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, and the second material includes natural graphite. The positive electrode active material includes one or more of a layered lithium transition metal oxide and its modified compounds, and the surface finish L of the negative electrode membrane facing away from the negative electrode current collector satisfies the following conditions: 40≤L≤50. Alternatively, the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and its modified compounds, and the surface finish L of the negative electrode membrane facing away from the negative electrode current collector satisfies the following conditions: 45≤L≤55.

Description

[0001] This application is a divisional application based on the invention with application number: 202080005576.0, application date: March 27, 2020, applicant: Contemporary Amperex Technology Co., Ltd., and invention name: “Secondary battery and device containing the same”. Technical Field

[0002] The present application belongs to the technical field of secondary batteries, and in particular 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 scope of application 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 urgently needs to be solved. Summary of the Invention

[0004] In a first aspect, the present application provides a secondary battery comprising a positive electrode sheet and a negative electrode sheet. The positive electrode sheet comprises a positive current collector and a positive electrode membrane disposed on at least one surface of the positive current collector and comprising a positive electrode active material. The negative electrode sheet comprises a negative current collector and a negative electrode membrane disposed on at least one surface of the negative current collector and comprising a negative electrode active material. The positive electrode active material comprises one or more of layered lithium transition metal oxides and modified compounds thereof. The negative electrode active material comprises a first material and a second material, the first material comprising artificial graphite and the second material comprising natural graphite. The surface finish L of the negative electrode membrane facing away from the negative current collector satisfies the following conditions: 40 ≤ L ≤ 50. Optionally, 43 ≤ L ≤ 48.

[0005] Surprisingly, the secondary battery of the present application has been found to achieve a higher energy density for the negative electrode sheet while effectively improving the active ion transport performance of the negative electrode sheet. Furthermore, the secondary battery can achieve both lower high-temperature cycling expansion and higher high-temperature cycling capacity retention.

[0006] 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 battery's energy density while further improving its power performance. Furthermore, an appropriate amount of natural graphite can also result in the battery having reduced high-temperature cycling expansion and improved high-temperature cycling performance.

[0007] In any embodiment of the first aspect of the present application, the powder compaction density of the negative electrode active material under a pressure of 30000N is 1.7g / cm 3 ~1.9g / cm 3 , optional 1.75g / cm 3 ~1.85g / cm 3 The compaction density of the negative electrode active material powder is within an appropriate range, which can improve the energy density of the battery.

[0008] In any embodiment of the first aspect of the present application, the powder compaction density of natural graphite under a pressure of 30000N is 1.85g / cm 3 ~1.95g / cm 3 , optional 1.90g / cm 3 ~1.95g / cm 3 The compaction density of natural graphite powder is within an appropriate range, which can improve the energy density of the battery.

[0009] In any embodiment of the first aspect of the present application, the powder compaction density of the artificial graphite under a pressure of 30000N is 1.75g / cm 3 ~1.85g / cm 3 , optional 1.77g / cm 3 ~1.80g / cm 3 When the compaction density of artificial graphite powder is within an appropriate range, the energy density of the battery can be improved.

[0010] In any embodiment of the first aspect of the present application, the degree of graphitization of the negative electrode active material is 92% to 96%, optionally 93% to 95%. A graphitization degree of the negative electrode active material within an appropriate range can further improve the low-temperature power performance of the battery. Furthermore, the negative electrode sheet using this negative electrode active material can also exhibit high cohesion and adhesion, improving the battery's low-cycle expansion performance.

[0011] In any embodiment of the first aspect of the present application, the degree of graphitization of the natural graphite is 95% to 98.5%, and can be optionally 97.5% to 98.5%. When the degree of graphitization of the natural graphite is within an appropriate range, the low-temperature power performance of the battery can be further improved.

[0012] In any embodiment of the first aspect of the present application, the degree of graphitization of the artificial graphite is 89% to 95%, and can be optionally 90% to 93%. When the degree of graphitization of the artificial graphite is within an appropriate range, the low-temperature power performance of the battery can be further improved.

[0013] 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, which is conducive to improving the battery's low-temperature power performance, energy density and high-temperature cycle performance.

[0014] In any embodiment of the first aspect of the present application, D of natural graphite v 50 is 15μm~20μm, 15μm~19μm can be selected, and 16μm~18μm can be selected. v 50 is appropriate, which is conducive to improving the battery's low-temperature power performance, energy density and high-temperature cycle performance.

[0015] In any embodiment of the first aspect of the present application, the D of the artificial graphite v 50 is 14μm~19μm, 14μm~18μm can be selected, and 15μm~17μm can be selected. v 50 is appropriate, which is conducive to improving the battery's low-temperature power performance, energy density and high-temperature cycle performance.

[0016] 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 extend the battery's high-temperature cycle life.

[0017] 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 / cm 2 , optional 10.5mg / cm 2 ~11.5mg / 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 battery's dynamic performance. In addition, the negative electrode sheet meets the above design requirements, which can further improve the high-temperature cycle performance of the battery.

[0018] In any embodiment of the first aspect of the present application, the modified compound includes a doping-modified compound and / or a surface-coating-modified compound.

[0019] A second aspect of the present application provides a secondary battery, which includes a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode collector and a positive electrode membrane arranged on at least one surface of the positive electrode collector and including a positive electrode active material, the negative electrode sheet includes a negative electrode collector and a negative electrode membrane arranged 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 smoothness L of the surface of the negative electrode membrane facing away from the negative electrode collector satisfies: 45≤L≤55; optionally, 48≤L≤52.

[0020] Surprisingly, the secondary battery of the present application has been found to achieve a higher energy density for the negative electrode sheet while effectively improving the active ion transport performance of the negative electrode sheet. Furthermore, the secondary battery can achieve both lower high-temperature cycling expansion and higher high-temperature cycling capacity retention.

[0021] In any embodiment of the second aspect of the present application, the weight percentage 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 can achieve a higher energy density while further improving the battery's power performance. Furthermore, an appropriate amount of natural graphite can result in a battery with lower high-temperature cycling expansion and higher high-temperature cycling performance.

[0022] In any embodiment of the second aspect of the present application, the powder compaction density of the negative electrode active material under a pressure of 30000N is 1.8g / cm 3 ~1.9g / cm 3 , optional 1.82g / cm 3 ~1.88g / cm 3 The compaction density of the negative electrode active material powder is within an appropriate range, which can improve the energy density of the battery.

[0023] In any embodiment of the first aspect of the present application, the powder compaction density of natural graphite under a pressure of 30000N is 1.85g / cm 3 ~1.95g / cm 3, optional 1.90g / cm 3 ~1.95g / cm 3 The compaction density of natural graphite powder is within an appropriate range, which can improve the energy density of the battery.

[0024] In any embodiment of the second aspect of the present application, the powder compaction density of the artificial graphite under a pressure of 30000N is 1.75g / cm 3 ~1.85g / cm 3 , optional 1.77g / cm 3 ~1.80g / cm 3 When the compaction density of artificial graphite powder is within an appropriate range, the energy density of the battery can be improved.

[0025] 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 low-temperature 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-cycle expansion performance.

[0026] In any embodiment of the second aspect of the present application, the degree of graphitization of the natural graphite is 95% to 98.5%, and can be optionally 97.5% to 98.5%. When the degree of graphitization of the natural graphite is within an appropriate range, the low-temperature power performance of the battery can be further improved.

[0027] In any embodiment of the second aspect of the present application, the degree of graphitization of the artificial graphite is 89% to 95%, and can be optionally 90% to 93%. When the degree of graphitization of the artificial graphite is within an appropriate range, the low-temperature power performance of the battery can be further improved.

[0028] 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 within an appropriate range, which is beneficial to improving the low-temperature power performance, energy density and high-temperature cycle performance of the battery.

[0029] In any embodiment of the second aspect of the present application, D of natural graphite v 50 is 15μm~20μm, 15μm~19μm can be selected, and 16μm~18μm can be selected. v 50 is appropriate, which is conducive to improving the battery's low-temperature power performance, energy density and high-temperature cycle performance.

[0030] In any embodiment of the second aspect of the present application, the D of the artificial graphite v50 is 14μm~19μm, 14μm~18μm can be selected, and 15μm~17μm can be selected. v 50 is appropriate, which is conducive to improving the battery's low-temperature power performance, energy density and high-temperature cycle performance.

[0031] 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 A negative electrode membrane compaction density within the given range can ensure a high energy density for the battery and further improve the battery's low-temperature power performance. Furthermore, a suitable compaction density can further extend the battery's high-temperature cycle life.

[0032] 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 battery's dynamic performance. In addition, the negative electrode sheet meets the above design requirements, which can further improve the high-temperature cycle performance of the battery.

[0033] In any embodiment of the second aspect of the present application, the modified compound includes a doping-modified compound and / or a surface-coating-modified compound.

[0034] In any embodiment of the first aspect and the second aspect of the present application, the smoothness L of the surface of the negative electrode diaphragm facing away from the negative electrode current collector is tested by the following method: the negative electrode is placed under the measuring port of a precision colorimeter, the measuring port of the precision colorimeter is brought close to the negative electrode diaphragm and the position is adjusted to achieve alignment, measurement is performed and the test results are recorded.

[0035] 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.

[0036] 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

[0037] 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.

[0038] Figure 1 This is a schematic diagram of one embodiment of a secondary battery.

[0039] Figure 2 yes Figure 1 Exploded diagram of .

[0040] Figure 3 is a schematic diagram of one embodiment of a battery module.

[0041] Figure 4 is a schematic diagram of one embodiment of a battery pack.

[0042] Figure 5 yes Figure 4 Exploded diagram of .

[0043] Figure 6 This is a schematic diagram of one embodiment of a device using a secondary battery as a power source. DETAILED DESCRIPTION

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] secondary batteries

[0049] 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.

[0050] [Positive electrode]

[0051] 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.

[0052] 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.

[0053] 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 a layered lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and modified compounds of the foregoing materials.

[0054] The “modification” in “modified compound” may be doping modification and / or surface coating modification of the material.

[0055] In some embodiments, the positive electrode active material includes one or more of a layered lithium transition metal oxide and a modified compound thereof. As a specific example, the positive electrode active material 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 a modified compound thereof. Alternatively, the positive electrode active material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and a modified compound thereof.

[0056] In some embodiments, the positive electrode active material includes Li a Ni b Co c Md 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 thereof. 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, and may optionally include one or more of Zr, Al, Zn and B; A is selected from one or more of N, F, S and Cl.

[0057] 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, and optionally 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, and optionally one or more of Zr, Zn, and B.

[0058] 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.

[0059] In some embodiments, the Li a Ni b Co c M d M' e O f A g 80% to 100% of the particle surface 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 particle has a coating layer.

[0060] In other embodiments, the positive electrode active material includes one or more of an olivine-structured lithium-containing phosphate and modified compounds thereof. Examples of the olivine-structured lithium-containing phosphate may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, and modified compounds thereof.

[0061] 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.

[0062] 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.

[0063] 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).

[0064] 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.

[0065] [Negative electrode]

[0066] 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.

[0067] 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.

[0068] The negative electrode membrane includes a negative electrode active material, which includes a first material and a second material, wherein the first material includes artificial graphite and the second material includes natural graphite. Surprisingly, it has been discovered that by controlling the surface finish L of the negative electrode membrane facing away from the negative electrode current collector within a specific range, the negative electrode membrane can achieve a higher energy density while effectively improving 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 cyclic capacity retention.

[0069] The smoothness L reflects the roughness of the surface of the negative electrode membrane facing away from the negative electrode current collector. The smoothness L can be adjusted to within a desired range by adjusting one or more of the selection of the first material, the selection of the second material, the ratio of the first and second materials, the proportion of natural graphite, the compaction density of the negative electrode membrane, the type of conductive agent, and the content of the conductive agent. The selection of the first and second materials may independently include, but are not limited to, one or more of their composition, degree of graphitization, particle size distribution, and surface coating modification.

[0070] The inventors further discovered that coordinating the design of the negative electrode sheet with the battery's positive electrode active material can enhance secondary battery performance. The inventors' in-depth research revealed that when the positive electrode sheet's positive active material includes one or more of layered lithium transition metal oxides and their modified compounds, the surface finish L of the negative electrode membrane facing away from the negative electrode current collector satisfies the following requirement: 40 ≤ L ≤ 50.

[0071] When the positive electrode active material includes one or more of layered lithium transition metal oxides and their modified compounds, the negative electrode active material of the negative electrode sheet includes both artificial graphite and natural graphite, and the smoothness L of the negative electrode film satisfies 40≤L≤50, the positive and negative electrode active materials can be effectively coordinated to give full play to the synergistic effect of the advantages of both. Under the premise of ensuring that the battery has a high energy density, the negative electrode sheet is also formed with a surface porosity suitable for electrolyte infiltration, and the solid phase diffusion rate of active ions in the negative electrode is increased, thereby further improving the transmission performance of active ions between the positive and negative electrodes. As a result, the kinetic performance of the battery is significantly improved. Even in a low temperature environment, the negative electrode can quickly receive active ions from the positive electrode, thereby improving the low temperature power performance of the secondary battery. The secondary battery is suitable for high rate charge and discharge, and the probability of lithium plating occurring during high rate charging at low temperature is significantly reduced. At the same time, the secondary battery can also have higher safety performance. In addition, the positive electrode active material includes one or more of layered lithium transition metal oxides and their modified compounds, which can make the positive electrode sheet have a higher surface density (for example, 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.

[0072] In these embodiments, optionally, the positive electrode active material is 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.

[0073] The inventors also discovered that when the positive electrode active material uses one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof, and the negative electrode active material contains both artificial graphite and natural graphite, and the smoothness L of the negative electrode membrane satisfies 40≤L≤50, the negative electrode plate can also have higher cohesion and adhesion, further reducing the expansion of the negative electrode during the battery cycle, and enhancing the electrolyte retention capacity in the plate, thereby enabling it to further improve the kinetic performance while maintaining a higher volume energy density.

[0074] In addition, the negative electrode sheet has high ion and electron transport performance, which can make the battery have low impedance. In addition, the negative electrode sheet contains both artificial graphite and natural graphite, and the smoothness L of the negative electrode membrane satisfies 40≤L≤50, for example, L is 40.5, 41.6, 42.3, 43.5, 44.8, 45.5, 46.0, 46.5, 47.0, 47.5, 48.0, 48.5 or 49.7. It can also reduce the loss of active material caused by poor particle contact and / or reduce the battery polarization phenomenon caused by uneven electrolyte distribution. Therefore, the battery can also have a high high-temperature cycle capacity retention rate.

[0075] In these embodiments, optionally, 43≤L≤48. When the smoothness L of the negative electrode membrane is within an appropriate range, the low-temperature power performance of the battery can be further improved, and the high-temperature cycle expansion of the battery can be further reduced and the high-temperature cycle capacity retention rate of the battery can be improved.

[0076] 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 smoothness L of the negative electrode membrane satisfies 40≤L≤50, if the negative electrode active material also meets one or more of the following conditions, the performance of the battery can be further improved.

[0077] 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%, and further optionally 15% to 25%, such as 15%, 17%, 19%, 20%, 21%, or 23%. The inclusion of an appropriate amount of natural graphite in the negative electrode active material can provide the negative electrode active material with a higher specific capacity, and in particular, can further increase the solid-phase diffusion rate of ions in the negative electrode pole piece, thereby providing the battery with a higher energy density while further improving the battery's power performance. Furthermore, an appropriate amount of natural graphite can improve the adhesion between the negative electrode active material particles and between the negative electrode active material and the negative electrode current collector, while providing the negative electrode pole piece with higher cohesion and adhesion, and reducing side reactions on the negative electrode surface, thereby enabling the battery to have lower high-temperature cycling expansion. The battery's high-temperature cycling performance can also be further improved.

[0078] In some optional embodiments, the powder compaction density of the negative electrode active material under a pressure of 30,000 N may be 1.7 g / cm 3 ~1.9g / cm 3 , optional 1.75g / cm 3 ~1.85g / cm 3 When the compaction density of the negative electrode active material powder is within an appropriate range, the negative electrode membrane can have a higher compaction density, thereby improving the energy density of the battery.

[0079] In order to make the powder compaction density of the negative electrode active material within the above-mentioned range, in some optional embodiments, the powder compaction density of natural graphite under a pressure of 30,000 N can be selected to be 1.75 g / cm 3 ~1.9g / cm 3 , 1.75g / cm2 is also available 3 ~1.85g / cm 3 The compacted density of artificial graphite powder under a pressure of 30,000 N can be selected as 1.7 g / cm 3 ~1.9g / cm 3 , 1.8g / cm2 is also available 3 ~1.9g / cm 3 .

[0080] 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 low-temperature 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-cycle expansion performance.

[0081] 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, and the smoothness L of the negative electrode membrane is within a specific range, the graphitization degree of the negative electrode active material is within the above range, which can also improve the high-temperature cycle performance of the battery.

[0082] In order to make the degree of graphitization of the negative electrode active material within the above-given range, in some optional embodiments, the degree of graphitization of natural graphite can be 95% to 98.5%, optionally 96% to 98%, and further optionally 96.5% to 97.6%; the degree of graphitization of artificial graphite can be 90% to 97.5%, optionally 90% to 95%, and further optionally 91% to 93.5%.

[0083] 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 50 is, the more particles there are on the surface of the negative electrode membrane, the greater the rebound of the electrode after cold pressing, the surface particles are embedded in each other, the surface roughness increases, and the surface finish of the negative electrode membrane is smaller, which can improve the electrolyte infiltration and the migration rate of ions in the negative electrode membrane. v When the ratio of the active ions to the negative electrode membrane is appropriately 50, the negative electrode membrane can have a higher active ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the active sites on the electrode surface, thereby improving the low-temperature power performance, energy density and high-temperature cycle performance of the battery.

[0084] In order to make the D of the negative electrode active material v 50 Within the above given range, in some optional embodiments, the D of natural graphite v 50 can be 10μm~16μm, optionally 10μm~14μm, and optionally 11μm~13μm; D of artificial graphite v 50 can be 12μm to 19μm, optionally 12μm to 16μm, and further optionally 13μm to 15μm.

[0085] In some optional embodiments, 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 , 1.68g / cm2 is also available 3 ~1.73g / cm 3. 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, the negative electrode active material includes artificial graphite and natural graphite, the smoothness L of the negative electrode diaphragm is within a specific range, and the compaction density of the negative electrode diaphragm is within the given range, the battery can be guaranteed to have a higher energy density; at the same time, the negative electrode diaphragm has higher electrolyte wettability and good ion solid phase diffusion rate, 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 sheet, reduce expansion and side reactions during the high-temperature cycle of the battery, and further improve the high-temperature cycle life of the battery.

[0086] In some optional embodiments, the surface density of the negative electrode film is 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 smoothness L of the negative electrode membrane 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 good 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 high-temperature cycle performance.

[0087] The inventors also found that when the positive active material of the positive electrode sheet includes one or more of olivine-structured lithium-containing phosphates and modified compounds thereof, optionally, the smoothness L of the surface of the negative electrode membrane facing away from the negative electrode current collector satisfies: 45≤L≤55.

[0088] When 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 of the negative electrode sheet includes both artificial graphite and natural graphite, and the surface finish L of the negative electrode film satisfies 45≤L≤55, an effective coordination between the positive and negative electrode active materials can be achieved, fully leveraging the synergistic effects of their respective advantages. While ensuring a high energy density of the battery, the solid-phase diffusion rate of active ions between the positive and negative electrodes is further increased. As a result, the battery's kinetic performance is further 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. This secondary battery is suitable for high-rate charging and discharging, and the probability of lithium plating occurring during high-rate charging at low temperatures is significantly reduced.

[0089] 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.

[0090] 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, and the negative electrode active material contains both artificial graphite and natural graphite, and the smoothness L of the negative electrode membrane satisfies 45≤L≤55, the expansion during the battery cycle can be further reduced. In addition, the electrolyte retention capacity between the active materials in the electrode is enhanced, which is conducive to improving the kinetic performance while maintaining a high volume energy density.

[0091] Furthermore, the negative electrode sheet has high ion and electron transport properties, resulting in a lower battery impedance. Furthermore, the inclusion of both artificial and natural graphite in the negative electrode sheet, along with a negative electrode film finish L that satisfies 45 ≤ L ≤ 55, can also reduce active material loss due to poor particle contact and / or battery polarization due to uneven electrolyte distribution. Consequently, the battery also exhibits high high-temperature cycle capacity retention.

[0092] In these embodiments, optionally, 48 ≤ L ≤ 52. For example, L is 48.5, 49.0, 49.5, 50.0, 50.5, 51.0, or 51.5. When the smoothness L of the negative electrode film is within an appropriate range, the low-temperature power performance of the battery can be further improved, and the high-temperature cycling expansion of the battery can be further reduced, thereby improving the high-temperature cycling capacity retention rate of the battery.

[0093] The inventors further discovered that when 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 both artificial graphite and natural graphite, and the smoothness L of the negative electrode membrane satisfies 45≤L≤55, if the negative electrode active material also meets one or more of the following conditions, the performance of the battery can be further improved.

[0094] 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 30%, 35%, 40%, 42%, 45%, 48%, or 50%. The inclusion of an appropriate amount of natural graphite in the negative electrode active material can give the negative electrode active material a higher gram capacity, and in particular, can further increase the solid-phase diffusion rate of ions in the negative electrode plate, thereby giving the battery a higher energy density while further improving the battery's power performance. In addition, an appropriate amount of natural graphite can improve the adhesion between the negative electrode active material particles and between the negative electrode active material and the negative electrode current collector, while giving the negative electrode plate higher cohesion and adhesion, and reducing side reactions on the negative electrode surface, thereby enabling the battery to have lower high-temperature cycle expansion. The high-temperature cycle performance of the battery can also be further improved.

[0095] In some optional embodiments, the powder compaction density of the negative electrode active material under a pressure of 30,000 N may be 1.8 g / cm 3 ~1.9g / cm 3 , 1.82g / cm2 is also available 3 ~1.88g / cm 3 When the compaction density of the negative electrode active material powder is within an appropriate range, the negative electrode membrane can have a higher compaction density, thereby improving the energy density of the battery.

[0096] In order to make the powder compaction density of the negative electrode active material within the above-mentioned range, in some optional embodiments, the powder compaction density of natural graphite under a pressure of 30,000 N can be selected to be 1.85 g / cm 3 ~1.95g / cm 3 , 1.90g / cm2 is also available 3 ~1.95g / cm 3 The compacted density of artificial graphite powder under a pressure of 30,000 N is 1.75 g / cm 3 ~1.85g / cm 3 , 1.77g / cm2 is also available 3 ~1.80g / cm 3 .

[0097] In some optional embodiments, the negative electrode active material has a degree of graphitization of 92% to 95%, optionally 93% to 94%. 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 low-temperature power performance. Furthermore, the negative electrode sheet using this negative electrode active material exhibits high cohesion and adhesion, improving the battery's low-cycle expansion performance.

[0098] The inventors also found that when the positive electrode active material includes one or more of olivine-structured lithium-containing phosphates and their modified compounds, the negative electrode active material includes artificial graphite and natural graphite, the smoothness L of the negative electrode membrane is within a specific range, and the graphitization degree of the negative electrode active material is within the above range, the high-temperature cycle performance of the battery can be further improved.

[0099] In order to make the graphitization degree of the negative electrode active material within the above-given range, in some optional embodiments, the graphitization degree of natural graphite can be 95% to 98.5%, optionally 97.5% to 98.5%; the graphitization degree of artificial graphite can be 89% to 95%, optionally 90% to 93%.

[0100] In some optional embodiments, 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 The smaller the 50 is, the more pores there are on the surface of the negative electrode membrane, and the surface finish of the negative electrode membrane is smaller, thereby improving the electrolyte infiltration and the migration rate of active ions in the negative electrode membrane. v When the ratio of the negative electrode to the negative electrode is 50, the negative electrode membrane can have a higher ion migration rate, while also increasing the gram capacity of the negative electrode active material and reducing the active sites on the electrode surface, thereby improving the low-temperature power performance, energy density and high-temperature cycle performance of the battery.

[0101] In order to make the D of the negative electrode active material v 50 Within the above given range, in some optional embodiments, the D of natural graphite v 50 can be 15μm~20μm, optionally 15μm~19μm, and optionally 16μm~18μm; D of artificial graphite v 50 can be 14μm to 19μm, optionally 14μm to 18μm, and further optionally 15μm to 17μm.

[0102] In some optional embodiments, the compaction density of the negative electrode membrane is 1.5 g / cm 3 ~1.7g / cm 3 , optional 1.55g / cm 3 ~1.65g / cm 3. When the positive electrode material includes one or more of the lithium-containing phosphates with an olivine structure and modified compounds thereof, the negative electrode active material includes both artificial graphite and natural graphite, the smoothness L of the negative electrode diaphragm is within a specific range, and the compaction density of the negative electrode diaphragm is within the given range, the battery can be guaranteed to have a higher energy density; at the same time, the negative electrode diaphragm has higher electrolyte wettability and good ion solid phase diffusion rate, thereby further improving the low-temperature 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 improving the cohesion and adhesion of the negative electrode sheet, reducing expansion and side reactions during the high-temperature cycle of the battery, and thus further improving the high-temperature cycle life of the battery.

[0103] In some optional embodiments, the surface density of the negative electrode film is 7 mg / cm 2 ~10mg / cm 2 , optional 7mg / cm 2 ~8mg / cm 2 When the positive electrode material includes one or more of olivine-structured lithium-containing phosphates and modified compounds thereof, the negative electrode active material includes artificial graphite and natural graphite, the smoothness L of the negative electrode membrane 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 good 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 high-temperature cycle performance.

[0104] 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.

[0105] 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).

[0106] In some embodiments, the negative electrode membrane may further include a thickener. For example, the thickener may be sodium carboxymethylcellulose (CMC-Na).

[0107] 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.

[0108] In this application, the smoothness L of the negative electrode film has a well-known meaning in the art and can be tested using methods known in the art. An exemplary test method is as follows: first, place the negative electrode film under the measurement port of a precision colorimeter (e.g., a multi-function precision colorimeter NR60CP), bring the measurement port of the precision colorimeter close to the negative electrode film and adjust the position to achieve alignment, perform measurements, and record the test results. To ensure the accuracy of the test results, 10 groups of samples to be tested can be taken, and the average value of these 10 groups of samples to be tested can be calculated.

[0109] In this application, the powder compaction density of the negative electrode active material has a meaning well known in the art and can be tested using methods known in the art. For example, it can be tested using an electronic pressure testing machine (such as UTM7305) with reference to GB / T 24533-2009. An exemplary test method is as follows: Place a powder sample to be tested with a mass of M on a special compaction mold (bottom area S), set different pressures (30000N is used in this application), maintain the pressure for 30s, release the pressure, wait for 10s, and read the thickness H of the powder after compaction under the pressure on the device; the powder compaction density of the negative electrode active material under the pressure = M / (H×S).

[0110] In this application, the surface density of the negative electrode film is a well-known meaning in the art and can be tested 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 surface density of the negative electrode film = (weight of the negative electrode sheet M1 - weight of the negative electrode current collector M0) / S1.

[0111] In this application, the compacted density of the negative electrode film is a well-known term 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), 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.

[0112] In this application, the D of the negative electrode active material v50 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 MasterSize 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%.

[0113] In this application, the graphitization degree of the negative electrode active material is a well-known meaning 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 K0131-1996 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 002 The 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.

[0114] [Electrolytes]

[0115] 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).

[0116] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0117] 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).

[0118] 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).

[0119] 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.

[0120] [Isolation film]

[0121] 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.

[0122] 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.

[0123] In some embodiments, the secondary battery may include an outer packaging for encapsulating the positive electrode sheet, the negative electrode sheet, and the electrolyte.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] Figure 3 4 is an example of a battery module. Figure 3 In 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.

[0129] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0130] 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.

[0131] 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.

[0132] [Preparation method]

[0133] The 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, a separator, and a 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 battery cell is then placed in an outer packaging, injected with electrolyte, and sealed to obtain a secondary battery.

[0134] 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.

[0135] 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.

[0136] Device

[0137] 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.

[0138] The device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0139] 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.

[0140] 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.

[0141] Example

[0142] 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.

[0143] Example 1

[0144] Positive electrode

[0145] 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 mass 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.

[0146] Negative electrode

[0147] 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 thoroughly stirred and mixed in an appropriate amount of deionized water in a mass ratio of 96.2:0.8:1.8:1.2 to form a uniform negative electrode slurry; the negative electrode slurry was coated on both surfaces of the negative electrode current collector copper foil, and after drying and cold pressing, the negative electrode sheet was obtained.

[0148] Isolation film

[0149] Use PP / PE composite isolation film.

[0150] Preparation of electrolyte

[0151] 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.

[0152] Preparation of secondary batteries

[0153] The positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound in order to obtain an electrode assembly; the electrode assembly is added to an outer package, and the above-prepared electrolyte is added. After packaging, standing, formation, aging and other processes, a secondary battery is obtained.

[0154] Examples 2 to 25 and Comparative Examples 1 to 12

[0155] The preparation method is similar to that of Example 1, except that the relevant parameters in the negative electrode sheet preparation step are adjusted 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, and the compacted density of the positive electrode film is 3.5 g / cm 3 , with a surface density of 18.5 mg / cm 2 The batteries of Examples 13 to 25 and Comparative Examples 7 to 12 in Table 2 all use lithium iron phosphate (LFP) as the positive electrode active material, and the compaction density of the positive electrode film is 2.3 g / cm 3 , with a surface density of 16.2 mg / cm 2 .

[0156] Test section

[0157] 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.

[0158] 1) Cyclic expansion rate test of negative electrode

[0159] The initial thickness of the negative electrode sheet in each embodiment and comparative example is recorded as H0. Then, the battery is subjected to a charge and discharge test in an environment of 25°C, and is discharged at a constant current of 1.0C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour) to the discharge cut-off voltage; then, it is charged at a constant current of 1.0C to the charge cut-off voltage, and the constant voltage charging is continued until the current reaches 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 of 100% DOD (100% depth of discharge, i.e., full charge followed by full discharge) in a Xinwei charger and discharger. When the number of cycles reaches 600, the cycle is stopped. Then charge the secondary battery to 100% SOC, disassemble the secondary battery and measure the thickness of the corresponding negative electrode sheet, which is recorded as H1. The cycle expansion rate of the negative electrode sheet after the battery is cycled at 45°C and 1C0 / 1C0 for 600 cycles is: (H1 / H0-1)×100%.

[0160] 2) Low temperature lithium deposition performance test of batteries

[0161] 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.

[0162] 3) High temperature cycle performance test of batteries

[0163] First, determine the actual capacity C0 of the battery at 1.0C according to the method in 1). Then, in an environment of 60°C, charge the battery with a charging current of 1.0C0 to the charging cut-off voltage, then charge it with a constant voltage to a current of 0.05C0, and then discharge it with a discharge current of 1.0C0 to the discharge cut-off voltage. This is one charge and discharge cycle, and the discharge capacity of this time is the discharge capacity of the first cycle. Subsequently, perform continuous charge and discharge cycles, record the discharge capacity value during the cycle, and calculate the capacity retention rate of each cycle. When the cycle capacity retention rate drops to 80% of the discharge capacity of the first cycle, record the number of cycles of the battery.

[0164]

[0165]

[0166] Comparison of Examples 1-12 with Comparative Examples 1-6 demonstrates that when the positive electrode sheet comprises one or more of layered lithium transition metal oxides and their modified compounds, the negative electrode sheet comprises both artificial graphite and natural graphite, and the surface finish L of the negative electrode membrane facing away from the negative electrode current collector satisfies 40 ≤ L ≤ 50, particularly 43 ≤ L ≤ 48, 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 and low-temperature power performance while maintaining a high energy density. Furthermore, the battery's capacity retention during high-temperature cycling is significantly improved.

[0167] Comparison of Examples 13-25 with Comparative Examples 7-12 demonstrates that when the positive electrode sheet comprises one or more of an olivine-structured lithium-containing phosphate and its modified compounds, the negative electrode sheet comprises both artificial graphite and natural graphite, and the surface finish L of the negative electrode membrane facing away from the negative electrode current collector satisfies 45 ≤ L ≤ 55, particularly 48 ≤ L ≤ 52, 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 and low-temperature power performance while maintaining a high energy density. Furthermore, the battery's capacity retention during high-temperature cycling is significantly improved.

[0168] 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, it is more conducive to making the battery simultaneously take into account higher low cycle expansion performance, low temperature power performance and high temperature cycle performance.

[0169] 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, it is more conducive to making the battery simultaneously take into account higher low cycle expansion performance, low temperature power performance and high temperature cycle performance.

[0170] 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, and the second material includes natural graphite; the volume average particle size D of the negative electrode active material v 50 is 11 μm ~ 15 μm; The smoothness L of the surface of the negative electrode membrane facing away from the negative electrode current collector satisfies: 40 ≤ L ≤ 50.

2. The secondary battery according to claim 1, wherein 43 ≤ L ≤ 48。 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 or 2, wherein The powder compaction density of the negative electrode active material under a pressure of 30000N is 1.7 g / cm 3 ~ 1.9 g / cm 3 .

6. The secondary battery according to claim 5, wherein The powder compaction density of the negative electrode active material under a pressure of 30000N is 1.75 g / cm 3 ~ 1.85 g / cm 3 .

7. The secondary battery according to claim 1 or 2, wherein The natural graphite powder compaction density under a pressure of 30000N is 1.85 g / cm 3 ~ 1.95 g / cm 3 ; and / or the artificial graphite powder compaction density under a pressure of 30000N is 1.75 g / cm 3 ~ 1.85 g / cm 3 .

8. The secondary battery according to claim 7, wherein The natural graphite powder compaction density under a pressure of 30000N is 1.90 g / cm 3 ~ 1.95 g / cm 3 .

9. The secondary battery according to claim 7, wherein The compacted density of the artificial graphite powder under a pressure of 30,000 N is 1.77 g / cm 3 ~ 1.80 g / cm 3 .

10. The secondary battery according to claim 1 or 2, wherein The graphitization degree of the negative electrode active material is 92% to 96%.

11. The secondary battery according to claim 10, wherein The graphitization degree of the negative electrode active material is 93% to 95%.

12. The secondary battery according to claim 1 or 2, wherein The degree of graphitization of the natural graphite is 95% to 98.5%; and / or the degree of graphitization of the artificial graphite is 89% to 95%.

13. The secondary battery according to claim 12, wherein The degree of graphitization of the natural graphite is 97.5% to 98.5%.

14. The secondary battery according to claim 12, wherein The degree of graphitization of the artificial graphite is 90% to 93%.

15. The secondary battery according to claim 1 or 2, wherein The volume average particle size D of the negative electrode active material v 50 is 12 μm ~ 14 μm.

16. The secondary battery according to claim 1 or 2, wherein The natural graphite D v 50 is 15 μm ~ 20 μm; and / or D of the artificial graphite v 50 is 14μm ~ 19 μm.

17. The secondary battery according to claim 16, wherein The natural graphite D v 50 is 15 μm ~ 19 μm.

18. The secondary battery according to claim 16, wherein The D of the artificial graphite v 50 is 14 μm ~ 18 μm.

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

20. The secondary battery according to claim 19, wherein The compaction density of the negative electrode membrane is 1.65 g / cm 3 ~1.75 g / cm 3 .

21. The secondary battery according to claim 19, wherein The surface density of the negative electrode membrane is 10.5 mg / cm 2 ~11.5 mg / cm 2 .

22. 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, and the second material includes natural graphite; the volume average particle size D of the negative electrode active material v 50 is 15 μm ~ 19 μm; The smoothness L of the surface of the negative electrode membrane facing away from the negative electrode current collector satisfies: 45 ≤ L ≤ 55.

23. The secondary battery according to claim 22, wherein 48 ≤ L ≤ 52。 24. The secondary battery according to claim 22, wherein The mass proportion of the natural graphite in the negative electrode active material is 10% to 50%.

25. The secondary battery according to claim 22, wherein The mass proportion of the natural graphite in the negative electrode active material is 35% to 50%.

26. The secondary battery according to claim 22 or 23, wherein The powder compaction density of the negative electrode active material under a pressure of 30000N is 1.8 g / cm 3 ~ 1.9 g / cm 3 .

27. The secondary battery according to claim 26, wherein The powder compaction density of the negative electrode active material under a pressure of 30,000 N is 1.82 g / cm 3 ~ 1.88 g / cm 3 .

28. The secondary battery according to claim 22 or 23, wherein The natural graphite powder compaction density under a pressure of 30000N is 1.85 g / cm 3 ~ 1.95 g / cm 3 ; and / or the artificial graphite powder compaction density under a pressure of 30000N is 1.75 g / cm 3 ~ 1.85 g / cm 3 .

29. The secondary battery according to claim 28, wherein The natural graphite powder compaction density under a pressure of 30000N is 1.90 g / cm 3 ~ 1.95 g / cm 3 .

30. The secondary battery according to claim 28, wherein The compacted density of the artificial graphite powder under a pressure of 30,000 N is 1.77 g / cm 3 ~ 1.80 g / cm 3 .

31. The secondary battery according to claim 22 or 23, wherein The graphitization degree of the negative electrode active material is 92% to 95%.

32. The secondary battery according to claim 31, wherein The graphitization degree of the negative electrode active material is 93% to 94%.

33. The secondary battery according to claim 22 or 23, wherein The degree of graphitization of the natural graphite is 95% to 98.5%; and / or the degree of graphitization of the artificial graphite is 89% to 95%.

34. The secondary battery according to claim 33, wherein The degree of graphitization of the natural graphite is 97.5% to 98.5%.

35. The secondary battery according to claim 33, wherein The degree of graphitization of the artificial graphite is 90% to 93%.

36. The secondary battery according to claim 22 or 23, wherein The volume average particle size D of the negative electrode active material v 50 is 16 μm ~ 18 μm.

37. The secondary battery according to claim 22 or 23, wherein The natural graphite D v 50 is 15 μm ~ 20 μm; and / or D of the artificial graphite v 50 is 14μm ~ 19 μm.

38. The secondary battery according to claim 37, wherein The natural graphite D v 50 is 15 μm ~ 19 μm.

39. The secondary battery according to claim 37, wherein The natural graphite D v 50 is 16 μm ~ 18 μm.

40. The secondary battery according to claim 37, wherein The D of the artificial graphite v 50 is 14 μm ~ 18 μm.

41. The secondary battery according to claim 37, wherein The D of the artificial graphite v 50 is 15 μm ~ 17 μm.

42. The secondary battery according to claim 22 or 23, wherein The compaction density of the negative electrode membrane is 1.5 g / cm 3 ~ 1.7 g / cm 3 ; and / or, the surface density of the negative electrode membrane is 7 mg / cm 2 ~ 10 mg / cm 2 .

43. The secondary battery according to claim 42, wherein The compaction density of the negative electrode membrane is 1.55 g / cm 3 ~1.65 g / cm 3 .

44. The secondary battery according to claim 42, wherein The surface density of the negative electrode membrane is 7 mg / cm 2 ~ 8mg / cm 2 .

45. The secondary battery according to claim 1 or 22, wherein The modified compound includes a doping-modified compound and / or a surface-coating-modified compound.

46. ​​A device comprising the secondary battery according to any one of claims 1 to 45.

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