Core-shell structured positive electrode material, positive electrode, sodium ion battery and electric equipment

Abstract

The invention provides a positive electrode material with a core-shell structure, a positive electrode, a battery and electric equipment, and relates to the technical field of batteries, the positive electrode material with the core-shell structure takes O3-phase layered oxide polycrystalline particles as a core, and takes whisker-shaped ferric sodium pyrophosphate phosphate crystals as a shell; and whisker-shaped ferric sodium pyrophosphate crystals partially positioned on the interface of the inner core and the shell are obliquely embedded into the inner core. The shell material part of the positive electrode material of the core-shell structure provided by the invention is obliquely inserted into the inner core to form an oblique pinning structure, so that the interface bonding strength of the shell and the inner core can be improved, and the expansion inhibition effect of the shell on the inner core is improved.

Description

Technical Field

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

[0002] Sodium is abundant and widely distributed, and compared to lithium-ion batteries, it is less prone to combustion and explosion, offering superior safety. Therefore, sodium-ion batteries have become a current research hotspot. However, layered oxides, one of the main cathode materials currently being researched, exhibit significant volume expansion with the continuous insertion and extraction of sodium ions, leading to rapid deterioration of the cycle performance of sodium-ion batteries and hindering their widespread application. While some expansion suppression can be achieved through coating, the interfacial bonding between conventional coating materials and layered oxides is insufficient, making it difficult to maintain the coating effect for extended periods, especially in high-temperature environments. Summary of the Invention

[0003] The purpose of this application is to provide a core-shell structured cathode material, cathode, battery, and electrical device. The outer shell material of the cathode material is inserted into the core at an angle to form an angled pinning structure, which can improve the interfacial bonding strength between the outer shell and the core and improve the effect of the outer shell on suppressing the expansion of the core.

[0004] One of the purposes of this application is to provide a positive electrode.

[0005] Another objective of this application is to provide a sodium-ion battery.

[0006] One of the purposes of this application is to provide an electrical appliance.

[0007] In a first aspect, in order to solve the above problems, this application provides a core-shell structured cathode material, wherein the core-shell structured cathode material has layered oxide polycrystalline particles of O3 phase as the core and whisker-shaped sodium iron pyrophosphate crystals as the outer shell. Some of the whisker-like sodium iron pyrophosphate crystals located at the interface between the core and the shell are obliquely embedded in the core.

[0008] Furthermore, in some embodiments of this application, the included angle formed when the sodium iron pyrophosphate whiskers are embedded in the core is 10-60°.

[0009] Furthermore, in some embodiments of this application, the included angle formed when the sodium iron pyrophosphate whiskers are embedded in the core is 15-35°.

[0010] Furthermore, in some embodiments of this application, the short diameter of the sodium iron pyrophosphate whiskers is 2nm-20nm.

[0011] Furthermore, in some embodiments of this application, the major diameter of the sodium iron pyrophosphate whiskers is 100-200 nm; The diameter of the layered oxide polycrystalline particles of the O3 phase is 5μm-40μm; the particle size of the crystals forming the layered oxide polycrystalline particles of the O3 phase is 100nm-1μm.

[0012] Furthermore, in some embodiments of this application, the outer shell formed by the whisker-like sodium iron pyrophosphate crystals is a continuous or discontinuous outer shell layer with a velvety structure on the outer surface.

[0013] Furthermore, in some embodiments of this application, the whisker-like sodium iron pyrophosphate crystals are distributed in clusters, and the diameter of each cluster of sodium iron pyrophosphate crystals is 500 nm-1 μm.

[0014] Furthermore, in some embodiments of this application, the sodium iron pyrophosphate phosphate accounts for 1%-10% of the core-shell structured cathode material; Fe-OP bonds are formed at the interface between the outer shell and the core.

[0015] Furthermore, in some embodiments of this application, the layered oxide of the O3 phase is a layered oxide containing nickel, and its chemical structural formula is: Na x Ni y Fe z Mn a M (1-y-z-a) O2; where x ranges from 0.9 to 1.0; y ranges from 0.1 to 0.5; z ranges from 0.1 to 0.5; and a ranges from 0.1 to 0.5. The core-shell structured cathode material is doped with tungsten, and the tungsten forms Ni-OWP bonds at the interface between the core layer and the outer shell layer.

[0016] Furthermore, in some embodiments of this application, the tungsten element is doped in the core-shell structured cathode material at an amount of 0.5-2.0 wt%; and / or The tungsten element is doped into the core-shell structured cathode material as hexavalent tungsten ions.

[0017] Secondly, this application also provides a positive electrode for a sodium-ion battery, comprising the core-shell structured positive electrode material described in the first aspect.

[0018] Thirdly, this application also provides a sodium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator; the separator and the electrolyte are disposed between the positive electrode and the negative electrode; The positive electrode is the positive electrode described in the second aspect.

[0019] Furthermore, in some embodiments of this application, the volume expansion rate after 1000 cycles at 60°C is no higher than 5%.

[0020] Fourthly, this application also provides a battery module, including the sodium-ion battery described in the third aspect, the positive electrode described in the second aspect, or the core-shell structured positive electrode material described in the first aspect.

[0021] This application provides a core-shell structured cathode material. This cathode material uses a polycrystalline layered oxide core and whisker-like NFPP crystals as the outer shell. The whisker structure of the NFPP is obliquely inserted between the grains of the layered oxide, forming a pinning structure. This improves the bonding strength between the shell and the core, enhances its ability to suppress volume changes during sodium ion insertion and extraction, reduces the volume expansion rate of the cathode material, and also improves the mechanical strength of the cathode material. Furthermore, the whisker-like NFPP crystals form a three-dimensional network structure on the core surface and in the embedded portion, which is beneficial to improving the ionic conductivity of the cathode material. Sodium-ion batteries based on the core-shell structured cathode material provided in this application can maintain a volume expansion rate of less than 5% after 1000 cycles at 60°C, which is far lower than that of sodium-ion batteries formed by layered oxides and layered oxides coated with other materials. Attached Figure Description

[0022] 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.

[0023] Figure 1 This is a scanning electron microscope image of the cathode material provided in Embodiment 1 of this application; Figure 2 This is an electron microscope image of the interface of the cathode material provided in Embodiment 1 of this application; Figure 3 The image shows a scanning electron microscope (SEM) image of the cathode material provided in Comparative Example 1 of this application. Detailed Implementation

[0024] The technical solutions of this application will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] In the description of this application, it should be understood that "multiple" means two or more, unless otherwise expressly and specifically limited.

[0026] The following disclosure provides many different embodiments or examples for implementing different structures of this application. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the scope of this application. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, various specific examples of processes and materials are provided in this application, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0027] One of the main factors contributing to the poor cycle performance of layered oxides with the O3 phase in sodium-ion cathode materials is the significant volume expansion that occurs during sodium-ion insertion and extraction. Existing technologies have proposed coating the surface of the O3-phase layered oxide with a coating layer to suppress this volume expansion and improve cycle performance. However, since the volume expansion of O3-phase layered oxides used in sodium-ion batteries is more pronounced than that used in lithium-ion batteries, conventional coatings offer limited improvement in suppressing volume expansion and are prone to damage, making it difficult to achieve long-term suppression, especially in high-temperature or high-rate charge-discharge environments. Furthermore, the framework structure of the O3-phase layered oxide is also prone to collapse due to lattice distortion, affecting its cycle performance and safety. Based on this, this application provides a core-shell structured cathode material. This cathode material uses whisker-shaped NFPP crystals, which can be inserted into the polycrystalline particles of layered oxides of the O3 phase, as a coating layer, and forms Fe-OP bonds between the NFPP and the layered oxides of the O3 phase. In addition, the applicant has discovered that when the whiskers of NFPP are tilted and inserted into the gaps between the crystals of the layered oxides, they can form a certain rake-like structure on the core of NFPP, which can further improve the suppression of volume expansion and lattice distortion of the layered oxides by the NFPP shell, significantly reduce the volume expansion rate of the cathode material, and improve its cycle performance. Furthermore, its ionic conductivity can be further optimized.

[0028] Specifically, the core-shell structured cathode material provided in this application has a core of layered oxide polycrystalline particles of O3 phase and a shell of whisker-shaped sodium iron pyrophosphate crystals. Some of the whisker-like sodium iron pyrophosphate crystals located at the interface between the core and the shell are obliquely embedded in the core.

[0029] It should be noted that the polycrystalline particles in this application are secondary particles formed by layered oxides; that is, the core of the positive electrode material provided in this application is a polycrystalline material of layered oxide with an O3 phase, and there are crystal interfaces between each crystal.

[0030] It should be noted that the whisker-like sodium iron pyrophosphate (hereinafter referred to as NFPP) crystals in this application refer to whisker-like primary or secondary particles with an aspect ratio of not less than 20 formed by the growth of NFPP crystals. NFPP whiskers are stacked in layers or networks on the core surface, exhibiting a nanofiber-like structure when viewed from the particle surface of the cathode material.

[0031] The applicant discovered that the core-shell structured cathode material provided in this application not only exhibits excellent cycle performance but also shows significant optimization in ion conduction rate. This may be because the tilted, whisker-like NFPP crystals not only improve the bonding strength between the outer shell and the core, enhancing the cohesiveness of the cathode material, but the villous structure on the outer surface also significantly increases the specific surface area of ​​the core-shell structured cathode material, expanding the contact area between the cathode material particles and the electrolyte, thus improving ion conduction speed. Furthermore, due to the villous structure formed by the whisker NFPP, several channels for sodium ions to pass through are easily formed on the outer shell, reducing the influence of the coating layer on the resistance of the cathode material. This results in the core-shell structured cathode material provided in this application exhibiting good mechanical strength, lower volume expansion rate, and better high-rate capacity retention. In addition, the high elastic modulus of NFPP whiskers, reaching 180 GPa, allows them to form a mechanical gradient with the O3 phase layered oxide, which may also be one of the reasons for inhibiting crack propagation.

[0032] Furthermore, since the outer shell layer provided in this application is formed by the growth and stacking of NFPP whiskers, it has a high aspect ratio. Under the same amount of coating layer, the outer shell formed by NFPP whiskers tends to form a mesh or layered coating layer, and its island coating accounts for a smaller proportion, which is more conducive to improving the suppression of volume expansion of the cathode material and more conducive to suppressing crack propagation.

[0033] It should be noted that, in this application, the interface between the core layer and the outer shell layer refers to the surface at or near the contact between the polycrystalline particles of the O3 phase layered oxide and the NFPP crystal. When the NFPP crystal is partially embedded between the crystal interfaces of the polycrystalline particles of the O3 phase layered oxide, the interface between the core layer and the outer shell layer in this application also includes the surface at or near the contact between the embedded NFPP crystal and the crystal of the O3 phase layered oxide.

[0034] Furthermore, the O3 phase layered oxide provided in this application is a layered oxide containing nickel and iron elements, and its specific structural formula is Na. x Ni y Fez Mn a M (1-y-z-a) A layered oxide of O2 in the O3 phase, wherein x ranges from 0.9 to 1.0; y ranges from 0.1 to 0.5; z ranges from 0.1 to 0.5; a ranges from 0.1 to 0.5; and the metallic element is selected from at least one of aluminum, titanium, vanadium, copper, zinc, tungsten, lanthanum, cerium, zirconium, and calcium.

[0035] Preferably, the included angle formed when the NFPP whiskers are embedded in the core is 10-60°. It should be noted that "the included angle formed when the NFPP whiskers are embedded in the core is 10-60°" means that the angle formed between the extension direction of the NFPP whiskers and the tangent to the surface of the core mainly falls within the range of 10-60°. Specifically, the amount of NFPP whiskers embedded in the core at an angle of 10-60° is at least 50% of the total amount of NFPP whiskers embedded in the core. Preferably, the included angle formed when the sodium iron pyrophosphate whiskers are embedded in the core is 15-35°.

[0036] In some embodiments, the minor diameter of the NFPP whiskers is 2-8 nm; the major diameter of the sodium iron pyrophosphate whiskers is 100-500 nm. The thickness of the shell formed by the NFPP whiskers is 100 nm-500 nm. It should be noted that the thickness of the shell formed by the NFPP whiskers here refers to the average of the maximum and minimum thicknesses of the shell layer formed by the NFPP whiskers.

[0037] In some embodiments, the diameter of the layered oxide polycrystalline particles of the O3 phase is 5 μm-40 μm; the particle size of the crystals forming the layered oxide polycrystalline particles of the O3 phase is 100 nm-1 μm.

[0038] The core-shell structured cathode material has a particle size of 5μm-40μm.

[0039] In some embodiments, the mass percentage of the NFPP whiskers in the cathode material is 3%-5%. The mass percentage of the NFPP whiskers in the cathode material should not be too low. If the mass percentage of the NFPP whiskers is too low, the NFPP whiskers will form on the surface of the core, which will easily lead to insufficient coating and will not be conducive to improving the ion conduction rate of the cathode material.

[0040] In other embodiments, the core-shell structure cathode material provided in this application is also doped with hexavalent tungsten, with a doping amount of 0.5-2.0 wt%. At least a portion of the doped hexavalent tungsten exists at the interface between the core and the shell, forming Ni-OWP bonds.

[0041] In this application, the doped tungsten element is present at the interface between the core and the shell layer, and may also be present in the core and / or the existing shell. Preferably, the doped tungsten element is enriched at the interface between the core and the shell, forming a large number of [Ni-OWP] charge bridges at the interface, which can better reduce the interface resistance. In other embodiments, the elemental concentration of the doped tungsten element decreases sequentially from the interface between the core and the shell towards the core and / or shell.

[0042] The tungsten doping amount in the cathode material is 0.5-2.0 wt%. For example, the tungsten doping amount in the core-shell structured cathode material can be any value within the range of 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, or 2.0 wt%. Preferably, the tungsten doping amount in the core-shell structured cathode material is 1%-1.5%. The tungsten doping amount should not be too high or too low. Excessive tungsten doping will adversely affect the charge-discharge capacity of the core-shell structured cathode material; while excessively low tungsten doping will not significantly reduce the interface resistance.

[0043] It should be noted that the tungsten doping in the core-shell structure cathode material provided in this application can not only reduce the interfacial resistance of the cathode material, but also further improve the cycle stability of the cathode material, especially the high-potential cycle stability.

[0044] To enable those skilled in the art to better implement this solution, this application also provides a method for preparing this tungsten-doped core-shell structured cathode material, including the following steps: (1) Preparation of layered oxide precursors Sodium, iron, nickel, and manganese sources, as well as dopants (added if present, not required if absent), are provided in a molar ratio of 0.9-1 : 0.2-0.4 : 0.2-0.4 : 0.2-0.4. The core raw materials are mixed with the first solution and dispersion, and the Zeta potential is adjusted to above +35mV by adding the dispersion. The mixture is then ground and spray-dried to obtain layered oxide precursor particles. (2) Preparation of sodium iron pyrophosphate precursor The outer shell material is provided in a molar ratio of sodium source, iron source and phosphorus source of 3.5-4:2.5-3:3.8-4.2. The outer shell material and the second solution are stirred in a water bath at 40-90℃ for 4-24 hours to obtain a transparent sol-like sodium iron pyrophosphate precursor. (3) The sodium iron pyrophosphate precursor and the layered oxide precursor particles are mixed at a mass ratio of 1:10~50 and ultrasonically dispersed for 30~180 min. The mixture is then concentrated at 80-85℃ until the sodium iron pyrophosphate precursor increases in weight by 5-8%, so that the sodium iron pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain the composite material. (4) Sintering The composite material was vacuum dried for 4-24 hours, heated to 400-450℃ in an environment of 91.5-95% N2 + 5-8.5% O2, and sintered at that temperature for 4-12 hours. The sintering atmosphere was then adjusted to have a carbon dioxide content of 3.2%-5.5% and a water vapor content of 1.0%-2.2wt%, and sintered at 600-900℃ for 4-12 hours, followed by annealing at 200-300℃ for 2-8 hours to obtain the core-shell structure cathode material of this application.

[0045] In step (1), the sodium source can be at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate. The iron source in step (1) can be at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The nickel source in step (1) can be at least one of nickel hydroxide, nickel nitrate, and nickel sulfate. The manganese source in step (1) can be at least one of manganese oxide, manganese trioxide, manganese hydroxide, and manganese sulfate. The dopant in step (1) can be at least one of copper, vanadium, titanium, and aluminum. The first solution can be at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol. The dispersion can be at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol.

[0046] In step (2), the sodium source can be at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate. The iron source in step (2) can be at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The second solution can be at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol.

[0047] Preferably, the sodium source in step (1) can be the same as that in step (2) to reduce the introduction of more impurities.

[0048] Secondly, this application also provides a positive electrode, comprising the positive electrode material described in the first aspect.

[0049] Thirdly, this application also provides a sodium-ion battery, including a positive electrode, a negative electrode, an electrolyte, and a separator; the separator and the electrolyte are disposed between the positive electrode and the negative electrode; the positive electrode is the positive electrode described in the second aspect.

[0050] The electrolyte can be a liquid electrolyte, a solid electrolyte, or other electrolytes. The separator can be a polymer separator or a polymer separator with a ceramic coating. The negative electrode can include a current collector and a coating applied to the surface of the current collector. The current collector may include a metal foil, such as aluminum foil or copper foil. The coating may include a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent. The negative electrode active material may be a carbon material or other material capable of sodium ion insertion / extraction. Similarly, the positive electrode may also include a current collector and a coating applied to the surface of the current collector. The coating may include the aforementioned positive electrode material and a binder.

[0051] Fourthly, this application also provides a battery device, including the sodium-ion battery described in the third aspect or including the positive electrode described in the second aspect or the positive electrode material described in the first aspect.

[0052] The electrical device provided in this application includes a battery module composed of multiple batteries, used to provide power to the device. Exemplary examples of the electrical device include mobile communication equipment, computers, electric vehicles, electric trains, energy storage systems, etc.

[0053] To facilitate a better understanding of the innovative aspects of this application by those skilled in the art, the technical solutions of this application are further described in detail below with reference to embodiments. The embodiments of this application described in detail below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0054] Example 1 This embodiment provides a cathode material, and the preparation method of the cathode material is as follows: (1) Preparation of layered oxide precursors Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, and manganese trioxide were provided in a molar ratio of Na:Ni:Fe:Mn = 1:0.33:0.33:0.34. The sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and ethanol solution were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The milling speed was 400 rpm, and the milling process was carried out for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical precursors for the preparation of layered oxides with an average particle size of D50: 10μm.

[0055] (2) Preparation of sodium iron pyrophosphate precursor Ferrous oxalate dihydrate and sodium dihydrogen phosphate were dissolved in 0.1 mol / L citric acid solution at a molar ratio of 3:4 and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thus obtaining a nano-micelle-like precursor of sodium iron pyrophosphate. (3) The layered oxide precursor described in the mass ratio of 1:10 is added to the transparent sol, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 50°C until the sol completely covers the particle surface (the weight gain is controlled within the range of 5-8%); the sodium iron pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain the composite material. (4) Sintering The composite material was vacuum dried at 100℃ for 12 hours; then heated to 400℃ and sintered for the first time in an atmosphere of 95% N2 + 5% O2 for 4 hours, followed by sintering at 600℃ for 6 hours while simultaneously introducing N2:CO2:H2O steam in a ratio of 96:3.5:0.5. Finally, it was annealed at 300℃ for 2 hours to obtain the cathode material NaNi in this application. 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of its sodium iron pyrophosphate whiskers is approximately 30°. (See reference...) Figure 1 , Figure 2 As shown.

[0056] Example 2 Compared to Example 1, in step 4 of this embodiment, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for the first time in a 93.5% N2 + 6.5% O2 atmosphere for 4 hours, followed by sintering at 600°C for 6 hours while simultaneously introducing N2:CO2:H2O vapor in a ratio of 96:3:1; finally, annealing at 300°C for 2 hours. The remaining steps are the same as in Example 1, resulting in the positive electrode material of this application. NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7) has a main insertion angle of approximately 35° for its sodium iron pyrophosphate whiskers.

[0057] Example 3 Compared to Example 1, in step 4 of this embodiment, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for the first time in a 95% N2 + 5% O2 atmosphere for 4 hours, followed by sintering at 600°C for 6 hours while simultaneously introducing N2:CO2:H2O vapor in a ratio of 96:3.8:0.2; finally, annealing is performed at 300°C for 2 hours. The remaining steps are the same as in Example 1, yielding the cathode material NaNi used in this application. 0.33Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7) has a main insertion angle of about 40° for its sodium iron pyrophosphate whiskers.

[0058] Example 4 Compared to Example 1, the molar ratio of sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, and manganese trioxide used in step (1) of this embodiment is 1:0.4:0.3:0.3. The remaining steps are the same as in Example 1, resulting in the positive electrode material of this application: NaNi 0.4 Fe 0.3 Mn 0.3 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of its sodium iron pyrophosphate whiskers is 30°.

[0059] Example 5 Compared to Example 1, the raw materials used in step (1) of this embodiment are: sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and tungsten trioxide, with a molar ratio of 1:0.3:0.3:0.3:0.1. The remaining steps are the same as in Example 1, resulting in the positive electrode material NaNi in this application. 0.3 Fe 0.3 Mn 0.3 W 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of its sodium iron pyrophosphate whiskers is about 30°.

[0060] Example 6 This embodiment differs from Embodiment 1 in that the amount of tungsten doping is changed. Specifically, in step (1), sodium dihydrogen phosphate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and tungsten trioxide are provided in a molar ratio of Na:Ni:Fe:Mn:W = 1:0.3:0.3:0.35:0.05. The remaining steps are the same as in Embodiment 1, resulting in a core-shell structured cathode material. NaNi 0.3 Fe 0.3 Mn 0.35 W 0.05 O2@1 / 11Na4Fe3(PO4)2(P2O7) has a main insertion angle of approximately 30° for its sodium iron pyrophosphate whiskers.

[0061] Example 7 Compared to Example 1, in this embodiment, the mixing mass ratio of the layered oxide precursor particles and the sodium iron pyrophosphate precursor is adjusted to 20:1. The remaining steps are the same as in Example 1, resulting in the cathode composite material: NaNi.0.33 Fe 0.33 Mn 0.34 O2@1 / 21Na4Fe3(PO4)2(P2O7), the main insertion angle of its sodium iron pyrophosphate whiskers is about 30°.

[0062] Example 8 Compared to Example 1, this embodiment provides 1 mole of sodium source, 0.3 mole of iron source, 0.3 mole of nickel source, 0.3 mole of manganese source, and 0.1 mole of Cu source in the step of preparing the layered oxide precursor. The remaining steps are the same as in Example 1, yielding the cathode material: NaNi. 0.3 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of its sodium iron pyrophosphate whiskers is 30°.

[0063] Comparative Example 1 Compared to Example 1, in step 4 of this comparative example, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for the first time in a 95% N2 + 5% O2 atmosphere for 4 hours, followed by sintering at 600°C for 6 hours while simultaneously introducing 98% N2: 2% CO2; finally, it is annealed at 300°C for 2 hours. The remaining steps are the same as in Example 1, yielding a core-shell structured cathode material: NaNi. 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7) has a relatively small main insertion angle of sodium iron pyrophosphate whiskers, mostly around 10°.

[0064] Comparative Example 2 Compared to Example 1, in step 4 of this comparative example, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C and sintered for the first time in a 95% N2 + 5% O2 atmosphere for 4 hours, then heated to 600°C and sintered for 6 hours while simultaneously introducing 100% N2, and finally annealed at 300°C for 2 hours. The remaining steps are the same as in Example 1, resulting in the positive electrode material of this application. NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7) has a relatively large main insertion angle of sodium iron pyrophosphate whiskers, mostly around 70°.

[0065] Comparative Example 3 Compared to Example 1, in step 4 of this comparative example, the atmosphere of the first calcination condition is nitrogen, and the atmosphere of the second calcination is the same as in Example 1, resulting in the following positive electrode material: NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), its morphology is as follows Figure 3 As shown, no whisker-like sodium ferric pyrophosphate coating layer was formed.

[0066] Comparative Example 4 Compared with Example 1, in step 2 of this comparative example, the preparation of the sodium iron phosphate precursor involves dissolving iron phosphate and sodium dihydrogen phosphate in a 1:1 molar ratio in a 0.1 mol / L citric acid solution and stirring in a water bath at 80°C for 4 hours to obtain a nano-micelle sodium iron phosphate precursor; (3) adding the sodium iron phosphate precursor to a transparent sol at a mass ratio of 1:10, ultrasonically dispersing for 30 minutes, and concentrating under reduced pressure at 50°C until the sol completely coats the particle surface (weight gain within the range of 5-8%); so that the sodium iron phosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material; the remaining steps are the same as in Example 1 to obtain a core-shell structured cathode material: NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na Ferric Phosphate.

[0067] Comparative Example 5 Compared to Example 1, in steps 3 and 4 of this comparative example, the NFPP precursor and the O3 phase precursor are calcined separately to form NFPP and O3 cathode materials, which are then mixed in a 1:10 ratio to form a homogenized slurry. The remaining steps are the same as in Example 1 to obtain a core-shell structured cathode material: NaNi. 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0068] Comparative Example 6 This comparative example, unlike the layered oxide of Example 5, does not contain Ni. Specifically, in step (1), sodium carbonate, ferrous oxalate dihydrate, manganese trioxide, and sodium tungstate are provided in a molar ratio of Na:Fe:Mn:W = 1:0.5:0.4:0.1. The remaining steps are the same as in Example 1, resulting in a core-shell structured cathode material. NaFe 0.5 Mn 0.4 W 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0069] To verify the superior performance of the core-shell structured cathode material provided in this application, the applicant used the core-shell structured cathode materials obtained in Examples 1 to 8 and the cathode materials in Comparative Examples 1 to 6 as cathode active materials to prepare corresponding sodium-ion batteries. The specific preparation methods are as follows: (1) Preparation of the positive electrode: The positive electrode material, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes were mixed and stirred in a mass ratio of 95:1:3:1. N-methylpyrrolidone solvent was added to adjust the slurry to a solid content of 60% and a viscosity of 6000 mPa·s. The positive electrode slurry was transferred and coated onto a 15 μm thick carbon-coated aluminum foil using a transfer coating method. The coated electrode was then rolled to a thickness of 3 mg / cm. 3 The compaction density is determined; the rolled electrode sheets are then die-cut into electrode sheets with a length of 48mm and a width of 38mm for later use.

[0070] (2) Preparation of negative electrode: Hard carbon material, conductive carbon black, carboxymethyl cellulose and styrene-butadiene rubber are mixed and stirred in a mass ratio of 92:3:2:3, deionized water is added, and the slurry is adjusted to a negative electrode slurry with a solid content of 45% and a viscosity of 5000 mPa·s. The negative electrode slurry is transferred and coated onto a 15 μm thick carbon-coated aluminum foil using a transfer coating method. The coated electrode is rolled to a compaction density of 1 mg / cm2. The rolled electrode is die-cut into electrode sheets with a length of 50 mm and a width of 40 mm for later use.

[0071] (3) Fabrication of sodium-ion batteries: The slit positive and negative electrode sheets are stacked on a stacking machine. The separator is made of PP / PE / PP three-layer material to form a soft-pack cell. Electrolyte (components: carbonate solvent and 1M sodium hexafluorophosphate) is injected, packaged, dried, and subjected to capacity testing to obtain a sodium-ion battery.

[0072] The sodium-ion batteries prepared above were subjected to morphology characterization tests, cycle performance tests, charge-discharge capacity tests, and rate performance tests; the specific test methods are as follows. (1) Cohesion test (2) The cohesive strength of the sample was tested according to the test method described in GB / T 19587-2004. Specific surface area test. The test was conducted according to GB / T 19587-2004, "Test Method for Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0073] (3) Cyclic performance test The room temperature cycling performance and the 60°C high temperature cycling performance were tested according to the method described in GB / T 33822-2017.

[0074] (4) Ionic conductivity test The ionic conductivity of the sample was tested according to the test method described in GB / T 31486-2015.

[0075] (5) Interface impedance test The interfacial impedance of the sample was tested according to the test method described in GB / T 39482.3-2020.

[0076] (6) Volume expansion rate test Cell volume test: The weight m0 of the water displaced by the cell is measured using a balance. The volume V0 of the displaced water is calculated using the formula: V0=m0 / ρ0, which is the volume of the battery. The same method can be used to test the volume V1 of a battery cell that has undergone 1000 cycles; The volume expansion rate can be calculated using the formula (V1-V0) / V0.

[0077] The test results are shown in Table 1.

[0078] from Figures 1-2 It can be seen that the morphology of the coating layer on the surface of the core-shell structured cathode material provided in this application is a whisker-like structure, and some of its whiskers are inclined to insert into the core, and a velvety structure is formed on its surface.

[0079] Table 1

[0080] As shown in Table 1, the cathode materials provided in Examples 1 to 8 of this application employ sodium iron pyrophosphate phosphate with a specific whisker structure to form a shell on the surface of layered oxides, and the NFPP whiskers are obliquely inserted into the core of the layered oxides. Compared with Comparative Examples 1 to 6, it can be seen that the cathode materials obtained in this application can maintain the cycle capacity retention rate and have lower interfacial impedance and lower volume expansion. In addition, as shown in Examples 1 and 7, the NFPP coating amount in the cathode material is preferably 8-15 wt%. Within this range, it is more beneficial to improve the ionic conductivity of the cathode material. This may be because when the NFPP content is within this range, an ion / electron bicontinuous network can be formed in the cathode material, which is conducive to ion migration.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A core-shell structured cathode material, characterized in that, The core-shell structured cathode material has a core of layered oxide polycrystalline particles of O3 phase and a shell of whisker-shaped sodium iron pyrophosphate crystals. Some of the whisker-like sodium iron pyrophosphate crystals located at the interface between the core and the shell are obliquely embedded in the core.

2. The core-shell structured cathode material according to claim 1, characterized in that, The included angle formed when the whisker-like sodium ferric pyrophosphate is embedded in the core is 10-60°.

3. The core-shell structured cathode material according to claim 2, characterized in that, The included angle formed when the whisker-like sodium iron pyrophosphate is embedded in the core is 15-35°.

4. The core-shell structured cathode material according to any one of claims 1 to 3, characterized in that, The whisker-shaped sodium ferric pyrophosphate has a short diameter of 2 nm to 20 nm.

5. The core-shell structured cathode material according to claim 4, characterized in that, The whisker-shaped sodium iron pyrophosphate has a major diameter of 100-200 nm. The diameter of the layered oxide polycrystalline particles of the O3 phase is 5μm-40μm; the particle size of the crystals forming the layered oxide polycrystalline particles of the O3 phase is 100nm-1μm.

6. The core-shell structured cathode material according to claim 1, characterized in that, The outer shell formed by the whisker-like sodium iron pyrophosphate crystals can be a continuous or discontinuous outer shell layer with a velvety structure on the outer surface.

7. The core-shell structured cathode material according to claim 1, characterized in that, The whisker-like sodium iron pyrophosphate crystals are distributed in clusters, with each cluster of sodium iron pyrophosphate crystals having a diameter of 500 nm to 1 μm.

8. The core-shell structured cathode material according to claim 1, characterized in that, The sodium iron pyrophosphate phosphate accounts for 1%-10% of the core-shell structured cathode material; Fe-OP bonds are formed at the interface between the outer shell and the core.

9. The core-shell structured cathode material according to any one of claims 1-3 and 5-8, characterized in that, The layered oxide of the O3 phase is a layered oxide containing nickel, and its chemical structural formula is: Na x Ni y Fe z Mn a M (1-y-z-a) O2; where x ranges from 0.9 to 1.0; y ranges from 0.1 to 0.5; z ranges from 0.1 to 0.5; and a ranges from 0.1 to 0.

5. The core-shell structured cathode material is doped with tungsten, and the tungsten forms Ni-OWP bonds at the interface between the core layer and the outer shell layer.

10. The core-shell structured cathode material according to claim 9, characterized in that, The tungsten element is doped in the core-shell structured cathode material at a concentration of 0.5-2.0 wt%; and / or The tungsten element is doped into the core-shell structured cathode material as hexavalent tungsten ions.

11. A positive electrode for a sodium-ion battery, characterized in that, The cathode material with a core-shell structure as described in any one of claims 1 to 10.

12. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, an electrolyte, and a separator; the separator and the electrolyte are disposed between the positive electrode and the negative electrode; The positive electrode is the positive electrode as described in claim 11.

13. The sodium-ion battery according to claim 12, characterized in that, The volume expansion rate after 1000 cycles at 60℃ is no higher than 5%.

14. A battery module, characterized in that, Includes the sodium-ion battery as described in claim 12 or 13, or the positive electrode as described in claim 11, or the core-shell structure positive electrode material as described in any one of claims 1 to 10.

Images