Electrode plate, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of energy storage. The invention discloses an electrode pole piece which comprises a current collector and at least three electrode material layers sequentially arranged on the surface of one side of the current collector. The porosity of each electrode material layer is gradually increased in a gradient manner along the direction vertical to the surface of the current collector and deviating from the current collector; wherein the conductive additive in the innermost electrode material layer in contact with the surface of the current collector comprises not less than 50wt% of a small-particle conductive agent, and the conductive additive in the outermost electrode material layer comprises not less than 80wt% of a one-dimensional carbon material. The other conductive additives in the electrode material layer located in the middle layer comprise a small-particle conductive agent and a one-dimensional carbon material, the content of the one-dimensional carbon material is lower than 80 wt%, and the content of the small-particle conductive agent is lower than 50 wt%. According to the electrode pole piece, a gradient pole piece design mode is adopted, so that the energy density of a battery is effectively improved, the rate capability of the battery is improved, and high-rate charging is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage, and specifically relates to an electrode plate and a preparation method thereof, and further relates to a secondary battery. Background Art

[0002] With the development of portable electronic products and electric vehicles, lithium batteries are required to have higher energy density and longer driving range. Without changing the basic chemical system of the battery, designing the electrode structure is a feasible and universal method to improve the energy density of lithium-ion batteries. Since the energy of the battery comes only from the electroactive substances in the electrodes, the core principle of the design of new battery structures is to minimize the proportion of inactive components while maintaining or improving the battery performance of active substances per unit mass or volume. At present, the loading capacity of lithium-sulfur electrodes is basically 10mg / cm 2 Below; the loading of lithium battery electrodes is basically 400mg / cm 2 The following needs to be improved.

[0003] Therefore, in order to obtain high-load batteries, it is necessary to conduct in-depth research and improvement on the electrodes. Summary of the invention

[0004] The present invention aims to solve one of the technical problems in the related art to at least a certain extent. To this end, the embodiments of the present invention provide an electrode plate and a preparation method thereof, which adopts a gradient plate design, which not only effectively improves the energy density of the battery, but also improves the rate performance of the battery, realizes high-rate charging, and adopts a dry method for preparation, without the use of organic solvents, reduces costs, and is more environmentally friendly.

[0005] The electrode plate of the embodiment of the present invention comprises a current collector and at least three electrode material layers sequentially arranged on one side surface of the current collector in a direction perpendicular to the current collector surface;

[0006] The porosity of each electrode material layer increases gradually along the direction perpendicular to the current collector surface and away from the current collector;

[0007] The electrode material layer includes electrode active materials, conductive additives and binders, wherein the conductive additives in the innermost electrode material layer contacting the surface of the current collector include not less than 50wt% of small-particle conductive agents, the conductive additives in the outermost electrode material layer include not less than 80wt% of one-dimensional carbon materials, and the conductive additives in other electrode material layers located in the middle layer include small-particle conductive agents and one-dimensional carbon materials, and the content of the one-dimensional carbon material is less than 80wt%, the content of the small-particle conductive agent is less than 50wt%, and the small-particle conductive agent has a particle size of not more than 50nm.

[0008] Advantages and technical effects brought by the electrode plates of the embodiments of the present invention: In the embodiments of the present invention, at least three electrode material layers are arranged on the current collector. By gradient designing the porosity of each material layer and designing different conductive agent compositions according to the different porosities of each electrode material layer, the transmission of ions and electrons is effectively improved, the reaction rate is increased, the manufactured battery can be quickly charged, and has a higher energy density, giving the battery excellent comprehensive performance.

[0009] In some embodiments, the electrode active material includes a positive electrode active material or a negative electrode active material; optionally, the positive electrode active material includes at least one of sulfur, lithium iron phosphate, lithium iron manganese phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material; the negative electrode active material includes at least one of graphite or silicon-based materials.

[0010] In some embodiments, the binder includes at least one of polyethylene oxide, polyvinylidene fluoride, styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylonitrile or sodium hydroxymethyl cellulose; and / or the one-dimensional carbon material includes at least one of carbon fiber or carbon nanotube.

[0011] In some embodiments, the small particle conductive agent includes at least one of Super P (conductive carbon black), acetylene black, and Ketjen black.

[0012] In some embodiments, the electrode material layer is three layers, and the porosities of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-30%, 30-40% and 40-50% respectively.

[0013] In some embodiments, when the electrode active material is at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material, the electrode active material loading of the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer is 50-100 mg / cm 2 100-200mg / cm 2 and 200-300mg / cm 2 ;

[0014] In some embodiments, when the electrode active material is sulfur, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer are 30-60 mg / cm 2 、60-120mg / cm 2 and 120-200mg / cm 2 .

[0015] In some embodiments, when the electrode active material is a negative electrode active material, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-40 mg / cm 2 40-80mg / cm 2 and 80-160mg / cm 2 .

[0016] In some embodiments, each electrode material layer is prepared by a dry method.

[0017] The embodiment of the present invention further provides a method for preparing an electrode sheet, comprising:

[0018] a. Mixing the electrode active material, the conductive additive and the binder according to the designed ratio, and rolling to form electrode material layers with different porosities;

[0019] b. The electrode material layers prepared in step a are stacked on the current collector in the order of porosity from low to high, and then rolled to obtain electrode sheets.

[0020] Advantages and technical effects brought by the method for preparing the electrode plate of the embodiment of the present invention: In the embodiment of the present invention, a dry process is adopted to prepare the electrode material layer, without the need for organic solvents, thus eliminating the electrode drying and solvent recovery steps, and is more environmentally friendly, suitable for large-scale production, and can effectively reduce manufacturing costs.

[0021] In some embodiments, in step a, the electrode material layers with different porosities are prepared by adjusting the rolling pressure.

[0022] In some embodiments, in the step a, the rolling pressure is 100-500MPa. Preferably, when the electrode material layer is three layers, the rolling pressure used to prepare the innermost electrode material layer is 350-500MPa, the rolling pressure used to prepare the middle electrode material layer is 200-350MPa, and the rolling pressure used to prepare the outermost electrode material layer is 100-200MPa; and / or, in the step b, the rolling pressure is 5-15MPa.

[0023] The embodiment of the present invention further provides a secondary battery, including the electrode plate of the embodiment of the present invention or the electrode plate prepared by the method of the embodiment of the present invention. Optionally, the secondary battery is a lithium battery, a sodium battery, a potassium battery or a zinc battery. The secondary battery of the embodiment of the present invention has all the advantages and technical effects that the electrode plate of the embodiment of the present invention can bring, which will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of an electrode plate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0026] In order to increase the load and energy density of the electrode plates, the commonly used improvement strategies in related technologies include: 1) using thinner and stronger materials to optimize battery packaging; 2) increasing the amount of electrolyte absorption to reduce the electrode porosity; 3) using thick electrodes. At present, the first method requires a breakthrough in battery packaging materials and requires careful evaluation of cost and safety parameters, which is difficult to improve in a short period of time. As for reducing the amount of electrolyte, it is also difficult to achieve in practical applications, because the proportion of electrolyte in commercial lithium-ion batteries is limited, and reducing the electrolyte may cause the battery performance to deteriorate with faster cycles. Based on the above considerations, thick electrode design has become the most promising solution for the structural design of high energy density lithium-ion batteries in the future. The thick electrode design method can significantly increase the load ratio of active materials at the battery level by reducing inactive components, thereby increasing the energy density of the battery and reducing costs.

[0027] However, conventional electrodes are formed by coating a slurry on a metal current collector and then drying it. Increasing the thickness of the electrode will inevitably lead to a proportional increase in the transport distance and resistance of charges (electrons and ions). The charge transfer kinetics of thick electrodes are poor, and more time is required for Li + The ions reach all storage locations in the electrode, which eventually leads to deterioration of rate performance and limited improvement in energy density. In addition, the fracture and delamination of thick electrodes during the drying process also pose great challenges to the manufacture of strong electrodes and stable battery performance.

[0028] The present invention is based on the inventor's discovery and understanding of the following facts and problems: In order to improve the energy density of the battery, researchers have proposed the preparation of high-load positive electrode sheets. Most solutions require the use of organic solvents to prepare positive electrode slurry, and the electrode production is completed through coating and other processes. Due to the slurry coating method, the electrode sheet needs to be dried and the solvent needs to be recovered, which consumes a lot of energy; some solutions require the use of electrospinning technology to make a three-dimensional self-supporting structure, which is costly and not conducive to industrial production; and these technical solutions do not have enough fast charging performance and cannot achieve high-rate charging.

[0029] An embodiment of the present invention provides an electrode plate, comprising a current collector and at least three electrode material layers sequentially arranged on a surface of one side of the current collector in a direction perpendicular to the surface of the current collector;

[0030] The porosity of each electrode material layer increases gradually along the direction perpendicular to the current collector surface and away from the current collector;

[0031] The electrode material layer includes electrode active materials, conductive additives and binders, wherein the conductive additives in the innermost electrode material layer contacting the surface of the current collector include not less than 50wt% of small-particle conductive agents, preferably 60-100%, and more preferably 80-100%; the conductive additives in the outermost electrode material layer include not less than 80wt% of one-dimensional carbon materials, preferably 85-100wt%, and more preferably 95-100%; the conductive additives in other electrode material layers located in the middle layer include small-particle conductive agents and one-dimensional carbon materials, and the content of the one-dimensional carbon material is less than 80wt%, and the content of the small-particle conductive agent is less than 50wt%, and the small-particle conductive agent is a conductive agent with a particle size not greater than 50nm.

[0032] The electrode plate of the embodiment of the present invention is provided with at least three electrode material layers on the current collector, and the porosity of the electrode material layer increases from low to high in a gradient design from the innermost layer close to the current collector to the outermost layer. In view of the different porosities of each electrode material layer, the embodiment of the present invention has made different designs for the composition of the conductive agent in the electrode material layer with different porosities. In the outermost electrode material layer that first receives metal ions and has the highest porosity, a high-content one-dimensional carbon material is used as a conductive agent, which can construct a long-range conductive network to achieve rapid reception of electrons transferred from the lower layer, and the higher porosity can not block the pores around the active material, which is conducive to ion transmission; in the middle electrode material layer with further reduced porosity, the conductive additive is added with a part of the small-particle conductive agent on the basis of the one-dimensional carbon material, which improves the conductivity around the single particle and is conducive to improving the reaction rate; in the innermost electrode material layer with the lowest porosity, the conductive additive uses a high-content small-particle conductive agent, which enhances the conductivity of the single particle, can quickly receive the electrons transferred by the current collector, and improves the reaction rate.

[0033] The electrode plate of the embodiment of the present invention effectively improves the transmission of ions and electrons and the reaction rate by gradient designing the porosity of each material layer and designing different conductive agent compositions according to the different porosities of each electrode material layer, so that the manufactured battery can be quickly charged and has a higher energy density, giving the battery excellent comprehensive performance.

[0034] In some embodiments, the electrode active material includes a positive electrode active material or a negative electrode active material; optionally, the positive electrode active material includes at least one of sulfur, lithium iron phosphate, lithium iron manganese phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material; the negative electrode active material includes at least one of graphite or silicon-based materials. The electrode plate of the embodiment of the present invention can be applied to both positive electrode plates and negative electrode plates.

[0035] In some embodiments, the binder includes at least one of polyoxyethylene, polyvinylidene fluoride, styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylonitrile or sodium hydroxymethyl cellulose.

[0036] In some embodiments, the one-dimensional carbon material includes at least one of carbon fibers or carbon nanotubes.

[0037] In some embodiments, the small particle conductive agent includes at least one of Super P (conductive carbon black), acetylene black, and Ketjen black.

[0038] In some embodiments, optionally, the conductive agent in the outermost electrode material layer is all one-dimensional carbon material, the conductive agent in the innermost electrode material layer is all small particle conductive agent, and the conductive agent in the other electrode material layers in the middle layer is a mixture of small particle conductive agent and one-dimensional carbon material. Further optionally, along the direction perpendicular to the current collector surface and away from the current collector, the mass content of the small particle conductive agent in each middle electrode material layer decreases gradually. In the embodiment of the present invention, the composition of the conductive additive in each electrode material layer is optimized, which is beneficial to the transmission of electrons and increases the reaction rate, thereby facilitating the fast charging performance of the battery.

[0039] In some embodiments, the electrode material layer includes 70 wt % to 95 wt % of electrode active material, 0.5 wt % to 15 wt % of conductive additive, and 1 wt % to 15 wt % of binder.

[0040] In some embodiments, the electrode material layer is three layers, and the porosity of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-30%, 30-40% and 40-50% respectively. In the embodiment of the present invention, the porosity of each layer is optimized when the three-layer electrode material layer design is adopted, which is beneficial to the transmission of ions, thereby improving the performance of the battery.

[0041] In some embodiments, when the electrode active material is at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material, the electrode active material loading of the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer is 50-100 mg / cm 2 100-200mg / cm 2 and 200-300mg / cm 2 ;

[0042] When the electrode active material is sulfur, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 30-60 mg / cm 2 、60-120mg / cm 2and 120-200mg / cm 2 .

[0043] When the electrode active material is a negative electrode active material, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-40 mg / cm 2 40-80mg / cm 2 and 80-160mg / cm 2 .

[0044] In the embodiment of the present invention, the active material loading of each layer when a three-layer electrode material layer design is adopted is further optimized, so that the total active material loading of the electrode plate can be greatly increased, and the obtained battery has excellent fast charging performance.

[0045] The embodiment of the present invention further provides a method for preparing an electrode sheet, comprising:

[0046] a. Mixing the electrode active material, the conductive additive and the binder according to the designed ratio, and rolling to form electrode material layers with different porosities;

[0047] b. The electrode material layers prepared in step a are stacked on the current collector in the order of porosity from low to high, and then rolled to obtain electrode sheets.

[0048] In the preparation method of the electrode plate of the embodiment of the present invention, a dry process is adopted to prepare the electrode material layer, which does not require an organic solvent, eliminates the electrode drying and solvent recovery steps, is more environmentally friendly, is suitable for large-scale production, and can effectively reduce manufacturing costs; in the embodiment of the present invention, a dry process is adopted to prepare the electrode material layer, which can greatly increase the loading capacity of the electrode active substance and improve the energy density of the battery. At the same time, a gradient stacking method of electrode material layers with different porosities is adopted on the current collector, which is beneficial to the transmission of metal ions and electrons during the charging and discharging process, so that the secondary battery made using the electrode plate has high loading capacity and excellent fast charging performance.

[0049] In some embodiments, in step a, the electrode material layers with different porosities are prepared by adjusting the rolling pressure. Optionally, the rolling pressure is 100-500MPa. Further optionally, when the electrode material layer is three layers, the rolling pressure used to prepare the innermost electrode material layer is 350-500MPa, the rolling pressure used to prepare the middle electrode material layer is 200-350MPa, and the rolling pressure used to prepare the outermost electrode material layer is 100-200MPa. In the embodiment of the present invention, in the process of preparing each layer of electrode material layer, electrode material layers with different porosities are obtained by using different rolling pressures in the rolling equipment.

[0050] In some embodiments, in step b, the rolling pressure is 5-15 MPa. In the embodiments of the present invention, the rolling pressure of laminating the current collector and each layer of electrode material to form an electrode sheet is further limited to 5-15 MPa, which is conducive to forming a stable electrode sheet.

[0051] An embodiment of the present invention further provides a secondary battery, comprising an electrode plate according to an embodiment of the present invention or an electrode plate prepared by a method according to an embodiment of the present invention. Optionally, the secondary battery is a lithium battery, a sodium battery, a potassium battery or a zinc battery.

[0052] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0053] 1. Preparation of positive electrode sheet

[0054] Example 1

[0055] 1. Preparation of the outermost electrode material layer: 85wt% of the positive electrode active material lithium iron phosphate, 10wt% of the conductive additive (composed of 95wt% carbon fiber and 5% conductive carbon black), and 5wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing, and then a porosity of 40% was prepared in a roller press at a pressure of 150MPa to obtain an electrode material layer, and the loading amount of lithium iron phosphate in the electrode material layer was 300mg / cm 2 .

[0056] 2. Preparation of the intermediate electrode material layer: 85wt% of the positive electrode active material lithium iron phosphate, 10wt% of the conductive additive (composed of 60wt% carbon fiber and 40% conductive carbon black), and 5wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing, and then a porosity of 30% was prepared in a roller press at a pressure of 250MPa. The loading amount of lithium iron phosphate in the electrode material layer was 100mg / cm 2 .

[0057] 3. Preparation of the innermost electrode material layer: 85wt% of the positive electrode active material lithium iron phosphate, 10wt% of the conductive additive (composed of 5wt% carbon fiber and 95% conductive carbon black), and 5wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing, and then a roll press was used to prepare an electrode material layer with a porosity of 20% at a pressure of 500MPa. The loading amount of lithium iron phosphate in the electrode material layer was 100mg / cm 2 .

[0058] 4. The current collector, the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer are stacked in sequence, and rolled under a pressure of 10 MPa to obtain an electrode sheet.

[0059] The total loading of the positive electrode sheet prepared in this embodiment is 500 mg / cm2 .

[0060] The schematic diagram of the positive electrode sheet prepared in this embodiment is as follows Figure 1 shown.

[0061] The electrode sheet, polyethylene diaphragm and lithium metal prepared in this embodiment are cut and wound in a glove box filled with argon gas, and the N / P ratio is controlled to be 1.05. Then, the battery is placed in the shell, spot welded, baked, injected, welded to the top cover, baked, injected, formed, aged, and divided into different volumes to obtain a square aluminum shell battery. Figure 1 .

[0062] Example 2

[0063] The method is the same as that of Example 1, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0064] Outermost electrode material layer: porosity is 45%, lithium iron phosphate loading is 250 mg / cm 2 ;

[0065] Intermediate electrode material layer: porosity is 35%, lithium iron phosphate loading is 150 mg / cm 2 ;

[0066] Innermost electrode material layer: porosity is 25%, lithium iron phosphate loading is 50 mg / cm 2 .

[0067] The total loading of the positive electrode sheet prepared in this embodiment is 450 mg / cm 2 .

[0068] Example 3

[0069] The method is the same as that of Example 1, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0070] Outermost electrode material layer: porosity 50%, lithium iron phosphate loading 200 mg / cm 2 ;

[0071] Intermediate electrode material layer: porosity is 40%, lithium iron phosphate loading is 200 mg / cm 2 ;

[0072] Innermost electrode material layer: porosity is 20%, lithium iron phosphate loading is 100 mg / cm 2 .

[0073] The total loading of the positive electrode sheet prepared in this embodiment is 500 mg / cm 2 .

[0074] Example 4

[0075] The method is the same as that of Example 1, except that the conductive agent in the outermost electrode material layer is composed of 80% carbon fiber and 20% conductive carbon black.

[0076] Example 5

[0077] The method is the same as that of Example 1, except that the conductive agent in the innermost electrode material layer is composed of 50% carbon fiber and 50% conductive carbon black.

[0078] Comparative Example 1

[0079] The method is the same as that of Example 1, except that the porosity of each electrode material layer is the same, which is 30%.

[0080] Comparative Example 2

[0081] The method is the same as that of Example 1, except that the conductive agent in each electrode material layer is the same, consisting of 60wt% carbon fiber and 40wt% conductive carbon black.

[0082] Comparative Example 3

[0083] The method is the same as that of Example 1, except that the conductive agent in each electrode material layer is the same, consisting of 95wt% carbon fiber and 5% conductive carbon black.

[0084] Comparative Example 4

[0085] The method is the same as that of Example 1, except that the conductive agent in each electrode material layer is the same, consisting of 5wt% carbon fiber and 95% conductive carbon black.

[0086] The performance of the batteries prepared in Examples 1-5 and Comparative Examples 1-4 was tested. The test results are shown in Table 1.

[0087] Area capacity: After welding the positive and negative poles of the above-mentioned square shell battery to the pole pieces, connect the charging and discharging equipment for testing. First, perform a small current constant capacity test on the battery, then fully charge the battery at 0.33C, and then test the 0.33C discharge capacity, divide it by the total area of ​​the pole piece (single side) to get the area capacity.

[0088] Rate performance: After welding the positive and negative poles of the above-mentioned square shell battery, connect the charging and discharging equipment for testing. First, perform a small current constant capacity test on the battery, then fully charge the battery at 0.33C, and then test the 0.33C discharge capacity. Then, fully charge the battery at 0.33C again, and then test the 4C discharge capacity.

[0089] Fast charging speed: Combine the three-electrode calibration results of the laminated soft-pack battery cell to obtain the fast charging spectrum, use a step-by-step charging current for fast charging, and test the charging time (0-80% charge).

[0090] Fast charge cycle performance: The battery is charged at 0-80% according to the above-mentioned stepped fast charge spectrum, charged at 80%-100% at an equivalent 1C current, and discharged at a constant current of 1C. The above steps are repeated until the battery capacity decays to 80% of the initial capacity (0.33C).

[0091] Table 1

[0092]

[0093] 2. Preparation of negative electrode sheet

[0094] Example 6

[0095] 1. Preparation of the outermost electrode material layer: 90wt% of negative electrode active material graphite, 6wt% of conductive additive (composed of 90wt% carbon fiber and 10% conductive carbon black), and 4wt% of binder polyvinylidene fluoride were mixed and sent to a jet mill for processing, and then a porosity of 40% was prepared in a roller press at a pressure of 80MPa to obtain an electrode material layer. The graphite loading in the electrode material layer was 160mg / cm 2 .

[0096] 2. Preparation of the intermediate electrode material layer: 90 wt% of negative electrode active material graphite, 6 wt% of conductive additive (composed of 55 wt% carbon fiber and 45% conductive carbon black), and 4 wt% of binder polyvinylidene fluoride were mixed and sent to a jet mill for processing. Then, an electrode material layer with a porosity of 30% was prepared in a roller press at a pressure of 150 MPa. The graphite loading in the electrode material layer was 80 mg / cm 2 .

[0097] 3. Preparation of the innermost electrode material layer: 90wt% of negative electrode active material graphite, 6wt% of conductive additive (composed of 5wt% carbon fiber and 95% conductive carbon black), and 4wt% of binder polyvinylidene fluoride were mixed and sent to a jet mill for processing. Then, an electrode material layer with a porosity of 20% was prepared in a roller press at a pressure of 260MPa. The graphite loading in the electrode material layer was 40mg / cm 2 .

[0098] 4. Stack the current collector, the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer in sequence, and roll-press them at a pressure of 5 MPa to obtain an electrode sheet.

[0099] The total loading of the negative electrode sheet prepared in this embodiment is 280 mg / cm 2.

[0100] Example 7

[0101] The method is the same as that of Example 6, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0102] Outermost electrode material layer: porosity 45%, graphite loading 100 mg / cm 2 ;

[0103] Intermediate electrode material layer: porosity is 35%, graphite loading is 80 mg / cm 2 ;

[0104] Innermost electrode material layer: porosity is 25%, graphite loading is 40 mg / cm 2 .

[0105] The total loading of the negative electrode sheet prepared in this embodiment is 220 mg / cm 2 .

[0106] Example 8

[0107] The method is the same as that of Example 6, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0108] Outermost electrode material layer: porosity is 50%, graphite loading is 150mg / cm 2 ;

[0109] Intermediate electrode material layer: porosity is 40%, graphite loading is 50 mg / cm 2 ;

[0110] Innermost electrode material layer: porosity is 20%, graphite loading is 30mg / cm 2 .

[0111] The total loading of the negative electrode sheet prepared in this embodiment is 230 mg / cm 2 .

[0112] The performance of the batteries prepared in Examples 6-8 was tested under the same test conditions as in Example 1. The test results are shown in Table 2.

[0113] Table 2

[0114]

[0115] 3. Preparation of lithium-sulfur batteries

[0116] Example 9

[0117] 1. Preparation of the outermost electrode material layer: 70wt% of the positive electrode active material sulfur, 25wt% of the conductive additive (composed of 98wt% carbon fiber and 2% conductive carbon black), and 5wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing, and then a porosity of 40% was prepared in a roller press at a pressure of 120MPa to obtain an electrode material layer with a sulfur loading of 200mg / cm 2 .

[0118] 2. Preparation of the intermediate electrode material layer: 70 wt% of the positive electrode active material sulfur, 25 wt% of the conductive additive (composed of 65 wt% of carbon fiber and 35% of conductive carbon black), and 5 wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing. Then, an electrode material layer with a porosity of 30% was prepared in a roller press at a pressure of 170 MPa. The sulfur loading in the electrode material layer was 120 mg / cm 2 .

[0119] 3. Preparation of the innermost electrode material layer: 70wt% of the positive electrode active material sulfur, 25wt% of the conductive additive (composed of 7wt% carbon fiber and 93% conductive carbon black), and 5wt% of the binder polyvinylidene fluoride were mixed and sent to a jet mill for processing. Then, an electrode material layer with a porosity of 20% was prepared in a roller press at a pressure of 260MPa. The sulfur loading in the electrode material layer was 60mg / cm 2 .

[0120] 4. The current collector, the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer are stacked in sequence, and rolled under a pressure of 8 MPa to obtain an electrode sheet.

[0121] The total loading of the positive electrode sheet prepared in this embodiment is 380 mg / cm 2 .

[0122] Example 10

[0123] The method is the same as that of Example 9, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0124] Outermost electrode material layer: porosity 45%, sulfur loading 150 mg / cm 2 ;

[0125] Intermediate electrode material layer: porosity 35%, sulfur loading 80 mg / cm 2 ;

[0126] Innermost electrode material layer: porosity 25%, sulfur loading 30 mg / cm 2 .

[0127] The total loading of the positive electrode sheet prepared in this embodiment is 260 mg / cm 2 .

[0128] Embodiment 11

[0129] The method is the same as that of Example 9, except that the porosity and active material loading of each electrode material layer are different, as follows:

[0130] Outermost electrode material layer: porosity 50%, sulfur loading 180 mg / cm 2 ;

[0131] Intermediate electrode material layer: porosity 40%, sulfur loading 100 mg / cm 2 ;

[0132] Innermost electrode material layer: porosity 20%, sulfur loading 40mg / cm 2 .

[0133] The total loading of the positive electrode sheet prepared in this embodiment is 320 mg / cm 2 .

[0134] The performance of the batteries prepared in Examples 9-11 was tested under the same test conditions as in Example 1. The test results are shown in Table 3.

[0135] Table 3

[0136]

[0137] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0138] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. An electrode plate, It is characterized in that It comprises a current collector and at least three electrode material layers sequentially arranged on one side surface of the current collector in a direction perpendicular to the surface of the current collector; The porosity of each electrode material layer increases gradually along the direction perpendicular to the current collector surface and away from the current collector; The electrode material layer includes electrode active materials, conductive additives and binders, wherein the conductive additives in the innermost electrode material layer contacting the surface of the current collector include not less than 50wt% of small-particle conductive agents, the conductive additives in the outermost electrode material layer include not less than 80wt% of one-dimensional carbon materials, and the conductive additives in other electrode material layers located in the middle layer include small-particle conductive agents and one-dimensional carbon materials, and the content of the one-dimensional carbon material is less than 80wt%, the content of the small-particle conductive agent is less than 50wt%, and the small-particle conductive agent has a particle size of not more than 50nm.

2. The electrode sheet according to claim 1, It is characterized in that The electrode active material includes a positive electrode active material or a negative electrode active material; preferably, the positive electrode active material includes at least one of sulfur, lithium iron phosphate, lithium iron manganese phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material; the negative electrode active material includes at least one of graphite or silicon-based material; And / or, the binder includes at least one of polyoxyethylene, polyvinylidene fluoride, styrene-butadiene rubber, styrene-acrylic emulsion, polyacrylonitrile or sodium hydroxymethyl cellulose; And / or, the one-dimensional carbon material includes at least one of carbon fiber or carbon nanotube; And / or, the small particle conductive agent includes at least one of Super P, acetylene black, and Ketjen black.

3. The electrode sheet according to claim 1, It is characterized in that The electrode material layer consists of three layers, and the porosities of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-30%, 30-40% and 40-50% respectively.

4. The electrode sheet according to claim 3, It is characterized in that When the electrode active material is at least one of lithium iron phosphate, lithium manganese iron phosphate, nickel cobalt manganese ternary positive electrode material, nickel cobalt manganese aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material, the electrode active material loading of the innermost electrode material layer, the middle electrode material layer, and the outermost electrode material layer is 50-100 mg / cm 2 100-200mg / cm 2 and 200-300mg / cm 2 ; And / or, when the electrode active material is sulfur, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 30-60 mg / cm 2 、60-120mg / cm 2 and 120-200mg / cm 2 .

5. The electrode sheet according to claim 3, It is characterized in that When the electrode active material is a negative electrode active material, the electrode active material loadings of the innermost electrode material layer, the middle electrode material layer and the outermost electrode material layer are 20-40 mg / cm 2 40-80mg / cm 2 and 80-160mg / cm 2 .

6. The electrode sheet according to any one of claims 1 to 3, It is characterized in that The electrode material layers are prepared by a dry method.

7. A method for preparing an electrode sheet according to any one of claims 1 to 6, It is characterized in that include: a. Mixing the electrode active material, the conductive additive and the binder according to the designed ratio, and rolling to form electrode material layers with different porosities; b. The electrode material layers prepared in step a are stacked on the current collector in the order of porosity from low to high, and then rolled to obtain electrode sheets.

8. The method for preparing the electrode sheet according to claim 7, It is characterized in that In the step a, the electrode material layers with different porosities are prepared by adjusting the rolling pressure.

9. The method for preparing the electrode sheet according to claim 7, It is characterized in that In the step a, the rolling pressure is 100-500MPa. Preferably, when the electrode material layer is three layers, the rolling pressure used to prepare the innermost electrode material layer is 350-500MPa, the rolling pressure used to prepare the middle electrode material layer is 200-350MPa, and the rolling pressure used to prepare the outermost electrode material layer is 100-200MPa. And / or, in the step b, the rolling pressure is 5-15 MPa.

10. A secondary battery, It is characterized in that The invention comprises an electrode sheet according to any one of claims 1 to 6 or an electrode sheet prepared by the method according to any one of claims 7 to 9.

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