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

Abstract

The present application relates to the field of battery technology and provides a secondary battery and its preparation method, energy storage system and electrical equipment, which are at least beneficial to improving the cycle performance of the secondary battery. The method includes: preparing a positive electrode sheet, the preparation steps including: preparing a composite lithium supplement material including boron-doped graphene, lithium tantalate and lithium tungstate by freeze-drying; preparing boron-lithium iron phosphate by electrostatic self-assembly; mixing the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive 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; applying the positive electrode slurry to the surface of the positive electrode current collector and drying it to obtain a positive electrode sheet; providing a negative electrode sheet and a separator, winding the positive electrode sheet, separator and negative electrode sheet or stacking them, and then placing them in a shell, and injecting electrolyte into the shell to obtain a secondary battery.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and a preparation method thereof, an energy storage system, and electrical equipment. Background Art

[0002] With the development of secondary batteries, the requirements for energy density, operating voltage, and cycle life are becoming increasingly stringent. Currently, the cathode materials for secondary batteries, such as lithium-ion batteries, primarily include lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary materials. Among them, lithium iron phosphate (LiFePO4) is widely used due to its high capacity, environmental friendliness, safety, and long cycle life. With the increasing adoption of electric vehicles, the pursuit of secondary batteries with even higher energy density is a common goal.

[0003] After prolonged cycling, traditional energy storage cells often experience increased internal resistance and capacity decay, severely impacting battery performance and lifespan. The primary causes of high internal resistance and low cycle life include the material and structural design of the positive electrode. For example, during the positive electrode manufacturing process, the composition and structure of the positive electrode are poorly chosen; and improper selection of lithium supplements can lead to unstable battery voltage during use, impacting battery performance and cycle life. Summary of the Invention

[0004] The embodiments of the present application provide a secondary battery and 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, an embodiment of the present application provides a method for preparing a secondary battery, including: preparing a positive electrode sheet, the preparation steps including: preparing a composite lithium supplement material, ultrasonically dispersing boron-doped graphene in a solvent to obtain a boron-doped graphene suspension; ultrasonically dispersing lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension; mixing the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, and 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 sequentially performing ultrasonic dispersion and centrifugation to obtain surface-modified elemental boron; adding lithium iron phosphate to a polar solvent and then adding a second surface modifier, and sequentially performing ultrasonic dispersion and centrifugation to obtain surface-modified phosphate Lithium iron phosphate, the charge of the second surface modifier is opposite to the charge of the first surface modifier; the surface-modified elemental boron and the surface-modified lithium iron phosphate are added to a polar solvent, and a self-assembled mixed solution is obtained through ultrasonic treatment or magnetic stirring; the self-assembled mixed solution is centrifuged and the precipitate is collected, and the precipitate is vacuum-dried to obtain boron-lithium iron phosphate; the composite lithium supplement material, boron-lithium iron phosphate, a conductive agent and an adhesive are mixed 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; the positive electrode slurry is coated on the surface of the positive electrode current collector and then dried to obtain a positive electrode sheet; a negative electrode sheet and a separator are provided, the positive electrode sheet, the separator and the negative electrode sheet are wound or stacked and then placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

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

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

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

[0009] In some embodiments, in the step of adding the surface-modified elemental boron and the surface-modified lithium iron phosphate to 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 polar solvent is 3:(100~200).

[0010] In some embodiments, the first surface modifier is polyethyleneimine or polydiallyldimethylammonium chloride; the second surface modifier is polyacrylic acid or polysulfonic acid.

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

[0012] According to some embodiments of the present application, on the other hand, the embodiments of the present application further 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, the material of the positive electrode material layer includes a composite lithium supplement material, boron-lithium iron phosphate, a conductive agent and an adhesive, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, the conductive agent and the adhesive is (1.5~2.2): (93.8~94.5): (0.8~1.2): (1.7~2.1), the composite lithium supplement material includes boron-doped graphene, lithium tantalate and lithium tungstate.

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

[0014] In some embodiments, the particle size when the cumulative distribution percentage of lithium iron phosphate in boron-lithium iron phosphate reaches 50% is 1 μm-2 μm; the particle size 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, another aspect of the embodiments of the present application further provides an energy storage system, comprising: a plurality of secondary batteries manufactured using the preparation method of the secondary battery in the above embodiment, or the secondary battery in the above embodiment.

[0016] According to some embodiments of the present application, on the other hand, embodiments of the present application further provide an electrical device, including a secondary battery manufactured by the secondary battery preparation method in the above embodiment; or including the secondary battery in the above embodiment; or including the energy storage system in the above embodiment.

[0017] The technical solution provided by the embodiments of the present application has at least the following advantages:

[0018] In the preparation method of the secondary battery provided in the embodiments of the present application, a freeze-drying method is used to prepare a composite lithium-supplementing material when preparing the positive electrode, and a boron-lithium iron phosphate method is used to prepare the positive electrode. The composite lithium-supplementing 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 and carboxyl groups), which can provide good attachment sites for lithium tantalate and lithium tungstate. Boron doping further enhances the conductivity and chemical activity of graphene, promoting the interfacial bonding between lithium tantalate and lithium tungstate and the boron-doped graphene. The conductive network constructed by the boron-doped graphene in the composite lithium-supplementing material can help improve the conductive performance of the positive electrode. Lithium tantalate and lithium tungstate can be used to replenish active lithium to compensate for the initial capacity loss of secondary batteries and the continuous capacity loss that occurs during their lifecycle. Lithium tungstate has a narrow chemical window but a high lithium content, while lithium tantalate has a wide electrochemical window and high stability. The combination of the two can balance stability and lithium replenishment efficiency. Lithium tantalate is a good ion conductor, and the crystal structure of lithium tungstate facilitates lithium ion migration. The combination of the two can form a more optimal lithium ion transmission channel, thereby improving the cycle performance of secondary batteries. Before freeze-drying, boron-doped graphene, lithium tantalate, and lithium tungstate are ultrasonically dispersed in a solvent separately, and then mixed and dispersed. This confines some of the lithium tantalate and lithium tungstate between the boron-doped graphene sheets, preventing agglomeration while facilitating the uniform dispersion of lithium tantalate and lithium tungstate. The freeze-drying method is beneficial for maintaining the porous structure and high specific surface area of ​​the boron-doped graphene, maintaining the uniformity of the composition and the stability of the interface bonding. Elemental boron is a semimetal with a certain degree of intrinsic conductivity. It can form conductive bridges between or on the surface of lithium iron phosphate particles, reducing interparticle contact resistance and enhancing electron transfer efficiency. The introduction of boron can also slightly alter the lattice interstices or surface lattice parameters of lithium iron phosphate, introducing dislocations or vacancies, increasing lithium ion diffusion channels, and thus improving the rate capability and cycle performance of secondary batteries. Preparing boron-lithium iron phosphate via electrostatic self-assembly ensures a uniform dispersion of boron and lithium iron phosphate, preventing agglomeration. Electrostatic self-assembly also results in a denser arrangement of boron and lithium iron phosphate, reducing voids and contributing to an increase in the tap density of the positive electrode. Low-temperature electrostatic self-assembly also prevents growth or phase transitions in lithium iron phosphate caused by high-temperature sintering, thereby improving the stability of the boron-lithium iron phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise stated, the pictures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 Flowchart corresponding to the steps for preparing the positive electrode sheet provided in the embodiment of the present application. DETAILED DESCRIPTION

[0021] As can be seen from the background technology, the main reasons for the high internal resistance and low cycle life of batteries include the material and structural design of the positive electrode sheet.

[0022] The embodiments of the present application provide a secondary battery and 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.

[0023] 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 number, specific order or primary and secondary relationship of the indicated technical features.

[0024] In the description of the embodiments of the present application, “multiple” means more than two, unless otherwise clearly and specifically defined.

[0025] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0026] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0027] In the description of the embodiments of the present application, when a component “includes” another component, unless otherwise stated, other components are not excluded, and other components may be further included.

[0028] The terms used in the description of the various embodiments described herein are for describing specific embodiments only and are not intended to be limiting. As used in the description of the various embodiments described and the appended claims, "components" are also intended to include plural forms unless the context clearly indicates otherwise.

[0029] The following detailed description of the various embodiments of the present application is provided in conjunction with the accompanying drawings. However, those skilled in the art will appreciate that many technical details are provided in the various embodiments of the present application to facilitate a better understanding of the present application. However, even without these technical details and the various variations and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0030] Figure 1 Flowchart corresponding to the steps for preparing the positive electrode sheet provided in the embodiment of the present application.

[0031] According to some embodiments of the present application, one aspect of the present application provides a method for preparing a secondary battery, and the preparation method is as follows.

[0032] S11, prepare the positive electrode sheet, refer to Figure 1 , the preparation steps are as follows.

[0033] S101. Prepare a composite lithium-replenishing 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 freeze-dry after ultrasonic dispersion or magnetic stirring to obtain a composite lithium-replenishing material.

[0034] In step S101, in the boron-doped graphene suspension, the concentration of the boron-doped graphene in the solvent is 1 mg / mL~5 mg / mL; in the lithium tantalate-lithium tungstate suspension, the concentration of the sum of lithium tantalate and lithium tungstate in the solvent is 10 mg / mL~50 mg / mL; in the mixed solution of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is (10~12):(6~8):(1~2). The concentrations of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension are within an appropriate range, which is beneficial to the subsequent compounding of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, so that the boron-doped graphene forms a good interface contact with the lithium tantalate and lithium tungstate. It is also beneficial to control the mass ratio of the boron-doped graphene, lithium tantalate and lithium tungstate. The mass ratio of the boron-doped graphene, lithium tantalate and lithium tungstate within an appropriate range is beneficial to reducing the agglomeration problem of the components.

[0035] The solvents of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension can both be selected from ethanol or deionized water.

[0036] In step S101 , freeze-drying conditions 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.

[0037] In step S101, the method for preparing boron-doped graphene includes mixing graphene and boric acid in a mass ratio of 1:(0.6-1.5), calcining at 800-1000°C for 1-4 hours, washing with concentrated sulfuric acid (95-98 wt% by weight) and deionized water, and drying to obtain the boron-doped graphene. The pH value of the washed boron-doped graphene is 6-7.

[0038] After step S101, the process may further include post-processing the composite lithium-supplementing material by heat-treating the composite lithium-supplementing material in an inert gas atmosphere at 300°C to 500°C for 1 to 2 hours. Heat treatment can improve the interfacial bonding strength and chemical stability of boron-doped graphene, lithium tantalate, and lithium tungstate, while also removing unreacted precursors and improving the purity of the composite lithium-supplementing material.

[0039] S102. Prepare boron-lithium iron phosphate, add elemental boron to a polar solvent, then add a first surface modifier, perform ultrasonic dispersion and centrifugal treatment in sequence to obtain surface-modified elemental boron; add lithium iron phosphate to a polar solvent, then add a second surface modifier, perform ultrasonic dispersion and centrifugal treatment in sequence to obtain surface-modified lithium iron phosphate, the charge of the second surface modifier is opposite to the charge of the first surface modifier; add the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, perform ultrasonic treatment or magnetic stirring to obtain a self-assembled mixed solution; centrifuge the self-assembled mixed solution and collect the precipitate, and vacuum dry the precipitate to obtain boron-lithium iron phosphate.

[0040] In step S102, when elemental boron is added to a polar solvent and then a first surface modifier is added, 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 lithium iron phosphate is added to a polar solvent and then a second surface modifier is added, 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).

[0041] The polar solvent can be selected from deionized water, ethanol or isopropanol.

[0042] In step S102 , after the surface-modified lithium iron phosphate and the surface-modified elemental boron are prepared, the surface-modified lithium iron phosphate and the surface-modified elemental boron can be washed 3 to 5 times with ethanol to remove unadsorbed modifiers.

[0043] In step S102, when the surface-modified elemental boron and the surface-modified lithium iron phosphate are added to 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 polar solvent is 3:(100~200).

[0044] In step S102 , the process parameters for vacuum drying the precipitate include: a temperature of 60° C. to 80° C. and a time of 12 hours to 24 hours.

[0045] In step S102 , the first surface modifier is polyethyleneimine or polydiallyldimethylammonium chloride; the second surface modifier is polyacrylic acid or polysulfonic acid.

[0046] S103. The composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive are mixed 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.

[0047] S104, coating the positive electrode slurry on the surface of the positive electrode current collector and drying it to obtain a positive electrode sheet.

[0048] S12. Provide a negative electrode sheet and a separator, wind or stack the positive electrode sheet, the separator and the negative electrode sheet, and place them in a shell, and inject an electrolyte into the shell to obtain a secondary battery.

[0049] In the preparation method of the secondary battery provided in the embodiments of the present application, a freeze-drying method is used to prepare a composite lithium-supplementing material when preparing the positive electrode, and a boron-lithium iron phosphate method is used to prepare the positive electrode. The composite lithium-supplementing 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 and carboxyl groups), which can provide good attachment sites for lithium tantalate and lithium tungstate. Boron doping further enhances the conductivity and chemical activity of graphene, promoting the interfacial bonding between lithium tantalate and lithium tungstate and the boron-doped graphene. The conductive network constructed by the boron-doped graphene in the composite lithium-supplementing material can help improve the conductive performance of the positive electrode. Lithium tantalate and lithium tungstate can be used to replenish active lithium to compensate for the initial capacity loss of secondary batteries and the continuous capacity loss that occurs during their lifecycle. Lithium tungstate has a narrow chemical window but a high lithium content, while lithium tantalate has a wide electrochemical window and high stability. The combination of the two can balance stability and lithium replenishment efficiency. Lithium tantalate is a good ion conductor, and the crystal structure of lithium tungstate facilitates lithium ion migration. The combination of the two can form a more optimal lithium ion transmission channel, thereby improving the cycle performance of secondary batteries. Before freeze-drying, boron-doped graphene, lithium tantalate, and lithium tungstate are ultrasonically dispersed in a solvent separately, and then mixed and dispersed. This confines some of the lithium tantalate and lithium tungstate between the boron-doped graphene sheets, preventing agglomeration while facilitating the uniform dispersion of lithium tantalate and lithium tungstate. The freeze-drying method is beneficial for maintaining the porous structure and high specific surface area of ​​the boron-doped graphene, maintaining the uniformity of the composition and the stability of the interface bonding. Elemental boron is a semimetal with a certain degree of intrinsic conductivity. It can form conductive bridges between or on the surface of lithium iron phosphate (LFP) particles, reducing interparticle contact resistance and enhancing electron transfer efficiency. Furthermore, the introduction of boron can slightly alter the lattice interstitial spaces or surface lattice parameters of the LFP, introducing dislocations or vacancies that increase lithium ion diffusion pathways, thereby improving the rate capability and cycling performance of the secondary battery. The preparation of boron-LFP via electrostatic self-assembly ensures a uniform dispersion of boron and LFP, preventing agglomeration. This electrostatic self-assembly also results in a denser arrangement of boron and LFP, reducing voids and contributing to an increase in the tap density of the positive electrode (tap density refers to the mass of active material per unit volume; a higher tap density indicates a higher energy density and battery life). Low-temperature electrostatic self-assembly also prevents growth or phase transitions in the LFP that can occur during high-temperature sintering, thereby enhancing the stability of the boron-LFP.

[0050] Accordingly, another embodiment of the present application further provides a secondary battery, which can be prepared using the secondary battery preparation method provided in the above embodiment. For the same or corresponding parts as the previous embodiment, please refer to the corresponding description of the previous embodiment, and will not be described in detail below.

[0051] The secondary battery includes: 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, the materials of the positive electrode material layer include a composite lithium supplement material, boron-lithium iron phosphate, a conductive agent and an adhesive, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive is (1.5~2.2): (93.8~94.5): (0.8~1.2): (1.7~2.1), the composite lithium supplement material includes boron-doped graphene, lithium tantalate and lithium tungstate.

[0052] In some embodiments, the particle size value of lithium tantalate when the cumulative distribution percentage reaches 50% is 8μm~17μm, for example, it can be 8μm, 10μm, 11μm, 13μm, 15μm or 17μm; the particle size value of lithium tungstate when the cumulative distribution percentage reaches 50% is 10μm~20μm, for example, it can be 10μm, 12μm, 14μm, 16μm, 18μm or 20μm; the sheet diameter of boron-doped graphene is 0.5μm~10μm, for example, it can be 0.5μm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm.

[0053] 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, 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, it can be 10μm, 15μm, 20μm, 25μm, 30μm, 35μm or 40μm.

[0054] In the secondary battery provided in the embodiments of the present application, the composite lithium-supplementing 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 and carboxyl groups), providing good attachment sites for lithium tantalate and lithium tungstate. Boron doping further enhances the conductivity and chemical activity of graphene, promoting interfacial bonding between lithium tantalate and lithium tungstate and the boron-doped graphene. The conductive network constructed by the boron-doped graphene in the composite lithium-supplementing material can improve the conductivity of the positive electrode. Lithium tantalate and lithium tungstate can be used to replenish active lithium to compensate for the initial capacity loss of the secondary battery and the continuous capacity loss that occurs during its lifecycle. Lithium tungstate has a narrow chemical window but a high lithium content, while lithium tantalate has a wide electrochemical window and high stability. The combination of the two can balance stability and lithium-supplementing efficiency. Lithium tantalate is a good ionic conductor, and the crystal structure of lithium tungstate facilitates lithium ion migration. The combination of the two can form a more optimized lithium ion transport channel, thereby improving the cycling performance of the secondary battery. Elemental boron is a semi-metal with certain intrinsic conductivity. Elemental boron can form conductive bridges between lithium iron phosphate particles or on the surface, reducing the contact resistance between particles and enhancing the efficiency of electron transfer. The introduction of elemental boron can slightly change the lattice gap or lattice parameters of the lithium iron phosphate surface, introduce dislocations or vacancies, increase the diffusion channels of lithium ions, and thus improve the rate performance and cycle performance of secondary batteries.

[0055] Correspondingly, another aspect of the embodiments of the present application further provides an energy storage system, comprising: a plurality of secondary batteries manufactured using the manufacturing method of the secondary battery in the above embodiment, or the secondary battery in the above embodiment.

[0056] Correspondingly, another aspect of the embodiments of the present application further provides an electrical device, including a secondary battery manufactured by the method for preparing a 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.

[0057] The following are specific examples of this application.

[0058] Example 1

[0059] S21. Prepare a positive electrode sheet. The preparation steps are as follows.

[0060] S201. Prepare a composite lithium-replenishing material, ultrasonically disperse boron-doped graphene in a solvent to obtain a boron-doped graphene suspension, wherein the concentration of the boron-doped graphene in the solvent is 5 mg / mL; ultrasonically disperse lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension, wherein the total concentration of lithium tantalate and lithium tungstate in the solvent is 50 mg / mL; mix the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, wherein the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is 11:7:1, and after ultrasonic dispersion, freeze-dry at -80°C and a vacuum degree of 0.05 mbar for 24 hours to obtain a composite lithium-replenishing material.

[0061] S202, preparing boron-lithium iron phosphate, adding elemental boron to a polar solvent ethanol, then adding a first surface modifier polyethyleneimine, sequentially performing ultrasonic dispersion and centrifugal treatment to obtain surface-modified elemental boron, wherein the mass ratio of elemental boron to the polar solvent is 1:150, and the mass ratio of the first surface modifier to elemental boron is 1:8; adding lithium iron phosphate to a polar solvent ethanol, then adding a second surface modifier polyacrylic acid, sequentially performing ultrasonic dispersion and centrifugal treatment to obtain surface-modified lithium iron phosphate, wherein the mass ratio of lithium iron phosphate to the polar solvent is 1:80, The mass ratio of the second surface modifier to lithium iron phosphate is 1:8; the surface-modified elemental boron and the surface-modified lithium iron phosphate are added to a polar solvent, and a self-assembled mixed liquid is obtained 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; the self-assembled mixed liquid is centrifuged and the precipitate is collected, and the precipitate is vacuum-dried at 60°C for 24 hours to obtain boron-lithium iron phosphate.

[0062] S203 , mixing the composite lithium supplement material, boron-lithium iron phosphate, a conductive agent, and a binder in a mass ratio of 2:94:1:2 to obtain a positive electrode slurry.

[0063] S204, coating the positive electrode slurry on the surface of the positive electrode current collector aluminum foil and drying it to obtain a positive electrode sheet.

[0064] 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 shell, inject electrolyte into the shell to obtain a secondary battery.

[0065] Example 2

[0066] The preparation method of Example 2 is substantially the same as that of Example 1, except that after S201, the prepared composite lithium supplement material is further post-treated by heat-treating the composite lithium supplement material at 350° C. for 1 hour in an inert gas atmosphere.

[0067] Example 3

[0068] The preparation method of Example 3 is basically the same as that of Example 1, except that in S201, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is 10:6:1.

[0069] Example 4

[0070] The preparation method of Example 4 is basically the same as that of Example 1, except that in S201, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is 12:8:2.

[0071] Example 5

[0072] The preparation method of Example 5 is basically the same as that of Example 1, except that in S202, the mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is 11:1, 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 3:160.

[0073] Example 6

[0074] The preparation method of Example 6 is basically the same as that of Example 1, except that in S202, the mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is 23:3, 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 3:130.

[0075] Example 7

[0076] The preparation method of Example 7 is basically the same as that of Example 1, except that in S202, the mass ratio of the surface-modified lithium iron phosphate to the surface-modified elemental boron is 23:2, 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 3:180.

[0077] Example 8

[0078] The preparation method of Example 8 is basically the same as that of Example 1, except that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive are prepared in a mass ratio of 1.5:93.8:0.8:1.7.

[0079] Example 9

[0080] The preparation method of Example 9 is basically the same as that of Example 1, except that in S203, the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive are prepared in a mass ratio of 2.2:94.5:1.2:2.1.

[0081] Example 10

[0082] The preparation method of Example 10 is substantially the same as that of Example 1, except that in S203 , the composite lithium supplement material, boron-lithium iron phosphate, conductive agent, and adhesive are prepared in a mass ratio of 2:94:1.2:2.1.

[0083] Comparative Example 1

[0084] The masses of the boron-doped graphene, lithium tantalate, lithium tungstate, lithium iron phosphate, boron, conductive agent and adhesive in Comparative Example 1 are the same as those of the boron-doped graphene, lithium tantalate, lithium tungstate, lithium iron phosphate, boron, conductive agent and adhesive in Example 1. In Comparative Example 1, the boron-doped graphene, lithium tantalate, lithium tungstate, lithium iron phosphate, boron, conductive agent and adhesive are directly prepared into a positive electrode slurry by ball milling and blending.

[0085] Comparative Example 2

[0086] 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 supplement material is added.

[0087] Comparative Example 3

[0088] The difference between Comparative Example 3 and Example 1 is that no composite lithium supplement material is added to the positive electrode slurry of Comparative Example 3.

[0089] Comparative Example 4

[0090] The difference between Comparative Example 4 and Example 1 is that the composite lithium supplement material of Comparative Example 4 only includes lithium tantalate.

[0091] Comparative Example 5

[0092] The difference between Comparative Example 5 and Example 1 is that the composite lithium supplement material of Comparative Example 5 only includes lithium tungstate.

[0093] Comparative Example 6

[0094] The difference between Comparative Example 6 and Example 1 is that the composite lithium supplement material of Comparative Example 6 only includes lithium tantalate and boron-doped graphene.

[0095] Comparative Example 7

[0096] The difference between Comparative Example 7 and Example 1 is that the composite lithium supplement material of Comparative Example 7 only includes lithium tungstate and boron-doped graphene.

[0097] Comparative Example 8

[0098] The difference between Comparative Example 8 and Example 1 is that in S203 , the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive is 0.5:94:1:2.

[0099] Comparative Example 9

[0100] The difference between Comparative Example 9 and Example 1 is that in S203 , the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive is 5:94:1:2.

[0101] Comparative Example 10

[0102] The difference between Comparative Example 10 and Example 1 is that in S203 , the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive is 2:90:1:2.

[0103] Comparative Example 11

[0104] 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 adhesive is 2:100:1:2.

[0105] Comparative Example 12

[0106] 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 adhesive are mixed in a mass ratio of 2:94:0.3:2.

[0107] Comparative Example 13

[0108] 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 adhesive are mixed in a mass ratio of 2:94:3:2.

[0109] Comparative Example 14

[0110] 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 adhesive are mixed in a mass ratio of 2:94:1:0.5.

[0111] Comparative Example 15

[0112] 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 adhesive are mixed in a mass ratio of 2:94:1:4.

[0113] The initial capacity, capacity retention rate after 500 cycles, 2C capacity retention rate and impedance of the above embodiments and comparative examples were tested. The test results are shown in Table 1 below.

[0114] Table 1

[0115]

[0116] Table 1 (continued)

[0117]

[0118] By comparing Examples 1 to 10 with Comparative Example 1, it can be found that the secondary battery and preparation method thereof provided in the embodiments of the present application, when preparing the positive electrode sheet, adopts a freeze-drying method to prepare the composite lithium supplement material, and adopts an electrostatic self-assembly method to prepare boron-lithium iron phosphate. Compared with the conventional ball milling blending method, it is beneficial to reduce the impedance of the secondary battery and improve the cycle performance and rate performance of the secondary battery.

[0119] By comparing Examples 1 to 10 and Comparative Examples 2 to 7, it can be found that the secondary battery and the preparation method thereof provided in the embodiments of the present application use boron-doped graphene, lithium tantalate and lithium tungstate as composite lithium supplement materials in the positive electrode sheet, and compound elemental boron with lithium iron phosphate, which can improve the cycle performance, rate performance and initial capacity of the secondary battery and reduce the impedance of the battery.

[0120] By comparing Example 1, Examples 8 to 10, and Comparative Examples 8 to 13, it can be found that too little composite lithium-supplementing material makes it difficult to achieve the lithium-supplementing effect, resulting in insignificant improvement in secondary battery performance, while too much may cause lithium deposition, affecting the overall performance of the secondary battery. Too little boron-lithium iron phosphate provides less active lithium, which is not conducive to the performance of the secondary battery. Due to the poor conductivity of boron-lithium iron phosphate, too much easily affects the conductivity of the electrode sheet, resulting in a decrease in the overall performance of the secondary battery. Too little conductive agent leads to a higher electrode sheet resistance. Although a larger amount can reduce the internal resistance of the secondary battery, the rate performance will also decrease. More adhesive is beneficial to improving the stability of the electrode sheet, but too much adhesive will lead to poor conductivity of the electrode sheet, affecting the overall performance of the secondary battery. Therefore, when the mass ratio of composite lithium-supplementing material, boron-lithium iron phosphate, conductive agent, and adhesive is (1.5-2.2): (93.8-94.5): (0.8-1.2): (1.7-2.1), the corresponding comprehensive performance of the secondary battery in terms of cycle performance, rate performance, initial capacity, and impedance is better.

[0121] Those skilled in the art will appreciate that the above-described embodiments are specific examples for implementing the present application, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined in the claims.

Claims

1. A method for preparing a secondary battery, characterized in that: include: Prepare the positive electrode sheet, the preparation steps include: Preparation of a composite lithium-replenishing material: ultrasonically dispersing boron-doped graphene in a solvent to obtain a boron-doped graphene suspension; ultrasonically dispersing lithium tantalate and lithium tungstate in a solvent to obtain a lithium tantalate-lithium tungstate suspension; mixing the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, ultrasonically dispersing or magnetically stirring, and then freeze-drying to obtain the composite lithium-replenishing material; Preparation of boron-lithium iron phosphate: adding elemental boron to a polar solvent, then adding a first surface modifier, sequentially performing ultrasonic dispersion and centrifugation to obtain surface-modified elemental boron; adding lithium iron phosphate to a polar solvent, then adding a second surface modifier, sequentially performing ultrasonic dispersion and centrifugation to obtain surface-modified lithium iron phosphate, wherein the charge of the second surface modifier is opposite to that of the first surface modifier; adding the surface-modified elemental boron and the surface-modified lithium iron phosphate to a polar solvent, performing ultrasonic treatment or magnetic stirring to obtain a self-assembled mixed solution; centrifuging the self-assembled mixed solution and collecting a precipitate, and vacuum drying the precipitate to obtain the boron-lithium iron phosphate; The composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive are mixed 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; The positive electrode slurry is applied to the surface of the positive electrode current collector and then dried to obtain the positive electrode sheet; A negative electrode sheet and a separator are provided, the positive electrode sheet, the separator and the negative electrode sheet are wound or stacked, and then placed in a shell, and an electrolyte is injected into the shell to obtain a secondary battery.

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

3. The method for preparing a secondary battery according to claim 1, wherein: In the boron-doped graphene suspension, the concentration of the 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 mixed solution of the boron-doped graphene suspension and the lithium tantalate-lithium tungstate suspension, the mass ratio of lithium tantalate, lithium tungstate and boron-doped graphene is (10-12): (6-8): (1-2).

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

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 into 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 polar solvent is 3:(100~200).

6. The method for preparing a secondary battery according to claim 1, wherein: The first surface modifier is polyethyleneimine or polydiallyldimethylammonium chloride; the second surface modifier is polyacrylic acid or polysulfonic acid.

7. The method for preparing a secondary battery according to claim 1, wherein: 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 800-1000°C for 1-4 hours, washing with concentrated sulfuric acid and deionized water in sequence, and drying to obtain the boron-doped graphene.

8. A secondary battery manufactured by the method for manufacturing a secondary battery according to any one of claims 1 to 7, characterized in that: include: A positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet includes: positive electrode current collector; A positive electrode material layer, the positive electrode material layer covers the surface of the positive electrode current collector, the materials of the positive electrode material layer include a composite lithium supplement material, boron-lithium iron phosphate, a conductive agent and an adhesive, the mass ratio of the composite lithium supplement material, boron-lithium iron phosphate, conductive agent and adhesive is (1.5~2.2): (93.8~94.5): (0.8~1.2): (1.7~2.1), the composite lithium supplement material includes boron-doped graphene, lithium tantalate and lithium tungstate.

9. The secondary battery according to claim 8, characterized in that The particle size of the lithium tantalate when the cumulative distribution percentage reaches 50% is 8μm~17μm; the particle size of the lithium tungstate when the cumulative distribution percentage 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 of the lithium iron phosphate in the boron-lithium iron phosphate when the cumulative distribution percentage reaches 50% is 1 μm-2 μm; the particle size of the elemental boron in the boron-lithium iron phosphate when the cumulative distribution percentage reaches 50% is 10 μm-40 μm.

11. An energy storage system, characterized in that: include: A plurality of secondary batteries manufactured using the method for manufacturing a secondary battery according to any one of claims 1 to 7, or a secondary battery according to any one of claims 8 to 10.

12. An electrical device, characterized in that: A secondary battery manufactured by the method for preparing a secondary battery according to any one of claims 1 to 7; or a secondary battery according to any one of claims 8 to 10; or an energy storage system according to claim 11.

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