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

Technical Field

[0001] 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

[0002] With the development of secondary battery research, the requirements for energy density, working voltage and cycle life are getting higher and higher. At present, the positive electrode materials of secondary batteries, such as lithium-ion batteries, mainly include lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, ternary materials, etc. Among them, lithium iron phosphate (LiFePO4) is widely used due to its high capacity, environmental friendliness, high safety and long cycle life. With the popularization of electric vehicles, the pursuit of secondary batteries with higher energy density is a common goal of people. Under many design concepts and material systems, further improving energy density often comes at the expense of battery safety, power and life. Summary of the invention

[0003] The embodiments of the present application provide a secondary battery and a method for preparing the same, which are at least beneficial to reducing the storage gas generation of the secondary battery.

[0004] According to some embodiments of the present application, on the one hand, the embodiments of the present application provide 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 in a molar ratio of (3~6):1 and dissolving them in deionized water, adding lithiated carbon nanotubes and spinning aids, 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 mixing them evenly to form a precursor solution; loading the precursor solution into a syringe of an electrospinning machine, and 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 at 300℃~400℃ for 2 hours~4 hours; The fiber body is calcined at 600°C to 800°C in an inert gas for 4 to 6 hours to transform the initial fiber membrane into a fiber lithium supplement layer, the fiber lithium supplement layer having a fiber network structure; a positive electrode material layer is coated 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, and the total mass ratio of lithium ferrite, lithium nickelate and lithiated carbon nanotubes in the fiber lithium supplement layer to lithium iron phosphate, the adhesive 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), and a positive electrode sheet is obtained after drying; a negative electrode sheet and a separator are provided, and 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.

[0005] In some embodiments, the particle size corresponding to the cumulative particle size distribution percentage of lithium ferrite reaches 50% is less than or equal to 12 μm; the particle size corresponding to 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~50 nm, and the average length is 1 μm~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-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.

[0007] In some embodiments, the preparation steps of lithium ferrite include: weighing a lithium source, an iron source and lithium fluoride, the molar ratio of the lithium source to the iron source is (4~6):1, and lithium fluoride accounts for 0.5%~1.5% of the total mass of the lithium source, the iron source and lithium fluoride; ball milling the lithium source, the iron source and lithium fluoride under the protection of an inert gas, the rotation speed is 200rpm~500rpm, and the time is 3 hours~6 hours; the mixture is pressed into a tablet, the pressure is 9Mpa~11Mpa, and the tablet diameter is 8mm~12mm; the tablet is placed in a microwave reaction cavity, the microwave power is 600W~1000W, the temperature is increased to 600℃ within 10 minutes, the temperature is kept for 30 minutes, and then quenched to room temperature; the product is sieved, the mesh size is 300 mesh~500 mesh; the sieved product is placed in a tubular furnace and annealed at 500℃~550℃ for 2 hours~3 hours.

[0008] In some embodiments, the preparation steps of lithium nickelate include: weighing a lithium source, a nickel source and citric acid, the molar ratio of the lithium source, the nickel source and the citric acid is (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; using a spray pyrolysis device to dry the mixed solution, and forming a lithium nickelate powder 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 in a volume ratio of 3:7, and the gas flow rate is 150mL / min~250mL / min; annealing the lithium nickelate powder at 600℃~650℃ for 4 hours~5 hours.

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

[0010] In some 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, wherein 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, the embodiments of the present application also provide a secondary battery, including: a shell, and a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte that are stacked and located in 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, 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 (3~6): 1, the ferrite The mass ratio of the total mass of lithium and lithium nickelate to the mass ratio of lithiated carbon nanotubes is (2~13):1; the positive electrode material layer, the positive electrode material layer covers 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, and the mass ratio of the total mass of lithium iron phosphate, lithium nickelate and lithiated carbon nanotubes in the fiber lithium supplement layer to lithium iron phosphate, a binder and a conductive agent is (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1).

[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] The technical solution provided by the embodiments of the present application has at least the following advantages: The present application provides a method for preparing a secondary battery, in which 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, and 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 can help promote the positive electrode sheet to absorb the electrolyte. On the other hand, the fiber lithium supplement 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 supplement layer include lithium ferrite, lithium nickelate and lithiated carbon nanotubes, all of which can make up for the loss of active lithium caused by SEI film. Lithium ferrite has good thermal stability, good cycle life, and low cost, but lithium ferrite has low energy density; lithium nickelate has high energy density, but short cycle life, poor thermal stability, rapid 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 fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a conductive structure of a spatial network. 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 high voltage activation potential to the positive electrode material. The materials of the positive electrode material layer include lithium iron phosphate, conductive agent and adhesive. Lithium iron phosphate provides active lithium to meet the performance of the battery. The addition of 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 adhesive is to better bond lithium iron phosphate with other substances and form a better adhesion effect for the fiber lithium supplement layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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 specified, 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 are 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 creative work.

[0016] Figure 1 A flowchart corresponding to a method for preparing a positive electrode sheet provided in an embodiment of the present application; Figure 2 A schematic diagram of the structure of a positive electrode sheet of a secondary battery provided in an embodiment of the present application; Figure 3A schematic structural diagram of a positive electrode sheet of another secondary battery provided in an embodiment of the present application. DETAILED DESCRIPTION

[0017] During the charging and discharging process, a SEI film (Solid Electrolyte Interface membrane) will form at the negative electrode of the secondary battery, resulting in the loss of active lithium, reducing the energy density and coulomb efficiency of the battery, and affecting the further development of the secondary battery. Battery lithium replenishment technology is an important means to improve the energy density of the battery. According to 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 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 replenishment.

[0018] 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, there will be storage gas production behavior in the prepared battery. The gas will affect the battery's cycle and high temperature performance.

[0019] 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.

[0020] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations 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. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0021] In the description of the embodiments of the present application, when a certain component “includes” another component, unless otherwise stated, other components are not excluded and other components may be further included.

[0022] The terms used in the description of the various described embodiments herein are used only to describe specific embodiments and are not intended to be limiting. As used in the description of the various described embodiments and the appended claims, "components" are also intended to include plural forms unless the context clearly indicates otherwise.

[0023] The following will describe the various embodiments of the present application in detail with reference to the accompanying drawings. However, it will be appreciated by those skilled in the art that in the various embodiments of the present application, many technical details are provided in order to enable the reader to better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solution claimed in the present application can be implemented.

[0024] According to some embodiments of the present application, the present application provides a method for preparing a secondary battery, comprising: 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.

[0025] 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.

[0026] The negative electrode material is selected from at least one of natural graphite, artificial graphite, soft carbon or hard carbon; the negative electrode current collector material includes copper foil; the negative electrode adhesive material is selected from at least one of sodium hydroxycellulose, polyvinylidene fluoride or styrene-butadiene rubber.

[0027] 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.

[0028] Figure 1 A flow chart corresponding to a method for preparing a positive electrode sheet provided in an embodiment of the present application.

[0029] refer to Figure 1 , the steps for preparing the positive electrode sheet include: S101: Prepare a precursor solution, mix lithium ferrite and lithium nickelate in a molar ratio of (3-6):1 and dissolve in deionized water, add lithiated carbon nanotubes and spinning aids, the total mass ratio of lithium ferrite and lithium nickelate to lithiated carbon nanotubes is (2-13):1, mix well to form a precursor solution.

[0030] In some embodiments, the molar ratio of lithium ferrite to lithium nickelate is specifically 3:1, 4:1, 5:1 or 6:1.

[0031] 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), polyether sulfone (PES), polyurethane (PU), polystyrene (PS), polyamide (PA), cellulose acetate (CA), chitosan (CS), silk fibroin (SF) or collagen.

[0032] 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, it can be 4:1, 5:1 or 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 (for example, it can be 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 rotation speed of 200 rpm~500rpm, the time is 3 hours~6 hours; the mixture is pressed into tablets, the pressure is 9Mpa~11Mpa, and the tablet diameter is 8mm~12mm; the tablets are placed in a microwave reaction chamber with a microwave power of 600W~1000W, the temperature is raised to 600℃ within 10 minutes, kept warm for 30 minutes, and then quenched to room temperature; the product is sieved with a mesh size of 300 mesh~500 mesh; the sieved product is placed in a tubular furnace and annealed at 500℃~550℃ for 2 hours~3 hours.

[0033] Lithium fluoride as a co-solvent can reduce the synthesis temperature of lithium source and iron source. Firstly, ball milling the lithium source, iron source and lithium fluoride can help improve the dispersion effect and contact area of ​​the reactants. Further, microwave reaction is used to synthesize lithium ferrite, which has the advantages of fast reaction speed, high energy utilization, high product purity, uniform particle size, simplified process, environmental friendliness and strong controllability. Screening the product after the reaction can help control the particle size of lithium ferrite. The uniform particle size is conducive to improving the electrochemical performance of lithium ferrite. Secondly, it is conducive to the formation of a fiber mesh structure with uniform thickness in the subsequent electrospinning process step.

[0034] In some embodiments, when the cumulative particle size distribution percentage of lithium ferrite reaches 50%, the corresponding particle size 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.

[0035] In some embodiments, the specific surface area of ​​lithium ferrite is 5m² / g to 50m² / g, for example, 5m² / g, 8m² / g, 10m² / g, 12m² / g, 14m² / g, 15m² / g, 18m² / g or 20m² / 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 larger specific surface area of ​​lithium ferrite is beneficial to increase the reactive sites, but an excessively large specific surface area will accelerate the decomposition of the electrolyte, resulting in gas production and capacity decay. Therefore, the specific surface area of ​​lithium ferrite needs to be within an appropriate range.

[0036] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate and lithium oxide.

[0037] The iron source is selected from one or more of ferric oxide, ferric oxide, ferric oxyhydroxide, ferric nitrate and ferric citrate.

[0038] 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 prepare 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 nickel oxide powder, 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 nickel oxide powder is annealed at 600℃~650℃ for 4 hours~5 hours.

[0039] Citric acid as a complexing agent can prevent component segregation and improve the stability of the reaction. Lithium source, nickel source and citric acid are synthesized into lithium nickelate by spray pyrolysis process, which 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.

[0040] In some embodiments, when the cumulative particle size distribution percentage of lithium nickelate reaches 50%, the corresponding particle size 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.

[0041] In some embodiments, the specific surface area of ​​lithium nickelate is 3m² / g to 15m² / g, for example, 3m² / g, 5m² / g, 8m² / g, 10m² / g, 13m² / g or 15m² / g. A lower specific surface area of ​​lithium nickelate can reduce the occurrence of side reactions, but it is easy to cause capacity decay. A larger specific surface area of ​​lithium nickelate is beneficial to improving rate performance and cycle stability, but an excessively large specific surface area is easy to cause accelerated decomposition of the electrolyte, resulting in gas production and battery capacity decay. Therefore, the specific surface area of ​​lithium nickelate needs to be within an appropriate range.

[0042] The lithium source is selected from one or more of lithium carbonate, lithium hydroxide monohydrate and lithium oxide.

[0043] 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.

[0044] In some embodiments, the preparation steps of lithiated carbon nanotubes include: treating activated carbon powder with hydrochloric acid; adding the activated carbon powder and lithium hydroxide powder treated 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.

[0045] In some embodiments, the average diameter of the lithiated carbon nanotubes is 10nm to 50nm, for example, 10nm, 13nm, 20nm, 25nm, 30nm, 34nm, 40nm, 46nm or 50nm; the average length is 1μm to 10μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm. Too thin carbon nanotubes are prone to insufficient mechanical strength or agglomeration problems, and too thick will 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, the longer length of the carbon nanotubes is conducive to the construction of a conductive network, but it is easy to cause poor dispersion. Although the short length of the carbon nanotubes is 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.

[0046] 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.

[0047] 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 speed of 150 rpm-250 rpm for 1.5 hours-3.5 hours; sieving the mixed solid to obtain a mixed solid with a particle size range of 9 μm-10 μm. Firstly, mixing lithium ferrite and lithium nickelate in a ball mill is beneficial for lithium ferrite and lithium nickelate to have a better dispersion effect in the spinning aid, and further beneficial for the fiber lithium supplement layer formed by electrospinning to have good uniformity. Sieving the mixed solid to obtain mixed particles with uniform particle size is beneficial for the formation of a fiber network structure with uniform thickness in the subsequent electrospinning process.

[0048] S102: Loading the precursor liquid into the syringe of the electrospinning machine, and depositing the precursor liquid 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.

[0049] The material of the positive electrode current collector includes aluminum foil.

[0050] In some embodiments, the parameters of electrospinning include: voltage of 15 kV to 30 kV, injection speed of 0.5 mL / h to 2 mL / h, and receiving distance of 10 cm to 20 cm.

[0051] 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.

[0052] 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.

[0053] S105: coating a positive electrode material layer on the surface of the fiber lithium supplement layer, wherein the materials of the positive electrode material layer include lithium iron phosphate, a conductive agent and an adhesive, and 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 adhesive and the conductive agent is (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1), and a positive electrode sheet is obtained after drying.

[0054] The conductive agent is selected from any one of conductive carbon black, graphite, conductive carbon tube or graphene, or a combination of at least two thereof.

[0055] In some embodiments, the lithium iron phosphate is pretreated before coating, and the pretreatment steps include: weighing trimethyl borate, dissolving the trimethyl borate in an ethanol and acetone solution with a volume ratio of 1:1; adding lithium iron phosphate and dispersing by ball milling at a rotation speed of 300rpm~350rpm for 4 hours~5 hours, and the total mass ratio of trimethyl borate to lithium iron phosphate and trimethyl borate is 1%~3%.

[0056] 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, wherein 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.

[0057] In the above two methods, trimethyl borate is coated on lithium iron phosphate by wet coating or trimethyl borate is adsorbed in the pores of the positive electrode by vacuum impregnation, so that trimethyl borate can be selectively oxidized in the positive electrode 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, 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.

[0058] The present application embodiment provides a method for preparing a secondary battery, in which 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 embedding and de-embedding efficiency of lithium ions, thereby improving the coulombic efficiency of the battery, and the fiber lithium supplement 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 supplement layer with a fiber network structure is conducive to gas overflow during the first charging process, avoiding storage gas production behavior, thereby avoiding the impact of storage gas production on battery performance. Among them, the materials of the fiber lithium supplement 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, good cycle life, and low cost, but lithium ferrite has low energy density; lithium nickelate has high energy density, but short cycle life, poor thermal stability, rapid 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 fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a conductive structure of a spatial network. 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 high voltage activation potential to the positive electrode material. The materials of the positive electrode material layer include lithium iron phosphate, conductive agent and adhesive. Lithium iron phosphate provides active lithium to meet the performance of the battery. The addition of 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 adhesive is to better bond lithium iron phosphate with other substances and form a better adhesion effect for the fiber lithium supplement layer.

[0059] Correspondingly, another embodiment of the present application also provides a secondary battery, which can be manufactured using the secondary battery manufacturing method in the above embodiment. The secondary battery provided by another embodiment of the present application will be described in detail below in conjunction with the accompanying drawings. For the parts that are the same or corresponding to the previous embodiment, refer to the corresponding description of the previous embodiment, and will not be described in detail below.

[0060] Figure 2 A schematic structural diagram of a positive electrode sheet of a secondary battery provided in an embodiment of the present application.

[0061] According to some embodiments of the present application, on the other hand, embodiments of the present application further provide a secondary battery, comprising: a shell, and a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte that are located in the shell and stacked.

[0062] refer to Figure 2The 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. 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 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).

[0063] In some embodiments, the thickness ratio of the fiber lithium supplement layer 202 to the thickness ratio of the positive electrode material layer 203 is 1:(8-16), for example, 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, and the fiber lithium supplement layer 202 is used to supplement lithium ions, so the thickness is thinner than the thickness of the positive electrode material layer 203. The thickness ratio of the fiber lithium supplement layer 202 to the thickness ratio of 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 supplement.

[0064] 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.

[0065] 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.

[0066] Figure 3 A schematic structural diagram of a positive electrode sheet of another secondary battery provided in an embodiment of the present application.

[0067] 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.

[0068] refer to Figure 3When 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.

[0069] The embodiment of the present application provides a secondary battery, wherein the positive electrode current collector 201 has a fiber lithium supplement layer 202 with a fiber network structure on its surface, and the fiber lithium supplement layer 202 has a positive electrode material layer 203 on its surface, and the fiber lithium supplement layer 202 is located on the side of the positive electrode material layer 203 close to the positive electrode current collector 201, and the electrolyte can be in direct contact with the positive electrode material layer 203 to improve the efficiency of lithium ion insertion and extraction, and the fiber lithium supplement layer 202 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, and on the other hand, the fiber lithium supplement layer 202 with a fiber network structure is conducive to gas overflow during the first charging process, avoiding storage gas production behavior, and thus avoiding the storage gas production from affecting the battery performance. Among them, the material of the fiber lithium supplement layer 202 includes lithium ferrite, lithium nickelate and lithiated carbon nanotubes, and the above materials can 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 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 fiber network structure, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are constructed into a conductive structure of a spatial network. 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 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 an adhesive. 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 adhesive is better to bond lithium iron phosphate with other substances and form a better adhesion effect for the fiber lithium supplement layer.

[0070] The following are specific embodiments of the present application: Example 1 Preparation of positive electrode sheet: preparing precursor solution, mixing lithium ferrite and lithium nickelate in a molar ratio of 5:1 and dissolving in deionized water, adding lithiated carbon nanotubes and polyvinyl pyrrolidone, the total mass ratio of lithiated carbon nanotubes to lithium ferrite and lithium nickelate is 1:8, mixing evenly to prepare precursor solution; loading the precursor solution into the syringe of the electrospinning machine, depositing 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-calcining 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-burned aluminum foil is calcined at 800°C in an inert gas for 4 hours to transform the initial fiber membrane into a fiber lithium supplement layer with a fiber network structure; a positive electrode material layer is coated 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 total mass ratio of lithium ferrite, lithium nickelate and lithiated carbon nanotubes in the fiber lithium supplement layer to the mass ratio of lithium iron phosphate, the adhesive and the conductive agent is 2:94:1:2, and a positive electrode sheet is obtained after drying.

[0071] The positive electrode sheet, the separator and the negative electrode sheet are wound and placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

[0072] 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.

[0073] 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.

[0074] 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 nickel oxide in Example 4 is 1:2.

[0075] 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 nickel oxide in Example 5 is 1:5.

[0076] 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 nickel oxide in Example 6 is 1:10.

[0077] 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 nickel oxide in Example 7 is 1:13.

[0078] The difference between Comparative Example 1 and Example 1 is that the preparation method of the positive electrode sheet of Comparative Example 1 is: lithium ferrite, lithium nickelate and lithiated carbon nanotubes are directly mixed with the positive electrode material layer and then coated on an aluminum foil.

[0079] The difference between Comparative Example 2 and Example 1 is that the preparation method of the positive electrode sheet of Comparative Example 2 is: firstly, lithium ferrite, lithium nickelate and lithiated carbon nanotubes are mixed with an adhesive and then coated on an aluminum foil, and then a positive electrode material layer is coated.

[0080] The preparation steps of Comparative Example 3 are substantially 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.

[0081] 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.

[0082] The preparation steps of Comparative Example 5 are substantially the same as those of Example 1, except that the total mass ratio of the lithiated carbon nanotubes to the lithium ferrite and lithium nickel oxide in Comparative Example 5 is 1:1.

[0083] 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 nickel oxide in Comparative Example 6 is 1:20.

[0084] The first coulombic efficiency, the maximum liquid level of the electrolyte during the first charging process, 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.

[0085] Table 1 shows the performance test results corresponding to Examples 1 to 7 and Comparative Examples 1 to 6.

[0086]

[0087] 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 is used to form a fiber lithium supplement layer on an aluminum foil by an electrospinning process and then coat the positive electrode material layer to prepare a positive electrode sheet, which 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 coulomb efficiency and energy density, and the cycle performance at high temperature is also improved.

[0088] 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 mass ratio of the total mass of lithium ferrite and lithium nickelate to the lithiated carbon nanotubes is in the range of (2-13):1, it is beneficial to maintain the battery with a high first coulombic efficiency, energy density and cycle performance at high temperature.

[0089] Those skilled in the art will appreciate that the above-mentioned embodiments are specific examples for implementing the present application, and in practical applications, various changes may be made to the embodiments in form and detail 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, and therefore the scope of protection of the present application shall be subject to the scope defined in 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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