A secondary battery and an electric device

Abstract

The application belongs to the technical field of new energy materials, and particularly relates to a secondary battery and an electric device. Compared with the prior art, the application can improve the energy density of the battery, and improve the first coulomb efficiency and the cycle performance by selecting the first active material and the second active material with a specific composition, and cooperating with the settings of the first coating and the second coating. Specifically, the energy density can be improved by using a soft carbon hard carbon composite material in combination with a graphite coated hard carbon material; the conductivity of the negative electrode sheet can be improved, the ion diffusion dynamics can be improved, and the sodium ion deintercalation can be facilitated by doping the active material with the first element and the second element; and the ion diffusion efficiency can be improved, the sodium ion deintercalation can be accelerated, the first coulomb efficiency can be improved, and the battery cycle performance can be improved by using the settings of the double active material coating.

Description

Technical Field

[0001] This application belongs to the field of new energy materials technology, specifically relating to a secondary battery and electrical equipment. Background Technology

[0002] Among various emerging clean energy sources, sodium-ion batteries have attracted widespread attention due to their advantages such as abundant raw materials and low production costs. Hard carbon, used as the negative electrode material in sodium-ion batteries, suffers irreversible capacity loss during the first discharge. This consumes a large amount of electrolyte and sodium ions released from the positive electrode, affecting the battery's discharge capacity and thus reducing its initial coulombic efficiency. Simultaneously, sodium-ion batteries also suffer from low cycle capacity retention and energy density. Therefore, improvements are needed to enhance their electrochemical performance and meet application requirements. Summary of the Invention

[0003] Therefore, the technical problem to be solved by this application is to overcome the defects of sodium-ion batteries in the prior art, such as low initial coulombic efficiency, low cycle capacity retention and low energy density, so as to provide a secondary battery and electrical device.

[0004] Therefore, this application provides the following technical solution:

[0005] According to one aspect of this application, a secondary battery is provided, including a negative electrode sheet, said negative electrode sheet comprising:

[0006] current collector;

[0007] A first coating is applied to at least one side surface of the current collector. The first coating contains a first active substance, which includes a soft carbon material and a first hard carbon material. The first active substance contains a first element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon.

[0008] The second coating is applied to the surface of the first coating. The second coating contains a second active substance, which includes a second hard carbon material. At least a portion of the surface of the second hard carbon material is covered with graphite material. The second active substance contains a second element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon.

[0009] In some alternative embodiments, the first element includes silicon, and at least one of nitrogen, sulfur, and phosphorus;

[0010] And / or, the second element includes silicon, and at least one of nitrogen, sulfur, and phosphorus.

[0011] In some optional embodiments, the mass percentage of the soft carbon material is 20% to 50% based on the total mass of the first active substance, and the mass percentage of the first element is 1% to 5%.

[0012] And / or, based on the total mass of the second active substance, the graphite material contains 10% to 40% by mass, and the second element contains 0.01% to 3% by mass.

[0013] In some optional embodiments, under a test pressure of 20 kN, the powder resistivity A of the first active material and the powder resistivity B of the second active material satisfy the following relationship:

[0014] 1.2≤A / B≤1.8.

[0015] In some alternative embodiments, the powder resistivity A of the first active material is 5 to 12 mΩ·cm;

[0016] And / or, the powder resistivity B of the second active material is 3 to 8 mΩ·cm.

[0017] In some alternative embodiments, the film resistance of the negative electrode is 1 to 3 mΩ.

[0018] In some optional embodiments, the Dv50 of the soft carbon material is 6–12 μm; the Dv50 of the first hard carbon material is 3–8 μm.

[0019] And / or, the Dv50 of the second active substance is 8–14 μm.

[0020] In some optional embodiments, thermogravimetric analysis is performed on the first and second active substances under an inert gas atmosphere, with a test temperature of 30–500°C and a heating rate of 10°C / min, satisfying the following:

[0021] The weight loss percentage of the first active substance in the temperature range of 200℃ to 400℃ is less than 5%;

[0022] The second active substance loses less than 3% of its weight in the temperature range of 200°C to 400°C.

[0023] In some alternative embodiments, the porosity of the first coating is 40% to 50%, and the porosity of the second coating is 20% to 30%.

[0024] And / or, based on the total thickness of the first coating and the second coating, the thickness of the first coating accounts for 55% to 70%.

[0025] According to another aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0026] The technical solution of this application has the following advantages:

[0027] The secondary battery provided in this application includes a negative electrode sheet, which comprises: a current collector; a first coating coated on at least one side surface of the current collector, the first coating containing a first active material, the first active material comprising a soft carbon material and a first hard carbon material, the first active material containing a first element, the first element comprising at least one of nitrogen, sulfur, phosphorus, and silicon; and a second coating coated on the surface of the first coating, the second coating containing a second active material, the second active material comprising a second hard carbon material, at least a portion of the surface of the second hard carbon material having graphite material distributed thereon, the second active material containing a second element, the second element comprising at least one of nitrogen, sulfur, phosphorus, and silicon. Compared with the prior art, this application, through the selection of a first active material and a second active material with specific compositions, and in conjunction with the arrangement of the first and second coatings, can improve the energy density of the battery and enhance the initial coulombic efficiency and cycle performance. Specifically, using soft carbon-hard carbon composite materials in combination with graphite-coated hard carbon materials can improve energy density; doping the active material with the first and second elements can improve the conductivity of the negative electrode, improve ion diffusion kinetics, and facilitate sodium ion insertion / extraction; at the same time, the use of a dual active material coating can improve ion diffusion efficiency, accelerate sodium ion insertion / extraction, improve initial coulombic efficiency, and enhance battery cycle performance.

[0028] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Detailed Implementation

[0029] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0030] As analyzed in the background section, existing sodium-ion batteries suffer from drawbacks such as low initial charge / discharge efficiency, low cycle capacity retention, and low energy density. To address these issues, this application provides a secondary battery and an electrical device.

[0031] According to one aspect of this application, a secondary battery is provided, including a negative electrode sheet, said negative electrode sheet comprising:

[0032] current collector;

[0033] A first coating is applied to at least one side surface of the current collector. The first coating contains a first active substance, which includes a soft carbon material and a first hard carbon material. The first active substance contains a first element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon.

[0034] The second coating is applied to the surface of the first coating. The second coating contains a second active substance, which includes a second hard carbon material. At least a portion of the surface of the second hard carbon material is covered with graphite material. The second active substance contains a second element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon.

[0035] Compared with existing technologies, this application, through the selection of a first active material and a second active material with specific compositions, and in conjunction with the arrangement of a first coating and a second coating, can improve the energy density of the battery, and enhance the initial coulombic efficiency and cycle performance. Specifically, using a soft carbon-hard carbon composite material combined with graphite-coated hard carbon material can improve energy density; doping the active material with a first element and a second element can improve the conductivity of the negative electrode sheet, improve ion diffusion kinetics, and facilitate sodium ion insertion / extraction; simultaneously, the use of a composite coating can improve ion diffusion efficiency, accelerate sodium ion insertion / extraction, improve initial coulombic efficiency, and enhance battery cycle performance.

[0036] In this application, the first active material can be obtained by ball milling a mixture of coal-based soft carbon and bio-based hard carbon to obtain a soft-hard carbon composite material with a microporous structure. The first element can be doped into the composite material by physical or chemical methods. For example, elements such as sulfur, phosphorus, silicon, and nitrogen can be obtained by mixing materials containing sulfur, phosphorus, silicon, or nitrogen elements as sulfur sources, phosphorus sources, silicon sources, or nitrogen sources with the composite material, such as sulfur powder, phosphorus pentoxide, silicon powder, urea, or any known substance containing the above elements, without limitation. The composite material with element doping is obtained by physical ball milling. The doping method of the second element in the second active material is the same as above. The second active material can be obtained by mixing the hard carbon doped with the second element with a graphitizable material and performing heat treatment at 2500℃ to 3000℃ or higher to obtain a graphite-coated hard carbon material; or, the hard carbon doped with the second element can be pretreated with a cationic surfactant and then mixed with graphite material to obtain a graphite-coated hard carbon material. Cationic surfactants can make the surface of hard carbon positively charged, enhancing the electrostatic adsorption between it and graphite, thereby achieving graphite coating of hard carbon. Simultaneously, surfactants also possess good dispersing properties, effectively preventing hard carbon agglomeration. Specifically, hard carbon can be added to cetyltrimethylammonium bromide (CTAB) at a concentration of 10 mg / mL to 20 mg / mL, stirred and soaked at room temperature for 6 to 10 hours, and then dried to obtain treated hard carbon. The pretreated hard carbon is then mixed with graphite and ball-milled to obtain graphite-coated hard carbon material.

[0037] In some alternative embodiments, the first element includes silicon, and at least one of nitrogen, sulfur, and phosphorus;

[0038] And / or, the second element includes silicon, and at least one of nitrogen, sulfur, and phosphorus.

[0039] In this application, by specifying that the first and second elements include silicon, the energy density can be further improved. This is because silicon has a theoretical specific capacity of up to 4200 mAh / g, making it a promising high-capacity anode material. Doping with silicon can provide more active sites for sodium ions, thereby further improving the battery's energy density.

[0040] In some alternative embodiments, the mass percentage of the soft carbon material is 20% to 50% based on the total mass of the first active material, and the mass percentage of the first element is 1% to 5%. For example, the mass percentage of the soft carbon material, based on the total mass of the first active material, can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any range thereof; the mass percentage of the first element can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any range thereof. This can increase the energy density of the first active material while reducing the adverse effects of volume expansion.

[0041] In some alternative embodiments, the mass percentage of graphite material is 10% to 40% based on the total mass of the second active material, and the mass percentage of the second element is 0.01% to 3%. For example, the mass percentage of graphite material based on the total mass of the second active material can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any range thereof; the mass percentage of the second element can be 0.01%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any range thereof. This can increase the energy density of the second active material while reducing the adverse effects of volume expansion.

[0042] In some optional embodiments, under a test pressure of 20 kN, the powder resistivity A of the first active material and the powder resistivity B of the second active material satisfy the following relationship:

[0043] 1.2≤A / B≤1.8.

[0044] As an example, the ratio A / B between the powder resistivity A of the first active material and the powder resistivity B of the second active material can be 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or within any of the above values.

[0045] In this application, by limiting the ratio between the powder resistivity A of the first active material and the powder resistivity B of the second active material, the insertion / extraction of sodium ions and the stability of the electrode structure can be improved. Specifically, when A / B meets the above-mentioned limiting range, the resistivity gradient formed inside the electrode is beneficial to guiding the insertion and extraction of sodium ions in the electrode. In addition, the upper and lower active materials can work together during charging and discharging, thereby enhancing the overall structural stability of the electrode.

[0046] In some optional embodiments, the powder resistivity A of the first active material is 5–12 mΩ·cm; as an example, the powder resistivity A of the first active material can be 5 mΩ·cm, 7 mΩ·cm, 9 mΩ·cm, 10 mΩ·cm, 12 mΩ·cm, or within any range of the above values. A powder resistivity A of the first active material within the above range allows for smoother electron transport in the first active material, which helps reduce the internal resistance of the battery. This not only reduces energy consumption by eliminating the need for higher voltages to overcome resistance during charging, but also increases the available output energy during discharge, thereby improving the battery's actual usable capacity and power density. And / or, in some optional embodiments, the powder resistivity B of the second active material is 3–8 mΩ·cm; as an example, the powder resistivity B of the second active material can be 3 mΩ·cm, 4 mΩ·cm, 5 mΩ·cm, 6 mΩ·cm, 7 mΩ·cm, 8 mΩ·cm, or within any range of the above values. The powder resistivity B of the second active material is within the above range, which allows for smoother electron transport in the second active material and helps reduce the internal resistance of the battery. This not only reduces the need for a higher voltage to overcome resistance during charging and lowers energy consumption, but also increases the available output energy during discharge, thereby improving the battery's actual usable capacity and power density.

[0047] In this application, the powder resistivity of the first active material and the second active material can be tested using methods and equipment known in the art. For example, the constant voltage method can be used for testing, using a ring electrode, and the measurement can be performed based on Ohm's law, i.e., R = U / I. The testing process uses a hydraulic method with a pressure of 20 kN.

[0048] This application improves sodium ion insertion / extraction by limiting the resistivity of the first and second active material powders, thereby ensuring energy density and overall battery performance.

[0049] In some alternative embodiments, the film resistance of the negative electrode is 1 to 3 mΩ; as an example, the film resistance of the negative electrode is 1 mΩ, 1.5 mΩ, 2 mΩ, 2.5 mΩ, 3 mΩ, or within any of the above values.

[0050] In this application, the dual-probe resistance method can be used for testing. Specifically, two probes are used to contact the electrode diaphragm. One probe applies current, and the other probe measures voltage. The resistance is calculated using Ohm's law.

[0051] In this application, the film resistance of the negative electrode is within the above-mentioned range, which can ensure that the electrode has good conductivity, allowing electrons to move quickly inside the electrode, reducing ohmic polarization, and at the same time preventing local overheating due to excessive resistance, thus avoiding structural changes in the electrode material.

[0052] In some alternative embodiments, the Dv50 of the soft carbon material is 6 to 12 μm; the Dv50 of the first hard carbon material is 3 to 8 μm; as an example, the Dv50 of the soft carbon material can be 6 μm, 8 μm, 10 μm, 12 μm, or within any range of the above values; the Dv50 of the first hard carbon material can be 3 μm, 5 μm, 7 μm, 8 μm, or within any range of the above values.

[0053] And / or, in some alternative embodiments, the Dv50 of the second active material is 8 to 14 μm; as an example, the Dv50 of the second active material can be 8 μm, 10 μm, 12 μm, 14 μm, or within any of the above values.

[0054] In this application, laser diffraction can be used for testing. Specifically, the powder sample to be tested can be dispersed in anhydrous ethanol, and then the dispersed sample can be placed in the sample cell of a laser diffractometer for testing. Data processing is performed through a built-in algorithm to determine Dv50.

[0055] By limiting the Dv50 of the first and second active materials, this application can improve the processing performance and electrochemical performance of the materials. Specifically, limiting the Dv50 of the active materials to the above-mentioned range can better mix the active materials, conductive agents and binders together, which is conducive to forming a stable electrode structure and facilitates sodium ion insertion / extraction.

[0056] In some optional embodiments, thermogravimetric analysis is performed on the first and second active substances under an inert gas atmosphere, with a test temperature of 30–500°C and a heating rate of 10°C / min, satisfying the following:

[0057] The weight loss percentage of the first active substance in the temperature range of 200℃ to 400℃ is less than 5%;

[0058] The second active substance loses less than 3% of its weight in the temperature range of 200°C to 400°C.

[0059] In this application, the above-mentioned limitations on the thermogravimetric analysis results of the first and second active materials demonstrate that the materials have high thermal stability, thereby improving the thermal stability and electrochemical performance of the battery. Specifically, limiting the thermogravimetric loss of the materials within the above-mentioned range can reduce the decline in battery performance and safety hazards caused by the decomposition or volatilization of active materials. The first active material has a slightly higher thermogravimetric loss (less than 5%), which can buffer the heat changes inside the battery to a certain extent, while the relatively lower thermogravimetric loss of the second active material (less than 3%) can provide better stability for the reaction interface of the battery.

[0060] In some alternative embodiments, the porosity of the first coating is 40% to 50%, and the porosity of the second coating is 20% to 30%. As an example, the porosity of the first coating can be 40%, 42%, 44%, 45%, 46%, 48%, 50%, or within any of the above values; the porosity of the second coating can be 20%, 22%, 24%, 25%, 26%, 28%, 30%, or within any of the above values.

[0061] In this application, the porosity can be tested using the hexadecane absorption method. Specifically, the pores of the electrode under test are wetted and filled with n-hexadecane, the mass change of the electrode before and after immersion in n-hexadecane is measured, and the pore volume filled by n-hexadecane is calculated in combination with the density of n-hexadecane, thereby obtaining the porosity of the electrode coating.

[0062] This application limits the porosity of the first coating and the second coating to the above-mentioned range, so that the inner layer of the negative electrode sheet has more pores than the outer layer of the electrode sheet, which is beneficial for the electrolyte to wet the electrode sheet. The outer layer of the electrode sheet has fewer pores, which can improve the electron transport rate, reduce the probability of the conductive path being cut off by the pores, avoid the obstruction of electron conduction, and reduce the ohmic internal resistance of the battery cell.

[0063] In some alternative implementations, the thickness of the first coating accounts for 55% to 70% of the total thickness of the first and second coatings. As an example, the thickness of the first coating can be 55%, 58%, 60%, 62%, 65%, 67%, 70%, or within any range of the above values, based on the total thickness of the first and second coatings.

[0064] In this application, by limiting the proportion of the first coating thickness, the ion transport path can be optimized; specifically, by limiting the proportion of the first coating thickness to the above range, more sodium ions can be accommodated, and the diffusion of sodium ions to the active site is more gradual, reducing polarization caused by excessive local concentration.

[0065] In some alternative embodiments, the first coating comprises 92% to 96% of a first active material, 2% to 4% of a first conductive agent, and 2% to 4% of a first binder, based on the total mass of the first coating.

[0066] And / or, by total mass of the second coating, it comprises 92% to 96% of a second active material, 2% to 4% of a second conductive agent, and 2% to 4% of a second binder.

[0067] In some optional embodiments, the first conductive agent includes one or more of acetylene black, carbon nanotubes, conductive carbon black, and composite conductive materials; optionally, the first conductive agent includes at least one of acetylene black and conductive carbon black.

[0068] And / or, the first adhesive comprises one or more of styrene-butadiene rubber latex, carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polyacrylate; optionally, the first adhesive comprises one or more of styrene-butadiene rubber latex, carboxymethyl cellulose, and polyacrylic acid.

[0069] In some optional embodiments, the second conductive agent includes one or more of acetylene black, carbon nanotubes, conductive carbon black, and composite conductive materials; optionally, the second conductive agent includes at least one of carbon nanotubes and composite conductive materials.

[0070] And / or, the second adhesive comprises one or more of styrene-butadiene rubber latex, carboxymethyl cellulose, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, and polyacrylate; optionally, the second adhesive comprises at least one of carboxymethyl cellulose and polyvinylidene fluoride.

[0071] In this application, the proportions of binder and conductive agent in the first and second coatings are within the above-mentioned range, which can make the upper and lower coatings of the negative electrode sheet have strong bonding strength. When subjected to external forces or environmental changes, the coating is not prone to delamination, peeling, or other problems, thus improving the stability and reliability of the coating. At the same time, it can simplify the preparation process, reduce production difficulty, and improve production efficiency. In the first coating, acetylene black and conductive carbon black are preferred conductive agents because they have good conductivity and can form a good conductive network, enhancing the conductivity between the first coating and the current collector. The preferred binders are styrene-butadiene rubber latex, carboxymethyl cellulose, and polyacrylic acid, which can effectively improve the adhesion between the first coating and the current collector. In the second coating, carbon nanotubes and composite conductive materials are preferred conductive agents. Carbon nanotubes have high conductivity, and composite conductive materials can improve conductivity through compositing, thereby improving the conductivity of the second coating. The preferred second binders are polyvinylidene fluoride (PVDF) and carboxymethyl cellulose. PVDF has a high dielectric constant, stable chemical properties, and can withstand the swelling and corrosion of the electrolyte, maintaining the adhesion between the first and second coatings. Carboxymethyl cellulose has a good dispersing effect on graphite, which is beneficial to the uniformity and stability of the graphite-coated hard carbon slurry.

[0072] Those skilled in the art will understand that the aforementioned secondary battery also includes a positive electrode, a separator, an electrolyte, and a casing. During the charging and discharging process, metal ions (sodium ions) are inserted and extracted back and forth between the positive and negative electrode. The electrolyte plays a role in conducting ions between the positive and negative electrode. The separator is disposed between the positive and negative electrode and mainly serves to prevent short circuits between the positive and negative electrodes, while allowing metal ions to pass through.

[0073] As an example, the negative electrode sheet includes a negative current collector and a negative active material layer. The negative current collector has two surfaces opposite each other in its own thickness direction, and the negative active material layer is disposed on either or both of the two opposite surfaces of the negative current collector.

[0074] The electrolyte used in the secondary battery of this application may include any technology disclosed in the prior art. As an example, it may include an organic solvent, a sodium salt, and additives, wherein the organic solvent includes, but is not limited to, at least one of ethylene carbonate, propylene carbonate, and diethyl carbonate; the lithium salt includes, but is not limited to, at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, and sodium hexafluoroarshophosphate; and the additives include, but are not limited to, at least one of film-forming additives, conductive additives, and flame-retardant additives.

[0075] According to another aspect of this application, an electrical device is provided, including the aforementioned secondary battery.

[0076] It is understood that the secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0077] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0078] Example 1

[0079] This embodiment provides a negative electrode sheet, the composition of which and its preparation method are as follows:

[0080] Preparation of the first active material: 3% by mass of silicon powder (based on the mass of silicon element in the silicon source) was ball-milled with coal-based soft carbon and bio-based hard carbon. The Dv50 of the coal-based soft carbon was 6 μm; the Dv50 of the biomass hard carbon material was 3 μm. The mass ratio of coal-based soft carbon to bio-based hard carbon was 2:8. The ball milling time was 10 h to disperse the silicon source in the mixed material. After ball milling, the material was heat-treated at 1000℃ to obtain a soft-hard carbon composite material, i.e., the first active material. Thermogravimetric analysis was performed on the first active material under an inert gas atmosphere. The test temperature was 30–500℃, and the heating rate was 10℃ / min. The weight loss percentage of the first active material in the temperature range of 200℃ to 400℃ was 3%.

[0081] Preparation of the second active material: Sulfur powder (based on the mass of sulfur in the sulfur source) and phosphorus pentoxide (based on the mass of phosphorus in the phosphorus source), each with a mass ratio of 1%, were ball-milled with bio-based hard carbon to disperse the sulfur and phosphorus sources in the hard carbon. The above material was heat-treated at 600℃ for 5 hours to obtain a hard carbon material containing 1% sulfur and 1% phosphorus. The doped hard carbon was added to a solution of 10 mg / mL hexadecyltrimethylammonium bromide (CTAB), stirred and soaked at room temperature for 6 hours, and then dried to obtain the treated hard carbon. The treated hard carbon material was mixed with graphite at a mass ratio of 9:1 to obtain the second active material. The Dv50 of the second active material was 8 μm. Thermogravimetric analysis of the second active material was performed under an inert gas atmosphere at a test temperature of 30–500℃ and a heating rate of 10℃ / min. The weight loss percentage of the second active material in the temperature range of 200℃ to 400℃ was 2%.

[0082] First coating: The first active material (with a tested powder resistivity of 7.8 mΩ·cm) doped with non-metallic elements prepared by the above method, acetylene black (as the first conductive agent), and styrene-butadiene rubber emulsion (as the first binder) are mixed in a mass ratio of 94:3:3. Deionized water is used as a solvent to make a slurry, which is then coated on the negative electrode current collector copper foil (thickness of 6 μm). The double-sided coating thickness is 112 μm (single-sided thickness is 56 μm). The thickness of the first coating is controlled to be 60.54% of the total thickness, and the porosity of the first coating is 40%.

[0083] Second coating: A second negative electrode active material doped with sulfur and phosphorus (with a tested powder resistivity of 6.4 Ω·cm), carbon nanotubes (as a second conductive agent), and carboxymethyl cellulose (as a second binder) are mixed in a mass ratio of 94:3:3, and a slurry is prepared using deionized water as a solvent. This slurry is then coated on top of the first coating. The total thickness of the second coating is controlled to be 73 μm (36.5 μm on one side), accounting for 39.46% of the total thickness of the first and second coatings. The porosity of the second coating is 20%.

[0084] The film resistance of the negative electrode provided in this embodiment is 1.5mΩ, according to the test results.

[0085] Examples 2-11

[0086] The difference between Examples 2-11 and Example 1 lies in the preparation of the first coating, as detailed in the table below.

[0087] Table 1

[0088]

[0089]

[0090] Examples 12-19

[0091] The difference between Examples 12-19 and Example 1 lies in the preparation of the second coating, as detailed in the table below.

[0092] Table 2

[0093]

[0094] Comparative Example 1

[0095] This comparative example provides a negative electrode sheet, which differs from Example 1 in that the negative electrode sheet is only coated with a first coating layer, and the thickness of the first coating layer is the same as the sum of the thicknesses of the first coating layer and the second coating layer in Example 1.

[0096] Comparative Example 2

[0097] This comparative example provides a negative electrode sheet, which differs from Example 1 in that no doping elements are added during the preparation of the first and second active materials.

[0098] Comparative Example 3

[0099] This comparative example provides a negative electrode sheet. The difference from Example 1 is that the first active material and the second active material in the first coating and the second coating are both artificial graphite (model BTR AGP S360). Everything else is the same as in Example 1.

[0100] Test case

[0101] The negative electrode sheets provided in the above embodiments and comparative examples are applied to sodium-ion batteries. The preparation method of the sodium-ion battery is as follows:

[0102] The positive electrode is prepared as follows: Weigh out NaNi, the positive electrode active material, in a mass ratio of 97.3:0.5:1.2:1.0. 1 / 3Fe 1 / 3 Mn 1 / 3 O2, SP, PVDF and carbon nanotubes are mixed with an appropriate amount of NMP to prepare a slurry, which is then coated on both sides of the positive electrode current collector aluminum foil and dried to obtain the positive electrode sheet with a double-sided coating thickness of 138μm.

[0103] A battery (2Ah capacity) was prepared by combining a positive electrode, a separator, an electrolyte (a 1mol / L NaPF6 solution, a mixture of ethylene carbonate, propylene carbonate and diethyl carbonate in a volume ratio of 1:1:1), and the negative electrode provided in the above examples and comparative examples. The battery was then subjected to cycle testing in a charge-discharge tester.

[0104] Electrical performance testing methods:

[0105] The batteries obtained by the above method were subjected to charge-discharge tests after being formed and capacity-controlled. They were charged and discharged at a constant current of 1C, with an operating voltage of 1.8–3.9V, a test temperature of 25℃, and cycled for 500 cycles at room temperature. After the cycle period, the capacity retention rate was calculated as: Capacity retention rate = (Discharge capacity of the Nth cycle / Initial discharge capacity) × 100%.

[0106] Initial coulombic efficiency = (initial discharge capacity / initial charge capacity) × 100%.

[0107] Energy density = First discharge energy / Battery mass.

[0108] The specific test results are shown in the table below:

[0109] Table 3

[0110] Group Energy density, Wh / kg First Coulomb efficiency, % 500-cycle capacity retention, % Example 1 150.5 90.6 98.0 Example 2 155.3 92.0 97.6 Example 3 150.4 93.3 96.3 Example 4 152.2 95.7 99.5 Example 5 151.0 92.4 98.5 Example 6 152.1 93.3 99.6 Example 7 153.2 94.5 99.3 Example 8 153.5 92.5 98.7 Example 9 149.8 94.2 97.4 Example 10 149.4 92.8 98.4 Example 11 151.5 94.2 99.6 Example 12 148.9 91.9 97.6 Example 13 149.7 93.2 98.3 Example 14 152.7 91.4 97.8 Example 15 154.9 92.6 99.8 Example 16 151.3 93.8 97.0 Example 17 152.6 93.0 99.4 Example 18 152.5 92.5 95.8 Example 19 152.1 93.4 99.5 Comparative Example 1 115.0 81.4 84.2 Comparative Example 2 128.2 85.5 88.6 Comparative Example 3 120.1 83.8 85.5

[0111] As can be seen from the test results in the table above, the negative electrode and sodium-ion battery provided in this application embodiment have high initial coulombic efficiency and energy density, as well as good cycle performance. The selection of specific active materials in the embodiment, combined with the double coating structure, can significantly improve the battery performance.

[0112] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A secondary battery, characterized in that, Includes a negative electrode sheet, wherein the negative electrode sheet comprises: current collector; A first coating is applied to at least one side surface of the current collector. The first coating contains a first active substance, which includes a soft carbon-hard carbon composite material obtained by mixing a soft carbon material and a first hard carbon material. The first active substance contains a first element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon. The second coating is applied to the surface of the first coating. The second coating contains a second active substance, which includes a second hard carbon material. At least a portion of the surface of the second hard carbon material is covered with graphite material. The second active substance contains a second element, which includes at least one of nitrogen, sulfur, phosphorus, and silicon. Under a test pressure of 20 kN, the powder resistivity A of the first active material and the powder resistivity B of the second active material satisfy the following relationship: 1.2≤A / B≤1.

8.

2. The secondary battery according to claim 1, characterized in that, The first element includes silicon, and at least one of nitrogen, sulfur, and phosphorus.

3. The secondary battery according to claim 1, characterized in that, The second element includes silicon, and at least one of nitrogen, sulfur, and phosphorus.

4. The secondary battery according to claim 1, characterized in that, Based on the total mass of the first active substance, the mass percentage of soft carbon material is 20% to 50%, and the mass percentage of the first element is 1% to 5%.

5. The secondary battery according to claim 1, characterized in that, Based on the total mass of the second active substance, the graphite material has a mass percentage of 10% to 40%, and the second element has a mass percentage of 0.01% to 3%.

6. The secondary battery according to claim 1, characterized in that, The powder resistivity A of the first active material is 5~12 mΩ·cm.

7. The secondary battery according to claim 1, characterized in that, The powder resistivity B of the second active material is 3~8 mΩ·cm.

8. The secondary battery according to claim 1, characterized in that, The film resistance of the negative electrode is 1~3mΩ.

9. The secondary battery according to claim 1, characterized in that, The Dv50 of the soft carbon material is 6~12μm; the Dv50 of the first hard carbon material is 3~8μm.

10. The secondary battery according to claim 1, characterized in that, The Dv50 of the second active substance is 8~14μm.

11. The secondary battery according to any one of claims 1 to 10, characterized in that, Thermogravimetric analysis was performed on the first and second active substances under an inert gas atmosphere. The test temperature was 30~500℃, and the heating rate was 10℃ / min, satisfying the following conditions: The weight loss percentage of the first active substance in the temperature range of 200℃ to 400℃ is less than 5%; The second active substance loses less than 3% of its weight in the temperature range of 200°C to 400°C.

12. The secondary battery according to claim 11, characterized in that, The porosity of the first coating is 40% to 50%, and the porosity of the second coating is 20% to 30%.

13. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 12.

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