Positive plate, preparation method thereof and lithium battery

Abstract

The invention discloses a positive plate, a preparation method thereof and a lithium battery. The positive plate comprises an electrode material layer, the electrode material layer comprises an active substance, a composite conductive agent and a binder, and the composite conductive agent comprises a polyester modified single-walled carbon nanotube and a zero-dimensional conductive carbon material. The composite conductivity of the polyester modified single-walled carbon nanotube and the zero-dimensional conductive carbon material is utilized, the composite conductivity is applied to the positive plate, the volume expansion of the electrode can be effectively inhibited, and the good electrochemical performance of the electrode is ensured.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery material technology, specifically relating to a positive electrode sheet, its preparation method, and a lithium battery. Background Technology

[0002] Electrodes (such as ternary cathodes) expand during cycling, causing some of the active material and the composite conductive agent network to decouple, reducing discharge capacity and cycle retention. For example, on the cathode side, at low SOC, the ternary cathode is close to its maximum expansion, resulting in numerous voids and insufficient utilization of the composite conductive agent. On the anode side, silicon undergoes significant volume expansion during charge and discharge, leading to large gaps between hard carbon and silicon and high impedance. For lithium batteries containing both a ternary cathode and a silicon-doped anode, the impedance is highest and the power output is worst at low SOC.

[0003] CN 112421018 A discloses a porous silicon-oxygen-carbon anode material for lithium batteries that suppresses volume expansion. This invention involves coating silicon-magnesium alloy powder with silica from silicate ester hydrolysis, then removing the internal magnesium to form a porous structure. During the melting of magnesium in high-temperature bismuth melt, some magnesium acts as a reducing agent to reduce the silica. Following this, carbon coating is applied, and during sintering, the silica coating layer further reacts with Si to form SiO₂. x The porous structure enables one-step molding of silicon-carbon particles. A silicon suboxide / carbon anode material is prepared using silicon-magnesium alloy particles as a framework. The internal silicon-based active material has high porosity, which can effectively mitigate the volume expansion caused by lithium intercalation.

[0004] CN117894962 A discloses a ternary cathode material with bulk multi-element doping and surface coating of lithium-ion conductors. It is prepared from lithium nickel cobalt manganese oxide with bulk doping of multiple elements and surface coating of lithium-ion conductors, or from lithium nickel cobalt aluminum oxide with bulk doping of multiple elements and surface coating of lithium-ion conductors. This invention effectively suppresses the formation of numerous microcracks within the secondary spherical particles of the ternary cathode material due to volume expansion during charging and discharging.

[0005] Existing technologies often employ coating methods to suppress volume expansion of positive or negative electrode active materials. However, this method inevitably hinders the insertion and extraction of lithium ions, leading to varying degrees of reduction in electrochemical performance.

[0006] Therefore, providing a strategy to effectively suppress the volume expansion of the electrode while ensuring its good electrochemical performance is a technical problem that urgently needs to be solved. Summary of the Invention

[0007] In view of the above-mentioned technical problems existing in the prior art, the purpose of the present invention is to provide a positive electrode sheet, a method for preparing the same, and a lithium battery.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising an electrode material layer, the electrode material layer comprising an active material, a composite conductive agent and a binder, wherein the composite conductive agent comprises polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials.

[0010] This invention utilizes the composite conductivity of polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials, applying them to positive electrode sheets to effectively suppress electrode volume expansion and ensure excellent electrochemical performance. The technical principles are as follows: First, single-walled carbon nanotubes have a high aspect ratio and, compared to multi-walled carbon nanotubes, higher conductivity and better flexibility, thus exhibiting better adhesion to the active material. Second, modifying single-walled carbon nanotubes to form polyester-modified single-walled carbon nanotubes increases their dispersibility in polar solvents. Simultaneously, polyester and ester solvents in the electrolyte follow the principle of "like dissolves like," which is beneficial for improving the ionic conductivity of single-walled carbon nanotubes. Moreover, polyester provides a certain swelling capacity, thus forming a buffer between the single-walled carbon nanotubes and the active material, reducing the stress on the single-walled carbon nanotubes and allowing them to better maintain the electronic conductivity of the electrode sheet during charging and discharging. Third, the synergistic use of polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials can form a composite conductive network, enhancing the conductivity of the composite conductive agent.

[0011] Preferably, the polyester-modified single-walled carbon nanotubes comprise carbon nanotubes and a polyester layer coating the surface of the carbon nanotubes.

[0012] Preferably, the mass ratio of the polyester layer to the carbon nanotube is (0.5~3):1, for example, it can be 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1 or 3:1, etc.

[0013] Preferably, the polyester layer is polymerized from monomers containing both carboxyl and hydroxyl groups. This method enables the formation of short-chain polyesters, which enhances dispersibility in polar solvents, improves compatibility with esters in electrolytes, and better reduces stress on single-walled carbon nanotubes.

[0014] Preferably, based on the total mass of the active material, composite conductive agent, and binder (100%), the mass percentage of the polyester-modified single-walled carbon nanotubes is 0.01% to 0.1%, for example, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.1%. If the content of polyester-modified single-walled carbon nanotubes is too low, it cannot suppress the deterioration of electrode resistance caused by expansion during cycling of the ternary cathode, leading to cycle decay; if the content of polyester-modified single-walled carbon nanotubes is too high, it cannot further improve cycle performance, and since the cost of single-walled carbon nanotubes is extremely high, excessive addition will significantly increase the cost of the battery cell.

[0015] Preferably, the mass percentage of the zero-dimensional carbon material is 1% to 3% based on the total mass of the active material, composite conductive agent and binder as 100%, for example, it can be 1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.8%, 2%, 2.1%, 2.3%, 2.5%, 2.6%, 2.8% or 3%, etc.

[0016] Preferably, the zero-dimensional conductive carbon material includes carbon black.

[0017] As a preferred embodiment of the positive electrode sheet of the present invention, the composite conductive agent further includes multi-walled carbon nanotubes.

[0018] Preferably, based on the total mass of the active material, composite conductive agent, and binder as 100%, the mass percentage of the multi-walled carbon nanotubes is 0.01% to 0.8%, for example, it can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, or 0.8%, etc.

[0019] By introducing multi-walled carbon nanotubes into the composite conductive agent, which includes polyester-modified single-walled carbon nanotubes and zero-dimensional carbon materials, an effective multi-level conductive network can be formed. Moreover, multi-walled carbon nanotubes have better hardness than single-walled carbon nanotubes and can play a good filling role, thereby improving the performance of the positive electrode.

[0020] Preferably, based on the total mass of the active material, composite conductive agent and binder as 100%, the mass percentage of the active material is 96% to 97.9%, for example, it can be 96%, 96.3%, 96.5%, 96.8%, 97%, 97.2%, 97.4%, 97.6%, 97.8% or 97.9%, etc.

[0021] Preferably, based on the total mass of the active material, composite conductive agent and binder as 100%, the mass percentage of the binder is 0.8% to 1%, for example, it can be 0.8%, 0.85%, 0.9%, 0.95% or 1%, etc.

[0022] This invention does not specifically limit the type of positive electrode sheet; it can be either a positive electrode sheet or a negative electrode sheet. When the positive electrode sheet is a positive electrode sheet, the electrode material layer is a positive electrode material layer, and the active material is a positive electrode active material; when the positive electrode sheet is a negative electrode sheet, the electrode material layer is a negative electrode material layer, and the active material is a negative electrode active material.

[0023] In a second aspect, the present invention provides a method for preparing a positive electrode sheet as described in the first aspect, the method comprising the following steps:

[0024] (1) Preparation of polyester-modified single-walled carbon nanotubes;

[0025] (2) Polyester-modified single-walled carbon nanotubes are dispersed in a solvent to form a solution of polyester-modified single-walled carbon nanotubes, and then mixed with other raw materials to obtain an electrode slurry; wherein, the other raw materials include active substances, binders and zero-dimensional carbon materials;

[0026] (3) The electrode paste is coated on the surface of the current collector and then post-processed to obtain a positive electrode sheet.

[0027] The preparation method of this invention is simple, easy to operate, and suitable for industrial production.

[0028] Preferably, the method for preparing polyester-modified single-walled carbon nanotubes in step (1) includes the following steps:

[0029] (A) Oxidation treatment of single-walled carbon nanotubes yields an intermediate;

[0030] (B) The intermediate is mixed with a monomer containing both carboxyl and hydroxyl groups and reacted to obtain the polyester-modified single-walled carbon nanotubes.

[0031] Preferably, the oxidation treatment method in step (A) includes: mixing single-walled carbon nanotube powder, hydrogen peroxide and concentrated acid, and refluxing under stirring conditions to obtain the intermediate;

[0032] Preferably, the mass ratio of the single-walled carbon nanotube powder, hydrogen peroxide, and concentrated acid is 1:(0.1~0.5):(3~5), wherein the hydrogen peroxide is selected from "0.1~0.5", for example, it can be 0.1, 0.2, 0.3, 0.4, or 0.5, etc. The concentrated acid is selected from "3~5", for example, it can be 3, 3.5, 4, 4.5, or 5, etc.

[0033] In one embodiment, the mass concentration of hydrogen peroxide is 20% to 35%, for example, it can be 20%, 22%, 25%, 27%, 30%, 31%, 32%, 33% or 35%, etc.

[0034] The present invention does not specifically limit the type of concentrated acid, including but not limited to at least one of concentrated sulfuric acid, concentrated nitric acid or concentrated hydrochloric acid.

[0035] In one embodiment, the concentrated acid is concentrated sulfuric acid with a mass fraction of 90% to 98% (e.g., 90%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%).

[0036] Preferably, the stirring speed is 100 rpm to 200 rpm, for example, it can be 100 rpm, 120 rpm, 140 rpm, 150 rpm, 160 rpm, 180 rpm or 200 rpm.

[0037] Preferably, the reflux temperature is 120℃~160℃, for example, it can be 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃ or 160℃. The reflux time is 1h~3h, for example, it can be 1h, 1.5h, 2h, 2.5h or 3h.

[0038] Preferably, in step (B), the monomer containing both carboxyl and hydroxyl groups includes α-hydroxycarboxylic acid, and more preferably includes at least one of glycolic acid, lactic acid and mandelic acid.

[0039] Preferably, in step (B), the mass ratio of the intermediate to the monomer containing both carboxyl and hydroxyl groups is (3~5):1, for example, it can be 3:1, 3.5:1, 4:1, 4.5:1 or 5:1, etc.

[0040] Preferably, the reaction in step (B) is carried out in the presence of a catalyst, which preferably includes stannous octoate.

[0041] Preferably, the reaction temperature in step (B) is 160℃~200℃, for example, it can be 160℃, 170℃, 180℃, 190℃, or 200℃. The reaction pressure in step (B) is 100Pa~300Pa, for example, it can be 100Pa, 120Pa, 140Pa, 160Pa, 180Pa, 200Pa, 220Pa, 240Pa, 260Pa, 280Pa, or 300Pa. The reaction time in step (B) is 20min~40min, for example, it can be 20min, 25min, 30min, 35min, or 40min.

[0042] Preferably, in step (2), the solvent is a PVDF solution dissolved in NMP to form an adhesive solution.

[0043] Preferably, the mass fraction of the polyester-modified single-walled carbon nanotubes in the solution is 0.2% to 0.5%, for example, it can be 0.2%, 0.3%, 0.4% or 0.5%.

[0044] Preferably, the other raw materials mentioned in step (2) also include multi-walled carbon nanotubes.

[0045] In step (3) of the present invention, the current collector is selected according to the type of positive electrode sheet prepared. For the positive electrode sheet, the current collector is selected as a positive electrode current collector, including but not limited to aluminum foil or carbon-coated aluminum foil; for the negative electrode sheet, the current collector is selected as a negative electrode current collector, including but not limited to copper foil or carbon-coated copper foil.

[0046] The present invention does not specifically limit the coating method described in step (3), including but not limited to transfer coating or extrusion coating.

[0047] Thirdly, the present invention provides a lithium battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that the positive electrode and / or the negative electrode are positive electrode sheets prepared by the method described in the first aspect or the method described in the second aspect.

[0048] Preferably, the electrolyte includes esters.

[0049] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0050] Compared with existing technologies, the present invention has the following beneficial effects:

[0051] (1) This invention utilizes the composite conductivity of polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials, and applies them to the positive electrode to effectively suppress the volume expansion of the electrode and ensure its good electrochemical performance.

[0052] (2) By introducing multi-walled carbon nanotubes into the composite conductive agent, which includes polyester-modified single-walled carbon nanotubes and zero-dimensional carbon materials, an effective multi-level conductive network can be formed. Moreover, multi-walled carbon nanotubes have better hardness than single-walled carbon nanotubes and can play a good filling role, thereby better improving the performance of the positive electrode. Detailed Implementation

[0053] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0054] The specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0055] Preparation Example 1

[0056] This preparation example provides a polyester-modified single-walled carbon nanotube, the preparation method of which includes the following steps:

[0057] (A) Take 100g of single-walled carbon nanotube powder and mix it in a 5L glass flask according to the mass ratio of single-walled carbon nanotube powder: hydrogen peroxide (mass fraction 30%): concentrated sulfuric acid (mass fraction 98%) = 1:0.2:3.8. Stir at 100rpm and reflux at 140℃ for 2h. After adding 3L of water to dilute, filter to obtain a single-walled carbon nanotube powder intermediate with partial surface oxidation.

[0058] (B) Take 80g of intermediate and mix them in a 250mL flask according to the mass ratio of intermediate to glycolic acid = 4:1. Add 1g of stannous octoate as a catalyst and dehydrate and condense for 30min at a temperature of 180℃ and a pressure of 200Pa. After washing the product three times with acetone to remove free monomers and oligomers, short-chain polyester-modified single-walled carbon nanotube powder is obtained.

[0059] The prepared short-chain polyester-modified single-walled carbon nanotube powder was homogenized in a PVDF NMP solution to form a solution of polyester-modified single-walled carbon nanotubes with a mass fraction of 0.4%.

[0060] The polyester-modified single-walled carbon nanotubes of this preparation example include carbon nanotubes and a polyester layer coating the surface of the carbon nanotubes; the mass ratio of the polyester layer to the carbon nanotubes is 1:1.

[0061] Preparation Example 2

[0062] This preparation example provides a polyester-modified single-walled carbon nanotube, the preparation method of which includes the following steps:

[0063] (A) Take 100g of single-walled carbon nanotube powder and mix it in a 5L glass flask according to the mass ratio of single-walled carbon nanotube powder: hydrogen peroxide (mass fraction 30%): concentrated sulfuric acid (mass fraction 98%) = 1:0.4:3.5. Stir at 200rpm and reflux at 120℃ for 3h. After diluting with 3L of water, filter to obtain a single-walled carbon nanotube powder intermediate with partial surface oxidation.

[0064] (B) Take 80g of intermediate and mix it in a 250mL flask according to the mass ratio of intermediate to glycolic acid = 3:1. Add 1.2g of stannous octoate as a catalyst and dehydrate and condense it for 22min at a temperature of 200℃ and a pressure of 240Pa. After washing the product three times with acetone to remove free monomers and oligomers, short-chain polyester-modified single-walled carbon nanotube powder is obtained.

[0065] The prepared short-chain polyester-modified single-walled carbon nanotube powder was homogenized in a PVDF NMP solution to form a solution of polyester-modified single-walled carbon nanotubes with a mass fraction of 0.25%.

[0066] The polyester-modified single-walled carbon nanotubes of this preparation example include carbon nanotubes and a polyester layer coating the surface of the carbon nanotubes; the mass ratio of the polyester layer to the carbon nanotubes is 0.5:1.

[0067] Preparation Example 3

[0068] This preparation example provides a polyester-modified single-walled carbon nanotube, the preparation method of which includes the following steps:

[0069] (A) Take 100g of single-walled carbon nanotube powder and mix it in a 5L glass flask according to the mass ratio of single-walled carbon nanotube powder: hydrogen peroxide (mass fraction 30%): concentrated sulfuric acid (mass fraction 98%) = 1:0.1:4.5. Stir at 150rpm and reflux at 160℃ for 1h. After diluting with 3L of water, filter to obtain a single-walled carbon nanotube powder intermediate with partial surface oxidation.

[0070] (B) Take 80g of intermediate and mix it in a 250mL flask according to the mass ratio of intermediate to glycolic acid = 5:1. Add 0.8g of stannous octoate as a catalyst and dehydrate and condense it for 35min at a temperature of 170℃ and a pressure of 150Pa. After washing the product three times with acetone to remove free monomers and oligomers, short-chain polyester-modified single-walled carbon nanotube powder is obtained.

[0071] The prepared short-chain polyester-modified single-walled carbon nanotube powder was homogenized in a PVDF NMP solution to form a solution of polyester-modified single-walled carbon nanotubes with a mass fraction of 0.5%.

[0072] The polyester-modified single-walled carbon nanotubes of this preparation example include carbon nanotubes and a polyester layer coating the surface of the carbon nanotubes; the mass ratio of the polyester layer to the carbon nanotubes is 3:1.

[0073] In the following embodiments, medium speed refers to a rotational speed in the range of 2000rpm to 3000rpm, such as 2000rpm, 2200rpm, 2400rpm, 2600rpm, 2800rpm or 3000rpm, etc. There are no special requirements for the specific value in this range in each embodiment, and all can be applied to achieve the same effect.

[0074] In the following embodiments, high speed refers to a rotational speed in the range of 3000rpm to 5000rpm, such as 3000rpm, 3250rpm, 3500rpm, 3700rpm, 4000rpm, 4500rpm or 5000rpm. There are no special requirements for the specific values ​​in this range in each embodiment, and all are applicable and can achieve the same effect.

[0075] Example 1

[0076] This embodiment provides a positive electrode sheet, including a positive current collector (aluminum foil) and a positive electrode material layer disposed on the surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode active material (LiNi). 0.8 Co 0.1 Mn 0.1 O2 (abbreviated as NCM811), composite conductive agent and binder (PVDF), wherein the composite conductive agent includes polyester modified single-walled carbon nanotubes (preparation example 1), multi-walled carbon nanotubes and zero-dimensional conductive carbon materials (carbon black).

[0077] The mass ratio of positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes and PVDF is 97.08:1.6:0.3:0.02:1.

[0078] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0079] (1) Add PVDF and NMP to a double planetary mixer, mix at medium speed, and disperse for 3 hours to obtain PVDF solution;

[0080] (2) Add PVDF adhesive, carbon black, multi-walled carbon nanotubes, and polyester-modified single-walled carbon nanotubes into a double planetary stirrer and disperse at medium speed for 1.5h. Add NCM811 and NMP in two batches. After each addition of NCM811, disperse at high speed for 2h. After the end, add an appropriate amount of NMP to adjust the viscosity of the slurry to obtain a suitable positive electrode slurry for coating.

[0081] In this embodiment, with the total mass of the positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes and PVDF as 100%, the mass ratio of NMP is 39%.

[0082] (3) The positive electrode paste is coated on both sides of the aluminum foil by transfer coating, with a coating surface density of 20 mg / cm³. 2 Then, it is baked in an oven at 100°C for 2 minutes to obtain the positive electrode sheet.

[0083] Example 2

[0084] This embodiment provides a positive electrode sheet, including a positive current collector (aluminum foil) and a positive electrode material layer disposed on the surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode active material (LiNi). 0.8 Co 0.1 Mn 0.1 O2 (abbreviated as NCM811), composite conductive agent and binder (PVDF), wherein the composite conductive agent includes polyester modified single-walled carbon nanotubes (preparation example 2), multi-walled carbon nanotubes and zero-dimensional conductive carbon materials (carbon black).

[0085] The mass ratio of positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes and PVDF is 97.15:1:0.8:0.05:1.

[0086] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0087] (1) Add PVDF and NMP to a double planetary mixer, mix at medium speed, and disperse for 3 hours to obtain PVDF solution;

[0088] (2) Add PVDF adhesive, carbon black, multi-walled carbon nanotubes, and polyester-modified single-walled carbon nanotubes into a double planetary stirrer and disperse at medium speed for 1.5h. Add NCM811 and NMP in two batches. After each addition of NCM811, disperse at high speed for 2h. After the end, add an appropriate amount of NMP to adjust the viscosity of the slurry to obtain a suitable positive electrode slurry for coating.

[0089] In this embodiment, with the total mass of positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes and PVDF as 100%, the mass ratio of NMP is 37%.

[0090] (3) The positive electrode paste was coated on both sides of the aluminum foil by transfer coating, with a coating surface density of 18 mg / cm³. 2 Then, it is baked in an oven at 105°C for 2 minutes to obtain the positive electrode sheet.

[0091] Example 3

[0092] This embodiment provides a positive electrode sheet, including a positive current collector (aluminum foil) and a positive electrode material layer disposed on the surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode active material (LiNi). 0.8 Co 0.1 Mn 0.1 O2 (abbreviated as NCM811), composite conductive agent and binder (PVDF), wherein the composite conductive agent includes polyester modified single-walled carbon nanotubes (preparation example 3), multi-walled carbon nanotubes and zero-dimensional conductive carbon materials (carbon black).

[0093] The mass ratio of the positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes, and PVDF is 96:3:0.1:0.1:0.8.

[0094] This embodiment also provides a method for preparing the above-mentioned positive electrode sheet, including the following steps:

[0095] (1) Add PVDF and NMP to a double planetary mixer, mix at medium speed, and disperse for 3 hours to obtain PVDF solution;

[0096] (2) Add PVDF adhesive, carbon black, multi-walled carbon nanotubes, and polyester-modified single-walled carbon nanotubes into a double planetary stirrer and disperse at medium speed for 1.5h. Add NCM811 and NMP in two batches. After each addition of NCM811, disperse at high speed for 2h. After the end, add an appropriate amount of NMP to adjust the viscosity of the slurry to obtain a suitable positive electrode slurry for coating.

[0097] In this embodiment, with the total mass of the positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes and PVDF as 100%, the mass ratio of NMP is 43%.

[0098] (3) The positive electrode paste is coated on both sides of the aluminum foil by transfer coating, with a coating surface density of 15 mg / cm². 2 Then, it is baked in an oven at 90°C for 3 minutes to obtain the positive electrode sheet.

[0099] Example 4

[0100] This embodiment provides a positive electrode sheet, the preparation method of which differs from that of Example 1 in that multi-walled carbon nanotubes are not added in step (2). The mass ratio of positive electrode active material, carbon black, polyester-modified single-walled carbon nanotubes and PVDF in the positive electrode sheet is 97.3:1.6:0.1:1.

[0101] In this embodiment, with the total mass of the positive electrode active material, carbon black, polyester-modified single-walled carbon nanotubes and PVDF as 100%, the mass ratio of NMP is 39%.

[0102] Example 5

[0103] This embodiment provides a positive electrode sheet whose preparation method differs from that of Example 1 in that the content of carbon black and polyester-modified single-walled carbon nanotubes is changed while keeping the total amount of carbon black and polyester-modified single-walled carbon nanotubes unchanged.

[0104] In this embodiment, the mass ratio of the positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes, and PVDF is 97.08:1.615:0.3:0.005:1. Example 6

[0105] This embodiment provides a positive electrode sheet whose preparation method differs from that of Example 1 in that the content of carbon black and polyester-modified single-walled carbon nanotubes is changed while keeping the total amount of carbon black and polyester-modified single-walled carbon nanotubes unchanged.

[0106] In this embodiment, the mass ratio of the positive electrode active material, carbon black, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes, and PVDF is 97.08:1.47:0.3:0.15:1.

[0107] Comparative Example 1

[0108] This comparative example provides a positive electrode sheet whose preparation method differs from that of Example 1 in that polyester-modified single-walled carbon nanotubes are not added in step (2). The mass ratio of positive electrode active material, carbon black, multi-walled carbon nanotubes and PVDF in the positive electrode sheet is 96.6:1.6:0.8:1.

[0109] In this embodiment, with the total mass of the positive electrode active material, carbon black, multi-walled carbon nanotubes and PVDF being 100%, the mass ratio of NMP is 39%.

[0110] Comparative Example 2

[0111] This comparative example provides a positive electrode sheet whose preparation method differs from that of Example 1 in that the polyester-modified single-walled carbon nanotubes in step (2) are replaced with unmodified single-walled carbon nanotubes.

[0112] Comparative Example 3

[0113] This comparative example provides a positive electrode sheet whose preparation method differs from that of Example 1 in that carbon black is not added in step (2). The mass ratio of the positive electrode active material, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes, and PVDF in the positive electrode sheet is 96.6:1.6:0.8:1.

[0114] In this embodiment, with the total mass of the positive electrode active material, multi-walled carbon nanotubes, polyester-modified single-walled carbon nanotubes, and PVDF being 100%, the mass ratio of NMP is 39%.

[0115] Comparative Example 4

[0116] This comparative example provides a positive electrode sheet whose preparation method differs from that of Example 1 in that multi-walled carbon nanotubes and polyester-modified single-walled carbon nanotubes are not added in step (2). The mass ratio of positive electrode active material, carbon black, and PVDF in the positive electrode sheet is 96:3:1.

[0117] In this embodiment, with the total mass of the positive electrode active material, carbon black and PVDF being 100%, the mass ratio of NMP is 39%.

[0118] Assemble lithium batteries:

[0119] The assembly of lithium batteries using the positive electrode sheets of Examples 1-6 and Comparative Examples 1-4 specifically includes the following steps:

[0120] (1) Preparation of negative electrode:

[0121] A negative electrode slurry was prepared using graphite and silicon carbide as the negative electrode active materials, carbon black and aqueous single-walled carbon nanotubes as composite conductive agents, CMC, PAA, and SBR as binders, and water as a solvent. The ratio of graphite:silicon carbide:carbon black:aqueous single-walled carbon nanotubes:CMC:PAA:SBR:water was 86:9.6:1:0.08:0.4:2.0:1:99. The preparation method of the negative electrode slurry is as follows:

[0122] 1) Add CMC and deionized water to a double planetary mixer and beat at medium speed for 1.5 hours to obtain CMC adhesive solution.

[0123] 2) Add CMC adhesive, PAA, aqueous single-walled carbon nanotubes and carbon black to a double planetary stirrer and disperse at medium speed for 2 hours. Then add graphite and deionized water in two batches and disperse at high speed for 3 hours. Add an appropriate amount of deionized water to adjust the viscosity of the slurry to a suitable range for coating. Add SBR and disperse at low speed for 30 minutes to obtain the negative electrode slurry.

[0124] 3) Coating is applied to both sides of the negative electrode current collector (copper foil or carbon-coated copper foil) by extrusion coating, with a coating surface density of 10 mg / cm³. 2 The sample was baked in an oven at 65℃ for 2 minutes and then rolled to a density of 1.58 g / cm³. 3 The compaction density was determined to obtain the negative electrode sheet, which was then baked at 100℃ for 24 hours before use.

[0125] The above-mentioned negative electrode sheet was assembled with the positive electrode sheet, separator and electrolyte (the solvent of the electrolyte includes esters) of Examples 1-6 and Comparative Examples 1-4 to form a soft-pack battery cell with a capacity of 5Ah.

[0126] In one embodiment of the present invention, a method for detecting electrode performance is also provided, comprising: performing a cycle test on a lithium battery assembled with a positive electrode sheet; after cleaning the empty positive electrode sheet after cycling, characterizing the electrode sheet resistivity; and determining the growth rate of the electrode sheet resistivity by comparing the resistivity of the electrode sheet with the resistivity of the positive electrode sheet after rolling.

[0127] Preferably, the cleaning method includes: soaking the circulated empty positive electrode sheet in a mixed solution of DMC and DEC for a period of time, then taking it out, washing it, and drying it to obtain the cleaned empty positive electrode sheet.

[0128] Preferably, in the mixed solution of DMC and DEC, the volume ratio of DMC to DEC is (40~60):(60~40).

[0129] Preferably, the time period is 20 min to 40 min, for example, it can be 20 min, 22 min, 25 min, 30 min, 33 min, 35 min or 40 min.

[0130] Existing technologies lack clear evaluation indicators for ternary cathodes. In the design of conductive agent formulations, simply testing the resistivity of the electrode after coating and rolling cannot correspond to the cyclic expansion of the ternary electrode, leading to a significant increase in electrode resistivity and a corresponding decrease in cyclic resistivity. The embodiments of this invention determine the growth rate of electrode resistivity using the method described above, which can accurately correspond to the cyclic expansion of the ternary electrode.

[0131] Specifically, the present invention conducted cycle tests on the batteries assembled with positive electrode sheets from Examples 1-6 and Comparative Examples 1-4 according to the following method. The number of cycles was 100. After cycling, the cells were disassembled to obtain empty positive electrode sheets. These empty positive electrode sheets were immersed in a mixed solution of DMC and DEC (DMC to DEC volume ratio 1:1) for 30 minutes, washed three times, and then dried at 60°C to obtain cleaned empty positive electrode sheets. Subsequently, the electrode resistivity was characterized, and the growth rate of electrode resistivity was determined by comparing this resistivity with that of the positive electrode sheet after rolling. The results are shown in Table 1.

[0132] The expansion ratio of the positive electrode sheet after 100 cycles was tested. The test method was as follows: Batteries assembled with positive electrode sheets from Examples 1-6 and Comparative Examples 1-4 were subjected to a cycle test according to the following method. The number of cycles was 100. After cycling, the cells were disassembled to obtain empty positive electrode sheets. The thickness of the positive electrode sheet was measured using a micrometer, and the average value was calculated to obtain d1. The thickness of the positive electrode sheet after rolling was taken as d0. The expansion ratio of the positive electrode sheet was calculated using the formula: Expansion ratio = d1 / d0 - 1. The results are shown in Table 1.

[0133]

[0134] The battery was tested using a multi-channel electrical performance tester, a high-precision constant temperature chamber with ±0.5℃ temperature control, and an automated LIMS system. Specific test items included HPPC (Hybrid Pulse Power Characterization) charge and discharge tests at 25℃ and -20℃ respectively, to evaluate the charge and discharge impedance at room temperature and low temperature.

[0135] The room temperature cycling performance test method is as follows: under 25℃ conditions, charge at 2C, discharge at 1C, cycle for 800 times, test the initial discharge capacity C1 and the discharge capacity C2 after 800 cycles, calculate the room temperature cycling capacity retention rate based on the discharge capacity after 800 cycles and the initial discharge capacity, and the cycling capacity retention rate = C2 / C1×100%.

[0136] The high-temperature cycling performance test method is as follows: under 45℃ conditions, charge at 2C, discharge at 1C, cycle for 800 times, test the initial discharge capacity C3 and the discharge capacity C4 after 800 cycles, calculate the high-temperature cycling capacity retention rate based on the discharge capacity after 800 cycles and the initial discharge capacity, and the cycling capacity retention rate = C4 / C3×100%.

[0137] The test results are shown in Table 2.

[0138]

[0139] As shown in Tables 1 and 2, the present invention utilizes the composite conductivity of polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials, and applies them to the positive electrode to effectively suppress the volume expansion of the electrode and ensure its good electrochemical performance.

[0140] Meanwhile, a comparison between Examples 1 and 4 shows that by further introducing multi-walled carbon nanotubes into the composite conductive agent, which includes polyester-modified single-walled carbon nanotubes and zero-dimensional carbon materials, an effective multi-level conductive network can be formed, which is beneficial to improving battery performance. The expansion ratios of Examples 4 and 1 after 100 cycles are similar, and the resistivity growth rate of Example 4 (excluding multi-walled carbon nanotubes) is higher, indicating lower cycle performance than Example 1.

[0141] A comparison between Example 1 and Example 5 shows that if the amount of polyester-modified single-walled carbon nanotubes is too small, it will lead to the deterioration of the electrode resistance caused by the expansion of the ternary cathode during cycling, resulting in an increase in the expansion ratio of the battery after cycling and performance degradation.

[0142] A comparison between Example 1 and Example 6 shows that if the amount of polyester-modified single-walled carbon nanotubes used is too high, it cannot further improve the cycle performance, and the single-walled nanotubes themselves are extremely expensive, which will significantly increase the manufacturing cost.

[0143] A comparison of Example 1 with Comparative Examples 1-4 shows that the lack of polyester-modified single-walled carbon nanotubes in Comparative Example 1, the absence of modification of single-walled carbon nanotubes in Comparative Example 2, the absence of carbon black in Comparative Example 3, and the absence of multi-walled carbon nanotubes and polyester-modified single-walled carbon nanotubes in Comparative Example 4 all resulted in varying degrees of deterioration in the rate of increase of battery resistivity, the expansion ratio after 100 cycles, and the cycle capacity retention rate. This indicates that the synergistic use of polyester-modified single-walled carbon nanotubes and carbon black is essential.

[0144] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes an electrode material layer, which comprises an active material, a composite conductive agent, and a binder. The composite conductive agent comprises polyester-modified single-walled carbon nanotubes and zero-dimensional conductive carbon materials.

2. The positive electrode sheet according to claim 1, characterized in that, The polyester-modified single-walled carbon nanotubes include carbon nanotubes and a polyester layer coating the surface of the carbon nanotubes. Preferably, the mass ratio of the polyester layer to the carbon nanotubes is (0.5~3):1; Preferably, the polyester layer is polymerized from monomers containing both carboxyl and hydroxyl groups.

3. The positive electrode sheet according to claim 1 or 2, characterized in that, Based on the total mass of the active material, composite conductive agent, and binder being 100%, the mass percentage of the polyester-modified single-walled carbon nanotubes is 0.01% to 0.1%. Preferably, the zero-dimensional carbon material accounts for 1% to 3% of the total mass of the active material, composite conductive agent, and binder, which is 100% of the total mass. Preferably, the zero-dimensional conductive carbon material includes carbon black.

4. The positive electrode sheet according to any one of claims 1-3, characterized in that, The composite conductive agent also includes multi-walled carbon nanotubes; Preferably, the mass percentage of the multi-walled carbon nanotubes is 0.01% to 0.8% based on the total mass of the active material, composite conductive agent and binder as 100%.

5. The positive electrode sheet according to any one of claims 1-4, characterized in that, Based on the total mass of the active material, composite conductive agent, and binder being 100%, the mass percentage of the active material is 96% to 97.9%. Preferably, the mass percentage of the binder is 0.8% to 1%, based on the total mass of the active material, composite conductive agent and binder as 100%.

6. A method for preparing a positive electrode sheet as described in any one of claims 1-5, characterized in that, The preparation method includes the following steps: (1) Preparation of polyester-modified single-walled carbon nanotubes; (2) Polyester-modified single-walled carbon nanotubes are dispersed in a solvent to form a solution of polyester-modified single-walled carbon nanotubes, and then mixed with other raw materials to obtain an electrode slurry; wherein, the other raw materials include active substances, binders and zero-dimensional carbon materials; (3) The electrode paste is coated on the surface of the current collector and then post-processed to obtain a positive electrode sheet.

7. The method for preparing the positive electrode sheet according to claim 6, characterized in that, The method for preparing polyester-modified single-walled carbon nanotubes in step (1) includes the following steps: (A) Oxidation treatment of single-walled carbon nanotubes yields an intermediate; (B) The intermediate is mixed with a monomer containing both carboxyl and hydroxyl groups and reacted to obtain the polyester-modified single-walled carbon nanotubes.

8. The method for preparing the positive electrode sheet according to claim 6 or 7, characterized in that, The oxidation treatment method in step (A) includes: mixing single-walled carbon nanotube powder, hydrogen peroxide and concentrated acid, and refluxing under stirring conditions to obtain the intermediate; Preferably, the mass ratio of the single-walled carbon nanotube powder, hydrogen peroxide, and concentrated acid is 1:(0.1~0.5):(3~5); Preferably, the stirring speed is 100 rpm to 200 rpm; Preferably, the reflux temperature is 120℃~160℃, and the reflux time is 1h~3h; Preferably, in step (B), the monomer containing both carboxyl and hydroxyl groups includes α-hydroxycarboxylic acid, and more preferably includes at least one of glycolic acid, lactic acid and mandelic acid; Preferably, in step (B), the mass ratio of the intermediate to the monomer containing both carboxyl and hydroxyl groups is (3~5):1; Preferably, the reaction in step (B) is carried out in the presence of a catalyst, which preferably includes stannous octoate; Preferably, the temperature of the reaction in step (B) is 160℃~200℃, the pressure of the reaction in step (B) is 100Pa~300Pa, and the reaction time in step (B) is 20min~40min.

9. The method for preparing the positive electrode sheet according to any one of claims 6-8, characterized in that, In step (2), the solvent is a PVDF solution dissolved in NMP to form an adhesive solution; Preferably, in the solution of polyester-modified single-walled carbon nanotubes, the mass fraction of polyester-modified single-walled carbon nanotubes is 0.2% to 0.5%; Preferably, the other raw materials mentioned in step (2) also include multi-walled carbon nanotubes.

10. A lithium battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode is a positive electrode sheet prepared by any one of claims 1-5 or any one of claims 6-9; Preferably, the electrolyte includes esters.

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