Secondary battery and preparation method thereof, energy storage system and electric equipment

Abstract

The invention relates to the technical field of batteries, and provides a secondary battery and a preparation method thereof, an energy storage system and electric equipment, which are at least beneficial to improving the cycle performance of the secondary battery. The method comprises the following steps: preparing a positive plate, wherein the preparation steps comprise: preparing a composite lithium supplementing material comprising boron-doped graphene, lithium tantalate and lithium tungstate by adopting a freeze-drying method; preparing boron-lithium iron phosphate by adopting an electrostatic self-assembly method; mixing the composite lithium supplementing material, boron-lithium iron phosphate, a conductive agent and an adhesive according to a mass ratio of (1.5 to 2.2): (93.8 to 94.5): (0.8 to 1.2): (1.7 to 2.1) to obtain positive electrode slurry; coating the surface of a positive current collector with the positive slurry, and drying to obtain a positive plate; and providing a negative plate and a diaphragm, carrying out winding treatment or lamination treatment on the positive plate, the diaphragm and the negative plate, putting into the shell, and injecting the electrolyte into the shell to obtain the secondary battery.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a secondary battery, a preparation method thereof, an energy storage system, and an electrical device. 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. At present, 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 of people.

[0003] After traditional energy storage battery cells are used in long-term cycles, problems such as increased internal resistance and capacity attenuation often occur, seriously affecting the performance and service life of the batteries. The main reasons for high battery internal resistance and low cycle life include the materials and structural design of the positive electrode plates. For example, during the production process of the positive electrode plates, the selection of the composition and structure of the positive electrode plates is unreasonable; the type of lithium supplementing agent is selected improperly, resulting in unstable voltage when the battery uses the lithium supplementing agent, affecting the battery performance and cycle life. Summary of the Invention

[0004] Embodiments of the present application provide a secondary battery, a preparation method thereof, an energy storage system, and an electrical device, which are at least beneficial to improving the cycle performance of the secondary battery.

[0005] According to some embodiments of the present application, on the one hand, a method for preparing a secondary battery is provided, including: preparing a positive electrode sheet, and the preparation steps include: preparing a composite lithium supplement material, dispersing boron-doped graphene in a solvent by ultrasonic treatment to obtain a boron-doped graphene suspension; dispersing lithium tantalate and lithium tungstate in a solvent by ultrasonic treatment to obtain a lithium tantalate-lithium tungstate suspension; mixing the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, and performing freeze-drying after ultrasonic dispersion or magnetic stirring to obtain a composite lithium supplement material; preparing boron-lithium iron phosphate, adding elemental boron to a polar solvent and then adding a first surface modifier, and performing ultrasonic dispersion and centrifugation treatment in sequence to obtain surface-modified elemental boron; adding lithium iron phosphate to a polar solvent and then adding a second surface modifier, and performing ultrasonic dispersion and centrifugation treatment in sequence to obtain surface-modified lithium iron phosphate, and the charge carried by the second surface modifier is opposite to that carried by the first surface modifier; adding the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, and obtaining a self-assembled mixture through ultrasonic treatment or magnetic stirring; centrifuging and separating the self-assembled mixture and collecting the precipitate, and vacuum-drying the precipitate to obtain boron-lithium iron phosphate; mixing the composite lithium supplement material, boron-lithium iron phosphate, a conductive agent, and a binder in a mass ratio of (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1) to obtain a positive electrode paste; coating the positive electrode paste on the surface of a positive electrode current collector and then drying to obtain a positive electrode sheet; providing a negative electrode sheet and a separator, winding or laminating the positive electrode sheet, the separator, and the negative electrode sheet and then placing them in a housing, and injecting an electrolyte into the housing to obtain a secondary battery.

[0006] In some embodiments, after preparing the composite lithium supplement material, it further includes: performing post-treatment on the composite lithium supplement material, and heat-treating the composite lithium supplement material in an inert gas atmosphere at 300°C to 500°C for 1h to 2h.

[0007] In some embodiments, in the boron-doped graphene suspension, the concentration of boron-doped graphene in the solvent is 1mg / mL to 5mg / mL; in the lithium tantalate-lithium tungstate suspension, the total concentration of lithium tantalate and lithium tungstate in the solvent is 10mg / mL to 50mg / mL; in the mixture of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, the mass ratio of lithium tantalate, lithium tungstate to boron-doped graphene is (10~12):(6~8):(1~2).

[0008] In some embodiments, the conditions for freeze-drying include: the temperature is -50°C to -80°C, the vacuum degree is 0.1mbar to 0.01mbar, and the time is 24h to 48h.

[0009] In some embodiments, in the step of adding surface-modified elemental boron and surface-modified lithium iron phosphate to a polar solvent, the mass ratio of surface-modified lithium iron phosphate to surface-modified elemental boron is (88 - 92):(8 - 12), and the mass ratio of the total mass of surface-modified lithium iron phosphate and surface-modified elemental boron to the mass of the polar solvent is 3:(100 - 200).

[0010] In some embodiments, the first surface modifier is one of a positively charged polar polymer or a negatively charged polar polymer, and the second surface modifier is the other of a positively charged polar polymer or a negatively charged polar polymer; the positively charged polar polymer is selected from at least one of polystyrene, polytetrafluoroethylene, polyacrylic acid, or polysulfonic acid; the negatively charged polar polymer is selected from at least one of polyethylene, polypropylene, polyethyleneimine, or polydiallyldimethylammonium chloride.

[0011] In some embodiments, the preparation method of boron-doped graphene includes: mixing graphene and boric acid at a mass ratio of 1:(0.6 - 1.5), calcining at 800 °C - 1000 °C for 1 h - 4 h, and sequentially washing with concentrated sulfuric acid and deionized water and drying to obtain boron-doped graphene.

[0012] According to some embodiments of the present application, on the other hand, the present application embodiments also provide a secondary battery, including: a positive electrode sheet, a negative electrode sheet, and a separator, the separator is located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet includes: a positive electrode current collector; a positive electrode material layer, the positive electrode material layer covers the surface of the positive electrode current collector, and the material of the positive electrode material layer includes a composite lithium supplement material, boron-lithium iron phosphate, a conductive agent, and a binder, and the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, the conductive agent, and the binder is (1.5 - 2.2):(93.8 - 94.5):(0.8 - 1.2):(1.7 - 2.1), and the composite lithium supplement material includes boron-doped graphene, lithium tantalate, and lithium tungstate.

[0013] In some embodiments, the particle size value when the cumulative distribution percentage of lithium tantalate reaches 50% is 8 μm - 17 μm; the particle size value when the cumulative distribution percentage of lithium tungstate reaches 50% is 10 μm - 20 μm; the sheet diameter of boron-doped graphene is 0.5 μm - 10 μm.

[0014] In some embodiments, the particle size value when the cumulative distribution percentage of lithium iron phosphate in boron-lithium iron phosphate reaches 50% is 1 μm - 2 μm; the particle size value when the cumulative distribution percentage of elemental boron in boron-lithium iron phosphate reaches 50% is 10 μm - 40 μm.

[0015] According to some embodiments of the present application, on the other hand, an energy storage system is further provided, including: a plurality of secondary batteries manufactured by using the manufacturing method of the secondary battery in the above embodiments, or the secondary battery in the above embodiments.

[0016] According to some embodiments of the present application, on yet another aspect, an electrical device is further provided, including a secondary battery manufactured by using the manufacturing method of the secondary battery in the above embodiments; or including the secondary battery in the above embodiments; or including the energy storage system in the above embodiments.

[0017] The technical solutions provided by the embodiments of the present application have at least the following advantages: In the method for preparing a secondary battery provided by an embodiment of the present application, when preparing a positive electrode sheet, a freeze-drying method is used to prepare a composite lithium supplement material, and an electrostatic self-assembly method is used to prepare boron-lithium iron phosphate. The composite lithium supplement material includes boron-doped graphene, lithium tantalate, and lithium tungstate. Graphene itself has a high specific surface area and abundant functional groups (such as hydroxyl groups and carboxyl groups), which can provide good attachment sites for lithium tantalate and lithium tungstate. Moreover, boron doping further enhances the conductivity and chemical activity of graphene, promoting the interfacial combination of lithium tantalate and lithium tungstate with boron-doped graphene. The conductive network constructed by boron-doped graphene in the composite lithium supplement material can help improve the conductivity of the positive electrode sheet. Lithium tantalate and lithium tungstate can be used to supplement active lithium to make up for the first-cycle capacity loss and the continuous capacity loss occurring during the life cycle of the secondary battery. The chemical window of lithium tungstate is narrow, but the lithium content is high. The electrochemical window of lithium tantalate is wide and the stability is relatively high. The combination of the two can balance stability and lithium supplement efficiency. Lithium tantalate is a good ionic conductor, and the crystal structure of lithium tungstate is conducive to the migration of lithium ions. The compounding of the two can form a better lithium ion transport channel, which is conducive to improving the cycle performance of the secondary battery. Before freeze-drying, boron-doped graphene, lithium tantalate, and lithium tungstate are respectively ultrasonically dispersed in a solvent, and then mixed and dispersed, so that part of lithium tantalate and lithium tungstate are confined between the layers of boron-doped graphene, avoiding agglomeration and being conducive to the uniform dispersion of lithium tantalate and lithium tungstate. The freeze-drying method is conducive to maintaining the porous structure and high specific surface area of boron-doped graphene, and maintaining the uniformity of components and the stability of interfacial combination. Elemental boron is a semi-metal with certain intrinsic conductivity. Elemental boron can form a conductive bridge between or on the surface of lithium iron phosphate particles, reducing the contact resistance between particles and enhancing the electron transport efficiency. Moreover, the introduction of elemental boron will slightly change the lattice gap or the lattice parameters on the surface of lithium iron phosphate, introducing dislocations or vacancies and increasing the diffusion channels of lithium ions, thereby improving the rate performance and cycle performance of the secondary battery. By preparing boron-lithium iron phosphate through electrostatic self-assembly, elemental boron and lithium iron phosphate can be evenly dispersed, avoiding agglomeration. Moreover, electrostatic self-assembly can make elemental boron and lithium iron phosphate arranged more densely, reducing voids, which is conducive to improving the tap density of the positive electrode sheet. The low-temperature electrostatic self-assembly can also avoid the problems of growth or phase change of lithium iron phosphate caused by high-temperature sintering, thereby improving the stability of boron-lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplary illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a scale limitation. To more clearly illustrate the technical solutions in the embodiments of the present application or in the prior art, the drawings required to be used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1 It is a flowchart corresponding to the preparation steps of the positive electrode sheet provided by the embodiment of the present application. Detailed implementation manners

[0020] As can be seen from the background art, the main reasons for the high internal resistance and low cycle life of the battery include the materials and structural designs of the positive electrode sheets.

[0021] The embodiment of the present application provides a secondary battery, its preparation method, an energy storage system, and an electrical device, which are at least beneficial to improving the cycle performance of the secondary battery.

[0022] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order, or primary-secondary relationship of the indicated technical features.

[0023] In the description of the embodiments of the present application, the meaning of "a plurality" is more than two, unless otherwise clearly and specifically defined.

[0024] Referring to "embodiments" herein means that the specific features, structures, or characteristics described in connection 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 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.

[0025] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: the existence of A, the simultaneous existence of A and B, and the existence of B. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0026] 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 further be included.

[0027] The terms used in the description of the various embodiments herein are only for the purpose of 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.

[0028] The embodiments of the present application will be described in detail below with reference to 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 provided to help the reader 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.

[0029] Figure 1 It is a flowchart corresponding to the preparation steps of the positive electrode sheet provided by the embodiments of the present application.

[0030] According to some embodiments of the present application, on the one hand, a method for preparing a secondary battery is provided, and the preparation method is as follows.

[0031] S11. Prepare a positive electrode sheet, refer to Figure 1 , and the preparation steps are as follows.

[0032] S101. Prepare a composite lithium supplement material. Ultrasonically disperse boron-doped graphene in a solvent to obtain a boron-doped graphene suspension; ultrasonically disperse lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension; mix the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, and after ultrasonic dispersion or magnetic stirring, perform freeze-drying to obtain a composite lithium supplement material.

[0033] In step S101, in the boron-doped graphene suspension, the concentration of boron-doped graphene in the solvent is 1 mg / mL to 5 mg / mL; in the lithium tantalate-lithium tungstate suspension, the total concentration of lithium tantalate and lithium tungstate in the solvent is 10 mg / mL to 50 mg / mL; in the mixture of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, the mass ratio of lithium tantalate, lithium tungstate to boron-doped graphene is (10 to 12):(6 to 8):(1 to 2). The concentrations of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension within an appropriate range are beneficial to the subsequent compounding of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, so that good interfacial contact is formed between boron-doped graphene and lithium tantalate and lithium tungstate. At the same time, it is also beneficial to control the mass ratio of boron-doped graphene, lithium tantalate and lithium tungstate. The appropriate range of the mass ratio of boron-doped graphene, lithium tantalate and lithium tungstate is beneficial to reducing the agglomeration problem of each component.

[0034] The solvents of both the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension can be selected from ethanol or deionized water.

[0035] In step S101, the conditions for lyophilization include: temperature of -50°C to -80°C, vacuum degree of 0.1 mbar to 0.01 mbar, and time of 24 h to 48 h.

[0036] In step S101, the method for preparing boron-doped graphene includes: mixing graphene and boric acid at a mass ratio of 1:(0.6 - 1.5), calcining at 800°C to 1000°C for 1 h to 4 h, and successively washing with concentrated sulfuric acid (weight percentage concentration of 95wt% - 98wt%) and deionized water and drying to obtain boron-doped graphene. Among them, the pH value of the boron-doped graphene after washing is 6 - 7.

[0037] After step S101, it may further include: post-treating the composite lithium supplement material, and heat-treating the composite lithium supplement material in an inert gas atmosphere at 300°C to 500°C for 1 h to 2 h. The heat treatment can help improve the interfacial bonding strength and chemical stability of boron-doped graphene, lithium tantalate, and lithium tungstate, and at the same time can remove unreacted precursors and improve the purity of the composite lithium supplement material.

[0038] S102. Prepare boron-lithium iron phosphate. Add elemental boron to a polar solvent and then add a first surface modifier, and successively perform ultrasonic dispersion and centrifugation to obtain surface-modified elemental boron; add lithium iron phosphate to a polar solvent and then add a second surface modifier, and successively perform ultrasonic dispersion and centrifugation to obtain surface-modified lithium iron phosphate, where the charge carried by the second surface modifier is opposite to that carried by the first surface modifier; add the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, and obtain a self-assembled mixture through ultrasonic treatment or magnetic stirring; centrifuge and separate the self-assembled mixture and collect the precipitate, and vacuum-dry the precipitate to obtain boron-lithium iron phosphate.

[0039] In step S102, when adding elemental boron to a polar solvent and then adding a first surface modifier, the mass ratio of elemental boron to the polar solvent is 1:(100 - 200), and the mass ratio of the first surface modifier to elemental boron is 1:(5 - 10); when adding lithium iron phosphate to a polar solvent and then adding a second surface modifier, the mass ratio of lithium iron phosphate to the polar solvent is 1:(50 - 100), and the mass ratio of the second surface modifier to lithium iron phosphate is 1:(5 - 10).

[0040] The polar solvent can be selected from deionized water, ethanol, isopropanol, etc.

[0041] In step S102, after preparing the surface-modified lithium iron phosphate and the surface-modified elemental boron, ethanol can be used to wash the surface-modified lithium iron phosphate and the surface-modified elemental boron 3 to 5 times to remove the unadsorbed modifiers.

[0042] In step S102, during the process of adding the surface-modified elemental boron and the surface-modified lithium iron phosphate into the polar solvent, the mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is (88 - 92):(8 - 12), and the mass ratio of the total mass of the surface-modified lithium iron phosphate and the surface-modified elemental boron to the mass of the polar solvent is 3:(100 - 200).

[0043] In step S102, the process parameters for vacuum drying the precipitate include: the temperature is 60°C - 80°C, and the time is 12h - 24h.

[0044] In step S102, the first surface modifier is one of a positively charged polar polymer or a negatively charged polar polymer, and the second surface modifier is the other of a positively charged polar polymer or a negatively charged polar polymer. Among them, the positively charged polar polymer can be selected from at least one of polystyrene, polytetrafluoroethylene, polyacrylic acid, or polysulfonic acid; the negatively charged polar polymer can be selected from at least one of polyethylene, polypropylene, polyethyleneimine, or polydiallyldimethylammonium chloride.

[0045] S103. Mix the composite lithium supplement material, boron-lithium iron phosphate, conductive agent, and binder in a mass ratio of (1.5 - 2.2):(93.8 - 94.5):(0.8 - 1.2):(1.7 - 2.1) to obtain the positive electrode slurry.

[0046] S104. Coat the positive electrode slurry on the surface of the positive electrode current collector and then dry it to obtain the positive electrode sheet.

[0047] S12. Provide a negative electrode sheet and a separator, wind or stack the positive electrode sheet, separator, and negative electrode sheet, then put them into a casing, and inject electrolyte into the casing to obtain a secondary battery.

[0048] In the method for preparing a secondary battery provided by an embodiment of the present application, when preparing the positive electrode sheet, a freeze-drying method is used to prepare a composite lithium supplement material, and an electrostatic self-assembly method is used to prepare boron-doped lithium iron phosphate. The composite lithium supplement material includes boron-doped graphene, lithium tantalate, and lithium tungstate. Graphene itself has a high specific surface area and abundant functional groups (such as hydroxyl groups and carboxyl groups), which can provide good attachment sites for lithium tantalate and lithium tungstate. Moreover, boron doping further enhances the conductivity and chemical activity of graphene, promoting the interfacial combination of lithium tantalate and lithium tungstate with boron-doped graphene. The conductive network constructed by boron-doped graphene in the composite lithium supplement material can help improve the conductivity of the positive electrode sheet. Lithium tantalate and lithium tungstate can be used to supplement active lithium to make up for the first-cycle capacity loss of the secondary battery and the continuous capacity loss occurring during the life cycle. The chemical window of lithium tungstate is narrow, but the lithium content is high. The electrochemical window of lithium tantalate is wide and the stability is relatively high. The combination of the two can balance stability and lithium supplementation efficiency. Lithium tantalate is a good ionic conductor, and the crystal structure of lithium tungstate is conducive to the migration of lithium ions. The compounding of the two can form a better lithium ion transport channel, thereby facilitating the improvement of the cycle performance of the secondary battery. Before freeze-drying, boron-doped graphene, lithium tantalate, and lithium tungstate are respectively ultrasonically dispersed in a solvent, and then mixed and dispersed, so that part of lithium tantalate and lithium tungstate are confined between the layers of boron-doped graphene, avoiding agglomeration and facilitating the uniform dispersion of lithium tantalate and lithium tungstate. The freeze-drying method is conducive to maintaining the porous structure and high specific surface area of boron-doped graphene, and maintaining the uniformity of the composition and the stability of the interfacial combination. Elemental boron is a semimetal with certain intrinsic conductivity. Elemental boron can form a conductive bridge between or on the surface of lithium iron phosphate particles, reducing the contact resistance between particles and enhancing the electron transport efficiency. Moreover, the introduction of elemental boron will slightly change the lattice parameters of the lattice gaps or surfaces of lithium iron phosphate, introducing dislocations or vacancies and increasing the diffusion channels of lithium ions, thereby improving the rate performance and cycle performance of the secondary battery. By preparing boron-doped lithium iron phosphate through electrostatic self-assembly, elemental boron and lithium iron phosphate can be evenly dispersed, avoiding agglomeration. Moreover, electrostatic self-assembly can make elemental boron and lithium iron phosphate arranged more densely, reducing voids, which is conducive to improving the tap density of the positive electrode sheet (the tap density refers to the mass of the active material per unit volume. The higher the tap density, the stronger the energy density and battery life of the secondary battery). The low-temperature electrostatic self-assembly can also avoid the problems of growth or phase change of lithium iron phosphate caused by high-temperature sintering, thereby improving the stability of boron-doped lithium iron phosphate.

[0049] Correspondingly, another embodiment of the present application also provides a secondary battery, which can be prepared by using the method for preparing a secondary battery provided by the above embodiment. 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.

[0050] The secondary battery includes: a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet. The positive electrode sheet includes: a positive electrode current collector; a positive electrode material layer covering the surface of the positive electrode current collector. The material of the positive electrode material layer includes a composite lithium supplementing material, boron-lithium iron phosphate, a conductive agent, and a binder. The mass ratio of the composite lithium supplementing material, boron-lithium iron phosphate, the conductive agent, and the binder is (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1). The composite lithium supplementing material includes boron-doped graphene, lithium tantalate, and lithium tungstate.

[0051] In some embodiments, the particle size value when the cumulative distribution percentage of lithium tantalate reaches 50% is 8μm~17μm, for example, specifically it can be 8μm, 10μm, 11μm, 13μm, 15μm or 17μm; the particle size value when the cumulative distribution percentage of lithium tungstate reaches 50% is 10μm~20μm, for example, specifically it can be 10μm, 12μm, 14μm, 16μm, 18μm or 20μm; the sheet diameter of boron-doped graphene is 0.5μm~10μm, for example, specifically it can be 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm.

[0052] In some embodiments, the particle size value when the cumulative distribution percentage of lithium iron phosphate in boron-lithium iron phosphate reaches 50% is 1μm~2μm, for example, specifically it can be 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm or 2μm; the particle size value when the cumulative distribution percentage of elemental boron in boron-lithium iron phosphate reaches 50% is 10μm~40μm, for example, specifically it can be 10μm, 15μm, 20μm, 25μm, 30μm, 35μm or 40μm.

[0053] In the secondary battery provided by the embodiment of the present application, the composite lithium supplement material includes boron-doped graphene, lithium tantalate and lithium tungstate. Graphene itself has a high specific surface area and abundant functional groups (such as hydroxyl groups and carboxyl groups), which can provide good attachment sites for lithium tantalate and lithium tungstate. Moreover, boron doping further enhances the conductivity and chemical activity of graphene, promotes the interfacial combination of lithium tantalate and lithium tungstate with boron-doped graphene. The conductive network constructed by boron-doped graphene in the composite lithium supplement material can help improve the conductivity of the positive electrode sheet. Lithium tantalate and lithium tungstate can be used to supplement active lithium to make up for the initial cycle capacity loss and the continuous capacity loss occurring during the life cycle of the secondary battery. The chemical window of lithium tungstate is narrow, but the lithium content is high. The electrochemical window of lithium tantalate is wide and the stability is relatively high. The combination of the two can balance stability and lithium supplement efficiency. Lithium tantalate is a good ionic conductor, and the crystal structure of lithium tungstate is beneficial to the migration of lithium ions. The compounding of the two can form a better lithium ion transmission channel, which is beneficial to improving the cycle performance of the secondary battery. Elemental boron is a semi-metal with certain intrinsic conductivity. Elemental boron can form a conductive bridge between or on the surface of lithium iron phosphate particles, reduce the contact resistance between particles, enhance the electron transport efficiency, and the introduction of elemental boron will slightly change the lattice gap or the lattice parameters on the surface of lithium iron phosphate, introducing dislocations or vacancies, increasing the diffusion channels of lithium ions, and thus improving the rate performance and cycle performance of the secondary battery.

[0054] Correspondingly, another aspect of the embodiment of the present application further provides an energy storage system, including: a plurality of secondary batteries manufactured by using the manufacturing method of the secondary battery in the above embodiment, or the secondary battery in the above embodiment.

[0055] Correspondingly, yet another aspect of the embodiment of the present application further provides an electrical device, including a secondary battery manufactured by using the manufacturing method of the secondary battery in the above embodiment; or including the secondary battery in the above embodiment; or including the energy storage system in the above embodiment.

[0056] The following are specific embodiments of the present application.

[0057] Embodiment 1 S21. Prepare a positive electrode sheet, and the preparation steps are as follows.

[0058] S201. Prepare a composite lithium supplement material. Ultrasonically disperse boron-doped graphene in a solvent to obtain a boron-doped graphene suspension with a concentration of 5 mg / mL of boron-doped graphene in the solvent. Ultrasonically disperse lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension with a total concentration of 50 mg / mL of lithium tantalate and lithium tungstate in the solvent. Mix the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension. Among them, the mass ratio of lithium tantalate, lithium tungstate to boron-doped graphene is 11:7:1. After ultrasonic dispersion, freeze-dry at -80 °C and a vacuum degree of 0.05 mbar for 24 h to obtain the composite lithium supplement material.

[0059] S202. Prepare boron-lithium iron phosphate. Add elemental boron to polar solvent ethanol, then add the first surface modifier polyethyleneimine, and perform ultrasonic dispersion and centrifugation in sequence to obtain surface-modified elemental boron. The mass ratio of elemental boron to polar solvent is 1:150, and the mass ratio of the first surface modifier to elemental boron is 1:8. Add lithium iron phosphate to polar solvent ethanol, then add the second surface modifier polyacrylic acid, and perform ultrasonic dispersion and centrifugation in sequence to obtain surface-modified lithium iron phosphate. The mass ratio of lithium iron phosphate to polar solvent is 1:80, and the mass ratio of the second surface modifier to lithium iron phosphate is 1:8. Add the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, and obtain a self-assembled mixture through ultrasonic treatment or magnetic stirring. The mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is 9:1, and the mass ratio of the total mass of the surface-modified lithium iron phosphate and the surface-modified elemental boron to the polar solvent is 1:50. Centrifuge and separate the self-assembled mixture and collect the precipitate. Vacuum-dry the precipitate at 60 °C for 24 h to obtain boron-lithium iron phosphate.

[0060] S203. Mix the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder in a mass ratio of 2:94:1:2 to obtain a positive electrode paste.

[0061] S204. Coat the positive electrode paste on the surface of the positive electrode current collector aluminum foil and dry to obtain a positive electrode sheet.

[0062] S22. Provide a negative electrode sheet and a separator. Wind or stack the positive electrode sheet, separator and negative electrode sheet and place them in a housing, and inject an electrolyte into the housing to obtain a secondary battery.

[0063] Example 2 The preparation method of Example 2 is basically the same as that of Example 1, except that after S201, the prepared composite lithium supplement material is post-treated, and the composite lithium supplement material is heat-treated at 350 °C for 1 h in an inert gas atmosphere.

[0064] Example 3 Example 3 has basically the same preparation method as Example 1, except that in S201, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is 10:6:1.

[0065] Example 4 Example 4 has basically the same preparation method as Example 1, except that in S201, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is 12:8:2.

[0066] Example 5 Example 5 has basically the same preparation method as Example 1, except that in S202, the mass ratio of surface-modified lithium iron phosphate to surface-modified elemental boron is 11:1, and the total mass of surface-modified lithium iron phosphate and surface-modified elemental boron to the mass of the polar solvent is 3:160.

[0067] Example 6 Example 6 has basically the same preparation method as Example 1, except that in S202, the mass ratio of surface-modified lithium iron phosphate to surface-modified elemental boron is 23:3, and the total mass of surface-modified lithium iron phosphate and surface-modified elemental boron to the mass of the polar solvent is 3:130.

[0068] Example 7 Example 7 has basically the same preparation method as Example 1, except that in S202, the mass ratio of surface-modified lithium iron phosphate to surface-modified elemental boron is 23:2, and the total mass of surface-modified lithium iron phosphate and surface-modified elemental boron to the mass of the polar solvent is 3:180.

[0069] Example 8 Example 8 has basically the same preparation method as Example 1, except that in S203, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder is 1.5:93.8:0.8:1.7.

[0070] Example 9 Example 9 has basically the same preparation method as Example 1, except that in S203, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder is 2.2:94.5:1.2:2.1.

[0071] Example 10 Example 10 has basically the same preparation method as Example 1, except that in S203, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder is 2:94:1.2:2.1.

[0072] Comparative Example 1 In Comparative Example 1, the masses of boron-doped graphene, lithium tantalate, lithium tungstate, lithium iron phosphate, boron, conductive agent, and binder are the same as those in Example 1. In Comparative Example 1, the boron-doped graphene, lithium tantalate, lithium tungstate, lithium iron phosphate, boron, conductive agent, and binder are directly prepared into a positive electrode slurry by ball milling and blending.

[0073] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that in S203, lithium iron phosphate is used instead of boron-lithium iron phosphate, and no composite lithium supplementing material is added.

[0074] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that no composite lithium supplementing material is added to the positive electrode slurry of Comparative Example 3.

[0075] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the composite lithium supplementing material of Comparative Example 4 only includes lithium tantalate.

[0076] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that the composite lithium supplementing material of Comparative Example 5 only includes lithium tungstate.

[0077] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the composite lithium supplementing material of Comparative Example 6 only includes lithium tantalate and boron-doped graphene.

[0078] Comparative Example 7 The difference between Comparative Example 7 and Example 1 is that the composite lithium supplementing material of Comparative Example 7 only includes lithium tungstate and boron-doped graphene.

[0079] Comparative Example 8 The difference between Comparative Example 8 and Example 1 is that in S203, the mass ratio of the composite lithium supplementing material, boron-lithium iron phosphate, conductive agent, and binder is 0.5:94:1:2.

[0080] Comparative Example 9 The difference between Comparative Example 9 and Example 1 is that in S203, the mass ratio of the composite lithium supplementing material, boron-lithium iron phosphate, conductive agent, and binder is 5:94:1:2.

[0081] Comparative Example 10 The difference between Comparative Example 10 and Example 1 is that in S203, the mass ratio of the composite lithium supplementing material, boron-lithium iron phosphate, conductive agent, and binder is 2:90:1:2.

[0082] Comparative Example 11 The difference between Comparative Example 11 and Example 1 is that in S203, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder is 2:100:1:2.

[0083] Comparative Example 12 The difference between Comparative Example 12 and Example 1 is that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder are mixed in a mass ratio of 2:94:0.3:2.

[0084] Comparative Example 13 The difference between Comparative Example 13 and Example 1 is that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder are mixed in a mass ratio of 2:94:3:2.

[0085] Comparative Example 14 The difference between Comparative Example 14 and Example 1 is that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder are mixed in a mass ratio of 2:94:1:0.5.

[0086] Comparative Example 15 The difference between Comparative Example 15 and Example 1 is that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder are mixed in a mass ratio of 2:94:1:4.

[0087] Test the initial capacity, capacity retention rate after 500 cycles, capacity retention rate at 2C and impedance of the above-mentioned examples and comparative examples. The test results are shown in Table 1 below.

[0088] Table 1

[0089] The continued table is shown on the next page.

[0090] Table 1 (continued)

[0091] By comparing Examples 1 to 10 with Comparative Example 1, it can be found that for the secondary battery and its preparation method provided by the embodiments of the present application, when preparing the positive electrode sheet, the composite lithium supplement material is prepared by the freeze-drying method, and boron-lithium iron phosphate is prepared by the electrostatic self-assembly method. Compared with the conventional ball milling and blending method, it is beneficial to reduce the impedance of the secondary battery and improve the cycling performance and rate performance of the secondary battery.

[0092] Comparing Comparative Examples 1 to 10 and Comparative Examples 2 to 7, it can be found that for the secondary battery and its preparation method provided in the embodiments of the present application, using boron-doped graphene, lithium tantalate, and lithium tungstate as the composite lithium supplement material in the positive electrode sheet, and compounding elemental boron with lithium iron phosphate can improve the cycle performance, rate performance, and initial capacity of the secondary battery, and reduce the impedance of the battery.

[0093] Comparing Comparative Example 1, Examples 8 to 10, and Comparative Examples 8 to 13, it can be found that if the amount of the composite lithium supplement material is too small, it is difficult to exert the lithium supplement effect, and the performance improvement of the secondary battery is not obvious. If it is too much, lithium plating may occur, affecting the comprehensive performance of the secondary battery; if the amount of boron-lithium iron phosphate is too small, the amount of active lithium provided is less, which is not conducive to the performance of the secondary battery. Since boron-lithium iron phosphate has poor conductivity, too much of it easily affects the conductivity of the electrode sheet, resulting in a decline in the comprehensive performance of the secondary battery; if the amount of the conductive agent is small, the resistance of the electrode sheet is large. When there is more conductive agent, although the internal resistance of the secondary battery can be reduced, the rate performance will also decline; more binder is beneficial to improving the stability of the electrode sheet, but too much binder will lead to poor conductivity of the electrode sheet, affecting the comprehensive performance of the secondary battery. Therefore, when the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent, and binder is (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1), the comprehensive performance of the secondary battery corresponding to the cycle performance, rate performance, initial capacity, and impedance is better.

[0094] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments 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, Including: Preparing a positive electrode sheet, and the preparation steps include: Preparing a composite lithium supplement material. Ultrasonically disperse boron-doped graphene in a solvent to obtain a boron-doped graphene suspension; ultrasonically disperse lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension; mix the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, and after ultrasonic dispersion or magnetic stirring, perform freeze-drying to obtain the composite lithium supplement material; Preparing boron-lithium iron phosphate. Add elemental boron to a polar solvent and then add a first surface modifier, and perform ultrasonic dispersion and centrifugation in sequence to obtain surface-modified elemental boron; add lithium iron phosphate to a polar solvent and then add a second surface modifier, and perform ultrasonic dispersion and centrifugation in sequence to obtain surface-modified lithium iron phosphate, and the charge carried by the second surface modifier is opposite to the charge carried by the first surface modifier; add the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, and obtain a self-assembled mixture through ultrasonic treatment or magnetic stirring; centrifuge and separate the self-assembled mixture and collect the precipitate, and vacuum-dry the precipitate to obtain the boron-lithium iron phosphate; Mix the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and binder in a mass ratio of (1.5~2.2):(93.8~94.5):(0.8~1.2):(1.7~2.1) to obtain a positive electrode slurry; Coat the positive electrode slurry on the surface of a positive electrode current collector and then dry to obtain the positive electrode sheet; Provide a negative electrode sheet and a separator, wind or laminate the positive electrode sheet, the separator and the negative electrode sheet, and then put them into a housing, and inject an electrolyte into the housing to obtain a secondary battery.

2. The method for preparing a secondary battery according to claim 1, wherein, After preparing the composite lithium supplement material, it further includes: performing post-treatment on the composite lithium supplement material, and heat-treating the composite lithium supplement material in an inert gas atmosphere at 300°C to 500°C for 1h to 2h.

3. The method for preparing a secondary battery according to claim 1, wherein In the boron-doped graphene suspension, the concentration of boron-doped graphene in the solvent is 1mg / mL to 5mg / mL; In the lithium tantalate-lithium tungstate suspension, the total concentration of lithium tantalate and lithium tungstate in the solvent is 10mg / mL to 50mg / mL; In the mixture of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, the mass ratio of lithium tantalate, lithium tungstate to boron-doped graphene is (10~12):(6~8):(1~2).

4. The method for preparing a secondary battery according to claim 1, wherein, The conditions for the freeze-drying include: the temperature is -50°C to -80°C, the vacuum degree is 0.1mbar to 0.01mbar, and the time is 24h to 48h.

5. The method for preparing a secondary battery according to claim 1, wherein, In the step of adding the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, the mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is (88~92):(8~12), and the mass ratio of the total mass of the surface-modified lithium iron phosphate and the surface-modified elemental boron to the mass of the polar solvent is 3:(100~200).

6. The preparation method of the secondary battery according to claim 1, wherein the first surface modifier is one of a positively charged polar polymer or a negatively charged polar polymer, and the second surface modifier is the other of a positively charged polar polymer or a negatively charged polar polymer; the positively charged polar polymer is selected from at least one of polystyrene, polytetrafluoroethylene, polyacrylic acid or polysulfonic acid; the negatively charged polar polymer is selected from at least one of polyethylene, polypropylene, polyethyleneimine or polydiallyldimethylammonium chloride.

7. The method for preparing a secondary battery according to claim 1, characterized in that, The preparation method of the boron-doped graphene includes: mixing graphene and boric acid in a mass ratio of 1:(0.6 - 1.5), calcining at 800°C - 1000°C for 1h - 4h, and sequentially washing with concentrated sulfuric acid and deionized water and drying to obtain the boron-doped graphene.

8. A secondary battery, characterized in that, Comprising: a positive electrode sheet, a negative electrode sheet and a separator, the separator is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet includes: a positive electrode current collector; a positive electrode material layer, the positive electrode material layer covers the surface of the positive electrode current collector, and the material of the positive electrode material layer includes a composite lithium supplementing material, boron-lithium iron phosphate, a conductive agent and a binder, and the mass ratio of the composite lithium supplementing material, boron-lithium iron phosphate, the conductive agent and the binder is (1.5 - 2.2):(93.8 - 94.5):(0.8 - 1.2):(1.7 - 2.1), and the composite lithium supplementing material includes boron-doped graphene, lithium tantalate and lithium tungstate.

9. The secondary battery according to claim 8, characterized in that, The particle size value when the cumulative distribution percentage of the lithium tantalate reaches 50% is 8μm - 17μm; the particle size value when the cumulative distribution percentage of the lithium tungstate reaches 50% is 10μm - 20μm; the sheet diameter of the boron-doped graphene is 0.5μm - 10μm.

10. The secondary battery according to claim 8, wherein The particle size value when the cumulative distribution percentage of the lithium iron phosphate in the boron-lithium iron phosphate reaches 50% is 1μm - 2μm; the particle size value when the cumulative distribution percentage of the elemental boron in the boron-lithium iron phosphate reaches 50% is 10μm - 40μm.

11. An energy storage system, characterized in that, Comprising: a plurality of secondary batteries manufactured by using the preparation method of the secondary battery according to any one of claims 1 - 7, or the secondary battery according to any one of claims 8 - 10.

12. An electrical device, characterized in that, Comprising a secondary battery manufactured by using the preparation method of the secondary battery according to any one of claims 1 - 7; or comprising the secondary battery according to any one of claims 8 - 10; or comprising the energy storage system according to claim 11.

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