive electrode current collector. As an example, the positive electrode current collector has two surfaces facing each other in the thickness direction, and the positive electrode membrane is laminated on either or both of the two surfaces of the positive electrode current collector.

[0045] The positive electrode current collector can be made of a material with good electrical conductivity and mechanical strength to perform both electrical conduction and current collection. In some embodiments, the negative electrode current collector can be made of aluminum foil.

[0046] The positive electrode membrane includes a positive electrode active material. The positive electrode active material can be any of those known in the art for use in secondary batteries. In some embodiments, the positive electrode active material can include one or more of layered lithium transition metal oxides and modified compounds thereof, olivine-structured lithium-containing phosphates and modified compounds thereof, and the like.

[0047] In the present application, the “modification” in “modified compound” may refer to doping modification and / or surface coating modification of the material.

[0048] In some embodiments, the positive electrode active material includes a layered lithium transition metal oxide and a modified compound thereof. As a specific example, the layered lithium transition metal oxide may include, but is not limited to, one or more 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 modified compounds thereof. Alternatively, the layered lithium transition metal oxide includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof.

[0049] In some optional embodiments, the positive electrode active material includes Li a Ni b Coc M d M' e O f A g and Li having a coating layer on at least a portion of its surface a Ni b Co c M d M' e O f A g One or more of the following: wherein 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 0≤e≤0.1, 1≤f≤2, 0≤g≤1; M is selected from one or more of Mn and Al; M' is selected from one or more of Zr, Al, Zn, Cu, Cr, Mg, Fe, V, Ti and B; A is selected from one or more of N, F, S and Cl.

[0050] In some embodiments, M is selected from Mn, and M' is selected from one or more of Zr, Al, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, optionally including one or more of Zr, Al, Zn, and B. Alternatively, M is selected from Al, and M' is selected from one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, optionally including one or more of Zr, Zn, and B.

[0051] The positive electrode active material includes a high-nickel ternary positive electrode active material, which can have a higher gram capacity, thereby improving the energy density of the battery.

[0052] In some optional embodiments, it is possible to a Ni b Co c M d M' e O f A g 80% to 100% of the surface of the material has a coating layer. a Ni b Co c M d M' e O f A g 90% to 100% of the surface of the material has a coating layer.

[0053] In other embodiments, the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and modified compounds thereof. As specific examples, the olivine-structured lithium-containing phosphate may include, but is not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, and a composite material of lithium manganese phosphate and carbon. Alternatively, the olivine-structured lithium-containing phosphate includes one or more of lithium iron phosphate and a composite material of lithium iron phosphate and carbon.

[0054] The composite material of lithium iron phosphate and carbon can be one or more of a coated composite material and an embedded composite material. The coated composite material is a lithium iron phosphate particle having a carbon coating layer on at least a portion of its surface. For example, the carbon coating layer is coated on 80% to 100% (such as 90% to 100%) of the surface of the lithium iron phosphate particle. The carbon coating layer may include one or more of graphite, hard carbon, soft carbon, carbon black, coke, etc. The embedded composite material is lithium iron carbonate dispersed in a carbon carrier. The carbon carrier may include one or more of graphite, hard carbon, soft carbon, carbon black, coke, etc.

[0055] The composite material of lithium manganese phosphate and carbon can be one or more of a coated composite material and an embedded composite material. The coated composite material is a material in which at least a portion of the surface of the lithium manganese phosphate particles has a carbon coating layer. For example, the carbon coating layer is coated on 80% to 100% (such as 90% to 100%) of the surface of the lithium manganese phosphate particles. The carbon coating layer may include one or more of graphite, hard carbon, soft carbon, carbon black, coke, etc. The embedded composite material is a material in which lithium manganese carbonate is dispersed in a carbon carrier. The carbon carrier may include one or more of graphite, hard carbon, soft carbon, carbon black, coke, etc.

[0056] In some embodiments, the positive electrode membrane may optionally include a binder. The type of binder is not specifically limited, and those skilled in the art may select the binder based on actual needs. For example, the binder for the positive electrode membrane may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0057] In some embodiments, the positive electrode diaphragm may optionally include a conductive agent. The type of conductive agent is not specifically limited, and those skilled in the art may select one based on actual needs. For example, the conductive agent used in the positive electrode diaphragm may include one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0058] [Negative electrode]

[0059] The negative electrode sheet includes a negative electrode current collector and a negative electrode membrane disposed on at least one surface of the negative electrode current collector. As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode membrane is laminated on either or both of the two surfaces of the negative electrode current collector.

[0060] The negative electrode current collector can be made of a material with good electrical conductivity and mechanical strength to perform both electrical conduction and current collection. In some embodiments, the negative electrode current collector can be made of copper foil.

[0061] The negative electrode membrane includes a negative electrode active material, which includes a first material and a second material. The first material includes artificial graphite, and the second material includes natural graphite. Surprisingly, it has been discovered that controlling the resistance R of the negative electrode membrane within a specific range can simultaneously achieve a higher energy density and effectively improve the active ion transport performance of the negative electrode membrane. This allows the secondary battery using this membrane to improve low-temperature power performance while maintaining a higher energy density. Furthermore, the secondary battery can also simultaneously achieve lower high-temperature cyclic expansion and higher high-temperature storage performance.

[0062] The inventors conducted in-depth research and found that when the positive electrode active material of the positive electrode plate includes one or more of layered lithium transition metal oxides and modified compounds thereof, the resistance R of the negative electrode plate satisfies: 6.0mΩ≤R≤12.0mΩ.

[0063] When the positive electrode active material includes one or more of layered lithium transition metal oxides and their modified compounds, and the negative electrode active material includes both artificial graphite and natural graphite, and the resistance R of the negative electrode plate satisfies 6.0mΩ≤R≤12.0mΩ, the positive and negative electrode active materials can be effectively coordinated, giving full play to the synergistic effect of the advantages of both. While ensuring that the battery has a high energy density, it can also improve the cohesion and adhesion of the negative electrode plate, reduce the expansion of the negative electrode during the battery cycle, and at the same time improve the solid-phase diffusion capacity of active ions in the negative electrode, thereby further improving the transmission performance of active ions between the positive and negative electrodes. In addition, while the negative electrode active material particles are in close contact, a pore network suitable for electrolyte infiltration is formed, thereby further improving the active ion transmission performance. As a result, the battery's kinetic performance is significantly improved. Even in low-temperature environments, the negative electrode can quickly receive active ions from the positive electrode, improving the low-temperature power performance of the secondary battery. Furthermore, the secondary battery is suitable for high-rate charge and discharge, and the probability of lithium plating on the negative electrode during high-rate charge is significantly reduced, while also enabling the secondary battery to have higher safety performance.

[0064] In addition, the positive electrode active material includes one or more of layered lithium transition metal oxides and modified compounds thereof, which can make the positive electrode sheet have a higher surface density (e.g. 14 mg / cm 2 ~20mg / cm 2 ) and compacted density (e.g. 3.3 g / cm 3 ~3.5g / cm 3 ), which enables the battery to have a higher energy density.

[0065] In these embodiments, optionally, 7.0 mΩ ≤ R ≤ 11.0 mΩ. Further optionally, 8.0 mΩ ≤ R ≤ 10.0 mΩ. For example, the resistance R of the negative electrode plate may be 8 mΩ, 8.5 mΩ, 9 mΩ, 9.5 mΩ, 9.8 mΩ, 10 mΩ, 10.5 mΩ, 11 mΩ, or 12 mΩ. When the resistance R of the negative electrode plate is within an appropriate range, the high-temperature cycling expansion of the battery can be further reduced and the low-temperature power performance of the battery can be improved.

[0066] In these embodiments, optionally, the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof, which can better exert the above-mentioned effects.

[0067] The inventors further discovered that when the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof, and the negative electrode active material includes both artificial graphite and natural graphite, and the resistance R of the negative electrode sheet satisfies 6.0 mΩ ≤ R ≤ 12.0 mΩ, the battery performance can be further improved if the negative electrode active material also meets one or more of the following conditions.

[0068] In some optional embodiments, the mass proportion of natural graphite in the negative electrode active material can be 10% to 50%, optionally 15% to 30%, further optionally 15% to 25%, further optionally 18% to 22%, such as 19%, 20%, or 21%. The inclusion of an appropriate amount of natural graphite in the negative electrode active material can enhance the specific capacity of the negative electrode active material. In particular, it can improve the adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector, thereby enhancing the cohesion and adhesion of the negative electrode sheet while reducing side reactions on the negative electrode surface, thereby further reducing battery cycle expansion. This can further improve the battery's high-temperature cycling performance and safety performance.

[0069] In some optional embodiments, the powder resistivity of natural graphite at a pressure of 8 MPa can be 10 mΩ·cm to 14 mΩ·cm, or can be 11 mΩ·cm to 13 mΩ·cm; such as 10.5 mΩ·cm, 11.5 mΩ·cm, 12.0 mΩ·cm, 12.5 mΩ·cm, 13.0 mΩ·cm, 13.5 mΩ·cm, or 13.8 mΩ·cm. Natural graphite powder resistivity within an appropriate range can further improve the active ion transport and electronic conductivity of the negative electrode during charge and discharge, thereby improving the power performance, energy density, and cycle performance of the battery.

[0070] In some optional embodiments, the powder resistivity of the artificial graphite at a pressure of 8 MPa is 11 mΩ·cm to 16 mΩ·cm, and may also be 13 mΩ·cm to 15 mΩ·cm; such as 12.0 mΩ·cm, 13.0 mΩ·cm, 13.5 mΩ·cm, 14.0 mΩ·cm, 14.3 mΩ·cm, or 14.6 mΩ·cm. When the powder resistivity of the artificial graphite is within an appropriate range, the active ion transport performance and electronic conductivity of the negative electrode during charge and discharge can be further improved, thereby improving the power performance, energy density, and cycle performance of the battery.

[0071] In some optional embodiments, the volume average particle size D of the negative electrode active material is v 50 can be 10 μm to 18 μm, optionally 11 μm to 15 μm, and further optionally 12 μm to 14 μm. v The smaller the 50 is, the smaller the powder resistivity is, and the lower the resistance of the negative electrode sheet is. v The smaller 50 is, the higher the migration rate of ions in the negative electrode sheet can be. v 50 is appropriate, so that it has a lower powder resistivity and a higher ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the side reaction of the electrolyte on the surface of the material, so that the battery can take into account higher power performance, cycle performance and energy density at the same time.

[0072] The inventors also found that when the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof, and the negative electrode active material includes artificial graphite and natural graphite, the resistance R of the negative electrode sheet is within a specific range, and the D of the negative electrode active material is v When the content of α-HgCl2 is within the above range, the battery can have a higher capacity retention rate during high-temperature storage, thereby improving the high-temperature storage performance.

[0073] In order to make the D of the negative electrode active material v In some embodiments, the volume average particle size D of natural graphite is within the above range. v 50 can be 10 μm to 16 μm, optionally 10 μm to 14 μm, and further optionally 11 μm to 13 μm. The volume average particle size D of artificial graphite v 50 can be 12μm to 19μm, optionally 12μm to 16μm, and further optionally 13μm to 15μm.

[0074] In some optional embodiments, the degree of graphitization of the negative electrode active material can be 92% to 96%, optionally 93% to 95%. A graphitization degree within an appropriate range can result in a low powder resistivity, reducing the resistance of the negative electrode sheet while also providing an interlayer spacing suitable for ion insertion and extraction, thereby further improving the battery's power performance. Furthermore, the negative electrode sheet using this negative electrode active material can also exhibit high cohesion and adhesion, improving the battery's low cyclic expansion performance.

[0075] The inventors also found that when the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and their modified compounds, the negative electrode active material includes artificial graphite and natural graphite, the resistance R of the negative electrode plate is within a specific range, and the degree of graphitization of the negative electrode active material is within the above range, the high-temperature storage performance of the battery can also be improved.

[0076] In order to ensure that the degree of graphitization of the negative electrode active material is within the above-mentioned range, in some embodiments, the degree of graphitization of natural graphite may be 95% to 98.5%, optionally 96% to 98%, or even 96.5% to 97.6%. The degree of graphitization of artificial graphite may be 90% to 97.5%, optionally 91% to 95%, or even 92% to 93.5%.

[0077] In some optional embodiments, the surface density of the negative electrode film can be 10 mg / cm 2 ~13mg / cm 2 , optional 10.5mg / cm 2 ~11.5mg / cm 2 When the positive electrode material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof, the negative electrode active material includes artificial graphite and natural graphite, the resistance R of the negative electrode sheet is within a specific range, and the surface density of the negative electrode membrane is within a given range, the battery can have a higher energy density; at the same time, the battery also has better active ion and electron transport properties, thereby further improving the battery's kinetic performance; in addition, when the battery meets the above design, polarization and side reactions can be reduced, thereby further improving the battery's cycle performance.

[0078] In some optional embodiments, the compaction density of the negative electrode film can be 1.6 g / cm 3 ~1.8g / cm 3 , optional 1.65g / cm 3 ~1.75g / cm 3 , 1.68g / cm2 is also available 3 ~1.73g / cm 3. When one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and their modified compounds, the negative electrode active material includes artificial graphite and natural graphite, the resistance R of the negative electrode pole piece is within a specific range, and the compaction density of the negative electrode membrane is within the given range, the battery can be guaranteed to have a higher energy density; at the same time, the close contact between the negative electrode active material particles can effectively reduce the resistance of the negative electrode membrane, thereby further improving the power performance of the battery. In addition, the appropriate compaction density can also protect the integrity of the negative electrode active material particle structure, which is beneficial to improve the cohesion and adhesion of the negative electrode pole piece, reduce expansion and side reactions during the battery cycle, and further improve the cycle life and safety performance of the battery.

[0079] The inventors also found that when the positive electrode active material of the positive electrode plate includes one or more of olivine-structured lithium-containing phosphates and modified compounds thereof, the resistance R of the negative electrode plate satisfies 3.0 mΩ≤R≤7.0 mΩ.

[0080] When the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and its modified compounds, and the negative electrode active material includes both artificial graphite and natural graphite, and the resistance R satisfies 3.0mΩ≤R≤7.0mΩ, an effective combination of the positive and negative electrode active materials can be formed, giving full play to the synergistic effect of the advantages of both. This can ensure that the battery has a high energy density while also improving the cohesion and adhesion of the negative electrode sheet, reducing the expansion of the negative electrode during the battery cycle, and at the same time improving the solid-phase diffusion capacity of the active ions in the negative electrode, thereby further improving the transmission performance of the active ions between the positive and negative electrodes. As a result, the kinetic performance of the battery is significantly improved, and even in a low-temperature environment, the negative electrode can quickly receive active ions from the positive electrode, improving the low-temperature power performance of the secondary battery. Furthermore, the secondary battery is suitable for high-rate charge and discharge, and the probability of negative electrode lithium plating occurring during high-rate charging is significantly reduced.

[0081] In these embodiments, optionally, 4.0mΩ≤R≤6.0mΩ, for example, R is 4.2mΩ, 4.5mΩ, 4.6mΩ, 4.7mΩ, 4.8mΩ, 5.0mΩ, 5.3mΩ, or 5.5mΩ. When the resistance R of the negative electrode sheet is within an appropriate range, the low-temperature power performance and high-temperature cycle performance of the battery can be further improved.

[0082] In these embodiments, optionally, the positive electrode active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and modified compounds thereof, which can better exert the above-mentioned effects.

[0083] The inventors further discovered that when the positive electrode active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and modified compounds thereof, and the negative electrode active material includes both artificial graphite and natural graphite, and the resistance R of the negative electrode sheet satisfies 3.0mΩ≤R≤7.0mΩ, if the negative electrode active material also meets one or more of the following conditions, the performance of the battery can be further improved.

[0084] In some optional embodiments, the mass proportion of natural graphite in the negative electrode active material can be 10% to 50%, optionally 15% to 50%, and further optionally 35% to 50%, such as 20%, 30%, 35%, 40%, 42%, 45%, 48% or 50%. The presence of an appropriate amount of natural graphite in the negative electrode active material can reduce the diaphragm resistance of the negative electrode sheet, thereby improving the internal resistance of the battery cell. At the same time, the ion transport performance and energy density of the negative electrode sheet are further improved. Therefore, the battery can simultaneously take into account high low-temperature power performance, energy density and high-temperature cycle performance.

[0085] In some optional embodiments, the powder resistivity of natural graphite at a pressure of 8 MPa can be 8 mΩ·cm to 12 mΩ·cm, and optionally 9.0 mΩ·cm to 11 mΩ·cm. Natural graphite powder resistivity within an appropriate range can further improve the active ion transport and electronic conductivity of the negative electrode during charge and discharge, thereby improving the power performance, energy density, and cycle performance of the battery.

[0086] In some optional embodiments, the powder resistivity of the artificial graphite at a pressure of 8 MPa can be 15 mΩ·cm to 20 mΩ·cm, and optionally 16 mΩ·cm to 18 mΩ·cm. Having the powder resistivity of the artificial graphite within an appropriate range can further improve the active ion transport and electronic conductivity of the negative electrode during charge and discharge, thereby improving the power performance, energy density, and cycle performance of the battery.

[0087] In some optional embodiments, the volume average particle size D of the negative electrode active material is v 50 can be 15μm to 19μm, and can be 16μm to 18μm. v 50 is appropriate, so that it has lower powder resistivity and higher lithium ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the side reaction of the electrolyte on the surface of the material, thereby improving the power performance, cycle performance and energy density of the battery.

[0088] The inventors also found that when the positive electrode active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and a modified compound thereof, and the negative electrode active material includes both artificial graphite and natural graphite, the resistance R of the negative electrode sheet is within a specific range, and the D of the negative electrode active material isv When the content of α-HgCl2 is within the above range, the battery can have a higher capacity retention rate during high-temperature storage, thereby improving the high-temperature storage performance.

[0089] In order to make the D of the negative electrode active material v In some embodiments, the volume average particle size D of natural graphite is within the above range. v 50 can be 15 μm to 20 μm, optionally 15 μm to 19 μm, and further optionally 16 μm to 18 μm. The volume average particle size D of artificial graphite v 50 can be 14μm to 19μm, optionally 14μm to 18μm, and further optionally 15μm to 17μm.

[0090] In some optional embodiments, the degree of graphitization of the negative electrode active material is 92% to 95%, optionally 93% to 94%. A graphitization degree of the negative electrode active material within an appropriate range can provide a suitable powder resistivity, thereby facilitating the negative electrode sheet's resistance to fall within the range specified in this application. It also provides an interlayer spacing suitable for active ion insertion and extraction, further improving the battery's power performance. Negative electrode sheets employing this negative electrode active material also exhibit high cohesion and adhesion, reducing battery expansion during cycling, thereby further improving the battery's cycling performance.

[0091] The inventors also found that when the positive electrode active material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and modified compounds thereof, the negative electrode active material includes both artificial graphite and natural graphite, the resistance R of the negative electrode plate is within a specific range, and the degree of graphitization of the negative electrode active material is within the above range, the high-temperature storage performance of the battery can also be improved.

[0092] In order to ensure that the degree of graphitization of the negative electrode active material is within the above-mentioned range, in some embodiments, the degree of graphitization of natural graphite can be 95% to 98.5%, optionally 95% to 98%, and further optionally 96% to 97%. The degree of graphitization of artificial graphite can be 89% to 95%, optionally 90% to 95%, and further optionally 91% to 93%.

[0093] In some optional embodiments, the surface density of the negative electrode membrane can be 7 mg / cm 2 ~10mg / cm 2 , optional 7mg / cm 2 ~8mg / cm 2. When the positive electrode material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and modified compounds thereof, the negative electrode active material includes both artificial graphite and natural graphite, the resistance R of the negative electrode pole piece is within a specific range, and the surface density of the negative electrode membrane is within a given range, the battery can have a higher energy density. At the same time, the battery also has better active ion and electron transport properties, thereby further improving the power performance of the battery. In addition, when the battery meets the above design, polarization and side reactions can be reduced, thereby further improving the cycle performance of the battery.

[0094] In some optional embodiments, the compaction density of the negative electrode film can be 1.5 g / cm 3 ~1.7g / cm 3 , optional 1.55g / cm 3 ~1.65g / cm 3 , 1.55g / cm2 is also available 3 ~1.6g / cm 3 . When the positive electrode material includes one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, and modified compounds thereof, the negative electrode active material includes both artificial graphite and natural graphite, the resistance R of the negative electrode pole piece is within a specific range, and the compaction density of the negative electrode membrane is within the given range, the battery can be guaranteed to have a higher energy density. At the same time, the close contact between the negative electrode active material particles can effectively reduce the resistance of the negative electrode membrane, thereby further improving the power performance of the battery. In addition, the appropriate compaction density can also protect the integrity of the particle structure of the negative electrode active material, which is beneficial to improve the cohesion and adhesion of the negative electrode pole piece, reduce expansion and side reactions during the battery cycle, and further improve the cycle life and safety performance of the battery.

[0095] In this application, a negative electrode sheet that satisfies a resistance R refers to a negative electrode sheet having a negative electrode membrane disposed on at least one surface of a negative electrode current collector, and more particularly, refers to a negative electrode sheet having negative electrode membranes disposed on two opposing surfaces of the negative electrode current collector. The resistance R of the negative electrode sheet can be adjusted to fall within a desired range by regulating one or more of the following: the selection of the first material, the selection of the second material, the ratio of the first material to the second material, the proportion of natural graphite, the compaction density of the negative electrode membrane, the type of conductive agent, the content of the conductive agent, etc. The selection of the first material and the second material may independently include, but are not limited to, one or more of their composition, powder resistivity, degree of graphitization, particle size distribution, surface coating modification, etc.

[0096] In any negative electrode sheet of the present application, the negative electrode active material may optionally include one or more of hard carbon, soft carbon, silicon-based materials, and tin-based materials. The silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys. The tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0097] In any negative electrode sheet of the present application, optionally, any two circular regions of equal area on the negative electrode sheet are designated as the first region and the second region, respectively. The center-to-center distance between the first region and the second region is 20 cm, and the sheet resistance R1 of the first region and the sheet resistance R2 of the second region satisfy the following relationship: |R1-R2|≤3. A small resistance difference between any two circular regions of equal area and 20 cm apart on the negative electrode sheet indicates minimal resistance fluctuation in the negative electrode sheet, i.e., good dispersion uniformity of the first and second materials in the negative electrode film. This improves the compaction density, cycling stability, and electrolyte distribution uniformity at all locations within the negative electrode sheet, thereby ensuring that the active ion transport and electron conductivity performance at different locations within the negative electrode sheet are substantially at the same level, thereby improving capacity utilization, cycle and storage life, and kinetic performance at all locations within the negative electrode sheet. The good overall consistency of the negative electrode sheet can further improve the energy density, high-temperature performance, and low-temperature power performance of the secondary battery. Optionally, |R1-R2|≤1.

[0098] In some embodiments, the negative electrode membrane may further include a binder. For example, the binder for the negative electrode membrane may be selected from one or more of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0099] In some embodiments, the negative electrode membrane may further include a thickener. For example, the thickener may be sodium carboxymethylcellulose (CMC-Na).

[0100] In some embodiments, the negative electrode membrane may further include a conductive agent. For example, the conductive agent for the negative electrode membrane may be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0101] In this application, the resistance R of the negative electrode sheet has a well-known meaning in the art and can be tested using methods known in the art. For example, a BER1300 multi-function electrode sheet resistance meter is used for testing. First, cut the negative electrode sheet into a test sample of a certain size; turn on the resistance meter, pressure display power supply and computer, open the air valve, and clean the upper and lower copper probes with dust-free paper soaked in alcohol. Click to open the computer software, select the port, pressure and test mode, and enter the terminal test area of ​​154.02mm 2 , place the test electrode between the two probes, click the run button on the software to start the test. After the test is completed, record the test results. To ensure the accuracy of the test results, you can take 5 groups of test samples at the same time and calculate the average value of these 5 groups of test samples.

[0102] The resistance R1 and R2 of any two circular areas of the same area and a center distance of 20 cm on the negative electrode plate can be tested by referring to the test method of resistance R, and the resistance difference |R1-R2| can be calculated.

[0103] The powder resistivity of natural graphite and artificial graphite at a pressure of 8 MPa is well known in the art and can be measured using methods known in the art, for example, using a PRCD1100 powder resistivity meter, in accordance with the national standard GB / T30835-2014.

[0104] The areal density of the negative electrode film is well known in the art and can be measured using methods known in the art. For example, take a negative electrode sheet that has been coated on one side and cold-pressed (if it is a double-sided coated sheet, the negative electrode film on one side can be wiped off first), punch it into small discs with an area of ​​S1, weigh it, and record it as M1. Then wipe off the negative electrode film of the weighed negative electrode sheet, weigh the weight of the negative electrode current collector, and record it as M0. The negative electrode film sheet is then measured as M0. The areal density of the negative electrode film = (weight of the negative electrode sheet M1 - weight of the negative electrode current collector M0) / S1.

[0105] The compacted density of a negative electrode film is well known in the art and can be measured using methods known in the art. For example, take a negative electrode film that has been coated on one side and cold-pressed (if it is a double-sided coated film, the negative electrode film on one side can be wiped off first), measure the thickness of the negative electrode film, and then measure the surface density of the negative electrode film using the above method. The compacted density of the negative electrode film = surface density of the negative electrode film / thickness of the negative electrode film.

[0106] D of negative electrode active material v 50 is a well-known meaning in the art and can be measured using methods known in the art. For example, it can be measured using a laser particle size analyzer (such as Malvern Master Size 3000) with reference to standard GB / T 19077.1-2016. v The physical definition of 50 is the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%.

[0107] The graphitization degree of the negative electrode active material is well known in the art and can be tested using methods known in the art. For example, an X-ray diffractometer (such as Bruker D8 Discover) can be used for testing. The test can refer to JIS K 0131-1996 and JB / T 4220-2011, and the d 002 The size of the G is then calculated according to the formula G = (0.344-d 002 ) / (0.344-0.3354)×100% to calculate the graphitization degree, where d 002The interlayer spacing in the graphite crystal structure is measured in nm. In X-ray diffraction analysis, CuK α The ray is the radiation source, and the wavelength of the ray is The scanning 2θ angle range was 20° to 80°, and the scanning rate was 4° / min.

[0108] [Electrolytes]

[0109] The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte; it can be selected based on needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte solution).

[0110] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0111] In some embodiments, the electrolyte salt may be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalatophosphate) and LiTFOP (lithium tetrafluorooxalatophosphate).

[0112] In some embodiments, the solvent can be selected from one or more 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).

[0113] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature performance.

[0114] [Isolation film]

[0115] Secondary batteries using electrolytes and some secondary batteries using solid electrolytes also include an isolation membrane. The isolation membrane is arranged between the positive electrode plate and the negative electrode plate to play an isolation role. The present application has no special restrictions on the type of isolation membrane, and any well-known porous structure isolation membrane with good chemical stability and mechanical stability can be selected. In some embodiments, the material of the isolation membrane can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different.

[0116] The present application has no particular restrictions on the shape of the secondary battery, which can be cylindrical, square or any other shape. Figure 1 The secondary battery 5 is a square structure as an example.

[0117] In some embodiments, the secondary battery may include an outer packaging for encapsulating the positive electrode sheet, the negative electrode sheet, and the electrolyte.

[0118] In some embodiments, the outer packaging of the secondary battery can be a soft bag, such as a pouch-type soft bag. The soft bag can be made of plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS). The outer packaging of the secondary battery can also be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell.

[0119] In some embodiments, reference Figure 2 The outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and side plates together form a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to seal the receiving cavity.

[0120] The positive electrode sheet, the negative electrode sheet, and the separator can be wound or laminated to form an electrode assembly 52. ​​The electrode assembly 52 is enclosed in the housing cavity. The electrolyte can be an electrolyte solution, which is impregnated into the electrode assembly 52. ​​The number of electrode assemblies 52 included in the secondary battery 5 can be one or more, and can be adjusted according to needs.

[0121] In some embodiments, secondary batteries can be assembled into a battery module. The battery module can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0122] Figure 3 4 is an example of a battery module. Figure 3In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of secondary batteries 5 may further be fixed by fasteners.

[0123] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0124] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0125] Figure 4 and Figure 5 The battery pack 1 is used as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple 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 an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0126] The inventors have further discovered that a coordinated design of the negative electrode plate in combination with the positive electrode active material used in the battery can enable the secondary battery to exhibit better performance.

[0127] [Preparation method]

[0128] A method for preparing a secondary battery may include assembling a negative electrode sheet, a positive electrode sheet, and an electrolyte to form a secondary battery. In some embodiments, the positive electrode sheet, separator, and negative electrode sheet may be sequentially wound or stacked, with the separator positioned between the positive and negative electrode sheets to provide isolation, to obtain an electrode assembly (i.e., a battery cell). The electrode assembly is then placed in an outer packaging, injected with electrolyte, and sealed to obtain a secondary battery.

[0129] In some embodiments, the preparation of a secondary battery may further include the step of preparing a positive electrode sheet. For example, the positive electrode active material, conductive agent, and binder may be dispersed in a solvent (e.g., N-methylpyrrolidone, NMP) to form a uniform positive electrode slurry. The positive electrode slurry is then coated on a positive electrode current collector, and the positive electrode sheet is obtained after drying, cold pressing, and other processes.

[0130] In some embodiments, the preparation of a secondary battery may further include the step of preparing a negative electrode sheet. For example, the negative electrode active material, a binder, and optionally a thickener and a conductive agent are dispersed in a solvent, which may be deionized water, to form a uniform negative electrode slurry. The negative electrode slurry is then coated on a negative electrode current collector, and after drying and cold pressing, the negative electrode sheet is obtained.

[0131] Device

[0132] The second aspect of the present application provides a device comprising the secondary battery of the first aspect of the present application. The secondary battery can be used as a power source for the device or as an energy storage unit for the device. The device can be, but is not limited to, a mobile device (e.g., a mobile phone, a laptop computer, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0133] The device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0134] Figure 6 This is an example device. The device is a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module can be used.

[0135] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0136] Example

[0137] The following examples describe the present disclosure in more detail and are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise indicated, all parts, percentages, and ratios reported in the following examples are by weight, and all reagents used in the examples are commercially available or synthesized according to conventional methods and used directly without further processing, and all instruments used in the examples are commercially available.

[0138] Example 1

[0139] Negative electrode

[0140] The negative electrode active material (75% by weight of artificial graphite and 25% by weight of natural graphite), conductive agent Super P, binder SBR, and thickener CMC-Na were mixed in a weight ratio of 96.2:0.8:1.8:1.2, and fully stirred in an appropriate amount of deionized water to form a uniform negative electrode slurry; the negative electrode slurry was applied to both surfaces of the negative electrode current collector copper foil, and after drying and cold pressing, the negative electrode sheet was obtained.

[0141] Positive electrode

[0142] The positive electrode active material LiNi 0.8 Co 0.1 Mn 0.1 O2 (abbreviated as NCM811), conductive agent Super P, and binder PVDF are fully stirred and mixed in an appropriate amount of NMP in a weight ratio of 96.5:1.5:2 to form a uniform positive electrode slurry; the positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode sheet is obtained.

[0143] Isolation film

[0144] Use PP / PE composite isolation film.

[0145] Preparation of electrolyte

[0146] 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 solution. The concentration of LiPF6 in the electrolyte solution was 1 mol / L.

[0147] Preparation of secondary batteries

[0148] The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in order to obtain an electrode assembly; the electrode assembly is placed in an outer package, and the above-prepared electrolyte is added. After packaging, standing, formation, aging and other processes, a secondary battery is obtained.

[0149] Examples 2 to 25 and Comparative Examples 1 to 12

[0150] The preparation method is similar to that of Example 1, except that: the relevant parameters in the negative electrode sheet preparation step are regulated to obtain the corresponding secondary batteries, as shown in Tables 1 and 2; and the batteries of Examples 1 to 12 and Comparative Examples 1 to 6 in Table 1 all use the positive electrode active material NCM811; the batteries of Examples 13 to 25 and Comparative Examples 7 to 12 in Table 2 all use the positive electrode active material lithium iron phosphate (abbreviated as LFP).

[0151] Test section

[0152] In the following tests, when the positive electrode active material is lithium nickel cobalt manganese oxide, the discharge cut-off voltage is 2.8V and the charge cut-off voltage is 4.2V; when the positive electrode active material is lithium iron phosphate, the discharge cut-off voltage is 2.5V and the charge cut-off voltage is 3.65V.

[0153] 1) High temperature cycle expansion rate test of negative electrode

[0154] The initial thickness of the negative electrode sheet is recorded as H0. The battery is charged and discharged in an environment of 25°C at 1.0C.

[0155] (i.e. the current value that completely discharges the theoretical capacity within 1h) is discharged at a constant current of 1.0C to the discharge cut-off voltage; then charged at a constant current of 1.0C to the charge cut-off voltage, and continued to charge at a constant voltage until the current is 0.05C. At this time, the battery is in a fully charged state, i.e. 100% SOC (State of Charge). After the fully charged battery is allowed to stand for 5 minutes, it is discharged at a constant current of 1.0C to the discharge cut-off voltage. The discharge capacity at this time is the actual capacity of the battery at 1.0C, recorded as C0. At 45°C, the secondary battery is subjected to a 1C0 / 1C0 charge and discharge cycle with 100% DOD (100% depth of discharge, that is, full charge followed by full discharge) in a Xinwei charger and discharger. When the number of cycles reaches 600, the cycle is stopped. The secondary battery is then charged to 100% SOC, the secondary battery is disassembled and the thickness of the corresponding negative electrode sheet is tested, recorded as H1. The expansion rate of the negative electrode after the battery is cycled at 45°C and 1C0 / 1C0 for 600 cycles is: (H1 / H0-1)×100%.

[0156] 2) Low temperature lithium deposition performance test of batteries

[0157] First, determine the actual capacity (C0) of the battery at 1.0C using the method in 1). Then, place the battery at -10°C and charge it at a constant current of xC0 to the charge cutoff voltage. Then, charge it at a constant voltage to a current of 0.05C0. After 5 minutes of rest, disassemble the battery and observe lithium deposition on the interface. If no lithium deposition occurs on the negative electrode surface, increase the charge rate and test again until lithium deposition occurs. Record the maximum charge rate at which no lithium deposition occurs on the negative electrode surface to determine the low-temperature lithium deposition rate.

[0158] 3) High temperature storage performance test of batteries

[0159] First, determine the battery's actual capacity (C0) at 1.0C using the method in 1). Then, charge the battery at 25°C at a constant current of 1C0 to the charge cutoff voltage. Continue charging at constant voltage until the current reaches 0.05C, at which point the battery is fully charged. Place the fully charged battery at 60°C for 30 days and then test the remaining capacity (C1) at 25°C. This constitutes one storage cycle, and the discharge capacity measured is the discharge capacity after the first storage cycle. Repeat the first storage test process until the discharge capacity reaches 90% of C0, recording the storage time at this point.

[0160]

[0161]

[0162] Comparison of Examples 1-12 with Comparative Examples 1-6 reveals that when the positive electrode sheet comprises one or more of layered lithium transition metal oxides and modified compounds thereof, and the negative electrode sheet comprises both artificial graphite and natural graphite, and the resistance R satisfies 6.0 mΩ ≤ R ≤ 12.0 mΩ, particularly 8.0 mΩ ≤ R ≤ 10.0 mΩ, the negative electrode sheet exhibits low cyclic expansion during charge and discharge, effectively improving the lithium ion transport performance of the negative electrode sheet. This allows the secondary battery to achieve improved low cyclic expansion performance and low-temperature power performance while maintaining a high energy density. Furthermore, the battery's capacity retention during high-temperature storage is significantly improved.

[0163] Comparison of Examples 13-25 with Comparative Examples 7-12 demonstrates that when the positive electrode sheet comprises one or more of olivine-structured lithium-containing phosphates and their modified compounds, and the negative electrode sheet comprises both artificial and natural graphite, and the resistance R satisfies 3.0 mΩ ≤ R ≤ 7.0 mΩ, and particularly 4.0 mΩ ≤ R ≤ 6.0 mΩ, the negative electrode sheet exhibits low cyclic expansion during charge and discharge, while effectively improving the lithium ion transport performance of the negative electrode sheet. This allows the secondary battery to achieve improved low cyclic expansion performance and low-temperature power performance while maintaining a high energy density. Furthermore, the battery's capacity retention during high-temperature storage is significantly improved.

[0164] In addition, it can be seen from the results of Examples 6-8 and Examples 18-21 that when the positive electrode plate adopts a specific positive electrode active material, the negative electrode plate contains both artificial graphite and natural graphite, and the mass proportion of natural graphite in the negative electrode active material is within a specific range, the battery's low cycle expansion performance, low temperature power performance and high temperature storage performance can be further improved.

[0165] It can be seen from the results of Examples 9-12 and Examples 22-25 that when the positive electrode plate adopts a specific positive electrode active material, the negative electrode plate contains both artificial graphite and natural graphite, and the compaction density and / or surface density of the negative electrode membrane is within a specific range, the battery's low cycle expansion performance, low temperature power performance and high temperature storage performance can be further improved.

[0166] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present application, and such modifications or substitutions should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A secondary battery comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode membrane disposed on at least one surface of the positive electrode current collector and comprising a positive electrode active material; the negative electrode sheet comprises a negative electrode current collector and a negative electrode membrane disposed on at least one surface of the negative electrode current collector and comprising a negative electrode active material, wherein: The positive electrode active material includes one or more of layered lithium transition metal oxides and modified compounds thereof; The negative electrode active material includes a first material and a second material, the first material includes artificial graphite, the second material includes natural graphite, and the degree of graphitization of the negative electrode active material is 92% to 96%; The resistance R of the negative electrode plate satisfies: 6.0 mΩ ≤ R ≤ 12.0 mΩ.

2. The secondary battery according to claim 1, wherein The resistance R of the negative electrode plate satisfies: 8.0 mΩ≤R≤10.0 mΩ.

3. The secondary battery according to claim 1, wherein The mass proportion of the natural graphite in the negative electrode active material is 10% to 50%.

4. The secondary battery according to claim 3, wherein The mass proportion of the natural graphite in the negative electrode active material is 15% to 30%.

5. The secondary battery according to claim 1, wherein The powder resistivity of the natural graphite at a pressure of 8 MPa is 10.0 mΩ·cm to 14.0 mΩ·cm; and / or, The powder resistivity of the artificial graphite under a pressure of 8 MPa is 11.0 mΩ·cm to 16.0 mΩ·cm.

6. The secondary battery according to claim 5, wherein The powder resistivity of the natural graphite at a pressure of 8 MPa is 11.0 mΩ·cm to 13.0 mΩ·cm; and / or, The powder resistivity of the artificial graphite under a pressure of 8 MPa is 13.0 mΩ·cm to 15.0 mΩ·cm.

7. The secondary battery according to claim 1, wherein The volume average particle size D of the negative electrode active material v 50 is 11μm ~ 15 μm.

8. The secondary battery according to claim 7, wherein The volume average particle size D of the negative electrode active material v 50 is 12μm ~ 14 μm.

9. The secondary battery according to claim 7, wherein The volume average particle size D of the natural graphite v 50 is 10 μm ~16 μm.

10. The secondary battery according to claim 9, wherein The volume average particle size D of the natural graphite v 50 is 10 μm~14 μm.

11. The secondary battery according to claim 7, wherein The volume average particle size D of the artificial graphite v 50 is 12 μm~19 μm.

12. The secondary battery according to claim 11, wherein The volume average particle size D of the artificial graphite v 50 is 12 μm~16 μm.

13. The secondary battery according to claim 1, wherein The graphitization degree of the negative electrode active material is 93% to 95%.

14. The secondary battery according to claim 13, wherein The degree of graphitization of the natural graphite is 95% to 98.5%.

15. The secondary battery according to claim 14, wherein The degree of graphitization of the natural graphite is 96% to 98%.

16. The secondary battery according to claim 13, wherein The degree of graphitization of the artificial graphite is 90% to 97.5%.

17. The secondary battery according to claim 16, wherein The degree of graphitization of the artificial graphite is 91% to 95%.

18. The secondary battery according to claim 1, wherein The surface density of the negative electrode membrane is 10 mg / cm 2 ~ 13 mg / cm 2 and / or, The compaction density of the negative electrode membrane is 1.6 g / cm 3 ~ 1.8 g / cm 3 .

19. The secondary battery according to claim 18, wherein The surface density of the negative electrode membrane is 10.5 mg / cm 2 ~ 11.5 mg / cm 2 and / or, The compaction density of the negative electrode membrane is 1.65 g / cm 3 ~ 1.75 g / cm 3 .

20. The secondary battery according to claim 1, wherein The layered lithium transition metal oxide includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof.

21. A secondary battery comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode membrane disposed on at least one surface of the positive electrode current collector and comprising a positive electrode active material; the negative electrode sheet comprises a negative electrode current collector and a negative electrode membrane disposed on at least one surface of the negative electrode current collector and comprising a negative electrode active material, wherein: The positive electrode active material includes one or more of olivine-structured lithium-containing phosphates and modified compounds thereof; The negative electrode active material includes a first material and a second material, the first material includes artificial graphite, the second material includes natural graphite, and the degree of graphitization of the negative electrode active material is 92% to 95%; The resistance of the negative electrode plate is 3.0 mΩ ≤ R ≤ 7.0 mΩ.

22. The secondary battery according to claim 21, wherein The resistance R of the negative electrode plate satisfies: 4.0 mΩ≤R≤6.0 mΩ.

23. The secondary battery according to claim 21, wherein The mass proportion of the natural graphite in the negative electrode active material is 10% to 50%.

24. The secondary battery according to claim 23, wherein The natural graphite accounts for 35% to 50% by mass of the negative electrode active material.

25. The secondary battery according to claim 21, wherein The powder resistivity of the natural graphite at a pressure of 8 MPa is 8.0 mΩ·cm to 12.0 mΩ·cm; and / or, The powder resistivity of the artificial graphite at a pressure of 8 MPa is 15 mΩ·cm to 20 mΩ·cm.

26. The secondary battery according to claim 25, wherein The powder resistivity of the natural graphite at a pressure of 8 MPa is 9.0 mΩ·cm to 11.0 mΩ·cm; and / or, The powder resistivity of the artificial graphite at a pressure of 8 MPa is 16 mΩ·cm to 18 mΩ·cm.

27. The secondary battery according to claim 21, wherein The volume average particle size D of the negative electrode active material v 50 is 15 μm ~ 19 μm.

28. The secondary battery according to claim 27, wherein The volume average particle size D of the negative electrode active material v 50 is 16 μm ~ 18 μm.

29. The secondary battery according to claim 27, wherein The volume average particle size D of the natural graphite v 50 is 15μm~20μm.

30. The secondary battery according to claim 29, wherein The volume average particle size D of the natural graphite v 50 is 15 μm~19 μm.

31. The secondary battery according to claim 27, wherein The volume average particle size D of the artificial graphite v 50 is 14μm~19μm.

32. The secondary battery according to claim 31, wherein The volume average particle size D of the artificial graphite v 50 is 14 μm~18 μm.

33. The secondary battery according to claim 21, wherein The graphitization degree of the negative electrode active material is 93% to 94%.

34. The secondary battery according to claim 33, wherein The degree of graphitization of the natural graphite is 95% to 98.5%.

35. The secondary battery according to claim 34, wherein The degree of graphitization of the natural graphite is 95% to 98%.

36. The secondary battery according to claim 33, wherein The degree of graphitization of the artificial graphite is 89% to 95%.

37. The secondary battery according to claim 36, wherein The degree of graphitization of the artificial graphite is 90% to 95%.

38. The secondary battery according to claim 21, wherein The surface density of the negative electrode membrane is 7 mg / cm 2 ~ 10mg / cm 2 and / or, The compaction density of the negative electrode membrane is 1.5 g / cm 3 ~ 1.7 g / cm 3 .

39. The secondary battery according to claim 38, wherein The surface density of the negative electrode membrane is 7 mg / cm 2 ~ 8mg / cm 2 and / or, The compaction density of the negative electrode membrane is 1.55 g / cm 3 ~ 1.65 g / cm 3 .

40. The secondary battery according to claim 21, wherein The lithium-containing phosphate with an olivine structure includes one or more of lithium iron phosphate and a composite material of lithium iron phosphate and carbon.

41. The secondary battery according to any one of claims 1 to 40, wherein Any two circular regions of the same area on the negative electrode plate are respectively recorded as the first region and the second region. The center distance between the first region and the second region is 20 cm. The resistance R1 of the first region and the resistance R2 of the second region satisfy: .

42. The secondary battery according to claim 41, wherein 。 43. A device comprising the secondary battery according to any one of claims 1 to 42.

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