Sodium-ion battery and electric device

Abstract

The application discloses a kind of sodium ion battery, electric device;Belong to battery technical field.The sodium ion battery provided in the application includes positive pole sheet, negative pole sheet, diaphragm and electrolyte, the positive pole sheet includes positive pole current collector and the positive pole active material layer being arranged on at least one surface of positive pole current collector, the surface of the positive pole active material layer away from current collector side is provided with protective layer;The positive pole active material layer includes positive pole active material, the positive pole active material includes layered oxide core, and cladding layer is arranged on the outer surface of layered oxide core, the cladding layer includes modified carbon nanotube, the contact angle of the modified carbon nanotube is ≥90 °;The protective layer includes fluorine-containing carbon nanotube, in the fluorine-containing carbon nanotube, the mass percentage of fluorine element is ≥5%.The sodium ion battery obtained by using the technical scheme of the application has excellent storage stability, rate performance and cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a sodium-ion battery and an electrical device. Background Technology

[0002] In the field of energy storage, sodium-ion batteries have gradually become a research and application hotspot due to their advantages such as abundant sodium resources and relatively low cost. However, sodium-ion battery electrodes face a significant challenge in practical applications: the high-nickel layered oxides currently used have limited Ni content on the material surface. 3+ It will spontaneously transform into Ni 2+ And release O 2- When the material is exposed to air, it reacts with CO2 and H2O in the air to generate Na2CO3 and NaOH. NaOH will react with the binder PVDF and the electrolyte NaPF6, affecting the battery processing technology and electrochemical performance. The formed Na2CO3 layer will hinder the diffusion of Na. At the same time, since the reaction consumes a lot of Na in the material, it will cause the material's capacity decay and poor kinetic performance.

[0003] The aforementioned challenges can cause sodium-ion battery positive electrode sheets to easily fail when stored in air, affecting the performance of subsequent sodium-ion batteries. Currently, the method used in production to store electrodes is to reduce the moisture content in the air, which significantly increases production costs and risks. Summary of the Invention

[0004] The purpose of this application is to solve the problems of short storage time and poor cycle performance of existing sodium-ion battery positive electrode sheets in practical applications, and to provide a sodium-ion battery and power device with excellent storage performance, rate performance and cycle performance.

[0005] To achieve the above objectives, a first aspect of this application provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. A protective layer is disposed on the surface of the positive active material layer away from the current collector. The positive active material layer includes a positive active material comprising a layered oxide core and a coating layer disposed on the outer surface of the layered oxide core. The coating layer includes modified carbon nanotubes, and the contact angle of the modified carbon nanotubes is ≥90°. The protective layer includes fluorinated carbon nanotubes, wherein the mass percentage of fluorine in the fluorinated carbon nanotubes is ≥5%.

[0006] As an embodiment of this application, the contact angle of the modified carbon nanotube is 90-100°.

[0007] As an embodiment of this application, the fluorine element in the fluorocarbon nanotube has a mass percentage of 5-30%.

[0008] As an embodiment of this application, the positive electrode sheet satisfies: 0.5≤M≤8, where M=A / B; Anm is the thickness of the coating layer, and B nm is the thickness of the protective layer.

[0009] As an embodiment of this application, the thickness A of the coating layer is 50-100 nm.

[0010] As an embodiment of this application, the thickness B of the protective layer is 50-100 nm.

[0011] As an embodiment of this application, the ratio of the thickness of the positive electrode active material layer to the thickness of the protective layer is 1000:(1~2).

[0012] As an embodiment of this application, the layered oxide core includes Na z Ni x M y O2, wherein 0.1 < x ≤ 1, x + y = 1, 0.5 ≤ z ≤ 1; M includes at least one of iron and manganese.

[0013] As an embodiment of this application, the modified carbon nanotubes include at least one of C12-C18 alkyl, C6-C10 aryl, C3-C6 perfluoroalkyl, C2-C4 ester, and C1-C2 haloalkyl.

[0014] As an embodiment of this application, the fluorocarbon nanotubes include at least one of C1 to C10 fluoroalkyl groups.

[0015] As an embodiment of this application, the positive electrode active material layer further includes a conductive agent and a binder.

[0016] As an embodiment of this application, the conductive agent includes at least one of carbon black, graphite, carbon fiber, carbon nanotubes, and graphene.

[0017] As an embodiment of this application, the adhesive includes polyvinylidene fluoride.

[0018] In a second aspect of this application, an electrical device is provided, including the aforementioned sodium-ion battery.

[0019] Compared with the prior art, the beneficial effects of this application are:

[0020] The sodium-ion battery provided in this application provides a protective layer on the surface of the positive electrode active material layer away from the current collector. The protective layer includes carbon nanotubes containing a specific mass percentage of fluorine, and modified carbon nanotubes with a specific contact angle range covering the core of the layered oxide included in the positive electrode active material layer. Both of these components can improve the storage capacity of the positive electrode in air while reducing the impact on the performance of the positive electrode, thereby improving the storage capacity of the sodium-ion battery and reducing gas generation during storage. Furthermore, the introduction of the above structure can improve the rate performance and cycle performance of the sodium-ion battery. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0023] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0024] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0025] In one embodiment of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. A protective layer is disposed on the surface of the positive active material layer away from the current collector. The positive active material layer includes a positive active material comprising a layered oxide core and a coating layer disposed on the outer surface of the layered oxide core. The coating layer includes modified carbon nanotubes, and the contact angle of the modified carbon nanotubes is ≥90°. The protective layer includes fluorinated carbon nanotubes, wherein the mass percentage of fluorine in the fluorinated carbon nanotubes is ≥5%.

[0026] This application research found that by setting a protective layer on the surface of the positive electrode active material layer away from the current collector, and defining the protective layer as including carbon nanotubes containing a specific mass percentage of fluorine, and modified carbon nanotubes with a specific contact angle range covering the core of the layered oxide included in the positive electrode active material layer, both can improve the hydrophobicity and carbon dioxide repellency of the positive electrode while reducing the impact on the performance of the positive electrode, thereby improving the storage capacity of the positive electrode in air, and further reducing gas generation in the sodium-ion battery during storage, thus improving the storage capacity of the sodium-ion battery; and the introduction of the above structure can improve the rate performance and cycle performance of the sodium-ion battery.

[0027] Specifically, the modified carbon nanotubes within a specific contact angle range not only possess excellent conductivity but also exhibit superior hydrophobicity, reducing the reaction between the positive electrode and water. The protective layer containing specific substances effectively reduces the adsorption of carbon dioxide by the coating layer, preventing carbon dioxide from affecting the positive electrode and thus reducing gas generation during sodium-ion battery storage. Simultaneously, this protective layer can also effectively improve the conductivity of the positive electrode, thereby enhancing the rate performance of the sodium-ion battery.

[0028] It should be noted that the test method for the contact angle of the modified carbon nanotubes is the nanofilm contact angle measurement method.

[0029] It should be noted that the modified carbon nanotubes can also be separated from the surface of the positive electrode active material. Because the modified carbon nanotubes are coated onto the surface of the positive electrode active material, the bonding ability between the modified carbon nanotubes and the positive electrode active material is very strong. The separation method is as follows: collect the positive electrode sheet, scrape off the protective layer on the surface of the positive electrode sheet, collect the active material in the positive electrode active material layer, place it in NMP, dissolve PVDF in an environment of 40-90℃, repeat 3 times, and collect the remaining powder; place the collected powder in NMP, ultrasonically clean it in a 300W-800W ultrasonic cleaner for 2-5 hours, after cleaning, centrifuge it at 5000-8000 rpm for 10-30 minutes, repeat ultrasonication and centrifugation 3-5 times, vacuum dry the collected sample in a vacuum drying oven at 80-120℃ for 10-15 hours, and collect the powder after drying.

[0030] It should be noted that the contact angle measurement method for nanofilms is based on the following principle: the contact angle refers to the angle between the tangent line drawn at the gas-liquid interface at the junction of the gas, liquid, and solid phases and the solid-liquid interface line on the liquid side. A contact angle measuring instrument can be used to test the contact angles of different liquids on different solid surfaces, thereby studying the wettability of different liquids on different solid surfaces, such as spreading, penetration, and absorption. Using the sitting drop method, a droplet is placed on the surface of a solid sample. The shape of the droplet is captured by a camera, and then a specific mathematical model (such as considering the droplet as part of a sphere, ellipse, or cone) is used to fit and calculate the contact angle using specific parameters (such as width and height). Double-sided tape is applied around the edges of a glass plate. The sample is cut into small pieces approximately 2cm x 8cm and then attached to the glass plate, ensuring the sample is as flat as possible and avoiding wrinkles. A syringe is connected to a liquid aspiration device, and approximately 10mL of electrolyte is poured into the cap. The position and height of the sample stage are adjusted, and the plastic bottle containing the electrolyte is placed under the needle, immersing the needle in the electrolyte. Click on the "Control" interface in the software, then click "Fill." After drawing an appropriate amount of electrolyte (approximately 1 / 3 of the syringe height), adjust the height of the needle and sample stage. Place the glass plate on the sample stage, and while pressing the dispensing device to the bottom, adjust the sample stage height so that the bottom of the needle is approximately 12mm from the electrode surface. Move the sample stage so that the area to be tested is below the needle, and prepare the stopwatch. When "Please drop the liquid onto the sample surface" is displayed, click "Record," and immediately press down the dispensing device to drop the liquid from the needle onto the solid surface.

[0031] It should be noted that the method for determining the mass percentage of fluorine in the fluorocarbon nanotubes is surface micro-area elemental detection. The testing principle is as follows: When a sample is bombarded with a focused electron beam of a certain energy in an analytical instrument such as a scanning electron microscope, the bombarded area emits characteristic X-rays of the elements contained in the sample. A semiconductor detector converts and amplifies the received signal, which is then further amplified, processed, and analyzed by a linear amplifier, pulse processor, and multichannel analyzer. This allows for the acquisition of the energy spectrum and intensity values ​​of the characteristic X-rays of each element. By comparing the energy spectrum with that of a standard sample of the corresponding element, and through correction calculations, the quantitative analysis results of the chemical composition of the tested sample can be obtained. Models: Field emission scanning electron microscope (SEM), model Merlin VP Compact; Energy dispersive spectrometer (EDS), model XFlash Detector 6 / 30. Test Method: For EDS powder testing, a sample size of approximately 2g is required; for electrode and separator samples, a size of 1cm x 1cm or larger is required. All samples must be dry and free of gel-like or liquid substances. Samples removed from the battery must be thoroughly dried before testing. EDS File Creation: Open the EDS testing software and input the relevant sample information according to the navigation map. Based on the actual condition of the sample, perform elemental identification using both reconstruction and peak-only methods. After identification, select the peak calibration editor and remove redundant element labels. The spectrum displays the position and height of the X-ray characteristic peaks of the tested elements. The test sample area contains the normalized weight percentage and atomic percentage of the elements.

[0032] For example, the contact angle of the modified carbon nanotube can be any point value or any two-point range value of ≥90°, such as 90~100°, 90~110°, 90~120°, 90~160°, or a range of one or any two of 90°, 100°, 110°, 120°, 130°, 140°, 150°, 160°.

[0033] For example, in the fluorocarbon nanotube, the mass percentage of fluorine can be any point value or any two points within a range of ≥5%, such as 5-30%, 5-20%, 5-50%, etc., or it can be one or any two of 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%.

[0034] In one embodiment, the contact angle of the modified carbon nanotube is 90–100°.

[0035] This study found that the contact angle of modified carbon nanotubes can reflect their hydrophobic properties to a certain extent. Furthermore, when the contact angle of the modified carbon nanotubes is further selected to be 90–100°, without affecting the electrochemical performance of the positive electrode, not only is the hydrophobicity of the resulting positive electrode better, but its interaction with carbon dioxide is also weakened. This achieves both good hydrophobicity and carbon dioxide repellency, improving the storage stability of the positive electrode and consequently enhancing the storage stability of the sodium-ion battery, reducing gas generation during storage, and also improving the cycle performance of the sodium-ion battery.

[0036] For example, the contact angle of the modified carbon nanotube can be any point value or any two-point range value between 90 and 100°, such as one or any two of 90°, 91°, 92°, 93°, 94°, 95°, 96°, 97°, 98°, 99°, and 100°.

[0037] In one embodiment, the contact angle of the modified carbon nanotube is 93–96°.

[0038] In one embodiment, the fluorine content in the fluorocarbon nanotube is 5-30% by mass.

[0039] This study found that the mass percentage of fluorine in fluorocarbon nanotubes affects their electronegativity, thus influencing not only their ability to prevent carbon dioxide from entering but also their conductivity. When the mass percentage of fluorine in the fluorocarbon nanotubes is further selected to be 5-30%, the resulting positive electrode exhibits better storage stability, leading to better storage stability and rate performance in the corresponding sodium-ion battery.

[0040] In one embodiment, the fluorine content in the fluorocarbon nanotube is 10-20% by mass.

[0041] In one embodiment, the positive electrode sheet satisfies: 0.5≤M≤8, where M=A / B; Anm is the thickness of the coating layer, and B nm is the thickness of the protective layer.

[0042] It should be noted that the thickness of the coating layer was obtained by testing using FIB focused ion beam technology; the thickness of the protective layer was obtained by polishing with an argon ion polisher (JEOL, model: IB-19520CCP) and then testing with a scanning electron microscope (Zeiss, model: Merlin VP Compact).

[0043] For example, M can be any point value or any two-point range value between 0.5 and 8, such as a range value of one or any two of 0.5, 1, 2, 3, 4, 5, 6, 7, 8.

[0044] This application research found that when the ratio M of the coating thickness to the protective layer thickness is within the range given in this application, it can ensure that carbon dioxide does not enter the positive electrode and can also prevent carbon dioxide molecules from accumulating, thereby improving the storage performance of the positive electrode and reducing the gas generation during storage in the corresponding sodium-ion battery; at the same time, M within the above range can also provide a suitable distance for ion diffusion, ensuring the high conductivity of the positive electrode and thus ensuring the excellent rate performance of the sodium-ion battery.

[0045] In one embodiment, the positive electrode sheet satisfies: 1 ≤ M ≤ 5. For example, M can be one or any two of the following values: 1, 1.3, 1.5, 1.8, 2, 2.3, 2.5, 2.8, 3, 3.3, 3.5, 3.8, 4, 4.3, 4.5, 4.8, 5. This application research has found that when M is further selected as 1 to 5, the overall performance of the resulting sodium-ion battery is superior.

[0046] In one embodiment, the thickness A of the coating layer is 50–100 nm.

[0047] For example, the thickness A of the coating layer can be any point value or any two-point range value between 50 and 100 nm, such as one or any two of 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, and 100 nm.

[0048] This study found that a coating layer within the 50–100 nm range can achieve a good balance between conductivity, hydrophobicity, and carbon dioxide repellency, resulting in excellent overall performance. Specifically, on the one hand, a coating layer within this thickness range can avoid the problem of poor electrical contact caused by an excessively thin coating layer, which would reduce the rate performance of the sodium-ion battery. It can also avoid the problem of weak hydrophobic and carbon dioxide repellency caused by an excessively thin coating layer, which would affect the storage performance of the positive electrode and the sodium-ion battery. On the other hand, a coating layer within this thickness range can also avoid the problem of an excessively thick coating layer, which would increase the migration distance between ions and the positive electrode active material, thus affecting the ion transport efficiency and consequently the rate performance of the sodium-ion battery.

[0049] In one embodiment, the thickness B of the protective layer is 50–100 nm.

[0050] For example, the thickness A of the protective layer can be any point value or any two-point range value between 50 and 100 nm, such as one or any two of 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, and 100 nm.

[0051] This study found that when the thickness of the protective layer is 50–100 nm, it can effectively prevent carbon dioxide from entering the positive electrode and avoid a significant increase in the ion diffusion distance that would affect the rate performance of sodium-ion batteries.

[0052] In one embodiment, the ratio of the thickness of the positive electrode active material layer to the thickness of the protective layer is 1000:(1-2).

[0053] It should be noted that the ratio of the thickness of the positive electrode active material layer to the thickness of the protective layer is a dimensional ratio. The thickness of the positive electrode active material layer is obtained by polishing with an argon ion polisher (JEOL, model: IB-19520CCP) and then measuring with a scanning electron microscope (Zeiss, model: Merlin VP Compact). For example, the ratio of the thickness of the positive electrode active material layer to the thickness of the protective layer can be any point value or any two-point range between 1000:(1-2), such as one or any two of 1000:1, 1000:1.2, 1000:1.4, 1000:1.6, 1000:1.8, 1000:2.

[0054] This study found that a key mechanism for carbon dioxide barrier function is steric hindrance. A thicker positive electrode layer exhibits stronger adsorption capacity for carbon dioxide from the air, while a thinner layer results in weaker adsorption. A ratio of 1000:(1-2) between the thickness of the positive electrode active material layer and the thickness of the protective layer can achieve both water and carbon dioxide barrier function while ensuring a good and suitable ion transport path. This improves the storage performance of the positive electrode layer, reduces gas generation during sodium-ion battery storage, and enhances the cycle performance and rate performance of the sodium-ion battery.

[0055] In one embodiment, the thickness of the positive electrode active material layer is 50–130 μm.

[0056] For example, the thickness of the positive electrode active material layer can be any point value or any two-point range value between 50 and 130 μm, such as one or any two of 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, and 130 μm.

[0057] This study found that a positive electrode active material layer with a suitable thickness range can achieve better cycle performance and rate performance in sodium-ion batteries.

[0058] In one embodiment, the layered oxide core comprises Na z Ni x M yO2, wherein 0.1 < x ≤ 1, x + y = 1, 0.5 ≤ z ≤ 1; M includes at least one of iron and manganese.

[0059] In one embodiment, the layered oxide core comprises Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2, NaNi 1 / 2 Mn 1 / 2 O2, Na 0.67 Ni 0.33 Mn 0.67 O2, Na 1.1 Mn 0.45 Ni 0.45 O2, NaNi 1 / 2 Fe 1 / 2 O2, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, NaNi 2 / 5 Fe 1 / 10 Mn 1 / At least one of 2O2.

[0060] In one embodiment, 0.6 < x ≤ 1.

[0061] This study found that when the layered oxide core is further selected to include the aforementioned substances, the resulting sodium-ion battery exhibits superior overall performance.

[0062] In one embodiment, the modified carbon nanotubes include at least one of C12-C18 alkyl, C6-C10 aryl, C3-C6 perfluoroalkyl, C2-C4 ester, and C1-C2 haloalkyl.

[0063] It should be noted that the test method for the functional groups in the modified carbon nanotubes is as follows: scrape the surface of the positive electrode sheet and collect the positive electrode active material layer material in the positive electrode sheet, and then characterize the modified functional groups in the coating layer by Raman test.

[0064] This application research found that when the above-mentioned groups are included in the modified carbon nanotubes, the contact angle of the modified carbon nanotubes can be made to be within the range given in this application.

[0065] In one embodiment, the modified carbon nanotubes include C3-C6 perfluoroalkyl groups.

[0066] This study found that fluorine atoms have strong electronegativity. When the carbon nanotubes are further modified to include C3-C6 perfluoroalkyl groups (i.e., modified with C3-C6 perfluoroalkyl groups), the carbon-fluorine bonds in these groups are highly polar, and the electronegativity of the fluorine atoms is high. This causes the electron cloud of the carbon-fluorine bonds to be strongly biased towards the fluorine atoms, resulting in lower surface energy for the perfluoroalkyl groups. Within the range of 3-6 carbon atoms, this carbon-fluorine structure effectively repels water molecules, exhibiting good hydrophobicity. The shorter carbon chain length allows for a more compact arrangement of the perfluoroalkyl groups on the material surface, further enhancing the hydrophobic effect. Furthermore, from a surface microstructure perspective, perfluoroalkyl groups within this carbon atom number range, after modifying the carbon nanotube surface, can form a relatively dense and fine micro-convex structure. This microstructure increases the surface roughness of the modified carbon nanotubes. According to the Cassie-Baxter theory, a rough surface structure enhances the hydrophobicity of the material, increasing the contact angle of water droplets on the material surface, thereby achieving a better hydrophobic effect. This improves the storage stability of the positive electrode, reduces gas generation during the storage process of sodium-ion batteries, and also enhances the rate performance and cycle performance of sodium-ion batteries.

[0067] In one embodiment, the modified carbon nanotubes can be obtained commercially or can be prepared in-house.

[0068] In one embodiment, the modified carbon nanotubes can be prepared by an ultrasonic-assisted method, in which carbon nanotubes, a modifier containing corresponding functional groups, and a solvent are placed in a reaction vessel, ultrasonically treated in an ultrasonic cleaner, and the reaction is promoted by the cavitation effect and mechanical stirring effect of ultrasound. The product is then collected by centrifugation.

[0069] In this invention, the contact angle of the modified carbon nanotubes can be achieved by adjusting the reaction mass ratio of the modifier and the carbon nanotubes or by adjusting the type of modifier.

[0070] In one embodiment, the fluorocarbon nanotubes include at least one of C1 to C10 fluoroalkyl groups.

[0071] It should be noted that the test method for the functional groups in the fluorocarbon nanotubes is as follows: scrape the surface of the positive electrode sheet and collect the protective layer material in the positive electrode sheet, and then characterize the modified functional groups in the protective layer by Raman spectroscopy.

[0072] In one embodiment, the fluorocarbon nanotubes include perfluorooctyl (-C8F) groups. 17 ) and perfluorohexyl (-C6F 13 ).

[0073] This application study found that when fluorocarbon nanotubes are further selected to include perfluorooctyl (-C8F) 17 ) and perfluorohexyl (-C6F 13When fluorocarbon nanotubes are used, the resulting fluorocarbon nanotubes exhibit better carbon dioxide repellency. On one hand, perfluorooctyl and perfluorohexyl nanotubes have high fluorine atom content, and fluorine atoms have strong electronegativity. The presence of a large number of fluorine atoms significantly alters the electron distribution of the molecules, leading to a weakening of the interaction between the fluorocarbon nanotubes and carbon dioxide molecules. On the other hand, the carbon chain length of perfluorooctyl and perfluorohexyl nanotubes is moderate, providing sufficient steric hindrance to hinder the approach and interaction of carbon dioxide molecules; thus improving the storage performance, rate performance, and cycle performance of sodium-ion batteries.

[0074] In one embodiment, the fluorocarbon nanotubes can be obtained commercially or can be prepared in-house.

[0075] In one embodiment, the preparation method of the fluorocarbon nanotubes can be an ultrasonic-assisted method, in which carbon nanotubes, a modifier containing corresponding functional groups, and a solvent are placed in a reaction vessel, ultrasonically treated in an ultrasonic cleaner, and the reaction is promoted by the cavitation effect and mechanical stirring effect of ultrasound, and then the product is collected by centrifugation.

[0076] In one embodiment, the positive electrode active material layer further includes a conductive agent and a binder.

[0077] In one embodiment, the conductive agent includes at least one of carbon black, graphite, carbon fiber, carbon nanotubes, and graphene.

[0078] In one embodiment, the adhesive comprises polyvinylidene fluoride.

[0079] In one embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector; the negative electrode active material layer includes a negative electrode active material. This application does not limit the negative electrode active material; any known negative electrode active material can be used.

[0080] In one embodiment, the electrolyte includes an organic solvent, a sodium salt, and an additive. This application does not limit the organic solvent, sodium salt, and additives in the electrolyte; any known organic solvent, sodium salt, and additive can be used.

[0081] For example, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, and dimethyl carbonate; the sodium salt includes at least one of sodium perchlorate and sodium hexafluorophosphate; and the additive includes at least one of film-forming additives, conductive additives, and flame-retardant additives.

[0082] In one embodiment, the separator of the secondary battery is disposed between the positive and negative electrodes.

[0083] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0084] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0085] Example 1

[0086] This application provides a secondary battery, the preparation method of which includes the following steps:

[0087] (1) Preparation of positive electrode sheet

[0088] 1) Preparation of modified carbon nanotubes:

[0089] S1. Carbon nanotube pretreatment: Weigh a certain amount of raw carbon nanotubes, put them in a crucible, and calcine them in a muffle furnace at 400℃ for 2 hours. After calcination, cool to room temperature, then add the carbon nanotubes to concentrated nitric acid and reflux at 80℃ for 6 hours. After reflux treatment, filter and repeatedly wash the filter residue with deionized water until neutral. Then dry in a vacuum drying oven at 80℃ for 12 hours to obtain pretreated carbon nanotubes for later use.

[0090] S2. Modification reaction: Accurately weigh 0.5g of pretreated carbon nanotubes and add them to 100mL of ethanol. Disperse ultrasonically for 30 minutes to form a uniform suspension. Add 12g of perfluorooctylpropyl alcohol to the suspension. Place the mixture in an ultrasonic cleaner and sonicate at 40℃ for 4 hours.

[0091] S3. Product Separation and Purification: After the ultrasonic treatment in step S2, the reaction solution was centrifuged at 8000 rpm for 15 minutes to remove the supernatant. The precipitate was washed repeatedly with ethanol three times, and finally the washed precipitate was dried in a vacuum drying oven at 80℃ for 12 hours to obtain perfluorooctyl modified carbon nanotubes with a contact angle of 95°.

[0092] 2) Preparation of fluorocarbon nanotubes:

[0093] S1. Carbon nanotube pretreatment: Weigh an appropriate amount of raw carbon nanotubes, place them in a crucible, and calcine them in a muffle furnace at 500℃ for 2.5 hours. After calcination, cool to room temperature, then add the carbon nanotubes to concentrated nitric acid and reflux at 85℃ for 5 hours. After reflux treatment, filter and repeatedly wash the filter residue with deionized water until neutral. Then dry in a vacuum drying oven at 70℃ for 11 hours to obtain pretreated carbon nanotubes for later use.

[0094] S2. Modification reaction: Accurately weigh 20g of pretreated carbon nanotubes and add them to 200mL of ethanol. Disperse the mixture using an ultrasonic cleaner for 25 minutes to form a uniform suspension. Add 15g of zinc perfluorooctanoate to the suspension. Place the mixture in an ultrasonic cleaner and sonicate at 40℃ for 4 hours.

[0095] S3. Product Separation and Purification: After the ultrasonic treatment in step S2, the reaction solution was centrifuged at 7000 rpm for 12 minutes, the precipitate was collected and washed repeatedly with ethanol three times. Finally, the washed precipitate was dried in a vacuum drying oven at 70℃ for 10 hours to obtain perfluorinated zinc-based modified carbon nanotubes. The mass percentage of fluorine in the obtained fluorinated carbon nanotubes was 20%.

[0096] 3) Preparation of positive electrode active material: using Na 2 / 3 Ni 1 / 3 Mn 2 / 3 Using O2 as the core of the positive electrode active material, the positive electrode material and modified carbon nanotubes were ball-milled in a planetary ball mill at a mass ratio of 10:1 at a speed of 400 rpm for 6 hours to obtain a positive electrode active material with a coating thickness of 60 nm.

[0097] 4) The positive electrode active material, conductive agent (Suepr P), and binder (PVDF) prepared in step 3) are mixed in NMP at a mass ratio of 96:1.5:2.5. After mixing, the slurry is uniformly coated on the aluminum foil current collector and dried to obtain a positive electrode active material layer with a thickness of 60 μm. Then, the fluorocarbon nanotubes prepared in step 2) are mixed with PVDF at a mass ratio of 97:3 in NMP solution. After mixing, the mixture is uniformly brushed onto one side of the dried electrode sheet and dried to obtain a protective layer with a thickness of 60 nm.

[0098] (2) Preparation of negative electrode sheet

[0099] Hard carbon anode material, conductive agent acetylene black (Super P), thickener (CMC) and binder (SBR) are mixed evenly in a mass ratio of 93:3:2:2. The mixed slurry is coated on copper foil current collector and dried to obtain a negative electrode sheet.

[0100] (3) Preparation of electrolyte

[0101] Sodium hexafluorophosphate was selected as the sodium salt. A solvent (a mixed solvent of ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio) was prepared, with 3% fluoroethylene carbonate as the additive. The concentration of sodium hexafluorophosphate was 1 mol / L. The weighed sodium hexafluorophosphate was slowly added to the mixed solvent, and stirred until fully dissolved for 2 hours. Then, 3% fluoroethylene carbonate (by volume of the total solvent) was added, and stirring continued for 30 minutes to obtain the electrolyte.

[0102] (4) Preparation of sodium-ion batteries

[0103] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator in the middle of the positive and negative electrode sheets. After winding, hot pressing and shaping, and electrode tab welding, a bare cell is obtained. The bare cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24 hours. The prepared negative electrode material is injected into the dried battery, and the battery is allowed to stand, form, and be capacity tested to complete the preparation of the sodium-ion soft pack battery.

[0104] Examples 2-4

[0105] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation process of the modified carbon nanotubes is adjusted to achieve the parameters in Table 1.

[0106] Examples 5-8

[0107] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation process of fluorocarbon nanotubes is adjusted to achieve the parameters in Table 1.

[0108] Examples 9-15

[0109] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the thickness of the coating layer and the thickness of the protective layer in the preparation of the positive electrode active material are adjusted to achieve the parameters in Table 1.

[0110] Example 16

[0111] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the thickness of the positive electrode active material layer is adjusted to achieve the parameters in Table 1.

[0112] Example 17

[0113] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the core of the positive electrode active material is adjusted to achieve the parameters in Table 1.

[0114] Example 18

[0115] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation of modified carbon nanotubes is adjusted to achieve the parameters in Table 1.

[0116] Example 19

[0117] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation of fluorocarbon nanotubes is adjusted to achieve the parameters in Table 1.

[0118] Example 20

[0119] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation of fluorocarbon nanotubes is adjusted to achieve the parameters in Table 1.

[0120] Example 21

[0121] This application provides a sodium-ion battery. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation of fluorocarbon nanotubes is adjusted to achieve the parameters in Table 1.

[0122] Comparative Example 1

[0123] This application provides a sodium-ion battery as a comparative example. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that the preparation process of the modified carbon nanotubes is adjusted to achieve the parameters in Table 1.

[0124] Comparative Example 2

[0125] This application provides a sodium-ion battery as a comparative example. The difference between the preparation method of the sodium-ion battery and Example 1 is that the preparation process of fluorocarbon nanotubes is adjusted to achieve the parameters in Table 1.

[0126] Comparative Example 3

[0127] This application provides a sodium-ion battery in a comparative example. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that no coating layer is provided.

[0128] Comparative Example 4

[0129] This application provides a sodium-ion battery in a comparative example. The difference between the preparation method of the sodium-ion battery and that of Example 1 is that no protective layer is provided.

[0130] The contact angle and modified groups of the modified carbon nanotubes in the coating layer, the mass percentage of fluorine and modified groups of the fluorinated carbon nanotubes in the protective layer, the thickness A of the coating layer, the thickness B of the protective layer, the value M of A / B, the ratio N of the thickness of the positive electrode active material layer to the thickness of the protective layer, and the layered oxide core in the examples and comparative examples are shown in Table 1.

[0131] Table 1. Parameters in sodium-ion batteries

[0132]

[0133]

[0134] The performance testing of the sodium-ion batteries prepared in the examples and comparative examples includes the following aspects:

[0135] (1) Storage stability: At 25°C, the soft-pack batteries obtained in each embodiment and comparative example were charged to 3.95V at a charging rate of 0.33C, left to stand for 5 minutes, and then discharged to 1.5V at a discharging rate of 0.33C. This process was repeated three times, followed by charging at a charging rate of 0.33C for 90 minutes to adjust to 50% SOC. At 50% SOC, the pressure after 30 days of storage was tested, in kPa, and recorded in Table 2.

[0136] (2) Rate performance: At 25°C, the soft-pack batteries obtained in each embodiment and comparative example were charged to 3.95V at a charging rate of 0.33C, left to stand for 5 minutes, and then discharged to 1.5V at a discharging rate of 0.33C. This process was repeated three times and the average value was taken. The soft-pack batteries were then charged to 3.95V at a charging rate of 0.33C, left to stand for 5 minutes, and then discharged to 1.5V at a discharging rate of 5C. This process was repeated three times and the average value was taken. The capacity retention rate at 5C rate was obtained and recorded in Table 2.

[0137] (3) Cyclic performance: The soft-pack batteries obtained in each embodiment and comparative example are charged to 3.95V at a charging rate of 1C, left to stand for 5 minutes, and then discharged to 1.5V at a discharging rate of 1C. The cycle is repeated 1000 times. The capacity retention rate after 1000 cycles can be obtained and recorded in Table 2.

[0138] Table 2 Performance test results of sodium-ion batteries

[0139]

[0140]

[0141] As can be seen from Table 2, when the technical solution provided in this application is adopted, the obtained sodium-ion battery has excellent storage stability, rate performance and cycle performance; specifically, the obtained sodium-ion battery has a pressure of less than 0.22 kPa after 30 days of storage at 50% SOC, a capacity retention rate of more than 85% at 5C rate, and a capacity retention rate of more than 81% after 1000 cycles.

[0142] As can be seen from Examples 1-18 and Comparative Example 1, when the contact angle of the modified carbon nanotubes used in the positive electrode active material coating layer is outside the range given in this application, the storage stability of the obtained sodium-ion battery deteriorates significantly. The main reason is that when the contact angle is outside the range, the positive electrode active material is hydrophilic and will absorb more moisture from the air, and the increase in moisture will lead to an increase in gas production. As can be seen from Examples 1-18 and Comparative Example 2, when the mass percentage of fluorine in the fluorinated carbon nanotubes used in the protective layer is outside the range given in this application, the storage stability of the obtained sodium-ion battery deteriorates significantly. The main reason is that when the fluorine content in the fluorinated carbon nanotubes is outside the range given in this application, the ability of the perfluoroalkyl group to isolate CO2 is weakened, and CO2 will react with the material to generate CO3. 2- This leads to increased gas production, thus affecting the electrochemical performance of sodium-ion batteries. As seen in Examples 1-18 and Comparative Example 3, when no coating layer is introduced into the positive electrode active material, the storage stability, rate performance, and cycle performance of the resulting sodium-ion batteries all deteriorate significantly. This is mainly because the coating layer enhances the electronic conductivity and hydrophobicity of the material. Electronic conductivity is strongly correlated with cycle and rate performance, while hydrophobicity is strongly correlated with storage stability. Without a coating layer, both electronic conductivity and hydrophobicity deteriorate significantly, resulting in a decrease in the storage stability, rate performance, and cycle performance of the sodium-ion battery. As seen in Examples 1-18 and Comparative Example 4, when no protective layer is introduced, the storage stability of the resulting sodium-ion battery deteriorates significantly. This is mainly because the protective layer's primary function is to isolate CO2 from the air and mitigate CO3 generation from the material. 2- This achieves the purpose of suppressing gas production. Therefore, removing the protective layer will cause more CO3 to be generated on the surface of the positive electrode active material. 2- This leads to a deterioration in storage performance.

[0143] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A sodium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, characterized by, The positive electrode tab comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and a protective layer arranged on the surface of the positive electrode active material layer away from the current collector side; The positive electrode active material layer comprises a positive electrode active material, the positive electrode active material comprises a layered oxide core and a cladding layer arranged on the outer surface of the layered oxide core, and the cladding layer comprises modified carbon nanotubes with a contact angle ≥90°. The protective layer comprises fluorine-containing carbon nanotubes, and the fluorine-containing carbon nanotubes have a mass percentage of fluorine element of 20%-30%.

2. The sodium-ion battery of claim 1, wherein, The contact angle of the modified carbon nanotubes is 90-100°.

3. The sodium-ion battery of claim 1, wherein, The positive electrode tab satisfies 0.5≤M≤8, wherein M=A / B; A nm is the thickness of the cladding layer, B nm is the thickness of the protective layer.

4. The sodium-ion battery of claim 1, wherein, At least one of the following conditions is satisfied: a. The thickness A of the cladding layer is 50-100 nm; b. The thickness B of the protective layer is 50-100 nm.

5. The sodium-ion battery of claim 1, wherein, The ratio of the thickness of the positive electrode active material layer to the thickness of the protective layer is 1000:(1-2).

6. The sodium-ion battery of claim 1, wherein, The layered oxide inner core comprises Na z Ni x M y O2, wherein 0.1 7. The sodium-ion battery of claim 1, wherein, The modified carbon nanotubes comprise at least one of C12-C18 alkyl, C6-C10 aryl, C3-C6 perfluoroalkyl, C2-C4 ester group, and C1-C2 halogenated alkyl.

8. The sodium-ion battery of claim 1, wherein, The fluorine-containing carbon nanotubes comprise at least one of C1-C10 fluorinated alkyl.

9. The sodium-ion battery of claim 1, wherein, The positive electrode active material layer further comprises a conductive agent and a binder.

10. The sodium-ion battery of claim 9, wherein, The conductive agent comprises at least one of carbon black, graphite, carbon fiber, carbon nanotube, and graphene; And / or, the binder comprises polyvinylidene fluoride.

11. An electrical device, characterized by The sodium-ion battery comprises the positive electrode tab as claimed in any one of claims 1-10.

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