Positive electrode sheet and sodium-ion secondary battery

Abstract

This application relates to the field of secondary battery technology and discloses a positive electrode sheet and a sodium-ion secondary battery. The positive electrode sheet includes a positive electrode active layer. In the positive electrode active layer, the first positive electrode active layer includes a single-crystal first sodium iron pyrophosphate material and a layered oxide material, and the second positive electrode active layer includes a polycrystalline second sodium iron pyrophosphate material. The particle size Dv50 of the first sodium iron pyrophosphate material is greater than that of the second sodium iron pyrophosphate material. The positive electrode active layer also includes a sodium supplementing agent, which includes at least one of the characteristic elements Cl, Br, and I. The mass content of the characteristic elements in the first and second positive electrode active layers is denoted as a and b, respectively, satisfying: a > b, 0.1% ≤ a ≤ 5%, and 30 ppm ≤ b ≤ 3000 ppm. The positive electrode sheet provided by this application can simultaneously improve the energy density, cycle stability, rate performance, and initial coulombic efficiency of sodium-ion secondary batteries.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, specifically to a positive electrode and 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, in practical applications, polyanionic sodium cathode materials are prone to problems such as poor capacity utilization and deteriorated kinetic performance, making it difficult for secondary batteries to simultaneously meet current market demands for energy density and rate performance. Furthermore, the formation of the positive electrolyte interface (CEI) film during the first charge and discharge cycle consumes a large amount of sodium ions, resulting in low first charge and discharge efficiency for polyanionic sodium cathode materials used in secondary batteries. Summary of the Invention

[0003] In view of this, in order to solve the problem that existing secondary batteries cannot simultaneously achieve good capacity performance, excellent dynamic performance and high first charge and discharge efficiency, this application provides a positive electrode and a sodium-ion secondary battery containing the positive electrode.

[0004] In a first aspect, this application provides a positive electrode sheet, including a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction; the positive active layer includes a first positive active layer and a second positive active layer stacked thereon, the first positive active layer being disposed between the positive current collector and the second positive active layer; The first positive electrode active layer includes a first positive electrode active material, which includes a first sodium iron pyrophosphate material and a layered oxide material. The second positive electrode active layer includes a second positive electrode active material, which includes a second sodium iron pyrophosphate material. The first sodium iron pyrophosphate material comprises single-crystal particles with a particle size Dv50 denoted as D1 μm; the second sodium iron pyrophosphate material comprises polycrystalline particles with a particle size Dv50 denoted as L1 μm; satisfying: D1>L1; The positive electrode active layer includes a sodium supplement agent, which includes a characteristic element, including at least one of Cl, Br, and I. The mass content of the characteristic element in the first positive electrode active layer is denoted as a, and the mass content of the characteristic element in the second positive electrode active layer is denoted as b, satisfying: a > b, 0.1% ≤ a ≤ 5%, and 30 ppm ≤ b ≤ 3000 ppm.

[0005] In a second aspect, this application provides a sodium-ion secondary battery, including a housing and an electrode assembly encapsulated inside the housing; the electrode assembly includes a positive electrode as described in the first aspect.

[0006] The technical solution of this application has the following advantages: 1. In this application, the active material in the first positive electrode active layer (also known as the bottom layer) near the positive electrode current collector is a single-crystal large-particle sodium iron pyrophosphate (NFPP) material and a layered oxide material, while the active material in the second positive electrode active layer (also known as the surface layer) away from the positive electrode current collector is a polycrystalline small-particle sodium iron pyrophosphate (NFPP) material. The single-crystal large-particle NFPP material has strong structural stability and more continuous and stable internal ion diffusion channels, enabling deep sodium removal without structural collapse. Furthermore, the large-particle NFPP material can effectively improve the overall compaction density of the electrode, thereby allowing the single-crystal large-particle NFPP material to fully utilize its high specific capacity advantage, and thus improve the energy density of the secondary battery. The layered oxide material in the first positive electrode active layer, due to its unique two-dimensional layered structure, can construct a continuous two-dimensional metal-oxygen-metal network, giving it high conductivity. This improves the electron and ion transport of the first positive electrode active layer, thereby contributing to improved rate performance and reduced internal resistance of the secondary battery. Furthermore, the layered oxide material helps to raise the overall voltage platform of the battery and maintain voltage stability during battery cycling. Its addition to the positive electrode active layer promotes increased battery energy density. The layered oxide material in the first positive electrode active layer synergistically works with the single-crystal large-particle sodium iron pyrophosphate (NFPP) material to jointly improve the structural stability, conductivity, and specific capacity of the first positive electrode active layer. Meanwhile, the introduction of polycrystalline small-particle NFPP material into the surface positive electrode active layer, with its numerous grain boundaries and pores, facilitates sodium ion transport and electrolyte wetting, ensuring excellent kinetic performance even when using thick electrode sheets in the secondary battery, thus improving the high-rate performance of the secondary battery.Furthermore, the sodium replenisher introduced into the positive electrode active layer in this application can replenish the sodium ions consumed during the formation of the positive electrode electrolyte interface (CEI) film during the first charge-discharge cycle, promoting the stable formation of the CEI film and improving the first coulombic efficiency of the secondary battery, thereby helping to improve the energy density of the secondary battery. On the other hand, controlling the mass content of the characteristic element (at least one of Cl, Br, I) contained in the sodium replenisher in the active layer, one is to control the mass content a of the characteristic element added to the first positive electrode active layer to be 0.1%~5%, which can form an interfacial phase rich in Na ions and characteristic element ions, suppressing the positive electrode Corrosion of the current collector helps to obtain a more resilient CEI film and replenish sodium lost by the layered oxide material during battery cycling, thereby improving the structural stability of the layered oxide material. Secondly, controlling the mass content b of the characteristic element in the second positive electrode active layer to 30ppm~3000ppm avoids the accumulation of characteristic element ions at the electrode-electrolyte interface, which could damage the CEI film on the electrode surface. This ensures the stability of the electrolyte interface (CEI) film on the electrode surface, thus improving the cycle stability of the battery, reducing internal resistance, and increasing the battery's energy density. This application introduces a sodium replenishing agent into the positive electrode active layer, combined with control of the characteristic element content in the first and second positive electrode active layers. This achieves sodium replenishment while improving the stability of the interface and active materials, avoiding damage to the CEI film caused by high levels of characteristic elements on the surface, reducing interfacial side reactions during cycling, and improving the overall structural stability of the positive electrode. This, in turn, improves the cycle stability, initial coulombic efficiency, and energy density of the secondary battery, and reduces internal resistance. 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 scanning electron microscope (SEM) image of the first sodium iron pyrophosphate material of Example 1 of this application.

[0009] Figure 2 This is a scanning electron microscope (SEM) image of the second phosphate sodium iron pyrophosphate material of Example 1 of this application.

[0010] Figure 3 This is a schematic diagram of the structure of the sodium-ion secondary battery provided in Embodiment 1 of this application.

[0011] 1. Electrode assembly; 2. Positive electrode tab; 3. Negative electrode tab; 4. Housing; X, first direction; Y, second direction; Z, third direction; α, dimension in the first direction; β, dimension in the second direction; γ, dimension in the third direction. Detailed Implementation

[0012] The following embodiments are provided to better understand this application. However, the following embodiments do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining the features of this application with other prior art, falls within the scope of protection of this application.

[0013] 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.

[0014] To address the problem that existing secondary batteries cannot simultaneously achieve good capacity performance, excellent kinetic performance, and high initial charge-discharge efficiency, this application proposes the following solution.

[0015] In a first aspect, this application provides a positive electrode sheet, including a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector in the thickness direction; the positive active layer includes a first positive active layer and a second positive active layer stacked thereon, the first positive active layer being disposed between the positive current collector and the second positive active layer; The first positive electrode active layer includes a first positive electrode active material, which includes a first sodium iron pyrophosphate material and a layered oxide material. The second positive electrode active layer includes a second positive electrode active material, which includes a second sodium iron pyrophosphate material. The first sodium iron pyrophosphate material comprises single-crystal particles with a particle size Dv50 denoted as D1 μm; the second sodium iron pyrophosphate material comprises polycrystalline particles with a particle size Dv50 denoted as L1 μm; satisfying: D1>L1; The positive electrode active layer further includes a sodium supplement agent, which includes a characteristic element, including at least one of Cl, Br, and I. The mass content of the characteristic element in the first positive electrode active layer is denoted as a, and the mass content of the characteristic element in the second positive electrode active layer is denoted as b, satisfying: a > b, 0.1% ≤ a ≤ 5%, and 30 ppm ≤ b ≤ 3000 ppm.

[0016] The positive electrode sheet of this application adopts a double-layer active layer structure. In the first positive electrode active layer (also known as the bottom layer) near the positive electrode current collector, monocrystalline large-particle sodium iron pyrophosphate (NFPP) material and layered oxide material are used as active materials. In the second positive electrode active layer (also known as the surface layer) away from the positive electrode current collector, polycrystalline small-particle sodium iron pyrophosphate (NFPP) material is used as the positive electrode active material. This design can simultaneously improve the energy density, cycle stability, and rate performance of the secondary battery, as well as reduce the internal resistance of the secondary battery. The monocrystalline large-particle NFPP material exhibits strong structural stability and more continuous and stable internal ion diffusion channels, enabling deep sodium removal without structural collapse. Furthermore, the large-particle NFPP material effectively increases the overall compaction density of the electrode sheet, allowing the monocrystalline large-particle NFPP material to fully utilize its high specific capacity, thereby improving the energy density of the secondary battery. The layered oxide material in the first positive electrode active layer, due to its unique two-dimensional layered structure, can construct a continuous two-dimensional metal-oxygen-metal network, giving it high conductivity. This improves the electron and ion transport of the first positive electrode active layer, thereby contributing to improved rate performance and reduced internal resistance of the secondary battery. Furthermore, the layered oxide material helps to raise the overall voltage platform of the battery and maintain voltage stability during battery cycling. Its addition to the positive electrode active layer promotes increased battery energy density. The layered oxide material in the first positive electrode active layer synergistically works with the single-crystal large-particle sodium iron pyrophosphate (NFPP) material to jointly improve the structural stability, conductivity, and capacity released per unit mass of active material in the first positive electrode active layer. Meanwhile, the introduction of polycrystalline small-particle NFPP material into the surface positive electrode active layer, with its numerous grain boundaries and pores, facilitates sodium ion transport and electrolyte wetting, ensuring excellent kinetic performance even when using thick electrode sheets in the secondary battery, thus improving the high-rate performance of the secondary battery. By introducing monocrystalline large-particle NFPP material and layered oxide into the first positive electrode active layer, and combining it with polycrystalline large-particle NFPP material into the second positive electrode active layer, the double-layer active layer structure and the positive electrode active material inside work together to improve the structural stability, electron transport performance, ion transport performance, electrolyte wettability, and capacity performance of the positive electrode sheet. This results in improved long-cycle stability, rate performance, and energy density of the secondary battery, as well as reduced internal resistance.

[0017] Furthermore, this application also introduces a sodium replenishing agent into the positive electrode active layer. On the one hand, the sodium replenishing agent in the active layer can replenish the sodium ions consumed when the positive electrode electrolyte interface (CEI) film is formed during the first charge-discharge cycle of the battery, promote the stable formation of the CEI film, improve the first coulombic efficiency of the secondary battery, and thus help improve the energy density of the secondary battery. On the other hand, controlling the mass content of characteristic elements (at least one of Cl, Br, and I) in the sodium replenisher in the active layer has two main effects: First, controlling the mass content (a) of the characteristic element added to the first positive electrode active layer to be 0.1% to 5% can form an interfacial phase rich in Na ions and characteristic element ions, inhibiting corrosion of the positive electrode current collector, helping to obtain a more resilient CEI film, and replenishing the sodium lost by the layered oxide material during battery cycling, thereby improving the structural stability of the layered oxide material. Second, controlling the mass content (b) of the characteristic element in the second positive electrode active layer to be 30ppm to 3000ppm can prevent the accumulation of a large number of characteristic elements in the second positive electrode active layer at the interface between the electrode sheet and the electrolyte, which would lead to damage to the CEI film on the electrode surface. This ensures the stability of the electrolyte interface (CEI) film on the electrode surface, which in turn helps to improve the cycle stability of the battery, reduce the battery internal resistance, and increase the battery energy density. This application introduces a sodium-supplementing agent into the positive electrode active layer and combines this with control over the content of characteristic elements in the first and second positive electrode active layers. This achieves sodium supplementation while improving the stability of the interface and active materials, avoiding damage to the CEI film caused by specific elements with high content on the surface, reducing the occurrence of interfacial side reactions during cycling, and improving the overall structural stability of the positive electrode sheet. As a result, it can improve the cycle stability, initial coulombic efficiency, and energy density of the secondary battery, as well as reduce the battery's internal resistance.

[0018] This application research found that if the particle size Dv50 of the first sodium iron pyrophosphate material is less than or equal to the particle size Dv50 of the second sodium iron pyrophosphate material, it means that the particle size Dv50 of the first sodium iron pyrophosphate material is too small, and the particle size Dv50 of the second sodium iron pyrophosphate material is too large. The specific capacity of the first sodium iron pyrophosphate 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 sodium iron pyrophosphate material with a particle size Dv50 that is too large is located in the second positive electrode 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.

[0019] This application also found that if the mass content of characteristic elements in the first positive electrode active layer is less than 0.1%, it means that the content of sodium replenishing agent in the first positive electrode active layer is too low, which cannot effectively replenish sodium to the electrode and cannot effectively improve the structural stability of the layered oxide material. It also leads to the inability to form an interface phase rich in sodium ions and characteristic element ions, which cannot effectively inhibit the corrosion of the current collector and the agglomeration of the active material, and is not conducive to improving the initial coulombic efficiency, cycle stability and energy density of the secondary battery. If the mass content of characteristic elements in the first positive electrode active layer is higher than 5%, it will lead to the sodium replenishing agent content in the electrode being too low. Excessive sodium content can easily decompose and form residues containing characteristic element ions, damaging the electrode structure and causing a decrease in the cycle stability of the secondary battery. If the mass content of characteristic elements in the second positive electrode active layer is less than 30 ppm, there will be insufficient sodium replenishment and uneven distribution of sodium replenishment, resulting in a decrease in the initial coulombic efficiency of the secondary battery. If the mass content of characteristic elements in the second positive electrode active layer is higher than 3000 ppm, it will cause excessive characteristic elements in the second positive electrode active layer to react with sodium salt, damaging the electrolyte interface (CEI) film on the electrode surface and causing a decrease in the cycle stability of the secondary battery.

[0020] This application also found that if the mass content of the characteristic elements in the first positive electrode active layer is lower than or equal to the mass content of the characteristic elements in the second positive electrode active layer, it will be impossible to achieve the desired sodium replenishment effect while simultaneously improving the structural stability of the first positive electrode active layer, suppressing current collector corrosion, and avoiding damage to the surface CEI film by the high content of characteristic ions in the sodium replenishment agent. This will lead to severe interfacial side reactions of the positive electrode sheet during battery cycling, affecting interfacial stability, which is not conducive to improving the long-cycle stability of the secondary battery, and will also lead to an increase in the internal resistance of the secondary battery.

[0021] In this application, the characteristic elements in the positive electrode active layer mainly originate from the sodium supplement containing the characteristic elements. Specifically, the characteristic elements in the first positive electrode active layer mainly originate from the characteristic elements in the sodium supplement, and the characteristic elements in the second positive electrode active layer can mainly originate from the sodium supplement added only during the preparation of the first positive electrode active layer. The sodium supplement in the first positive electrode active layer migrates to the second positive electrode active layer during the preparation of the secondary battery (especially during the battery formation stage), thus the second positive electrode active layer contains trace amounts of characteristic elements; or, it can also originate from the sodium supplement containing the characteristic elements added directly to the second positive electrode active layer, that is, the same or different types of sodium supplements are added during the preparation of both the first and second positive electrode active layers.

[0022] It should be noted that the mass content 'a' of the characteristic elements in the first positive electrode active layer can be obtained by disassembling the secondary battery, removing the positive electrode sheet, immersing and rinsing it in dimethyl carbonate (DMC) solvent, and then drying it to obtain the treated positive electrode sheet. The material of the first positive electrode active layer can be separated from the treated positive electrode sheet and tested using energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma (ICP). The mass content 'b' of the characteristic elements in the second positive electrode active layer can also be obtained by energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma (ICP). For example, the mass content 'a' of the characteristic element in the first positive electrode active layer can be, for example, 0.1%, 0.5%, 1.5%, 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 'b' of the characteristic element in the second positive electrode active layer can be, for example, 30ppm, 50ppm, 100ppm, 200ppm, 400ppm, 600ppm, 800ppm, 1000ppm, 1200ppm, 1400ppm, 1600ppm, 1800ppm, 2000ppm, 2200ppm, 2400ppm, 2600ppm, 2800ppm, 3000ppm, or a value within the range of any two of the above values.

[0023] It should be noted that in this application, the particle size Dv50 of the first sodium iron pyrophosphate material can be obtained through conventional testing processes. For example, it can be obtained by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), or by separating the first sodium iron pyrophosphate material from the first positive electrode active layer or by directly using the first sodium iron pyrophosphate material raw material for laser particle size analysis. Specifically, the testing process using SEM combined with EDS can be as follows: based on the SEM image of the positive electrode active layer and its EDS area scan image, at least 100 pieces of the first sodium iron pyrophosphate material are selected as samples. The particle size of each piece of the first sodium iron pyrophosphate in the samples is tested, and a cumulative volume particle size distribution curve is plotted. The particle size Dv50 of the first sodium iron pyrophosphate is obtained by taking the particle size value corresponding to the cumulative volume distribution reaching 50%. Similarly, the particle size Dv10 and Dv90 of the first sodium iron pyrophosphate are obtained by taking the particle size values ​​corresponding to the cumulative volume distribution reaching 10% and 90%, respectively. The sodium iron pyrophosphate material can be separated using conventional methods in the art, for example, by the following process: disassemble the secondary battery and remove the positive electrode sheet, soak and rinse it in DMC solvent, and then dry it to obtain the treated positive electrode sheet. Then, use adhesive tape to peel off the second positive electrode active layer in the positive electrode active layer, scrape off the material of the first positive electrode active layer on the positive electrode current collector, remove the binder and conductive agent in the material, and then use an organic weak acid (such as citric acid, ascorbic acid, etc.) to remove the layered oxide material in the material. Finally, at least 100 sodium iron pyrophosphate materials are separated by centrifugation, filtration and drying.

[0024] Similarly, the particle size Dv50 of the second sodium iron pyrophosphate material can be obtained through conventional testing processes. For example, it can be obtained by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), or by separating the second sodium iron pyrophosphate material from the second positive electrode active layer, or by directly using the second sodium iron pyrophosphate material raw material for laser particle size analysis. The testing process using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) is basically the same as that for the first sodium iron pyrophosphate material described above. The second sodium iron pyrophosphate material can be separated from the second positive electrode active layer using conventional methods in the art. For example, it can be separated by the following process: disassembling the secondary battery and removing the positive electrode sheet, immersing and rinsing it in DMC solvent, and then drying it to obtain the treated positive electrode sheet. Then, the second positive electrode active layer is coated, and the binder and conductive agent in the coated material are removed to separate at least 100 particles of the first sodium iron pyrophosphate material.

[0025] 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.

[0026] In some embodiments, the sodium supplement includes at least one of NaCl, NaBr, and NaI.

[0027] Optionally, in some embodiments, the first positive electrode active layer includes a first sodium supplement.

[0028] Optionally, in other embodiments, the first positive electrode active layer includes a first sodium replenishing agent, and the second positive electrode active layer includes a second sodium replenishing agent. The first sodium replenishing agent and the second sodium replenishing agent may be of the same or different types, and independently include at least one of NaCl, NaBr, and NaI.

[0029] In some embodiments, the layered oxide material includes an O3-phase layered oxide material and a P2-phase layered oxide material. Thus, the higher sodium ion content in the O3-phase layered oxide material is more conducive to increasing the electrode capacity, while the higher conductivity of the P2-phase layered oxide material can further improve the electron transport of the thick electrode. The synergistic effect of the O3-phase and P2-phase layered oxide materials can further reduce polarization, decrease the battery's internal resistance, and further improve the energy density and rate performance of the secondary battery. Furthermore, introducing a mixed-phase layered oxide material into the first positive electrode active layer can also suppress the phase transition of the layered active material, further improving the long-cycle stability of the secondary battery.

[0030] Furthermore, in some embodiments, the mass ratio of the O3 phase layered oxide material, the P2 phase layered oxide material, and the first sodium iron pyrophosphate material in the first positive electrode active material is (0.2~1.5):(0.3~2.5):(6~9.5). This further improves the capacity, conductivity, and structural stability of the positive electrode active layer, thereby better improving the cycle stability, rate performance, and energy density of the secondary battery, and further reducing the battery's internal resistance.

[0031] It should be noted that the mass content of the O3 phase layered oxide material, the P2 phase layered oxide material, and the first phosphate iron sodium pyrophosphate material in the first positive electrode active material can be obtained by X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). For example, the mass fraction of the O3 phase layered oxide material in the first positive electrode active material can be, for example, 0.2, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or values ​​within any two of the above ranges; the mass fraction of the P2 phase layered oxide material in the first positive electrode active material can be, for example, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1. 7, 1.9, 2.1, 2.3, 2.5, etc., or values ​​within the range of any two of the above values; the mass fraction of the first sodium iron pyrophosphate material in the first positive electrode active material can be, for example, 6.0, 6.2, 6.4, 6.6, 6.8, 7.0, 7.2, 7.4, 7.6, 7.8, 8.0, 8.2, 8.4, 8.6, 8.8, 9.0, 9.2, 9.4, 9.5, etc., or values ​​within the range of any two of the above values.

[0032] Furthermore, in some embodiments, the specific surface area of ​​the O3 phase layered oxide material is denoted as S1 m. 2 / g, the specific surface area of ​​the P2 phase layered oxide material is denoted as S2 m 2 / g, satisfying: S1 < S2. Thus, the O3 phase layered oxide material has a lower surface area, which can further improve the specific capacity of the electrode and is more conducive to improving the energy density of the secondary battery. The P2 phase layered oxide material has a higher specific surface area, which can further improve the ion insertion / extraction capability and provide more insertion / extraction channels during charge and discharge. This further compensates for the poor kinetic performance of the first phosphate iron sodium pyrophosphate material with large single crystal particles, further improves the kinetic performance of the first positive electrode active layer, and is more conducive to improving the rate performance and cycle stability of the secondary battery.

[0033] It should be noted that the specific surface area of ​​the O3 phase layered oxide material and the specific surface area of ​​the P2 phase layered oxide material can be obtained by using O3 phase layered oxide material and P2 phase layered oxide material separated from the first positive electrode active layer, or by directly using O3 phase layered oxide material and P2 phase layered oxide material raw materials. The isothermal adsorption-desorption curve is obtained through N2 isothermal adsorption-desorption test and calculated according to the Brunauer-Emmett-Teller (BET) model. The process for separating O3-phase layered oxide material and P2-phase layered oxide material from the first positive electrode active layer can be as follows: The positive electrode sheet is removed from the secondary battery, immersed in DMC solvent and rinsed, and then dried to obtain the processed positive electrode sheet. Then, the first and second positive electrode active layers are separated using ion milling or a peeling process with adhesive tape. The material of the first positive electrode active layer is scraped off, and the binder and conductive agent in the material are removed to obtain a particle mixture of the first positive electrode active material. A small amount of the first positive electrode active material particles are placed on different conductive adhesives, and at least 2g of O3-phase layered oxide material and at least 2g of P2-phase layered oxide material are separated from the particle mixture using energy-dispersive X-ray spectroscopy (EDS). The specific surface area of ​​the O3-phase layered oxide material and the P2-phase layered oxide material is measured using a N2 isothermal adsorption-desorption combined with BET testing.

[0034] In some embodiments, the general chemical formula of the O3 phase layered oxide material is: Na x Fe y Ni z Mn r M l O2; wherein M includes at least one of Ti, Cu, Mg, Ca, Cr, Co, Ce, Zn, Pd, Al, and Mo; 0.8≤x<1, 0.20≤y≤0.35, 0.23≤z≤0.35, 0.24≤r≤0.36, and 0≤l≤0.2.

[0035] For example, x can be 0.80, 0.82, 0.84, 0.86, 0.88, 0.90, 0.92, 0.94, 0.96, 0.98, 0.99, or a value within the range of any two of the above values; y can be 0.20, 0.22, 0.24, 0.26, 0.28, 0.30, 0.32, 0.34, 0.35, or a value within the range of any two of the above values; z can be 0.23, 0.24, 0.26, 0. 28, 0.30, 0.32, 0.34, 0.35, etc., or values ​​within the range of any two of the above values; r can be, for example, 0.24, 0.26, 0.28, 0.30, 0.32, 0.34, 0.36, etc., or values ​​within the range of any two of the above values; l can be, for example, 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, etc., or values ​​within the range of any two of the above values.

[0036] Optionally, in some embodiments, the chemical formula of the O3 phase layered oxide material satisfies: y+z+r+l=1.

[0037] In some implementations, the following condition is satisfied: 5.5 ≤ D² ≤ 7.5. This further improves the capacity performance of the electrode, while simultaneously shortening the sodium ion transport path, enhancing electrode kinetics, and thus better improving both the rate performance and energy density of the secondary battery.

[0038] It should be noted that the particle size Dv50 of the O3 phase layered oxide material can be obtained by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), or by separating the O3 phase layered oxide material from the first positive electrode active layer and performing laser particle size analysis, or by directly using the O3 phase layered oxide material raw material for laser particle size analysis. Specifically, the testing process using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) involves: based on the SEM image of the positive electrode active layer and its EDS area scan image, selecting at least 100 O3 phase layered oxide materials, testing their particle size through image analysis, and plotting a normal distribution map of the particle size, then taking the cumulative volume distribution. The particle size Dv50 of the O3 phase layered oxide material corresponding to a 50% concentration can be obtained by selecting at least 100 O3 phase layered oxide materials multiple times and plotting the normal distribution region of the particle size to obtain multiple particle size Dv50 values ​​and taking the average. Similarly, by taking the cumulative volume distribution to reach 90% and 10%, the particle size Dv90 and particle size Dv10 can be obtained. For example, the particle size Dv50 of the O3 phase layered oxide material can be, for example, 5.5μm, 5.6μm, 5.8μm, 6.0μm, 6.2μm, 6.4μm, 6.6μm, 6.8μm, 7.0μm, 7.2μm, 7.4μm, 7.5μm, etc., or a value within the range of any two of the above values.

[0039] In some embodiments, the span value of the O3 phase layered oxide material is 0.5~2.2. This ensures that the particle size of the O3 phase layered oxide material has good uniformity, which is more conducive to improving the compaction density of the electrode and enhancing its kinetic performance, thereby further improving the energy density and rate performance of the secondary battery.

[0040] In this application, the particle size span value is defined as (particle size Dv90 - particle size Dv10) / particle size Dv50.

[0041] For example, the span value of the particle size of the O3 phase layered oxide material can be, for example, 0.50, 0.60, 0.80, 1.00, 1.20, 1.40, 1.60, 1.80, 2.00, 2.20, or a value within the range of any two of the above values.

[0042] Optionally, in some embodiments, the particle size Dv90 of the O3 phase layered oxide material is 8 μm to 14 μm. Exemplarily, the particle size Dv90 of the O3 phase layered oxide material can be, for example, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or a value within the range of any two of the above values.

[0043] Optionally, in some embodiments, the particle size Dv10 of the O3 phase layered oxide material is 2 μm to 5 μm. Exemplarily, the particle size Dv10 of the O3 phase layered oxide material can be, for example, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, or a value within the range of any two of the above values.

[0044] In some embodiments, the specific surface area of ​​the O3 phase layered oxide material satisfies: 0.4 ≤ S1 ≤ 0.8. This allows for improved capacity performance of the cathode while also achieving superior kinetic performance, further enhancing sodium ion transport in the first cathode active layer, reducing impedance, and ultimately improving both the energy density and rate performance of the secondary battery.

[0045] For example, the specific surface area of ​​the O3 phase layered oxide material may be, for example, 0.40 m². 2 / g, 0.45m 2 / g, 0.50m 2 / g, 0.55m 2 / g, 0.60m 2 / g, 0.65m 2 / g, 0.70m 2 / g, 0.75m 2 / g, 0.80m 2 / g or values ​​within the range of any two of the above values.

[0046] In some embodiments, the general chemical formula of the P2 phase layered oxide material is: Na w Fe t Ni u Mn v Cu s Q q O2, wherein Q includes at least one of Mg, Ca, Cr, Co, Ce, Zn, Pd, Ti, Al and Mo; 0.2≤w<0.8, 0≤t≤0.3, 0≤u≤0.3, 0<v≤0.8, 0.2≤s≤0.6, 0≤q≤0.2.

[0047] For example, w can be 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.79, 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, 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; v can be 0.01, 0.05, 0.10, ... The values ​​0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, etc., or values ​​within the range of any two of the above values; the value s can be, for example, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, etc., or values ​​within the range of any two of the above values; the value q can be, for example, 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, etc., or values ​​within the range of any two of the above values.

[0048] Optionally, in some embodiments, the chemical formula of the P2 phase layered oxide material satisfies: t+u+v+s+q=1.

[0049] In some implementations, the following condition is satisfied: 4.6 ≤ D3 ≤ 7. Thus, while the kinetic performance of the first positive electrode active layer can be further improved by using P2 phase layered oxide materials, the compaction density of the first positive electrode active layer can be further increased by using particles with a larger particle size Dv50, thereby further contributing to the simultaneous improvement of the energy density and rate performance of the secondary battery.

[0050] It should be noted that the particle size Dv50 of the P2 phase layered oxide material can be obtained by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), or by separating the P2 phase layered oxide material from the first positive electrode active layer and then performing laser particle size analysis, or by directly testing the P2 phase layered oxide material raw material. The testing process using scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) involves: based on the SEM image of the positive electrode active layer combined with its… The EDS area scan image is used to select at least 100 P2 phase layered oxide materials. The particle size is tested using graphic analysis software, and a normal distribution map of the particle size is plotted. The particle size Dv50 of the P2 phase layered oxide material corresponding to a cumulative volume distribution reaching 50% is taken, which can be either particle size Dv90 or particle size Dv10. Alternatively, multiple particle size Dv50 values ​​can be obtained by selecting at least 100 P2 phase layered oxide materials and plotting the normal distribution region of the particle size, and then averaging the values. Similarly, values ​​can be taken when the cumulative volume distribution reaches 90% and 10%. For example, the particle size Dv50 of the P2 phase layered oxide material can be, for example, 4.6 μm, 4.8 μm, 5.0 μm, 5.2 μm, 5.4 μm, 5.6 μm, 5.8 μm, 6.0 μm, 6.2 μm, 6.4 μm, 6.6 μm, 6.8 μm, 7.0 μm, or values ​​within any two of the above ranges.

[0051] In some embodiments, the particle size span of the P2 phase layered oxide material is 1.7 to 2.8. Thus, the P2 phase layered oxide material has a wide-ranging particle size distribution, which can further improve the material's dispersibility, thereby facilitating further improvement in the electrode compaction density and ultimately enhancing the energy density of the secondary battery.

[0052] For example, the span value of the particle size of the P2 phase layered oxide material can be, for example, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, or a value within the range of any two of the above values.

[0053] Optionally, in some embodiments, the particle size Dv90 of the P2 phase layered oxide material is 14 μm to 18 μm. Exemplarily, the particle size Dv90 of the P2 phase layered oxide 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, or a value within the range of any two of the above values.

[0054] Optionally, in some embodiments, the particle size Dv10 of the P2 phase layered oxide material is 1.5 μm to 2.8 μm. Exemplarily, the particle size Dv10 of the P2 phase layered oxide 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, or a value within the range of any two of the above values.

[0055] In some implementations, the following condition is satisfied: 0.62 ≤ S2 ≤ 0.95. Thus, the specific surface area of ​​the P2 phase layered oxide material is approximately 0.62 m². 2 / g~0.95m 2 Within the range of / g, it can provide a transport channel for ion insertion / extraction, which is more conducive to improving the kinetic performance of the electrode and can further improve the rate performance of the secondary battery.

[0056] For example, the specific surface area of ​​the P2 phase layered oxide material can be, for instance, 0.62 m². 2 / g, 0.64m 2 / g, 0.66m 2 / g, 0.68m 2 / g, 0.70m 2 / g, 0.72m 2 / g, 0.74m 2 / g, 0.76m 2 / g, 0.78m 2 / g, 0.80m 2 / g, 0.82m 2 / g, 0.84m 2 / g, 0.86m 2 / g, 0.88m 2 / g, 0.90m 2 / g, 0.92m 2 / g, 0.94m 2 / g, 0.95m 2 / g or values ​​within the range of any two of the above values.

[0057] In some embodiments, the chemical formula of the first sodium iron pyrophosphate material is Na. f Fe g E c (PO4) hP2O7, wherein 3≤f≤6, 2≤g≤5, 0≤c≤0.3, 1≤h≤4, and E includes at least one of Mg, Ca, Al, Cr, Mn, W, and V. For example, f 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; g 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; c 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; and h 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.

[0058] In some embodiments, the following condition is satisfied: 6.4 ≤ D1 ≤ 9.6. Thus, the particle size Dv50 of the first sodium iron pyrophosphate material is within the range of 6.4 μm to 9.6 μm, which can effectively improve the specific capacity of the first sodium iron pyrophosphate material and increase the compaction density of the electrode. Simultaneously, it avoids excessively large particle sizes of the sodium iron pyrophosphate 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. This is more conducive to simultaneously improving the energy density and kinetic performance of the secondary battery.

[0059] For example, the particle size Dv50 of the first sodium iron pyrophosphate material can be, for example, 6.4 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 9.6 μm, or a value within the range of any two of the above values.

[0060] In some embodiments, the particle size span of the first sodium iron pyrophosphate material is 1.08~2.83. This results in better particle size uniformity of the first sodium iron pyrophosphate material, which is more conducive to improving the compaction density of the first positive electrode active layer while simultaneously improving its kinetic performance, thereby further enhancing the overall energy density and rate performance of the secondary battery.

[0061] For example, the particle size span value of the first sodium iron pyrophosphate material can be, for example, 1.08, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, 2.83, or a value within the range of any two of the above values.

[0062] Optionally, in some embodiments, the particle size Dv90 of the first sodium iron pyrophosphate material is 14.0 μm to 20.0 μm. Exemplarily, the particle size Dv90 of the first sodium iron pyrophosphate 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.

[0063] Optionally, in some embodiments, the particle size Dv10 of the second sodium iron pyrophosphate material is 1.5 μm to 3.9 μm. Exemplarily, the particle size Dv10 of the second sodium iron pyrophosphate 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.

[0064] In some embodiments, the specific surface area of ​​the first sodium iron pyrophosphate material is 2.1 m². 2 / g~5.5m 2 / g. This can further increase the compaction density of the first positive electrode active layer, which is more conducive to improving the energy density of the secondary battery.

[0065] It should be noted that the specific surface area of ​​the first sodium iron pyrophosphate material can be obtained by BET-nitrogen adsorption method. For example, the specific surface area of ​​the first positive electrode active material can be, for instance, 2.1 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 or values ​​within the range of any two of the above values.

[0066] In some embodiments, the chemical formula of the second sodium iron pyrophosphate material is Na. j Fe k R d (PO4) iP2O7, wherein 3≤j≤6, 2≤k≤5, 0≤d≤0.3, 1≤i≤4, and R includes at least one of Mg, Ca, Al, Cr, Mn, W, and V. For example, j 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; k 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; d can be 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, etc., or a value within the range of any two of the above values; and i can be 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, etc., or a value within the range of any two of the above values.

[0067] In some implementations, the following condition is satisfied: 3.8 ≤ L1 ≤ 6.2. 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 further enhancing both the energy density and kinetic performance of the secondary battery.

[0068] For example, the particle size Dv50 of the second sodium iron pyrophosphate material can be, for example, 3.8 μm, 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, 6.2 μm, or a value within the range of any two of the above values.

[0069] In some embodiments, the particle size span of the second sodium iron pyrophosphate material is 2.59~4.24. This results in a wide particle size distribution for the second sodium iron pyrophosphate 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 energy density and rate performance of the secondary battery.

[0070] For example, the particle size span value of the second sodium iron pyrophosphate material can be, for example, 2.59, 2.60, 2.80, 3.00, 3.20, 3.40, 3.60, 3.80, 4.20, 4.24, or a value within the range of any two of the above values.

[0071] Optionally, in some embodiments, the particle size Dv90 of the second sodium iron pyrophosphate material is 13.0 μm to 18.0 μm. Exemplarily, the particle size Dv90 of the second sodium iron pyrophosphate 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.

[0072] Optionally, in some embodiments, the particle size Dv10 of the second sodium iron pyrophosphate material is 1.0 μm to 2.5 μm. Exemplarily, the particle size Dv10 of the second sodium iron pyrophosphate material can be, for example, 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 the range of any two of the above values.

[0073] In some embodiments, the specific surface area of ​​the second sodium iron pyrophosphate material is 7.2 m². 2 / g~12.8m 2 / g. This can better optimize 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.

[0074] It should be noted that the specific surface area of ​​the second sodium iron pyrophosphate material can be obtained by BET-nitrogen adsorption method. For example, the specific surface area of ​​the second sodium iron pyrophosphate material can be, for instance, 7.2 m². 2 / g, 8.0m 2 / g, 9.0m 2 / g, 10.0m 2 / g, 11.0m 2 / g, 12.0m 2 / g, 12.8m 2 / g or values ​​within the range of any two of the above values.

[0075] In some embodiments, the layered oxide material includes Ni and Mn elements, and the total mass content A of Ni and Mn elements is 1% to 15% based on the total mass of the first positive electrode active layer. Thus, the total mass content of Ni and Mn elements in the first active layer is within the range of 1% to 15%, ensuring the appropriate content of Ni and Mn elements in the layered oxide material. Ni and Mn elements in the layered oxide material can synergistically improve the overall operating voltage, reversible capacity, and structural stability of the battery, thereby further improving the energy density and cycle stability of the secondary battery. On the other hand, Ni elements in the layered oxide material can further improve the capacity of the layered oxide material, while Mn elements are beneficial for improving the structural stability of the layered oxide material. By controlling the mass content of Ni and Mn elements in the first positive electrode active layer, the material can be further guaranteed to have superior capacity performance and structural stability.

[0076] It should be noted that the total mass content of Ni and Mn elements in the first positive electrode active layer can be obtained by energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma (ICP) testing. For example, the total mass content of Ni and Mn elements in the first positive electrode active layer can be 1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 15%, or a value within any two of the above ranges.

[0077] In this application, the Ni and Mn elements in the first positive electrode active layer mainly originate from the O3 phase layered oxide material and the P2 phase layered oxide material, and may also originate from the Mn element selectively doped in the first sodium iron pyrophosphate material.

[0078] In this application, by controlling the content of O3 phase layered oxide material and P2 phase layered oxide material in the first positive electrode active layer, and further controlling the total mass content of Ni and Mn elements in the first positive electrode active layer, the migration content of Ni and Mn elements from the first positive electrode active layer to the second positive electrode active layer during battery charge-discharge cycles can be controlled. This avoids the impact of excessive Ni and Mn elements migrating to the second positive electrode active layer or even the surface of the second positive electrode active layer during charge-discharge cycles on the active layer structure, thereby ensuring that the secondary battery has excellent cycle performance.

[0079] In this application, the O3 phase layered oxide material and the P2 phase layered oxide material can be obtained commercially or prepared using conventional techniques in the art.

[0080] In some embodiments, the thickness ratio of the second positive electrode active layer to the first positive electrode active layer is (0.2~0.9):1. By controlling the thickness ratio of the second positive electrode active layer to the first positive electrode active layer within the range of (0.2~0.9):1, the matching between the first and second positive electrode active layers is ensured, thereby further improving the overall energy density, cycle stability, and rate performance of the secondary battery, and also being more conducive to reducing the battery's internal resistance. Furthermore, controlling the thickness ratio of the first positive electrode active layer also ensures that the positive electrode sheet has an appropriate amount of sodium supplementer, thereby further improving the initial coulombic efficiency and long-term cycle stability of the secondary battery.

[0081] It should be noted that the thickness ratio of the second positive electrode active layer to the first positive electrode active layer can be obtained by combining cross-sectional scanning electron microscopy images of the positive electrode active layer with image analysis. For example, the thickness ratio of the second positive electrode active layer to the first positive electrode active layer can be 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, or a value within any two of the above ranges.

[0082] Furthermore, in some embodiments, the total thickness of the positive electrode active layer on one side is 100 μm to 170 μm. Thus, the positive electrode sheet provided in this application has a relatively thick positive electrode active layer on one side, enabling its application in large-size batteries and further improving the overall energy density, cycle stability, rate performance, and initial coulombic efficiency of the secondary battery.

[0083] It should be noted that the total thickness of the positive electrode active layer on one side can be measured using a micrometer screw gauge, or obtained by combining a cross-sectional scanning electron microscope image of the positive electrode active layer with image analysis. For example, the total thickness of the positive electrode active layer on one side can be 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 ranges.

[0084] In this application, the term "thickness" refers to the average thickness. That is, it can be obtained by averaging the thickness of the active layer at any of at least 10 locations.

[0085] In some embodiments, the porosity of the first positive electrode active layer is denoted as p, and the porosity of the second positive electrode active layer is denoted as φ, satisfying: 18%≤p≤30%, 30%≤φ≤50%, and 1.1≤φ / p≤2.7. Thus, a porosity of 18%~30% in the first positive electrode active layer is beneficial for achieving high capacity and low impedance while also ensuring superior ion transport efficiency and electrolyte retention, further improving the overall energy density and rate performance of the battery. A porosity of 30%~50% in the second positive electrode active layer facilitates rapid ion insertion / extraction and electrolyte wetting, further reducing internal resistance and improving the rate performance and cycle stability of the secondary battery. Furthermore, controlling the ratio of the porosity of the second positive electrode active layer to that of the first positive electrode active layer to be 1.1~2.7 ensures that the porosities of the two active layers are matched, enabling the secondary battery to achieve superior energy density, rate performance, and cycle stability.

[0086] It should be noted that the porosity can be obtained through the following process: 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 at least 10 times, and the average value T is calculated, followed by the calculation of its apparent volume V0 = S × T; the weight of the electrode sheet is measured using an electronic balance, 3 times, and the average value B is obtained; the true density ρ of the material in the active layer is measured using a true density meter, and the true volume V1 = B / ρ of the material in the active layer is calculated; the porosity of the electrode sheet is calculated according to the following formula. This method is also applicable to testing the porosity of the separator. Specifically, in this application, the porosity of the entire positive electrode sheet is tested first, and then the porosity of the first positive electrode active layer is tested after removing the second positive electrode active layer from the surface of the electrode sheet. The porosity of the second positive electrode active layer can be calculated based on the above two porosities. Porosity (%) = (V0) / (S × T) V1) / V1×100%. For example, the porosity p of the first positive electrode active layer can be, for example, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or a value within any two of the above values; the porosity φ of the second positive electrode active layer can be, for example, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, 50%, 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 can be, for example, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, 2.10, 2.20, 2.30, 2.40, 2.50, 2.60, 2.70, or a value within any two of the above values.

[0087] In some embodiments, the surface of the first sodium iron pyrophosphate material is further provided with a first coating layer, which covers at least a portion of the surface of the first sodium iron pyrophosphate material, and the first coating layer comprises a carbon material; the surface of the second sodium iron pyrophosphate material is further provided with a second coating layer, which covers at least a portion of the surface of the second sodium iron pyrophosphate material, and the second coating layer comprises a carbon material. Thus, coating the surface of the first sodium iron pyrophosphate material with carbon material, and coating the surface of the second sodium iron pyrophosphate material with carbon material, can improve the conductivity of the sodium iron pyrophosphate material, thereby further improving the kinetic performance of the material and making it more beneficial for improving the rate performance of the secondary battery.

[0088] Optionally, in some embodiments, the carbon material in the first coating layer includes nano-carbon.

[0089] Optionally, in some embodiments, the carbon material in the second coating layer includes nano-carbon.

[0090] In this application, the source of the carbon-coated first phosphate, iron 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, pyrophosphate, phosphate, iron, sodium, and carbon sources are mixed according to the formula, and then mixed further 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 iron pyrophosphate with carbon coating is obtained. The carbon source used for the nano-carbon coating layer can be an organic carbon source, such as glucose or sucrose.

[0091] In this application, the source of the carbon-coated sodium ferric pyrophosphate second phosphate is not specifically limited. It can be obtained from commercial sources or prepared through processing. The specific preparation process can employ conventional methods in the art. For example, pyrophosphate, phosphoric acid, iron, sodium, and carbon sources are mixed according to the formula, and further mixed in an optional medium and additives. Then, the mixture undergoes drying, heat treatment, cooling, crushing, and sieving processes, controlling the process and its parameters to prepare polycrystalline small particles of sodium ferric pyrophosphate second phosphate with carbon coating. The carbon source used for the nano-carbon coating layer can be an organic carbon source, such as glucose or sucrose.

[0092] In this application, both the first sodium ferric pyrophosphate and the second sodium ferric pyrophosphate satisfy the following: when the sodium ferric pyrophosphate contains a carbon coating layer, the relevant parameters of the particle size and specific surface area of ​​the sodium ferric pyrophosphate refer to the corresponding parameter values ​​of the sodium ferric pyrophosphate containing the carbon coating layer.

[0093] In some embodiments, the mass content of the first coating layer is denoted as m, based on the total mass of the first sodium iron pyrophosphate material and the first coating layer; and the mass content of the second coating layer is denoted as n, based on the total mass of the second sodium iron pyrophosphate material and the second coating layer, satisfying: 1.8%≤m≤4%, 1%≤n≤3%, and 0.25≤n / m≤1.5. Thus, by coating the surface of the large-particle, single-crystal sodium iron pyrophosphate material with a specific amount of carbon coating, this application can effectively compensate for the poor conductivity of the large-particle, single-crystal sodium iron pyrophosphate material, further improving the conductivity of the first sodium iron pyrophosphate material. Meanwhile, the small-particle, polycrystalline sodium iron pyrophosphate material exhibits superior kinetic properties, and a carbon coating with a mass content of 1%~3% can achieve excellent electronic conductivity, further resulting in a second sodium iron pyrophosphate material with excellent both electronic and ionic conductivity. This application controls the mass content of the second coating layer to be within the range of 0.25 to 1.5% of the mass content of the first coating layer, thereby improving the compatibility between the carbon coating content in the first and second sodium iron pyrophosphate materials. This further ensures that the positive electrode active material in the positive electrode active layer has excellent conductivity, while also avoiding the impact of excessive carbon coating layer on the specific capacity of the positive electrode active material. As a result, it can further improve the overall performance of the secondary battery in terms of rate capability and energy density.

[0094] It should be noted that the mass content of the first coating layer in the first sodium iron pyrophosphate material can be obtained by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS); the mass content of the second coating layer in the second sodium iron pyrophosphate material can be obtained by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS). For example, the mass content of the first coating layer in the first sodium iron pyrophosphate material may be, for example, 1.8%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, or a value within the range of any two of the above values; the mass content of the second coating layer in the second sodium iron pyrophosphate material may be, for example, 1.0%, 1.2%, 1.4%, 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; the ratio n / m of the mass content of the second coating layer in the second sodium iron pyrophosphate material to the mass content of the first coating layer in the first sodium iron pyrophosphate material may be, for example, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or a value within the range of any two of the above values.

[0095] In some embodiments, the first positive electrode active layer further includes a first conductive agent, which comprises conductive carbon black and a first carbon nanotube, wherein the aspect ratio of the first carbon nanotube is 800-5500. Thus, by incorporating carbon nanotubes with an aspect ratio of 800-5500 in the first positive electrode active layer, these nanotubes can form a continuous electronic conduction network with the single-crystal large-particle sodium iron pyrophosphate material and layered oxides in the 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, thereby further improving the electronic conductivity of the positive electrode, further reducing the battery's internal resistance, reducing polarization during battery charging and discharging, and further improving interface stability, ultimately enhancing the battery's long-cycle stability.

[0096] It should be noted that the aspect ratio of the first carbon nanotube can be obtained by selecting at least 100 carbon nanotubes in the first positive electrode active layer from the cross-sectional scanning electron microscope (SEM) image of the 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 first carbon nanotube can be 800, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, etc., or a value within any two of the above ranges.

[0097] In some embodiments, the second positive electrode active layer further includes a second 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 sodium iron pyrophosphate material, further improving the electronic conductivity of the electrode, reducing internal resistance, reducing polarization during battery charging and discharging, further improving the interface stability of the electrode, and further enhancing the long-cycle stability of the secondary battery.

[0098] Furthermore, in some embodiments, the second conductive agent further includes a second carbon nanotube with an aspect ratio of 100 to 900. Thus, by selecting carbon nanotubes with an aspect ratio of 100 to 900 for the second positive electrode active layer, it is possible to form a more continuous and uniform conductive network structure with the polycrystalline small particles of sodium iron pyrophosphate and the conductive carbon black, thereby further reducing the battery's internal resistance and improving its long-cycle stability.

[0099] It should be noted that the aspect ratio of the second carbon nanotube can be obtained by selecting at least 100 carbon nanotubes in the second positive electrode active layer from the cross-sectional scanning electron microscope (SEM) image of the 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 second carbon nanotube can be 100, 200, 300, 400, 500, 600, 700, 800, 900, etc., or a value within any two of the above ranges.

[0100] In some embodiments, the lateral surface density of the positive electrode active layer is 0.0190 g / cm³. 2 ~0.0350g / cm 2 Thus, the single-sided surface density of the positive electrode active layer in the positive electrode sheet provided in this application is within the above-mentioned range. On the one hand, a higher 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 also avoid the serious polarization and powder shedding of the positive electrode sheet during battery cycling caused by excessive coating surface density, thereby further reducing the internal resistance of the battery and improving the cycle stability of the battery.

[0101] It should be noted that the lateral surface 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 can be cut off, the mass of the cut positive electrode active layer sample can be weighed, and the areal density of the positive electrode active layer can be calculated by substituting the measured area and coating mass into the formula: areal density = coating mass / coating area. For example, the lateral surface density of the positive electrode active layer can be, for example, 0.0190 g / cm³. 2 0.0200g / cm 2 0.0220g / cm 2 0.0240g / cm 2 0.0260g / cm 2 0.0280g / cm 2 0.0300g / cm 2 0.0320 g / 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.

[0102] Optionally, in some embodiments, the first positive electrode active layer further includes a first binder and a first conductive agent. Based on the total mass content of the first positive electrode active layer, the mass content of the first positive electrode active material is 85% to 97%, the mass content of the first binder is 1% to 3%, the mass content of the first conductive agent is 1% to 4%, and the mass content of the sodium supplement is 0.1% to 9%.

[0103] Optionally, in some embodiments, the second positive electrode active layer further includes a second binder and a second conductive agent. Based on the total mass content of the second positive electrode active layer, the mass content of the second positive electrode active material is 93% to 97%, the mass content of the second binder is 1% to 3%, the mass content of the second conductive agent is 1% to 4%, and the mass content of the sodium supplement is 0% to 0.6%.

[0104] Secondly, this application provides a sodium-ion secondary battery, including a housing and an electrode assembly encapsulated inside the housing; the electrode assembly includes a positive electrode as described in the first aspect. Thus, the sodium-ion secondary battery provided by this application contains a specific positive electrode, and through a first positive electrode active layer and a second positive electrode active layer with a specific composition in the positive electrode, the cycle stability, rate performance, energy density, and initial coulombic efficiency of the sodium-ion secondary battery can be jointly improved, as well as the internal resistance of the sodium-ion secondary battery can be reduced.

[0105] Furthermore, in some embodiments, the positive electrode sheet includes a 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; The thickness direction of the electrode assembly is a first direction, the extension direction of the positive electrode tab is a second direction, and the direction perpendicular to both the first and second directions is a third direction. The sodium-ion secondary battery has a dimension of 40mm~100mm in the first direction, 100mm~250mm in the second direction, and 100mm~350mm in the third direction. Thus, the secondary battery provided in this application can be applied to large-size structures. The positive electrode, constructed with a thick active layer of specific composition and structure introduced inside the battery, maintains high energy density, high initial coulombic efficiency, excellent cycle stability and rate performance, and low internal resistance even when designed as a large-size secondary battery.

[0106] It should be noted that the dimensions of the sodium-ion secondary battery in the first direction can be, for example, 40mm, 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 can 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 can 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.

[0107] In some embodiments, the sodium-ion secondary battery further includes an electrolyte comprising sodium salts, organic solvents, and additives.

[0108] Furthermore, in some embodiments, the density of the electrolyte is 1.0 g / cm³ at 25°C. 3 ~1.5g / cm 3 This further improves the wettability of the electrolyte to the electrodes, provides channels for the liquid-phase transport and transfer of ions and electrons, further reduces battery polarization, and is more conducive to improving the rate performance and cycle stability of the battery, as well as further reducing the internal resistance of the battery.

[0109] It should be noted that the density of the electrolyte can be calculated by measuring the mass of a unit volume of electrolyte at 25°C. For example, the density of the electrolyte could be 1.00 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 / cm 3 1.35g / cm 3 1.40 g / cm 3 1.45g / cm 3 1.50g / cm 3 Values ​​equal to or within the range of any two of the above values.

[0110] Furthermore, in some embodiments, the conductivity of the electrolyte is 7.0 ms / cm to 10.0 ms / cm at 25°C.

[0111] It should be noted that the conductivity of the electrolyte can be obtained by measuring the conductivity at 25°C using a conductivity meter. For example, the conductivity of the electrolyte can be 7.0 ms / cm, 7.5 ms / cm, 8.0 ms / cm, 8.5 ms / cm, 9.0 ms / cm, 9.5 ms / cm, 10.0 ms / cm, or a value within any two of the above ranges.

[0112] Furthermore, in some embodiments, the sodium salt comprises sodium hexafluorophosphate and sodium difluorooxalate borate. Based on the total mass of the electrolyte, the mass content of sodium hexafluorophosphate is denoted as δ%, and the mass content of sodium difluorooxalate borate is denoted as ε%, satisfying: 0.1 ≤ ε / δ ≤ 1. Thus, the introduction of sodium hexafluorophosphate into the electrolyte offers significant cost advantages. Simultaneously, it possesses high solubility, providing higher ionic conductivity, and exhibits superior chemical and electrochemical stability, further contributing to improved cycle stability and rate performance of the secondary battery. Sodium difluorooxalate borate can participate in interface regulation, promoting the formation of a dense and stable CEI film rich in inorganic matter, further enhancing the battery's long-term cycle stability. Additionally, by controlling the mass ratio of sodium difluorooxalate borate to sodium hexafluorophosphate to be 0.1~1, a stable CEI film can be formed on the surface of the positive electrode particles, while also exhibiting good ion-liquid phase transport performance, which is beneficial for stable charge-discharge performance and further improves the battery's cycle performance.

[0113] Optionally, in some implementations, the following condition is satisfied: 3≤δ≤15.

[0114] Optionally, in some implementations, the following condition is satisfied: 1 ≤ ε ≤ 10.

[0115] It should be noted that the mass content of sodium hexafluorophosphate and sodium difluorooxalate borate can be obtained by ion chromatography-mass spectrometry (IC-MS). For example, the ratio ε / δ of the mass content of sodium difluorooxalate borate to the mass content of sodium hexafluorophosphate 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 above values.

[0116] Furthermore, in some embodiments, the organic solvent includes a first solvent, which comprises ethylene glycol dimethyl ether and / or ethylene glycol diethyl ether, and the mass content of the first solvent is 40% to 60% based on the total mass of the electrolyte. Thus, by using a specific content of ether-based solvent, this application can obtain an electrolyte with low viscosity and high conductivity, promoting faster wetting of the electrolyte into the interior of the electrode, reducing the migration resistance of sodium ions, and thereby further improving the rate performance and long-cycle stability of the secondary battery.

[0117] It should be noted that the mass content of the first solvent in the electrolyte can be obtained by gas chromatography-mass spectrometry (GC-MS). For example, the mass content of the first solvent in the electrolyte can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, or a value within any two of the above ranges.

[0118] Furthermore, in some embodiments, the organic solvent further includes a second 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.

[0119] Furthermore, in some embodiments, the additive includes at least one of a first additive, a second additive, and a third additive.

[0120] Furthermore, in some embodiments, the first additive comprises methyltriphenylphosphine bromide, and the mass content of the first additive in the electrolyte is 1% to 10%. Thus, by introducing the methyltriphenylphosphine bromide (MTPPB) additive into the electrolyte, this application can form anionic-cation cross-coordination with sodium hexafluorophosphate, thereby increasing the Br content in the methyltriphenylphosphine bromide. - Participating in Na + The solvation structure weakens Na + The interaction with the solvent lowers the coordination number and desolvation energy barrier, thereby further improving the kinetic performance of the battery and thus enhancing the rate performance of the secondary battery. On the other hand, the methyltriphenylphosphine bromide additive also exhibits a π-π conjugated bridging effect with the surface of the negative electrode active material, preferentially adsorbing to form a physical isolation layer, while MTPP... + With PF6 - Electrostatic interactions between them induce PF6 -The process involves targeted reduction to generate an ultrathin and stable electrolyte interface film, thereby suppressing electrolyte side reactions and active sodium loss, which further improves the cycle stability and initial coulombic efficiency of the secondary battery. Simultaneously, this application controls the mass content of the first additive to ensure that methyltriphenylphosphine bromide achieves the aforementioned effects, while avoiding excessive methyltriphenylphosphine bromide content, which could lead to excess methyltriphenylphosphine bromide adhering to the particle surface, resulting in impaired overall kinetic performance, increased internal resistance, and deterioration of charge-discharge and cycle performance.

[0121] Furthermore, in some embodiments, the second additive comprises tri-(2,2,2-trifluoroethyl)-phosphate, and the mass content of the second additive in the electrolyte is 1% to 10%. Thus, tri-(2,2,2-trifluoroethyl)-phosphate (TFEP), as a co-solvent, can enhance the interaction between anions and cations in the phosphate electrolyte, promote the reorganization of the ion-solventized structure, and form a stable anion-induced ion-solvent coordination structure, achieving high compatibility and stability at low salt concentrations. On the other hand, TFEP can also reduce and decompose the NaF-rich inorganic electrolyte interface (CEI) film generated. This CEI film has good interfacial compatibility, which helps ensure the stability of the electrolyte and is more conducive to improving the long-cycle stability of the secondary battery. Meanwhile, this application controls the mass content of the second additive within the range of 1% to 10%, which ensures that there is sufficient tri-(2,2,2-trifluoroethyl)-phosphate to achieve the above-mentioned effects. At the same time, it avoids excessively high mass content of tri-(2,2,2-trifluoroethyl)-phosphate, which would lead to increased electrolyte viscosity, deterioration of kinetic performance, and affect charge-discharge and cycle performance; it would also lead to excessively thick film, increased impedance, low first-time efficiency, and affect energy density.

[0122] Furthermore, in some embodiments, the third additive includes at least one of vinylene carbonate and fluoroethylene carbonate, and the mass content of the third additive in the electrolyte is 0.5% to 5%. This is more conducive to film formation, inhibits the decomposition of the solvent in the electrolyte, and further improves the long-cycle stability of the secondary battery.

[0123] It should be noted that the mass content of the first additive in the electrolyte can be obtained by ion chromatography-mass spectrometry (IC-MS); the mass content of the second additive in the electrolyte can be obtained by ion chromatography-mass spectrometry (IC-MS); and the mass content of the third additive in the electrolyte can be obtained by gas chromatography-mass spectrometry (GC-MS). For example, the mass content of the first additive in the electrolyte can be, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within any two of the above values; the mass content of the second additive in the electrolyte can be, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a value within any two of the above values; and the mass content of the third additive in the electrolyte can be, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, or a value within any two of the above values.

[0124] In some embodiments, the battery cell further includes a negative electrode sheet, which includes a negative current collector and a negative active layer disposed on at least one side surface of the negative current collector in the thickness direction. The negative active layer includes a negative active material, which is a porous carbon material. The pore structure of the porous carbon material includes mesopores and micropores, with the mesopores accounting for 20% to 30% of the total pore volume and the micropores accounting for 13% to 37% of the total pore volume. Thus, the micropores in the porous carbon negative electrode material can provide a large number of storage sites, the mesopores can provide transport channels, shorten the diffusion path, and the hierarchical pore structure can improve the wettability of the electrolyte, further reduce interfacial impedance, and further contribute to improving the energy density and rate performance of the secondary battery. Meanwhile, controlling the pore volume ratio of mesopores to 20% to 30% and the pore volume ratio of micropores to 13% to 37% can achieve a better balance between sodium storage capacity and ion transport efficiency, which is more conducive to improving the rate performance and energy density of the secondary battery.

[0125] It should be noted that the percentage of mesopore and micropore volume to the total pore volume can be obtained by combining gas (CO2) adsorption with the Advanced PSD (pore size distribution) module of NLDFT (Non-local density functional theory). For example, the percentage of mesopore volume to the total pore volume can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or values ​​within any two of the above ranges; the percentage of micropore volume to the total pore volume can be 13%, 15%, 20%, 25%, 30%, 35%, 37%, or values ​​within any two of the above ranges.

[0126] Optionally, in some embodiments, the pore size of the micropore is less than 2 nm.

[0127] Optionally, in some embodiments, the pore size of the mesopore is 2nm to 50nm.

[0128] Optionally, in some embodiments, the porous carbon material further includes macropores in its pore structure.

[0129] Optionally, in some embodiments, the pore size of the macropore is greater than 50 nm.

[0130] Optionally, in some embodiments, the volume ratio of the macropore is 33% to 67%.

[0131] In some embodiments, the porosity of the negative electrode active layer is 30% to 40%. This provides sufficient channels for sodium ion transport and electrolyte wetting, which is more conducive to improving the kinetic performance of the thick electrode sheet, thereby further improving the rate performance of the secondary battery.

[0132] It should be noted that the porosity of the negative electrode sheet can be obtained by measuring the thickness and areal density of the negative electrode active layer, calculating the apparent density of the negative electrode active layer (=areal density / thickness), calculating the true density of the mixture of active material, conductive agent, and binder in the negative electrode active layer (weighted by the density and mass fraction of each component), and calculating the ratio of apparent density to true density. For example, the porosity of the negative electrode sheet can be, for example, 30%, 32%, 34%, 36%, 38%, 40%, or a value within any two of the above ranges.

[0133] 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.

[0134] Optionally, in some embodiments, the mass content of the negative electrode active material is 91% to 97% based on the total mass of the negative electrode active layer.

[0135] Optionally, in some embodiments, the negative electrode active layer further includes a negative electrode binder and a negative electrode conductive agent.

[0136] Optionally, in some embodiments, the mass content of the negative electrode binder is 1% to 3% and the mass content of the negative electrode conductive agent is 3% to 6% based on the total mass of the negative electrode active layer.

[0137] Optionally, in some embodiments, the negative electrode binder includes at least one of polyurethane, acrylic-acrylonitrile copolymer, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylic acid (PAA), sodium carboxymethyl cellulose, and styrene-butadiene rubber (SBR).

[0138] Optionally, in some embodiments, the negative electrode conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, acetylene black, and Ketjen black.

[0139] In some embodiments, the cell further includes a separator, the separator comprising a substrate layer and a functional coating disposed on at least one surface of the substrate layer in the thickness direction.

[0140] Furthermore, in some embodiments, the substrate layer comprises polyethylene and / or polypropylene; and / or, the material of the functional coating comprises a nitrogen-containing compound. Thus, the amino groups in the nitrogen-containing compound of the separator functional coating participate in a dehydration condensation reaction to construct a triazine ring-based donor-acceptor (DA) polymer structure. The triazine ring unit has sodium affinity and can strongly interact with sodium ions, simplifying the solvation structure and lowering the desolvation energy barrier, thereby accelerating the migration kinetics of sodium ions in the electrolyte and improving ionic conductivity, which is more conducive to improving the rate performance and cycle stability of the secondary battery.

[0141] 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.

[0142] Optionally, in some embodiments, the thickness of the substrate layer is 10 μm to 18 μm.

[0143] Optionally, in some embodiments, the thickness of the functional coating is 2μm to 4μm.

[0144] Optionally, in some embodiments, the porosity of the diaphragm is 25% to 40%.

[0145] 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, sodium supplement NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes with an aspect ratio of 3400 were mixed in a mass ratio of 92.8:2.2:2:2.5:0.5. The first positive electrode active material consisted of O3 phase layered oxide material, P2 phase layered oxide material, and sodium iron pyrophosphate material in a mass ratio of 1:2:7. N-methylpyrrolidone was added and the mixture was further mixed. The surface of the sodium iron pyrophosphate material was coated with a nano-carbon coating layer to obtain the first positive electrode mixed slurry.

[0146] Sodium iron pyrophosphate pyrophosphate, used as the second positive electrode active material, was mixed with sodium supplement NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes with an aspect ratio of 640 in a mass ratio of 94.8:0.2:2:2.5:0.5. N-methylpyrrolidone was added and mixing continued. The surface of the sodium iron pyrophosphate pyrophosphate material was coated with a nano-carbon coating layer to obtain the second positive electrode mixed slurry.

[0147] The first positive electrode slurry and the second positive electrode slurry are sequentially coated onto both sides of the aluminum foil current collector. After drying and rolling, positive electrode active layers are formed on both sides of the aluminum foil current collector. The positive electrode active layer on one side consists of the first positive electrode active layer and the second positive electrode active layer. The first positive electrode active layer is located between the positive electrode current collector and the second positive electrode active layer. After coating, the coating is dried and rolled to obtain an electrode sheet of qualified thickness. Then, it is laser-divided into positive electrode sheets of fixed size with positive electrode tabs to obtain the positive electrode sheet.

[0148] Figure 1 This is a scanning electron microscope image of sodium iron pyrophosphate, the first phosphate. Figure 1 The image shows only a portion of the sodium iron pyrophosphate (SOP) material. As can be seen from the image, it contains large-particle single-crystal SOP material.

[0149] Figure 2 This is a scanning electron microscope image of sodium iron pyrophosphate (Fe2PO4) material. Figure 2 The image shows only a portion of the sodium iron pyrophosphate pyrophosphate material, which appears to be a small-particle polycrystalline sodium iron pyrophosphate pyrophosphate material.

[0150] (2) Preparation of negative electrode: Hard carbon, a negative electrode active material, a binder (styrene-butadiene rubber and sodium carboxymethyl cellulose in a mass ratio of 0.8:1), and a conductive agent (conductive carbon black) are mixed in a mass ratio of 93:3:4. Deionized water is added as a solvent and the mixture is further mixed to obtain a first negative electrode slurry. This first negative electrode slurry is then coated onto both sides of an aluminum foil. After drying and rolling, the foil is laser-diced to a specific size and shape to obtain a negative electrode sheet containing negative electrode tabs. The hard carbon, the negative electrode active material, is a porous material with a pore structure including mesopores, micropores, and macropores. The micropore volume accounts for 29.62%, the mesopore volume accounts for 25.05%, and the macropore volume accounts for 45.33%. The porosity of the negative electrode sheet is 35%.

[0151] (3) Preparation of electrolyte: Sodium hexafluorophosphate, sodium difluorooxalate borate, ethylene glycol dimethyl ether, diethyl carbonate, propylene carbonate, methyltriphenylphosphine bromide, tri-(2,2,2-trifluoroethyl)-phosphate, vinylene carbonate, and fluoroethylene carbonate were mixed in a mass ratio of 8:4:45:18:18:2:2:1:2 to obtain an electrolyte. Based on the total mass of the electrolyte, the ratio of the mass content ε of sodium difluorooxalate borate to the mass content δ of sodium hexafluorophosphate was 0.5.

[0152] (4) Preparation of the diaphragm An alumina-containing ceramic layer with a thickness of 2 μm is formed on one side of a 9 μm thick polyethylene substrate layer. A melamine-containing functional coating with a thickness of 2 μm is formed on the other side of the polyethylene substrate layer, forming a separator with a porosity of 35%. During 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.

[0153] (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 a square shell 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.

[0154] Figure 3This 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 4 and an electrode assembly 1 disposed inside the housing 4. The positive electrode and negative electrode in the electrode assembly 1 extend outward with positive electrode tabs 2 and negative electrode tabs 3. In the sodium-ion secondary battery, the thickness direction of the electrode assembly 1 is taken as the first direction X, the extension direction of the positive electrode tab 2 is taken as the second direction Y, and the direction perpendicular to the first direction X and the second direction Y is taken as the third direction Z. The dimension α of the housing 4 in the first direction is 73.05 mm, the dimension β in the second direction is 216.3 mm, and the dimension γ in the third direction is 286 mm.

[0155] The preparation methods and parameter settings of the remaining embodiments and comparative examples are basically the same as those of Example 1, with the differences shown in Tables 1-4. In Tables 1-4, " "" indicates: the same as in Example 1 or a value with reasonable deviations due to testing process and equipment. " / " indicates: not present. D1 indicates: particle size Dv50 of the first sodium iron pyrophosphate material; m indicates: the mass content of the carbon coating layer in the first positive electrode active layer, based on the total mass of the first sodium iron pyrophosphate material and the carbon coating layer thereon; D2 indicates: particle size Dv50 of the O3 phase layered oxide material; S1 indicates: specific surface area of ​​the O3 phase layered oxide material; D3 indicates: particle size Dv50 of the P2 phase layered oxide material; S2 indicates: specific surface area of ​​the P2 phase layered oxide material; a indicates: mass content of Cl element in the first positive electrode active layer; A indicates: the mass content of the first positive electrode active layer. The total mass content of Ni and Mn elements in the active layer; p represents the porosity of the first positive electrode active layer; h1 represents the thickness of the first positive electrode active layer; L1 represents the particle size Dv50 of the second sodium iron pyrophosphate material; n represents the mass content of the carbon coating layer in the second positive electrode active layer, based on the total mass of the second sodium iron pyrophosphate material and its carbon coating layer; b represents the mass content of Cl element in the second positive electrode active layer; φ represents the porosity of the second positive electrode active layer; h2 represents the thickness of the second positive electrode active layer; δ represents the mass content of sodium hexafluorophosphate based on the total mass of the electrolyte; ε represents the mass content of sodium difluorooxalate borate based on the total mass of the electrolyte.

[0156] Table 1

[0157] Table 2

[0158] Table 3

[0159] Table 4

[0160] In addition to the differences in Tables 1 to 4 above, some of the above embodiments also differ from Embodiment 1 in the following ways: The difference between Example 8 and Example 1 lies in the following: the first positive electrode active material, sodium supplement NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 94.8:0.2:2:2.5:0.5; the second positive electrode active material, sodium iron pyrophosphate, NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 95:0:2:2.5:0.5. The mass content of Cl ions in the second positive electrode active layer originates from the migration of NaCl added to the first positive electrode active layer during the sodium-ion secondary battery formation process. The rest of the contents are the same as in Example 1.

[0161] The difference between Example 9 and Example 1 lies in the following: the first positive electrode active material, sodium supplement NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 86.1:8.9:2:2.5:0.5; the second phosphate pyrophosphate sodium material, NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 94.48:0.52:2:2.5:0.5. All other contents are the same as in Example 1.

[0162] The difference between Example 40 and Example 1 is that the sodium supplement in the first positive electrode active layer is replaced with an equal mass of NaBr, and the mass content of Br element in the first positive electrode active layer is 1.6%; the sodium supplement in the second positive electrode active layer is replaced with an equal mass of NaBr, and the mass content of Br element in the second positive electrode active layer is 1131 ppm. All other contents are the same as in Example 1.

[0163] The difference between Example 41 and Example 1 is that the sodium supplement in the first positive electrode active layer is replaced with an equal mass of NaI, and the mass content of element I in the first positive electrode active layer is 1.7%; the sodium supplement in the second positive electrode active layer is replaced with an equal mass of NaI, and the mass content of element I in the second positive electrode active layer is 1205 ppm. All other contents are the same as in Example 1.

[0164] The difference between Comparative Example 1 and Example 1 lies in the following: the first positive electrode active material, sodium supplementer - NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 94.9:0.1:2:2.5:0.5; the second phosphate material, sodium iron pyrophosphate, NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 95:0:2:2.5:0.5. The mass content of Cl ions in the second positive electrode active layer originates from the migration of NaCl added to the first positive electrode active layer during the sodium-ion secondary battery formation process. All other aspects are the same as in Example 1.

[0165] The difference between Comparative Example 2 and Example 1 is that the first positive electrode active material, sodium supplement - NaCl, polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are in a mass ratio of 85.5:9.5:2:2.5:0.5. All other contents are the same as in Example 1.

[0166] The difference between Comparative Example 3 and Example 1 is that the mass ratio of sodium iron pyrophosphate (Fe2PO4), NaCl, polyvinylidene fluoride (PVDF), conductive carbon black, and carbon nanotubes is 94.45:0.55:2:2.5:0.5. All other contents are the same as in Example 1.

[0167] The difference between Comparative Example 6 and Example 1 is that the first phosphate, sodium iron pyrophosphate, is a polycrystalline particle. All other aspects are the same as in Example 1.

[0168] The difference between Comparative Example 7 and Example 1 is that the sodium iron pyrophosphate phosphate material in Comparative Example 7 is a single crystal particle. All other aspects are the same as in Example 1.

[0169] 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.

[0170] (2) Test process for initial coulombic efficiency: The sodium-ion secondary batteries provided in the above embodiments and comparative examples were charged at 25±2℃ with a constant current and constant voltage of 0.2C to 3.65V, and then cut off at 0.02C; the charging capacity was recorded as C1; after resting for 10 minutes, they were discharged at a constant current of 0.2C to 1.5V, and the discharge capacity was recorded as D1; ​​the initial coulombic efficiency was calculated as D1 / C1×100%.

[0171] (3) 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 initial discharge specific capacity was recorded as C0. (3) Steps (1) to (2) were repeated until 2000 cycles were reached. The discharge specific capacity after 2000 cycles was recorded as C2. The cycle capacity retention rate was calculated as C2 / C0×100%.

[0172] (4) 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℃.

[0173] (5) 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.

[0174] The test results are shown in Table 5.

[0175] Table 5

[0176] As can be seen from Tables 1-5, this application introduces single-crystal large-particle NFPP material and layered oxide into the first positive electrode active layer, and polycrystalline large-particle NFPP material into the second positive electrode active layer. This collectively achieves a positive electrode sheet with better structural stability, excellent electron transport performance, ion transport performance, electrolyte wettability, and higher capacity performance. This improves the long-cycle stability, rate performance, and energy density of the secondary battery, while reducing internal resistance. Simultaneously, this application introduces a sodium supplement agent into the positive electrode active layer and controls the content of characteristic elements in both the first and second positive electrode active layers. This achieves sodium supplementation while improving the structural stability of the first positive electrode active layer and preventing damage to the CEI film by ions from the sodium supplement agent. This reduces interfacial side reactions during cycling and improves the overall structural stability of the positive electrode active layer, thereby enhancing the cycle stability and initial coulombic efficiency of the secondary battery and reducing internal resistance.

[0177] 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 application.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive active layer disposed on at least one surface of the positive current collector in the thickness direction; characterized in that, The positive electrode active layer includes a first positive electrode active layer and a second positive electrode active layer stacked together, wherein the first positive electrode active layer is disposed between the positive electrode current collector and the second positive electrode active layer; The first positive electrode active layer includes a first positive electrode active material, which includes a first sodium iron pyrophosphate material and a layered oxide material. The second positive electrode active layer includes a second positive electrode active material, which includes a second sodium iron pyrophosphate material. The first sodium iron pyrophosphate material comprises single-crystal particles with a particle size Dv50 denoted as D1 μm; the second sodium iron pyrophosphate material comprises polycrystalline particles with a particle size Dv50 denoted as L1 μm; satisfying: D1>L1; The positive electrode active layer further includes a sodium supplement agent, which includes a characteristic element, including at least one of Cl, Br, and I. The mass content of the characteristic element in the first positive electrode active layer is denoted as a, and the mass content of the characteristic element in the second positive electrode active layer is denoted as b, satisfying: a > b, 0.1% ≤ a ≤ 5%, and 30 ppm ≤ b ≤ 3000 ppm.

2. The positive electrode sheet according to claim 1, characterized in that, The sodium supplement includes at least one of NaCl, NaBr, and NaI; And / or, the layered oxide material includes O3 phase layered oxide material and P2 phase layered oxide material; Preferably, in the first positive electrode active material, the mass ratio of the O3 phase layered oxide material, the P2 phase layered oxide material, and the first sodium iron pyrophosphate material is (0.2~1.5):(0.3~2.5):(6~9.5); Preferably, the specific surface area of ​​the O3 phase layered oxide material is denoted as S1 m. 2 / g, the specific surface area of ​​the P2 phase layered oxide material is denoted as S2 m 2 / g, satisfying: S1<S2.

3. The positive electrode sheet according to claim 2, characterized in that, At least one of the following conditions must be met: (1) The general chemical formula of the O3 phase layered oxide material is: Na x Fe y Ni z Mn r M l O2; wherein M includes at least one of Ti, Cu, Mg, Ca, Cr, Co, Ce, Zn, Pd, Al, and Mo; 0.8≤x<1, 0.20≤y≤0.35, 0.23≤z≤0.35, 0.24≤r≤0.36, 0≤l≤0.2; (2) The particle size Dv50 of the O3 phase layered oxide material is denoted as D2 μm, which satisfies: 5.5≤D2≤7.5; (3) The particle size span of the O3 phase layered oxide material is 0.5~2.2; （4）0.4≤S1≤0.8； (5) The general chemical formula of the P2 phase layered oxide material is: Na w Fe t Ni u Mn v Cu s Q q O2, wherein Q includes at least one of Mg, Ca, Cr, Co, Ce, Zn, Pd, Ti, Al, and Mo; 0.2≤w<0.8, 0≤t≤0.3, 0≤u≤0.3, 0<v≤0.8, 0.2≤s≤0.6, 0≤q≤0.2; (6) The particle size Dv50 of the P2 phase layered oxide material is denoted as D3 μm, which satisfies: 4.6≤D3≤7; (7) The span value of the particle size of the P2 phase layered oxide material is 1.7~2.8; （8）0.62≤S2≤0.95。 4. The positive electrode sheet according to claim 1, characterized in that, The first sodium iron pyrophosphate material satisfies at least one of the following conditions: (A) 6.4≤D1≤9.6; (B) Particle size span value is 1.08~2.83; (C) Specific surface area is 2.1 m² 2 / g~5.5m 2 / g; And / or, the second sodium iron pyrophosphate material satisfies at least one of the following conditions: (a) 3.8 ≤ L1 ≤ 6.2; (b) Particle size span values ​​range from 2.59 to 4.24; (c) Specific surface area is 7.2 m² 2 / g~12.8m 2 / g.

5. The positive electrode sheet according to claim 1, characterized in that, The layered oxide material includes Ni and Mn elements, and the total mass content A of Ni and Mn elements is 1% to 15% based on the total mass of the first positive electrode active layer.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized in that, The thickness ratio of the second positive electrode active layer to the first positive electrode active layer is (0.2~0.9):1; And / or, the total thickness of the positive electrode active layer on one side is 100μm~170μm; And / or, the porosity of the first positive electrode active layer is denoted as p, and the porosity of the second positive electrode active layer is denoted as φ, satisfying: 18%≤p≤30%, 30%≤φ≤50%, 1.1≤φ / p≤2.7; And / or, the surface of the first sodium iron pyrophosphate material is further provided with a first coating layer, the first coating layer covering at least a portion of the surface of the first sodium iron pyrophosphate material, the first coating layer comprising a carbon material; the surface of the second sodium iron pyrophosphate material is further provided with a second coating layer, the second coating layer covering at least a portion of the surface of the second sodium iron pyrophosphate material, the second coating layer comprising a carbon material. Based on the total mass of the first sodium iron pyrophosphate material and the first coating layer, the mass content of the first coating layer is denoted as m; Based on the total mass of the second sodium iron pyrophosphate material and the second coating layer, the mass content of the second coating layer is denoted as n, which satisfies the following conditions: 1.8%≤m≤4%, 1%≤n≤3%, 0.25≤n / m≤1.5; And / or, the first positive electrode active layer further includes a first conductive agent, the first conductive agent including conductive carbon black and a first carbon nanotube, the first carbon nanotube having an aspect ratio of 800~5500; And / or, the second positive electrode active layer further includes a second conductive agent, the second conductive agent including conductive carbon black; preferably, the second conductive agent further includes a second carbon nanotube, the aspect ratio of the second carbon nanotube being 100~900; And / or, the lateral surface density of the positive electrode active layer is 0.0190 g / cm³. 2 ~0.0350g / cm 2 .

7. A sodium-ion secondary battery, comprising a housing and an electrode assembly encapsulated within the housing; characterized in that, The electrode assembly includes the positive electrode sheet according to any one of claims 1 to 6; Preferably, the positive electrode includes a positive tab extending from the positive current collector, and the extending direction of the positive tab is perpendicular to the thickness direction of the positive current collector; The thickness direction of the electrode assembly is the first direction, the extension direction of the positive electrode tab is the second direction, and the direction perpendicular to the first and second directions is the third direction; the size of the sodium-ion secondary battery is 40mm~100mm in the first direction, 100mm~250mm in the second direction, and 100mm~350mm in the third direction.

8. The sodium-ion secondary battery according to claim 7, characterized in that, The sodium-ion secondary battery further includes an electrolyte, which comprises a sodium salt, an organic solvent, and additives, and the electrolyte satisfies at least one of the following conditions: (I) The density of the electrolyte is 1.0 g / cm³ at 25°C. 3 ~1.5g / cm 3 ; (II) At 25°C, the conductivity of the electrolyte is 7 mS / cm to 10.0 mS / cm; (III) The sodium salt includes sodium hexafluorophosphate and sodium difluorooxalate borate. Based on the total mass of the electrolyte, the mass content of sodium hexafluorophosphate is denoted as δ%, and the mass content of sodium difluorooxalate borate is denoted as ε%, satisfying: 0.1≤ε / δ≤1; (IV) The organic solvent includes a first solvent, which includes ethylene glycol dimethyl ether and / or ethylene glycol diethyl ether, and the mass content of the first solvent is 40% to 60% based on the total mass of the electrolyte; Preferably, the organic solvent further includes a second 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. (V) The additive includes at least one of a first additive, a second additive, and a third additive; the first additive includes methyltriphenylphosphine bromide, and the mass content of the first additive in the electrolyte is 1% to 10%; and / or, the second additive includes tri-(2,2,2-trifluoroethyl)-phosphate, and the mass content of the second additive in the electrolyte is 1% to 10%; and / or, the third additive includes at least one of vinylene carbonate and fluoroethylene carbonate, and the mass content of the third additive in the electrolyte is 0.5% to 5%.

9. The sodium-ion secondary battery according to claim 7 or 8, characterized in that, The electrode assembly further includes a negative electrode sheet, which 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 and satisfies at least one of the following conditions: (i) The negative electrode active material includes a porous carbon material, the pore structure of which includes mesopores and micropores, the pore volume of the mesopores accounting for 20% to 30% of the total pore volume, and the pore volume of the micropores accounting for 13% to 37% of the total pore volume; (ii) The porosity of the negative electrode sheet is 30%~40%.

10. The sodium-ion secondary battery according to claim 7, characterized in that, The electrode assembly further includes a diaphragm, the diaphragm comprising a substrate layer and a functional coating disposed on at least one surface of the substrate layer in the thickness direction; The substrate layer comprises polyethylene and / or polypropylene; and / or, the material of the functional coating comprises a nitrogen-containing compound; 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.

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