Secondary battery and method for manufacturing the same

Abstract

The invention relates to the technical field of lithium batteries, and provides a secondary battery and a preparation method thereof, which are at least beneficial to improving the performance of the secondary battery. The preparation method comprises the following steps: carrying out winding treatment or lamination treatment on a positive plate, a diaphragm and a negative plate, then putting the positive plate, the diaphragm and the negative plate into a shell, and injecting an electrolyte into the shell to obtain the secondary battery, the preparation method of the positive plate comprises the following steps: preparing lithium ferrite, lithium nickelate, lithiated carbon nanotubes and a spinning auxiliary agent into a precursor solution; depositing the precursor solution on the surface of a positive electrode current collector through electrostatic spinning so as to form an initial fiber membrane on the surface of the positive electrode current collector; the positive electrode current collector with the initial fiber membrane is pre-sintered in air and then calcined in inert gas, so that the initial fiber membrane is converted into a fiber lithium supplementing layer, and the fiber lithium supplementing layer is of a fiber network structure; and coating a positive electrode material layer on the surface of the fiber lithium supplementing layer, and drying to obtain the positive plate.

Description

[0001] This application is a divisional application of the patent application with application date of April 11, 2025, application number 2025104576013, and invention name “Secondary battery and preparation method thereof”. Technical Field

[0002] The present application relates to the technical field of lithium batteries, and in particular to a secondary battery and a preparation method thereof. Background Art

[0003] With the development of secondary batteries, the requirements for energy density, operating voltage, and cycle life are becoming increasingly stringent. Currently, the cathode materials for secondary batteries, such as lithium-ion batteries, primarily include lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary materials. Among them, lithium iron phosphate (LiFePO4) is widely used due to its high capacity, environmental friendliness, high safety, and long cycle life. With the increasing adoption of electric vehicles, the pursuit of higher energy density secondary batteries is a common goal. Under many design concepts and material systems, further increases in energy density often come at the expense of battery safety, power, and life. Summary of the Invention

[0004] The embodiments of the present application provide a secondary battery and a method for preparing the same, which at least helps to reduce the storage gas generation of the secondary battery.

[0005] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a method for preparing a secondary battery, including: preparing a positive electrode sheet, the preparation steps including: preparing a precursor solution, mixing lithium ferrite and lithium nickelate and dissolving them in a solvent, adding lithiated carbon nanotubes and a spinning aid, and mixing them evenly to form the precursor solution; loading the precursor solution into a syringe of an electrospinning machine, and depositing the precursor solution on the surface of a positive electrode collector by electrospinning to form an initial fiber film on the surface of the positive electrode collector; pre-calcining the positive electrode collector having the initial fiber film and then calcining it to transform the initial fiber film into a fiber lithium-supplementing layer, and the fiber lithium-supplementing layer has a fiber network structure; coating a positive electrode material layer on the surface of the fiber lithium-supplementing layer, the material of the positive electrode material layer including lithium iron phosphate, a conductive agent and an adhesive, and drying to obtain the positive electrode sheet; providing a negative electrode sheet and a separator, winding or stacking the positive electrode sheet, the separator and the negative electrode sheet and placing them in a shell, and injecting an electrolyte into the shell to obtain a secondary battery.

[0006] In some embodiments, in the step of preparing the precursor solution, the molar ratio of the lithium ferrite and the lithium nickelate is (3-6):1; the mass ratio of the total mass of the lithium ferrite and the lithium nickelate to the mass of the lithiated carbon nanotubes is (2-13):1.

[0007] In some embodiments, the lithium iron phosphate is pretreated before coating, and the pretreatment step includes: weighing trimethyl borate and dissolving the trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; adding the lithium iron phosphate and then ball milling and dispersing, and the total mass ratio of the trimethyl borate to the lithium iron phosphate and the trimethyl borate is 1% to 3%.

[0008] In some embodiments, after obtaining the positive electrode sheet, the positive electrode sheet is post-processed, and the post-processing step includes: immersing the positive electrode sheet in a mixed solution of trimethyl borate and ethanol, wherein the mass ratio of trimethyl borate to the total mass of trimethyl borate and ethanol is 3% to 8%, and then drying under vacuum conditions.

[0009] According to some embodiments of the present application, another aspect of the embodiments of the present application provides a secondary battery, comprising: a shell, and a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte located in the shell and stacked, the positive electrode sheet comprising: a positive electrode current collector; a fiber lithium-supplementing layer, the fiber lithium-supplementing layer covering the surface of the positive electrode current collector, the fiber lithium-supplementing layer having a fiber network structure, the material of the fiber lithium-supplementing layer comprising lithium ferrite, lithium nickelate and lithiated carbon nanotubes; a positive electrode material layer, the positive electrode material layer covering the surface of the fiber lithium-supplementing layer, the material of the positive electrode material layer comprising lithium iron phosphate, a conductive agent and an adhesive.

[0010] In some embodiments, the mass ratio of the total mass of the lithium ferrite and the lithium nickelate to the lithiated carbon nanotubes is (2-13):1.

[0011] In some embodiments, the molar ratio of the lithium ferrite to the lithium nickelate is (3-6):1.

[0012] In some embodiments, the ratio of the thickness of the fiber lithium supplement layer to the thickness of the positive electrode material layer is 1:(8-16).

[0013] In some embodiments, the thickness of the fiber lithium supplement layer is 10 μm to 20 μm; the thickness of the positive electrode material layer is 140 μm to 160 μm.

[0014] In some embodiments, the particle size corresponding to the cumulative particle size distribution percentage of the lithium ferrite reaches 50% is less than or equal to 12 μm; the particle size corresponding to the cumulative particle size distribution percentage of the lithium nickelate reaches 50% is less than or equal to 15 μm; the average diameter of the lithiated carbon nanotubes is 10 nm to 50 nm, and the average length is 1 μm to 10 μm.

[0015] The technical solution provided by the embodiments of the present application has at least the following advantages:

[0016] The embodiment of the present application provides a method for preparing a secondary battery, in which an electrostatic spinning process is used to form a fiber lithium replenishment layer on the positive electrode current collector and a positive electrode material layer is coated on the fiber lithium replenishment layer. The fiber lithium replenishment layer is located on the side of the positive electrode material layer close to the positive electrode current collector. The electrolyte can be in direct contact with the positive electrode material layer to improve the efficiency of lithium ion insertion and extraction. The fiber lithium replenishment layer has a fiber network structure. On the one hand, the fiber network structure has a large porosity, which can help promote the positive electrode sheet to absorb the electrolyte. On the other hand, the fiber lithium replenishment layer with a fiber network structure is conducive to gas overflow during the first charging process, avoiding storage gas production behavior, and thus avoiding the impact of storage gas production on battery performance. Among them, the materials of the fiber lithium replenishment layer include lithium ferrite, lithium nickelate and lithiated carbon nanotubes, all of which can make up for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, a long cycle life, and low cost, but lithium ferrite has a low energy density; lithium nickelate has a high energy density, but a short cycle life, poor thermal stability, rapid performance degradation at high temperatures, and high cost. The combination of lithium ferrite and lithium nickelate can complement each other to combine the advantages of both. In addition, based on the fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a spatial network conductive structure. The presence of lithiated carbon nanotubes improves the conductivity of the fiber lithium supplement layer, while increasing the chemical activity of lithium ferrite and lithium nickelate, reducing the decomposition potential of lithium ferrite and lithium nickelate, and reducing the damage of high voltage activation potential to the positive electrode material. The materials of the positive electrode material layer include lithium iron phosphate, a conductive agent and a binder. Lithium iron phosphate provides active lithium to meet the performance of the battery. The addition of the conductive agent improves the weak conductivity of lithium iron phosphate, forming a better conductive network and improving the performance of the battery. The binder can better bond the lithium iron phosphate to other substances and form a better adhesion effect on the fiber lithium supplement layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise stated, the pictures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments will be briefly introduced below. 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 paying any creative work.

[0018] Figure 1 A flowchart corresponding to a method for preparing a positive electrode sheet provided in an embodiment of the present application;

[0019] Figure 2 A schematic structural diagram of a positive electrode sheet of a secondary battery provided in an embodiment of the present application;

[0020] Figure 3 A schematic structural diagram of a positive electrode sheet of another secondary battery provided in an embodiment of the present application. DETAILED DESCRIPTION

[0021] During the charge and discharge process, a SEI (Solid Electrolyte Interface membrane) film forms on the negative electrode of a secondary battery, resulting in a loss of active lithium, reducing the battery's energy density and coulombic efficiency, and hindering the further development of secondary batteries. Battery lithium replenishment technology is an important means of improving battery energy density. Based on the technical route, lithium replenishment can be divided into two categories: positive electrode lithium replenishment and negative electrode lithium replenishment. Among them, positive electrode lithium replenishment technology adds lithium replenishment materials to the positive electrode of the secondary battery. The added lithium replenishment materials decompose and release active lithium during the battery charging process, thereby compensating for the irreversible active lithium loss caused by the growth of the negative electrode SEI film, achieving the desired lithium replenishment effect.

[0022] Usually, the positive electrode lithium supplement additive is usually added by mixing it with the positive electrode main material during homogenization. Since the lithium supplement agent releases gas during the charging and discharging process, the prepared battery will have storage gas production behavior, and the gas will affect the battery's cycle and high-temperature performance.

[0023] The embodiments of the present application provide a secondary battery and a method for preparing the same, which are at least beneficial to improving the performance of the secondary battery.

[0024] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0025] In the description of the embodiments of the present application, when a component “includes” another component, unless otherwise stated, other components are not excluded, and other components may be further included.

[0026] The terms used in the description of the various embodiments described herein are for describing specific embodiments only and are not intended to be limiting. As used in the description of the various embodiments described and the appended claims, "components" are also intended to include plural forms unless the context clearly indicates otherwise.

[0027] The following detailed description of the various embodiments of the present application is provided in conjunction with the accompanying drawings. However, those skilled in the art will appreciate that many technical details are provided in the various embodiments of the present application to facilitate a better understanding of the present application. However, even without these technical details and the various variations and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0028] According to some embodiments of the present application, on one hand, a method for preparing a secondary battery is provided, comprising:

[0029] A negative electrode sheet, a positive electrode sheet and a separator are provided, the positive electrode sheet, the separator and the negative electrode sheet are wound or stacked, and then placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

[0030] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer covering the surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode material and a negative electrode binder. The negative electrode material is dispersed in the negative electrode binder.

[0031] The negative electrode material is selected from at least one of natural graphite, artificial graphite, soft carbon or hard carbon; the material of the negative electrode current collector includes copper foil; and the material of the negative electrode adhesive is selected from at least one of sodium hydroxycellulose, polyvinylidene fluoride or styrene-butadiene rubber.

[0032] The material of the diaphragm is selected from at least one of a polyolefin diaphragm, a modified polyolefin diaphragm, a non-woven diaphragm or a ceramic composite diaphragm.

[0033] The preparation steps of the positive electrode sheet include: preparing the positive electrode sheet, the preparation steps include: preparing a precursor solution, mixing lithium ferrite and lithium nickelate and dissolving them in a solvent, adding lithiated carbon nanotubes and spinning aids, and mixing them evenly to form a precursor solution; loading the precursor solution into the syringe of the electrospinning machine, and depositing the precursor solution on the surface of the positive electrode collector by electrospinning to form an initial fiber film on the surface of the positive electrode collector; pre-burning the positive electrode collector with the initial fiber film and then calcining it to transform the initial fiber film into a fiber lithium replenishing layer, and the fiber lithium replenishing layer has a fiber network structure; coating a positive electrode material layer on the surface of the fiber lithium replenishing layer, the material of the positive electrode material layer includes lithium iron phosphate, a conductive agent and an adhesive, and obtaining a positive electrode sheet after drying; providing a negative electrode sheet and a separator, winding the positive electrode sheet, the separator and the negative electrode sheet and placing them in a shell after a stacking process, and injecting an electrolyte into the shell to obtain a secondary battery.

[0034] Figure 1 A flowchart corresponding to a method for preparing a positive electrode sheet provided in an embodiment of the present application.

[0035] refer to Figure 1 , the preparation steps of the positive electrode sheet include:

[0036] S101: Prepare a precursor solution, mix lithium ferrite and lithium nickelate in a molar ratio of (3-6):1 and dissolve them in deionized water, add lithiated carbon nanotubes and spinning aids, and the mass ratio of the total mass of lithium ferrite and lithium nickelate to the mass of lithiated carbon nanotubes is (2-13):1, and mix them evenly to form a precursor solution.

[0037] In some embodiments, the molar ratio of lithium ferrite to lithium nickelate is 3:1, 4:1, 5:1 or 6:1.

[0038] The spinning aid is selected from at least one of polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polycaprolactone (PCL), polylactic acid (PLA), polyethersulfone (PES), polyurethane (PU), polystyrene (PS), polyamide (PA), cellulose acetate (CA), chitosan (CS), silk fibroin (SF) or collagen.

[0039] In some embodiments, the preparation steps of lithium ferrite include: weighing a lithium source, an iron source and lithium fluoride, wherein the molar ratio of the lithium source to the iron source is (4-6):1 (for example, specifically 4:1, 5:1 or 6:1), and lithium fluoride accounts for 0.5% to 1.5% of the total mass of the lithium source, the iron source and the lithium fluoride (for example, specifically 0.5%, 0.7%, 0.9%, 1%, 1.2%, 1.4% or 1.5%); ball milling the lithium source, the iron source and the lithium fluoride under the protection of an inert gas at a speed of 200 The method comprises the following steps: heating the mixture at a speed of 9 MPa to 500 rpm for 3 to 6 hours; pressing the mixture into tablets at a pressure of 9 MPa to 11 MPa and a tablet diameter of 8 mm to 12 mm; placing the tablets in a microwave reaction chamber with a microwave power of 600 W to 1000 W, heating the tablets to 600° C. within 10 minutes, keeping the temperature for 30 minutes, and then quenching the tablets to room temperature; sieving the product with a sieve mesh of 300 to 500 mesh; placing the sieved product in a tube furnace, and annealing the product at 500° C. to 550° C. for 2 to 3 hours.

[0040] Lithium fluoride, as a cosolvent, can lower the synthesis temperature of the lithium and iron sources. Ball-milling the lithium, iron, and lithium fluoride prior to synthesis can improve the dispersion and contact area of ​​the reactants. Further, microwave-assisted synthesis of lithium ferrite offers advantages such as rapid reaction speed, high energy efficiency, high product purity, uniform particle size, simplified process, environmental friendliness, and strong controllability. Sieving the reaction product can help control the particle size of the lithium ferrite. This uniform particle size improves the electrochemical performance of the lithium ferrite and facilitates the formation of a uniform fiber network structure in the subsequent electrospinning process.

[0041] In some embodiments, the particle size corresponding to the cumulative particle size distribution percentage of lithium ferrite reaching 50% is less than or equal to 12 μm, for example, it can be 12 μm, 11 μm, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm or 5 μm.

[0042] In some embodiments, the specific surface area of ​​lithium ferrite is 5 m 2 / g~50m 2 / g, for example, it can be 5m 2 / g、8m 2 / g、10m 2 / g、12m 2 / g、14m 2 / g、15m 2 / g、18m 2 / g or 20m 2 / g. A too low specific surface area of ​​lithium ferrite will reduce the migration rate of lithium ions, resulting in capacity decay. A larger specific surface area of ​​lithium ferrite is beneficial for increasing the number of reactive sites, but an excessively large specific surface area will accelerate the decomposition of the electrolyte, causing gas production and capacity decay. Therefore, the specific surface area of ​​lithium ferrite needs to be within an appropriate range.

[0043] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate and lithium oxide.

[0044] The iron source is selected from one or more of ferric oxide, ferric oxide, ferric oxyhydroxide, ferric nitrate and ferric citrate.

[0045] In some embodiments, the preparation steps of lithium nickelate include: weighing a lithium source, a nickel source and citric acid, wherein the molar ratio of the lithium source, the nickel source and the citric acid is (1.05-1.1):1:1 (for example, it can be 1.05:1:1, 1.06:1:1, 1.07:1:1, 1.08:1:1, 1.09:1:1 or 1.1:1:1), dissolving the lithium source and the nickel source in deionized water to form a 0.5 mol / L solution, and then adding citric acid and stirring for 2 hours until transparent; using a spray pyrolysis device The mixed solution is dried and heat-treated to form lithium nickelate powder, with an atomization pressure of 0.3 MPa to 0.5 MPa, a droplet diameter of 2.5 μm to 3.5 μm, a pyrolysis temperature of 750° C. to 850° C., a heating rate of 40° C. / min to 60° C. / min, a residence time of 8 seconds to 15 seconds, an atmosphere of oxygen and nitrogen in a volume ratio of 3:7, and a gas flow rate of 150 mL / min to 250 mL / min; and the lithium nickelate powder is annealed at 600° C. to 650° C. for 4 to 5 hours.

[0046] Citric acid as a chelating agent can prevent component segregation and improve the stability of the reaction. Lithium nickelate is synthesized by spray pyrolysis process using lithium source, nickel source and citric acid. It has the advantages of efficient reaction, uniform product, high purity, controllable morphology, simple process, energy saving and environmental protection, wide application range and high crystallinity.

[0047] In some embodiments, the particle size corresponding to the cumulative particle size distribution percentage of lithium nickelate reaching 50% is less than or equal to 15 μm, for example, it can be 15 μm, 14 μm, 13 μm, 12 μm, 11 μm, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm or 5 μm.

[0048] In some embodiments, the specific surface area of ​​lithium nickelate is 3 m 2 / g~15m 2 / g, for example, it can be 3m 2 / g、5m 2 / g、8m 2 / g、10m 2 / g、13m 2 / g or 15m 2 / g. A low specific surface area of ​​lithium nickelate can reduce side reactions, but it can easily cause capacity decay. A large specific surface area of ​​lithium nickelate can improve rate performance and cycle stability, but an excessively large specific surface area can accelerate electrolyte decomposition, leading to gas production and battery capacity decay. Therefore, the specific surface area of ​​lithium nickelate needs to be within an appropriate range.

[0049] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate and lithium oxide.

[0050] The nickel source is selected from one or more of nickel oxide, nickel peroxide, nickel trioxide, nickel oxalate, nickel acetate, nickel carbonate and nickel hydroxide.

[0051] In some embodiments, the preparation steps of lithiated carbon nanotubes include: pickling activated carbon powder with hydrochloric acid; adding the activated carbon powder and lithium hydroxide powder that have been pickled with hydrochloric acid to deionized water, stirring for 100 minutes, adding oxalic acid to adjust the pH value of the mixture to 7±1, and filtering and drying to obtain lithiated carbon nanotubes.

[0052] In some embodiments, the average diameter of the lithiated carbon nanotubes is 10 nm to 50 nm, for example, 10 nm, 13 nm, 20 nm, 25 nm, 30 nm, 34 nm, 40 nm, 46 nm, or 50 nm; and the average length is 1 μm to 10 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. Too thin carbon nanotubes can easily lead to insufficient mechanical strength or agglomeration problems, while too thick can lead to reduced conductivity and specific surface area. Therefore, the average diameter of the carbon nanotubes needs to be within an appropriate range. At the same time, longer carbon nanotubes are conducive to building a conductive network, but are prone to poor dispersion. Although shorter carbon nanotubes are conducive to dispersion, the constructed conductive network may be incomplete. Therefore, the average length of the carbon nanotubes needs to be within an appropriate range.

[0053] Since the sizes of different single lithiated carbon nanotubes may be different, the average diameter refers to the average diameter of multiple lithiated carbon nanotube particles per unit mass, and the average length refers to the average length of multiple lithiated carbon nanotube particles per unit mass.

[0054] In some embodiments, mixing lithium ferrite and lithium nickelate in a molar ratio of (3 to 6):1 includes: mixing the lithium ferrite and lithium nickelate in a ball mill at a rotation speed of 150 rpm to 250 rpm for 1.5 hours to 3.5 hours; and sieving the mixed solid to obtain a mixed solid with a particle size range of 9 μm to 10 μm. First, mixing the lithium ferrite and lithium nickelate in a ball mill is beneficial for better dispersion of the lithium ferrite and lithium nickelate in the spinning aid, thereby facilitating good uniformity of the lithium-supplemented fiber layer formed by electrospinning. Sieving the mixed solid to obtain mixed particles with uniform particle size is beneficial for forming a fiber network structure with uniform thickness in the subsequent electrospinning process.

[0055] S102: The precursor liquid is loaded into the syringe of the electrospinning machine, and the precursor liquid is deposited onto the surface of the positive electrode current collector by electrospinning to form an initial fiber film on the surface of the positive electrode current collector.

[0056] The material of the positive electrode current collector includes aluminum foil.

[0057] In some embodiments, the electrospinning parameters include: a voltage of 15 kV to 30 kV, an injection speed of 0.5 mL / h to 2 mL / h, and a receiving distance of 10 cm to 20 cm.

[0058] S103: pre-calcining the positive electrode current collector with the initial fiber membrane in air at 300° C. to 400° C. for 2 to 4 hours. The purpose of the low-temperature pre-calcination is to remove organic components such as spinning aids.

[0059] S104: calcining the pre-burned positive electrode current collector in an inert gas at 600°C to 800°C for 4 to 6 hours to transform the initial fiber membrane into a fiber lithium replenishing layer. The fiber lithium replenishing layer has a fiber network structure. High temperature calcination makes the fiber network structure more stable.

[0060] S105: Coating a positive electrode material layer on the surface of the fiber lithium supplement layer. The materials of the positive electrode material layer include lithium iron phosphate, a conductive agent and an adhesive. The mass ratio of the total mass of lithium ferrite, lithium nickelate and lithiated carbon nanotubes in the fiber lithium supplement layer to the lithium iron phosphate, the adhesive and the conductive agent is (1.5-2.2): (93.8-94.5): (0.8-1.2): (1.7-2.1). After drying, a positive electrode sheet is obtained.

[0061] The conductive agent is selected from any one of conductive carbon black, graphite, conductive carbon tubes or graphene, or a combination of at least two thereof.

[0062] In some embodiments, the lithium iron phosphate is pretreated before coating, and the pretreatment steps include: weighing trimethyl borate and dissolving the trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; adding lithium iron phosphate and ball milling dispersion at a rotation speed of 300 rpm to 350 rpm for 4 to 5 hours, and the total mass ratio of trimethyl borate to the lithium iron phosphate and trimethyl borate is 1% to 3%.

[0063] In other embodiments, after obtaining the positive electrode sheet, the positive electrode sheet is post-processed, and the post-processing steps include: immersing the positive electrode sheet in a mixed solution of trimethyl borate and ethanol, where the mass ratio of trimethyl borate to the total mass of trimethyl borate and ethanol is 3% to 8%, and maintaining it under vacuum conditions for 30 minutes to 60 minutes; taking out the positive electrode sheet and drying it at 60°C.

[0064] In the above two methods, the wet coating method is used to coat the lithium iron phosphate with trimethyl borate, or the vacuum impregnation method is used to adsorb trimethyl borate in the pores of the positive electrode sheet. This can enable the selective oxidation of trimethyl borate in the positive electrode sheet to form a stable low-impedance surface film, inhibiting the decomposition of the electrolyte and the dissolution of transition metal ions from the positive electrode sheet, thereby reducing the gas production problem caused by the decomposition of the electrolyte, and further avoiding the storage gas production behavior of the secondary battery.

[0065] The embodiment of the present application provides a method for preparing a secondary battery, in which an electrostatic spinning process is used to form a fiber lithium replenishment layer on the positive electrode current collector and a positive electrode material layer is coated on the fiber lithium replenishment layer. The fiber lithium replenishment layer is located on the side of the positive electrode material layer close to the positive electrode current collector. The electrolyte can be in direct contact with the positive electrode material layer to improve the insertion and extraction efficiency of lithium ions, thereby improving the coulombic efficiency of the battery. The fiber lithium replenishment layer has a fiber network structure. On the one hand, the fiber network structure has a large porosity, which can help promote the positive electrode sheet to absorb the electrolyte. On the other hand, the fiber lithium replenishment layer with a fiber network structure is conducive to gas overflow during the first charging process, avoiding storage gas production behavior, and thereby avoiding the impact of storage gas production on battery performance. Among them, the materials of the fiber lithium replenishment layer include lithium ferrite, lithium nickelate and lithiated carbon nanotubes, all of which can make up for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, a long cycle life, and low cost, but lithium ferrite has a low energy density; lithium nickelate has a high energy density, but a short cycle life, poor thermal stability, rapid performance degradation at high temperatures, and high cost. The combination of lithium ferrite and lithium nickelate can complement each other to combine the advantages of both. In addition, based on the fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a spatial network conductive structure. The presence of lithiated carbon nanotubes improves the conductivity of the fiber lithium supplement layer, while increasing the chemical activity of lithium ferrite and lithium nickelate, reducing the decomposition potential of lithium ferrite and lithium nickelate, and reducing the damage of high voltage activation potential to the positive electrode material. The materials of the positive electrode material layer include lithium iron phosphate, a conductive agent and a binder. Lithium iron phosphate provides active lithium to meet the performance of the battery. The addition of the conductive agent improves the weak conductivity of lithium iron phosphate, forming a better conductive network and improving the performance of the battery. The binder can better bond the lithium iron phosphate to other substances and form a better adhesion effect on the fiber lithium supplement layer.

[0066] Accordingly, another embodiment of the present application further provides a secondary battery, which can be manufactured using the secondary battery manufacturing method of the above embodiment. The secondary battery provided by another embodiment of the present application will be described in detail below with reference to the accompanying drawings. For parts that are identical or corresponding to the previous embodiment, please refer to the corresponding description of the previous embodiment and will not be described in detail below.

[0067] The secondary battery includes: a shell, and a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte that are stacked inside the shell. The positive electrode sheet includes: a positive electrode current collector; a fiber lithium supplement layer, the fiber lithium supplement layer covers the surface of the positive electrode current collector, the fiber lithium supplement layer has a fiber network structure, and the material of the fiber lithium supplement layer includes lithium ferrite, lithium nickelate and lithiated carbon nanotubes; a positive electrode material layer, the positive electrode material layer covers the surface of the fiber lithium supplement layer, and the material of the positive electrode material layer includes lithium iron phosphate, a conductive agent and an adhesive.

[0068] Figure 2A schematic structural diagram of a positive electrode sheet of a secondary battery provided in an embodiment of the present application.

[0069] According to some embodiments of the present application, another aspect of the present application further provides a secondary battery, comprising: a housing, and a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte that are stacked and located inside the housing.

[0070] refer to Figure 2 The positive electrode sheet includes: a positive electrode current collector 201, a fiber lithium supplement layer 202 and a positive electrode material layer 203. The fiber lithium supplement layer 202 covers the surface of the positive electrode current collector 201. The fiber lithium supplement layer 202 has a fiber network structure. The material of the fiber lithium supplement layer 202 includes lithium ferrite, lithium nickelate and lithiated carbon nanotubes. The molar ratio of lithium ferrite to lithium nickelate is (3-6):1, and the mass ratio of the total mass of lithium ferrite and lithium nickelate to the lithiated carbon nanotubes is (2-13):1; the positive electrode material layer 203 covers the surface of the fiber lithium supplement layer 202. The material of the positive electrode material layer 203 includes lithium iron phosphate, a conductive agent and an adhesive. The mass ratio of the total mass of lithium ferrite, lithium nickelate and lithiated carbon nanotubes in the fiber lithium supplement layer to the lithium iron phosphate, the adhesive and the conductive agent is (1.5-2.2):(93.8-94.5):(0.8-1.2):(1.7-2.1).

[0071] In some embodiments, the ratio of the thickness of the fiber lithium-supplementing layer 202 to the thickness of the positive electrode material layer 203 is 1:(8-16), for example, 1:8, 1:10, 1:11, 1:13, 1:15, or 1:16. The lithium iron phosphate in the positive electrode material layer 203 is primarily used to provide active lithium to meet battery performance requirements. The fiber lithium-supplementing layer 202 is used to supplement lithium ions, and therefore is relatively thin relative to the thickness of the positive electrode material layer 203. The ratio of the thickness of the fiber lithium-supplementing layer 202 to the thickness of the positive electrode material layer 203 needs to be within an appropriate range to facilitate the fiber lithium-supplementing layer 202 to fully exert its lithium-supplementing function.

[0072] In some embodiments, the thickness of the fiber lithium supplement layer is 10 μm to 20 μm, for example, 10 μm, 13 μm, 15 μm, 18 μm or 20 μm.

[0073] In some embodiments, the thickness of the positive electrode material layer is 140 μm to 160 μm, for example, 140 μm, 145 μm, 150 μm, 155 μm or 160 μm.

[0074] Figure 3 A schematic structural diagram of a positive electrode sheet of another secondary battery provided in an embodiment of the present application.

[0075] In some embodiments, since the fiber lithium supplement layer 202 has a fiber network structure, the positive electrode material layer 203 may partially penetrate into the pores of the fiber network structure during the coating process, thereby forming a Figure 3 In the structure shown, there is no obvious boundary between the fiber lithium supplement layer 202 and the positive electrode material layer 203.

[0076] refer to Figure 3 When the positive electrode material layer 203 partially penetrates into the pores of the fiber network structure, the thickness of the fiber lithium supplement layer 202 refers to the average value of the distance between the point on the surface of the fiber lithium supplement layer 202 away from the positive electrode current collector 201 and the surface of the positive electrode current collector 201; the thickness of the positive electrode material layer 203 refers to the average value of the distance between the point on the surface of the positive electrode material layer 203 close to the positive electrode current collector 201 and the positive electrode material layer 203 away from the surface of the positive electrode current collector 201.

[0077] The present application provides a secondary battery in which a positive electrode current collector 201 has a fiber-network-like structure of a fiber lithium-replenishing layer 202, and a positive electrode material layer 203 is provided on the surface of the fiber lithium-replenishing layer 202. The fiber lithium-replenishing layer 202 is located on the side of the positive electrode material layer 203 close to the positive electrode current collector 201. The electrolyte can directly contact the positive electrode material layer 203 to improve the efficiency of lithium ion insertion and extraction. The fiber lithium-replenishing layer 202 has a fiber-network-like structure. On the one hand, the fiber-network-like structure has a large porosity, which can facilitate the absorption of electrolyte by the positive electrode sheet. On the other hand, the fiber lithium-replenishing layer 202 with a fiber-network-like structure is conducive to gas overflow during the first charge process, avoiding storage gas generation behavior, and thus avoiding the impact of storage gas generation on battery performance. Among them, the materials of the fiber lithium-replenishing layer 202 include lithium ferrite, lithium nickelate and lithiated carbon nanotubes, all of which can compensate for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, good cycle life, and low cost, but lithium ferrite has a low energy density; lithium nickelate has a high energy density, but a short cycle life, poor thermal stability, rapid performance degradation at high temperatures, and high cost. The combination of lithium ferrite and lithium nickelate can complement each other to combine the advantages of the two. In addition, based on the fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a spatial network conductive structure. The presence of lithiated carbon nanotubes improves the conductivity of the fiber lithium supplement layer, while increasing the chemical activity of lithium ferrite and lithium nickelate, reducing the decomposition potential of lithium ferrite and lithium nickelate, and reducing the harm of high voltage activation potential to the positive electrode material. The materials of the positive electrode material layer 203 include lithium iron phosphate, a conductive agent and a binder. Lithium iron phosphate provides active lithium to meet the performance of the battery. The addition of the conductive agent improves the problem of weak conductivity of lithium iron phosphate, forms a better conductive network, and improves the performance of the battery. The binder can better bond the lithium iron phosphate to other substances and form a better adhesion effect on the fiber lithium supplement layer.

[0078] The following are specific embodiments of this application:

[0079] Example 1

[0080] Preparation of the positive electrode: Prepare a precursor solution, mix lithium ferrite and lithium nickelate in a molar ratio of 5:1 and dissolve it in deionized water, add lithiated carbon nanotubes and polyvinyl pyrrolidone, and the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate is 1:8, and mix them evenly to form a precursor solution; load the precursor solution into the syringe of the electrospinning machine, and deposit the precursor solution on the surface of the aluminum foil by electrospinning to form an initial fiber film on the surface of the aluminum foil; pre-sinter the aluminum foil with the initial fiber film at 300°C in air for 3 hours to remove organic components such as polyvinyl pyrrolidone; the pre-calcined aluminum foil is calcined at 800°C in an inert gas for 4 hours to transform the initial fiber membrane into a fiber lithium-replenishing layer with a fiber network structure; a positive electrode material layer is coated on the surface of the fiber lithium-replenishing layer, wherein the materials of the positive electrode material layer include lithium iron phosphate, a conductive agent and a binder, and the mass ratio of the total mass of lithium ferrite, lithium nickelate and lithiated carbon nanotubes in the fiber lithium-replenishing layer to the lithium iron phosphate, the binder and the conductive agent is 2:94:1:2, and the positive electrode sheet is obtained after drying.

[0081] The positive electrode sheet, separator and negative electrode sheet are wound and placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

[0082] The preparation steps of Example 2 are basically the same as those of Example 1, except that the molar ratio of lithium ferrite to lithium nickelate in Example 2 is 3:1.

[0083] The preparation steps of Example 3 are basically the same as those of Example 1, except that the molar ratio of lithium ferrite to lithium nickelate in Example 3 is 6:1.

[0084] The preparation steps of Example 4 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Example 4 is 1:2.

[0085] The preparation steps of Example 5 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Example 5 is 1:5.

[0086] The preparation steps of Example 6 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Example 6 is 1:10.

[0087] The preparation steps of Example 7 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Example 7 is 1:13.

[0088] The difference between Comparative Example 1 and Example 1 is that the positive electrode sheet of Comparative Example 1 is prepared by mixing lithium ferrite, lithium nickelate and lithiated carbon nanotubes directly with the positive electrode material layer and then coating the mixture on an aluminum foil.

[0089] The difference between Comparative Example 2 and Example 1 is that the positive electrode sheet of Comparative Example 2 is prepared by mixing lithium ferrite, lithium nickelate and lithiated carbon nanotubes with an adhesive and then coating the mixture on aluminum foil, and then coating the positive electrode material layer.

[0090] The preparation steps of Comparative Example 3 are basically the same as those of Example 1, except that the molar ratio of lithium ferrite to lithium nickelate in Comparative Example 3 is 1:1.

[0091] The preparation steps of Comparative Example 4 are substantially the same as those of Example 1, except that the molar ratio of lithium ferrite to lithium nickelate in Comparative Example 4 is 10:1.

[0092] The preparation steps of Comparative Example 5 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Comparative Example 5 is 1:1.

[0093] The preparation steps of Comparative Example 6 are basically the same as those of Example 1, except that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Comparative Example 6 is 1:20.

[0094] The first coulombic efficiency, the highest liquid level of the electrolyte during the first charge, the capacity retention rate after 1000 cycles at 45°C, and the energy density of Examples 1 to 7 and Comparative Examples 1 to 6 were tested.

[0095] Table 1 shows the performance test results corresponding to Examples 1 to 7 and Comparative Examples 1 to 6.

[0096]

[0097] According to the test results of Example 1, Comparative Example 1 and Comparative Example 2 in Table 1, compared with the conventional process method of adding a lithium supplement agent to the positive electrode material layer, the preparation method provided in the embodiment of the present application, in which a fiber lithium supplement layer is formed on an aluminum foil by an electrospinning process and then coated with a positive electrode material layer, prepares a positive electrode sheet. This can help reduce the gas production problem of the lithium supplement agent, thereby reducing the impact of storage gas production on battery performance, maintaining the battery with a high first coulombic efficiency and energy density, and improving the cycle performance at high temperature.

[0098] According to the test results of Examples 1 to 7 and Comparative Examples 3 to 6 in Table 1, when the molar ratio of lithium ferrite and lithium nickelate is in the range of (3 to 6): 1, and the mass ratio of the total mass of lithium ferrite and lithium nickelate to the lithiated carbon nanotubes is in the range of (2 to 13): 1, it is beneficial to maintain the battery with a high first coulombic efficiency, energy density and cycle performance at high temperature.

[0099] Those skilled in the art will appreciate that the above-described embodiments are specific examples for implementing the present application, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined in the claims.

Claims

1. A method for preparing a secondary battery, characterized in that: include: Prepare the positive electrode sheet, the preparation steps include: A precursor solution is prepared by mixing lithium ferrite and lithium nickelate and dissolving the mixture in a solvent, adding lithiated carbon nanotubes and a spinning aid, and mixing the mixture evenly to prepare the precursor solution; The precursor solution is loaded into a syringe of an electrospinning machine, and the precursor solution is deposited onto the surface of the positive electrode current collector by electrospinning to form an initial fiber film on the surface of the positive electrode current collector; Pre-calcining the positive electrode current collector having the initial fiber membrane and then calcining it to transform the initial fiber membrane into a fiber lithium replenishing layer, wherein the fiber lithium replenishing layer has a fiber network structure; Coating a positive electrode material layer on the surface of the fiber lithium supplement layer, wherein the material of the positive electrode material layer includes lithium iron phosphate, a conductive agent and an adhesive, and drying to obtain the positive electrode sheet; A negative electrode sheet and a separator are provided, the positive electrode sheet, the separator and the negative electrode sheet are wound or stacked, and then placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

2. The method for preparing a secondary battery according to claim 1, wherein: In the step of preparing the precursor solution, the molar ratio of the lithium ferrite and the lithium nickelate is (3-6):1; the mass ratio of the total mass of the lithium ferrite and the lithium nickelate to the lithiated carbon nanotubes is (2-13):

1.

3. The method for preparing a secondary battery according to claim 1, wherein: The lithium iron phosphate is pretreated before coating, and the pretreatment steps include: Weigh trimethyl borate and dissolve it in an ethanol and acetone solution with a volume ratio of 1:1; After the lithium iron phosphate is added and dispersed by ball milling, the mass ratio of the trimethyl borate to the total mass of the lithium iron phosphate and the trimethyl borate is 1% to 3%.

4. The method for preparing a secondary battery according to claim 1, wherein: After obtaining the positive electrode sheet, the positive electrode sheet is post-processed, and the post-processing steps include: The positive electrode sheet is immersed in a mixed solution of trimethyl borate and ethanol, wherein the mass ratio of trimethyl borate to the total mass of the trimethyl borate and the ethanol is 3% to 8%, and the solution is kept under vacuum and then dried.

5. A secondary battery, characterized in that: include: A housing, and a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte stacked in the housing, wherein the positive electrode sheet comprises: positive electrode current collector; A fiber lithium-supplementing layer, the fiber lithium-supplementing layer covering the surface of the positive electrode current collector, the fiber lithium-supplementing layer having a fiber network structure, and the material of the fiber lithium-supplementing layer includes lithium ferrite, lithium nickelate and lithiated carbon nanotubes; A positive electrode material layer is covered on the surface of the fiber lithium supplement layer. The material of the positive electrode material layer includes lithium iron phosphate, a conductive agent and an adhesive.

6. The secondary battery according to claim 5, characterized in that The mass ratio of the total mass of the lithium ferrite and the lithium nickelate to the lithiated carbon nanotubes is (2-13):

1.

7. The secondary battery according to claim 5, characterized in that The molar ratio of the lithium ferrite to the lithium nickelate is (3-6):

1.

8. The secondary battery according to claim 5, characterized in that The ratio of the thickness of the fiber lithium supplement layer to the thickness of the positive electrode material layer is 1:(8-16).

9. The secondary battery according to claim 5, characterized in that The thickness of the fiber lithium supplement layer is 10 μm to 20 μm; the thickness of the positive electrode material layer is 140 μm to 160 μm.

10. The secondary battery according to claim 5, characterized in that When the cumulative particle size distribution percentage of the lithium ferrite reaches 50%, the corresponding particle size is less than or equal to 12 μm; when the cumulative particle size distribution percentage of the lithium nickelate reaches 50%, the corresponding particle size is less than or equal to 15 μm; the average diameter of the lithiated carbon nanotubes is 10 nm to 50 nm, and the average length is 1 μm to 10 μm.

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