Secondary battery and method for manufacturing the same

Abstract

This application 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 includes: winding or laminating a positive electrode sheet, a separator, and a negative electrode sheet and then placing them in a housing, and injecting an electrolyte into the housing to obtain a secondary battery; wherein the preparation steps of the positive electrode sheet include: preparing a precursor solution by mixing lithium ferrite, lithium nickelate, lithiated carbon nanotubes, and a spinning aid; depositing the precursor solution on 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-burning the positive electrode current collector with the initial fiber film in air and then calcining it in an inert gas to transform the initial fiber film into a fiber lithium supplement layer, and the fiber lithium supplement layer has a fiber network structure; coating a positive electrode material layer on the surface of the fiber lithium supplement layer, and drying to obtain the positive electrode sheet.

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries, and particularly relates to a secondary battery and a preparation method thereof. Background Art

[0002] With the research and development of secondary batteries, the requirements for their energy density, working voltage, and cycle life are getting higher and higher. Currently, the cathode materials of secondary batteries, such as lithium-ion batteries, mainly include lithium iron phosphate, lithium cobaltate, lithium manganate, ternary materials, etc. Among them, lithium iron phosphate (LiFePO4) is widely used due to its advantages such as high capacity, environmental friendliness, high safety, and long cycle life. With the popularization of automotive electrification, pursuing secondary batteries with higher energy density is a common goal for people. Under many design concepts and material systems, further improving the energy density often comes at the cost of battery safety, power, and life. Summary of the Invention

[0003] The embodiments of this application provide a secondary battery and a preparation method thereof, which are at least beneficial to reducing the gas generation during the storage of the secondary battery.

[0004] According to some embodiments of this application, on the one hand, the embodiments of this application provide a preparation method of a secondary battery, including: preparing a positive electrode sheet, and the preparation steps include: configuring a precursor solution, mixing lithium iron phosphate and lithium nickelate according to a molar ratio of (3 - 6):1 and dissolving them in deionized water, adding lithiated carbon nanotubes and a spinning aid, and the mass ratio of the total mass of lithium iron phosphate and lithium nickelate to the mass of lithiated carbon nanotubes is (2 - 13):1, mixing evenly to form a precursor solution; loading the precursor solution into the syringe of an electrospinning machine, and depositing the precursor solution onto the surface of a positive electrode current collector through electrospinning to form an initial fiber film on the surface of the positive electrode current collector; pre-burning the positive electrode current collector with the initial fiber film in air at 300°C - 400°C for 2 hours - 4 hours; calcining the pre-burned positive electrode current collector in an inert gas at 600°C - 800°C for 4 hours - 6 hours to transform the initial fiber film into a fiber lithium compensation layer, and the fiber lithium compensation layer has a fiber network structure; coating a positive electrode material layer on the surface of the fiber lithium compensation layer, and 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 iron phosphate, lithium nickelate, and lithiated carbon nanotubes in the fiber lithium compensation layer to the mass of lithium iron phosphate, the binder, and the conductive agent is (1.5 - 2.2):(93.8 - 94.5):(0.8 - 1.2):(1.7 - 2.1), and after drying, a positive electrode sheet is obtained; providing a negative electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then putting them into a housing, and injecting an electrolyte into the housing to obtain a secondary battery.

[0005] In some embodiments, the particle size corresponding to when the cumulative particle size distribution percentage of lithium ferrite reaches 50% is less than or equal to 12 μm; the particle size corresponding to when the cumulative particle size distribution percentage of 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.

[0006] In some embodiments, mixing lithium ferrite and lithium nickelate in a molar ratio of (3 - 6):1 includes: mixing 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; sieving the mixed solid to obtain a mixed solid with a particle size range of 9 μm to 10 μm.

[0007] In some embodiments, the preparation steps of lithium ferrite include: weighing a lithium source, an iron source, and lithium fluoride, with the molar ratio of the lithium source to the iron source being (4 - 6):1, and lithium fluoride accounting for 0.5% to 1.5% of the total mass of the lithium source, the iron source, and lithium fluoride; ball-milling and mixing the lithium source, the iron source, and lithium fluoride under the protection of an inert gas at a rotation speed of 200 rpm to 500 rpm for 3 hours to 6 hours; pressing the mixture into tablets with 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 cavity with a microwave power of 600 W to 1000 W, heating to 600 °C within 10 minutes, holding for 30 minutes, and then quenching to room temperature; sieving the product with a sieve mesh size of 300 to 500 meshes; placing the sieved product in a tubular furnace and annealing at 500 °C to 550 °C for 2 hours to 3 hours.

[0008] In some embodiments, the preparation steps of lithium nickelate include: weighing a lithium source, a nickel source, and citric acid, with the molar ratio of the lithium source, the nickel source, and citric acid being (1.05 - 1.1):1:1, dissolving the lithium source and the nickel source in deionized water to prepare a 0.5 mol / L solution, and then adding citric acid and stirring for 2 hours until transparent; drying the mixed solution using a spray pyrolysis device and forming a powder of lithium nickelate through heat treatment, 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 with a volume ratio of 3:7, and a gas flow rate of 150 mL / min to 250 mL / min; annealing the powder of lithium nickelate at 600 °C to 650 °C for 4 hours to 5 hours.

[0009] In some embodiments, lithium iron phosphate is pretreated before coating. The pretreatment steps include: weighing trimethyl borate, dissolving trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; adding lithium iron phosphate and then ball milling and dispersing at a rotation speed of 300 rpm to 350 rpm for 4 hours to 5 hours, and the mass ratio of trimethyl borate to the total mass of lithium iron phosphate and trimethyl borate is 1% to 3%.

[0010] In some embodiments, after obtaining the positive electrode sheet, post-treatment is performed on the positive electrode sheet. The post-treatment 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.

[0011] According to some embodiments of the present application, on the other hand, the present application embodiments also provide a secondary battery, including: a housing, and a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte that are stacked and arranged inside the housing. 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. The molar ratio of lithium ferrite to lithium nickelate is (3 to 6):1, and the total mass ratio of lithium ferrite and lithium nickelate to the mass of lithiated carbon nanotubes is (2 to 13):1; 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 a binder. The total mass ratio of lithium ferrite, lithium nickelate, and lithiated carbon nanotubes in the fiber lithium supplement layer to the mass of lithium iron phosphate, the binder, and the conductive agent is (1.5 to 2.2):(93.8 to 94.5):(0.8 to 1.2):(1.7 to 2.1).

[0012] In some embodiments, the thickness ratio of the fiber lithium supplement layer to the positive electrode material layer is 1:(8 to 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] The technical solutions provided by the embodiments of the present application have at least the following advantages:

[0015] In the preparation method of a secondary battery provided by an embodiment of the present application, an electrostatic spinning process is used to form a fiber lithium supplement layer on the positive electrode current collector, and a positive electrode material layer is coated on the fiber lithium supplement layer. The fiber lithium supplement 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 supplement layer has a fiber network structure. On the one hand, the fiber network structure has a large porosity, which is conducive to promoting the absorption of the electrolyte by the positive electrode sheet. On the other hand, the fiber lithium supplement layer with the fiber network structure is conducive to the gas overflow during the first charging process, avoiding the behavior of gas generation during storage, and thus avoiding the impact of gas generation during storage on the battery performance. Among them, the materials of the fiber lithium supplement layer include lithium ferrite, lithium nickelate and lithiated carbon nanotubes, and the above materials can all make up for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, good cycle life and low cost, but the energy density of lithium ferrite is low; lithium nickelate has a high energy density, but a short cycle life, poor thermal stability, rapid performance decay at high temperatures, and high cost. The compounding 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, simultaneously improves the chemical activity of lithium ferrite and lithium nickelate, reduces the decomposition potential of lithium ferrite and lithium nickelate, and reduces the harm of the 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 problem of weak conductivity of lithium iron phosphate, forms a better conductive network, and improves the performance of the battery. The role of the binder is to better bond lithium iron phosphate with other substances and form a better adhesion with the fiber lithium supplement layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] One or more embodiments are illustrated by way of example in the accompanying drawings corresponding thereto. These illustrative descriptions do not limit the embodiments unless otherwise stated. The figures in the accompanying drawings do not constitute a scale limitation; in order to more clearly illustrate the technical solutions in the embodiments of the present application or in the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following described drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0017] Figure 1 It is a flowchart corresponding to a method for preparing a positive electrode sheet provided by an embodiment of the present application;

[0018] Figure 2 It is a schematic structural diagram of a positive electrode sheet of a secondary battery provided by an embodiment of the present application;

[0019] Figure 3 This is a schematic structural diagram of the positive electrode sheet of another secondary battery provided by the embodiments of the present application. Detailed implementation manners

[0020] During the charge and discharge process of a secondary battery, an SEI film (Solid Electrolyte Interface membrane) will be formed on the negative electrode, resulting in the loss of active lithium, reducing the energy density and Coulomb efficiency of the battery, and affecting the further development of secondary batteries. The battery lithium supplementation technology is an important means to improve the energy density of the battery. According to the technical route, lithium supplementation can be divided into two categories: positive electrode lithium supplementation and negative electrode lithium supplementation; among them, the positive electrode lithium supplementation technology adds a lithium supplementation material to the positive electrode of the secondary battery. The added lithium supplementation material will 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 and achieving the effect of lithium supplementation.

[0021] Generally, the usage method of the positive electrode lithium supplementation agent additive is usually to mix and add it with the main positive electrode material during homogenization. Since the lithium supplementation agent will release gas during the charge and discharge process, there will be gas storage and gas generation behaviors in the prepared battery, and the gas will affect the cycle and high-temperature performance of the battery.

[0022] The embodiments of the present application provide a secondary battery and a preparation method thereof, which are at least beneficial to improving the performance of the secondary battery.

[0023] Referring to "embodiments" in this text means that the specific features, structures or characteristics described in combination with the embodiments can be included in at least one embodiment of the present application. The appearance of this phrase in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0024] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise specified, other components are not excluded, and other components may further be included.

[0025] The terms used in the description of the various embodiments herein are only for describing specific embodiments and are not intended to be limiting. As used in the description of the various embodiments and the appended claims, "component" is also intended to include the plural form unless the context clearly indicates otherwise.

[0026] The following will elaborate on the embodiments of the present application in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in the embodiments of the present application, many technical details are presented to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0027] According to some embodiments of the present application, on the one hand, a method for preparing a secondary battery is provided, including:

[0028] Providing a negative electrode sheet, a positive electrode sheet, and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then placing them into a housing, and injecting an electrolyte into the housing to obtain a secondary battery.

[0029] 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, and the negative electrode material is dispersed in the negative electrode binder.

[0030] 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; the materials of the negative electrode binder are all selected from at least one of sodium carboxymethyl cellulose, polyvinylidene fluoride, or styrene-butadiene rubber.

[0031] The material of the separator is selected from at least one of a polyolefin separator, a modified polyolefin separator, a non-woven separator, or a ceramic composite separator.

[0032] Figure 1 It is a flowchart corresponding to a method for preparing a positive electrode sheet provided by an embodiment of the present application.

[0033] Reference Figure 1 The preparation steps of the positive electrode sheet include:

[0034] 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 a spinning aid. The mass ratio of the total mass of lithium ferrite and lithium nickelate to the mass of lithiated carbon nanotubes is (2 - 13):1. After mixing evenly, a precursor solution is made.

[0035] In some embodiments, the molar ratio of lithium ferrite to lithium nickelate is specifically 3:1, 4:1, 5:1, or 6:1.

[0036] The spinning aid is selected from at least one of polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyvinylpyrrolidone (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.

[0037] In some embodiments, the preparation steps of lithium ferrite include: weighing a lithium source, an iron source, and lithium fluoride, where 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% - 1.5% of the total mass of the lithium source, the iron source, and lithium fluoride (for example, specifically 0.5%, 0.7%, 0.9%, 1%, 1.2%, 1.4%, or 1.5%); ball-milling and mixing the lithium source, the iron source, and lithium fluoride under the protection of an inert gas, with a rotation speed of 200 rpm - 500 rpm and a time of 3 hours - 6 hours; pressing the mixture into tablets, with a pressure of 9 Mpa - 11 Mpa and a tablet diameter of 8 mm - 12 mm; placing the tablets in a microwave reaction chamber, with a microwave power of 600 W - 1000 W, heating to 600 °C within 10 minutes, maintaining the temperature for 30 minutes, and then quenching to room temperature; sieving the product, with a sieve mesh size of 300 mesh - 500 mesh; placing the sieved product in a tubular furnace and annealing at 500 °C - 550 °C for 2 hours - 3 hours.

[0038] Lithium fluoride, as a co-solvent, can reduce the synthesis temperature of the lithium source and the iron source. First, ball-milling and mixing the lithium source, the iron source, and lithium fluoride can help improve the dispersion effect and contact area of the reactants. Further, synthesizing lithium ferrite by microwave reaction has advantages such as fast reaction speed, high energy utilization rate, high product purity, uniform particle size, simplified process, environmental friendliness, and strong controllability. Sieving the reaction product can help control the particle size of lithium ferrite. Uniform particle size is beneficial for improving the electrochemical performance of lithium ferrite and, secondly, for forming a fiber network structure with uniform thickness in the subsequent electrospinning process steps.

[0039] 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, specifically 12 μm, 11 μm, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm, or 5 μm.

[0040] In some embodiments, the specific surface area of lithium ferrite is 5 m² / g to 50 m² / g. For example, it can specifically be 5 m² / g, 8 m² / g, 10 m² / g, 12 m² / g, 14 m² / g, 15 m² / g, 18 m² / g, or 20 m² / g. If the specific surface area of lithium ferrite is too low, the migration rate of lithium ions will be reduced, resulting in capacity decay. A relatively large specific surface area of lithium ferrite is beneficial for increasing the reactive active sites, but an overly large specific surface area will accelerate the decomposition of the electrolyte, causing gas generation and capacity decay. Therefore, the specific surface area of lithium ferrite needs to be within an appropriate range.

[0041] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate, and lithium oxide.

[0042] The iron source is selected from one or more of iron(III) oxide, iron(II,III) oxide, iron(III) hydroxide, iron(III) nitrate, and iron(III) citrate.

[0043] In some embodiments, the preparation steps of lithium nickelate include: weighing the lithium source, nickel source, and citric acid. The molar ratio of the lithium source, nickel source, and citric acid is (1.05 - 1.1):1:1 (for example, it can specifically 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). Dissolve the lithium source and nickel source in deionized water to prepare a 0.5 mol / L solution, and then add citric acid and stir for 2 hours until it becomes transparent; use a spray pyrolysis device to dry the mixed solution, and form the powder of lithium nickelate through heat treatment. The atomization pressure is 0.3 Mpa to 0.5 Mpa, the droplet diameter is 2.5 μm to 3.5 μm, the pyrolysis temperature is 750 °C to 850 °C, the heating rate is 40 °C / min to 60 °C / min, the residence time is 8 seconds to 15 seconds, the atmosphere is oxygen and nitrogen with a volume ratio of 3:7, and the gas flow rate is 150 mL / min to 250 mL / min; anneal the powder of lithium nickelate at 600 °C to 650 °C for 4 hours to 5 hours.

[0044] As a complexing agent, citric acid can prevent component segregation and improve the stability of the reaction. Synthesizing lithium nickelate from the lithium source, nickel source, and citric acid using the spray pyrolysis process has advantages such as efficient reaction, uniform products, high purity, controllable morphology, simple process, energy conservation and environmental protection, wide application range, and high crystallinity.

[0045] 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 specifically be 15 μm, 14 μm, 13 μm, 12 μm, 11 μm, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm, or 5 μm.

[0046] In some embodiments, the specific surface area of lithium nickelate is 3 m² / g to 15 m² / g. For example, it can specifically be 3 m² / g, 5 m² / g, 8 m² / g, 10 m² / g, 13 m² / g, or 15 m² / g. A lower specific surface area of lithium nickelate can reduce the occurrence of side reactions, but it is prone to capacity fade. A larger specific surface area of lithium nickelate is beneficial to improving rate performance and cycle stability, but an overly large specific surface area is prone to accelerating the decomposition of the electrolyte, resulting in gas generation and attenuation of battery capacity. Therefore, the specific surface area of lithium nickelate needs to be within an appropriate range.

[0047] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate, and lithium oxide.

[0048] The nickel source is selected from one or more of nickel oxide, nickel peroxide, nickel sesquioxide, nickel oxalate, nickel acetate, nickel carbonate, and nickel hydroxide.

[0049] 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 after hydrochloric acid pickling treatment into deionized water, stirring for 100 minutes, then adding oxalic acid to adjust the pH value of the mixture to 7 ± 1, and filtering and drying to obtain lithiated carbon nanotubes.

[0050] In some embodiments, the average diameter of lithiated carbon nanotubes is 10 nm to 50 nm. For example, it can specifically be 10 nm, 13 nm, 20 nm, 25 nm, 30 nm, 34 nm, 40 nm, 46 nm, or 50 nm; the average length is 1 μm to 10 μm. For example, it can specifically be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. If the carbon nanotubes are too thin, it is prone to insufficient mechanical strength or agglomeration problems. If they are too thick, it will lead to a decrease in electrical conductivity and specific surface area. Therefore, the average diameter of the carbon nanotubes needs to be within an appropriate range. At the same time, a longer length of the carbon nanotubes is beneficial to constructing a conductive network, but it is prone to poor dispersibility. If the length of the carbon nanotubes is short, although it is beneficial to dispersion, the constructed conductive network may be incomplete. Therefore, the average length of the carbon nanotubes needs to be within an appropriate range.

[0051] 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.

[0052] In some embodiments, mixing lithium ferrite and lithium nickelate in a molar ratio of (3 to 6):1 includes: mixing lithium ferrite and lithium nickelate in a ball mill at a rotational speed of 150 rpm to 250 rpm for a time of 1.5 hours to 3.5 hours; sieving the mixed solid to obtain a mixed solid with a particle size range of 9 μm to 10 μm. First, mixing lithium ferrite and lithium nickelate through a ball mill is beneficial to achieving a better dispersion effect of lithium ferrite and lithium nickelate in the spinning aid, and thus beneficial to the fiber lithium supplement layer formed by electrospinning having good uniformity. Sieving the mixed solid to obtain mixed particles with uniform particle size is beneficial to forming a fiber network structure with uniform thickness during the subsequent electrospinning process.

[0053] S102: Load the precursor solution into the syringe of an electrospinning machine, and deposit the precursor solution onto the surface of the positive current collector through electrospinning to form an initial fiber membrane on the surface of the positive current collector.

[0054] The material of the positive current collector includes aluminum foil.

[0055] In some embodiments, the parameters of electrospinning 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.

[0056] S103: Pre-bake the positive current collector with the initial fiber membrane in air at 300°C to 400°C for 2 hours to 4 hours. The purpose of the low-temperature calcination pre-baking is to remove organic components such as the spinning aid.

[0057] S104: Calcinate the pre-baked positive current collector in an inert gas at 600°C to 800°C for 4 hours to 6 hours to transform the initial fiber membrane into a fiber lithium supplement layer. The fiber lithium supplement layer has a fiber network structure, and the high-temperature calcination makes the fiber network structure more stable.

[0058] S105: Coat a positive electrode material layer 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 a binder. The total mass ratio of lithium ferrite, lithium nickelate, and lithiated carbon nanotubes in the fiber lithium supplement layer to the mass of lithium iron phosphate, the binder, and the conductive agent is (1.5 to 2.2):(93.8 to 94.5):(0.8 to 1.2):(1.7 to 2.1). After drying, a positive electrode sheet is obtained.

[0059] The conductive agent is selected from any one or a combination of at least two of conductive carbon black, graphite, carbon nanotubes, or graphene.

[0060] In some embodiments, the lithium iron phosphate is pretreated before coating. The steps of the pretreatment include: weighing trimethyl borate, dissolving trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; adding lithium iron phosphate and then ball-milling and dispersing at a rotation speed of 300 rpm to 350 rpm for 4 hours to 5 hours. The mass ratio of trimethyl borate to the total mass of lithium iron phosphate and trimethyl borate is 1% to 3%.

[0061] In other embodiments, after the positive electrode sheet is obtained, post-treatment is performed on the positive electrode sheet. The steps of the post-treatment 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.

[0062] In the above two methods, by using the wet coating method to coat lithium iron phosphate with trimethyl borate or using the vacuum impregnation method to adsorb trimethyl borate in the pores of the positive electrode sheet, a stable low-impedance surface film can be formed by the selective oxidation of trimethyl borate in the positive electrode sheet, inhibiting the decomposition of the electrolyte and the dissolution of transition metal ions from the positive electrode sheet, thereby reducing the gas generation problem caused by the decomposition of the electrolyte and further avoiding the gas generation behavior during the storage of secondary batteries.

[0063] In the preparation method of a secondary battery provided by an embodiment of the present application, an electrostatic spinning process is used to form a fiber lithium supplement layer on the positive current collector, and a positive electrode material layer is coated on the fiber lithium supplement layer. The fiber lithium supplement layer is located on the side of the positive electrode material layer close to the positive 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, thereby improving the Coulomb efficiency of the battery. The fiber lithium supplement layer has a fiber network structure. On the one hand, the fiber network structure has a large porosity, which is conducive to promoting the absorption of the electrolyte by the positive electrode sheet. On the other hand, the fiber lithium supplement layer with the fiber network structure is conducive to the gas overflow during the first charging process, avoiding the gas generation behavior during storage, and thus avoiding the impact of the stored gas on the battery performance. Among them, the materials of the fiber lithium supplement layer include lithium ferrite, lithium nickelate, and lithiated carbon nanotubes, and the above materials can all make up for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, good cycle life, and low cost, but the energy density of lithium ferrite is low; lithium nickelate has a high energy density, but a short cycle life, poor thermal stability, fast performance decay at high temperatures, and high cost. The compounding 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 improving the chemical activity of lithium ferrite and lithium nickelate, reducing the decomposition potential of lithium ferrite and lithium nickelate, and reducing the harm of the 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 problem of weak conductivity of lithium iron phosphate, forms a better conductive network, and improves the performance of the battery. The role of the binder is to better bond lithium iron phosphate with other substances and form a better adhesion with the fiber lithium supplement layer.

[0064] Correspondingly, another embodiment of the present application further provides a secondary battery, which can be manufactured by using the preparation method of the secondary battery in 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 the same or corresponding parts as the previous embodiment, reference can be made to the corresponding description of the previous embodiment, and details will not be repeated below.

[0065] Figure 2 It is a schematic structural diagram of a positive electrode sheet of a secondary battery provided by an embodiment of the present application.

[0066] According to some embodiments of the present application, on the other hand, the present application embodiment further provides a secondary battery, including: a housing, and a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte that are stacked and arranged inside the housing.

[0067] 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 mass of 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 a binder. The mass ratio of the total mass of lithium ferrite, lithium nickelate, and lithiated carbon nanotubes in the fiber lithium supplement layer to the mass of lithium iron phosphate, the binder, and the conductive agent is (1.5 - 2.2):(93.8 - 94.5):(0.8 - 1.2):(1.7 - 2.1).

[0068] In some embodiments, the thickness ratio of the fiber lithium supplement layer 202 to the positive electrode material layer 203 is 1:(8 - 16), for example, specifically it can be 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 mainly used to provide active lithium to meet the performance of the battery. The fiber lithium supplement layer 202 is used to supplement lithium ions. Therefore, the thickness is relatively thin compared to the thickness of the positive electrode material layer 203. The thickness ratio of the fiber lithium supplement layer 202 to the positive electrode material layer 203 needs to be within a suitable range to facilitate the fiber lithium supplement layer 202 to fully play the role of lithium supplementation.

[0069] In some embodiments, the thickness of the fiber lithium supplement layer is 10μm - 20μm, for example, specifically it can be 10μm, 13μm, 15μm, 18μm, or 20μm.

[0070] In some embodiments, the thickness of the positive electrode material layer is 140μm - 160μm, for example, specifically it can be 140μm, 145μm, 150μm, 155μm, or 160μm.

[0071] Figure 3 This is a schematic structural diagram of the positive electrode sheet of another secondary battery provided by the embodiments of the present application.

[0072] In some embodiments, due to the fiber network structure of the fiber lithium supplement layer 202, during the coating process of the positive electrode material layer 203, part of it may penetrate into the pores of the fiber network structure, thus forming a structure as shown in Figure 3 , and at this time, there is no obvious boundary between the fiber lithium supplement layer 202 and the positive electrode material layer 203.

[0073] Reference Figure 3When a part of the positive electrode material layer 203 penetrates into the pores of the fibrous network structure, the thickness of the fibrous lithium supplement layer 202 refers to the average value of the distance between the points on the surface of the fibrous 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 points on the surface of the positive electrode material layer 203 close to the positive electrode current collector 201 and the surface of the positive electrode material layer 203 away from the positive electrode current collector 201.

[0074] In an embodiment of the present application, a secondary battery is provided. A fibrous lithium supplement layer 202 with a fibrous network structure is provided on the surface of a positive electrode current collector 201. A positive electrode material layer 203 is provided on the surface of the fibrous lithium supplement layer 202. The fibrous lithium supplement 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 be in direct contact with the positive electrode material layer 203 to improve the efficiency of lithium ion insertion and extraction. The fibrous lithium supplement layer 202 has a fibrous network structure. On the one hand, the fibrous network structure has a large porosity, which is beneficial to promoting the absorption of the electrolyte by the positive electrode sheet. On the other hand, the fibrous lithium supplement layer 202 with the fibrous network structure is beneficial to the gas overflow during the first charging process, avoiding the behavior of gas storage, and further avoiding the impact of gas storage on the battery performance. Among them, the material of the fibrous lithium supplement layer 202 includes lithium ferrite, lithium nickelate, and lithiated carbon nanotubes. The above materials can all make up for the loss of active lithium caused by the SEI film. Lithium ferrite has good thermal stability, good cycle life, and low cost, but the energy density of lithium ferrite is relatively low; lithium nickelate has a high energy density, but short cycle life, poor thermal stability, fast performance decay at high temperature, and high cost. The compounding of lithium ferrite and lithium nickelate can complement each other to combine the advantages of the two. In addition, based on the fibrous 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 fibrous lithium supplement layer, while improving the chemical activity of lithium ferrite and lithium nickelate, reducing the decomposition potential of lithium ferrite and lithium nickelate, and reducing the harm of the high-voltage activation potential to the positive electrode material. The material of the positive electrode material layer 203 includes 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 role of the binder is to better bond lithium iron phosphate with other substances and form a better adhesion with the fibrous lithium supplement layer.

[0075] The following are specific embodiments of the present application:

[0076] Example 1

[0077] Preparation of the positive electrode sheet: Prepare the precursor solution. Mix lithium ferrite and lithium nickelate with a molar ratio of 5:1 and dissolve them in deionized water. Add lithiated carbon nanotubes and polyvinylpyrrolidone. The total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate is 1:8. After mixing evenly, make the precursor solution. Load the precursor solution into the syringe of an electrospinning machine and deposit the precursor solution onto the surface of the aluminum foil through electrospinning to form an initial fiber membrane on the aluminum foil surface. Pre-bake the aluminum foil with the initial fiber membrane in air at 300 °C for 3 hours to remove organic components such as polyvinylpyrrolidone. Calcinate the pre-baked aluminum foil in an inert gas at 800 °C for 4 hours to transform the initial fiber membrane into a fiber lithium supplement layer with a fiber network structure. Coat 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 a binder. The total mass ratio of lithium ferrite, lithium nickelate, and lithiated carbon nanotubes in the fiber lithium supplement layer to the mass of lithium iron phosphate, the binder, and the conductive agent is 2:94:1:2. After drying, obtain the positive electrode sheet.

[0078] Wind the positive electrode sheet, the separator, and the negative electrode sheet and put them into the housing, and inject electrolyte into the housing to obtain a secondary battery.

[0079] The preparation steps of Example 2 are basically the same as those of Example 1, except that the molar ratio of lithium ferrite and lithium nickelate in Example 2 is 3:1.

[0080] The preparation steps of Example 3 are basically the same as those of Example 1, except that the molar ratio of lithium ferrite and lithium nickelate in Example 3 is 6:1.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] The difference between Comparative Example 1 and Example 1 is that the preparation method of the positive electrode sheet in Comparative Example 1 is: directly mix lithium ferrite, lithium nickelate, and lithiated carbon nanotubes with the positive electrode material layer and then coat them on the aluminum foil.

[0086] The difference between Comparative Example 2 and Example 1 is that the preparation method of the positive electrode sheet in Comparative Example 2 is as follows: first, lithium ferrite, lithium nickelate, and lithiated carbon nanotubes are mixed with a binder and then coated on an aluminum foil, and then a positive electrode material layer is coated.

[0087] The preparation steps of Comparative Example 3 are basically the same as those of Example 1, and the difference is that the molar ratio of lithium ferrite to lithium nickelate in Comparative Example 3 is 1:1.

[0088] The preparation steps of Comparative Example 4 are basically the same as those of Example 1, and the difference is that the molar ratio of lithium ferrite to lithium nickelate in Comparative Example 4 is 10:1.

[0089] The preparation steps of Comparative Example 5 are basically the same as those of Example 1, and the difference is that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Comparative Example 5 is 1:1.

[0090] The preparation steps of Comparative Example 6 are basically the same as those of Example 1, and the difference is that the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate in Comparative Example 6 is 1:20.

[0091] Test the initial 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.

[0092] Table 1 shows the performance test results corresponding to Examples 1 to 7 and Comparative Examples 1 to 6.

[0093]

[0094] 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 in the positive electrode material layer, the positive electrode sheet prepared by using the preparation method provided in the embodiment of the present application, in which a fiber lithium supplement layer is formed by an electrospinning process on an aluminum foil and then a positive electrode material layer is coated, can help reduce the gas generation problem of the lithium supplement agent, thereby reducing the impact of storage gas generation on the battery performance, maintaining a high initial Coulombic efficiency and energy density of the battery, and also improving the cycling performance at high temperature.

[0095] 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 to lithium nickelate is in the range of (3~6):1, and the total mass ratio of lithium ferrite and lithium nickelate to lithiated carbon nanotubes is in the range of (2~13):1, it is beneficial to maintain a high initial Coulombic efficiency, energy density, and cycling performance at high temperature of the battery.

[0096] Those of ordinary skill in the art can understand that the above embodiments are specific examples for implementing the present application. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.

Claims

1. A method for preparing a secondary battery, characterized in that: include: Prepare a positive electrode sheet, the preparation steps include: Preparing a precursor solution, mixing lithium ferrite and lithium nickelate in a molar ratio of (3-6):1 and dissolving them in deionized water, adding lithiated carbon nanotubes and a spinning aid, wherein the total mass ratio of the lithium ferrite and the lithium nickelate to the lithiated carbon nanotubes is (2-13):1, and mixing them evenly to prepare the precursor solution; The precursor solution is loaded into a syringe of an electrospinning machine, and the precursor solution is deposited on 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 in air at 300° C. to 400° C. for 2 hours to 4 hours; calcining the pre-calcined positive electrode current collector in an inert gas at 600° C. to 800° C. for 4 to 6 hours, so that the initial fiber membrane is transformed into a fiber lithium supplement layer, and the fiber lithium supplement layer has a fiber network structure; A positive electrode material layer is coated 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 the mass ratio of the total mass of the lithium ferrite, the lithium nickelate and the 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), and the positive electrode sheet is obtained after drying; 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, 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~50 nm, and the average length is 1 μm~10 μm.

3. The method for preparing a secondary battery according to claim 1 or 2, characterized in that: Mixing the lithium ferrite and the lithium nickelate in a molar ratio of (3-6):1 includes: mixing the lithium ferrite and the lithium nickelate in a ball mill at a rotation speed of 150 rpm-250 rpm for 1.5 hours-3.5 hours; and sieving the mixed solid to obtain a mixed solid with a particle size range of 9 μm-10 μm.

4. The method for preparing a secondary battery according to claim 1, characterized in that: The preparation steps of the lithium ferrite include: Weigh 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, and the lithium fluoride accounts for 0.5%-1.5% of the total mass of the lithium source, the iron source and the lithium fluoride; Under the protection of inert gas, the lithium source, the iron source and the lithium fluoride are ball-milled and mixed at a rotation speed of 200 rpm to 500 rpm for 3 hours to 6 hours; The mixture is tableted at a pressure of 9Mpa~11Mpa and a tablet diameter of 8mm~12mm; The pressed tablets were placed in a microwave reaction chamber with a microwave power of 600W-1000W, heated to 600°C within 10 minutes, kept at this temperature for 30 minutes, and then quenched to room temperature; The product is sieved with a mesh size of 300-500 mesh; The sieved product is placed in a tube furnace and annealed at 500°C to 550°C for 2 hours to 3 hours.

5. The method for preparing a secondary battery according to claim 1, characterized in that: The preparation steps of the lithium nickelate include: Weigh 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, dissolve the lithium source and the nickel source in deionized water to prepare a 0.5 mol / L solution, then add the citric acid and stir for 2 hours until transparent; The mixed solution is dried by a spray pyrolysis device, and the powder of the lithium nickelate is formed by heat treatment, the atomization pressure is 0.3Mpa~0.5Mpa, the droplet diameter is 2.5μm~3.5μm, the pyrolysis temperature is 750℃~850℃, the heating rate is 40℃ / min~60℃ / min, the residence time is 8 seconds~15 seconds, the atmosphere is oxygen and nitrogen with a volume ratio of 3:7, and the gas flow rate is 150mL / min~250mL / min; The lithium nickelate powder is annealed at 600° C. to 650° C. for 4 to 5 hours.

6. The method for preparing a secondary battery according to claim 1, characterized in that: The lithium iron phosphate is pretreated before coating, and the pretreatment steps include: Weigh trimethyl borate, and dissolve the trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; After adding the lithium iron phosphate, the mixture is dispersed by ball milling at a rotation speed of 300 rpm to 350 rpm for 4 to 5 hours, and the mass ratio of the trimethyl borate to the total mass of the lithium iron phosphate and the trimethyl borate is 1% to 3%.

7. The method for preparing a secondary battery according to claim 1, characterized in that: 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, wherein the mass ratio of trimethyl borate to the total mass of trimethyl borate and ethanol is 3% to 8%, and maintaining the mixture under vacuum for 30 minutes to 60 minutes; The positive electrode sheet was taken out and dried at 60°C.

8. A secondary battery, characterized in that: include: A shell, and a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte that are stacked and located in the shell, wherein the positive electrode sheet comprises: 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, the material of the fiber lithium supplement layer includes lithium ferrite, lithium nickelate and lithiated carbon nanotubes, the molar ratio of the lithium ferrite to the lithium nickelate is (3-6):1, and the mass ratio of the total mass of the lithium ferrite and the lithium nickelate to the lithiated carbon nanotubes is (2-13):1; A positive electrode material layer, wherein the positive electrode material layer covers 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, and the mass ratio of the total mass of the lithium ferrite, the lithium nickelate and the 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).

9. The secondary battery according to claim 8, 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).

10. The secondary battery according to claim 8, 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.

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