Negative pole piece, secondary battery, battery assembly and electric device

Abstract

The invention discloses a negative pole piece, a secondary battery, a battery assembly and an electric device, relates to the technical field of batteries, and aims to solve the problem that the internal resistance of the secondary battery is relatively large when the surface density of a battery pole piece of the secondary battery is increased. The negative pole piece comprises a current collector and a negative pole material arranged on at least one surface of the current collector. And the negative electrode material comprises a negative electrode active material and a conductive agent. Wherein the conductive agent comprises a carbon nano tube and a graphene sheet. The average tube diameter of the carbon nano tube ranges from 20 nm to 80 nm; the average sheet diameter of the graphene sheets ranges from 0.5 [mu] m to 5 [mu] m. According to the negative electrode material provided by the embodiment of the invention, a compact conductive network can be formed, the internal contact resistance in the negative electrode material can be reduced, the conductivity is improved, the internal resistance of the secondary battery is further reduced, and the charging performance of the secondary battery is improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a negative electrode plate, a secondary battery, a battery assembly and an electrical device. Background Art

[0002] Secondary batteries are widely used in electric vehicles, electric motorcycles, aerospace and other fields. As the application range of secondary batteries becomes wider and wider, higher requirements are placed on the performance of secondary batteries. However, as the surface density of the battery pole of the secondary battery increases, the internal resistance of the secondary battery increases significantly, resulting in a decrease in the conductivity of the secondary battery, thereby reducing the charging speed of the secondary battery and affecting the charging performance of the secondary battery. Summary of the invention

[0003] The purpose of the present application is to provide a negative electrode plate, a secondary battery, a battery assembly and an electrical device. When the surface density of the battery plate increases, the internal resistance of the secondary battery is large, which affects the charging speed of the secondary battery.

[0004] In order to achieve the above objectives, this application adopts the following technical solutions:

[0005] In the first aspect, an embodiment of the present application provides a negative electrode plate. The negative electrode plate includes: a current collector and a negative electrode material disposed on at least one surface of the current collector. The negative electrode material includes: a negative electrode active material and a conductive agent. Among them, the conductive agent includes: carbon nanotubes and graphene sheets. The average tube diameter of the carbon nanotubes ranges from 20nm to 80nm; the average sheet diameter of the graphene sheets ranges from 0.5μm to 5μm.

[0006] The negative electrode material provided in the embodiment of the present application is used for the negative electrode sheet of the secondary battery. The average diameter of the carbon nanotubes in the conductive agent is in the range of 20nm to 80nm, which can make the carbon nanotubes form a continuous conductive network and improve the diffusion rate of active ions in the secondary battery in the carbon nanotubes. The diameter of the graphene sheet in the conductive agent is in the range of 0.5μm to 5μm, which can improve the dispersion of the graphene sheet in the negative electrode material and the close connection performance with other components of the negative electrode material, and is used to improve the compaction performance and flexibility of the negative electrode sheet. Furthermore, by setting the average tube diameter of the carbon nanotubes to be in the range of 20nm to 80nm and the average sheet diameter of the graphene sheets to be in the range of 0.5μm to 5μm, the carbon nanotubes with a linear structure and the graphene sheets with a lamellar structure can complement each other to form a entangled conductive network, so that the negative electrode material can form a more uniform and dense microstructure, increase the rate of transmission and diffusion of active ions between and inside the carbon nanotubes, and improve the compaction performance and flexibility of the negative electrode sheet, reduce the internal resistance of the secondary battery, and thus improve the charge and discharge performance of the battery.

[0007] In some embodiments, the average tube length of the carbon nanotubes is in the range of 0.5 μm to 30 μm.

[0008] In some embodiments, the ratio of the mass of the carbon nanotubes to the mass of the graphene sheets ranges from 0.1 to 10.

[0009] In some embodiments, the ratio of the mass of the carbon nanotubes to the mass of the graphene sheets ranges from 0.1 to 3.

[0010] In some embodiments, the ratio of the mass of carbon nanotubes to the negative electrode active material is in the range of 0.0008 to 0.03.

[0011] In some embodiments, the ratio of the mass of carbon nanotubes to the negative electrode active material is in the range of 0.0008 to 0.02.

[0012] In some embodiments, the conductive agent further includes at least one of conductive carbon black and carbon fiber material.

[0013] In some embodiments, the negative electrode material further includes a thickener and a binder, wherein the ratio of the sum of the mass of the thickener and the binder to the sum of the mass of the negative electrode active material, the mass of the conductive agent, the mass of the thickener and the mass of the binder is in the range of 0.025 to 0.035.

[0014] In a second aspect, an embodiment of the present application further provides a secondary battery. The secondary battery comprises: a positive electrode sheet and a negative electrode sheet as in any of the above embodiments. The positive electrode sheet and the negative electrode sheet are spaced apart.

[0015] It can be understood that the beneficial effects that can be achieved by the secondary battery provided in the above embodiments of the present application can refer to the beneficial effects of the negative electrode plate mentioned above, which will not be repeated here.

[0016] In a third aspect, an embodiment of the present application further provides a battery assembly. The battery assembly comprises: a secondary battery as described in the above embodiment and an assembly housing. The secondary battery is disposed in the assembly housing.

[0017] It can be understood that the beneficial effects that can be achieved by the battery assembly provided in the above embodiments of the present application can refer to the beneficial effects of the negative electrode plate mentioned above, and will not be repeated here.

[0018] In a fourth aspect, an embodiment of the present application further provides an electric device, which includes: at least one of the secondary battery in the above embodiment and the battery assembly in the above embodiment.

[0019] It can be understood that the beneficial effects that can be achieved by the electrical device provided in the above embodiments of the present application can refer to the beneficial effects of the negative electrode plate mentioned above, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. 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 these drawings without paying any creative work.

[0021] Figure 1 Schematic diagram of a negative electrode sheet in some embodiments;

[0022] Figure 2 Schematic diagram of negative electrode materials in some embodiments. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0024] In the description of this application, it should be understood that the terms "upper", "lower", "left", "right", "front", "back", "inside", "outside", etc. indicate directions or positional relationships based on the directions or relative positional relationships shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on this application. Unless otherwise specified, the above-mentioned directional description can be flexibly set in the process of actual application under the condition that the relative positional relationship shown in the accompanying drawings is met.

[0025] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0026] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection. It can be directly connected, or indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0027] In the present application, the terms "comprises", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, article or device. In the absence of further restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, article or device including the element.

[0028] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary" or "for example" is intended to present related concepts in a specific way.

[0029] In the description of this specification, specific features, structures, materials or characteristics may be combined in an appropriate manner in any one or more embodiments or examples.

[0030] In order to make the technical problems, technical solutions and beneficial effects solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0031] In some embodiments, the secondary battery includes: a positive electrode sheet, a negative electrode sheet, a shell, an electrolyte and a separator. The positive electrode sheet, the negative electrode sheet, the electrolyte and the separator are placed in the shell. During the charging and discharging process of the secondary battery, the active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, and at the same time to allow active ions to pass through.

[0032] The speed at which active ions are inserted and removed back and forth between the positive electrode and the negative electrode has an important influence on the charging performance of the secondary battery. If the diffusion rate of active ions is faster, the charging speed of the secondary battery is faster. Conversely, if the diffusion rate of active ions is slower, the charging speed of the secondary battery is slower.

[0033] For example, the negative electrode sheet is loaded with negative electrode materials, which have an important influence on the charging speed of the secondary battery. The influence of the negative electrode materials on the charging speed of the secondary battery may include the following three aspects.

[0034] First, the negative electrode material can affect the diffusion rate of active ions. The negative electrode material is the main storage place for active ions. The faster the active ions are embedded in and out of the negative electrode material, the faster the diffusion rate of the active ions is. This can reduce the transmission resistance of the active ions in the material, reduce the contact internal resistance of the negative electrode material, and then reduce the internal resistance of the secondary battery, and increase the charging speed of the secondary battery.

[0035] Secondly, the negative electrode material can affect the compaction performance of the negative electrode sheet. The compaction performance of the negative electrode sheet can affect the conductivity of the active ions. A higher compaction performance can reduce the internal resistance of the secondary battery, thereby reducing the internal resistance of the secondary battery and increasing the charging speed of the secondary battery.

[0036] Thirdly, the negative electrode material can affect the flexibility of the negative electrode sheet. During the charge and discharge process of the secondary battery, the negative electrode sheet may break or pulverize due to volume change. The flexibility of the negative electrode sheet can reduce the stress on the negative electrode sheet due to volume change, maintain the structural stability of the negative electrode sheet, and reduce the problem of breakage or pulverization of the negative electrode sheet, thereby maintaining the negative electrode sheet with stable interface contact, which helps to reduce the internal resistance of the secondary battery.

[0037] Based on this, some embodiments of the present application provide a negative electrode plate 10. Figure 1 As shown, the negative electrode sheet 10 includes: a current collector 11 and a negative electrode material 12 disposed on at least one surface of the current collector 11. Figure 2 As shown, the negative electrode material 12 includes: a negative electrode active material 1 and a conductive agent 2. The conductive agent 2 includes: carbon nanotubes 21 and graphene sheets 22. The average tube diameter of the carbon nanotubes 21 ranges from 20nm to 80nm; the average sheet diameter of the graphene sheets 22 ranges from 0.5μm to 5μm.

[0038] The current collector 11 mainly collects and conducts current. During the charge and discharge process of the secondary battery, the current collector 11 is responsible for effectively conducting the current generated on the negative electrode material and transferring it to the external circuit.

[0039] It should be noted that the active material is an important component of the negative electrode plate 10. During the charge and discharge process of the secondary battery, the negative electrode active material 1 plays an important role in transferring electrons. The negative electrode active material 1 can also reversibly de- / embed active ions and store energy, and is a key material for secondary batteries.

[0040] For example, the negative electrode active material 1 includes a carbon material, such as natural graphite, artificial graphite, hard carbon, secondary silicon carbon, and the like.

[0041] The conductive agent 2 can reduce the contact resistance of the negative electrode plate 10, thereby accelerating the movement rate of electrons and improving the charge and discharge efficiency of the secondary battery. During the multiple charge and discharge processes of the secondary battery, the insertion and extraction of active ions will cause the volume expansion of the negative electrode material 12, reducing the conductivity of the negative electrode material 12, and the conductive agent 2 can help alleviate the volume expansion of the negative electrode material 12, thereby improving the cycle performance of the secondary battery.

[0042] For example, the average diameter of the carbon nanotubes 21 may be 20 nm, 35 nm, 50 nm, 65 nm, or 80 nm, etc., which is not limited here.

[0043] For example, the average sheet diameter of the graphene sheet 22 may be 0.5 μm, 1 μm, 1.5 μm, 2.5 μm, 3.5 μm or 5 μm, etc., which is not limited here.

[0044] Based on this solution, firstly, the carbon nanotube 21 has a high specific surface area, which can provide a large number of active ion active sites, which is conducive to the embedding and extraction of active ions; and the cylindrical configuration of the carbon nanotube 21 can embed active ions from both the outer wall and the inner wall, so it can effectively improve the diffusion capacity of active ions in the negative electrode material 12, enhance the conductivity of the negative electrode plate 10, and help reduce the internal resistance of the secondary battery. In addition, the carbon nanotube 21 has good flexibility, and the compressed carbon nanotube 21 forms a crest-shaped knot, which can be elastically relaxed after unloading and restore the original shape, and can resist the distortion caused by torsional force.

[0045] When the average diameter of the carbon nanotubes 21 is relatively thick, for example, the average diameter of the carbon nanotubes 21 is greater than 80nm, gaps will be formed in the negative electrode sheet, resulting in a lower compaction performance of the negative electrode sheet 10; moreover, the thicker diameter may also make the distribution of the carbon nanotubes 21 in the negative electrode sheet 10 uneven, further affecting the compaction performance of the negative electrode sheet 10. When the average diameter of the carbon nanotubes 21 is relatively thin, for example, when the average diameter of the carbon nanotubes 21 is less than 5nm, they can only be coated on the surface of the negative electrode material 12, and cannot form an effective three-dimensional conductive network, and cannot form sufficient contact points, which affects the transmission and diffusion of active ions between and inside the carbon nanotubes 21, and thus cannot achieve a good connection of the negative electrode conductive path.

[0046] Secondly, the high conductivity of the graphene sheet 22 enables electrons to be transmitted more quickly in the negative electrode sheet 10, and the graphene sheet 22 has a two-dimensional layered structure and a large specific surface area, and can form a relatively dense network structure with the carbon nanotubes 21, thereby increasing the compaction density of the negative electrode sheet 10, so that the internal resistance of the secondary battery can be reduced. At the same time, the graphene sheet 22 has excellent flexibility, and can maintain its structural integrity and stability even when subjected to external forces. During the charge and discharge process of the secondary battery, the negative electrode sheet 10 can resist its volume change and prevent the negative electrode sheet 10 from breaking or pulverizing; and the graphene sheet 22 can serve as a "bridge" in the negative electrode material 12, connecting the negative electrode materials 12 of different components, forming a more stable three-dimensional network structure, thereby further improving the flexibility of the negative electrode sheet 10.

[0047] However, when the average sheet diameter of the graphene sheet 22 is large, for example, the average sheet diameter of the graphene sheet 22 is greater than 5 μm, the graphene sheet 22 may form larger agglomerates in the negative electrode material 12, and the dispersion is reduced, thereby increasing the number of voids in the negative electrode material on the negative electrode sheet 10 and reducing the compaction performance of the negative electrode sheet 10. Moreover, the graphene sheet 22 with an average sheet diameter that is too large will increase the brittleness of the negative electrode sheet 10 and reduce the flexibility of the negative electrode sheet 10. During the charge and discharge process of the secondary battery, the negative electrode sheet 10 may be broken or pulverized due to volume changes.

[0048] When the average sheet diameter of the graphene sheet 22 is small, for example, less than 0.5 μm, it may affect the formation of a continuous and uniform contact network between the components of the negative electrode material 12, resulting in the graphene sheet 22 not being stacked tightly in the negative electrode material 12, thereby affecting the compaction performance of the negative electrode sheet 10. Moreover, the graphene sheet 22 with a small average sheet diameter may increase the contact resistance between the graphene sheets 22, resulting in an increase in the internal resistance of the negative electrode sheet 10.

[0049] In summary, on the one hand, by setting the average diameter of the carbon nanotubes 21 in the range of 20nm to 80nm, the carbon nanotubes 21 can be more easily contacted and overlapped with each other, forming a continuous conductive network, so that active ions can be more easily transported and diffused between and inside the carbon nanotubes 21, and the diffusion rate of active ions in the carbon nanotubes 21 is increased, thereby reducing the internal resistance of the secondary battery. Further, the carbon nanotubes 21 can also improve the compaction performance of the negative electrode sheet 10, further reduce the internal resistance of the secondary battery, and improve the stability of the secondary battery. By setting the average sheet diameter of the graphene sheet 22 in the range of 0.5μm to 5μm, the dispersibility of the graphene sheet 22 in the negative electrode material 12 can be improved, so as to improve the close connection performance of the graphene sheet 22 and other components in the negative electrode material 12, thereby improving the compaction performance and flexibility of the negative electrode sheet 10.

[0050] On the other hand, by setting the average tube diameter of the carbon nanotubes 21 to be in the range of 20nm to 80nm and the average sheet diameter of the graphene sheets 22 to be in the range of 0.5μm to 5μm, the carbon nanotubes 21 with a linear structure and the graphene sheets 22 with a lamellar structure can complement each other to form a entangled conductive network, so that the negative electrode material 12 can form a more uniform and dense microstructure, which can further improve the rate of transmission and diffusion of active ions between and inside the carbon nanotubes 21, as well as improve the compaction performance and flexibility of the negative electrode plate 10, further reduce the internal resistance of the secondary battery, and thus improve the charging performance of the secondary battery.

[0051] For example, the characteristics of the conductive agent 2 on the surface and cross-section of the negative electrode sheet 10 can be directly characterized by a focused ion beam scanning electron microscope (FIB-SEM).

[0052] Exemplarily, the carbon nanotubes 21 may be prepared by a moving bed / fixed bed growth process.

[0053] For example, the carbon nanotubes 21 may be prepared by a fixed bed growth process using a nickel-based catalyst.

[0054] For example, the average diameter of the carbon nanotubes 21 and the average diameter of the graphene sheets 22 can be tested by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the test standard can refer to the national standard GB / T 30544.13-2018. The specific method is: dilute with ethanol after separation, disperse in a microgrid copper mesh, randomly select 100 carbon nanotubes 21 for diameter statistics, perform statistics, and take the average value.

[0055] Exemplarily, the carbon nanotubes 21 and the graphene sheet 22 in the negative electrode material 12 on the negative electrode plate 10 are separated, and the average diameter of the carbon nanotubes 21 and the catalyst information are tested.

[0056] Exemplarily, the following steps can be used to separate the carbon nanotubes 21 and graphene sheets 22 in the negative electrode material 12 on the negative electrode plate 10. The negative electrode plate 10 is placed in a certain concentration of HCl solution, and the dissolution is accelerated by stirring or ultrasound to remove the current collector 11 (such as copper foil) and other soluble components in the plate, and then the solution and the insoluble components are separated by filtration or centrifugation. The insoluble components are dried and ground to be free of particles, and then mixed with an appropriate amount of solvent or dispersant, placed in an ultrasonic bath or ultrasonically treated using an ultrasonic probe to form a uniform suspension.

[0057] During the ultrasonic treatment, due to the different characteristics of the negative electrode active material 1 and the carbon nanotubes 21, slight aggregation and dispersion phenomena will occur. After the ultrasonic treatment, the sample is centrifuged, and the centrifugal rate is adjusted to separate the carbon nanotubes 21, the graphene sheets 22 and the negative electrode active material 1, and tested using a scanning electron microscope (SEM) and a transmission electron microscope (TEM) respectively. The specific method is as follows: after separation, it is diluted with ethanol and dispersed in a micro-grid copper mesh, 100 graphene sheets 22 are randomly selected for tube diameter statistics, and statistics are performed to take the average value.

[0058] In some embodiments, the average tube length of the carbon nanotubes 21 ranges from 0.5 μm to 30 μm.

[0059] For example, the average tube length of the carbon nanotubes 21 may be 0.5 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm or 30 μm, etc., which is not limited here.

[0060] When the average tube length of the carbon nanotubes 21 is long, for example, the average tube length of the carbon nanotubes 21 is greater than 30 μm, the carbon nanotubes 21 will have a large curvature, and when the carbon nanotubes 21 are subjected to external forces, the instability of their internal structure will increase. The long average tube length of the carbon nanotubes 21 may also cause discontinuity of the conductive network and increase the internal resistance of the negative electrode sheet 10. The long average tube length of the carbon nanotubes 21 causes pores to exist in the negative electrode material 12, resulting in a low compaction performance of the negative electrode sheet 10.

[0061] When the average tube length of the carbon nanotubes 21 is small, for example, the average tube length of the carbon nanotubes 21 is less than 0.5 μm, electrons may encounter obstacles more easily when transmitting in the negative electrode material 12, or may deviate from the original path more easily, thereby affecting the embedding and extraction speed of active ions, and ultimately affecting the charging performance of the secondary battery.

[0062] Therefore, by setting the average tube length of the carbon nanotubes 21 to a range of 0.5μm to 20μm, the transmission resistance of electrons inside the negative electrode material 12 can be reduced, so that the negative electrode material 12 can provide a better electron transmission path and form an effective conductive network, so that the active ions can diffuse more rapidly and evenly in the negative electrode material 12, thereby reducing the internal resistance of the negative electrode plate 10, and then reducing the internal resistance of the secondary battery, so as to improve the charging performance of the secondary battery.

[0063] In some embodiments, the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheet 22 is in the range of 0.1-10.

[0064] For example, the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheets 22 may be 0.1, 2, 4, 6, 8 or 10, etc., which is not limited here.

[0065] By setting the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheet 22 in the range of 0.1 to 10, the conductive network of the negative electrode plate 10 can be made denser, so that the electron transmission is more efficient, thereby improving the diffusion performance of active ions, reducing the contact internal resistance inside the negative electrode material 12, and improving the conductivity, thereby reducing the internal resistance of the secondary battery; it can also improve the compaction performance and flexibility of the negative electrode plate 10, forming a more stable and continuous conductive channel, and further reducing the internal resistance of the secondary battery.

[0066] In some embodiments, the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheet 22 is in the range of 0.1-3.

[0067] By setting the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheet 22 to be in the range of 0.1 to 3, the conductive network of the negative electrode plate 10 can be made denser, further reducing the internal resistance of the secondary battery.

[0068] In some embodiments, the ratio of the mass of the carbon nanotubes 21 to the mass of the negative electrode active material 1 is in the range of 0.0008 to 0.03.

[0069] For example, the ratio of the mass of the carbon nanotubes 21 to the mass of the negative electrode active material 1 may be 0.0008, 0.001, 0.0015, 0.002, 0.0025 or 0.03, etc., which is not limited here.

[0070] By setting the ratio of the mass of the carbon nanotubes 21 to the mass of the negative electrode active material 1 to a range of 0.0008 to 0.03, the carbon nanotubes 21 can more effectively fill the gaps between the particles of the negative electrode active material 1 to form a denser conductive channel, which is conducive to the rapid transmission of electrons in the negative electrode material 12, and can also ensure that the secondary battery maintains stable performance during the charge and discharge process, and further reduce the internal resistance of the secondary battery.

[0071] In some embodiments, the ratio of the mass of the carbon nanotubes 21 to the mass of the negative electrode active material 1 is in the range of 0.0008 to 0.002.

[0072] By setting the ratio of the mass of the carbon nanotubes 21 to the mass of the negative electrode active material 1 to be in the range of 0.0008 to 0.02, it can further facilitate the rapid transmission of electrons in the negative electrode material 12 and further reduce the internal resistance of the secondary battery.

[0073] In some embodiments, the conductive agent 2 further includes: at least one of conductive carbon black and carbon fiber material.

[0074] It can be understood that any one or more of the above conductive agents 2 can improve the charging performance of the secondary battery.

[0075] In some embodiments, the negative electrode material 12 further includes: a thickener and a binder; the ratio of the sum of the mass of the thickener and the mass of the binder to the sum of the mass of the negative electrode active material 1, the mass of the conductive agent 2, the mass of the thickener and the mass of the binder is in the range of 0.025 to 0.035.

[0076] For example, the ratio of the sum of the mass of the thickener and the mass of the binder to the sum of the mass of the negative electrode active material 1, the mass of the conductive agent 2, the mass of the thickener and the mass of the binder can be 0.025, 0.03 or 0.035, etc., which is not limited here.

[0077] The main function of the thickener is to increase the viscosity of the negative electrode slurry, improve the rheological properties of the negative electrode slurry, and make the negative electrode material 12 easier to shape, thereby improving the formability and adhesion of the negative electrode sheet 10.

[0078] The main function of the binder is to adhere the negative electrode material 12 to the current collector 11 and maintain the structural integrity and stability of the negative electrode sheet 10. The binder can also improve the wettability of the electrolyte and promote the transmission of active ions at the interface between the negative electrode and the electrolyte.

[0079] In a second aspect, some embodiments of the present application further provide a secondary battery, which comprises a positive electrode sheet and a negative electrode sheet 10 as in any of the above embodiments, wherein the positive electrode sheet and the negative electrode sheet 10 are spaced apart from each other.

[0080] Exemplarily, the positive electrode sheet includes: a positive electrode current collector 11 and a positive electrode material disposed on at least one surface of the positive electrode current collector 11 .

[0081] For example, the positive electrode current collector 11 may be a metal foil or a composite current collector 11. For example, aluminum foil may be used as the metal foil. The composite current collector 11 may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer.

[0082] Illustratively, the positive electrode material includes a positive electrode active material, a conductive agent 2 and a binder.

[0083] Exemplarily, the positive electrode active material may be a positive electrode active material for a battery known in the art. For example, the positive electrode active material may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. The present application takes lithium iron phosphate (LiFePO4) as an example of a positive electrode active material.

[0084] Exemplarily, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0085] For example, the conductive agent 2 may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes 21, graphene sheets 22, and carbon nanofibers.

[0086] In some embodiments, the negative electrode sheet 10 can be prepared in the following manner: the components used to prepare the negative electrode sheet 10, such as the negative electrode active material 1, the conductive agent 2, the thickener and the binder are dispersed in a solvent to form a negative electrode slurry; the positive electrode slurry is coated on the positive electrode collector 11, and after drying, cold pressing and other processes, the negative electrode sheet 10 can be obtained.

[0087] In some embodiments, the electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate 10. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0088] In some embodiments, the electrolyte includes an electrolyte salt and a solvent.

[0089] Exemplarily, the electrolyte salt may be lithium hexafluorophosphate.

[0090] Illustratively, the solvent may be a combination of dimethyl carbonate and ethyl methyl carbonate.

[0091] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0092] In some embodiments, the material of the isolation membrane can be selected from at least one 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, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0093] In some embodiments, the positive electrode sheet, the negative electrode sheet 10 and the separator can be made into an electrode assembly by a winding process or a lamination process.

[0094] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. Figure 1 This is a secondary battery with a square structure as an example.

[0095] In a third aspect, an embodiment of the present application further provides a battery assembly. The battery assembly comprises: a secondary battery as described in the above embodiment and an assembly housing. The secondary battery is disposed in the assembly housing.

[0096] In a fourth aspect, an embodiment of the present application further provides an electric device, which includes: at least one of the secondary battery in the above embodiment and the battery assembly in the above embodiment.

[0097] The secondary battery or battery assembly can be used as a power source for the electrical device or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, and energy storage systems.

[0098] As the electrical device, a secondary battery or a battery assembly can be selected according to its usage requirements.

[0099] As another example, the electric device may be a mobile phone, a tablet computer, a notebook computer, etc. The electric device is usually required to be light and thin, and a secondary battery may be used as a power source.

[0100] The present application is further described in detail below using specific experiments as examples.

[0101] Example 1

[0102] The method for preparing a secondary battery includes steps S11 to S14.

[0103] S11: adding a positive electrode active material, a conductive agent and a binder in a mass ratio of 97:1:2 into a solvent to form a mixture, mixing and stirring into a positive electrode slurry using a homogenizer, and evenly coating the mixture on an aluminum foil to form a positive electrode sheet.

[0104] Exemplarily, lithium iron phosphate is selected as the positive electrode active material.

[0105] Exemplarily, the conductive agent is conductive carbon black.

[0106] Exemplarily, the binder is polyvinylidene fluoride.

[0107] Exemplarily, the solvent is methyl pyrrolidone.

[0108] S12: negative electrode active material 1, conductive agent 2, thickener and binder in a mass ratio of 96:1:1:2 are mixed, stirred by a homogenizer to form negative electrode slurry and coated on a copper foil in a single layer to form a negative electrode sheet 10 .

[0109] Exemplarily, the negative electrode active material 1 is artificial graphite.

[0110] Exemplarily, the thickener is sodium carboxymethyl cellulose.

[0111] Exemplarily, the binder is styrene-butadiene rubber.

[0112] Exemplarily, the conductive agent 2 is selected from carbon nanotubes 21, graphene sheets 22 and carbon black.

[0113] The mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black is 1:1:10; the average tube diameter of the carbon nanotubes 21 is 20 nm, and the average tube length is 20 μm; and the average sheet diameter of the graphene sheets 22 is 0.5 μm.

[0114] S13: Ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate are mixed in a volume ratio of 1:1:1, and lithium hexafluorophosphate is added and mixed and stirred to form a lithium hexafluorophosphate electrolyte with a molar concentration of 1M.

[0115] S14: The positive electrode sheet and the isolation film formed in step S11 and the negative electrode sheet 10 formed in step S12 are stacked and assembled into a 1.7Ah secondary battery, and the electrolyte is the electrolyte prepared in step S13.

[0116] Exemplarily, the material of the isolation membrane is polypropylene.

[0117] Example 2

[0118] The preparation method of the secondary battery of Example 2 is the same as that of Example 1, with the only difference being that the mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black in the conductive agent 2 of S12 is 2:2:10; the average tube diameter of the carbon nanotubes 21 is 80 nm, and the average tube length is 0.5 μm; and the average sheet diameter of the graphene sheets 22 is 5 μm.

[0119] Example 3

[0120] The preparation method of the secondary battery of Example 3 is the same as that of Example 1, with the only difference being that the mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black in the conductive agent 2 of S12 is 2:2:6; the average tube diameter of the carbon nanotubes 21 is 40 nm, and the average tube length is 2 μm; and the average sheet diameter of the graphene sheets 22 is 1 μm.

[0121] Example 4

[0122] The preparation method of the secondary battery of Example 4 is the same as that of Example 1, except that the mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black in the conductive agent 2 of S12 is 1:10:1.

[0123] Example 5

[0124] The preparation method of the secondary battery of Example 5 is the same as that of Example 1, except that the mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black in S12 is 9:3:1.

[0125] Example 6

[0126] The preparation method of the secondary battery of Example 6 is the same as that of Example 1, except that the mass ratio of the carbon nanotubes 21, the graphene sheets 22 and the carbon black in S12 is 1:5:5.

[0127] Example 7

[0128] The preparation method of the secondary battery of Example 7 is the same as that of Example 1, except that the conductive agent 2 composed of carbon nanotubes 21, graphene sheets 22 and carbon black in S12 is replaced by a conductive agent 2 composed of carbon nanotubes 21 and graphene sheets 22 in equal amounts, wherein the mass ratio of the nanotubes 21 to the graphene sheets 22 is 1:1.

[0129] Comparative Example 1

[0130] The preparation method of the secondary battery of Comparative Example 1 is the same as that of Example 1, except that the conductive agent 2 in S12 is conductive carbon black.

[0131] Comparative Example 2

[0132] The preparation method of the secondary battery of Comparative Example 1 is the same as that of Example 1, except that the mass ratio of the negative electrode active material 1, the conductive agent 2, the thickener and the binder in S12 is 95:2:1:2, wherein the conductive agent 2 is conductive carbon black.

[0133] Comparative Example 3

[0134] The preparation method of the secondary battery of Comparative Example 3 is the same as that of Example 1, except that the average tube diameter of the carbon nanotubes 21 in S12 is 7 nm, and the average tube length is 20 μm; and the average sheet diameter of the graphene sheet 22 is 0.5 μm.

[0135] Comparative Example 4

[0136] The preparation method of the secondary battery of Comparative Example 4 is the same as that of Example 1, except that the average tube diameter of the carbon nanotubes 21 in S12 is 90 nm, and the average tube length is 20 μm; and the average sheet diameter of the graphene sheet 22 is 0.5 μm.

[0137] Comparative Example 5

[0138] The preparation method of the secondary battery of Comparative Example 5 is the same as that of Example 1, except that the average tube diameter of the carbon nanotubes 21 in S12 is 20 nm, and the average tube length is 20 μm; and the average sheet diameter of the graphene sheet 22 is 0.2 μm.

[0139] Comparative Example 6

[0140] The preparation method of the secondary battery of Comparative Example 6 is the same as that of Example 1, except that the average tube diameter of the carbon nanotubes 21 in S12 is 20 nm, and the average tube length is 20 μm; and the average sheet diameter of the graphene sheet 22 is 10 μm.

[0141] The following introduces the performance test data of the batteries provided by Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2.

[0142] DC internal resistance test (abbreviated as: DCIR), the test method is as follows:

[0143] (1) At a temperature of 25°C, perform 0.2C charge and discharge to calibrate the battery capacity;

[0144] (2) Charge at 0.2C to 50% state of charge and 90% state of charge;

[0145] (3) Set the charging time to 2C for 30 seconds, record the final voltage and final current of each process, and calculate the DCIR. The results are shown in Table 1.

[0146] Table 1 DC internal resistance of secondary batteries of embodiments and comparative examples (mΩ)

[0147]

[0148]

[0149] It can be seen from Table 1 that the DC internal resistance of the batteries provided by Examples 1 to 7 is smaller than the DC internal resistance of the secondary batteries provided by Comparative Examples 1 to 6, indicating that the secondary batteries formed by the negative electrode sheets 10 provided in the embodiments of the present application have better charging performance.

[0150] The DC internal resistance of the secondary batteries provided by Examples 4 and 5 is less than that of the secondary battery provided by Example 1, indicating that when the ratio of the mass of the carbon nanotubes 21 to the mass of the graphene sheet 22 is in the range of 0.1 to 3, the secondary battery has better charging performance.

[0151] Combined with the fact that the DC internal resistance of the secondary battery provided by Comparative Example 1 is less than the DC internal resistance of the secondary batteries provided by Comparative Examples 3 to 6, it is explained that in the embodiments of the present application, when the negative electrode material 12 formed by the conductive agent with an average tube diameter range of 20nm to 80nm for the carbon nanotubes 21 and an average sheet diameter range of 0.5μm to 5μm for the graphene sheets 22 is used in a secondary battery, the internal resistance of the secondary battery can be reduced. This is related to the fact that the average tube diameter range of 5nm to 80nm for the carbon nanotubes 21 and the average sheet diameter range of 0.5μm to 5μm for the graphene sheets 22, which can improve the diffusion performance, compaction performance and flexibility of the ions of the negative electrode sheet 10.

[0152] The above are only specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A negative electrode plate, characterized in that: include: a current collector and a negative electrode material disposed on at least one surface of the current collector; The negative electrode material comprises: a negative electrode active material and a conductive agent; Wherein, the conductive agent includes: carbon nanotubes and graphene sheets; The average diameter of the carbon nanotubes is in the range of 20 nm to 80 nm; The average sheet diameter of the graphene sheet is in the range of 0.5 μm to 5 μm.

2. The negative electrode sheet according to claim 1, characterized in that: The average tube length of the carbon nanotubes ranges from 0.5 μm to 30 μm.

3. The negative electrode sheet according to claim 1, characterized in that: The ratio of the mass of the carbon nanotubes to the mass of the graphene sheets ranges from 0.1 to 10.

4. The negative electrode sheet according to claim 3, characterized in that: The ratio of the mass of the carbon nanotubes to the mass of the graphene sheets is in the range of 0.1 to 3.

5. The negative electrode sheet according to claim 1, characterized in that: The ratio of the mass of the carbon nanotubes to the mass of the negative electrode active material is in the range of 0.0008 to 0.

03.

6. The negative electrode sheet according to claim 1, characterized in that: The ratio of the mass of the carbon nanotubes to the mass of the negative electrode active material is in the range of 0.0008 to 0.

02.

7. The negative electrode sheet according to claim 1, characterized in that: The conductive agent also includes: at least one of conductive carbon black and carbon fiber material.

8. The negative electrode sheet according to any one of claims 1 to 7, characterized in that: Also includes: Thickener and binder; the ratio of the sum of the mass of the thickener and the mass of the binder to the mass of the negative electrode material is in the range of 0.025 to 0.

035.

9. A secondary battery, characterized in that: include: The negative electrode sheet according to any one of claims 1 to 8; The positive electrode plate is spaced apart from the negative electrode plate.

10. A battery assembly, characterized in that: include: The secondary battery as claimed in claim 9; A component housing, wherein the secondary battery is disposed in the component housing.

11. An electrical device, characterized in that: include: At least one of the secondary battery as claimed in claim 9 and the battery assembly as claimed in claim 10.

Images