TWI547000B - Cathode of lithium-ion battery and method for making same - Google Patents

Description

Lithium ion battery positive electrode and preparation method thereof

The invention relates to a positive electrode of a lithium ion battery and a preparation method thereof, in particular to a positive electrode of a lithium ion battery based on a carbon nanotube and a preparation method thereof.

Lithium-ion battery is a new type of green chemical power source. Compared with traditional nickel-cadmium batteries and nickel-hydrogen batteries, it has the advantages of high voltage, long life and high energy density. Since Sony introduced the first generation of lithium-ion batteries in 1990, it has been rapidly developed and widely used in a variety of portable devices.

The positive electrode of the lithium ion battery includes a positive electrode material, and the positive electrode material is mainly composed of a positive electrode active material. There are two kinds of structures for the positive electrode of a lithium ion battery, one is directly using a positive electrode material having a self-supporting structure as a positive electrode of a lithium ion battery, and the other is obtained by coating or fixing a positive electrode material on a current collector. The positive electrode active material is generally selected from an intercalation compound, and commonly used are lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, etc. The active materials of other positive electrode materials include iron oxides and other metal oxides. However, since the positive electrode active material itself has poor conductivity, the internal resistance of the electrode is large, and the depth of discharge is insufficient. As a result, the utilization rate of the positive electrode active material is low, and the residual capacity of the electrode is large, so that the active material and the current collector are improved. And the conductivity between the active material particles is critical to the performance of the positive electrode of the lithium ion battery. Therefore, in practical applications, it is generally required to add a conductive agent to improve the electrical conductivity of the active material.

The type and amount of the conductive agent have a great influence on the specific capacity of the electrode of the active material and the discharge behavior of the rate. Graphite, acetylene black and carbon fiber have good electrical conductivity, low density, stable structure and stable chemical properties, and are often used as conductive agents for lithium ion battery cathode materials. In order to make full use of the active material, the mass percentage of these conductive agents in the positive electrode material is usually 10%, which increases the mass and volume of the positive electrode of the lithium ion battery.

Therefore, it is provided with a lower internal resistance, better charge and discharge performance under the premise of the same quality and volume compared with the prior art, or has a light weight under the premise of equivalent internal resistance and charge and discharge performance compared with the prior art. A small-sized lithium ion battery positive electrode and a preparation method thereof are really necessary.

The positive electrode of the lithium ion battery includes: a positive electrode active material, a binder, wherein the positive electrode of the lithium ion battery further comprises a plurality of carbon nanotubes, the plurality of carbon nanotubes are uniformly dispersed in the lithium ion battery, and the plurality The mass percentage of the carbon nanotubes is 0.001 wt% or more and less than 1 wt%.

A method for preparing a positive electrode of a lithium ion battery, comprising the steps of: providing a paste mixture comprising a positive active material of a lithium ion battery and a binder; rolling the paste mixture into a sheet structure; The surface of the structure uniformly forms a carbon nanotube layer structure to form a positive electrode preform; the positive electrode preform is crimped, the curled positive electrode preform is rolled into a positive electrode of a lithium ion battery; and the lithium ion battery is dried Positive electrode sheet.

Compared with the prior art, in the positive electrode of the lithium ion battery provided by the invention, the carbon nanotubes are uniformly distributed, and the carbon nanotubes have a positive electrode of the lithium ion battery at a mass percentage of more than 0.001 wt% and less than 1 wt%. Lower internal resistance; at the same time, due to nanocarbon The mass content of the tube is small, and the mass ratio of the active material in the positive electrode of the lithium ion battery can be improved under the condition of the same quality as the lithium ion battery in the prior art, so that the charge and discharge performance of the lithium ion battery is better; When the mass of the positive electrode active material and the conductive agent in the lithium ion battery is constant, since the mass content of the carbon nanotube is small and the conductivity of the positive electrode of the lithium ion battery can be remarkably improved, it is not necessary to add other conductive particles, so The mass and volume of the positive electrode of the lithium ion battery are small, and the mass and volume of the lithium ion battery using the positive electrode of the lithium ion battery are also reduced.

Compared with the prior art, the method for preparing a positive electrode of a lithium ion battery provided by the present invention forms a layer structure of a carbon nanotube on a surface of a sheet structure including a positive electrode active material, and then rolls it, without solving the problem of the carbon nanotube in the positive electrode. The problem of dispersion in the active material is simple to operate.

1 is a flow chart of a method for preparing a positive electrode of a lithium ion battery according to a first embodiment of the present invention.

2 is a scanning electron micrograph of a cross section of a positive electrode of a lithium ion battery provided by the present invention.

3 is a comparison diagram of charge and discharge performance of a positive electrode of a lithium ion battery provided by the present invention and a positive electrode of a lithium ion battery in which only carbon black is added as a conductive agent.

The embodiments of the present invention will be further described in detail below with reference to the drawings and specific embodiments.

Referring to FIG. 1 , an embodiment of the present invention provides a method for preparing a positive electrode of a lithium ion battery. The preparation method of the positive electrode of the lithium ion battery of the embodiment of the invention mainly comprises the following steps: Step 1. A paste mixture is provided, the paste mixture comprising a lithium ion battery positive active material and a binder.

The positive electrode active material may be lithium iron phosphate (LiFePO ₄ ), lithium nickel cobalt (LiNi _0.8 Co _0.2 O ₂ ), lithium nickel cobalt manganese (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), cobalt acid One or more of lithium (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium manganate (LiMn ₂ O ₄ ). In the present embodiment, lithium iron phosphate is selected as the positive electrode active material.

The binder includes an adhesive material and a solvent. The binder material may be a fluorine-containing resin or a polyolefin compound, and the polyolefin compound may be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), or styrene-butadiene rubber (SBR).

The method of preparing the paste mixture comprises the steps of first providing an adhesive comprising a binding material and a solvent.

The binder is uniformly dispersed in the solvent to form a binder in a suspension state or a binder in a solution state. The solvent may be one or more of ethanol, ethylene glycol, N-methylpyrrolidone (NMP), and water. The binder has a mass concentration of the binder in the range of 5% to 60%. The solvent in the embodiment is water, the bonding material is polytetrafluoroethylene, and the polytetrafluoroethylene is dispersed in water to form a binder in a suspension state, and the mass concentration of the binder in the binder is 20%.

Next, a lithium ion battery positive active material powder is added to the binder and stirred uniformly to form a paste mixture.

In the paste mixture, the mass ratio of the positive active material of the lithium ion battery to the binding material is 5:1 to 20:1. It is understood that in order to form a paste mixture by adding a positive electrode active material to the binder, the amount of the positive electrode active material added is related to the concentration of the binder. When the concentration of the binder is large, the amount of the positive electrode active material required is small when bonding When the concentration of the agent is small, the amount of the positive electrode active material required is large. In this embodiment, the mass ratio of lithium iron phosphate to polytetrafluoroethylene is 8:1.

Alternatively, a small amount of carbon black powder may be added to the positive electrode material powder before the positive electrode material powder is added to the binder, and after mixing uniformly, the mixed powder is added to the binder, the carbon black and the lithium ion battery. The mass ratio of the positive electrode active material is 0.1:1 or less, preferably 0.02:1.

Step 2. The paste mixture is rolled into a sheet structure.

The thickness of the sheet structure is not limited, and is preferably 10 μm to 1 mm. The shape of the sheet structure is not limited and may be square, circular or irregular.

The paste mixture can be pressed into a sheet-like structure by means of rolling or vertical pressing on a sheet rolling machine. The paste mixture may be directly rolled into a sheet structure, or the paste mixture may be wrapped in a flexible sheet and then rolled. The flexible sheet may be an aluminum foil, a plastic film or paper. In this embodiment, the process of rolling the paste mixture comprises the following steps: first, wrapping the paste mixture on an aluminum foil sheet; secondly, placing the aluminum foil sheet wrapped with the cream mixture into a sheet rolling machine The middle rolling forms a sheet-like structure, and the rolling mill includes two rolling wheels; finally, the aluminum foil is peeled off from the sheet structure. By wrapping the paste mixture in a flexible sheet and then rolling the sheet, the paste mixture can be prevented from being contaminated by the sheet rolling machine, and the paste mixture can be prevented from sticking to the sheet rolling machine during the sheet rolling, resulting in waste of material.

Step 3: uniformly forming a carbon nanotube layer structure on the surface of the sheet structure to form a positive electrode preform.

The carbon nanotube layer structure may be formed by spraying or coating a slurry containing a carbon nanotube on the surface of the sheet structure, or directly laminating the carbon nanotube layer structure. It is provided on the surface of the sheet structure. Preferably, the carbon nanotube layered structure is a self-supporting structure. The so-called self-supporting means that the carbon nanotube layered structure can be self-supported to maintain a layer shape without the need of a matrix support. The self-supporting carbon nanotube structure can be formed on the surface of the sheet structure by laying. When the self-supporting carbon nanotube layered structure is used, it is not necessary to form a slurry of the carbon nanotubes, and it is not necessary to solve the problem of dispersion of the carbon nanotubes, and no other impurities are introduced into the positive electrode of the lithium ion battery. Moreover, the self-supporting carbon nanotube layered structure can form the carbon nanotube layered structure on the surface of the sheet structure by laying, and the operation is simple.

The self-supporting carbon nanotube layered structure comprises at least one layer of carbon nanotube film. When the self-supporting carbon nanotube layered structure comprises two or more layers of carbon nanotube film, the plurality of layers of carbon nanotube film may be stacked or juxtaposed. The carbon nanotube membrane comprises a plurality of uniformly distributed carbon nanotubes. The carbon nanotube film can be composed only of a carbon nanotube tube, and the carbon nanotube film itself has a certain viscosity, so that the carbon nanotube layer structure composed of a carbon nanotube film can directly adhere to the sheet structure. Surface, no adhesive required. The carbon nanotube layered structure has a thickness of 10 nm to 100 microns. The carbon nanotube film may be a carbon nanotube film, a carbon nanotube film or a carbon nanotube film. The carbon nanotube layered structure may be composed of any one of the above three types of carbon nanotube films or any combination of two or more types of films stacked or arranged in parallel.

(1) A method for preparing a carbon nanotube film comprises the steps of: firstly, providing a carbon nanotube array formed on a growth substrate, the array being a super-aligned carbon nanotube array.

The method for preparing the carbon nanotube array adopts a chemical vapor deposition method, and the specific steps thereof include: (a) providing a flat growth substrate, the growth substrate may be a P-type or N-type germanium growth substrate, or an oxide layer may be formed.矽 growth substrate, the embodiment of the invention is excellent It is selected to use a 4-inch germanium growth substrate; (b) a catalyst layer is uniformly formed on the surface of the growth substrate, and the catalyst layer material may be selected from iron (Fe), cobalt (Co), nickel (Ni) or any combination thereof. (c) annealing the growth substrate on which the catalyst layer is formed in air at 700 ° C to 900 ° C for about 30 minutes to 90 minutes; (d) placing the treated growth substrate in a reaction furnace in a protective gas atmosphere The mixture is heated to 500 ° C to 740 ° C, and then passed through a carbon source gas for about 5 minutes to 30 minutes to grow to obtain a carbon nanotube array. The carbon nanotube array is a plurality of pure carbon nanotube arrays formed of carbon nanotubes that are parallel to each other and perpendicular to the growth substrate. The aligned carbon nanotube array contains substantially no impurities, such as amorphous carbon or residual catalyst metal particles, etc., by controlling the growth conditions described above.

Secondly, a drawing tool is used to pull the carbon nanotubes from the carbon nanotube array to obtain at least one carbon nanotube film, which specifically comprises the following steps: (a) from the super-sequential carbon nanotube array Selecting one or a plurality of carbon nanotubes having a certain width, preferably adopting a tape, a braid or a clip having a certain width to contact the array of carbon nanotubes to select one or a plurality of carbon nanotubes having a certain width; (b The selected carbon nanotubes are drawn at a certain speed to form a plurality of carbon nanotube fragments connected end to end, thereby forming a continuous carbon nanotube film. The pull direction is substantially perpendicular to the growth direction of the nanotube array.

In the above stretching process, the plurality of carbon nanotube segments are gradually separated from the growth substrate in the stretching direction under the tensile force, and the selected plurality of carbon nanotube segments are respectively combined with the other due to the van der Waals force. The carbon nanotube segments are continuously pulled out end to end to form a continuous, uniform, and wide-width carbon nanotube film.

The width of the carbon nanotube film is related to the size of the carbon nanotube array, the carbon nanotube The length of the film is not limited and can be made according to actual needs. When the area of the carbon nanotube array is 4 inches, the width of the carbon nanotube film is 0.5 nm to 10 cm, and the thickness of the carbon nanotube film is 0.5 nm to 100 μm. For the preparation method of the carbon nanotube film, please refer to Fan Shoushan et al., filed on February 12, 1996, in the Republic of China, No. 96110016, published on August 16, 1997, Taiwan Patent Application "Nano Carbon Tube" Membrane structure and preparation method thereof, Applicant: Tsinghua University, Hongfujin Precision Industry (Shenzhen) Co., Ltd. In order to save space, only the above is cited, but all the technical disclosures of the above application are also considered as part of the technical disclosure of the present application.

(2) The preparation method of the carbon nanotube flocculation membrane comprises the following steps: First, a carbon nanotube raw material is provided.

The carbon nanotube raw material may be a carbon nanotube prepared by various methods such as chemical vapor deposition, graphite electrode constant current arc discharge deposition or laser evaporation deposition.

The aligned carbon nanotube arrays are scraped off the substrate using a blade or other tool to obtain a carbon nanotube material. Preferably, in the carbon nanotube raw material, the length of the carbon nanotube is greater than 100 micrometers.

Next, the above carbon nanotube raw material is added to a solvent and subjected to flocculation treatment to obtain a nano carbon tube floc structure, and the above carbon nanotube floc structure is separated from the solvent, and the carbon nanotube is separated. The flocculated structure is shaped to obtain a carbon nanotube flocculation film.

The solvent can be selected from water, a volatile organic solvent, and the like. The flocculation treatment can be carried out by a method such as ultrasonic dispersion treatment or high-intensity stirring. Preferably, the embodiment of the invention uses ultrasonic dispersion for 10 minutes to 30 minutes. Due to the extremely large specific surface area of the carbon nanotubes, there is a large van der Waals force between the intertwined carbon nanotubes. The above flocculation treatment does not completely disperse the carbon nanotubes in the carbon nanotube raw material in the solvent, and the nanometer The carbon tubes are attracted and entangled by van der Waals forces to form a network structure.

The method for separating the carbon nanotube floc structure specifically comprises the steps of: pouring the solvent containing the carbon nanotube floc structure into a funnel with a filter paper; and drying for a period of time to obtain a separation. Nano carbon tube floc structure.

The shaping treatment process of the nano carbon tube floc structure specifically comprises the steps of: placing the above carbon nanotube floc structure in a container; spreading the nano carbon tube floc structure according to a predetermined shape; applying A certain pressure is applied to the expanded carbon nanotube floc structure; and the residual solvent in the nano carbon tube floc structure is dried or the solvent is naturally volatilized to obtain a carbon nanotube flocculation film.

It will be appreciated that the present invention controls the thickness and areal density of the carbon nanotube flocculation membrane by controlling the area over which the carbon nanotube floc is spread. The larger the area spread by the carbon nanotube floc structure, the smaller the thickness and areal density of the carbon nanotube flocculation film.

In addition, the step of separating and shaping the carbon nanotube floc structure can also be directly carried out by suction filtration, and specifically includes the following steps: providing a pore filter membrane and an air suction funnel; and the above-mentioned carbon nanotube containing The solvent of the structure is poured into the suction funnel through the pore filter membrane; after suction filtration and drying, a carbon nanotube flocculation membrane is obtained. The pore L-gap filter is a filter having a smooth surface and a size of 0.22 μm. Since the suction filtration method itself will provide a large gas pressure on the carbon nanotube floc structure, the carbon nanotube floc structure directly forms a uniform carbon nanotube flocculation membrane by suction filtration. Moreover, since the pore filter membrane surface is smooth, the carbon nanotube flocculation membrane is easily peeled off, and a self-supporting carbon nanotube flocculation membrane is obtained.

It can be understood that the carbon nanotube flocculation membrane has a certain thickness, and the carbon nanotube flocculation can be controlled by controlling the area of the carbon nanotube floc structure and the pressure. The thickness of the film. The carbon nanotube flocculation membrane can be used as a carbon nanotube layer structure, or at least two layers of carbon nanotube flocculation membranes can be stacked or arranged side by side to form a carbon nanotube layer structure.

(3) The preparation method of the carbon nanotube rolled film includes the following steps: First, an array of carbon nanotubes is provided on a growth substrate, the array being an array of aligned carbon nanotubes.

The carbon nanotube array is preferably a super-aligned array of carbon nanotubes. The carbon nanotube array is prepared in the same manner as the above-described carbon nanotube array.

Next, a carbon nanotube array is extruded by using a pressing device to obtain a carbon nanotube rolled film, wherein the pressing device applies a certain pressure to the carbon nanotube array. During the pressing process, the carbon nanotube array is separated from the growth substrate by pressure to form a carbon nanotube rolled film having a self-supporting structure composed of a plurality of carbon nanotubes, and The plurality of carbon nanotubes are substantially parallel to the surface of the carbon nanotube rolled film.

The pressing device is an indenter, the surface of the indenter is smooth, and the shape and extrusion direction of the indenter determine the arrangement of the carbon nanotubes in the prepared carbon nanotube rolled film. Preferably, when the planar indenter is pressed in a direction perpendicular to the growth substrate of the carbon nanotube array, the carbon nanotubes are obtained as isotropically arranged carbon nanotubes; when the roller is used When the indenter is rolled in a certain fixed direction, a carbon nanotube film which is aligned along the fixed direction of the carbon nanotubes can be obtained; when the roller-shaped indenter is rolled in different directions, the nanometer can be obtained. A carbon nanotube rolled film in which carbon tubes are aligned in different directions.

It can be understood that when the above-mentioned carbon nanotube array is extruded in the above different manner, the carbon nanotubes are poured under the action of pressure and passed through the van der Waals with the adjacent carbon nanotubes. The force attracts and connects with each other to form a carbon nanotube film with a self-supporting structure composed of a plurality of carbon nanotubes.

It can be understood that the carbon nanotube rolled film has a certain thickness, and the thickness can be controlled by the height of the carbon nanotube array and the pressure. Therefore, the carbon nanotube rolled film can be directly used as a carbon nanotube layered structure. In addition, at least two layers of carbon nanotube rolled films may be stacked or arranged side by side to form a carbon nanotube layered structure.

In this embodiment, the method for uniformly forming a carbon nanotube layer structure on the surface of the sheet structure is to directly lay a self-supporting carbon nanotube layer structure on the surface of the sheet structure. The self-supporting carbon nanotube layered structure comprises five layers of carbon nanotube film laminated on each other.

Step 4: The positive electrode preform is crimped, and the crimped positive electrode preform is rolled to form a positive electrode of a lithium ion battery.

The manner of crimping is not limited, and the direction is not limited, and may be curled in one direction or may be curled in a plurality of directions. Preferably, the carbon nanotube layered structure is wrapped in the positive electrode preform in the direction of the surface of the positive electrode preform provided with the carbon nanotube layer structure. This crimping method prevents the carbon nanotube layered structure from being destroyed during rolling. The crimped positive electrode material preform may be directly rolled into a sheet-like structure, or the crimped positive electrode material preform may be wrapped in a flexible sheet and then rolled. In this embodiment, the process of rolling the crimped positive electrode material preform into the positive electrode of the lithium ion battery comprises the following steps: first, wrapping the crimped positive electrode material preform on an aluminum foil; secondly, wrapping The aluminum foil having the curled positive electrode material preform is rolled into a sheet rolling machine to form a positive electrode of a lithium ion battery, the rolling mill comprising two rolling wheels; and finally, the aluminum foil is taken from the positive electrode of the lithium ion battery. Uncover it.

Alternatively, the third step and the fourth step may be repeated a plurality of times, that is, the formation of the carbon nanotube layer structure on the surface of the sheet structure is sequentially performed to form the positive electrode preform, and the positive electrode preform is curled, and the curled positive electrode is prefabricated. Body rolled lithium ion battery positive electrode. After a plurality of layers of carbon nanotubes are formed, the content of the carbon nanotubes in the positive electrode of the lithium ion battery increases after multiple rolling, and the carbon nanotubes can be more uniformly dispersed in the positive electrode of the lithium ion battery. In the layered structure of the carbon nanotubes laid each time, the number of layers of the carbon nanotube film may be the same or different. The thickness of the positive electrode sheets of the lithium ion battery formed after each rolling may be the same or different.

Alternatively, the step four, that is, the crimped positive electrode preform, may be repeated a plurality of times, and the step of rolling the crimped positive electrode preform may be repeated. The positive electrode preform is subjected to a plurality of crimping and multiple rolling steps to form a positive electrode of a lithium ion battery. This multiple crimping and multiple rolling method allows the carbon nanotubes to be uniformly dispersed in the positive electrode of the lithium ion battery. The thickness of the positive electrode sheets of the lithium ion battery formed after each rolling may be the same or different.

In the lithium implementation of this embodiment, steps 3 and 4 are repeated four times in succession, and the layered carbon nanotube layer structure includes 5 layers of carbon nanotube film and 4 layers of carbon nanotube layer structure. Includes 20 layers of carbon nanotube film.

After the positive electrode sheet of the lithium ion battery is formed, the positive electrode sheet of the lithium ion battery can be further cut to form a predetermined size.

Step 5: drying the positive electrode of the lithium ion battery to form a positive electrode of the lithium ion battery.

The process of drying the positive electrode sheet of the lithium ion battery is to dry the positive electrode sheet of the lithium ion battery at a temperature of 100 ° C to 150 ° C for 10 to 40 hours. The drying step is to charge lithium ions Solvent removal in the binder in the positive electrode of the cell.

The size of the positive electrode of the lithium ion battery is not limited, and the thickness of the positive electrode of the lithium ion battery is 10 micrometers to 2 millimeters. When the positive electrode of the lithium ion battery has a rectangular parallelepiped structure, the length is from 100 micrometers to 300 millimeters and the width is from 50 micrometers to 100 millimeters. When the positive electrode of the lithium ion battery has a circular structure, its diameter is 1 mm to 10 cm. When the lithium ion battery positive electrode is used as a micro lithium ion battery, the lithium ion battery has a thickness of 10 micrometers to 100 micrometers, a length of 100 micrometers to 1 millimeter, and a width of 50 micrometers to 0.5 millimeters. In this embodiment, the positive electrode of the lithium ion battery is a circular structure having a thickness of 0.3 mm and a diameter of 8 mm.

The positive electrode of the lithium ion battery prepared by the above method includes a positive electrode active material, a binder material, and a plurality of carbon nanotubes. The carbon nanotubes include single-walled carbon nanotubes, double-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the carbon nanotubes is from 1 nm to 200 nm, and the length of the carbon nanotubes is from 10 μm to 1 cm. Preferably, the length of the carbon nanotubes is greater than 300 microns, and when the length of the carbon nanotubes is greater than 300 microns, the positive electrode of the lithium ion battery has better electrical conductivity. Referring to FIG. 2, in the positive electrode of the lithium ion battery, the positive electrode active material, the binding material and the carbon nanotubes are uniformly mixed together, wherein the carbon nanotubes are uniformly dispersed in the positive electrode of the lithium ion battery, that is, In different regions of the positive electrode of a lithium ion battery, the number of carbon nanotubes in an equal area or volume is substantially equal. By controlling the number of layers of the carbon nanotube layered structure and the thickness of the layer of the carbon nanotube layer, the mass percentage of the carbon nanotubes in the positive electrode of the lithium ion battery can be controlled. The mass percentage of the plurality of carbon nanotubes in the positive electrode of the lithium ion battery is 0.001 wt% or more and less than 1 wt%, and preferably, the mass percentage of the plurality of carbon nanotubes in the positive electrode of the lithium ion battery is 0.1 wt% or less. . In the positive electrode of the lithium ion battery, the mass percentage of the positive electrode active material is 83% by weight or more and 95% by weight or less. Preferably, the mass percentage of the positive electrode active material is greater than or equal to 89% by weight or less. 95wt%. In this embodiment, the mass ratio of the positive electrode active material, the binder material, and the carbon nanotubes in the positive electrode of the lithium ion battery is 8:1:0.01. The plurality of carbon nanotubes are uniformly distributed in the lithium ion battery and are isotropic, so that the positive resistivity of the positive electrode of the lithium ion battery is uniform. The isotropic means that the distribution density of the carbon nanotubes in the positive electrode of the lithium ion battery is substantially the same, that is, the mass percentage of the carbon nanotubes per unit volume of the lithium ion battery is substantially the same.

Table 1 is a comparison table of the resistivity of the positive electrode of the lithium ion battery prepared by the six different examples and the resistivity of the positive electrode of the lithium ion battery prepared in the comparative test by the lithium ion battery preparation method provided by the present invention. In each of the embodiments, the test size of the positive electrode of the lithium ion battery is 20 mm × 8 mm × 0.34 mm, and the positive active material of the lithium ion battery is lithium iron phosphate, and the bonding materials are polytetrafluoroethylene, lithium iron phosphate and poly The mass ratio of tetrafluoroethylene is 8:1, and the number of layers of the carbon nanotube film and the number of laying of the carbon nanotube layer structure in the carbon nanotube layer structure are different, but in each embodiment, the carbon carbon The mass of the tube is less than 0.1% by weight in the positive electrode of the lithium ion battery. The positive electrode of the lithium ion battery in Comparative Experiment 1 only included lithium iron phosphate and polytetrafluoroethylene, and the mass ratio was 8:1. The lithium ion battery in Comparative Test 2 included lithium iron phosphate, polytetrafluoroethylene and carbon black, and the mass ratio of lithium iron phosphate, polytetrafluoroethylene and carbon black was 8:1:1.

Table 1 Comparison of resistivity of positive electrode of lithium ion battery

It can be seen from Table 1 that the resistivity of the positive electrode of the lithium ion battery prepared by the preparation method provided by the embodiment of the present invention can be reduced by two orders of magnitude compared with the resistivity of the lithium ion battery provided by the comparative test 1, that is, the embodiment of the present invention The conductivity of the positive electrode of the lithium ion battery prepared by the preparation method is obviously improved. In the positive electrode of the lithium ion battery prepared by the preparation method provided by the embodiment of the invention, the mass percentage of the carbon nanotubes is less than 0.1 wt%, and in the comparative test 2, the mass percentage of the carbon black is 10 wt%, the embodiment of the invention The resistivity of the positive electrode of the lithium ion battery prepared by the preparation method is similar to that of the positive electrode of the lithium ion battery in Comparative Experiment 2, and may even be lower than the resistivity of the positive electrode of the lithium ion battery in Comparative Test 2.

Further, the specific capacities of the positive electrodes of the lithium ion batteries in Example 1 and Comparative Test 2 were respectively measured, and the specific capacities of the positive electrodes of the two lithium ion batteries were both 160-170 mAh/g (mAh/g). However, in the positive electrode of the lithium ion battery prepared by the preparation method provided by the embodiment of the present invention, the mass ratio of the carbon nanotubes is less than 0.1 wt%, which is negligible, so that the mass ratio of the active materials in the positive electrode of the lithium ion battery can be made. Compared with Comparative Test 2, it can be increased by 10%, that is, under the condition of the same specific capacity and total capacity, by the first embodiment The battery provided by the positive electrode of the lithium ion battery has a smaller mass and volume.

It can also be seen from Table 1 above that in the positive electrode of the lithium ion battery, as the total number of layers of the carbon nanotube film increases, the resistivity becomes smaller, so the content of the carbon nanotube can be in the lithium ion battery. The mass percentage in the positive electrode is adjusted in the range of 0.001 wt% to 1 wt% as needed.

It can be understood that the positive electrode of the lithium ion battery prepared by the above method may further include a small amount of carbon black, that is, the positive electrode of the lithium ion battery includes both a carbon nanotube and a carbon black, and the quality of the carbon black in the positive electrode of the lithium ion battery. The percentage is less than 10%, preferably, less than or equal to 1% by weight, and the mass percentage of the carbon nanotube in the positive electrode of the lithium ion battery is 0.001% by weight or more and less than 1% by weight. When the positive electrode of the lithium ion battery includes a small amount of carbon black, since the carbon black exists in the form of particles, and the carbon nanotube is a continuous linear material, the carbon black particles can be better filled in the carbon nanotube and the lithium ion battery cathode material. Between the particles of the positive electrode material of the lithium ion battery, the conductivity of the positive electrode of the lithium ion battery is further improved, and the specific capacity of the lithium ion battery is increased. Referring to FIG. 3, in the positive electrode of the lithium ion battery No. 1, the mass ratio of lithium cobaltate, carbon black, polytetrafluoroethylene and carbon nanotubes is 8:0.1:1:0.005; in the positive electrode of the No. 2 lithium ion battery, The mass ratio of lithium cobaltate, carbon black and polytetrafluoroethylene is 8:0.1:1. The positive electrode of lithium ion battery No. 2 does not include carbon nanotubes, and the positive electrodes of lithium ion batteries No. 1 and No. 2 are circular structures. The diameter of the test was 8 cm. As can be seen from Fig. 3, the positive electrode of the lithium ion battery containing both carbon black and carbon nanotubes has better charge and discharge cycle performance and a larger specific capacity.

In the positive electrode of the lithium ion battery provided by the invention, the carbon nanotubes are evenly distributed, and the carbon nanotubes have a lower internal resistance in the case of a mass percentage of 0.001 wt% or more and less than 1 wt%. At the same time, due to the small mass content of the carbon nanotubes, in the case of a certain quality of the lithium ion battery, the positive electrode of the lithium ion battery can be The mass ratio of the active material is improved, so that the charge and discharge performance of the lithium ion battery is better; in addition, in the case where the mass of the positive electrode active material and the conductive agent in the lithium ion battery is constant, since the mass content of the carbon nanotube is small, The lithium ion battery positive electrode has a small mass and volume, so the mass and volume of the lithium ion battery using the lithium ion battery positive electrode are also reduced.

The method for preparing a positive electrode of a lithium ion battery provided by the invention adopts a layered structure of a carbon nanotube tube as a raw material, and lays a layered structure of a carbon nanotube tube on a surface of a sheet-like structure including a positive electrode active material, and then rolls it, without solving the solution. The problem of dispersion of the carbon nanotubes in the positive electrode active material is simple, and the carbon nanotubes in the prepared carbon nanotube positive electrode do not agglomerate, and the carbon nanotubes are evenly distributed, so the content of the carbon nanotubes is small. In the case of the lithium ion battery, the conductivity of the cathode material can be significantly improved, thereby improving the charge and discharge performance of the lithium ion battery.

In summary, the present invention has indeed met the requirements of the invention patent, and has filed a patent application according to law. However, the above description is only a preferred embodiment of the present invention, and it is not possible to limit the scope of the patent application of the present invention. Equivalent modifications or variations made by persons skilled in the art in light of the spirit of the invention are intended to be included within the scope of the following claims.

Claims (14)

A method for preparing a positive electrode of a lithium ion battery, comprising the steps of: providing a paste mixture comprising a positive active material and a binder; rolling the paste mixture into a sheet structure; in the sheet structure The surface uniformly forms a carbon nanotube layer structure to form a positive electrode preform, the carbon nanotube layer structure includes a plurality of carbon nanotubes; the positive electrode preform is crimped, and the crimped positive electrode preform is rolled Forming a positive electrode of a lithium ion battery, the plurality of carbon nanotubes are uniformly dispersed in the positive electrode of the lithium ion battery; and drying the positive electrode of the lithium ion battery. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the method for preparing the paste mixture comprises the steps of: providing a binder comprising a binder material and a solvent; The active material is placed in the binder and stirred uniformly to form the paste mixture. The method for preparing a positive electrode of a lithium ion battery according to claim 2, wherein the binder has a mass concentration of the binder in the range of 5% to 60%. The method for preparing a positive electrode of a lithium ion battery according to claim 2, further comprising adding a small amount of carbon black to the binder, and the mass ratio of the carbon black to the positive electrode active material is 0.01 or less. The method for preparing a positive electrode of a lithium ion battery according to claim 2, wherein a mass ratio of the positive electrode active material to the binder material is 5:1 to 20:1. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the positive electrode active material comprises lithium iron phosphate, lithium nickel cobalt, lithium nickel cobalt manganese, lithium cobalt oxide, lithium nickelate and lithium manganate. One or several. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the process of rolling the paste mixture into a sheet structure specifically comprises the steps of: wrapping the paste mixture in a flexible sheet; The flexible sheet wrapped with the paste mixture is rolled into a sheet rolling machine to form a sheet-like structure; and the flexible sheet is peeled off from the sheet structure. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the carbon nanotube layered structure is formed on the surface of the sheet structure by coating or spraying. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the carbon nanotube layered structure is a self-supporting structure, and the carbon nanotube layered structure is directly laid on the surface of the sheet structure. The method for preparing a positive electrode of a lithium ion battery according to claim 9, wherein the carbon nanotube layered structure comprises at least one layer of carbon nanotube film, and the method for preparing the carbon nanotube film comprises the following steps: Providing a carbon nanotube array formed on a growth substrate, the array being a super-aligned carbon nanotube array; and, using a stretching tool, pulling a carbon nanotube from the carbon nanotube array to obtain a nanometer Carbon tube membrane. The method for producing a positive electrode of a lithium ion battery according to claim 1, wherein the step of crimping the positive electrode preform and rolling the rolled positive electrode preform into a positive electrode of a lithium ion battery is repeated. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the carbon nanotube layered structure is formed on the surface of the sheet structure to form a positive electrode preform, and the positive electrode preform is crimped, and the curled The step of rolling the positive electrode of the lithium ion battery with the positive electrode preform is sequentially repeated. The method for preparing a positive electrode of a lithium ion battery according to claim 1, further comprising the step of cutting the positive electrode of the lithium ion battery. The method for preparing a positive electrode of a lithium ion battery according to claim 1, wherein the temperature of the positive electrode of the lithium ion battery is from 100 ° C to 150 ° C for 10 hours to 40 hours.