A kind of core-shell structure positive electrode material, positive electrode, sodium ion battery and electric equipment

Abstract

The application provides a core-shell structure positive electrode material, a positive electrode, a battery and an electric device, and relates to the technical field of batteries.The core-shell structure positive electrode material has a layered oxide polycrystal particle in O3 phase as an inner core and a whisker-shaped sodium iron pyrophosphate phosphate crystal as an outer shell; and the whisker-shaped sodium iron pyrophosphate phosphate crystals located on the interface between the inner core and the outer shell are inclinedly embedded in the inner core.The outer shell material of the core-shell structure positive electrode material provided by the application is partially inclinedly inserted into the inner core to form an inclined pinning structure, which can improve the interface bonding strength of the outer shell and the inner core and improve the expansion inhibition effect of the outer shell on the inner core.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a core-shell structure positive electrode material, a positive electrode, a sodium ion battery and an electric device. BACKGROUND

[0002] Sodium is abundant in reserves and widely distributed, and compared with lithium ion batteries, it is not prone to combustion and explosion, and has better safety performance. Therefore, sodium ion batteries have become a research hotspot at present. However, one of the positive electrode materials as the main research hotspot in the prior art, layered oxide, although has a large volume expansion rate with the continuous intercalation and deintercalation of sodium ions, which leads to rapid deterioration of the cycle performance of sodium ion batteries, and is difficult to be widely applied. Although a certain expansion inhibition can be achieved by coating, the interface bonding force between the conventional coating material and the layered oxide is insufficient, and it is difficult to maintain its long-term coating effect, especially in a high temperature environment. SUMMARY

[0003] The present application aims to provide a core-shell structure positive electrode material, a positive electrode, a battery and an electric device, wherein the shell material of the core-shell structure positive electrode material is partially inserted into the inner core in an inclined manner, forming an inclined pinning structure, which can improve the interface bonding strength between the shell and the inner core, and improve the expansion inhibition effect of the shell on the inner core.

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

[0005] Another purpose of the present application is also to provide a sodium ion battery.

[0006] One of the purposes of the present application is also to provide an electric device.

[0007] In a first aspect, the present application provides a core-shell structure positive electrode material to solve the above problems, wherein the core-shell structure positive electrode material takes a layered oxide polycrystalline particle in O3 phase as an inner core, and takes a whisker-shaped sodium iron pyrophosphate phosphate crystal as a shell.

[0008] Part of the whisker-shaped sodium iron pyrophosphate phosphate crystal located on the interface between the inner core and the shell is inserted into the inner core in an inclined manner.

[0009] Further, in some embodiments of the present application, the included angle formed when the sodium iron pyrophosphate phosphate whisker is inserted into the inner core is 10-60°.

[0010] Further, in some embodiments of the present application, the included angle formed when the sodium iron pyrophosphate phosphate whisker is inserted into the inner core is 15-35°.

[0011] Further, in some embodiments of the present application, the short diameter of the sodium iron pyrophosphate phosphate whisker is 2nm-20nm.

[0012] Further, in some embodiments of the present application, the length-diameter ratio of the sodium iron pyrophosphate whisker is 100-200 nm.

[0013] The diameter of the O3 phase layered oxide polycrystal particle is 5-40 μm; and the particle size of the crystal of the O3 phase layered oxide polycrystal particle is 100 nm-1 μm.

[0014] Further, in some embodiments of the present application, the shell formed by the whisker-shaped sodium iron pyrophosphate crystal is a continuous or discontinuous shell layer with a villiform structure on the outer surface.

[0015] Further, in some embodiments of the present application, the whisker-shaped sodium iron pyrophosphate crystal is distributed in clusters, and the diameter of each cluster of sodium iron pyrophosphate crystal is 500 nm-1 μm.

[0016] Further, in some embodiments of the present application, the proportion of the sodium iron pyrophosphate in the core-shell structure positive electrode material is 1%-10%.

[0017] The interface between the shell and the core forms a Fe-O-P bond.

[0018] Further, in some embodiments of the present application, the O3 phase layered oxide is a layered oxide containing nickel element, and the chemical structure formula is: x Ni y Fe z Mn a M (1-y-z-a) O2; wherein, the value range of x is 0.9-1.0; the value range of y is 0.1-0.5; the value range of z is 0.1-0.5; and the value range of a is 0.1-0.5.

[0019] The core-shell structure positive electrode material is doped with tungsten element, and the tungsten element forms a Ni-O-W-P bond on the interface between the core layer and the shell layer.

[0020] Further, in some embodiments of the present application, the doping amount of the tungsten element in the core-shell structure positive electrode material is 0.5-2.0 wt%; and / or

[0021] The tungsten element is doped into the core-shell structure positive electrode material in the form of hexavalent tungsten ion.

[0022] In a second aspect, the present application further provides a positive electrode of a sodium ion battery, comprising the core-shell structure positive electrode material of the first aspect.

[0023] In a third aspect, the application further provides a sodium ion battery, comprising a positive electrode, a negative electrode, an electrolyte and a separator; the separator and the electrolyte are arranged between the positive electrode and the negative electrode.

[0024] The positive electrode is the positive electrode of the second aspect.

[0025] Further, in some embodiments of the application, the volume expansion rate is not higher than 5% after 1000 cycles at 60℃.

[0026] In a fourth aspect, the application further provides a battery module, comprising the sodium ion battery of the third aspect or the positive electrode of the second aspect or the positive electrode material of the core-shell structure of the first aspect.

[0027] The application provides a positive electrode material of a core-shell structure, which uses a polycrystalline layered oxide as an inner core and a whisker-shaped NFPP crystal as an outer shell material. The whisker structure of the NFPP is inserted between the crystal grains of the layered oxide at an angle to form a pinning structure, thereby improving the bonding strength between the outer shell and the inner core, improving the inhibition of the volume change of the inner core during sodium ion insertion and extraction, reducing the volume expansion rate of the positive electrode material, and also improving the mechanical strength of the positive electrode material. Moreover, the whisker-shaped NFPP crystal forms a three-dimensional network structure on the surface of the inner core and the part inserted into the inner core, which is conducive to improving the ionic conductivity of the positive electrode material. The sodium ion battery based on the positive electrode material of the core-shell structure provided by the application can maintain a volume expansion rate of less than 5% after 1000 cycles at 60℃, which is much lower than the sodium ion battery formed by the layered oxide and the layered oxide coated with other materials. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the drawings needed in the description of the specific embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the application, and those skilled in the art can also obtain other drawings based on these drawings without creative labor.

[0029] Figure 1 The scanning electron microscope image of the positive electrode material provided in Example 1 of the application;

[0030] Figure 2 The interface electron microscope image of the positive electrode material provided in Example 1 of the application;

[0031] Figure 3 The scanning electron microscope image of the positive electrode material provided in Comparative Example 1 of the application. DETAILED DESCRIPTION

[0032] The technical solutions of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of the present application.

[0033] In the description of the present application, it is understood that the meaning of "multiple" is two or more, unless otherwise explicitly and specifically limited.

[0034] The following disclosure provides many different embodiments or examples for implementing different structures of the present application. For the purpose of simplifying the present application, the components and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. In addition, the present application can repeatedly refer to numbers and / or letters in different examples. Such repetition is for the purpose of simplification and clarity, and in itself does not indicate the relationship between the various embodiments and / or settings discussed. In addition, the present application provides examples of various specific processes and materials, but those of ordinary skill in the art can realize the application of other processes and / or the use of other materials.

[0035] One of the main factors affecting the poor cycle performance of O3 phase layered oxides in sodium ion positive electrode materials is the significant volume expansion that occurs during the sodium ion intercalation and deintercalation process. To address this, the prior art proposes coating a layer of coating on the surface of O3 phase layered oxides to suppress the volume expansion and improve the cycle performance. However, since the volume expansion of O3 phase layered oxides for sodium ion batteries is more significant than that of layered oxides for lithium ion batteries, the conventional coating layer has limited effect on suppressing the volume expansion, and is prone to damage, making it difficult to achieve long-term suppression effect, especially in high-temperature or high-rate charging and discharging environments. In addition, the framework structure of O3 phase layered oxides is also prone to collapse due to lattice distortion, affecting the cycle performance and safety performance. Based on this, the present application provides a core-shell structured positive electrode material, which uses a NFPP crystal in the form of a whisker of a polycrystalline particle of O3 phase layered oxides as a coating layer, and forms a Fe-O-P bond between the NFPP and the O3 phase layered oxides; in addition, the applicant has found that when the whisker of NFPP is inserted obliquely into the gap between the crystals of the layered oxides, it can form a certain harrow type structure on the inner core, which can further improve the suppression of the volume expansion and lattice distortion of the layered oxides by the NFPP shell, significantly reduce the volume expansion rate of the positive electrode material, and improve its cycle performance; in addition, it can further optimize its ionic conductivity.

[0036] Specifically, the application provides a core-shell structure positive electrode material, wherein the core-shell structure positive electrode material has an O3 phase layered oxide polycrystal particle as an inner core and a whisker-shaped sodium iron pyrophosphate crystal as an outer shell.

[0037] The whisker-shaped sodium iron pyrophosphate crystal partially located on the interface between the inner core and the outer shell is tilted and embedded in the inner core.

[0038] It should be noted that the polycrystal particle in the application is a secondary particle formed by a layered oxide; that is, the inner core in the positive electrode material provided by the application is a polycrystal material of the O3 phase layered oxide, and there is a crystal interface between each crystal.

[0039] It should be noted that the whisker-shaped sodium iron pyrophosphate (hereinafter referred to as NFPP) crystal in the application refers to a whisker-shaped primary particle or secondary particle with an aspect ratio of not less than 20 formed by growth of the NFPP crystal. The NFPP whisker is stacked in a layered or networked manner on the surface of the inner core, and appears as a nano-fluff structure from the surface of the positive electrode material particle.

[0040] The applicant finds that the core-shell structure positive electrode material provided by the application not only has good cycle performance, but also has obvious optimization in ion conduction rate, and the reason may be that the tilted and embedded whisker-shaped NFPP crystal not only improves the bonding strength of the outer shell and the inner core and the cohesion of the positive electrode material, but also the fluff structure on the outer surface can significantly improve the specific surface area of the core-shell structure positive electrode material, expand the contact area of the positive electrode material particle and the electrolyte, and improve the ion conduction speed. Moreover, due to the fluff structure formed by the NFPP whisker, a plurality of channels through which sodium ions can pass can be formed on the outer shell, reducing the influence of the coating layer on the resistance of the positive electrode material, so that the core-shell structure positive electrode material provided by the application exhibits good mechanical strength, lower volume expansion rate and more excellent high-rate capacity retention rate. In addition, the elastic modulus of the NFPP whisker is high, which can reach 180 GPa, and it can form a mechanical gradient with the O3 phase layered oxide, which may be one of the reasons for inhibiting crack propagation.

[0041] In addition, since the outer shell layer provided by the application is formed by growth and stacking of the NFPP whisker, the aspect ratio is high, and under the same amount of coating layer, the outer shell formed by the NFPP whisker tends to form a networked or layered coating layer, and the proportion of island-shaped coating is smaller, which is more conducive to improving the inhibition of volume expansion of the positive electrode material and more conducive to inhibiting crack propagation.

[0042] It should be noted that the interface between the core layer and the shell layer in the present application refers to the surface of the polycrystalline particles of the O3 phase layered oxide in contact with or near the contact with 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 shell layer in the present application also includes the surface of the embedded NFPP crystal in contact with the O3 phase layered oxide and the surface near the contact.

[0043] In addition, the O3 phase layered oxide provided by the present application is a layered oxide containing nickel and iron elements, and the specific structure general formula is Na x Ni y Fe z Mn a M (1-y-z-a) O2, wherein, the value range of x is 0.9-1.0; the value range of y is 0.1-0.5; the value range of z is 0.1-0.5; the value range of a is 0.1-0.5; the metal element is selected from at least one of aluminum, titanium, vanadium, copper, zinc, tungsten, lanthanum, cerium, zirconium and calcium.

[0044] Preferably, the included angle formed when the NFPP whisker is embedded in the core is 10-60°. It should be noted that the "included angle formed when the NFPP whisker is embedded in the core is 10-60°" herein refers to the included angle formed between the extension direction of the NFPP and the tangent of the surface of the core when the NFPP whisker is embedded in the core at an angle. Specifically, the amount of NFPP whisker embedded in the core at an angle of 10-60° is at least 50% of the amount of NFPP whisker embedded in the core at an angle. Preferably, the included angle formed when the sodium iron phosphate pyrophosphate whisker is embedded in the core is 15-35°.

[0045] In some embodiments, the short diameter of the NFPP whisker is 2-8 nm; the long diameter of the sodium iron phosphate pyrophosphate whisker is 100-500 nm. The thickness of the shell formed by the NFPP whisker is 100 nm-500 nm. It should be noted that the thickness of the shell formed by the NFPP whisker herein refers to the average of the maximum and minimum thickness of the shell layer formed by the NFPP whisker.

[0046] In some embodiments, the diameter of the O3 phase layered oxide polycrystalline particle is 5 μm-40 μm; the particle size of the crystal forming the O3 phase layered oxide polycrystalline particle is 100 nm-1 μm.

[0047] The particle size of the core-shell structure formed anode material is 5 μm-40 μm.

[0048] In some embodiments, the mass percentage of the NFPP whisker in the positive electrode material is 3%-5%. The mass percentage of the NFPP whisker in the positive electrode material should not be too low. If the mass percentage of the NFPP whisker is too low, the NFPP whisker is formed on the surface of the inner core, which is easy to cause insufficient coating degree, and is not conducive to improving the ion conduction rate of the positive electrode material.

[0049] In some other embodiments, the positive electrode material of the core-shell structure is further doped with hexavalent tungsten element, and the doping amount of the hexavalent tungsten element is 0.5-2.0wt%. At least part of the doped hexavalent tungsten element exists on the interface between the inner core and the shell, forming a Ni-O-W-P bond.

[0050] In the present application, the doped tungsten element exists on the interface between the inner core and the shell, and in addition, can exist in the inner core and / or in the shell. Preferably, the doped tungsten element is enriched on the interface between the inner core and the shell, so that it forms a large number of [Ni-O-W-P] charge bridges on the interface between the inner core and the shell, which can better reduce the interface resistance. In some other embodiments, the element concentration of the doped tungsten element decreases in turn from the interface between the inner core and the shell to the inner core and / or the shell.

[0051] In some other embodiments, the positive electrode material of the core-shell structure is further doped with hexavalent tungsten element, and the doping amount of the hexavalent tungsten element is 0.5-2.0wt%. At least part of the doped hexavalent tungsten element exists on the interface between the inner core and the shell, forming a Ni-O-W-P bond.

[0052] It should be noted that the tungsten element doped in the positive electrode material of the core-shell structure provided in the present application not only can reduce the interface resistance of the positive electrode material, but also can further improve the cycle stability of the positive electrode material, especially the high-potential cycle stability.

[0053] In order to enable those skilled in the art to better implement the present application, the present application further provides a preparation method of the tungsten-doped positive electrode material of the core-shell structure, which comprises the following steps:

[0054] (1) Preparation of layered oxide precursor

[0055] The core raw materials, such as a sodium source, an iron source, a nickel source, a manganese source, and a dopant (if any, no dopant is needed if not) 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 the dispersion liquid, and the Zeta potential is adjusted to +35 mV or more by the added dispersion liquid, grinding, spray drying, to obtain the layered oxide precursor particles;

[0056] (2) Preparation of sodium iron phosphate pyrophosphate precursor

[0057] The shell layer raw materials are provided in a molar ratio of 3.5-4:2.5-3:3.8-4.2 for the sodium source, the iron source, and the phosphorus source, and the shell layer raw materials are stirred with the second solution in a water bath at 40-90℃ for 4-24h to obtain a transparent sol-like sodium iron phosphate pyrophosphate precursor;

[0058] (3) The sodium iron phosphate pyrophosphate precursor and the layered oxide precursor particles are mixed in a mass ratio of 1:10-50, ultrasonic dispersion for 30-180min, and concentrated at 80-85℃ to increase the weight of the sodium iron phosphate pyrophosphate precursor by 5-8%, so that the sodium iron phosphate pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles, to obtain a composite material;

[0059] (4) Sintering

[0060] The composite material is vacuum dried for 4-24h, and heated to 400-450℃ in an environment of 91.5-95% N2+5-8.5% O2, and sintered for 4-12h, and then the sintering atmosphere is adjusted to 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 4-12h, and annealed at 200-300℃ for 2-8h, to obtain the core-shell structure positive electrode material in the present application.

[0061] The sodium source in step (1) 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, diiron trioxide, iron 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, dimanganese 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-methyl pyrrolidone, and ethanol. The dispersion liquid can be at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol.

[0062] The sodium source in step (2) 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, ferrous 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-methyl pyrrolidone, and ethanol.

[0063] Preferably, the sodium source in step (1) can be the same as in step (2) to reduce the introduction of more impurities.

[0064] In a second aspect, the present application also provides a positive electrode including the positive electrode material of the first aspect.

[0065] In a third aspect, the present 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 sheet is the positive electrode sheet of the second aspect.

[0066] The electrolyte can be a liquid electrolyte, a solid electrolyte, or other electrolytes. The separator can be a polymer separator or a polymer separator provided with a ceramic coating. The negative electrode can include a current collector and a coating layer coated on the surface of the current collector. The current collector can include a metal foil such as an aluminum foil, a copper foil, or the like. The coating layer can include a negative electrode active material, a negative electrode binder, a negative electrode conductive agent, or the like. The negative electrode active material can be a carbon material or other material capable of deintercalating sodium ions. Similarly, the positive electrode can also include a current collector and a coating layer coated on the surface of the current collector. The coating layer can include the positive electrode material and a binder described above.

[0067] In a fourth aspect, the present application also provides a battery device including the sodium-ion battery of the third aspect or including the positive electrode of the second aspect or the positive electrode material of the first aspect.

[0068] The power-consuming device provided by the present application includes a battery module composed of a plurality of batteries to provide power for the device. The power-consuming device can be, for example, a mobile communication device, a computer, an electric vehicle, an electric train, an energy storage system, or the like.

[0069] In order to better understand the innovations of the present application, the technical solutions of the present application are described in further detail below in combination with embodiments. The embodiments of the present application described in detail below are exemplary and are used to explain the present application, and cannot be understood as a limitation of the present application.

[0070] Embodiment 1

[0071] The present embodiment provides a positive electrode material, and a preparation method thereof is as follows:

[0072] (1) Preparation of layered oxide precursor

[0073] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, and manganese sesquioxide were provided in a molar ratio of Na:Ni:Fe:Mn = 1:0.33:0.33:0.34, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide, and ethanol solution were added to a planetary ball mill, an ammonium polyacrylate dispersant was added, the Zeta potential was adjusted to +40 mV or more, the rotation speed was 400 rpm, and grinding was performed for 24 hours to obtain a wet mill base; the wet mill base was spray dried under the following conditions: inlet air temperature: 220-250°C, outlet air temperature: 80-100°C, atomization pressure: 0.3-0.5 MPa, nozzle diameter: 0.5 mm, and peristaltic pump control feed rate: 10 mL / min to obtain spherical layered oxide precursor with an average particle size of D50: 10 μm.

[0074] (2) Preparation of sodium iron pyrophosphate phosphate precursor

[0075] Ferrous oxalate dihydrate and sodium dihydrogen phosphate were dissolved in a 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, thereby obtaining a nanomicellar sodium iron pyrophosphate phosphate precursor;

[0076] (3) The layered oxide precursor was added to the transparent sol at a mass ratio of 1:10, ultrasonic dispersion was performed for 30 minutes, and the sol was concentrated at 50°C under reduced pressure until the sol completely coated the surface of the particles (control the weight gain in the range of 5-8%); the sodium iron pyrophosphate phosphate precursor was uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material;

[0077] (4) Sintering

[0078] The composite material was vacuum dried at 100°C for 12 hours; then the temperature was increased to 400°C, and the first sintering was performed in an atmosphere of 95% N2+5% O2 for 4 hours, then the temperature was increased to 600°C for sintering for 6 hours, while N2:CO2:H2O vapor = 96:3.5:0.5 was introduced, and finally the temperature was annealed at 300°C for 2 hours to obtain the positive electrode material NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7) in the present application, the main insertion angle of the sodium iron pyrophosphate phosphate whisker is about 30°, as shown in Figure 1 、 Figure 2 .

[0079] Example 2

[0080] The embodiment is compared with embodiment 1, and in step 4, the composite material is vacuum dried at 100 DEG C for 12 hours; then heated to 400 DEG C, and the first sintering is carried out in a 93.5% N2+6.5% O2 atmosphere for 4 hours, then heated to 600 DEG C for sintering for 6 hours, while N2:CO2:H2O vapor = 96:3:1 is introduced, finally annealed at 300 DEG C for 2 hours, and the remaining steps are the same as those of embodiment 1, to obtain the positive electrode material in the application:

[0081] NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of sodium iron phosphate pyro-phosphate whisker is about 35 DEG.

[0082] Example 3

[0083] The embodiment is compared with embodiment 1, and in step 4, the composite material is vacuum dried at 100 DEG C for 12 hours; then heated to 400 DEG C, and the first sintering is carried out in a 93.5% N2+6.5% O2 atmosphere for 4 hours, then heated to 600 DEG C for sintering for 6 hours, while N2:CO2:H2O vapor = 96:3.8:0.2 is introduced, finally annealed at 300 DEG C for 2 hours, and the remaining steps are the same as those of embodiment 1, to obtain the positive electrode material in the application: 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of sodium iron phosphate pyro-phosphate whisker is about 40 DEG.

[0084] Example 4

[0085] The embodiment is compared with embodiment 1, and in step (1), the molar ratio of sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide and manganese trioxide is 1:0.4:0.3:0.3, and the remaining steps are the same as those of embodiment 1, to obtain the positive electrode material in the application:

[0086] NaNi 0.4 Fe 0.3 Mn 0.3 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of sodium iron phosphate pyro-phosphate whisker is 30 DEG.

[0087] Example 5

[0088] The raw materials used in step (1) of this embodiment compared to embodiment 1 are: sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide, and tungsten trioxide, in a molar ratio of 1:0.3:0.3:0.3:0.1, and the remaining steps are the same as in embodiment 1, to obtain the positive electrode material in this application: NaNi 0.3 Fe 0.3 Mn 0.3 W 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of the sodium iron phosphate pyrophosphate whisker is about 30° or so.

[0089] Example 6

[0090] This embodiment changes the doping amount of tungsten element compared to embodiment 1, which is to provide sodium dihydrogen phosphate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide, and tungsten trioxide in a molar ratio of Na:Ni:Fe:Mn:W = 1:0.3:0.3:0.35:0.05 in step (1), and the remaining steps are the same as in embodiment 1, to obtain a positive electrode material with a core-shell structure:

[0091] NaNi 0.3 Fe 0.3 Mn 0.35 W 0.05 O2@1 / 11Na4Fe3(PO4)2(P2O7), the main insertion angle of the sodium iron phosphate pyrophosphate whisker is about 30° or so.

[0092] Example 7

[0093] This embodiment adjusts the mixing mass ratio of the layered oxide precursor particles and the sodium iron phosphate pyrophosphate precursor to 20:1 compared to embodiment 1, and the remaining steps are the same as in embodiment 1, to obtain a positive electrode composite material: NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 21Na4Fe3(PO4)2(P2O7), the main insertion angle of the sodium iron phosphate pyrophosphate whisker is about 30° or so.

[0094] Example 8

[0095] This embodiment provides 1 mole of sodium source, 0.3 moles of iron source, 0.3 moles of nickel source, 0.3 moles of manganese source, and 0.1 moles of Cu source in the step of preparing the layered oxide precursor compared to embodiment 1. The remaining steps are the same as in embodiment 1, to obtain a positive electrode material: NaNi 0.3 Fe 0.3 Mn 0.3 Cu 0.1O2@1 / 11Na4Fe3(PO4)2(P2O7), which has a main insertion angle of 30° for the sodium iron phosphate pyrophosphate whisker.

[0096] Comparative Example 1

[0097] Comparing with Example 1, in step 4 of the present comparative example, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C, and first sintered in a 95% N2+5% O2 atmosphere for 4 hours, then heated to 600°C for 6 hours while passing 98% N2:2% CO2, and finally annealed at 300°C for 2 hours, and the remaining steps are the same as Example 1, to obtain a core-shell structured positive electrode material: NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), which has a main insertion angle of 30° for the sodium iron phosphate pyrophosphate whisker.

[0098] Comparative Example 2

[0099] Comparing with Example 1, in step 4 of the present comparative example, the composite material is vacuum dried at 100°C for 12 hours; then heated to 400°C, and first sintered in a 95% N2+5% O2 atmosphere for 4 hours, then heated to 600°C for 6 hours while passing 98% N2:2% CO2, and finally annealed at 300°C for 2 hours, and the remaining steps are the same as Example 1, to obtain a core-shell structured positive electrode material: NaNi

[0100] NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), which has a main insertion angle of 30° for the sodium iron phosphate pyrophosphate whisker.

[0101] Comparative Example 3

[0102] Comparing with Example 1, in step 4 of the present comparative example, the first calcination condition is nitrogen, and the second calcination condition is the same as Example 1, to obtain a positive electrode material:

[0103] NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7), which has a main insertion angle of 30° for the sodium iron phosphate pyrophosphate whisker. Figure 3

[0104] Comparative Example 4

[0105] ​Comparative Example 2: The sodium iron phosphate precursor in step 2 is prepared by dissolving iron phosphate and sodium dihydrogen phosphate in a 0.1 mol / L citric acid solution at a 1:1 molar ratio and stirring in a water bath at 80°C for 4 hours to obtain a nanomicellar sodium iron phosphate precursor; (3) The sodium iron phosphate precursor is added to the transparent sol at a mass ratio of 1:10, ultrasonic dispersion for 30 minutes, and concentrated at 50°C under reduced pressure until the sol completely coats the particle surface (with a weight gain of 5-8%); 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 positive electrode material:

[0106] NaNi 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0107] Comparative Example 5

[0108] Comparative Example 2: The sodium iron phosphate precursor in step 2 is prepared by dissolving iron phosphate and sodium dihydrogen phosphate in a 0.1 mol / L citric acid solution at a 1:1 molar ratio and stirring in a water bath at 80°C for 4 hours to obtain a nanomicellar sodium iron phosphate precursor; (3) The sodium iron phosphate precursor is added to the transparent sol at a mass ratio of 1:10, ultrasonic dispersion for 30 minutes, and concentrated at 50°C under reduced pressure until the sol completely coats the particle surface (with a weight gain of 5-8%); 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 positive electrode material: 0.33 Fe 0.33 Mn 0.34 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0109] Comparative Example 6

[0110] Comparative Example 2: The sodium iron phosphate precursor in step 2 is prepared by dissolving iron phosphate and sodium dihydrogen phosphate in a 0.1 mol / L citric acid solution at a 1:1 molar ratio and stirring in a water bath at 80°C for 4 hours to obtain a nanomicellar sodium iron phosphate precursor; (3) The sodium iron phosphate precursor is added to the transparent sol at a mass ratio of 1:10, ultrasonic dispersion for 30 minutes, and concentrated at 50°C under reduced pressure until the sol completely coats the particle surface (with a weight gain of 5-8%); 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 positive electrode material:

[0111] NaFe 0.5 Mn 0.4 W 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0112] To verify the excellent performance of the core-shell structured positive electrode material provided in the present application, the applicant prepared sodium ion batteries using the core-shell structured positive electrode materials obtained in Examples 1-8 and the positive electrode materials of Comparative Examples 1-6 as positive electrode active materials, which were prepared as follows:

[0113] (1) Preparation of the positive electrode: the positive electrode material, conductive carbon black, polyvinylidene fluoride, and nanometer carbon tube are mixed, stirred, and kneaded according to a mass ratio of 95:1:3:1, N-methyl pyrrolidone solvent is added, the slurry is adjusted to a positive electrode slurry with a solid content of 60% and a viscosity of 6000 mPa·s; the positive electrode slurry is transferred and coated onto a 15 μm thick carbon-coated aluminum foil using a transfer coating method; the coated electrode sheet is rolled to a compaction density of 3 mg / cm 3 ; the rolled electrode sheet is die-cut into an electrode sheet with a length of 48 mm and a width of 38 mm for standby use.

[0114] (2) Preparation of the negative electrode sheet: the hard carbon material, conductive carbon black, carboxymethyl cellulose, and butadiene-styrene rubber are mixed, stirred, and kneaded according to 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 sheet is rolled to a compaction density of 1 mg / cm2; the rolled electrode sheet is die-cut into an electrode sheet with a length of 50 mm and a width of 40 mm for standby use.

[0115] (3) Preparation of the sodium ion battery: the striped positive electrode sheet and the negative electrode sheet are stacked on a stacker, the separator is a PP / PE / PP three-layer material, a soft package battery cell is formed, electrolyte (composition: carbonate solvent and 1M sodium hexafluorophosphate) is injected, and the sodium ion battery is obtained after packaging, drying, formation, and capacity distribution.

[0116] The above-prepared sodium ion battery is subjected to morphology characterization test, cycle performance test, charge-discharge capacity test, and rate performance test; the specific test methods are as follows:

[0117] (1) Cohesive force test

[0118] (2) The cohesive force of the sample is tested according to the test method recorded in GB / T 19587-2004.

[0119] The specific surface area test is performed according to the test method for determining the specific surface area of solid substances by gas adsorption BET method in GB / T 19587-2004.

[0120] (3) Cycle performance test

[0121] The normal temperature cycle performance and the high temperature cycle performance at 60°C are tested according to the method recorded in GB / T 33822-2017.

[0122] (4) Ion conductivity test

[0123] The ion conductivity of the sample is tested according to the test method recorded in GB / T 31486-2015.

[0124] (5) Interface impedance test

[0125] The interface impedance of the sample was tested according to the test method described in GB / T 39482.3-2020.

[0126] (6) Volume expansion rate test

[0127] Battery volume test: the weight m0 of the battery after expelling water was measured by using an electronic balance, and the volume V0 of the battery after expelling water was calculated by the formula: V0 = m0 / ρ0.

[0128] The volume V1 of the battery after 1000 cycles was tested by the same method.

[0129] The volume expansion rate can be calculated by the formula (V1-V0) / V0.

[0130] The test results are shown in Table 1.

[0131] From Figures 1-2 It can be seen that the morphology of the coating layer on the surface of the core-shell structure positive electrode material provided by the application is a whisker structure, and part of the whiskers are inserted into the inner core obliquely, and a fluff structure is formed on the surface.

[0132] Table 1

[0133]

[0134] As can be seen from Table 1, the positive electrode material provided by the application examples 1-8 adopts the specific structure of the whisker of the sodium iron pyrophosphate phosphate coating on the surface of the layered oxide to form a shell layer, and the NFPP whisker is inserted into the inner core of the layered oxide obliquely. Compared with Comparative Examples 1-6, it can be seen that the positive electrode material obtained by the application can maintain the cycle capacity retention rate, and has lower interface impedance and lower volume expansion. In addition, as can be seen from Example 1 and Example 7, the coating amount of NFPP in the positive electrode material is preferably 8-15wt%, which is more conducive to the improvement of the ionic conductivity of the positive electrode material. The reason may be that when the content of NFPP is in this range, an ion / electron double continuous network can be formed in the positive electrode material, which is beneficial to ion migration.

[0135] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the application, and not to limit them; although the application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions described in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the 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 whisker-like sodium iron pyrophosphate crystals located at the interface between the core and the shell are obliquely embedded in the core; The included angle formed when the whisker-like sodium ferric pyrophosphate is embedded in the core is 10-60°.

2. The core-shell structured cathode material according to claim 1, characterized in that, The included angle formed when the whisker-like sodium iron pyrophosphate is embedded in the core is 15-35°.

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

4. The core-shell structured cathode material according to claim 3, 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.

5. 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.

6. 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.

7. 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.

8. The core-shell structured cathode material according to any one of claims 1-2 and 4-7, 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.

9. The core-shell structured cathode material according to claim 8, 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.

10. 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 9.

11. 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 according to claim 10.

12. The sodium-ion battery according to claim 11, characterized in that, The volume expansion rate after 1000 cycles at 60℃ is no higher than 5%.

13. A battery module, characterized in that, The invention includes a sodium-ion battery according to claim 11 or 12, or a positive electrode material including the positive electrode of claim 10 or any one of claims 1 to 9 with a core-shell structure.

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