Conductive agent, negative electrode sheet, secondary battery, and electric device

Abstract

The invention relates to a conductive agent, a negative pole piece, a secondary battery and an electric device, the conductive agent comprises crystalline flake graphite, the volume average particle size Dv50 of the crystalline flake graphite is 3-12 [mu] m, and the specific surface area of the crystalline flake graphite is 10-12 m < 2 > / g. Through the design, the cycle performance of the secondary battery can be improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a conductive agent, a negative electrode sheet, a secondary battery and a power utilization device. BACKGROUND

[0002] The statements herein merely provide background information related to the present application and do not necessarily constitute the prior art.

[0003] In recent years, with the increasingly wide application range of lithium ion batteries, lithium ion batteries are widely used in energy storage power supply systems such as hydraulic, thermal, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, etc. Due to the great development of lithium ion batteries, higher requirements are put forward for their cycle performance, so how to further improve the cycle performance of secondary batteries is a technical problem to be solved at present. SUMMARY

[0004] The present application provides a conductive agent, a negative electrode sheet, a secondary battery and a power utilization device, aiming to improve the cycle performance of secondary batteries.

[0005] In a first aspect of the present application, a conductive agent is provided, which comprises flaky graphite, the volume average particle size Dv50 of the flaky graphite is 3 μm to 12 μm, the specific surface area of the flaky graphite is 10 m 2 / g to 12 m 2 / g.

[0006] Controlling the volume average particle size Dv50 and the specific surface area of the flaky graphite in the conductive agent within the above range makes the surface structure of the flaky graphite more stable, the side reactions occurring in the charging and discharging process are less, and the flaky graphite is more easily dispersed in the negative electrode slurry, so as to be able to build an excellent conductive network between the negative electrode active materials, enhance the contact between the negative electrode active materials, and promote the transmission of active ions. At the same time, the above flaky graphite has good lubricity, the stress distribution of the negative electrode sheet containing the flaky graphite is more uniform in the cold pressing process, so that the contact between the flaky graphite and the negative electrode active material is better, thereby the cycle performance of the secondary battery can be improved.

[0007] Since the negative active material expands or shrinks during the charging and discharging process, if the volume average particle size Dv50 of the flake graphite is too small, the contact with the negative active material can be reduced, the gap is increased, the conductive stability of the negative active material is affected, and the cycle performance of the secondary battery is deteriorated; if the volume average particle size Dv50 of the flake graphite is too large, the gap between part of the negative active materials cannot accommodate the flake graphite, the contact between the negative active materials is insufficient, and the cycle performance of the secondary battery is limited. When the specific surface area of the flake graphite is too small, the wettability of the electrolyte to the negative electrode sheet is poor, which affects the cycle performance of the secondary battery; and when the specific surface area is too large, the surface structure is not stable, and the side reactions occurring during the charging and discharging process increase, which deteriorates the cycle performance of the secondary battery.

[0008] In some embodiments, the volume average particle size Dv50 of the flake graphite is 3.6 μm to 10 μm. In this way, the cycle performance of the secondary battery can be further improved.

[0009] In some embodiments, the number particle size distribution Dn10 of the flake graphite is 0.6 μm to 0.9 μm. By controlling the number particle size distribution Dn10 of the flake graphite to be in the above range, the expansion of the negative electrode sheet can be reduced, the stability of the conductive network can be improved, and the cycle performance of the secondary battery can be further improved.

[0010] In some embodiments, the volume particle size distribution Dv99 of the flake graphite is 16 μm to 21 μm. In this way, the expansion of the negative electrode sheet can be further reduced and the cycle performance of the secondary battery can be improved.

[0011] In some embodiments, the purity of the flake graphite is 99.9% or higher.

[0012] In some embodiments, the conductive agent further includes a carbon black type conductive agent, and the mass ratio of the flake graphite to the carbon black type conductive agent in the conductive agent is (0.5-2):1. The combination of the flake graphite and the carbon black type conductive agent can improve the expansion of the negative electrode sheet and improve the cycle performance of the secondary battery.

[0013] In some embodiments, the conductive agent further includes a carbon black type conductive agent, and the mass ratio of the flake graphite to the carbon black type conductive agent in the conductive agent is (1-1.5):1. The combination of the flake graphite and the carbon black type conductive agent can improve the expansion of the negative electrode sheet and improve the cycle performance of the secondary battery.

[0014] In some embodiments, the carbon black type conductive agent includes Super P. The combination of the above flake graphite and Super P can further improve the cycle performance of the secondary battery.

[0015] In a second aspect of the present application, a negative electrode tab is provided, which comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the conductive agent according to the first aspect of the present application.

[0016] The negative electrode tab of the present application comprises the conductive agent provided by the present application, and thus has at least the same advantages as the conductive agent.

[0017] In some embodiments, the mass percentage of the conductive agent in the negative electrode active material layer is 0.1% to 2.5%. In this way, the cycle performance of the secondary battery can be further improved.

[0018] In some embodiments, the mass percentage of the conductive agent in the negative electrode active material layer is 0.4% to 1.5%. In this way, the cycle performance of the secondary battery can be further improved.

[0019] In a third aspect of the present application, a secondary battery is provided, which comprises the negative electrode tab according to the second aspect of the present application.

[0020] The secondary battery of the present application comprises the negative electrode tab provided by the present application, and thus has at least the same advantages as the negative electrode tab.

[0021] In some embodiments, the secondary battery further comprises a positive electrode tab, wherein the positive electrode active material comprises one or more of a lithium transition metal oxide, an olivine structure lithium-containing phosphate and a modified compound thereof.

[0022] In a fourth aspect of the present application, an electric device is provided, which comprises the secondary battery according to the third aspect of the present application.

[0023] The electric device of the present application comprises the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery.

[0024] The details of one or more embodiments of the present application are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present application will be apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to better describe and illustrate the embodiments or examples provided by the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed application, the presently described embodiments or examples, and any one of the best modes of these applications presently understood. Moreover, the same reference numbers are used to represent the same components throughout the drawings. In the drawings:

[0026] Figure 1A schematic view of a battery cell according to an embodiment of the present application.

[0027] Figure 2 A perspective view of a battery cell according to an embodiment of the present application. Figure 1 An exploded view of a battery cell according to an embodiment of the present application.

[0028] Figure 3 A schematic view of a battery module according to an embodiment of the present application.

[0029] Figure 4 A schematic view of a battery pack according to an embodiment of the present application.

[0030] Figure 5 A perspective view of a battery pack according to an embodiment of the present application. Figure 4 An exploded view of a battery pack according to an embodiment of the present application.

[0031] Figure 6 A schematic view of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0032] Figure 7 A scanning electron microscope image of flake graphite prepared in Example 1 of the present application.

[0033] Figure 8 A scanning electron microscope image of flake graphite prepared in Example 1 of the present application at another magnification.

[0034] Figure 9 A partial enlarged view of Figure 8

[0035] BRIEF DESCRIPTION OF DRAWINGS

[0036] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 case; 52 electrode assembly; 53 cover plate; 6 electric device. DETAILED DESCRIPTION

[0037] Some embodiments of the conductive agent, the negative electrode sheet, the secondary battery, and the electric device according to the present application are described below in detail with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed explanations are omitted. For example, there are cases where detailed explanations of matters that are already well known, repeated explanations of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0038] ​The "range" disclosed in this application can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be combined arbitrarily, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0" and "5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when describing a parameter as an integer ≥ 2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 for that parameter. For instance, when describing a parameter as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0039] In this application, the terms "multiple" or "various" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0040] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0041] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0042] It is understood by those skilled in the art that the order of writing each step in the method of each embodiment or example does not mean a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. If not specifically stated, all steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can also comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0043] In the present application, the open technical features or technical solutions described by the words "containing", "including", "comprising" and the like do not exclude additional members other than the listed members, and can be regarded as providing both a closed feature or solution composed of the listed members and an open feature or solution including additional members in addition to the listed members. For example, A includes a1, a2 and a3, and unless otherwise specified, it can also include other members or can not include additional members, and can be regarded as providing both the feature or solution that "A is composed of a1, a2 and a3" and the feature or solution that "A includes a1, a2 and a3, and also includes other members". In the present application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0044] In the present application, "optionally", "optional" and "optional" mean that it can or can not be present, that is, it can be selected from either of the two parallel solutions "yes" or "no". If there are multiple "options" in a technical solution, unless otherwise specified, and there is no contradiction or mutual restriction, each "option" is independent.

[0045] An embodiment of the present application provides a conductive agent, which comprises flaky graphite, the volume average particle size Dv50 of the flaky graphite is 3 μm-12 μm, the specific surface area of the flaky graphite is 10 m 2 / g-12 m 2 / g.

[0046] The volume average particle size Dv50 and the specific surface area of the flake graphite in the conductive agent are controlled within the above range, so that the flake graphite has higher electrical conductivity, more stable surface structure, fewer side reactions occurring during the charging and discharging process, is easier to disperse in the negative electrode slurry, and thus can build an excellent conductive network between the negative electrode active materials, enhance the contact between the negative electrode active materials, and promote the transmission of active ions, while the flake graphite has better lubricity, the stress distribution of the negative electrode sheet containing the flake graphite is more uniform during cold pressing, so that the contact between the flake graphite and the negative electrode active material is better, and thus the cycle performance of the secondary battery can be improved.

[0047] Due to the expansion or shrinkage of the negative electrode active material during the charging and discharging process, if the volume average particle size Dv50 of the flake graphite is too small, the contact between the flake graphite and the negative electrode active material may be reduced, the gap is increased, the conductive stability of the negative electrode active material is affected, and the cycle performance of the secondary battery is deteriorated; if the volume average particle size Dv50 of the flake graphite is too large, the gap between part of the negative electrode active materials cannot accommodate the flake graphite, the contact between the negative electrode active materials is not enough, and the cycle performance of the secondary battery is limited. When the specific surface area of the flake graphite is too small, the wettability of the electrolyte to the negative electrode sheet is poor, and the cycle performance of the secondary battery is affected; and when the specific surface area of the flake graphite is too large, the surface structure is not stable enough, and the number of side reactions occurring during the charging and discharging process is increased, which deteriorates the cycle performance of the secondary battery.

[0048] It can be understood that the volume average particle size Dv50 of the flake graphite includes but is not limited to 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, and the specific surface area of the flake graphite includes but is not limited to 10 m 2 / g, 10.5 m 2 / g, 11 m 2 / g, 11.5 m 2 / g, 12 m 2 / g.

[0049] In some embodiments, the volume average particle size Dv50 of the flake graphite is 3.6 μm to 10 μm. In this way, the cycle performance of the secondary battery can be further improved.

[0050] In some embodiments, the flake graphite has a number particle size distribution Dn10 of 0.6 μm to 0.9 μm. Controlling the number particle size distribution Dn10 of the flake graphite to be within the above range allows the flake graphite to fill in the small gaps of the negative active material, and due to the layered structure of the flake graphite, the flake graphite can absorb the expansion force generated by the negative active material during the charging and discharging process to a greater extent, thereby reducing the expansion of the negative electrode sheet and improving the stability of the conductive network and further improving the cycle performance of the secondary battery. It is understood that the number particle size distribution Dn10 of the flake graphite includes but is not limited to 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm.

[0051] In some embodiments, the flake graphite has a volume particle size distribution Dv99 of 16 μm to 21 μm. Such design allows the volume particle size distribution of the flake graphite to be more concentrated, which can increase the filling amount of the flake graphite in the gaps of the negative active material, on the one hand, can further absorb the expansion force generated by the negative active material during the charging and discharging process, on the other hand, can construct a more abundant conductive network in the negative active material, thereby further reducing the expansion of the negative electrode sheet and improving the cycle performance of the secondary battery.

[0052] In the present application, Dv50 represents the particle size corresponding to the cumulative volume distribution percentage of 50%; Dv99 represents the particle size corresponding to the cumulative volume distribution percentage of 99%; Dn10 represents the particle size corresponding to the cumulative number distribution percentage of 10%. The Dv50, Dv99, Dn10 of the flake graphite can be tested by methods known in the art.

[0053] As an example, the Dv50, Dv99, Dn10 of the flake graphite can be conveniently determined by referring to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method, using a laser diffraction particle size distribution measuring instrument, for example, a Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK.

[0054] In some embodiments, the purity of the flake graphite is 99.9% or higher. Exemplarily, the purity of the flake graphite includes but is not limited to 99.9%, 99.91%, 99.92%, 99.93%, 99.94%, 99.95%, 99.96%, 99.97%, 99.98%, 99.99%.

[0055] In the present application, the purity of the flake graphite has the meaning known in the art and can be tested by methods known in the art, for example, a carbon-sulfur analyzer (such as Deike HCS-140) can be used for testing.

[0056] In some embodiments, the conductive agent further comprises a carbon black type conductive agent, and the mass ratio of the flaky graphite to the carbon black type conductive agent in the conductive agent is (0.5-2):1. The combination of the flaky graphite and the carbon black type conductive agent can improve the expansion of the negative electrode sheet and improve the cycle performance of the secondary battery. It can be understood that the mass ratio of the flaky graphite to the carbon black type conductive agent in the conductive agent includes but is not limited to 0.5:1, 1:1, 1.5:1, and 2:1. Further, the mass ratio of the flaky graphite to the carbon black type conductive agent in the conductive agent is (1-1.5):1.

[0057] In some embodiments, the carbon black type conductive agent comprises Super P. The combination of the flaky graphite and Super P described above can further improve the cycle performance of the secondary battery. Alternatively, the carbon black type conductive agent further comprises Ketjen black.

[0058] In some embodiments, the conductive agent described above can be used as a negative electrode conductive agent.

[0059] Another embodiment of the present application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, and the negative electrode active material layer comprises the conductive agent described above. The negative electrode sheet of the present application comprises the conductive agent provided by the present application, and thus has at least the same advantages as the conductive agent.

[0060] As a non-limiting example, the negative electrode current collector has two opposite surfaces in the thickness direction of the negative electrode current collector, and the negative electrode active material layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0061] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector can comprise a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base layer. Non-limiting examples of the metal material in the negative electrode current collector can include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, and the like. Non-limiting examples of the polymer material base layer in the negative electrode current collector can include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), and the like.

[0062] In some embodiments, the mass fraction of the conductive agent in the negative electrode active material layer is 0.1%-2.5%. In this way, the cycle performance of the secondary battery can be further improved. Further, the mass fraction of the conductive agent in the negative electrode active material layer is 0.4%-1.5%.

[0063] In some embodiments, the negative active material layer further comprises a negative active material. The negative active material can be any negative active material known in the art for use in batteries. By way of non-limiting example, the negative active material can include one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and lithium titanate, etc. The silicon-based material can include one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can include one or more of elemental tin, tin oxide compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as negative active materials for batteries can also be used. The negative active material can be used alone or in combination with two or more.

[0064] In some embodiments, the negative active material layer further optionally comprises a binder. The binder can include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0065] In some embodiments, the negative active material layer further optionally comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0066] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; coating the negative electrode slurry on at least one side surface of the negative current collector, and after processes such as drying, cold pressing, and the like, the negative electrode sheet can be obtained. The surface of the negative current collector coated with the negative electrode slurry can be a single surface of the negative current collector, or both surfaces of the negative current collector. The solid content of the negative electrode slurry can be 40wt%-60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s-10000mPa·s. When coating the negative electrode slurry, the coating unit area density (excluding the solvent) can be 75g / m 2 -220g / m 2 . The compaction density of the negative electrode sheet can be 1.0g / cm 3 -1.8g / cm 3 .

[0067] Further, another embodiment of the present application provides a secondary battery comprising the above-mentioned negative electrode sheet of the present application. The secondary battery of the present application comprises the negative electrode sheet provided by the present application, and thus has at least the same advantages as the negative electrode sheet.

[0068] In some embodiments, the secondary battery further includes a positive electrode tab including a positive electrode active material, the positive electrode active material including one or more of a lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and a modified compound of each thereof.

[0069] In some embodiments, the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. Optionally, the positive electrode active material has a mass percentage of 90% to 98% in the positive electrode active material layer.

[0070] In some embodiments, examples of the lithium transition metal oxide can include, but are not limited to, one or more of lithium cobalt oxide (e.g., LiCoO2), 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, etc. Non-limiting examples of the lithium cobalt oxide can include LiCoO2; non-limiting examples of the lithium nickel oxide can include LiNiO2; non-limiting examples of the lithium manganese oxide can include LiMnO2, LiMn2O4, etc.; non-limiting examples of the lithium nickel cobalt manganese oxide can include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be referred to simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be referred to simply as NCM 811 ), etc. Non-limiting examples of the lithium nickel cobalt aluminum oxide can include LiNi 0.8 Co 0.15 Al 0.05 O2.

[0071] In some embodiments, non-limiting examples of the olivine-structured lithium-containing phosphate can include, but are not limited to, one or more of lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon, etc.

[0072] It can be understood that the battery will be accompanied by lithium (Li) deintercalation and consumption during charging and discharging, and the content of Li in the positive plate is different when the battery is discharged to different states. In the enumeration of the positive active material in this application, the content of Li is the initial state of the material unless otherwise stated. When the positive active material is applied to the positive plate in the battery system, the content of Li in the positive active material contained in the plate will usually change after charging and discharging cycles. Among them, the content of Li can be quantified by molar content, but is not limited thereto. As for "the content of Li is the initial state of the material", the initial state of the material refers to the state before being put into the positive slurry. It can be understood that the new material obtained by properly modifying the listed positive active material is also within the scope of the positive active material, and the foregoing proper modification refers to acceptable modification of the positive active material, and non-limiting examples include coating modification and / or doping modification.

[0073] In the enumeration of the positive active material in this application, the content of oxygen (O) is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual content of O will fluctuate. Among them, the content of O can be quantified by molar content, but is not limited thereto.

[0074] In some embodiments, the positive plate further optionally comprises a binder. As a non-limiting example, the binder can include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic ester resin.

[0075] In some embodiments, the positive plate further optionally comprises a conductive agent. As a non-limiting example, the conductive agent can include one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0076] In some embodiments, the positive plate comprises a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. Optionally, the above-mentioned positive active material, conductive agent and binder are all located in the positive active material layer.

[0077] As a non-limiting example, the positive current collector has two opposite surfaces in the thickness direction of itself, and the positive active material layer is disposed on any one or both of the two opposite surfaces of the positive current collector.

[0078] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be obtained by forming a metal material on a polymer material base material. Non-limiting examples of the metal material in the positive current collector can include one or more of aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, a silver alloy, and the like. Non-limiting examples of the polymer material base material in the positive current collector can include one or more of a polypropylene (PP), a polyethylene terephthalate (PET), a polybutylene terephthalate (PBT), a polystyrene (PS), a polyethylene (PE), and the like.

[0079] In some embodiments, the positive electrode sheet can be prepared by dispersing the components for preparing the positive electrode sheet described above, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry on at least one side surface of the positive current collector, and then drying, cold-pressing, or the like to obtain the positive electrode sheet. The type of the solvent can be selected from, but is not limited to, any of the types described in the foregoing embodiments, such as N-methylpyrrolidone (NMP). The surface of the positive current collector to which the positive electrode slurry is coated can be a single surface of the positive current collector or both surfaces of the positive current collector. The surface of the positive current collector to which the positive electrode slurry is coated can be a single surface of the positive current collector or both surfaces of the positive current collector. The solid content of the positive electrode slurry can be 40 wt% to 80 wt%. The viscosity of the positive electrode slurry at room temperature can be adjusted to 5,000 mPa·s to 25,000 mPa·s. When the positive electrode slurry is coated, the coating unit area density, in terms of dry weight (excluding the solvent), can be 15 mg / cm 2 - 35 mg / cm 2 . The positive electrode sheet can have a compacted density of 3.0 g / cm 3 - 3.6 g / cm 3 , and optionally 3.3 g / cm 3 - 3.5 g / cm 3 .

[0080] In some embodiments, the secondary battery further includes an electrolyte.

[0081] The electrolyte has a function of conducting ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application and can be selected as needed.

[0082] In some embodiments, the electrolyte employs an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0083] In some embodiments, the electrolyte salt can include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorophosphate (LiPO2F2), lithium difluoro-oxalato-borate (LiDFOB), lithium bis-oxalato-borate (LiBOB), lithium difluoro-bis-oxalato-phosphate (LiDFOP), and lithium tetrafluoro-oxalato-phosphate (LiTFOP).

[0084] In some embodiments, the solvent can include one or more of ethylene carbonate (EC, ), propylene carbonate (PC, ), methyl ethyl 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, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, butyrolactone sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0085] In some embodiments, the electrolyte solution can further optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0086] In some embodiments, the additive in the electrolyte solution can include, but is not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethyl ethylene carbonate (TFPC), etc.

[0087] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0088] In some embodiments, the material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0089] In some embodiments, the thickness of the separator is 6 μm to 40 μm, and can be 12 μm to 20 μm.

[0090] In some embodiments, the positive electrode tab, the negative electrode tab, and the separator film can be made into an electrode assembly through a winding process or a stacking process.

[0091] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0092] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and further, non-limiting examples of the plastic can include one or more of polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.

[0093] The secondary battery includes at least one battery cell. The secondary battery can include one or more battery cells.

[0094] In the present application, unless otherwise specified, a “battery cell” refers to a basic unit capable of realizing mutual conversion between chemical energy and electrical energy, and further, generally includes at least a positive electrode tab, a negative electrode tab, and an electrolyte. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode tab and the negative electrode tab. The electrolyte plays a role in conducting active ions between the positive electrode tab and the negative electrode tab.

[0095] The shape of the battery cell is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure battery cell 5 as an example.

[0096] In some embodiments, referring to Figure 2 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator film can be made into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by a person skilled in the art according to actual needs.

[0097] The secondary battery can be a battery module 4 or a battery pack 1.

[0098] The battery module includes at least one battery cell. The number of battery cells contained in the battery module can be one or more, which can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0099] Figure 3 is a battery module 4 as an example. Refer to Figure 3 In the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, the arrangement can be in any other manner. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0100] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0101] In some embodiments, the above-mentioned battery module can be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the skilled in the art can select a suitable number according to the application and capacity of the battery pack.

[0102] Figure 4 and Figure 5 is a battery pack 1 as an example. Refer to Figure 4 and Figure 5 In the battery pack 1, a battery box and a plurality of battery modules 4 arranged in the battery box can be included. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0103] In addition, the application also provides a power utilization device including the above-mentioned secondary battery. The power utilization device of the application includes the secondary battery provided by the application, and thus at least has the same advantages as the secondary battery.

[0104] The secondary battery can be used as a power source of a power utilization device, or as an energy storage unit of a power utilization device. The power utilization device can include a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto. Among them, the mobile device can be a mobile phone, a notebook computer, etc.; the electric vehicle can be 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., but is not limited thereto.

[0105] As a power utilization device, the secondary battery can be selected according to the use requirement thereof.

[0106] Figure 6 is a power utilization device 6 as an example. The power utilization device 6 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the secondary battery for the power utilization device, a battery pack or a battery module can be used.

[0107] As another example, the device can be a mobile phone, a tablet, a notebook computer, etc. The device generally requires thinning, and a secondary battery can be used as a power source.

[0108] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are for the purpose of explanation of the present application and should not be understood as a limitation of the present application. In the embodiments, the technology or conditions not mentioned are performed according to the technology or conditions described in the literature in the art or according to the product manual. The reagents or instruments not mentioned are all conventional products that can be obtained commercially.

[0109] Example 1

[0110] (1) The flake graphite was prepared as follows:

[0111] The natural graphite was physically beneficiated, and then further purified by a hydrofluoric acid method. Specifically, the natural graphite and hydrofluoric acid were mixed, the impurities in the natural graphite and the hydrofluoric acid reacted for a period of time to produce soluble substances or volatile substances, and the impurities were removed after washing.

[0112] After purification, the purity of the obtained flake graphite reached more than 99.9%. The flake graphite after purification was crushed, and the magnetic separation and sieving were performed to obtain the flake graphite product. The volume average particle size Dv50 of the flake graphite was 3.6 μm, the specific surface area was 11.8 m2 / g, the number particle size distribution Dn10 was 0.6 μm, and the volume particle size distribution Dv99 was 16.9 μm. 2

[0113] (2) Preparation of the positive electrode sheet

[0114] The positive electrode active material lithium iron phosphate LiFePO4, the conductive agent acetylene black, and the binder PVDF were mixed at a mass ratio of 96:2:2, and the solvent NMP was added. The system was stirred under the action of a vacuum stirrer until it became uniform to obtain a positive electrode slurry. The positive electrode slurry was uniformly coated on the double-sided surfaces of the positive electrode current collector aluminum foil, and after air drying at room temperature, it was transferred to an oven for continuous drying, and then cold pressing and slitting were performed to obtain the positive electrode sheet.

[0115] (3) Preparation of the negative electrode sheet

[0116] The negative electrode active material artificial graphite, the conductive agent (flake graphite prepared in step (1)), the thickening agent CMC, and the binder SBR were mixed at a mass ratio of 96.4:0.8:1.3:1.5, and the solvent deionized water was added. The system was stirred under the action of a vacuum stirrer until it became uniform to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on the double-sided surfaces of the negative electrode current collector copper foil, and after air drying at room temperature, it was transferred to an oven for continuous drying, and then cold pressing and slitting were performed to obtain the negative electrode sheet containing the negative electrode active material layer. The mass ratio of the conductive agent in the negative electrode active material layer was 0.80%.​

[0117] (4) Preparation of electrolyte

[0118] Vinyl carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then a fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0119] (5) Preparation of separator

[0120] A polyethylene film was selected as the separator.

[0121] (6) Preparation of lithium ion secondary battery

[0122] The above positive electrode sheet, separator, and negative electrode sheet were stacked in order with the separator between the positive and negative electrode sheets to play a separating role, and then wound to obtain a bare cell; the bare cell was placed in an outer packaging shell, dried, and then injected with electrolyte, and then subjected to vacuum packaging, standing, formation, shaping, and other processes to obtain a lithium ion secondary battery.

[0123] Example 2-6

[0124] The preparation method of the secondary battery was basically the same as that of Example 1, except that the product parameters of the flake graphite prepared in step (1) were different, as shown in Table 1.

[0125] Example 7

[0126] The preparation method of the secondary battery was basically the same as that of Example 1, except that in step (3), flake graphite and Super P (SP) with a mass ratio of 2:1 were used as the conductive agent when preparing the negative electrode sheet.

[0127] Example 8-10

[0128] The preparation method of the secondary battery was basically the same as that of Example 7, except that in step (3), the mass ratio of flake graphite and Super P (SP) in the conductive agent was changed when preparing the negative electrode sheet, as shown in Table 1.

[0129] Example 11-14

[0130] The preparation method of the secondary battery was basically the same as that of Example 1, except that in step (3), the mass ratio of flake graphite and Super P (SP) in the conductive agent was changed when preparing the negative electrode sheet, as shown in Table 1.

[0131] Specifically, in the preparation of the negative electrode sheet in step (3) of Example 11, the negative electrode active material artificial graphite, the conductive agent, the thickening agent CMC, and the binder SBR are mixed in a mass ratio of 96.4:0.4:1.7:1.5, and a solvent deionized water is added, and the system is stirred to be uniform under the action of a vacuum stirrer, to obtain a negative electrode slurry.

[0132] In the preparation of the negative electrode sheet in step (3) of Example 12, the negative electrode active material artificial graphite, the conductive agent, the thickening agent CMC, and the binder SBR are mixed in a mass ratio of 96.4:1.5:0.8:1.3, and a solvent deionized water is added, and the system is stirred to be uniform under the action of a vacuum stirrer, to obtain a negative electrode slurry.

[0133] In the preparation of the negative electrode sheet in step (3) of Example 13, the negative electrode active material artificial graphite, the conductive agent, the thickening agent CMC, and the binder SBR are mixed in a mass ratio of 96.4:0.1:2.0:1.5, and a solvent deionized water is added, and the system is stirred to be uniform under the action of a vacuum stirrer, to obtain a negative electrode slurry.

[0134] In the preparation of the negative electrode sheet in step (3) of Example 14, the negative electrode active material artificial graphite, the conductive agent, the thickening agent CMC, and the binder SBR are mixed in a mass ratio of 95.0:2.5:1.0:1.5, and a solvent deionized water is added, and the system is stirred to be uniform under the action of a vacuum stirrer, to obtain a negative electrode slurry.

[0135] Comparative Examples 1-4

[0136] The preparation method of the secondary battery is basically the same as that of Example 1, except that the product parameters of the flake graphite prepared in step (1) are different, and the specific parameters are shown in Table 1.

[0137] Test Examples

[0138] (1) Dv50, Dv99 and Dn10 test

[0139] The flake graphite sample to be tested is uniformly dispersed in a dispersion medium (such as water, alcohol, or NMP (N-methyl pyrrolidone)), and a laser particle size analyzer (such as Mastersizer 2000E) is used for testing. In order to achieve uniform dispersion, the test can be performed after appropriate ultrasonic treatment. The Dv50, Dv99 and Dn10 of the flake graphite are obtained by referring to GB / T 19077-2016 Particle Size Distribution-Laser Diffraction Method.

[0140] (2) Specific surface area test

[0141] The specific surface area of the flake graphite is tested by nitrogen adsorption and desorption method. Specifically, the specific surface area analyzer (Tri star II) is used to measure the specific surface area of the flake graphite, and the GB / T 19587-2004 standard is referred to.

[0142] (3) Purity test

[0143] The purity of the flake graphite is tested by using a carbon-sulfur analyzer. Specifically, the flake graphite sample is burned in oxygen to convert the carbon element into CO2; the combustion product enters the absorption cell and is converted into a corresponding signal by the detector, and is sampled by the computer, and after linear correction, it is converted into a value proportional to the CO2 concentration. Then the values of the entire analysis process are added up. After the analysis is completed, the cumulative value is divided by the weight value, multiplied by the correction coefficient, and the blank is deducted to obtain the carbon content of the flake graphite, i.e. the purity of the flake graphite.

[0144] (4) Cycle performance test of the battery

[0145] At 25°C, the lithium ion batteries prepared from each example and the comparative example are charged to a voltage of 2V at a rate of 1C, discharged to a voltage of 3.65V at a rate of 1C, and full charge and full discharge cycle test is carried out until the capacity of the lithium ion battery is attenuated to 80% of the initial capacity, and the cycle number is recorded, i.e. the cycle performance of the battery.

[0146] (5) Swelling performance test of the negative electrode sheet

[0147] The thickness h1 of the negative electrode sheet before winding is measured and recorded. After the lithium ion battery is prepared, the lithium ion battery is fully charged at a rate of 1C to a voltage of 3.65V, and then the battery is disassembled to obtain a fully charged negative electrode sheet, and its thickness h2 is measured and recorded. The full charge swelling rate (%) of the lithium ion battery negative electrode sheet = (h2-h1) / h1*100%.

[0148] The product parameters and test results of each example and each comparative example are shown in Table 1. In Table 1, " / " represents the absence of the substance or the parameter.

[0149] Table 1

[0150]

[0151] Figure 7 The scanning electron microscope image of the flake graphite prepared in Example 1 of the present application is shown in FIG. 1, Figure 8 The scanning electron microscope image of the flake graphite prepared in Example 1 of the present application at another magnification is shown in FIG. 2, Figure 9 is a partial enlarged view of Figure 8 , and the scales in FIG. 1, Figures 7 to 9 are 10 μm, 2 μm, and 1 μm, respectively. From FIG. 1, Figures 7 to 9It can be seen that the flake graphite prepared in Example 1 has a typical sheet structure.

[0152] It can be seen from the data in Table 1 that the full charge expansion rate of the negative electrode sheet of Examples 1-14 is lower, and the cycle performance of the battery is better, compared with Comparative Examples 1-4, which indicates that the volume average particle size Dv50 of the flake graphite in the conductive agent is controlled to be 3 μm-12 μm, the specific surface area of the flake graphite is controlled to be 10 m 2 / g-12 m 2 / g, which can improve the cycle performance of the battery and reduce the expansion of the negative electrode sheet.

[0153] The above description of each embodiment tends to emphasize the differences between each embodiment, and the same or similar parts can be mutually referred to, and for the sake of brevity, will not be repeated herein.

[0154] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are only examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the configuration elements of the embodiments are also included in the scope of the present application.

Claims

1. A conductive agent, characterized in that, The conductive agent comprises flake graphite, wherein the volume average particle size Dv50 of the flake graphite is 3μm~12μm, and the specific surface area of ​​the flake graphite is 10m². 2 / g~12m 2 / g.

2. The conductive agent according to claim 1, characterized in that, The volume average particle size Dv50 of the flake graphite is 3.6 μm to 10 μm.

3. The conductive agent according to claim 1 or 2, characterized in that, The number and particle size distribution of the flake graphite, Dn10, is 0.6 μm to 0.9 μm.

4. The conductive agent according to any one of claims 1 to 3, characterized in that, The volumetric particle size distribution (Dv99) of the flake graphite is 16 μm to 21 μm.

5. The conductive agent according to any one of claims 1 to 4, characterized in that, The purity of the flake graphite is above 99.9%.

6. The conductive agent according to any one of claims 1 to 5, characterized in that, The conductive agent also includes a carbon black-based conductive agent, and the mass ratio of the flake graphite to the carbon black-based conductive agent in the conductive agent is (0.5~2):

1.

7. The conductive agent according to any one of claims 1 to 6, characterized in that, The conductive agent also includes a carbon black-based conductive agent, and the mass ratio of the flake graphite to the carbon black-based conductive agent in the conductive agent is (1~1.5):

1.

8. The conductive agent according to claim 6 or 7, characterized in that, The carbon black conductive agent includes Super P.

9. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer contains the conductive agent as described in any one of claims 1 to 8.

10. The negative electrode sheet according to claim 9, characterized in that, The conductive agent accounts for 0.1% to 2.5% of the mass of the negative electrode active material layer.

11. The negative electrode sheet according to claim 9 or 10, characterized in that, The conductive agent accounts for 0.4% to 1.5% of the mass of the negative electrode active material layer.

12. A secondary battery, characterized in that, Includes the negative electrode sheet according to any one of claims 9 to 11.

13. The secondary battery according to claim 12, characterized in that, It also includes a positive electrode sheet containing a positive electrode active material, which includes one or more of lithium transition metal oxides, lithium phosphates with an olivine structure, and their respective modified compounds.

14. An electrical appliance, characterized in that, The secondary battery includes any one of claims 12 to 13.

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