Negative electrode sheet, positive electrode sheet, and battery

Abstract

The application relates to the field of batteries, and discloses a negative electrode sheet, a positive electrode sheet and a battery. The negative electrode sheet comprises a negative electrode current collector and a negative electrode material layer coated on the surface of the negative electrode current collector, the negative electrode material layer comprises a negative electrode active material, a conductive agent and a binder, the mass proportion of the conductive agent in the negative electrode material layer is 1.5%-4%, the conductive agent comprises carbon nanotubes and conductive carbon black, the mass ratio of the carbon nanotubes to the conductive carbon black is 0.1-0.83, and the aspect ratio of the carbon nanotubes is greater than or equal to 10000. The negative electrode sheet can solve the problem of increased discharge voltage of the battery caused by the carbon nanotubes piercing the diaphragm.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a negative electrode sheet, a positive electrode sheet and a battery. BACKGROUND

[0002] In the current performance indicators of batteries, the rate of change of open-circuit voltage with storage time K is a core parameter for measuring the storage stability of the battery. Existing research shows that the abnormal K value of lithium batteries is mainly caused by defects in the negative electrode system. In the existing conductive agent of negative electrode material, carbon nanotubes are usually short and thick structures with a small aspect ratio, such as the aspect ratio of carbon nanotubes in the existing conductive agent, which is usually less than 1000, the length is generally 5-10 microns, and the tube diameter is generally 20-150 nm. The present inventors found in the research that carbon nanotubes of this size are prone to pierce the separator during the rolling process of the electrode sheet, and even pierce the separator, resulting in high self-discharge of the battery and high K value. If the use of carbon nanotubes is reduced, the internal resistance of the electrode sheet will increase, affecting the heat generation of the battery and increasing the risk of thermal runaway of the battery. SUMMARY

[0003] The present application discloses a negative electrode sheet, a positive electrode sheet and a battery to solve the problem of increased discharge voltage of the battery caused by carbon nanotubes piercing the separator in the existing negative electrode sheet.

[0004] In a first aspect, the present application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode material layer coated on the surface of the negative electrode current collector, the negative electrode material layer comprising a negative electrode active material, a conductive agent and a binder, the mass fraction of the conductive agent in the negative electrode material layer being 1.5%-4%.

[0005] The conductive agent comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 0.1-0.83; the aspect ratio of the carbon nanotubes is greater than or equal to 10,000.

[0006] The carbon nanotubes and the conductive carbon black in the negative electrode material layer of the negative electrode sheet of the present application can form a "linear-spherical" composite conductive network in the negative electrode material layer, and the filling effect of the conductive carbon black can reduce the contact probability of the carbon nanotubes and the separator, wherein the mass ratio of the carbon nanotubes to the conductive carbon black needs to be controlled within the range of 0.1-0.83 to prevent the increase of the internal resistance of the negative electrode sheet caused by the low content of the carbon nanotubes, and also to prevent the increase of the contact probability of the carbon nanotubes and the separator caused by the high content of the carbon nanotubes. In the present application, the aspect ratio of the carbon nanotubes is greater than or equal to 10,000, which can significantly reduce the internal stress of the negative electrode sheet and improve the flexibility of the carbon nanotubes, thereby avoiding the piercing of the separator by the carbon nanotubes. The negative electrode sheet of the present application uses the negative electrode material layer composed of the above components, which can not only improve the conductivity of the negative electrode sheet, but also reduce the self-discharge voltage of the battery and improve the stability of the battery.

[0007] In a second aspect, the application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode material layer coated on the surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a conductive agent and a binder, and the mass percentage of the conductive agent in the positive electrode material layer is 1.5% to 4%.

[0008] The conductive agent comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 0.1 to 0.83; and the aspect ratio of the carbon nanotubes is greater than or equal to 10,000.

[0009] In the positive electrode sheet of the application, the carbon nanotubes and the conductive carbon black in the positive electrode material layer can form a "linear-spherical" composite conductive network in the positive electrode material layer, and the contact probability of the carbon nanotubes with the separator can be reduced by the filling effect of the conductive carbon black. The mass ratio of the carbon nanotubes to the conductive carbon black needs to be controlled in the range of 0.1 to 0.83 to prevent the increase of the internal resistance of the positive electrode sheet caused by the too low content of the carbon nanotubes, and also to prevent the increase of the contact probability of the carbon nanotubes with the separator caused by the too high content of the carbon nanotubes. In the application, the aspect ratio of the carbon nanotubes is greater than or equal to 10,000, which can significantly reduce the internal stress of the positive electrode sheet and improve the flexibility of the carbon nanotubes, thereby avoiding the puncture of the separator by the carbon nanotubes. The positive electrode sheet of the application uses the positive electrode material layer with the above composition, which can improve the conductivity of the positive electrode sheet and also improve the stability of the battery.

[0010] In a third aspect, the application provides a battery, which comprises a negative electrode sheet, a separator, an electrolyte and a positive electrode sheet, wherein the negative electrode sheet is the negative electrode sheet of the application, and / or the positive electrode sheet is the positive electrode sheet of the application; the positive electrode sheet and the negative electrode sheet are oppositely arranged and stacked, the separator is arranged between the positive electrode sheet and the negative electrode sheet, the electrolyte immerses the positive electrode sheet, the separator and the negative electrode sheet, and the thickness of the separator is 7 to 20 μm.

[0011] The battery of the application has the characteristics of low self-discharge voltage, high stability, low internal resistance and low heat generation because it comprises the negative electrode sheet of the application. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 The structure schematic diagram of the negative electrode sheet provided by an embodiment of the application is shown.

[0013] Reference signs:

[0014] 10-negative electrode sheet; 11-negative electrode current collector; 12-negative electrode material layer; 20-separator;

[0015] 121-negative electrode active material; 122-carbon nanotube; 123-conductive carbon black. DETAILED DESCRIPTION

[0016] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0017] It should be noted that: in the present application, all the embodiments and preferred implementation methods mentioned in the present application can be combined to form new technical solutions, unless otherwise specified. In the present application, all the technical features and preferred features mentioned in the present application can be combined to form new technical solutions, unless otherwise specified. In the present application, unless otherwise specified, percentage (%) or part refers to the percentage by weight or weight part of the composition. In the present application, unless otherwise specified, each component or its preferred component can be combined to form a new technical solution. In the present application, unless otherwise specified, the numerical range "a~b" represents a shortcut of any real number combination between a and b, where a and b are real numbers. For example, the numerical range "6~22" represents that all real numbers between "6~22" have been listed in the present application, and "6~22" is only a shortcut of these numerical combinations. The lower limit and upper limit of the range disclosed in the present application can be one or more lower limits and one or more upper limits, respectively. In the present application, unless otherwise specified, each reaction or operation step can be performed sequentially or according to the sequence. Preferably, the reaction method in the present application is performed sequentially.

[0018] Unless otherwise specified, the professional and scientific terms used in the present application have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to the described content can also be applied in the present application.

[0019] In the negative plate of the existing battery, carbon nanotubes are usually added as conductive agents. The inventors of the present application found in practical application that the open circuit voltage of the existing battery has a high change rate K with the standing time, which affects the cycle performance, safety performance and capacity of the battery. Through research, it is found that the existing carbon nanotubes have a short and thick structure, such as a length-diameter ratio less than 1000, a tube diameter generally of 20~150nm, and a length generally of 5~10 microns. The inventors of the present application found that the carbon nanotubes with the above-mentioned size will gradually pierce into the separator during the charging and discharging process of the battery, resulting in a larger self-discharge voltage and thus deteriorating the performance of the battery.

[0020] Based on this, the embodiments of the present application provide a negative plate. Figure 1 is a structural schematic diagram of the negative plate of an embodiment. As shown in Figure 1As shown, the negative electrode sheet 10 includes a negative electrode current collector 11 and a negative electrode material layer 12 coated on the surface of the negative electrode current collector 11, the negative electrode material layer 12 including a negative electrode active material 121, a conductive agent, and a binder (not shown in the figure). Among them, the negative electrode material layer 12 is used to contact the separator 20.

[0021] The negative electrode current collector can be a metal foil. The negative electrode current collector is an electrically conductive base material for collecting and conducting electrons generated by the electrode, including but not limited to copper foil, carbon-coated copper foil, etc.

[0022] The negative electrode active material in the negative electrode material layer participates in the negative electrode electrochemical reaction, can realize the processes of intercalation and deintercalation of active ions such as lithium ions, or deposition and dissolution, etc., and is accompanied by electron transfer, thereby being a key substance for storing and releasing electric energy. The negative electrode active material includes but is not limited to carbon materials, silicon-carbon materials, etc. In the embodiments of the present application, the negative electrode active material is a carbon material, which includes at least one of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, or fibrous carbon, as an exemplary illustration.

[0023] In the embodiments of the present application, the mass fraction of the negative electrode active material in the negative electrode material layer can be, for example, 90% to 95%. Exemplarily, the mass fraction of the negative electrode active material in the negative electrode material layer can be, for example, 90%, 91%, 92%, 93%, 94%, or 95%, or any value between any two of the above values.

[0024] The binder is a high-molecular material with adhesion, which plays a key role in firmly bonding different components such as the negative electrode active material and the conductive agent in the negative electrode material layer, and tightly attaching the entire negative electrode material layer to the surface of the negative electrode current collector, forming a negative electrode sheet with stable structure. In the embodiments of the present application, the binder includes but is not limited to polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), polymerized styrene butadiene rubber (SBR), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), etc.

[0025] In an embodiment, the binder comprises at least one of styrene butadiene rubber, polyacrylic acid or carboxymethyl cellulose. When the binder contains styrene butadiene rubber, the weight average molecular weight of the styrene butadiene rubber is 100000-500000, and the polymerization degree of the styrene butadiene rubber is 350-10000. When the binder contains polyacrylic acid, the weight average molecular weight of the polyacrylic acid is 100000-500000, and the polymerization degree of the polyacrylic acid is 1000-7000. When the binder contains carboxymethyl cellulose, the weight average molecular weight of the carboxymethyl cellulose is 100000-1000000, and the polymerization degree of the carboxymethyl cellulose is 400-4000.

[0026] In an embodiment, the mass percentage of the binder in the negative electrode material layer is, for example, 2-4%, and exemplarily, for example, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8% or 4% or any value between any two of the above values.

[0027] The conductive agent is a material with good electronic conductivity, and mainly functions to build a continuous conductive network in the electrode and improve the electronic conductivity of the electrode.

[0028] Continuing to refer to Figure 1 In an embodiment, the mass percentage of the conductive agent in the negative electrode material layer 12 is 1.5%-4%. The conductive agent comprises carbon nanotubes 122 and conductive carbon black 123, and the mass ratio of the carbon nanotubes 122 to the conductive carbon black 123 is 0.1-0.83. The aspect ratio of the carbon nanotubes 122 is greater than or equal to 10000.

[0029] The mass percentage of the conductive agent in the negative electrode material layer is controlled in the range of 1.5%-4%, which can help improve the conductivity of the negative electrode sheet and avoid affecting the capacity of the negative electrode sheet. When the mass ratio of the carbon nanotubes to the conductive carbon black is in the range of 0.1-0.83, the carbon nanotubes can be used to fully connect the conductive agent and form a point-like and line-like conductive network to improve the conductivity of the negative electrode sheet. Meanwhile, the conductive carbon black can be used to separate the carbon nanotubes from the separator and reduce the contact probability of the carbon nanotubes with the separator. The aspect ratio of the carbon nanotubes is controlled to be greater than or equal to 10000, which can improve the flexibility of the carbon nanotubes and reduce the stress concentration when the carbon nanotubes are stressed, thereby reducing the probability of the carbon nanotubes piercing the separator.

[0030] Exemplarily, in an embodiment, the mass percentage of the conductive agent in the negative electrode material layer is, for example, 1.5%, 1.7%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%, 3.2%, 3.5%, 3.8% or 4% or any value between any two of the above values.

[0031] Exemplarily, in the embodiments of the present application, the mass ratio of the carbon nanotubes to the conductive carbon black may be, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.83, or a value between any two of the above values.

[0032] In the embodiments of the present application, the mass percentage of the carbon nanotubes in the negative electrode material layer is 0.3% to 1.0%, and the mass percentage of the conductive carbon black in the negative electrode material layer is 1.2% to 3.0%. The carbon nanotubes with the above-mentioned ratio can connect multiple conductive carbon black particles as a bridging structure, thereby improving the conductivity efficiency. Exemplarily, the mass percentage of the conductive carbon black in the negative electrode material layer may be, for example, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, or 3.0%, or a value between any two of the above values.

[0033] The carbon nanotubes and the conductive carbon black in the negative electrode material layer of the negative electrode sheet of the present application can form a "linear-spherical" composite conductive network in the negative electrode material layer, and the filling effect of the conductive carbon black can reduce the contact probability of the carbon nanotubes and the separator. The mass ratio of the carbon nanotubes to the conductive carbon black needs to be controlled within the range of 0.1 to 0.83 to prevent the increase of the internal resistance of the negative electrode sheet caused by the too low content of the carbon nanotubes, and also to prevent the increase of the contact probability of the carbon nanotubes and the separator caused by the too high content of the carbon nanotubes. In the present application, the aspect ratio of the carbon nanotubes is greater than or equal to 10,000, which can significantly reduce the internal stress of the negative electrode sheet and improve the flexibility of the carbon nanotubes, thereby avoiding the puncture of the separator by the carbon nanotubes. The negative electrode material layer with the above-mentioned composition used in the negative electrode sheet of the present application can improve the conductivity of the negative electrode sheet, and also can reduce the self-discharge voltage of the battery and improve the stability of the battery.

[0034] In an embodiment of the present application, the tube diameter of the carbon nanotubes is 1 to 2 nm, and the length of the carbon nanotubes is 15 to 60 μm. The tube diameter and the length of the carbon nanotubes are controlled within the above-mentioned ranges, and the overall structure of the carbon nanotubes presents an elongated structure, thereby further improving the flexibility, reducing the high stress sites, and reducing the probability of the carbon nanotubes puncturing the separator and the self-discharge voltage of the battery. Exemplarily, the tube diameter of the carbon nanotubes may be, for example, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, or 2 nm, or a value between any two of the above values. The length of the carbon nanotubes may be, for example, 15 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, or a value between any two of the above values.

[0035] In an embodiment of the present application, the carbon nanotubes are single-walled carbon nanotubes. Single-walled carbon nanotubes have better flexibility, which can help reduce the probability of carbon nanotubes piercing the separator.

[0036] In an embodiment of the present application, the specific surface area of the single-walled carbon nanotubes is 240-310 m 2 / g. Controlling the specific surface area of the single-walled carbon nanotubes within the range of 240-310 m 2 / g can increase the contact area of the single-walled carbon nanotubes with the conductive carbon black, thereby improving the conductivity of the negative electrode material layer and helping to achieve the preparation of high-power batteries.

[0037] In an embodiment of the present application, the particle size of the conductive carbon black is 20-40 nm, and the specific surface area of the conductive carbon black is 60-100 m 2 / g. If the particle size of the conductive carbon black is too small, it will affect the dispersibility of the conductive carbon black, thereby reducing the electrical performance of the negative electrode material layer. If the particle size of the conductive carbon black is too large, it will cause the reduction of point contact between particles, the decrease of conductivity, and the increase of direct current resistance (DCR) of the battery. If the specific surface area of the conductive carbon black is too small, it will affect the contact area with the negative electrode active material. If the specific surface area of the conductive carbon black is too large, the surface energy will increase, the particles will easily agglomerate and be difficult to disperse, and the battery cannot be normally coated, thereby increasing the manufacturing cost. Therefore, by optimizing and controlling the particle size and specific surface area of the conductive carbon black, the particle size and specific surface area of the conductive carbon black can be matched with the size of the carbon nanotubes, which can reduce the manufacturing cost while reducing the K value and DCR of the battery and improving the electrical performance of the battery.

[0038] In an embodiment of the present application, the D50 particle size of the carbon material is 4-20 μm. The negative electrode active material is a carbon material. Controlling the D50 particle size of the carbon material within the range of 4-20 μm can match the size of the carbon nanotubes and the particle size and specific surface area of the conductive carbon black, thereby improving the electron conduction ability. In an embodiment of the present application, the compaction density of the negative electrode material layer in the negative electrode sheet is 1.2-2 g / cm 3 By optimizing the compaction density of the negative electrode material layer, the ion transmission channel can be ensured, and the structure of the active material can be prevented from being damaged due to excessive compaction.

[0039] Based on the same technical purpose, an embodiment of the present application also provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode material layer coated on the surface of the positive electrode current collector. The positive electrode material layer comprises a positive electrode active material, a conductive agent, and a binder. The mass ratio of the conductive agent in the positive electrode material layer is 1.5%-4%. The conductive agent comprises carbon nanotubes and conductive carbon black. The mass ratio of the carbon nanotubes to the conductive carbon black is 0.1-0.83. The aspect ratio of the carbon nanotubes is greater than or equal to 10,000.

[0040] The positive current collector is an electrically conductive substrate for collecting and conducting electrons generated by the electrode, including but not limited to aluminum foil, carbon-coated aluminum foil, etc.

[0041] In the positive electrode material layer, the positive active material participates in the positive electrode electrochemical reaction, can realize active ion such as lithium ion intercalation / deintercalation, or deposition / dissolution, etc., and is accompanied by electron transfer, thereby storing and releasing electric energy. The positive active material includes but is not limited to lithium cobaltate (LiCoO2), lithium iron phosphate (LiFePO4), ternary material (such as LiNiCoMnO2), etc. The binder is a high molecular material with adhesion, and its core function is to firmly bond different components such as the positive active material, the conductive agent, etc. in the positive electrode together, and at the same time, tightly attach the entire positive electrode material layer to the surface of the positive current collector, forming a positive electrode sheet with stable structure, including but not limited to polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc. The conductive agent is a material with good electronic conductivity, and its main function is to build a continuous conductive network inside the electrode, improve the electronic conductivity of the electrode, including but not limited to conductive carbon black, carbon nanotube, graphene, etc.

[0042] In the embodiments of the present application, the mass fraction of the positive active material in the positive electrode material layer may be, for example, 85% to 95%. For example, the mass fraction of the positive active material in the positive electrode material layer may be, for example, 85%, 88%, 90%, 91%, 92%, 93%, 94% or 95%, or any value between any two of the above values.

[0043] The binder is a high molecular material with adhesion, and its core function is to firmly bond different components such as the positive active material, the conductive agent, etc. in the positive electrode material layer together, and at the same time, tightly attach the entire positive electrode material layer to the surface of the positive current collector, forming a positive electrode sheet with stable structure. In the embodiments of the present application, the binder includes but is not limited to polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), polymerized styrene butadiene rubber (SBR), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), etc.

[0044] In an embodiment, the binder comprises at least one of styrene butadiene rubber, polyacrylic acid or carboxymethyl cellulose. When the binder contains styrene butadiene rubber, the weight average molecular weight of the styrene butadiene rubber is 100000-500000, and the polymerization degree of the styrene butadiene rubber is 350-10000. When the binder contains polyacrylic acid, the weight average molecular weight of the polyacrylic acid is 100000-500000, and the polymerization degree of the polyacrylic acid is 1000-7000. When the binder contains carboxymethyl cellulose, the weight average molecular weight of the carboxymethyl cellulose is 100000-1000000, and the polymerization degree of the carboxymethyl cellulose is 400-4000.

[0045] In an embodiment, the mass percentage of the binder in the positive electrode active material layer may, for example, be 2-4%, and may, for example, be 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8% or 4% or any value between any two of the foregoing values.

[0046] The conductive agent is a material with good electronic conductivity, and mainly functions to build a continuous conductive network in the electrode and improve the electronic conductivity of the electrode.

[0047] In an embodiment of the present application, the mass percentage of the conductive agent in the positive electrode material layer is 1.5%-4%. The conductive agent comprises carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 0.1-0.83. The aspect ratio of the carbon nanotubes is greater than or equal to 10000.

[0048] The mass percentage of the conductive agent in the positive electrode material is controlled to be in the range of 1.5%-4%, which can help to improve the conductivity of the positive electrode sheet and also avoid affecting the capacity of the positive electrode sheet. When the mass ratio of the carbon nanotubes to the conductive carbon black is in the range of 0.1-0.83, the carbon nanotubes can be used to fully connect the conductive agent, and the point-like and line-like conductive networks formed can improve the conductivity of the positive electrode sheet. Meanwhile, the conductive carbon black can be used to separate the carbon nanotubes from the separator, thereby reducing the contact probability of the carbon nanotubes with the separator. The aspect ratio of the carbon nanotubes is controlled to be greater than or equal to 10000, which can improve the flexibility of the carbon nanotubes and reduce the stress concentration when the carbon nanotubes are stressed, thereby reducing the probability of the carbon nanotubes penetrating into the separator.

[0049] For example, in an embodiment of the present application, the mass percentage of the conductive agent in the positive electrode material layer may, for example, be 1.5%, 1.7%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%, 3.2%, 3.5%, 3.8% or 4% or any value between any two of the foregoing values.

[0050] Exemplarily, in the embodiments of the present application, the mass ratio of the carbon nanotubes to the conductive carbon black may be, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.83, or a value between any two of the above values.

[0051] In the embodiments of the present application, the mass percentage of the carbon nanotubes in the positive electrode material layer is 0.3% to 1.0%, and the mass percentage of the conductive carbon black in the positive electrode material layer is 1.2% to 3.0%. The carbon nanotubes with the above-mentioned ratio can connect multiple conductive carbon black particles as a bridging structure, thereby improving the conductivity efficiency. Exemplarily, the mass percentage of the conductive carbon black in the positive electrode material layer may be, for example, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, or 3.0%, or a value between any two of the above values.

[0052] The positive electrode sheet of the present application can form a "linear-spherical" composite conductive network in the positive electrode material layer by using the carbon nanotubes and the conductive carbon black, and reduce the contact probability of the carbon nanotubes and the separator by using the filling effect of the conductive carbon black. The mass ratio of the carbon nanotubes to the conductive carbon black needs to be controlled in the range of 0.1 to 0.83 to prevent the increase of the internal resistance of the positive electrode sheet caused by the too low content of the carbon nanotubes, and also prevent the increase of the contact probability of the carbon nanotubes and the separator caused by the too high content of the carbon nanotubes. In the present application, the aspect ratio of the carbon nanotubes is greater than or equal to 10,000, which can significantly reduce the internal stress of the positive electrode sheet and improve the flexibility of the carbon nanotubes, thereby avoiding the piercing of the separator by the carbon nanotubes. The positive electrode sheet of the present application uses the positive electrode material layer with the above-mentioned composition, which can improve the conductivity of the positive electrode sheet and also improve the stability of the battery.

[0053] In an embodiment of the present application, the tube diameter of the carbon nanotubes is 1 to 2 nm, and the length of the carbon nanotubes is 15 to 60 μm. The tube diameter and the length of the carbon nanotubes are controlled in the above-mentioned ranges, and the overall structure of the carbon nanotubes presents an elongated structure, thereby further improving the flexibility, reducing the high stress sites, and reducing the probability of the piercing of the separator by the carbon nanotubes and the self-discharge voltage of the battery. Exemplarily, the tube diameter of the carbon nanotubes may be, for example, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, 2 nm, or a value between any two of the above values. The length of the carbon nanotubes may be, for example, 15 μm, 20 μm, 30 μm, 40 μm, 50 μm, or 60 μm, or a value between any two of the above values.

[0054] In an embodiment of the present application, the carbon nanotubes are single-walled carbon nanotubes. The single-walled carbon nanotubes have better flexibility, which can help reduce the probability of the carbon nanotubes piercing the separator.

[0055] In an embodiment of the present application, the specific surface area of the single-walled carbon nanotubes is 240-310 m 2 / g. Controlling the specific surface area of the single-walled carbon nanotubes within the range of 240-310 m 2 / g can increase the contact area of the single-walled carbon nanotubes with the conductive carbon black, thereby improving the conductivity of the positive electrode material layer and helping to achieve the preparation of high-power batteries.

[0056] In an embodiment of the present application, the particle size of the conductive carbon black is 20-40 nm, and the specific surface area of the conductive carbon black is 60-100 m 2 / g. If the particle size of the conductive carbon black is too small, the dispersibility of the conductive carbon black will be affected, thereby reducing the electrical performance of the positive electrode material layer. If the particle size of the conductive carbon black is too large, the point contact between the particles will decrease, the conductivity will decrease, and the direct current resistance (DCR) of the battery will increase. If the specific surface area of the conductive carbon black is too small, the contact area with the positive electrode active material will be affected. If the specific surface area of the conductive carbon black is too large, the surface energy will increase, the particles will easily agglomerate and be difficult to disperse, and the conductive carbon black cannot be normally coated, thereby increasing the manufacturing cost. Therefore, by optimizing and controlling the particle size and specific surface area of the conductive carbon black, the particle size and specific surface area of the conductive carbon black can be matched with the size of the carbon nanotubes, which can reduce the manufacturing cost, reduce the K value and DCR of the battery, and improve the electrical performance of the battery.

[0057] Based on the same technical purpose, the present application also provides a battery. The battery of the present application comprises a negative electrode sheet, a separator, an electrolyte, and a positive electrode sheet. The negative electrode sheet is the negative electrode sheet of the above-mentioned embodiments of the present application, and / or the positive electrode sheet is the positive electrode sheet of the above-mentioned embodiments of the present application. The positive electrode sheet and the negative electrode sheet are oppositely arranged and stacked, the separator is arranged between the positive electrode sheet and the negative electrode sheet, the electrolyte immerses the positive electrode sheet, the separator, and the negative electrode sheet, and the thickness of the separator is 7-20 μm.

[0058] For example, in an embodiment of the present application, the thickness of the separator is 9 μm, wherein the thickness of the separator substrate can be 7 μm, and the thickness of the coating layers on both sides of the separator substrate can each be 1 μm. In another embodiment of the present application, the thickness of the separator is 14 μm, wherein the thickness of the separator substrate can be 9 μm, the thickness of the coating layer on one side of the separator substrate can be 3 μm, and the thickness of the coating layer on the other side of the separator substrate can be 2 μm.

[0059] In an embodiment of the present application, the relationship between the thickness d1 of the diaphragm, the length L of the carbon nanotube, and the thickness d2 of the negative electrode material layer satisfies: d1 / (L-d2) is greater than 0.4, for example, greater than or equal to 0.5, greater than or equal to 0.6, or greater than or equal to 0.8. In an embodiment, d1 / (L-d2) can be less than or equal to 10, or less than or equal to 6, or less than or equal to 4. The thickness d2 of the negative electrode material layer is the thickness of the single-layer negative electrode material layer on one side surface of the negative electrode current collector.

[0060] The battery of the embodiment of the present application can obtain a battery with high capacity and fast charging and discharging speed by adding single-wall carbon nanotubes with a high aspect ratio and reducing the thickness of the negative electrode material layer and the thickness of the diaphragm, so as to meet the development of future 3C and power batteries. When the thickness of the negative electrode material layer is too thin, the amount of carbon nanotubes exposed on the surface of the negative electrode sheet increases, and if the diaphragm is also too thin, the self-discharge (K value) of the battery will increase. In the embodiment of the present application, the thickness of the negative electrode material layer, the thickness of the diaphragm, and the length of the carbon nanotube are comprehensively set to meet the requirements of high capacity and fast charging while reducing the self-discharge speed of the battery, such as reducing the K value of the battery.

[0061] The electrolyte in the battery of the embodiment of the present application is filled between the positive electrode sheet and the negative electrode sheet and infiltrates the diaphragm. The electrolyte includes an electrolyte and a solvent. The electrolyte can be a lithium salt, such as lithium hexafluorophosphate. The solvent can be a mixture of diethyl carbonate, methyl ethyl carbonate, and ethylene carbonate mixed at a volume ratio of 1:1:3. The concentration of the electrolyte can be 0.8-1.5 mol / L.

[0062] In an embodiment, the battery liquid retention amount is controlled to be 5-15 g / Ah. Controlling the battery liquid retention amount to be in the range of 5-15 g / Ah can ensure the ion conduction rate while avoiding excessive electrolyte, which can reduce the energy density of the battery and pose a safety risk. For example, the battery liquid retention amount can be 5 g / Ah, 6 g / Ah, 7 g / Ah, 8 g / Ah, 9 g / Ah, 10 g / Ah, 11 g / Ah, 12 g / Ah, 13 g / Ah, 14 g / Ah, 15 g / Ah, or any value between any two of the above values.

[0063] The battery of the embodiment of the present application will be specifically explained in combination with specific embodiments and comparative examples.

[0064] Embodiment 1

[0065] This embodiment is a high-energy-density lithium-ion power battery, and the manufacturing process is as follows:

[0066] S1, Preparation of the positive electrode sheet: LiFePO4 is used as the positive electrode active material, PVDF (polyvinylidene fluoride) is used as the binder, and SP (conductive carbon black) is used as the conductive agent in this embodiment; LiFePO4, PVDF, and SP are poured into a mixing kettle in a ratio of 93%:3.0%:4%, and after dry mixing at a speed of 30 RPM for 30 minutes, an appropriate amount of N-methylpyrrolidone (NMP) is added to adjust the solid content to about 70%, and then stirring and kneading are continued at a speed of 20 RPM for 2 hours; after the completion of the kneading, an appropriate amount of NMP is added to adjust the solid content to 65%, and then dispersion is started at a shear speed of 6 m / s, stirring is started at 30 RPM, and after 45 minutes of continuous stirring, the slurry is further added with an appropriate amount of NMP to adjust the solid content to about 60%, dispersion is started at a shear speed of 8 m / s, stirring is started at 30 RPM, and vacuum is applied to ≤-0.085 MPa, and dispersion is continued for 30 minutes, so that the required slurry is prepared; three-stage addition of NMP can make the substances in the slurry fully dispersed and mixed; the required slurry is uniformly coated on an aluminum current collector by a coating machine, and the coating surface density is controlled to be 180 g / m 2 ; a positive electrode sheet with a compacted density of 2.0 g / cm 3 is prepared by rolling, and then cut with a special mold to obtain the corresponding positive electrode sheet.

[0067] S2, Preparation of the negative electrode sheet: graphite is used as the negative electrode active material, SBR (styrene butadiene rubber), PAA (polyacrylic acid), and CMC (carboxymethyl cellulose) are used as the binder (SBR:PAA:CMC=2:1:1), and SP (conductive carbon black) and CNT-B (single-walled carbon nanotube, parameters as shown in Table 1) are used as the conductive agent; the proportions of graphite, SBR, PAA, CMC, SP, and CNT-B in the formula are 93.0%:2.0%:1.0%:1.0%:2.3%:0.7%, and the specific steps are as follows: graphite, CMC, and SP are poured into a mixing kettle in the above proportions, and dry mixing is performed at a speed of 30 RPM for 30 minutes; then a fixed proportion of PAA and CNT-B and an appropriate amount of water are added, the solid content is adjusted to about 65%, and then stirring and kneading are continued at a speed of 20 RPM for 2 hours; after the completion of the kneading, an appropriate amount of water is added to adjust the solid content to 55%, and then dispersion is started at a shear speed of 7 m / s, stirring is started at 30 RPM, and continuous stirring is continued for 45 minutes, then SBR and an appropriate amount of water are added, the solid content is adjusted to 50%, dispersion is started at a shear speed of 6 m / s, stirring is started at 30 RPM, and vacuum is applied to ≤-0.085 MPa, and dispersion is continued for 30 minutes, so that the required slurry is prepared; three-stage addition of water can make the substances in the slurry fully dispersed and mixed; the required slurry is uniformly coated on a copper current collector by a coating machine, and the coating surface density is controlled to be 90 g / m 2 ; a negative electrode sheet with a compacted density of 1.3 g / cm 3The negative electrode sheet is cut by a specific die to obtain a corresponding negative electrode sheet.

[0068] S3, battery preparation: the positive electrode sheet and the negative electrode sheet are stacked in a Z shape to form an electric core, and after welding the positive and negative electrode tabs, the electric core is dried by hot pressing and then packaged in an aluminum plastic film. After injection, standing, formation, and constant volume, a soft package power battery is obtained. The electrolyte is a mixture of diethyl carbonate, methyl ethyl carbonate, and ethylene carbonate in a ratio of 1:1:3 as a solvent, lithium salt is lithium hexafluorophosphate, the concentration is 1.0 mol / L, and the liquid retention amount is controlled at 8 g / Ah.

[0069] Examples 2-10 and Comparative Examples 1-2 are each a battery, and the manufacturing process can refer to the specific manufacturing process of Example 1. The composition of the conductive agent in each example and comparative example is listed in Table 1. In Examples 1-10, the carbon nanotubes are single-walled carbon nanotubes, and in Comparative Example 1, the carbon nanotubes are multi-walled carbon nanotubes. In Comparative Example 2, the conductive agent contains only SP without carbon nanotubes, and the content is 3%.

[0070] Table 1

[0071]

[0072] Note: In Table 1, the thickness of the separator is 9+3+2, which means that the thickness of the separator substrate is 9 μm, and the thickness of one side of the coating layer on both sides of the separator substrate is 3 μm, and the thickness of the other side of the coating layer is 2 μm. The thickness of the separator is 7+1+1, which means that the thickness of the separator substrate is 7 μm, and the thickness of one side of the coating layer on both sides of the separator substrate is 1 μm, and the thickness of the other side of the coating layer is 1 μm.

[0073] The alternating current resistance (ACR) value and open circuit voltage of each example and comparative example battery were tested, and the K value of the battery was calculated. The number of test samples of each example and comparative example battery is 10. The test results are listed in Table 2.

[0074] The battery is tested by the blue light test system CT3002A at 1C, and the discharge voltage is the ratio of the total discharge power and discharge current. After constant volume, the battery is adjusted to 20% SOC, and the BT3562 battery resistance tester is used to record the open circuit voltage OCV and alternating current resistance ACR value. The voltage is tested every 1 day (i.e. 24H), and the cumulative test is 10 days (240H). K value=(OCV3-OCV 10 ) / 168 (i.e. the difference between the open circuit voltage on the third day and the open circuit voltage on the tenth day divided by the time interval); ACR (unit: mΩ), discharge voltage (unit: V), and K value (unit: mV / h) are the average values of 10 batteries.

[0075] Table 2

[0076]

[0077] From the test data in Table 2, the ACR of the batteries of Examples 1-10 is lower than that of Comparative Examples 1 and 2, and the discharge voltage is higher than that of Comparative Examples 1 and 2, and the K value of Examples 1-10 is also lower than that of Comparative Example 1. Among them, in the battery of Comparative Example 1, because the aspect ratio of the carbon nanotube is low, the internal resistance and the K value of the battery are high. In the battery of Comparative Example 2, because no carbon nanotube is added, the internal resistance ACR is high, and the discharge voltage is low.

[0078] From the above comparison, the battery of the present application can have the following advantages:

[0079] K value control double mechanism: by adding high aspect ratio single-walled carbon nanotubes to reduce micro-short circuit, and by combining single-walled carbon nanotubes with conductive carbon black to optimize the conductive network, the K value of Example 5 is reduced by more than 25% compared with the low aspect ratio multi-walled carbon nanotube scheme of Comparative Example 1.

[0080] Performance synergistic improvement: the ACR of the battery of Example 5 is reduced by more than 10% compared with the battery of Comparative Example 1, and the discharge voltage is increased by more than 1.5%, and compared with Comparative Example 2 without adding carbon nanotubes, the ACR is reduced by more than 13%, and the discharge voltage is increased by more than 6%, achieving low internal resistance-high uniformity at the same time.

[0081] Strong process compatibility: no new equipment is needed, only the type and ratio of conductive agent need to be adjusted, which is suitable for existing lithium battery production lines.

[0082] Examples 11-19 and Comparative Examples 3-8

[0083] Examples 11-19 and Comparative Examples 3-8 are each a battery, and the K values of the batteries under different thicknesses of the separator, different lengths of the carbon nanotube, and different thicknesses of the negative electrode material layer are compared. The specific composition is listed in Table 3. Among them, the negative electrode sheet material composition, the positive electrode sheet material composition, the separator, and the material composition of the electrolyte of the batteries of Examples 11-19 and Comparative Examples 3-8 are the same, and the differences are listed in Table 3.

[0084] Table 3

[0085]

[0086] As shown in Table 3, d1 / (L-d2) is denoted by a, when a≤0.4, the measured battery K value will increase sharply. Taking the carbon nanotube with a length of 60 μm as an example, when a>0.4, such as when a is greater than 0.5, the setting ratio of the thickness of the separator and the thickness of the negative electrode material layer can ensure that the carbon nanotube forms a continuous conductive path on the surface or inside of the negative electrode material layer (supports large current transmission), and can also avoid the negative electrode material layer and the separator being too thin, which causes the carbon nanotube to penetrate the separator, resulting in an increase in the battery self-discharge (K value). If a≤0.4, the thickness of the negative electrode material layer and the thickness of the separator cannot be selected to reduce the effect of the battery self-discharge. Among them, the thickness of the negative electrode material layer and the thickness of the separator are too low, which increases the probability of the carbon nanotube penetrating the separator, resulting in an increase in the K value.

[0087] In summary, in the design of the high-capacity high-rate battery "thin negative electrode material layer, low thickness of the separator", the relationship between the thickness of the negative electrode material layer and the separator and the length of the carbon nanotube can be used to set the battery, reduce the battery self-discharge, and thus ensure the high-rate performance while optimizing the capacity retention and cycle stability.

[0088] Obviously, those skilled in the art can make various modifications and variations to the embodiments of the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application belong to the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these modifications and variations.

Claims

1. A battery, characterized in that, Includes negative electrode, separator, electrolyte and positive electrode; The positive electrode and the negative electrode are arranged opposite to each other and stacked, the separator is disposed between the positive electrode and the negative electrode, the electrolyte is immersed in the positive electrode, the separator and the negative electrode, and the thickness of the separator is 7~20μm; The negative electrode sheet includes a negative electrode current collector and a negative electrode material layer coated on the surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material, a conductive agent, and a binder. The conductive agent accounts for 1.5% to 4% of the mass of the negative electrode material layer. The conductive agent comprises carbon nanotubes and conductive carbon black, wherein the mass ratio of carbon nanotubes to conductive carbon black is 0.1 to 0.83; and the aspect ratio of the carbon nanotubes is greater than or equal to 10000. The relationship between the thickness d1 of the diaphragm, the length L of the carbon nanotube, and the thickness d2 of the negative electrode material layer satisfies: d1 / (L-d2) is greater than 0.4; The carbon nanotubes have a diameter of 1-2 nm and a length of 15-60 μm.

2. The battery according to claim 1, characterized in that, The carbon nanotubes are single-walled carbon nanotubes.

3. The battery according to claim 2, characterized in that, The specific surface area of ​​the single-walled carbon nanotubes is 240~310 m². 2 / g.

4. The battery according to any one of claims 1-3, characterized in that, The conductive carbon black has a particle size of 20-40 nm and a specific surface area of ​​60-100 m². 2 / g.

5. The battery according to claim 4, characterized in that, The conductivity of the conductive carbon black is 10. 2 ~10 3 S / m.

6. The battery according to any one of claims 1-3, characterized in that, In the negative electrode sheet, the compaction density of the negative electrode material layer is 1.2~2 g / cm³. 3 .

7. The battery according to any one of claims 1-3, characterized in that, The negative electrode active material is a carbon material with a particle size of 4~20μm.

8. The battery according to claim 7, characterized in that, The carbon material includes at least one of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, or fibrous carbon.

9. The battery according to any one of claims 1-3, characterized in that, The carbon nanotubes account for 0.3% to 1.0% of the mass of the negative electrode material layer, and the conductive carbon black accounts for 1.2% to 3.0% of the mass of the negative electrode material layer.

10. The battery according to any one of claims 1-3, characterized in that, The positive electrode sheet includes a positive current collector and a positive electrode material layer coated on the surface of the positive current collector. The positive electrode material layer includes a positive electrode active material, a conductive agent, and a binder. The conductive agent accounts for 1.5% to 4% of the mass of the positive electrode material layer. In the positive electrode material layer, the conductive agent includes carbon nanotubes and conductive carbon black, and the mass ratio of the carbon nanotubes to the conductive carbon black is 0.1~0.83; the aspect ratio of the carbon nanotubes is greater than or equal to 10000.

11. The battery according to claim 10, characterized in that, In the cathode material layer, the diameter of the carbon nanotubes is 1~2nm and the length of the carbon nanotubes is 15~60μm.

12. The battery according to claim 11, characterized in that, In the cathode material layer, the carbon nanotubes are single-walled carbon nanotubes.

13. The battery according to claim 12, characterized in that, In the cathode material layer, the specific surface area of ​​the single-walled carbon nanotubes is 240~310 m². 2 / g.

14. The battery according to claim 10, characterized in that, In the positive electrode material layer, the conductive carbon black has a particle size of 20-40 nm and a specific surface area of ​​60-100 m². 2 / g.

15. The battery according to claim 14, characterized in that, In the positive electrode material layer, the conductivity of the conductive carbon black is 10. 2 ~10 3 S / m.

16. The battery according to any one of claims 1-3, characterized in that, The electrolyte retention capacity of the battery is 5~15g / Ah.

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