A sodium-ion secondary battery

Abstract

This application relates to the field of secondary battery technology and discloses a sodium-ion secondary battery. The positive electrode includes a current collector, a bottom carbon layer, a first positive electrode active layer, and a second positive electrode active layer. The first positive electrode active material in the first positive electrode active layer is a monocrystalline polyanionic material, and the second positive electrode active material in the second positive electrode active layer is a polycrystalline polyanionic material. The particle size Dv50 of the first positive electrode active material is larger than that of the second positive electrode active material. The mass content of sodium salt in the electrolyte is A; the single-sided thicknesses of the bottom carbon layer and the positive electrode active layer are G μm and H μm, respectively, satisfying: 10.15 ≤ G / H × 100 + A ≤ 16.4. The sodium-ion secondary battery provided by this application combines high energy density, excellent cycle stability and rate performance, and low internal resistance.

Description

Technical Field

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

[0002] Polyanionic cathode materials for sodium-ion secondary batteries are widely used in commercial energy storage due to their stable structure, low cost, and excellent cycle life. However, most polyanionic sodium cathode materials currently exhibit problems such as poor cycle stability, inadequate capacity utilization, and deteriorated kinetic performance in practical applications of sodium-ion secondary batteries. This makes it difficult for the long-cycle stability, rate performance, and energy density of secondary batteries to simultaneously meet the demands of today's market. Summary of the Invention

[0003] In view of this, this application provides a sodium-ion secondary battery to solve the problem that existing polyanion cathode materials cannot simultaneously achieve good cycle stability, high energy density, and excellent kinetic performance when applied to secondary batteries.

[0004] According to an embodiment of this application, in a first aspect, this application provides a sodium-ion secondary battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive current collector and a bottom carbon layer and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction, wherein the bottom carbon layer is located between the positive current collector and the positive active layer. The positive active layer includes a first positive active layer and a second positive active layer stacked thereon, wherein the first positive active layer is disposed between the bottom carbon layer and the second positive active layer. The first positive electrode active layer includes a first positive electrode active material, the first positive electrode active material includes a first polyanionic material, and the first polyanionic material includes single crystal particles; The second positive electrode active layer includes a second positive electrode active material, the second positive electrode active material includes a second polyanionic material, and the second polyanionic material includes polycrystalline particles; Let D1 be the particle size Dv50 of the first positive electrode active material and L1 be the particle size Dv50 of the second positive electrode active material, satisfying: D1 > L1; The first polyanionic material and the second polyanionic material independently comprise sodium iron pyrophosphate and / or doped sodium iron pyrophosphate. The electrolyte includes sodium salt; the mass content of the sodium salt is denoted as A% based on the total mass of the electrolyte; the thickness of the bottom carbon layer on one side is denoted as G μm; and the total thickness of the positive electrode active layer on one side is denoted as H μm, satisfying: 10.15≤G / H×100+A≤16.4.

[0005] The technical solution of this application has the following advantages: 1. The positive electrode sheet of this application adopts a double-layer active layer structure. The active material in the first positive electrode active layer (also known as the inner layer) near the positive electrode current collector is a single-crystal large-particle doped or undoped sodium iron pyrophosphate polyanionic positive electrode material (for ease of description, the doped or undoped sodium iron pyrophosphate polyanionic positive electrode material will be referred to as NFPP-type polyanionic material in the following text). The active material in the second positive electrode active layer (also known as the outer layer) away from the positive electrode current collector is a polycrystalline small-particle doped or undoped sodium iron pyrophosphate polyanionic positive electrode material. The single-crystal large-particle NFPP-type polyanionic material has strong structural stability and inner layer... The ion diffusion channels in the first positive electrode layer are more continuous and stable, enabling deep sodium removal without structural collapse. Furthermore, the large-particle NFPP-type polyanionic material effectively increases the overall compaction density of the electrode, resulting in higher specific capacity and improved energy density of the secondary battery. A polycrystalline small-particle NFPP-type polyanionic material is then placed on the surface of the positive electrode active layer containing the large-particle NFPP-type polyanionic material. This polycrystalline small-particle NFPP-type polyanionic material, with its numerous grain boundaries and pores, facilitates sodium ion transport and electrolyte wetting, improving the overall kinetic performance of the electrode. This application combines the use of large-particle NFPP-type polyanionic material in the first positive electrode active layer with the use of polycrystalline small-particle NFPP-type polyanionic material in the second positive electrode active layer. These two materials work synergistically to improve the cycle stability, capacity performance, and kinetic performance of the positive electrode, thereby contributing to a sodium-ion secondary battery with high cycle stability, energy density, and good rate performance.

[0006] On the other hand, this application provides a bottom carbon layer between the positive electrode active layer with a specific structure and composition and the positive electrode current collector. First, the bottom carbon layer can tightly connect the positive electrode active layer and the positive electrode current collector to form an ohmic contact, reduce the interfacial electron transport barrier, thereby reducing the interfacial internal resistance of the secondary battery and improving the rate performance of the battery. Second, the bottom carbon layer can alleviate the cumulative stress generated by the repeated insertion and extraction of sodium in the positive electrode active material during battery cycling, improve the structural stability of the electrode, thereby improving the cycle stability of the battery. Furthermore, even if microcracks are generated in the positive electrode active layer due to the volume change of the internal active material, the bottom carbon layer can still maintain a good electron pathway and prevent a sudden drop in battery capacity. Meanwhile, this application controls the thickness of the bottom carbon layer, the positive electrode active layer, and the mass content of sodium salt in the electrolyte to satisfy: 10.15≤G / H×100+A≤16.4, so that the bottom carbon layer has a thickness that matches the thickness of the positive electrode active layer, and controls the sum of the thickness percentage of the two and the mass content of sodium salt in the electrolyte to be within the range of 10.15~16.4, so that there is excellent matching between the electrolyte and the positive electrode active layer structure. The two can better cooperate with each other to improve the electron transport effect inside the positive electrode sheet of NFPP-type polyanionic material containing single crystal large particles and polycrystalline small particles, reduce the interfacial internal resistance, promote ion liquid phase transport in the electrolyte, reduce liquid phase transport impedance, and improve the ionic conductivity and electronic conductivity of the battery. This is beneficial to improve the rate performance of sodium-ion secondary batteries and improve the cycle stability of secondary batteries, while ensuring that the secondary battery has a high energy density. 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 front cross-sectional structure of the positive electrode sheet in Embodiment 1 of this application; Figure 2 This is a top view of the positive electrode sheet of Embodiment 1 of this application; Figure 3 This is a schematic diagram of the structure of the sodium-ion secondary battery of Embodiment 1 of this application.

[0009] In the figure, 1 is the positive current collector; 2 is the bottom carbon layer; 3 is the positive active layer; 3-1 is the first positive active layer; 3-2 is the second positive active layer; 4 is the positive electrode tab; 5 is the insulating layer; 6 is the electrode assembly; 7 is the negative electrode tab; 8 is the housing; X is the first direction; Y is the second direction; Z is the third direction; α is the dimension in the first direction; β is the dimension in the second direction; γ is the dimension in the third direction. Detailed Implementation

[0010] 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.

[0011] It should be noted in the description of this application that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the technical features involved in the different embodiments of this application described below may be combined with each other as long as they do not conflict with each other.

[0012] To address the problem that existing polyanion cathode materials cannot simultaneously achieve good cycle stability, high energy density, and excellent kinetic performance when applied to secondary batteries, this application proposes the following solution.

[0013] In a first aspect, this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a bottom carbon layer and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction, wherein the bottom carbon layer is located between the positive current collector and the positive active layer. The positive active layer includes a first positive active layer and a second positive active layer stacked thereon, wherein the first positive active layer is disposed between the bottom carbon layer and the second positive active layer. The first positive electrode active layer includes a first positive electrode active material, the first positive electrode active material includes a first polyanionic material, and the first polyanionic material includes single crystal particles; The second positive electrode active layer includes a second positive electrode active material, the second positive electrode active material includes a second polyanionic material, and the second polyanionic material includes polycrystalline particles; Let D1 be the particle size Dv50 of the first positive electrode active material and L1 be the particle size Dv50 of the second positive electrode active material, satisfying: D1 > L1; The first polyanionic material and the second polyanionic material independently comprise sodium iron pyrophosphate and / or doped sodium iron pyrophosphate. The electrolyte includes sodium salt; the mass content of the sodium salt is denoted as A% based on the total mass of the electrolyte; the thickness of the bottom carbon layer on one side is denoted as G μm; and the total thickness of the positive electrode active layer on one side is denoted as H μm, satisfying: 10.15≤G / H×100+A≤16.4.

[0014] The positive electrode sheet of this application adopts a double-layer active layer structure. The active material in the first positive electrode active layer (also called the inner layer) near the positive electrode current collector is a single-crystal large-particle doped or undoped sodium iron pyrophosphate polyanionic positive electrode material (for ease of description, the doped or undoped sodium iron pyrophosphate polyanionic positive electrode material will be referred to as NFPP-type polyanionic material in the following text). The active material in the second positive electrode active layer (also called the outer layer) away from the positive electrode current collector is a polycrystalline small-particle doped or undoped sodium iron pyrophosphate polyanionic positive electrode material. The single-crystal large-particle NFPP-type polyanionic material has strong structural stability and internal... The ion diffusion channels are more continuous and stable, enabling deep sodium removal without structural collapse. Furthermore, large-particle NFPP-type polyanionic materials effectively increase the overall compaction density of the electrode, resulting in higher specific capacity and improved energy density of the secondary battery. A positive electrode active layer containing polycrystalline small-particle NFPP-type polyanionic materials is then placed on the surface of the positive electrode active layer containing large-particle NFPP-type polyanionic materials. These polycrystalline small-particle NFPP-type polyanionic materials have numerous grain boundaries and pores, facilitating sodium ion transport and electrolyte wetting, thus improving the overall kinetic performance of the electrode. This application combines the use of large-particle NFPP-type polyanionic materials in the first positive electrode active layer with the use of polycrystalline small-particle NFPP-type polyanionic materials in the second positive electrode active layer. These two materials work synergistically to improve the cycle stability, capacity performance, and kinetic performance of the positive electrode, thereby contributing to a secondary battery with high cycle stability, energy density, and good rate performance.

[0015] On the other hand, this application provides a bottom carbon layer between the positive electrode active layer with a specific structure and composition and the positive electrode current collector. First, the bottom carbon layer can tightly connect the positive electrode active layer and the positive electrode current collector to form an ohmic contact, reduce the interfacial electron transport barrier, thereby reducing the interfacial internal resistance of the secondary battery and improving the rate performance of the battery. Second, the bottom carbon layer can alleviate the cumulative stress generated by the repeated insertion and extraction of sodium in the positive electrode active material during battery cycling, improve the structural stability of the electrode, thereby improving the cycle stability of the battery. Furthermore, even if microcracks are generated in the positive electrode active layer due to the volume change of the internal active material, the bottom carbon layer can still maintain a good electron pathway and prevent a sudden drop in battery capacity. Meanwhile, this application controls the thickness of the bottom carbon layer, the positive electrode active layer, and the mass content of sodium salt in the electrolyte to satisfy: 10.15≤G / H×100+A≤16.4, so that the bottom carbon layer has a thickness that matches the thickness of the positive electrode active layer, and controls the sum of the thickness percentage of the two and the mass content of sodium salt in the electrolyte to be within the range of 10.15~16.4, so that there is excellent matching between the electrolyte and the positive electrode active layer structure. The two can better cooperate with each other to improve the electron transport effect inside the positive electrode sheet containing NFPP-type polyanionic materials with single crystal large particles and polycrystalline small particles, reduce the interfacial internal resistance, promote ion liquid phase transport in the electrolyte, reduce liquid phase transport impedance, and improve the ionic conductivity and electronic conductivity of the battery. This is beneficial to improve the rate performance and cycle stability of the secondary battery, and at the same time, it can ensure that the secondary battery has a high energy density.

[0016] This application research found that if the particle size Dv50 of the first positive electrode active material is less than or equal to the particle size Dv50 of the second positive electrode active material, it means that the particle size Dv50 of the first positive electrode active material is too small and the particle size Dv50 of the second positive electrode active material is too large. The specific capacity of the first positive electrode active material with a particle size Dv50 that is too small is low, which is not conducive to improving the energy density of the secondary battery. On the other hand, the second positive electrode active material with a particle size Dv50 that is too large is located in the outer active layer, which will affect the transport of sodium ions in the positive electrode active layer and the wetting of the electrolyte, resulting in a decrease in the overall kinetic performance of the electrode, which is not conducive to improving the rate performance of the secondary battery and increases the internal resistance of the battery.

[0017] This application also found that if the sum of the thickness percentages of the bottom carbon layer and the positive electrode active layer and the mass content of sodium salt in the electrolyte is less than 10.15, the matching between the thickness of the bottom carbon layer, the thickness of the positive electrode active layer, and the mass content of sodium salt in the electrolyte will deteriorate. This means that either the thickness percentage of the bottom carbon layer and the positive electrode active layer is too low, or the mass content of sodium salt in the electrolyte is too low. A low thickness percentage means the bottom carbon layer is insufficient to match the thickness of the positive electrode active layer, failing to effectively connect the positive electrode active layer and the positive electrode current collector. This results in poor contact between the various layers within the positive electrode sheet, increasing the interfacial resistance of the secondary battery. Furthermore, a low thickness percentage also fails to effectively alleviate the stress generated by the repeated sodium insertion / extraction within the positive electrode active layer during battery cycling, thus affecting the structural stability of the electrode sheet and hindering improvements in battery cycle stability. In addition, the thinner bottom carbon layer compared to the positive electrode active layer cannot maintain a good electronic pathway, easily leading to a sudden drop in battery capacity. If the sodium salt content in the electrolyte is too low, it will affect the diffusion effect of ions in the positive electrode, resulting in a decrease in the ionic conductivity of the secondary battery and poor stability of the positive electrode electrolyte interface (CEI) film, which will affect the rate performance and cycle stability of the secondary battery. Conversely, if the sum of the thickness percentage of the bottom carbon layer and the positive electrode active layer and the mass content of sodium salt in the electrolyte is higher than 16.4%, it indicates that the thickness percentage of the bottom carbon layer and the positive electrode active layer is too high, or that the mass content of sodium salt in the electrolyte is too high. An excessively high thickness percentage of the bottom carbon layer and the positive electrode active layer means that the thickness of the bottom carbon layer, which matches the thickness of the positive electrode active layer, is too high. This will lead to a decrease in the proportion of active material in the electrode, resulting in a longer diffusion path for sodium ions, increased resistance, and impaired sodium ion transport in the positive electrode. Consequently, the energy density and rate performance of the secondary battery will decrease. Furthermore, an excessively thick bottom carbon layer will also increase the manufacturing cost of the secondary battery. On the other hand, an excessively high sodium salt content in the electrolyte increases the electrolyte viscosity, hinders ion mobility, reduces the ionic conductivity of the electrode, and decreases the wetting effect of the electrolyte on the electrode, increasing local polarization. This will affect the rate performance and cycle stability of the secondary battery, and also increase the battery's cycle resistance.

[0018] It should be noted that the particle size Dv50 of the first positive electrode active material can be obtained by measuring the particle size of the first positive electrode active material using a scanning electron microscope image combined with image analysis or a laser particle size analyzer; the particle size Dv50 of the second positive electrode active material can be obtained by measuring the particle size of the second positive electrode active material using a scanning electron microscope image combined with image analysis or a laser particle size analyzer; the mass content A% of sodium salt in the electrolyte can be obtained by GC-MS (gas chromatography-mass spectrometry); the thickness G μm of the bottom carbon layer can be obtained by measuring the thickness of the bottom carbon layer at any at least 10 locations in the cross-sectional SEM image of the positive electrode sheet using an image analyzer and calculating the average value; the total thickness H μm of the positive electrode active layer can be obtained by measuring the total thickness of the positive electrode active layer at at least 10 locations in the cross-sectional SEM image of the positive electrode sheet and calculating the average value.

[0019] For example, G / H×100+A can be 10.15, 11.00, 12.00, 13.00, 14.00, 15.00, 16.00, 16.40, or a value within the range of any two of the above values.

[0020] In one optional embodiment, the following condition is satisfied: 6.5μm≤D1≤9.5μm. Thus, the particle size Dv50 of the first positive electrode active material is within the range of 6.5μm to 9.5μm, which can effectively improve the specific capacity of the first positive electrode active material and increase the compaction density of the electrode sheet. Simultaneously, it avoids excessively large particle sizes of the active material in the first positive electrode active layer, which would lead to longer sodium ion diffusion particle sizes, increased electrode polarization, and negative impacts on the kinetic performance of the electrode sheet. This is more conducive to simultaneously improving the energy density and rate performance of the secondary battery, as well as reducing the battery's internal resistance.

[0021] For example, the particle size Dv50 of the first positive electrode active material may be 6.5μm, 6.6μm, 6.8μm, 7.0μm, 7.2μm, 7.4μm, 7.8μm, 8.0μm, 8.2μm, 8.4μm, 8.6μm, 8.8μm, 9.0μm, 9.2μm, 9.4μm, 9.5μm, or a value within the range of any two of the above values.

[0022] In some embodiments, the particle size Dv10 of the first positive electrode active material is 1.5 μm to 3.9 μm. Exemplarily, the particle size Dv10 of the first positive electrode active material can be, for example, 1.5 μm, 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, 3.9 μm, or a value within the range of any two of the above values.

[0023] In some embodiments, the particle size Dv90 of the first positive electrode active material is 14.0 μm to 20.0 μm. Exemplarily, the particle size Dv90 of the first positive electrode active material can be, for example, 14.0 μm, 14.5 μm, 15.0 μm, 15.5 μm, 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, or a value within the range of any two of the above values.

[0024] It should be noted that the particle size Dv10 and particle size Dv90 of the first positive electrode active material can be obtained by combining the scanning electron microscope image of the first positive electrode active layer with image analysis to test the particle size values ​​of multiple first positive electrode active materials and then plotting the cumulative distribution curve of the particle size, or by testing the first positive electrode active material with a laser particle size analyzer.

[0025] In some embodiments, the particle size span of the first positive electrode active material is 1.1 to 2.8. This results in better particle size uniformity of the first positive electrode active material, which is more conducive to improving the compaction density of the first positive electrode active layer. Simultaneously, it also improves its kinetic performance, thereby further enhancing the overall energy density and rate performance of the secondary battery.

[0026] It should be noted that the particle size span value = (particle size Dv90 - particle size Dv10) / particle size Dv50. For example, the span value of the first positive electrode active material can be 1.10, 1.20, 1.40, 1.60, 1.80, 2.00, 2.20, 2.40, 2.60, 2.80, etc., or a value within the range of any two of the above values.

[0027] In this application, "particle size Dv50" refers to the particle size corresponding to 50% of the total volume when the particles are arranged in ascending order of size; "particle size Dv10" refers to the particle size corresponding to 10% of the total volume when the particles are arranged in ascending order of size; and "particle size Dv90" refers to the particle size corresponding to 90% of the total volume when the particles are arranged in ascending order of size.

[0028] In some embodiments, the specific surface area of ​​the first positive electrode active material is 2.2 m². 2 / g~5.6m 2 / g. In this way, the first positive electrode active layer can be guaranteed to have a high compaction density, which is more conducive to improving the energy density of the secondary battery. At the same time, it can also avoid the impact of the low specific surface area of ​​the first positive electrode active material on the electrode dynamic performance.

[0029] It should be noted that the specific surface area of ​​the first positive electrode active material can be obtained by testing using the BET-nitrogen adsorption method. For example, the specific surface area of ​​the first positive electrode active material could be 2.2 m². 2 / g, 2.5m 2 / g, 3.0m 2 / g, 3.5m 2 / g, 4.0m 2 / g, 4.5m 2 / g, 5.0m 2 / g, 5.5m 2 / g, 5.6m 2 / g or values ​​within the range of any two of the above values.

[0030] In some implementations, the following condition is satisfied: 4.0 μm ≤ L1 ≤ 6.0 μm. This allows for the provision of a small-particle-size second positive electrode active material, improving the overall kinetic performance of the electrode while ensuring a high compaction density, thus making it more advantageous to obtain a secondary battery with high energy density and high kinetic performance.

[0031] For example, the particle size Dv50 of the second positive electrode active material may be 4.0 μm, 4.2 μm, 4.4 μm, 4.6 μm, 4.8 μm, 5.0 μm, 5.2 μm, 5.4 μm, 5.6 μm, 5.8 μm, 6.0 μm, or a value within the range of any two of the above values.

[0032] In some embodiments, the particle size Dv10 of the second positive electrode active material is 1.0 μm to 2.5 μm. For example, the particle size Dv10 of the second positive electrode active material can be 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.4 μm, 2.5 μm, or a value within any two of the above ranges.

[0033] In some embodiments, the particle size Dv90 of the second positive electrode active material is 13.0 μm to 18.0 μm. Exemplarily, the particle size Dv90 of the second positive electrode active material can be, for example, 13.0 μm, 13.5 μm, 14.0 μm, 14.5 μm, 15.0 μm, 15.5 μm, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, or a value within the range of any two of the above values.

[0034] It should be noted that the particle size Dv10 and particle size Dv90 of the second positive electrode active material can be obtained by combining the scanning electron microscope image of the second positive electrode active layer with image analysis to test the particle size values ​​of multiple second positive electrode active materials and then plotting the cumulative distribution curve of the particle size, or by testing the second positive electrode active material with a laser particle size analyzer.

[0035] In some embodiments, the particle size span of the second positive electrode active material is 2.5 to 4.2. This results in a wider particle size distribution for the second positive electrode active material, which can further improve the kinetic performance of the second positive electrode active layer while also improving its compaction density, thereby enhancing the overall rate performance and energy density of the secondary battery.

[0036] For example, the particle size span value of the second positive electrode active material may be 2.50, 2.60, 2.80, 3.00, 3.20, 3.40, 3.60, 3.80, 4.00, 4.20, or a value within the range of any two of the above values.

[0037] In some embodiments, the specific surface area of ​​the second positive electrode active material is 7.0 m². 2 / g~13.0m 2 / g. Thus, the higher specific surface area of ​​the second positive electrode active material is more conducive to optimizing the ion transport effect, electrolyte wetting effect, and electrolyte retention capacity in the second positive electrode active layer, thereby improving the kinetic performance of the electrode and further enhancing the rate performance of the secondary battery.

[0038] It should be noted that the specific surface area of ​​the second positive electrode active material can be obtained by testing using the BET-nitrogen adsorption method. For example, the specific surface area of ​​the second positive electrode active material could be, for instance, 7.0 m². 2 / g, 8.0m 2 / g, 9.0m 2 / g, 10.0m 2 / g, 11.0m 2 / g, 12.0m 2 / g, 13.0m 2 / g or values ​​within the range of any two of the above values.

[0039] In some embodiments, both the first and second positive electrode active materials include Fe and P elements. The molar ratio of Fe to P in the first positive electrode active material is denoted as m, and the molar ratio of Fe to P in the second positive electrode active material is denoted as n, satisfying the condition: 1.0 ≤ m / n ≤ 1.15. Thus, this application controls the molar ratio of Fe to P in the first positive electrode active material (for ease of description, the molar ratio of Fe to P can also be simply referred to as the Fe / P ratio below) to the molar ratio of Fe to P in the second positive electrode active material (Fe / P ratio) to be within the range of 1.0 to 1.15. This ensures excellent compatibility between the Fe / P ratio in a specific first polyanionic material and the Fe / P ratio in a specific second polyanionic material. A higher Fe / P ratio in the specific first polyanionic material can introduce more sodium storage space and Fe... 2+ / Fe 3+ The redox electron pair, along with the ability to extend the high-voltage plateau and suppress voltage decay during cycling, further contributes to improving the material's energy output. The lower Fe / P ratio in the specific second polyanionic material makes the sodium ion migration path smoother and more stable, improving the material's electronic conductivity and enhancing crystal structure stability, thus further improving the material's cycle stability and kinetic performance. By controlling the Fe / P ratio of the first and second positive electrode active materials within a specific range, it is possible to simultaneously promote further improvements in the energy density and rate performance of the secondary battery. This application combines the use of monocrystalline large-particle NFPP-type polyanionic materials in the first positive electrode active layer and polycrystalline small-particle NFPP-type polyanionic materials in the second positive electrode active layer with controlling the Fe / P ratio of the inner and outer positive electrode active materials within the range of 1.0 to 1.15, thereby jointly improving the cycle stability, capacity performance, and kinetic performance of the positive electrode sheet, thus making it more conducive to obtaining a secondary battery with high cycle stability, high energy density, and good rate performance.

[0040] This application's research found that if the ratio (m / n) of the Fe / P ratio in the first positive electrode active material to that in the second positive electrode active material is less than 1.0, it means that the Fe / P ratio in the first positive electrode active material is too low, the Fe / P ratio in the second positive electrode active material is too high, the Fe / P ratio in the first positive electrode active layer is too low, and the iron content in the inner active layer is too low, resulting in Fe... 2+ / Fe 3+A decrease in the number of redox electron pairs is detrimental to increasing electrode capacity, thus affecting the energy density of the secondary battery. Conversely, an excessively high Fe / P ratio in the second positive electrode active layer hinders ion transport within the active layer, impacting the rate performance of the secondary battery. If the ratio (m / n) of the Fe / P ratio in the first positive electrode active material to that in the second positive electrode active material is higher than 1.2, it indicates that the Fe / P ratio in the first positive electrode active material is too high, and the Fe / P ratio in the second positive electrode active material is too low. An excessively high Fe / P ratio in the first positive electrode active layer leads to an excessively high iron content in the active layer, making iron dissolution more likely and reducing the cycle stability of the secondary battery. Conversely, an excessively low Fe / P ratio in the second positive electrode active layer results in a lower overall iron content in the active layer, leading to a significant decrease in the energy density of the secondary battery.

[0041] It should be noted that the molar ratio m of Fe and P elements in the first positive electrode active material is calculated after testing the elemental content using energy-dispersive X-ray spectroscopy (EDS) or inductively coupled plasma (ICP); the molar ratio n of Fe and P elements in the second positive electrode active material can also be calculated after testing the elemental content using energy-dispersive X-ray spectroscopy (EDS) or inductively coupled plasma (ICP). For example, the ratio m / n of the molar ratio of Fe and P elements in the first positive electrode active material to that in the second positive electrode active material can be, for example, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, or a value within any range of two of the above values. Furthermore, the molar ratio of Fe to P in the first positive electrode active material satisfies: 0.73 ≤ m ≤ 0.79. Thus, controlling the molar ratio of Fe to P in the first positive electrode active material to be 0.73~0.79 allows for a higher Fe / P ratio, which helps to further improve the capacity of the active material, thereby increasing the energy density of the battery. Simultaneously, it avoids the phenomenon of iron dissolution that can easily occur when the Fe / P molar ratio is too high, ensuring superior cycle stability.

[0042] For example, the molar ratio of Fe to P in the first positive electrode active material may be 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, or a value within any two of the above values.

[0043] Furthermore, in some embodiments, the molar ratio of Fe to P in the second positive electrode active material satisfies: 0.68 ≤ n ≤ 0.73. Thus, controlling the molar ratio of Fe to P in the second positive electrode active material to be 0.68~0.73 allows for a higher content of doped elements in the active material, which is more conducive to reducing the sodium ion diffusion barrier, improving the electronic conductivity of the material, and enhancing the stability of the crystal structure, thereby further improving the cycle stability and rate performance of the secondary battery.

[0044] For example, the molar ratio of Fe to P in the second sodium iron pyrophosphate material can be, for example, 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, or a value within any two of the above values.

[0045] In some embodiments, the doped sodium iron pyrophosphate includes at least one of Mg, Ca, Al, Cr, Mn, W, and V.

[0046] In some embodiments, the doped sodium ferric pyrophosphate includes sulfate ions.

[0047] Optionally, in some embodiments, the chemical formula of the first polyanionic material is Na. e Fe f T t (PO4) j (SO4) i P₂O₇, wherein 3≤e≤6, 2≤f≤5, 0≤t≤0.4, 1≤j≤4, 0≤i≤0.4, and T includes at least one of Mg, Ca, Al, Cr, Mn, W, and V. For example, e can be 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, etc., or a value within the range of any two of the above values; f can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, etc., or a value within the range of any two of the above values; t can be 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, etc. 0.35, 0.40, etc., or values ​​within the range of any two of the above values; j can be, for example, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, etc., or values ​​within the range of any two of the above values; i can be, for example, 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, etc., or values ​​within the range of any two of the above values.

[0048] Optionally, in some embodiments, the chemical formula of the second polyanionic material is Na.k Fe p R q (PO4) v (SO4) u P2O7, wherein 3≤k≤6, 2≤p≤5, 0≤q≤0.3, 1≤v≤4, 0≤u≤0.3, and R includes at least one of Mg, Ca, Al, Cr, Mn, W, and V. For example, k can be 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, or a value within the range of any two of the above values; p can be 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, or a value within the range of any two of the above values; q can be 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or a value within the range of any two of the above values; v can be 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or a value within the range of any two of the above values; u can be 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or a value within the range of any two of the above values.

[0049] In this application, the molar ratio of Fe to P in the positive electrode active material is adjusted by changing the stoichiometric coefficients between the doped cations and anions in the chemical formula of the polyanionic material. Specifically, by doping the cation sites with metal dopants (e.g., Mg, Ca, Al, Cr, Mn, W, V), the molar number of iron in the polyanionic material is reduced, thus decreasing the molar ratio of Fe to P. By doping the polyanionic material with sulfate ions, the molar number of P in the polyanionic material is reduced, thus increasing the molar ratio of Fe to P. That is, by using doped sodium iron pyrophosphate, the type and amount of metal cation doping, as well as the type and amount of sulfate anion doping, are controlled to control the molar ratio of Fe to P in the positive electrode active material. Furthermore, the doping situation in the first and second polyanionic materials is adjusted by the above doping methods to control the molar ratios m and n of Fe to P in the first and second positive electrode active materials, respectively, thereby controlling the m / n value.

[0050] In this application, the sources of the first and second positive electrode active materials are not specifically limited; they can be obtained commercially or prepared using conventional processes. The doping and substitution process for the metal element at the cation site of the polyanionic material can employ conventional techniques in the art, such as adding a prescribed amount of a dopant metal source to the raw materials. Similarly, the doping and substitution process for the sulfate ions in the polyanionic material can employ conventional techniques in the art, such as introducing a prescribed amount of a sulfate-containing substance into the raw materials.

[0051] In some embodiments, the first positive electrode active material further includes a first coating layer, which coats at least a portion of the surface of the first polyanionic material, and the first coating layer is a carbon material; the second positive electrode active material further includes a second coating layer, which coats at least a portion of the surface of the second polyanionic material, and the second coating layer is a carbon material. Thus, coating the surface of the first polyanionic material with carbon material and coating the surface of the second polyanionic material with carbon material, as described in this application, can further improve the conductivity of NFPP-type polyanionic materials, thereby further improving the kinetic performance of the active material and the positive electrode sheet, and is more conducive to improving the rate performance of the secondary battery.

[0052] Optionally, in some embodiments, the carbon material in the first coating layer includes nano-carbon.

[0053] Optionally, in some embodiments, the carbon material in the second coating layer includes nano-carbon.

[0054] Furthermore, in some embodiments, the mass content of the first coating layer is denoted as x, based on the total mass of the first positive electrode active material; and the mass content of the second coating layer is denoted as y, based on the total mass of the second positive electrode active material, satisfying: 2%≤x≤5%, 0.5%≤y≤2%, and 0.13≤y / x≤0.9. Thus, by coating the surface of the large-particle single-crystal NFPP-type polyanionic material with a specific amount of carbon coating layer, this application can effectively compensate for the poor conductivity of large-particle single-crystal NFPP-type polyanionic materials, thereby further improving the conductivity of the first positive electrode active material. Meanwhile, the small-particle polycrystalline NFPP-type polyanionic material exhibits superior kinetic performance, and a carbon coating layer with a mass content of 0.5%~2% can achieve excellent electronic conductivity while avoiding the impact of excessively high carbon content on battery energy density. This application controls the mass content of the second coating layer to be within the range of 0.13 to 0.9% of the mass content of the first coating layer. This ensures that the positive electrode active material in the positive electrode active layer has excellent conductivity, while also avoiding the influence of excessive carbon coating layer on the material's specific capacity. This further improves the energy density and rate performance of the secondary battery.

[0055] It should be noted that the mass content of the first coating layer in the first positive electrode active material can be obtained by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) or thermogravimetric analysis of the first positive electrode active material; the mass content of the second coating layer in the second positive electrode active material can be obtained by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) or thermogravimetric analysis of the first positive electrode active material. For example, the mass content of the first coating layer in the first positive electrode active material may be, for example, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, or a value within the range of any two of the above values; the mass content of the second coating layer in the second positive electrode active material may be, for example, 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, or a value within the range of any two of the above values; the ratio of the mass content of the second coating layer in the second positive electrode active material to the mass content of the first coating layer in the first positive electrode active material may be, for example, 0.13, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or a value within the range of any two of the above values.

[0056] In this application, the source of the carbon-coated sodium ferric pyrophosphate is not specifically limited. It can be commercially available or prepared. The specific preparation process can employ conventional methods in the art. For example, a pyrophosphate source, a phosphate source, an iron source, a sodium source, a carbon source, and optionally an M source and a sulfate-containing substance are mixed according to the formula amount, and further mixed in an optional medium and additives. The mixture is then subjected to drying, at least one or more stages of heat treatment, cooling, crushing, and sieving processes. By controlling the preparation process and parameters, single-crystal large-particle sodium ferric pyrophosphate or doped sodium ferric pyrophosphate with carbon coating can be prepared. The carbon source used for the nano-carbon coating layer can be an organic carbon source, such as glucose or sucrose.

[0057] In this application, the source of the carbon-coated sodium ferric pyrophosphate is not specifically limited. It can be commercially available or prepared. The specific preparation process can employ conventional methods in the art. For example, a pyrophosphate source, a phosphate source, an iron source, a sodium source, a carbon source, and optionally an M source and a sulfate-containing substance are mixed according to a formula. The mixture is then further mixed in an optional medium and additives, followed by drying, heat treatment, cooling, crushing, and sieving processes. By controlling the process and its parameters, polycrystalline small particles of carbon-coated sodium ferric pyrophosphate or doped sodium ferric pyrophosphate can be prepared. The carbon source used for the nano-carbon coating layer can be an organic carbon source, such as glucose or sucrose.

[0058] In some embodiments, the total thickness H μm of the positive electrode active layer on one side is 100 μm to 170 μm.

[0059] It should be noted that the total thickness of the positive electrode active layer can be obtained by combining a cross-sectional SEM image of the positive electrode sheet with an image analyzer. For example, the total thickness H μm of the positive electrode active layer on one side can be, for example, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, or a value within any two of the above values.

[0060] In this application, the thickness refers to the average thickness, which is the average of the thickness values ​​at multiple arbitrary locations or at multiple equally spaced locations of the test sample during the thickness test.

[0061] In some embodiments, the thickness of the first positive electrode active layer on one side is denoted as h1 μm, and the thickness of the second positive electrode active layer on one side is denoted as h2 μm, satisfying: 0.1≤h2 / h1≤1, h2+h1=H. Thus, controlling the ratio of the thickness of the first positive electrode active layer to the thickness of the second positive electrode active layer within the range of 0.1 to 1 allows for a matching of the thicknesses of the first and second positive electrode active layers. This provides sufficient single-crystal large-particle positive electrode active material and sufficient polycrystalline small-particle positive electrode active material for compatibility, ensuring a better balance between the compaction density and kinetic performance of the electrode, thereby further contributing to obtaining a secondary battery with high energy density and high rate performance.

[0062] It should be noted that the thickness of the first positive electrode active layer can be obtained by measuring the dimensions of the first positive electrode active layer along the thickness direction at at least 10 locations in the cross-sectional SEM image of the positive electrode sheet using an image analyzer, and then calculating the average value. Similarly, the thickness of the second positive electrode active layer can be obtained by measuring the dimensions of the second positive electrode active layer along the thickness direction at at least 10 locations in the cross-sectional SEM image of the positive electrode sheet, and then calculating the average value. For example, the ratio h2 / h1 of the thickness of the second positive electrode active layer to the thickness of the first positive electrode active layer can be, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or a value within any range of two of the aforementioned values.

[0063] Optionally, in some embodiments, the thickness of the first positive electrode active layer on one side is 50 μm to 150 μm.

[0064] Optionally, in some embodiments, the thickness of the second positive electrode active layer on one side is 10 μm to 80 μm.

[0065] In some embodiments, the sphericity of the first positive electrode active material is denoted as 'a', and the sphericity of the second positive electrode active material is denoted as 'b', satisfying: 85%≤a≤95%, 40%≤b≤90%, and 5%≤|ab|≤45%. Thus, the large single-crystal particles of the first positive electrode active material in the first positive electrode active layer adopt a spherical or near-spherical structure, which is beneficial for increasing the specific surface area of ​​the positive electrode active material and facilitating ion transport into the active layer. This ensures an increase in the energy density of the secondary battery while further improving its rate performance. Meanwhile, the polycrystalline small particles of the second positive electrode active material have a sphericity in the range of 40% to 90%, which improves electrode dynamics and effectively increases the material's compaction density, thereby further enhancing the energy density of the secondary battery.

[0066] It should be noted that the sphericity 'a' of the first positive electrode active material can be tested using conventional methods in the art. For example, using image processing software (e.g., Image Pro Plus), at least 20 particles of the first positive electrode active material are selected in a scanning electron microscope (SEM) image of the first positive electrode active material at a certain magnification. The perimeter and area of ​​each particle are measured, and the equivalent radius of perimeter r1 and the equivalent radius of area r2 of each particle are calculated respectively. The sphericity is then r2 / r1, and the average value is taken to obtain the sphericity of the first positive electrode active material. Similarly, the sphericity 'a' of the second positive electrode active material can be tested using conventional methods in the art. For example, using image processing software (e.g., Image Pro Plus), at least 20 particles of the second positive electrode active material are selected in a scanning electron microscope (SEM) image of the second positive electrode active material at a certain magnification. The perimeter and area of ​​each particle are measured, and the equivalent radius of perimeter r1 and the equivalent radius of area r2 of each particle are calculated respectively. The sphericity is then r2 / r1, and the average value is taken to obtain the sphericity of the second positive electrode active material. For example, the sphericity 'a' of the first positive electrode active material can be, for example, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, etc., or a value within the range of any two of the above values; the sphericity 'b' of the second positive electrode active material can be, for example, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc., or a value within the range of any two of the above values; the absolute value of the difference between the sphericity of the first positive electrode active material and the sphericity of the second positive electrode active material can be, for example, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, etc., or a value within the range of any two of the above values.

[0067] In this application, "|ab|" refers to the absolute value of the difference between the sphericity of the first positive electrode active material and the sphericity of the second positive electrode active material. That is, when the sphericity of the first positive electrode active material is greater than or equal to the sphericity of the second positive electrode active material, "|ab|" is the difference between the sphericity of the first positive electrode active material and the second positive electrode active material; when the sphericity of the first positive electrode active material is less than the sphericity of the second positive electrode active material, "|ab|" is the negative of the difference between the sphericity of the first positive electrode active material and the second positive electrode active material.

[0068] In some embodiments, the porosity of the first positive electrode active layer is denoted as c, and the porosity of the second positive electrode active layer is denoted as d, satisfying: 20%≤c≤29%, 30%≤d≤58%, and 1.2≤d / c≤2.5. Thus, the ratio of the porosity of the second positive electrode active layer to that of the first positive electrode active layer is 1.2~2.5. The higher porosity of the second positive electrode active layer can further improve the wetting of the positive electrode sheet by the electrolyte and the transport of sodium ions within the positive electrode sheet, while the lower porosity of the first positive electrode active layer ensures that the electrode sheet has a high compaction density. The matching porosities between the first and second positive electrode active layers allow the secondary battery to achieve both superior energy density and rate performance.

[0069] It should be noted that the porosity can be calculated as follows: The electrode sheet is cut according to a template, and the area S of the cut sample is measured. Then, the thickness of the sample is measured 10 times, and the average value B is calculated. The apparent volume V0 = S × B 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 then calculated using the following formula. This method is also applicable to testing the porosity of the separator. Specifically, in this application, the porosity of the entire positive electrode sheet is first tested. Then, after removing the second active material layer on the electrode sheet surface, the porosity of the first active material layer is tested. The porosity of the second active material layer can be calculated based on these two porosities. Porosity = (V0 - V1) / V1 × 100%. For example, the porosity of the first positive electrode active layer may be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or a value within any two of the above values; the porosity of the second positive electrode active layer may be 30%, 35%, 40%, 45%, 50%, 55%, 58%, or a value within any two of the above values; the ratio of the porosity of the second positive electrode active layer to the porosity of the first positive electrode active layer is 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.5, or a value within any two of the above values.

[0070] In some embodiments, the compaction density of the positive electrode active layer is 1.8 g / cm³. 3 ~2.3g / cm 3 .

[0071] For example, the compaction density of the positive electrode active layer may be 1.80 g / cm³. 3 1.85g / cm 3 1.90g / cm 3 1.95g / cm 3 2.00g / cm 3 2.05g / cm 3 2.10 g / cm 3 2.15g / cm 3 2.20g / cm 3 2.25g / cm 3 2.30g / cm 3 Values ​​equal to or within the range of any two of the above values.

[0072] In some embodiments, the lateral surface density of the positive electrode active layer is 0.0200 g / cm³. 2~0.0350g / cm 2 Thus, the coating surface density of the active material in the positive electrode provided in this application is within the above-mentioned range. On the one hand, a higher coating surface density can provide higher capacity, which is more conducive to improving the energy density of the secondary battery. On the other hand, it can avoid the positive electrode from being severely polarized and producing powder shedding during battery cycling due to excessively high coating surface density, thereby further reducing the internal resistance of the battery and improving the cycle stability of the battery.

[0073] It should be noted that the areal density of the positive electrode active layer can be obtained by conventional methods in the art. For example, a certain area of ​​the positive electrode active layer is cut off, the mass of the cut positive electrode active layer sample is weighed, and the areal density of the positive electrode active layer is calculated by substituting the measured area and the mass of the positive electrode active layer into the formula: areal density = mass of positive electrode active layer / area of ​​positive electrode active layer. For example, the total areal density of the positive electrode active layer coating on one side can be, for example, 0.0200 g / cm³. 2 0.0220g / cm 2 0.0240g / cm 2 0.0260g / cm 2 0.0280g / cm 2 0.0300g / cm 2 0.0320g / cm 2 0.0340 g / cm 2 0.0350g / cm 2 Values ​​equal to or within the range of any two of the above values.

[0074] Optionally, in some embodiments, the mass content of the first positive electrode active material is 93% to 97% based on the total mass of the first positive electrode active layer. For example, the mass content of the first positive electrode active material in the first positive electrode active layer may be 93.0%, 93.5%, 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97.0%, or a value within the range of any two of the above values.

[0075] Optionally, in some embodiments, the first positive electrode active layer further includes a first binder, which includes at least one selected from 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). Based on the total mass of the first positive electrode active layer, the mass content of the first binder is 1.5% to 3%. For example, the mass content of the first binder in the first positive electrode active layer may be, for example, 1.5%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, or a value within the range of any two of the above values.

[0076] Optionally, in some embodiments, the mass content of the second positive electrode active material is 93% to 97% based on the total mass of the second positive electrode active layer. For example, the mass content of the second positive electrode active material in the second positive electrode active layer may be 93.0%, 93.5%, 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97.0%, or a value within any two of the above ranges.

[0077] Optionally, in some embodiments, the second positive electrode active layer further includes a second binder, wherein the second binder is selected from 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), and the mass content of the second binder is 1.5% to 3% based on the total mass of the second positive electrode active layer. For example, the mass content of the second binder in the second positive electrode active layer may be, for example, 1.5%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, or a value within any two of the above ranges.

[0078] In some embodiments, the thickness G μm of the bottom carbon layer is 0.3 μm to 3 μm. This facilitates a tighter connection between the positive electrode active layer and the positive electrode current collector, further reducing the interfacial resistance of the secondary battery and improving its rate performance. Simultaneously, it effectively alleviates the stress generated during sodium insertion / extraction in the positive electrode active layer during battery cycling, further improving the battery's cycle stability and preventing a sudden drop in battery capacity. Furthermore, it avoids a decrease in the energy density of the secondary battery due to an excessively thick bottom carbon layer.

[0079] For example, the thickness of the bottom carbon layer may be 0.3μm, 0.4μm, 0.6μm, 0.8μm, 1.0μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2.0μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm, 3.0μm, or a value within the range of any two of the above values.

[0080] Optionally, in some embodiments, the bottom carbon layer comprises a carbon material, which may be a conventional carbon material for bottom carbon layers in the art, such as at least one of conductive carbon black, graphene, carbon nanotubes, amorphous carbon, and Ketjen black.

[0081] In some embodiments, the positive electrode further includes a positive tab and an insulating layer. The positive tab extends from the positive current collector, and the extension direction of the positive tab is perpendicular to the thickness direction. In the extension direction of the positive tab, the insulating layer is located between the positive tab and the positive active layer. The thickness of the insulating layer on one side is denoted as J μm, satisfying: 0.55≤J / H≤0.80. Thus, this application provides an insulating layer on one side of the positive active layer. As an insulator, the insulating layer can effectively isolate the direct contact between the positive electrode edge and the negative electrode, preventing burrs at the edge of the positive electrode from piercing the separator and causing an internal short circuit. The insulating layer can also serve as a dense physical barrier, isolating the electrolyte from the edge active material, mitigating side reactions between the electrode and the electrolyte. Furthermore, the insulating layer can regulate the interfacial impedance at the edge of the positive active layer, preventing overcharging or over-charging at the edge, contributing to uniform current distribution, further improving the cycle stability and rate performance of the secondary battery, and helping to reduce the battery's internal resistance. Meanwhile, this application also controls the ratio of the thickness of the insulating layer to the thickness of the positive electrode active layer to be within the range of 0.55 to 0.8, which can ensure that the proportion of positive electrode active material in the positive electrode sheet is high enough. While effectively improving the energy density of the secondary battery, it can also ensure that the insulating layer that matches the positive electrode active layer can play its advantageous role, and more effectively improve the cycle stability, rate performance and reduce the internal resistance of the secondary battery.

[0082] It should be noted that the thickness of the insulating layer can be obtained by measuring the thickness of the insulating layer at at least 10 locations in a cross-sectional SEM image of the positive electrode using image processing software (such as Image Pro Plus) and calculating the average value, or by measuring with a micrometer. For example, the ratio of the thickness of the insulating layer to the total thickness of the positive electrode active layer can be, for example, 0.55, 0.56, 0.58, 0.60, 0.62, 0.64, 0.66, 0.68, 0.70, 0.72, 0.74, 0.76, 0.78, 0.80, or a value within any range of two of the above values.

[0083] Optionally, in some embodiments, the thickness J μm of the insulating layer on one side satisfies: 50 ≤ J ≤ 150.

[0084] Optionally, in some embodiments, the insulating layer comprises an inorganic material, which includes at least one of alumina, silicon carbide, silicon dioxide, glass fiber, titanium dioxide, zirconium dioxide, magnesium hydroxide, aluminum hydroxide, boehmite, barium sulfate, barium titanate, aluminum titanate, zinc oxide, boron nitride, aluminum nitride, magnesium nitride, attapulgite, zinc phosphate, and zinc borate.

[0085] Optionally, in some embodiments, the insulating layer further includes an organic polymer selected from at least one of polyterephthalate, polyamide (PA), polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), poly(p-phenylene terephthalamide), polypropylene, acrylonitrile-butadiene-styrene copolymer, polyvinyl alcohol formal, polyvinyl butyral, polyurethane, polyacrylonitrile, polyvinyl acetate, polyoxymethylene, phenolic resin, epoxy resin, acrylic resin, urea-formaldehyde resin, amino resin, formaldehyde resin, furan resin, chloroprene rubber, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate, polysulfone, polyethersulfone, or polyphenylene ether. The polymer may also be selected from at least one of the derivatives, crosslinks, or copolymers of the above polymers.

[0086] Furthermore, in some embodiments, the thickness direction of the positive electrode sheet is taken as the first direction, the extension direction of the positive electrode tab is taken as the second direction, and the direction perpendicular to the first direction and the second direction is taken as the third direction; the size of the sodium-ion secondary battery is 50mm~100mm in the first direction, 100mm~250mm in the second direction, and 100mm~350mm in the third direction.

[0087] It should be noted that the dimensions of the sodium-ion secondary battery in the first direction may be, for example, 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, etc., or values ​​within any two of the above values; the dimensions of the sodium-ion secondary battery in the second direction may be, for example, 100mm, 120mm, 140mm, 160mm, 180mm, 200mm, 220mm, 240mm, 250mm, etc., or values ​​within any two of the above values; the dimensions of the sodium-ion secondary battery in the first direction may be, for example, 100mm, 120mm, 140mm, 160mm, 180mm, 200mm, 220mm, 240mm, 260mm, 280mm, 300mm, 320mm, 340mm, 350mm, etc., or values ​​within any two of the above values.

[0088] In this application, the "thickness direction of the positive electrode sheet", the "thickness direction of the positive electrode current collector", and the "thickness direction of the positive electrode active layer" are all the same.

[0089] In some embodiments, the first positive electrode active layer further includes a first positive electrode conductive agent, which includes conductive carbon black and carbon nanotubes, wherein the aspect ratio of the carbon nanotubes is 1000-5000. Thus, by incorporating carbon nanotubes with an aspect ratio of 1000-5000 into the first positive electrode active layer, they can form a continuous electronic conduction network with the single-crystal large-particle first positive electrode active material. Combined with the introduction of conductive carbon black, this further facilitates the construction of a continuous and uniform conductive network structure, further improving the electronic conductivity of the electrode, reducing the battery's internal resistance, reducing polarization during battery charging and discharging, and improving interface stability, thereby further enhancing the long-cycle stability of the battery.

[0090] It should be noted that the aspect ratio of the carbon nanotubes in the first positive electrode active layer 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 positive 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 carbon nanotubes in the first positive electrode active layer can be, for example, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, etc., or values ​​within any two of the above ranges.

[0091] In some embodiments, the second positive electrode active layer further includes a second positive electrode conductive agent, which includes conductive carbon black. Thus, the conductive carbon black can form a continuous and uniform conductive network structure with the polycrystalline small-particle second positive electrode active material, further improving the electronic conductivity of the electrode, reducing internal resistance, reducing polarization during battery charging and discharging, further improving the cross-sectional stability of the electrode, and further enhancing the long-cycle stability of the secondary battery.

[0092] Furthermore, in some embodiments, the second positive electrode conductive agent further includes carbon nanotubes with an aspect ratio of 200 to 1000. Thus, by selecting carbon nanotubes with an aspect ratio of 200 to 1000 for the second positive electrode active layer, a more continuous and uniform conductive network structure can be formed with the polycrystalline small particles of the second positive electrode active material and the conductive carbon black, thereby further reducing the battery's internal resistance and improving its long-cycle stability.

[0093] It should be noted that the aspect ratio of the carbon nanotubes in the second positive electrode active layer 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 positive 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 carbon nanotubes in the second positive electrode active layer can be, for example, 200, 300, 400, 500, 600, 700, 800, 900, 1000, or a value within any two of the above ranges.

[0094] In some embodiments, the density of the electrolyte is 1.0 g / cm³ at 25°C. 3 ~1.5g / cm 3 And / or, at 25°C, the conductivity of the electrolyte is 7 mS / cm to 9.0 mS / cm. This further improves the wettability of the electrolyte to the electrode, provides a channel for the liquid-phase transport and transfer of ions and electrons, further reduces polarization, and is more conducive to improving the rate performance and cycle stability of the battery.

[0095] It should be noted that the density of the electrolyte can be obtained by weighing a unit volume of electrolyte at 25°C; the conductivity of the electrolyte can be obtained by measuring the conductivity of a conductivity meter at 25°C. For example, the density of the electrolyte may be 1.00 g / cm³. 3 1.05g / cm 3 1.10 g / cm 3 1.15g / cm 3 1.20g / cm 3 1.25g / cm 3 1.30g / cm3 1.35g / cm 3 1.40 g / cm 3 1.45g / cm 3 1.50g / cm 3 The conductivity of the electrolyte may be, for example, 7.0 ms / cm, 7.2 ms / cm, 7.4 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, etc., or a value within the range of any two of the above values.

[0096] In some embodiments, the sodium salt content in the electrolyte is 10% to 15% by mass. This allows for better matching with the thickness ratio of the bottom carbon layer and the positive electrode active layer, which is more conducive to improving ion transport in the liquid phase, further reducing liquid phase transport impedance, further improving the ionic conductivity of the secondary battery, and thus further improving the rate performance of the secondary battery.

[0097] It should be noted that the mass content of the sodium salt in the electrolyte can be obtained by GC-MS (gas chromatography-mass spectrometry) or ion chromatography (IC). The mass content of the sodium salt in the electrolyte can be, for example, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%, 15.0%, or a value within any two of the above ranges.

[0098] Optionally, in some embodiments, the sodium salt in the electrolyte includes at least one of NaPF6, NaBF4, NaClO4, NaAsF6, and NaCF3SO3.

[0099] In some embodiments, the electrolyte further 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.

[0100] 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.

[0101] In some embodiments, the substrate layer is made of polyethylene and / or polypropylene; the thickness of the substrate layer is 5 μm to 12 μm.

[0102] 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.

[0103] In some embodiments, the material of the functional coating includes a nitrogen-containing compound, and the thickness of the functional coating is 2 μm to 4 μm.

[0104] 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.

[0105] In some embodiments, the porosity of the diaphragm is 25% to 40%.

[0106] It should be noted that the porosity of the membrane can be calculated by 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.

[0107] Furthermore, in some embodiments, the diaphragm further includes a ceramic layer disposed on at least one side surface of the substrate layer in the thickness direction; the functional coating is disposed on the surface of the ceramic layer away from the substrate layer, and / or the functional coating is disposed on the surface of the substrate layer and disposed opposite to the ceramic layer.

[0108] Further, in some embodiments, the nitrogen-containing compound includes at least one selected from melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazin-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, cyanuric chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-dimethoxy-1,3,5-triazine.

[0109] In some embodiments, the negative electrode sheet includes 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 includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode active material includes a hard carbon material.

[0110] In some embodiments, the negative electrode conductive agent includes conductive carbon black and / or carbon nanotubes.

[0111] In some embodiments, the negative electrode binder includes at least one of polyacrylic acid, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0112] Optionally, in some embodiments, based on the total mass of the negative electrode active layer, the mass content of the negative electrode active material is 91% to 97%, the mass content of the negative electrode conductive agent is 3% to 6%, and the mass content of the negative electrode binder is 1% to 3%.

[0113] For example, the mass content of the negative electrode active material in the negative electrode active layer may be 91.0%, 91.5%, 92.0%, 92.5%, 93.0%, 93.5%, 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, 96.5%, 97.0%, or a value within any two of the above values; the mass content of the negative electrode conductive agent in the negative electrode active layer may be 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, or a value within any two of the above values; the mass content of the negative electrode binder in the negative electrode active layer may be 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, or a value within any two of the above values.

[0114] Optionally, 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 allows for a tight bond between the negative electrode current collector and the negative electrode active layer, improving the structural stability of the electrode, reducing the interfacial impedance of the electrode, and thus further improving the cycle stability of the battery.

[0115] Example 1 This embodiment provides a method for preparing a sodium-ion secondary battery, including the following steps: (1) Preparation of the positive electrode: The first positive electrode active material, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes with an aspect ratio of 3162 were mixed in a mass ratio of 95:2:2:1. N-methylpyrrolidone was added and mixing continued to obtain the first mixed slurry. The first positive electrode active material is a first polyanionic material with a nano-carbon coating layer. This first polyanionic material is a single-crystal first iron pyrophosphate material with the chemical formula Na4Fe3(PO4)2P2O7, and the molar ratio of Fe to P is 0.75. The particle size of the first positive electrode active material is Dv50 of 8.6 μm, Dv10 of 2.1 μm, Dv90 of 16.2 μm, with a particle size span of 1.64 and a specific surface area of ​​3.41 m². 2 / g, sphericity is 91%, and the mass content of the nano-carbon coating layer in the first positive electrode active material is 3.2%.

[0116] The second positive electrode active material, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes with an aspect ratio of 624 were mixed in a mass ratio of 95:2:2:1. N-methylpyrrolidone was then added and mixing continued to obtain a second mixed slurry. The second positive electrode active material is a second polyanionic material with a nano-carbon coating layer. This second polyanionic material is a polycrystalline, doped sodium iron pyrophosphate material with the chemical formula Na₄Fe₂O₃. 2.84 Mg 0.16 (PO4)2P2O7, with a Fe to P molar ratio of 0.71; the second positive electrode active material has a particle size Dv50 of 5.0 μm, a particle size Dv10 of 1.6 μm, a particle size Dv90 of 14.4 μm, a particle size span of 2.56, and a specific surface area of ​​9.32 m². 2 / g, sphericity is 62%, and the mass content of the nano-carbon coating layer in the second positive electrode active material is 1.4%.

[0117] Conductive carbon black and polyvinylidene fluoride were mixed at a mass ratio of 3:1, and N-methylpyrrolidone was added and mixed further to obtain a third mixed slurry.

[0118] Next, a third mixed slurry is coated on both sides of the aluminum foil current collector and dried to form a bottom carbon layer on both sides of the aluminum foil current collector along the thickness direction. Then, a first mixed slurry and a second mixed slurry are sequentially coated on the surfaces of the aluminum foil current collector containing the bottom carbon layer on both sides, and a mixed slurry (composed of aluminum oxide, polyvinylidene fluoride, and N-methylpyrrolidone, with a mass ratio of aluminum oxide to polyvinylidene fluoride of 97:3) is coated on the surface of the bottom carbon layer away from the aluminum foil current collector. The first positive electrode mixed slurry is in direct contact with the bottom carbon layer and is located between the bottom carbon layer and the second positive electrode mixed slurry. The insulating layer is coated on both sides of the aluminum foil current collector along the thickness direction. Then, after drying and rolling, the rolled electrode sheet is laser-cut to form a positive electrode tab, thus obtaining the positive electrode sheet.

[0119] The schematic diagram of the main cross-sectional structure of the positive electrode obtained by the above process is shown below. Figure 1 As shown, the top view of the obtained positive electrode structure is as follows: Figure 2 ,Depend on Figure 1 and Figure 2 As shown, the positive electrode sheet includes a positive current collector 1, a bottom carbon layer 2, a positive active layer 3, a positive tab 4, and an insulating layer 5. The thickness direction of the positive electrode sheet is denoted as the first direction X. The bottom carbon layer 2 and the positive active layer 3 are disposed on both sides of the positive current collector 1 in the first direction X. In the first direction X, the bottom carbon layer 2 is located between the positive current collector 1 and the positive active layer 3. The positive active layer includes a first positive active layer 3-1 and a second positive active layer 3-2 stacked together. In the first direction X, the first positive active layer 3-1 is located between the bottom carbon layer 2 and the second positive active layer 3-2. The positive tab 4 extends from the positive current collector 1. The extension direction of the positive tab 4 in the positive electrode sheet is denoted as the second direction Y. The second direction Y is perpendicular to the first direction X. In the second direction Y, the insulating layer 5 is located between the positive tab 4 and the positive active layer 3.

[0120] (2) Preparation of negative electrode: The negative electrode active material hard carbon, binder and conductive agent are mixed in a mass ratio of 93:3:4. Deionized water is added as a solvent and the mixture is continued to be mixed to obtain the first negative electrode slurry. The binder is composed of styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 2:1. The conductive agent is conductive carbon black. Then, the first negative electrode slurry is coated on the surface of both sides of aluminum foil. After drying and rolling, negative electrode tabs are formed by laser cutting to obtain the negative electrode sheet.

[0121] (3) 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%.

[0122] (4) Preparation of the diaphragm Polymethyl acrylate (PMMA) and alumina were mixed with deionized water at a mass ratio of 2:8 to form a slurry with a solid content of 50%. This slurry was coated onto one side of a 9 μm thick polyethylene substrate layer and then dried and shaped in a multi-section oven at 60°C to obtain a 2 μm thick ceramic layer. Next, melamine cyanurate (MDI) and polymethyl methacrylate (PMMA) were mixed in deionized water at a mass ratio of 9:1 and thoroughly stirred to obtain a mixed slurry with a solid content of 25%. This mixed slurry was coated onto the other side of the substrate layer using a gravure roller. A functional coating with a thickness of 2 μm was then applied to the surface of the substrate layer, opposite to the ceramic layer, resulting in a separator with a porosity of 35%. In the assembly of the sodium-ion secondary battery, the ceramic layer is positioned closer to the positive electrode, and the functional coating is positioned closer to the negative electrode.

[0123] (5) Assembly of sodium-ion secondary batteries: The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are stacked in a Z-shape to form an electrode assembly. The assembly is then encapsulated in an aluminum-plastic film and injected with the electrolyte prepared in step (3). After standing, the assembly is charged, and after two sealing processes, the assembly is sorted and tested with OCV to obtain a sodium-ion secondary battery.

[0124] Figure 3 This is a schematic diagram of the structure of a sodium-ion secondary battery provided in Embodiment 1 of this application. As shown in the figure, the sodium-ion secondary battery includes a housing 8 and an electrode assembly 6 disposed inside the housing 8. The positive electrode and negative electrode in the electrode assembly 6 extend outward with positive electrode tabs 4 and negative electrode tabs 7. In the sodium-ion secondary battery, the thickness direction of the positive electrode is denoted as the first direction X, the extension direction of the positive electrode tab 4 in the positive electrode is denoted as the second direction Y, and the direction that is perpendicular to both the first direction X and the second direction Y is denoted as the third direction Z. The dimension α of the housing 8 in the first direction is 76.5 mm, the dimension β in the second direction is 237.8 mm, and the dimension γ in the third direction is 294.3 mm.

[0125] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Example 1. The differences are shown in Tables 1, 2, and 3. In Tables 1-3, " "" indicates: the same as in Example 1 or a value with reasonable deviation due to the testing process. " / " indicates: not present. D1 indicates: particle size Dv50 of the first positive electrode active material; m indicates: the molar ratio of Fe and P elements in the first positive electrode active material; x indicates: the mass content of the carbon coating layer in the first positive electrode active material based on the total mass of the first positive electrode active material; a indicates: the sphericity of the first positive electrode active material; c indicates: the porosity of the first positive electrode active layer; h1 indicates: the thickness of the first positive electrode active layer on one side; L1 indicates: the second positive electrode... The particle size of the active material is Dv50; n represents the molar ratio of Fe and P elements in the second positive electrode active material; y represents the mass content of the carbon coating layer in the second positive electrode active material based on the total mass of the second positive electrode active material; b represents the sphericity of the second positive electrode active material; d represents the porosity of the second positive electrode active layer; h2 represents the thickness of the second positive electrode active layer on one side; H represents the total thickness of the positive electrode active layer on one side; G represents the thickness of the bottom carbon layer on one side; J represents the thickness of the insulating layer on one side; A represents the mass content of sodium salt in the electrolyte.

[0126] Table 1

[0127] Table 2

[0128] Table 3

[0129] Test example: The sodium-ion secondary batteries provided in the above embodiments and comparative examples were tested as follows: (1) Energy density testing process: The sodium-ion secondary 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 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 V (length × height × width), and the volumetric energy density was calculated as E / V, with the unit being Wh / L.

[0130] (2) Testing process for cycle capacity retention: The sodium-ion secondary 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 specific capacity of the processed discharge was recorded as C0. (3) Steps (1) to (2) were repeated until 2000 cycles were reached. The specific capacity of the discharge after 2000 cycles was recorded as C1. The cycle capacity retention rate was calculated as C1 / C0×100%.

[0131] (3) Internal resistance testing process: At 25℃, the sodium-ion secondary battery was charged to 3.65V at a constant current and constant voltage of 0.2C, and 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 secondary battery was tested using a Metrohm PGSTAT302N chemical workstation in the range of 100KHz-0.1mHz at 25℃.

[0132] (4) Testing process for rate performance: The sodium-ion secondary 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 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 density 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 at 2C was recorded. The percentage ratio of the discharge specific capacity at 2C to the discharge specific capacity at 0.2C is the rate performance of the battery.

[0133] The test results are shown in Table 4.

[0134] Table 4

[0135] As can be seen from Tables 1 to 4, this application improves the cycle stability, capacity performance, and kinetic performance of the positive electrode by using single-crystal large-particle NFPP material in the first positive electrode active layer and polycrystalline small-particle NFPP material in the second positive electrode active layer. This is beneficial for obtaining a sodium-ion secondary battery with high cycle stability, high energy density, and good rate performance. Simultaneously, this application controls the thickness of the bottom carbon layer and the positive electrode active layer, as well as the mass content of sodium salt in the electrolyte, to meet the following condition: 10.15 ≤ G / H × 100 + A ≤ 16.4. This ensures that the bottom carbon layer has a thickness matching that of the positive electrode active layer, and that the sum of the thickness percentages of both layers and the mass content of sodium salt in the electrolyte is within the range of 10.15 to 16.4. This improves the electron transport effect within the electrode, reduces interfacial resistance, promotes ion transport in the electrolyte, reduces liquid phase transport impedance, and jointly improves the ionic conductivity and electronic conductivity of the battery. This enhances the rate performance and cycle stability of the sodium-ion secondary battery, while ensuring a high energy density.

[0136] 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 sodium-ion secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode sheet includes a positive current collector and a bottom carbon layer and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction, wherein the bottom carbon layer is located between the positive current collector and the positive active layer; the positive active layer includes a first positive active layer and a second positive active layer stacked thereon, wherein the first positive active layer is disposed between the bottom carbon layer and the second positive active layer. The first positive electrode active layer includes a first positive electrode active material, the first positive electrode active material includes a first polyanionic material, and the first polyanionic material includes single crystal particles; The second positive electrode active layer includes a second positive electrode active material, the second positive electrode active material includes a second polyanionic material, and the second polyanionic material includes polycrystalline particles; Let D1 be the particle size Dv50 of the first positive electrode active material and L1 be the particle size Dv50 of the second positive electrode active material, satisfying: D1 > L1; The first polyanionic material and the second polyanionic material independently comprise sodium iron pyrophosphate and / or doped sodium iron pyrophosphate. The electrolyte includes sodium salt; the mass content of the sodium salt is denoted as A% based on the total mass of the electrolyte; the thickness of the bottom carbon layer on one side is denoted as G μm; and the total thickness of the positive electrode active layer on one side is denoted as H μm, satisfying: 10.15≤G / H×100+A≤16.

4.

2. The sodium-ion secondary battery according to claim 1, characterized in that, The first positive electrode active material satisfies at least one of the following conditions: (A) 6.5μm≤D1≤9.5μm; (B) Particle size Dv10 is 1.5μm~3.9μm; (C) Particle size Dv90 is 14.0 μm~20.0 μm; (D) Particle size span value is 1.1~2.8; (E) a specific surface area of 2.2 m 2 / g ~ 5.6 m 2 / g.

3. The sodium-ion secondary battery according to claim 1, characterized in that, The second positive electrode active material satisfies at least one of the following conditions: (a) 4.0μm≤L1≤6.0μm; (b) Particle size Dv10 is 1.0 μm to 2.5 μm; (c) Particle size Dv90 is 13.0 μm to 18.0 μm; (d) Particle size span value: 2.5~4.2; (e) Specific surface area is 7.0 m² 2 / g~13.0m 2 / g.

4. The sodium-ion secondary battery according to claim 1, characterized in that, Both the first positive electrode active material and the second positive electrode active material include Fe and P elements. The molar ratio of Fe to P elements in the first positive electrode active material is denoted as m, and the molar ratio of Fe to P elements in the second positive electrode active material is denoted as n. The two satisfy: 1.0≤m / n≤1.15; preferably, they satisfy: 0.73≤m≤0.79, and / or 0.68≤n≤0.

73. And / or, the doped sodium iron pyrophosphate includes at least one of Mg, Ca, Al, Cr, Mn, W, and V; And / or, the doped sodium ferric pyrophosphate includes sulfate ions; And / or, the first positive electrode active material further includes a first coating layer, the first coating layer coating at least a portion of the surface of the first polyanionic material, the first coating layer comprising a carbon material; the second positive electrode active material further includes a second coating layer, the second coating layer coating at least a portion of the surface of the second polyanionic material, the second coating layer comprising a carbon material; Based on the total mass of the first positive electrode active material, the mass content of the first coating layer is denoted as x; based on the total mass of the second positive electrode active material, the mass content of the second coating layer is denoted as y, satisfying: 2%≤x≤5%, 0.5%≤y≤2%, 0.13≤y / x≤0.

9.

5. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, The positive electrode active layer satisfies at least one of the following conditions: (1) The total thickness H μm of the positive electrode active layer on one side is 100 μm to 170 μm; (2) The thickness of the first positive electrode active layer on one side is denoted as h1 μm, and the thickness of the second positive electrode active layer on one side is denoted as h2 μm: satisfying: 0.1≤h2 / h1≤1, h2+h1=H; (3) The sphericity of the first positive electrode active material is denoted as a, and the sphericity of the second positive electrode active material is denoted as b, satisfying: 85%≤a≤95%, 40%≤b≤90%, 5%≤|ab|≤45%; (4) The porosity of the first positive electrode active layer is denoted as c, and the porosity of the second positive electrode active layer is denoted as d, satisfying: 20%≤c≤29%, 30%≤d≤45%, 1.2≤d / c≤2.2; (5) The compaction density of the positive electrode active layer is 1.8 g / cm³. 3 ~2.3g / cm 3 ; (6) The lateral surface density of the positive electrode active layer is 0.0200 g / cm³. 2 ~0.0350g / cm 2 .

6. The sodium-ion secondary battery according to claim 5, characterized in that, The thickness G μm of the bottom carbon layer on one side is 0.3 μm to 3 μm; And / or, the positive electrode further includes a positive electrode tab and an insulating layer, the positive electrode tab extending from the positive current collector, the extension direction of the positive electrode tab being perpendicular to the thickness direction of the positive current collector; in the extension direction of the positive electrode tab, the insulating layer is located between the positive electrode tab and the positive active layer; the thickness of the insulating layer on one side is denoted as J μm, satisfying: 0.55≤J / H≤0.80; Preferably, the thickness direction of the positive electrode sheet is taken as the first direction, the extension direction of the positive electrode tab is taken as the second direction, and the direction perpendicular to the first direction and the second direction is taken as the third direction; the size of the sodium-ion secondary battery is 50mm~100mm in the first direction, 100mm~250mm in the second direction, and 100mm~350mm in the third direction.

7. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, The first positive electrode active layer further includes a first positive electrode conductive agent, which includes conductive carbon black and carbon nanotubes, wherein the aspect ratio of the carbon nanotubes is 1000~5000. And / or, the second positive electrode active layer further includes a second positive electrode conductive agent, the second positive electrode conductive agent including conductive carbon black; preferably, the second positive electrode conductive agent further includes carbon nanotubes, the aspect ratio of the carbon nanotubes being 200~1000.

8. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, At 25°C, the density of the electrolyte is 1.0 g / cm³. 3 ~1.5g / cm 3 ; And / or, at 25°C, the conductivity of the electrolyte is 7 mS / cm to 9.0 mS / cm; And / or, the sodium salt in the electrolyte has a mass content of 10% to 15%; And / or, the electrolyte further includes an organic solvent, the organic solvent including 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.

9. The sodium-ion secondary battery according to any one of claims 1 to 4, 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 substrate layer is made of polyethylene and / or polypropylene; the thickness of the substrate layer is 5μm~12μm. The material of the functional coating includes nitrogen-containing compounds, and the thickness of the functional coating is 2μm~4μm; Preferably, the porosity of the diaphragm is 25% to 40%; Preferably, the diaphragm further includes a ceramic layer disposed on at least one surface in the thickness direction of the substrate layer; the functional coating is disposed on the surface of the ceramic layer away from the substrate layer, and / or the functional coating is disposed on the surface of the substrate layer and disposed opposite to the ceramic layer; Preferably, the nitrogen-containing compound includes at least one of melamine cyanurate, melamine polyphosphate, melamine thiocyanate, melamine, 2,4,6-tris(aminohexanoic acid)-1,3,5-triazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazin-2-yl)guanidine, 2,4-diamino-6-dimethylamino-1,3,5-triazine, melamine chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-dimethoxy-1,3,5-triazine.

10. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, The negative electrode sheet includes 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 includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode sheet satisfies at least one of the following conditions: (I) The negative electrode active material includes hard carbon material; (II) The negative electrode conductive agent includes conductive carbon black and / or carbon nanotubes; (III) The negative electrode binder includes at least one of polyacrylic acid, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

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