Negative electrode sheet and sodium-ion battery

Abstract

The application relates to the secondary battery technical field and discloses a negative electrode sheet and a sodium ion battery, wherein the negative electrode active layer in the negative electrode sheet comprises a first negative electrode active layer and a second negative electrode active layer, the first negative electrode active material in the first negative electrode active layer comprises a first hard carbon material; the second negative electrode active material in the second negative electrode active layer comprises a second hard carbon material; the particle size Dv50 of the first negative electrode active material is recorded as D1 mu m, the particle size Dv50 of the second negative electrode active material is recorded as L1 mu m, and the following conditions are met: 2 <= D1 / L1 <= 6; the cross-section filling rate of the first negative electrode active material is recorded as A1, the cross-section filling rate of the second negative electrode active material is recorded as A2, and the following conditions are met: 0 < A1-A2 <= 20%. The negative electrode sheet provided by the application has good cycle stability, excellent kinetic performance and large compaction density, and the cycle stability, rate performance and energy density of the sodium ion battery are improved.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a negative electrode and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries, due to their abundant resources, low cost, and excellent safety, have broad application prospects in large-scale energy storage, low-speed electric vehicles, and portable electronic devices, and have become one of the research hotspots in the new energy field. Energy density (ED), as one of the core performance indicators of sodium-ion batteries, directly determines the expansion of their application scenarios and market competitiveness. Therefore, improving the energy density of sodium-ion batteries has become an important research direction in the industry.

[0003] Currently, sodium-ion batteries generally suffer from low energy density, which restricts their promotion and application in mid-to-high-end scenarios. To improve the energy density of sodium-ion batteries, the industry has proposed various technical solutions to achieve high energy density. However, high-energy-density sodium-ion batteries suffer from deteriorated internal kinetic performance, difficulty in effectively buffering the volume expansion and contraction of active materials, and a higher risk of active material shedding and electrode cracking. These issues further degrade the cycle stability and rate performance of high-energy-density sodium-ion batteries. Summary of the Invention

[0004] In view of this, this application provides a negative electrode and a sodium-ion battery to solve the problem that existing sodium-ion batteries cannot simultaneously achieve high energy density, good kinetic performance, and excellent cycle stability.

[0005] In a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector in the thickness direction, the negative electrode active layer including a first negative electrode active layer and a second negative electrode active layer, the first negative electrode active layer being disposed between the negative electrode current collector and the second negative electrode active layer. The first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes a first hard carbon material; The second negative electrode active layer includes a second negative electrode active material, and the second negative electrode active material includes a second hard carbon material; The particle size Dv50 of the first negative electrode active material is denoted as D1 μm, and the particle size Dv50 of the second negative electrode active material is denoted as L1 μm, satisfying: 2≤D1 / L1≤6; The cross-sectional filling rate of the first negative electrode active material is denoted as A1, and the cross-sectional filling rate of the second negative electrode active material is denoted as A2, satisfying: 0 < A1 - A2 ≤ 20%.

[0006] The technical solution of this application has the following advantages: 1. The negative electrode sheet provided in this application adopts a double-layer active layer structure. The first negative electrode active layer uses large-particle hard carbon material as the active material, which can improve the ultimate compaction density of the electrode sheet and provide a higher specific capacity, thereby improving the energy density of the battery. The large-particle hard carbon material is located at the bottom layer of the negative electrode active layer (i.e., the layer structure close to the negative electrode current collector). While improving the ultimate compaction density of the electrode sheet, it can also form a more robust conductive framework, providing a low-resistance transport channel for electrons from the bottom layer to the negative electrode current collector, and improving the stability of the active layer, avoiding the shedding of active material and cracking of the electrode sheet, which is beneficial to improving the cycle stability of the battery. Furthermore, the large-particle hard carbon material can provide a larger and more interconnected pore structure, which can improve the electrolyte retention capacity of the electrode sheet and the stable and continuous transport of sodium ions into the active layer. The second negative electrode active layer uses small-particle hard carbon material as the active material, which can shorten the transport path of sodium ions, increase the contact area with the electrolyte and the affinity for surface wetting, thereby improving the kinetic performance of the electrode sheet. Furthermore, the small-particle hard carbon material located on the surface of the negative electrode active layer (i.e., the layer structure away from the negative electrode current collector) ensures that the electrolyte can more quickly and uniformly wet the surface of the electrode, improving the rapid insertion and extraction of sodium ions, reducing the electrochemical polarization of the electrode, and improving the kinetic performance of the battery. Simultaneously, this application controls the ratio of the particle size Dv50 of the first negative electrode active material to that of the second negative electrode active material to be 2-6, achieving optimal results in electrode compaction density, kinetic performance, and cycle stability, reaching a balance among these three aspects, and ensuring that the battery simultaneously possesses high overall energy density, excellent rate performance, and long-term cycle stability. On the other hand, this application also controls the difference between the cross-sectional filling rate of the first negative electrode active material in the first negative electrode active layer and the cross-sectional filling rate of the second negative electrode active material in the second negative electrode active layer to satisfy: 0 < A1 - A2 ≤ 20%. The cross-sectional filling rate of the first negative electrode active material is greater than that of the second negative electrode active material. The higher cross-sectional filling rate of the first negative electrode active material is beneficial to improving the overall compaction density and capacity of the electrode sheet. The lower cross-sectional filling rate of the second negative electrode active material is beneficial to the rapid and stable intercalation and deintercalation of sodium ions in the second negative electrode active layer, thereby improving the kinetic performance and cycle stability of the electrode sheet interface. Attached Figure Description

[0007] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0008] Figure 1 This is a schematic diagram of the cross-sectional structure of the negative electrode active material provided in one embodiment of this application.

[0009] Figure 1 In the middle, 1. solid part; 2. pores.

[0010] Figure 2 This is a partial cross-sectional scanning electron microscope image of the negative electrode active layer provided in one embodiment of this application. Detailed Implementation

[0011] The following embodiments are provided to better understand this application. However, the following embodiments 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 the features of this application with other prior art, falls within the scope of protection of this application.

[0012] To address the issue that secondary batteries in related technologies cannot simultaneously achieve high energy density, good kinetic performance, and excellent cycle stability, this application proposes the following solution.

[0013] In a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector in the thickness direction, the negative electrode active layer including a first negative electrode active layer and a second negative electrode active layer, the first negative electrode active layer being disposed between the negative electrode current collector and the second negative electrode active layer. The first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes a first hard carbon material; The second negative electrode active layer includes a second negative electrode active material, and the second negative electrode active material includes a second hard carbon material; The particle size Dv50 of the first negative electrode active material is denoted as D1 μm, and the particle size Dv50 of the second negative electrode active material is denoted as L1 μm, satisfying: 2≤D1 / L1≤6; The cross-sectional filling rate of the first negative electrode active material is denoted as A1, and the cross-sectional filling rate of the second negative electrode active material is denoted as A2, satisfying: 0 < A1 - A2 ≤ 20%.

[0014] The negative electrode sheet provided in this application adopts a double-layer active layer structure. The first negative electrode active layer uses large-particle hard carbon material as the active material, which can improve the ultimate compaction density of the electrode sheet and provide a higher specific capacity, thereby improving the energy density of the battery. The large-particle hard carbon material is located at the bottom layer of the negative electrode active layer (i.e., the layer structure close to the negative electrode current collector). While improving the ultimate compaction density of the electrode sheet, it can also form a more robust conductive framework, providing a low-resistance transport channel for electrons from the bottom layer to the negative electrode current collector, and improving the stability of the active layer, avoiding the shedding of active material and cracking of the electrode sheet, which is beneficial to improving the cycle stability of the battery. Furthermore, the large-particle hard carbon material can provide a larger and more interconnected pore structure, which can improve the electrolyte retention capacity of the electrode sheet and the stable and continuous transport of sodium ions into the active layer. The second negative electrode active layer uses small-particle hard carbon material as the active material, which can shorten the transport path of sodium ions, increase the contact area with the electrolyte and the affinity for surface wetting, thereby improving the kinetic performance of the electrode sheet. Furthermore, the small-particle hard carbon material located on the surface of the negative electrode active layer (i.e., the layer structure away from the negative electrode current collector) can ensure that the electrolyte wets the surface of the electrode more quickly and evenly, improve the rapid insertion and extraction of sodium ions, reduce the electrochemical polarization of the electrode, and improve the kinetic performance of the battery.

[0015] Meanwhile, this application controls the ratio of the particle size Dv50 of the first negative electrode active material to the particle size Dv50 of the second negative electrode active material to be 2 to 6, which can achieve the best results in electrode compaction density, kinetic performance and cycle stability, and reach the balance point of the three, ensuring that the battery has high overall energy density, excellent rate performance and long cycle stability at the same time.

[0016] On the other hand, this application also controls the difference between the cross-sectional filling rate A1 of the first negative electrode active material in the first negative electrode active layer and the cross-sectional filling rate A2 of the second negative electrode active material in the second negative electrode active layer to satisfy: 0 < A1 - A2 ≤ 20%. The cross-sectional filling rate of the first negative electrode active material is greater than that of the second negative electrode active material. The higher cross-sectional filling rate of the first negative electrode active material is beneficial to improving the overall compaction density and capacity of the electrode sheet. The lower cross-sectional filling rate of the second negative electrode active material is beneficial to the rapid and stable intercalation and deintercalation of sodium ions in the second negative electrode active layer, thereby enhancing the dynamics of the electrode interface. To improve kinetic performance and cycle stability, and to ensure that the difference in cross-sectional fill rate between the first and second negative electrode active materials is less than or equal to 20%, the electrode can have a higher compaction density, thereby improving the battery's energy density. At the same time, maintaining a suitable cross-sectional fill rate for the active material in the outer layer of the active layer (the second negative electrode active layer) can result in higher kinetic performance and cycle stability. Furthermore, it can also prevent the storage of a large number of sodium metal clusters in the pores of the particles due to an excessively small cross-sectional fill rate of the second negative electrode active material, which would lead to increased irreversible capacity loss and affect the overall energy density and cycle life of the battery.

[0017] This study found that if the ratio of the particle size Dv50 of the first negative electrode active material to that of the second negative electrode active material is less than 2, it means that the particle size Dv50 of the first negative electrode active material is too low and the particle size Dv50 of the second negative electrode active material is too high. The first negative electrode active material with a low particle size Dv50 will affect the overall performance improvement of the battery's energy density and cycle stability, while the second negative electrode active material with a high particle size Dv50 will lead to a decrease in the sodium ion transport efficiency in the second negative electrode active layer and a worsening of the electrolyte wetting effect, which is detrimental to improving the kinetic performance of the electrode and thus to improving the battery's rate performance. Conversely, if the ratio of the particle size Dv50 of the first negative electrode active material to that of the second negative electrode active material is less than 2, the ratio of the particle size Dv50 of the second negative electrode active material will be less than 2. The ratio of particle size Dv50 of the materials is higher than 6, which indicates that the particle size Dv50 of the first negative electrode active material is too high and the particle size Dv50 of the second negative electrode active material is too low. The excessively high particle size Dv50 of the first negative electrode active material will result in an excessively long diffusion path of sodium ions in the second negative electrode active layer, which is not conducive to the rapid insertion and extraction of sodium ions, increases battery polarization, and results in insufficient capacity utilization. It will also lead to an ineffective improvement in the compaction density of the electrode sheet, which is not conducive to improving the rate performance and energy density of the battery. The excessively low particle size Dv50 of the second negative electrode active material results in high surface activity of ultrafine particles, large electrolyte consumption, and dense packing of small particles, which will affect the wetting of electrolyte and the transport of sodium ions, which is also not conducive to improving the rate performance of the battery. Furthermore, if the ratio of the particle size Dv50 of the first negative electrode active material to that of the second negative electrode active material is not within the range of 2 to 6, it will also lead to a deterioration in the matching performance between the first and second negative electrode active materials, which is not conducive to improving the overall electrochemical performance of the battery in terms of energy density, rate performance and cycle stability.

[0018] This application research found that if the difference between the cross-sectional filling rate of the first negative electrode active material in the first negative electrode active layer and the cross-sectional filling rate of the second negative electrode active material in the second negative electrode active layer is greater than 20%, the cross-sectional filling rate of the second negative electrode active material will be too small, resulting in more or larger pore structures inside the material. This makes it easy for sodium ions to form quasi-metallic sodium clusters in the large pores. Excessive accumulation of sodium clusters makes it difficult for them to escape, leading to a significant decrease in battery capacity and affecting the cycle performance of the battery. If the cross-sectional filling rate of the first negative electrode active material is equal to that of the second negative electrode active material, it will be impossible to achieve the optimal balance between electrode compaction density, electrode kinetic performance and cycle stability, which is not conducive to improving the energy density and rate performance of the battery.

[0019] It should be noted that cross-sectional fill rate is an important indicator for evaluating the performance of individual particles in negative electrode active materials, such as... Figure 1 As shown, the cross-sectional filling rate of the negative electrode active material is actually the proportion of the solid portion in the cross-section of the negative electrode active material. Figure 1The ratio between the area of ​​the shaded region and the area of ​​the closed region enclosed by the outer contour of the negative electrode active material particles. Figure 1 The solid fill (i.e., the gray area) in the image represents solid part 1, while the blank area represents pores 2. In this application, the cross-sectional filling rate of the negative electrode active material can be obtained through the following testing process: A cross-section along the thickness direction of the electrode sheet is cut using a plasma beam. A cross-sectional scanning electron microscope (SEM) image or a high-resolution SEM image of the negative electrode sheet is obtained. At least 30 negative electrode active materials are selected, and the area of ​​the solid fill (excluding internal pores, which refer to the statistical analysis of pore structures with a diameter of 6 nm or larger) in the negative electrode active material is measured using image analysis and denoted as S1μm. 2 The area of ​​the closed region (including internal pores) enclosed by the measured outer contour curve of the negative electrode active material is denoted as S² μm. 2 The cross-sectional fill rate of a single negative electrode active material is obtained using the formula: S1 / S2 × 100%. The average cross-sectional fill rate of multiple negative electrode active materials is then calculated, which is the cross-sectional fill rate of the negative electrode active material. Both the first and second negative electrode active materials are tested using the above method. For example, A1-A2 can be values ​​such as 0.1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or values ​​within any two of the above ranges.

[0020] It is understandable that the higher the cross-sectional fill rate of the negative electrode active material, the higher its internal density, while the lower the cross-sectional fill rate, the more internal pores the negative electrode active material has.

[0021] Figure 2 To satisfy the partial scanning electron microscope image of the negative electrode active layer provided in one embodiment of this application, it can be seen from the figure that the negative electrode active hard carbon material in the negative electrode active layer contains a porous structure.

[0022] It should be noted that the particle sizes Dv50 of the first and second negative electrode active materials can be obtained by laser particle size analyzer, or by using image processing software to statistically analyze the particle sizes of the first and second negative electrode active materials from a scanning electron microscope image of the negative electrode active layer, arranging the particle sizes from smallest to largest, and taking the particle size value that accumulates to 50% of the volume as the particle size Dv50 of the polymer particles. For example, the ratio of the particle size Dv50 of the first negative electrode active material to the particle size Dv50 of the second negative electrode active material can be, for example, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, or a value within any range of two of the above values.

[0023] It is understood that in this application, the statement that "large-particle hard carbon materials, as active materials, can increase the compaction density of the electrode and provide higher specific capacity, thereby improving the energy density of the battery" refers to the fact that large-particle hard carbon materials can withstand a greater limit compaction density (maximum compaction density) in the negative electrode, which is more conducive to obtaining a negative electrode with high compaction density and high capacity, thereby promoting the improvement of battery energy density.

[0024] In this application, the particle sizes Dv10, Dv50, and Dv90 have the conventional meaning in the art, referring to the following: when the particles are arranged from smallest to largest, the particle size corresponding to a cumulative volume of 10% of the total volume is Dv10, the particle size corresponding to a cumulative volume of 50% of the total volume is Dv50, and the particle size corresponding to a cumulative volume of 90% of the total volume is Dv90.

[0025] In some embodiments, the particle size Dv50 of the first negative electrode active material satisfies: 7≤D1≤12. This allows the negative electrode active material in the first negative electrode active layer to have a suitable particle size Dv50, which can further improve the compaction density and capacity performance of the negative electrode sheet, and ensure that the first negative electrode active layer has a more stable and continuous transport channel, thereby further improving the energy density and rate performance of the battery.

[0026] In some embodiments, the particle size Dv10 of the first negative electrode active material is 2 μm to 6 μm.

[0027] In some embodiments, the particle size Dv90 of the first negative electrode active material is 16 μm to 24 μm.

[0028] It should be noted that the particle size Dv10 and particle size Dv90 of the first negative electrode active material can be obtained by testing with a laser particle size analyzer, or by using image processing software to statistically analyze the particle size of the first negative electrode active material in the scanning electron microscope image of the negative electrode active layer, arranging the particle size from smallest to largest, and taking the particle size value that reaches 50% of the total volume as the particle size Dv50 of the polymer particles. For example, the particle size Dv50 of the first negative electrode active material can be, for example, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, 12.0 μm, or a value within any two of the above values; the particle size Dv10 of the first negative electrode active material can be, for example, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, The particle size Dv90 of the first negative electrode active material can be, for example, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, 18.5 μm, 19.0 μm, 19.5 μm, 20.0 μm, 20.5 μm, 21.0 μm, 21.5 μm, 22.0 μm, 22.5 μm, 23.0 μm, 23.5 μm, 24.0 μm, or a value within the range of any two of the above values.

[0029] In some embodiments, the particle size span of the first negative electrode active material is 0.9 to 3.1. This matching of particle size with the first negative electrode active material helps to improve the battery's energy density and forms a more robust conductive framework, enhancing the stability of the negative electrode active layer and further preventing the shedding of active material and cracking of the electrode sheet, thereby further improving the battery's cycle stability.

[0030] It should be noted that the particle size span value of the first negative electrode active material is calculated as (particle size Dv90 - particle size Dv10) / particle size Dv50. For example, the particle size span value of the first negative electrode active material can be 0.9, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.1, or a value within any two of the above ranges.

[0031] In some embodiments, the cross-sectional fill rate of the first negative electrode active material satisfies: 85% ≤ A1 ≤ 99%. Thus, the first negative electrode active material has a high cross-sectional fill rate, which can further improve the overall compaction density and capacity of the negative electrode sheet.

[0032] This study found that if the cross-sectional filling rate of the first negative electrode active material is less than 85%, it indicates that the proportion of the solid part in the first negative electrode active material is too low, the internal density is low, the capacity of the material decreases, which is not conducive to improving the energy density of the battery. Furthermore, the low cross-sectional filling rate of the first negative electrode active material will also lead to a decrease in the structural stability of the first negative electrode active layer. The first negative electrode active material located in the inner layer is prone to breakage, pulverization and flaking, which will lead to a decrease in the cycle stability of the battery.

[0033] For example, the cross-sectional filling rate A1 of the first negative electrode active material may be 85%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99%, or a value within the range of any two of the above values.

[0034] In some embodiments, the specific surface area of ​​the first negative electrode active material is 3.2 m². 2 / g~5.2m 2 / g. Thus, the first negative electrode active material has a suitable specific surface area. On the one hand, it can further improve the compaction density of the first negative electrode active layer, thereby improving the overall compaction density and capacity of the electrode, which is more conducive to obtaining a high energy density battery. On the other hand, ensuring that the negative electrode active material has a suitable specific surface area is beneficial to improving the kinetic performance of the first negative electrode active layer containing large-particle active materials, which is more conducive to improving the overall rate performance of the electrode.

[0035] It should be noted that the specific surface area of ​​the first negative electrode active material was obtained through isothermal adsorption-desorption testing with nitrogen gas, yielding isothermal adsorption-desorption curves, and calculated using the Brunauer-Emmett-Teller (BET) model. For example, the specific surface area of ​​the first negative electrode active material could be, for instance, 3.2 m². 2 / g, 3.4m 2 / g, 3.6m 2 / g, 3.8m 2 / g, 4.0m 2 / g, 4.2m 2 / g, 4.4m 2 / g, 4.6m 2 / g, 4.8m 2 / g, 5.0m 2 / g, 5.2m 2 / g or values ​​within the range of any two of the above values.

[0036] In some embodiments, the particle size Dv50 of the second negative electrode active material is 1.6 μm to 5 μm. Thus, the second negative electrode active material has a suitable particle size Dv50, which can further improve the sodium ion transport efficiency and electrolyte wetting effect, and is more conducive to improving the kinetic performance of the electrode, further improving the rate performance of the battery. At the same time, it avoids a particle size Dv50 of less than 1.6 μm, which would lead to the dense packing of small particles affecting ion transport and electrolyte wetting, and would also cause a decrease in the overall compaction density of the electrode, thereby hindering the improvement of the battery's energy density and rate performance.

[0037] In some embodiments, the particle size Dv10 of the second negative electrode active material is 0.7 μm to 2.0 μm.

[0038] In some embodiments, the particle size Dv90 of the second negative electrode active material is 5.0 μm to 8.0 μm.

[0039] It should be noted that the particle size Dv10 and particle size Dv90 of the second negative electrode active material can be obtained by testing with a laser particle size analyzer, or by using image processing software to statistically analyze the particle size of the second negative electrode active material in the scanning electron microscope image of the negative electrode active layer, arranging the particle size from smallest to largest, and taking the particle size value that reaches 50% of the total volume as the particle size Dv50 of the polymer particles. For example, the particle size Dv50 of the second negative electrode active material can be, for example, 1.6 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3.0 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4.0 μm, 4.2 μm, 4.4 μm, 4.6 μm, 4.8 μm, 5.0 μm, or a value within the range of any two of the above values; the particle size Dv10 of the second negative electrode active material can be, for example, 0.7 μm, 0.8 μm, 1.0 μm, or 0.5 μm. The particle size Dv90 of the second negative electrode active material can be, for example, 5.0 μm, 5.2 μm, 5.4 μm, 5.6 μm, 5.8 μm, 6.0 μm, 6.2 μm, 6.4 μm, 6.6 μm, 6.8 μm, 7.0 μm, 7.2 μm, 7.4 μm, 7.6 μm, 7.8 μm, 8.0 μm, or a value within the range of any two of the above values.

[0040] In some embodiments, the particle size span of the second negative electrode active material is 0.6 to 3.6. Thus, the second negative electrode active material with a particle size span within this range is matched to its particle size, which can further improve the kinetic performance of the negative electrode sheet, thereby further contributing to improving the rate performance of the battery.

[0041] It should be noted that the particle size span value of the second negative electrode active material is calculated as (particle size Dv90 - particle size Dv10) / particle size Dv50. For example, the particle size span value of the second negative electrode active material can be 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 3.2, 3.4, 3.6, or a value within any range of two of the aforementioned values.

[0042] In some implementations, the following condition is met: 79% ≤ A2 ≤ 94%. Thus, controlling the cross-sectional fill rate of the second negative electrode active material within the range of 79% to 94% can further improve the insertion / extraction rate of sodium ions in the second negative electrode active layer, further enhance the interfacial dynamics of the electrode, and better contribute to improving the rate performance of the battery. Simultaneously, maintaining the cross-sectional fill rate within the aforementioned range ensures the structural stability of the second negative electrode active layer, preventing excessively low cross-sectional fill rates that could lead to poor structural stability of the negative electrode active material and affect the cycle stability of the battery.

[0043] For example, the cross-sectional filling rate A2 of the second negative electrode active material can be, for example, 79%, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, etc., or a value within the range of any two of the above values.

[0044] In some embodiments, the specific surface area of ​​the second negative electrode active material is 5.5 m². 2 / g~9.0m 2 / g. Thus, the second negative electrode active material has a high and suitable specific surface area, which can better facilitate the rapid transport of sodium ions in the second negative electrode active layer and further improve the wettability of the electrolyte to the second negative electrode active layer, thereby better improving the kinetic performance of the electrode and further improving the rate performance of the battery.

[0045] It should be noted that the specific surface area of ​​the second negative electrode active material was obtained through isothermal adsorption-desorption testing under nitrogen, yielding isothermal adsorption-desorption curves, and calculated using the Brunauer-Emmett-Teller (BET) model. For example, the specific surface area of ​​the second negative electrode active material could be, for instance, 5.5 m². 2 / g, 5.6m 2 / g, 5.8m 2 / g, 6.0m 2 / g、6.2m 2 / g, 6.4m 2 / g, 6.6m 2 / g, 6.8m 2 / g, 7.0m 2 / g, 7.2m 2 / g, 7.4m 2 / g, 7.6m 2 / g, 7.8m 2 / g, 8.0m 2 / g、8.2m 2 / g, 8.4m 2 / g, 8.6m 2 / g, 8.8m 2 / g, 9.0m 2 / g or values ​​within the range of any two of the above values.

[0046] In some embodiments, the porosity of the first negative electrode active layer is denoted as b, and the porosity of the second negative electrode active layer is denoted as c, satisfying: 25%≤b≤40%, 40%≤c≤50%, and 1≤c / b≤2. Thus, the porosity of the bottom first negative electrode active layer, within the range of 25%~40%, can further increase the compaction density of the bottom active layer, which is more conducive to obtaining an electrode with high compaction density, thereby further improving the energy density of the battery. Furthermore, the porosity of the inner layer within the above range can also ensure the effective transport of sodium ions to the deeper structures inside the electrode and improve the electrolyte retention capacity of the inner active layer structure, further contributing to improving the kinetic performance of the electrode. The porosity of the surface second negative electrode active layer, within the range of 40%~50%, helps to provide sufficient pore structure for sodium ion transport and electrolyte wetting, further contributing to improving the kinetic performance of the electrode and further improving the rate performance of the battery. Meanwhile, controlling the ratio of the porosity of the first negative electrode active layer to the porosity of the second negative electrode active layer within the range of 1 to 2 can further improve the matching between the first and second negative electrode active layers, enabling the kinetic performance, cycle stability, and compaction density of the electrode to reach the optimal balance point, thus further contributing to obtaining batteries with high energy density, excellent rate performance, and cycle stability.

[0047] It should be noted that the porosity b of the first negative electrode active layer and the porosity c of the second negative electrode active layer can be obtained through the following testing process: The electrode sheet is cut according to a template, and the area S3 of the cut sample is measured. Then, the thickness of the sample is measured 10 times, and the average value T is calculated. The apparent volume V0 = S3 × T is then calculated. The weight of the electrode sheet is measured using an electronic balance 3 times, and the average value M is obtained. The true density ρ of the material is measured using a true density meter, and the true volume V1 = M / ρ is calculated. The porosity of the electrode sheet is calculated according to the following formula. This method is also applicable to testing the porosity of the separator. Specifically, in this application, the porosity of the entire negative electrode active layer is tested first, and then the porosity of the first negative electrode active layer is tested after removing the second negative electrode active layer from the electrode sheet surface. The porosity of the second negative electrode active layer can be calculated based on these two porosities. Porosity = (V0 - V1) / V0 × 100%; For example, the porosity b of the first negative electrode active layer can be, for example, 25%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, etc., or a value within the range of any two of the above values; the porosity c of the second negative electrode active material can be, for example, 40%, 42%, 44%, 46%, 48%, 50%, etc., or a value within the range of any two of the above values. The ratio of the porosity of the first negative electrode active material to the porosity c / b of the second negative electrode active material can be, for example, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, etc., or a value within the range of any two of the above values.

[0048] In some embodiments, the one-sided surface density of the negative electrode active layer is 0.007 g / cm³. 2 ~0.018g / cm 2 This increases the capacity of the electrode, which helps improve the energy density of the battery. At the same time, it avoids the problem of insufficient kinetic performance caused by excessively high density on one side of the negative electrode active layer, which can lead to a drop in cycle performance.

[0049] It should be noted that the lateral surface density of the negative electrode active layer can be obtained by conventional methods in the art. For example, a certain area of ​​the lateral surface density of the negative electrode active layer can be cut off, the mass of the cut negative electrode active layer sample can be weighed, and the areal density of the negative electrode active layer can be calculated by substituting the measured area and the mass of the negative electrode active layer into the formula: areal density = mass of negative electrode active layer / area of ​​negative electrode active layer. For example, the lateral surface density of the negative electrode active layer can be 0.007 g / cm³. 2 0.008g / cm 2 0.009g / cm 2 0.010g / cm 2 0.011 g / cm2 0.012g / cm 2 0.013g / cm 2 0.014 g / cm 2 0.015g / cm 2 0.016g / cm 2 0.017g / cm 2 0.018g / cm 2 Values ​​equal to or within the range of any two of the above values.

[0050] In some embodiments, the thickness h1 of the first negative electrode active layer on one side is 55 μm to 100 μm, and the thickness h2 of the second negative electrode active layer on one side is 10 μm to 50 μm, satisfying: 55 ≤ h1 ≤ 100; and / or, 10 ≤ h2 ≤ 50; and / or, 0.1 ≤ h2 / h1 ≤ 0.8. Thus, by controlling the thickness ratio of the second negative electrode active layer to the first negative electrode active layer within the range of 0.1 to 0.8, this application can improve the matching between the first and second negative electrode active layers, ensuring that the electrode contains a sufficiently thick first negative electrode active layer. This results in a sufficient amount of large-particle negative electrode active material in the electrode, leading to a higher compaction density. Simultaneously, it ensures that the electrode contains a sufficiently thick second negative electrode active layer, resulting in a sufficient amount of small-particle second negative electrode active material. This further improves the kinetic performance of the electrode. Controlling the thickness ratio of the second to the first negative electrode active layer within the aforementioned range is more conducive to obtaining a negative electrode with the best overall effect in terms of compaction density and kinetic performance, thereby further contributing to improving the energy density and rate performance of the battery.

[0051] It should be noted that the single-sided thickness h1 of the first negative electrode active layer can be obtained by measuring the dimensions of the first negative electrode active layer along the thickness direction at at least 10 locations in the cross-sectional SEM image of the negative electrode sheet using an image analyzer, and then calculating the average value; the single-sided thickness h2 of the second negative electrode active layer can be obtained by measuring the dimensions of the second negative electrode active layer along the thickness direction at at least 10 locations in the cross-sectional SEM image of the negative electrode sheet using an image analyzer, and then calculating the average value. For example, the single-sided thickness h1 of the first negative electrode active layer can be, for example, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, 100μm, or a value within the range of any two of the above values; the single-sided thickness h2 of the second negative electrode active layer can be, for example, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, or a value within the range of any two of the above values; the ratio of the single-sided thickness h2 of the second negative electrode active layer to the single-sided thickness h1 of the first negative electrode active layer is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or a value within the range of any two of the above values.

[0052] In some embodiments, the total thickness of the negative electrode active layer on one side is denoted as H μm, satisfying: 90 ≤ H ≤ 140. Thus, a negative electrode active layer with a single-sided thickness of 90 μm to 140 μm can further improve the energy density of the battery. Simultaneously, the negative electrode active layer provided in this application, through its dual-layer active layer structure design and the selection of active materials in the first and second negative electrode active layers, is suitable for thicker negative electrode active layers and can also ensure that the thick electrode sheet has superior structural stability and kinetic performance, further improving the overall performance of the battery in terms of energy density, rate performance, and cycle stability.

[0053] It should be noted that the total thickness of the negative electrode active layer on one side can be obtained by measuring the thickness of the negative electrode active layer at at least 10 locations in the cross-sectional SEM image of the negative electrode sheet using image processing software (such as Image Pro Plus) and calculating the average value, or by measuring with a micrometer. For example, the total thickness of the negative electrode active layer on one side can be 90μm, 95μm, 100μm, 105μm, 110μm, 115μm, 120μm, 125μm, 130μm, 135μm, 140μm, etc., or a value within the range of any two of the above values.

[0054] In some embodiments, the total thickness of the negative electrode sheet is J μm, and the thickness of the negative electrode current collector is h³ μm, satisfying: 4 ≤ J / h³ ≤ 25. This improves the processing toughness of the electrode sheet, ensures superior ion migration and electron conduction within the electrode, and further contributes to improving the cycle stability and kinetic performance of the battery.

[0055] For example, the ratio of the total thickness of the negative electrode sheet to the thickness of the negative electrode current collector, J / h3, can be, for example, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 25, or a value within the range of any two of the above values.

[0056] Optionally, in some implementations, the following condition is satisfied: 150≤J≤300.

[0057] Optionally, in some implementations, the following condition is satisfied: 5≤h3≤20.

[0058] In some embodiments, in the cross-sectional scanning electron microscope image along the thickness direction of the negative electrode active layer, the maximum pore size within an arbitrary 50 μm × 50 μm range is denoted as k μm, and the particle size of the negative electrode active material including the maximum pore size is denoted as d μm, satisfying: 0.005 ≤ k / d ≤ 0.15, k ≥ 0.1. The negative electrode active material includes the first negative electrode active material and the second negative electrode active material. This allows the negative electrode particles to have a high compaction density while avoiding the problem of sodium ion diffusion and aggregation leading to a decrease in electrode capacity and an increase in irreversible side reactions caused by excessively large pore sizes in the negative electrode active layer.

[0059] It should be noted that the cross-sectional scanning electron microscope (SEM) image of the negative electrode active layer along its thickness direction is obtained by scanning electron microscopy of the interface after cutting the electrode sheet along its thickness direction with a plasma beam. When there are two or more negative electrode active materials containing the largest pore size, the smallest particle size is selected as d μm. If no particles with a maximum pore size ≥ 0.1 μm exist within the selected 50 μm × 50 μm region, it indicates that the local region of this particle is dominated by small-sized mesopores, which is effective in improving the material's plateau capacity; in this case, k is set to 0.

[0060] For example, k can be 0.1μm, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1.0μm, or a value within the range of any two of the above values; k / d can be 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, or a value within the range of any two of the above values.

[0061] Optionally, in some embodiments, in the cross-sectional scanning electron microscope image along the thickness direction of the negative electrode active layer, a 50μm × 50μm area at the junction of the first negative electrode active layer and the second negative electrode active layer is taken as a first region. The thickness ratio of the first negative electrode active layer to the second negative electrode active layer in the first region is (1~3):(1~3). The negative electrode active layer includes a first pore, which is a pore structure with a pore diameter of 0.1μm or more in the negative electrode active material. The negative electrode active material includes the first negative electrode active material and the second negative electrode active material. The number of the first pores in the first region is 5 to 120.

[0062] It should be noted that the number of pores in the first region is measured using a scanning electron microscope (SEM): Under a scanning electron microscope (SEM), the number of first pores in the negative electrode active material within the first region of the negative electrode active layer is recorded; this operation is repeated 5 times and the average value is taken, wherein only the first pores with complete cross-sectional morphology in the first region are counted. For example, the number of first pores in the first region can be, for example, 5, 10, 20, 40, 60, 80, 100, 120, etc., or a value within any two of the above ranges.

[0063] In some embodiments, the first negative electrode active layer further includes a first conductive agent, the first conductive agent including at least a first carbon nanotube, the first carbon nanotube having an aspect ratio of 1000 to 5000; the second negative electrode active layer further includes a second conductive agent, the second conductive agent including conductive carbon black and / or a second carbon nanotube, the second carbon nanotube having an aspect ratio of 200 to 1000. Thus, using carbon nanotubes with an aspect ratio of 1000-5000 in the first negative electrode active layer allows them to form a continuous electronic conduction network with the large particles of the first negative electrode active material. This is more conducive to constructing a continuous and uniform conductive network structure, further improving the electronic conductivity of the electrode, further reducing the battery's internal resistance, reducing polarization during battery charging and discharging, and further improving interface stability, which in turn is more conducive to improving the battery's long-cycle stability. The second negative electrode active layer uses carbon nanotubes or conductive carbon black with an aspect ratio of 200-1000, which can also form a continuous and uniform conductive network structure with the small particles of the second negative electrode active material, thereby further reducing the battery's internal resistance and improving the battery's long-cycle stability.

[0064] It should be noted that the aspect ratio of the first carbon nanotube can be obtained by selecting at least 100 carbon nanotubes in the first positive electrode active layer from the cross-sectional scanning electron microscope (SEM) image of the negative electrode active layer, measuring their length and diameter using image analysis tools, and calculating the aspect ratio by averaging the values. For example, the aspect ratio of the first carbon nanotube can be 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, or a value within any two of the above ranges.

[0065] It should be noted that the aspect ratio of the second carbon nanotube can be obtained by selecting at least 100 carbon nanotubes in the second positive electrode active layer from the cross-sectional scanning electron microscope (SEM) image of the negative electrode active layer, measuring their length and diameter using image analysis tools, and calculating the aspect ratio by averaging the values. For example, the aspect ratio of the second carbon nanotube can be 200, 300, 400, 500, 600, 700, 800, 900, 1000, or a value within any two of the above ranges.

[0066] Furthermore, in some embodiments, the ratio of the aspect ratio of the first carbon nanotube to that of the second carbon nanotube is 1.5 to 20. This improves the matching of electronic conductivity between the first and second negative electrode active layers, ensuring that both layers possess excellent electronic conductivity, further reducing polarization during battery charging and discharging, further improving interface stability, and ultimately contributing to improved battery cycle stability.

[0067] For example, the ratio of the aspect ratio of the first carbon nanotube to the aspect ratio of the second carbon nanotube can be, for example, 1.5, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, or a value within the range of any two of the above values.

[0068] In some embodiments, the negative electrode current collector is a corona-treated current collector, and / or a carbon layer is further disposed between the negative electrode current collector and the negative electrode active layer. This improves the adhesion between the negative electrode active layer and the negative electrode current collector, further enhancing the cycle stability of the battery.

[0069] In some embodiments, the first hard carbon material includes a first biomass-based hard carbon material.

[0070] In some embodiments, the second hard carbon material includes a second biomass-based hard carbon material.

[0071] The first biomass-based hard carbon material provided in this application can be obtained commercially or prepared using conventional processes in the art. Exemplarily, the first biomass-based hard carbon material can be prepared using the following process: A polysaccharide (including at least one of starch, glucose, and cellulose) is selected as a precursor and dispersed in anhydrous ethanol with an esterifying agent (e.g., citric acid, adipic acid, acetic acid, phosphoric acid, or maleic anhydride). The mixture is stirred at a constant temperature of 50°C to 80°C for 2 to 4 hours, filtered, washed with deionized water until neutral, and vacuum dried to obtain a modified precursor. The modified precursor is placed in a tube furnace, and an inert atmosphere is introduced to purge the air from the tube furnace. The temperature is increased to 400°C to 500°C at a rate of 1°C / min to 10°C / min and held for 1 to 2 hours. The mixture is then allowed to cool naturally and removed. Next, under an inert atmosphere, the temperature is increased to 1200°C to 1400°C at a rate of 1°C / min to 10°C / min and held for 1 to 3 hours, with the gas flow rate of the inert atmosphere maintained at 30 mL / min to 50 mL / min throughout the process, to obtain a carbonized product. The carbonized product is ball-milled and sieved to obtain a first biomass-based hard carbon material.

[0072] In this application, the inert atmosphere includes at least one of argon atmosphere, nitrogen atmosphere, neon atmosphere, and helium atmosphere.

[0073] Among them, the particle size-related parameters of the obtained biomass-based hard carbon material are controlled by adjusting the ball milling and sieving processes, such as adjusting the ball milling speed or time and the sieve mesh size; the cross-sectional filling rate of the biomass-based hard carbon material is controlled by controlling the heat treatment process of the tube furnace, such as the temperature and time of each stage of heat treatment.

[0074] The second biomass-based hard carbon material provided in this application can be obtained commercially or prepared using conventional processes in the art. For example, the second biomass-based hard carbon material can be prepared using the following process: Bamboo shavings are sieved to obtain uniform bamboo powder, which is then reacted with a 7wt%~15wt% acid reagent (e.g., hydrochloric acid) at 80℃~90℃ for 20min~60min at a solid-liquid ratio of 1:(15~30). The solid material is then filtered out, washed with deionized water until neutral, and dried to obtain the first powder. The first powder is then soaked in a 7wt%~15wt% alkaline solution (e.g., sodium hydroxide) at 20℃~30℃ for 2h~6h at a solid-liquid ratio of 1:(15~30). After filtration, the solid product is taken out, washed with deionized water until neutral, and dried to obtain the second powder. The second powder is placed in an inert atmosphere and first pyrolyzed at 400℃~600℃ for 4h~8h, then the temperature is further increased to 1200℃~1500℃ and held for 1h~4h to obtain a carbide. The carbide is then ball-milled and sieved to obtain the second biomass hard carbon material.

[0075] Among them, the particle size-related parameters of the obtained biomass-based hard carbon material are controlled by adjusting the ball milling and sieving processes, such as adjusting the ball milling speed or time and the sieve mesh size; the cross-sectional filling rate of the biomass-based hard carbon material is controlled by adjusting the acid washing process, alkali treatment process, and sintering process, such as the concentration of reagents used in acid washing and alkali treatment, treatment time, sintering temperature, and sintering time.

[0076] In some embodiments, the negative electrode sheet further includes a negative electrode binder, which includes at least one of polyvinylidene fluoride, polyacrylic acid, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0077] Optionally, in some embodiments, based on the total mass of the first negative electrode active layer, the mass content of the first negative electrode active material is 91% to 97%, the mass content of the first negative electrode conductive agent is 1% to 6%, and the mass content of the negative electrode binder is 1% to 5%.

[0078] Optionally, in some embodiments, based on the total mass of the second negative electrode active layer, the mass content of the second negative electrode active material is 91% to 97%, the mass content of the second negative electrode conductive agent is 1% to 6%, and the mass content of the negative electrode binder is 1% to 5%.

[0079] Secondly, this application provides a sodium-ion battery, comprising the negative electrode sheet described in the first aspect, as well as a positive electrode sheet, a separator, and an electrolyte; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction. Thus, the sodium-ion battery provided by this application contains a specific negative electrode sheet, which can improve the cycle stability, rate performance, and energy density of the sodium-ion battery.

[0080] In some embodiments, the negative electrode includes a plurality of negative tabs extending from a negative current collector, the extension direction of which is perpendicular to the thickness direction of the negative current collector; the positive electrode includes a plurality of positive tabs extending from the positive current collector, the extension direction of which is perpendicular to the thickness direction of the positive current collector; the number of negative tabs in the negative electrode is denoted as e, and the number of positive tabs in the positive electrode is denoted as f, satisfying: e ≥ f. Thus, the number of negative tabs in the negative electrode is higher than or equal to the number of positive tabs in the positive electrode, which can further reduce the electron transport internal resistance on the negative electrode during battery cycling, further reduce the heat generation on the negative electrode side, thereby promoting better cycle performance of the battery at high rates.

[0081] Optionally, in some embodiments, the number of negative electrode tabs in the negative electrode sheet is 110 to 140.

[0082] Optionally, in some embodiments, the number of positive tabs in the positive electrode sheet is 100 to 130.

[0083] In some embodiments, the capacity ratio of the negative electrode to the positive electrode is 1.12 to 1.40. This further improves the overall energy density and cycle life of the battery.

[0084] It should be noted that the capacity ratio of the negative electrode to the positive electrode can be calculated by sintering a certain mass M0 of electrode, grinding it, centrifuging to separate the active particle material, calculating the mass M1 of the active particles, and thus determining the weight percentage of the active material in the electrode. The active material is then assembled into a button cell, and its reversible specific capacity is measured. Therefore, the capacity ratio of the negative electrode to the positive electrode is calculated as (negative electrode areal density × weight percentage of negative electrode active material × negative electrode reversible specific capacity) / (positive electrode areal density × weight percentage of positive electrode active material × positive electrode reversible specific capacity). For example, the capacity ratio of the negative electrode to the positive electrode can be 1.12, 1.14, 1.16, 1.18, 1.20, 1.22, 1.24, 1.26, 1.28, 1.30, 1.32, 1.34, 1.36, 1.38, 1.40, or a value within any two of the above ranges.

[0085] In some embodiments, the positive current collector is a corona-treated current collector, and / or a carbon layer is further disposed between the positive current collector and the positive active layer.

[0086] Optionally, in some embodiments, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector along the thickness direction, the positive active layer including a positive active material, a positive conductive agent and a positive binder.

[0087] Optionally, in some embodiments, the positive electrode active material includes at least one of layered transition metal oxides, polyanionic compounds, and Prussian blue analogues.

[0088] Optionally, in some embodiments, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), nitrile rubber (NBR), aqueous acrylic resin, polyvinyl alcohol, polyvinyl butyral, polyurethane, fluorinated rubber, carboxymethyl cellulose (CMC), and polyacrylic acid (PAA).

[0089] Optionally, in some embodiments, the positive electrode conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, and Ketjen black.

[0090] Optionally, in some embodiments, based on the total mass content of the positive electrode active layer, the mass content of the positive electrode active material is 93% to 97%, the mass content of the positive electrode binder is 1% to 5%, and the mass content of the positive electrode conductive agent is 1% to 5%.

[0091] Optionally, in some embodiments, the density of the electrolyte is 1.1 g / cm³ at 25°C. 3 ~1.3g / cm 3 .

[0092] It should be noted that the density of the electrolyte can be calculated by measuring the mass of a unit volume of electrolyte at 25°C. For example, the density of the electrolyte could be 1.10 g / cm³. 3 1.15g / cm 3 1.20g / cm 3 1.25g / cm 3 1.30g / cm 3 Values ​​equal to or within the range of any two of the above values.

[0093] Optionally, in some embodiments, the conductivity of the electrolyte is 7.5 ms / cm to 9.0 ms / cm at 25°C.

[0094] It should be noted that the conductivity of the electrolyte can be obtained by measuring the conductivity at 25°C using a conductivity meter. For example, the conductivity of the electrolyte can be 7.5 ms / cm, 7.6 ms / cm, 7.8 ms / cm, 8.0 ms / cm, 8.2 ms / cm, 8.4 ms / cm, 8.6 ms / cm, 8.8 ms / cm, 9.0 ms / cm, or a value within any two of the above ranges.

[0095] In some embodiments, the electrolyte comprises an organic solvent, which includes at least one of acetonitrile, tetrahydrofuran, ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, propyl propionate, ethyl propionate, propyl acetate, ethyl acetate, methyl ethyl carbonate, and dimethyl sulfoxide.

[0096] Optionally, in some embodiments, the electrolyte further includes a sodium salt, which includes at least one of NaPF6, NaBF4, NaClO4, NaAsF6, and NaCF3SO3.

[0097] In some embodiments, the diaphragm includes a substrate layer and a functional coating disposed on at least one side surface of the substrate layer in the thickness direction.

[0098] In some embodiments, the thickness of the substrate layer is 5 μm to 12 μm.

[0099] For example, the thickness of the substrate layer may be 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, or a value within the range of any two of the above values.

[0100] Optionally, in some embodiments, the substrate layer is made of polyethylene and / or polypropylene.

[0101] In some embodiments, the material of the functional coating includes inorganic particles.

[0102] Optionally, in some embodiments, the inorganic particles include at least one of alumina, boehmite, lanthanum aluminum zirconate, lanthanum aluminum titanate, lithium aluminum titanium phosphate, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zirconium oxide, zinc oxide, calcium oxide, magnesium hydroxide, aluminum hydroxide, barium hydroxide, barium sulfate, calcium silicate, and titanium dioxide.

[0103] Optionally, in some embodiments, the diaphragm further includes an adhesive layer.

[0104] Optionally, in some embodiments, the adhesive layer includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, polymethyl methacrylate (PMMA), and polydopamine.

[0105] In some embodiments, the thickness of the functional coating is 2 μm to 4 μm.

[0106] For example, the thickness of the functional coating may be 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3.0μm, 3.2μm, 3.4μm, 3.6μm, 3.8μm, 4.0μm, or a value within the range of any two of the above values.

[0107] In some embodiments, the porosity of the diaphragm is 25% to 40%.

[0108] It should be noted that the porosity of the membrane can be obtained by testing the ratio of density to true density calculated from the thickness. For example, the porosity of the membrane can be 25%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, or a value within any two of the above values.

[0109] Example 1 This embodiment provides a method for preparing a sodium-ion battery, including the following steps: (1) Preparation of negative electrode A first biomass-based hard carbon material, conductive carbon black, carbon nanotubes with an aspect ratio of 3567, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of 95:2:0.5:1.3:1.2, and deionized water was added as a solvent to continue mixing, resulting in a first negative electrode slurry. A second biomass-based hard carbon material, conductive carbon black, carbon nanotubes with an aspect ratio of 942, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed in a mass ratio of 95:2:0.5:1.3:1.2, and deionized water was added as a solvent to continue mixing, resulting in a second negative electrode slurry. The first and second negative electrode slurries were sequentially coated onto both sides of the negative electrode current collector-aluminum foil along the thickness direction. Then, after drying and rolling, the rolled electrode sheet was laser-cut to form negative electrode tabs, resulting in a negative electrode sheet.

[0110] The negative electrode sheet consists of a negative electrode current collector, negative electrode active layers disposed on both sides of the negative electrode current collector in the thickness direction, and negative electrode tabs. Each side of the negative electrode active layer consists of a first negative electrode active layer and a second negative electrode active layer, with the first negative electrode active layer disposed between the negative electrode current collector and the second negative electrode active layer. The negative electrode tabs extend from the negative electrode current collector. In the cross-sectional scanning electron microscope image of the negative electrode active layer in the thickness direction, the maximum pore size within an arbitrary 50μm × 50μm range is denoted as k, and the particle size of the negative electrode active material containing the maximum pore size is denoted as d, satisfying: k / d = 0.0448, k = 0.247μm, and d = 5.51μm. The negative electrode active material is a first biomass-based hard carbon material and a second biomass-based hard carbon material. Other parameters of the negative electrode sheet are shown in Tables 1-3 below.

[0111] (2) Preparation of positive electrode sheet The positive electrode active material (chemical formula: Na4Fe3(PO4)2P2O7), conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5. N-methylpyrrolidone was added and mixing continued to obtain a positive electrode slurry. This slurry was then coated onto both sides of the positive electrode current collector (aluminum foil) along its thickness direction. After drying and rolling, the positive electrode active layer (with a single-sided density of 0.029 g / cm³) was obtained. 2 Then, the rolled electrode sheet is laser-cut to form a positive electrode tab, thus obtaining the positive electrode sheet.

[0112] (3) Preparation of the diaphragm A functional coating is formed on one side of a polyethylene substrate layer. Then, a polyvinylidene fluoride (PVDF) adhesive layer is formed on both the side of the polyethylene substrate layer without the functional coating and the side of the functional coating away from the polyethylene substrate layer to obtain a diaphragm. The polyethylene substrate layer has a thickness of 9 μm, the functional coating includes alumina particles and has a thickness of 3 μm, the PVDF adhesive layer has a thickness of 1 μm, and the porosity of the resulting diaphragm is 32.4%.

[0113] (4) Preparation of electrolyte A solvent consisting of propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (EMC) in a volume ratio of 1:1:1 was mixed. Ethylene carbonate, succinate, and sodium salt NaPF6 were then added and mixed further to obtain an electrolyte. Based on the total mass of the electrolyte, the mass content of the ethylene carbonate was 5%, the mass content of the succinate was 3%, and the mass content of NaPF6 was 12%.

[0114] (5) Assembly of sodium-ion batteries The positive electrode sheet prepared in step (2), the separator prepared in step (3), and the negative electrode sheet prepared in step (1) are wound into a core. The functional coating in the separator is set facing the positive electrode sheet to form an electrode assembly. The assembly is encapsulated in a square shell and injected with the electrolyte prepared in step (4). After standing, the electrolyte is charged. After two sealings are completed, the assembly is sorted and tested with OCV to obtain a sodium-ion battery.

[0115] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Example 1, with differences shown in Tables 1, 2, and 3. Among them, " "" indicates: the same as in Example 1 or a value with reasonable deviation due to the testing process; " / " indicates: not present; A1 indicates: the cross-sectional filling rate of the first negative electrode active material (i.e., the first biomass-based hard carbon material); D1 indicates: the particle size Dv50 of the first negative electrode active material; b indicates: the porosity of the first negative electrode active layer; h1 indicates: the thickness of one side of the first negative electrode active layer; A2 indicates: the cross-sectional filling rate of the second negative electrode active material (i.e., the second biomass-based hard carbon material); L1 indicates: The following values ​​are represented as follows: Dv50, the particle size of the second negative electrode active material; c, the porosity of the second negative electrode active layer; h2, the thickness of one side of the second negative electrode active layer; H, the total thickness of the one-side negative electrode active layer; J, the total thickness of the negative electrode sheet; h3, the thickness of the negative electrode current collector; k, the maximum pore size within a 50μm × 50μm range in the cross-sectional scanning electron microscope image along the thickness direction of the negative electrode active layer; and d, the particle size of the negative electrode active material including the maximum pore size k.

[0116] Table 1

[0117] Table 2

[0118] Table 3

[0119] Compared with Example 1, Comparative Example 6 has the same total thickness of negative electrode active layer, omits the second negative electrode active layer, and only has the first negative electrode active layer.

[0120] Compared with Example 1, Comparative Example 7 has the same total thickness of negative electrode active layer, but the first negative electrode active layer is omitted and the negative electrode active layer is only the second negative electrode active layer.

[0121] Test example: The sodium-ion batteries provided in the above embodiments and comparative examples were tested as follows: (1) Energy density testing process: The sodium-ion batteries provided in the above embodiments and comparative examples were charged to 3.65V at a constant current and constant voltage of 0.2C at an environment of 25±2℃, and then cut off at 0.02C. After resting for 10 minutes, they were discharged to 1.5V at a constant current of 0.2C, and the discharge energy E was recorded. The volume of the test electrode assembly was V2 (length × height × width), and the volumetric energy density was calculated as E / V2, with the unit being Wh / L.

[0122] (2) Testing process for cycle capacity retention: The sodium-ion batteries provided in the above examples and comparative examples were placed in an environment of 25±2℃. (1) They were charged at a constant current and constant voltage of 0.2C to the upper limit voltage of 3.65V, with a cutoff current of 0.02C, and left to stand for 10 minutes. (2) They were discharged at a constant current of 0.2C to 1.5V, left to stand for 10 minutes, and the initial discharge specific capacity was recorded as C0. (3) Steps (1) to (2) were repeated until 2000 cycles were reached. The discharge specific capacity of the 2000th cycle was recorded as C1. The cycle capacity retention rate was calculated as C1 / C0×100%.

[0123] (3) Internal resistance testing process: At 25°C, the sodium-ion battery was charged to 3.65V at a constant current and constant voltage of 0.2C, and then cut off at 0.02C. After resting for 10 minutes, it was discharged to 50% SOC at a constant current of 0.2C. The AC impedance of the charged and discharged sodium-ion battery was tested using a Metrohm PGSTAT302N chemist in the range of 100KHz~0.1mHz at 25°C.

[0124] (4) Testing process for rate performance: The sodium-ion batteries provided in the above embodiments and comparative examples were charged at a constant current density of 0.2C to 3.65V in an environment of 25±2℃, then charged at a constant voltage of 3.65V with a cutoff current of 0.02C. After resting for 10 minutes, they were discharged at a constant current density of 0.2C to 1.5V and rested for 10 minutes. The discharge specific capacity C2 at 0.2C was recorded. The batteries were then charged at a constant current density of 0.2C to 3.65V again, then charged at a constant voltage of 3.65V with a cutoff current of 0.02C. After resting for 10 minutes, they were discharged at a constant current density of 2C to 1.5V and rested for 10 minutes. The discharge specific capacity C3 at 2C was recorded. The percentage ratio of the discharge specific capacity C3 at 2C to the discharge specific capacity C2 at 0.2C is the rate performance of the battery, i.e., rate performance (%) = C3 / C2 × 100%.

[0125] The test results are shown in Table 4.

[0126] Table 4

[0127] As can be seen from Tables 1 to 4, in the negative electrode sheet provided in this application, the first negative electrode active layer uses large-particle hard carbon material, and the second negative electrode active layer uses small-particle hard carbon material. The ratio of the particle size Dv50 of the negative electrode active material in the first negative electrode active layer to that in the second negative electrode active layer is controlled to be 2 to 6. At the same time, the difference in cross-sectional filling rate between the first negative electrode active material and the second negative electrode active material is controlled to be greater than 0 and less than or equal to 20%. A negative electrode sheet with good cycle stability, excellent kinetic performance and high compaction density is obtained, thereby improving the cycle stability, rate performance and energy density of sodium-ion batteries.

[0128] 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 negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer disposed on at least one surface of the negative electrode current collector in the thickness direction, characterized in that, The negative electrode active layer includes a first negative electrode active layer and a second negative electrode active layer, wherein the first negative electrode active layer is disposed between the negative electrode current collector and the second negative electrode active layer; The first negative electrode active layer includes a first negative electrode active material, and the first negative electrode active material includes a first hard carbon material; The second negative electrode active layer includes a second negative electrode active material, and the second negative electrode active material includes a second hard carbon material; The particle size Dv50 of the first negative electrode active material is denoted as D1 μm, and the particle size Dv50 of the second negative electrode active material is denoted as L1 μm, satisfying: 2≤D1 / L1≤6; The cross-sectional filling rate of the first negative electrode active material is denoted as A1, and the cross-sectional filling rate of the second negative electrode active material is denoted as A2, satisfying: 0 < A1 - A2 ≤ 20%.

2. The negative electrode sheet according to claim 1, characterized in that, The first negative electrode active material satisfies at least one of the following conditions: (A) 7≤D1≤12; (B) Particle size Dv10 is 2μm~6μm; (C) Particle size Dv90 is 16μm~24μm; (D) Particle size span value is 0.9~3.1; (E) 85% ≤ A1 ≤ 99%; (F) Specific surface area is 3.2 m² 2 / g~5.2m 2 / g.

3. The negative electrode sheet according to claim 1, characterized in that, The second negative electrode active material satisfies at least one of the following conditions: (a) 1.6 ≤ L1 ≤ 5; (b) Particle size Dv10 is 0.7 μm to 2.0 μm; (c) Particle size Dv90 is 5.0 μm to 8.0 μm; (d) Particle size span value is 0.6~3.6; (e) 79% ≤ A2 ≤ 94%; (f) Specific surface area is 5.5 m² 2 / g~9.0m 2 / g.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that, The porosity of the first negative electrode active layer is denoted as b, and the porosity of the second negative electrode active layer is denoted as c, satisfying: 25%≤b≤40%, 40%≤c≤50%, 1≤c / b≤2; And / or, the density of one side of the negative electrode active layer is 0.007 g / cm³. 2 ~0.018g / cm 2 .

5. The negative electrode sheet according to any one of claims 1 to 3, characterized in that, The negative electrode sheet satisfies at least one of the following conditions: (1) The thickness of the first negative electrode active layer on one side is denoted as h1 μm, and the thickness of the second negative electrode active layer on one side is denoted as h2 μm, satisfying: 55≤h1≤100; and / or, 10≤h2≤50; and / or, 0.1≤h2 / h1≤0.8; (2) The total thickness of the negative electrode active layer on one side is denoted as H μm, which satisfies: 90≤H≤140; (3) The total thickness of the negative electrode sheet is J μm, and the thickness of the negative electrode current collector is h3 μm, satisfying: 4≤J / h3≤25; (4) In the cross-sectional scanning electron microscope image of the negative electrode active layer in the thickness direction, the maximum pore size within a range of 50μm×50μm is denoted as k μm, and the particle size of the negative electrode active material including the maximum pore size is denoted as d μm, satisfying: 0.005≤k / d≤0.15, k≥0.1; the negative electrode active material includes the first negative electrode active material and the second negative electrode active material; (5) The first negative electrode active layer further includes a first conductive agent, the first conductive agent including at least a first carbon nanotube, the aspect ratio of the first carbon nanotube being 1000~5000; the second negative electrode active layer further includes a second conductive agent, the second conductive agent including conductive carbon black and / or a second carbon nanotube, the aspect ratio of the second carbon nanotube being 200~1000; preferably, the ratio of the aspect ratio of the first carbon nanotube to the aspect ratio of the second carbon nanotube is 1.5~20; (6) The negative electrode current collector is a current collector treated with corona discharge, and / or, a carbon layer is provided between the negative electrode current collector and the negative electrode active layer; (7) The first hard carbon material includes a first biomass-based hard carbon material; (8) The second hard carbon material includes a second biomass-based hard carbon material.

6. A sodium-ion battery, characterized in that, The sodium-ion battery includes a negative electrode sheet as described in any one of claims 1 to 5, as well as a positive electrode sheet, a separator, and an electrolyte; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction.

7. The sodium-ion battery according to claim 6, characterized in that, The negative electrode plate includes a plurality of negative electrode tabs, which extend from the negative electrode current collector, and the extension direction of the negative electrode tabs is perpendicular to the thickness direction of the negative electrode current collector; the positive electrode plate includes a plurality of positive electrode tabs, which extend from the positive electrode current collector, and the extension direction of the positive electrode tabs is perpendicular to the thickness direction of the positive electrode current collector; the number of negative electrode tabs in the negative electrode plate is denoted as e, and the number of positive electrode tabs in the positive electrode plate is denoted as f, satisfying: e≥f.

8. The sodium-ion battery according to claim 6, characterized in that, The capacity ratio of the negative electrode to the positive electrode is 1.12 to 1.

40.

9. The sodium-ion battery according to any one of claims 6 to 8, characterized in that, The electrolyte includes an organic solvent, which includes at least one of acetonitrile, tetrahydrofuran, ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, propyl propionate, ethyl propionate, propyl acetate, ethyl acetate, methyl ethyl carbonate, and dimethyl sulfoxide.

10. The sodium-ion battery according to any one of claims 6 to 8, characterized in that, The diaphragm includes a substrate layer and a functional coating disposed on at least one surface of the substrate layer in the thickness direction; The thickness of the substrate layer is 5μm~12μm; The functional coating comprises inorganic particles; The thickness of the functional coating is 2μm~4μm; The porosity of the diaphragm is 25%~40%.

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