A positive electrode sheet, a method for manufacturing the same, a secondary battery, and an electric device

Abstract

The application provides a positive electrode sheet, a preparation method thereof, a secondary battery and an electric device. The positive electrode sheet comprises a substrate and a positive active material distributed in the substrate; the substrate is a porous structure, and the ratio of the average pore size of a first surface of the substrate to the average pore size of a second surface of the substrate is (5-10):(1-2). The substrate with a specific porous structure is used as a self-supporting structure of the positive active material, then the positive active material is distributed in the substrate with the specific porous structure to obtain the positive electrode sheet, the specific porous structure can improve the initial discharge capacity and the cycle stability of the battery at the same time; the positive electrode sheet can be directly used without being loaded on an aluminum foil, the use of the binder is reduced, the situation that the cycle life is short due to the falling of the binder in the battery charge-discharge cycle process is avoided, and the cycle stability of the battery is further improved.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a positive electrode sheet and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Lithium manganese iron phosphate is a commonly used positive electrode active material in secondary batteries. Compared with lithium iron phosphate, it has a wider voltage plateau and higher energy density. However, with the addition of manganese ions, the electron transition band gap in lithium manganese iron phosphate is as high as 2eV, which is basically an insulator. Compared with the 0.2eV of lithium iron phosphate, it has poorer electronic conductivity and lower ion mobility, which limits its development and application.

[0003] Currently, the electrochemical activity of lithium manganese iron phosphate (LFP) is typically improved through methods such as nano-sizing, carbon coating, metal ion doping, and combining it with other cathode materials with good conductivity. However, these methods are complex or technically immature.

[0004] Therefore, it is necessary to develop a positive electrode that simultaneously possesses good conductivity, ion transport rate, and excellent charge-discharge cycle performance. Summary of the Invention

[0005] The purpose of this invention is to provide a positive electrode sheet comprising a matrix framework with a specific pore structure distribution in the thickness direction and a positive electrode active material. Furthermore, this positive electrode sheet can be used directly as a positive electrode sheet without being loaded onto aluminum foil, reducing the use of binders and avoiding the short cycle life caused by binder detachment during battery charge-discharge cycles. This significantly improves the cycle stability and cycle life of the resulting secondary battery. Using this positive electrode sheet, positive electrode sheets with excellent conductivity and charge-discharge cycle performance can be prepared.

[0006] To achieve the above objectives, in a first aspect of the present invention, the present invention provides a positive electrode sheet, comprising a substrate and a positive electrode active material distributed in the substrate; the substrate has a porous structure, and the ratio of the average pore size of the first surface of the substrate to the average pore size of the second surface of the substrate is (5-10):(1-2).

[0007] As an embodiment of the present invention, the average pore size of the second surface of the substrate is 1 to 10 μm; the pore size of the first surface is 20 to 100 μm.

[0008] As an embodiment of the present invention, the density of the matrix is ​​2.9–3.1 g / cm³. 3 .

[0009] As an embodiment of the present invention, the thickness of the substrate is 0.12 to 0.14 mm.

[0010] As an embodiment of the present invention, the Dv of the positive electrode active material 50 The wavelength is 300–500 nm.

[0011] As an embodiment of the present invention, the weight ratio of the substrate to the positive electrode active material is 1:(10-20).

[0012] As an embodiment of the present invention, the matrix comprises carbon fiber; the positive electrode active material comprises lithium iron phosphate (LiFePO4). a Mn 1-x Fe x PO4, 0 < x < 1, 0.8 < a < 1.2), doped lithium iron phosphate (Li a Mn 1-x-y Fe x M y PO4, with doping element M including but not limited to at least one of Mn, W, Mo, Zr, Sr, Nb, Ce, In, Ta, Y, Si, Sn, Ba or La, 0 < x < 1, 0 ≤ y < 1, 0.8 < a < 1.2).

[0013] As an embodiment of the present invention, the positive electrode sheet further includes at least one of a conductive agent and a binder.

[0014] In a second aspect, the present invention provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0015] The matrix is ​​obtained by layering and coating slurries containing matrix raw materials with different solid content concentrations and drying them; the matrix absorbs the slurry containing the positive electrode active material, and the positive electrode sheet is obtained by vacuum drying and compaction.

[0016] In a third aspect, the present invention provides a secondary battery comprising the above-described positive electrode, negative electrode, separator, and electrolyte.

[0017] In a fourth aspect, the present invention provides an electrical device including the aforementioned secondary battery, wherein the secondary battery serves as a power supply for the electrical device.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] This invention designs a substrate with a specific three-dimensional network porous structure, which can simultaneously improve battery capacity and cycle stability. In the substrate, the average pore size decreases along the thickness direction from the first surface to the second surface. This invention uses the substrate with the specific three-dimensional network porous structure as a self-supporting framework for the positive electrode active material, and then distributes the positive electrode active material into this substrate with the specific porous structure to obtain a positive electrode sheet. The specific porous structure can simultaneously improve the initial discharge capacity and cycle stability of the battery; it also eliminates the need for loading onto aluminum foil, allowing it to be directly used as a positive electrode sheet, reducing the use of binders and avoiding the short cycle life caused by binder detachment during battery charge-discharge cycles, further significantly improving the battery's cycle stability. Furthermore, the positive electrode sheet preparation process of this application is simple and can reduce manufacturing costs. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the substrate structure of the positive electrode sheet described in an embodiment of the present invention. Detailed Implementation

[0021] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments do not limit the invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in this invention are conventional reagents, methods, and equipment in this technical field. Unless otherwise specified, all reagents and materials used in this invention are commercially available.

[0022] An embodiment of the present invention provides a positive electrode sheet, comprising a substrate and a positive electrode active material distributed in the substrate; the substrate has a porous structure, and the ratio of the average pore size of the first surface of the substrate to the average pore size of the second surface of the substrate is (5-10):(1-2).

[0023] This invention uses a matrix with a specific three-dimensional network-like porous structure as the framework self-supporting structure of the positive electrode active material. The positive electrode active material is then distributed into this matrix with the specific porous structure to obtain a positive electrode sheet. In the positive electrode sheet, the porous structure of the matrix allows for a greater loading capacity of the positive electrode active material. Simultaneously, the porous structure also improves the wettability of the positive electrode sheet to the electrolyte, reduces polarization, and increases the ion transport rate. Furthermore, the first and second surfaces of the positive electrode sheet have different average pore sizes. The side with a larger average pore size exhibits good ion transport characteristics but relatively poor electronic conductivity, while the side with a smaller average pore size exhibits poor ion transport characteristics but good electronic conductivity. The positive electrode sheet of this application combines both, thus balancing ion transport and electronic conductivity. The smaller pore size in the second surface makes the framework structure more compact, which can, to some extent, protect the CEI film, improve the stability of the interface between the positive electrode sheet and the electrolyte, reduce the dissolution of transition metals, and protect the structure of the positive electrode active material from damage. Accordingly, the battery prepared using the above-mentioned positive electrode sheet has a higher specific capacity, better cycle stability, and higher rate performance.

[0024] Furthermore, the matrix framework of the present invention can also be used as a current collector, eliminating the need to coat the positive electrode active material onto the aluminum foil, reducing the use of binders, and avoiding the occurrence of short cycle life caused by binder detachment during battery charge and discharge cycles, thereby significantly improving the cycle stability and cycle life of the obtained secondary battery.

[0025] In some embodiments, the ratio of the average pore size of the first surface and the second surface of the substrate is (8-10):(1-1.5). When the ratio of the average pore size of the first surface and the second surface is within the above range, it is beneficial to improve the energy density of the battery, while also improving the mechanical strength of the substrate, preventing the electrode structure from collapsing during battery cycling, and avoiding a decrease in battery cycle stability and cycle life.

[0026] In some embodiments, the average pore size of the second surface of the substrate is 1–10 μm, and the average pore size of the first surface is 20–150 μm. A smaller pore size on the second surface is beneficial for a denser framework structure, protecting the positive electrode active material within the framework; however, when the average pore size of the second surface becomes too small, it can also lead to poor electrolyte wettability and decreased ion conductivity, thereby reducing the charge / discharge capacity of the battery. When the average pore size of the first surface is within the aforementioned range, it further improves the wettability of the positive electrode sheet.

[0027] It should be further explained that the first surface and the second surface mentioned in this invention are two surfaces of the substrate in the thickness direction. When the substrate is thick, it can be composed of multiple layers in the thickness direction. In the substrate, the size of the pores decreases in the thickness direction from the first surface to the second surface.

[0028] In some embodiments, the density of the matrix is ​​2.9–3.1 g / cm³. 3 Within the suitable density range of this invention, more positive electrode active material can be accommodated, while the matrix has high mechanical strength, resulting in better cycle stability of the battery.

[0029] In some embodiments, the amount of the positive electrode active material contained in the matrix is ​​such that the weight ratio of the matrix to the positive electrode active material is 1:(10-20). Within this range, the secondary battery has a superior energy density.

[0030] In some embodiments, the thickness of the substrate is 0.12–0.14 mm. Within this thickness range, deformation of the pore structure is less likely to occur during substrate compaction. In addition, it can ensure better dispersion of the positive electrode active material in the middle layer of the substrate, avoid agglomeration or voids, and improve the cycle stability of the battery.

[0031] It should be noted that this invention does not limit the type of positive electrode active material; conventional positive electrode active materials in the art can be used in this invention, including but not limited to lithium iron phosphate (LiFePO4). a Mn 1-x Fe x PO4, 0 < x < 1, 0.8 < a < 1.2), doped lithium iron phosphate (Li a Mn 1-x-y Fe x M y PO4, with doping element M including but not limited to at least one of Mn, W, Mo, Zr, Sr, Nb, Ce, In, Ta, Y, Si, Sn, Ba or La, 0 < x < 1, 0 ≤ y < 1, 0.8 < a < 1.2).

[0032] In some embodiments, the particle size Dv of the positive electrode active material 50 The particle size is 300–500 nm. In this invention, the particle size of the positive electrode active material is smaller than the pore size of the matrix, which ensures that the positive electrode active material can smoothly enter the matrix. In addition, selecting the above-mentioned particle size range can simultaneously improve the battery capacity and cycle life.

[0033] Conventional fiber materials can be used to prepare the matrix in this invention. In some embodiments, the fiber material is preferably bacterial fiber, which is carbonized to obtain a corresponding carbon fiber matrix. This carbon fiber matrix can further improve the conductivity of the positive electrode and reduce the need for conductive carbon black. Furthermore, this carbon fiber matrix has a certain degree of flexibility, making it easy to process and applicable to various flexible devices, greatly expanding its application areas.

[0034] In this invention, the positive electrode sheet further includes a conductive agent and a binder. Conventional conductive agents and binders can be used in this invention.

[0035] The conductive agent includes, but is not limited to, conductive carbon black.

[0036] The adhesive includes, but is not limited to, polyvinylidene fluoride (PVDF).

[0037] In some embodiments, the positive electrode does not contain a metal foil current collector. The metal foil is an aluminum foil.

[0038] In some embodiments, the conductivity of the positive electrode is 0.35 to 0.65 S / cm.

[0039] An embodiment of the present invention provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0040] A slurry containing matrix raw materials with different solid content concentrations is layered and coated, and then dried to obtain the matrix; the matrix absorbs the slurry containing the positive electrode active material, and the positive electrode sheet is obtained after vacuum drying.

[0041] In this invention, the positive electrode slurry is prepared by adding solvent to positive electrode raw materials, and the solvent includes, but is not limited to, N-methylpyrrolidone (NMP).

[0042] In some embodiments, during the layered coating process, the slurries containing matrix raw materials with different solid content concentrations are arranged in a gradient according to their solid content concentration.

[0043] In some embodiments, the slurry containing the matrix raw material has a solid content of 60-90 wt%.

[0044] In some embodiments, the slurry containing the positive electrode active material has a solid content of 60-75 wt%. A suitable solid content can improve the dispersibility and stability of the positive electrode active material in the matrix.

[0045] In some embodiments, the vacuum drying temperature is -10 to -20°C. Vacuum drying allows the positive electrode active material to be uniformly loaded into the substrate, improving the cycle stability of the battery.

[0046] One embodiment of the present invention also provides a secondary battery, the secondary battery comprising the above-mentioned positive electrode, negative electrode, separator and electrolyte.

[0047] In this invention, the negative electrode sheet contains a negative electrode active material. Conventional negative electrode active materials can be used in this invention, such as at least one of graphite, soft carbon, hard carbon, carbon fiber, silicon-based materials or tin-based materials.

[0048] Conventional membrane materials and electrolytes can be used in this invention.

[0049] The diaphragm includes, but is not limited to, at least one of polyethylene, polypropylene, or polyvinylidene fluoride.

[0050] The electrolyte includes an electrolyte salt and an organic solvent: the electrolyte salt includes, but is not limited to, at least one of lithium hexafluorophosphate, lithium organoborate, lithium perchlorate, and lithium sulfonylimide salts; the organic solvent includes, but is not limited to, at least one of cyclic carbonates and carboxylic acid esters.

[0051] The following embodiments are provided to facilitate understanding of the invention. These embodiments are not intended to limit the scope of the claims.

[0052] Example 1

[0053] This embodiment provides a positive electrode sheet, which is prepared by a method including the following steps:

[0054] S1. Preparation of the matrix

[0055] A bacterial cellulose slurry with a solid content of 90 wt% was spread on a polytetrafluoroethylene plate and named L1 layer. It was then placed in a freeze dryer for freeze curing. A second bacterial cellulose slurry with a solid content of 70 wt% (named L2 layer) was spread on top of L1 layer and then placed in a freeze dryer for freeze drying to prepare a matrix aerogel (bacterial cellulose aerogel).

[0056] The matrix aerogel was compacted under a roller press to control its thickness to 0.20 mm. It was then calcined at 1000 °C for 1 h in a nitrogen (N2) atmosphere to carbonize and obtain a carbonized bacterial cellulose matrix with a thickness of 0.12 mm.

[0057] S2. Preparation of positive electrode sheet

[0058] LiMn, the positive electrode active material 0.6 Fe 0.4 PO4(D v50 A positive electrode active material slurry with a solid content of 60wt% was prepared by mixing 400nm conductive carbon black, PVDF binder, and N-methylpyrrolidone in a weight ratio of 88:4:8.0.

[0059] The matrix material obtained in step S1 is immersed in the positive electrode active material slurry and left to stand for 3 hours under sealed conditions to allow the matrix to fully absorb the positive electrode active material. Then, the matrix fully absorbing the positive electrode active material is placed in an oven at 80°C and dried to constant weight to remove moisture, thereby obtaining the positive electrode sheet.

[0060] It should be noted that in this invention, the thickness of the substrate is the same as the thickness of the prepared positive electrode sheet.

[0061] Example 2

[0062] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that the solid content of the slurry coating the L2 layer in step S1 is 90 wt%.

[0063] Example 3

[0064] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that: the solid content of the slurry coating L2 layer in step S1 is 85wt%, and after uniform coating, it is placed in a freeze dryer for freeze curing; then a bacterial cellulose slurry layer with a solid content of 80wt% (named L3 layer) is coated on the surface of L2 layer, and the subsequent steps are the same as in Example 1.

[0065] Example 4

[0066] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, except that: in step S1, the slurry coating L2 layer has a solid content of 85 wt%, and after uniform coating, it is placed in a freeze dryer for freeze curing; then, a bacterial cellulose slurry layer with a solid content of 80 wt% (named L3 layer) is coated on the surface of L2 layer, and after uniform coating, it is placed in a freeze dryer for freeze curing; a bacterial cellulose slurry layer with a solid content of 75 wt% (named L4 layer) is coated on the surface of L3 layer, and the subsequent steps are the same as in Example 1.

[0067] Example 5

[0068] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that the solid content of the slurry in the L1 layer and the L2 layer in step S1 is 1.05 times that of Example 1, that is, the solid content of the slurry in the L1 layer is 95wt% and the solid content of the slurry in the L2 layer is 74wt%.

[0069] Example 6

[0070] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that the solid content of the slurry in the L1 layer and the L2 layer in step S1 is 0.8 times that of Example 1, the solid content of the slurry in the L1 layer is 72wt%, and the solid content of the slurry in the L2 layer is 56wt%.

[0071] Example 7

[0072] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, except that the solid content of the positive electrode active material slurry in step S2 is 50 wt%.

[0073] Example 8

[0074] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that the solid content of the positive electrode active material slurry in step S2 is 70 wt%.

[0075] Example 9

[0076] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, but differs from Example 1 in that: the D of the positive electrode active material in step S2 is... v50 The particle size is 300 nm.

[0077] Example 10

[0078] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, but differs from Example 1 in that: the D of the positive electrode active material in step S2 is... v50 The particle size is 500 nm.

[0079] Example 11

[0080] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that in step S1, the amount of bacterial cellulose slurry is adjusted so that the thickness of the matrix after compaction is 0.22 mm.

[0081] Example 12

[0082] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that in step S1, the amount of bacterial cellulose slurry is adjusted so that the thickness of the matrix after compaction is 0.18 mm.

[0083] Example 13

[0084] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, except that the positive electrode active material used in step S2 is lithium iron phosphate (D...).v50 (400nm).

[0085] Example 14

[0086] This embodiment provides a positive electrode sheet, which is prepared according to the method of Example 1. The difference from Example 1 is that the solid content of the slurry in the L1 layer and L2 layer in step S1 is 0.6 times that of Example 1.

[0087] Example 15

[0088] This embodiment provides a positive electrode sheet, prepared according to the method of Example 1, except that the amount of bacterial cellulose slurry is adjusted so that the thickness of the matrix after compaction is 0.3 mm. Comparative Example 1

[0089] This comparative example provides a positive electrode sheet. The matrix material of this comparative example has a relatively consistent average pore size in the thickness direction. It is prepared according to the method of Example 1. The difference from Example 1 is that in step S1, a bacterial cellulose slurry with a solid content of 80 wt% is spread on a polytetrafluoroethylene plate and freeze-dried in a freeze dryer to prepare a matrix aerogel (bacterial cellulose aerogel); the remaining steps are the same as in Example 1.

[0090] Comparative Example 2

[0091] This comparative example provides a positive electrode sheet, prepared according to the method of Example 1, the difference being that the solid content of the slurry coating the L2 layer in step S1 is 50wt%.

[0092] Performance Testing and Applications

[0093] The performance of the positive electrode sheets obtained in the above embodiments and comparative examples, and the secondary batteries (applications) prepared from them, were tested. The specific test items, test methods, and results are as follows:

[0094] 1. Characterization of matrix materials:

[0095] 1) Average pore size (μm): The average pore size of the substrate surface was measured using a scanning electron microscope (SEM). The side with the larger average pore size was defined as the first surface, with an average pore size of D1; the side with the smaller average pore size was defined as the second surface, with an average pore size of D2.

[0096] In this invention, the average pore size is determined by selecting five sample points on the surface of the substrate membrane material using the "five-point sampling method". With each sample point as the center, a range of 50μm×50μm is taken, and the average pore size within this area is tested and calculated. Then, the average pore size of the five sample points is selected and calculated again to obtain the average pore size of the first and second surfaces.

[0097] 2) Matrix density (g / cm³) 3 According to the formula "Matrix density = Matrix material mass (g) / [PTFE sheet area (cm²)]", the matrix density is calculated as follows: 2 The result is calculated as [] × matrix thickness (cm).

[0098] 2. Characterization of the positive electrode sheet:

[0099] 1) Ratio of matrix to positive electrode active material: The mass of the matrix material before adsorbing the positive electrode active material is denoted as m1, and the mass of the positive electrode sheet obtained after adsorbing the positive electrode active material is denoted as m2. Then the mass of the positive electrode active material is (m2-m1). Then calculate the ratio of matrix to positive electrode active material.

[0100] 3. Application of the positive electrode and electrochemical performance testing of secondary batteries

[0101] 1) Application

[0102] Application Example 1: The positive electrode sheet prepared in the above examples and comparative examples is assembled with the negative electrode sheet, electrolyte and separator to prepare a secondary battery. The specific preparation process includes the following steps:

[0103] Negative electrode preparation: The negative electrode active material graphite, conductive agent CNT, thickener CMC, and binder SBR were mixed at a mass ratio of 96.5:0.8:0.9:1.8. Deionized water was added as solvent, and the mixture was stirred under vacuum until homogeneous, obtaining a negative electrode slurry. This slurry was then evenly coated onto the upper and lower surfaces of the negative electrode current collector copper foil. After air-drying at room temperature (25℃), it was transferred to an oven for further drying until a compacted density of 1.7 g / cm³ was achieved. 3 The negative electrode sheet is obtained after cold pressing and slitting under the conditions;

[0104] Secondary battery preparation: The positive electrode, negative electrode, polyethylene membrane, and separator prepared in the above examples and comparative examples are stacked in the order of "positive electrode - separator - negative electrode". The surface of the positive electrode with a larger average pore size is in contact with the separator. The cells are wound up to obtain a battery cell. After drying, an electrolyte is added (the electrolyte composition is: a mixed organic solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1, and 1 mol / L lithium hexafluorophosphate). The secondary battery is obtained through vacuum sealing, standing, formation, and capacity testing.

[0105] 2) Battery performance test:

[0106] The electrochemical performance of the battery prepared in Case 1 was tested:

[0107] Electrical conductivity (S / cm): The overall resistivity of the electrode is directly measured using the two-probe method. The test probe is installed on a material mechanical property testing device. By applying current and resistance through the probe, the electrical conductivity of the electrode is obtained by the formula: electrical conductivity ρ = measured resistance value * contact area / electrode thickness.

[0108] Initial discharge capacity: Charge and discharge tests were conducted using a charge and discharge test system at 25±0.5℃. Charge and discharge conditions: charging termination voltage 4.3V; discharging termination voltage 2.8V; charge and discharge current density: 0.1C.

[0109] Cycling performance: Charge and discharge tests were conducted using a charge and discharge test system at 25±0.5℃. Charge and discharge conditions: charging termination voltage 4.3V; discharging termination voltage 2.8V; charge and discharge current density: 1C. The cycle performance is the capacity retention rate after 500 cycles under 1C conditions.

[0110] The test results are detailed in Tables 1 and 2.

[0111] Table 1

[0112]

[0113] Table 2

[0114]

[0115] The results above show that:

[0116] The secondary battery prepared by this invention simultaneously exhibits high conductivity, high initial discharge capacity, and excellent cycle stability. Specifically, the conductivity is above 0.3 S / cm, reaching as high as 0.65 S / cm; the initial discharge capacity is above 140 mAh / g, reaching as high as 161 mAh / g; and the capacity retention rate after 500 cycles is above 90%, reaching as high as 97.1%. As can be seen from the examples and comparative data, when the ratio of the average pore size of the first surface to the second surface of the substrate of the positive electrode is not in the range of (5-10):(1-2), the electrochemical performance of the secondary battery significantly decreases.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the positive electrode sheet comprises a substrate and a positive electrode active material distributed in the substrate; the substrate is a carbon fiber with a porous structure, the ratio of the average pore size of the first surface of the substrate to the average pore size of the second surface of the substrate is (5-10):(1-2); the thickness of the substrate is 0.12-0.14 mm; the average pore size of the second surface of the substrate is 1-10 μm; and the average pore size of the first surface of the substrate is 20-150 μm.

2. The secondary battery according to claim 1, characterized by The density of the base body is 2.9 to 3.1 g / cm3 3 .

3. The secondary battery according to claim 1, characterized by The Dv50 of the positive electrode active material is 5 to 15 pm. 50 is 300 to 500 nm.

4. The secondary battery according to claim 1, characterized by The weight ratio of the substrate to the positive electrode active material is 1:(10-20).

5. The secondary battery according to any one of claims 1 to 4, characterized by The preparation method of the positive electrode sheet comprises the following steps: stacking and coating a slurry containing a substrate raw material with different solid content concentrations, carbonizing at high temperature after drying to obtain the substrate; absorbing a slurry containing the positive electrode active material by using the substrate, vacuum drying and compacting to obtain the positive electrode sheet.

6. An electric device characterized by comprising: The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the positive electrode sheet comprises a substrate and a positive electrode active material distributed in the substrate; the substrate is a carbon fiber with a porous structure, the ratio of the average pore size of the first surface of the substrate to the average pore size of the second surface of the substrate is (5-10):(1-2); the thickness of the substrate is 0.12-0.14 mm; the average pore size of the second surface of the substrate is 1-10 μm; and the average pore size of the first surface of the substrate is 20-150 μm. The weight ratio of the substrate to the positive electrode active material is 1:(10-20). The preparation method of the positive electrode sheet comprises the following steps: stacking and coating a slurry containing a substrate raw material with different solid content concentrations, carbonizing at high temperature after drying to obtain the substrate; absorbing a slurry containing the positive electrode active material by using the substrate, vacuum drying and compacting to obtain the positive electrode sheet. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the positive electrode sheet comprises a substrate and a positive electrode active material distributed in the substrate; the substrate is a carbon fiber with a porous structure, the ratio of the average pore size of the first surface of the substrate to the average pore size of the second surface of the substrate is (5-10):(1-2); the thickness of the substrate is 0.12-0.14 mm; the average pore size of the second surface of the substrate is 1-10 μm; and the average pore size of the first surface of the substrate is 20-150 μm. The weight ratio of the substrate to the positive electrode active material is 1

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