Negative plate, hole impedance control method thereof and lithium ion battery

Abstract

The invention provides a negative electrode plate, a hole impedance control method thereof and a lithium ion battery, the negative electrode plate comprises a negative electrode current collector and negative electrode active material layers coating the surfaces of the two sides of the negative electrode current collector, and the negative electrode active material layers comprise 2 wt% of a binder. The binder comprises one or more of styrene butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid and derivatives thereof and polyaniline, the negative plate is a low-pore-impedance negative plate subjected to plasma treatment, and parameters in the plasma treatment process are as follows: discharge voltage V volt and discharge time t seconds, the hole impedance Z of the negative plate is as follows: Z (V, t) = 3.43 + 3.25 / (1 + (V / 82.3) (1.05 * (1 + 0.00001 V)) e (-0.1 t) + (t / 8.45) 1.12 e (-0.001 V)), V is more than or equal to 10 and less than 400, and t is more than or equal to 1 and less than or equal to 40; 400 < = V < = 500, and 1 < = t < 5. And through a formula Z (V, t), carrying out matching to obtain an optimal plasma processing condition and a negative electrode plate hole impedance regulation effect, and carrying out modification processing on the negative electrode plate under the optimal plasma processing condition to obtain the negative electrode plate of which the hole impedance is obviously reduced.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion battery manufacturing, and in particular to a negative electrode sheet, a pore impedance control method thereof, and a lithium-ion battery. Background Art

[0002] The diffusion rate of lithium ions in the electrochemical system of lithium-ion batteries is an important factor affecting the overall rate performance of the battery. During the battery charging process, the diffusion path of lithium ions is: escaping from the lattice of the positive electrode active material, passing through the diaphragm through the electrolyte, and then embedding into the lattice of the negative electrode active material. At this time, the pore network structure of the negative electrode active material layer is the core factor affecting the lithium ion insertion rate. In the specific implementation process, the pore impedance of the negative electrode sheet can be experimentally measured to evaluate the transmission resistance of the pore network of the negative electrode active material layer to lithium ions.

[0003] In the existing technology, the porosity of the negative electrode sheet is mainly increased by adding pore-forming agents, compounding single-walled carbon nanotubes, or plasma treatment to achieve the effect of reducing pore impedance. The basic principle of the plasma treatment method is to use highly excited substances such as oxygen anions and reactive oxygen free radicals contained in the plasma to bombard the binder on the surface of the negative electrode active material layer. Through physical etching and chemical oxidation, the binder is decomposed into small volatile molecules such as CO2 and H2O, thereby releasing more active sites, effectively increasing the electrochemically active surface area of ​​the negative electrode sheet and reducing pore impedance.

[0004] However, the plasma treatment process is difficult to monitor, and its effects are unpredictable. If the plasma treatment intensity is too high, the negative electrode active material layer will be destroyed due to excessive erosion; if the plasma treatment intensity is insufficient, the pore network of the negative electrode active material layer cannot be effectively improved, and the pore impedance control effect is poor. Therefore, in the actual implementation process, it is difficult to control the intensity of the plasma treatment and the effect of the plasma treatment on the pore impedance of the negative electrode sheet. Summary of the Invention

[0005] In order to solve the above problems, the pore impedance of the negative electrode sheet after plasma treatment is measured by plasma treatment parameters. The first aspect of the present application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer coated on both sides of the negative electrode current collector. The negative electrode active material layer contains a binder with a mass percentage of 2wt%, and the binder includes one or more of styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid and its derivatives and polyaniline. The negative electrode sheet is plasma treated. A negative electrode sheet with low pore impedance is prepared by plasma treatment. The parameters of the negative electrode sheet during the plasma treatment process are: discharge voltage V volts and discharge time t seconds. The pore impedance Z of the negative electrode sheet is: Z(V,t)=3.43+3.25 / (1+(V / 82.3)^(1.05×(1+0.00001V))e^(-0.1t)+(t / 8.45)^1.12e^(-0.001V)), 10≤V<400, 1≤t≤40; 400≤V≤500, 1≤t<5.

[0006] In some optional embodiments, the negative electrode active material layer comprises the following solid materials in the following mass ratios: 96 wt% of the negative electrode active material, 1.0 wt% of the conductive agent, 1 wt% of the thickener, and 2 wt% of the binder. The surface compaction density of the negative electrode active material layer is 1-2 g / cm 3 The thickness of the negative electrode active material layer on one side is 50-100 μm.

[0007] A second aspect of the present application provides a method for controlling the pore impedance of a negative electrode sheet, wherein the negative electrode sheet is any one of the negative electrode sheets described above, and the method for controlling the pore impedance of the negative electrode sheet comprises the following steps:

[0008] S1: rolling and baking the negative electrode current collector coated with the negative electrode active material layer to obtain a negative electrode substrate; S2: placing the substrate in a discharge space of a dielectric barrier plasma device; and

[0009] S3: Setting the discharge voltage of the dielectric barrier plasma device to V volts and the discharge time to t seconds, performing plasma treatment on the substrate to prepare the negative electrode sheet with low pore impedance, and the pore impedance Z of the negative electrode sheet satisfies:

[0010] Z(V,t)=3.43+3.25 / (1+(V / 82.3)^(1.05×(1+0.00001V))e^(-0.1t)+ (t / 8.45)^1.12e^(-0.001V)), 10≤V<400, 1≤t≤40; 400≤V≤500, 1≤t<5.

[0011] In some optional embodiments, the dielectric barrier plasma device includes a power supply, a high-voltage electrode and a ground electrode respectively extending from the two poles of the power supply, and an insulating medium respectively arranged at the discharge ends of the high-voltage electrode and the ground electrode, and the discharge space is separated by the two insulating media, and the width of the discharge space in the direction of the connection between the two high-voltage electrodes is: 0.2-1 cm.

[0012] In some optional embodiments, the substrate is arranged parallel to the insulating medium and located at the center of the discharge space, and the negative electrode active material layer is completely exposed in the discharge channel of the discharge space.

[0013] In some optional embodiments, the plasma emitted by the dielectric barrier plasma device penetrates the negative electrode active material layer to a depth d less than 200 nm.

[0014] In some optional embodiments, the AC frequency of the power supply is 50 Hz-1 MHz, the power of the dielectric barrier plasma device is 100-2000 W, and the discharge space is an air atmosphere at normal pressure.

[0015] In some optional embodiments, the insulating medium is made of glass, quartz, ceramic, thin enamel or organic polymer.

[0016] In some optional embodiments, the preparation of the substrate further comprises the following steps:

[0017] S11: taking copper foil as the negative electrode current collector;

[0018] S12: preparing solid materials: 96 wt% of negative electrode active material, 1.0 wt% of conductive agent, 1 wt% of thickener and 2 wt% of binder, adding the solid materials to deionized water, and stirring to form a negative electrode slurry with a solid content of 50 wt%; and

[0019] S13: Apply the negative electrode slurry on both sides of the copper foil, dry it, roll-press it at a pressure of 1-5T, and bake it at a temperature of 65-105°C for 1-4 hours to obtain the substrate. The thickness of the negative electrode active material layer on one side is 50-100 μm.

[0020] A third aspect of the present application provides a lithium-ion battery comprising a positive electrode sheet, a separator and a negative electrode sheet, characterized in that the negative electrode sheet is a negative electrode sheet as described in any of the above items after being plasma treated according to the pore impedance control method of the negative electrode sheet as described in any of the above items.

[0021] This application has at least the following technical effects:

[0022] 1) The first aspect of the present application provides a negative electrode sheet, which matches the optimal plasma treatment conditions and the negative electrode sheet pore impedance control effect through the formula Z(V,t), and modifies the negative electrode sheet under the optimal plasma treatment conditions to obtain a negative electrode sheet with significantly reduced pore impedance.

[0023] 2) The second aspect of the present application provides a method for controlling the pore impedance of a negative electrode sheet, establishes a formula Z(V, t), and associates the voltage V parameter of the plasma treatment process with the discharge time parameter t and the functional relationship with the pore impedance Z of the negative electrode sheet. After clarifying the specific parameters of voltage and discharge time, the size of the pore impedance Z can be calculated, which facilitates matching the optimal plasma treatment conditions and the negative electrode sheet pore impedance control effect.

[0024] 3) The third aspect of the present application provides a lithium-ion battery, which adopts the pore impedance control method of the negative electrode sheet described in any of the above items to produce a negative electrode sheet with significantly reduced pore impedance, thereby improving the diffusion efficiency of lithium ions during the charging process and increasing the battery's first-cycle charging capacity and discharge capacity retention rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 A schematic diagram of a dielectric barrier plasma device according to an optional embodiment of the method for controlling pore impedance of a negative electrode sheet of the present application;

[0027] Figure 2 This is a scatter plot of the negative electrode sheet pore impedance and discharge time at a discharge voltage of 100 volts for Examples 1 to 5 of the present application;

[0028] Figure 3 These are scatter plots of the pore impedance and discharge voltage for Examples 1, 6, 19, and 19 of the present application when the discharge duration t is 1 second and t is 5 seconds, respectively. DETAILED DESCRIPTION

[0029] The following describes in detail embodiments of this embodiment. In this description, "several" means one or more, "many" means more than two, "greater than," "less than," and "exceed" are understood to exclude the number itself, while "above," "below," and "within" are understood to include the number itself. The terms "first" and "second" are used solely to distinguish technical features and are not to be construed as indicating or implying relative importance, or as implicitly specifying the number or order of the technical features.

[0030] In the description of this embodiment, unless otherwise clearly defined, terms such as setting, installing, and connecting should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in this embodiment based on the specific content of the technical solution.

[0031] The common structure of existing lithium-ion batteries includes a casing with an opening at one end, a winding core assembled into the casing through the opening, an electrolyte injected into the casing, and a cap positioned over the casing opening. It is understood that the lithium-ion battery can be a cylindrical lithium-ion battery or a prismatic lithium-ion battery.

[0032] The core is formed by winding a stacked positive electrode sheet, a separator and a negative electrode sheet. The positive electrode sheet includes a positive electrode collector and a positive electrode active material layer coated on both sides of the positive electrode collector. The negative electrode sheet includes a negative electrode collector and a negative electrode active material layer coated on both sides of the negative electrode collector. Lithium ions complete the conversion between chemical energy and electrical energy through insertion or deinsertion reactions on the surfaces of the positive electrode active material layer and the negative electrode active material layer.

[0033] In a first aspect, the present application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer coated on both sides of the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active substance and a binder, wherein the binder comprises one or more of styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid and its derivatives, and polyaniline. The negative electrode sheet is a low-pore impedance negative electrode sheet that has been treated with plasma. The parameters of the negative electrode sheet during the plasma treatment process are: discharge voltage V volts and discharge time t seconds. The pore impedance Z of the negative electrode sheet is: Z(V, t) = 3.43 + 3.25 / (1 + (V / 82.3) ^ (1.05 × (1 + 0.00001 V)) e ^ (-0.1 t) + (t / 8.45) ^ 1.12 e ^ (-0.001 V)), 10 ≤ V < 400, 1 ≤ t ≤ 40; 400 ≤ V ≤ 500, 1 ≤ t < 5. Through the formula Z(V,t), the optimal plasma treatment conditions and the negative electrode sheet pore impedance control effect are matched, and the negative electrode sheet is modified under the optimal plasma treatment conditions to obtain a negative electrode sheet with significantly reduced pore impedance.

[0034] Specifically, during the plasma treatment, the active groups or molecular segments in the binder are attacked by reactive oxygen free radicals (·O), ultimately decomposing into small volatile molecules such as CO2 and H2O, thereby releasing more active sites in the negative electrode active material layer. Examples include the benzene rings and butadiene segments in styrene-butadiene rubber, the carboxylmethyl and hydroxyl groups, and the cellulose backbone containing glycosidic bonds in sodium carboxymethyl cellulose, the polymer segments in polyvinylidene fluoride, the carboxyl groups, ester groups, and polymer segments in polyacrylic acid and its derivatives, and the benzene rings and amine polymer segments in polyaniline.

[0035] Furthermore, the negative electrode active material layer includes the following solid materials in the following mass ratios: 96 wt% of the negative electrode active material, 1.0 wt% of the conductive agent, 1 wt% of the thickener and 2 wt% of the binder, illustratively, 96 wt% of artificial graphite, 1.0 wt% of Super-P, 1 wt% of carboxymethyl cellulose and 2% of styrene-butadiene rubber, and the surface compaction density of the negative electrode active material layer is: 1-2 g / cm 3 The thickness of the negative electrode active material layer on a single side is 50-100 μm. During the plasma treatment process, reactive oxygen radicals (·O) attack the benzene rings and butadiene segments in the styrene-butadiene rubber, ultimately decomposing into small volatile molecules such as CO2 and H2O. This releases more active sites in the negative electrode active material layer, effectively increasing the electrochemically active surface area of ​​the negative electrode sheet and reducing pore impedance.

[0036] It can be understood that the intensity of the plasma treatment is correlated with the degree of decomposition of the binder. In the present application, the parameters of the plasma discharge voltage and discharge time are controlled to control the degree of decomposition of the binder, so as to ensure the mechanical strength of the negative electrode active material layer and release as many active sites as possible. Preferably, the depth of plasma penetration into the negative electrode active material layer d <200nm, and the activated thickness of the negative electrode active material layer on each side accounts for 0.4% of the total thickness, which can better balance the structural stability and electrochemical activity of the negative electrode sheet.

[0037] The present application provides two methods for controlling the pore impedance of a negative electrode sheet, the methods comprising the following steps:

[0038] S1: rolling and baking the negative electrode current collector coated with the negative electrode active material layer to obtain a negative electrode substrate; S2: placing the substrate in a discharge space of a dielectric barrier plasma device; and

[0039] S3: Setting the discharge voltage of the dielectric barrier plasma device to V volts and the discharge time to t seconds, performing plasma treatment on the substrate to prepare the negative electrode sheet with low pore impedance, and the pore impedance Z of the negative electrode sheet satisfies:

[0040] Z(V,t)=3.43+3.25 / (1+(V / 82.3)^(1.05×(1+0.00001V))e^(-0.1t)+ (t / 8.45)^1.12e^(-0.001V)), 10≤V<400, 1≤t≤40; 400≤V≤500, 1≤t<5.

[0041] Further, see Figure 1 The dielectric barrier plasma device includes a power supply, a high-voltage electrode and a ground electrode respectively extending from the two poles of the power supply, and an insulating medium respectively arranged at the discharge ends of the high-voltage electrode and the ground electrode. The discharge space is separated by the two insulating media, and the width of the discharge space in the direction of the connection between the two high-voltage electrodes is: 0.2-1 cm.

[0042] In a specific implementation process, the power supply is an AC power supply with a sinusoidal waveform.

[0043] Specifically, the insulating medium is made of glass, quartz, ceramic, thin enamel or organic polymer, wherein the organic polymer can be polyimide, polytetrafluoroethylene, silicone rubber or the like.

[0044] Furthermore, the negative electrode sheet is arranged parallel to the insulating medium and located at the center of the discharge space, and the negative electrode active material layer is completely exposed within the discharge channel of the discharge space. Specifically, the negative electrode active material layers on both sides of the negative electrode sheet should face the discharge surface of the insulating medium, and the discharge surface of the insulating medium should be larger than the negative electrode active material layer, so that the negative electrode active material layer is completely exposed within the discharge channel formed between the two discharge surfaces of the insulating medium.

[0045] In some optional embodiments, the plasma treatment penetrates the negative electrode active material layer to a depth d less than 200 nm.

[0046] In practice, when the thickness of the negative electrode active material layer bombarded by excited species during plasma treatment exceeds 200nm, the structure of the negative electrode active material layer becomes loose due to excessive decomposition of the binder. During the subsequent electrochemical reaction, the negative electrode active material layer is unable to withstand the impact of lithium ion insertion and extraction, and is prone to layer collapse and shedding, making normal charge and discharge impossible. Limiting the thickness that plasma treatment can penetrate can better balance the structural stability and electrochemical activity of the negative electrode sheet.

[0047] Furthermore, the AC frequency of the power supply is 50 Hz-1 MHz, the power of the dielectric barrier plasma device is 100-2000 W, and the discharge space is an atmospheric pressure air atmosphere.

[0048] In some optional implementation methods, the preparation of the negative electrode sheet further includes the following steps:

[0049] S11: taking copper foil as the negative electrode current collector;

[0050] S12: preparing solid materials: 96 wt% of negative electrode active material, 1.0 wt% of conductive agent, 1 wt% of thickener and 2 wt% of binder, adding the solid materials to deionized water, and stirring to form a negative electrode slurry with a solid content of 50 wt%; and

[0051] S13: coating the negative electrode slurry on both sides of the copper foil, drying it, rolling it with a pressure of 1-5T, and baking it at a temperature of 65-105°C for 1-4h to obtain the substrate.

[0052] Furthermore, the negative electrode active material can be artificial graphite, natural graphite, silicon oxide material, silicon carbon material, etc., the conductive agent can be Super-P, acetylene black, carbon nanotubes, graphene, etc., the thickener can be carboxymethyl cellulose, hydroxypropyl methyl cellulose, etc., and the binder can be styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid and its derivatives, polyaniline, etc.

[0053] Furthermore, the surface compaction density of the negative electrode active material layer is 1-2 g / cm 3 The thickness of the negative electrode active material layer on one side is 50-100 μm.

[0054] A third aspect of the present application provides a lithium-ion battery, wherein the negative electrode sheet is a negative electrode sheet that has been plasma-treated according to any of the aforementioned methods for controlling the pore impedance of a negative electrode sheet. Using any of the aforementioned methods for controlling the pore impedance of a negative electrode sheet produces a negative electrode sheet with significantly reduced pore impedance, thereby improving the diffusion efficiency of lithium ions during charging and increasing the battery's first-cycle charge capacity and discharge capacity retention rate.

[0055] The technical solution of the present application is described below with reference to Examples 1-19 and Comparative Examples 1-4.

[0056] Example 1 provides a negative electrode sheet and a battery comprising the negative electrode sheet, comprising the following specific steps:

[0057] 1. Production of negative electrode sheet:

[0058] The negative electrode sheet includes a copper foil and a negative electrode slurry coated on both sides of the copper foil. The solid materials are prepared as follows: 96wt% artificial graphite, 1.0wt% Super-P, 1wt% carboxymethyl cellulose and 2% styrene-butadiene rubber. The above solid materials are added to deionized water and stirred to form a negative electrode slurry with a solid content of 50wt%. The negative electrode slurry is applied to both sides of the copper foil, dried, rolled and baked to obtain the negative electrode sheet.

[0059] 2. Production of positive electrode:

[0060] The positive electrode sheet includes a positive electrode current collector aluminum foil and a positive electrode slurry coated on the surface of the aluminum foil. The solid materials are prepared as follows: 96.5% nickel cobalt manganese oxide ternary positive electrode (NCM), 0.5% single-walled carbon nanotubes, 2% Super-P and 1% polyvinylidene fluoride. The above solid materials are added to N-methylpyrrolidone (NMP) and stirred to form a positive electrode slurry with a solid content of 60wt%. The positive electrode slurry is applied to both sides of the aluminum foil, dried, rolled and baked to obtain the positive electrode sheet.

[0061] 3. Preparation of electrolyte:

[0062] Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 1:1:1 to obtain an organic solvent, 2 wt% of vinylene carbonate (VC) film-forming additive is added, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L.

[0063] 4. Plasma treatment of negative electrode sheet:

[0064] The prepared negative electrode sheet was placed in the discharge space of a dielectric barrier plasma device, and a voltage of 100 volts was applied between the two high-voltage electrodes of the dielectric barrier plasma device for 5 seconds to prepare a plasma-treated negative electrode sheet.

[0065] 5. Preparation of button batteries:

[0066] Coin cells were used to evaluate the electrical properties of the negative electrode active material layer, including its lithium intercalation and deintercalation capacity and first coulombic efficiency. The negative electrode sheet was punched into small discs with a diameter of 14 mm using a punch. The discs were weighed using a balance. The small negative electrode discs were assembled with a ceramic separator, a lithium sheet, and a CR2032 button cell case. The electrolyte was then added dropwise to form the button cell.

[0067] 6. Preparation of symmetrical soft pack batteries:

[0068] A symmetrical battery was used to evaluate the hole impedance of the negative electrode sheet. The negative electrode sheet was punched into a 7 cm x 5 cm electrode sheet using a sheet press. The electrode sheet thickness was measured. The electrode sheet, ceramic separator, and aluminum-plastic film were packaged, electrolyte was injected, and air was evacuated to produce a symmetrical soft-pack battery.

[0069] 7. Preparation of all-electric soft-pack batteries

[0070] The effect of plasma treatment of the negative electrode sheet on the battery's rate performance was evaluated using a fully electric pouch cell. The positive and negative electrode sheets were punched into 7 cm × 5 cm sheets using a sheet press. The stamped sheets, ceramic separator, and aluminum-plastic film were then packaged, electrolyte injected, and evacuated to create the fully electric pouch cell.

[0071] Compared with Example 1, the discharge voltage and / or discharge duration parameters during the plasma treatment of the negative electrode sheet were changed in Examples 2 to 19 and Comparative Examples 1 to 4. For specific parameter changes, see Table 1:

[0072] Table 1

[0073]

[0074]

[0075] Performance testing:

[0076] (1) First-cycle discharge capacity and first-cycle coulombic efficiency test: A battery tester was used to perform charge and discharge tests on button cells prepared from the negative electrode sheets of each embodiment and comparative example to evaluate the electrochemical performance of the negative electrode sheets. The discharge cutoff voltage was set to 0.005V and the discharge rate was 0.01C; the charge cutoff voltage was set to 1.5V and the charge rate was 0.1C. The first-cycle coulombic efficiency of the negative electrode sheet was calculated using the following formula:

[0077]

[0078] (2) Negative Electrode Pore Impedance Test: Symmetrical soft-pack batteries prepared using the negative electrode sheets of each Example and Comparative Example were used to evaluate the pore impedance of the negative electrode sheets. Electrochemical impedance spectroscopy (EIS) was performed on the symmetrical soft-pack batteries of each Example and Comparative Example at open circuit voltage using an electrochemical instrument with a frequency range of 100 kHz to 0.1 Hz.

[0079] (3) Rate performance test: The rate performance of each fully electric soft pack battery was evaluated using the negative electrode sheets of each embodiment and comparative example. First, each fully electric soft pack battery was subjected to formation aging, and then a rate discharge test was performed. In the rate discharge test, the charge cut-off voltage was 4.0V and the charge rate was 0.1C; the discharge cut-off voltage was 2.5V, and the discharge was performed at rates of 0.1C, 0.5C, 1C, 2C, 4C, 6C and 8C respectively; the retention rate of the 4C rate discharge capacity was calculated by the following formula:

[0080]

[0081] The test results of the first-cycle discharge capacity in grams, the first-cycle charge capacity in grams, the discharge capacity retention rate, the first-cycle coulombic efficiency, and the pore impedance of the negative electrode sheet for Examples 1-19 and Comparative Examples 1-4, as well as the calculated results and the errors of the calculated values ​​relative to the measured values, are shown in Table 2:

[0082] Table 2

[0083]

[0084]

[0085] From the results of Examples 1-5 and Comparative Example 2 in Table 2, it can be seen that when the discharge voltage V is fixed at 100V, the longer the discharge time of the plasma treatment of the negative electrode sheet is, the lower the measured pore impedance of the negative electrode sheet is. Figure 2 The scatter plot in also reflects this trend, which is consistent with the trend of change in the calculation results of the function Z(V, t) proposed in this application. The optimal discharge time can be predicted based on the calculated value of the pore impedance Z of the negative electrode.

[0086] In Table 2, the results of Example 1, Examples 6-10, and Comparative Example 3 are compared with the results of Examples 11-19 and Comparative Example 4. It can be seen that when the discharge time t is fixed at 1 second and 5 seconds, the measured pore impedance of the negative electrode sheet decreases with the increase of the discharge voltage. Moreover, at the same discharge voltage, the longer the discharge time, the lower the pore impedance of the negative electrode sheet. Figure 3 The scatter plot in also reflects this trend. The longer the discharge time, the lower the limit value of the adapted discharge voltage.

[0087] In addition, Table 2 also reflects the correlation effect between the pore impedance of the negative electrode sheet and the first-cycle charge capacity. When the pore impedance of the negative electrode sheet is smaller, the first-cycle charge capacity will increase accordingly. This also confirms that plasma treatment can improve the pore network of the negative electrode active material layer, allowing lithium ions to be more efficiently inserted and inserted into the negative electrode active material layer during the charging process.

[0088] In summary, the Z(V, t) function of the present application can accurately predict the changing trend of the pore impedance with the discharge voltage V parameter and the discharge time t parameter of the plasma treatment. It can be seen from the calculated and measured values ​​of the pore impedance of Examples 1-19 in Table 2 that the calculation error of the pore impedance of the negative electrode sheet by the Z(V, t) function model is less than 10%. In the specific implementation process, according to the actual situation of the equipment and production needs, the discharge voltage V or the discharge time t is determined within the V, t interval applicable to the function Z(V, t), and substituted into the Z(V, t) function model to obtain the corresponding (Z, t) value or several groups of (Z, t) sets, or the corresponding (Z, V) value or (Z, V) set, and the optimal discharge voltage V and the optimal discharge time t can be easily screened out to prepare the negative electrode sheet with the lowest pore impedance Z at the selected discharge voltage V or discharge time t. Combined with the optimal discharge voltage and the optimal discharge time, it can be used to guide the adjustment of the battery manufacturing process.

[0089] Although examples of the present embodiment have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and intent of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer coated on both sides of the negative electrode current collector, characterized in that: The negative electrode active material layer contains a binder with a mass percentage of 2wt%, and the binder includes one or more of styrene-butadiene rubber, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid and its derivatives, and polyaniline. The negative electrode sheet is a low-pore impedance negative electrode sheet that has been treated with plasma. The parameters of the negative electrode sheet during the plasma treatment process are: discharge voltage V volts and discharge time t seconds. The pore impedance Z of the negative electrode sheet is: Z(V, t) = 3.43 + 3.25 / (1 + (V / 82.3) ^ (1.05 × (1 + 0.00001V)) e ^ (-0.1t) + (t / 8.45) ^ 1.12 e ^ (-0.001V)), 10 ≤ V < 400, 1 ≤ t ≤ 40; 400 ≤ V ≤ 500, 1 ≤ t < 5.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material layer includes the following solid materials in the following mass ratios: 96 wt% of the negative electrode active material, 1.0 wt% of the conductive agent, 1 wt% of the thickener, and 2 wt% of the binder. The surface compaction density of the negative electrode active material layer is 1-2 g / cm 3 The thickness of the negative electrode active material layer on one side is 50-100 μm.

3. A method for controlling the hole impedance of a negative electrode sheet, characterized in that: The negative electrode sheet is the negative electrode sheet according to claim 1 or 2, comprising the following steps: S1: rolling and baking the negative electrode current collector coated with the negative electrode active material layer to obtain a negative electrode substrate; S2: placing the substrate in a discharge space of a dielectric barrier plasma device; and S3: Setting the discharge voltage of the dielectric barrier plasma device to V volts and the discharge time to t seconds, performing plasma treatment on the substrate to prepare the negative electrode sheet with low pore impedance, and the pore impedance Z of the negative electrode sheet satisfies: Z(V,t)=3.43+3.25 / (1+(V / 82.3)^(1.05×(1+0.00001V))e^(-0.1t)+ (t / 8.45)^1.12e^(-0.001V)), 10≤V<400, 1≤t≤40; 400≤V≤500, 1≤t<5.

4. The method for controlling the hole impedance of a negative electrode sheet according to claim 3, wherein: The dielectric barrier plasma device includes a power supply, a high-voltage electrode and a ground electrode respectively extending from the two poles of the power supply, and an insulating medium respectively arranged at the discharge ends of the high-voltage electrode and the ground electrode. The two insulating media separate the discharge space, and the width of the discharge space in the direction of the connection between the two high-voltage electrodes is: 0.2-1 cm.

5. The method for controlling the hole impedance of a negative electrode sheet according to claim 4, wherein: The substrate is arranged parallel to the insulating medium and located at the center of the discharge space, and the negative electrode active material layer is completely exposed in the discharge channel of the discharge space.

6. The method for controlling the hole impedance of a negative electrode sheet according to claim 4, wherein: The plasma emitted by the dielectric barrier plasma device penetrates the negative electrode active material layer to a depth d less than 200 nm.

7. The method for controlling the hole impedance of a negative electrode sheet according to claim 4, wherein: The AC frequency of the power supply is 50 Hz-1 MHz, the power of the dielectric barrier plasma device is 100-2000 W, and the discharge space is an air atmosphere at normal pressure.

8. The method for controlling the hole impedance of a negative electrode sheet according to claim 4, wherein: The insulating medium is made of glass, quartz, ceramic, thin enamel or organic polymer.

9. The method for controlling the pore impedance of a negative electrode sheet according to claim 3, wherein: The preparation of the substrate further comprises the following steps: S11: taking copper foil as the negative electrode current collector; S12: preparing solid materials: 96 wt% of negative electrode active material, 1.0 wt% of conductive agent, 1 wt% of thickener and 2 wt% of binder, adding the solid materials to deionized water, and stirring to form a negative electrode slurry with a solid content of 50 wt%; and S13: Apply the negative electrode slurry on both sides of the copper foil, dry it, roll-press it at a pressure of 1-5T, and bake it at a temperature of 65-105°C for 1-4 hours to obtain the substrate. The thickness of the negative electrode active material layer on one side is 50-100 μm.

10. A lithium-ion battery comprising a positive electrode, a separator and a negative electrode, characterized in that: The negative electrode sheet is the negative electrode sheet according to claim 1 or 2 after being subjected to plasma treatment according to the pore impedance control method for a negative electrode sheet according to any one of claims 3 to 9.

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