CN116111101B

CN116111101B - Positive electrode sheet, secondary battery, and electronic device

Info

Publication number: CN116111101B
Application number: CN202310384555.XA
Authority: CN
Inventors: 韩冬冬; 刘晓欠; 王可飞; 陶兴华
Original assignee: Ningde Amperex Technology Ltd
Current assignee: Ningde Amperex Technology Ltd
Priority date: 2023-04-11
Filing date: 2023-04-11
Publication date: 2023-08-18
Anticipated expiration: 2043-04-11
Also published as: CN116111101A

Abstract

The application provides a positive plate, a secondary battery and electronic equipment, wherein the positive plate comprises a positive active material layer, and the positive active material layer comprises a positive active material, conductive particles, carbon nano tube clusters and a binder; wherein the carbon nanotube cluster consists of a plurality of carbon nanotube units which are arranged in a bundle shape, and the diameter of the carbon nanotube cluster is larger than 0.2 mu m; the conductive particles, the carbon nanotube clusters and the binder form a conductive composition in the gaps of the positive electrode active material, and the maximum continuous length Y of the conductive composition in the positive electrode active material layer is more than or equal to 5 mu m. The conductive composition can form a long and wide conductive path in the positive electrode active material layer, and can reduce the resistance of the positive electrode sheet, thereby reducing charge polarization and improving the high-rate charging performance of the battery.

Description

Positive electrode sheet, secondary battery, and electronic device

Technical Field

The application relates to the technical field of batteries, in particular to a positive plate, a secondary battery and electronic equipment.

Background

Secondary batteries represented by lithium ion batteries are widely used in products such as digital electronic products, energy storage, unmanned aerial vehicles, electric tools, and electric vehicles due to their characteristics such as high energy density, long cycle life, high safety, and rapid charging capability. High-rate charging of the battery cannot be achieved because polarization of the battery occurs during charging. Therefore, there is a need to improve the high-rate charging performance of the battery.

Disclosure of Invention

The application provides a positive plate, a secondary battery and electronic equipment, which aim to reduce the resistance of the positive plate and improve the high-rate charging performance of the battery by increasing a conductive path in a positive active material layer.

In a first aspect, the present application provides a positive electrode sheet including a positive electrode active material layer including a positive electrode active material, conductive particles, carbon nanotube clusters, and a binder; wherein the carbon nanotube cluster consists of a plurality of carbon nanotube units which are arranged in a bundle shape, and the diameter of the carbon nanotube cluster is larger than 0.2 mu m; the conductive particles, the carbon nanotube clusters and the binder form a conductive composition in the gaps of the positive electrode active material, and the maximum continuous length Y of the conductive composition in the positive electrode active material layer is more than or equal to 5 mu m.

According to the application, in the positive electrode active material layer, the carbon nanotube clusters which are formed by a plurality of carbon nanotube units arranged in a bundle shape and have the diameter larger than 0.2 mu m can form a long-range conductive network, meanwhile, the carbon nanotube clusters are matched with the binder mixed with conductive particles, the conductive composition is formed in a gap of the positive electrode active material together, and the maximum continuous length Y of the conductive composition in the positive electrode active material layer is not smaller than 5 mu m, so that the conductive path in the positive electrode active material layer can be effectively prolonged and widened, the resistance of the positive electrode plate is reduced, the charging polarization is reduced, and the high-rate charging performance of the battery is improved.

In some embodiments, the conductive particles have a particle size of 80% of no more than 2 μm.

In some embodiments, the conductive particles comprise metallic conductive particles and/or conductive carbon particles; the metal conductive particles comprise one or more of simple substances Au, ag, al, ni, co, li, fe, cu.

In some embodiments, the carbon nanotube cluster satisfies at least one of the following conditions: 1) The average diameter d of the carbon nano tube unit is more than or equal to 3nm and less than or equal to 40nm; 2) The carbon nanotube unit is a multiwall carbon nanotube unit; 3) The average diameter D of the carbon nano tube clusters is more than 0.2 mu m; 4) The average length L of the carbon nano tube bundle is more than or equal to 3 mu m.

In some embodiments, the carbon nanotube cluster satisfies at least one of the following conditions: 5) The average diameter d of the carbon nano tube unit is more than or equal to 5nm and less than or equal to 20nm; 6) The average diameter D of the carbon nano tube bundle is more than or equal to 0.5 mu m and less than or equal to 3 mu m; 7) The average length L of the carbon nano tube bundle is more than or equal to 5 mu m and less than or equal to 30 mu m.

In some embodiments, the binder comprises a fibrous binder comprising one or more of polyacrylic acid, polyacrylate, polyamide-imide, polyvinyl alcohol, carboxymethyl cellulose salt.

In some embodiments, the weight average molecular weight of the binder is 300,000 to 3,000,000.

In some embodiments, the cohesion of the positive electrode active material layer is not less than 5N/m.

In some embodiments, the positive electrode sheet satisfies at least one of the following conditions: 8) The mass percentage of the positive electrode active material in the positive electrode active material layer is 89% to 99.3%; 9) The mass percentage of the conductive particles in the positive electrode active material layer is 0.1-3%; 10 The mass percentage of the carbon nano tube bundle in the positive electrode active material layer is 0.1 to 3 percent; 11 The mass percentage of the binder in the positive electrode active material layer is 0.5% to 5%.

In a second aspect, the present application provides a secondary battery comprising: a negative electrode sheet, a separator, an electrolyte, and a positive electrode sheet according to any one of the embodiments of the first aspect.

In a third aspect, the present application provides an electronic device, comprising: the secondary battery according to any one of the embodiments of the first aspect.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and together with the description, serve to explain the principles of the application.

Fig. 1 is a scanning electron microscope image of a cross section of a positive electrode active material layer according to an embodiment of the present application.

Detailed Description

Each example or embodiment in this specification is described in a progressive manner, each example focusing on differences from other examples.

In the description of the present specification, reference to the terms "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the application. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present application, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise. In the description of the present application, "normal temperature" means 20 to 25 ℃.

In the present application, the battery may include a lithium ion secondary battery, a lithium sulfur battery, a sodium lithium ion, a sodium ion battery, a magnesium ion battery, or the like, which is not limited in the embodiment of the present application. The battery may be in the shape of a cylinder, a flat body, a rectangular parallelepiped, or other shapes, etc., and the embodiment of the present application is not limited thereto.

As described above in the background art, with the widespread use of secondary batteries, there is a higher demand for their charging performance. The polarization phenomenon exists in the charging process of the battery, particularly when the battery is charged at a high rate, the larger the current is, the more obvious the polarization phenomenon is, so that the high rate charging performance of the battery is poor.

One main reason for the occurrence of polarization phenomenon is that the battery has a certain internal resistance, particularly the internal resistance of the positive electrode plate has a larger influence on the internal resistance of the battery, and the positive electrode plate has a higher internal resistance due to the poor conductive performance of the positive electrode active material, so that the internal resistance of the battery can be reduced by reducing the internal resistance of the positive electrode plate, thereby reducing the polarization of the battery and improving the high-rate charging performance of the battery.

At present, the resistance of the positive electrode sheet is mainly reduced by adding a conductive agent into the positive electrode active material layer, but the conventional conductive agent, such as conductive carbon black, graphene, discrete carbon nano tubes and other conductive materials, can reduce the resistance of the positive electrode sheet to a certain extent, but the conductive path formed in the positive electrode active material layer is short and narrow, and the discontinuous conductive path is not conductive to electrons between the positive electrode active materials, so that the effect of reducing the resistance of the positive electrode sheet is limited.

Based on the above, the application provides a positive plate, a secondary battery and an electronic device, wherein the positive active material layer of the positive plate is provided with a longer conductive path, so that the positive active material layer has lower internal resistance, the battery with lower internal resistance can be obtained, the polarization phenomenon of the battery is reduced, and the high-rate charging performance of the battery is improved. Embodiments of the present application are described in detail below.

According to the application, firstly, a plurality of carbon nanotube clusters which are formed by bundle-shaped carbon nanotube units and have diameters larger than 0.2 mu m are used in the positive electrode active material layer, and the carbon nanotube clusters are different from the discrete carbon nanotubes used in the prior art, generally, the discrete carbon nanotubes are used as a conductive agent, and are easy to wind on the surface of the positive electrode active material due to small diameter and large flexibility, and the carbon nanotube clusters are hard to wind due to the diameters larger than 0.2 mu m, so that the carbon nanotube clusters are hard to wind in a bending way and are difficult to wind, and a long-range conductive network with a certain strength is formed in the positive electrode active material layer, and can bridge different positive electrode active materials and current collectors, so that the problem that the internal resistance of the positive electrode sheet cannot be effectively reduced due to discontinuous conductive paths in the prior art can be effectively overcome.

Further, the conductive particles are included in the positive electrode active material layer, because the long-range conductive network is formed by carbon nanotube clusters, the carbon nanotube clusters are bonded by an adhesive, and meanwhile, the positive electrode active material is also required to be bonded with the long-range conductive network by the adhesive, and because the conductivity of the adhesive is generally poor, the bonding position connected by the adhesive is not beneficial to the conduction of electrons, and the resistance of the positive electrode sheet is affected. Based on this, conductive particles are added in the positive electrode active material layer, and these conductive particles can be dispersed in the binder, so that the problem that the above-mentioned bonding place is not easy to conduct electrons can be effectively overcome; meanwhile, the conductive particles can be bonded on the surface of the carbon nano tube bundle through the adhesive, so that the width of a conductive path in a long-range conductive network is widened; the conductive particles, the carbon nanotube clusters and the binder can jointly form a conductive composition with good conductivity between gaps of the positive electrode active material, and the conductive composition can further prolong and widen a conductive path in the positive electrode active material layer compared with a long-range conductive network formed by singly using the carbon nanotube clusters, namely, the conductive composition can form a conductive path longer and wider than the long-range conductive network in the positive electrode active material layer, and can further reduce the resistance of the positive electrode sheet.

It can be understood that the larger the maximum continuous length Y of the conductive composition in the positive electrode active material layer, the longer the conductive path that can be formed in the positive electrode active material layer, so that the resistance of the positive electrode sheet can be further reduced, the internal resistance of the battery can be further reduced, the polarization of the battery can be reduced, and the high-rate charging performance of the battery can be improved.

In the context of the present application, the conductive composition refers to, in particular, an integral structure in which carbon nanotube clusters and conductive particles are directly bonded by a binder in the positive electrode active material layer, as shown in fig. 1. If the carbon nanotube clusters or the conductive particles are bonded to the positive electrode active material layer by the binder alone, the carbon nanotubes and the conductive particles are not directly bonded by the binder, and therefore, the conductive composition is not included.

In the application, the maximum continuous length Y of the conductive composition in the positive electrode active material layer is not less than 5 mu m, and the positive electrode active material layer has a denser and smooth conductive network, so that the positive electrode plate has smaller internal resistance, and the battery has better high-rate charging performance. For example, the maximum continuous length Y of the conductive composition in the positive electrode active material layer may be 5 μm,6 μm,7 μm,8 μm,9 μm,10 μm,11 μm,12 μm,13 μm,14 μm,15 μm,16 μm,17 μm,18 μm,19 μm,20 μm,21 μm,22 μm,23 μm,24 μm,25 μm,26 μm,27 μm,28 μm,29 μm,30 μm,31 μm,32 μm,33 μm,34 μm,35 μm,36 μm,37 μm,38 μm,39 μm,40 μm,41 μm,42 μm,43 μm,44 μm,45 μm, or a range composed of any of the above values.

The maximum continuous length Y of the conductive composition in the positive electrode active material layer is related to the mass percentage of carbon nanotube clusters, conductive particles, binders and components, and the preparation method of the positive electrode active material layer, and the maximum continuous length Y of the conductive composition in the positive electrode active material layer can be controlled by reasonable selection.

In the context of the present application, "carbon nanotube cluster" refers to a structure composed of a plurality of carbon nanotube units arranged in bundles and bonded, wherein long axes of the carbon nanotube units are bonded in parallel to each other to form a diameter of more than 0.2 μm. "discrete carbon nanotubes" refers to structures in which a single carbon nanotube unit or multiple carbon nanotube units are bonded to each other and have a diameter much less than 0.2 μm. Prior to the present application, carbon nanotubes have been used as a conductive agent in an electrode active material layer. Since the carbon nanotubes have a very large aspect ratio and specific surface area, agglomeration easily occurs. Thus, conventional carbon nanotube feedstock is typically provided in the form of agglomerates. According to the related teachings of the present application before, in order to exert the conductive effect of the carbon nanotubes, it is required that the carbon nanotubes are uniformly dispersed in the electrode active material layer in the form of single carbon nanotube units. For this reason, a dispersion of the carbon nanotube conductive agent in the dispersant is generally prepared first, and the dispersion conditions used are such that the carbon nanotube units are sufficiently dispersed in the dispersant that no or substantially no carbon nanotube clusters are formed (i.e., the content of carbon nanotube clusters is very low even if carbon nanotube clusters similar to those provided by the present application are unintentionally formed) in the dispersion; such carbon nanotube conductive agent dispersion is then thoroughly mixed with an electrode active material and other additives to form an electrode active paste, which is then coated and dried to form an electrode active material layer. As described above, in the electrode active material layer thus formed, the carbon nanotubes are uniformly dispersed in the electrode active material layer substantially in the form of carbon nanotube units, and perform a conductive function in the form of discrete carbon nanotubes, without or substantially without (i.e., the mass percentage of carbon nanotube clusters in the electrode active material layer does not meet the requirements of the present application even if carbon nanotube clusters like those provided by the present application are unintentionally formed) having a diameter of greater than 0.2 μm.

In the present application, the maximum continuous length Y of the conductive composition in the positive electrode active material layer can be detected by the following method, unless otherwise specified:

a. disassembling the finished battery to obtain a positive plate;

b. airing the positive plate in the step a for 30min in an environment with humidity less than or equal to 15% and temperature of 25+/-3 ℃;

c. b, taking the positive plate in the step b, and obtaining the cross section of the positive material active layer on the positive plate in a liquid nitrogen brittle failure mode;

d. the cross section obtained in c was observed under a Scanning Electron Microscope (SEM), and the maximum continuous length Y of the conductive composition composed of the conductive particles, the carbon nanotube clusters, and the binder in the positive electrode active material layer was tested.

In some of the above embodiments, the conductive particles may have a particle size of 80% or less than 2 μm, because if the particle size of the conductive particles is too large, the number of conductive particles is small under the condition that the mass percentage of the conductive agent in the positive electrode active material layer is constant, which is disadvantageous for the extension of the length of the conductive composition, and in addition, too large conductive particles are disadvantageous for adhesion to the long-range conductive network, which cannot effectively widen the width of the conductive path; in addition to the influence on the maximum continuous length Y of the conductive composition in the positive electrode active material layer, conductive particles may be not easily dispersed in the binder, and thus conduction of electrons at the bonding sites between carbon nanotube clusters and at the bonding sites between the positive electrode active material and the long-range conductive network cannot be effectively promoted, affecting the conductivity of the conductive composition itself; the two effects are combined, so that the resistance of the positive plate is larger, the polarization of the battery is not reduced, and the high-rate charging performance of the battery is reduced, and the particle size of 80% of the conductive particles can be controlled to be not more than 2 mu m. For example, 80% of the conductive particles may have a particle diameter of not more than 0.01 μm,0.02 μm,0.03 μm,0.04 μm,0.05 μm,0.06 μm,0.07 μm,0.08 μm,0.09 μm,0.1 μm,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,1.3 μm,1.4 μm,1.5 μm,1.6 μm,1.7 μm,1.8 μm,1.9 μm,2 μm, or a range composed of any of the above values.

In some of the above embodiments, the conductive particles may include metal conductive particles and/or conductive carbon particles, and it is understood that the specific kind of conductive particles is not particularly limited, and only needs to be satisfied that the conductive particles have good conductivity, and any conductive particles in the prior art may be selected according to actual needs by those skilled in the art. The better the conductivity of the conductive particles, the more advantageous is to reduce the resistance of the positive electrode sheet, while the stability of the conductive particles in the battery needs to be considered to ensure the stability of the conductive composition in forming the conductive path.

In the above embodiment, a few metal elements having good conductivity are further exemplified, and one or more of them can be selected according to actual needs by those skilled in the art.

In some embodiments, the carbon nanotube cluster satisfies at least one of the following conditions: 1) The average diameter d of the carbon nano tube unit is more than or equal to 3nm and less than or equal to 40nm; 2) The carbon nanotube unit is a multi-wall carbon nanotube unit; 3) The average diameter D of the carbon nano tube cluster is more than 0.2 mu m; 4) The average length L of the carbon nanotube clusters is more than or equal to 3 mu m.

In some embodiments, the carbon nanotube cluster satisfies at least one of the following conditions: 5) The average diameter d of the carbon nano tube units is more than or equal to 5nm and less than or equal to 20nm; 6) The average diameter D of the carbon nano tube bundle is more than or equal to 0.5 mu m and less than or equal to 3 mu m; 7) The average length L of the carbon nano tube bundle is more than or equal to 5 mu m and less than or equal to 30 mu m.

In some of the above embodiments, the specification size of the carbon nanotube cluster is further defined. Wherein the average diameter d of the carbon nanotube units constituting the carbon nanotube cluster may be 3nm to 40nm. The carbon nano tube cluster is formed by mutually combining carbon nano tube units, when the average diameter of the carbon nano tube units is too small, the carbon nano tube units with too small diameters are softer in the process of manufacturing the carbon nano tube cluster, and can be clustered and wound with other carbon nano tube units, so that the obtained carbon nano tube cluster has insufficient relative strength; when the average diameter of the carbon nano tube units is too large, the carbon nano tube units with the too large diameter are easy to deform or even break in the process of manufacturing the carbon nano tube clusters, so that the formation of a long-range conductive network is not facilitated; in addition, the average diameter of the carbon nano tube units is too large, so that the conductivity of the carbon nano tube clusters is reduced, and the resistance of the positive plate is increased. For example, the average diameter d of the carbon nanotube units may be 3nm,4nm,5nm,8nm,10nm,12nm,14nm,16nm,18nm,20nm,25nm,30nm,35nm,40nm or within a range consisting of any of the above values. Further preferably, the average diameter d of the carbon nanotube unit may be 5nm to 20nm.

The carbon nanotube unit may be a multiwall carbon nanotube unit. Because the single-wall carbon nano tube can be described as a seamless hollow cylinder formed by rolling a single-layer graphene sheet, the diameter of the single-wall carbon nano tube is generally 1nm to 2nm, the single-wall carbon nano tube with larger diameter can cause unstable self structure, the defect number is increased, and the length of the single-wall carbon nano tube is generally in a micron level, the single-wall carbon nano tube has very high length-diameter ratio, thus the single-wall carbon nano tube has very high flexibility, and in the process of manufacturing the carbon nano tube cluster, agglomeration and winding are very easy to occur, and the carbon nano tube cluster with the diameter larger than 0.2 mu m is difficult to obtain; on the other hand, the inventor finds that when the carbon nanotube cluster consisting of single-wall carbon nanotube units is applied to the positive plate, the carbon nanotube cluster is easy to wind in other carbon nanotube clusters, and a long-range conductive network is not easy to form. The multi-wall carbon nanotubes can be regarded as concentric arrangement of single-wall carbon nanotubes, namely a tubular structure rolled up by a plurality of graphene sheets in a seamless way, and the multi-wall carbon nanotubes have larger diameter and certain strength, are not easy to bend, twist, kink or bend, so that agglomeration and winding are not easy to occur, carbon nanotube clusters with the diameter larger than 0.2 μm are more easily produced, and the beneficial effects of the positive electrode sheet are more beneficial to realization.

The carbon nanotube clusters have an average diameter D greater than 0.2 μm. When the average diameter of the carbon nanotube clusters is larger than 0.2 μm, the carbon nanotube clusters have enough strength to form a long-range conductive network in the positive electrode active material layer, so that corresponding effects are realized, if the average diameter is too small, the carbon nanotube clusters are not rigid enough, bending and winding are easy to occur, and the long-range conductive network is likely to be incapable of being formed, and is a basic structure of the conductive composition in the application, if the long-range conductive network is incapable of being formed, the maximum continuous length of the conductive composition in the positive electrode active material layer is smaller, and the electronic path of the positive electrode active material layer is reduced, so that the resistance of the positive electrode sheet cannot be effectively reduced, and the high-rate charging performance of the battery is affected. For example, the average diameter D of the carbon nanotube clusters may be 0.22 μm,0.24 μm,0.26 μm,0.28 μm,0.3 μm,0.4 μm,0.5 μm,1 μm,1.5 μm,2 μm,2.5 μm,3 μm,3.5 μm,4 μm,4.5 μm,5 μm,5.5 μm,6 μm,6.5 μm, or within a range consisting of any of the above values.

Further preferably, the average diameter D of the carbon nanotube clusters may be 0.5 μm to 3 μm. The carbon nano tube clusters are easy to agglomerate if the diameters of the carbon nano tube clusters are too large, so that more carbon nano tube clusters need to be added to be uniformly dispersed in the positive electrode active material layer to form a long-range conductive network, and meanwhile, the carbon nano tube clusters with the too large diameters are easy to crack in the processing process, so that the production and processing difficulty is increased, and the cost is increased; in addition, the average diameter of the carbon nano tube clusters is further increased, and the influence on the resistance of the positive plate is small, so that the average diameter of the carbon nano tube clusters is not excessively large; meanwhile, the average diameter of the carbon nano tube clusters is properly increased, the strength of a long-range conductive network can be increased, the width of a conductive path is widened, the stability of the conductive network can be further improved, the stable conductive path is maintained in the positive electrode active material layer, and the resistance of the positive electrode plate is effectively reduced. The average diameter D of the carbon nanotube clusters may be 0.5 μm to 3 μm, in which case the maximum continuous length of the conductive composition in the positive electrode active material layer is large, and the high-rate charge performance of the battery is good.

The average length L of the carbon nanotube clusters may be not less than 3 μm. This is because if the length of the carbon nanotube cluster is too short, a long-range conductive network is not easily formed, which is unfavorable for continuous extension of the conductive composition in the positive electrode active material layer, so that a long and wide conductive path cannot be formed, which may result in a large resistance of the positive electrode sheet and affect the high-rate charging performance of the battery. For example, the average length L of the carbon nanotube clusters may be 3 μm,4 μm,5 μm,6 μm,7 μm,8 μm,9 μm,10 μm,12 μm,14 μm,16 μm,18 μm,20 μm,25 μm,30 μm,35 μm,40 μm, or within a range consisting of any of the above values.

Further preferably, the average length L of the carbon nanotube clusters may be 5 μm to 30 μm. It can be understood that in general, the longer the average length, the longer the maximum continuous length of the conductive composition in the positive electrode active material layer, the more advantageous the conduction of electrons, and the smaller the resistance of the positive electrode sheet; however, if the average length is too long, the carbon nanotube clusters may be easily agglomerated in the preparation process, and particle scratches or positive salient points are formed in the preparation process of the positive plate, in addition, the rigid carbon nanotube clusters are easily broken in the processing and utilizing processes, so that the length of the carbon nanotube clusters is further increased, and the production cost is not reduced; on the other hand, the continuous increase in the average length of the carbon nanotube bundles has no significant effect on the maximum continuous length of the conductive composition in the positive electrode active material layer, because the longer the carbon nanotube bundles are, the more advantageous the conductive composition is in extending in the positive electrode active material layer, but since the longer the carbon nanotube bundles are, the greater the aspect ratio thereof is, the more the bending moment is increased, which may cause bending, kinking, winding to occur, and thus the maximum continuous length of the conductive composition cannot be significantly increased, and thus the average length L of the carbon nanotube bundles may be 5 μm to 30 μm. It is also understood that, since bending, kinking, and twisting may occur when the average length of the carbon nanotube clusters is excessively long, the maximum continuous length of the formed conductive composition in the positive electrode active material layer may be smaller than the average length of the carbon nanotube clusters. The average diameter D, the average length L, and the average diameter D of the carbon nanotube units of the carbon nanotube cluster can be measured by the following methods, unless otherwise specified:

a. Disassembling the battery to be tested to obtain a positive plate;

b. soaking the positive plate in DMC (dimethyl carbonate) at normal temperature for 60min, taking out, and airing at normal temperature;

c. b, taking the positive plate in the step b, and obtaining the cross section of the positive active material layer on the positive plate in a liquid nitrogen brittle failure mode;

d. and c, observing the section obtained in the step c under SEM, testing at least 5 different positions, and respectively calculating the average value of the diameters and the lengths of the carbon nanotube clusters and the diameters of the carbon nanotube units which are not less than 15 in total, thus obtaining the average diameter D, the average length L and the average diameter D of the carbon nanotube clusters.

In some of the above embodiments, it is further defined that the fibrous binder is included in the binder, because the fibrous binder may be entangled on the surface of the carbon nanotube clusters, thereby promoting the formation of a long-range conductive network, which serves as a host structure of the conductive composition, facilitating the extension of the length of the conductive composition in the positive electrode active material layer. In addition, the fibrous binder can promote the dispersion of the conductive particles in the positive electrode active material layer, and is more beneficial to the electronic conduction between carbon nano tube clusters and at the bonding position between the positive electrode active material and the long-range conductive network. Therefore, the binder containing the fibrous binder is used, so that electrons of the positive plate can be effectively reduced, and the high-rate charging performance of the battery can be improved.

The above embodiments also provide several fibrous binders commonly used in the art, it being understood that fibrous binders include, but are not limited to, the several materials described above, and those skilled in the art may choose according to actual needs.

In the context of the present application, unless otherwise specified, "fibrous binder" refers to the appearance form of the corresponding binder in the positive electrode active material layer, which is confirmed by a scanning microscope, and the binder dispersed in the positive electrode active material layer in a fibrous or chain form between the conductive agents is "fibrous binder"; in contrast, the binder may be further classified into a "spot-type binder" according to the appearance of the binder in the positive electrode active material layer, and the binder between the active material and the conductive agent is dispersed in the positive electrode active material layer in the form of spots or particles as a "spot-type binder".

In some of the above embodiments, the weight average molecular weight of the binder is further limited, and if the weight average molecular weight of the binder is too small, the binding performance of the binder is poor at this time, and effective binding between components of the positive electrode active material layer cannot be performed, so that the length extension of the conductive composition in the positive electrode active material layer is affected, which is not beneficial to reducing the resistance of the positive electrode sheet; if the weight average molecular weight of the binder is too large, the binder may not be easily dissolved during the preparation of the positive electrode active slurry, resulting in difficulty in processing, and difficulty in uniform dispersion of the binder in the positive electrode active layer, thereby also being unfavorable for the binder to exert its binding effect. Therefore, the weight average molecular weight of the binder may be 300,000 to 3,000,000, and at this time, the binder has a good binding effect, and the conductive composition has a long maximum continuous length in the positive electrode active material layer, so that the resistance of the positive electrode sheet can be effectively reduced, and the battery has good high-rate charging performance.

For example, the weight average molecular weight of the binder may be 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, 1,500,000, 1,600,000, 1,700,000, 1,800,000, 1,900,000, 2,000,000, 2,100,000, 2,200,000, 2,300,000, 2,400,000, 2,500,000, 2,600,000, 2,700,000, 2,800,000, 2,900,000, 3,000,000, or within a range consisting of any of the above.

In the present application, the weight average molecular weight and crystallinity of the binder have the meanings known in the art, and the weight average molecular weight means the molecular weight of the binder as a result of statistics by weighting the mass, and can be measured by methods and instruments known in the art. For example, it can be measured by the following method:

a. disassembling the battery to be tested to obtain a positive plate;

b. taking the positive plate in the step a, soaking the positive plate in a corresponding solvent for film removal, and uniformly dispersing the film layer in the solvent by using a disperser to obtain slurry;

c. taking the slurry in the step b, and obtaining a binder in the film layer by adopting a centrifugal method;

d. the weight average molecular weight of the binder in c was determined by Gel Permeation Chromatography (GPC).

In some of the above embodiments, the cohesive force of the positive electrode active material layer is further defined, and it is understood that since the carbon nanotube clusters can form a long-range conductive network in the positive electrode active material layer, the cohesive force of the positive electrode active material layer can be improved, the larger the cohesive force is, the smaller the volume change rate of the positive electrode active material layer with the charge and discharge of the battery is, and thus the structure of the positive electrode active material layer is more stable, the continuous structure of the conductive composition in the positive electrode active material layer is less likely to break, and thus the conductive stability of the positive electrode sheet is better. The cohesive force of the positive electrode active material layer is related to the structural size and content of the carbon nanotube clusters, the kind and content of the binder, and thus the cohesive force of the positive electrode active material layer can be adjusted by adjusting the above parameters. In order to provide good stability to the structure of the conductive composition in the positive electrode active material layer so that the positive electrode sheet has good conductive stability, it is necessary to control the cohesive force of the positive electrode active material layer to 5N/m or more. For example, the cohesive force of the positive electrode active material layer may be 5N/m,6N/m,7N/m,8N/m,9N/m,10N/m,11N/m,12N/m,13N/m,15N/m,17N/m,20N/m, or a range composed of any of the above values.

Note that, the cohesive force of the positive electrode active material layer may be measured by the following method, except for the specific description:

a. disassembling the finished battery to obtain a positive plate;

b. soaking the positive plate in DMC (dimethyl carbonate) at normal temperature for 60min, taking out, and airing at room temperature;

c. taking the positive plate in the step b, and testing cohesive force by adopting a high-speed rail tension machine and a 90-degree angle method which are commonly used in the lithium battery industry: the part of the negative plate coated with the second positive electrode active material layer is made into a strip shape, and one part of the positive plate is adhered to the steel plate through double-sided adhesive tape from one end of the positive plate along the length direction; and then fixing the steel plate at the corresponding position of the high-speed rail tensile machine, pulling up an adhesive tape with one end adhered to the steel plate, putting the adhesive tape into a chuck through a connector or directly for clamping, and when the tension of a clamping opening is more than 0kgf and less than 0.02kgf, starting to test by the high-speed rail tensile machine, wherein the average value of the finally measured tension is recorded as the cohesive force of the positive electrode active material layer.

In some embodiments, the positive electrode sheet satisfies at least one of the following conditions: 8) The mass percentage of the positive electrode active material in the positive electrode active material layer is 89% to 99.3%; 9) The mass percentage of the conductive particles in the positive electrode active material layer is 0.1 to 3 percent; 10 0.1 to 3% by mass of the carbon nanotube clusters in the positive electrode active material layer; 11 The mass percentage of the binder in the positive electrode active material layer is 0.5% to 5%.

In some of the above embodiments, the mass percentage of the positive electrode active material in the positive electrode active material layer may be 89% to 99.3%, and if the content of the positive electrode active material is too high, it may result in a decrease in the energy density of the battery; if the content of the binder, the carbon nanotube clusters and the conductive particles is too high, the content of the binder, the carbon nanotube clusters and the conductive particles is reduced, which is not beneficial to the improvement of the maximum continuous length of the conductive composition in the positive electrode active material layer, and meanwhile, the cohesive force of the positive electrode active material layer is reduced, so that the resistance and the stability of the positive electrode sheet are both reduced, and the high-rate charging performance of the battery is affected. The mass percentage of the positive electrode active material in the positive electrode active material layer can be controlled within the above range.

The mass percentage of the conductive particles in the positive electrode active material layer can be 0.1 to 3 percent, if the content of the conductive particles is too low, on one hand, the conductive particles are unfavorable for the extension of the conductive composition in the positive electrode active material layer, a long and wide conductive path cannot be formed, and on the other hand, the too low conductive particles can cause the reduction of the conductivity of the formed conductive composition, and the reduction of the resistance of the positive electrode sheet is unfavorable; if the content of the conductive particles is too high, the content of the carbon nanotube clusters, the binder or the positive electrode active material may be reduced, which is not advantageous for improving the energy density of the battery, and also may reduce the cohesive force of the positive electrode active material layer, which is disadvantageous for the stability of the battery. The mass percentage of the conductive particles in the positive electrode active material layer can be controlled within the above range. For example, the amount may be 0.1%,0.15%,0.2%,0.3%,0.4%,0.5%,0.6%,0.7%,0.8%,0.9%,1%,1.1%,1.2%,1.3%,1.4%,1.5%,1.6%,1.7%,1.8%,1.9%,2%,2.1%,2.2%,2.3%,2.4%,2.5%,2.6%,2.7%,2.8%,2.9%,3%, or a range composed of any of the above values.

The mass percentage of the carbon nanotube clusters in the positive electrode active material layer can be 0.1-3%, if the content of the carbon nanotube clusters is too low, a stable long-range conductive network can not be formed in the positive electrode active material layer, meanwhile, the long-range conductive network Cheng Daodian is used as a framework of the conductive composition, and the unstable long-range conductive network can influence the extension of the conductive composition in the positive electrode active material layer and the conductivity of the conductive composition; if the carbon nanotube cluster content is too high, the cost may be increased, and the energy density of the battery may be affected. The mass percentage of the carbon nanotube clusters in the positive electrode active material layer can be controlled within the above range. For example, the amount may be 0.1%,0.15%,0.2%,0.3%,0.4%,0.5%,0.6%,0.7%,0.8%,0.9%,1%,1.1%,1.2%,1.3%,1.4%,1.5%,1.6%,1.7%,1.8%,1.9%,2%,2.1%,2.2%,2.3%,2.4%,2.5%,2.6%,2.7%,2.8%,2.9%,3%, or a range composed of any of the above values.

The mass percentage of the binder in the positive electrode active material layer can be 0.5-5%, the binder plays a role in binding each component of the positive electrode active material layer and the current collector, if the content of the binder is too low, the cohesive force of the positive electrode active material layer is lower, the positive electrode active material layer is easy to peel off from the current collector, the contact resistance between the positive electrode active material layer and the current collector is larger, and the high-rate charging performance of the battery is affected; if the content of the binder is too high, on one hand, the energy density of the battery is reduced, and on the other hand, the conductivity of the conductive composition is affected by too much binder due to poor conductivity of the binder, which is not beneficial to reducing the resistance of the positive plate and also is not beneficial to improving the high-rate charging performance of the battery. The mass percentage of the binder in the positive electrode active material layer can be controlled within the above range. For example, it may be 0.5%,0.6%,0.7%,0.8%,0.9%,1%,1.1%,1.2%,1.3%,1.4%,1.5%,1.6%,1.7%,1.8%,1.9%,2%,2.1%,2.2%,2.3%,2.4%,2.5%,2.6%,2.7%,2.8%,2.9%,3%,3.1%,3.2%,3.3%,3.4%,3.5%,3.6%,3.7%,3.8%,3.9%,4%,4.1%,4.2%,4.3%,4.4%,4.5%,4.6%,4.7%,4.8%,4.9%,5%, or a range composed of any of the above values.

In the present application, the binder, the positive electrode active material may be commonly used in the art, and there is no particular limitation on the specific type.

For example, the binder may use one or more of polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose, styrene-butadiene rubber, fluororubber, or various copolymers thereof. These binders may be used alone or in combination of two or more.

For example, the positive electrode active material may use one or more of lithium-containing phosphates including lithium transition metal oxides, olivine structures, and their respective modified compounds. The modifying compound for each positive electrode active material may be a doping modification, a surface coating modification, or a doping and surface coating modification of the positive electrode active material. As an example, the lithium transition metal oxide may include one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof. As an example, the olivine-structured lithium-containing phosphate may include 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, and modified compounds thereof. These positive electrode active materials may be used alone or in combination of two or more.

The positive plate provided by the application can comprise a current collector, the current collector is not limited, and a metal foil, a porous metal plate or a composite current collector can be used. The composite current collector may include a polymeric material base layer and a metal layer formed on at least one surface of the polymeric material base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel alloy, titanium alloy, silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). As one example, the current collector is aluminum foil.

In some embodiments, the current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode active material layer may be disposed on one surface of the current collector or may be disposed on both surfaces of the current collector at the same time. For example, the current collector has two surfaces opposing in the thickness direction thereof, and the positive electrode active material layer is provided on either one or both of the two opposing surfaces of the current collector.

Method for manufacturing positive electrode sheet

The application also provides a method for manufacturing the positive plate, which can comprise the following steps:

s10: preparing a dispersion of carbon nanotube clusters and conductive particles;

s20: and adding the carbon nanotube cluster dispersion, the conductive particle dispersion, the positive electrode active material and the binder into a solvent to obtain positive electrode active slurry.

[ preparation of carbon nanotube Cluster Dispersion ]

In some embodiments, step S10 may specifically include:

s11: adding a conventional carbon nano tube raw material and a dispersing agent into a dispersing medium to obtain a mixed solution;

s12: the dispersion of the carbon nanotube clusters is obtained by dispersing a conventional carbon nanotube raw material by applying a shearing force to the mixed solution.

In some embodiments, in step S11, the dispersion medium may include Dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (N-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols such as one or more of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1, 3-propanediol, 1, 3-butanediol, 1, 5-pentanediol, hexanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, acetone, methyl ethyl ketone, methyl propyl ketone, cyclopentanone, ethyl acetate, gamma-butyrolactone, and epsilon-propiolactone. These dispersion media may be used alone or in combination of two or more. As an example, the dispersion medium may be N-methylpyrrolidone (NMP). The conventional carbon nanotube raw material and the carbon nanotube cluster have good dispersibility in the above dispersion medium.

In some embodiments, in step S11, the conventional carbon nanotube feedstock may be a bond or an aggregate of carbon nanotube units, and as one example, the conventional carbon nanotube feedstock may be an aggregate of multi-walled carbon nanotube units.

In some embodiments, in step S11, the mass percentage of the conventional carbon nanotube material in the mixed solution may be 1% to 4%. Under this condition, the conventional carbon nanotube raw material is dispersed in the mixed solution at an appropriate level to form carbon nanotube clusters of an appropriate specification. As an example, the mass percentage of the conventional carbon nanotube raw material in the mixed solution may be 1.5%.

In some embodiments, in step S11, the mass ratio of the conventional carbon nanotube raw material to the dispersant may be 1:0.1 to 10. Under the condition, the conventional carbon nanotube raw material is dispersed in the mixed solution at a proper level to form carbon nanotube clusters of proper specification, and meanwhile, the dispersion stability of the carbon nanotube clusters is improved. As one example, the mass ratio of conventional carbon nanotube feedstock to dispersant may be 1:2.

In some embodiments, in step S11, the solids content in the mixed solution is 1.5wt% to 20wt%. Under the condition, the conventional carbon nanotube raw material is dispersed in the mixed solution at a proper level to form carbon nanotube clusters of proper specification, and meanwhile, the dispersion stability of the carbon nanotube clusters is improved.

In some embodiments, in step S12, the dispersion of carbon nanotube clusters may be obtained by dispersing the conventional carbon nanotube raw material by applying a shearing force to the mixed solution using a mixing device such as a homogenizer, a bead mill, a ball mill, a sand mill, a basket mill, an attritor, a universal stirrer, a transparent mixer, a pin mill, a TK mixer, or an ultrasonic dispersing device. In particular, the diameter of the carbon nanotube clusters can be controlled by ball milling, so as to meet the requirements of the carbon nanotube clusters in any embodiment of the first aspect of the present application.

In some embodiments, step S12 may specifically include: adding the mixed solution into a container containing sand grinding balls, rotating the container to obtain a dispersion of carbon nanotube clusters,

wherein, the average diameter of the sand grinding balls can be 0.5mm to 2.5mm, the rotating speed of the container can be 500rpm to 6000rpm, and the ball milling time can be 0.5h to 2h. Under this condition, the structure of the carbon nanotube unit can be not destroyed and the diameter of the carbon nanotube cluster can be properly controlled. The time of ball milling refers to the total time of using ball milling, for example, if ball milling is performed a plurality of times, the time of ball milling refers to the total time of ball milling a plurality of times.

The above ball milling conditions are used for properly dispersing the conventional carbon nanotube raw material, and particularly, the conditions for dispersing the conventional carbon nanotube raw material into carbon nanotube clusters or single-chain carbon nanotubes having a diameter of not more than 0.2 μm are excluded. I.e., ball milling conditions are used to properly disperse conventional carbon nanotube raw materials to form carbon nanotube clusters in which carbon nanotube units are bonded to each other side by side to have a diameter of greater than 0.2 μm. This can be achieved only by strictly controlling the composition of the mixed solution and the conditions of the dispersing step.

The average diameter of the carbon nanotube clusters is mainly controlled by the average diameter, the rotating speed and the ball milling time of the sand grinding balls, and in general, the average diameter of the carbon nanotube clusters is improved on the premise that the conventional carbon nanotube raw materials are dispersed to obtain the carbon nanotube clusters by properly improving the diameter of the sand grinding balls and reducing the rotating speed and the ball milling time. In addition, the average length of the carbon nanotube clusters and the average diameter of the carbon nanotube units are mainly determined by the length and the diameter of the carbon nanotube units in the conventional carbon nanotube raw material. The person skilled in the art can make corresponding selective adjustments as required.

[ preparation of conductive particle Dispersion ]

After preparing the mixed solution containing the conductive particle raw material, the dispersion medium and the dispersing agent of the embodiment of the first aspect, the conductive particle raw material may be dispersed by applying a shearing force to the mixed solution using a mixing device such as a homogenizer, a bead mill, a ball mill, a sand mill, a basket mill, an attritor, a general-purpose stirrer, a transparent mixer, a pin mill, a TK mixer, or an ultrasonic dispersion device to obtain the dispersion of the conductive particles. The dispersion medium and the dispersant may be the same as those used in the preparation of the above-described carbon nanotube cluster dispersion, and a repetitive description thereof will not be given here.

The solvent may include Dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (N-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol or octanol; diols such as one or more of ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1, 3-propanediol, 1, 3-butanediol, 1, 5-pentanediol, hexanediol, glycerol, trimethylolpropane, pentaerythritol, sorbitol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monobutyl ether, acetone, methyl ethyl ketone, methyl propyl ketone, cyclopentanone, ethyl acetate, gamma-butyrolactone, and epsilon-propiolactone. These solvents may be used alone or in combination of two or more. The solvent may be the same as or different from the dispersion medium. As one example, the solvent may be N-methylpyrrolidone (NMP).

In some embodiments, the solid content in the positive electrode active slurry is 50wt% to 80wt%.

The method of manufacturing the positive electrode sheet may further include: and drying the positive electrode active slurry to obtain a positive electrode active material layer. Specifically, the positive electrode active material layer may be formed by a method of coating a positive electrode active slurry on a current collector and then drying the coated current collector, or may be formed by a method of casting a positive electrode active slurry on a separate support and then laminating a film separated from the support on the current collector. Further, the areal density of the positive electrode active material layer is adjusted by controlling the amount of additive of the positive electrode active slurry coated on the current collector.

If necessary, after the positive electrode active material layer is formed by the above-described method, a rolling process may be further performed. In this case, in consideration of physical properties of the finally prepared positive electrode sheet, such as the thickness of the active material layer in the positive electrode sheet, drying and rolling may be performed under appropriate conditions, without particular limitation.

The positive electrode sheet in the present application does not exclude other additional functional layers than the positive electrode active material layer. For example, in some embodiments, the positive electrode sheet of the present application further includes a conductive undercoat layer (e.g., composed of a conductive agent and a binder) interposed between the current collector and the positive electrode active material layer, disposed on the surface of the current collector.

According to the present application, since the positive electrode sheet of any one of the embodiments of the first aspect is included in the secondary battery, the secondary battery has the advantageous effects of the first aspect.

The secondary battery provided by the application has higher total capacity when charged at 25 ℃ under the 2C multiplying power, which indicates that the secondary battery has better high multiplying power charging performance.

[ Positive electrode sheet ]

The positive electrode sheet used in the secondary battery of the present application is the positive electrode sheet of any one of the embodiments of the first aspect of the present application. The embodiments of the positive electrode sheet have been described and illustrated in detail above and are not repeated here.

[ negative electrode sheet ]

The material, composition and manufacturing method of the negative electrode sheet used in the secondary battery of the present application may include any technique known in the art.

The negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one surface of the current collector and including a negative electrode active material. As an example, the current collector has two surfaces opposing in the thickness direction thereof, and the anode active material layer is provided on either one or both of the two opposing surfaces of the current collector. The current collector is not limited by the present application, and the current collector provided according to the first aspect is selected. As one example, the current collector is copper foil.

The specific kind of the anode active material is not particularly limited, and may be selected according to the need. For example, the anode active material may use one or more of a carbonaceous material, a metal compound that can be alloyed with lithium, a metal oxide that can be doped and undoped with lithium, and a composite including a metal compound and a carbonaceous material. As an example, the carbonaceous material may include one or more of artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; the metal compound which can be alloyed with lithium may include one or more of silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), si alloy, sn alloy, or Al alloy; the metal oxide which may be doped and undoped with lithium may include one or more of SiOv (0 < v < 2), snO2, vanadium oxide, and lithium vanadium oxide; the composite comprising the metal compound and the carbonaceous material may comprise a Si-C composite and/or a Sn-C composite. These negative electrode active materials may be used alone or in combination of two or more.

In some embodiments, the anode active material layer further optionally includes a binder. The binder may be at least one selected from 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).

In some embodiments, the anode active material layer further optionally includes a conductive agent. The conductive agent may include at least one of conductive carbon black, acetylene black, discrete carbon nanotubes, carbon fibers, ketjen black, and graphene.

In some embodiments, the anode active material layer may also optionally include other adjuvants, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)), and the like.

However, the present application is not limited to the above materials, and other known materials that can be used as a negative electrode active material, a conductive agent, a binder, and a thickener may be used as the negative electrode sheet of the present application.

The negative electrode sheet of the present application may be prepared according to a conventional method in the art. For example, the negative electrode active material, the conductive agent, the binder and the thickener are dispersed in a solvent, wherein the solvent can be N-methyl pyrrolidone (NMP) or deionized water to form uniform negative electrode slurry, the negative electrode slurry is coated on a negative electrode current collector, and the negative electrode active material layer is obtained after drying and cold pressing, so as to obtain the negative electrode sheet.

The negative electrode sheet in the present application does not exclude other additional functional layers than the negative electrode active material layer. For example, in some embodiments, the negative electrode sheet of the present application further includes a conductive undercoat layer (e.g., composed of a conductive agent and a binder) interposed between the current collector and the negative electrode active material layer, disposed on the surface of the current collector.

[ MEANS FOR PROBLEMS ]

The diaphragm is arranged between the positive plate and the negative plate, mainly plays a role in preventing the positive and negative electrodes from being short-circuited, and can enable active ions to pass through. The type of the separator is not particularly limited, and any known porous separator having good chemical stability and mechanical stability may be used.

In some embodiments, the material of the separator may be one or more selected from glass fiber, non-woven fabric, polyethylene, polypropylene, polyvinylidene fluoride, but is not limited thereto. Optionally, the material of the separator may include polyethylene and/or polypropylene. The separator may be a single-layer film or a multilayer composite film. When the separator is a multilayer composite film, the materials of the layers are the same or different. In some embodiments, a ceramic coating, a metal oxide coating may also be provided on the separator.

[ electrolyte ]

The electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet. The secondary electrolyte that can be used in the present application may be an electrolyte known in the art.

In some embodiments, the electrolyte may include an organic solvent, an electrolyte salt, and optional additives, and the types of the organic solvent, the lithium salt, and the additives are not particularly limited and may be selected according to the needs.

In some embodiments, the secondary battery is a lithium ion battery, and the electrolyte salt may include a lithium salt. As an example, the lithium salt includes, but is not limited to LiPF ₆ Lithium hexafluorophosphate, liBF ₄ Lithium tetrafluoroborate, liClO ₄ (lithium perchlorate), liFeSI (lithium bis-fluorosulfonyl imide), liTFSI (lithium bis-trifluoromethanesulfonyl imide), liTFS (lithium trifluoromethanesulfonate), liDFOB (lithium difluorooxalato borate), liBOB (lithium bisoxalato borate), liPO ₂ F ₂ Lithium difluorophosphate, liDFOP (difluorodioxaoxalophosphate)Lithium) and LiTFOP (lithium tetrafluorooxalate phosphate). The lithium salts may be used singly or in combination of two or more.

In some embodiments, the secondary battery is a sodium ion battery, and the electrolyte salt may include a sodium salt. As an example, the sodium salt may be selected from NaPF ₆ 、NaClO ₄ 、NaBCl ₄ 、NaSO ₃ CF ₃ Na (CH) ₃ )C ₆ H ₄ SO ₃ At least one of them.

In some embodiments, the organic solvent includes, by way of example, but is not limited to at least one of Ethylene Carbonate (EC), propylene Carbonate (PC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), butylene Carbonate (BC), fluoroethylene carbonate (FEC), methyl Formate (MF), methyl Acetate (MA), ethyl Acetate (EA), propyl Acetate (PA), methyl Propionate (MP), ethyl Propionate (EP), propyl Propionate (PP), methyl Butyrate (MB), ethyl Butyrate (EB), 1, 4-butyrolactone (GBL), sulfolane (SF), dimethylsulfone (MSM), methylsulfone (EMS), and diethylsulfone (ESE). The organic solvents may be used singly or in combination of two or more. Alternatively, two or more of the above organic solvents are used simultaneously.

In some embodiments, the additives may include negative film-forming additives, positive film-forming additives, and may also include additives that improve certain properties of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high or low temperature performance of the battery, and the like.

As an example, the additive includes, but is not limited to, at least one of fluoroethylene carbonate (FEC), vinylene Carbonate (VC), vinyl Ethylene Carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, ethylene Sulfite (ES), 1, 3-Propane Sultone (PS), 1, 3-Propane Sultone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris (trimethylsilane) phosphate (TMSP), tris (trimethylsilane) borate (TMSB).

The electrolyte may be prepared according to a conventional method in the art. For example, the organic solvent, electrolyte salt, and optional additives may be uniformly mixed to obtain the electrolyte. The order of addition of the materials is not particularly limited, and for example, electrolyte salt and optional additives are added into an organic solvent and mixed uniformly to obtain an electrolyte; or adding electrolyte salt into the organic solvent, and then adding optional additives into the organic solvent to be uniformly mixed to obtain the electrolyte.

Electronic equipment

In a third aspect, the present application provides an electronic device, comprising: the secondary battery according to any one of the embodiments of the second aspect.

According to the present application, since the electronic device includes the secondary battery of any one of the embodiments of the second aspect, the electronic device has the advantageous effects of the second aspect.

The electronic device of the present application is not particularly limited, and may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copier, a portable printer, a headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD-player, a mini-compact disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable audio recorder, a radio, a backup power source, a motor, an automobile, a motorcycle, a power assisted bicycle, a lighting fixture, a toy, a game machine, a clock, an electric tool, a flash light, a camera, a household large battery, a lithium ion capacitor, and the like.

Hereinafter, embodiments of the present application are described. The following examples are illustrative only and are not to be construed as limiting the application. The examples are not to be construed as limiting the specific techniques or conditions described in the literature in this field or as per the specifications of the product. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.

Preparation of carbon nanotube cluster dispersions：

Mixing a conventional carbon nanotube raw material consisting of carbon nanotube units having an average diameter of 2nm or more and an average length of 2 μm or more with hydrogenated nitrile rubber in NMP to obtain a mixed solution, wherein the solid content in the mixed solution is 1.5 to 20wt%, and the mass ratio of the conventional carbon nanotube raw material to the hydrogenated nitrile rubber is 1:0.1 to 10.

The mixed solution is added into a container containing sand grinding balls, the container is rotated to obtain a dispersion of carbon nanotube clusters, wherein the average diameter of the sand grinding balls can be 0.5mm to 2.5mm, the rotating speed of the container can be 500rpm to 6000rpm, and the ball milling time can be 0.5h to 2h.

The carbon nano tube cluster dispersoids with different specifications are obtained for standby by controlling the specifications of the conventional carbon nano tube raw materials and the ball milling conditions.

Testing of the charging capacity ratio of secondary cell 2C：

a. Fully placing the secondary battery at 25 ℃ and 0.5C multiplying power to a set value (LCO voltage is 3.0V for the positive electrode active material);

b. the battery in a is fully charged to the design voltage at the rate of 0.2C, then is charged to the cut-off current of 0.05C at a constant voltage of the design voltage, is kept stand for 5min, and is discharged to the set value at the current of 0.5C;

c. b, fully charging the battery in the step b to the designed voltage at the rate of 2C, and recording the charging capacity as C1 at the moment; then charging with a designed voltage constant voltage until the cut-off current is 0.05 ℃, standing for 5min, and recording the charging capacity in the whole process as C0;

d. The charge capacity of the secondary battery 2C was C1/C0X 100%.

Example 1-1

Preparation of a positive plate:

mixing lithium cobaltate, carbon nano tube clusters, conductive particles and a binder in a mass ratio of 97.6:0.5:0.5:1.4 in NMP to obtain positive electrode active slurry, wherein the average diameter D of the carbon nano tube clusters, the average length L and the average diameter D of carbon nano tube cluster units contained in the carbon nano tube clusters are shown in a table 1, the conductive particles are conductive carbon black, the particle diameter Dn80 of the cumulative 80% of the conductive particles is 0.5 mu m, and the binder is polyvinylidene fluoride with a weight average molecular weight of 1,000,000; coating the positive electrode active slurry on an aluminum foil, drying the aluminum foil at 95 ℃, cold pressing, cutting, slitting and drying for 4 hours under the vacuum condition at 85 ℃ to obtain the positive electrode plate.

Preparing a negative plate:

mixing an artificial graphite anode active material, a conductive agent Super P, a thickener sodium carboxymethyl cellulose (CMC-Na) and a binder Styrene Butadiene Rubber (SBR) in a mass ratio of 96.4:1.5:0.5:1.6 in deionized water to obtain anode active slurry, wherein the solid content of the anode active slurry is 54wt%; coating the negative electrode active slurry on a copper foil, drying the copper foil at 85 ℃, cold pressing, cutting, slitting and drying for 12 hours under the vacuum condition at 80 ℃ to obtain the negative electrode plate.

Preparation of electrolyte: conventional lithium hexafluorophosphate electrolyte with the concentration of 1.5mol/L is selected.

Preparation of a separation film: a 7 μm thick Polyethylene (PE) barrier film substrate was chosen to be coated with a 3 μm ceramic coating.

Preparation of a lithium ion battery: sequentially stacking the positive plate, the isolating film and the negative plate, enabling the isolating film to be positioned between the positive plate and the negative plate to play a role of isolation, and then winding to obtain a bare cell; and (3) placing the bare cell in an aluminum plastic film of an outer packaging foil after welding the tab, injecting the prepared electrolyte into the dried bare cell, and performing procedures such as vacuum packaging, standing, formation, shaping, capacity testing and the like to obtain the soft-package lithium ion battery.

The obtained charge capacity ratio of the lithium ion battery 2C was measured, and the cohesion of the positive electrode active material layer in the positive electrode sheet in the lithium ion battery and the maximum continuous length Y of the conductive composition in the positive electrode active material layer were measured, and the results are shown in table 1.

Examples 1-2 to 1-40 and comparative examples 1-1, comparative examples 1-2

The preparation of the positive plate, the negative plate, the separator, the electrolyte and the lithium ion battery are similar to those of example 1-1, except that: the average diameter D, the average length L, and the average diameter D of the carbon nanotube clusters and the average diameter Dn80 of the carbon nanotube cluster units contained therein, the particle diameter Dn80 of the conductive particles accumulated by 80%, and the weight average molecular weight Mw of the binder are different, see table 1 for details.

Comparative examples 1 to 3

The preparation of the positive plate, the negative plate, the separator, the electrolyte and the lithium ion battery are similar to those of example 1-1, except that: instead of carbon nanotube clusters, discrete carbon nanotubes with an average tube diameter of 10nm and an average tube length of 0.3 μm were used.

TABLE 1

Note that: in table 1 "\" indicates that this parameter is not contained.

According to table 1, the lithium ion batteries obtained in each example have a higher 2C charge capacity ratio than the comparative example, indicating that the battery provided by the application has good high-rate charge performance. In comparative example 1-1, the conductive particles used had too large a particle size, which was not conducive to the formation of a conductive composition with good conductivity, and was not conducive to the extension of the conductive composition in the positive electrode active material layer, and Y was small, which was not effective in reducing the internal resistance of the positive electrode sheet, and thus the high-rate charging performance of the obtained battery was poor. In comparative examples 1 to 2, the binder used had too small a weight average molecular weight and low adhesion, resulting in a smaller maximum continuous length Y of the conductive composition in the positive electrode active material layer, failing to effectively reduce the internal resistance of the positive electrode sheet, and thus the high-rate charging performance of the resulting battery was poor. In comparative examples 1 to 3, discrete carbon nanotubes were used instead of carbon nanotube clusters, which failed to form long-range conductive networks, and also failed to form the conductive composition of the present application, and the discrete carbon nanotubes and conductive particles each acted in the positive electrode active material layer, failed to form long and wide conductive paths, failed to effectively reduce the internal resistance of the positive electrode sheet, and further the high-rate charging performance of the obtained battery was poor.

As can be seen from comparative examples 1-1 to 1-10, the average length L of the carbon nanotube clusters has a certain influence on the high-rate charging performance of the battery, and the 2C charging capacity of the battery is relatively high and the high-rate charging performance of the battery is relatively good under the condition that L is not less than 3 μm; preferably, under the condition that L is more than or equal to 5 mu m and less than or equal to 30 mu m, the 2C charging capacity of the battery is higher in proportion, and the high-rate charging performance of the battery is better. Although the 2C charge capacity of the battery is higher by further increasing L, the degree of increase is not large and the cost is increased, so that L is more preferably 5 μm and L is more preferably 30 μm.

As is clear from comparative examples 1-11 to examples 1-15 and examples 1-6, the average diameter D of the carbon nanotube cluster has a certain influence on the high-rate charging performance of the battery, and under the condition that D > 0.2 μm, the 2C charging capacity of the battery occupies a relatively high proportion, and the high-rate charging performance of the battery is relatively good; preferably, under the condition that D is more than or equal to 0.5 mu m and less than or equal to 3 mu m, the 2C charging capacity of the battery is higher in proportion, and the high-rate charging performance of the battery is better. In the present application, the carbon nanotube cluster has a diameter of more than 0.2 μm, and in order to facilitate comparative analysis, in the practical experimental process, a carbon nanotube structure having a diameter of not less than 0.18 μm and formed by bundling and combining a plurality of carbon nanotube units is used as the carbon nanotube cluster for statistical analysis, wherein the long axes of the carbon nanotube units are combined in parallel with each other.

As can be seen from comparative examples 1-16 to examples 1-23 and examples 1-6, the average diameter d of the carbon nanotube units in the carbon nanotube cluster has a certain influence on the high-rate charging performance of the battery, and under the condition that d is not less than 3nm and not more than 40nm, the 2C charging capacity of the battery is relatively high, and the high-rate charging performance of the battery is relatively good; preferably, under the condition that d is less than or equal to 5nm and less than or equal to 20nm, the 2C charging capacity of the battery is higher in proportion, and the high-rate charging performance of the battery is better.

As is clear from comparative examples 1 to 24 to comparative examples 1 to 32 and examples 1 to 6, the particle diameter of the conductive particles has a certain influence on the high-rate charging performance of the battery, and under the condition that the particle diameter of 80% of the conductive particles does not exceed 2 μm, the maximum continuous length Y of the conductive composition in the positive electrode active material layer is longer, the conductivity of the conductive composition is better, a positive electrode sheet having a smaller resistance can be obtained, and a battery having a better high-rate charging performance can be obtained.

As is clear from comparative examples 1 to 33 to examples 1 to 40 and examples 1 to 6, the weight average molecular weight of the binder has a certain influence on the high-rate charge performance of the battery, and a longer maximum continuous length of the conductive composition in the positive electrode active material layer can be ensured under the condition that the weight average molecular weight of the binder is 300,000 to 3,000,000 to obtain a positive electrode sheet having a smaller resistance, resulting in a battery having a better high-rate charge performance.

The possible reasons for the above results are described in detail above and are not described here again.

Examples 2-1 to 2-10 and comparative examples 2-1, comparative examples 2-2

The preparation of the positive plate, the negative plate, the separator, the electrolyte and the lithium ion battery are similar to those of examples 1 to 6, except that: the positive electrode active material composition contains at least one component of different mass percentage and conductive particle type, and is specifically shown in table 2.

The obtained charge capacity ratio of the lithium ion battery 2C was measured, and the cohesion of the positive electrode active material layer in the positive electrode sheet in the lithium ion battery and the maximum continuous length Y of the conductive composition in the positive electrode active material layer were measured, and the results are shown in table 2.

TABLE 2

Note that: in table 2 "\" indicates that this parameter is not contained.

According to table 2, the lithium ion batteries obtained in the respective examples have better high-rate charging performance than the comparative examples. In comparative example 2-1, only the conductive particles were used as the conductive agent, a long-range conductive network could not be formed in the positive electrode active material layer, and the conductive composition of the present application could not be formed, a long and wide conductive network could not be obtained, and the resistance of the positive electrode sheet could not be effectively reduced, so that the high-rate charging performance of the battery was poor. In comparative example 2-2, only carbon nanotube clusters were used as the conductive agent, and although a long-range conductive network could be formed, the conductive path formed by the conductive composition provided by the present application was short and narrow, and the conductivity was inferior to that of the conductive composition, so that the high-rate charge performance of the resulting battery was poor.

As is clear from comparative examples 2-1 to 2-8, the mass percentage of each component in the positive electrode active material layer has a certain effect on the high-rate charging performance of the battery, and the content of each component can be appropriately adjusted so that the maximum continuous length of the conductive composition in the positive electrode active material layer is large, and a positive electrode sheet with small resistance is obtained, so that a battery with good high-rate charging performance is obtained.

As can be seen from comparative examples 1 to 6, examples 2 to 9 and examples 2 to 10, the use of different kinds of conductive particles can effectively reduce the resistance of the positive electrode sheet, and those skilled in the art can select according to actual needs.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present application, and not for limiting the same; although the application has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the application.

Claims

1. A positive electrode sheet, characterized by comprising a positive electrode active material layer including a positive electrode active material, conductive particles, carbon nanotube clusters, and a binder;

wherein the carbon nanotube cluster consists of a plurality of carbon nanotube units which are arranged in a bundle shape, and the diameter of the carbon nanotube cluster is larger than 0.2 mu m;

the conductive particles, the carbon nano tube clusters and the binder form a conductive composition in the gaps of the positive electrode active material, the conductive composition is an integral structure formed by bonding the carbon nano tube clusters and the conductive particles directly through the binder, and the maximum continuous length Y of the carbon nano tube clusters and the conductive particles in the conductive composition, which is bonded directly through the binder, is more than or equal to 5 mu m.

2. The positive electrode sheet according to claim 1, wherein 80% of the conductive particles have a particle diameter of not more than 2 μm.

3. The positive electrode sheet according to claim 1, wherein the conductive particles comprise metallic conductive particles and/or conductive carbon particles;

the metal conductive particles comprise one or more of simple substances Au, ag, al, ni, co, li, fe, cu.

4. The positive electrode sheet according to claim 1, wherein the carbon nanotube clusters satisfy at least one of the following conditions:

1) The average diameter d of the carbon nano tube unit is more than or equal to 3nm and less than or equal to 40nm;

2) The carbon nanotube unit is a multiwall carbon nanotube unit;

3) The average diameter D of the carbon nano tube clusters is more than 0.2 mu m;

4) The average length L of the carbon nano tube bundle is more than or equal to 3 mu m.

5. The positive electrode sheet according to claim 1, wherein the carbon nanotube clusters satisfy at least one of the following conditions:

5) The average diameter d of the carbon nano tube unit is more than or equal to 5nm and less than or equal to 20nm;

6) The average diameter D of the carbon nano tube bundle is more than or equal to 0.5 mu m and less than or equal to 3 mu m;

7) The average length L of the carbon nano tube bundle is more than or equal to 5 mu m and less than or equal to 30 mu m.

6. The positive electrode sheet according to claim 1, wherein the binder comprises a fibrous binder comprising one or more of polyacrylic acid, polyacrylate, polyamide-imide, polyvinyl alcohol, and carboxymethyl cellulose salt.

7. The positive electrode sheet according to claim 1, wherein the weight average molecular weight of the binder is 300,000 to 3,000,000.

8. The positive electrode sheet according to claim 1, wherein the cohesive force of the positive electrode active material layer is not less than 5N/m.

9. The positive electrode sheet according to claim 1, wherein the positive electrode sheet satisfies at least one of the following conditions:

8) The mass percentage of the positive electrode active material in the positive electrode active material layer is 89% to 99.3%;

9) The mass percentage of the conductive particles in the positive electrode active material layer is 0.1-3%;

10 The mass percentage of the carbon nano tube bundle in the positive electrode active material layer is 0.1 to 3 percent;

11 The mass percentage of the binder in the positive electrode active material layer is 0.5% to 5%.

10. A secondary battery, characterized by comprising: a negative electrode sheet, a separator, an electrolyte, and the positive electrode sheet according to any one of claims 1 to 9.

11. An electronic device, comprising: the secondary battery according to claim 10.