Negative electrode material, negative electrode plate, lithium ion battery and preparation method of lithium ion battery

Abstract

The invention provides a negative electrode material, a negative electrode plate, a lithium ion battery and a preparation method of the negative electrode material, and aims to solve the technical problem that the amount of lithium difluorophosphate directly added into an electrolyte of the lithium ion battery is limited. The negative electrode material is used for the lithium ion battery, the lithium ion battery contains lithium hexafluorophosphate, the negative electrode material comprises a negative electrode active substance and aluminum oxide, and the aluminum oxide is attached to the surface of the negative electrode active substance; and in the negative electrode material, the mass ratio of the negative electrode active substance to the aluminum oxide is 1: (0.001-0.03).

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and specifically to a negative electrode material, a method for preparing a negative electrode material, a negative electrode sheet, a method for preparing a negative electrode sheet, a lithium-ion battery, and a method for preparing a lithium-ion battery. Background Art

[0002] The electrode / electrolyte interface has a crucial impact on the performance of lithium-ion batteries. This interface determines the lithium-ion transmission efficiency, the cycle stability and safety of lithium-ion batteries. Therefore, optimizing the properties of the electrode / electrolyte interface is one of the key ways to improve the performance of lithium-ion batteries.

[0003] In order to improve the performance of lithium-ion batteries, the properties of the electrode / electrolyte interface can be improved by using electrolyte additives. Lithium difluorophosphate, as a commonly used electrolyte additive, can be reduced and decomposed on the surface of the negative electrode to form a layer of thermodynamically and mechanically stable solid electrolyte membrane, which can not only inhibit the further reduction and decomposition of the electrolyte at the negative electrode, but also effectively improve the cycle stability. However, due to the limited solubility of lithium difluorophosphate in the electrolyte, adding too much lithium difluorophosphate will cause the viscosity of the electrolyte to increase sharply, thereby reducing or even worsening the effect of its performance improvement. Summary of the invention

[0004] The embodiments of the present application provide a negative electrode material, a negative electrode plate, a lithium-ion battery and a preparation method thereof, so as to improve the technical problem of the limited amount of lithium difluorophosphate directly added to the electrolyte of the lithium-ion battery.

[0005] In a first aspect, an embodiment of the present application provides a negative electrode material for a lithium ion battery, wherein the lithium ion battery comprises lithium hexafluorophosphate. The negative electrode material comprises a negative electrode active material and aluminum oxide, wherein the aluminum oxide is attached to the surface of the negative electrode active material; in the negative electrode material, the mass ratio of the negative electrode active material to the aluminum oxide is 1:(0.001-0.03).

[0006] In one embodiment, in the aluminum oxide, the volume fraction of aluminum oxide particles with a particle size of 10 nm to 30 nm is greater than or equal to 20 vol%.

[0007] In one embodiment, the negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon and silicon-oxygen.

[0008] In a second aspect, an embodiment of the present application provides a method for preparing a negative electrode material, which is used to prepare the above-mentioned negative electrode material. The method for preparing the negative electrode material comprises: mixing a negative electrode active material with aluminum oxide to obtain a mixture, and ball milling the mixture so that the aluminum oxide adheres to the surface of the negative electrode active material to obtain the negative electrode material.

[0009] In one embodiment, the ball milling treatment lasts for 3 hours to 10 hours, and the rotation speed is 80 r / min to 150 r / min.

[0010] In a third aspect, an embodiment of the present application provides a negative electrode plate, comprising a negative electrode current collector and a negative electrode film layer arranged on the negative electrode current collector, wherein the negative electrode film layer comprises the above-mentioned negative electrode material, or the negative electrode material prepared by the above-mentioned method for preparing the negative electrode material.

[0011] In one embodiment, the content of aluminum in the negative electrode film layer is 1800ppm to 56000ppm.

[0012] In one embodiment, the negative electrode film layer further includes a binder, a conductor and a thickener. In the negative electrode film layer, the mass ratio of the negative electrode material, the binder, the conductor and the thickener is (95-98): (0.8-2.5): (0.5-3.5): (0.7-2.0).

[0013] In a fourth aspect, an embodiment of the present application provides a method for preparing a negative electrode sheet, which is used to prepare the above-mentioned negative electrode sheet. The method for preparing the negative electrode sheet comprises: providing a negative electrode slurry, wherein the negative electrode slurry comprises a negative electrode material and a solvent; performing a film-forming process on the negative electrode slurry on a negative electrode current collector to obtain a wet film layer; and performing a drying process on the wet film layer to obtain a negative electrode film layer.

[0014] In one embodiment, the drying process is vacuum high temperature drying, and the temperature of the vacuum high temperature drying is 60°C to 100°C.

[0015] In a fifth aspect, an embodiment of the present application provides a lithium-ion battery, comprising: an electrode assembly and a shell, wherein a cavity is formed in the shell, the electrode assembly is arranged in the cavity, the cavity is also loaded with an electrolyte, the electrolyte infiltrates the electrode assembly, the electrolyte contains lithium hexafluorophosphate, the electrode assembly comprises a positive electrode sheet and a negative electrode sheet arranged opposite to each other, the negative electrode sheet is the above-mentioned negative electrode sheet, or a negative electrode sheet prepared by the above-mentioned method for preparing the negative electrode sheet.

[0016] In one embodiment, in the lithium-ion battery, the molar ratio of the lithium hexafluorophosphate to the aluminum oxide is 1:(0.005-0.15).

[0017] In one embodiment, the electrolyte further comprises an organic solvent, and the organic solvent comprises at least one of ethylene carbonate, methylpropyl carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0018] In a sixth aspect, an embodiment of the present application provides a method for preparing a lithium-ion battery, comprising:

[0019] Providing an electrode assembly, the electrode assembly comprising a positive electrode sheet and a negative electrode sheet arranged opposite to each other, the negative electrode sheet being the above-mentioned negative electrode sheet, or a negative electrode sheet prepared by the above-mentioned method for preparing the negative electrode sheet;

[0020] Providing a housing, wherein a cavity is formed in the housing, and the electrode assembly is placed in the cavity;

[0021] An electrolyte is injected into the cavity so that the electrolyte infiltrates the electrode assembly to obtain a lithium-ion battery, wherein the electrolyte contains lithium hexafluorophosphate.

[0022] In one embodiment, the method for preparing the lithium-ion battery further includes: subjecting the lithium-ion battery to a static treatment.

[0023] In one embodiment, the static treatment is performed at a temperature of 40°C to 80°C.

[0024] In one embodiment, the standing time of the standing treatment is 12 hours to 96 hours.

[0025] In one embodiment, the method for preparing the lithium-ion battery further comprises: performing a formation treatment on the lithium-ion battery after the static treatment.

[0026] Beneficial effects of the embodiments of the present application:

[0027] In the negative electrode material provided in the embodiment of the present application, aluminum oxide is attached to the surface of the negative electrode active material. When the negative electrode material is applied to a lithium ion battery containing lithium hexafluorophosphate, aluminum oxide can react with lithium hexafluorophosphate and generate a coating layer containing lithium difluorophosphate and aluminum fluoride on the surface of the negative electrode active material, thereby effectively increasing the content of lithium difluorophosphate in the lithium ion battery. Since lithium difluorophosphate is generated in situ on the surface of the negative electrode active material in the embodiment of the present application, it is possible to avoid adding too much lithium difluorophosphate to the electrolyte to increase the viscosity of the electrolyte, thereby causing the electrolyte to have poor wettability to the pole piece and battery performance to decay.

[0028] In addition, during the first charging process, the lithium difluorophosphate on the surface of the negative electrode active material will be reduced and decomposed and combined with aluminum fluoride to form a relatively stable composite solid electrolyte membrane, which can not only inhibit the reduction and decomposition of the electrolyte on the negative electrode surface during the cycle, but also improve the ionic conductivity of the solid electrolyte membrane at the interface between the negative electrode active material and the electrolyte, effectively reduce the electrode / electrolyte interface impedance, and improve the battery's rate performance and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 This is an ion chromatogram of the negative electrode sheet of the lithium-ion battery provided in Example 1 of the present application after standing at high temperature but before being formed. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of this application.

[0032] In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application. In the present application, unless otherwise stated, the directional words used, such as "upper" and "lower", generally refer to the upper and lower parts of the device in actual use or working state, specifically the drawing direction in the accompanying drawings; while "inner" and "outer" refer to the outline of the device.

[0033] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0034] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or mutual communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0035] The terms "comprises," "includes," or any other variation thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0036] In the description of the embodiments of the present application, words such as "example" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "example" or "for example" in the embodiments of the present application is not to be interpreted as being more preferred or having more advantages than another embodiment or design. The use of words such as "example" or "for example" is intended to present relative concepts in a clear manner.

[0037] To facilitate understanding of the scheme of the present application, the spline curves and arrows used in the drawings are explained here: the components indicated by the spline curves without arrows are solid components, that is, components with solid structures; the components indicated by the spline curves with arrows are virtual components, that is, components without solid structures.

[0038] In a first aspect, an embodiment of the present application provides a negative electrode material, which is used in a lithium-ion battery. Specifically, the negative electrode material is used in a negative electrode plate of a lithium-ion battery. The lithium-ion battery contains lithium hexafluorophosphate (LiPF6). As an example, the lithium-ion battery includes an electrolyte, and the lithium hexafluorophosphate is contained in the electrolyte.

[0039] Specifically, the negative electrode material includes a negative electrode active material and aluminum oxide, and the aluminum oxide is attached to the surface of the negative electrode active material. In the negative electrode material, the mass ratio of the negative electrode active material to the aluminum oxide is 1:(0.001-0.03).

[0040] In the negative electrode material provided in the embodiment of the present application, aluminum oxide is attached to the surface of the negative electrode active material. When the negative electrode material is applied to a lithium ion battery containing lithium hexafluorophosphate, aluminum oxide can react with lithium hexafluorophosphate and generate a coating layer containing lithium difluorophosphate and aluminum fluoride on the surface of the negative electrode active material, thereby effectively increasing the content of lithium difluorophosphate in the lithium ion battery. Since lithium difluorophosphate is generated in situ on the surface of the negative electrode active material in the embodiment of the present application, it is possible to avoid adding too much lithium difluorophosphate to the electrolyte to increase the viscosity of the electrolyte, thereby causing the electrolyte to have poor wettability to the pole piece and battery performance to decay.

[0041] During the first charging process, the lithium difluorophosphate on the surface of the negative electrode active material will be reduced and decomposed and combined with aluminum fluoride to form a relatively stable composite solid electrolyte membrane, which can not only inhibit the reduction and decomposition of the electrolyte on the negative electrode surface during the cycle, but also improve the ionic conductivity of the solid electrolyte membrane at the interface between the negative electrode active material and the electrolyte, effectively reduce the electrode / electrolyte interface impedance, and improve the battery's rate performance and cycle stability.

[0042] Generally, the content of aluminum oxide in the negative electrode material will affect the amount of lithium difluorophosphate generated by the reaction in the lithium-ion battery, so the content of aluminum oxide should not be too low. However, the conductivity of aluminum oxide (Al2O3) is poor. If the content of aluminum oxide in the negative electrode material is too high, the conductivity of the negative electrode material will decrease, thereby increasing the internal resistance of the lithium-ion battery. In addition, too high alumina content will also lead to an increase in the consumption of lithium hexafluorophosphate, which will have an adverse effect on the cycle life of the lithium-ion battery.

[0043] By controlling the content of aluminum oxide in the negative electrode material, the mass ratio of the negative electrode active material to aluminum oxide is made to be 1:(0.001-0.03). Within this range, the electrochemical performance of the lithium-ion battery is better.

[0044] As an example, in the negative electrode material, the mass ratio of the negative electrode active material to aluminum oxide is 1:0.001, 1:0.002, 1:0.003, 1:0.004, 1:0.005, 1:0.006, 1:0.007, 1:0.008, 1:0.009, 1:0.01, 1:0.012, 1:0.014, 1:0.016, 1:0.018, 1:0.02, 1:0.022, 1:0.024, 1:0.026, 1:0.028 or 1:0.03.

[0045] In some embodiments, in the alumina, the volume fraction of alumina particles with a particle size of 10nm to 30nm is greater than or equal to 20vol%. It can be understood that the alumina in the negative electrode material is granular, that is, alumina particles, and calculated by volume content, at least 20vol% of the alumina particles in the alumina have a particle size of 10nm to 30nm. Generally, the smaller the particle size of the alumina in the negative electrode material, the easier it is for the alumina to react with lithium hexafluorophosphate and generate lithium difluorophosphate. However, since the alumina is attached to the surface of the negative electrode active material, the smaller the particle size of the alumina will make it more difficult for the electrolyte to penetrate into the negative electrode active material, affecting mass transfer, thereby reducing the probability of contact between the alumina and lithium hexafluorophosphate, and reducing the amount of lithium difluorophosphate generated. As an example, the particle size of the alumina particles is 10nm, 15nm, 20nm, 25nm or 30nm. As an example, the volume percentage of aluminum oxide particles with a particle size of 10 nm to 30 nm in aluminum oxide is 20 vol%, 30 vol%, 40 vol%, 50 vol%, 60 vol%, 70 vol%, 80 vol%, 90 vol% or 100 vol%. In addition, too small a particle size of aluminum oxide in the negative electrode material will also increase the production cost of the negative electrode material.

[0046] In some embodiments, the negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon and silicon oxide. It should be noted here that soft carbon and hard carbon are two different carbon materials, and the main difference lies in the ability to graphitize at high temperatures. Soft carbon can be graphitized at high temperatures above 2500°C, while hard carbon is difficult to graphitize. Soft carbon usually has a low and stable charge and discharge potential platform, large charge and discharge capacity and high efficiency, while hard carbon has a stable structure and a long charge and discharge cycle life.

[0047] In some embodiments, the chemical formula of silicon oxygen is SiO x , and 0<x<2. It can be understood that the silicon oxygen here is an incomplete oxide of silicon. Here x represents the oxygen content in the silicon oxygen, and x can be any value between 0 and 2, such as 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 1.99. As an example, the silicon oxygen includes silicon oxide.

[0048] In a second aspect, an embodiment of the present application further provides a method for preparing a negative electrode material, and the method for preparing a negative electrode material is used to prepare the above-mentioned negative electrode material.

[0049] Specifically, the method for preparing the negative electrode material includes:

[0050] S11, mixing the negative electrode active material with aluminum oxide to obtain a mixture;

[0051] S12, ball milling the mixed material so that aluminum oxide adheres to the surface of the negative electrode active material to obtain a negative electrode material.

[0052] The method for preparing the negative electrode material provided in the embodiment of the present application is simple to operate, easy to implement, and has no special environmental control requirements.

[0053] In some embodiments, the negative electrode active material is mixed with alumina to obtain a mixture, specifically by mixing the negative electrode active material solid particles with the alumina solid particles in a dry manner. Of course, in other embodiments, the negative electrode active material solid particles and the alumina solid particles can also be mixed in a wet manner. Here, the dry method refers to not using a solvent during the mixing process, while the wet method refers to using a solvent during the mixing process. Compared with the wet method, the dry method is simpler and does not require drying to remove the solvent, thereby simplifying the process and reducing production costs.

[0054] In some embodiments, the ball milling time is 3h to 10h, and the rotation speed is 80r / min to 150r / min. Within this parameter range, aluminum oxide can be uniformly coated on the surface of the negative electrode active material. As an example, the ball milling time is 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h, and the rotation speed is 80r / min, 90r / min, 100r / min, 110r / min, 120r / min, 130r / min, 140r / min or 150r / min.

[0055] In a third aspect, the present application also provides a negative electrode sheet, which includes a negative electrode current collector and a negative electrode film layer, wherein the negative electrode film layer is disposed on the negative electrode current collector. The negative electrode film layer includes the above-mentioned negative electrode material, or the negative electrode material prepared by the above-mentioned method for preparing the negative electrode material.

[0056] In some embodiments, the content of aluminum in the negative electrode film layer is 1800ppm to 56000ppm. Within this range, the amount of lithium difluorophosphate generated by the reaction in the lithium-ion battery and the conductivity of the negative electrode plate can be taken into account, and the consumption of lithium hexafluorophosphate can also be alleviated. As an example, the content of aluminum in the negative electrode film layer is 1800ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 6000ppm, 8000ppm, 10000ppm, 20000ppm, 30000ppm, 40000ppm, 50000ppm or 56000ppm.

[0057] In some embodiments, the negative electrode film layer further includes a binder, a conductive agent and a thickener. In the negative electrode film layer, the mass ratio of the negative electrode material, the binder, the conductive agent and the thickener is (95-98): (0.8-2.5): (0.5-3.5): (0.7-2.0). As an example, the conductive agent includes at least one of conductive carbon black, carbon fiber, carbon nanotubes (CNT) and graphene. As an example, the conductive carbon black includes at least one of acetylene black, Super P (SP for short) and Ketjen black. As an example, the carbon nanotubes include at least one of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). As an example, the binder includes at least one of polyacrylic acid (PAA) and polystyrene butadiene copolymer (SBR). As an example, the thickener includes carboxymethyl cellulose (CMC).

[0058] In some embodiments, the negative electrode current collector is a copper foil, and the thickness of the copper foil is 4 μm to 12 μm.

[0059] In a fourth aspect, an embodiment of the present application further provides a method for preparing a negative electrode sheet, and the method for preparing a negative electrode sheet is used to prepare the above-mentioned negative electrode sheet.

[0060] Specifically, the method for preparing the negative electrode sheet includes:

[0061] S21, providing negative electrode slurry, wherein the negative electrode slurry includes a negative electrode material and a solvent;

[0062] S22, performing film-forming treatment on the negative electrode slurry on the negative electrode current collector to obtain a wet film layer;

[0063] S23, drying the wet film layer to obtain a negative electrode film layer.

[0064] The method for preparing the negative electrode plate provided in the embodiment of the present application is simple to operate, easy to implement, and has no special environmental control requirements.

[0065] In some embodiments, the solvent in the negative electrode slurry includes one of deionized water and N-methylpyrrolidone (NMP).

[0066] In some embodiments, the negative electrode slurry is subjected to a film-forming treatment on the negative electrode current collector to obtain a wet film layer. Specifically, the negative electrode slurry is coated on the negative electrode current collector to obtain a wet film layer. It is understood that the wet film layer contains a solvent. Optionally, the coating method includes but is not limited to at least one of gravure coating, micro-gravure coating, spraying and electrostatic spinning technology.

[0067] The drying process for the wet film layer specifically involves removing the solvent in the wet film layer.

[0068] In some embodiments, the drying process is vacuum high temperature drying, and the temperature of vacuum high temperature drying is 60° C. to 100° C. As an example, the temperature of vacuum high temperature drying is 60° C., 70° C., 80° C., 90° C. or 100° C.

[0069] In a fifth aspect, an embodiment of the present application provides a lithium-ion battery, which includes: an electrode assembly and a housing. A cavity is formed in the housing, and the electrode assembly is arranged in the cavity. The cavity is also loaded with an electrolyte, and the electrolyte infiltrates the electrode assembly. The electrolyte contains lithium hexafluorophosphate. The electrode assembly includes a positive electrode sheet and a negative electrode sheet arranged opposite to each other, and the negative electrode sheet is the above-mentioned negative electrode sheet, or a negative electrode sheet prepared by the preparation method of the above-mentioned negative electrode sheet.

[0070] In some embodiments, in the lithium ion battery, the molar ratio of lithium hexafluorophosphate to aluminum oxide is 1:(0.005-0.15). If the molar ratio of lithium hexafluorophosphate to aluminum oxide is too large, that is, the content of aluminum oxide in the negative electrode material is too low, the amount of lithium difluorophosphate generated by the reaction in the lithium ion battery will be reduced, but if the molar ratio of lithium hexafluorophosphate to aluminum oxide is too small, that is, the content of aluminum oxide in the negative electrode material is too high, the consumption of lithium hexafluorophosphate will increase. As an example, in a lithium ion battery, the molar ratio of lithium hexafluorophosphate to aluminum oxide is 1:0.005, 1:0.006, 1:0.007, 1:0.008, 1:0.009, 1:0.01, 1:0.02, 1:0.03, 1:0.04, 1:0.05, 1:0.06, 1:0.07, 1:0.08, 1:0.09, 1:0.10, 1:0.11, 1:0.12, 1:0.13, 1:0.14 or 1:0.15.

[0071] In some embodiments, the electrolyte further comprises an organic solvent, in which the lithium hexafluorophosphate is dissolved. The organic solvent comprises at least one of ethylene carbonate (EC), methyl propyl carbonate (MSDS), propylene carbonate (PC), butylene carbonate (CBC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC).

[0072] In some embodiments, the electrode assembly further includes a separator, which is disposed between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet to prevent the positive electrode sheet from contacting the negative electrode sheet and causing a short circuit. As an example, the separator includes at least one of a polyethylene (PE) film and a polypropylene (PP) film.

[0073] In some embodiments, the positive electrode plate includes a positive current collector and a positive film layer, and the positive film layer is disposed on the positive current collector. As an example, the positive electrode film layer contains a positive active material. As an example, the positive electrode active material includes at least one of lithium nickel cobalt manganese oxide (NMC), lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4, LFP), lithium manganese oxide (LiMn2O4, LMO) and lithium nickel cobalt aluminum oxide (NCA). Optionally, the positive electrode film layer also includes a conductive agent, which includes at least one of carbon black, conductive graphite, graphene and carbon nanotubes. Optionally, the positive electrode film layer also includes a binder, which includes at least one of organic polymers such as polyacrylic acid (PAA) and polyvinylidene fluoride (PVDF).

[0074] In some embodiments, the positive electrode current collector is an aluminum foil, and the thickness of the aluminum foil is 6 μm to 15 μm.

[0075] In a sixth aspect, an embodiment of the present application provides a method for preparing a lithium-ion battery, comprising:

[0076] S31, providing an electrode assembly, the electrode assembly comprising a positive electrode sheet and a negative electrode sheet arranged opposite to each other, the negative electrode sheet being the above-mentioned negative electrode sheet, or a negative electrode sheet prepared by the above-mentioned method for preparing the negative electrode sheet;

[0077] S32, providing a housing, wherein a cavity is formed in the housing, and the electrode assembly is placed in the cavity;

[0078] S33, injecting electrolyte into the cavity so that the electrolyte infiltrates the electrode assembly to obtain a lithium-ion battery, wherein the electrolyte contains lithium hexafluorophosphate.

[0079] The preparation method of the lithium-ion battery provided in the embodiment of the present application is simple to operate, easy to implement, and has no special environmental control requirements.

[0080] In some embodiments, the method for preparing a lithium ion battery further comprises:

[0081] S34, the lithium ion battery is placed in a static state. The reaction rate of lithium hexafluorophosphate and aluminum oxide is relatively slow. By placing the lithium ion battery obtained after the injection in a static state, the lithium hexafluorophosphate can react with the effective aluminum oxide during the static state to generate a sufficient amount of lithium difluorophosphate.

[0082] In some embodiments, the standing temperature of the standing treatment is 40°C to 80°C. The reaction rate of lithium hexafluorophosphate and aluminum oxide can be accelerated by increasing the standing temperature, but the standing temperature should not be too high, otherwise it will affect the stability of other structures or components in the lithium ion battery. For example, the stability of lithium hexafluorophosphate decreases at high temperatures, and it is easy to decompose. As an example, the standing temperature is 40°C, 50°C, 60°C, 70°C or 80°C.

[0083] In some embodiments, the standing time of the standing treatment is 12 hours to 96 hours. By controlling the standing time, it is possible to ensure that lithium hexafluorophosphate and aluminum oxide fully react. Generally, if the standing temperature of the standing treatment increases, the standing time can be shortened, thereby accelerating production efficiency. As an example, the standing time is 12 hours, 20 hours, 30 hours, 40 hours, 50 hours, 60 hours, 70 hours, 80 hours, 90 hours, or 96 hours.

[0084] In some embodiments, the method for preparing a lithium ion battery further comprises:

[0085] S35, after the static treatment, performing a formation treatment on the lithium-ion battery.

[0086] During the formation process, the lithium difluorophosphate on the surface of the negative electrode active material will be reduced and decomposed and combined with aluminum fluoride to form a relatively stable composite solid electrolyte membrane, which can not only inhibit the reduction and decomposition of the electrolyte on the negative electrode surface during the cycle, but also improve the ionic conductivity of the solid electrolyte membrane at the interface between the negative electrode active material and the electrolyte, effectively reduce the electrode / electrolyte interface impedance, and improve the battery's rate performance and cycle stability.

[0087] In some embodiments, the lithium ion battery is a liquid battery.

[0088] The following is a description with reference to specific embodiments.

[0089] Example 1

[0090] 1. Preparation of negative electrode materials:

[0091] The negative electrode active material graphite and alumina (both are solid particles) are mixed at a mass ratio M of 1:0.01 and then ball-milled for 5 hours at a rotation speed of 100 r / min, so that the surface of the negative electrode active material is evenly coated with alumina, and the volume of solid particles with a particle size of 20 nm in the alumina accounts for 50 vol%, to obtain a negative electrode material.

[0092] 2. Preparation of negative electrode sheet:

[0093] The prepared negative electrode material, binder SBR, thickener CMC, and conductive agent SP are added to the solvent deionized water, stirred and dispersed evenly to obtain negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector copper foil, and vacuum high-temperature drying is performed to obtain a negative electrode film layer bonded to the negative electrode current collector, and then a negative electrode sheet is obtained. Among them, the mass ratio of negative electrode material, binder, thickener and conductive agent in the negative electrode film layer of the negative electrode sheet is 96:2.0:1.3:0.7, and the vacuum high-temperature drying temperature is 90°C and the time is 12h.

[0094] 3. Lithium-ion battery preparation:

[0095] The prepared negative electrode sheet and the corresponding positive electrode sheet and the separator are stacked in the order of positive electrode sheet-separator-negative electrode sheet, so that the separator is between the positive electrode sheet and the negative electrode sheet to play an isolating role, and a bare battery cell is obtained. The bare battery cell is placed in a shell, and a fixed amount of prepared electrolyte (1.2M LiPF6, the solvent includes EC, DMC and EMC, wherein the volume ratio is EC:DMC:EMC=4:3:3) is injected and then packaged to obtain a lithium-ion battery after injection. The separator is a PE film, and the composition of the positive electrode sheet is LFP:PVDF:CNT:SP=97:1.8:0.5:0.7;

[0096] The lithium-ion battery after injection is placed at high temperature, the temperature of the high temperature placement is 60° C., and the placement time is 48 hours, so that the aluminum oxide on the surface of the negative electrode active material reacts with the lithium hexafluorophosphate in the electrolyte, and a layer of lithium difluorophosphate and aluminum fluoride coating is formed on the surface of the negative electrode active material, wherein the molar ratio N of lithium hexafluorophosphate to aluminum oxide is 1:0.05;

[0097] After standing at high temperature, the lithium-ion battery is then formed and divided into different volumes to obtain the final product.

[0098] Example 2

[0099] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0.001, and the molar ratio N of lithium hexafluorophosphate to aluminum oxide in the lithium ion battery is 1:0.005.

[0100] The rest is the same as Example 1.

[0101] Example 3

[0102] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0.005, and the molar ratio N of lithium hexafluorophosphate to aluminum oxide in the lithium ion battery is 1:0.025.

[0103] The rest is the same as Example 1.

[0104] Example 4

[0105] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0.008, and the molar ratio N of lithium hexafluorophosphate to aluminum oxide in the lithium ion battery is 1:0.04.

[0106] The rest is the same as Example 1.

[0107] Example 5

[0108] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0.012, and the molar ratio N of lithium hexafluorophosphate to aluminum oxide in the lithium ion battery is 1:0.06.

[0109] The rest is the same as Example 1.

[0110] Example 6

[0111] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite and aluminum oxide is 1:0.03, and the molar ratio N of lithium hexafluorophosphate and aluminum oxide in the lithium ion battery is 1:0.15.

[0112] The rest is the same as Example 1.

[0113] Example 7

[0114] The only difference from Example 4 is that when preparing the negative electrode material, silicon is used instead of graphite as the negative electrode active material.

[0115] The rest is the same as Example 4.

[0116] Example 8

[0117] The only difference from Example 5 is that silicon is used instead of graphite as the negative electrode active material during the preparation of the negative electrode material.

[0118] The rest is the same as Example 5.

[0119] Example 9

[0120] The only difference from Example 1 is that during the preparation of the lithium-ion battery, the high-temperature static temperature is 40°C.

[0121] The rest is the same as Example 1.

[0122] Example 10

[0123] The only difference from Example 1 is that during the preparation of the lithium-ion battery, the high-temperature static temperature is 80°C.

[0124] The rest is the same as Example 1.

[0125] Embodiment 11

[0126] The only difference from Example 1 is that the volume proportion of solid particles with a particle size of 10 nm in the aluminum oxide is 50 vol%.

[0127] The rest is the same as Example 1.

[0128] Example 12

[0129] The only difference from Example 1 is that the volume proportion of solid particles with a particle size of 30 nm in the aluminum oxide is 50 vol%.

[0130] The rest is the same as Example 1.

[0131] Embodiment 13

[0132] The only difference from Example 1 is that the volume proportion of solid particles with a particle size of 20 nm in the aluminum oxide is 20 vol%.

[0133] The rest is the same as Example 1.

[0134] Embodiment 14

[0135] The only difference from Example 1 is that the volume proportion of solid particles with a particle size of 20 nm in the aluminum oxide is 90 vol%.

[0136] The rest is the same as Example 1.

[0137] Comparative Example 1

[0138] The only difference from Example 1 is that when preparing the negative electrode material, no alumina is added, that is, the mass ratio M of the negative electrode active material graphite and alumina is 1:0. In this way, when preparing the lithium ion battery, the molar ratio N of lithium hexafluorophosphate in the electrolyte and alumina in the negative electrode is also 1:0.

[0139] Comparative Example 2

[0140] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite to aluminum oxide is 1:0.0005, and the molar ratio N of lithium hexafluorophosphate to aluminum oxide in the lithium ion battery is 1:0.0025.

[0141] The rest is the same as Example 1.

[0142] Comparative Example 3

[0143] The only difference from Example 1 is that when preparing the negative electrode material, the mass ratio M of the negative electrode active material graphite and aluminum oxide is 1:0.04, and the molar ratio N of lithium hexafluorophosphate and aluminum oxide in the lithium ion battery is 1:0.20.

[0144] The rest is the same as Example 1.

[0145] Comparative Example 4

[0146] The only difference from Example 1 is that when preparing the negative electrode material, silicon is used instead of graphite, and no aluminum oxide is added, that is, the mass ratio M of the negative electrode active material silicon and aluminum oxide is 1:0. In this way, when preparing the lithium-ion battery, the molar ratio N of lithium hexafluorophosphate in the electrolyte and aluminum oxide in the negative electrode is also 1:0.

[0147] Table 1

[0148]

[0149]

[0150] The following tests were performed on each embodiment and comparison, and the test results are recorded in Table 2:

[0151] 1. Test the content of aluminum in the negative electrode film layer of the negative electrode plate. The test method includes:

[0152] Weigh 0.5g of the negative electrode film sample and grind it into fine powder, put it into a microwave digestion tank, add 3ml nitric acid and 9ml hydrochloric acid, heat it to 200℃ and digest it for 30min, filter it after cooling, transfer the filtrate to a 100ml volumetric flask, and dilute it to the mark line with deionized water. Introduce the prepared solution into an inductively coupled plasma spectrometer (ICP spectrometer for short) for analysis, record the detection intensity or percentage content, use the standard curve to convert the signal intensity of the sample into element concentration, and calculate the content of aluminum in the sample.

[0153] 2. Perform ion chromatography test on the negative electrode of the lithium-ion battery after high temperature standing and before formation. The test method includes:

[0154] Take a 2cm*2cm negative electrode sheet after high temperature standing and place it in 10ml acetonitrile to soak for 4h, filter the soaking liquid, take 1ml of the above filtrate and dilute it 10 times, take 5ml of the dilution liquid for ion chromatography test, and obtain the peak intensity of the characteristic peak of 7min to 9min (the characteristic peak represents lithium difluorophosphate). Among them, the detection conditions of ion chromatography are as follows: the chromatographic column is an anion chromatographic column, the mobile phase is a 3.6mmol / L sodium carbonate solution (the solvent is an acetonitrile aqueous solution with a volume concentration of 40%), and the detector is a conductivity detector.

[0155] 3. Test the 3C rate discharge capacity retention rate of lithium-ion batteries. The test methods include:

[0156] The lithium-ion battery was charged to 100% SOC (State of Charge) at 0.1C at 25°C, then discharged to 0% SOC at 0.1C, and the discharge capacity C1 was recorded. Then, the battery was charged to 100% SOC at 0.1C, and then discharged to 0% SOC at 3C, and the discharge capacity C2 was recorded. 3C rate discharge capacity retention rate = C2 / C1.

[0157] 4. Test the cycle performance of lithium-ion batteries. The test methods include:

[0158] The lithium ion battery was charged and discharged 500 times at a constant current rate of 1.0 C (nominal capacity) at 45° C. Capacity retention rate after 500 cycles (C%) = (discharge capacity at the 500th cycle / initial discharge capacity) × 100%.

[0159] Table 2

[0160]

[0161] Figure 1 The ion chromatogram of the negative electrode sheet of the lithium-ion battery provided in Example 1 before formation after high temperature standing, from Figure 1 It can be seen that there are obvious characteristic peaks at the position of 7min to 9min in the spectrum, which indicates that lithium difluorophosphate exists in the negative electrode plate of the lithium-ion battery. Since lithium difluorophosphate is not added in the process of preparing the lithium-ion battery, the lithium difluorophosphate is mainly generated by the reaction of aluminum oxide in the negative electrode plate and lithium hexafluorophosphate in the electrolyte when the lithium-ion battery is standing at high temperature.

[0162] It can be seen from Table 1 that as the mass ratio of the negative electrode active material to alumina in the negative electrode material decreases, that is, the content of alumina in the negative electrode material increases (comparative example 1, comparative example 2, examples 2 to 4, example 1, examples 5 to 6, comparative example 3), the content of aluminum in the negative electrode film layer of the negative electrode plate increases. Since the injection amount of the electrolyte in each example is the same when preparing the lithium ion battery, the molar ratio of lithium hexafluorophosphate to alumina in the lithium ion battery also decreases, that is, the amount of alumina increases. After the lithium ion battery is left to stand at high temperature, the concentration of lithium difluorophosphate in the negative electrode plate increases. However, the 3C rate discharge capacity retention rate and the capacity retention rate after 500 cycles of the lithium ion battery first increase and then decrease. This is because during the first charging process, lithium difluorophosphate will be reduced and decomposed and composited with aluminum fluoride to form a relatively A stable composite solid electrolyte membrane, as the mass ratio of the negative electrode active material to alumina decreases and the concentration of lithium difluorophosphate increases, the thickness and density of the composite solid electrolyte membrane are improved, thereby improving the stability of the composite solid electrolyte membrane, so that the composite solid electrolyte membrane can effectively inhibit the reduction and decomposition of the electrolyte on the negative electrode surface during the cycle process, thereby improving the cycle performance of the lithium-ion battery; in addition, the ionic conductivity of the solid electrolyte membrane will also be improved, thereby reducing the electrode / electrolyte interface impedance and improving the rate performance of the lithium-ion battery; however, if the mass ratio of the negative electrode active material to alumina is too small, the thickness of the solid electrolyte membrane will be too large, which will increase the electrode / electrolyte interface impedance and reduce the rate performance of the lithium-ion battery, and if the content of alumina is too large, the lithium hexafluorophosphate in the electrolyte will be excessively consumed, which will in turn reduce the cycle performance of the lithium-ion battery.

[0163] By comparing Comparative Example 4, Example 7 and Example 8, it can be seen that when the negative electrode active material is silicon, the use of alumina to coat silicon can also improve the cycle performance and rate performance of the lithium-ion battery; however, compared with when the negative electrode active material is graphite, the cycle performance and rate performance of the ion battery are slightly worse. This is because silicon is easier to expand than graphite, thereby affecting the performance of the ion battery.

[0164] By comparing Example 9, Example 1 and Example 10, it can be seen that with the increase of the standing temperature during high-temperature standing, the concentration of lithium difluorophosphate first increases and then decreases, and the 3C rate discharge capacity retention rate of the lithium ion battery and the capacity retention rate after 500 cycles also show that they first increase and then decrease. This is because, with the increase of the standing temperature, the reaction of aluminum oxide and lithium hexafluorophosphate to generate lithium difluorophosphate can be accelerated. However, too high a standing temperature will cause the lithium hexafluorophosphate to be unstable, thereby affecting the amount of lithium difluorophosphate generated by the reaction of aluminum oxide and lithium hexafluorophosphate, and ultimately affecting the performance of the lithium ion battery.

[0165] By comparing Example 11, Example 1 and Example 12, it can be seen that the content of aluminum element in the negative electrode film layer of the negative electrode plate is relatively close, but with the increase of the particle size of aluminum oxide, the concentration of lithium difluorophosphate in the negative electrode plate decreases successively after the lithium-ion battery is left to stand at high temperature. This is because with the increase of the particle size of aluminum oxide, the specific surface area of ​​aluminum oxide decreases, which leads to a decrease in the probability of contact between aluminum oxide molecules and lithium hexafluorophosphate molecules, and then a decrease in the amount of lithium difluorophosphate generated, and ultimately the rate performance and cycle performance of the lithium-ion battery decrease.

[0166] By comparing Example 13, Example 1 and Example 14, it can be seen that the content of aluminum element in the negative electrode film layer of the negative electrode plate is relatively close, but as the volume fraction of aluminum oxide with a particle size of 20 nm increases, the concentration of lithium difluorophosphate in the negative electrode plate of the lithium ion battery increases successively after high-temperature standing. This is because when the particle size of aluminum oxide is 20 nm, a sufficiently large gap can be formed between the aluminum oxide particles to ensure that the electrolyte containing lithium hexafluorophosphate effectively penetrates and infiltrates the aluminum oxide, and the specific surface area of ​​the aluminum oxide particles of this particle size is large enough. The dual effects jointly increase the probability of aluminum oxide molecules contacting with lithium hexafluorophosphate molecules, and as the content of aluminum oxide particles with a particle size of 20 nm increases, the probability of aluminum oxide molecules contacting with lithium hexafluorophosphate molecules further increases, thereby increasing the amount of lithium difluorophosphate generated, and ultimately improving the rate performance and cycle performance of the lithium ion battery.

[0167] The embodiments of the present application are introduced in detail above. Specific examples are used in this article to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea. At the same time, for technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A negative electrode material for a lithium ion battery, wherein the lithium ion battery comprises lithium hexafluorophosphate, characterized in that: The negative electrode material comprises a negative electrode active material and aluminum oxide, wherein the aluminum oxide is attached to the surface of the negative electrode active material; in the negative electrode material, the mass ratio of the negative electrode active material to the aluminum oxide is 1:(0.001-0.03).

2. The negative electrode material according to claim 1, characterized in that In the aluminum oxide, the volume fraction of aluminum oxide particles with a particle size of 10 nm to 30 nm is greater than or equal to 20 vol%; and / or the negative electrode active material includes at least one of graphite, soft carbon, hard carbon, silicon and silicon oxide.

3. A method for preparing a negative electrode material, for preparing the negative electrode material as claimed in claim 1 or 2, characterized in that: include: A negative electrode active material is mixed with aluminum oxide to obtain a mixture, and the mixture is ball-milled so that the aluminum oxide adheres to the surface of the negative electrode active material to obtain a negative electrode material; optionally, the ball milling time is 3h to 10h, and the rotation speed is 80r / min to 150r / min.

4. A negative electrode plate, characterized in that: It comprises a negative electrode current collector and a negative electrode film layer arranged on the negative electrode current collector, wherein the negative electrode film layer comprises the negative electrode material as claimed in claim 1 or 2, or the negative electrode material prepared by the method for preparing the negative electrode material as claimed in claim 3.

5. The negative electrode sheet according to claim 4, characterized in that: The content of aluminum element in the negative electrode film layer is 1800ppm~56000ppm; and / or, the negative electrode film layer also includes a binder, a conductive agent and a thickener, and in the negative electrode film layer, the mass ratio of the negative electrode material, the binder, the conductive agent and the thickener is (95~98): (0.8~2.5): (0.5~3.5): (0.7~2.0).

6. A method for preparing a negative electrode sheet, used for preparing the negative electrode sheet as claimed in claim 4 or 5, characterized in that: include: A negative electrode slurry is provided, wherein the negative electrode slurry includes a negative electrode material and a solvent; the negative electrode slurry is subjected to a film-forming treatment on a negative electrode current collector to obtain a wet film layer; the wet film layer is subjected to a drying treatment to obtain a negative electrode film layer; optionally, the drying treatment is vacuum high-temperature drying, and the temperature of the vacuum high-temperature drying is 60° C. to 100° C.

7. A lithium ion battery, characterized in that: include: An electrode assembly and a shell, wherein a cavity is formed in the shell, the electrode assembly is arranged in the cavity, the cavity is also loaded with an electrolyte, the electrolyte infiltrates the electrode assembly, the electrolyte contains lithium hexafluorophosphate, the electrode assembly includes a positive electrode sheet and a negative electrode sheet arranged oppositely, the negative electrode sheet is the negative electrode sheet as described in claim 4 or 5, or the negative electrode sheet prepared by the method for preparing the negative electrode sheet as described in claim 6.

8. The lithium-ion battery according to claim 7, characterized in that: In the lithium-ion battery, the molar ratio of the lithium hexafluorophosphate to the aluminum oxide is 1:(0.005-0.15); and / or the electrolyte further comprises an organic solvent, and the organic solvent comprises at least one of ethylene carbonate, methyl propyl carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

9. A method for preparing a lithium ion battery, characterized in that: include: An electrode assembly is provided, wherein the electrode assembly comprises a positive electrode sheet and a negative electrode sheet arranged opposite to each other, wherein the negative electrode sheet is the negative electrode sheet as claimed in claim 4 or 5, or the negative electrode sheet prepared by the method for preparing the negative electrode sheet as claimed in claim 6; Providing a housing, wherein a cavity is formed in the housing, and the electrode assembly is placed in the cavity; An electrolyte is injected into the cavity so that the electrolyte infiltrates the electrode assembly to obtain a lithium-ion battery, wherein the electrolyte contains lithium hexafluorophosphate.

10. The method for preparing a lithium ion battery according to claim 9, characterized in that: The method for preparing the lithium-ion battery further includes: performing a standing treatment on the lithium-ion battery; optionally, the standing temperature of the standing treatment is 40° C. to 80° C., and / or the standing time of the standing treatment is 12 h to 96 h; optionally, the method for preparing the lithium-ion battery further includes: performing a formation treatment on the lithium-ion battery after the standing treatment.

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